Hetero-atom doped graphene for marvellous hydrogen storage: unveiling recent advances and future pathways

Abstract

Hydrogen energy and storage are gaining significant attention due to their potential to address various energy and environmental challenges. The storage of hydrogen in solid-state media is an area of significant interest and research in comparison to compressed gas or liquefied hydrogen storage in high-pressure and large-volume storage tanks. Solid materials can store hydrogen in a condensed form, allowing more hydrogen to be stored within a smaller volume or mass. In addition, solid-state materials offer safer hydrogen storage compared to gaseous or liquid hydrogen where hydrogen is stored at very high pressure (350–700 bar). Extensive research has been carried out on graphene-based materials doped with heteroatoms, showing promise as potential candidates for hydrogen storage. This is attributed to their distinctive properties and improved ability to adsorb hydrogen. Several heteroatoms, such as nitrogen, boron, and sulfur, have been extensively studied for their effects on graphene's hydrogen storage performance. As far as the authors know, there is no critical review reported on heteroatom-doped graphene for hydrogen storage. This article is a comprehensive review of the recent progress of various types of heteroatom (e.g., N, B, Pd, Ni, Ti, Ca, Al, Be, Li, etc.) doped graphene-based materials used for hydrogen storage. The doping level, type of heteroatom, and specific synthesis methods, which greatly influence hydrogen storage performance, are discussed in detail. Lastly, the current challenges and prospects of heteroatom-doped graphene as an effective material for hydrogen fuel cell technology are discussed.

---

Shankar Ghotia

Mr Shankar Ghotia is a senior research fellow at the Academy of Scientific and Innovative Research (AcSIR), CSIR-Advanced Material Processes and Research Institute, Bhopal, India. He post-graduated in Physics (2019) from the Department of Physics, Central University of Rajasthan, India, in 2019 with a master's degree in physics. His current research focuses on synthesizing 2D materials such as graphene, MXene, and their composites for hydrogen storage applications.

Tripti Rimza

Ms Tripti Rimza is currently a lecturer at Government College, Ujjain, Madhya Pradesh, India. She worked as a research scholar at CSIR-AMPRI, under the supervision of Dr Pradip Kumar. She has completed her Master's in Physics from Vikram University Ujjain, M.P. India. Her main research interest focuses on graphene, MXenes, and other 2D materials and composites for energy storage applications. Apart from this, her research interest includes the study of waves and instabilities in plasma and astrophysical plasma.

Shiv Singh

Dr Shiv Singh is currently working as a Scientist and Assistant Professor at CSIR-AMPRI, Bhopal, India. He received his PhD (2015) in chemical engineering from IIT Kanpur, India. He has done post-doctoral research at KIMS, South Korea. Currently, he is working on electrode materials for bio/electrochemical reduction of CO2 to value-added products and bio-energy, hydrogen generation and electrochemical sensors. Dr Singh also received Seal of Excellence certificates from Marie Skłodowska-Curie actions call H2020-MSCA-IF-the European Commission and the DST INSPIRE faculty award. He is also the community board member of RSC-Materials Horizons and an early career board member of Springer Nano-Micro Letters.

Neeraj Dwivedi

Dr Neeraj Dwivedi is presently a Principal Scientist and Associate Professor at the CSIR-Advanced Materials and Processes Research Institute, India and the Academy of Scientific and Innovative Research, India, respectively. His research interests include carbon materials, interface engineering, carbon nanocoatings, and 2D materials such as graphene-based materials, MXenes, metal oxides and nitrides for various applications including energy, sensors, tribology, corrosion, and antimicrobial.

Avanish Kumar Srivastava

Dr Avinash Kumar Srivastava is currently a Director of CSIR-Advanced Materials and Processes Research Institute and an Outstanding Professor of AcSIR, India. He has 410 peer-reviewed research publications and transferred about 15 technologies/know-how. He is instrumental in manufacturing the country's first high-end Make-in-India class-2 Raman Spectrometer under PPP. He is the Former President of the EMSI. He received awards like the MRSI Medal, Metallurgists of the Year, GoI, NRDC societal innovation, and CII Waste to Worth. He is a Fellow of the INAE, IIM, and EMSI. He served as the Director (Additional Charge) of CSIR – NML Jamshedpur for one year.

Pradip Kumar

Dr Pradip Kumar is currently working as a Senior Scientist and Assistant Professor at the CSIR-Advanced Materials and Processes Research Institute, India and AcSIR, India, respectively. He received his PhD degree in Physics (2012) from the School of Physical Sciences, Jawaharlal Nehru University, New Delhi. He did his postdoctoral research at KAIST and KIST, South Korea. He received the DST Inspire Faculty award in 2016. His current research interest focuses on 2D materials and composites for hydrogen energy storage, catalysts, thermal management and EMI shielding applications. He has published 65 peer-reviewed publications, several book chapters and a patent.

---

1. Introduction

The world is facing a complicated energy problem of unparalleled depth and complexity as a result of the ongoing rise in energy consumption. Fossil fuels have played a dominant role in global energy for many decades. Currently, conventional fossil fuels account for almost 86% of the primary energy requirements in the world. Meanwhile fossil fuels are facing several challenges, including finite resources, greenhouse gas emissions, and climate change. For example, the CO₂ concentration has increased by 28% since the industrial revolution and the sea level has been rising by ∼3.2 mm yearly.^1–4 Therefore, shifting from fossil fuels to renewable energy drives innovation in clean technologies and energy storage systems. Renewable sources of energy, including solar, water power, geothermal power, wind, hydrogen, and biomass, can reduce our dependence on conventional energy with minimal greenhouse gas emissions compared to fossil fuels. Thus, advancements in renewable energy technologies can lead to breakthroughs in energy efficiency, grid integration, and energy management, benefiting various sectors of the economy.^5,6

Among various sources of renewable energy, hydrogen serves as a versatile medium for energy transport that can be generated from renewable sources such as water through electrolysis, making it a clean and sustainable energy option. When hydrogen is used in fuel cells, it produces electricity with only water vapour as a by-product, emitting no greenhouse gases or pollutants.^7–10 Also, hydrogen, being abundant in nature, does not face seasonal fluctuations like solar or wind power. In addition, hydrogen has a significantly higher energy content (142 MJ kg⁻¹) than other energy sources.^11–13 Hydrogen can be integrated into multiple sectors, including transportation, industry, power generation, and heating. It has the potential to serve as a fuel source for vehicles equipped with fuel cells, feedstock for industrial processes, a replacement for natural gas in heating systems, and a clean alternative for power generation in stationary fuel cells. Hydrogen can also act as a secondary energy carrier and can be used to store electricity from intermittent renewable energies such as wind power.¹⁴ Thus, hydrogen offers the potential for greater energy independence and security. Fig. 1 illustrates the multiple merits of hydrogen energy such as being renewable and eco-friendly and having high calorific value or high energy density, helping lower the carbon economy, and providing better energy security.

Fig. 1 Merits of hydrogen energy.

The development of hydrogen technologies and infrastructure is driving innovation in the field of hydrogen generation, storage, and utilization technologies, as well as advancements in fuel cell technology.^4,15–17 These advancements can lead to more efficient and cost-effective hydrogen solutions. Among various innovations, hydrogen storage is an important aspect of hydrogen-based energy systems, and various methods have been studied for their potential as hydrogen storage media.^18–21 Hydrogen storage in solid-state materials offers the possibility of safer storage compared to liquefied or compressed gaseous forms of hydrogen.²² Storing hydrogen in a solid-state medium reduces the risks associated with hydrogen leakage, as solid materials typically have lower vapour pressures and are less prone to leaks or explosions. In solid-state materials, hydrogen can adsorb on the surface via physisorption or absorb within host materials via chemisorption. Fig. 2 shows the various solid-state materials such as metal–organic frameworks (MOFs), covalent organic frameworks (COFs), metal hydrides, hydrocarbons, MXenes, and carbon-based materials investigated for hydrogen storage under different temperatures and pressures.^23–44 Some have shown outstanding hydrogen storage capacity (HSC) of ≥6.5 wt% under ambient conditions.⁴⁵

Fig. 2 Overview of solid-state materials investigated for hydrogen storage.

MOFs, COFs, and zeolites are competitive materials for hydrogen storage due to their porous nature and high surface area. DUT-32 exhibits an HSC of 14.21 wt% at 77 K and 82 bar, MOF-650 revealed an HSC of 10 wt% at 77 K and 20 bar hydrogen pressure, and COF-108 has an HSC of 18.9 wt% at 77 K.^46–48 These materials show good hydrogen storage capacity at low temperatures by physical adsorption of hydrogen molecules but are still challenging under ambient conditions.^49–51 They have a low adsorption enthalpy and have a complex production task.²⁴ Other hydrogen storage materials, like metal hydrides, complex hydrides, etc., are bulkier and highly dense. These materials show sluggish adsorption–desorption kinetics under ambient conditions. Also, these materials have reversibility and cycling stability issues. After a few cycles, they cannot maintain their hydrogen storage performance and structural integrity. In addition, these materials also need high ease of activation and thermal management. The enthalpy of adsorption for hydrogen to these materials is very high, which shows a strong interaction between hydrogen and metals.^52–54 However, these materials have faced many challenges, such as materials synthesis, system cost, durability/operability, charging/discharging rate, system life-cycle, and lack of understanding of hydrogen adsorption/desorption kinetics to use hydrogen energy storage technologies in mobile transportation and power station applications. Fig. 3 illustrates various solid-state materials, including graphene, which have been explored for hydrogen storage in terms of HSC and sorption temperature.

Fig. 3 Overview of solid-state materials studied for hydrogen storage application.

Graphene, a two-dimensional (2D) material, possesses unique properties such as large surface area, tunable pore size, superior electrical and thermal stability, and porosity, making it an attractive candidate for energy storage.^55–70 The potential of heteroatom-doped graphene has been reviewed for various applications such as batteries, electromagnetic (EMI) shielding, capacitors, solar cells, and sensors. In one study, Lee et al. overviewed the progress and challenges of heteroatom-doped graphene-based materials in the energy field, including batteries, supercapacitors, hydrogen production, etc., and highlighted the material's electrochemical properties.⁷¹ In another study, Kumar et al. disclosed the potential of heteroatom-doped graphene-based materials in energy storage and conversion and the various synthesis approaches of heteroatom-doped graphene.⁷² In another study, Hamed et al. overviewed the advances in heteroatom-doped graphene for Li-ion batteries.⁷³ They summarised the relevant mechanisms and electrochemical performance of lithium-ion battery anodes.⁷³ In another study, Cui et al. overviewed the electrocatalytic behaviour of zinc and lithium–air batteries. This study also discusses the working mechanisms, future challenges, and future perspectives of heteroatom-doped graphene for fuel cells and metal-battery fields.⁷⁴ In another study, Kumar et al. reviewed the advances in EMI shielding using heteroatom-doped graphene-based materials. This study shows a detailed discussion of doping strategies in graphene for EMI shielding. Also, it provides the type of heteroatom, which is very prominent for EMI shielding.⁷⁵ Further, a review article by Wang et al. describes the synthesis strategies, properties, and application of heteroatom-doped graphene for sensors, solar cells, and other applications.⁷⁶ In another study, Ngidi et al. reviewed heteroatom-doped graphene as a counter electrode for dye-sensitized solar cells. This study overviewed how dye-sensitized solar cells, photovoltaic performance, and power conversion efficiency change using heteroatom-doped graphene as a counter electrode.⁷⁷ Despite the potential of heteroatom-doped graphene in many fields, including hydrogen storage, recent progress and the future scope of heteroatom-doped graphene have not been reviewed, which could offer innovations in the field and a broader context for future research.

In graphene-based materials, hydrogen storage can occur through physisorption or chemisorption.⁷⁸ The 2D nature of graphene allows rapid hydrogen transport within its lattice, enabling faster hydrogen uptake and release.⁷⁹ Theoretical and experimental studies suggested that graphene doping with hetero-atoms or functionalizing its surface can modify its properties and boost hydrogen storage performance.^80–84 Several heteroatoms, such as nitrogen (N), boron (B), palladium (Pd), titanium (Ti), calcium (Ca), aluminium (Al), lithium (Li), and sulfur (S), have been extensively studied for their effects on graphene's hydrogen storage capabilities. The heteroatom doping/decoration of graphene sheets creates a new anchoring site and introduces additional binding sites for hydrogen adsorption with fast charging–discharging kinetics.^72,85–89 These dopants can create localized charge imbalances, create defects, or modify the electronic structure of graphene, enabling stronger hydrogen adsorption and enhancing the storage capacity. The heteroatom-doped and decorated materials exhibit an excellent rate of uptake/release of hydrogen and long-term cycling stability, with full discharge capability and no memory drawback, making them adequately engineered materials. Also, the lightweight and compact nature of heteroatom-doped graphene makes it advantageous for applications where weight and space considerations are crucial, such as fuel-cell electric vehicles.

Numerous heteroatom-doped and decorated graphene-based materials have met the targets set by the US DoE for gravimetric and volumetric hydrogen storage criteria in many aspects, such as storage capacity, and temperature–pressure conditions for hydrogen charging and discharging to fulfill the energy requirements. The enthalpy of adsorption for heteroatom-doped materials mainly lies in the range of 10–50 kJ mol⁻¹.²⁴ The adsorption–desorption kinetics, material cycling stability, manufacturing cost, and HSC give heteroatom-doped materials an advantage over other hydrogen storage materials. Some heteroatom-doped graphene-based materials are in queue to achieve these targets as depicted in Fig. 3. The illustration shows that most heteroatom-doped graphene-based materials are best suited to achieve the target of the US DoE criteria. Fig. 4 presents the number of articles obtained from the Web of Science using the search phrase “graphene for hydrogen storage”. Significant research is being done on developing heteroatom-doped graphene for hydrogen storage, but there is no critical review of the progress of heteroatom-doped graphene for hydrogen storage applications.

Fig. 4 The number of research articles on graphene (a) and heteroatom graphene (b) for hydrogen storage published per year between 2000 and 2023 from the “Web of Science”.

In this review article, for the first time, we critically reviewed the recent advances in heteroatom-doped graphene for hydrogen storage, their challenges, limitations, and prospects. The hydrogen storage performance depends on many factors, such as the type and concentration of dopant, doping method, storage temperature, pressure, and surface area of the host material. Detailed hydrogen storage mechanisms in graphene and heteroatom-doped graphene-based materials are discussed to understand the core principles and their potential for practical applications. Hydrogen storage in graphene strongly depends on the dopant type. Herein, the hydrogen storage properties of all types of dopants, like non-metal and semi-metal, transition metal, alkaline earth metal, and alkali metal doped graphene, are discussed in detail. Their current challenges, limitations, commercial points of view, and future research directions are also presented. So, this comprehensive review will enrich the scientific understanding and significantly impact graphene-based materials for hydrogen storage applications.

2. Hydrogen storage mechanism in graphene and heteroatom-doped graphene materials

There are two primary ways through which solid-state materials may store hydrogen: physisorption and chemisorption. Physisorption refers to the van der Waals interaction between hydrogen molecules and the host material, while chemisorption involves the formation of chemical bonds between hydrogen and the host material.^11,90 Both mechanisms contribute to hydrogen storage in solid-state materials, but chemisorption offers higher storage capacities. When the hydrogen atoms form a strong covalent bond, it is referred to as chemisorption (binding energy >10 kJ mol⁻¹). To get ultrahigh HSC, the system needs lower temperatures and some amount of energy during hydrogenation and dehydrogenation, respectively. In actuality, chemisorption requires a moderate temperature to surpass the activation energy needed for the formation of chemical bonds. This process generally involves the splitting of a hydrogen molecule into two separate hydrogen atoms.⁹¹ Conversely, in physisorption (binding energy <10 kJ mol⁻¹), the hydrogen adsorbs in molecular form through van der Waal forces and provides shallow hydrogen storage at room temperature. Typically, physisorption occurs at lower temperatures because hydrogen maintains molecular form at lower temperatures and adsorbs into the material via weak van der Waals bonds. Hydrogen storage in pristine graphene is dominated by the physisorption mechanism, which is always a persistent issue of low binding energy between hydrogen molecules and graphene substrates. The recommended binding energy range falls between 20 and 30 kJ mol⁻¹, where there is an equilibrium between HSC and the expenses associated with maintaining hydrogenation/dehydrogenation temperatures.⁹²

In heteroatom-doped graphene, doping and decoration automatically alter the graphene electron charge density, Fermi level properties, catalytic activity, and surface properties. The dopant atoms having different sizes and electronegativity break the neutrality of the graphene network.^{75,76,93–95} This process creates more charge sites and improves the conductivity of the graphene substrate, resulting in a stronger bonding between the heteroatom and graphene sheets.^72,96 This strong interaction increases the enthalpy of adsorption of the hydrogen molecules on the graphene sheets. The substitutional heteroatoms in the carbon network enhance the hydrogen adsorption on graphene at their sites and nearby sites. Further, this heteroatom doping creates additional imperfections and pores in the graphene lattice structure. These imperfections improve the surface area of graphene, provide more anchoring sites for hydrogen adsorption, and enhance hydrogen uptake capacity. The categorization of interactions between hydrogen molecules/atoms and any host graphene material is based on the strength of the bond formed between them. Fig. 5 illustrates the mechanisms that govern hydrogen storage in heteroatom-doped graphene-based materials: physisorption, chemisorption, spillover mechanism, and Kubas-type interaction.⁹⁷ The enthalpy of adsorption for hydrogen for the Kubas-type interaction and spillover activity lies in the 10–50 kJ mol⁻¹ range.²⁴ Both these mechanisms are predominant in heteroatom-doped/decorated graphene for hydrogen adsorption under ambient conditions. The Kubas-type interaction and spillover mechanism facilitate resolving the coalescing issue with the help of heteroatoms and help to improve the hydrogen uptake capacity.^{88,90,98–101} Both these phenomena enable better hydrogen adsorption by increasing the binding energy between hydrogen atoms and the graphene substrate.

Fig. 5 Possible hydrogen storage mechanism in graphene and heteroatom-doped graphene-based materials.

Foreign metal element doped/decorated graphene faces the coalescing and clustering issues of metal nanoparticles onto a graphene substrate because of their higher cohesive energy. The aggregation of metal nanoparticles onto the graphene surface reduces the hydrogen uptake capacity. The substitutional heteroatoms minimize this clustering issue, resulting in a uniform metal nanoparticle distribution. This uniform dispersion of metal nanoparticles favours more spillover activity and Kubas interaction and improves the hydrogen uptake performance. In the spillover mechanism, hydrogen storage is enhanced by three governing factors.^90,102–104 The first is the foreign element used for hydrogen dissociation, and the second one is the substrate, which is graphene that has a vast specific surface area and helps to make more anchoring sites available for hydrogen adsorption. The third one is the chemistry between the foreign elements and the graphene substrate. The significant impact of all these factors boosts the binding energy of hydrogen molecules/atoms on the graphene substrate. Moreover, hydrogen storage is influenced by the combination of the low binding energy between the metal atom and graphene support and the robust interaction between the metal atom and hydrogen molecule. For instance, doping with transition metals increases the overall mass of the system, which significantly reduces hydrogen storage. Thus, the homogeneous distribution of metal atoms on graphene sheets strengthens the binding energy and improves hydrogen storage. Recent experimental and theoretical studies have proposed the Kubas-type interaction between hydrogen molecules and heteroatoms (mainly transition metals) to overcome the binding energy problem.⁹² In this mechanism, the occupied σ orbital of hydrogen interacts with the vacant d-orbital of the metal atom with simultaneous π back donation from the occupied d orbital of the metal atom to the σ* orbital of hydrogen. Such a back donation is essential for lengthening the H–H bond length as well as stabilizing the bonding.

3. Hydrogen storage in heteroatom-doped graphene

The hydrogen storage properties of heteroatom-doped graphene can be influenced by various factors, such as the morphology and size of graphene sheets, the presence of defects or functional groups, and the concentration and nature of dopants. These factors can be manipulated to optimize the hydrogen storage capacity, adsorption/desorption kinetics, and stability of the material.

3.1. Non-metal and semi-metal doped graphene

3.1.1. Nitrogen.
3.1.1.1. Nitrogen-doped graphene for hydrogen storage. Hydrogen storage in nitrogen-doped graphene is a subject of active research and shows potential for improving hydrogen storage capabilities.¹⁰⁵ Nitrogen (N) doping involves introducing N atoms into the graphene lattice, which can modify its physical and structural properties.¹⁰⁶ Three types of bonding can occur when nitrogen is doped into graphene, i.e., graphitic N, pyridinic N, and pyrrolic N. Several experimental and theoretical investigations reveal that nitrogenated graphene is a remarkable approach for hydrogen storage. The presence of N atoms improves π-electron density in graphene's carbon network as well as catalytic activity on the graphene surface, enabling better hydrogen dissociative adsorption.^107–112 Ariharan et al. have developed an N-doped graphene layered material using the hydrothermal method. Fig. 6(a) and (b) show the high-resolution scanning electron microscopy and high-resolution transmission electron microscopy images of the N-doped graphene. Further, X-ray photoelectron spectroscopy was carried out to investigate the valence states and elemental components. Fig. 6(c–e) depict the core level spectra of C1s, N1s, and O1s. The C1s core level spectra are resolved into three peaks at 284.1, 285.3, and 288.1 eV, corresponding to C–C/C=C, C–O/C=O, and C–N/O–C=O bonding, respectively. N1s spectra analysis indicates the predominant incorporation of nitrogen within graphene sheets, resulting in the formation of different bonds. Further the hydrogen storage performance of the respective samples was determined as shown in Fig. 6(f) and found that the HSC of the N-doped graphene reached up to ∼1.5 wt% at room temperature and 90 bar pressure. The findings suggest that the presence of nitrogen increases the enthalpy of adsorption and also takes part in hydrogen adsorption at room temperature.¹⁰⁷ In another study, Ao et al. demonstrated that applying a perpendicular electric field serves as a switch for the uptake and release of hydrogen in N-doped graphene by following dissociative hydrogen adsorption and diffusion of hydrogen atoms with lower energy barriers. The HSC of N-doped graphene was determined to be 6.73 wt%. However, by removing electric field, the stored hydrogen is released under practical conditions, when the hydrogen amount is higher than 0.5 wt%, and this residual amount (0.5 wt%) could be further eliminated by raising the temperature.¹¹³ Similar computation using DFT is done by Ao and Peeters by synthesizing hydrogenated graphene. They found that the doping of N atoms into the graphene layer in the presence of an electric field perpendicular to the graphene surface boosts dissociative hydrogen adsorption by reducing the energy barrier from 2.7 to 0.88 eV under an electric field of 0.005 au.¹¹⁴

Fig. 6 (a) SEM and (b) TEM images, (c–e) core level spectra of C1s, N1s, and O1s, and (f) hydrogen storage performance of N-doped graphene. Adapted with permission from ref. 107 Copyright 2017, Scientific Research. Discharge capacity of (g) graphene, (h) N-doped graphene, and (i) NS-graphene, at 1 mA. Adapted with permission from ref. 118 Copyright 2019, Elsevier.

3.1.1.2. Nitrogen co-doped graphene for hydrogen storage. Doping of other heteroatoms in graphene, including nitrogen, creates synergistic effects, enhancing hydrogen storage capabilities. Nitrogen doping helps in the uniform dispersion of another heteroatom by altering the electronic properties of the graphene sheets. For instance, dual-doped graphene (e.g., N and Pd) has demonstrated 272% higher hydrogen storage performance compared to single N-doped graphene at operating temperature and 2 MPa pressure.¹⁰⁹ The nitrogen doping was done by plasma treatment and Pd decoration using a chemical method. The nitrogen doping in graphene changes its electronic structure, and the d-orbital of Pd plays a vital role in Pd uniform dispersion. This interaction is constructive for the improvement in hydrogen adsorption. The Pd nanoparticles attached to C–C bridge sites could form Pd–C bonds. There is an exchange of electrons between the 4d orbital of palladium and the 2s orbital of carbon, which is responsible for their attachment. Also, there is a back bonding between C 2p to Pd 5s orbitals, and after hydrogen adsorption, Pd–H bonds were formed. However, Pd–C and C–C bonds become weaker, as described using DFT theory.¹⁰⁸ In other work, a synergistic effect of transition metals (TMs) such as Sc, Ti, and V on the hydrogen storage behaviour of N-doped graphene (PNG) has been studied.¹¹⁰ The N-doping creates defects and generates highly localized states close to the Fermi level. This doping will help in the strong interaction and binding of TMs and facilitate the hydrogen adsorption in PNG. These strong TM bindings minimize the metal aggregation, improve the material's stability, and show promising potential for hydrogen adsorption and desorption under ambient conditions.^110,115 Further, the impact of biaxial strain on the hydrogen uptake and release in Co–N-doped graphene has been reported. The computational result predicts that biaxially strained and unstrained Co–N-doped graphene has an excellent gravimetric hydrogen storage performance of 11.36 wt% and 5.03 wt% at room temperature and 12 bar pressure. This material is also thermally stable at 300 K. The study also revealed effective regulation of hydrogen adsorption behaviour through the application of biaxial strain, serving as a novel reversible switch for controlling hydrogen uptake and release. It was estimated that under 10% strain at 1 atm pressure, hydrogen adsorption in unstrained Co–N-doped graphene is increased to 6 wt%.¹¹⁶ Further, the effect of an external electric field on the HSC of lithium (Li) decorated N-doped graphene has been studied using first-principles theory. The electric field was considered an effective medium to control the storage capacity and reversibility of the doped material under near ambient conditions.¹¹⁷ Further, a study based on electrochemical hydrogen storage is done considering N and sulfur(S) co-doped graphene (NSG), which is synthesized by a simple hydrothermal method.¹¹⁸ Urea and thiourea are used as precursors for the N and S-doping. The hydrogen spillover mechanism in such kinds of pyridinic and pyrrolic N-doped graphene was studied considering DFT. It was found that N and S co-doped graphene acts as an electrode material that could promote hydrogen migration even at room temperature. It can be seen in Fig. 6(g) that the discharge capacity for simple graphene is 653 mA h g⁻¹, 1663 mA h g⁻¹ for N-doped graphene (Fig. 6(h)), and 2418 mA h g⁻¹ for NSG (Fig. 6(i)). The findings suggest that the doping of N and NS increases the discharge capacity. This enhancement in performance can be due to the increased electrical conductivity due to foreign elements doping.

Another study by Sathishkumar et al. reported that N/B-doped penta-graphene is also an efficient material for hydrogen storage.¹¹⁹ These dopants provide more anchoring sites for hydrogen adsorption and storage. In this study, they varied the amount of dopant from 4.2 at% to 8.3 at% at different doping sites. The enthalpy of adsorption increases after the doping of N and B. The N/B-doped penta-graphene can dissociate the hydrogen molecule into hydrogen atoms. The finding suggests that the N/B-doped penta-graphene's activation energy for hydrogen dissociation and diffusion is lower than that of pristine penta-graphene. The activation energy of the hydrogen molecule dissociation decreases to 0.45 to 0.70 eV for 4.2 at% B-doped penta-graphene and 0.32 to 0.64 eV for 4.2 at% N-doped penta-graphene.

Similarly, the activation energy of 8.3 at% B-doped pentagraphene and N-doped pentagraphene is lower from 0.13 to 0.87 and 0.13 to 0.83 eV, respectively. In hydrogen adsorption behaviour, it is observed that hydrogen atoms tend to favour adsorption near the dopant sites due to the electron-deficient characteristics of B and N atoms.¹¹⁹ Recently, Bakhshi and Farhadian have reported a simulation study on Pd-decorated N-doped graphene using molecular dynamics and grand canonical Monte-Carlo simulation. While performing simulations it was noticed that the Pd atoms are more efficient to capture the hydrogen atoms compared with Pd nanoparticles. Improved hydrogen storage by 437% as compared to pure graphene is obtained.¹²⁰ Another study reveals that the defect created in N-doped graphene connects with small Pd clusters that could enhance hydrogen storage via a spillover mechanism.¹²¹ In this study, small Pd clusters are decorated with pyridinic and pyrrolic N-doped graphene. The authors observed that the pyridinic and pyrrolic defects are the superior way to investigate N-doped graphene's doping effects, electronic features, and physical and chemical characteristics. The pyridinic and pyrrolic defects act as nucleation sites for Pd clusters because, during the decoration process, the Pd clusters are tightly bonded onto the vacancies. This strong bonding prevents the Pd clusters from aggregating and dissociating the hydrogen molecules with dissociation energy less than 13.8 kJ mol⁻¹. When the Pd₄ cluster was saturated with hydrogen adsorption, the dissociation energy for the pyridinic and pyrrolic vacancies reached 18.4 and 11.5 kJ mol⁻¹, respectively. Their study also reveals that the hydrogen migration is an endothermic reaction that increases the system's entropy. The dissociation and migration processes of hydrogen both occur at a time under ambient conditions.

3.1.2. Aluminium-doped graphene for hydrogen storage. Theoretically, it is estimated that Al-doped graphene can gain better catalytic decomposition of different materials and is useful for hydrogen storage and gas sensors, too.^122,123 It is exposed that Al-doped graphene is an exciting material for hydrogen storage as it can reduce the energy barrier of hydrogen adsorption by forming different kinds of lattice sites and defect sites between layers that could promote adsorbing more hydrogen atoms. The HSC is improved in Al-doped graphene because the adsorbed Al on graphene sheets changes the electron distribution of both hydrogen molecules and graphene sheets and interconnects their electron cloud, resulting in stronger adsorption of hydrogen molecules. A novel approach for the synthesis of Al-doped graphene is proposed by Ullah et al., using the chemical vapor deposition technique. The HRTEM result shows various types of bonding between aluminium and graphene carbon atoms.¹²² While storing hydrogen, Al-doped graphene forms aluminium hydrides (AlH_x) that have a large HSC of ∼10 wt%.^124,125 These materials have been kinetically stable for years with little hydrogen loss. However, they are not thermodynamically stable under operating conditions. The type of bond form is Al–H bonds, which are strong mixed ionic and covalent bonds that could make desorption impractical.^125–127 Many studies have been done on synthesizing Al-doped graphene.

A significantly high HSC is obtained considering Al-doped graphene in which Al is doped on both surfaces of the graphene layer, which could bring about a promising hydrogen storage wt% of ∼13.79 with an average adsorption energy of −0.193 eV per H₂ and met the targets (6 wt%) set by the US DoE.¹²⁸ The calculations are done using DFT theory. Ni and Al-doped graphene composites were synthesized, demonstrating a HSC of 5.7 wt% under conditions of 473 K temperature and 5 MPa pressure.¹²⁹ Further, based on DFT theory, the HSC of Al-doped porous graphene has significantly reached up to 10.5 wt%, which is found by studying the density of states and electronic distribution of the structure with and without porosity.¹³⁰ It is concluded that the presence of pores makes electron transfer easy from aluminium to graphene, later polarizing the adsorbed hydrogen molecule and strengthening hydrogen adsorption. Moreover, Al-doped graphene is anticipated to exhibit approximately 5.13 wt% hydrogen adsorption at room temperature and 0.1 GPa, accompanied by an adsorption energy of −0.26 eV/H₂.¹³¹

3.1.3. Boron-doped graphene for hydrogen storage. Boron (B) doping is an efficient method to enhance the properties of the parent material. B-doping tailors the electronic properties and changes the surface chemistry of the host material.^112,132,133 Flamina et al. explored B-doped reduced graphene oxide and its various composites at different temperatures. They found that B-doping improves the enthalpy of adsorption of molecular hydrogen on graphene.¹³² Using DFT calculations, many authors have studied the impact of catalysts on the hydrogen storage properties of B-doped graphene. The theoretical model explains that B-doping in graphene enhances the adsorption energy for both metal clusters and hydrogen molecules on the substrate, which could promote the spillover mechanism.¹³⁴ The larger size of the B atom compared to carbon enables more vital interaction with hydrogen molecules, increasing adsorption wt%.¹³⁵ The total energy investigation of B-doped graphene was done by Miwa et al., and they concluded that in B-doped graphene, the B atoms help in tuning the formation of hydrogen clusters on the graphene substrate. The binding energy of hydrogen ad-atoms is more as compared with isolated hydrogen molecules.¹³⁶ In another study, it was found that B-doped graphene has excellent HSC, which is found to be 10.10 wt% (when the B atom is doped on one side of the graphene sheet) and 16.95 wt% (when the B atom is doped on both surfaces of the graphene sheet).¹³⁷ The calculations are done using the first principles.

3.1.4. Silicon-doped graphene for hydrogen storage. Incorporating silicon (Si) into graphene has attracted much interest for its potential application in hydrogen storage. The Si-doped graphene offers an exciting platform due to its unique characteristics that could enhance hydrogen adsorption and storage capabilities.^138,139 Ganji et al. investigated Si-decorated graphene as a promising hydrogen storage medium using DFT calculations.¹⁴⁰ Calculations reveal that Si-decorated graphene exhibits high energy for adsorption and significant charge transfer figures. This study observed that the Si is chemically adsorbed onto the graphene sheet with an adsorption energy of −1.93 eV, creating anchoring sites for hydrogen adsorption. The hydrogen molecules are physically adsorbed into the graphene sheets with an adsorption energy of −0.19 eV. Further, analysis shows that Si atoms provide positive charges that show their polarization features and improve the interaction between Si-doped graphene and hydrogen molecules. The single-side decorated Si graphene can bind eight hydrogen molecules with a binding energy of −0.16 eV/H₂ under near ambient conditions. Furthermore, a maximum of sixteen hydrogen molecules can bind when Si atoms decorate both sides of the graphene sheet, and a HSC of up to 15 wt% was revealed.

3.2. Transition metal-doped graphene for hydrogen storage

Transition metal (TM) doped and decorated graphene are considered to have enormous potential for hydrogen storage because of their extremely high gravimetric density and increased host–hydrogen molecule interaction. TM atoms are recommended to build up the synergy between hydrogen molecules/atoms and the graphene substrate.^88,141–144 Doping and decoration of TM atoms improve the bonding propensity of hydrogen due to the hybridization of TM atoms' d-orbitals with the hydrogen σ or σ* orbitals. In ambient temperature hydrogen storage, TM doping increases the adsorption energy of hydrogen molecules, which is followed by a Kubas-type interaction (15–20 kJ mol⁻¹) and a spillover activity.^98–101 TMs have high cohesive energy, so they make clusters, or TM–TM interaction is more robust than TM–graphene interaction, which is sometimes an issue with TMs.¹⁴⁵ This clustering tendency reduces the hydrogen storage capacity. Therefore, doping with other atoms (such as nitrogen and boron) reduces the clustering of TM atoms on TM-doped graphene. It promotes the ability to adsorb more hydrogen and increases the hydrogen storage capacity.¹⁴⁶

3.2.1. Palladium.
3.2.1.1. Palladium-doped graphene for hydrogen storage. Palladium (Pd) doped graphene is a widely explored medium for hydrogen storage. The uniformly dispersed Pd nanoparticles facilitate more hydrogen molecules in graphene. Pd atoms provide more surface area for hydrogen adsorption into the host material and have more binding affinity to hydrogen adsorption. Therefore, the spillover effect and activated hydrogen molecules adsorbed on Pd atoms and uniform dispersion are liable for the improvement in the HSC of the Pd-doped graphene.^147–150 Upon the interaction of the d-orbital of Pd nanoparticles with the σ and σ* orbital of hydrogen, it enhances the binding energy and promotes the absorption of the hydrogen molecule in Pd-doped graphene. Chen et al. proposed that the primary reason for hydrogen storage in Pd-doped graphene nanocomposites is the adsorption of spillover hydrogen on the graphene receptor. The HSC of the Pd-doped graphene/activated carbon composite (Pd-GS/AC) was enhanced by 49% at room temperature and 8 MPa, followed by the spillover mechanism.¹⁵¹ Using the chemical method, Hu et al. developed graphene/Pd nanoparticle composites using a combination of graphite oxide and Pd(en)₂Cl₂, followed by chemical reduction with NaBH₄. After reducing the samples, they observed that the Pd atoms intercalated between the graphene sheets. Subsequently, they reported an HSC of 3.4 wt% at 77 K and 0.11 MPa.¹⁵² In another study, Zhou et al. developed a Pd/graphene nanocomposite using a simple chemical method.¹⁵³ In this study, the hydrogen uptake was measured by thermal gravimetric analysis (TGA) in an argon atmosphere and the four different hydrogen storage modes that occur during the reaction were predicted: (i) hydrogen molecule–Pd atom binding; (ii) hydrogen atom–Pd nanoparticle bonding as PdH_x hydrides; (iii) atomic hydrogen at the vacant position at graphene; (iv) molecular form hydrogen confined in the perforated graphene substrate.^154–158 The spillover mechanism mainly governs hydrogen storage. The scanning electron microscopy (SEM) observation depicted in Fig. 7(a) shows the uniform dispersion of Pd atoms onto the graphene sheets. In Fig. 7(b), the transmission electron microscopy (TEM) image depicts the morphology of both the graphene sheet and the Pd/graphene nanocomposite, showcasing a uniform distribution of Pd nanoparticles. Fig. 7(c) shows the mechanism for hydrogen storage in the Pd/graphene system. Based on the measurements, the HSC of 1% and 5% Pd/graphene nanocomposites reached up to 8.67 wt% and 7.16 wt% at a pressure of 60 bar, respectively, as shown in Fig. 7(d).

Fig. 7 Microscopy structure: SEM observations of (a) the Pd/graphene composite, TEM images of (b) a graphene sheet (left) and the 1% Pd/graphene nanocomposite (right), (c) hydrogen storage mechanism of the developed composite, and (d) hydrogen uptake capacities of the composites as a result of pressure varying from 10 bar to 60 bar. Adapted with permission from ref. 153 Copyright 2016, American Chemical Society.

In another study, the HSC of Pd-doped functionalized hydrogen exfoliated graphene (Pd/f-HEG) developed via the ethylene glycol reduction technique has been studied. The chemical treatment of graphene creates adsorption sites for the even distribution of the Pd nanoparticles on graphene. This study reveals that, in transition metal-doped graphene, spillover activity has a very significant role in hydrogen uptake. Further, it also revealed that the type of contact or energy barrier between Pd and graphene sheets also affects hydrogen adsorption. The HSC of HEG was 0.5 wt%, and the gravimetric HSC of Pd/f-HEG improved by 69%, reaching up to 1.76 wt% at 25 °C and 2 MPa.¹⁰¹ As the pressure increases, the HSC increases, and as temperature increases, the HSC decreases. But at room temperature, the adsorption–desorption of hydrogen is reversible. The isosteric heat of adsorption for Pd/f-HEG lies in the 7.0 to 12.5 kJ mol⁻¹ range, which agrees with desirable potential hydrogen storage.

In another study, Vinayan et al. disclosed that the HSC of Pd-decorated graphene nanoplatelets (Pd/GNPs) reached up to 1.21 wt% at 25 °C and 3.2 MPa. They follow the chemical route to prepare the samples. They concluded that the consistent dispersion of the Pd nanoparticles on graphene nanoplatelets increases the HSC through the spillover mechanism.¹⁵⁹ López-Corral et al. disclosed the dimerization effect of Pd atoms on graphene to achieve better HSC of the composite, and it recommends different adsorption sites of Pd atoms on the graphene sheet to adsorb hydrogen in both molecular form and dissociative form.¹⁶⁰ Throughout the adsorption phase, the C–Pd bond is established via C 2p_z and Pd 5s, 5p_z, and 4d_z² orbitals. Moreover, the hydrogen is stored through the Kubas-type mechanism and leads to reinforcing the H–H bond, not the PdH₂ complex.¹⁰⁸ In another study, Kumar et al. fabricated Pd-embedded 3D porous graphene via a microwave irradiation method with a nanohole structure.¹⁶¹ This study reveals that Pd nanoparticles generate nanoholes on the basal plane of graphene sheets. The edges of these nanoholes are lined with carbon atoms of graphene sheets, and these carbon atoms possess unsaturated bonds. These unsaturated bonds are chemically reactive, making them perfect for hydrogen storage. Also, the authors observed that the larger Pd nanoparticles penetrate the few-layered graphene surfaces, while smaller Pd nanoparticles adhere to the graphene surface. The nanoholes formed by bigger Pd nanoparticles result in defects in graphene on the outer surface as well as nearby it. These nanoholes in the graphene play a pivotal role in hydrogen uptake. At a pressure of 7.5 MPa, the nanostructure had an HSC of 5.7 wt%, which suggests that the enhanced HSC is caused by the nanoholes on the graphene surface.

In addition to experimental investigations, there have been a few theoretical studies on Pd-doped graphene. The first principles calculation done by Pantha et al. using DFT suggests that the Pd atom can be absorbed on a graphene sheet via three sites. The first one is directly above the carbon atom, the second one is at the center of the hollow hexagon, and the third one is situated halfway down the C–C bond bridge. This study found that these three sites adsorb at most seven hydrogen molecules and have an HSC of 4.19 wt%.¹⁶² In another study, Tian et al. used first-principles calculations to analyze a graphene–Pd(T)–graphene sandwich structure for hydrogen storage.¹⁶³ Their findings indicated that a sandwich structure could sustainably adsorb three hydrogen molecules with an adsorption energy of 0.22 eV. Moreover, the binding energy of Pd atoms in bilayer graphene (4.16 eV) surpasses their cohesive energy (3.89 eV), diminishing the propensity of Pd atoms to cluster on graphene sheets. Another theoretical study suggests that the Pd cluster supported by graphene mono-vacancies results in robust hybridization between carbon and Pd atoms and enhances the hydrogen uptake of graphene.¹⁶⁴

3.2.1.2. Palladium decorated N/B/S/P-doped graphene for hydrogen storage. Heteroatoms, such as N, B, etc., improve the binding energy between Pd nanoparticles and the graphene substrate. They also help in uniform dispersion of the Pd nanoparticles. As the dispersion of the Pd nanoparticles improves, the hydrogen adsorption automatically increases due to the spillover activity shown by Pd atoms. The increase in atomic hydrogen spillover and good adhesion between Pd atoms and graphene sheets, achieved by heteroatom doping, leads to enhanced hydrogen uptake. This heteroatom doping in graphene creates defects in graphene sheets, which is helpful for the deposition of Pd nanoparticles. The N–H bond formation is more favoured than that of the C–H bond. In N-doped graphene, hydrogen molecules interact strongly with carbon atoms of graphene due to the polarization ability of the N atoms compared to bare graphene. For Pd decorated N-doped graphene, N atoms work as sites for Pd atom nucleation. Also, the N-doped graphene sheets facilitate strong Pd–C bonds via back bonding due to improved π-electron density. A bond between Pd nanoparticles and graphene carbon atoms is formed in the back donation when electron donation between Pd 4d and C 2s orbitals occurs. However, the graphene carbon atom's increased electron density is donated back from the Pd 5s orbital to the C 2s orbital, leading to a strong bond formation.^108,165 Vinayan et al. synthesized Pd-decorated N-doped graphene nanoplatelets (Pd/N-GNPs) using thermal exfoliation, plasma techniques, and chemical methods. N-doping generates certain defects that serve as deposition sites for Pd nanoparticles, thereby augmenting the binding energy between Pd atoms and GNPs. The HSC of GNP, N-GNP, and Pd/N-GNPs reached up to 0.27, 0.42, and 1.25 wt% under 32 bar and at 25 °C, respectively. The findings demonstrate an enhancement factor of 0.83 wt% in Pd/N-GNPs compared to N-GNP. The increased HSC in Pd/N-GNPs is attributed to the heightened spillover activity and catalytic effects induced by nitrogen doping. The isosteric heat of adsorption was calculated using the Clausius–Clapeyron equation, and it was found to be 6.7, 7.59, and 9.62 kJ mol⁻¹ for GNP, N-GNP, and Pd/N-GNP, respectively.¹⁶⁶ In another study, Pd-decorated N-doped graphene (Pd/N-G) was developed by a solar light-assisted method, in which three steps were involved: graphene substrate formation, N-doping, and Pd precursor reduction. All the steps occur together. The HSC of such a hybrid composite reached up to 4.3 wt% at 4 MPa and 3 wt% at 2 MPa at 25 °C. The N-doping increases adsorption sites on the graphene sheet and boosts the binding energy between Pd atoms and the graphene substrate.¹⁶⁷ In another study, a Pd-decorated N-doped graphene (Pd/N-rGO) composite is synthesized via a high-temperature thermal reduction method.¹⁶⁸ This nanocomposite attained an HSC of 0.46 wt% at 1 MPa and 2.9 wt% at 4 MPa at 25 °C. The storage capacity falls to 0.8 wt% at 4 MPa and 25 °C after three absorption–desorption cycles, a 72% reduction due to the clustering and shedding of the Pd nanoparticles, which reduces the hydrogen spillover activity. In the XRD spectra of Pd/N-rGO after 3 cycles, the half-width of the peaks of Pd nanoparticles is increased compared to pristine Pd/N-rGO samples, which shows that the size of the Pd nanoparticles decreased after cycling performance. Also, the surface of the cycled Pd/N-rGO samples is decreased, revealing the shedding of the Pd nanoparticles. Further, Raman spectroscopy also confirmed that the defects in the cycled Pd/N-rGO decrease compared to pristine Pd/N-rGO, which also deteriorates the samples' hydrogen uptake. Therefore, the confinement of the Pd nanoparticles on graphene sheets needs another strategy. Faye et al. studied single and dual-sided Pd functionalized graphene (Pd-fG) and NH-doped Pd functionalized graphene (NH/Pd-fG) for hydrogen storage. For Pd-fG on both sides, eight hydrogen molecules can adsorb at 0 K with a binding energy of 0.652 eV/H₂ and with an HSC of 3.62 wt%. In NH-doped, the NH dopant serves as a binder and boosts the binding capacity of hydrogen molecules on the Pd-fG sheets. The binding mechanism of hydrogen molecules on Pd-fG follows the molecular polarization between Pd and a hydrogen atom and numerous σ bonds between hydrogen molecules and the d-orbital of Pd atoms.¹⁶⁹ In another study, Ma et al. theoretically investigated the HSC of Pd-decorated graphene with nitrogen defects.¹⁷⁰ After decorating double-sided graphene with Pd and introducing pyrrolic and pyridinic N defects, it reveals the gravimetric capacity of 1.99 wt%, followed by Kubas interaction. Based on binding energy calculations, the Pd-decorated graphene with nitrogen defects is more stable than Pd-decorated graphene. Further, in another study, Habibullah et al., using DFT calculation, revealed the viability of using Pd₄ and Pd₃P clusters for hydrogen storage. They focused on four different graphene-modified samples. They found that decoration of Pd₄ and Pd₃P clusters on the phosphorus-doped single vacancy graphene (PSVG), pristine graphene (PG), and phosphorus-doped pristine graphene (PPG) is not beneficial for hydrogen adsorption. The binding energy for Pd₃P-SVG and Pd₄-SVG is −4.49 eV and −5.37 eV, respectively. These materials are highly stable at 500 K. This high binding energy decreases the desorption of hydrogen from PdH_x hydrides. However, Pd₃P-SVG shows 5.74 wt% hydrogen storage, which is in the recommended range with an adsorption energy of −0.25 eV/H₂. The spillover activity occurs under ambient conditions within 68 seconds with the energy barrier decreased to 0.75 eV from 0.86 eV.¹⁷¹ This type of system is very useful for hydrogen technologies.

3.2.2. Titanium.
3.2.2.1. Titanium-doped graphene for hydrogen storage. Titanium (Ti) is considered a very suitable dopant in graphene for various applications. The Ti doping in graphene is challenging due to its clustering tendency.¹⁴² External compressive strain effects are often required to mitigate the tendency to form clusters and boost the nanocomposite hydrogen storage capacity.¹⁷² According to DFT studies, the Ti atoms do exhibit clustering at moderate concentrations but do so at higher concentrations.¹⁷³ An external electric field helps to reduce the aggregation of Ti atoms on graphene and stabilize the distribution of Ti atoms on graphene by changing the electron transfer between Ti atoms and carbon rings. Bajestani et al. developed Ti-decorated graphene (Ti-RGO) for hydrogen storage. In this study, the well-known Hummer's technique was used to develop reduced graphene oxide followed by Ti decoration by a wet impregnation technique. The HSC was measured at room temperature and a pressure of 10 bar. It is significantly improved from 1.3 wt% to 1.4 wt% after the decoration of Ti atoms over graphene sheets by chemical interactions. This study suggests that the chemisorption of hydrogen molecules takes place instead of molecular adsorption.¹⁷⁴ Guo et al. have reported a theoretical investigation on hydrogen adsorption on Ti-coated graphene, and the results show that hydrogen adsorption is enhanced by using an external electric field. A negative electric field helps the Ti atoms' dispersion over the graphene and minimizes the Ti aggregation. The charge transfer between Ti atoms and graphene's carbon atoms was regulated by the external electric field, which improves the interaction between Ti and carbon atoms and avoids the accumulation of Ti atoms. The hydrogen adsorption and desorption are also effectively regulated by the external electric field.¹⁷⁵ Another study found that Ti-decorated porous graphene is a promising material for hydrogen adsorption. Yuan et al. used DFT simulations to conduct this study. The possible Ti adsorption sites and electronic properties of Ti-decorated porous graphene are also discussed in this study. This study reveals that Ti atoms adsorb at the center of the hexagonal graphene site with a binding energy of 3.65 eV. They decorate only one Ti atom on a single side of porous graphene to avoid clustering, and four hydrogen molecules are attached to this single Ti atom. Hydrogen adsorption is governed by polarization and hybridization mechanisms. For both sides of Ti-decorated porous graphene, eight hydrogen molecules are adsorbed with an adsorption energy of −0.457 eV having a HSC of 6.11 wt%.¹⁷⁶ Further, Liu et al. also performed DFT calculations for Ti-decorated graphene. They concluded that the binding energy of Ti increases by 0.30 eV to 0.60 eV on Ti-decorated graphene. This energy fulfills the requirement of hydrogen molecules to adsorb or desorb on the graphene surface under ambient conditions.¹⁷⁷ This calculation suggests a Kubas-type interaction. The increased binding energy is attributed to the interaction between the surface dipole and the dipole of the polarized hydrogen molecule.¹⁷⁸

Chakraborty et al. have done a DFT study and discovered that a recyclable single Ti atom on ψ-graphene combines with nine hydrogen molecules with a mean binding energy of −0.30 eV/H₂ and HSC of 13.14 wt% with a suitable dehydrogenation temperature of 387 K.¹⁷⁹ The reason behind such an uptake capacity is an orbital interaction between the Ti 3d-orbital and C 2p-orbital and Kubas-type interaction. However, Ti decorated ψ-graphene was found to have a desirable storage capacity and a quite optimum acceptable desorption temperature. In another study, Tan et al. explored Ti-decorated irida-graphene. Irida-graphene, a carbon isomer in a 2D form, comprises triangular, hexagonal, and octagonal carbon rings. The Ti atoms are attached to the hexagonal ring. A unit cell of Ti-decorated irida-graphene holds five hydrogen molecules with −0.41 eV/H₂ adsorption energy having a stable gravimetric HSC of 7.7 wt%. They also revealed that the Kubas-type interaction is responsible for hydrogen adsorption with a diffusion barrier of 5.0 eV for Ti–Ti clustering. This Ti-decorated irida-graphene is thermally stable up to 600 K with 524 K dehydrogenation temperature. These exceptional features show that this system is very stable for hydrogen charging and discharging.¹⁸⁰ Another study shows that Ti-doped zigzag graphene nanoribbons (GNRs) are another inspiring candidate for hydrogen storage. Using a density functional calculation, Lebon et al. revealed that Ti/GNRs had an effective HSC of more than 6 wt% and that a maximum of four hydrogen molecules can bind with a single Ti atom at each site.¹⁸¹ Further, a study on double-sided Ti-decorated double vacancy graphene was performed. In double vacancy graphene, Ti atoms are attached above and below the graphene sheets, with binding energy higher than Ti–Ti cohesive energy. There is no bonding during hydrogen adsorption, revealing an HSC of 6.67 wt% with −0.25 eV/H₂ adsorption energy. The analysis shows an orbital hybridization between the C 2p and Ti 3d orbitals.¹⁸² Further, the Ti-decorated polycrystalline graphene was explored for hydrogen adsorption. In this study, the authors work on the effect of the grain size and mechanical features of hydrogen adsorption. The grain boundaries lower the mechanical properties of polycrystalline graphene, but Ti decoration does not significantly change the mechanical properties of polycrystalline graphene. The results show that the hydrogen uptake of polycrystalline graphene is 57% more than bare graphene at 300 K and 50 bar, which exhibits a strong indication of physisorption. In the case of 1% Ti-decorated polycrystalline graphene, the HSC reaches 3.2 wt% at 300 K and 9.9 wt% at 77 K, both under a 100 bar pressure.¹⁸³

3.2.2.2. Titanium-decorated N/B-doped graphene for hydrogen storage. In Ti-decorated heteroatom (N, B, etc.) doped graphene, these N and B suppress the aggregation tendency of the Ti atoms. The electron transfer between Ti atoms and graphene is enhanced by the gaining tendency of the p-orbital of the heteroatoms. The enthalpy of adsorption of Ti atoms on B or N-doped graphene is larger than Ti–Ti atoms' cohesive energy, preventing coalescing. A study has been done on the Ti-decorated B-doped porous graphene. The findings of this study reveal that B atoms improve the interaction between Ti atoms and porous graphene and prevent agglomeration of Ti atoms. The HSC of the Ti-decorated B-doped porous graphene reached 8.58 wt% at 25 °C and 30 bar, and worthwhile desorption takes place at 100 °C and 3 bar. Thermodynamic calculation also reveals the useable capacity of the material.¹⁸⁴ Intayot et al. revealed that Ti₄-decorated B/N-doped graphene is also a promising reversible hydrogen storage medium.¹⁸⁵

3.2.3. Nickel.
3.2.3.1. Nickel-doped graphene for hydrogen storage. The HSC of the composite is significantly higher than that of metals and pristine graphene. The inclusion of nickel (Ni) in graphene serves solely as a catalyst and increases the volumetric and gravimetric HSC of the graphene composite via a spillover phenomenon. The decoration of Ni atoms in graphene provides a shorter diffusion path for atomic hydrogen to store between graphene layers. Ismail et al. synthesized a Ni and Pd-doped graphene nanocomposite and verified the existence of Ni in hydroxide and oxide forms using an X-ray diffractometer.¹⁸⁶ The even dispersion of Ni and Pd over graphene sheets was confirmed by microscopy techniques. Fig. 8(a–c) depict the HRTEM images of reduced graphene oxide (RGO), and 5 wt% nanocomposites of Ni-RGO and Pd-RGO, respectively, and reveal the uniform dispersion of Ni and Pd over rGO. Fig. 8(d) depicts the HSC of pristine RGO, which is 0.25 wt% at 300 K and 20 bar. While the HSC of 5 wt% Pd-RGO is slightly more than that of RGO, the HSC of the 10 wt% Pd-RGO nanocomposite reached up to 0.4 wt%. At a lower temperature of 80 K, the HSC was enhanced in all the samples. The storage capacity of RGO reached 2 wt%, while for the 5 wt% Pd-RGO and 10 wt% Pd-RGO nanocomposites, the HSC reached up to 2.5 wt% and 2.8 wt%, respectively, at 20 bar as depicts in Fig. 8(e). The improvement is ascribed to the effective catalytic activity of Pd.

Fig. 8 TEM image of (a) RGO, 5 wt% composites of (b) Pd-RGO and (c) Ni-RGO, hydrogen storage performance of Pd-RGO nanocomposites (d) at 300 K and (e) at 80 K, hydrogen storage performance of Ni-RGO nanocomposites (f) at 300 K and (g) at 80 K, and rate of hydrogen adsorption of nanocomposites (h) at 300 K and (i) 80 K. Adapted with permission from ref. 186 Copyright 2015, Elsevier.

In the case of Ni-RGO, the HSC of the 5 wt% and 10 wt% Ni-RGO nanocomposites at 300 K and 20 bar reaches up to 0.24% and 0.23 wt%, respectively, which is lower than pristine RGO hydrogen uptake as shown in Fig. 8(f). This may be due to the availability of Ni oxide and hydroxide forms, which restricts hydrogen adsorption at 300 K. However, at a lower temperature, the HSC of 5% and 10% Ni-RGO nanocomposites increased to 2.5% and 2.7%, respectively, as illustrated in Fig. 8(g). This is because Ni increases the storage of hydrogen in its molecular form more effectively. It concludes that doping with Ni increases hydrogen storage capacity at 80 K, but it has a deleterious effect at 300 K. This unusual behavior of Ni could be due to its oxide and hydroxide forms, which may impede the gravimetric hydrogen uptake at 300 K. Fig. 8(h and i) show the rate of adsorption at 300 K and 80 K, which shows a high rate of adsorption in the nanocomposite compared to RGO.¹⁸⁶

In another study, a Ni/graphene composite was synthesized using graphene oxide and nickel acetate by Zhou et al. using a chemical method. The available defects, vacancies, and oxygen-containing functional groups on graphene facilitate the uniform dispersion of Ni, and further close contact between Ni particles and graphene controls their agglomeration. The HSC of the composite was 0.14 wt% at room temperature and 1 bar pressure and revealed 1.18 wt% at 60 bar hydrogen pressure. Most of the dehydrogenation occurs below 150 °C and complete dehydrogenation of the composite occurs at 250 °C. This study suggests that physisorption and chemisorption take place for hydrogen storage.¹⁸⁷ In another study, Wei et al. noted that a spillover mechanism improves the hydrogen storage performance of Ni-doped rGO and its metal composite-based nanocomposite. The reversible HSC at 295 K and 1 bar pressure for Ni/rGO, Ni/Pd/rGO, and Ni/Ag/Pd/rGO is 0.007 wt%, 0.13 wt%, and 0.055 wt%, respectively. According to the findings, the nanocomposite possesses a higher HSC than GO and rGO. In the Ni/rGO hybrid, hydrogen uptake performance is not that active because Ni shows very little spillover activity.¹⁸⁸ The enhanced HSC observed in Ni/Pd/rGO and Ni/Ag/Pd/rGO nanocomposites can be attributed to the effective spillover function of Pd atoms. Additionally, the substantial surface area of the composite ensures optimal contact with hydrogen molecules, contributing to superior performance. The metal mixture composition is also an important factor for spillover activity.¹⁸⁹ Further, Lee et al. found that a Ni-GO/MIL hybrid composite shows spillover activity and serves as a promising option for hydrogen storage.¹⁹⁰ Theoretical investigations indicate that the introduction of Ni onto pristine graphene sheets improves the system's HSC. This enhancement is attributed to the ability of a single Ni atom to accommodate up to five hydrogen molecules.¹⁹¹ This study reveals that hydrogen adsorption in Ni/graphene systems occurs without the formation of hydrides. As a result, the HSC of this system attained up to 2.8 wt%. Castillo et al. conducted an extensive detailed study of defective graphene nano-platelet doped Ti₄ and Ni₄.¹⁹² The robust chemistry between the 1 s orbital of hydrogen and the 3d orbital of Ti₄ leads to the generation of metal hydrides, and notably, the HSC reached 3.4 wt%. However, the Ni₄-doped defective graphene prefers the Kubas-type interaction in which the 1s orbital of hydrogen interacts with the 4s orbital of nickel with only 0.30 wt% hydrogen uptake. The Ti₄-doped graphene has high HSC because of its planar geometry. Thus, it covers more graphene layers, allowing all the atoms to contribute to hydrogen adsorption. Controlling the defect in the graphene sheet may increase the gravimetric hydrogen uptake.

3.2.4. Platinum-doped graphene for hydrogen storage. Platinum (Pt) doped graphene effectively increases hydrogen adsorption by a spillover phenomenon. The even distribution and minimal aggregation of Pt atoms increase the HSC of the Pt-doped graphene. D. Puthusseri et al. developed Pt-decorated hydrogen exfoliated graphene (Pt-HEG) using the hydrogen exfoliation and polyol reduction methods. The HSC of the Pt-decorated hydrogen exfoliated graphene is 1.4 wt% at 298 K and 3 MPa, which is twice the HEG at the same variables. The isosteric heat of adsorption of Pt-HEG is 14.7 kJ mol⁻¹, much more than HEG. The enhancement in the HSC is directly evident in the spillover mechanism, which occurs at room temperature.¹⁹³ In another study, Huang et al. used the electroless deposition technique to decorate Pt nanoparticles on graphene. They observed that the Pt and Pd nanoparticles were evenly dispersed over the graphene sheets and increased the HSC to 0.155 wt% at 303 K and 5.7 MPa via spillover activity.⁸⁰ This experimental finding demonstrates that the HSC of the doped graphene is twice that of the pristine graphene system. In another study, the bioinspired graphene foam for hydrogen storage was studied by Jung et al. In the bioinspired hydrogen storage mechanism, the surface area of the graphene foam, distribution, and the amount of decorated Pt were regulated by polydopamine employed as a reducing agent. The functional group present in polydopamine, i.e., catechol and amine, works as a more robust Pt binder, resulting in a uniform dispersion of Pt. Therefore, polydopamine functionalization can control the decoration of Pd nanoparticles. The HSC of the Pt-decorated polydopamine functionalized graphene was 3.19 wt% at room temperature and 100 bar, which is 513% more than that of reduced graphene oxide.¹⁹⁴

Further, plasma-treated graphene with a Pt decorated surface through thermal reduction of a Pt precursor under a hydrogen flow was derived by Kostoglou et al.¹⁹⁵ The plasma treatment changes the physical and chemical properties by ablation, functionalization, etc., depending on the gas precursor and power of the plasma reactor. This study measured the HSC of pristine graphene and graphene adorned with Pt at 77 K and 20 bar, resulting in values of 1.71 wt% and 1.64 wt%, respectively. Similarly, at 298 K and 20 bar, the hydrogen uptakes for pristine graphene and Pt-decorated graphene were 0.084 wt% and 0.131 wt%, respectively. The hydrogen uptake in Pt-decorated few-layer graphene increased to 56% at room temperature. At room temperature, the presence of Pt nanoparticles enhances the hydrogen storage characteristics and reversibility of graphene adorned with Pt when compared to pristine graphene. Most of the desorption of hydrogen takes place under ambient conditions. The desorption of the remaining chemisorbed hydrogen needs external heating inputs. This improvement is attributed to a gentle chemisorption mechanism or robust spillover behaviour.

Gueye et al. used first-principles calculations to examine the interaction between hydrogen molecules, Pt-decorated graphene, and NH–Pt-decorated graphene.¹⁹⁶ This study unveiled that the favorable interaction between a Pt atom and graphene occurs when Pt is positioned at the bridge site, exhibiting a binding energy of 2.611 eV. In contrast, the stability of the interaction between Pt and a hydrogen molecule is heightened when Pt is situated in a top site, accommodating the adsorption of three hydrogen molecules with an adsorption energy of 3.049 eV. The freely available Ti atom binds four hydrogen molecules with a binding energy range of 1.753–3.560 eV. They also noted that charge transfer is between Ti atoms and graphene by Mulliken charge analysis. Introducing NH radicals into Pt-decorated graphene amplifies the interaction between Pt and graphene by improving their binding energies. The stability of the interaction between Pt and hydrogen molecules is enhanced when the Pt atom settles at a top site, with an adsorption energy of 3.389 eV. They also noticed that Pt-decorated NH-doped graphene is unable to adsorb three hydrogen molecules like Pt-decorated graphene.

3.2.5. Vanadium-doped graphene for hydrogen storage. Vanadium (V) can also serve as a catalyst for hydrogen storage in carbon-based materials. V-doped or decorated graphene is a very emerging material for hydrogen storage under ambient conditions. The V atoms interact with the graphene via the Dewar interaction and stored hydrogen molecules in V-doped graphene via the Kubas interaction with good adsorption energy. To prevent the clustering tendency of the V atoms, Kureshi et al. explored V-decorated graphene oxide instead of V-decorated graphene using DFT calculations. They observed that available functional groups in graphene oxide, such as hydroxyl, epoxy, etc., provide anchoring sites for the uniform dispersion of V atoms by minimizing coalescing issues. The calculation revealed that V atoms are attached to the graphene oxide sheets with a binding energy of ∼6.2 eV, which is more than the cohesive energy of V atoms. This study reveals that V-decorated graphene oxide is a promising material for hydrogen storage technology.¹⁹⁷ L. Yuan et al. examined the electronic features and hydrogen storage behavior of V-decorated porous graphene (V-PG). They observed that the single V atom is firmly attached to the central site of the PG hexagonal structure. In V-PG, hydrogen adsorption is governed by two phenomena: the polarization of hydrogen molecules produced by an electric field generated by V–PG interaction and the orbital hybridization among V atoms, C atoms, and hydrogen molecules. They revealed that V-PG is a stable material with the potential for adequate hydrogen storage. The finding of this study suggests that the unit cell of V-PG can adsorb six hydrogen molecules with an HSC of 4.58 wt% at 300 K and without external hydrogen pressure.¹⁹⁸ Further, using first principles calculation, V-decorated boron–nitrogen co-doped graphene (V3/BNDG) was explored for hydrogen storage applications.¹⁹⁹ This study found that single-sided V₃-decorated BNDG can adsorb eight hydrogen molecules with an energy of −0.49 eV, and double-sided V₃-decorated BNDG can adsorb eighteen hydrogen molecules with an adsorption energy of −0.45 eV at lower temperatures, respectively. This study also examined the impact of an electric field on hydrogen adsorption and revealed that electric fields strengthen hydrogen adsorption. The gravimetric HSC of the V₃ double-sided BNDG reached up to 6.43 wt%. This V₃/BNDG exhibits promising results in a higher-pressure environment.

3.2.6. Other transition metal doped graphene.
3.2.6.1. Chromium-doped graphene for hydrogen storage. The chromium (Cr) atom is also chosen to modify graphene-based materials. The 3d orbital of Cr atoms and the p-orbital of graphene C atoms interaction are responsible for the modification of graphene. This interaction brings about half metallicity and magnetic nature in graphene, which is helpful for many applications.²⁰⁰ The Cr-doped graphene and penta-graphene show remarkable potential for hydrogen storage applications.²⁰¹ Xiang et al. investigated Cr-doped graphene nanoflakes using theoretical calculations for hydrogen storage usage. They observed that one Cr atom doped graphene nanoflake can store up to three hydrogen molecules with a binding energy of −0.574 eV. Two (or three) Cr atom doped graphene nanoflakes resulted in a notable increase in the quantity of the stored hydrogen molecules, reaching six (or nine) molecules.²⁰²
3.2.6.2. Copper-doped graphene for hydrogen storage. Further, due to its lightweight, copper (Cu) is also a dopant of interest in graphene.²⁰³ A study by A. Choudhary et al. explored the effect of defects on hydrogen storage in Cu-decorated graphene using first-principles calculations.²⁰³ Working on various structure combinations, they found a stable position of the Cu atom on pristine graphene and single vacancy graphene. The Cu atom strongly binds with defect-free graphene and single vacancy graphene with binding energies of −0.07 eV and −3.36 eV, respectively. In Cu-decorated single vacancy graphene, the binding energy of Cu atom attachment is more than the cohesive energy of Cu atoms, which controls its clustering and improves hydrogen adsorption. Faye et al. performed a detailed, in-depth study on the structural and electronic characteristics of bare Cu, Cu-functionalized graphene, and B-doped Cu-functionalized graphene using DFT. They noted two possible scenarios when hydrogen interacts with a bare Cu atom, i.e., in hydrogen's molecular and atomic forms. The molecular form of hydrogen is more stable than the atomic form, having a binding energy of 0.287 eV. In contrast, an energy barrier of 0.523 eV is needed to pass from atomic to molecular form. They also revealed that molecular hydrogen adsorption on bare Cu atoms is inefficient for hydrogen storage due to the robust interaction between hydrogen molecules and bare Cu atom surfaces. In Cu-functionalized graphene, molecular hydrogen adsorption occurs; during this process, the transfer of Cu atoms occurs from one position to another with some energy release. The HSC of Cu-functionalized B-doped graphene measured up to 4.231 wt% when hydrogen molecules were attached to both sides. In Cu-functionalized B-doped graphene, hydrogen is stored in atomic and molecular form. This finding shows that Cu-doped graphene could be a prominent option for hydrogen storage.²⁰⁴
3.2.6.3. Iron-doped graphene for hydrogen storage. Iron (Fe) can also be a dopant for graphene-based hydrogen storage applications.⁸⁸ The Kubas-type interaction is more favourable for hydrogen adsorption in Fe-doped graphene systems. M. Sterlin Leo Hudson et al. investigated the HSC of the Fe nanocluster-decorated graphene material.²⁰⁵

Materials, including graphene sheets and Fe-nanocluster-decorated graphene sheets, were developed using pure graphite rods with an arc reactor. Another graphite rod impregnated with Fe nanopowder acts as an anode for Fe nanocluster decoration. The pure graphite rod works as a cathode. A direct current source is used to generate the arc. After completion of the arcing process, samples were collected from the walls of the arc chamber. The hydrogen storage measurement shows that the developed materials show good hydrogen storage behavior. The HSC of thermally reduced graphene oxide, chemically reduced graphene oxide, and Fe-nanocluster decorated graphene sheets reached up to 0.32, 0.1, and 0.27 wt% at 300 K and 50 bar pressure, and 2.07, 0.54, and 2.16 wt% at 77 K and 50 bar, respectively. Their analysis shows that hydrogen uptake depends on the samples' surface area and defect concentration.²⁰⁵

3.2.6.4. Yttrium-doped graphene for hydrogen storage. Yttrium (Y) decorated graphene also shows potential as a carrier for the storage of hydrogen.²⁰⁶ According to DFT calculations, Liu et al. examined the HSC of Y-decorated and Y-decorated boron-doped graphene.²⁰⁷ They observed that the cohesive energy (4.33 eV) of Y atoms is more than the binding energy of Y-decorated graphene, resulting in Y atoms' aggregation in Y-decorated graphene. From this behavior, the HSC of Y-decorated graphene is insignificant. To further resolve this problem, Y-decorated B-doped graphene has been explored. The doping of B-atoms improves the binding energy of Y-decorated graphene and the resulting binding energy is more than cohesive energy of Y-atoms. The boron doping also helps in the uniform distribution of the Y atoms on the graphene sheets. The HSC of the double-sided Y-decorated boron-doped graphene reached up to 5.78 wt% with an average binding energy of −0.568 eV. The interaction between the Y 5d orbital and the H 1s orbital plays a pivotal role in the enhancement of the binding energy of hydrogen. In another study, Y-decorated ψ-graphene was explored by Chakraborty et al. for hydrogen storage, particularly for fuel cell vehicle applications.²⁰⁸ They proposed that this system is highly stable at elevated temperatures. The Y atoms firmly interact with ψ-graphene, with a binding energy of −3.06 eV. The aggregation of Y atoms is also prevented due to the available diffusion barrier of 0.4–0.7 eV. They evidenced the interaction between the Y 4d orbital and C 2p orbital, responsible for the hydrogen adsorption. The Y atoms strongly bind with the graphene and store 8.31 wt% hydrogen with a remarkable −0.39 eV energy figure. This system follows the Kubas-type mechanism via charge donation and back donation along with a dehydrogenation temperature of 496.55 K, which is highly conducive for hydrogen-based vehicular applications.
3.2.6.5. Scandium-doped graphene for hydrogen storage. Scandium (Sc), i.e., the lightest transition metal, having fewer d-orbit electrons, is best suited for holding hydrogen when decorated on graphene-based materials. Sc has one d-orbital electron to facilitate strong physisorption interaction with hydrogen, i.e., Kubas interaction. Hydrogen binding in Sc-decorated graphene is enhanced by Kubas interaction more than simple physisorption. However, it cannot dissociate hydrogen molecules and form Sc–H chemical bonds. The moderate binding between Sc and H lies in between physisorption and chemisorption, fulfilling the binding energy criterion of the US DOE. The Sc-decorated graphene performs well in reversible hydrogen storage. The Sc edge-decorated graphene nanoribbons are also promising for hydrogen adsorption. The binding energy of SC atoms with graphene nanoribbons is more than the cohesive energy of bulk Sc atoms. The predicted HSC of this system is >9 wt%, which fulfills the US DOE targets. Also, an external electric field can regulate the adsorption energy of hydrogen molecules.^209,210 Based on a DFT study, Chen et al. disclosed the hydrogen storage features of Sc-decorated porous graphene. The single-sided Sc-decorated graphene shows noteworthy hydrogen storage performance. It can adsorb four hydrogen molecules having a HSC of 3.94 wt% with −0.429 eV/H₂ adsorption energy. The most noteworthy results were achieved in the double-sided Sc-decorated graphene system. This system can accommodate twelve hydrogen molecules with an HSC of 9.09 wt%, with an average of −0.296 eV/H₂ binding energy. The intense interaction between carbon, hydrogen, and Sc atoms is responsible for the outstanding results.²¹¹ In another investigation, Sc-decorated B-doped porous graphene was explored by Wang et al. via first-principles calculations for hydrogen storage.²¹² They found that B atoms facilitate the uniform distribution of the Sc-atoms on graphene sheets. This Sc-decorated B-doped porous graphene is a reliable material for hydrogen storage under practical conditions. The single-side Sc-decorated B-doped porous graphene exhibits an HSC of 4.91 wt% with an adsorption energy of −0.515 eV/H₂ by the adsorption of five hydrogen molecules. The double-sided Sc-decorated B-doped porous graphene is more stable than single-side decoration. In double-side decoration, the Sc atom decoration binding energy is more than their cohesive energy. The double-side decorated system exhibits an HSC of 9.13 wt%, with a remarkable average binding energy of −0.225 eV/H₂. The presence of the boron atom significantly enhances the adsorption energy of both the Sc atoms and hydrogen molecules, leading to an overall improvement in the hydrogen storage efficiency of the material. The orbital interaction between hydrogen and Sc atoms and the coulombic attraction between the positively charged Sc atom and negatively charged hydrogen molecules are noted, which are very prominent for hydrogen storage. In another study, Cui et al. investigated the hydrogen adsorption energies of a sandwich graphene(N)–Sc–graphene(N) system and electronic features using first principles calculations. The doping of nitrogen in graphene prevents the accumulation of Sc atoms. They found that this system can store ten hydrogen molecules with 0.24 eV binding energy, revealing its applicability for hydrogen storage.²¹³ In another study, Sc-decorated ψ-graphene was explored for hydrogen storage. The material is reversible and stable at room temperature and may be helpful in light fuel cell vehicles. The single side Sc atom decoration can hold five hydrogen molecules with a favorable adsorption energy of ∼0.2–0.4 eV. Both sides of the Sc decorated system adsorb nine hydrogen molecules with an adsorption energy of 0.3 eV and a capacity of 14.46 wt% hydrogen. At a temperature of ∼400 K, complete dehydrogenation takes place. The Kubas interaction is responsible for such outstanding results for hydrogen adsorption.²⁰⁹
3.2.6.6. Zirconium-doped graphene for hydrogen storage. Zirconium (Zr) doped graphene is also an excellent material for hydrogen storage. In Zr-doped graphene, Kubas interaction plays a prominent role in hydrogen adsorption. The Zr-atom decorated graphene shows a magnetic character. The uniform decoration of Zr atoms of graphene increases the magnetic moment of the system. This increased magnetic moment elevated the dehydrogenation temperature of the Zr-decorated graphene. The increased magnetic moment influences the charge transfer between Zr atoms and hydrogen molecules, affecting the hydrogen molecules' binding energy. This binding energy is directly related to the dehydrogenation temperature of the material. The less-magnetic or non-magnetic systems typically exhibits uniform charge transfer and binding energy. A DFT study on Zr-doped graphene revealed that the desorption temperature for the adsorbed first molecule is very high due to the magnetic moment of the Zr-doped graphene. In this study, the authors also observed that single Zr atom doped graphene could store nine hydrogen molecules with a binding energy of 0.34 eV and a dehydrogenation temperature of 433 K. However, both sides of doped Zr-doped graphene's HSC reached 11 wt%, and the system stabilized up to 900 K. The authors concluded that the material should be less magnetic and have a lower magnetic moment to get a low dehydrogenation temperature.²¹⁴ Further, A. S. Shajahan et al. decorated Zr atoms on an advanced carbon allotrope, i.e., penta-graphene.²¹⁵ The pentagraphene has a five-membered carbon ring, which provides more space for the Zr decoration. The Zr atom is firmly attached to the pentagraphene with an energy of −3.41 eV. This Zr-decorated penta-graphene can store up to eleven hydrogen molecules with −0.42 eV energy. This system follows the Kubas-type interaction for hydrogen adsorption and reveals a storage capacity of up to 14.8 wt%. In another study, Nair et al. investigated the fact that Zr-decorated psi-graphene is also a promising hydrogen storage material. The polymerization of the carbon skeleton of s-indacenes is used to develop psi-graphene. Mechanical features and vacancy defects in psi-graphene is better than graphene. The binding energy of the Zr atom on psi-graphene is −3.54 eV. The Zr-decorated psi-graphene can hold nine hydrogen molecules with an adsorption energy of 0.38 eV/H₂ and is thermally stable at room temperature. Preferably, this Zr-decorated psi-graphene governed Kubas interaction for hydrogen absorption.²¹⁶
3.2.6.7. Molybdenum-doped graphene for hydrogen storage. Molybdenum (Mo), as an additive in many hydrogen storage materials, has a significant role in hydrogen adsorption. Mo-doped graphene also provides insightful results for hydrogen storage. The Mo atoms on graphene improve hydrogen adsorption by dissociating the hydrogen molecules. Karde et al. explored Mo-doped graphene for hydrogen storage. They observed that the center of the hexagonal graphene sheet is the most favorable site for the adsorption of the Mo atom, with a resulting adsorption energy of −2.96 eV. The cohesive energy of Mo atoms is −3.82 eV, indicating that Mo atoms tend to form clusters. This Mo-doped graphene can store up to four hydrogen molecules with a −0.534 to −0.626 eV/H₂ binding energy range. The s-orbital of hydrogen and d-orbital of Mo atoms overlap via orbital hybridization interaction, and increases the hydrogen adsorption.²¹⁷
3.2.6.8. Niobium-doped graphene for hydrogen storage. Niobium (Nb)-decorated graphene is also a prominent option for hydrogen storage for mobile applications. Nb-decorated graphene is non-magnetic, which influences the charge transfer properties of the system. In one study, Nb atoms and Nb-decorated graphene were explored for hydrogen storage. The Nb atoms bind six hydrogen molecules with a binding energy range of 0.228–0.630 eV, and complete dehydrogenation occurs at 466 K. The Nb atoms like to settle at hollow sites of the graphene with a binding energy of 1.783 eV. The diffusion energy for the Nb atoms is 0.435 eV to diffuse from one hollow site to another, which is more than the thermal vibrational energy of Nb atoms at 300 K. The single-side Nb-decorated graphene prefers molecular adsorption of hydrogen with a binding energy of 0.640 eV. The Nb-decorated graphene doped with 7.25 at% nitrogen can adsorb twelve hydrogen molecules with a binding energy of 0.410 eV with a dehydrogenation temperature of 520 K. They also revealed that increased nitrogen doping lowers the dehydrogenation temperature. Both sides of Nb-decorated nitrogen-doped graphene's HSC reached up to 8 wt%, which shows that nitrogen doping improves hydrogen adsorption.²¹⁸

3.3. Alkaline earth metal-doped graphene for hydrogen storage

Doping and decorating graphene with alkaline earth metals have the potential to be a viable material for hydrogen storage technologies. The uniform dispersion of alkaline earth metals over graphene sheets is facilitated by their low cohesive energy. The intercalation and even distribution of these metals between graphene layers generate additional sites for hydrogen adsorption, enhancing the binding energy of the system. The electronic transition between alkaline earth metals and graphene via ionic bonding reveals that doping of alkaline earth metals on graphene changes its electronic and magnetic properties up to a certain degree of success.^219–222

3.3.1. Beryllium-doped graphene for hydrogen storage. Beryllium (Be) is a lightweight dopant and found to be the most stable and lowest in energy, and it supports achieving high hydrogen gravimetric density. Theoretically, it is claimed that Be-doped graphene may act as a solid-state hydrogen storage material. The adsorption of hydrogen molecules on Be-decorated graphene sheets has been calculated using first principles and concluded that, in comparison to pristine graphene, the Be-decorated graphene has more interaction and higher attraction between adsorbed hydrogen molecules and graphene sheets. Further, a molecular dynamics simulation confirms that hydrogen molecules can adsorb onto graphene sheets at room temperature with specific adsorption energy. The synergy between Be-decorated graphene and adsorbed hydrogen molecules is further enhanced by the application of an external electric field.²²³ D. Li et al. reported a DFT study on Be atoms decorated on B-doped graphene.²²⁴ The sp² hybridized B-atoms bonded with the nearest three carbon atoms of graphene. After doping B-atoms, Be atoms have three sites available for the adsorption: at the center of the hexagon, in the middle of the C–C bond, and above the C or B-atoms. The Be atom is decorated at the center of the hexagonal structure of graphene with a binding energy of −2.498 eV. The reasonable distance between two Be atoms, and the strong interaction between Be atoms and graphene minimize the clustering. Both side decorations of Be atoms increase the surface area for hydrogen adsorption. In both side decorations, the Be atoms are attached to the top of the B atoms with a binding energy of −1.553 eV. Six hydrogen molecules can be adsorbed for single-sided Be-decorated B-doped graphene with an HSC of 10.36 wt% with a binding energy of −0.289 eV/H₂. In double-sided adsorption, a maximum of ten hydrogen molecules can be adsorbed with a HSC of 15.1 wt% having an adsorption energy of −0.298 eV/H₂, which lies in the required operating range of −0.2 to −0.6 eV/H₂. Such an HSC results from the change in electron distribution.²²⁴ In another study, Mahenderan et al. did a DFT study on Be-decorated N-doped graphene for hydrogen storage. They found that Be atoms are homogeneously dispersed on graphene sheets. In the scenario of Be-decorated 585 DCV (double carbon vacancy) graphene, solely two hydrogen molecules undergo adsorption through the Kubas-type mechanism. Upon N-doping, the composite captures four hydrogen molecules via chemisorption, facilitated by the interaction between the p-orbital of Be atoms and the σ orbital of hydrogen. The HSC of this composite was measured to be 1.9 wt% under ambient conditions. This calculation suggests that Be decorated N-doped 585 DCV graphene offers a viable option for hydrogen storage.²²⁵

3.3.2. Calcium-doped graphene for hydrogen storage. Calcium (Ca), the alkaline earth metal atom with a vacant 3d-orbital, is thought to be superior to proposed metal doping elements for boosting the HSC of graphene. Ca atoms have low cohesive energy, discouraging the desire to cluster and leading to a uniform distribution over the graphene substrate. In Ca-doped graphene, the work function is reduced by the Ca-doping.^219,226,227 Wang et al. studied van der Waals functional theory and claimed that the Ca atoms adsorb on both sides of the graphene. van der Waals chemistry plays a pivotal factor in the adsorption of Ca atoms on graphene and in the binding of the hydrogen molecule in the Ca decorated graphene. According to this study, each Ca atom binds only one hydrogen molecule with a binding energy of −62 meV. 33.4% covered Ca-decorated graphene's (Ca₂C₆) calculated hydrogen storage capacity was 2.6 wt%.²²⁷ In another study, based on plane wave calculations, Ataca et al. reported that at room temperature, Ca-decorated graphene is useful as a bi-directional hydrogen storage medium.²²⁸ When Ca is adsorbed through the chemisorption mechanism (the 4s orbital of Ca atoms donates an electron to the π* orbital of graphene), graphene switches from a semi-metallic state to metallic, and the adsorbed Ca atom gets positively charged. The Ca atom to graphene binding is affected by the hybridization of the graphene carbon orbital with the d orbital of the Ca atom. The attractive Coulomb interaction between negatively charged hydrogen and positively charged Ca atoms and weak van der Waals interaction result in mixed bonding between hydrogen molecules and Ca atoms adsorbed on graphene. On both sides, the Ca decorated system's binding energy of the Ca atom is more than that of a single-sided Ca atom decoration. The occupancies of the 3d orbital on both sides decorated Ca atoms are more than that of a single side Ca decoration. An adsorbed Ca atom binds a maximum of four hydrogen molecules on a single side. In the case of Ca atoms bound on both sides, the HSC reached 8.4 wt%.

In another study, first-principles total energy calculations were performed by Gao et al. for hydrogen storage practices of Ca-decorated graphene.²²⁹ According to this study, the system's stability is promoted when electrons from hydrogen atoms fill the electronic state of Ca to the top-filled level. Further, this study suggests that hydrogen storage or adsorption onto Ca-decorated graphene is not attainable by physisorption, but it is possible via the spillover mechanism and capacity reached up to 7.7 wt%. The adsorption energy of the hydrogen atom increases when more hydrogen atoms adsorb on the Ca-decorated graphene sheets, and this process favors spillover activity. The Ca atom 4s orbital is bonded with the graphene carbon 2p orbital. From the Ca atom to the carbon atom, electron transfer occurs, and Dirac points of graphene shift toward lower energy. The Ca atom adsorbed on graphene was also observed to be positively charged, and ionic bonding between the Ca atom and graphene was shown. They also revealed that electric field formation between the Ca atom and graphene polarizes the hydrogen molecule and contributes to hydrogen adsorption. Simultaneously, the partially occupied Ca 3d orbital improves the hydrogen adsorption by Ca 3d interaction with the hydrogen electron. In another investigation, Ca-decorated graphene nanoribbons (Ca/GNRs) were exposed for hydrogen storage. Different edge geometries of graphene were explored, such as zigzag armchair GNRs, boron-doped zigzag armchair GNRs, and large vacancy defect graphene. Lee et al. reported that Ca atoms adhere to the irregular edges of graphene without clustering. The single Ca atom can bind six hydrogen molecules with 0.2 eV/H₂ adsorption energy. This Ca-decorated irregular graphene nanoribbon system claimed to have an HSC of up to 5 wt%.¹⁴¹ Further, a DFT-based theoretical investigation by Beheshti et al. revealed that Ca-decorated B-doped graphene exhibits a remarkable HSC. B-doping prevents the aggregation of Ca atoms. In Ca-decorated B-doped graphene, two mechanisms appear for hydrogen adsorption. The proposed binding mechanisms are as follows: the interaction of the 3d orbitals of Ca atoms with the σ orbitals of hydrogen and alignment of the hydrogen molecule in an electric field produced by Ca–graphene dipoles. The bilaterally Ca-decorated B-doped graphene attained a gravimetric HSC of 8.38 wt% with −0.4 eV/H₂ binding energy under ambient conditions.²³⁰ Another study shows how the topological defects in graphene affect its ability to store hydrogen. This study explored Ca-decorated single vacancy, 585 double carbon vacancy, and 555–777 double carbon vacancy defective graphene. These topologically defective graphene geometries can securely store a maximum of six hydrogen molecules and exhibit a binding energy range of 0.17–0.39 eV/H₂. Theoretically, the hydrogen storage capacity of the both-sided Ca-decorated topologically 585 double carbon vacancy and 555–777 double carbon vacancy defective graphene reached up to 5.2 wt%.²³¹ Further, another first-principles calculation was performed by Kim et al. on Ca-decorated polygon graphene structures (biphenylene and Ψ-graphene). Both biphenylene and Ψ-graphene in the pristine form weakly bind the hydrogen molecules. After Ca-doping, biphenylene adsorbed five hydrogen molecules, and Ψ-graphene adsorbed six hydrogen molecules with reversible room-temperature hydrogen storage. The HSC of Ca-decorated biphenylene and Ψ-graphene reached 6.8 and 4.2 wt% levels with a binding energy of 0.30 eV with dehydrogenation at 300 K and 380 K, respectively.²³² Recently, Ca-decorated HOT-graphene has been explored for hydrogen storage. HOT-graphene, a novel 2D semimetallic carbon allotrope with a Dirac cone, is confirmed by the convergence of energy bands and the vanishing DOS at the Fermi level. This HOT graphene structure consists of tetragon, hexagon, and octagon carbon rings. The Ca atom is strongly bonded with HOT graphene and has an enormous binding energy, and it can store six hydrogen molecules with reversible hydrogen storage. The HSC of double-sided Ca-decorated HOT graphene reached up to 13.71 wt% under ambient conditions. The dehydrogenation temperature is also practical under working conditions. The finding also revealed good charge transfer between the Ca atom and HOT graphene. The charge transfer disclosed a strong interaction between the Ca atom and HOT graphene. The adsorption of hydrogen molecules is facilitated by the polarization of hydrogen molecules, which leads to electrostatic and van der Waals interaction. All these outstanding findings make Ca-decorated HOT graphene a viable material for hydrogen storage.²³³

3.3.3. Magnesium.
3.3.3.1. Magnesium-doped graphene for hydrogen storage. As a dopant, magnesium increases the interaction between graphene sheets and hydrogen molecules/atoms. The multi-laminates of reduced graphene oxide-magnesium (rGO-Mg) were explored for safe and selective hydrogen storage.²³⁴ The rGO-Mg multilaminates show environmental stability and remarkable HSC with reversibility. rGO thin sheets provide environmental protection to the Mg nanocrystals and prevent degradation. The Sievert setup studied the hydrogen adsorption–desorption behavior of rGO-Mg. This material exhibits a high HSC of 6.5 wt% and 0.105 kg H₂ per liter at 15 bar. The rGO-Mg composite releases 6.12 wt% hydrogen readily and shows good reversibility. In a theoretical exploration, Amaniseyed et al. studied hydrogen molecule adsorption on both charged and uncharged graphene decorated with Mg using DFT and van der Waals interactions. The findings of this study revealed that the overall interaction between hydrogen molecules and graphene sheets intensifies with an increase in the number of Mg atoms and hydrogen molecules. Also, hydrogen molecules are pivotal in influencing the interaction between Mg atoms. The Mg atom tends to be stored in the hollow hexagonal position of graphene at a specific distance from the surface in both charged and uncharged scenarios. Nine hydrogen molecules get adsorbed with a binding energy of 0.134 eV/H₂ in Mg-doped charged graphene. The analysis shows that in uncharged graphene, electron transfer occurs from hydrogen molecules to the Mg atoms. Further analysis revealed that positive charge concentrated on Mg atoms in Mg-charged graphene and nH₂–Mg-charged graphene. After the adsorption of four hydrogen molecules, charge transfer shifts from Mg atoms to hydrogen molecules and graphene, enhancing the interaction between Mg and the charged graphene surface.²³⁵ In another study, Chen et al. investigated the HSC of Mg-doped graphene oxide using first-principles calculation. They found that it can store 5.6 wt% hydrogen at 200 K without external pressure. This study investigates the role of functional groups, particularly oxygen, in hydrogen storage. Typically, the hydroxyl group lowers the HSC by making water when it reacts with graphene. However, in this study, the hydroxyl groups were reduced by Mg doping on the graphene oxide surface. The Mg atoms are attached to the surface of the graphene oxide by epoxy groups. In this system, Mg atoms and oxygen separately produce an electric field to adsorb hydrogen. The binding energy for hydrogen adsorption for oxygen is 0.15 eV/H₂; for the Mg atom, it is 0.25 eV/H2; for pure graphene oxide, it is 0.08 eV/H₂. The Mg atom binding energy for hydrogen adsorption is more than that of others individually. In contrast, a more powerful electric field is generated in the mid-region of the Mg atom and oxygen, providing a hydrogen binding energy of 0.38 eV/H₂ due to the combined effect of Mg and oxygen simultaneously polarizing the hydrogen molecule in all possible directions.²³⁶
3.3.3.2. Magnesium-decorated N/B-doped graphene for hydrogen storage. For the uniform decoration of Mg atoms and to prevent aggregation on the graphene substrate, doping heteroatoms such as boron and nitrogen becomes necessary. These heteroatoms improve the adsorption of Mg atoms on graphene sheets by altering the binding structure of the graphene substrate. Using first-principles-based DFT calculations, Lone et al. explored Mg decorated B-doped graphene for hydrogen storage media.²³⁷ The B-doping is a practical approach to modify the binding configuration and improve the adsorption of the dissociated hydrogen atoms. The analysis revealed that in Mg-decorated B-doped graphene, the adsorption energy of the Mg atom is −4.48 eV/Mg, and the cohesive energy of Mg atoms is 4.26 eV per atom. As a result, the B-doping prevents the aggregation of Mg atoms. The charge transfer between the B atom and C atom leads to improved adsorption energy of Mg atoms on graphene due to vacant p-orbital in graphene's C atom. Further, this study suggests that up to four hydrogen molecules can be stored with the Mg atom in B-doped graphene with an HSC of 8.26 wt% and −0.566 to −0.687 eV/H₂ binding energy.

Further, Tang et al. investigated nitrogen-doped Mg decorated graphene. They observed that after the decoration of the Mg atom, N-doping, and introducing vacancy defects, semimetallic graphene was converted into a metallic state with good thermodynamic stability. The Mg-decorated N-doped graphene adsorbs up to seven hydrogen molecules with an adsorption energy range of −0.15 to −0.21 eV, which is in the recommended range. The hydrogen molecule polarization, and the orbital interaction between hydrogen molecule and decorated/doped graphene primarily attributed to the hydrogen adsorption. The Mg-decorated N-doped graphene shows reversible hydrogen storage with a dehydrogenation temperature above 206 K.²³⁸

3.4. Alkali metal-doped graphene for hydrogen storage

Graphene materials doped with alkali metals are becoming popular contenders for hydrogen storage applications. The introduction of alkali metals into the graphene sheets alters the surface chemistry of the graphene substrate, making it an effective adsorption site for hydrogen. The present section briefly demonstrates the study where the graphene monolayer is functionalized with alkali elements such as lithium, sodium, and potassium to enhance hydrogen adsorption. The alkali metals reside above each other on the carbon C₄ and C₆ rings due to their lower charge as compared to alkaline earth metals.²³⁹ These alkali metals primarily exhibit Kubas-type interaction with the graphene substrate and do not form clusters.²⁴⁰ The various arrangements in which alkali metals form with graphene have been explored. Many studies have been done to calculate the interaction energy of alkali metals with hydrogen over the graphene substrate.

3.4.1. Lithium.
3.4.1.1. Lithium-doped graphene for hydrogen storage. Graphene doped with lithium (Li) has the potential to be used as a hydrogen storage material.^241–243 Numerous methods exist for binding graphene and Li atoms. The binding energy between Li atoms and the graphene sheet in the case of 2 × 2 graphene is 1.67 eV. However, in the case of 3 × 3 graphene, the energy increases dramatically to 2.09 eV.²⁴⁴ A few properties of Li are useful for hydrogen storage applications, such as its inability to form a cluster when mixed with graphene. It can make graphene charged. Li-doped graphene nano-flakes are also promising candidates for hydrogen storage.²⁴⁵ So many studies have been done considering first-principles calculations that could estimate a remarkable HSC in Li-doped graphene. Zhou et al. have examined the hydrogen storage mechanism of Li-doped graphene using a first-principles study showing that graphene could adopt metallic properties by charge transfer from Li atoms to π* bonds of graphene. The Li atoms become positively charged and can adsorb four hydrogen molecules by polarizing them. The HSC reached up to 16 wt% when the Li atom was decorated on both sides of graphene.²⁴⁶ An et al., based on first-principles calculation, showed that Li-doped B₂C (carbon–boron mixed structure) graphene can be used as a hydrogen storage material with a gravimetric HSC of 7.54 wt%.²⁴⁷ The strong binding of the Li atom on graphene comes from the interaction of the B-2p and C-2p orbitals with the Li-2p orbital. Findings suggest that hydrogen adsorption involves contributions from orbital hybridization and polarization mechanisms with a 0.12–0.22 eV/H₂ binding energy range. This system is reversible for hydrogen adsorption and desorption at room temperature. S. Seenithurai et al. have shown that Li-decorated double carbon vacancy graphene (DVG) is prominent for hydrogen storage, as a first-principles study claims.²⁴⁴ The binding energy of the Li atom in DVG is 4.04 eV, which is greater than that of bare graphene. On one side, Li-decorated DVG adsorbs a maximum of four hydrogen molecules, with an adsorption energy of 0.23 eV/H₂ and an HSC of 3.89 wt%.

On the other hand, double-sided Li-decorated DVG's HSC reached 7.26 eV/H₂ with an adsorption energy of 0.26 eV/H₂. This system's dehydrogenation takes place under mild conditions.²⁴⁴ In another DFT study, 3D B-doped graphene decorated with Li was explored for hydrogen adsorption. The decorated Li atom binding energy is 2.64 eV, more significant than the Li atoms' cohesive energy. This 3D Li decorated B-doped graphene is thermally stable, and gravimetric and volumetric HSC reached up to 5.9 wt% and 52.6 g L⁻¹, respectively, at 100 bar and 298 K.²⁴⁸ Further, porous graphene decorated with Li (PG) has been studied.²⁴⁹ MD simulations show that the HSC of Li-decorated porous graphene is 10.89 wt% and 10.79 wt% for the two different systems at 300 K with no external hydrogen pressure. Pillared graphene oxide doped with Li is also an efficient option for hydrogen storage. The Li atoms replace the hydroxyl H with –OLi groups. The material is thermally stable and easy to develop. The hydrogen adsorption occurs through physisorption on the –OLi groups. The pore size and –OLi groups' surface density affect the HSC. The HSC of Li-pillared graphene oxide at 77 K and 100 bar, surpassing 10 wt% and 55 g L⁻¹.²⁵⁰

In another study, Du et al. revealed the HSC of Li-doped multifunctional porous graphene up to 12 wt% using first principles calculations.²⁵¹ In another study, Zheng et al. studied the HSC of Li-decorated graphene nanoribbons using DFT theory. The Li atom covalently bonded at the hollow center of the graphene hexagon ring with an adsorption energy of −0.807 eV. The four hydrogen molecules chemically adsorbed at the Li atom in Li-decorated graphene nanoribbons with an adsorption energy of −0.235 eV. Nanoribbons with decorated Li atoms on both sides adsorb eight hydrogen molecules. This study concluded that modifying graphene nanoribbons with metal atoms can improve hydrogen storage performance.²⁵² In another study, a graphene-BBC structure decorated with Li showed an HSC of 4.26 wt%, taking 160 lithium atoms (at 298 K and 100 bar pressure).²⁵³ In another study, a defective graphene system doped with Li is used to study the hydrogen storage performance using DFT calculations.²⁵⁴ This study considers four different stable graphene structures with defective geometry and states that Li-doping does not form clusters. In this system, three hydrogen molecules were adsorbed per Li atom with an adsorption energy of nearly 0.2–0.4 eV that could maintain adsorption as well as desorption in an ambient temperature range.²⁵⁴ The study also suggests that boron doping and Li can yield better results. Further, a study on Li-doped defective graphene has been done by Eisapour et al.²⁵⁵ This study shows that despite changing the electrical properties of the defective graphene, Li does not influence its structural stability. These investigations show that Li-doped defective graphene is a very prospective hydrogen storage material.

3.4.1.2. Lithium-doped graphene allotropes for hydrogen storage. The different carbon allotropes in graphene configuration decorated with Li doping are also promising materials for hydrogen storage. Ye et al. have studied the HSC of a novel T-graphene (2D carbon allotrope with tetrarings) material doped with Li atoms.²⁵⁶ Theoretical calculations suggest a pronounced hybridization potential between the C-2p and Li-2p orbitals in T-graphene and Li atoms. This system shows a noteworthy HSC of 7.7 wt%, coupled with a suitable adsorption energy of 0.19 eV/H₂ under ambient temperature and pressure conditions. Two main things govern hydrogen binding: polarization and orbital interaction. In polarization, a charge transfer exists between the Li atom and T-graphene. As a result, the Li atom gets positively charged and polarizes the hydrogen molecule. In orbital interaction, the hydrogen s-orbital is hybridized with the Li 2p orbital, exhibiting that hydrogen s-orbital and Li 2p orbital electron transfer occurs. Another study revealed that Li-decorated polymerized as indacenes (PAI) graphene is also promising for reversible hydrogen storage. The authors observed that a single unit of PAI-graphene can be decorated with eight Li atoms. Each Li atom adsorbs four hydrogen molecules, revealing an HSC of 15.7 wt%. The Li atom gets ionized by donating its 2s valence electron to the PAI-graphene. The coalescing of Li atoms does not occur due to the diffusion energy barrier. The adsorption energies of the hydrogen molecules that bind with Li atoms lie within the recommended range, which is reversible and valuable for practical conditions. This study also shows that electrostatic and van der Waal interactions are responsible for hydrogen adsorption.²⁵⁷ The Li-decorated 2D irida-graphene is also a potential material for reversible hydrogen storage. Irida graphene is a metallic material exhibiting a Dirac cone positioned above the Fermi level of its band structure. It is made up of a fused ring containing 3–6–8 carbon atoms. The Li atom is attached to the top of the octagonal carbon ring of irida graphene. Up to sixteen and twenty-four hydrogen molecules with an adsorption energy of −0.230 eV/H₂ and −0.276 eV/H₂ adsorb by the single and both-sided Li-decorated irida graphene, respectively. The HSC reached up to 7.06 wt%, and adsorption energies lie in the reversible recommended range for onboard applications.²⁵⁸

3.4.2. Sodium-doped graphene for hydrogen storage. Recently, many studies have been done to explore sodium (Na) for hydrogen storage applications.^259,260 Na is quite heavier than Li, but being inexpensive, Na can be considered an alternative to Li for hydrogen storage. The chemical properties of Na and Li are similar. They do not form clusters in carbon nanostructures.²⁶⁰ Tachikawa et al. have optimized various geometrical structures of Na-doped graphene. Using DFT, they did a comparative study considering Li- and Na-based graphene structures. The binding energies for n = 4, Li, and Na-based graphene are 2.20 kcal mol⁻¹ and 2.34 kCal mol⁻¹, respectively, implying that Na-doped graphene binds hydrogen more strongly than Li. However, the hydrogen adsorption percentage is not mentioned in the study.²⁵⁹ Another study suggests that Na-adsorbed graphene can bind 2–5 hydrogen molecules, resulting in 4.02 wt% hydrogen storage.²⁶⁰ The calculations are done using a first-principles study. Fig. 9 shows the different numbers of hydrogen molecules on Na-adsorbed graphene.

Fig. 9 Adsorption of a varying number of H₂ molecules on a Na-adsorbed graphene system. Adapted with permission from ref. 260 Copyright 2015, Springer.

3.4.3. Potassium-doped graphene for hydrogen storage. Some authors have studied the potential of potassium for hydrogen storage. Tapia et al. have theoretically studied the behavior of K-adsorbed graphene using DFT. They also revealed how the bonding, magnetic and structural properties change when graphene interacts with the K atom and hydrogen. The binding energy for hydrogen adsorption in graphene and K-adsorbed graphene is 0.85 eV per atom and 1.22 eV per atom, respectively. There is a 43.5% improvement in hydrogen adsorption energy. When a hydrogen atom is adsorbed into the K-doped graphene, a charge transfer occurs from the K atom to the graphene surface, and the hybridization of carbon atoms changes from sp² to sp³.²⁶¹

3.5. Miscellaneous metal-doped graphene for hydrogen storage

3.5.1. Phosphorus-doped graphene for hydrogen storage. Phosphorus (P) doped graphene is also explored for hydrogen storage.^262,263 Doping of P atoms significantly changes the features such as Fermi level, electrical, thermal, and mechanical properties, charge transport, etc. of the graphene substrate. Also, the P atom has a higher ability for electron donation. There are several approaches to dope P-atoms into graphene, such as chemical methods, chemical vapour deposition, thermal annealing, electrochemical erosion method, etc. Generally, H₃PO₄ is used as a phosphorus source. The HSC of the P-doped graphene reached up to nearly 2.2 wt% at 298 K and 100 bar. This hydrogen uptake is manifold times higher than that of graphene without phosphorus. In another study, Ikot et al., using a DFT study, revealed that Al, Ca, Mg, Ni, and Zr decorated phosphorus-doped graphene is a promising material for hydrogen storage.⁸⁶ Many other studies also revealed that miscellaneous metals or alloys decorated with P-doped graphene composites are also useful for hydrogen storage. Jin et al. reported P-doped graphene decorated a Pd₃P composite for hydrogen storage. The thermal reduction method was used to synthesize the composite. At 298 K and a pressure of 4 MPa, the Pd₃P decorated P-doped graphene's HSC measured 3.66 wt%, as depicted in Fig. 10(a). Its HSC improves to 3.94 wt% at a temperature of 253 K. At 298 K and 5.5 MPa pressure, meanwhile, it holds 4.7 wt% hydrogen as displayed in Fig. 10(b). The D-band of Pd atoms shifts downwards, followed by P-doping. As a result, hydrogen adsorption energy on the Pd atom decreases. This phenomenon facilitates the hydrogen spillover activity. Furthermore, the Pd atom's electronegativity is less than the P atom's electronegativity, suggesting that P–H configuration stability is more significant. Hence, it is concluded that the stability of the Pd–P–H configuration is higher than that of the Pd–H structure throughout many hydrogenation and dehydrogenation reversible cycles. This composite offers hydrogen adsorption both chemically and physically throughout the adsorption process. This outstanding feature shows that transition metal phosphide decoration on graphene reveals good hydrogen storage performance.²⁶⁴

Fig. 10 Hydrogen storage profile for (a) Pd₃P decorated P-doped graphene at 4 MPa and 298 K and (b) at 5.5 MPa and 298 K, adapted with permission from ref. 264 Copyright 2022, Elsevier. Hydrogen storage curve for (c) MgNi/G, (d) MgNi/BG, and (e) MgNi/NG, adapted with permission from ref. 267 Copyright 2021, Elsevier. (f) Pressure-composition isotherm of Pd₃Co, (g) Pd₃Co-NG, and (h) Pd₃Co-BG, adapted with permission from ref. 268 Copyright 2018, Elsevier. (i) The hydrogen adsorption mechanism in Pd₃P decorated P-doped graphene, adapted with permission from ref. 264 Copyright 2022, Elsevier.

3.5.2. Metal-alloy doped graphene for hydrogen storage. Metal alloys, along with graphene, play an important role in efficient and practical hydrogen storage applications. To some extent, the doping of alloys into graphene overcomes the problems related to HSC, reversibility, kinetics, etc. Wang et al. conducted a study in which hydrogen storage was accomplished by Ni–B nano-alloy doped 3D graphene. The HSC of the graphene composite having 0.83 wt% Ni and 1.09 wt% B reached 4.4 wt% at a temperature of 77 K and a pressure of 106 kPa, which is reversible. This value is thrice that of pure graphene. Physical adsorption of hydrogen and the spillover phenomenon governed the hydrogen storage performance, which is confirmed by the specific surface area and pore volume.²⁶⁵ Similarly, in another investigation, Ni–B nano-alloy doped 2D graphene with 0.14 wt% Ni and 0.63 wt% B had a gravimetric HSC of 2.81 wt% at a pressure of 106 kPa and temperature of 77 K. This capacity value is twice that of the neat graphene substrate.²⁶⁶ The physisorption and dissociative chemisorption show a plausible role in hydrogen adsorption. Samantaray et al. conducted an experimental and theoretical study of an alloy, i.e., a MgNi nanocomposite decorated over boron and nitrogen-doped graphene (MgNi/BG and MgNi/NG), for hydrogen storage.²⁶⁷ Doping was used in this study to solve the issue of hydrogen's weak interaction with the graphene substrate. Fig. 10(c) depicts the HSC of the MgNi doped graphene at up to 30 bar pressure and three different temperatures. The hydrogen gravimetric uptake of MgNi/BG was 3.5 wt% at 25 °C and for MgNi/NG was 5.4 wt% at 25 °C and 3 MPa (which is close to the US DoE target) as illustrated in Fig. 10(d and e). The uniform distribution, porosity, and particle size of alloys are the critical factors for hydrogen enhancement compared to pure graphene (hydrogen uptake 0.5 wt%). Physisorption by simple dipole–dipole interaction, via Kubas interaction, and the spillover phenomenon involving chemical dissociation of hydrogen molecules into atoms are responsible for hydrogen adsorption. This study's theoretical insights also support the experimental investigation of the hydrogen adsorption mechanism.

In another study, Samantaray et al. developed Pd₃Co decorated graphene, Pd₃Co decorated N-doped graphene and B-doped graphene nanocomposites.²⁶⁸ The graphene substrate with N and B-doping helps to promote more spillover activity to the nanocomposite. At room temperature and 30 bar pressure, the corresponding nanocomposite's HSC was 3 wt%, 4.2 wt%, and 4.6 wt%, respectively, as depicted in Fig. 10(f–h). Another study examined the electrochemical HSC of Co–B doped graphene synthesized using the ball-milling process. The HSC of this system under operating conditions was 718.4 mA h g⁻¹ (2.68 wt%) at 100 mA g⁻¹ current density. This capacity is preferably favored by the Kubas-type interaction.²⁶⁹ In another study, manganese (Mn) and manganese vanadium (Mn–V) decorated graphene were developed via an in situ wet reduction method. There is a significant improvement in the HSC after Mn decoration over graphene. The hydrogen uptake reaches up to 0.25 wt% to 0.36 wt% at 4 MPa pressure and operating temperature after Mn decoration over graphene. Moreover, the HSC of the Mn–V decorated graphene was measured to be 1.81 wt% under the same operating conditions. The enhanced hydrogen uptake is attributed to the spillover activity by combining Mn and V and their interaction with the graphene substrate.²⁷⁰Fig. 10(i) displays the hydrogen adsorption mechanism via spillover activity in Pd₃P nanoparticles decorated with P-doped graphene.

The experimentally measured and theoretically calculated HSCs of the heteroatom-doped and decorated graphene-based materials are summarized in Tables 1 and 2, respectively.

Table 1 State of the art of experimental hydrogen storage performance in heteroatom-doped and decorated graphene materials

S. no.	Material	Doping method	Pressure (bar)	Temp. (K)	H2 storage (wt%)	Enthalpy of Ad. (kJ mol^−1)	Ref.
a RT: room temperature; —: data not available.
1	20% doped Pt/graphene-MWCNT	CVD/chemical method	20	RT	1.79	—	99
2	20% doped Pt/graphene-MWCNT	CVD/chemical method	40	RT	2.67	—	99
3	Pd-f-HEG	Chemical method	20	298	1.76	—	101
4	N-doped graphene	Chemical method (high temperature reduction)	90	RT	1.50	—	107
5	Ni/Al-doped graphene	Chemical method	50	473	5.70	—	129
6	B-doped rGO	Chemical method (high temperature reduction)	20	77	8.20	—	132
7	B-doped rGO	Chemical method (high temperature reduction)	20	273	0.58	—	132
8	B-doped rGO	Chemical method (high temperature reduction)	20	298	0.24	—	132
9	Ni/B-doped rGO	Chemical method	20	77	6.90	—	132
10	Ni/B-doped rGO	Chemical method	20	273	0.41	—	132
11	Ni/B-doped rGO	Chemical method	20	298	0.16	—	132
12	Pd-decorated graphene sheets/activated carbon	Chemical method (wet impregnation method)	80	RT	0.82	−14 to −10	151
13	Pd/graphene	Chemical method	1.1	77	3.40	—	152
14	Pd/graphene	Chemical method	60	RT	8.67	—	153
15	Pd-decorated graphene nanoplatelets	Chemical method	32	298	1.21	—	159
16	Pd-embedded 3D porous graphene	Microwave-assisted method	75	77	5.40	—	161
17	Pd-decorated N-doped graphene nanoplatelets	Sputtering/chemical method	32	298	1.25	—	166
18	Pd-decorated N-doped graphene	Solar light assisted/chemical method	40	298	4.30	—	167
19	Pd-decorated N-doped graphene (Pd/N-rGO)	Chemical method	40	298	2.99	—	168
20	Ti-decorated graphene	Thermal reduction method	10	RT	1.40	—	174
21	Ni-doped rGO	Chemical/microwave-assisted method	20	300	0.23	—	186
22	Ni-doped rGO	Chemical/microwave-assisted method	20	80	2.70	—	186
23	Ni-doped rGO	Chemical/microwave-assisted method	1	RT	0.007	—	186
24	Ni/Pd/rGO	Chemical/microwave-assisted method	1	RT	0.13	—	186
25	Ni/Ag/Pd/rGO	Chemical/microwave-assisted method	1	RT	0.055	—	186
26	Ni/graphene	Chemical method	60	RT	1.18	—	187
27	Ni/Pd-rGO	Hydrothermal method	40	RT	2.65	−0.2 to −0.6 eV	189
28	Ni-GO	Chemical method	1	77	0.32	—	190
29	Ni-GO/MIL	Hydrothermal method		77	1.64	—	190
30	Pt-decorated plasma-derived graphene	Thermal reduction method	20	77	1.64	—	195
31	Pt-decorated plasma-derived graphene	Thermal reduction method	20	298	0.13	—	195
32	Fe-decorated graphene sheets	Arc discharge method	50	300	0.27	—	205
33	Fe-decorated graphene sheets	Arc discharge method	50	77	2.16	—	205
34	Multi-laminate rGO-Mg	Chemical method	15	—	6.50	—	234
35	Pd3P/P-graphene	Hydrothermal method	40	298	3.79	−0.59 eV/H2	263
36	Pd3P decorated P-doped graphene	Pyrolysis method	40	298	3.66	—	264
37	Pd3P decorated P-doped graphene	Pyrolysis method	40	253	3.94	—	264
38	Pd3P decorated P-doped graphene	Pyrolysis method	55	298	4.70	—	264
39	Ni–B nanoalloy doped 3D graphene	Chemical method	1.06	77	4.40	1	265
40	Ni–B nanoalloy doped 2D graphene	Chemical method	1.06	77	2.84	—	266
41	MgNi/B-doped graphene	Thermal method	30	RT	3.50	19.74	267
42	MgNi/N-doped graphene	Thermal method	30	RT	5.40	20.93	267
43	Pd3Co alloy decorated N-doped graphene	Mechanical/chemical method	30	RT	4.20	17.12	268
44	Pd3Co alloy decorated B-doped graphene	Mechanical/chemical method	30	RT	4.60	15.12	268
45	Co–B doped graphene hybrid	Mechanical method	—	RT	2.68	—	269
46	Mn-decorated graphene	Chemical method	40	RT	0.36	—	270
47	Mn–V decorated graphene	Chemical method	40	RT	1.81	—	270
48	Li^+ doped MIL-100(Fe)/GO composite	Hydrothermal method	50	298	2.02	7.33	271
49	Pd-doped hydrogenated GO	Chemical method	1	RT	17.35	—	272

---

Table 2 State of the art of theoretically predicted hydrogen storage capacity in various heteroatom-doped and decorated graphene materials

S. no.	Material	Analysis method	Pressure (bar)	Temp. (K)	H2 storage (wt%)	Adsorption energy	Ref.
a DFT: density functional theory; —: data not available.
50	N-doped graphene	DFT	—	—	6.73	—	113
51	Co/N-doped graphene	DFT	12	RT	5.03	−5.11 eV	116
52	Al-doped graphene	DFT	1000	RT	5.13	−0.0260 eV/H2	126
53	Al-doped graphene	DFT	—	—	13.79	−0.193 eV/H2	128
54	Al-doped porous graphene	DFT	—	—	10.50	−1.11 to −0.41 eV/H2	130
55	B-doped graphene	DFT	—	—	5.30	0.58 eV	133
56	B-doped graphene	DFT	—	—	10.10	−0.209 eV/H2	137
57	Si-doped graphene	Monte Carlo	100	RT	2.40	18–19 kJ mol^−1	138
58	Si-decorated graphene	DFT	—	—	15.00	−0.19 eV	140
59	Ca-decorated zigzag graphene nanoribbons	First principles	—	—	5.00	0.2 eV/H2	141
60	Ni–Ti–Mg decorated B-doped graphene	DFT	—	—	6.50	−0.41 eV/H2	146
61	Pd cluster doped graphene	DFT	—	—	0.60	−0.085 eV	164
62	NH-doped Pd-functionalized graphene	DFT	—	0	3.62	0.65 eV/H2	169
63	Pd-decorated N-doped graphene	DFT	—	—	1.99	1.22 eV	170
64	Pd3P/single vacancy graphene	DFT	—	—	5.74	−0.25 eV/H2	171
65	Ti-decorated porous graphene	DFT	1	27	6.11	−0.457 eV	176
66	Ti-doped ψ-graphene	DFT	—	—	13.14	−0.30 eV	179
67	Ti-decorated irida-graphene	DFT	—	—	7.70	−0.41 eV/H2	180
68	Ti-decorated B-doped porous graphene	DFT	30	298	8.58	−2.96 eV	184
69	Ti-decorated double vacancy graphene	DFT	—	—	6.67	−0.25 eV/H2	182
70	Ti-decorated polycrystalline graphene	DFT	100	300	3.20	—	183
71	Ti-decorated polycrystalline graphene	DFT	100	77	9.90	—	183
72	Ni/graphene	DFT	—	—	2.80	—	191
73	Ni4/Graphene	DFT	Moderate	RT	0.30	0.1 eV	192
74	V-decorated porous graphene	DFT	No external pressure	300	4.58	−0.564 eV	198
75	V3 decorated B–N-doped graphene	DFT	—	—	6.43	−0.45 eV	199
76	Cu functionalized graphene	DFT	—	—	4.231	—	204
77	Y-decorated graphene	DFT	—	—	5.78	−0.568 eV	207
78	Y-decorated ψ-graphene	DFT	—	—	8.31	−0.39 eV/H2	208
79	Sc-decorated ψ graphene	DFT	1	RT	14.46	−0.36 eV/H2	209
80	Sc-decorated porous graphene	DFT	—	—	9.09	−0.296 eV/H2	211
81	Sc-decorated B-doped porous graphene	DFT	—	—	9.13	−0.225 eV/H2	212
82	Zr-decorated penta-graphene	DFT	—	—	14.8	−0.420 eV/H2	215
83	Zr-decorated psi-graphene	DFT	—	RT	11.3	−0.38 eV/H2	216
84	Nb-decorated N-doped graphene	DFT	—	—	8.00	0.410 eV	218
85	Be-decorated B-doped graphene	DFT	—	—	15.10	−0.298 eV/H2	224
86	Be-decorated N-doped graphene	DFT	—	—	1.90	−0.430 eV/H2	225
87	Ca-decorated B-doped graphene	DFT	—	—	8.38	−0.4 eV/H2	230
88	Ca-decorated graphene	DFT	—	RT	8.40	—	228
89	Ca-decorated defective graphene	DFT	—	—	5.20	0.39 eV/H2	231
90	Ca-decorated biphenylene graphene	DFT	—	—	6.80	0.30 eV	232
91	Ca-decorated ψ graphene	DFT	—	—	4.20	0.30 eV	232
92	Mg-decorated B-doped graphene	DFT	—	—	8.26	0.676 eV/H2	237
93	Mg-doped graphene oxide	DFT	No pressure	200	5.60	0.25 eV/H2	236
94	Li-doped graphene	DFT	—	—	12.80	—	241
95	Li-decorated phagraphene	DFT	100	77	15.88	15–25 kJ mol^−1	242
96	Li-decorated ψ graphene	DFT	—	—	15.15	−0.31 eV/H2	243
97	Li-doped B2C graphene (boron–carbon mixed)	DFT	—	—	7.54	0.22 eV/H2	247
98	Li-decorated porous graphene	DFT	No external pressure	300	10.89	0.245 eV	249
99	Li-doped pillared graphene oxide	DFT	100	77	10.00	—	250
100	Li-decorated multifunctional porous graphene	LDA	—	—	12.00	0.243 eV	251
101	Li-dispersed graphene-BBC structure	DFT	100	298	4.26	—	253
102	Novel T graphene material doped with Li atoms	DFT	Ambient	Ambient	7.70	0.19 eV/H2	256
103	Li-decorated 2D irida graphene	DFT	—	—	7.06	−0.27 eV/H2	258
104	Na adsorbed graphene	DFT	—	—	4.02	0.192 eV	260
105	A small concentration lithium doping on graphene	DFT	—	—	3.23	0.29 eV	273
106	Li-decorated penta-octa-graphene	DFT	—	—	9.90	0.14–0.95 eV	274
107	Ti-decorated penta-octa-graphene	DFT	—	Ambient	6.50	0.14–0.95 eV	274

---

4. Current challenges and future research direction

Various heteroatom-doped graphene-based materials have been investigated for hydrogen storage. Many of them show very promising results. However, practical applications of these materials still face some challenges and limitations in synthesis and storage conditions. The challenges mainly relate to their synthesis method optimization, doping control, scalability, doping mechanisms, and hydrogen storage capacity under ambient conditions. The current challenges and future research directions of heteroatom-doped graphene for hydrogen storage applications are as follows:

(1) Controlled doping and uniform dopant dispersion in graphene are the main challenges. It requires optimizing parameters to control the doping level and precise dopant concentration and ensure uniform dispersion over the graphene sheet to improve the hydrogen adsorption–desorption kinetics, reproducibility, and long-term cycling stability under practical conditions. Therefore, future research should aim to solve this challenge by selecting the type of heteroatom, optimizing doping parameters and methods, etc., that can improve hydrogen adsorption and interaction in graphene.

(2) Heteroatom doping/decoration creates defects and disorders in graphene sheets. This is generally helpful for many applications. However, creating more defects and disorders may change the physical properties of graphene sheets. Therefore, controlling the defects and disorders by novel synthesis approaches in heteroatom-doped graphene is essential for achieving good hydrogen storage performance. So, future research should focus on precise imperfections and engineering disorders to enhance hydrogen adsorption and kinetics.

(3) Scalable mass production and cost-effectiveness are also significant challenges to heteroatom-doped graphene. Therefore, future research should focus on large-scale production methods and cost-effective techniques, which can make it economically viable for large-scale application in hydrogen-based technologies. Also, future endeavours will focus on converting laboratory-scale achievements into real-world practical implementation.

(4) An in-depth understanding and analysis of the interactions between hydrogen molecules/atoms and dopant atoms within the graphene lattice are still needed. Future research needs to be focused on understanding the underlying specific mechanisms responsible for the observed improvement in hydrogen storage for heteroatom-doped graphene.

(5) Multi-heteroatom doping and integrating heteroatom-doped graphene with other advanced materials is also challenging. Therefore, future research should explore the multi-heteroatom-doped graphene mechanism and the in-depth analysis of the comprehensive effects of co-dopants on the hydrogen storage performance of the material. This approach may result in a synergistic effect to enhance overall hydrogen storage performance.

(6) The joint effort of materials scientists, chemists, engineers, and industry experts is essential for advancing the understanding and application of heteroatom-doped graphene in hydrogen storage. This evolving field requires interdisciplinary approaches that draw on various professionals' expertise to address the complex challenges of developing efficient and practical hydrogen storage solutions.

Hence, the ongoing efforts in research, development, and exploration of heteroatom-doped graphene-based materials have the potential to contribute significantly to the shift toward sustainable and efficient hydrogen-based energy systems. The findings show that heteroatom-doped graphene-based materials are highly suitable for hydrogen storage applications. As a final note, the future of hydrogen storage in heteroatom-doped graphene looks promising, but it is still an active area of research. Further studies are needed to overcome challenges such as storage capacity, adsorption/desorption kinetics, scalability, cost-effectiveness, and long-term stability. Nevertheless, the potential benefits of using heteroatom-doped graphene for hydrogen storage make it an exciting avenue for future developments in clean energy technology.

5. Conclusion

This comprehensive review article explores the potential of heteroatom-doped graphene for hydrogen storage applications. Over the last decade, heteroatom-doped graphene has opened new doors for hydrogen storage technologies under ambient conditions. The continued research on heteroatom-doped graphene significantly impacts the design and development of hydrogen storage solid-state materials. A brief introduction of various hydrogen storage materials is provided, and heteroatom-doped graphene-based materials, their advantages over other materials, and their hydrogen storage mechanisms are discussed in detail. The specific synthesis methods, doping level, and type of dopant that greatly influence hydrogen storage performance are discussed in detail. Next, a detailed discussion on the hydrogen storage performance of different heteroatom-doped graphene is carried out. Experimentally, the HSC of heteroatom-doped graphene was measured to be 8.67 wt%. The theoretical outcomes showed the HSC of heteroatom-doped graphene up to 15.88 wt% under normal conditions. However, the commercialization of heteroatom-doped graphene for hydrogen storage faces some technological, economic, and societal hurdles. Technological hurdles include optimizing mass production methods and long-term material stability under varying conditions, determining the mechanism of interaction of heteroatoms with the graphene substrate, and further finding the optimal concentrations of heteroatoms for hydrogen adsorption–desorption at moderate temperatures and pressures. The cost considerations are also crucial in the competitive landscape of hydrogen storage solutions. Various economic issues include precursor material costs, materials production costs, feasibility, and material durability. Scaling up production from the laboratory to the industrial level also often requires high-class infrastructure and investment. Therefore, supportive policies are required to overcome these economic barriers from a commercial point of view. Societal acceptance is also an important parameter from a commercial point of view. Social acceptance of heteroatom-doped graphene technology is essential for its safe and responsible implementation. Regulatory frameworks, clear guidelines, and outreach efforts are needed for societal acceptance to address safety concerns and build public trust. Therefore, a multidisciplinary approach involving researchers, industry players, regulators, and public support is required to unlock the full potential of heteroatom-doped graphene-based materials for hydrogen storage technology. Overall, resolving these challenges is necessary to realize the full commercial potential of heteroatom-doped graphene in hydrogen storage applications.

Author contribution

Shankar Ghotia: designed the project, data curation, writing-original draft, and investigation. Tripti Rimza: data curation, writing, review, and editing. Shiv Singh: review, editing, and visualization. Neeraj Dwivedi: editing and formal analysis and provided constructive inputs. Avinash Kumar Srivastava: editing, reviewing, and provided constructive inputs. Pradip Kumar: conceived the idea, designed the project, supervision, formal analysis, review, and editing. All the authors critically reviewed the manuscript and helped to improve the manuscript draft.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the Council of Scientific and Industrial Research (CSIR), Senior Research Fellowship (SRF), and SERB-CRG (CRG/2023/000125, GAP 140), India. The authors gratefully acknowledge the Director of CSIR-Advanced Materials Processes Research Institute, Bhopal, India, for furnishing the necessary infrastructure.

References

1. J. L. Holechek, H. M. E. Geli, M. N. Sawalhah and R. Valdez, Sustainability, 2022, 14, 4792.
2. K. J. Warner and G. A. Jones, Energies, 2017, 10, 1197.
3. X. Zhang, Y. Sun, S. Ju, J. Ye, X. Hu, W. Chen, L. Yao, G. Xia, F. Fang, D. Sun and X. Yu, Adv. Mater., 2023, 35, 2206946.
4. G. Singh, K. Ramadass, V. D. B. C. DasiReddy, X. Yuan, Y. Sik Ok, N. Bolan, X. Xiao, T. Ma, A. Karakoti, J. Yi and A. Vinu, Prog. Mater. Sci., 2023, 135, 101104.
5. X. Li, W. Xu, Y. Fang, R. Hu, J. Yu, H. Liu and W. Zhou, SusMat, 2023, 3, 160–179.
6. S. Rezaie, D. M. J. Smeulders and A. Luna-Triguero, Chem. Eng. J., 2023, 476, 146525.
7. M. Momirlan and T. N. Veziroglu, Renewable Sustainable Energy Rev., 2002, 6, 141–179.
8. O. Czakkel, B. Nagy, G. Dobos, P. Fouquet, E. Bahn and K. László, Int. J. Hydrogen Energy, 2019, 44, 18169–18178.
9. L. Schlapbach and A. Züttel, Nature, 2001, 414, 353–358.
10. M. Yue, H. Lambert, E. Pahon, R. Roche, S. Jemei and D. Hissel, Renewable Sustainable Energy Rev., 2021, 146, 111180.
11. A. Gupta, G. V. Baron, P. Perreault, S. Lenaerts, R.-G. Ciocarlan, P. Cool, P. G. M. Mileo, S. Rogge, V. Van Speybroeck, G. Watson, P. Van Der Voort, M. Houlleberghs, E. Breynaert, J. Martens and J. F. M. Denayer, Energy Storage Mater., 2021, 41, 69–107.
12. Y. Zhang, Y. Li, G. Wu, C. Fan, L. Zhang and S. Han, Chem. Eng. J., 2023, 477, 147190.
13. P. Sharma, J. Han, J. Park, D. Y. Kim, J. Lee, D. Oh, N. Kim, D.-H. Seo, Y. Kim, S. J. Kang, S. M. Hwang and J.-W. Jang, JACS Au, 2021, 1, 2339–2348.
14. M. Ball and M. Wietschel, Int. J. Hydrogen Energy, 2009, 34, 615–627.
15. C. Sun, C. Wang, T. Ha, J. Lee, J. H. Shim and Y. Kim, Nano Energy, 2023, 113, 108554.
16. Y. Luo, L. Sun, F. Xu and Z. Liu, J. Mater. Chem. A, 2018, 6, 7293–7309.
17. D. Wei, X. Shi, R. Qu, K. Junge, H. Junge and M. Beller, ACS Energy Lett., 2022, 7, 3734–3752.
18. M. R. Usman, Renewable Sustainable Energy Rev., 2022, 167, 112743.
19. G. Nazir, A. Rehman, S. Hussain, S. Aftab, K. Heo, M. Ikram, S. A. Patil and M. Aizaz Ud Din, Adv. Sustainable Syst., 2022, 6, 2200276.
20. X. Yu, Z. Tang, D. Sun, L. Ouyang and M. Zhu, Prog. Mater. Sci., 2017, 88, 1–48.
21. Y. Meng, J. Zhang, S. Ju, Y. Yang, Z. Li, F. Fang, D. Sun, G. Xia, H. Pan and X. Yu, J. Mater. Chem. A, 2023, 11, 9762–9771.
22. J. Zheng, C.-G. Wang, H. Zhou, E. Ye, J. Xu, Z. Li and X. J. Loh, Research, 2021, 2021, 3750689.
23. A. Züttel, Naturwissenschaften, 2004, 91, 157–172.
24. E. Boateng and A. Chen, Mater. Today Adv., 2020, 6, 100022.
25. T. Rimza, S. Saha, C. Dhand, N. Dwivedi, S. S. Patel, S. Singh and P. Kumar, ChemSusChem, 2022, 15, e202200281.
26. R. Bhattacharyya and S. Mohan, Renewable Sustainable Energy Rev., 2015, 41, 872–883.
27. M. Hirscher, V. A. Yartys, M. Baricco, J. Bellosta von Colbe, D. Blanchard, R. C. Bowman, D. P. Broom, C. E. Buckley, F. Chang, P. Chen, Y. W. Cho, J.-C. Crivello, F. Cuevas, W. I. F. David, P. E. de Jongh, R. V. Denys, M. Dornheim, M. Felderhoff, Y. Filinchuk, G. E. Froudakis, D. M. Grant, E. M. Gray, B. C. Hauback, T. He, T. D. Humphries, T. R. Jensen, S. Kim, Y. Kojima, M. Latroche, H.-W. Li, M. V. Lototskyy, J. W. Makepeace, K. T. Møller, L. Naheed, P. Ngene, D. Noréus, M. M. Nygård, S.-i. Orimo, M. Paskevicius, L. Pasquini, D. B. Ravnsbæk, M. Veronica Sofianos, T. J. Udovic, T. Vegge, G. S. Walker, C. J. Webb, C. Weidenthaler and C. Zlotea, J. Alloys Compd., 2020, 827, 153548.
28. P. Kumar, S. Singh, S. A. R. Hashmi and K.-H. Kim, Nano Energy, 2021, 85, 105989.
29. J. Park, A. Adhikary and H. R. Moon, Coord. Chem. Rev., 2023, 497, 215402.
30. X. Gao, Z. Zhong, L. Huang, Y. Mao, H. Wang, J. Liu, L. Ouyang, L. Zhang, M. Han, X. Ma and M. Zhu, Nano Energy, 2023, 118, 109038.
31. G. Xia, Y. Tan, F. Wu, F. Fang, D. Sun, Z. Guo, Z. Huang and X. Yu, Nano Energy, 2016, 26, 488–495.
32. B. W. J. Chen and M. Mavrikakis, Nano Energy, 2019, 63, 103858.
33. Z. Ding, Y. Lu, L. Li and L. Shaw, Energy Storage Mater., 2019, 20, 24–35.
34. G. Xia, L. Zhang, X. Chen, Y. Huang, D. Sun, F. Fang, Z. Guo and X. Yu, Energy Storage Mater., 2018, 14, 314–323.
35. J. Liu, L. Sun, J. Yang, D. Guo, D. Chen, L. Yang and P. Xiao, RSC Adv., 2022, 12, 35744–35755.
36. M. Mohan, V. K. Sharma, E. A. Kumar and V. Gayathri, Energy Storage, 2019, 1, e35.
37. S. Liu, J. Liu, X. Liu, J. Shang, L. Xu, R. Yu and J. Shui, Nat. Nanotechnol., 2021, 16, 331–336.
38. B. P. Tarasov, A. A. Arbuzov, A. A. Volodin, P. V. Fursikov, S. A. Mozhzhuhin, M. V. Lototskyy and V. A. Yartys, J. Alloys Compd., 2022, 896, 162881.
39. N. L. Rosi, J. Eckert, M. Eddaoudi, D. T. Vodak, J. Kim, M. O'Keeffe and O. M. Yaghi, Science, 2003, 300, 1127–1129.
40. Y. H. Hu and L. Zhang, Adv. Mater., 2010, 22, E117–E130.
41. Y. Wang, X. Chen, H. Zhang, G. Xia, D. Sun and X. Yu, Adv. Mater., 2020, 32, 2002647.
42. A. F. Dalebrook, W. Gan, M. Grasemann, S. Moret and G. Laurenczy, Chem. Commun., 2013, 49, 8735–8751.
43. E. Klontzas, E. Tylianakis and G. E. Froudakis, Nano Lett., 2010, 10, 452–454.
44. S. Ghotia, A. Kumar, V. Sudarsan, N. Dwivedi, S. Singh and P. Kumar, Int. J. Hydrogen Energy, 2024, 52, 100–107.
45. P. Chen and M. Zhu, Mater. Today, 2008, 11, 36–43.
46. R. Grünker, V. Bon, P. Müller, U. Stoeck, S. Krause, U. Mueller, I. Senkovska and S. Kaskel, Chem. Commun., 2014, 50, 3450–3452.
47. S. Yu, G. Jing, S. Li, Z. Li and X. Ju, Int. J. Hydrogen Energy, 2020, 45, 6757–6764.
48. S. Ghosh and J. K. Singh, Int. J. Hydrogen Energy, 2019, 44, 1782–1796.
49. S. P. Shet, S. Shanmuga Priya, K. Sudhakar and M. Tahir, Int. J. Hydrogen Energy, 2021, 46, 11782–11803.
50. A. Ahmed, S. Seth, J. Purewal, A. G. Wong-Foy, M. Veenstra, A. J. Matzger and D. J. Siegel, Nat. Commun., 2019, 10, 1568.
51. P. García-Holley, B. Schweitzer, T. Islamoglu, Y. Liu, L. Lin, S. Rodriguez, M. H. Weston, J. T. Hupp, D. A. Gómez-Gualdrón, T. Yildirim and O. K. Farha, ACS Energy Lett., 2018, 3, 748–754.
52. N. Klopčič, I. Grimmer, F. Winkler, M. Sartory and A. Trattner, J. Energy Storage, 2023, 72, 108456.
53. A. Schneemann, J. L. White, S. Kang, S. Jeong, L. F. Wan, E. S. Cho, T. W. Heo, D. Prendergast, J. J. Urban, B. C. Wood, M. D. Allendorf and V. Stavila, Chem. Rev., 2018, 118, 10775–10839.
54. P. Larpruenrudee, N. S. Bennett, Y. Gu, R. Fitch and M. S. Islam, Sci. Rep., 2022, 12, 13436.
55. S. Gadipelli and Z. X. Guo, Prog. Mater. Sci., 2015, 69, 1–60.
56. G. Srinivas, Y. Zhu, R. Piner, N. Skipper, M. Ellerby and R. Ruoff, Carbon, 2010, 48, 630–635.
57. S. Dwivedi, Int. J. Hydrogen Energy, 2022, 47, 41848–41877.
58. K. S. Subrahmanyam, P. Kumar, U. Maitra, A. Govindaraj, K. P. S. S. Hembram, U. V. Waghmare and C. N. R. Rao, Proc. Natl. Acad. Sci. U. S. A., 2011, 108, 2674–2677.
59. M. K. Yadav, N. Panwar, S. Singh and P. Kumar, Int. J. Hydrogen Energy, 2020, 45, 19561–19566.
60. Y. Jiang, F. Guo, Y. Liu, Z. Xu and C. Gao, SusMat, 2021, 1, 304–323.
61. T. H. Kim, J. Bae, T. H. Lee, J. Hwang, J. H. Jung, D. K. Kim, J. S. Lee, D. O. Kim, Y. H. Lee and J. Ihm, Nano Energy, 2016, 27, 402–411.
62. B. Wang, T. Ruan, Y. Chen, F. Jin, L. Peng, Y. Zhou, D. Wang and S. Dou, Energy Storage Mater., 2020, 24, 22–51.
63. R. Nagar, B. P. Vinayan, S. S. Samantaray and S. Ramaprabhu, J. Mater. Chem. A, 2017, 5, 22897–22912.
64. F. Li, J. Gao, J. Zhang, F. Xu, J. Zhao and L. Sun, J. Mater. Chem. A, 2013, 1, 8016–8022.
65. A. Baird and J. Andrews, Int. J. Hydrogen Energy, 2023, 48, 27944–27959.
66. H. Tachikawa, Y. Izumi, T. Iyama and K. Azumi, ACS Omega, 2021, 6, 7778–7785.
67. Y.-J. Park, H. Lee, H. L. Choi, M. C. Tapia, C. Y. Chuah and T.-H. Bae, npj 2D Mater. Appl., 2023, 7, 61.
68. F. Bonaccorso, L. Colombo, G. Yu, M. Stoller, V. Tozzini, A. C. Ferrari, R. S. Ruoff and V. Pellegrini, Science, 2015, 347, 1246501.
69. A. Macili, Y. Vlamidis, G. Pfusterschmied, M. Leitgeb, U. Schmid, S. Heun and S. Veronesi, Appl. Surf. Sci., 2023, 615, 156375.
70. L. Liu, L. Jiao and X. Huang, J. Mol. Model., 2023, 29, 185.
71. S. J. Lee, J. Theerthagiri, P. Nithyadharseni, P. Arunachalam, D. Balaji, A. Madan Kumar, J. Madhavan, V. Mittal and M. Y. Choi, Renewable Sustainable Energy Rev., 2021, 143, 110849.
72. R. Kumar, S. Sahoo, E. Joanni, R. K. Singh, K. Maegawa, W. K. Tan, G. Kawamura, K. K. Kar and A. Matsuda, Mater. Today, 2020, 39, 47–65.
73. H. Aghamohammadi, N. Hassanzadeh and R. Eslami-Farsani, Ceram. Int., 2021, 47, 22269–22301.
74. H. Cui, Z. Zhou and D. Jia, Mater. Horiz., 2017, 4, 7–19.
75. R. Kumar, S. Sahoo, E. Joanni, R. K. Singh, W. K. Tan, S. A. Moshkalev, A. Matsuda and K. K. Kar, Crit. Rev. Solid State Mater. Sci., 2022, 47, 570–619.
76. X. Wang, G. Sun, P. Routh, D.-H. Kim, W. Huang and P. Chen, Chem. Soc. Rev., 2014, 43, 7067–7098.
77. N. P. D. Ngidi, M. A. Ollengo and V. O. Nyamori, Int. J. Energy Res., 2019, 43, 1702–1734.
78. Y. Xia, Z. Yang and Y. Zhu, J. Mater. Chem. A, 2013, 1, 9365–9381.
79. A. C. Lokhande, I. A. Qattan, C. D. Lokhande and S. P. Patole, J. Mater. Chem. A, 2020, 8, 918–977.
80. C.-C. Huang, N.-W. Pu, C.-A. Wang, J.-C. Huang, Y. Sung and M.-D. Ger, Sep. Purif. Technol., 2011, 82, 210–215.
81. F. Darkrim Lamari and D. Levesque, Carbon, 2011, 49, 5196–5200.
82. V. Tozzini and V. Pellegrini, Phys. Chem. Chem. Phys., 2013, 15, 80–89.
83. O. K. Alekseeva, I. V. Pushkareva, A. S. Pushkarev and V. N. Fateev, Nanotechnol. Russ., 2020, 15, 273–300.
84. G. E. Ashna, K. J. Sivasankar, C. P. Kala, R. M. Hariharan and D. J. Thiruvadigal, Mater. Today Commun., 2023, 37, 107319.
85. V. Bhaghavathi Parambath, R. Nagar and R. Sundara, J. Mater. Chem., 2013, 1, 11192–11199.
86. I. J. Ikot, P. O. Olagoke, H. Louis, D. E. Charlie, T. O. Magu and A. E. Owen, Int. J. Hydrogen Energy, 2023, 48, 13362–13376.
87. T. H. Nguyen, D. Yang, B. Zhu, H. Lin, T. Ma and B. Jia, J. Mater. Chem. A, 2021, 9, 7366–7395.
88. K. Raja K, T. Anusuya and V. Kumar, Phys. Chem. Chem. Phys., 2023, 25, 262–273.
89. J.-W. Chen, S.-H. Hsieh, S.-S. Wong, Y.-C. Chiu, H.-W. Shiu, C.-H. Wang, Y.-W. Yang, Y.-J. Hsu, D. Convertino, C. Coletti, S. Heun, C.-H. Chen and C.-L. Wu, ACS Energy Lett., 2022, 7, 2297–2303.
90. H. Cheng, L. Chen, A. C. Cooper, X. Sha and G. P. Pez, Energy Environ. Sci., 2008, 1, 338–354.
91. M. Bartolomei, M. I. Hernández, J. Campos-Martínez, R. Hernández-Lamoneda and G. Giorgi, Carbon, 2021, 178, 718–727.
92. T. K. A. Hoang and D. M. Antonelli, Adv. Mater., 2009, 21, 1787–1800.
93. P. A. Denis, ACS Omega, 2022, 7, 45935–45961.
94. J. Wang and W.-Q. Han, Adv. Funct. Mater., 2022, 32, 2107166.
95. J. Gallagher, Nat. Rev. Chem, 2018, 2, 0138.
96. S. Kaushal, M. Kaur, N. Kaur, V. Kumari and P. P. Singh, RSC Adv., 2020, 10, 28608–28629.
97. Y. S. Al-Hamdani, A. Zen, A. Michaelides and D. Alfè, Phys. Rev. Mater., 2023, 7, 035402.
98. G. J. Kubas, Science, 2006, 314, 1096–1097.
99. V. Bhaghavathi Parambath and R. Sundara, Nano Commun., 2013, 1, 1–7.
100. D. S. Pyle, E. M. Gray and C. J. Webb, Int. J. Hydrogen Energy, 2016, 41, 19098–19113.
101. V. B. Parambhath, R. Nagar, K. Sethupathi and S. Ramaprabhu, J. Phys. Chem. C, 2011, 115, 15679–15685.
102. H. Shen, H. Li, Z. Yang and C. Li, Green Energy Environ., 2022, 7, 1161–1198.
103. G. M. Psofogiannakis and G. E. Froudakis, Chem. Commun., 2011, 47, 7933–7943.
104. R. T. Yang and Y. Wang, J. Am. Chem. Soc., 2009, 131, 4224–4226.
105. F. Chen, X. Zhang, X. Guan, S. Gao, J. Hao, L. Li, Y. Yuan, C. Zhang, W. Chen and P. Lu, Appl. Surf. Sci., 2023, 622, 156895.
106. R. Muhammad, Y. Shuai and H.-P. Tan, Phys. E, 2017, 88, 115–124.
107. A. Ariharan, B. Viswanathan and V. Nandhakumar, Graphene, 2017, 6, 41–60.
108. I. López-Corral, E. Germán, A. Juan, M. A. Volpe and G. P. Brizuela, J. Phys. Chem. C, 2011, 115, 4315–4323.
109. V. B. Parambhath, R. Nagar and S. Ramaprabhu, Langmuir, 2012, 28, 7826–7833.
110. G. Kim, S.-H. Jhi and N. Park, Appl. Phys. Lett., 2008, 92, 013106.
111. S. Pal, A. Agrawal, S. K. Nippani and G. Anand, Procedia Mater. Sci., 2015, 10, 103–110.
112. H. Wang, T. Maiyalagan and X. Wang, ACS Catal., 2012, 2, 781–794.
113. Z. M. Ao, A. D. Hernández-Nieves, F. M. Peeters and S. Li, Phys. Chem. Chem. Phys., 2012, 14, 1463–1467.
114. Z. M. Ao and F. M. Peeters, J. Phys. Chem. C, 2010, 114, 14503–14509.
115. G. Kim, S.-H. Jhi, N. Park, S. G. Louie and M. L. Cohen, Phys. Rev. B: Condens. Matter Mater. Phys., 2008, 78, 085408.
116. X. Liang, S.-P. Ng, N. Ding and C.-M. L. Wu, Appl. Surf. Sci., 2019, 473, 174–181.
117. S. Lee, M. Lee and Y.-C. Chung, Phys. Chem. Chem. Phys., 2013, 15, 3243–3248.
118. Z. Ramezani and H. Dehghani, Int. J. Hydrogen Energy, 2019, 44, 13613–13622.
119. N. Sathishkumar, S.-Y. Wu and H.-T. Chen, Int. J. Energy Res., 2019, 43, 4867–4878.
120. F. Bakhshi and N. Farhadian, Int. J. Hydrogen Energy, 2019, 44, 13655–13665.
121. E. Rangel, E. Sansores, E. Vallejo, A. Hernández-Hernández and P. A. López-Pérez, Phys. Chem. Chem. Phys., 2016, 18, 33158–33170.
122. S. Ullah, Y. Liu, M. Hasan, W. Zeng, Q. Shi, X. Yang, L. Fu, H. Q. Ta, X. Lian, J. Sun, R. Yang, L. Liu and M. H. Rümmeli, Nano Res., 2022, 15, 1310–1318.
123. Y. L. He, D. X. Liu, Y. Qu and Z. Yao, Adv. Mater. Res., 2012, 507, 61–64.
124. X. Li, A. Grubisic, S. T. Stokes, J. Cordes, G. F. Ganteför, K. H. Bowen, B. Kiran, M. Willis, P. Jena, R. Burgert and H. Schnöckel, Science, 2007, 315, 356–358.
125. J. Graetz, S. Chaudhuri, Y. Lee, T. Vogt, J. T. Muckerman and J. J. Reilly, Phys. Rev. B: Condens. Matter Mater. Phys., 2006, 74, 214114.
126. Z. M. Ao, Q. Jiang, R. Q. Zhang, T. T. Tan and S. Li, J. Appl. Phys., 2009, 105, 074307.
127. A. Fukushima, A. Sawairi, K. Doi, M. Senami, L. Chen, H. Cheng and A. Tachibana, J. Phys. Soc. Jpn., 2011, 80, 074705.
128. Z. M. Ao and F. M. Peeters, Phys. Rev. B: Condens. Matter Mater. Phys., 2010, 81, 205406.
129. J. Gu, X. Zhang, L. Fu and A. Pang, Int. J. Hydrogen Energy, 2019, 44, 6036–6044.
130. Z. Ao, S. Dou, Z. Xu, Q. Jiang and G. Wang, Int. J. Hydrogen Energy, 2014, 39, 16244–16251.
131. L. Jiao, L. Zhang, X. Wang, G. Diankov and H. Dai, Nature, 2009, 458, 877–880.
132. A. Flamina, R. M. Raghavendra, A. Gupta and A. Subramaniam, Appl. Surf. Sci. Adv., 2023, 13, 100371.
133. X. Tan, H. A. Tahini and S. C. Smith, ACS Appl. Mater. Interfaces, 2016, 8, 32815–32822.
134. H.-Y. Wu, X. Fan, J.-L. Kuo and W.-Q. Deng, J. Phys. Chem. C, 2011, 115, 9241–9249.
135. L. Firlej, B. Kuchta, C. Wexler and P. Pfeifer, Adsorption, 2009, 15, 312–317.
136. R. H. Miwa, T. B. Martins and A. Fazzio, Nanotechnology, 2008, 19, 155708.
137. J. Li, X. Wang, K. Liu, Y. Sun and L. Chen, Solid State Commun., 2012, 152, 386–389.
138. J. H. Cho, S. J. Yang, K. Lee and C. R. Park, Int. J. Hydrogen Energy, 2011, 36, 12286–12295.
139. M. Ghanbari, S. Afshari and S. A. Nabavi Amri, Int. J. Hydrogen Energy, 2020, 45, 23048–23055.
140. M. D. Ganji, S. N. Emami, A. Khosravi and M. Abbasi, Appl. Surf. Sci., 2015, 332, 105–111.
141. H. Lee, J. Ihm, M. L. Cohen and S. G. Louie, Nano Lett., 2010, 10, 793–798.
142. L. Wang, K. Lee, Y.-Y. Sun, M. Lucking, Z. Chen, J. J. Zhao and S. B. Zhang, ACS Nano, 2009, 3, 2995–3000.
143. D. Grasseschi, W. C. Silva, R. d. Souza Paiva, L. D. Starke and A. S. do Nascimento, Coord. Chem. Rev., 2020, 422, 213469.
144. M. Malček, D. N. Sredojević, O. Tkáč and L. Bucinsky, Diamond Relat. Mater., 2023, 139, 110335.
145. Y. Li, Y. Mi and G. Sun, J. Mater. Sci. Chem. Eng., 2015, 03, 87–94.
146. S. Nachimuthu, P.-J. Lai, E. G. Leggesse and J.-C. Jiang, Sci. Rep., 2015, 5, 16797.
147. I. Cabria, M. J. López, S. Fraile and J. A. Alonso, J. Phys. Chem. C, 2012, 116, 21179–21189.
148. J. A. Alonso, A. Granja, I. Cabria and M. J. López, AIP Conf. Proc., 2015, 1702, 050002.
149. F. Ferrante, A. Prestianni, M. Bertini and D. Duca, Catalysts, 2020, 10, 1306.
150. S. K. Konda and A. Chen, Mater. Today, 2016, 19, 100–108.
151. C.-H. Chen, T.-Y. Chung, C.-C. Shen, M.-S. Yu, C.-S. Tsao, G.-N. Shi, C.-C. Huang, M.-D. Ger and W.-L. Lee, Int. J. Hydrogen Energy, 2013, 38, 3681–3688.
152. Z. L. Hu, Y. F. Chen, N. Li, W. Zhang, H. Chen and W. Q. Gong, Adv. Mater. Res., 2013, 772, 349–354.
153. C. Zhou and J. A. Szpunar, ACS Appl. Mater. Interfaces, 2016, 8, 25933–25940.
154. C. I. Contescu, K. van Benthem, S. Li, C. S. Bonifacio, S. J. Pennycook, P. Jena and N. C. Gallego, Carbon, 2011, 49, 4050–4058.
155. J. Huot, in Hydrogen Technology: Mobile and Portable Applications, ed. A. Léon, Springer Berlin Heidelberg, Berlin, Heidelberg, 2008, pp. 471–500,  DOI:10.1007/978-3-540-69925-5_19.
156. S. Mukherjee, B. Ramalingam and S. Gangopadhyay, J. Mater. Chem. A, 2014, 2, 3954–3960.
157. R. Bhowmick, S. Rajasekaran, D. Friebel, C. Beasley, L. Jiao, H. Ogasawara, H. Dai, B. Clemens and A. Nilsson, J. Am. Chem. Soc., 2011, 133, 5580–5586.
158. V. P. Ting, A. J. Ramirez-Cuesta, N. Bimbo, J. E. Sharpe, A. Noguera-Diaz, V. Presser, S. Rudic and T. J. Mays, ACS Nano, 2015, 9, 8249–8254.
159. B. P. Vinayan, K. Sethupathi and S. Ramaprabhu, Trans. Indian Inst. Met., 2011, 64, 169.
160. I. López-Corral, E. Germán, A. Juan, M. A. Volpe and G. P. Brizuela, Int. J. Hydrogen Energy, 2012, 37, 6653–6665.
161. R. Kumar, J.-H. Oh, H.-J. Kim, J.-H. Jung, C.-H. Jung, W. G. Hong, H.-J. Kim, J.-Y. Park and I.-K. Oh, ACS Nano, 2015, 9, 7343–7351.
162. N. Pantha, A. Khaniya and N. P. Adhikari, Int. J. Mod. Phys. B, 2015, 29, 1550143.
163. W. Tian, Y. Zhang, Y. Wang, T. Liu and H. Cui, Int. J. Hydrogen Energy, 2020, 45, 12376–12383.
164. C. M. Ramos-Castillo, J. U. Reveles, R. R. Zope and R. de Coss, J. Phys. Chem. C, 2015, 119, 8402–8409.
165. B. P. Vinayan, R. Nagar, N. Rajalakshmi and S. Ramaprabhu, Adv. Funct. Mater., 2012, 22, 3519–3526.
166. B. Vinayan, K. Sethupathi and S. Ramaprabhu, J. Nanosci. Nanotechnol., 2012, 12, 6608–6614.
167. B. P. Vinayan, R. Nagar and S. Ramaprabhu, J. Mater. Chem. A, 2013, 1, 11192–11199.
168. J. Li, C. Jin, R. Qian, C. Wu, Y. Wang, Y. Yan and Y. Chen, Prog. Nat. Sci.: Mater. Int., 2021, 31, 514–520.
169. O. Faye, J. A. Szpunar, B. Szpunar and A. C. Beye, Appl. Surf. Sci., 2017, 392, 362–374.
170. L. Ma, J.-M. Zhang and K.-W. Xu, Appl. Surf. Sci., 2014, 292, 921–927.
171. Habibullah, W. Cen, Y. Wang, Y. Yan, Y. Chen and C. Wu, Int. J. Hydrogen Energy, 2024, 50, 659–669.
172. M. Zhou, Y. Lu, C. Zhang and Y. P. Feng, Appl. Phys. Lett., 2010, 97, 103109.
173. S. Li, H.-m. Zhao and P. Jena, Front. Phys., 2011, 6, 204–208.
174. Z. Gohari Bajestani and Y. Yurum, Progress in Clean Energy, 2015, vol. 2, pp. 863–871.
175. Y. Guo, J. Cao, B. Xu, Y. Xia, J. Yin and Z. Liu, Comput. Mater. Sci., 2013, 68, 61–65.
176. L. Yuan, L. Kang, Y. Chen, D. Wang, J. Gong, C. Wang, M. Zhang and X. Wu, Appl. Surf. Sci., 2018, 434, 843–849.
177. Y. Liu, L. Ren, Y. He and H.-P. Cheng, J. Phys.: Condens. Matter, 2010, 22, 445301.
178. D. M. P. Mingos, J. Organomet. Chem., 2001, 635, 1–8.
179. B. Chakraborty, P. Ray, N. Garg and S. Banerjee, Int. J. Hydrogen Energy, 2021, 46, 4154–4167.
180. Y. Tan, X. Tao, Y. Ouyang and Q. Peng, Int. J. Hydrogen Energy, 2023, 50, 738–748.
181. A. Lebon, J. Carrete, L. J. Gallego and A. Vega, Int. J. Hydrogen Energy, 2015, 40, 4960–4968.
182. K. Ma, E. Lv, D. Zheng, W. Cui, S. Dong, W. Yang, Z. Gao and Y. Zhou, Energies, 2021, 14, 6845.
183. N. Luhadiya, S. I. Kundalwal and S. K. Sahu, Appl. Phys. A: Mater. Sci. Process., 2022, 128, 49.
184. Y. Huo, Y. Zhang, C. Wang, Y. Fang, K. Li and Y. Chen, Int. J. Hydrogen Energy, 2021, 46, 40301–40311.
185. R. Intayot, C. Rungnim, S. Namuangruk, N. Yodsin and S. Jungsuttiwong, Dalton Trans., 2021, 50, 11398–11411.
186. N. Ismail, M. Madian and M. S. El-Shall, J. Ind. Eng. Chem., 2015, 30, 328–335.
187. C. Zhou, J. A. Szpunar and X. Cui, ACS Appl. Mater. Interfaces, 2016, 8, 15232–15241.
188. L. Wei and Y. Mao, Int. J. Hydrogen Energy, 2016, 41, 11692–11699.
189. Y. Chen, Habibullah, G. Xia, C. Jin, Y. Wang, Y. Yan, Y. Chen, X. Gong, Y. Lai and C. Wu, Inorganics, 2023, 11, 251.
190. S. Y. Lee and S. J. Park, J. Nanosci. Nanotechnol., 2013, 13, 443–447.
191. A. Sigal, M. I. Rojas and E. P. M. Leiva, Int. J. Hydrogen Energy, 2011, 36, 3537–3546.
192. C. M. Ramos-Castillo, J. U. Reveles, M. E. Cifuentes-Quintal, R. R. Zope and R. de Coss, J. Phys. Chem. C, 2016, 120, 5001–5009.
193. D. Puthusseri and S. Ramaprabhu, Phys. Chem. Chem. Phys., 2014, 16, 26725–26729.
194. H. Jung, K. T. Park, M. N. Gueye, S. H. So and C. R. Park, Int. J. Hydrogen Energy, 2016, 41, 5019–5027.
195. N. Kostoglou, C.-W. Liao, C.-Y. Wang, J. N. Kondo, C. Tampaxis, T. Steriotis, K. Giannakopoulos, A. G. Kontos, S. Hinder, M. Baker, E. Bousser, A. Matthews, C. Rebholz and C. Mitterer, Carbon, 2021, 171, 294–305.
196. E. Hadji Oumar Gueye, A. Ndiaye Dione, A. Dioum, B. Modou Ndiaye, P. Douta Tall and A. Chédikh Beye, Am. J. Nanomater., 2019, 7, 30–38.
197. S. Kureshi, A. Tokarev, M. Cannon, G. Quan and E. Kjeang, ECS Meeting Abstracts, 2017, MA2017-01, p. 780.
198. L. Yuan, D. Wang, J. Gong, C. Zhang, L. Zhang, M. Zhang, X. Wu and L. Kang, Chem. Phys. Lett., 2019, 726, 57–61.
199. S. Nachimuthu, L. He, H.-J. Cheng, R. D. Tiono and J.-C. Jiang, Sustainable Energy Fuels, 2021, 5, 2159–2168.
200. O. Dyck, M. Yoon, L. Zhang, A. R. Lupini, J. L. Swett and S. Jesse, ACS Appl. Nano Mater., 2020, 3, 10855–10863.
201. J. I. G. Enriquez and A. R. C. Villagracia, Int. J. Hydrogen Energy, 2016, 41, 12157–12166.
202. C. Xiang, A. Li, S. Yang, Z. Lan, W. Xie, Y. Tang, H. Xu, Z. Wang and H. Gu, RSC Adv., 2019, 9, 25690–25696.
203. A. Choudhary, L. Malakkal, R. K. Siripurapu, B. Szpunar and J. Szpunar, Int. J. Hydrogen Energy, 2016, 41, 17652–17656.
204. O. Faye, U. Eduok, J. Szpunar, B. Szpunar, A. Samoura and A. Beye, Int. J. Hydrogen Energy, 2017, 42, 4233–4243.
205. M. Sterlin Leo Hudson, H. Raghubanshi, S. Awasthi, T. Sadhasivam, A. Bhatnager, S. Simizu, S. G. Sankar and O. N. Srivastava, Int. J. Hydrogen Energy, 2014, 39, 8311–8320.
206. S. Desnavi, B. Chakraborty and L. M. Ramaniah, AIP Conf. Proc., 2014, 1591, 1775–1777.
207. W. Liu, Y. Liu and R. Wang, Appl. Surf. Sci., 2014, 296, 204–208.
208. B. Chakraborty, A. Vaidyanathan, M. Kandasamy, V. Wagh and S. Sahu, J. Appl. Phys., 2022, 132, 065002.
209. A. Vaidyanathan, M. Kandasamy, L. M. Ramaniah, V. Wagh and B. Chakraborty, Int. J. Hydrogen Energy, 2024, 52, 376–389.
210. M. Wu, Y. Gao, Z. Zhang and X. C. Zeng, Nanoscale, 2012, 4, 915–920.
211. Y.-H. Chen, J. Wang, Y. Lihua, M. Zhang and C.-R. Zhang, Materials, 2017, 10, 894.
212. J. Wang, Y. Chen, L. Yuan, M. Zhang and C. Zhang, Molecules, 2019, 24, 2382.
213. H. Cui, W. Tian, Y. Zhang, T. Liu, Y. Wang, P. Shan, Y. Chen and H. Yuan, Int. J. Hydrogen Energy, 2020, 45, 33789–33797.
214. A. Yadav, B. Chakraborty, A. Gangan, N. Patel, M. R. Press and L. M. Ramaniah, J. Phys. Chem. C, 2017, 121, 16721–16730.
215. A. S. Shajahan, N. Kalarikkal, N. Garg, Y. Kawazo and B. Chakraborty, Int. J. Hydrogen Energy, 2022, 47, 36190–36203.
216. H. T. Nair, P. K. Jha and B. Chakraborty, Int. J. Hydrogen Energy, 2023, 48, 37860–37871.
217. R. Karde and B. Lone, Int. J. Sci. Res. Sci. Technol., 2023, 9, 238–245.
218. O. Faye and J. A. Szpunar, J. Phys. Chem. C, 2018, 122, 28506–28517.
219. A. C. F. Serraon, J. A. D. Del Rosario, P.-Y. Abel Chuang, M. N. Chong, Y. Morikawa, A. A. B. Padama and J. D. Ocon, RSC Adv., 2021, 11, 6268–6283.
220. A. C. F. Serraon, A. A. B. Padama, J. A. D. del Rosario and J. D. Ocon, ECS Trans., 2017, 77, 629.
221. K. Gopalsamy and V. Subramanian, Int. J. Hydrogen Energy, 2014, 39, 2549–2559.
222. H. Luo, H. Li and Q. Fu, Chem. Phys. Lett., 2017, 669, 238–244.
223. Y. Liu, Y. Zhou, S. Yang, H. Xu, Z. Lan, J. Xiong, Z. Wang and H. Gu, Int. J. Hydrogen Energy, 2021, 46, 5891–5903.
224. D. Li, Y. Ouyang, J. Li, Y. Sun and L. Chen, Solid State Commun., 2012, 152, 422–425.
225. M. Mahendran, B. Rekha, S. Seenithurai, R. K. Pandyan and S. V. Kumar, Funct. Mater. Lett., 2017, 10, 1750023.
226. Y. Gao, Z. Li, P. Wang, W.-G. Cui, X. Wang, Y. Yang, F. Gao, M. Zhang, J. Gan, C. Li, Y. Liu, X. Wang, F. Qi, J. Zhang, X. Han, W. Du, J. Chen, Z. Xia and H. Pan, Nat. Commun., 2024, 15, 928.
227. V. Wang, H. Mizuseki, H. P. He, G. Chen, S. L. Zhang and Y. Kawazoe, Comput. Mater. Sci., 2012, 55, 180–185.
228. C. Ataca, E. Aktürk and S. Ciraci, Phys. Rev. B: Condens. Matter Mater. Phys., 2009, 79, 041406.
229. Y. Gao, N. Zhao, J. Li, E. Liu, C. He and C. Shi, Int. J. Hydrogen Energy, 2012, 37, 11835–11841.
230. E. Beheshti, A. Nojeh and P. Servati, Carbon, 2011, 49, 1561–1567.
231. L. Ma, J.-M. Zhang, K.-W. Xu and V. Ji, Phys. E, 2014, 63, 45–51.
232. J. Kim, H. Kim, J. Kim, H. Bae, A. Singh, T. Hussain and H. Lee, ACS Appl. Energy Mater., 2023, 6, 6807–6813.
233. M. Cheng, D. Chen, R. Chen, W. Liu, Q. Lin and Z. Zhu, Int. J. Hydrogen Energy, 2023, 48, 34164–34179.
234. E. S. Cho, A. M. Ruminski, S. Aloni, Y.-S. Liu, J. Guo and J. J. Urban, Nat. Commun., 2016, 7, 10804.
235. Z. Amaniseyed and Z. Tavangar, Int. J. Hydrogen Energy, 2019, 44, 3803–3811.
236. C. Chen, J. Zhang, B. Zhang and H. Ming Duan, J. Phys. Chem. C, 2013, 117, 4337–4344.
237. B. Lone, Int. J. Res. Appl. Sci. Eng. Technol., 2020, 8, 181–190.
238. L. Tang, S. Shi, C. Yao, S. Zhang, Y. Liu, Z. Duan, J. Jiang and D. Chen, Appl. Surf. Sci., 2024, 648, 159078.
239. T. Hussain, M. Hankel and D. J. Searles, J. Phys. Chem. C, 2017, 121, 14393–14400.
240. P. Panigrahi, S. R. Naqvi, M. Hankel, R. Ahuja and T. Hussain, Appl. Surf. Sci., 2018, 444, 467–473.
241. C. Ataca, E. Aktürk, S. Ciraci and H. Ustunel, Appl. Phys. Lett., 2008, 93, 043123.
242. M.-M. Zhang, F. Zhang, Q. Wu, X. Huang, W. Yan, C.-M. Zhao, W. Chen, Z.-H. Yang, Y.-H. Wang and T.-T. Wu, Chin. Phys. B, 2023, 32, 066803.
243. J. Dewangan, V. Mahamiya, A. Shukla and B. Chakraborty, Int. J. Hydrogen Energy, 2023, 48, 37908–37920.
244. S. Seenithurai, R. K. Pandyan, S. V. Kumar, C. Saranya and M. Mahendran, Int. J. Hydrogen Energy, 2014, 39, 11016–11026.
245. H. Tachikawa and T. Iyama, J. Phys. Chem. C, 2019, 123, 8709–8716.
246. W. Zhou, J. Zhou, J. Shen, C. Ouyang and S. Shi, J. Phys. Chem. Solids, 2012, 73, 245–251.
247. H. An, C.-s. Liu, Z. Zeng, C. Fan and X. Ju, Appl. Phys. Lett., 2011, 98, 173101.
248. Y. Wang, Z. Meng, Y. Liu, D. You, K. Wu, J. Lv, X. Wang, K. Deng, D. Rao and R. Lu, Appl. Phys. Lett., 2015, 106, 063901.
249. F. Wang, T. Zhang, X. Hou, W. Zhang, S. Tang, H. Sun and J. Zhang, Int. J. Hydrogen Energy, 2017, 42, 10099–10108.
250. E. Tylianakis, G. M. Psofogiannakis and G. E. Froudakis, J. Phys. Chem. Lett., 2010, 1, 2459–2464.
251. A. Du, Z. Zhu and S. C. Smith, J. Am. Chem. Soc., 2010, 132, 2876–2877.
252. N. Zheng, S. Yang, H. Xu, Z. Lan, Z. Wang and H. Gu, Vacuum, 2020, 171, 109011.
253. Z. Öztürk, Int. J. Hydrogen Energy, 2021, 46, 11804–11814.
254. Y. Zhou, W. Chu, F. Jing, J. Zheng, W. Sun and Y. Xue, Appl. Surf. Sci., 2017, 410, 166–176.
255. E. Eisapour, S. M. Hashemianzadeh and S. Ketabi, Appl. Chem., 2016, 10, 63–70.
256. X.-J. Ye, C.-S. Liu, W. Zhong, Z. Zeng and Y.-W. Du, J. Appl. Phys., 2014, 116, 114304.
257. V. Mahamiya, A. Shukla and B. Chakraborty, Int. J. Hydrogen Energy, 2023, 48, 37898–37907.
258. Y.-F. Zhang and J. Guo, Int. J. Hydrogen Energy, 2024, 50, 1004–1014.
259. H. Tachikawa, H. Yi, T. Iyama, S. Yamasaki and K. Azumi, Hydrogen, 2022, 3, 43–52.
260. N. Pantha, K. Belbase and N. P. Adhikari, Appl. Nanosci., 2015, 5, 393–402.
261. A. Tapia, C. Acosta, R. A. Medina-Esquivel and G. Canto, Comput. Mater. Sci., 2011, 50, 2427–2432.
262. A. Arjunan, B. Viswanathan and V. Nandhakumar, Graphene, 2016, 5, 39–50.
263. Y. Chen, Habibullah, G. Xia, C. Jin, Y. Wang, Y. Yan, Y. Chen, X. Gong, Y. Lai and C. Wu, Materials, 2023, 16, 4219.
264. C. Jin, J. Li, K. Zhang, Habibullah, G. Xia, C. Wu, Y. Wang, W. Cen, Y. Chen, Y. Yan and Y. Chen, Nano Energy, 2022, 99, 107360.
265. Y. Wang, C. X. Guo, X. Wang, C. Guan, H. Yang, K. Wang and C. M. Li, Energy Environ. Sci., 2011, 4, 195–200.
266. Y. Wang, J. Liu, K. Wang, T. Chen, X. Tan and C. M. Li, Int. J. Hydrogen Energy, 2011, 36, 12950–12954.
267. S. S. Samantaray, P. Anees, V. Bhaghavathi Parambath and R. S, Acta Mater., 2021, 215, 117040.
268. S. S. Samantaray, V. Sangeetha, S. Abinaya and S. Ramaprabhu, Int. J. Hydrogen Energy, 2018, 43, 8018–8025.
269. X. Li, S. Sun, J. Zhang, K. Luo, P. Gao, T. Wu, S. Du, Y. Wang, X. Zhou, L. Sha, Y. Yang, P. Yang, Y. Wang and Y. Chen, RSC Adv., 2016, 6, 93238–93244.
270. P. Pei, M. B. Whitwick, W. L. Sun, G. Quan, M. Cannon and E. Kjeang, Nanoscale, 2017, 9, 4143–4153.
271. C. Liu, D. Shen, Z. Tu and S. Li, Int. J. Hydrogen Energy, 2022, 47, 5393–5402.
272. D. Rout, P. Senapati, H. Sutar, D. Chandra and R. Murmu, 2019,  DOI:10.4236/graphene.2019.83003.
273. T. Hussain, B. Pathak, T. Adit Maark, C. Moyses Araujo, R. H. Scheicher and R. Ahuja, EPL, 2011, 96, 27013.
274. L. Bi, Z. Miao, Y. Ge, Z. Liu, Y. Xu, J. Yin, X. Huang, Y. Wang and Z. Yang, Int. J. Hydrogen Energy, 2022, 47, 32552–32564.

---