Graphene-based 2D materials for rechargeable batteries and hydrogen production and storage: a critical review

Abstract

Batteries and hydrogen energy devices are considered the most critical technologies for achieving zero carbon dioxide emissions. However, they still suffer from several limitations, including low efficiency, short cycling life, low storage, and poor safety. With their strong mechanical strength (flexibility), chemical inertness, large surface area, remarkable thermal stability, and excellent electrical and high ion conductivity, graphene can overcome some of the issues associated with batteries and hydrogen energy devices. The properties of various two-dimensional (2D) materials make them potential candidates for a wide range of applications (batteries and hydrogen energy devices), thereby gaining considerable interest. Similarly, graphene has the potential for efficient hydrogen production and storage because of its large surface area and adjustable porosity. Graphene/2D composite materials are promising electrodes for lithium batteries, hydrogen storage, and production applications. This review provides a comprehensive overview of graphene/2D composite materials for lithium batteries and hydrogen storage and production applications.

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1. Introduction

The expanding population and growing industrialization have increased the need for energy globally to an unprecedented degree. However, most of our energy comes from fossil fuels, which increase greenhouse gas emissions (GHGs) and contribute to worldwide climate change.^1–4 The rising temperature of the earth's has prompted scientists to explore alternative energy sources. Reducing the dependency on fossil fuels and utilizing renewable energy to build new technologies are urgent. As a result, hydrogen (H₂) and batteries are considered clean and sustainable energy sources for the future.^5–12

Batteries can store intermittent energy; thus, they are becoming increasingly popular as renewable sources.^13–15 Lithium-ion batteries are the most researched rechargeable batteries owing to their excellent energy density and storage capacity, which are advantageous for various applications and daily energy demands.^16–21 Electrode materials are crucial to lithium rechargeable batteries, but the sluggish kinetics and significant volume changes still hinder further applications.^16,22–24

H₂ is an effective and clean energy source that substitutes fossil fuels.^25–27 Hydrogen energy has a gravimetric density roughly seven times higher than that of fossil fuels.²⁸ Hydrogen, a readily available element and a flexible energy carrier, can transform several industries and sectors, including manufacturing, transportation, and power generation. The potential of hydrogen to provide energy without producing hazardous pollutants makes it significant. Since water is the only byproduct of using hydrogen as fuel, hydrogen is a clean and green choice.²⁹ However, even with its enormous promise, there are still some obstacles that must be overcome before hydrogen energy is widely used.³⁰ The main issues are linked to the production and storage of H₂ energy.³¹ Regarding H₂ production, novel catalysts and materials, such as nanostructured catalysts, are being investigated to speed up reactions and increase the efficiency for maximizing hydrogen generation.²⁹ Furthermore, the lower volumetric density of hydrogen necessitates huge storage capacities or high pressure. Therefore, advanced catalysts and materials can significantly improve the efficacy and effectiveness of hydrogen storage.³² The common materials used to store hydrogen energy are sodium aluminium hydride (NaAlH₄), lithium aluminium hydride (LiAlH₄), lithium borohydride (LiBH₄), magnesium nickel hydride (Mg₂NiH₄), sodium borohydride (NaBH₄), aluminium hydride (AlH₃), zeolites, metal–organic frameworks (MOFs), and covalent organic frameworks (COFs).^33–40 However, the drawback of hydrogen adsorption–desorption exists as it lacks reversibility because of its strong bonding force and slow kinetics, necessitating high temperatures for desorption. For the production of hydrogen, titanium dioxide (TiO₂), molybdenum disulfide (MoS₂), tungsten(VI) oxide (WO₃), zinc oxide (ZnO), iron oxide (α-Fe₂O₃), bismuth vanadate (BiVO₄), copper oxide (Cu₂O), cadmium sulfide (CdS), and copper/zinc/tungsten (Cu/Zn/W) sulfide are widely employed as electrodes.^41–48 However, certain stability and low-efficiency challenges persist, leading to their poor photoelectrochemical (PEC) activity for hydrogen production.^49,50 Thus, innovative strategies are needed to address these issues and increase hydrogen generation methods' safety, economy, and efficiency.

Lithium-ion batteries, hydrogen storage, and production electrode material capabilities are found in several carbon-based substrates including porous carbons (PCs), carbon nanotubes (CNTs), MXenes and nanofibres (NFs) with different geometries.^51–56 Among all the recently discovered and synthesized nanostructured carbonaceous species, graphene stands out as the most extraordinary material. Their remarkable properties including their enormous surface area, mechanical toughness, and electrical conductivity have revolutionized the study of materials. Consequently, compared to other carbon compounds (PCs, CNTs, MXenes, and NFs), graphene has been used advantageously as prospective substitute electrode materials in many applications for improving particular technical domains, particularly those related to energy generation and storage. Because of their large theoretical SSA (2630 m² g⁻¹) and ease of functionalization by heteroatoms such as N atoms, graphene offers more electrochemical reaction active sites than other carbon compounds (PCs, CNTs, MXenes, and NFs).

Graphenes were isolated in 2004 and used in energy storage due to their high thermal conductivity, high electrical conductivity, high elasticity and flexibility, and high hardness. Particularly in lithium-ion batteries, they serve as electrodes, conductive materials, and controllers for volume expansion in the electrode material. However, Li ions may ideally be accommodated on both sides of graphene, and their theoretical capacity is at least double that of other carbon-based materials (PCs, CNTs, MXenes, and NFs).^57,58 The graphene is a promising substitute for improving H₂ generation efficiency. Because of graphene's particular two-dimensional conjugated framework and electrical characteristics, it can be used to increase the photocatalytic efficiency.⁵⁹ Moreover, the graphene, composed of carbon atoms arranged in a honeycomb lattice, holds the potential for hydrogen storage, giving rise to new hopes for creating an effective solid-state hydrogen storage device.⁶⁰ In recent years, several papers have been published on graphene composites with 2D materials in batteries and hydrogen storage and production applications. This review aims to assess graphene composites with 2D materials for lithium batteries, hydrogen storage, and production applications.

2. Lithium rechargeable batteries

2.1 Graphene-based 2D materials for lithium-ion batteries (LIBs)

2.1.1. Graphenes for lithium-ion batteries (LIBs). Numerous portable devices that are essential to modern life are powered by rechargeable batteries. Rechargeable battery technologies with high power and energy densities are also currently being investigated for the electrification of autos.^61,62 The market for rechargeable batteries used for energy storage is dominated by LIBs, a type of battery that operates on the intercalation and de-intercalation of Li ions due to many fascinating features, such as high energy and power density, long cycle life, low self-discharging, and no memory effect.⁶³ As seen in Fig. 1, lithium-ion batteries typically consist of the following four components: the cathode, anode, electrolyte, and separator.⁶⁴ Graphitic materials are typically used as the anode material in commercial LIBs, while Li metal oxides or phosphates such as LiCoO₂ and LiFePO₄ are commonly used as the cathode material.⁶⁵ A separator keeps the chemical potentials of the cathode and anode apart. During the charging and discharging processes, Li ions can flow between the cathode and the anode via an organic electrolyte based on carbonate, including a salt containing Li, such as lithium hexafluorophosphate (LiPF₆). During discharging, electrons move from the anode to the cathode while Li ions move through the electrolyte to keep the charge balanced. An external electrical power source forces the electrons and Li ions to go in the opposite direction during the charging process.

Fig. 1 Schematic of the structure and working mechanism of LIBs.⁶⁶

“Anode” refers to the negative electrode that releases an electron into the external circuit. An anode collects lithium ions and converts them into active materials. Some characteristics that the anode materials should have include a large energy capacity, excellent charge/ion storage, ion release, long cycle rate, ease of manufacture, safety in use, and cost-effectiveness.⁶⁷ Since the early 1990s, Sony has been commercially launching LIBs, with graphite was the anode material. Currently, graphite, the most popular anode material in LIBs, has a relatively low theoretical capacity of about 372 mA h g⁻¹ and safety issues related to the insertion of Li⁺ into the anode structure.⁶⁸ A graphite anode's limited capacity will prevent it from meeting the demands of the rapidly expanding industries.⁶⁹ To improve LIB performance and energy density, research is required to fabricate new anode materials with a capacity greater than that of graphite.⁷⁰

A 2D nanomaterial called graphene is made up of hexagon-shaped carbon atoms with sp² hybrid orbitals. Graphene has garnered considerable interest across numerous applications since its isolation in 2004.^71–77 Graphene, a single layer of graphite, is attracting a lot of attention as a potential anode material due to its enormous specific surface area (2620 m² g),⁷⁸ mechanical strength, excellent thermal conductivity (5000 W m⁻¹ K⁻¹),⁷⁹ electrical conductivity, rapid charge–discharge cycles and a more remarkable ability to hold Li ions and higher capacity (three times that of graphite).^80–83 Because of its high energy storage capacity of 669 mA h g⁻¹, good cycling stability, longer interlayer distance of 0.615 nm with mild van der Waals forces, and simple intercalation and deintercalation procedure, graphene is highly suitable for LIB anode applications.⁸⁴ Numerous scientists have noted that Li⁺ ion attachments may be facilitated by extra paths provided by graphene sheet defects. The enormous challenge of fabricating LIBs that can provide high power and energy density outputs during charging and discharging may have an answer in this feature. However, porous graphene's high ionic conductivity, surface characteristics, and electric conductivity have attracted considerable attention in battery research.⁸⁵ Ordered mesoporous graphene nanosheets were synthesized by Fang et al. using a regulated low-concentration mono micelle close-packing assembly technique.⁸⁶ This anode can hold 833 mA h g⁻¹ at different current densities after multiple cycles. It can give a high reversible capacity of 1040 mA h g⁻¹ at 100 mA g⁻¹. About 255 mA h g⁻¹ of reversible capacity is kept even at a high current density of 5 A g⁻¹. For instance, magnesium oxide and methane were used as starting ingredients in the chemical vapor deposition process to synthesize a hierarchical porous graphene nanomaterial. The Brunauer–Emmett–Teller (BET) surface area and pore size were changed, and this affected the electrochemical properties of LIBs.⁸⁷ In particular, the heteroatom-doped graphene has demonstrated advances in the electrochemical performance of LIBs. Using the differential electroneutrality of doped nitrogen (N), phosphorus (P), sulfur (S), and boron (B) heteroatoms, a method was devised to increase the relative specific capacity (900 mA h g⁻¹) of LIBs.^81,88–92 However, extensive research has revealed that the ionic-steric effect and decreased stability of graphene make it impractical to use it directly as an anode active material in LIBs.^93,94 Furthermore, with several advantageous effects, the synthesis of graphene-based composites has been one of the most important avenues for LIB anode active materials. The graphene, particularly with a 2D material, has emerged as a potential anode active material for LIB applications.

2.1.2. Graphene/graphitic carbon nitride (C₃N₄) anodes for lithium-ion batteries. Because of their comparable layered architectures, graphene and graphitic carbon nitride (C₃N₄) are known to be modifiable. Synergistic effects between graphene and g-C₃N₄ are caused by highly exposed N-active sites on conductive graphene sheets, which increase the electrochemical characteristics. Nevertheless, these active spots become inactive as soon as graphene aggregation occurs. Thus, structural exploration of g-C₃N₄/graphene-based materials could help design a stable 2D structure with many active sites. Shuguang Wang et al. prepared a g-C₃N₄@reduced graphene oxide composite (g-C₃N₄@RGO) in a cost-effective way and applied it as an LIB anode (Fig. 2a).⁹⁵ For practical applications, the coin-type half batteries are fabricated with g-C₃N₄@RGO and connected in series to a tandem red light-emitting diode (LED) and a watch. The g-C₃N₄@RGO electrode delivered a stable cycle performance with a 595.1 mA h g⁻¹ capacity even after 1000 cycles at a high current density of 1 A g⁻¹. Marlies Hankel and co-workers designed a C₃N₄/graphene heterostructure and conducted scanning electron microscopy (SEM) analysis to observe its morphology (Fig. 2b). However, the Density Functional Theory (DFT) study showed superior conductivity of the designed C₃N₄/graphene heterostructure for LIB applications. The C₃N₄/graphene heterostructure electrode exhibited an excellent electrochemical capacity of 862.2 mA h g⁻¹ after 90 cycles with a coulombic efficiency of 90.4%.⁹⁶

Fig. 2 (a) Schematic of the synthesis procedure of g-C₃N₄@RGO. (b) SEM image and DFT calculations of the C₃N₄/graphene heterostructure. (c) Hall bar device with the h-BN/graphene/h-BN heterostructure. (d) Vacuum filtration process for preparation, transmission electron microscopy (TEM) image and electrochemical study of the GO/BN composite film. (e) Bilayer BN_xGr_y/PP separator and electrochemical characterization of LIBs.^95–99

2.1.3. Graphene/hexagonal boron nitride (h-BN) for lithium-ion batteries. With its strong mechanical strength, chemical inertness, remarkable thermal stability, and high ion conductivity, h-BN has emerged as a potential solution for various energy storage and conversion issues. Generally speaking, h-BN can function as an ideal modifier to increase battery safety by enhancing separators' mechanical strength, providing superior thermal conductivity, and prolonging the cycle life of Li-metal batteries by safeguarding the solid-state electrolyte. During charge–discharge cycling, lithium metal experiences a constant growth of Li dendrites, which results in low coulombic efficiency and early failure. After extensive cycle experiments, John Hong et al. reported a freestanding h-BN nanosheet separator with exceptional physical rigidity against dendritic Li development and exceptional thermal endurance in high-temperature settings.¹⁰⁰ Pol et al. prepared bilayer separators and used them in lithium-ion batteries. A polypropylene (PP) membrane covered with a layer of boron nitride-graphene (BN_xGr_y) dramatically lowers polarization and impedance while greatly enhancing cell performance and stability. The full cell fabricated with the modified BN_xGr_y/PP separator delivered a discharge capacity of 114 mA h g⁻¹ at 1C rate even after 1000 cycles (Fig. 2e).⁹⁷ Using the Hall potentiometry approach, Frank Zhao et al. reported the electrochemical intercalation of Li into graphene enclosed between h-BN layers (Fig. 2c).⁹⁸ This work presents an electrochemical method for the controlled intercalation of lithium ions into a range of van der Waals heterostructures composed of hexagonal boron nitride and graphene. Tong Teo and colleagues successfully made the composite films of reduced graphene oxide (rGO)/boron nitride (BN) using simple vacuum filtration and subsequent heat treatment (Fig. 2d).⁹⁹ The thin 2D nanosheet morphologies of rGO and BN were well retained following their combination, according to the morphological analysis of the generated rGO/BN SEM images. The electrochemical analysis indicates that the composite film electrode with rGO/BN (rGO/BN-2%) exhibits superior rate performance. Even at a high current density of 1 A g⁻¹, the rGO/BN (rGO/BN-2%) electrode delivers a specific capacity of 121 mA h g⁻¹, which is higher than that of the rGO film electrode.

2.1.4. Graphene/transition metal dichalcogenides (TMDs) for lithium-ion batteries. Because of their unique lamellar structure, TMDs have a superior electrochemical active interface, excellent mechanical properties, higher specific capacity, and improved cycling stability compared to typical carbon anodes, and hence, LIBs frequently use them. However, TMDs are a viable option for energy storage applications because of their huge intercalation gap, which can encourage the quick interlayer migration of lithium ions. TMDs have the structure MX₂, where X can be either tellurium (Te), selenium (Se), or S, and M can be any transition metal element (e.g., titanium (Ti), tungsten (W), vanadium (V), and molybdenum (Mo)). Typical two-dimensional TMDs are molybdenum disulfide (MoS₂), vanadium sulfide (VS₂), vanadium selenide (VSe₂), tungsten disulfide (WS₂), and so forth. The obstacles of the TMDs for commercial application volume expansion and during the charge–discharge process are the van der Waals interaction between TMD sheets, which lowers the utilization rate. TMDs with layered structures such as graphene are easier to exfoliate and facilitate quick ion diffusion. With its huge specific surface area, great mechanical capabilities, and excellent electrochemical stability, TMD/graphene heterogeneous composites are perfect energy storage materials. The hollow MoS₂ nanocages/RGO nanocomposites fabricated by Dafang He et al. are assembled using a straightforward solvothermal-assisted assembly technique in conjunction with freeze-drying and annealing (Fig. 3a).¹⁰¹ The graphene and MoS₂ are tightly bound via molecular links, contributing to the structural stability, improved electrical conductivity, and increased lithium storage capacity of the MoS₂ anode material. Thus, at a high current density of 3 A g⁻¹ (Fig. 3b), this new nanocomposite demonstrates good rate capability and significant potential as an anode nanocomposite for improved lithium-ion batteries. By a simple hydrothermal process, Xinglan Zhou et al. fabricated 1T MoS₂ nanosheets on graphene oxide (GO) as an anode in lithium-ion batteries. The synthesized 1T MoS₂/GO (1 g) electrode exhibited the first specific charging/discharging capacity of 1612 mA h g⁻¹ at a current density of 0.1 A g⁻¹. It maintained a specific capacity of 774 mA h g⁻¹ even after 60 cycles at a current density of 1 A g⁻¹.¹⁰⁵ WS₂ one more TMD was reported for lithium-ion battery anode materials due to its excellent electrochemical properties.¹⁰⁶ Yong-Lin Wu et al. synthesized a WS₂/graphene nanosheet composite via ball milling followed by calcination to overcome the van der Waals interactions between the interlayer of graphite and WS₂.¹⁰⁷ At a current density of 250 mA g⁻¹, the produced WS₂/GNS composite anode material exhibited a 95% capacity retention rate and a high reversible specific capacity of 705 mA h g⁻¹ even after 200 cycles.

Fig. 3 (a) Synthesis process and (b) rate performance of the MoS₂/RGO nanocomposite and MoS₂ electrode. (c) Fabrication and (d) electrochemical performance of the MXene, rGO and MXene-rGO films at 1 A g⁻¹. (e) Fabrication and coulombic efficiencies of the MG electrode at a current density of 0.5 mA cm⁻² with a Li deposition amount of 5 mA h cm⁻² for 2700 h. (f) Lithiophilic functional groups result in the formation of a uniform SEI layer (decomposition of LiN(CF₃SO₂)₂); cryo-TEM images of equally dispersed LiF and ordered layered Li₂O and superior cycling lifespan over 900 cycles with a capacity retention of 77.7% at 30C of Ti₃CNT_x/G/Li electrode.^101–104

2.1.5. Graphene/MXenes for lithium-ion batteries. Another class of electrode materials that have recently attracted much attention are MXenes. MXenes is a collective term for a novel class of 2D carbonitride, nitride, and carbide materials that was coined in 2011. MXenes have demonstrated significant promise in using LIB electrodes due to their distinct 2D structure and abundant active sites on their surface. However, the noticeable restacking effects of MXene layers and low electric conductivity cannot be used in battery applications. Thus, MXenes combined with graphene composites have proven to be highly effective electrodes for Li-ion batteries, and they have been successfully fabricated. Yun-Ting Du et al. methodically examined the electrochemical characteristics of MXene/graphene heterostructures and their potential applications in energy storage through first-principles calculations.¹⁰⁸ The results show that adding graphene improves the mechanical stiffness, Li adsorption strength, and electric conductivity and prevents the restacking effects of MXene layers. Zhiying Ma et al. synthesized a MXene (titanium carbide (Ti₃C₂)) and graphene hybrid film in different ratios and applied it for lithium-ion storage applications (Fig. 3c).¹⁰² The hybrid film has a high specific capacity of 335.5 mA h g⁻¹ at 0.05 A g⁻¹ and is used as an electrode material for lithium-ion batteries. Because of its stable structure, the film electrode also shows good cycling stability, with no capacity loss even after 1000 cycles at high rates (1 A g⁻¹) (Fig. 3d). However, MXenes/graphene also use a dendrite-free Li metal anode in lithium-ion batteries. The synthesized MXene/graphene electrode by Zhong-Shuai Wu and colleagues has a record lifespan of 2700 hours and high coulombic efficiency of ∼99% (Fig. 3e).¹⁰³ Comparably, the conductive scaffold was employed by MXene (Ti₃CNT_x) and graphene composites to control Li nucleation and solid-electrolyte interphase (SEI) production on the Li metal anode. With a capacity retention of 77.7% at 30C, the Ti₃CNT_x-G-Li electrode-based complete cells with LiFePO₄ have an exceptional cycling lifespan of over 900 cycles (Fig. 3f).¹⁰⁴ Carbon nanotubes (CNTs) and graphene with varying weight ratios of MXene to carbon, synthesized by Motahare S. Mohseni-Salehi et al., were hybridized with the nanohybrids.¹⁰⁹ With a coulombic efficiency of 98%, the V₂CT_x MXene/G showed an impressive specific discharge capacity of 460 mA h g⁻¹ at a current rate of 100 mA g⁻¹.

2.1.6. Graphene/transition metal oxides (TMOs) for lithium-ion batteries. Over the past few decades, transition metal oxides (TMOs) have been the subject of extensive research because of their intriguing charge transfer and storage properties. However, TMOs as LIB anodes are still far from commercial applications due to noticeable volume expansion (∼300%) observed during the cycling process. To improve the anode material's performance and get around these problems, TMOs and graphenes can be combined.^110,111 The graphene is an ideal conductive carbon coating for TMO anode materials to mitigate the volumetric expansion.¹¹² Therefore, to overcome the aforementioned drawback, a number of methods have been proposed as practical solutions for extending the cycle life and reducing the volume expenditure of LIB anode materials. One such method is the preparation of nanocomposite materials, which capitalize on the synergy between combined materials.^113–116 By a straightforward sonication process, MnO₂ nanotubes were able to integrate the components via the electrostatic interaction of the MnO₂ nanotubes and graphene functional groups. It was discovered that the incorporation of graphene oxide improved the interfacial characteristics and structural stability during charge/discharge cycling while reducing volume expansion and charge transfer resistance.¹¹⁷

Cobalt oxide (Co₃O₄) has been studied as an anode material since it was first used as the anode electrode for LIBs because of its advantages, which include a large theoretical specific capacity of approximately 890 mA h g⁻¹, wide availability, ease of manufacture, and stable chemical properties. Liang Tan et al. used a simple hydrothermal method to make a Co₃O₄/graphene composite, which was subsequently tested at very low temperatures (Fig. 4a).¹¹⁸ The Co₃O₄/G anode exhibits a noticeably higher capacity of 605 mA h g⁻¹ at 0.5 A g⁻¹ (Fig. 4b) than that of other anodes at temperatures below zero thanks to the material's creative nanostructure, high conductivity, and extremely good lithiation and delithiation potentials. The inexpensive metal oxide Fe₂O₃ can displace graphite as the primary anode material for lithium-ion batteries. The redox conversion reaction from Fe₂O₃ to a combination of Li₂O and metallic iron gives a theoretical capacity of up to 1006 mA h g⁻¹. Yu Huang et al. used a simple and effective one-pot hydrothermal process to create a 2D nanostructured Fe₂O₃/N-rGO composite (Fig. 4c). When tested as LIB anodes, the 2D nanostructured Fe₂O₃/N-rGO composite demonstrated an exceptional high-rate capacity of 652 mA h g⁻¹ at 15 A g⁻¹ (Fig. 4c).¹¹⁹ A similar Fe₂O₃/multilayer graphene was synthesized by Junming Xu et al. by a chemical deposition method, which when used as the anode for LIBs exhibited a specific capacity of 1050 mA h g⁻¹ at 0.1C even after 100 cycles.¹²³ Due to their noteworthy characteristics, tin oxide (SnO₂) nanoparticles have been thoroughly investigated for various energy-related applications, including LIBs. Naiqing Zhang and coworkers reported hierarchical SnO₂ nanoclusters anchored on the graphene sponges (SnO₂/GS) via a solvothermal approach with an average diameter of ∼110 nm, as shown in Fig. 4d.¹²⁰ These hierarchical SnO₂/GSs retained a higher reversible capacity of 600 mA h g⁻¹ after over 600 cycles at 4000 mA g⁻¹ (Fig. 4d). Based on a conversion reaction from nickel oxide (NiO) to metallic nickel and Li₂O, the estimated theoretical capacity for NiO is 718 mA h g⁻¹. The NiO also suffers from poor rate performance and cycling stability similar to those of other conversion-based anode nanomaterials. NiO composite with graphene is used in the LIB applications to address these obstacles. Ju Fu et al. synthesized hierarchically porous NiO micro-flowers/graphene papers (fNiO/GP) by a facile hydrothermal method followed by annealing in air at 400 °C for 40 min (Fig. 4e).¹²¹ Even after 600 cycles at a current density of 1000 mA g⁻¹, fNiO/GP demonstrated high reversible specific capacities of 359 mA h g⁻¹ in lithium-storage applications (Fig. 4e). Similarly, graphene–(Ni–NiO)–C hybrid was hydrothermally treated at 180 °C for 24 h and then carbonized at 700 °C for three hours by Jian-Guo Zhao and colleagues.¹²⁴ When it was used as the anode material for LIBs, the initial capacity was 711.6 mA h g⁻¹, and after 300 cycles, it increased to 772.1 mA h g⁻¹. Manganese dioxide (MnO₂) is plentiful in the Earth's crust, which is a more environmentally friendly metal oxide than other oxides. As an anode for LIBs, MnO₂ has a high theoretical capacity of 1232 mA h g⁻¹, enabling a quick charge/discharge rate over a broad potential window. As anode materials for Li-ion batteries, Pedda Masthanaiah Ette et al. revealed that two distinct mesoporous MnO₂ were produced from silica templates (MCM-41 and MCM-48) and integrated with graphenes (Fig. 4f).¹²² Both composites demonstrate improved rate performance, large discharge capacity, and strong cycling stability. When used as LIB electrodes, M8-MnO₂-G exhibits a consistent discharge capacity of 1494 mA h g⁻¹ at a rate of 5C (Fig. 4f). The synergistic effects of graphene-integrated mesoporous MnO₂ are responsible for the composites' high specific capacities and long cycling performance. LiNi_0.8Mn_0.1Co_0.1O₂ as a cathode studied the full cell tests with an M8-MnO₂-G composite material anode. The full cell shows a stable capacity up to 200 cycles at a current density of 0.1C.¹²²

Fig. 4 (a) Schematic representation of the preparation and (b) long-cycling performance of Co₃O₄@G at −10 °C and different current rates. (c) High-resolution transmission electron microscopic (HRTEM) image of the Fe₂O₃/N-rGO sample and rate capability of Fe₂O₃/N-rGO and pure Fe₂O₃ at different current densities. (d) TEM image and long cycling performance of the SnO₂@graphene sponges (GS) at a current density of 5C. (e) NiO micro-flowers/graphene paper electrode rate capability at different current densities and long cycling at 1 A g⁻¹. (f) Long cycling performance of the M1-MnO₂-G and M8-MnO₂-G composite anode material and full cell study of the M8-MnO₂-G composite anode with the LiNi_0.8Mn_0.1Co_0.1O₂ cathode.^118–122

2.2 Graphene-based 2D materials for lithium–sulfur batteries (LSBs)

Lithium–sulfur batteries are considered the most promising candidates among the battery technologies now used to act as the batteries of the future generation with superior electrochemical performance. The high specific capacity (∼1675 mA h g⁻¹) and high energy density (∼2600 W h kg⁻¹) of lithium–sulfur batteries (LSBs) have gained interest in recent years.¹²⁵ The present bottlenecks of LSBs include the severe shuttle effect of soluble lithium polysulfides (LiPSs) and slow redox kinetics, which lead to capacity fading and poor rate performance.¹²⁶ Due to the severe capacity fading (poor cycling stability) and low coulombic efficiency that these devices exhibit, the effective practical implementation of these devices has been put off. Numerous approaches have been proposed to address these problems, some of which involve using 2D-based materials to produce a better component architecture, as will be covered in the following discussion.

2.2.1. Graphenes/hexagonal boron nitride (h-BN) for lithium–sulfur batteries. With its Li-friendly surface and high stability, h-BN can reduce Li dendrites without sacrificing the Li-ion conductivity. Furthermore, h-BN can act as Lewis acidic sites to attract polysulfide anions in the electrolyte, increasing the Li–S battery capacity. Thanks to their synergistic impact, h-BN and graphenes have been validated as improved cathode materials in LSBs. This combination can efficiently adsorb LiPSs and speed up the redox kinetics for LiPS conversion, even under challenging conditions. According to Deng et al., the G/BN composite cathode demonstrated a capacity fading <0.04% per cycle at 70 °C and reached a steady capacity of 888 mA h g⁻¹ at 2C after 300 cycles. However, graphene/BN is also constituted by functional separators or interlayers blocking barriers for LiPS in LSBs. Recently, an extremely flexible LSB with a cathode made of graphene/sulfur composites, an anode made of lithium cloth, an interlayer made of functionalized BN nanosheets and graphenes, and a separator made of functionalized polysulfone have also been produced (Fig. 5a–c).

Fig. 5 (a and b) SEM images of BN/graphene. (c) Long cycling performance of BN/graphene at 70 °C temperature. (d) Lithium cloth anodes, PSU-Celgard separators, and free-standing graphene/sulfur cathodes shielded by an FBN/G interlayer are used to build flexible Li−S complete cells. (e) Charge/discharge curves and cycling performance of the Li–S pouch cells in the flat state and bent state, respectively, of the FBN/graphene interlayer. (f) Scheme of the multi-functional ion-sieve constructed by three 2D materials (gC₃N₄, BN and graphene). (g) Coulombic efficiency and discharge capacity of the cell with the BN/graphene interlayer at rates of 1C and 2C.^127–129

Consequently, the resulting flexible Li–S full-cells exhibit excellent electrochemical performance at a high areal capacity of 5.13 mA h cm⁻² and a high volumetric and gravimetric energy density of 468 W h L⁻¹ in the folded state, up to 800 cycles (Fig. 5d and e). In contrast to separators, an efficient interlayer must have a high conductivity to maximize the use of active sulfur and boost rate capability. With a polypropylene membrane covered with g-C₃N₄/h-BN/G, the LSB was built, and after 500 cycles at 1C, it released a discharge capacity of around 600 mA h g⁻¹. The capacity attenuation was less than 0.01% per cycle with a 6 mg cm⁻² area S-loading (Fig. 5f and g).

2.2.2. Graphene/graphitic carbon nitride for lithium–sulfur batteries. The organic n-type visible light-active semiconductor graphitic carbon nitride (g-C₃N₄) photocatalyst attracted a lot of attention in various applications due to its 2D structure, composition devoid of metals, and excellent chemical and thermal stability. Strong affinity to Li₂S_n is demonstrated by graphitic carbon nitride (g-C₃N₄), a 2D structure similar to graphene, which is explained by the chemical interaction between Li atoms and pyridinic-N sites. Juan Zhang et al. used a microemulsion technique to fabricate a 3D porous sulfur/graphene@g-C₃N₄ (S/GCN) hybrid sponge that may be used as a cathode for Li–S batteries with improved electrochemical characteristics. The outcomes of these combined effects are superior performances of 3D S/GCN in terms of high specific capacity, advantageous high-rate capability, and exceptional long-term cycling stability. Specifically, a low-capacity fading rate of 0.017% per cycle over 800 cycles at 0.3C and a high energy density of up to 1493 W h kg⁻¹ are attained.¹³⁰ Furthermore, this one-pot method is straightforward to use. It eliminates the need for unwanted Li₂S_n catalysts, which simplifies the process of creating sulfur/carbon hybrid electrodes (Fig. 6a and b). Significantly, in another study published by Qu, Long, et al., polysulfides were trapped using the 2D g-C₃N₄/graphene sheet composite (g-C₃N₄/GS) as an interlayer for a sulfur/Ketjen black carbon (S/KB) cathode. The interlayer of g-C₃N₄/GS combined with the sulfur-based cathode produced an exceptionally high reversible capacity of 1191.7 mA h g⁻¹ at 0.1C after 100 cycles, which is significantly greater than the reversible capacity of the sulfur-based cathode alone (625.8 mA h g⁻¹). It also demonstrated extended cycling stability with a reversible capacity of 612.4 mA h g⁻¹ at 1C after 1000 cycles (Fig. 6c and d).¹³¹ Xiaoyu Wu et al. group synthesized an S@C-NC/GN/g-C₃N₄ cathode and performed electrochemical characterization at a rate of 0.5C. After 500 consecutive electrochemical cycles, the manufactured S@C-NC/GN/g-C₃N₄ cathode exhibits an outstanding cycling performance, maintaining a high reversible capacity of around 1130 mA h g⁻¹ (Fig. 6e and f).¹³²

Fig. 6 (a) Schematic of the procedure for preparing S/GCN hybrid sponge. (b) Long-term cycling performance of S/GCN, GCN-Li₂S_n, and S-GCN cathodes at a low current density of 0.5 A g⁻¹ (0.3C). (c) Schematic of cell configuration with a laminated structure of the g-C₃N₄/GS cathode interlayer. (d) Cycling performance and capacity retention of the S/KB@C₃N₄/GS cathode at 1 and 2C for 1000 cycles. (e) Free-standing 3D network cathode (N-doped carbon/graphene/g-C₃N₄) preparation. (f) Long cycling performance of the S@C-NC, S@C-NC/GN and S@C-NC/GN/g-C₃N₄ cathodes at 0.5C rate.^130–132

2.2.3. Graphenes/MXenes for lithium–sulfur batteries. MXene is a 2D transition metal carbide or carbonitride emerging from the MAX phase. Many sulfur atoms can be loaded into the unusual structural variation of MXenes, and the addition of heteroatoms increases the chemisorption of polysulfide. Furthermore, the material's superior electrical conductivity and lithium-ion transport capacity enable its direct application as a sulfur carrier. The electrochemical performance of the LSBs can be noticeably enhanced with the MXene@S cathode. Engineered MXenes can help with material agglomeration and stacking issues, but the procedure is highly time-consuming and costly. Consequently, scientists have improved the MXene carbon composite material.^133–135 Ti₃C₂ is the MXene derivative that is investigated most frequently among all of the them.¹³⁶ According to Luo Yuanzheng et al., a freestanding graphene-supported N-doped Ti₃C₂T_x MXene@S cathode was successfully manufactured by the co-assembly of rGO flakes onto the sulfur particles in a reduction process via a simple sacrificial templating approach (Fig. 7a). The battery was able to achieve a high specific capacity of 721.7 mA h g⁻¹ at 0.1C and a capacity retention of 579.6 mA h g⁻¹ after 300 cycles with the optimal N–Ti₃C₂T_x@S/G cathode despite a comparatively high loading amount of 5.2 mg cm⁻² (Fig. 7b).¹³⁷ A functionally antagonistic Janus composite made of sulfur, graphene oxide, and Ti₃C₂T_x MXene was reported by Sanghee Nam et al.¹³⁸ While the GO side offers significant porosity and nonelectrical conductivity, the MXene side retains mechanical qualities and metallic-like conductivity. As a result, GO and MXenes were employed as the selective separator and current collector, respectively. The permselectivity test in a 50 mL H-type electrolytic cell with a GO/MXene bilayer composite (GM) and a commercial PP separator is shown in Fig. 7c. The color of these two cells' right chambers was assessed after 12, 24, and 36 hours. Dark brown gradually altered in the right chamber using a PP (Fig. 7c). As seen in Fig. 7c, however, in the instance of the GM, LiPSs did not diffuse to the right chamber, and the right chamber's color remained colorless. As seen in Fig. 7d, the charge–discharge tests were carried out at different current densities ranging from 0.1 to 2.0C. With a cyclic retention of around 85.1%, the specific capacity demonstrated exceptional stability at low C rates. The particular capacities at 1.0 and 2.0C were adequately maintained despite minor variations. At 0.1 and 0.2, respectively, the initial discharge capacities were 1424.52 and 1241.67 mA h g⁻¹. At different C rates of 0.2, 0.5, 1.0, 2.0, 3.0, and 6.0C, respectively, specific capacities of 1, 046.73, 731.85, 617.86, 461.3, and 422.94 mA h g⁻¹ were achieved (Fig. 7d).

Fig. 7 (a) Schematic of the synthesis of N–Ti₃C₂T_x MXene nanosheets; corresponding SEM and TEM images of Ti₃C₂T_x flakes, the melamine–formaldehyde (MF) nanosphere and the Ti₃C₂T_x-wrapped MF nanosphere and schematic of the one-step synthesis process of the freestanding NMXene@S/G composite. (b) Long-term cycling performances of the Li–S cell with NMSG at 1C for 300 cycles. (c) Optical images of the visualized lithium polysulfide diffusion test with a commercially available PP separator and a GO/MXene bilayer composite at 0, 12, 24, and 36 h, consisting of Li₂S₆ (Left), GM (Middle), and DOL/TEGDME (Right) in a H-type electrolytic cell and electrochemical measurements; long-term cycling test at 0.1, 0.2, 1.0, and 2.0C of GSM and S8/PP cell. (d) Charge–discharge profile at a C rate of 0.1C (200 mA g⁻¹) from the 1st to the 100th cycle; charge–discharge profiles at different C rates (0.1 to 6C).^137,138

2.2.4. Graphenes/transition metal dichalcogenides for lithium–sulfur batteries. Transition metal dichalcogenides (TMDs) with layered structures such as graphenes are easier to exfoliate and facilitate quick ion diffusion. TMDs exhibit multiple structural phases, with the most prevalent in the octahedral (1T) and trigonal prismatic (2H) phases. Numerous new characteristics are present in these various TMD groupings. As a result, they have been extensively employed in studying different ion battery electrode materials. In recent years, TMD and TMD/graphene composite materials have brought new life to the design of LSBs for enhanced performance. He et al. fabricated a graphene/1T-MoS₂ heterostructure with high-efficiency electrocatalytic properties, suppressing the polysulfide shuttle, to enhance the performance of LSBs. A few layers of graphene nanosheets were sandwiched between metallic, hydrophilic 1T-MoS₂ multilayers with many active sites to form the heterostructure (Fig. 8a). While a high conductivity of 1T-MoS₂ and graphene facilitates electron transmission, the porous 3D structure and hydrophilicity aid in the electrolyte penetration and Li-ion transfer. As shown in Fig. 8b, these benefits result in the graphene/1T-MoS₂-containing LSBs having good electrochemical performance, with a reversible discharge capacity of 1181 mA h g⁻¹ and a capacity retention of 96.3% after 200 cycles.¹³⁹ Similarly, MoTe₂@graphene composites were prepared quickly and easily via microwave synthesis by Zhen Wei et al. and utilized as cathode host materials for LSBs to reduce polysulfide shuttling and enhance interfacial electron/ion transport. The MoTe₂@graphene@carbon cloth electrode demonstrated exceptional rate performance, good cycling stability (98.7% capacity retention after 100 cycles at 0.2C), and a high initial discharge capacity of 1246 mA h g⁻¹ at 0.2C for the first galvanostatic cycle (Fig. 8c).¹⁴⁰ A MoS₂@ carbonized melamine foam (CF)-nitrogen-doped reduced graphene oxide (NRGO) composite was fabricated by Jiayu Zhan et al. and used in Li–S batteries as a modified polypropylene separator layer (MoS₂@CF-NRGO/PP) (Fig. 8d). The composite preserved a three-dimensional carbon matrix structure, which improved Li⁺ shuttling conditions and improved the LiPS filtration efficiency (Fig. 8e–g). Li–S batteries featuring a separator modified by MoS₂@CF-NRGO demonstrated an exceptional rate performance of 615 mA h g⁻¹ at 5C, surpassing the CF-NRGO and PP configurations (Fig. 8l).

Fig. 8 (a) SEM images of 1T MoS₂ on the graphene aerogel ((three-dimensional graphene) 3DG/1T MoS₂ (TM)). (b) Cycling stability of electrocatalytically active 3DG, 3DG/1H MoS₂ (HM), and 3DG/TM as working electrodes vs. Li/Li⁺ with a catholyte consisting of 1 M Li₂S₆ at 0.1C rate.¹³⁹ (c) Polysulfide conversion ability of polar MoTe₂@graphene composite as a cathode material for LSBs.¹⁴⁰ (d) Schematic of the preparation stages of MoS₂@CF-NRGO. SEM images of (e) carbonized MF and (f and g) MoS₂@CF-NRGO. (h) TEM and (i and j) HR-TEM images of MoS₂@CF-NRGO. (k) EDS mapping images of MoS₂@CF-NRGO and the corresponding images of Mo, S, and N. (l) Rate capacity and cycling capabilities at 0.2C of a cell with the MoS₂@CF-NRGO/PP separator, CF-NRGO/PP, and PP separators and cycling performance at 0.5C before and after rest of the MoS₂@CF-NRGO/PP and PP separators,¹⁴¹ (m) digital photographs of MoS₂ and MoS₂/G composites in a Li₂S₆ solution for 0 and 24 h; schematic of the lithium–sulfur battery without and with the MoS₂/G interlayer. (n) Cycling performance and coulombic efficiency of the carbon nanotube (CNT)/S@MoS₂/G and CNT/S cathodes at 0.1C, cycling performance and coulombic efficiency of the CNT/S@ MoS₂/G at 0.5C and rate capability.¹⁴²

Additionally, after 200 cycles, the MoS₂@CF-NRGO modified separator exhibits a 99.6% average coulombic efficiency, demonstrating an outstanding capacity retention. After ten charge/discharge cycles at 0.5C, Li–S batteries were allowed to rest for five days to examine their self-discharge behavior. Following a five-day rest period, the MoS₂@CF-NRGO configuration's discharge capacity drops from 1015.8 to 927.9 mA h g⁻¹, with an average self-discharge value of only 8.6% (Fig. 8l). The molybdenum disulfide/graphene (MoS₂/G) interlayer was constructed by Mingang Zhang and his team using a straightforward coating technique, and it was utilized in an LSB separator.¹⁴² The color change of the mixed solution can be used to directly compare and evaluate the polysulfide adsorption capacity of composites (Fig. 8m). At 0.1C, the CNT/S@MoS₂/G cathode has an initial discharge capacity of 1537.2 mA h g⁻¹. At 0.5C, the CNT/S@MoS₂/G cathode has a discharge capacity of 984.9 mA h g⁻¹ (Fig. 8n). With an ultralow capacity decay of 0.061% every cycle, the capacity drops to 713.8 mA h g⁻¹ after 450 cycles. The CNT/S@MoS₂/G cathode delivered excellent rate capability. The MoS₂/G interlayer, which sits between the cathode and the separator, is thought to have more excellent cycling stability because it confines polysulfide to the cathode and lessens the shuttle effect.

2.2.5. Graphenes/transition metal oxides for lithium–sulfur batteries. Metal oxide-decorated graphenes both reduce the shuttle effect that polysulfides cause and improve their chemical adsorption, because of their strong surface polarity. As a result, research on graphene and metal oxide complex composites has been extensive. A double-shelled TiO₂–graphene heterostructure (H–TiO₂/rGO) with a high degree of exposure on the active plane and a lot of oxygen vacancies has been described by Yanqi Feng et al. as an enhanced host material for LSBs.¹⁴³ The electrode (S–TiO₂/rGO-1) combines the beneficial properties of strong anchoring and catalysing LiPSs to provide a product with excellent rate performance, a long lifespan (1000 cycles at 1C, 0.023% capacity loss per cycle), and high coulombic efficiency (Fig. 9a). The polysulfide shuttle in the LSBs was regulated by the thin TiO₂/graphene interlayer, as demonstrated by Zhubing Xiao et al. (Fig. 9b).¹⁴⁴ It was discovered that the porous carbon nanotube (PCNT)–S cathode could provide a reversible specific capacity of ≈1040 mA h g⁻¹ over 300 cycles at 0.5C thanks to the application of the graphene/TiO₂ sheet as an interlayer (Fig. 9c). Porous graphenes provided an extra electrically conducting network and physically confined S and PS in this well-thought-out design. As an added benefit, TiO₂ in the graphene/TiO₂ barrier film further chemically suppressed PS's dissolution and lessened the undesired shuttle effect. Youlong Xu group reported that ferric oxide nanoparticles on an N, S co-doped graphene (Fe₃O₄–N, S/rGO) sheet structure may have a major impact on the development of Li–S batteries with high energy density and extended cycling life by controlling LiPSs and improving the conversion kinetics.¹⁴⁵ Using a straightforward microwave hydrothermal process (Fig. 9d), ferric oxide nanoparticles on an N, S co-doped graphene (Fe₃O₄–N, S/rGO) sheet structure were fabricated and used as the cathode for LSBs. In the end, the built Fe₃O₄–N, S/rGO–S batteries provide consistent long-term cycle life and high-rate capability. The Fe₃O₄–N, S/rGO–S electrode continues to provide a high discharge specific capacity of 1004 mA h g⁻¹ even after 200 cycles at 0.5C (Fig. 9e).

Fig. 9 (a) Schematic of the formation and long-term cycling performances at a current density of 1C of H–TiO₂/rGO-1.¹⁴³ (b) Schematic of the electrode configuration for the Li–S battery with a graphene/TiO₂ coating film and conventional Li–S battery. (c) Cycling stability of PCNTs–S, PCNTs–S@G, and PCNTs–S@G/3% TiO₂ cathodes at 0.5C, and a current density of 0.86 mA cm⁻².¹⁴⁴ (d) Schematic of the synthesis process of the Fe₃O₄–N, S/rGO composite. (e) Cycling performance of the Fe₃O₄–N, S/rGO–S and Fe₃O₄-rGO–S electrode at 0.5C after 200 cycles.¹⁴⁵

2.3 Graphene-based 2D materials for lithium–O₂ and lithium–CO₂ batteries

Rechargeable Li–O₂ and Li–CO₂ batteries, which offer a higher theoretical energy density than that of Li-ion batteries, have recently attracted a lot of attention.^146,147 Li–O₂ batteries have been developed to fulfil the increased demand for electric energy in today's world. The formation of a lithium peroxide (Li₂O₂) product, which has a substantially higher energy storage capacity than that of other energy storage systems, is responsible for the high energy density of Li–O₂ batteries.¹⁴⁸ The high band gap of solid Li₂O₂, which is probably covering the catalytic sites, makes charge transfer necessary for the breakdown of Li₂O₂ during the charge process difficult.¹⁴⁹ This results in a slow charging process that raises the potentials needed for Li₂O₂ breakdown, which lowers the battery's energy efficiency and increases the danger of electrolyte degradation.^150,151 Furthermore, the size and shape of the Li₂O₂ product can affect the charge potential. Developing Li–O₂ batteries requires improving the materials used in each component of the battery in order to address knotting problems. Two-dimensional materials have recently attracted attention due to their potential to improve the Li–O₂ battery performance. This is because of their distinct designs, which have the potential to effectively address a number of issues related to the electrode materials (cathode and anode), solid-state electrolyte, and separator in Li–O₂ batteries.^152–155 A unique type of MnO₂@rGO nanocomposite material was described by Lihua Zhu et al.; it is very effective for Li–O₂ batteries and consists of extremely few nano-sized MnO₂ grains on the surface of rGO (Fig. 10a). In Li–O₂ batteries, MnO₂@rGO composites were used as cathode catalysts with a cut-off capacity of 1000 mA h g⁻¹ and a current density of 200 mA g⁻¹ (Fig. 10b).¹⁵⁶ Over 60 cycles, batteries based on MnO₂@rGO demonstrated a consistent discharge capacity. Strong synergistic effects between the rGO framework and the nano-sized MnO₂ particles on its surface are responsible for the exceptional performance. Functionalized graphenes with an optimal bimodal porous structure were produced by Jie Xiao et al. for Li–O₂ battery operation (Fig. 10c). A novel air electrode that uses an uncommon hierarchical configuration of functionalized graphene sheets produces the highest capacity of 15 000 mA h g⁻¹ at around 2.7 V.⁸⁵ Yuejiao Li and colleagues described the facet engineering of an ultrathin, two-dimensional Mn₃O₄ nanosheet on graphene (Mn₃O₄ NS/G) with the dominating (101) crystal planes. The goal of this catalyst is to generate durable, effective oxygen for high-performance Li–O₂ batteries that exhibit ultrahigh capacity and long-term stability, offering a lower charge overpotential (Fig. 10d).¹⁵⁷

Fig. 10 (a) Schematic of the structure catalyzed by MnO₂@rGO for the Li–O₂ battery. (b) Discharge/charge capacity of MnO₂@rGO for the Li–O₂ battery at a current density of 200 mA g⁻¹. (c) An optimal bimodal porous structure on a functionalized graphene sheet (FGS) and the discharge curve of a Li–O₂ cell using the FGS. (d) Two-dimensional Mn₃O₄ nanosheets on graphene with charge/discharge voltages. (e) Li–CO₂ battery's schematic, optical picture and flexible optical picture of the TiVC/rGO aerogel (TVGA) cathode sheet. (f) LEDs with the flexible Li–CO₂ battery. (g) Variation in the terminal polarization of Li–CO₂ batteries with different cathodes at a current density of 200 mA h g⁻¹.^85,156–158

Similar to Li–O₂ batteries, the Li–CO₂ battery is another advanced technology because of its high theoretical energy density of 1876 W h kg⁻¹. Li–CO₂ batteries offer a novel method for capturing CO₂ and advancing the development of electrochemical energy storage systems of the next generation by using CO₂ as the active material at the cathode. Rechargeable Li–CO₂ batteries store energy by breaking down the discharge products when exposed to external electricity during the charging process. During discharge, this process produces electricity via a redox reaction between the Li anode and the CO₂ cathode, namely Li₂CO₃.^159–161 Zhang Zhang et al. reported the first introduced graphene as a cathode material to improve the performance of Li–CO₂ batteries.¹⁶² Wei Yu et al. synthesized N, O-diatomic graphene carbon aerogels by a straightforward, one-step thermal approach and used it as a cathode Li–CO₂ battery. The resulting Li–CO₂ batteries demonstrated an impressive cycling stability of more than 1500 h at 20 μA cm⁻² and a significantly improved initial energy efficiency of about 78.46%.¹⁶⁰ MXene-based materials have been used as the cathode in Li–CO₂ batteries because of their superior interlayer ion transport channels, strong electrical conductivity, and chemical composition's tunability. Wentian Zhao prepared porous TVGA via a hydrothermal reaction and used it as a cathode in Li–CO₂ batteries (Fig. 10e). The TVGA cathode has flexible nature, and hence, a flexible Li–CO₂ battery was fabricated and used in different bending angles. Light-emitting diodes (LEDs) can be steadily lit by a Li–CO₂ battery without any adjustments (Fig. 10f). Fig. 10g compares the cycling performance of Li–CO₂ batteries with different cathodes. At a current density of 200 mA g⁻¹, TVGA displays a comparatively low overpotential of 1.5 V.¹⁵⁸ The battery has a 91 times stable cycling capacity at 200 mA g⁻¹, with an over potential of 1.5 V for both charging and discharging. These exceptional electrochemical performances are due to the special synergistic interaction between Ti and V in TiVC, which improves the reversible formation and breakdown of the discharge product Li₂CO₃ by enhancing the interfacial chemical bonding ability, realizing the full exposure of active sites and promoting the catalytic interfacial structural reorganization. In the meantime, the aerogel's rich pore structure helps lower the mass and charge transfer resistance by promoting the diffusion of CO₂ and electrolytes in addition to helping to enhance ion transport.

3. Lithium metal anode modification with graphenes

Since lithium metal has the lowest working potential (−3.04 vs. conventional hydrogen electrode) and the largest specific capacity (3860 mA h g⁻¹), it has been considered the best option for lithium-ion battery anodes. The lithium metal anode, a crucial component of the next generation of high-energy density lithium metal batteries (LMBs), has gained significant interest from both industry and academics. However, because of its innate propensity to generate dendrites during cycling, the lower cycling stability and safety concerns have long hindered the adoption of lithium metal anodes. The excessive parasitic interactions between the Li metal dendritic growth and the liquid electrolyte result in the irreversible consumption of both the electrolyte and active lithium. Consequently, the rapid capacity decline and catastrophic cell failure of LMBs are explained by the production of Li dendrites. Several methods have been devised to enhance the lithium metal anode's cycling stability and controlling the Li dendrites.^163–169 According to David Mitlin and colleagues, a significant degree of structural and chemical imperfection in graphene leads to unstable SEI formation. This is associated with the rapid decline of coulombic efficiency and the production of filament-like Li dendrites, as well as the consumption of the fluoroethylene carbonate (FEC) additive in the carbonate electrolyte.¹⁶⁴ Pinxian Jiang et al. described the graphene hybrid architecture as a lithiophilic host for a high-performance Li metal anode. They utilized it in the LMB to synthesize a nanosheet/graphene heterostructure composite via a polymer-assisted sonochemical process. The Li metal anode that results from the graphene hybrid architectural scaffold shows stable cycling over 200 cycles at 0.5 mA cm⁻² (2 mA h) and provides a high energy density of 402 W h kg⁻¹.¹⁷⁰

4. Hydrogen production and storage

4.1 Sustainable green hydrogen production approaches

Hydrogen (H₂) is recognized as an efficient energy carrier as it is an eco-friendly, non-toxic, renewable, and less costly energy source. Currently, H₂ is generated from renewable and non-renewable sources, where only 95% of H₂ is from non-renewable fossil fuels, and 5% is from renewable sources such as solar and wind.⁴¹ The H₂ production approaches dependent on non-renewable resources are associated with CO₂ emissions, are expensive and energy-intensive, and involve harsh operating conditions.¹⁷¹ Thus, researchers are exploring approaches to generate sustainable, less costly, and zero carbon-emitting H₂ energy.¹⁷² In terms of renewable energy, the role of abundantly existing solar energy is significant, and two approaches for H₂ production can be employed, namely, (1) photocatalytic and (2) photoelectrochemical. Photocatalytic H₂ production by water splitting using semiconductor catalysts is a widely researched, less costly, eco-friendly, and effective approach for green H₂ generation.^173,174 Moreover, PEC water splitting for H₂ production is another approach that uses solar-irradiated photoelectrodes and an external bias voltage to suppress charge recombination, leading to excellent overall performance.¹⁷⁵

4.1.1. Mechanism of photocatalytic and photoelectrochemical H₂ production. Generally, an efficient photocatalytic setup needs a catalyst capable of absorbing visible irradiation as a major part of sunlight constitutes irradiation within the visible range, a suitable photoreactor, and a sacrificial agent (Table 1).¹⁹¹ Moreover, an effective interaction between all the components of the photocatalytic setup is needed for significant performance improvement.¹⁹² Overall, water splitting into H₂ and O₂ is not favorable in terms of thermodynamics under normal temperature and pressure conditions because of the existence of strong bonds in H₂O and its low power to ionize. Therefore, at room temperature, the entire process needs 237 kJ mol⁻¹, i.e., 2.46 eV per H₂O molecule of Gibbs free energy (ΔG).¹⁹³ The photocatalytic H₂ generation process initiates after the semiconductor catalyst absorbs the solar irradiation, having a wavelength greater than or equal to the semiconductor band gap to generate electron–hole pairs followed by the separation of charges where electrons jump to the conduction band (CB) of the semiconductor and holes are left in the valence band (VB). Then, a redox reaction occurs, where the electrons play a role in the reduction of H⁺ to H₂, and the holes are responsible for oxidizing H₂O into O₂.¹⁹⁴Eqn (1)–(3) describe the steps involved in overall photocatalytic H₂ generation. Moreover, for the photocatalytic process, the energy level of the semiconductor CB should be at a potential less than 0 V (E(H⁺/H₂)) for the reduction process, and the VB energy level should be at a higher potential than 1.23 V (E(O₂/H₂O)) to conduct oxidation reactions.^195,196Fig. 11a illustrates the photocatalytic H₂ generation mechanism over photocatalysts.

2H⁺ + 2e⁻ → H₂ (1)

2H₂O + 4h⁺ → O₂ + 4H⁺ (2)

2H₂O + 2O₂ → 2H⁺ (3)

Table 1 Summary of the recent 2D graphene-based nanocomposites as photocatalysts for photocatalytic H₂ production together with the yield of H₂ and parameters, including loading of catalyst, source of light, and sacrificial agent

Photocatalyst	Catalyst loading (mg)	Light source	Sacrificial agent	H2 production (μmol g^−1 h^−1)	Ref.
TiO2/rGO/MoS2	50	3 W 4 LEDs (λ 365 nm)	10 vol% methyl alcohol	880	176
2D CN@graphene@CN	10	300 W Xe lamp with 420 nm cut-off filter	10 vol% TEOA	5580	177
Few-layer graphene (FLG)/g-C3N4	0.15 g L^−1	4 LEDs of 880 W m^−2 each (420 nm)	10% v/v ethylenediaminetetraacetic acid (EDTA) and 3 wt% Pt	1274	178
0D-2D-2D CdSe QD/B-rGO/O-gC3N4	30	Xe arc lamp (λ > 400 nm)	Triethanolamine (TEOA)	1435	179
Ni2P/rGO/g-C3N4	10	300 W Xe lamp (λ > 420 nm)	10 vol% TEOA	2921	180
CdS@h-BN/rGO	10	Simulated solar light	10 vol% triethanolamine (TEA)	6465.33	181
P.RGO/MoS2	2	400 W Xe lamp	20% (v/v) TEOA and 1 mL eosin Y	11 230	182
2D/2D MoS2/g-C3N4-Au	200	300 W Xe lamp 420 nm cutoff filter	20 mL methanol	1100	183
Molybdenum sulfoselenide (MoSSe)@RGO	—	400 W Xe lamp (λ ≥ 420 nm)	Eosin Y and TEOA	5016	184
GQD/TpPa-1-COF	20	300 W Xe lamp	0.1 mol per L ascorbic acid	487	185
2D/2D Cu-tetrahydroxyquinone MOF (CuTHQ)/N-doped graphene (NG)	20	150 W Xe lamp	Methanol (MeOH)	164	186
Co-BDC MOF/GO	—	150 W halogen lamp	0.2 M TEOA and EY photosensitizer	14.5	187
2D/2D PRGO/CdS-diethylenetriamine (DETA)	50	0.35 M Na2S, 0.25 M Na2SO3 and Pt cocatalyst	300 W Xe lamp with 400 nm cut-off filter	10 500	188
Cu2O/RGO/BiVO4	50	30 mL tetracycline	300 W Xe arc lamp (λ > 420 nm)	5.95	189
rGO-coupled C3N4/C3N5	50	10 vol% TEOA	300 W Xe lamp (λ ≥ 400 nm)	6380	190

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Fig. 11 (a) Illustrative overview of the general photocatalytic H₂ generation mechanism. (b) Representation of photoanodes and cathode semiconductors for PEC H₂ generation. (c) Mechanism of PEC water splitting for H₂ generation. (d) Characteristics of graphene for photocatalytic and PEC H₂ production.^197–199

The PEC H₂ production utilizes a PEC cell comprising two or three electrodes acting as semiconductors for photoelectrodes submerged in an aqueous electrolyte, with the electrodes separated by a membrane.¹⁹⁵ Out of the electrodes used, one or two electrodes should have the potential to be activated under irradiation. Normally, the PEC comprises three electrodes for water splitting: a photoanode and a cathode act as active electrodes, while Ag/AgCl acts as a reference electrode. As shown in Fig. 11b, n-type semiconductors have excess electrons and thus are used as photoanodes. Conversely, p-type semiconductors function as photocathodes due to their excess electron–hole pairs. Electrons are excited and move from the VB to the CB when the photoanode is exposed to light with an energy higher than its energy bandgap, followed by the movement of electrons toward the cathode to carry out a reduction reaction, while the holes left behind are involved in the process of oxidation.^200,201 With the need for an overpotential for each half-reaction, electrode resistance, loss from charge carriers' recombination, and potential decline at contacts, the energy required for PEC water splitting in practical systems can approach 2 V. To compensate for these losses and supply the energy needed for water splitting, an external bias is frequently used.²⁰²Fig. 11c illustrates the steps involved in the PEC water splitting mechanism for H₂ generation.

4.1.2. Overview of photocatalysts and 2D graphenes for green H₂ production. The overall water splitting involving the generation of H₂ and O₂ is an uphill process, requiring 1.23 eV energy. Therefore, the major prerequisite of a suitable catalyst is the band gap, which should be higher than 1.23 eV. Since 1972, after the exploration of water splitting by Honda and Fujishima, where a photoanode comprising TiO₂ and Pt cathode was used,²⁰³ various catalysts including nitrides, oxides, sulfides, and carbides have been extensively explored for H₂ production via water splitting. The efficiency of all the explored catalysts is dependent on the solar light absorption ability, generation of charge carriers, separation of charge carriers, redox ability, more exposed surface area and stability. Moreover, the non-toxic nature and low cost are also the requirements for selecting a suitable catalyst for H₂ generation.^{197,204–206}

Earlier, various semiconductor materials such as TiO₂,²⁰⁷ BiVO₄,²⁰⁸ WO₃,²⁰⁹ perovskites,²¹⁰ g-C₃N₄,²¹¹ LDHs,²¹² ZrO₂,²¹³ and CdS²¹⁴ as photocatalysts have been researched for photocatalytic H₂ generation; however, they show poor efficiency towards photocatalytic H₂ generation mainly due to lower stability, wide band gap, and charge carrier recombination, whereas the photocatalysts having a narrow band gap suffer from a lower efficiency based on their surface area.²¹⁵ The utilization of metal-based photocatalysts is also explored extensively; however, metals are toxic and at the same time expensive.²¹⁶ In this regard, 2D materials as semiconductors for photocatalysis have attracted substantial attention as they offer excellent optical characteristics, good mechanical features, large surface area and less distance for the movement of electron–hole pairs to the solid–water interface.²¹⁷ Particularly, the heterojunction formation of 2D graphenes having a honeycomb structure with other semiconductor photocatalysts is considered to improve the overall photocatalytic H₂ production efficiency.²¹⁸ This is associated with the existence of an exceptional sp² hybrid C framework, making it highly conductive for thermal energy, i.e., about 500 W m⁻¹ K⁻¹ and depicts an exceptional charge migration under ambient temperature condition, i.e., about 200 000 cm² V⁻¹ s⁻¹.²¹⁹ Besides, it shows a zero band gap and an enhanced surface area of about 2600 m² g⁻¹. Graphene-based photocatalysts depict improved transfer and separation of electron–hole pairs.²²⁰ Numerous published works used rGO as one of the standard semiconductor materials for improving the performance. RGO enhances the photocatalytic activity of these composites by facilitating the separation of electron–hole pairs across the catalyst/RGO contact. The photocatalytic H₂ production of the nanocomposites is largely dependent on the type of interface and defects present in the produced rGO.²²¹ rGO is unique as it possesses a variety of qualities that promote photocatalytic H₂ production efficiency. These properties include increased light utilization, charge carrier separation, intrinsic catalytic activity, and stability under operating conditions. While other photocatalysts could need co-catalysts or extra modifications to operate on par, their appropriateness varies depending on the demands of the given application and optimization techniques.²²² The efficient photocatalytic H₂ generation by graphene-based photocatalyst materials is due to the following mechanisms (Fig. 11d): (1) reduction of charge carrier recombination, (2) presence of catalytic active sites for reactions, (3) improved adsorption, (4) tunable band gap, and (5) potential to act as photosensitizers and cocatalysts.⁵⁹ Graphene derivatives including graphene oxide (GO) and reduced graphene oxide (rGO) contain epoxy, hydroxyl, and carboxyl surface groups having O₂, acting as catalytic reaction sites and also as supports for the uniform dispersion of metallic ions for nucleation and growth.²²³ The dark colors of both GO and rGO lead to better absorption of light, expanding the absorption from the UV to the visible region.²²⁴

In the case of PEC H₂ production, photoelectrodes must possess the following features: (1) efficient absorption in the visible region, (2) appropriate band gap and positions for band edge to carry out redox reactions, and (3) stable nature in solutions of different pH values.⁴⁹ PEC H₂ production has been extensively studied for photocatalytic semiconductors such as TiO₂, MoS₂, WO₃, ZnO, α-Fe₂O₃, BiVO₄, Cu₂O, CdS, and Cu/Zn/W sulfide; however, certain challenges persist, leading to their poor PEC activity. For instance, TiO₂,²²⁵ WO₃,²²⁶ and ZnO²²⁷ constitute wide band gaps and charge recombination, whereas α-Fe₂O₃ suffers from the demerits of short lifetime of charges, electron–hole pair recombination and less conductivity,²²⁸ BiVO₄ also shows charge recombination²²⁹ while Cu-based electrodes face the issue of photo corrosion, leading to less efficient PEC performance.²³⁰ Lately, C-based materials such as graphene derivatives and rGO were studied to act as supports for developing different photoanodes due to their high surface area, excellent conductivity, charge movement, and stability.²³¹ Along with other semiconductor materials, they are known to show improved overall migration of charges, improve the photocurrent response and inhibit the process of photo corrosion.²³²

Moreover, regarding heterojunction formation, 2D/2D heterostructures are known to depict excellent interface, leading to an improved surface area and less resistant electron transfer. Compared to the conventional interface, the 2D/2D interface provides a faster transport of charges which reduced their recombination.²³³ Moreover, 2D/2D heterojunctions help in tuning the band edges for efficient redox reactions.²³⁴ The heterojunction also leads to a stable heterostructure, less agglomeration and less photo-corrosion phenomena.²³⁵

4.1.3. Graphene-based 2D nanocomposites for photocatalytic hydrogen production. Regarding the utilization of graphene-based nanocomposites, due to the efficient electro movement in GO, MoS₂ were grown onto rGO followed by TiO₂ coupling. The H₂ production under UV light was 88 folds more than the H₂ production from pure TiO₂ because of the synergistic effect based on the ability of rGO towards efficient electron migration and MoS₂ acting as active sites.¹⁷⁶ The Ti₃C₂/TiO₂/rGO nanocomposite was studied for H₂ generation and the nanocomposite generated 808.11 μmol g⁻¹ h⁻¹ of H₂ within the 1st h, associated with the presence of the rutile phase of TiO₂ and rGO, which offered the pathway for the migration of electrons and minimized the recombination of charges.²³⁶ A 2D carbon nitride (CN)@graphene@CN loaded with Pt was explored for photocatalytic H₂ generation, where under light irradiation, the electrons from the CB of CN were transferred to the graphene. The graphene aided in trapping the electrons and at the same time helped in their movement to the edges for their reaction with protons (Fig. 12a). The efficient activity was concluded to be dependent on the enhanced visible light absorption, excellent transfer of electrons across the interface, spatially divided redox active sites and the presence of excess active sites.¹⁷⁷ In another study, 2 wt% few-layer graphene (FLG)/graphitic carbon nitride (GCN) depicted 4-times 1274 μmol g⁻¹ h⁻¹ of H₂ under visible light, as shown in Fig. 12b and c along with an excellent stability over 4 consecutive cycles. As shown in Fig. 12d, electron–hole pairs were generated in CN, while the layered and 2D structure of CN aided in electron transfer from CN to FLG. FLG constitutes good electric conductivity, thus providing pathways for the movement of electrons, inhibiting electron–hole pair recombination. Moreover, the greater work function of the Pt metal enabled the transfer of electrons from FLG to Pt, for H₂ generation, while holes in GCN were quenched by the sacrificial agent.¹⁷⁸ Ni₂P/rGO/g-C₃N₄ was explored for efficient visible light-induced H₂ production and stability, which was associated with the active sites provided by Ni₂P and the synergistic effect of rGO towards the migration of electrons, leading to less charge recombination.¹⁸⁰ As shown in Fig. 12e, a solution-phase electrostatic assembly of polymer-functionalized rGO and boron carbon nitride (BCN) having positive and negative charges, respectively was used to synthesize polymer rGO (P.rGO) and polymer BCN (P.BCN). The 2D nanocomposite P.rGO/MoS₂ generated the highest yield of H₂ based on the presence of active sites for the reaction and improved the migration of charges within the layered structure.¹⁸² In a current study, graphene quantum dots (GQDs)/TpPa-1-COF were studied for photocatalytic H₂ production, where the GQDs acted as photosensitizers for improving the light absorption and acted as a trap for electrons, leading to reduced recombination of electron–hole pairs. Moreover, the movement of electrons was aided due to the presence of an ohmic interaction among the covalent organic frameworks (COF) and GQDs.¹⁸⁵ In another work, the need for a noble metal as a cocatalyst was diminished while using 2D/2D Cu-tetrahydroxyquinone MOF (CuTHQ)/N-doped graphene (NG) nanocomposites for H₂ production. This was linked to the efficient migration of electrons between both photocatalysts, leading to less charge recombination. Therefore, the 2D/2D heterojunction helped in an increased yield of H₂ but at the same time played a vital role in separating electron–hole pairs, leading to noble metal-free photocatalytic H₂ generation.¹⁸⁶ Another noble metal-free photocatalytic H₂ generation was achieved by the fabrication of eosin Y (EY) dye-sensitized earth-abundant cobalt (Co-BDC) MOF with the addition of GO, where GO played a vital role as compared to EY in improving the H₂ generation under visible light irradiation. As shown in Fig. 12f, the irradiation is absorbed by the EY dye for the generation of singlet excited state, followed by quenching by TEOA or formation of ³EY* and leading to quenching by amines for the generation of radical EY anions. Then, an electron migration from EY to GO occurs followed by migration to the MOF, which was considered the main reason for efficient H₂ generation.¹⁸⁷

Fig. 12 (a) Illustrative overview of the charge transfer mechanism for the photocatalytic H₂ production process using 2D CN@graphene@CN loaded with Pt. (b) Rate of photocatalytic H₂ production and (c) stability over 4 consecutive cycles using FLG/GCN nanocomposites. (d) Charge transfer mechanism in the FLG/GCN nanocomposite. (e) Overview of the solution-phase electrostatic assembly to synthesize P.rGO and P.BCN. (f) Photocatalytic H₂ production mechanism in EY dye-sensitized Co-BDC MOF/GO. (g) Two-dimensional LDH@graphene and g-C₃N₄ heterojunctions for PEC H₂ generation. (h) Overview of the suitable Fermi level alignment and PEC H₂ generation in Ag-deposited MoS₂/GO.^{177,178,182,187,237,238}

In conclusion, GO-based photocatalytic nanocomposites showed efficient and economic H₂ generation based on the improved surface area, migration of charges, separation of charges and provision of active sites. The incorporation of GO as a cocatalyst was explored to have the potential of replacing noble metals for photocatalytic H₂ generation. The 2D/2D GO/g-C₃N₄ heterojunction incorporated into other semiconductors was extensively studied for H₂ generation based on its simple fabrication, non-toxicity, efficient charge separation and transfer and active sites.

4.1.4. Graphene-based 2D nanocomposites for photoelectrochemical hydrogen production. LDHs are known to have a layered structure and good physio-chemical characteristics, but they are less conductive, have slow kinetics and are not stable in acids. The PEC H₂ production ability of the LDH was improved by forming 2D LDH@graphene and g-C₃N₄ heterojunction with MoS₂ as a co-catalyst (Fig. 12g). The heterojunction showed good production and stability for PEC H₂ production and also effectively separated the charges.²³⁷ In another work, the MoS₂ was fabricated onto graphenes with Ag, which showed a photoconversion of 2.2% at −0.58 V for PEC water splitting. This is due to the plasmonic effect of Ag, leading to improved visible light absorption, and due to the heterojunction, which promoted charge segregation and migration. The Fermi level of pure graphene was positioned among the MoS₂ CB and the ITO, leading to electron withdrawal to the cathode, as indicated by Fig. 12h.²³⁸ MoS₂/crumpled N-doped graphene p–n nanojunction was explored for PEC H₂ generation with an overpotential of approximately 100 mV vs. reversible hydrogen electrode (RHE). The N doped onto graphenes acted as support for the anchorage of MoS₂ and the presence of crumpled graphenes minimized the accumulation. Overall, the heterojunction helped in electron–hole pair separation.²³⁹ Two-dimensional RGO wrapped in nanotubes of TiO₂ with decorated 2D MoS₂ was synthesized for H₂ generation, which was linked to the efficient charge transfer and the control on immobilized active sites.²⁴⁰ A hybrid Ag@g-C₃N₄/r-GO nanocomposite was synthesized for PEC H₂ generation under visible irradiation. The nanocomposite showed durability, efficient activity and adaptability for PEC reaction. An apparent quantum yield of 1.69 and a rate of 954 μmol g⁻¹ h⁻¹ for photocatalytic H₂ generation were achieved in the presence of simulated solar light while an overpotential value of 0.484 V at a current density of 10 mA cm⁻² was achieved in PEC H₂ generation activity.²⁴¹ In another study, a graphene/WS₂ nanocomposite was explored as a photoanode for PEC H₂ generation. The nanocomposite depicted an efficient absorption of 98% under visible irradiations. The increased visible light absorption resulted in excellent generation of electrons with improved movement of charges.²⁴²

In conclusion, the literature to date has shown that adding graphenes at certain interfaces can improve the PEC efficiency of photoelectrodes towards PEC H₂ generation. The high electric conductivity of graphenes, which enhances the charge carrier movement across the heterojunction components, is responsible for the significant increases in the efficiency; however, in literature for PEC H₂ production using 2D/2D graphene and its derivatives.

4.2 Hydrogen storage using graphene-based nanomaterials

Graphene-based 2D nanomaterials are found to be promising candidates for practical solid-state hydrogen storage applications due to their exceptionally large surface area (2600 m² g⁻¹), porous structure, and high chemical and thermal stabilities.²⁴³ Pristine graphenes generally have low hydrogen storage capacities while functionalizing graphene nanosheets drastically improve their performance.^244,245 The functionalization includes doping with hetero atoms such as boron (B) and nitrogen (N), decorating with metal atoms, and defect engineering in the nanosheets (Table 2). Moreover, the graphene nanosheets allow surface curvature tuning, which would help develop a reversible hydrogen storage system with enhanced kinetics.⁶⁰

Table 2 Hydrogen storage using various graphene-based nanomaterials

Material	Pressure (bar)	Temperature (K)	Hydrogen storage (wt%)	Reference
Graphene nanosheets	100	298	0.9–1.5	244 and 245
	10	77	1.2
Hierarchical graphene	1	77	4.0	246
Graphene oxide	50	298	1.4	247
Functionalized graphene with ClSO3H	2	77	1.6	248
Functionalized graphene with oleum	2	77	1.4
Functionalized graphene with Pt nanoparticles	57	303	0.15	249
Functionalized graphene with Pd nanoparticles	40	298	0.81	250
N-doped graphene	90	298	1.5	251
Li-decorated N-doped penta graphene	—	—	7.88	252
Pd3Co-decorated nitrogen/boron-doped graphene	30	298	4.6	253
Phosphorous doped graphene	100	—	2.4	251
Li-decorated holey graphyne	—	—	12.8 (simulation result)	254
Lithium decorated ψ-graphene	—	—	15.15 (simulation result)	255
Ti doped ψ-graphene	—	—	13.14 (simulation result)	256
Zr decorated ψ-graphene	—	—	11.3 (simulation result)	257
Zr-decorated γ-graphyne monolayer	—	—	7.95 (simulation result)	258
Nitrogenated holey graphene (C2N) with titanium clusters	—	—	6.8	259
3D-graphene	1	77	1.4	260
Transition metal-decorated boron doped twin-graphene	1	77	4.95	261
Lithium doped fullerene pillared graphene nanocomposites (Li-FPGNs)	—	—	9.1	262
Ni/Pd co-modified graphene	4	298	2.65	263
Gold decorated graphene nanosheet	—	—	4.65	264
Diatom frustule-graphene	20	298	4.83	265
Fe–Ti decorated defective graphene	—	—	5.1	266
Ca on graphene	—	—	5–6
Mg2.3Y0.1Ni alloy with graphene	—	—	3.25	267

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The adsorption of hydrogen molecules onto graphene sheets can occur either by physisorption (van der Waals interactions) or by chemisorption (chemical bonds with carbon atoms). The theoretical energy requirement for the physisorption of hydrogen molecules (H₂) on single-layer graphenes was evaluated to be in the range of 0.01–0.06 eV (Fig. 13). Even though the energy requirement for hydrogen adsorption onto the graphene sheet is quite low, it still requires low temperatures and high pressures to achieve reasonable hydrogen storage capacities. Chemisorption requires the dissociation of hydrogen molecules, and consequently, the energy requirement is much higher (1.5 eV) (Fig. 13) than that of physisorption. Hydrogen storage capacities of various graphene-based 2D nanomaterials are summarized in Table 2.

Fig. 13 Energy level diagram of hydrogen adsorption–desorption on the graphene monolayer.⁶⁰ Adapted with permission from RSC.

A significant progress has been made in the field of hydrogen storage using graphene-based 2D materials. Among various graphene-based 2D nanomaterials, alkali, alkaline earth, and transition metal nanoparticle-decorated graphene doped with hetero atoms are found to be potential candidates for this purpose. In addition, the functionalized graphene nanosheets are distinctly advantageous than other carbon allotropes such as fullerene, nanotubes, and porous carbons, as shown in Fig. 14. Nevertheless, the adsorption and desorption properties of hydrogen molecules in 2D nanomaterials are still unclear. Therefore, hydrogen storage at ambient temperatures and pressures using functionalized graphene nanosheets even remains a challenge.

Fig. 14 Summary of solid-state hydrogen storage using different carbon-based materials.²⁴⁴ Adapted with permission from MDPI.

5. Summary and outlook

As society's energy needs increase, lithium batteries for energy storage require efficient electrode materials to store energy and the ability to be charged more quickly. Moreover, hydrogen, with the current rate of industrialization leading to greenhouse gas (GHG) emissions, is considered a potential energy carrier that can replace fossil fuels. However, the materials for hydrogen production and storage show challenges of inefficiency and cost-effectiveness. The development of 2D materials may offer a solution to these issues. Graphene is a two-dimensional substance that exhibits remarkable mechanical strength, exceptional conductivity, superb flexibility, high surface area and excellent chemical stability. With these properties, graphenes are desirable for energy storage, hydrogen production and electronic applications (Fig. 15). The synergistic effect of graphenes with 2D composite materials is appropriate for fabricating electrodes of lithium batteries because of their high interlayer adsorption energy and a large specific surface area. As graphenes and 2D composite materials have special features, they can be employed to create stable SEI layers, serve as anode hosts, alter the separator interface (LSB), and change the electrolyte (manage the polysulfides). Additionally, graphenes can enhance the electrical conductivity of two-dimensional materials and lessen volume fluctuations. Apart from this, in terms of H₂ production, GO-based photocatalytic nanocomposites show efficient and economic H₂ generation based on the improved surface area, efficient migration of charges, separation of charges, and provision of active sites. Moreover, in the case of PEC H₂ generation, graphenes show high electric conductivity for efficient charge carrier movement.

Fig. 15 Advantages of the graphene nanomaterial over other carbon-based nanomaterials. Adapted with permission from RSC.²⁶⁸

Data availability

The data that support the findings of this study are available from the corresponding author.

Conflicts of interest

The manuscript was written with contributions from all authors. All authors have approved the final version of the manuscript.

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