Zinc-based materials for electrocatalytic reduction reactions: progress and prospects

Abstract

The persistent energy crisis and environmental pollution pose significant challenges for modern society. Developing efficient methods for electrochemical energy conversion presents a promising solution to address these pressing issues. In the past few years, various electrocatalytic reduction reactions such as the hydrogen evolution reaction (HER), oxygen reduction reaction (ORR), nitrogen reduction reaction (NRR), nitrate reduction reaction (NO₃⁻RR), and carbon dioxide reduction reaction (CO₂RR) have been investigated to create a pollution free green society and environment. Zn-based materials have garnered significant attention as potential candidates in the electrocatalytic reduction reactions owing to their precisely tuned structural and electronic properties, three-dimensional architectures, large surface areas, abundant active sites, high stability, and enhanced mass transport and diffusion capabilities. Numerous studies have been published investigating the potential of Zn-based materials in various electrocatalytic reduction reactions. However, there is a lack of comprehensive reviews systematically exploring the use of Zn-based materials in electrocatalytic reduction reactions. This review explores the structure–property–performance correlations of zinc-based catalysts, emphasizing their role in various electrocatalytic reduction reactions. We discuss the influence of structural modifications, such as doping, alloying, heterostructure formation, and morphological control, on the catalytic activity, stability, and selectivity of these materials. Special focus is given to the electronic structure modulation, active site optimization, and charge transfer mechanisms that underpin their performance. Recent advancements in synthesis techniques and characterization methods are highlighted to illustrate how tailored design strategies enhance catalytic efficiency. By presenting a comprehensive overview of zinc-based catalysts, this review aims to provide insights into their structure–performance relationships and offer guidance for the rational design of next-generation electrocatalysts for sustainable energy and chemical production.

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Baghendra Singh

Dr. Baghendra Singh earned his PhD in 2023 from the Indian Institute of Technology (BHU) Varanasi, under the guidance of Dr. Arindam Indra. He is currently serving as a National Post-Doctoral Fellow at the Department of Chemistry at IIT Kanpur, working under the mentorship of Dr. Apparao Draksharapu. Over the course of his academic journey, he has been the recipient of several prestigious fellowships awarded by the Government of India, including the INSPIRE Fellowship and the NPDF. In recognition of his scientific contributions, Dr. Singh was selected by the Japan Society for the Promotion of Science (JSPS) to participate in the 14th HOPE Meeting with Nobel Laureates, representing India among a selected group of young researchers. His ongoing research focuses on the development of advanced electrocatalysts for water splitting, hybrid water electrolysis, and energy storage applications.

Apparao Draksharapu

Dr. Apparao Draksharapu obtained his master's degree from the University of Hyderabad, India, where he conducted research on emulsion polymerization. He earned his PhD in 2013 from the University of Groningen, Netherlands, under the supervision of Prof. Wesley R. Browne, focusing on the electrochemical and photochemical properties of inorganic metal complexes with relevance to biological systems. Following his doctoral studies, he pursued postdoctoral research at the University of Minnesota, USA, working with Prof. Lawrence Que, Jr., on bioinspired iron complexes. Currently, Dr. Draksharapu is an Assistant Professor at the Department of Chemistry at IIT Kanpur. His research is primarily directed toward the development of high-valent 3d-metal transient species for catalytic applications and energy conversion processes.

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1. Introduction

Electrocatalytic reduction reactions play a pivotal role in energy conversion technologies, critical for transitioning towards sustainable and renewable energy systems.^1–8 These reactions are essential in processes such as hydrogen production, fuel cells, carbon capture, and the synthesis of valuable chemicals, all of which contribute to the broader goals of energy efficiency, decarbonization, and environmental sustainability. The electrocatalytic reduction reactions mainly involve the hydrogen evolution reaction (HER), oxygen reduction reaction (ORR), nitrogen reduction reaction (NRR), nitrate reduction reaction (NO₃⁻RR), and carbon dioxide reduction reaction (CO₂RR).^1–4,9 The HER is essential for creating hydrogen gas (H₂), a clean fuel that may be utilized in combustion engines and fuel cells, among other energy applications.⁹ Hydrogen is often referred to as a “green” energy carrier when produced through electrocatalytic water splitting using electricity. This process offers a sustainable alternative to fossil fuels. The ORR is the central process for the operation of fuel cells and metal–air batteries.^6,7 These devices are key components of energy conversion systems. The ORR, which converts chemical energy into electricity with high efficiency and minimal emissions, takes place at the cathode in solid oxide and proton exchange membrane fuel cells. In zinc–air batteries (ZABs), lithium–air batteries (LiABs), aluminum air batteries (AlABs), and sodium air batteries (NaABs), the ORR is responsible for the discharge process.^6,7 These batteries are attractive for storing energy at a practical level because of their large energy density and potential for grid-scale renewable energy storage.

The CO₂RR is essential for mitigating climate change through carbon capture and utilization (CCU).² The CO₂RR aims to convert CO₂, a greenhouse gas, into value-added products and fuels such as carbon monoxide (CO), methane (CH₄), or formic acid (HCOOH). By converting CO₂ into useful products, this reaction helps address the growing concern of global warming and contributes to climate mitigation strategies.² The NRR allows for the production of ammonia (NH₃) electrochemically, which is essential for fertilizers and other industrial processes.^1,3,4 Traditional ammonia production, via the Haber–Bosch process, is energy-intensive and contributes significantly to the world's CO₂ emissions. The electrocatalytic NRR offers a sustainable path for ammonia production. The NRR can be driven by renewable electricity, drastically reducing the energy input and greenhouse gas emissions associated with ammonia synthesis.^3,4 The NO₃⁻RR is vital for environmental remediation, particularly in reducing nitrate contamination in water sources, a significant environmental and public health issue.⁵ Nitrate pollution, commonly caused by agricultural runoff and industrial processes, contaminates water. The NO₃⁻RR can convert harmful nitrates into benign products like nitrogen gas (N₂) or ammonia (NH₃).⁵ Electrocatalytic reduction reactions are key in transitioning away from fossil fuels by enabling renewable and sustainable alternatives for energy production, chemical synthesis, and storage.^1,2,4,6

However, an active electrocatalyst is needed to drive these electrocatalytic reduction reactions efficiently. Researchers have explored a wide range of transition and noble metal-based materials for electrocatalytic reduction reactions in this context.^10–20 Over the past few years, zinc-based materials have garnered much interest in electrocatalytic reduction processes because of their readily available, inexpensive, environmentally friendly, and customizable structural and electrical characteristics. These materials, whether in the form of zinc-based oxides, sulfides, phosphides, metal–organic frameworks (MOFs), or alloys, show promise across several key electrocatalytic reduction reactions (HER, ORR, NRR, NO₃⁻RR, and CO₂RR) that are critical for energy conversion.^10–20 In all these reduction reactions, Zn-based materials have shown excellent performance.^21–30 Zinc-based materials have gained significant attention in electrocatalysis due to their unique properties, cost-effectiveness, and versatility. These materials play a crucial role in various reduction reactions, including the CO₂ reduction reaction, NRR, and ORR, contributing to sustainable energy and chemical production.

Zinc is widely available, reducing reliance on expensive and scarce noble metals such as platinum or palladium.^{21–28,30–37} Zinc-based catalysts provide an economically viable alternative for industrial applications, especially in large-scale energy and chemical production. Zinc-based materials include oxides, phosphides, sulfides, MOFs, and alloys, each tailored for specific reactions.^{21–28,30–37} Zinc materials readily accommodate structural defects, which improve adsorption sites and reaction kinetics. Zinc-based catalysts can be functionalized with other elements or compounds to enhance activity, stability, and selectivity. Combining Zn with conductive materials (e.g., carbon nanotubes or graphene) improves charge transfer and catalytic efficiency. Incorporation of transition metals like Fe, Co, or Ni into Zn-based structures fine-tunes electronic properties and catalytic behavior. Zinc-based materials are indispensable for advancing electrocatalytic reduction reactions attributed to their unique structural and electronic properties.^{21–28,30–37} Their abundance, tunability, and compatibility with green technologies make them a cornerstone for sustainable energy and chemical production.

However, a fundamental understanding of the structure–property–performance correlation in zinc-based materials is essential for their rational design and optimization. The intrinsic properties of zinc, such as its electronic structure, coordination environment, and redox behavior, significantly influence its catalytic activity and stability. Additionally, structural modifications, including doping, alloying, heterostructure formation, and nanoscale engineering, have been employed to fine-tune these materials for enhanced performance.^{21–28,30–37} These modifications can alter active site availability, improve charge transfer, and modulate binding energies of reaction intermediates, all of which are critical for catalytic efficiency. While Zn-based materials have been extensively studied for various electrocatalytic reduction reactions, much of the research has focused on their structural, morphological, and electronic tuning.^{21–28,30–37} The challenges remain in achieving a comprehensive understanding of the mechanisms by which zinc-based materials catalyze reduction reactions and in translating their laboratory-scale performance to practical applications. Recent progress in synthesis techniques, advanced characterization methods, and computational modeling has provided valuable insights into the interplay between material structure and catalytic behavior.

However, a thorough review that systematically addresses their design, properties, applications, and challenges in electrocatalytic reduction reactions is still missing. This review aims to fill that gap by focusing on Zn-based materials and critically analyzing their design, properties, and applications in water-splitting processes (Fig. 1). It highlights various Zn-based materials, such as oxides, phosphides, chalcogenides, alloys, and MOFs, with a special focus on their structure, morphology, and electronic properties. This overview aims to thoroughly grasp the properties and uses of various materials through important examples. This review also discusses recent advancements, persistent challenges, and the intricate relationship between structure, activity, and stability while outlining future research directions.

Fig. 1 Pictorial depiction showing the various objectives of this review.

This review aims to provide a comprehensive overview of zinc-based materials for electrocatalytic reduction reactions, focusing on their structure–property–performance relationships. We discuss the impact of various structural modifications on catalytic activity, selectivity, and durability, highlighting key advancements in this field. By synthesizing knowledge from experimental and theoretical studies, this review seeks to guide the development of next-generation zinc-based electrocatalysts for sustainable energy and chemical production.

2. Why should zinc be chosen for material design?

Materials based on zinc have become more significant in electrocatalytic reduction reactions due to their unique properties, including abundant availability, low cost, environmental friendliness, and versatile electrochemical behavior.^{21–28,30–37} They serve as promising candidates in various reduction processes. Zn is an abundant and relatively inexpensive metal compared to other metals.^38,39 Its widespread availability makes it an economically viable choice for large-scale applications in designing Zn-based materials for electrocatalytic reduction reactions, particularly for sustainable energy technologies.^{21–28,30–37} The leaching of zinc by evaporation at high temperatures during the synthesis of catalytic materials can significantly influence the final material's structure, morphology, and catalytic properties.^40–42

During synthesis, zinc leaching is often a controlled process and can result in beneficial modifications to the material's structure.^40–42 Zinc leaching can create porous structures within the catalyst. When removed during synthesis, zinc leaves voids or pores that can expand the material's surface area, giving catalytic processes additional active sites. The development of metal oxide/hydroxide phases due to leaching might enhance catalytic activity. For example, in zinc–cobalt materials, the leaching of zinc during synthesis can expose more active cobalt sites.^40,41 Leaching of zinc can alter the electronic properties of the material, optimizing its catalytic behavior.

The charge distribution, band gap, and electron density in the catalyst can be tuned, improving its ability to adsorb and activate reactants. Zinc can be combined with other metals to create bimetallic or multimetallic catalysts that exhibit synergistic effects.^{21–28,30–37} When Zn is introduced with other metals, it provides the available binding sites for the process intermediates by changing the electronic states of the different metals.^{21–28,30–37} Overall, zinc is a versatile element used in the design of various catalytic materials due to its abundance, cost-effectiveness, tunable electronic properties, and ability to form stable compounds.^{21–28,30–37}

3. Properties of Zn-based materials

Owing to its unique properties, Zn has been employed to design a series of various materials such as oxides, phosphides, and chalcogenides for electrocatalytic reduction reactions.^{21–28,30–37} Zn-based materials offer altered electronic structure, large surface area, tunable properties, and desirable morphology (Fig. 2). In most materials, Zn can be evaporated during pyrolysis causing a porous structure. The porosity provides exposed active sites improving the adsorption and release of reactive intermediates and products, respectively.^{21–28,30–37} Intrinsic and extrinsic defects in Zn-based materials, such as oxygen vacancies in ZnO or Zn-doping in other materials, enhance catalytic activity by modulating electronic properties and creating more reactive sites. Zinc-based materials can be tailored into various structural forms, such as nanorods, nanowires, nanosheets, and nanoparticles. These designs enhance catalytic performance by increasing surface area and exposing more active sites.^{21–28,30–37} Porous zinc-based materials, in particular, improve mass transport and provide abundant reactive sites, benefiting catalytic processes. Zinc-based phosphides, on the other hand, exhibit high electrical conductivity and excellent water reduction activity. Moreover, Zn-based metal–organic frameworks (MOFs) are valued for their high porosity, customizable structures, and potential as precursors for derivative materials.^{21–28,30–37} Zinc-based materials, especially oxides and hydroxides, often have poor electrical conductivity, which hampers charge transfer and reduces overall catalytic efficiency. The need for conductive substrates or additional conductive additives complicates material design and increases production costs. Zinc-based materials can suffer from structural instability under harsh reaction conditions, including acidic or alkaline media. Zinc tends to dissolve or undergo phase changes, particularly during prolonged operation, leading to catalyst degradation and reduced durability.^{21–28,30–37} Many zinc-based catalysts have limited active sites for catalytic reactions. This results in low turnover frequencies (TOF) and reduced overall catalytic activity. Achieving high selectivity for desired products (e.g., specific reduction products in CO₂ or N₂ reduction) can be challenging due to competing side reactions, such as hydrogen evolution.^{21–28,30–37} Tailoring zinc-based materials for specific reaction pathways remains a key challenge.

Fig. 2 Schematic illustration depicting the properties of Zn-based materials for electrocatalytic reduction reactions.

Incorporation of conductive supports (e.g., carbon-based materials) in Zn-based materials can improve conductivity. Doping with other metals or non-metals enhances stability and active site density.^{21–28,30–37} Designing hybrid structures or composites mitigates structural instability and poisoning effects. In-depth mechanistic studies and computational modeling are very helpful in rational catalyst design. Compared to other emerging electrocatalysts such as transition metal chalcogenides, single-atom catalysts (SACs), and metal–nitrogen–carbon (M–N–C) systems, Zn-based materials exhibit several unique advantages and trade-offs. Zn's moderate binding energy with key intermediates makes it particularly selective for reactions like the CO₂RR to CO and NRR, while its low hydrogen evolution activity enhances faradaic efficiency. However, Zn-based catalysts generally suffer from lower intrinsic conductivity and limited multi-electron product selectivity. In contrast, transition metal chalcogenides offer higher electrical conductivity and tunable layered structures that are beneficial for the HER and the ORR but often require stabilization strategies under long-term operation. Single-atom catalysts, including Zn–N₄ moieties, demonstrate high atom utilization and excellent selectivity, though their synthesis and scalability remain challenging. Meanwhile, M–N–C systems, particularly those based on Fe, Co, or Ni, have demonstrated superior activity and durability in the ORR and CO₂RR, attributed to their rich active sites and conductive carbon matrix. Overall, while Zn-based materials show promising selectivity and earth abundance, further advancements in structure engineering and electronic modulation are required to match the performance benchmarks set by more mature catalyst families like SACs and M–N–C materials.

4. Design of Zn-based materials

The design of Zn-based materials for electrochemical reduction reactions involves careful selection of composition, morphology, and structure.

4.1. Approaches of design

Zn-based materials are designed based on several structural, electronic, and morphological approaches so that one can get superior electrocatalytic performance (Fig. 3).

Fig. 3 Schematic illustration depicting the designing of Zn-based materials using various approaches.

4.1.1. Structural design approach. It is well established that Zn-based MOFs offer high porosity, tunable structure, and abundant active sites. Therefore, bimetallic MOFs are designed by introducing transition metals (e.g., Zn–Co, Zn–Fe, and Zn–Ni) to improve catalytic activity and stability.^21–28 Furthermore, Zn-based MOF-derived materials are synthesized from Zn-MOFs. Doping with heteroatoms (N, S, and P) modifies their electronic structure, introducing localized charge density variations and improving electron transfer kinetics. Nitrogen, sulfur, and phosphorus can donate or withdraw electrons, creating electroactive sites and enhancing electrical conductivity to improve the reactivity. Zn-based oxides are prepared using air calcination to create porous and hollow structures improving the surface adsorption properties.^21–28 Porous and hollow structures provide a high surface area-to-volume ratio, exposing more active sites for reactant interaction. These architectures also allow efficient mass transport of ions and gases through the catalyst, minimizing diffusion limitations and improving overall catalytic efficiency. Templating and controlled synthesis of Zn-based hydroxides leads to the formation of layered hydroxides with enhanced electrochemical properties.^21–28 Tailored interlayer spacing improves ion accessibility and diffusion. The layered structures also offer more exposed basal planes and edge sites, which are known to be catalytically active.

The formation of heterostructures by integrating Zn-based materials with other materials or conductive substances is also adopted to get excellent catalytic activity and stability. Combining Zn-based materials with conductive or catalytically active phases creates synergistic interfaces that facilitate charge separation, improved conductivity, and optimized adsorption energies of intermediates. These interfacial effects often lower energy barriers and enhance reaction kinetics. Surface engineering such as coating with carbon layers and defect engineering such as vacancy creation are the main approaches utilized to design Zn-based materials. Surface engineering provides unsaturated coordination sites, which often act as active centers by increasing the binding affinity for reactants or intermediates. These defects also modulate the local electronic environment, facilitating charge redistribution and lowering reaction energy barriers.

4.1.2. Morphological design approach. Zn-based materials are designed with varying morphology including 0D, 1D, 2D, and 3D architectures. The morphological tuning offers increased active surface area by designing hierarchical structures.^21–28 The core–shell nanostructures are also designed with shell modifications to create a high surface area. As already mentioned, porous morphology provides excellent surface adsorption properties and exposed active sites; designing Zn-based materials with a highly porous nature is adopted to ensure efficient mass diffusion in catalytic processes.^30–37

4.1.3. Electronic structure design approach. During the synthesis of Zn-based materials, one should take care of the modulation of electronic structure.^30–37 The Zn-based materials with altered electronic features provide excellent catalytic performance. Doping or substitution of the other metals with Zn atoms alters the local electronic environment and redistributes electron density. Doping can modulate the d-band center relative to the Fermi level. A closer d-band center (toward the Fermi level) typically enhances adsorption strength for intermediates, which can be beneficial or detrimental depending on the reaction. The introduction of Zn²⁺ modifies the local charge density, affecting the electrostatic potential at active sites and thus reactivity.^30–37 Doping can introduce new energy states in the bandgap or decrease the bandgap altogether, leading to enhanced conductivity, which is essential for electron transport in electrocatalysis. Introducing vacancies disrupts the periodicity of the lattice, altering local coordination environments and electronic states. Oxygen vacancies create shallow donor levels within the bandgap, facilitating charge carrier generation and improving electronic conductivity.^30–37 Vacancies cause uneven electron distribution, resulting in localized charge polarization that can serve as active centers for adsorption and activation of small molecules. The presence of defects changes the symmetry and occupancy of d-orbitals, potentially shifting the d-band center and modifying the binding strength with intermediates. Introducing non-metal heteroatoms into Zn-based catalysts alters the electronegativity landscape, introducing asymmetry in electronic distribution. N, S, or P atoms have different electronegativities and introduce localized dipoles, which affect charge delocalization and the electronic affinity of neighboring Zn centers. The Zn–N or Zn–S coordination creates localized electronic states that tune adsorption energies of reaction intermediates.^30–37 Heteroatoms can conjugate with carbon supports, forming sp²-hybridized networks with enhanced π-electron delocalization, leading to better electron mobility. Alloying Zn with transition metals (e.g., Cu, Co, and Ni) combines two or more metals at the atomic level, creating new electronic states through orbital hybridization. Alloying introduces the lattice strain effect that alters the crystal field and electronic orbital overlap, which can either enhance or suppress catalytic activity. Adjacent metal atoms may participate in dual-site catalysis, enabling multi-step electron transfer reactions or stabilizing key intermediates through ensemble effects.^30–37

4.2. Methods of design

The synthesis of zinc-based materials can be achieved through a variety of methods, each designed to regulate the morphology, structure, and properties of the desired materials. These techniques are tailored to optimize the performance of zinc-based materials in electrocatalytic reduction reactions.

4.2.1. Hydrothermal/solvothermal methods. The hydrothermal synthesis of Zn-based materials involves heating of metal precursors in an autoclave with different reagents like sulfur, phosphorus, selenium, and others. Various solvents, such as water and ethanol, have been employed to facilitate the synthesis of Zn-based materials. Hydrothermal processes usually take place in autoclaves at high temperatures and pressures (Fig. 4).^43,44 This method promotes the growth of zinc-based materials with well-defined morphologies, such as nanorods, nanowires, and nanosheets.^43,44 Solvothermal synthesis is similar to hydrothermal synthesis but uses non-aqueous solvents. It is beneficial for preparing zinc chalcogenides and other complex zinc-based materials. In this regard, Zn_xCo_1−xSe₂ was developed using the ZnCo-MOF as the precursor and the hydrothermal method.⁴⁵ The Zn_xCo_1−xSe₂ catalyst was created by heating the mixture to 200 °C in an autoclave and keeping it there for 12 h. Similarly, ZnInS was also created using the hydrothermal method (120 °C for 4 h).⁴⁶

Fig. 4 Pictorial depiction showing the hydrothermal synthesis of Zn-based materials.

In a different work, Nam and coworkers prepared ZnS using a simple hydrothermal method following a specific procedure.⁴⁷ Initially, a solution of 5 mM Zn(NO₃)₂·6H₂O was prepared in water following the addition of 5 mmol of S-source such as C₂H₅NS or CH₄N₂S. After being cleaned, Zn-foil was heated in an autoclave in a convection oven for 6 h at 120 °C. Following the hydrothermal process, the autoclave was left to cool naturally to ambient temperature, resulting in ZnS. In further works, Ti-ZnCoS was also fabricated using a hydrothermal technique.⁴⁸

4.2.2. Chemical vapor deposition (CVD) method. Zn-based compounds may be effectively synthesized via chemical vapor deposition (CVD).^49–51 Zn-based precursors are treated at high temperatures in an inert environment in a furnace, frequently with reagents such as phosphorus, nitrogen, sulfur, or selenium to form Zn-based chalcogenides, phosphides, or nitrides (Fig. 5). It should be noted that the high-temperature heating of the Zn-precursor leads to the evaporation of zinc, creating a highly porous structure.^49–51 Therefore, the synthesis temperature can be carefully regulated based on the desired materials and their specific properties. To fine-tune attributes for catalytic applications, temperature control is essential in establishing the final material's crystallinity, porosity, and chemical makeup.^49–51 In this context, Dai et al. explored the ZnS@C material for electrocatalytic ORR reactivity.⁵¹ First, they prepared the Zn-precursor in water and ethanol. The Zn-precursor was then heated to 700 °C for two hours at a rate of 5 °C per minute while being exposed to an Ar flow. After cooling, the resulting sample was the ZnS@C catalyst. In another work, the ZnSe/NiSe heterostructured material was also developed employing the CVD technique.⁵²

Fig. 5 Pictorial depiction showing the CVD process for the synthesis of Zn-based materials.

Heating N,C-containing Zn-MOFs under inert conditions results in the formation of N-doped carbon frameworks embedded in the final material. For example, Lim and coworkers first prepared a ZnCo-ZIF and then utilized the CVD method for the sulfidation to produce ZnCoS/NCS.⁴⁹ The ZnCo-ZIF was treated with thiourea in a tubular furnace under an inert atmosphere. The ZnCo-ZIF was pyrolyzed at 700 °C for 2 h with the rate of 10 °C min⁻¹ under Ar-gas. After the reaction, ZnCoS integrated with N-, and S-doped carbon was synthesized. The N-doped carbon was in situ integrated with the sulfide by the pyrolysis of a MOF precursor having a nitrogenous ligand. Similarly, Hu and coworkers developed Cu–ZnSe@NC using a ZnCu-ZIF under CVD.⁵⁰ In an N₂ environment, the ZIF and Se powder were employed and reacted at 500 °C for four hours. The in situ synthesis under CVD resulted in Cu–ZnSe@NC.

4.2.3. Air calcination method. In a related process known as “air calcination,” Zn-precursors are exposed to an oxygen-rich environment, where they are heated to convert into their corresponding oxide forms.^43,44 During this process, the precursors thermally decompose, forming highly porous zinc oxide materials. For example, Liang et al. demonstrated the C–ZnO material for hydrogen evolution.⁵³ To produce C–ZnO, the ZIF was calcined at different temperatures, speeds, and durations in an air environment muffle furnace. The calcination procedure involved setting temperatures at 350, 400, 450, and 500 °C and calcining at a rate of 2 °C per minute for three hours. The obtained samples were given the corresponding designations ZnO-350, ZnO-400, ZnO-450, and ZnO-500. As an alternative, a two-step calcination method was used to create C–ZnO. The first phase was heating at 350 °C for two hours at a rate of 2 °C per minute and then heating at 400 °C and 450 °C for one hour at a rate of 1 °C per minute. The resulting samples were designated as ZnO-350–400 and ZnO-350–450, respectively. Similarly, Buckley and coworkers developed ZnO for electrocatalytic hydrogen evolution using air calcination.⁵⁴ The Zn-precursor was calcined in the air within a muffle furnace for 90 minutes at 600 °C with a 5 °C min⁻¹ rate.

Although the above-mentioned three methods are used for the synthesis of Zn-based materials, there are pros and cons of each method (Table 1).^{43,44,49–51} Hydrothermal synthesis operates at relatively moderate temperatures and pressures and enables the synthesis of a wide range of structures (nanorods, nanowires, etc.) by adjusting reaction parameters. The hydrothermal technique produces materials with high phase purity.^{43,44,49–51} The hydrothermal method is suitable for large-scale synthesis with appropriate reactor designs and also allows uniform doping and incorporation of heteroatoms. Besides these merits, the hydrothermal method also has demerits. The hydrothermal method often requires hours to days to complete the reaction and requires specialized high-pressure vessels (autoclaves). This method is not ideal for synthesizing materials that require extreme temperatures or vapor-phase reactions.^{43,44,49–51}

Table 1 Summary of the advantages and disadvantages of various synthesis methods of Zn-based materials

Sr. no.	Synthesis method	Advantages	Disadvantages
1	Hydrothermal or solvothermal method	Need for moderate temperature, adjustable reaction parameters, controlled structure design, easier synthesis of various materials	Requires high-pressure autoclaves, the danger of exploding
2	Chemical vapor deposition (CVD) method	Produces highly crystalline materials, precise control over temperature and gas flow, forms highly porous materials with rough surfaces	Requires high temperature and expensive equipment, needs input of gases like N2 and Ar, releases toxic gases like phosphine
3	Air calcination method	Need for minimum equipment, no external gas flow, open-air heating, produces highly porous and crystalline materials	Requires high temperature, poor control over structure and morphology of the designed materials

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The CVD method is suitable for a wide range of materials, including semiconductors and composites, and produces uniform and high-purity thin films or coatings.^{43,44,49–51} The CVD technique enables precise control over material thickness, composition, and nanostructure. In contrast to these merits, CVD requires expensive equipment and precursors. CVD often operates at elevated temperatures and requires precise control of gas flow rates and temperature. CVD uses toxic, flammable, or hazardous gases in some cases.^{43,44,49–51} Air-calcination is a straightforward process requiring minimal equipment. This method is suitable for achieving phase transitions or crystallinity in materials and is used for preparing oxides, phosphates, and other stable materials.^{43,44,49–51} The air calcination method has poor control over nanostructures and surface area and requires high temperatures, leading to significant energy consumption. Each technique has its unique advantages and limitations, making it suitable for specific applications based on the desired material properties and end-use requirements.^{43,44,49–51}

5. Hydrogen evolution reaction (HER)

The electrochemical hydrogen evolution reaction involving the reduction of water (in alkaline) or protons (in acidic) into molecular hydrogen by a two-electron transfer process.^55,56 The reaction mechanism of the HER includes the Volmer Tafel and Volmer Heyrovsky pathways (Fig. 6).^43,44,57 In most noble metal-based materials, the catalysts reduce water by the Volmer Tafel mechanism. On the other hand, materials based on transition metals reduce water using the Volmer Heyrovsky route (Fig. 6).^43,44,57 In recent years, most investigations have focused on designing proficient catalysts in which zinc-based materials have been thoroughly explored for the electrocatalytic HER.

Fig. 6 Reaction mechanism of the electrocatalytic water reduction reaction in alkaline and acidic electrolytes. Reproduced from ref. 56 with permission from Wiley-VCH.

Mirabella and coworkers synthesized a ZnOHF-ZnO catalyst using chemical bath deposition.⁵⁸ After casting onto graphene paper, this catalyst was assessed as a potential material for a full water-splitting cell. At a current density of 10 mA cm⁻², it displayed an overpotential (η₁₀ = overpotential required at 10 mA cm⁻² current density) of 181 mV. Upon Pt decoration, the catalyst achieved a cell voltage (E₁₀ = cell voltage required at 10 mA cm⁻² current density) of 1.7 V at 10 mA cm⁻² for overall water splitting. Kumar et al. explored a heterostructured CuO/ZnO catalyst, finding that adding ZnO significantly boosted CuO's reactivity.⁵⁹ For the OER, CuO/ZnO exhibited an onset potential of 1.40 V, lower than CuO's 1.74 V. For the HER, CuO/ZnO showed an onset voltage of 0.15 V, while CuO alone reached 0.24 V. Furthermore, Kim and coworkers developed a ternary oxide consisting of Gd₂O₃, In₂O₃, and ZnO, which displayed a HER η₁₀ of 271 mV, indicating promising catalytic potential.⁶⁰

In another study, Zn-doped CoP was synthesized to enhance water-splitting performance.⁶¹ This catalyst was prepared on cobalt foam through a two-step hydrothermal process followed by phosphating (Fig. 7a). The unchanged PXRD peaks confirmed the successful incorporation of Zn into the lattice. Compared to CoP, the Zn–CoP's Co 2p spectrum showed a negative shift of approximately 1.72 eV, indicating that Zn doping influenced cobalt's electronic state, reducing the Co³⁺ amount (Fig. 7b). The P 2p spectrum displayed a negative shift of ∼0.15 eV compared to CoP, suggesting a strong electronic interaction between Zn and Co (Fig. 7c). The presence of Co³⁺ and Zn²⁺ in Zn–CoP further indicated electronic interactions between these metal ions.

Fig. 7 (a) Pictorial depiction for the synthesis of Zn–CoP using the CVD method; (b) Co 2p XPS of CoP and Zn–CoP showing the negative shift in the peak of Co³⁺ in Zn–CoP; (c) P 2p XPS of CoP and Zn–CoP; (d) electronic interaction between Co 3d and Zn 3d orbitals; (e) electrocatalytic HER reactivity of Zn–CoP compared with CoP, Pt/C and bare CF manifesting the best HER activity of Zn–CoP; (f) CA stability plot of Zn–CoP for the HER; (g) free energy change profile for Zn–CoP and CoP; and (h) PDOS plot of the CoP and Zn–CoP showing the d-band center. Reproduced from ref. 61 with permission from the Royal Society of Chemistry; (i) LSV curves showing the improved HER activity of CoP after Zn-doping. Reproduced from ref. 62 with permission from Elsevier; (j) LSV profiles indicating the enhanced HER activity of P–Zn–CoNiP-2 among the studied catalysts. Reproduced from ref. 63 with permission from Wiley-VCH; and (k) catalytic HER activity of Zn–CoNiP compared with CoNiP showing the best activity of Zn–CoNiP. Reproduced from ref. 64 with permission from Elsevier.

The electronic configurations of Zn and Co were examined to clarify these interactions. Co³⁺, with a valence electron configuration of 3d⁶ (t⁴_2ge²_g), had two unpaired electrons in the t_2g orbitals, which interact with O²⁻via π-donation (Fig. 7d). In contrast, Zn²⁺, with a configuration of 3d¹⁰ (t_2g⁶e_g⁴), had fully occupied t_2g orbitals, resulting in electron–electron repulsion with O²⁻. When Co and Zn are combined, the π-interaction between Co and O was strengthened by the repulsion between O²⁻ and Zn²⁺, causing partial charge transfer. In Zn–CoP, this redistribution of valence electrons adjusted the binding energies of hydrogen and oxygen intermediates, aligning with the Sabatier principle. The Zn doping altered the electronic structure of CoP and improved the charge transport, significantly boosting the catalytic activity.⁶¹

The Zn–CoP catalyst exhibited an η₁₀₀ (η₁₀₀ = overpotential required at 100 mA cm⁻² current density) of just 166 mV for hydrogen evolution, a notably low value compared to other materials studied (Fig. 7e). A water electrolyzer was assembled using Zn–CoP as both the cathode and anode, achieving an E₁₀₀ (E₁₀₀ = cell voltage required at 100 mA cm⁻² current density) of 1.71 V and an E₁₀₀₀ (E₁₀₀₀ = cell voltage required at 1000 mA cm⁻² current density) of 2.01 V. Tafel plot analysis indicated that Zn–CoP had the fastest reaction kinetics for the HER among the catalysts tested. Furthermore, the catalyst demonstrated excellent stability of 60 h for the HER during CA testing (Fig. 7f). The Gibbs free energy change profile determined through DFT suggested that the free energy change decreased after Zn-doping, which was closer to zero, suggesting the improved HER reactivity in the case of Zn–CoP (Fig. 7g). Furthermore, density of state (DOS) analysis revealed a higher density of states near the Fermi level, indicating the metallic characteristics of CoP (Fig. 5h).

In a separate study, a highly effective HER electrocatalyst was synthesized through a straightforward approach. The Zn–CoP catalyst featured a 3D hierarchical nanostructure, providing a large surface area and abundant catalytic sites to facilitate efficient mass transport of reactants and electrolytes.⁶² Its unique one-dimensional porous architecture further enhanced electrocatalytic activity by offering numerous active sites for the HER. Zn–CoP demonstrated impressive HER performance in alkaline media with an η₁₀ of 138 mV (Fig. 7i). Cai et al. investigated Zn-doped CoNiP for overall water splitting, noting that Zn doping modified the electronic properties of Ni and Co sites.⁶³ This catalyst achieved a HER η₁₀₀ of 177 mV, with an E₅₀ of 1.71 V for concurrent O₂ and H₂ production, respectively (Fig. 7j). Similarly, Wang and coworkers developed a Zn–CoNiP catalyst for overall water splitting, achieving remarkable HER activity with an η₁₀ of 73 mV under acidic and 34 mV under alkaline conditions (Fig. 7k).⁶⁴ The catalyst required only an E₁₀ of 1.51 V for total water splitting. These above-mentioned studies showed that Zn doping altered the electronic features of Co and Ni sites improving the catalytic HER performance.

To enhance the catalytic performance of Zn-based phosphides, conductive materials have been incorporated to form N- and C-containing phosphides. In several studies, researchers directly integrate N-doped carbons, nanotubes, or graphene into the phosphide catalyst. In most cases, metal–organic frameworks (MOFs) serve as precursors for phosphide synthesis. MOFs with N- and C-containing linkers are phosphorized under high-temperature conditions, creating conductive N-doped carbon materials within the phosphide structure in situ. For Zn-based MOFs, the high-temperature phosphidation process causes zinc to evaporate, resulting in a highly porous structure.

Li et al. demonstrated the Co₂P/CoP–NC and NiP/Co₂P/CoP–NC catalysts using a Zn-based MOF precursor, where Zn evaporated during synthesis, forming the phosphide heterostructure.⁶⁵ Additionally, NC was integrated into the heterostructured phosphide catalyst during this process. The NC support facilitated rapid charge transfer in the Co₂P/CoP–NC catalyst, and its hierarchical structure provided a large surface area with numerous channels for mass transport, enhancing diffusion in electrochemical reactions.

Improved catalytic kinetics and overall activity were observed due to electronic interactions between the NC nanosheets and dual-phased phosphides, which enhanced charge transfer and lowered reaction barriers for intermediates. The Co₂P/CoP hetero-interfaces further optimized electronic topologies, creating multiple pathways for charge movement. The Co₂P/CoP–NC-0.1 catalyst exhibited excellent HER activity, with η₁₀ values of 84 mV in acidic and 91 mV in alkaline electrolytes, while CoP–NC showed higher η₁₀ values of 144 mV and 188 mV under the same conditions.

As shown theoretically and experimentally in the study, heterostructured catalysts provide superior catalytic performance compared to individual materials; forming heterostructures involves combining two or more distinct phases or materials to enhance reaction efficiency.^66–70 These heterostructures create interfaces between materials that enable the effective movement of charge carriers, facilitating improved charge transfer between phases and enhancing overall catalytic activity.^66–70 The materials in a heterostructure often possess complementary electronic properties, such as band structure or work function, which can reduce energy barriers, improve electronic conductivity, and tailor the electronic environment for better reactant interaction.^66–70 Additionally, the combination of materials within a heterostructure increases the density of active sites, resulting in more efficient catalytic processes and higher reaction rates for the HER and the OER.

Zhang and coworkers conducted an in-depth study to enhance electrochemical HER activity by incorporating conductive polyaniline (PANI) into ZnS nanoparticles.⁷¹ PANI was integrated into ZnS in varied quantities (10, 50, 90, and 110 mg), yielding samples denoted as ZnS-10 mg, ZnS-50 mg, ZnS-90 mg, and ZnS-110 mg. Among these, ZnS-90 mg showed an optimal synergistic effect, achieving an η₁₀ of 168 mV to enable the HER, significantly lower than the 244 mV overpotential required by unmodified ZnS. This comparative study established that PANI notably enhanced charge transfer properties by elevating conductivity, thereby improving HER activity. The mechanism behind PANI's effectiveness lies in its conductive nature, which facilitates electron mobility within the ZnS matrix, thus lowering charge transfer resistance and providing a more favorable electronic environment. This interaction underscores the potential of conductive polymers like PANI in boosting the catalytic properties of semiconductors, highlighting a promising pathway for enhancing efficiency in HER and other electrocatalytic applications.

An intriguing study conducted by Pan et al. focused on the design of CnZnS on copper foam for total water splitting.⁷² The catalyst was synthesized using a one-step hydrothermal method. SEM images of CoZnS showed the presence of radial nanoflocs and angular nanoparticles. TEM pictures revealed lattice spacings that corresponded to the planes of CoS. The CoZnS achieved an η₁₀ of 130 mV for the HER, showcasing superior reactivity compared to CoS and ZnS. Additionally, CoZnS exhibited an E₁₀ of 1.53 V, indicating its effectiveness in producing both O₂ and H₂ in a two-electrode system.

In a related study, Zn doping was applied to CoNiS to enhance its HER activity.⁷³ The incorporation of Zn optimized the electronic properties of the catalyst, resulting in an E₁₀ of 1.50 V in a two-electrode configuration. Building on this, researchers have also developed heterostructured Zn-based chalcogenides for water-splitting applications. In this context, Wei and coworkers created a heterostructured polyoxometalate (POM)/ZnCoS catalyst to improve overall water-splitting efficiency.⁷⁴ The POM@ZnCoS nanowires were synthesized on a 3D spongy Ni substrate using a simple and scalable two-step hydrothermal method.

ZnCoS nanowires were initially synthesized through a reaction between ZnCo-hydroxide and Na₂S during hydrothermal processes. Following this, PW₁₂ nanoparticles were anchored onto the ZnCoS nanowires, forming POM@ZnCoS nanowires. SEM pictures of POM@ZnCoS revealed a morphology similar to that of ZnCoS but with noticeable differences. The nanowires appeared thicker, rougher, and shorter, exhibiting increased roughness compared to the original ZnCoS. This significant alteration in shape is likely due to the attachment of PW₁₂ on the surface of the nanowires. TEM pictures of ZnCoS showed surface roughness with consistent size, thickness, and diameter of the nanowires. In contrast, the TEM images of POM@ZnCoS revealed a distinct morphology, where PW₁₂ nanoparticles were anchored onto the ZnCoS nanowires, resulting in a rough structure. HRTEM images displayed clear lattice fringes with observable lattice spacings attributed to the planes of Zn_0.76Co_0.24S and PW₁₂. Selected area electron diffraction (SAED) further confirmed the crystalline nature and identified various planes within the material.

The ZnCoS catalyst required an η₂₀ of 240 mV to achieve the OER. In contrast, the POM@ZnCoS demonstrated a lower η₂₀ of 200 mV. Furthermore, POM@ZnCoS exhibited total water splitting with an E₁₀ of 1.56 V, which was lower than that of ZnCoS. CA stability tests indicated that POM@ZnCoS maintained stability for 20 hours in a two-electrode setup. The remarkable OER activity of POM@ZnCoS can be attributed to the synergistic effects resulting from the heterostructure formation between POM and ZnCoS nanowires, which facilitates significant electronic and structural modulation of the catalyst. Additionally, various studies have reported a series of heterostructured catalysts, such as ZnFeS/MoS₂,⁷⁵ Zn_0.76Co_0.24S/CoS₂,⁷⁶ and ZnCoS/MoS₂,⁷⁷ all demonstrating efficient water-splitting activity.

In recent developments, doping a second metal along with a conductive substance has been employed in zinc selenide to enhance HER reactivity. The Cu–ZnSe@NC catalyst was synthesized using a MOF as a precursor for water splitting, employing a CVD method.⁵⁰ SEM images of Cu–ZnSe@NC displayed cubic characteristics, while HRSEM images revealed a rough surface, indicating the successful formation of a porous nanostructure. TEM images further confirmed that the Cu–ZnSe nanoparticles were well-dispersed within an amorphous carbon matrix. HRTEM images indicated a d-value of 0.327 nm, corresponding to the (111) facet of Cu–ZnSe, thereby providing provisional evidence for the formation of the composite catalyst.

Raman data revealed three prominent peaks corresponding to the G, D, and 2D bands at 1575.9 cm⁻¹, 1333.2 cm⁻¹, and 2807.9 cm⁻¹, respectively. Notably, the peaks for Cu–ZnSe@NC shifted to lower wavenumbers compared to those of pristine ZnSe@NC, indicating that the incorporation of copper may have altered the electrical properties of Cu–ZnSe@NC. The Se 3d spectra showed a shift to lower binding energies by approximately 0.2 eV in Cu–ZnSe@NC compared to ZnSe@NC, suggesting an increase in electron density due to Cu doping. Similarly, the Cu–C 1s spectra exhibited shifts to higher binding energies, implying that electron depletion occurred following Cu incorporation. Deconvolution of the N 1s spectra identified three peaks. As expected, Cu–ZnSe@NC exhibited remarkable hydrogen evolution reactivity with an η₁₀ of 84 mV, significantly outperforming pristine ZnSe@NC, which required 222 mV. Additionally, various studies have reported the introduction of conductive substances into zinc selenide to enhance water-splitting performance.

These studies concluded that Zn-based materials exhibit exceptional reactivity for the electrocatalytic HER due to their altered electronic properties (Table 2).^{22,31,32,78,79} Despite promising attributes such as earth abundance, tunable electronic properties, and structural versatility, zinc-based materials face several challenges in the HER. Key limitations include relatively low intrinsic conductivity, moderate catalytic activity compared to noble metals, and structural instability under prolonged electrochemical operation, particularly in acidic media where Zn is prone to corrosion and dissolution. Moreover, the limited density of active sites and sluggish reaction kinetics hinder their practical performance. Looking forward, overcoming these barriers require innovative strategies such as electronic structure modulation through doping or alloying, development of Zn-based heterostructures with conductive supports, and stabilization via protective coatings or hybrid architectures. Integration with computational screening and in situ/operando techniques is essential for understanding reaction pathways and guiding rational design.

Table 2 Comparison of electrocatalytic HER performance of various Zn-based materials reported in the literature

Sr. no.	Catalysts	Overpotential (mV)	Current density (mA cm^−2)	Ref.
1	ZnOHF–ZnO	181	10	58
2	Gd2O3–In2O3–ZnO	271	10	60
3	Zn–CoP	166	100	61
4	Zn–CoP	138	10	62
5	Zn–CoNiP	177	100	63
6	Zn–CoNiP	34	10	64
7	Co2P/CoP–NC	91	10	65
8	ZnS/PANI	168	10	71
9	CoZnS	130	10	72
10	Zn–CoNiS	138	10	73
11	POM/ZnCoS	200	20	74
12	ZnFeS/MoS2	145	10	75
13	Zn0.76Co0.24S/CoS2	238	20	76
14	MoS2@Zn0.76Co0.24S	169	20	77
15	Cu–ZnSe@NC	84	10	50

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6. Oxygen reduction reaction (ORR)

One important electrochemical process that takes place in a variety of energy conversion devices is the ORR.^2,7,8,16,19 In these systems, the ORR is crucial for generating electricity by reducing O₂ molecules at the cathode. Systems that seek to efficiently transform chemical energy into electrical energy depend on this process. The quantity of electrons transported during the reaction is the primary factor distinguishing the several paths by which the ORR might progress.^2,7,8,16,19

Two primary mechanisms are widely studied for the oxygen reduction reaction (ORR): (i) the four-electron pathway and (ii) the two-electron pathway (Fig. 8).^142,143 The O₂ is immediately reduced in the 4e⁻ route to OH⁻ in an alkaline medium or H₂O in an acidic electrolyte. The 4e⁻ is the most efficient pathway, as it produces a large amount of energy by immediately reducing oxygen to water. In the 2e⁻ process, oxygen is reduced to either O₂²⁻ in an alkaline medium or H₂O₂ in an acidic medium, which can be further reduced to water.^2,7,8,16,19 This pathway is less efficient and can degrade the performance of the electrochemical device. The ORR is inherently slow and requires highly efficient catalysts to proceed at a reasonable rate, especially in the four-electron pathway.

Fig. 8 (a) Associative mechanism of oxygen reduction reaction; and (b) dissociative mechanism of oxygen reduction reaction. Reproduced from ref. 61 with permission from the Royal Society of Chemistry.

Precious metal catalysts like platinum are often used, but alternative catalysts based on transition metals and carbon-based materials are being explored due to high cost and scarcity of noble metals. The mode of oxygen adsorption on the metal surface plays a crucial role in determining the reaction pathway.¹⁴² In the four-electron mechanism, oxygen undergoes reduction via a bidentate adsorption mode, leading to the formation of water (H₂O). In contrast, the two-electron mechanism involves an end-on adsorption mode, resulting in the production of hydrogen peroxide (H₂O₂).^143,144 The four-electron pathway is preferred in energy conversion applications, such as fuel cells and metal–air batteries, due to its higher efficiency. Meanwhile, the two-electron pathway is primarily utilized in industrial processes for H₂O₂ production.¹⁴⁴

The two-electron ORR process differs under alkaline and acidic conditions. In alkaline media, the active catalytic site initially binds to molecular O₂, which is subsequently reduced through the transfer of an electron and a proton, forming the hydroperoxide intermediate (*OOH).^{2,7,8,16,19,61} This intermediate undergoes further reduction by accepting an additional electron, producing HO₂⁻, which then desorbs from the catalyst surface, completing the reaction cycle. In acidic media, the mechanism diverges due to the high concentration of free H⁺ ions. Here, the *OOH intermediate readily reacts with protons, directly yielding hydrogen peroxide (H₂O₂) without forming the HO₂⁻ species.^{2,7,8,16,19,61} The four-electron transfer mechanism also differs in acidic and alkaline media. In an acidic medium, molecular O₂ adsorbs onto the active catalytic site of the electrode. Furthermore, O₂ undergoes the first proton-coupled electron transfer, forming a hydroperoxide intermediate (*OOH). A second proton-coupled electron transfer reduces *OOH, breaking the O–O bond and forming adsorbed oxygen (*O) and water (H₂O).^{2,7,8,16,19,61} The *O is further reduced by sequential proton-coupled electron transfer, forming adsorbed hydroxyl (*OH) and then water (H₂O). The final product, H₂O, desorbs from the catalyst surface, completing the cycle. In an alkaline medium, O₂ adsorbs onto the catalytic active site. After that, the first electron transfer occurs, followed by protonation from water, forming *OOH.

The *OOH undergoes further reduction and splits into adsorbed oxygen (*O) and hydroxide (OH⁻). The *O is reduced to *OH through another proton-coupled electron transfer, followed by a reduction to OH⁻. Finally, OH⁻ desorbs from the active site, regenerating the catalyst surface.^{2,7,8,16,19,61} The distinction between the two-electron and four-electron oxygen reduction pathways arises from variations in oxygen adsorption modes, which influence the reaction mechanism. These differences are primarily governed by the binding strength between O₂ and the active catalytic site. When the active site exhibits a strong affinity for O₂, the molecule is adsorbed in a parallel orientation, interacting with two adjacent active centers to form a bridging oxygen configuration (A–O–O–A).^{2,7,8,16,19,61} This adsorbed O₂ undergoes proton coupling to form an A–OH intermediate, which is subsequently reduced to water (H₂O). In contrast, when O₂ is adsorbed in an end-on configuration, it undergoes a proton-coupled electron transfer to form a MOOH intermediate. During this pathway, successive electron transfers lead to the formation of A–O and A–OH intermediates, eventually resulting in H₂O. The end-on adsorption mode is associated with higher reaction efficiency and faster kinetics compared to the bridging oxygen configuration. These insights are crucial for the rational design of catalysts with enhanced activity and performance in oxygen reduction processes.^{2,7,8,16,19,61}

In this regard, zinc-based materials are well-studied for the electrochemical ORR. Mostly, zinc-based oxides and alloys have been reported for the electrochemical ORR.^{2,7,8,16,19,61} However, introducing N-doped carbon (NC) with Zn-based materials has been observed to offer excellent advantages in the ORR. When nitrogen atoms are added to the carbon lattice, catalytically active sites are formed, which makes it easier for oxygen to be adsorbed and reduced. Nitrogen's higher electronegativity than carbon attracts electron density, leading to a more efficient interaction with oxygen molecules.^{2,7,8,16,19,61} Nitrogen atoms can donate electron density to adjacent carbon atoms, creating localized charge density that enhances the adsorption of oxygen intermediates. Nitrogen doping can change the spin density of carbon atoms, making the catalyst more effective in facilitating the 4-electron pathway for more efficient ORR.

In Zn-based materials with NC, four different types of nitrogen have been observed to be coordinated with carbon (Fig. 9).⁸⁰ Pyridinic nitrogen provides lone pairs of electrons that can participate directly in the reduction process, contributing significantly to the catalytic activity. It is known to facilitate the reduction of O₂ by interacting with intermediates. Pyrrolic nitrogen contributes to the structural stability of the carbon framework. Graphitic nitrogen alters the conductivity of the carbon matrix, improving electron transfer rates, which is essential for raising the ORR's general effectiveness.⁸⁰

Fig. 9 Schematic illustrations showing four types of nitrogen present in Zn-based materials having N-doped carbon.

Oxidized nitrogen (quaternary) can increase the hydrophilicity of the catalyst, promoting better interaction with the electrolyte. To fully capitalize on the advantages of both oxides and N-doped carbon materials and improve electrochemical performance, a layered crystalline spinel nanostructure was hybridized with nitrogen-doped graphene (NG) nanosheets. This hybridization aimed to enhance the catalyst's activity and stability, making it more effective for the ORR. This led to the effective synthesis of a hybrid Zn-based catalyst called Zn₂Mn₃O₈ fixed on NG sheets.²³ A hydrothermal technique and post-annealing treatment were used to create the catalyst. During synthesis, the amount of precursor salts may be changed to precisely regulate the hybrid's size, shape, and morphology. A potential contender for ORR applications, the hybrid showed remarkable electrocatalytic activity, excellent cycle stability, and great methanol endurance in alkaline circumstances.

Zn₂Mn₃O₈/NG was first established by the PXRD analysis, which manifested the spinel structure of Zn₂Mn₃O₈ with the peak of NG having significant intensity. The D and G bands indicative of graphitic carbon substrates were represented by two distinctive peaks in the Raman data of the studied materials, which were located at approximately 1350 cm⁻¹ and 1590 cm⁻¹.²³ These bands were key indicators of the material's structural properties. While the G band correlated with the in-plane vibration of sp²-connected carbon atoms, the D band was linked to defects in the carbon lattice. Defect concentration in graphitic nanostructures was measured by the D to G band intensity ratio. For NG, the I_D/I_G ratio was 0.98, notably higher than that of GO (0.90), indicating an increase in defect density. The I_D/I_G values were further elevated in the hybrid materials: ZnO–NG showed a ratio of 1.01, Mn₃O₄–NG had 1.0, and Zn₂Mn₃O₈–NG reached 1.02. Due to the stabilizing impact of dense metal nanoparticles on the graphene surface, this rise indicated a higher degree of distortion inside the graphitic composition.²³

The electrocatalytic performance of the Zn₂Mn₃O₈–NG hybrid for the ORR was initially assessed using CV in alkaline electrolyte, both under N₂ and O₂-saturated conditions. The CV curve for the catalyst exhibited a well-defined reduction peak at −0.14 V, indicating superior ORR activity in an alkaline environment. The Zn₂Mn₃O₈–NG hybrid was the most notable catalyst when comparing the CV plots of other catalysts since it showed the greatest peak current and the strongest positive peak potential. In contrast, the E_peak values were −0.213 V for rGO, −0.187 V for NG, −0.184 V for ZnO–NG, and −0.142 V for Mn₃O₄–NG. These outcomes demonstrated the outstanding catalytic reactivity of the Zn₂Mn₃O₈–NG hybrid. Further analysis using LSV underscored the good reactivity of the Zn₂Mn₃O₈–NG hybrid, showing the highest onset potential (V_os = −0.013 V) and half-wave potential (V_hw = −0.12 V). These metrics outperformed the other catalysts. The uniform distribution of active nanomaterial over graphene's enormous surface area and excellent conductivity significantly enhanced the catalyst's performance. This uniform distribution improved interfacial charge transfer, increased the number of adsorption sites, and created more active centers for the ORR.

In similar studies, ZnFe₂O₄/rGO²⁵ and Co–ZnO@NC/CNT³³ have been employed for the ORR. According to these investigations, ZnO was essential in improving ORR performance by catalyzing the formation of critical active species such as metallic Co⁰, Co²⁺ ions, and nitrogen species like pyridinic and graphitic N, all of which were essential for ORR activity. These species provide active sites facilitating oxygen reduction, boosting the overall catalytic efficiency. Like oxides, zinc–cobalt sulfide fabricated on N,S-doped carbon was reported for the electrochemical ORR.⁴⁹ The exceptional ORR efficacy of the synthesized Zn–Co–S@NSC in an alkaline medium was facilitated by the binary metal system's distinct structure and synergistic effects. This catalyst exhibited impressive electrocatalytic activity, having a V_os of 0.955 V vs. RHE and a V_hw of 0.831 V vs. RHE, comparable to the traditional Pt/C material. DFT analysis supported these observations, indicating that the binary metal system effectively enhanced the ORR activity through the optimization of the free energy, leading to more efficient oxygen reduction. Furthermore, Huang et al. developed ZnS/Co-NSCNTs for the electrocatalytic ORR.⁸¹ They also observed a similar effect of N-doped carbon material, as mentioned in the previous literature.

As further progress of Zn-based materials, Zn-based alloys have been well studied in the literature for the electrocatalytic ORR. For example, a work presented by Fu et al. demonstrated the creation of Zn and Fe on the NC material (Zn/Fe–NC) for efficient ORR reactivity.⁸² This method produced dual-atom Zn and Fe reactive sites that were stabilized inside a carbon framework with a porous structure resembling a honeycomb. As a result of this thermal treatment, a significant portion of the zinc evaporated, and Fe-infused ZIF-8 produced a microporous carbon structure that included both Zn and Fe after breaking down. In the interim, a carbon-based structure was produced as a result of the PVP matrix carbonizing.

There were two noticeable peaks of diffraction for the graphitic carbon planes in the PXRD data of Zn–NC and Zn/Fe–NC. The efficient dispersion of Zn and Fe atoms throughout the NC matrix was confirmed by the notable absence of PXRD peaks associated with any other phases having Zn and Fe. Two separate peaks were visible in the Raman data of materials at 1350 cm⁻¹ for the D band and 1580 cm⁻¹ for the G band. For Zn–NC, Zn/Fe–NC, and Zn/Fe–NC-50, the I_D/I_G values were determined to be 0.97, 0.94, and 0.96, respectively. According to these ratios, the Zn/Fe–NC sample had a somewhat more significant graphitization level than the rest.⁸²

XAS was examined to learn more about the conditions for coordination in Zn/Fe–NC catalysts. The Zn K-edge value of Zn/Fe–NC was situated in the middle of the metallic Zn foil and ZnO spectra, suggesting that the Zn's chemical valence state lay somewhere in between (Fig. 10a). The Fe K-edge value in Zn/Fe–NC was found to be closer to that of FeO than Fe₂O₃, indicative of the +2 state of Fe (Fig. 10b). At 1.47 Å, the Zn–N scattering channel was identified as the source of the significant peak in the Zn R space plot of Zn/Fe–NC (Fig. 10c). Crucially, no signal corresponding to the Zn–Zn link was seen, which further supported the idea that atomically distributed Zn species are present and rule out metallic Zn. With no discernible peak for the Fe–Fe bonding, the Fe R space plot showed a prominent peak at 1.50 Å, which corresponded to the Fe–N scattering channel (Fig. 10d). The fact that there was no Fe–Fe signal suggests that Fe was atomically distributed across the Zn/Fe–NC material. These results strongly support the effective creation of atomically scattered Zn and Fe sites in NC.

Fig. 10 (a) Zn K edge XANES plot for Zn/Fe–NC, ZnO and ZnPC; (b) Fe K edge XANES plot of Zn/Fe–NC, FeO and FePC; (c) Zn R space profile of Zn/Fe–NC, ZnO and ZnPC; (d) Fe R space profile of Zn/Fe–NC, ZnO and ZnPC; (e) CV profile of Zn/Fe–NC in N₂ and O₂ containing 0.1 M KOH; (f) LSV curves of Pt/C and Zn/Fe–NC in O₂-saturated 0.1 M KOH electrolyte; (g) free energy change profile at U = 0 V; (h) free energy change profile at U = 1.23 V. Reproduced from ref. 82 with permission from Elsevier; and (i) ORR activity profile of A-SAC(Fe,Ni,Zn)/NC, SAC(Fe,Ni,Zn)/NC, and commercial Pt/C in O₂-saturated 0.1 M KOH. Reproduced from ref. 83 with permission from Elsevier.

Significantly higher ORR activity was shown by the Zn/Fe–NC as the peak of O₂ reduction emerged at 0.810 V (Fig. 10e). This potential was noticeably more positive than the studied materials, including the commercial Pt/C catalyst. The Zn/Fe–NC manifested a V_os of 1.081 V and a V_hw of 0.875 V. These measures performed noticeably better than those of pure Zn–NC, Fe–NC, and commercial Pt/C (Fig. 10f). This improvement demonstrated how the Zn and Fe atoms' synergistic collaboration in the Zn/Fe–NC improved the ORR reactivity. As demonstrated by the Zn/Fe–NC-50, which had a V_os and V_hw of 0.994 V and 0.855 V, respectively, a higher Fe content was observed to impair ORR reactivity.

The reactive sites and their combined impact on the ORR were investigated using DFT. All reaction paths showed that the electron transfer process in all the materials happened exothermically at the input of U = 0 V (Fig. 10g). At 1.08 eV energy, the reduction of OOH* was the potential-determining step (PDS) for the Zn–NC when U climbed to 1.23 V (Fig. 10h). The overpotentials of Zn/Fe–NC and Fe–NC, on the other hand, were considerably 0.282 eV and 0.438 eV, respectively, as the PDS changed to the reduction of OH*. The addition of zinc increased the reactivity of the Fe atom in the Zn/Fe–NC, as evidenced by the difference in the PDS and the significantly reduced overpotential for the Zn/Fe–NC. The electronic features of the Fe and Zn in Zn/Fe–NC and the Zn–NC and Fe–NC were examined to investigate the source of the synergistic effect in more detail.⁸²

The Zn/Fe–NC catalyst exhibited modest electron deficiency around Zn, similar to Zn–NC, while differential charge density analysis revealed increased electron concentration near Fe compared to Fe–NC. Electron transfer from Zn to Fe led to a downshift in the Fe d-band center, reducing the adsorption strength of *OH. The d-band centers of Zn–NC, Zn/Fe–NC, and Fe–NC were −7.00 eV, −2.28 eV, and −1.60 eV, respectively, with a 0.68 eV shift in Zn/Fe–NC enhancing ORR activity. Partial density of states (PDOS) analysis indicated electronic synergy between Zn and Fe, highlighting the cooperative effect of the dual-atom configuration in improving electrocatalytic performance.

Following the above work, Tsai et al. introduced a third metal, Ni, in a ZnFe/NC based catalyst and developed a single atom catalyst consisting of Fe, Zn, and Ni in NC named A-SAC (Fe,Ni,Zn)/NC for the ORR.⁸³ They observed that introducing a trimetallic catalyst with N-doped carbon significantly boosted the performance. Additionally, the study investigated the effectiveness of Fe–N_x, Ni–N_x, and Zn–N_x as ORR reactive sites. Their total catalytic efficacy was greatly improved by the three neighboring Fe, Ni, and Zn-SACs working in concert inside the NC framework. As a result, they obtained a V_hw of 0.88 V and a voltage differential of 0.75 V (Fig. 10i).

In an intriguing study, Lin et al. used a straightforward pyrolysis process of a meticulously crafted MOF precursor for the ORR to establish an efficient ZnCoFe–N–C SAC.⁸⁴ The XAS analysis specifically elaborated on the formation of SAC. The Zn K-edge XANES data manifested that the absorption edges for Zn–NC and ZnP_c were closely aligned, suggesting that Zn was in the +2-valence state. The Zn–N coordination was indicated by a noticeable peak at 1.56 Å that emerged in the Zn R space plot of ZnCoFe–N–C. Importantly, no peak associated with the Zn–Zn bonding was seen, suggesting the absence of Zn clusters within ZnCoFe–NC. The Co K-edge absorption edge for ZnCoFe–N–C was positioned between the edges of CoO and Co₂O₃, indicating that Co had an electronic state between +2 and +3. The R space plot data for cobalt showed only a peak at about 1.58 Å, confirming Co–N coordination and supporting the conclusion that cobalt atoms were atomically dispersed within the material.⁸⁴ The absorption edge of ZnCoFe–NC was located in between FeO and Fe₂O₃ as seen in XANES data, indicating that the electronic state of Fe is between +2 and +3.

The EXAFS for iron displayed a dominant peak around 1.48 Å, which likely corresponded to Fe–N coordination. Notably, no signals indicating Fe–Fe bonding at 2.16 Å were observed, further confirming the atomic level distribution of Fe. The LSV test indicated that the ZnCoFe–N–C afforded the largest V_hw of 0.878 V. This performance surpassed Zn–NC, Co–NC, Fe–NC, ZnCo–NC, and ZnFe–NC, having the V_hw of 0.736 V, 0.838 V, 0.841 V, 0.841 V, and 0.823 V, respectively.

Interestingly, the V_hw of ZnCoFe–NC was 28 mV higher than the V_hw of 0.850 V for the Pt/C. The existence of M–N_x reactive sites and the synergistic impact amongst the three metal single atoms were credited with this remarkable performance. In further reports, a series of Zn-based trimetallic catalysts with N-doped carbon such as ZnCoFe/NC,⁸⁵ FeCoZn@PANI,⁸⁶ Ni–Co–Zn–N–PC,⁸⁷ and so on have been studied for the electrocatalytic ORR.

The literature studies have established that doping heteroatoms such as boron, sulfur, and phosphorus can significantly enhance the electrocatalytic performance of N-doped carbon materials. The electronic features of materials having carbon may be altered by heteroatom doping, which increases the electrical conductivity of certain materials. The chemical characteristics of N-doped carbon may be precisely adjusted by changing the kind and quantity of heteroatoms present.⁸⁶ This allows for the optimization of materials for specific applications, such as enhancing the selectivity and activity in chemical reactions. Heteroatom doping can improve the structural stability of N-doped carbon materials under operating conditions. In this regard, Liu et al. presented an innovative design featuring atomically distributed Zn and Co dual metals on N,S co-doped 3D dendritic carbon structure optimized for the high-performance ORR.⁸⁸ The addition of S, which had a bigger atomic size and a smaller electronegativity than N, successfully changed the reactive sites and electronic features. The XAS analysis verified that the electronic characteristics were adjusted by introducing S and N in C. The resulting (Zn,Co)/NSC catalyst exhibited exceptional ORR reactivity. The (Zn,Co)/NSC demonstrated V_hw, which was 67 mV greater than commercial Pt/C. The A-Zn@NSG has been thoroughly investigated for the ORR in subsequent developments.⁸⁹

Although the approaches mentioned above in developing Zn-based catalysts improve the catalytic ORR performance, few researchers have demonstrated the introduction of Au nanoparticles in Zn-based catalysts to attain impressive performance. Consequently, they obtained efficient ORR activity with Au-doped Zn-based materials. Introducing Au is primarily done to serve as a location for nucleation to lay down other metal nanoparticles, creating catalysts with high surface area and active site density. Furthermore, Au can improve the conductivity of Zn-based materials. Its high electrical conductivity can facilitate better charge transfer during the ORR, contributing to enhanced catalytic efficiency. A multilayer core–shell-shaped Au@ZnFe/C material was created by Lu et al. and utilized for ORR reactivity.⁹⁰ The ZnFe-MOFs and C material were used in an experiment to wrap a single Au nanoparticle in a porous carbon shell embedded with Zn–Fe compounds. Spectroscopy and microscopy were used to study the core–shell structure of Au@ZnFe/C. The Au@ZnFe/C material manifested a V_os of +0.94 V vs. RHE for ORR reactivity.

In contrast to Pt/C, a follow-up investigation showed an O–PdZn catalyst with multiple Pd atomic layers to triple the mass reactivity for the ORR. Furthermore, adding Au to the O–PdZn (Au–O–PdZn) significantly increased its longevity; after 30 000 potential cycles, the mass reactivity decreased by less than 10%.⁹¹ The stabilizing effect of the Au atoms and the meticulously planned orderly structure were credited with this improvement. Using spectroscopic techniques, it was shown that Au not only entered the PdZn lattice and produced a homogeneous distribution across the particles but also substituted Pd and Zn at their surfaces through galvanic interchange.

It is evident from the aforementioned research that Zn-based materials may be a viable option for the ORR (Table 3).^92–107 However, Zn-based materials face several challenges in terms of the ORR, which include insufficient intrinsic catalytic activity, poor electrical conductivity, and low tolerance to intermediate poisoning (e.g., by peroxide species), particularly in acidic environments. Furthermore, Zn-based catalysts often exhibit limited stability, especially when used in practical devices such as zinc–air batteries. To address these challenges, future strategies may include designing atomically dispersed Zn active sites embedded in conductive carbon matrices (e.g., Zn–N–C structures), engineering Zn-based hybrid systems with transition metals to modulate the d-band center, and introducing structural defects or vacancies to boost active site density.

Table 3 Comparison of the electrocatalytic ORR activity of Zn-based materials reported in the literature

Sr. no.	Catalysts	Onset potential (V)	Half-wave potential (V)	Ref.
1	Zn2Mn3O8/NG	−0.013	−0.12	23
2	ZnFe2O4/rGO	−0.08	—	25
3	Co–ZnO@NC/CNT	0.90	0.86	33
5	Zn–Co–S@NSC	0.955	0.831	49
6	ZnS/Co–NSCNTs	—	0.871	81
7	Zn/Fe–NC	1.0801	0.875	82
8	FeNiZn/NC	0.92	0.88	83
9	ZnCoFe–N–C	—	0.878	84
10	ZnCoFe/NC	0.95	0.878	85
11	Ni–Co–Zn–N–PC	—	0.864	87
12	Zn–Co/N,S–C	1.07	0.893	88
13	A-Zn@NSG	1.040	0.905	89
14	Au@ZnFe/C	0.94	—	90

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7. Nitrogen reduction reaction (NRR)

The nitrogen reduction reaction (NRR) is the process of reducing molecular nitrogen to ammonia, which is a six-electron transfer process.^1,4,13–15 It is a crucial reaction in natural and industrial processes, especially for producing ammonia, an essential chemical for fertilizers and various industrial applications. The NRR mimics the biological nitrogen fixation process carried out by certain bacteria. However, electrochemical or catalytic setup aims to reduce nitrogen more sustainably and efficiently than current industrial methods such as the Haber–Bosch process.^1,4,13–15 The electrocatalytic NRR follows two pathways, namely associative and dissociative pathways (Fig. 11). In the first one, both nitrogen atoms are reduced simultaneously. In the later one, the nitrogen–nitrogen bond breaks, and each nitrogen atom is reduced independently.^1,4,13–15 Although various advancements have been achieved in the electrocatalytic NRR process, several challenges remain.^13–15 The NRR is often hindered by the competing HER, which dominates under standard electrocatalytic conditions. Zn-based materials, due to their unique properties and tunability, help mitigate HER and enhance NRR selectivity through several strategies.^13–15 Zn-based materials often exhibit low hydrogen adsorption energy (H*), making the HER less favorable compared to N₂ adsorption and activation. Zinc-based active sites promote π-backdonation from d-orbitals to the antibonding orbitals of N₂, facilitating selective N₂ reduction over H₂ evolution.

Fig. 11 Reaction pathways illustrating key electrochemical nitrogen reduction reaction (NRR) mechanisms: (a) dissociative mechanism, (b) alternating associative mechanism, (c) distal associative mechanism, and (d) Mars–van Krevelen (MvK) mechanism.

Defect-rich Zn-based materials preferentially bind and activate N₂ molecules rather than protons, suppressing the HER.^13–15 Zn-based materials doped with heteroatoms (e.g., N,S) create unique coordination environments that lower the activation barrier for N₂ reduction while reducing HER activity. Zn-based materials with polar surfaces can stabilize adsorbed N₂ through dipole interactions, favoring NRR pathways. In non-alkaline media, Zn-based materials often demonstrate weaker HER kinetics, allowing the NRR to dominate.

The triple bond in molecular nitrogen (N≡N) is one of the strongest chemical bonds, requiring significant energy to break. Overcoming this challenge mildly and efficiently is the biggest task for the NRR.^13–15 Competing side reactions, especially the HER, often dominate during the NRR. Since the electrochemical potential for the HER is close to that for the NRR, catalysts can preferentially reduce protons to H₂, lowering ammonia yield. Efficient catalysts for the NRR are still under development. Transition metal-based catalysts and non-metal catalysts like boron nitride and carbon-based materials are being explored for the NRR due to their ability to bind and activate nitrogen. However, achieving high selectivity, efficiency, and stability remains a challenge. In most systems, the faradaic efficiency is low due to the strong competition from side reactions, particularly the HER.

In this context, Wen and coworkers explored oxygen vacancy containing Zn-doped Co₃O₄ for the electrocatalytic NRR.¹⁰⁸ The Zn–Co₃O₄ was created using a ZIF under air calcination. The Zn–Co₃O₄ displayed diffraction peaks similar to those of pure Co₃O₄, which corresponded to the cubic Co₃O₄. This showed that the catalysts were effectively created by a low-temperature calcination procedure. A low carbon content inside the catalyst was also shown by the PXRD data of Zn–Co₃O₄-x (here x denotes 0, 5, 10, 15, 20), which matched the typical spectrum of Co₃O₄ with a few peaks from C components (Fig. 12a). Furthermore, as the Zn doping level increased, the Zn–Co₃O₄-x PXRD peaks shifted slightly toward higher angles, indicating a reduction in the d-spacing of Co₃O₄ due to zinc incorporation (Fig. 12a).

Fig. 12 (a) PXRD analysis of Zn–Co₃O₄ and Co₃O₄; (b) Co 2p XPS of Zn–Co₃O₄ and Co₃O₄; (c) O 1s XPS of Zn–Co₃O₄ and Co₃O₄; (d) Zn 2p XPS of Zn–Co₃O₄; (e) ESR spectra of Zn–Co₃O₄ and Co₃O₄; (f) NH₃ production rates and FE of Zn–Co₃O₄ across a range of applied potentials. Reproduced from ref. 108 with permission from the American Chemical Society; (g) Zn R space profiles of Zn Pc and Zn/NC NSs showing the absence of the Zn–Zn peak for Zn/NC NSs; (h) NH₃ production rates and FE of Zn/NC NSs across a range of applied potentials; and (i) NH₃ production rates and FE of Zn/NC NSs for various cycles. Reproduced from ref. 26 with permission from Wiley-VCH.

XPS analysis confirmed Zn incorporation in Zn–Co₃O₄, indicated by an additional peak absent in pristine Co₃O₄. The XPS analysis revealed Co³⁺ and Co²⁺ oxidation states (Fig. 12b). A shift to lower binding energies in Zn–Co₃O₄ suggests electron transfer from Zn to Co, increasing Co's electron density. This shift, along with a rise in Co²⁺ concentration, created oxygen vacancies (V_o) as some O atoms escape to balance the charge, creating donor–acceptor pairings of Zn and Co that may improve NRR reactivity by encouraging ammonia desorption, nitrogen binding, and dissociation.

In the O 1s XPS spectrum of Zn–Co₃O₄, three peaks were noted corresponding to O present in the lattice, V_o, and adsorbed oxygen (Fig. 12c).¹⁰⁸ The V_o peak was significantly enhanced with Zn doping, indicating increased oxygen vacancies. The Zn 2p spectra verified Zn²⁺ (Fig. 12d). ESR measurements further explored V_o density in Co₃O₄ as well as Zn–Co₃O₄ (Fig. 12e). At g = 2.0, the Zn–Co₃O₄-10 showed the brightest ESR peak, indicating the highest V_o density. This resulted in a large number of Lewis acid sites that made nitrogen adsorption easier. The bimetallic ZIF-x exhibited weak ESR signals, indicating minimal V_o presence. Raman spectroscopy revealed five characteristic peaks of Co₃O₄, with increased V_o concentration causing peak broadening and redshift, suggesting structural modifications. Zn–Co₃O₄ showed the most pronounced broadening and shift, enhancing nitrogen binding, which benefits subsequent hydrogenation for ammonia production.

Under nitrogen-saturated conditions, a marked increase in current density was observed at voltages below −0.1 V, suggesting the significant NRR reactivity of Zn–Co₃O₄. Furthermore, in contrast to Co₃O₄, Zn–Co₃O₄ displayed a more pronounced difference in the polarization curve's current between nitrogen and argon conditions, indicating superior NRR performance for the Zn-doped material. During NRR testing at various applied potentials over a 2-hour duration, the time-dependent current density curves for Zn–Co₃O₄ showed minimal decay, indicating good catalyst durability in a 0.1 M HCl solution. Additionally, UV-vis absorption spectra confirmed the NRR reactivity of Zn–Co₃O₄ when voltage varied from −0.1 to −0.5 V. The best results were obtained at −0.3 V, offering a FE of 11.9% and an outstanding rate of 22.71 μg h⁻¹ mg⁻¹ to give ammonia, indicating the most efficient potential of Zn–Co₃O₄ for the NRR (Fig. 12f).

In an interesting study, atomically dispersed zinc supported on N-doped carbon nanosheets (Zn/NC NSs) was analyzed for electrocatalytic NRR applications.²⁶ XAS results manifested the absence of a peak for Zn–Zn, suggesting the atomic dispersion of Zn and the formation of a single-atom catalyst (Fig. 12g). The Zn/NC NSs delivered a substantial ammonia production rate of 46.62 μg h⁻¹ mg⁻¹ at −0.85 V vs. RHE and an impressive FE of 95.8% at −0.70 V vs. RHE (Fig. 12h). Furthermore, the Zn/NC demonstrated excellent stability and selectivity, showing consistent NH₃ production rates and FEs across multiple cycles (Fig. 12i). Structural characterization confirmed that Zn–N₄ sites within the catalyst were the key active centers driving the nitrogen reduction reaction.

They conducted DFT calculations to investigate the NRR pathway and the enhanced performance of atomically dispersed Zn/NC. The DFT analysis showed the alternate and distal NRR pathways. The Gibbs free energy profiles for both pathways highlighted a notably lower energy barrier for the alternate pathway, indicating that the NRR in Zn/NC likely followed this route. In the alternate pathway, the hydrogenation of *N₂ to form *NNH was identified as the PDS. The energy barrier for *N₂ reduction in Zn/NC was significantly lower than in NC. This indicated that the Zn–N₄ sites in Zn/NC NSs effectively lowered the energy barrier for forming *NNH intermediates, which enhanced *N₂ activation and promoted subsequent hydrogenation steps, ultimately accelerating the NH₃ production rate. Based on spectroscopy and theory, this study has confirmed that introducing conductive N-doped carbon substances provided several advantages to improve the catalytic NRR reactivity. Furthermore, it has been shown that ZnFe/NC,¹⁰⁹ PdZn–NPs/NHCP,¹¹⁰ and Zn–N₂S₂-MOF¹¹¹ exhibit competent reactivity for the NRR.

Historically, Zn-based compounds have predominantly been used in electrochemical energy storage due to Zn's d¹⁰ electronic configuration. However, recent defect and interface engineering advances have sparked interest in ZnS as an electrocatalyst, given its low cost and environmental benefits. For example, Feng and coworkers used thiourea in situ sulfurizing Zn foil to provide N-doped-ZnS containing sulfur vacancies (S_v) for the NRR.¹¹² With a FE of 7.92%, the N–ZnS produced NH₃ having the rate of 2.42 × 10⁻¹⁰ mol s⁻¹ cm⁻², which was ascribed to S_v and Zn–N reactive sites. Further studies conducted by Zhao and coworkers explored defect-rich ZnS supported on reduced graphite oxide (DR ZnS–rGO), created utilizing a Zn-salt and a S-source in a solvothermal process.¹¹³ In contrast to pristine ZnS, the ZnS with defects had a much greater NH₃ output and FE, owing to enhanced stability and recyclability in nitrogen and argon-saturated environments due to rGO. According to DFT data, the first hydrogenation was the step that determined the potential, and DR ZnS–rGO with two S_v had a lower barrier of energy. The NRR used associative distal and alternative routes with similar energy barriers to go forward. Chen and coworkers developed a ZnS nanoarray via self-organized growth under solvothermal conditions.¹¹⁴ This ZnS@Ni structure benefits from a large surface area and strong adhesion to the 3D Ni foam substrate, resulting in an excellent rate of 5.27 × 10⁻¹⁰ mol s⁻¹ cm⁻² to give ammonia having 5.62% FE. Furthermore, Zhang et al. created FeS₂/ZnS–NC for proficient NRR reactivity by utilizing 2-methylimidazole to create FeZn-LDH in situ and then converted it to FeS₂/ZnS–NC.¹¹⁵ SEM revealed evenly spaced FeS₂ nanosheets, generating a large number of active sites. The FeS₂/ZnS–NC showed potential for good NRR reactivity, producing ammonia at 58.52 μg h⁻¹ mg⁻¹ having 46.84% FE.

It has been discovered that using Zn-based alloys effectively increases the NRR reactivity. By altering the electrical characteristics, researchers have significantly enhanced the NRR reactivity of Zn-based materials by integrating conductive N-doped carbon. The literature-reported studies have demonstrated that Zn-based materials offer excellent NRR activity due to their altered electronic features (Table 4). However, inherently low electrical conductivity, poor activation of the strong N≡N triple bond due to limited d-orbital participation, and low faradaic efficiency arising from competing HER, especially under aqueous conditions are the main concerns with Zn-based materials. Additionally, the identification of true NRR activity is complicated by contamination and the extremely low yield rates of ammonia, making reproducibility and verification a challenge. Looking ahead, strategies such as atomically dispersed Zn coordination, dual-site engineering, and integration with defect-rich carbon supports could enhance N₂ adsorption and activation. Furthermore, advanced electrolyte systems and in situ/operando techniques will be critical for mechanistic elucidation and real-time tracking of intermediates.

Table 4 Comparison of the electrocatalytic NRR activity of Zn-based materials reported in the literature (potentials have been presented against RHE)

Sr. no.	Catalysts	Potential	FE	NH3 yield rate	Ref.
1	Zn–Co3O4	−0.3 V	11.9%	22.71 μg h^−1 mg^−1	108
2	Zn/NC	−0.85 V	95.8%	46.62 μg h^−1 mg^−1	26
3	PdZn–NPs	−0.2 V	16.9%	5.28 μg mg^−1 h^−1	110
4	Zn–N2S2-MOF	−0.3 V	44.5%	25.07 μg h^−1 cm^−2	111
5	N–ZnS	−0.6 V	7.92%	2.42 × 10^−10 mol s^−1 cm^−2	112
6	ZnS–rGO	−0.15 V	28.2%	51.2 μg h^−1 mg^−1	113
7	ZnS@Ni	−0.5 V	10.8%	5.27 × 10^−10 mol s^−1 cm^−2	114
8	FeS2/ZnS–NC	−0.5 V	46.8%	58.52 μg h^−1 mg^−1	115

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8. Nitrate reduction reaction (NO₃⁻RR)

The nitrate reduction reaction (NO₃⁻RR) is the process in which nitrate (NO₃⁻) is reduced to various nitrogen-containing compounds, such as nitrite (NO₂⁻), ammonia (NH₃), or nitrogen gas (N₂), depending on the conditions and the catalyst used.^5,10–12,34 This reaction is vital in both environmental processes, such as denitrification in wastewater treatment and energy-related electrochemical systems, where nitrate reduction is studied for its potential in nitrogen recovery and conversion to valuable chemicals.^5,10–12,34 Nitrate reduction can proceed through multiple pathways, each leading to different products (Fig. 13).

Fig. 13 Schematic illustration for the reaction mechanism of the electrocatalytic nitrate reduction reaction. Reproduced from ref. 34 with permission from OAE Publishing Inc.

The overall reaction pathways vary depending on the intended result and the quantity of electrons involved. The reduction to nitrite (NO₂⁻) is the first step in most nitrate reduction processes, involving a two-electron transfer. In certain catalytic systems, nitrate can be reduced all the way to ammonia, involving an eight-electron transfer.^5,10–12,34 Nitrate ions can also be reduced to nitrogen gas (N₂), the most stable form of nitrogen. This pathway involves a multi-step process with intermediate species like nitrite (NO₂⁻), nitric oxide (NO), and nitrous oxide (N₂O), requiring ten electrons. Selectively and efficiently reducing nitrate is an active area of research. Transition metal catalysts and bimetallic systems are commonly employed to achieve this goal. These catalysts must balance activity, selectivity, and stability to perform effectively under the harsh conditions often present in nitrate reduction.

Owing to their excellent properties, Zn-based materials have been well-studied for the electrocatalytic NO₃⁻RR. For instance, Huang and coworkers discovered the efficient nitrate reduction activity of a flower-shaped zinc-based ZnCo₂O₄ catalyst.¹¹⁶ They adopted a simple calcination procedure to develop ZnCo₂O₄ and further identified the structure of ZnCo₂O₄ using PXRD (Fig. 14a). The PXRD analysis established the creation of a zinc cobaltite phase that was well aligned with reference data (Fig. 14b). The SEM and TEM analyses showed flower shapes with various planes of cobaltite in HRTEM (Fig. 14c–f). Elemental mapping images clarified the even distribution of the constituting elements (Fig. 14g). As Zn was added to the cobalt oxide material, it was observed to introduce electronic modification in Co sites. These electronic modifications were tracked by XPS and UPS spectroscopy. XPS analysis confirmed Zn²⁺ incorporation in ZnCo₂O₄, with Zn 2p spectra. The Co 2p peaks indicated Co³⁺ and Co²⁺ oxidation states. A positive shift in Co₃O₄ peaks after Zn²⁺ doping suggested electron migration from Co to Zn, reducing Co electron density. Oxygen species in ZnCo₂O₄ were also observed. ZnCo₂O₄ showed a higher V_o content (37.6%) than Co₃O₄ (33.1%). UPS analysis revealed a lower work function (6.37 eV vs. 6.62 eV), enhancing electron transport and NO₃⁻RR catalytic activity.

Fig. 14 (a) Synthesis of ZnCo₂O₄; (b) PXRD data of ZnCo₂O₄; (c) SEM image of ZnCo₂O₄; (d) and (e) TEM images of ZnCo₂O₄; (f) HRTEM image of ZnCo₂O₄; (g) elemental mapping pictures of ZnCo₂O₄; (h) Co 2p XPS of ZnCo₂O₄ and Co₃O₄; (i) O 1s XPS of Co₃O₄ and ZnCo₂O₄; (j) UPS valence band plot of ZnCo₂O₄ and Co₃O₄; (k) LSV profile for electrocatalytic nitrate reduction reaction with ZnCo₂O₄ in 0.1 M KOH solution with or without nitrate; (l) CA test with ZnCo₂O₄ for electrocatalytic nitrate reduction at various potentials; and (m) NH₃ production rates of ZnCo₂O₄ and Co₃O₄ across a range of applied potentials. Reproduced from ref. 116 with permission from Wiley-VCH.

The fact that ZnCoO₄ successfully reduced NO₃⁻ was proven by the increased current density. The onset of NO₃⁻ reduction was started at the voltage of −0.2 V vs. RHE, while the NO₃⁻RR reactivity was also assessed at varying potentials between −0.2 and −0.6 V vs. RHE (Fig. 14h). As the test voltage increased, both ZnCo₂O₄ and Co₃O₄ demonstrated higher current densities for the NO₃⁻RR, although ZnCo₂O₄ exhibited a significantly greater current density than Co₃O₄, indicating that Zn ion doping enhanced reduction current density (Fig. 14h). The NH₃ yield rate was increased with increasing voltage, while the FE for NH₃ followed a volcano-shaped trend, peaking at −0.4 V. The CA test at varying potentials was also conducted to determine the maximum yield. The ZnCo₂O₄ outperformed Co₃O₄, manifesting 95.4% FE to give an excellent rate of 2101.2 μg mg⁻¹ h⁻¹ for ammonia formation, in contrast to 83.2% and 1200 μg mg⁻¹ h⁻¹ for Co₃O₄, respectively. This enhanced performance highlighted the impact of Zn doping on the catalytic efficacy of ZnCo₂O₄ for the NO₃⁻RR.

In a similar type of analysis, Dong and coworkers reported ZnCr₂O₄ (denoted as 2Z-ZCO) with oxygen vacancies for nitrate reduction.¹¹⁷ However, there was a difference that the Dong group performed the nitrate reduction in a buffer solution with a neutral pH. They also adjusted the composition of Zn and Cr to get an optimized composition. Furthermore, they performed DFT, explaining that oxygen vacancies significantly altered the d-band center and improved catalytic reactivity. The existence of Zn⁰ in 2Z-ZCO was verified by XAS analysis, where the magnitude of the signal in 2Z-ZCO was lower compared to ZnO but greater in contrast to Zn foil. The Zn⁰ arrangement was further confirmed by the Zn–Zn bonding signal and the Zn–O coordination peak.

The LSV data showed a noticeable rise in current density in electrolytes with NO₃⁻, suggesting active nitrate electroreduction (Fig. 14i). The 2Z-ZCO had the shortest radius among the analyzed materials, according to EIS Nyquist plots, indicating proficient charge transport. Furthermore, 2Z-ZCO exhibited a minimal slope value compared to the others, suggesting a faster reaction rate. Nitrate reduction on 2Z-ZCO was evaluated over a varying voltage, where NH₃ yield progressively increased with increasing voltage, affording a rate of 30.32 mg h⁻¹ mg⁻¹ (Fig. 14j). The FE followed a volcano-shaped trend, recorded highest at −1.2 V. Beyond this potential, the FE decreased slightly.

Thus, −1.2 V was adopted as the ideal potential for assessing selectivity and stability. Ion chromatography (IC) detected an NH₃ concentration of 50.8 mg L⁻¹, which closely matched the 51.7 mg L⁻¹ obtained through UV-vis. Compared to other forms, 2Z-ZCO, with the highest concentration of oxygen vacancies, demonstrates superior performance for nitrate reduction.

With a ΔG_(NO3⁻) of −2.28 eV, the ZCO (220) surface demonstrated significant contact with nitrate ions. Electron transport from Cr and N to O was shown by the charge density difference profile, strengthening the Cr–O binding while weakening the N–O binding. The free energy of the following stage (*NO₃⁻ → *NH₃) decreased steadily, necessitating no extra energy for upcoming steps. With an energy boost of 0.62 eV, the NH₃ desorption process [*NH → NH₃(g)] presented a potential reaction barrier.

As demonstrated by the charge density difference, the ΔG_(NO3⁻) dropped to −1.25 eV when the Cr–O pair in ZnCr₂O₄ was substituted with Zn. This was due to minimal orbital interaction between the O and Zn. Interestingly, the free energy of ammonia desorption was reduced to 0.36 eV, indicating that NH₃ was more readily formed and desorbed, supporting a continuous reaction flow. Posing an energy demand of 0.72 eV, the PDS for the ZnO (002) surface happened during the *NO → *N process.

With the introduction of oxygen vacancies (V_o), the PDS changed from *NO₂ to *NO. The weaker Zn–O involvement, compared to Cr–O bonds, further facilitated NH₃ desorption, enhancing the overall reaction efficiency. The formation energy of oxygen vacancies for ZCO showed a considerable decrease compared to ZnO. This indicated that oxygen vacancies were more readily formed in ZnCr₂O₄, making them more favorable in this structure. The above two studies have clearly explained based on experimental and theoretical analyses that the catalytic reactivity was significantly increased by O-vacancies. In a recent report, Ye et al. developed a reduced cobalt-based spinel oxide to enhance the electrocatalytic NO₃⁻RR.¹¹⁸ Comprehensive structural characterization and electrochemical assessments showed that incorporating zinc and applying a reduction treatment created electron-deficient Co²⁺ sites in tetrahedral coordination. The modified R-ZnCo₂O₄ demonstrated impressive ammonia production capabilities, achieving high selectivity and FE for ammonia, having the proficient rate of 26.75 mg_NH3 h⁻¹ mg_cat⁻¹. DFT calculations and in situ data provided insights into the NO₃⁻RR mechanism on R-ZnCo₂O₄. Additionally, the excellent NO₃⁻RR performance of R-ZnCo₂O₄ was examined within a Zn–NO₃⁻ battery, demonstrating an OCV of 1.46 V having 1.40 mW cm⁻² power density. These results underscored the material's potential for the NO₃⁻RR.

In an interesting study conducted by Du et al., Zn–CuO has been created as an efficient catalyst for excellent NO₃⁻RR reactivity.¹¹⁹ This was because Zn doping modified the electronic features of CuO, which greatly improved charge transport. By subjecting commercial brass to an alkaline medium, where it underwent successive corrosion, oxidation, and structural reformation, these Zn–CuO NAs were created. Firstly, the brass was dealloyed, generating nanoscale Cu, rapidly oxidizing to form Cu₂O with a lower valence. At the same time as Zn₂O was incorporated, this CuO was further oxidized to CuO and reassembled into nanosheets. For the NO₃⁻RR, Zn–CuO demonstrated exceptional performance, achieving an efficient rate of 945.1 μg h⁻¹ cm⁻² for ammonia creation. This efficiency placed Zn–CuO among the top-performing catalysts, showcasing a straightforward and scalable synthesis method that enabled efficient NH₃ production from NO₃⁻ reduction. In further work, Yang and coworkers developed Ti/CuZn₅O for electrocatalytic nitrate reduction with efficient ammonia yield.¹²⁰

As already studied, in the final step of NO₃⁻ reduction to N₂, N–N coupling is necessary and can only occur when two N* species occupy adjacent active sites. Supported single-atom catalysts, which feature isolated active metal centers without neighboring active sites, may hinder this N–N coupling step, thus reducing the selectivity toward N₂ production. In this context, Zhao and coworkers developed a Zn-based ZnSA-MNC as an efficient and selective catalyst for the NO₃⁻RR.¹²¹ This catalyst achieved a NO₃⁻ conversion rate of 97.2%, with an NH₃ selectivity of 94.9%, producing ammonia at the rate of 2.31 mmol h⁻¹ mg cat⁻¹. DFT calculations suggested that the positively charged Zn atoms, coordinated with N atoms, promoted the NO₃⁻RR, aiding in reaction pathway clarification. Two primary reaction pathways guided NO₃⁻ conversion, involving Zn–O and Zn–N bond formation that led to *ONO and *NOO intermediates. The rate-limiting step for NO₃⁻ to NH₃ conversion on ZnSA-MNC was the initial hydrogenation (*NO₃ → *NO₃H), validating the energy demand of 0.58 eV. In further developments, Zn-doped Cu nanosheets and Zn₁/MnO₂ were demonstrated for efficient nitrate reduction reactions. Besides these studies, numerous Pt/calcined CuZnAl hydrotalcite,¹²² Cu/Zn/TiO₂,¹²³ Fe/ZnO,¹²⁴ Cu@ZnO,¹²⁵ and Zn–Cu catalyst¹²⁶ have been designed for the electrocatalytic NO₃⁻RR. The successful application of zinc-based catalysts for nitrate reduction addresses environmental concerns associated with nitrate pollution and presents opportunities for resource recovery and the generation of value-added products (Table 5).

Table 5 Comparison of the electrocatalytic NO₃⁻RR activity of Zn-based materials reported in the literature (potentials have been presented against RHE)

Sr. no.	Catalysts	Potential	FE	NH3 yield rate	Ref.
1	ZnCo2O4	−0.4 V	95.4%	2101.2 μg mg^−1 h^−1	116
2	ZnCr2O4	−1.2 V	90.21%	30.32 mg h^−1 mg^−1	117
3	ZnCo2O4	−0.3 V	92.2%	26.75 mg h^−1 mg^−1	118
4	Zn–CuO	−0.7 V	95.6%	945.1 μg h^−1 cm^−2	119
6	ZnSA-MNC	−1.0 V	97.2%	2.31 mmol h^−1 mg^−1	121
9	Fe/ZnO	−0.7 V	87.0%	31 nmol s^−1 cm^−2	124
10	Cu@ZnO	−0.6 V	89.14%	6.03 mg cm^−2 h^−1	125
11	ZnCu-alloy	−0.85 V	98.4%	5.8 mol h^−1 g^−1	126

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9. Carbon dioxide reduction reaction (CO₂RR)

The carbon dioxide reduction reaction (CO₂RR) is a chemical process where carbon dioxide (CO₂) is electrochemically reduced to produce valuable products such as fuels and chemicals.^6,18,29 This reaction is of significant interest in the context of addressing climate change and promoting sustainable energy because it can help mitigate CO₂ emissions by converting them into valuable products, utilizing renewable energy in the process. The reaction requires the presence of a catalyst to lower the energy barriers and drive the reduction of CO₂.^6,18,29 Electrocatalysts are often metals like copper, silver, zinc, or gold. However, non-metallic materials like carbon-based catalysts or metal–organic frameworks (MOFs) are also being explored. The products of CO₂RR depend on the catalyst used, the applied potential, and the reaction conditions. They range from carbon monoxide (CO), formic acid (HCOOH), and methane (CH₄) to more complex molecules like ethylene (C₂H₄), ethanol (C₂H₅OH), or other hydrocarbons and alcohols (Fig. 15).^6,18,29 The CO₂RR process involves multiple electron transfer steps and intermediate stages, making it complex.

Fig. 15 Electrocatalytic CO₂RR shows the various products formed after CO₂ reduction and their reaction mechanisms. Reproduced from ref. 18 with permission from Wiley-VCH.

Specifically, Zn sites, especially those in oxidized states, exhibit moderate binding to CO₂, facilitating its activation without excessive binding that would hinder desorption.^6,18,29 Zn's electronic configuration supports partial electron transfer to the CO₂ molecule, bending the linear CO₂ geometry and forming reactive intermediates like CO₂⁻ or CO. Zn has an optimal binding energy for CO, stabilizing *CO intermediates without poisoning the catalyst. This is critical for the subsequent coupling reactions that form C₂₊ products. In Zn oxides or hydroxides, the presence of surface hydroxyl groups can stabilize intermediates like *COOH and *CH₂, facilitating multi-step reductions. Zn surfaces or Zn atoms anchored in a nitrogen-doped carbon matrix (Zn–N₄ sites) provide active sites where *CO intermediates are brought into close proximity, increasing the likelihood of C–C coupling.^6,18,29 The Zn's coordination environment, particularly in Zn-doped carbon structures, can polarize the adsorbed *CO, facilitating the coupling of two *CO molecules to form C₂ intermediates like *OCCO. Hybrid Zn systems (e.g., Zn–Cu or Zn with co-catalysts) enable synergistic effects, where Zn stabilizes intermediates, and neighboring active sites drive C–C coupling. After C–C coupling to form *OCCO, hydrogenation steps catalyzed by Zn sites can lead to *CH₂CHO, which further reduces to ethanol. Zn sites can also promote the dehydration of intermediates like *CH₂CH₂OH, leading to ethylene formation. Hydrophobic surfaces on Zn-based materials can enhance ethylene selectivity by destabilizing polar intermediates, favoring desorption.

The CO₂ can be reduced through different pathways, producing various products. CO is one of the simplest and most common products, requiring 2 electrons. Formic acid requires 2 electrons and protons.^6,18,29 Methane is a more reduced product that requires 8 electrons and protons. Ethylene formation requires 12 electrons and protons. The selectivity of the catalyst determines which pathway dominates. Controlling the production of specific products is challenging due to the multiple reaction pathways. Achieving high faradaic is critical. Catalysts can degrade over time, and maintaining long-term performance is a major research focus. Enhancing CO₂RR's energy efficiency is crucial to making the process commercially feasible because it uses much energy.^6,18,29

Exploring the Zn-based oxides, Shviro and coworkers demonstrated ZnO with varying morphologies for efficient CO₂RR reactivity.¹²⁷ The ZnO nanoparticles were prepared in diverse morphologies, such as nanorods, nanosheets, nanoparticles, and random shapes. They observed that the change in the morphology also altered the electronic states of Zn. As a result, ZnO nanorods offered excellent reactivity for the CO₂RR. The PXRD studies showed the creation of ZnO hexagonal wurtzite structure for all the morphologies. The Zn 2p XPS manifested that the XPS peaks of nanorods were moved in the positive direction by 0.7 eV in contrast to nanoparticles, suggesting the electronic property modification. This resulted from significant variations in the material's shape, crystallinity, and O_v inside its lattice. In terms of reactivity, all ZnO samples demonstrated excellent CO selectivity with a CO FE of approximately 80%. The Zn nanorods and nanosheets achieved the most outstanding FE values, reaching 90% at higher negative potentials, equivalent to Ag nanoparticle's reactivity. Notably, ZnO nanorods had the greatest CO current density out of all the studied materials. When compared to other ZnO-based gas diffusion electrodes, ZnO nanorods demonstrated the most promising result, affording −150 mA cm⁻² current density with the input of −1.2 V voltage while retaining a strong FE of 83%.

Furthermore, Chen and coworkers established the CO₂RR mechanism in zinc-based electrocatalysts, a set of thermally oxidized zinc foils prepared to establish a connection between the catalyst's chemical state and product selectivity.¹²⁸ Findings from in situ Raman spectroscopy, XAS, and PXRD indicated that surface Zn(II) species primarily produced carbon monoxide, while Zn(0) species were linked to formate production. Notably, thermal oxidation, which disrupted a dense oxide layer on the zinc foil surface, resulted in a fourfold increase in the FE for formate in the CO₂RR. In situ analyses highlighted that the chemical state of zinc electrocatalysts played a critical role in determining product distribution, offering a viable approach for tuning selectivity in zinc-based CO₂RR electrocatalysts. Therefore, it is clear that not only morphology but also the valence state of Zn has a significant impact on CO₂RR reactivity.

To reduce the impact of the Zn substrate in XRD measurements, Qin and coworkers investigated a range of Zn catalysts with different ratios of Zn(002) to Zn(101) crystal facets.¹²⁹ A detailed investigation of catalytic reactivity revealed that the CO/H₂ ratio was influenced by the particular electrochemical reduction circumstances as well as the Zn crystal facet ratio. To confirm surface changes on the electrode during the CO₂RR, ATR-IRAS analyses were conducted. The CO₂ was represented by the signal at 2358 cm⁻¹, whose intensity progressively increased in the early stages, indicating progressive CO₂ adsorption on the electrode's surface. After 20 minutes, this intensity stabilized due to CO₂ adsorption saturation. The C=O of formate was associated with the 1723 cm⁻¹ signal, whereas COO⁻ was associated with the 1436 and 1596 cm⁻¹ peaks. Furthermore, the signals for the –OH group in COOH and free –OH were also witnessed in Raman data.

As electrolysis occurred, these grouping intensities surged, signifying their accumulation on the electrode surface and alteration of the electrolytic environment. The CO adsorbed on Zn was identified as the cause of two small signals in Raman. Adsorbed over the electrode surface, CO₂ and HCOOO⁻ are first interconverted. Then, (i) a reversible transformation from HCOOO⁻ to CO₃²⁻ radicals took place, serving as a proton source; (ii) CO₂ accepted electrons to become CO₂*⁻; (iii) the CO₂*⁻ radical underwent further electrochemical reduction by accessing electrons from (i), forming COOH* intermediate; (iv) a small fraction of COOH* was reduced to COOH⁻, desorbing over the catalyst generating HCOOH; and (v) the majority of COOH* was reduced, producing CO*, which then desorbed from Zn to produce CO, the primary product.

In the CO₂RR to CO, the energy necessity of COOH* and CO* was verified using DFT analysis. The process of converting CO₂ to COOH* was the rate-regulating phase. The CO₂RR was inhibited if the relative energy between CO₂ and COOH* was too high. As the potential decreased, the CO₂RR was initiated over the (101) facet of Zn with −(η + 0.2) V, where the CO₂ to COOH* energy was −0.015 eV. On the (002) facet of Zn, however, this energy barrier stayed high at 0.343 eV until the CO₂RR happened at −(η + 0.6) V, with the energy of CO₂ to COOH* at −0.015 eV.¹²⁹

DFT data evidenced that the (101) facet of Zn favored the CO₂RR at a lower reduction potential due to a 0.358 eV lower CO₂ to COOH* relative energy compared to Zn(002). This aligned with experimental observations in the CO₂RR. At the height of the FEs of CO, Zn-1 and Zn-2 needed potentials of −1.2 and −1.1 V, respectively, but Zn-3, which had a higher percentage of Zn(101) facets, needed a potential of −0.9 V. Experimental findings and DFT simulations both verified that CO₂RR to CO performance could be further enhanced by increasing Zn(101) facets.

In a work, for the efficient CO₂RR reactivity, nanoporous ZnO was created by hydrothermal synthesis followed by thermal breakdown.¹³⁰ The ZnO was converted to Zn throughout the electrochemical conditions used for the CO₂RR, according to in situ XAS. Having a very good CO FE of 92.0%, the ZnO showed noticeably better reactivity than Zn foil. This improvement suggested that the nano-porosity of the catalyst boosted the CO₂RR reactivity, possibly due to the enormous surface area and a higher number of coordination-unsaturated surface atoms. In another report, Yeo's group reported ZnO nanoparticles for electrocatalytic CO₂RR reactivity.

The development of heterostructures and heterointerfaces based on zinc-based materials was crucial to improving CO₂RR performance. To comprehend the link between methanol selectivity and structure in the Cu–ZnO system for the CO₂RR, a thorough structural and catalytic investigation of ZnO/Cu/Al₂O₃ catalysts was carried out.¹³¹ Two different synthetic procedures were applied to produce ZnO/Cu with varying morphologies, resulting in distinct outcomes in terms of methanol selectivity and production. Smaller copper particles were created by the incipient-wetness impregnation technique, which produced a greater distribution of ZnO on both Cu and Al₂O₃. On the other hand, zinc oxide was directly deposited over the pre-existing Cu via CVD, forming more organized ZnO and Cu structures.¹³¹ Analysis through spectroscopy and microscopy showed that chemical vapor deposition provides a more advantageous alignment of zinc oxide with copper, enhancing ZnO–Cu interface formation, which was essential for selective CO₂ conversion to methanol. Meanwhile, the highly dispersed Cu–ZnO phases in the impregnated sample resulted in less effective contact between phases, decreasing both reactivity to generate CH₃OH, preferring the reverse water–gas-shift process, and producing more CO. These results demonstrated that, in the Cu–ZnO system, obtaining high reactivity toward methanol required an ideal ZnO–Cu phase interaction, which outweighed the advantage of greater Cu dispersion.

In addition to these studies, researchers have developed bimetallic Zn-based materials for electrocatalytic CO₂RR reactivity. In this regard, oxide-based bimetallic CuZn_x catalysts have been suggested as viable substitutes for attaining efficient selectivity for a range of essential products.¹³² The CuO was synthesized using a co-precipitation technique with varying concentrations of ZnO dopants. An aqueous carbonate solution was used to evaluate the electrochemical reactivity. Ethanol production and FE were assessed for various CuO–ZnO_x (where x denotes 5, 10, 15, and 20 weight percent). Among the catalysts studied, CuO–ZnO₁₀ produced C₂H₅OH at a rate of around 121 μmol h⁻¹ L⁻¹, with a maximum FE of 22.27%. The CuO–ZnO₁₀ showed long-term stability for a minimum of 12 h. The catalyst's reactive sites were better understood by further analysis, which showed that CO₂ reduction took place on reduced reactive sites instead of on the metal oxides.

After these efforts, a ZnTe-MOF-based ZnTe/ZnO@C material supported by NC was demonstrated with efficient CO₂RR reactivity.²⁷ The catalyst was fabricated using the solvothermal method followed by calcination. The XRD patterns manifested diffraction peaks corresponding to ZnTe and ZnO in ZnTe/ZnO@C and ZnO@C. Raman analysis was performed to evaluate the graphitization level in the created materials. Both displayed a distinctive D band at 1336 cm⁻¹ and a G band at 1585 cm⁻¹. The I_D/I_G content in ZnTe/ZnO@C suggested a high level of graphitization in the C-layer, which was beneficial for electron transport. The ZnTe/ZnO@C demonstrated significantly improved CO₂RR reactivity to give formate, attaining an 86% selectivity and stable current density. Compared to previous Zn-based materials, ZnTe/ZnO@C showed greater reactivity and selectivity. According to DFT research, the stabilization of the essential HCOO* on ZnTe promoted the selective production of formate.

In recent developments, Zn-single atom catalysts were studied for the electrocatalytic CO₂RR. Han et al. created an electrocatalyst named SA-Zn/MNC for the electroreduction of CO₂ to methane to enable good electron transport.¹³³ TEM and HRTEM pictures manifested the creation of SA-Zn/MNC (Fig. 16a and b). Raman spectra and specific surface area analysis indicated that incorporating zinc into the MNC led to an increase in structural defects and consequently enhanced the creation of micropores. According to the Zn K-edge XANES, the extent of the first signal for SA-Zn/MNC after the edge was comparable to that of ZnCl₂ but higher in contrast to Zn foil. The pre-edge characteristic also moved to a larger energy value than the reference, more closely matching ZnCl₂. These findings indicated the electronic state of Zn to be +2 (Fig. 16c). The Zn–N interaction was responsible for the noticeable signal at 1.55 Å in the Zn R space plot of SA-Zn/MNC (Fig. 16d and e). Moreover, SA-Zn/MNC showed no scattering peaks suggestive of Zn–Zn coordination. These findings validated Zn's atomic dispersion. The N 1s showed the peak for Zn–N bonding in SA-Zn/MNC (Fig. 16f).

Fig. 16 (a) and (b) TEM pictures of SA-Zn/MNC; (c) Zn K edge XANES spectra of SA-Zn/MNC compared with ZnO and ZnCl₂; (d) Zn R space profile of SA-Zn/MNC compared with ZnO and ZnCl₂; (e) fitted Zn R space profile of SA-Zn/MNC; (f) N 1s XPS of SA-Zn/MNC compared with MNS; (g) LSV profile for the CO₂RR activity of SA-Zn/MNC compared with MNC and Zn-powder; (h) potential dependent FE of SA-Zn/MNC compared with MNC and Zn-powder; and (i) methane yield rate with SA-Zn/MNC compared with MNC and Zn-powder. Reproduced from ref. 133 with permission from the American Chemical Society.

The LSV was examined to reveal the CO₂RR reactivity of SA-Zn/MNC, MNC, and Zn powder (Fig. 16g). The results showed that SA-Zn/MNC afforded current density for the CO₂RR that exceeded those of both MNC and Zn powder, indicating it had the highest activity for this reaction. The total FE for methane, carbon monoxide, and hydrogen was nearly 100% throughout the tested potential range, with no other byproducts observed. The FE of 85% was recorded with SA-Zn/MNC for CH₄ generation, which was remarkably high among the highest values documented in aqueous environments (Fig. 16h).¹³³ Moreover, 158 ± 4 μmol h⁻¹ cm⁻² was determined to be the creation rate of CH₄ (Fig. 16i). Higher quantities of KHCO₃ were found to increase the production rate of CH₄, according to an analysis of the effect of electrolyte concentration on CO₂RR reactivity.

This connection between CH₄ generation and the amount of Zn in SA-Zn/MNC demonstrated how crucial atomically distributed Zn–N sites were to the CO₂RR process. These studies highlighted the role of the N-doped carbon support in improving catalytic reactivity. In further efforts, a series of electrocatalysts having N-doped carbon have been designed for efficient CO₂RR reactivity. In this context, ZnO,¹³⁴ CuZn/rGO,¹³⁵ Zn/NG,¹³⁶ N-ZnNiO,¹³⁷ and Zn-NHPC³⁰ were demonstrated by different research groups exploring the efficient reactivity in terms of the CO₂RR. In further advancements, ZnCu-alloy,¹³⁸ Ag/Zn-alloy,¹³⁹ CuZn nanoparticles,¹⁴⁰ Pd/ZnO,¹⁴¹ Cu₅Zn₈ catalyst,¹⁴² In–Zn-alloy,¹⁴³ CuZn-catalyst,¹⁴⁴ CuZn-alloy,¹⁴⁵ CuZn-alloy,¹⁴⁶ SnZnO_x,¹⁴⁷ and CuO/ZnO¹⁴⁸ have been demonstrated for efficient reactivity for the CO₂RR.

With a fully occupied shell orbital and an electronic structure of d¹⁰, Zn²⁺ is categorized as a moderately hard acid.³⁷ It may easily coordinate with atoms such as S, N, or O. As a result, Zn-based MOFs might have coordination numbers between 4 and 6. Given their potential for a multitude of uses, these MOFs have been the subject of much research. They have recently drawn interest as proficient CO₂RR catalysts. Han and coworkers investigated the Zn-BTC MOF's capacity to reduce CO₂ in ionic liquid (IL).¹⁴⁹ Their results showed that current density and selectivity for CH₄ were strongly influenced by the nature of electrodes and the shape of the MOFs. They thoroughly examined how ILs influenced CO₂ reduction effectiveness, observing that the viscosity of the liquid and the kind of counterion both impacted ionic liquids' efficiency.

Their research showed that the electrolytes' composition and characteristics affected the reaction route and the end product's activity and selectivity. During the CO₂RR, the Zn-MOF was essential for promoting electron transport. Their investigation demonstrated that imidazolium cations first adsorbed onto the Zn-MOF surface during electrolysis. The CO₂˙⁻ intermediates were created when one electron was moved to the CO₂ the ILs had captured via Zn-MOF. Because CO adsorbed more strongly than CH₄ over the Zn-MOF, these intermediates obtained another electron to create CO, which was transformed into CH₄ through a six electron transfer process. Therefore, the specific CO₂RR to produce CH₄ depended on the synergistic action of ILs and the Zn-MOF.

Beyond organic electrolytes, using inorganic counterions has also been shown to impact the reactivity of Zn-MOF. Wang and associates used different zinc sources to create different ZIF-8 and investigated how these sources and electrolytes affected CO₂RR reactivity.¹⁵⁰ Having a FE of 65%, they discovered that ZIF-8, which was derived from ZnSO₄, had the best catalytic activity for reducing CO₂ to CO. Weaker contacts between the SO₄²⁻ and the Zn accounted for the increased reactivity of ZIF-8 including SO₄²⁻. This facilitated anion exchange for charge balancing. Furthermore, a drop in the electrolyte's hydrated anionic radius (such as NaCl) was associated with a rise in FE for converting CO₂ to CO and a corresponding fall in current density. This implied that smaller hydrated anions restricted the formation of hydrogen while enhancing anion exchange for charge balance. According to CV investigations, ZIF-8 showed a redox peak that matched the Zn center's redox reaction. While the metal nodes of ZIF-8 were considered electrocatalytic active sites, it is important to confirm this since Zn(II) is a d¹⁰ system and typically does not undergo redox reactions under the given conditions. Nonetheless, the research suggests that electrocatalytic CO₂ reduction using Zn-MOFs is feasible with the appropriate solvent.

Therefore, the ability of zinc-based catalysts to effectively convert CO₂ into useful products addresses pressing environmental concerns related to greenhouse gas emissions and contributes to the development of sustainable energy solutions.^151–154 Although Zn-based materials have emerged as promising candidates for the electrochemical CO₂RR, particularly for the selective production of CO and formate due to their moderate binding strength with *COOH intermediates and high hydrogen evolution suppression in neutral and alkaline media, several key limitations persist. These include limited multi-carbon (C₂₊) product selectivity, relatively low intrinsic activity compared to noble metals, and instability under long-term electrolysis due to catalyst degradation or restructuring. Moreover, the underlying reaction pathways and intermediate stabilization mechanisms at Zn active sites remain insufficiently understood. To overcome these challenges, future directions may focus on precise tuning of the electronic structure via heteroatom doping or alloying, constructing Zn-based heterostructures with synergistic interfaces and anchoring single Zn atoms on conductive nitrogen-doped carbon frameworks to enhance selectivity and atom utilization. Additionally, the integration of machine learning-guided design and in situ spectroscopic techniques is crucial for improving selectivity, stability, and scalability.

10. Conclusion and future perspectives

Zinc-based materials have emerged as promising candidates for electrocatalytic reduction reactions, demonstrating remarkable potential in the reduction of H₂O, CO₂, O₂, N₂, and NO₃⁻ into valuable chemicals. The unique properties of zinc, including its favorable coordination chemistry, low cost, and abundance, make it an attractive element for developing efficient catalysts. Recent advancements in the synthesis and characterization of zinc-based nanostructured materials have shown significant improvements in catalytic performance, selectivity, and stability. The exploration of innovative synthetic strategies has allowed researchers to tailor zinc-based catalysts' morphology, electronic structure, and surface characteristics, enhancing their efficiency in electrocatalytic processes. Notably, the integration of zinc with other metal components and the development of heterostructures have further boosted catalytic activity, leading to high faradaic efficiencies for target products.

Numerous studies have investigated the potential of zinc-based materials for various electrocatalytic reduction reactions, including the HER, ORR, NRR, NO₃⁻RR, and CO₂RR. However, there is noticeable absence of comprehensive reviews that systematically examine their applications in these electrocatalytic processes. This review provides an in-depth analysis of recent advancements in zinc-based materials for key reduction reactions, emphasizing the reaction pathways involved. It discusses the structural, morphological, and electronic characteristics of these materials and how optimizing these features can improve catalytic performance. Additionally, this review investigates the relationship between structure, properties, and performance, focusing on enhancing activity, stability, and efficiency in reduction reactions. It also addresses the challenges faced in the field and outlines future directions, underscoring the significant potential of zinc-based materials.

Besides electrocatalytic reduction reactions, Zn-based materials hold potential for several applications in electrocatalysis beyond the commonly discussed areas such as water splitting and CO₂ reduction. Their versatile electronic properties, tunable surface characteristics, and stability in various environments make them suitable for other emerging applications like hydrogen storage, organic pollutant degradation, and more. ZnO-based materials can act as catalysts for generating reactive oxygen species (ROS), such as hydroxyl radicals (*OH), in electrochemical systems, which are effective for breaking down organic pollutants. Zn-based catalysts can degrade complex organics like dyes, pharmaceuticals, and pesticides in wastewater through ROS-driven oxidation. Zn-based semiconductors (e.g., ZnO and ZnS) have demonstrated excellent photocatalytic activity under UV or visible light, making them suitable for removing organic pollutants. Zn-based materials offer immense potential across various electrocatalytic applications due to their versatility, tunable properties, and cost-effectiveness. While some of these applications, like pollutant degradation and nitrogen fixation, are still emerging, ongoing research into material design and performance optimization could unlock new opportunities, making Zn-based materials increasingly relevant for addressing global energy and environmental challenges.

The Zn-based materials also play a key role in commercial applications. For example, zinc oxides are used as co-catalysts with other metals in methanol synthesis and the water–gas shift reaction. These catalysts are critical for large-scale hydrogen production and methanol plants. Zinc oxide (ZnO) is widely applied for photocatalytic degradation of organic pollutants in wastewater treatment. Zn-based sorbents and catalysts are used to remove sulfur-containing compounds from fuels, meeting environmental standards for cleaner energy. Zinc chalcogenides and Zn-doped materials are explored for the HER and OER, offering potential for scalable hydrogen production. Catalysts based on Zn/Fe–NC enhance the ORR efficiency in ZABs, making them suitable for next-generation energy storage devices. Zinc-based electrocatalysts are utilized to convert CO₂ into value-added chemicals like CO and formate, aiding in carbon-neutral technology. Zinc ions serve as active centers in enzymes like carbonic anhydrase, which catalyzes the hydration of CO₂. Zinc-based catalysts facilitate the two-electron ORR pathway for hydrogen peroxide production, a critical chemical in pulp bleaching, disinfectants, and industrial oxidation processes. These catalysts are cost-effective and scalable for industrial H₂O₂ synthesis.

Although Zn-based materials have been well studied for a series of valuable applications, there are several challenges with Zn-based materials. As mentioned in the previous sections, stability is a critical challenge for catalytic materials in real-world applications, as harsh operational environments, such as extreme pH, high temperatures, or prolonged cycling, can lead to material degradation. For zinc-based materials, the main stability issues include dissolution in acidic or alkaline media, surface passivation, and structural degradation under electrochemical conditions. Addressing these issues requires careful design and modification of the catalytic materials to achieve a balance between stability and activity. Various strategies like surface engineering, doping and alloying, formation of heterostructures, and morphological tuning have been employed to enhance the stability of Zn-based materials. While enhancing stability, maintaining or even improving catalytic activity is a crucial challenge. The key is to ensure that structural modifications do not block the active sites or impede electron/proton transfer. By adopting these strategies, zinc-based catalytic materials can achieve the necessary balance of activity and durability for real-world applications, paving the way for their widespread use in energy conversion and storage systems. Furthermore, the efficiency of stability tests conducted for Zn-based electrocatalysts to evaluate industrial applicability depends on several factors, including the duration of the tests, the operating conditions, and the evaluation metrics used. The degradation mechanism can be identified during the stability test. Generally, long-term durability tests are conducted under industrially relevant conditions. During this test, in situ techniques (e.g., XPS, TEM, and operando spectroscopy) can be used to monitor structural and chemical changes during operation. Post-test analysis can be performed by ex situ analyses (e.g., SEM, XRD, and ICP-MS) to identify leaching, structural changes, and surface modifications. Moreover, computational methods can be used to predict degradation pathways and correlate them with experimental data.

If the stability tests conducted for Zn-based electrocatalysts did not thoroughly replicate industrial conditions or identify degradation mechanisms, their efficiency for evaluating industrial applicability is questionable. Incorporating comprehensive testing protocols and explicitly addressing degradation mechanisms such as leaching, structural collapse, and passivation can significantly enhance the reliability and applicability of Zn-based electrocatalysts in industrial settings.

The intrinsic catalytic activity of Zn-based materials is often lower compared to noble metal catalysts. The moderate binding energy of intermediates (e.g., *O, *OH, or *OOH) leads to suboptimal catalytic performance in reactions like the ORR and NRR. Achieving high selectivity for desired products (e.g., formate in CO₂ reduction or NH₃ in NRR) remains a significant challenge due to competing side reactions. Zinc-based materials generally exhibit poor electronic conductivity, which restricts charge transfer and overall catalytic efficiency. Scalable and reproducible synthesis of Zn-based materials with controlled morphology, size, and defect density remains difficult. However, these challenges can be tackled with the help of various techniques. The different techniques such as formation of heterostructures and composites, doping and alloying, and defect engineering can be utilized for the designing of advanced electrocatalysts. A special focus can be provided on understanding reaction pathways and intermediate binding energies through in situ and operando techniques. Zinc-based catalysts are integral to diverse commercial applications, spanning chemical synthesis, environmental protection, energy conversion, and beyond. Ongoing advancements in catalyst design and scalability will unlock their full potential for sustainable industrial processes.

After the significant progress made in the field of Zn-based materials, several outstanding questions are available preventing further progress. The questions are: How can the stability of Zn-based materials be enhanced in acidic media? Zinc leaching remains a critical bottleneck for their use in acidic environments. What are the most effective strategies for improving conductivity? Can advanced hybridization techniques or conductive frameworks make Zn-based materials competitive with noble metals? What governs the selectivity of Zn-based catalysts in multi-electron reduction reactions? What role do defects, doping, and morphology play in optimizing Zn-based catalysts? Can Zn-based materials achieve industrial-level performance?

These questions can be resolved by adopting advanced characterization tools, computational techniques, appropriate electrochemical testing, and creating collaborative platforms. Techniques like X-ray absorption spectroscopy (XAS), Fourier-transform infrared (FTIR) spectroscopy, and Raman spectroscopy can provide insights into reaction intermediates and catalyst behavior under working conditions. Tools like scanning electron microscopy (SEM), transmission electron microscopy (TEM), and atomic force microscopy (AFM) allow visualization of surface morphology and defects. Density functional theory (DFT) is essential for understanding reaction mechanisms, adsorption energies, and electronic structure. Machine learning (ML) provides predictive models that can identify promising Zn-based catalysts and optimize synthesis parameters. Extensive electrochemical investigations like Tafel and EIS analysis should be used to reveal the reaction kinetics and charge transfer properties. Open databases and collaborative research networks can accelerate the discovery of new materials. Zn-based materials hold immense potential for electrocatalytic reduction reactions due to their tunable properties and abundance. However, challenges such as stability, conductivity, and selectivity must be addressed. By leveraging advanced tools, fostering interdisciplinary collaborations, and focusing on sustainable design, future research can unlock the full potential of Zn-based materials for large-scale, efficient, and sustainable energy and chemical production. Overall, the progress in zinc-based electrocatalysts holds significant promise for advancing the field of electrocatalysis and addressing global energy and environmental challenges.

Author contributions

Dr. Baghendra Singh conducted the literature survey and contributed to the manuscript preparation. Dr. Baghendra Singh and Dr. Apparao conceptualized and edited the manuscript.

Conflicts of interest

The authors declare no conflict of interest.

Data availability

No primary research results, software or code have been included and no new data were generated or analysed as part of this review.

Acknowledgements

This work is gratefully supported by SERB (CRG/2023/001112) and CSIR (01(3050)/21/EMR-II). Baghendra Singh acknowledges the financial support from the Science and Engineering Research Board National Post-Doctoral Fellowship, Govt. of India (PDF/2023/001766) for providing financial assistance.

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