Unveiling the potential of bismuth-based catalysts for electrochemical CO₂ reduction

Abstract

Electrochemical CO₂ reduction has favorable industrial relevance due to its integrability with renewable energies and controllable product generation. Bismuth-based catalysts have emerged as promising candidates in this regard due to their intriguing electrochemical properties and cost-effectiveness. This review focuses on recent advances in bismuth-based catalysts for the electrochemical reduction of CO₂, including synthesis methods and approaches for performance improvements. Insights into product formations using Bi-based catalysts are also presented, where in situ FTIR and Raman spectroscopic studies are highlighted to understand the structural evolution of the catalysts and to decipher the mechanisms of CO₂ reduction. Further, recent progress of electrochemical CO₂ reduction from an industrial perspective and strategies for further development of the bismuth-based catalysts with high activity, selectivity and stability towards practical applications are discussed.

Keywords: Electrochemical CO₂ reduction; Bismuth; Nanomaterials; Electrocatalysts; In situ spectroscopy.

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Negar Sabouhanian

Negar Sabouhanian is a PhD candidate at the Electrochemical Technology Centre, Department of Chemistry, University of Guelph, Canada under the co-supervision of Professor Aicheng Chen and Professor Jacek Lipkowski. She received her MSc degree in Materials Engineering, specializing in Corrosion and Protection of Materials, from the University of Tehran, Iran in 2018. Her current research focuses on developing advanced nanomaterials and electrocatalysts for clean energy technologies, particularly electrochemical reduction of carbon dioxide. With a strong passion for sustainable energy solutions, Negar is contributing to advancing green technologies through her innovative work in electrochemistry.

Jacek Lipkowski

Jacek Lipkowski is a Professor Emeritus at the University of Guelph, Ontario, Canada. He received his PhD and DSc from the University of Warsaw, Poland and moved to Canada in 1983. His research interests span several areas such as electrochemistry, surface spectroscopy and biomimetics. He has authored ∼280 papers and several book chapters. Dr Lipkowski is Fellow of the Royal Society of Canada, Killam Fellow and Fellow of the International Society of Electrochemistry (ISE). He is recipient of several awards including Gold Medal, Jacques Tacussel Prize, Bioelectrochemistry Award and Frumkin Medal of ISE, Alcan Medal and John Polanyi Award of the Canadian Society for Chemistry, E. Yeager Award of ECS and Alexander von Humboldt Stiftung Research Award.

Aicheng Chen

Aicheng Chen is a Professor of Chemistry, Director of the Electrochemical Technology Centre at the University of Guelph, and Tier 1 Canada Research Chair in Electrochemistry and Nanoscience. His research interests span the areas of electrochemistry, photoelectrochemistry, biosensors, and nanoscience. He has published ∼300 journal articles. Dr Chen is Fellow of the Chemical Institute of Canada, the Royal Society of Canada, the Royal Society of Chemistry (UK) and the International Society of Electrochemistry. He is recipient of the Ontario Premier's Research Excellence Award; the Lash Miller Award and the R.C. Jacobsen Award of the ECS; the Fred Beamish Award, the Keith Laidler Award, the W.A.E. McBryde Medal and the Ricardo Aroca Award of the Canadian Society for Chemistry.

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1. Introduction

The incessant increase in atmospheric carbon dioxide (CO₂) levels is widely recognized as the primary driver of climate change/global warming.^1,2 Potential strategies for carbon capture and utilization (CCU) to mitigate CO₂ emissions have attracted considerable interest.^3–5 The electrochemical CO₂ reduction reaction (CO₂RR) has significant potential for industrial applications as it can integrate renewable energies as power sources.^6–8 In this context, reports in the literature have revealed the intensive investigation of diverse electrocatalysts and CO₂RR mechanisms.^8–10 Despite extensive research, implementing CO₂ electroreduction remains constrained due to the excessive costs involved.¹¹

Formate is one of the products of CO₂RR having substantial market potential as a valuable industrial feedstock.¹² The formic acid/formate market is anticipated to reach one megaton per year by 2030 with various applications in medical, agriculture, and textile industries.¹³ They are regarded as a vital intermediate for synthesizing valuable oxygen-containing compounds such as alcohols, esters, and acids in syngas catalysis. Additionally, the high density of formic acid (1.22 kg L⁻¹) gives it a significant volumetric hydrogen capacity of 53 g H₂ per liter, making it a promising candidate as a liquid hydrogen carrier.¹⁴ It may be utilized in energy production and storage systems such as hydrogen fuel cells.¹³ Formate/formic acid formation by electrochemical reduction of CO₂ is a promising approach due to its integrability with renewable energies, relatively low cost, practical operation and being environmentally friendly.¹⁵ Among various metal-based catalysts, bismuth-based materials have exhibited significant potential for the electrochemical reduction of CO₂ to formate with high selectivity.¹⁶ Bismuth is mainly found on the ores bismuthinite (bismuth sulfide) and bismite (bismuth oxide) as well as Bi crystals with an oxide layer. Bismuth is not very reactive and can sometimes be found as a native metal.¹⁷ Bi-based catalysts exhibited a lower overpotential and higher faradaic efficiency for formate formation compared with other metals. The high formate selectivity can be attributed to the low energy barrier of *COO⁻ intermediate formation. In addition, hydrogen evolution reaction (HER) as a competitive reaction exhibits a relatively high overpotential at Bi-based catalysts.¹⁸ These factors along with the cost-effectiveness, low toxicity and high abundance in nature in contrast to other metals make Bi-based catalysts promising for CO₂ reduction to formate in large-scale applications.^19–24 Different nanostructured monometallic Bi catalysts (e.g., nanoparticles, nanorods, nanodendrites, and nanosheets) have been synthesized and investigated for CO₂ reduction.²⁵ However, the weaker *OCHO binding energy makes it difficult to boost formate generation.²⁶ It has been shown that the addition of secondary elements to form bimetallic Bi-based catalysts might effectively improve their catalytic performance.²⁷ The binding energy of oxygen in *OCHO may be adjusted by introduction of another metal (e.g., CuBi,²⁸ BiSn,²⁹ InBi,³⁰ and ZnBi (ref. 31)), which may enhance the *OCHO adsorption through compressive strain and changes of surface electronic bands structure.²⁶ Further, it is revealed that bimetallic Bi-based catalysts not only electrochemically reduce CO₂ to formate, but also lead to the creation of other products such as propane and ethylene.^32,33In situ characterization techniques have been developed to study the reaction mechanisms and identify the key intermediates formed during CO₂ reduction. Great efforts have been made to attain industrial-compatible current densities and high stability for CO₂ reduction to formate on Bi-based catalysts using flow cells or membrane electrode assemblies (MEAs). However, achieving ampere level current densities and high electrode stability remains challenging.

In this review, we discuss the recent advances in the development of bismuth-based catalysts with a focus on synthesis methods and strategies for performance enhancement. The possibility of additional product generation using Bi-based catalysts reported in the recent literature is featured. The detection of key intermediates and monitoring of product formation via in situ FTIR and Raman spectroscopy are highlighted. Finally, recent achievements from an industrial perspective are described, and future research directions are proposed.

2. Monometallic Bi-based catalysts

2.1. Synthesis methods

Several strategies have been employed to fabricate monometallic Bi-based catalysts, including top-down exfoliation techniques, chemical reduction reactions, and the electroreduction of pre-synthesized materials (e.g., BiOX where X is Cl, Br, or I,³⁴ (BiO)₂CO₃,³⁵ Bi₂O₃,²¹ and Bi-metal organic frameworks (Bi-MOFs)).³⁶ The morphologies²¹ and coordination environments³⁷ of Bi-containing precursors significantly affect the final structure of the Bi-based catalyst. Various surface engineering techniques have been applied to improve the faradaic efficiency (FE) of formate production, including defect engineering, heteroatom doping, and the active site reconstitution in Bi nanosheets (Bi NSs).^38,39 In this section, recent advances in strategies for monometallic Bi-based catalysts synthesis, and proposed strategies for enhancing their catalytic performance are reviewed.

2.1.1. In situ electrochemical transformation. Traditional strategies for the production of monometallic Bi-based catalysts, whether through top-down or bottom-up approaches, face challenges when it comes to scaling up their production. Thus, indirect methods may offer a promising alternative for the synthesis of Bi-based catalysts,⁴⁰ with nanosheet morphologies getting significant attention.⁴¹ Numerous unsaturated Bi atom coordination sites are available in two-dimensional (2D) Bi NSs comprised of tens of atomically thin layers, which provide ample reactive sites for CO₂RR.^40,42 The in situ (electro)chemical transformation of Bi-containing pre-catalysts has gained considerable interest as an indirect method for the synthesis of Bi NS catalysts.⁴³ Changes in chemical compositions and/or structures have been widely observed during (electro)chemical transformation, giving an efficient catalyst with desirable reactive sites for CO₂RR.^44,45 Different Bi-based pre-catalysts can endow bismuth materials with unique surface properties and local environments, strongly impacting their performance for CO₂ reduction.⁴⁶ For instance, it was demonstrated that the morphologies, particle sizes, thicknesses,⁴⁰ and compositions⁴⁷ of Bi-based precursors altered the activities of Bi-based catalysts. Wang et al.⁴⁷ prepared differently oriented bismuth oxyiodide nanosheets to adjust the ratio of basal and edge planes in derived Bi NSs. They found that during the electroreduction process, iodine and oxygen atoms were dynamically removed, leaving behind a bismuth framework. They suggested that the original bismuth oxyiodide nanosheets served as templates for catalysts of CO₂RR. Their finding showed that the (004) oriented Bi₅O₇I NSs and (102) oriented BiOI NSs were transformed by electroreduction into basal- and edge-oriented Bi NSs, respectively. Basal-oriented Bi NSs exhibited higher selectivity for CO₂RR and lower activity for HER, in contrast to edge-oriented Bi NSs. It was revealed that the (003) basal plane played an important role in enhancing the FE due to its low activity for the competing HER. The highest FE of formate (98.0%) was achieved at −0.68 V vs. RHE for the basal-oriented Bi NSs. In another study conducted by Zheng et al.,⁴³ 2D bismuth oxyiodide (BiOI) was synthesized through the assembly of 1D BiOI nanotubes, which was further converted to metallic Bi via electroreduction. It was demonstrated that the abundant edge sites resulting from the distinct 2D Bi nanostructure ensured high selectivity for formate production. A series of BiOX nanoplates were also synthesized by Liu and coworkers⁴⁰ and further electroreduced by in situ transformations, showing that the thickness of the initial BiOX nanoplate precursor was important to maintain the 2D structure during the electroreduction. During preparation, the thickness of the BiOX nanoplates was controlled through the addition of polyvinylpyrrolidone (PVP) and mannitol. The Bi NSs derived from the electroreduction of BiOX nanoplates with an optimized thickness of ∼30–40 nm exhibited a maximum FE of formate (95%) at −0.9 V vs. RHE. The thickness of Bi NSs is also an important factor, as thinner Bi NSs exhibited greater selectivity as they exposed a greater number of edge sites.³⁶ Bi NSs were synthesized from Bi₂MoO₆ and BiOIO₃ precursors with two different thicknesses via cathodic reduction. A schematic of the preparation procedure is illustrated in Fig. 1a.³⁶ In the first stage, the Bi-containing precursors with different structures and compositions were prepared by a hydrothermal method, which then underwent long-term cathodic reduction at low potential. Interestingly, thinner Bi NSs (1.5 nm) were obtained using Bi₂MoO₆ as an initial precursor. A corresponding SEM image of the Bi NSs produced from Bi₂MoO₆ is presented in Fig. 1b, which shows dense arrays of large nanosheets. The electrochemical performance of the Bi NSs was investigated with linear scanning voltammetry (LSV) in a CO₂ and Ar saturated 0.5 M KHCO₃ solution as the electrolyte. Fig. 1c depicts the LSV of the Bi NSs obtained with two different thicknesses. The thinner Bi NSs exhibited a higher current density compared to the thicker Bi NSs and showed better catalytic activity. It was revealed that thinner nanosheets generated coordination-unsaturated pits, which in turn created more edge sites leading to the higher FE of formate. Jiang et al.⁴⁸ reported on the electrochemical reconstruction of bismuth oxide formate nanowires (BiOCOOH NWs) into Bi/BiO_x NSs and Bi NWs, dependent on the electrochemical reduction conditions. They demonstrated the formation of the intermediate state of Bi₂O₂CO₃ during the electroconversion of the initial precursor in a CO₂-saturated KHCO₃ solution after 500 s, which resulted in the conversion of nanowires to 2D nanosheets. The structural evolution of the initial precursor originated from the solvent-assisted ligand exchange and further dissociation of Bi³⁺ to form Bi₂O₂CO₃, which served as a sacrificial template to be further electrochemically converted to Bi/BiO_x NSs. This enabled easier electron transport through the 2D structure and surface oxide layer of the Bi/BiO_x NSs. Their findings revealed that the presence of Bi-O structures in Bi/BiO_x NSs promoted the adsorption of *CO₂⁻ and *OCHO, which facilitated CO₂ activation. Similarly, the electrochemical transformation of (BiO)₂CO₃ to Bi NSs was reported by Peng et al.³⁵ A schematics of the preparation of Bi NSs is presented in Fig. 1d. The initial (BiO)₂CO₃ precursor with a persimmon-like morphology was synthesized by a hydrothermal method, which served as a self-template to be electrochemically converted to Bi NSs. The electron transfer capacity was evaluated by the electrochemical impedance spectroscopy (EIS) as shown in Fig. 1e. The smaller charge transfer resistance of Bi NSs was due to the reduced distance for electron transport in the integrative intercrossed network structure. Fig. 1f reveals that the formate yield rate was significantly higher for Bi NSs compared with commercial Bi nanopowders, due to more exposed active sites.

Fig. 1 (a) Schematic of the preparation of ultrathin Bi NSs; (b) SEM image of 1.5 nm Bi NSs; (c) LSV curves for 3.1 nm Bi NSs and 1.5 nm Bi NSs in a CO₂ and Ar saturated 0.5 M KHCO₃ solution³⁶ (copyright 2023 Elsevier); (d) schematic of the preparation of Bi NSs; (e) Nyquist plots and (f) formate yielded rates for CO₂RR over Bi NSs and Bi nanopowders³⁵ (copyright 2021 John Wiley and Sons).

2.1.2. Metal–organic framework (MOF). Metal–organic frameworks (MOFs) represent a class of materials comprising an organized and extended structure of metal ions linked together by rigid organic ligands.^49,50 The organic linkers in the MOF structure can alter the adsorption behavior of water and CO₂ reduction intermediates, to improve the selectivity of CO₂RR.⁵¹ Further, the permanent porosity of MOFs makes them interesting materials for an extensive range of applications. Covalent organic frameworks (COFs) may act as electron current collectors and enhance the adsorption of intermediates.⁵² Electrochemical CO₂ reduction has been reported in recent publications using Bi-containing MOFs.⁵³ Bi-MOFs were utilized directly as catalysts or indirectly as pre-catalysts for further reconstruction to form highly active and selective Bi-based materials for CO₂RR. For example, a cost-effective TAL-33 MOF material was synthesized followed by optimized carbonization and used directly as a catalyst with metallic Bi sites for the conversion of CO₂RR to formate, with the FE attaining 100%.¹³ Bi-BTC MOF (CAU-17) was prepared and investigated for CO₂RR in another study, which exhibited a 92.2% FE of formate at −0.9 V vs. RHE, with over 30 h of stability. In contrast to other Bi-based catalysts (e.g., Bi NSs) that were rapidly and completely reduced from Bi(+3) to metallic Bi(0), it was suggested that the MOF structure could preserve Bi in the +3 oxidation state during CO₂RR based on the in situ X-ray absorption near-edge structure (XANES) spectroscopic measurments.⁵¹ In another study by Liu et al.,⁵⁴ the Bi-MOF was coated on a gas diffusion electrode and used as a catalyst for CO₂RR. A solvothermal method was employed to prepare the Bi-MOF using Bi(NO₃)₃·5H₂O and H₃BTC, which were added into methanol and dimethylformamide (DMF) solutions. Following CO₂RR, surface reconstruction was observed where the initially smooth surface was converted to perpendicular sheets with nanoscale flakes on the surface. A study of the phase evolution revealed that the Bi-MOF was partially reduced to metallic Bi and partially converted to the Bi₂O_2.5 phase. It was proposed that the Bi/Bi–O interface facilitated the adsorption of intermediates, which resulted in improving CO₂RR activities and HCOOH selectivity. Recent studies have reported that Bi–O bonds can facilitate CO₂ adsorption and *OOCH intermediate formation; thus, enhancing the FE of formate. Consequently, Bi-based catalysts containing Bi–O bonds such as bismuth oxides (Bi₂O₃) and Bi₂O₂CO₃ have been synthesized and evaluated for CO₂RR.²⁵ It was reported that some Bi-MOFs were in situ reconstructed to Bi₂O₂CO₃, which is the active material for CO₂RR.⁵⁵ Cao et al.⁵⁶ synthesized a Bi-based MOF material Bi₂(BDC)₃ (referred to as Bi-BDC) via a solvothermal method at an optimized temperature (120 °C) to create sphere-like structures assembled by small nanowires. It exhibited a higher current density compared to commercial Bi₂O₃ catalysts, indicating superior catalytic activities. The structural evolution during the CO₂RR was investigated and it was found that the catalyst was transformed into a pore-rich volcanic stone-like morphology after electrolysis. Their results confirmed that the Bi-BDC MOF catalyst was converted to Bi₂O₂CO_3. The Bi–O bonds of Bi-carboxylate MOF can be destroyed by bicarbonate ions in solution, leading to its in situ reconstruction to Bi₂O₂CO₃. The formate FE was ∼90% after 36 h of CO₂RR, showing long-term stability for formate generation, because Bi₂O₂CO₃ could maintain active Bi–O sites in the structure. Based on density-functional theory (DFT) calculations, it was revealed that the initial hydrogenation step was the rate-determining step, and the CO₂ reduction on the Bi-BDC catalyst occurred through proton-coupled electron transfer (PCET) via the *OCHO intermediate, leading to the high selectivity of formate generation. Huang et al.⁵⁷ also reported the reconstruction of Bi-MOF (Bi-BTC) to Bi₂O₂CO₃. The XRD results shown in Fig. 2a confirmed the formation of Bi₂O₂CO₃ after 5 min of electrolysis. The assigned peaks of Bi₂O₂CO₃ gradually sharpened through prolonged electrolysis, due to the creation of Bi₂O₂CO₃ with a well-defined crystal shape. The SEM images of the Bi-BTC MOF before and following electrolysis are presented in Fig. 2b and c. The transformation of the Bi-MOF with a nanocolumn morphology to Bi NSs possessing an ultrathin nanosheet structure after electrolysis could be clearly observed. This morphological transition was due to the charge attraction between hard base ions (HCO₃⁻) and the intermediate acid (Bi³⁺) that could destroy Bi–O bonds in the Bi-MOF.

Fig. 2 (a) XRD patterns of Bi-BTC after different electrolysis times (0 min, 5 min, 10 min, 20 min, 1 h, and 2 h); SEM image of Bi-BTC (b) before and (c) following electrolysis⁵⁷ (copyright 2024 American Chemical Society).

2.1.3. Other methods. Solution-based synthesis strategies including solvothermal or chemical reduction have been proposed for the preparation of Bi-based catalysts. NaBH₄ has been used as a reductant to chemically reduce Bi(NO₃)₃ (ref. 38) or BiCl₃ (ref. 58) as Bi precursors, to synthesize Bi NSs with an FE of formate of >90%. It was shown that the reducing capacity of the solvents and reductants could alter the morphology of the synthesized Bi nanomaterials. For example, Yu et al.⁵⁹ controlled the reduction rate of Bi(NO₃)₃·5H₂O, and realized that the slow reduction rate facilitated by the solvothermal method resulted in the formation of porous Bi NSs. However, a fast reduction rate through the addition of NaBH₄ resulted in the formation of Bi nanoparticles. The feasibility and scalability of wet chemical methods made them interesting for the synthesis of Bi-based catalysts.

2.2. Strategies for improving performance

Defect engineering has been employed as an effective strategy to enhance CO₂RR performance by improving selectivity, activity and stability of the catalyst. The grain boundaries in polycrystalline materials may create sites with improved activity for CO₂ reduction in comparison to the competing HER.⁶⁰ The selectivity of Bi-based catalysts is enhanced by improving the affinity to the *OCHO intermediate. It has been shown that the adequate exposure of edge sites and defects is a successful approach for improving the selectivity of the Bi-based catalysts in CO₂RR.^16,34,36 The defects of reconstructed Bi strongly depend on the initial morphology and coordination environment of Bi-based pre-catalysts.³⁷ For example, bismuth oxide nanosheets could be converted into porous bismuth nanosheets (Bi PNSs) with abundant kink sites on the pore walls,⁶¹ or converted to Bi nanoribbons by an in-plane confined hydrogen-reduction strategy with abundant Bi–O edge sites.⁶² Bismuth sulfide (Bi₂S₃) nanorods were electroreduced to defect-rich metallic Bi.⁶³ The major impacts of the preferential exposure sites and defect engineering were studied by Xu and coworkers.³⁴ The authors synthesized BiOBr nanosheets as Bi-containing precursors using a hydrothermal method, which was then converted to Bi NSs by topotactic transformation. As distinct Bi sources, cetyltrimethylammonium bromide (CTAB) and KBr were used during the hydrothermal process to control crystallization and form BiOBr NS with rich edge and terrace sites, respectively. Following topotactic transformation, preferential exposure sites were maintained, and a certain quantity of defect sites was also produced. DFT calculations were performed to evaluate the effects of edge sites, terrace sites, and defects on *OCHO intermediate formation as the most energetically favored reaction pathway toward formate creation. It was revealed that the formation energy of *OCHO on the Bi-edge sites was lower than that of the terrace sites, and defects on the edge sites could further decrease its Gibbs-free energy. It was determined that the edge/defect-rich Bi NS with a dramatically enlarged surface area exhibited high performance for CO₂RR with a current density of up to 870 mA cm⁻² at −1.08 V vs. RHE and FE higher than 90% for formate generation. In another study conducted by Wang et al.,⁶⁴ metal Bi with abundant defects (Bi-D) was synthesized via a solvothermal method and showed a 93.9% FE of formate at −0.9 V vs. RHE. The presence of amorphous phases introduced abundant defects and unsaturated active sites that could enhance the FE compared with commercial Bi powder. The introduction of oxygen vacancies as defects was shown by Ren et al.⁶⁵ to increase the activity and selectivity in a wide potential window, where the 2D Bi/Bi₂O₃ catalyst that possessed abundant oxygen vacancies (Bi/Bi₂O₃–O_v) exhibited high selectivity for the generation of formate with an FE of >90% in a wide potential range of −0.7 to −1.35 V (RHE). It was shown that the modified adsorbing-desorbing property was due to the abundant oxygen vacancies provided adsorption sites for CO₂ molecules on the catalyst surface. CO₂ molecules were thus ready to be reduced even at high negative potentials (up to −1.35 V (RHE)). The FE of formate remained at around 90% with no obvious decrease after 200 h at −0.8 and −0.85 V (RHE), showing the high stability of Bi/Bi₂O₃–O_v catalyst. In addition to oxygen vacancies, amorphous regions as indicated in Fig. 3a (red dotted circles) were observed. The Bi/Bi₂O₃–O_v catalyst showed a higher formate FE than the same catalysts without oxygen vacancies (Fig. 3b). DFT calculations revealed that the Bi/Bi₂O₃–O_v catalyst exhibited a lower energy barrier for *OCHO formation than the Bi/Bi₂O₃ catalyst (Fig. 3c), confirming the higher activity and selectivity of Bi/Bi₂O₃–O_v for CO₂RR to formate in a wide range of potential. The numerous O vacancies were suggested to create frustrated Lewis pairs (FLPs) on the surface to promote CO₂RR.

Fig. 3 (a) HRTEM image of Bi/Bi₂O₃–O_v; (b) FE of formate for Bi/Bi₂O₃ and Bi/Bi₂O₃–O_v catalysts measured at different potentials in flow cells using 1 M KOH electrolyte; (c) free energy diagrams of CO₂RR to formate using Bi/Bi₂O₃ and Bi/Bi₂O₃–O_v catalysts⁶⁵ (copyright 2022 John Wiley and Sons); (d) schematic of CO₂ reduction mechanism using Bi-S2 catalyst; (e) HR-TEM image for Bi-S2 catalysts⁶⁶ (copyright 2023 Elsevier).

The creation of disordered metal sites has been proposed by Wang et al.¹⁹ as an efficient defect engineering strategy to activate CO₂ and enhance activity for formate formation. It has been suggested that CO₂ molecules tended to be adsorbed to and activated on the disorder-engineered Bi sites. It was observed that the distorted metal sites could enhance localized electron transfer to the antibonding π* orbital of adsorbed CO₂ to bend the linear CO₂ molecules. The richly lattice distorted Bi NSs exhibited a high value of formate FE, reaching ∼100% at a current density of 200 mA cm⁻².

It was confirmed that the introduction of p-block atoms into bismuth could modify its electronic structure and alter the energy required for the generation of intermediates.⁶⁷ The introduction of appropriate heteroatoms may modify the electronic density of Bi p-orbitals, thus enhancing the adsorption of the *OCHO intermediates and improving the intrinsic activity for CO₂ reduction.⁶⁸ For instance, the edge defects of Bi NSs may be coordinated by heteroatom dopants such as sulfur,^37,66 titanium,⁶⁹ or copper³⁹ to enhance CO₂RR selectivity, while suppressing the competing HER. Chen et al.⁶⁷ showed that boron doping could induce the formation of electron-rich Bi, thus facilitating the reduction of *OCHO. A similar electron enrichment effect was also observed by Ti doping, which enhanced interactions between the active sites and *OCHO intermediates. For pure Bi, *OCHO intermediates are strongly adsorbed on the surface, making them difficult to desorb as formate.⁶⁷ It was revealed that the electron enrichment of Bi could weaken the binding strengths between the active metal centers and oxygen atoms, thereby lowering the barrier for generating *OCHO intermediate.⁶⁹ Furthermore, the formation of H* as a key intermediate for HER may be strongly suppressed by doping.⁶⁷ A recent study conducted by Wang et al.⁶⁶ demonstrated that sulfur doping not only induced charge redistribution around Bi atoms but also activated water molecules to provide sufficient for CO₂RR rather than HER (Fig. 3d) to optimize the reaction pathway toward formate formation. The HR-STEM image of S-doped Bi in Fig. 3e revealed a profusion of defects due to the breaking of Bi–O bonds in the initial Bi-containing precursor following topotactic transformation. The effects of sulfur doping on the catalytic activity of Bi NSs were investigated using DFT calculations as reported by Lv et al.,³⁷ indicating that sulfur dopants existed primarily at the edge sites of Bi NSs. This translated to the strong adsorption capacities of *OCHO intermediates while inhibiting the production of CO and H₂.

Recent studies have claimed that the bismuth subcarbonate (Bi₂O₂CO₃) is a stable Bi phase under CO₂RR that may serve as the active phase for the generation of formate.⁷⁰ Bi₂O₂CO₃ can provide high carrier mobility due to its thin thickness and limits nanoparticle growth, preventing disordered aggregation and preserving the number of active sites.²⁶ The establishment of the Bi–Bi₂O₂CO₃ interface has been suggested as an efficient strategy to promote CO₂ activation and the formation of key intermediates.⁶³ Liu et al.²⁵ synthesized flower-like Bi NSs and confirmed the formation of stable Bi–Bi₂O₂CO₃ interfaces after exposure to air. It was shown that the electrode containing Bi–Bi₂O₂CO₃ interfaces exhibited improved CO₂ reduction activity in contrast to the bulk Bi electrode, and the FE of formate could be enhanced up to 89% at −1.07 V vs. RHE. Two-dimensional nanoflake Bi₂O₂CO₃ was utilized as a substrate to load InO_x nanodots for efficient CO₂ reduction to formate. A good performance of the catalysts with a high FE of 90.83% at a current density of 200 mA cm⁻² was attributed to the exposure of the active sites.²⁶

3. Bimetallic Bi-based catalysts

Bimetallic electrocatalysts typically exhibit higher activity and selectivity for CO₂ reduction in contrast to monometallic catalysts since it is an effective approach for adjusting the composition, stabilizing key intermediates, optimizing the electronic structure, and suppressing competing reactions.^20,71,72 The careful and rational design of the alloy composition and structure can enhance the selective adsorption of intermediates at active sites, lowering activation barriers and favoring desired reaction pathways. In this section, recently reported Bi-based bimetallic catalysts for the generation of formate and other products are reviewed.

3.1. Formate generation

Cu and Bi have been widely investigated in bimetallic systems to augment the generation of formate.⁷³ The introduction of Cu into Bi enables the reduction of the energy barriers for intermediate formation through effective electronic structure modifications, facilitating the creation of formate.^74,75 Yang et al.⁷⁶ studied the effect of electronic structure modifications on the CO₂ adsorption at Bi, Bi/Cu₉S₅, and Bi/Cu₉S₈ catalysts. The projected density of states (PDOS) results showed that the interaction of *OCHO with Bi was mainly due to the s-p orbital hybridization. However, the heterojunctions Bi/Cu₉S₅ and Bi/Cu₉S₈ provided more Bi orbital hybridization mainly due to the p-d orbital hybridization, enhancing the adsorption of *OCHO intermediate. Recently, differently structured CuBi catalysts were synthesized utilizing various techniques (e.g., electrodeposition,⁷² hydrothermal,⁷⁷ derivations from MOFs,⁷⁸ and galvanic exchange reactions¹¹). Liu et al.⁷⁹ synthesized BiCu on a Cu foam by coupling a hydrothermal reaction followed by electrochemical transformation. Cu²⁺ and Bi³⁺ ions were co-deposited on a Cu foam to create a BiCu pre-catalyst (BiCu/CF-p), which had close interactions between Bi and Cu. After further electrochemical transformation, the obtained bimetallic BiCu catalyst on Cu foam (BiCu/CF) exhibited an unexpectedly high formate current density. Fig. 4a illustrates their synthesis procedures, which show that the delocalization of Bi p-orbitals induced by nearby metal Cu atoms enhanced the CO₂RR pathway. This interaction facilitated the hybridization of orbitals of Bi atoms and *OCHO intermediates, which created additional anti-bonding orbitals. Consequently, the *OCHO intermediates were stabilized and the thermodynamic barrier of CO₂RR was reduced. Similarly, Lou et al.⁷³ successfully co-electrodeposited CuBi bimetallic catalysts on a derived copper foam using complexing agents like trisodium citrate dehydrate in the electrolyte solution. In this work, the researchers confirmed that the applied potential for co-electrodeposition had a significant impact on the growth mode of the catalyst, which altered its performance to selectively convert CO₂ to formate. It was observed that a needle-like bimetallic CuBi structure (Fig. 4b) was formed at −0.6 V (Ag/AgCl), which had irregular coverage and showed the highest formate FE (94.4%) at −0.97 V (RHE). The tip of the needles could enhance the concentration of the adsorbed CO₂ on the catalyst surface due to the field-induced reagent concentration (FIRC) effect. In another study conducted by Xue et al.⁷⁸ a novel Cu/Bi bimetallic catalyst with a cylindrical morphology containing bimetallic nanoparticles was derived from MOFs (Fig. 4c). The FE of formate attained 93% at −0.94 V (RHE), which was attributed to the stronger adsorption of CO₂⁻ intermediates. The electron transfer of Cu to Bi could tune the binding strength of CO₂⁻ intermediates, leading to improving electrocatalytic selectivity toward formate. A new strategy for the synthesis of 3D Cu–Bi nanofoam electrodes was reported by Yang et al.,⁸⁰ who employed a rapid thermal shock synthesis technique followed by porosity engineering via acid etching and electroreduction.

Fig. 4 (a) Schematic of fabrication pathway for a BiCu/CF electrode⁷⁹ (copyright 2022 Elsevier); (b) SEM image of CuBi bimetallic catalyst electrode synthesized at −0.6 V⁷³ (copyright 2022 Elsevier); (c) SEM image of the optimized Cu₁–Bi/Bi₂O₃@C catalyst⁷⁸ (copyright 2022 John Wiley and Sons); (d) FE of CO₂RR over Cu–Bi electrocatalysts at −0.9 V (vs. RHE)⁸¹ (copyright 2022 John Wiley and Sons); (e) free energy diagrams of various pathways of CO₂RR to formate on a CuBi75 (211) plane and (f) proposed mechanism of CO₂ reduction on the CuBi75 catalyst⁸² (copyright 2021 Elsevier).

In addition to the morphologies of the bimetallic CuBi catalysts, their compositions played key roles in determining their activities⁷⁷ and selectivities.³² Li et al.⁸¹ prepared self-supporting Cu–Bi aerogel catalysts at different molar ratios (Fig. 4d) and showed that the selectivity could be modified by altering the molar ratio of Cu/Bi. A high formate FE (96.57%) was achieved using a Cu₁Bi₂ catalyst. They suggested that the 3D self-supporting structure and high surface area of Cu–Bi aerogels could facilitate electron transfer through more transport channels. As a result, it enhanced the reaction of the intermediates with protons/electrons, leading to higher CO₂RR catalytic efficiency. In another study, a composite Cu₁Bi₁ bimetallic catalyst with a ginger root-like structure (CuO/CuBi₂O₄) was synthesized by Ren et al.,⁸³ which exhibited a high FE (98.07%) for formate at −0.98 V vs. RHE. It was demonstrated that the Cu–Bi interface could provide abundant active sites for CO₂RR, and the presence of bismuth–oxygen bonds stabilized the adsorbed CO₂^*− intermediate. According to the literature, it was demonstrated that the conversion of CO₂ to formate on CuBi catalysts was more favorable through the HCOO* pathway since it was considerably downhill in energy (−0.66 eV). The corresponding free energy diagram of different pathways and the proposed mechanism of CO₂ reduction are shown in Fig. 4e and f.⁸² CO₂ was preferably converted to formate with the HCOO* intermediate.

Recently, bimetallic bismuth-based catalysts integrated with other metals such as Sn, In, and Zn have been explored. For instance, bimetallic Bi/Sn catalysts were prepared by a two-step electrodeposition method,²⁹ and it was found that the morphology and catalytic activity could be greatly affected by the deposition time. The needle-like structure in Fig. 5a was created through the deposition of metallic Bi for 5 min followed by the deposition of metallic Sn for 60 min (Bi₅Sn₆₀) on a copper mesh substrate. It was demonstrated that Bi₅Sn₆₀ had a high formate production rate (634.3 μmol cm⁻² h⁻¹) at −1.0 V (vs. RHE). This remarkable formate generation rate was related to the modification of the electronic structure, which enhanced the interactions between the active sites and *OCHO intermediates. A facile co-electrodeposition method was suggested by Yang et al.⁸⁴ for the synthesis of bimetallic SnBi catalysts. They utilized the specifically formulated electrolyte solution with an adjusted pH of 8–10 at the temperature of 60 °C to obtain a desirable alloy with the best performance for CO₂ reduction. This group reported a high formate FE of 96.1% at −1.06 V vs. RHE for a Sn_0.5Bi catalyst (0.5 Sn²⁺/Bi³⁺ molar ratio), which was attributed to a large surface area with an abundance of active sites and defects. It was reported that the tin metal oxide/bismuth metal oxide interface stabilized the CO₂⁻ intermediate and suppressed HER. Also, the electronic coupling at the interfaces of Sn and Bi led to the *OCHO formation pathway, thus promoting the formate generation. Xu et al.⁸⁵ prepared a Sn-doped Bi nanowire bundle (NB) through the in situ reconstruction of Sn-doped Bi₂S₃ precursors. By optimizing the doping concentration, a remarkable performance for CO₂RR to formate could be achieved. The Sn_1/24-Bi NBs showed a high FE of formate >90% over a wide potential window of −0.5 to −1.9 V vs. RHE. The excellent activity of the Sn_1/24-Bi NBs catalysts originated from the electron-rich surface, as well as a lower reaction kinetic barrier. The calculation of the Fermi level of both Bi and Sn_1/24-Bi catalysts exhibited the availability of more free electrons on the surface of Sn_1/24-Bi catalyst, improving the CO₂⁻ adsorption.

Fig. 5 (a) SEM image of Bi₅Sn₆₀ catalyst²⁹ (copyright 2022 Elsevier); (b) the measured FE of formate for InBi catalysts with different compositions at −0.94 V (solid bar) and the DFT-calculated WF for In, In₁₆Bi₈₄, and Bi (orange line)³⁰ (copyright 2022 American Chemical Society); (c) SEM image of the ZnBi-3 electrode at 3000× magnification. (d) LSV curves of Zn and ZnBi electrodes recorded at a 20 mV s⁻¹ scan rate for CO₂RR (solid lines) and HER (dashed lines)⁸⁶ (copyright 2024 American Chemical Society).

Zhou et al.⁸⁷ synthesized a defective BiIn catalyst and studied the influences of defective surfaces on the promotion of HCOOH formation, showing that the introduction of indium to bismuth is an effective approach to enhance the FE of formate via the optimization of the binding energy of *OCHO. Phosphorus-doped BiIn was used as a pre-catalyst to create defective surfaces during the self-reconstruction. It was demonstrated that the defective sites increased *OH adsorption, promoted water dissociation, and enhanced CO₂RR kinetics. The Bi : In ratio in BiIn catalysts can be optimized to attain high activity and selectivity. For this purpose, Wang et al.⁸⁸ prepared Bi-In₂O₃ nanoflower catalysts with different Bi/In ratios and investigated the relationship between the catalyst composition and CO₂RR performance. It was found that the catalyst with a Bi/In ratio of 6 : 94 had a high FE of formate (88.1%) at −0.7 V vs. RHE. Likewise, indium–bismuth nanosphere catalysts with different compositions were synthesized by Tan et al.³⁰ and tested for CO₂RR to formate. The In₁₆Bi₈₄ had the highest FE (∼100%) of formate at −0.94 V vs. RHE as seen in Fig. 5b. It was concluded that the presence of Bi enabled electrons to flow from Bi to In and provided additional active sites for CO₂RR. Improvements in the performance of bimetallic BiIn catalysts via defect engineering were proposed by Yang et al.⁸⁹ The researchers showed that the oxygen vacancies originating from the lattice mismatches of Bi₂O₃ and In₂O₃ could reduce the CO₂ activation energy. DFT calculations confirmed that CO₂ molecules were mainly adsorbed by the oxygen vacancies, and the dominant pathway was through oxygen vacancies. MOF-derived Bi/In bimetallic oxide nanoparticles embedded in carbon networks showed excellent selectivity for formate due to the high surface area, desirable pore size distribution, and high electrical conductivity of the carbon network as well as the synergistic effect of Bi and In bimetallic components.⁹⁰

As a nonprecious earth-abundant metal, Zn is a promising electrocatalyst for CO₂RR. Recently, the synergistic effects of Zn and Bi have been recognized, and bimetallic ZnBi catalysts have been studied for the CO₂RR to formate.⁹¹ Wang et al.³¹ synthesized a Zn–Bi bimetallic catalyst (Zn-Bi₂O₃/CN) and revealed that the surface Bi–O structure and synergistic Zn–Bi effect might enhance the CO₂RR. DFT calculations indicated that the presence of Zn reduced the energy barrier of HCOO* formation, thus facilitating the production of formate. A hydrothermal process was employed by Zhang et al.⁹¹ to prepare bimetallic ZnBi catalysts with a formate FE of 94% at −0.8 V vs. RHE. A feasible and cost-effective strategy was proposed by Sabouhanian et al.⁸⁶ to grow bismuth nanodendrites on the Zn surface. The ZnBi catalysts were synthesized by immersing the electrodeposited Zn in a bismuth nitrate solution. Due to differences in the reduction potentials of Zn²⁺ and Bi³⁺, galvanic replacement took place and Bi nanodendrites grew uniformly on the surface (Fig. 5c). Fig. 5d depicts the activities of Zn and ZnBi electrodes for CO₂RR (solid lines) and HER (dashed lines). It was observed that the CO₂RR activity increased significantly after 60 s of immersion (ZnBi-1) and was boosted further at 90 s (ZnBi-2). After 90 s, the CO₂RR was only slightly improved for 120 s of the galvanic replacement time (ZnBi-3). Furthermore, the additional incorporation of Bi decreased the onset potential for CO₂RR. In contrast, the HER activity remained almost the same by introducing Bi as a secondary element. In situ IR spectroscopy determined that CO₂ reduction at the ZnBi catalysts proceeded through the generation of the adsorbed *COO⁻ intermediate.

3.2. Generation of other products

Although most of the reported bimetallic CuBi catalysts exhibited high selectivity for the production of formate, recent research has suggested the feasibility of generating other products.³² Azenha et al.⁹² reported the deposition of Bi on CuO NWs under various charge levels passed through the substrate; the formed catalyst demonstrated an exceptionally high selectivity of CO₂RR for the generation of propane in a 0.1 M KHCO₃ electrolyte, which achieved an FE of 85.4% (Fig. 6a). This remarkable catalytic performance was attributed to enhanced CO₂ and CO adsorption capacities due to abundant oxygen defects. In another study, Wang et al.³² developed bimetallic Cu_xBi aerogel catalysts by simultaneously reducing CuCl₂·2H₂O and BiCl₃ with NaBH₄, and controlled the composition of Cu_xBi aerogels where X was 5, 10, 50, and 100. Interestingly, the product selectivity varied from CO to CH₄, C₂H₄, or formate. A schematic for the synthesis of Cu_xBi aerogels with different selectivity is presented in Fig. 6b. It was demonstrated that the introduction of varying amounts of Bi resulted in changes to the Cu(II)/Cu(I) ratios on the catalyst surface; thus, regulating the hydrogenation capacities of intermediates. Similarly, Cu–Bi NPs were prepared with different stoichiometric ratios via chemical reduction, which revealed that the FE of CH₄ and C₂H₄ was sensitive to the quantity of Bi.⁹³ Cu₇Bi₁ had a high FE (70.6%) for CH₄ at −1.2 V (vs. RHE). Further, lowering the C–C coupling energy barrier to enhance the FE of C₂H₄ was verified through the integration of single Bi atoms and oxygen vacancies with CuO (Bi–CuO (V_O)).³³ Interestingly, Bi–CuO (V_O) with a FE that exceeded 48% of C₂H₄ at −1.05 V (vs. RHE) significantly outperformed the other Cu-based electrocatalysts. Cao et al.⁹⁴ further showed the C–C coupling ability of a single Bi atom-decorated Cu alloy (BiCu-SAA) by employing operando FTIR based on a synchrotron radiation (SR-FTIR) technique. The appearance of an absorption peak at 1563 cm⁻¹ corresponded to the *COCOH species, which verified the C–C coupling for C₂ products.

Fig. 6 (a) Faradaic efficiencies of CO₂RR on CuBi catalysts prepared with different charge values passed through the substrate using 0.1 M KHCO₃ catholyte⁹² (copyright 2022 Elsevier); (b) schematic of synthesis procedure for Cu_xBi aerogels and different product selectivity³² (copyright 2022 Elsevier).

4. In situ spectroscopic studies

The Bi-based catalysts were often synthesized through the in situ electrochemical transformation of the initial Bi-containing precursors. Thus, it is vital to monitor and understand the in situ structural reconstruction during the CO₂RR. In situ Raman spectroscopy has been employed to monitor the dynamic reconstruction and evolution of Bi-based catalysts under CO₂RR conditions.⁹⁵ The transformation of the initial Bi-containing precursors to Bi NSs can be explored via in situ Raman spectroscopy through changes in the Raman peaks assigned to the vibration of the Bi–M or Bi–O bonds.³⁶ Shen et al.³⁹ showed the structural reconstruction of the CuS–Bi₂S₃ precursor to metallic Bi. In situ Raman spectra revealed that the band assigned to Bi₂S₃ and CuS quickly disappeared and two broad bands at 72 and 96 cm⁻¹ appeared, which were ascribed to the E_g and A_1g stretching modes of Bi–Bi bonds. Although most studies have shown the complete reduction of Bi₂O₃ to metallic Bi during the CO₂ reduction, Deng et al.⁹⁶ demonstrated the partial reduction of Bi₂O₃ by in situ Raman spectroscopy. They reported that the presence of Bi–O structure at the near surface was the main incentive for the selective conversion of CO₂ to formate. The Bi–O structure could enhance CO₂ adsorption and improve the stabilization of CO₂⁻ intermediate.

The formation of bismuth subcarbonate (Bi₂O₂CO₃) was reported and detected by in situ Raman spectroscopy during the CO₂RR, contingent on the electrolyte and initial composition of the Bi precursor.⁹⁵ An et al.⁹⁷ reported that the formation of Bi₂O₂CO₃ originated from the formation of a surface oxide layer (Bi³⁺) or oxidized metal Bi exposed to air (BiO⁺). In the case of Bi³⁺ formation, bismuth hydroxide (Bi(OH)₃) was generated by the reaction of the Bi³⁺ and OH⁻ from local alkalinity. However, the Bi(OH)₃ was not stable and reacted with CO₂ to form Bi₂O₂CO₃ as shown in reaction (1) and (2). In the case of BiO⁺ formation, it further reacted with carbonate that was present in the highly alkaline CO₂-purged electrolyte to form Bi₂O₂CO₃ species as shown in reaction (3) and (4).

Bi³⁺ + OH⁻ ↔ Bi(OH)₃ (1)

2Bi(OH)₃ + CO₂ ↔ Bi₂O₂CO₃ + 3H₂O (2)

4Bi + 3O₂ + 2H₂O ↔ 4BiO⁺ + 4OH⁻ (3)

2BiO⁺ + CO₃²⁻ ↔ Bi₂O₂CO₃ (4)

The existence of Bi₂O₂CO₃ species during the CO₂RR process was detected using the in situ shell-isolated nanoparticle enhanced Raman spectroscopy (SHINERS) method. The appearance of the Bi–O stretching vibration of Bi₂O₂CO₃ located at 182 cm⁻¹ confirmed the formation of Bi₂O₂CO₃ on the electrode surface.⁹⁵ Wu et al.³⁸ demonstrated that a thin layer of Bi₂O₂CO₃ initially formed on the surface; however, it could be completely diminished and converted to metallic Bi prior to the occurrence of CO₂RR. As shown in Fig. 7a, the peak at 162 cm⁻¹ assigned to the Bi=O vibration mode of the Bi₂O₂CO₃ appeared at the open circuit potential (OCP), which verified the formation of the Bi₂O₂CO₃. With the application of more negative potentials from OCP to −0.9 V, the peak intensity decreased and finally disappeared. Similarly, the transformation of Bi₂O₃ microcrystals to Bi₂O₂CO₃ under electrochemical conditions was reported by Zeng et al.,⁹⁸ it was further reduced to metallic Bi at potentials higher than −0.6 V vs. RHE. Bi₂O₂CO₃ has also been directly employed as a catalyst for CO₂RR.^57,70 The abundant oxygen vacancies in Bi₂O₂CO₃ nanosheets (V_O-BOC-NS) served as durable electrocatalysts for CO₂ reduction to formate with an FE of >95% at −0.62 V vs. RHE. The stability of the V_O-BOC-NS catalyst under CO₂ reduction conditions was characterized using in situ Raman spectroscopy in a 0.5 M KCl electrolyte. No notable change of the intensity of peak at 1066 cm⁻¹ derived from CO₃²⁻ in V_O-BOC-NS was observed even at more negative potentials, showing excellent stability.⁹⁹

Fig. 7 (a) In situ Raman spectra for the transformation of the Bi₂O₃ precursor from Bi₂O₂CO₃ to Bi during the CO₂RR³⁸ (copyright 2022 American Chemical Society); potential-dependent (b) and time-dependent (c) IR spectra measured in a CO₂-saturated 0.1 M K₂SO₄ solution for the ZnBi-3 electrode⁸⁶ (copyright 2024 American Chemical Society).

In addition to studying the structural evolution of materials, the detection of key intermediates and the elucidation of reaction pathways are vital for gaining better insights into reaction mechanisms, and further improving performance by modifying the binding energy of intermediates.¹⁰⁰ It was reported that the generation of formate often occurs through the oxygen-bridged *OCHO intermediate on Bi-based catalysts.^57,76 The stability of the adsorbates with Bi–O bonds is systematically higher than those with C–Bi bonds. Thus, the *OCHO pathway is dominant compared with *COOH pathway, leading to higher selectivity of formate generation. Some studies have shown that the formation of the *OCHO intermediate is accompanied by the adsorption of HCO₃⁻ groups.¹⁰¹ Sabouhanian et al.⁸⁶ studied the CO₂ reduction mechanism at ZnBi catalysts using an in situ electrochemical ATR-FTIR technique. As shown in Fig. 7b, the peaks assigned to formate at 1305 cm⁻¹ (C–H bending mode), 1360 cm⁻¹ (C–O symmetric stretch), and 1660 cm⁻¹ (C=O asymmetric stretch) were observed. The signal that appeared at 1614 cm⁻¹ was attributed to the asymmetric stretch of the adsorbed *COO⁻ intermediate, revealing that the CO₂ molecule was adsorbed by the carbon atom in a monodentate orientation. The time-dependent FTIR spectra of the ZnBi catalyst are presented in Fig. 7c to monitor the formation and consumption of species over time. It was observed that the peaks assigned to the symmetric and asymmetric stretches of formate followed the same trend as the peak allocated to *COO⁻, and they became stronger as the reaction progressed. This proves that more *COO⁻ species resulted in the production of more formate.

5. Industrial perspective

Although Bi-based catalysts exhibit a high FE and selectivity for formate, the required high activity, high stability, and low overpotential hinder them from being employed on an industrial scale. Consequently, multiple challenges need to be resolved in terms of their stabilities and activities for industrial applications.^34,39 To meet the required criteria for commercialization, it is necessary to achieve current densities of >200 mA cm⁻² and long-term (>100 h) stability.¹⁶ Encouraging progress has been made recently toward upgrading the electrochemical stability of Bi-based catalysts, while maintaining high activity. Flow cells and membrane electrode electrolyzers were developed for scalable CO₂RR systems as they can address mass transport issues.^77,102,103 The gas diffusion electrode (GDE) configuration allowed CO₂ to access the electrode surface as a gas and facilitated its mass transportation. There are some pending patents using Bi-based catalysts for CO₂ reduction to formate.^104,105 A superior high current density of 2.0 A cm⁻² with 93% FE of formate at −0.95 V vs. RHE (Fig. 8a) was achieved by Lin et al.⁶³ in a flow cell. This group synthesized a Bi₂S₃ precursor that underwent structural evolution and created a nanocomposite catalyst containing Bi⁰ clusters and Bi₂O₂CO₃ nanosheets. They showed that the FE of formate increased from 66% at −0.38 V vs. RHE to 96% at −0.52 V vs. RHE. This catalyst maintained a stable and industrial-level current density for 100 h. Interestingly, Shen et al.³⁹ prepared a Bi₂S₃-containing precursor through the integration of CuS. The CuS–Bi₂S₃ nano-heterojunction precursor was observed to be reconstructed to Cu-doped Bi (CDB) nanosheet electrocatalysts. An industrial-compatible current density of −1132 mA cm⁻² at −0.86 V vs. RHE was recorded in a flow cell. Moreover, the long-term stability of over 100 h at −400 mA cm⁻² was attained in a membrane electrode assembly.

Fig. 8 (a) The measured FE of each product and total current density for CO₂ reduction over a Bi₂S₃-derived catalyst using 1 M KOH electrolyte in a flow cell⁶³ (copyright 2023 John Wiley and Sons); (b) schematic of the degradation of a conventional Bi catalyst and limited CO₂RR dynamics; (c) schematic of structural stabilization using a novel Bi-DC catalyst with a special sp²/sp³ carbon hybridization structure for efficient and stable formate formation¹⁰⁶ (copyright 2024 American Chemical Society).

In addition to the activity, stability is the most important issue for Bi-based catalysts due to thermodynamically driven aggregation of nanoparticles, the dissolution of reactive species, and structural reconstruction during CO₂RR.^39,45,106 Only a few research papers have reported a stability time of ∼100 h. As catalytic reactions take place on the catalyst's surface, surface modifications can greatly influence the catalytic reactions. The microenvironmental modulation using the molecularly modified surface layers can impact the adsorption behavior of ions and molecules on the catalyst, improving the stability during CO₂ reduction.¹⁰⁷ For example, HER on Cu nanoneedles was suppressed by a hydrophobic polytetrafluoroethylene (PTFE) coating, leading to the high C₂ selectivity with 47% FE. Moreover, the Cu nanoneedle structures were well maintained during the CO₂ reduction due to the PTFE coating, resulting in high stability.¹⁰⁸ Li et al.¹⁰⁶ recorded the most prolonged stability (120 h at 0.4 A) in a membrane electrode assembly cell. A CO₂-philic defective carbon (DC) was formed over a Bi catalyst (Bi-DC), where the presence of sp³-hybrid defects in the carbon exhibited a unique sieving effect on CO₂ molecules. Thus, it could effectively suppress the degradation and structural evolution of active Bi species during CO₂RR. The DC species facilitated the formation of an interconnected carbon network, which enhanced the dispersion of the active Bi component. This resulted in a larger electrochemical surface area (ECSA) and, consequently, a higher current density. Schematics of the degradation of a conventional Bi catalyst and structural stabilization using a novel Bi-DC catalyst are presented in Fig. 8b and c, respectively. It has been reported that the stability of the Bi-based catalysts might be affected by the oxygenated species from the electrolyte leading to poisoning of the active sites. Zhu et al.¹⁰⁷ introduced a molecular passivation layer of oxyphilic ascorbic acid to inhibit the poisoning of the hydroxyl. So, the free OH⁻ preferred to bind to the outer ascorbic acid layer, and the possibility of binding to defective Bi sites was reduced. As a result, the high stability of over 120 hours was achieved at 50 mA cm⁻². It has been reported that liquid metals as catalysts might have higher stability compared with solid ones due to dynamic surface properties and surface renewal ability. Liquid Bi alloy catalysts were generated by Guo et al.,¹⁰⁹ showing 98% FE of formate over 80 h. They overcame the problem of deterioration of the traditional solid-state Bi-based catalysts during CO₂ reduction. Compared with the liquid Bi alloy, the solid one exhibited a lower formate FE and less stability. To date, alkaline electrolytes have exhibited the highest activity for CO₂RR to formate using Bi-based catalysts.¹¹⁰ For example, Peng et al.¹¹¹ demonstrated that KOH solutions had improved activities over KHCO₃ due to their lower solution resistance, which was confirmed by EIS. However, CO₂ molecules can react with OH⁻ ions in alkaline electrolytes and be converted to carbonate, which leads to carbonate deposition and the consumption of CO₂ feedstocks.¹¹² More importantly, the formation of carbonates can significantly threaten the stability of CO₂RR, as they may obstruct the porous channels required for CO₂ transport in the gas diffusion electrode, accelerate electrolyte leakage and raise cell resistance. These challenges greatly constrain the industrial potential of an alkaline or neutral CO₂RR system.¹¹³ Recently, Bi-based catalysts have been utilized for CO₂RR in acidic electrolytes to overcome the aforementioned issues. Formic acid is generated in acidic electrolytes rather than formate, which can reduce the industrial expenses associated with subsequent separation and purification from the electrolyte.²⁰ However, HER typically dominates under acidic conditions. For example, in a strong acid with a pH of 1 or lower, the FE for CO₂RR products is nearly zero.¹¹³ Recently, a strategy for the engineering of the local microenvironment has been proposed to enhance the CO₂RR under acidic conditions. It was found that the HER in acidic electrolytes could be suppressed by creating a hydrophobic interfacial environment,¹¹² or reducing proton coverage¹¹³ on the catalyst surface. The FE of formate up to 92.2% at a current density of −237.1 mA cm⁻² was reported for CO₂ reduction over Bi NSs in acidic electrolytes.¹¹³ The recently reported Bi-based catalysts with an industrially compatible current density of formate are summarized in Table 1, showing that over 90% FE was achieved.

Table 1 Recently reported Bi-based catalysts with industrially compatible current densities of formate

Catalyst	Electrolyte	Potential (V vs. RHE)	Current density (mA cm^−2)	Stability (h)	FE (%)	Ref.
Ti–Bi NS	1 M KOH	−1.01	224.1	12	96.3	69
Heterophase Bi	1 M KOH	−0.68	140	10	95.2	114
Cu@Bi NW/Cu	0.5 M KHCO3	−1.07	129	6	98.7	42
BOC@GDY	1 M KOH	−1.1	200	10	93.5	115
Bi NS	1 M KOH	−0.63	400	26	>90	19
Bi-ene-NW	1 M KOH	−0.7	200	110	>90	16
In/Bi-750	1 M KOH	−1.2	200	13	90.8	26
Bi-GDE	1 M KOH	−1.2	355	60	94	98
BiIn@P	1 M KOH	−0.92	500	17	97.3	87
BOC-NS	1 M KOH	−1.55	1000	24	93	116
Copper-doped bismuth	5 M KOH	−0.86	1132	100	>90	39
Sn1/24-Bi	1 M KOH	−0.5–1.9	>200	84	>90	85
Bi2S3-derived	1 M KOH	−0.95	2000	100	93	63

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6. Conclusions and perspectives

The recent advances in Bi-based catalysts for CO₂ reduction are compared in Table 2 and Fig. 9, which highlight various morphologies and structures obtained for Bi-based catalysts along with their FE of formate. Bi-based catalysts have shown a high FE and selectivity for the generation of formate and may serve in the future as a potential electrocatalyst for CO₂RR to formate for industrial applications. Through innovative synthesis techniques and the careful engineering of catalyst structures, researchers have achieved remarkable advancements to improve the activity and selectivity. Further, the introduction of secondary elements to synthesize bimetallic Bi-based catalysts has been extensively studied to decrease the energy barrier for intermediate formation. Not only formate but also other products such as propane, methane, and ethylene could be produced using CuBi catalysts by regulating the hydrogenation capacities of intermediates, enhancing CO₂ and CO adsorption capacities, and lowering the C–C coupling energy barrier. Significant advances have been made using flow cell and membrane electrode assembly electrolyzers utilizing gas-diffusion electrodes to achieve high current densities owing to rapid mass transfer. Selectivity of up to 100% and current densities higher than −200 mA cm⁻² have been reached in most recent studies. However, achieving ampere-level current densities and long-term stabilities of >100 h remains challenging. Consequently, despite several notable achievements, there is still a need to direct future research as follows to narrow the gap between laboratory and industrial scales.

Table 2 Comparison between the performance of different Bi-based catalysts recently reported in the literature for the conversion of CO₂RR to formate

	Catalyst	Synthesis method	Electrolyte	Potential (RHE)	Stability (h)	FE (%)	Ref.
Mono-metallic Bi-based	Bi NS	Electroreduction	0.5 M KHCO3	−0.8–−1.2	22	90	38
	OD-BiNS	Electrochemical transformation	0.5 M KHCO3	−0.95	10	93	21
	Bi-PNS	Solvothermal	0.5 M KHCO3	−1.0	9	95	59
	Bi NS	Electroreduction	0.5 M KHCO3	−0.98	12	94.5	36
	Bi–S	Electrochemical transformation	0.5 M KHCO3	−0.9	35	96.7	66
	Bi PNS	Electroreduction	0.1 M KHCO3	−1.2	10	95.31	61
	Bi2S3/CNTs	Sonochemical	0.5 M KHCO3	−0.91	78	99.3	117
	Bi/BiOx NS	Electrochemical conversion	0.5 M KHCO3	−0.78–−1.18	10	94	48
Bi-metallic Bi-based	CuBi	Co-deposition	0.5 M KHCO3	−0.97	20	94.4	73
	Bi–Cu (2 : 1)	Ion-assisted codeposition	0.1 M KHCO3	−1.0	20	94.1	74
	Cu–Bi aerogel	One-step reduction	0.5 M KHCO3	−0.9	36	96.57	81
	Cu0.8Bi0.2	Thermal evaporation	1 M KOH	∼−0.72	24	>95	118
	Bi3Cu1	Hydrothermal	0.5 M KOH	−0.75	20	95.1	77
	P–Cu–BiNF	Fast reduction	0.5 M KHCO3	−0.78–−1.08	12	90	119
	Bi–Cu	Electrochemical deposition	0.5 M KHCO3	−0.91	50	94.37	111
	CuBi	CuBi-MOF	0.5 M KHCO3	−0.77	24	100	82
	Bi on Cu foil	Electrodeposition	0.1 M KHCO3	−0.65	24	100	120
	Cu@Bi nanocone	Electrodeposition	0.5 M KHCO3	−0.95	10	96.9	98
	Bi/Cu foam	Electrodeposition	0.1 M KHCO3	−1.0	20	92	97
	Cu1Bi1	Co-precipitation	0.5 M KHCO3	−0.98	60	98.07	83
	Bi5Sn60	Electrodeposition	0.1 M KHCO3	−1.0	20	94.8	29
	Bi–Sn aerogel	Chemical reduction	0.1 M KHCO3	−1.0	10	93.9	99
	Sn-doped Bi2O3 NSs	Solvothermal	0.5 M KHCO3	−0.97	8	93.4	121
	SnBi	Electrodeposition	0.5 M KHCO3	−1.06	100	96.1	84
	Sn–Bi	Hydrothermal	0.5 M KHCO3	−0.74–−1.14	160	>90	122
	Sn1−xBix	Co-reduction	0.5 M KHCO3	−0.67–−0.92	50	>90	123
	In/Bi-750	Electrochemical transformation	0.5 M KHCO3	−1.0	48	97.17	26
	BiIn5-500@C	MOF	0.5 M KHCO3	−0.86	15	97.5	124
	Bi-In2O3	Wet chemical	0.5 M KHCO3	−0.7	10	88.1	88
	In16Bi84 NS	Liquid-polyol	0.5 M KHCO3	−0.94	10	∼100	30
	In2O3/Bi2O3	MOF	0.5 M KHCO3	−0.4–−1.6	30	99.9	89
	Zn–Bi	Hydrothermal	0.5 M NaHCO3	−0.8	7	94	91

---

Fig. 9 SEM images of (a) Bi NSs³⁸ (copyright 2022 American Chemical Society); (b) P-nanoplates-Bi catalysts⁴⁰ (copyright 2021 Elsevier); (c) nitrogen-doped Bi NSs¹²⁵ (copyright 2022 MDPI); (d) Cu@Bi nanocone¹²⁶ (copyright 2020 Elsevier); (e) CuBi₃ (ref. 71) (copyright 2023 Elsevier); (f) Bi–Sn aerogel¹²⁷ (copyright 2021 John Wiley and Sons); (g) ZnBi (ref. 91) (copyright 2024 American Chemical Society); (h) In₁₆Bi₈₄ (ref. 30) (copyright 2022 American Chemical Society).

Continued efforts should be made to enhance the stability of Bi-based catalysts for industrial applications. It is also necessary to conduct further research into scalable synthesis techniques and cost-effective catalyst design for large-scale applications. It is vital to gain insights into the kinetics of CO₂RR and identify intermediates in real-world applications. Thus, in situ characterization techniques should be developed to be compatible with high current densities. Further research should be conducted to optimize the conditions and designs of cells to generate concentrated formic acid to reduce the expenses of product purification and separation. Integrating Bi-based catalysts with renewable energy sources such as solar and wind power should be considered. The coupling of electrochemical CO₂ reduction with renewable energy will reduce the reliance on fossil fuels and contribute to a greener energy landscape. Finally, CO₂RR should be assessed regarding the economic perspective and environmental impact. Building a pilot-scale system that allows for real-world testing is crucial to evaluating the stability of the catalysts, the costs and the efficiency of the CO₂ reduction process. It would provide valuable data on operational expenses, overall system performance, and long-term durability, offering key insights into the economic viability and potential scalability.

Data availability

This is a review article. All the data are extracted from the published papers, which are specially described in the related figures and tables.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This research was funded by Discovery Grants from the Natural Sciences and Engineering Research Council of Canada (J.L.: RGPIN-2022-03958; A.C.: RGPIN-2022-04238). A.C. acknowledges the NSERC and Canada Foundation for Innovation (CFI) for the Canada Research Chair Award in Electrochemistry and Nanoscience.

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