Recent development in catalysts for the production, storage and release of hydrogen

Abstract

The rising demand for energy and growing environmental concerns have intensified the search for sustainable alternatives to fossil fuels, establishing hydrogen as a promising clean energy carrier. However, its widespread adoption is currently limited by its reliance on a fossil-based production method, as well as by persistent challenges in storage and transport. Recent advancements in CO₂ hydrogenation, catalytic design, and machine learning-driven optimisation offer ways to enhance both the sustainability and efficiency of hydrogen production. Among these advancements, formic acid (FA) is emerging as a sustainable energy source due to its ease of handling compared to solid or gaseous materials. This review explores the development of state-of-the-art catalysts for CO₂ hydrogenation and FA dehydrogenation, contributing to the advancement of a hydrogen-based economy. It also delves into the methanol economy as an alternative strategy. Particular emphasis has been placed on transition metal-based complexes, which are recognised for their high catalytic activity associated with a well-defined mechanism, including Pd-based heterogeneous and non-noble metal homogeneous catalysts. Furthermore, this review demonstrates how machine learning can accelerate catalyst innovation. By addressing the key challenges of hydrogen storage, efficiency, and scalability, this review contributes to the development of practical, cost-effective, and environmentally friendly hydrogen energy solutions.

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Introduction

The surge in global population and economic expansion, coupled with rising living standards and efforts to replace conventional crude oil due to environmental concerns, has ignited a global debate on energy sustainability.^1,2 Although coal remains a key component of the global economy as a primary energy source, significantly reducing coal consumption is essential for mitigating climate change as it is a leading contributor to anthropogenic CO₂ emissions.³ Various strategies are being pursued to lower CO₂ emissions and reduce dependence on fossil fuels.^4–6 Since the advent of catalysis, chemists have explored the molecular conversion of CO₂ into value-added products.^7–10 Recent data show that fossil fuels account for 85% of the world's energy supply, with petroleum contributing 34%, coal 27%, and natural gas 24%. Alternative energy sources, including solar, wind, ocean (tidal, wave, and thermal), biomass, nuclear, and geothermal, provide the additional capacity.^11,12 As of 2025, nuclear power continues to supply approximately 9% of the world's electricity. Although its share has declined slightly in recent years, nuclear energy remains a significant source of low-carbon electricity, contributing to global energy needs – though it does not dominate the energy market.¹³ Moving toward a sustainable and eco-friendly future will require the use of a mix of fossil fuels and multiple renewable energy sources.¹⁴

Hydrogen (H₂) gas is a storable energy carrier that can be used on demand, offering a practical solution to address fluctuations in renewable energy availability.^15,16 It has the potential to drive the global transition toward a sustainable, low-carbon future by integrating renewable energy sources and mitigating their intermittency.^17,18 Among the different types of H₂, “green” H₂ is particularly promising for minimizing environmental impact.^16,19 For instance, using H₂ in fuel cells produces only water as a byproduct. However, a shift in global H₂ production from fossil fuel-based methods (primarily natural gas) to processes using water or biomass as feedstock is imperative.²⁰

A key challenge associated with H₂ is its low density: although molecular H₂ has the highest energy density by weight (120 MJ kg⁻¹), its energy density by volume is one of the lowest (0.0108 MJ L⁻¹).²¹ This makes efficient handling and storage essential for its widespread use as an energy source. Consequently, the practical development of H₂ as an energy carrier faces significant challenges related to production, storage, transportation, and conversion. Developing safe, cost-effective, and efficient H₂ production and storage technologies remains a primary goal of researchers and is crucial for establishing a sustainable clean energy system. Currently, most industrial H₂ storage depends on high-pressure gaseous or low-temperature liquid H₂, both of which require specialized equipment and raise safety concerns.²²

Chemical compounds with a high H₂ content are stable, cost-effective, and easy to transport, making them attractive for H₂ storage. Among these, liquid organic hydrogen carrier (LOHC) systems show strong potential for large-scale adoption because they generally entail lower costs and energy consumption, provide higher H₂ storage density and improved storage and transportation efficiency compared to high-pressure gaseous storage.^23–25 However, developing safe and cost-effective H₂ storage suitable for long-distance transport remains a significant challenge for realising the H₂ economy.^26,27 Ideal H₂ storage materials should exhibit high gravimetric capacity, low viscosity, and the ability to produce high-purity gas. They should also enable reversible H₂ storage and release, remain stable throughout charge–discharge cycles, and be non-toxic.

Currently, around 48% of H₂ is produced from natural gas, 30% from heavy oils and naphtha, 18% from coal, and only 4% via water electrolysis (WE).^28,29 Fossil fuel-based H₂ production significantly contributes to CO₂ emissions and environmental acidification, while thermochemical cycles pose additional ecological risks. Achieving the widespread adoption of H₂ demands both advancements in production technologies and a reduction in costs.³⁰ Consequently, current research focuses on economically viable, practical, reliable, sustainable, and environmentally friendly H₂ production methods. By integrating with renewable energy sources and mitigating their intermittency, H₂ can support the global transition to a low-carbon future.^19,31

With a focus on feedstock costs, which ultimately determine the pricing of hydrogenation products together with catalytic processes, we begin with a concise overview of the main CO₂ capture technologies from point sources.³² We then address sustainable H₂ production methods before examining the history and current state of various catalysts for the direct hydrogenation of CO₂ into valuable chemicals and energy carriers. Numerous studies have explored the use of CO₂ as a feedstock under photoirradiation and electrochemical conditions, as well as in the production of syngas by reforming natural gas with CO₂ to generate molecules such as CO and formic acid (FA). Nevertheless, all alternatives face technological challenges compared to commercial production, particularly CO₂ electroreduction, which remains far from industrial applications. In this review, we discuss recent achievements in H₂ production and storage, highlight the essential developmental steps needed to establish a formic acid-based H₂ economy, and consider the potential challenges ahead.

Hydrogen production and storage technologies

Conventional H₂ production methods rely on fossil fuels, resulting in substantial CO₂ emissions and creating an unsustainable scenario for future energy systems.¹¹ Although current industrial H₂ production from natural gas reforming or coal gasification is cost-effective (around 1 USD kg⁻¹ H₂),³³ molecular H₂ can be obtained from multiple feedstocks, including fossil fuels, biomass, and water. Each of these requires a continuous and abundant energy source.³⁴ The choice of H₂ production method largely depends on whether fossil fuels or renewable energy sources are used as the raw materials. Hydrocarbon reforming and pyrolysis remain the primary approaches for converting fossil fuel to H₂ and currently satisfy most of the demand for H₂.²⁷ H₂ production at the megawatt scale is achievable through alkaline WE.³⁵ However, electrolysis is generally more expensive than fossil fuel-based methods due to the associated costs of Pt catalyst electrodes, water purification, and electricity consumption. Solar power plants can provide the energy for water splitting, enabling sustainable electrolysis. Environmentally friendly WE technologies, such as proton-exchange membranes, solid oxide electrolysers, and carbon-assisted electrolysis, are under evaluation.³⁶ Industrial-scale polymer electrolyte membrane electrolysis requires further improvements in stack design, noble-metal loading, and long-term stability. Solid oxide electrolysers can achieve 100% faradaic efficiency for H₂ production, indicating their industrial potential, if ceramics and steam/hydrogen electrodes can withstand high temperatures. Electrolytes, electrodes, and supports must remain chemically stable under both oxidising and reducing conditions at elevated temperatures.³⁷

H₂ can be generated from both renewable technologies and fossil fuels. Fossil fuel-based options include steam reforming, partial oxidation, autothermal oxidation, and gasification. Renewable pathways include biomass or biofuel gasification and water splitting powered by solar or wind energy. Biological H₂ production, performed under ambient conditions, lowers energy requirement and is attracting increasing attention. It also promotes waste recycling by leveraging renewable energy and waste feedstocks. The five main biological processes, direct biophotolysis, indirect biophotolysis, the biological water gas shift (WGS) reaction, photo-fermentation, and fermentation, each offer distinct advantages and disadvantages. A concise summary of the key H₂ production methods can be found in Table 1.

Table 1 Assessment of various hydrogen production methodologies

Process	Advantages	Disadvantages	Efficiency (%)
Steam reforming	(i) Primarily developed for industrial processes with existing infrastructure	CO2 byproduct, depending on fossil fuels	70–85
	(ii) Does not require oxygen
	(iii) Operates at low temperatures
	(iv) Provides a favorable H2/CO ratio
Partial oxidation	(i) Low desulphurization requirements	(v) Low H2/CO ratio	60–75
	(ii) No catalyst needed	(vi) High operating temperature
	(iii) Low methane slip	(vii) Complex handling process
	(iv) Utilizes existing infrastructure and technology
Autothermal reforming	(i) Lower process temperature compared with that required for partial oxidation method	(i) Requires air and O2	60–75
	(ii) Low methane slip	(ii) New technology
	(iii) Utilizes existing infrastructure and technology
Biomass pyrolysis	(i) Abundant and cheap feedstock	(i) Tar formation	35–50
	(ii) CO2 neutral	(ii) H2 content depends on feedstock availability and impurities
Biomass gasification	(i) Abundant and inexpensive feedstock	(i) Tar formation	35–50
	(ii) CO2 neutral	(ii) H2 content depends on feedstock availability and impurities
Bio-photolysis	(i) Byproduct is O2; CO2 is consumed	(i) Requires sunlight	0.5–10
	(ii) Operates under mild conditions	(ii) Low H2 rates and yields
	(iii) Abundant supply	(iii) Expensive raw material
		(iv) O2 sensitive
		(v) Requires large reactor volume
Dark fermentation	(i) Produce H2 without light	(vii) Fatty acid removal	60–80
	(ii) Contribute to waste recycling	(viii) Low H2 yields and rates
	(iii) No O2 limitation	(ix) Low conversion efficiency
	(iv) CO2-neutral	(x) Requires large reactor volume
	(v) Simple reactor	(xi) Low COD (chemical oxygen demand
	(vi) High production rates
Photo-fermentation	(i) Uses different organic wastes and wastewaters	(i) Requires sunlight	0.1
	(ii) CO2 neutral	(ii) Low H2 rates and yields
	(iii) Contributes to waste recycling	(iii) Low conversion efficiency
	(iv) Nearly complete substrate conversion	(iv) O2 sensitive
		(v) Requires large reactor volume
Electrolysis	(i) Absence of pollution with renewable source	(i) Efficiency very low with high cost	40–60
	(ii) Established technology and existing infrastructure
	(iii) Sufficient feedstock
	(iv) Oxygen as the sole byproduct
	(v) Facilitates the integration of renewable energy sources as an electricity storage solution
Photo-electrolysis	(i) Abundant feedstock	(i) Low conversion efficiency	0.06
	(ii) Emission-free	(ii) Requires sunlight
	(iii) Byproduct is O2	(iii) Non-effective photocatalytic materials

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Hydrogen generation from renewable sources

Recently, renewable sources of H₂ have become more significant in the production of green H₂. Reducing the global carbon foot print is essential to limiting the rise in global temperature to below 2 °C, as outlined by international climate goals.³⁸ Water is the most abundant H₂ source and can be split into oxygen and H₂ without producing any harmful byproducts, provided a clean energy input is available. Electrolysis is one of the most straightforward and widely studied methods for water splitting. Alternative techniques such as such as thermolysis, photoelectrolysis, and biophotolysis also offer promising pathways for sustainable H₂ production. This section provides a concise overview of these environmentally friendly technologies for H₂ generation.

Electrolysis

H₂ production via water splitting technologies can be broadly categorised into four main types utilizing either renewable or conventional electrical sources: proton exchange membrane electrolysis (PEM),³⁹ solid oxide electrolysis (SOE),⁴⁰ alkaline water electrolysis (AWE),⁴¹ and anion-exchange membrane electrolysis (AEM).⁴² Among these, AWE is the oldest and most industrialised method due to its simplicity, reliability, and relatively low cost.⁴³ Currently, only a small portion – approximately 4% – of global H₂ production is derived from water electrolysis. Nevertheless, electrolysis remains a promising and efficient method for producing high-purity H₂. Operational costs account for up to 80% of the total costs of H₂ production,⁴⁴ and in addition, electrolysers require sufficient space for effective operation and typically consist of an electrolyte, an asbestos diaphragm, a cathode and an anode. At low temperatures, it is essential to maintain pressure equilibrium between the anode and cathode compartments to prevent the cross-diffusion of H₂ and O₂ gases.

PEM electrolysis technology offers several advantages, including high conversion efficiency and ease of integration with renewable energy sources. However, the high cost of precious metal catalysts, such as platinum and iridium, remains a significant drawback. PEM electrolysis cells are compact and well-suited to being coupled with solar, wind, and photovoltaic systems, making them a promising solution for decentralised green H₂ production. The core components of a PEM system include a polymer-based cation-exchange membrane and an electrolytic cell. Despite these benefits, achieving scalability for large-scale industrial applications remains a major challenge.^45–47

SOECs require specialised materials due to the high temperatures and pressures at which they operate. Cathodes are usually made of nickel-based cermets, while anodes are typically composed of perovskite materials that incorporate rare-earth elements and nickel oxide.⁴⁸

In contrast, AEM electrolysis operates at low temperatures and is still in the process of being developed. This technology uses an AEM for ion conduction and shares several operational similarities with PEM electrolysers. In AEM systems, water is introduced at the cathode, where fast activation occurs. One of the main advantages of AEM electrolysis is its potential for using metal electrocatalysts, similar to those employed in AWE, thereby reducing overall system costs. In contrast, PEM electrolysers typically rely on precious metals such as platinum and iridium. Considerable research has focused on developing efficient and stable AEM-compatible electrocatalysts. In alkaline environments, NiFe-layered double hydroxide (LDH)-based materials have emerged as some of the most promising candidates for the oxygen evolution reaction (OER).^49,50

Single-atom catalysis (SAC) is a highly promising approach in catalysis, offering exceptional catalytic activity, selectivity, and cost effectiveness.⁵¹ SACs utilise isolated metal atoms dispersed on suitable supports, thereby maximising atomic utilisation and enabling precise control over reaction pathways. Platinum-based SACs are widely recognised as the most effective electrocatalysts for the H₂ evolution reaction (HER).⁵² However, the high cost and limited availability of platinum hinder the large-scale commercialisation of such systems. In order to enable a sustainable H₂ economy, it is critical to develop active, stable, and cost-effective electrocatalysts for water splitting. SACs have demonstrated exceptional performance in various conventional heterogeneous catalytic reactions and are increasingly being investigated for use in electrochemical applications. Notably, SACs bridge the gap between homogeneous and heterogeneous catalysis by providing well-defined, isolated active sites on solid supports. This unique feature enhances their potential for industrial-scale applications, establishing SACs as a transformative technology in advancing next-generation energy systems.

Photocatalysis system

Photocatalytic water splitting is an effective method for producing H₂ that involves a semiconductor photocatalyst absorbing light and generating electro–hole pairs to drive redox reactions. A landmark discovery in this field was made in 1972 by Fujishima and Honda, who demonstrated that H₂ could be produced via photocatalysis using TiO₂ electrode under ultraviolet light.⁵³ The ability of photocatalytic systems to convert solar energy directly into chemical energy makes this technology promising for producing green H₂. In addition to water, researchers have explored alternative feedstocks such as alcohols and biomass for photocatalytic H₂ generation.^54–56 Various strategies have been employed to enhance the efficiency of these systems, including catalyst engineering, bandgap tuning, and surface modification. However, large-scale commercialisation remains limited, partly due to the low radiation efficiency of available light sources and the lack of optimised reactor designs. Further research into reactor and device engineering is essential to realise practical applications.

Metal–organic frameworks (MOFs) have recently emerged as a promising class of material for use in photocatalytic applications. Current efforts focus on constructing Z-scheme heterojunctions within MOFs in order to emulate natural photosynthesis. This approach aims to improve light absorption, spatially separate reductive and oxidative sites spatially, and retain strong redox capabilities, thereby enhancing the overall photocatalytic performance of MOFs.^57–60

Hydrogen storage system

Bockris first proposed H₂ as an energy storage medium in 1972, though handling limitations posed major challenges at the time.⁶¹ Since then, the safe and efficient storage, handling, and distribution of H₂ have emerged as critical barriers to developing a sustainable H₂-based economy. Conventional storage technologies, such as high-pressure compression and/or cryogenic liquefaction, involve trade-offs between storage density and efficiency.⁶² In response, ambient liquid-phase H₂ storage via chemical hydrides, such as borohydrides, hydrazine, and FA, has attracted growing interest.^39,63 A “physical storage system” typically refers to three technologies: compressed gaseous H₂ at 35–70 MPa and ambient temperature, liquid H₂ at 0.1–1 MPa and approximately −253 °C, and cryo-adsorption on high-surface-area materials at 0.2–0.5 MPa and approximately −193 °C (Fig. 1). In these systems, there are no significant chemical bonds, such as covalent or ionic interactions, between H₂ and the host compound. However, interstitial metallic hydrides are occasionally included in this category. The most advanced physical storage technologies are compressed gaseous and liquid H₂; most fuel-cell-powered car prototypes use one of these systems,^64–66 with 70-MPa compressed gaseous H₂ being considered the leading technology. Recent advancements in electric vehicle technology require a range of at least 500 km, demanding 5–6 kg of H₂ stored in a 35–70-MPa system.^64,67 However, due to insufficient energy content makes lower levels of storage are impractical. When pressures exceed 70 MPa, the gas behaviour deviates significantly, making the increased demand on pressure vessels disproportionate to the marginal increase in energy content.⁶²

Fig. 1 Schematic overview of technologies for hydrogen production, transportation, storage, and utilisation. This figure is reproduced from ref. 17 with permission from Elsevier, copyright 2024.

The low density of H₂ presents a significant challenge for vehicle integration, even at a compression of 70 MPa (Fig. 2). Cylindrical vessels are required for type I and II, as well as for the most other storage systems. The energy density per volume, pressure vessel cost and H₂ production rate limit these H₂ technologies. Because of its low density, H₂ must be stored in high-pressure vessels. Compressed and cryogenic tanks are more expensive due to the use of composite materials, such as carbon fibre, which account for 75% of the tank's cost. There is also high energy consumption during compression and liquefaction.⁶⁸ In addition, safety concerns arise when using these vessels in densely populated areas.⁶⁹

Fig. 2 H₂ density in liquid H₂ systems under various conditions. Reproduced from ref. 62 with permission from Royal Society of Chemistry, copyright 1999.

Metal hydrides for hydrogen storage

Metal hydrides (MHs) have attracted significant interest as a potential material for H₂ storage,^27,70 with a storage capacity ranging from 5% to 7%. The strong interaction between MHs and H₂ necessitates a temperature range of 120–200 °C for H₂ desorption. Fig. 3 describes the key advantages of MHs. MHs are generally classified into two main types: metallic and complex hydrides. These hydrides consist of two distinct elements: A- and B-elements. The A-element in MHs forms a stable hydride and is typically composed of a rare earth or an alkaline metal, while the B-element forms unstable hydrides, usually consisting of transition metals. For the B-element, Ni is commonly used due to its excellent catalytic properties for H₂ dissociation. Specific ratios of B : A, such as x = 0.5, 1, 2, and 5, are known to facilitate the formation of hydrides, with the hydrogen-to-metal ratio reaching as high as 2. In MH-based systems, the metal atoms serve as a host lattice, with hydrogen atoms occupying interstitial sites within this lattice. These hydrogen atoms occupy octahedral or tetrahedral sites within the metal lattice, or a combination of both. The hydrogen atoms possess a partial negative charge, which varies depending on the type of metal. Recent research has focused on light metals, especially Mg, along with Na, Li, and Be.⁷¹ Mg is economically advantageous and has a high H₂ storage capacity by weight. Moreover, its hydrides exhibit excellent heat resistance, vibration absorption, reversibility, and recyclability. Fig. 4 summarises Mg-based hydrides. According to published studies, increasing the surface area of Mg-based MHs through ball-milling methods can increase H₂ absorption by up to ten times compared with that of unmilled materials.⁷² The ball-milling method is considered to be a cost-effective way of producing H2 with high-performance MHs. This method is regarded as an efficient and affordable way of enhancing the H₂ storage capacity in MHs. Light metals from Groups 1, 2, and 3 (e.g., Li, Mg, B, and Al) form a variety of complex hydrides, which are of particular interest due to their low density and high hydrogen-to-metal atom ratio, typically two. Among these, LiBH₄ exhibits the highest known gravimetric H₂ density at room temperature, reaching 18 mass%, making it a promising H₂ storage material for mobile applications.⁷³ However, its high thermodynamic stability results in an excessively high decomposition temperature. To address this challenge, the addition of MgH₂ and TiCl₃ as catalysts has been shown to enhance H₂ storage capacity to 8–10% at temperatures between 315 and 400 °C.⁷⁴ Complex hydrides represent an innovative class of H₂ storage materials. Despite extensive research in this area in recent years, many new compounds remain unexplored. Borides in particular are noteworthy for their exceptional gravimetric and volumetric H₂ densities.

Fig. 3 Key advantages of metallic hydrides.

Fig. 4 Overview of Mg-, Li-, and Na-based hydrides as metal hydride materials for solid hydrogen storage. This figure is reproduced from ref. 67 with permission from Elsevier, copyright 2018.

Hydrides present significant challenges, particularly due to their strong reactivity with air, which can irritate the skin and eyes. During the uptake of H₂ in storage tanks, impurities are absorbed, which complicates the counteraction and recycling processes. Since impurities occupy H₂ storage sites, the tank's lifespan is reduced. Desorbing H₂ from MgH₂ in the presence of air and oxygen requires high temperatures, resulting in slow kinetics and increased reactivity. In addition, complex hydrides decompose into stable elements, making them difficult to handle safely.⁷⁵

Liquid organic hydrogen carriers (LOHCs)

The ongoing global warming crisis has promoted many countries to prioritise renewable energy sources, necessitating an accelerated transition towards sustainable alternatives. As H₂ is regarded as a clean energy carrier, it is essential to establish its role and develop a strategic approach to overcoming future energy challenges. The U.S. Department of Energy (DOE) has set specific targets for on-board H₂ storage, requiring gravimetric and volumetric H₂ capacities of 5.5 wt% and 62 kg m⁻³, respectively (Fig. 5).^76,93 To meet these objectives, LOHCs have emerged as a promising technology for large-scale and long-distance H₂ transportation.⁹² The key advantages of LOHCs include high storage capacity, effective carbon cycling, long-term usability, and enhanced safety. Continuous advancements in LOHC technology are underway, and modern developments have demonstrated significant potential, making LOHCs a commercially viable solution for H₂ storage (Table 2).^77–88

Fig. 5 Energy storage density and H₂ content in organic chemical hydrides, compressed H₂ tanks, and metal hydrides. Reproduced from ref. 93 with permission from Elsevier, copyright 2016.

Table 2 Hydrogen capacities of liquid organic hydrogen carriers and their dehydrogenation temperature

Liquid organic hydrogen carriers	Hydrogen capacity (wt%)	Dehydrogenation temperature (K)	Ref.
Cyclohexane ⇌ Benzene	7.2	300–320	77
Methylcyclohexane ⇌ Toluene	6.2	300–350	78 and 95
Decalin ⇌ Naphthalene	7.3	320–340	79
Perhydro-dibenzyltoluene ⇌ Dibenzytoluene	6.2	260–310	80 and 81
Bicyclohexyl ⇌ Biphenyl	7.27	310–330	106
Dicyclohexylmethane ⇌ Diphenylmethane	6.66	340–360	82
Indoline ⇌ Indole	1.6	110	83
4-Aminopiperdine ⇌ 4-Aminopyridine	6.1	170	84
Perhydrodibenzofuran ⇌ Dibenzofuran	6.7	200	85
Dodecahydrocarbazole ⇌ Carbazole	6.7	100	86 and 87
Dodecahydro-N-ethylcarbazole ⇌ N-Ethylcarbazole	5.8	170	88

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LOHC technology operates through a reversible mechanism that enables the storage and release of H₂ using unsaturated/saturated organic liquid chemicals as H₂ carriers. The most commonly used LOHCs for H₂ storage are toluene, naphthalene, and N-ethylcarbazole, while H₂ carriers include methylcyclohexane, decalin, and dodecahydro-N-ethylcarbazole.^89–91 The fundamental principle of hydrogenation in LOHCs is an exothermic reaction, in which an organic H₂ storage liquid is combined with raw H₂ in a reactor. Upon heating in the presence of a catalyst, the system produces a saturated hydride, forming H₂ carriers.^92,93 During this process, H₂ catalyses the conversion of unsaturated bonds into saturated bonds, while the catalyst facilitates H₂ extraction from LOHCs within the dehydrogenation apparatus. The energy disparity between hydrogen atom dissociation and C–H bond activation leads to the absorption of external heat. Cycloalkanes offer certain advantages over other LOHCs, mainly due to their high boiling points and comparatively greater H₂ capacity; however, H₂ release occurs at elevated temperatures.^94,95 Recent research has shown that substituting heteroatoms, such as N atoms, for C atoms significantly reduces both the enthalpy and required temperature for H₂ release. Organic compounds with a higher N content exhibit increased dehydrogenation temperature.^96,97 The practical application of cyclohexane requires elevated temperatures and a stable catalyst to prevent coking during dehydrogenation. Due to their cost-effectiveness, high H₂ purity, and widespread availability, cycloalkanes are considered ideal for large-scale H₂ production and long-distance transportation.

The application of MOFs for LOHCs has garnered significant attention in recent years. Separating a LOHC mixture is difficult due to the analogous physical properties of saturated and unsaturated LOHCs. Porous materials are advantageous in separation processes due to their substantial free volume and elevated penetration.^98–101 Porous materials are emerging for complex organic liquid separations, offering processes that utilize less energy than conventional thermal separation due to their precisely controlled pore environments.¹⁰² Among the diverse materials exhibiting selectivity for LOHCs, two MOFs, CUB-5 and CUB-30, have been proposed to generate a distinctive pore environment that selectively interacts with LOHCs.¹⁰³ Consequently, there exists the potential to engineer a MOF material with enhanced size-sieving capabilities and functional groups for effective LOHC separation.

Current storage alternatives

There are currently various methods of storing H₂ available, such as compressed H₂ (CH₂),¹⁰⁴ liquefied H₂ (LH₂),¹⁰⁵ liquid organic H₂ carriers (LOHCs), and underground storage. However, several drawbacks mean that they are not the complete solution. Some countries have advanced implementation. Examples include the Japanese JFE all-steel bottle container and the US carbon fibre-wound H₂ storage container with a steel liner are used at H₂ refuelling stations. Researchers are studying bottle leakage, liner and interface sealing, and transportable hydrogen cylinder design.

Circular hydrogen carriers

Formic acid (FA)

FA's significant volumetric capacity of 53 g H₂/L, coupled with its low toxicity and flammability under ambient conditions, makes it an attractive option for transporting H₂. It decomposes into CO₂ and H₂, and can be regenerated, functioning as a storage medium for H₂ and chemical energy storage. FA is primarily sourced from fossil feedstocks; however, the use of CO₂ as a H₂ carrier through hydrogenation results in the generation of FA, and subsequent dehydrogenation of FA produces H₂, with the released CO₂ being recycled to complete the carbon cycle. The formation of FA via the direct hydrogenation of CO₂ is thermodynamically unfavourable in the gas phase, as shown in eqn (1):

CO₂(g) + H₂(g) ⇌ HCOOH, ΔG⁰ = 32.9 kJ mol⁻¹ (1)

However, in the presence of a base, the CO₂ hydrogenation step becomes thermodynamically favourable.

The H₂ content in FA is approximately 4.4%, making it a promising energy storage material when compared with other state-of-the-art options. However, for fuel cell applications, FA can face adverse effects due to CO production from the indirect decomposition of FA or catalyst poisoning. In the case of its use as a fuel in proton exchange membrane fuel cells (PEMFCs), FA decomposes directly into CO₂ and H₂. H₂ generation from FA occurs via two separate reaction mechanisms, as illustrated in Fig. 6. These two steps are closely interconnected via the water–gas shift (WGS) and reverse water–gas shift (RWGS) reactions at high temperatures. While the full implementation of this concept is still on the horizon, recent advances in FA decomposition catalysis are making the FA-based production of H₂, particularly for mobile applications, increasingly achievable. In this review, we will explore the promising decomposition of FA through heterogeneous and homogeneous catalysts, as well as the generation of H₂ from renewable resources.

Fig. 6 Potential reaction pathways for formic acid decomposition: dehydrogenation and dehydration.

Heterogeneous and homogeneous catalysts for FA dehydrogenation

The production of H₂ from FA in the presence of heterogeneous catalysts has been extensively studied since the early 1930s. Numerous types of heterogeneous catalysts have been investigated in detail and discussed in several excellent reviews.^107–110 Despite the excellent decomposition activity of these catalysts, their effectiveness diminishes at high reaction temperatures (>100 °C).

In the context of FA decomposition using high-performance catalysts, noble metals such as Pd, Au, Ag, and Pt have demonstrated exceptional performance, which was characterised by long-term stability and high selectivity, particularly from an economic perspective. Liu et al. demonstrated the synergistic effect of a Schiff base-based Au nanocatalyst at high FA concentrations, achieving a turnover frequency (TOF) of 2882 h⁻¹.²⁴

Pd-based catalysts are among the most active metals used for H₂ production from FA. Single-metal Pd catalysts or Pd doped with other metals exhibit high dispersion and stability. For example, a Pd/N-MNC-30-based nanocatalyst demonstrated 100% selectivity for FA–sodium formate decomposition at 333 K, with a TOF of 8414 h⁻¹.¹¹¹ Similarly, the AP-SiO₂@PDA-NGO@Pd nanocatalyst exhibited a higher activity compared with that of single Pd nanoclusters.¹¹²

Bimetallic catalysts consist of various metal elements, where the specific metal species and their molar ratios play a crucial role in establishing effective synergy and stability. The Ag@ZIF-8 catalyst exhibits minimal activity; however, this performance significantly improves upon alloying with Pd. At an Ag : Pd ratio of 18 : 82, the molecular relationship between the AgPd alloys and the carrier is optimised, resulting in the highest catalytic activity.¹¹³

Wang et al. reported that an Au–Pt/CeO₂ catalyst demonstrated remarkable selectivity for H₂, with no CO production detected during decomposition, achieving a TOF of 1637 h⁻¹ at 433 K.¹¹⁴ Introducing an appropriate amount of one metal into another induces significant electronic interactions between them, thereby altering the electronic structure of the metal surface and enhancing its catalytic activity.¹¹⁵

Core–shell-based heterogeneous catalysts exhibit exceptional stability, superior recyclability, and elevated catalytic activity, making them highly promising for various applications. Choi et al. synthesised PdAg@Pd core–shell nanocrystal (PdAg@PdONC) catalysts with a Pd/Ag ratio of 3.5 : 1 and Pd shell atomic layer thickness of 1.1, demonstrating outstanding catalytic performance. In an FA–sodium formate (SF) solution at 323 K, the catalyst achieved a remarkable TOF of 21 500 h⁻¹ with 100% H₂ selectivity.¹¹⁶ Qian et al. reported that by modifying the Pd content, the TOF of the Ag@AgPd_0.8 catalyst reached 667.08 h⁻¹ in an FA solution at 333 K.¹¹⁷

From an economic perspective, substituting noble metals with non-noble metals is a practical approach for optimising the use of natural resources. Liang et al. reported that the optimized Co–N/U-CNx catalyst produces 19 668.3 mL g_Co⁻¹ h⁻¹, making it one of the most effective heterogeneous non-noble-metal catalysts for FA dehydrogenation.¹¹⁸

A study examining the activity of KIT-6 supported Ni and Co catalysts investigated the impact of Co incorporation into the Ni@KIT-6 catalyst during FA dehydrogenation, where complete FA conversion was observed at temperatures ranging from 200 to 350 °C.¹¹⁹ The incorporation of non-precious metal elements into various alloys serves as a method to decrease dependence on precious metals. Yang et al. employed a non-precious metal sacrificial method to incorporate AuPd alloy nanoparticles on reduced graphene oxide, creating a (Co₃)EAu_0.6Pd_0.4/rGO catalyst with high catalytic activity, stability, and selectivity. At 323 K, the catalyst achieved a turnover frequency of 4840 h⁻¹ with 100% H₂ selectivity and no CO impurity.¹²⁰ While pure non-noble metal catalysts can enhance the economic feasibility of H₂ production, their catalytic activity remains lower than that of noble metals and precious metal-enhanced catalysts. Developing high-activity, low-cost non-noble metal catalysts remains an exciting challenge.

A number of homogeneous catalysts have been studied for the FA dehydrogenation, and Fig. 7 shows the selected homogenous catalysts. As early as 1967, Coffey reported that soluble Pt, Ru, and Ir phosphine complexes were selectively used and IrH₂Cl(PPh₃)₃ showed the highest TOF of 1187 h⁻¹ of FA dehydrogenation.¹²¹ In recent decades, significant efforts have been dedicated to developing high-performance homogeneous catalysts for FA dehydrogenation. Puddephatt et al. investigated the binuclear [Ru₂(μ-CO)(CO)₄(μ-dppm)₂] complex for the FA dehydrogenation in an acetone solution, demonstrating its efficiency in the reversible reaction between HCOOH and CO₂/H₂.¹²² Beller et al. studied the FA dehydrogenation using Ru-based catalysts, which exhibited stability and continuous operation, achieving TOF and turn over number TON values of 900 h⁻¹ and 260 000, respectively, under mild conditions.¹²³ The same group further reported a selective FA dehydrogenation process using commercially available Ru precursors, including [RuCl₂(PPh₃)₃], [RuCl₂(p-cymene)]₂, [RuCl₃(benzene)]₂, and RuBr₃·× H₂O, in combination with triphenylphosphine-type ligands in an FA/NEt₃ mixture.¹²⁴ In addition, integrating the system with a fuel cell enabled direct H₂ utilisation for electricity generation without CO contamination.

Fig. 7 Molecular catalysts used for the dehydrogenation of neat FA.

Despite their low cost, low toxicity, and ease of handling, simple organic salts have been rarely studied for this purpose. The H₂ storage and release system using HCOOK and KHCO₃ in this study demonstrated long-term stability and applicability. In 2024, Beller et al. reported H₂ release rates of 9.3 L h⁻¹ using ppm-level quantities of a Ru-based complex to induce hydrogenation within the same catalyst system, achieving a TON of 9650. The H₂ storage–release cycles were successfully repeated 40 times.¹²⁵

Systems for generating H₂ gas from neat FA without solvents or additives remain scarce. Milstein et al. demonstrated the substantial catalytic efficacy of a Ru 9H-acridine pincer complex, which displayed exceptional stability and durability in pure FA, even at elevated temperatures for over a month, attaining a TON of 1701 150. The system produced elevated H₂/CO₂ gas pressures from pure FA, with experiments performed at pressures reaching 100 bars.¹²⁶

Despite significant advancements, most catalytic systems developed thus far require dilute solutions with a solvent and, in some cases, a significant quantity of additives to achieve efficient dehydrogenation.^127,128 Solvents and additives, while effective, reduce the system's net volumetric and gravimetric H₂-storage capacities, which are theoretically limited to 53 g(H₂) L⁻¹ and 4.4 wt% H₂, respectively, for pure FA. Moreover, volatile solvents and additives, such as amines, can contaminate the gaseous products, resulting in higher gas purification costs before fuel cell applications.¹²⁹

Williams et al. developed a P,N-ligated Ir complex as a dehydrogenation (pre)catalyst in pure FA with a base such as HCO₂Na (5 mol%, relative to FA). Fischmeister et al. developed Cp*Ir^III-derivatives that effectively dehydrogenate neat formic acid, achieving an initial TOF of up to 13 292 h⁻¹ at 100 °C.¹³⁰ Xiao presents impressive TOF values soaring to 147 000 h⁻¹, accomplished in quick reaction times, for the dehydrogenation of neat 5 : 2 azeotropic HCOOH/NEt₃ mixtures through the use of their Cp*Ir-systems.¹³¹ Gelman et al. showed neat FA dehydrogenation at 70 °C with TON 383 000 and maximum TOF 11 760 h⁻¹.¹³² Casado and his co-worker report that CNC-pincer based rhodium complex provided maximum TOF 10 150 h⁻¹ in the FA/SF mixture.¹³³ Yebra et al. demonstrated that a trazole based Ir-complex exhibited a maximum of TOF 10 703 h⁻¹ and TOF value 26 879 after 6 times addition of an FA/SF mixture in the presence of absence of an external solvent.¹³⁴ The catalytic reaction at 90 °C produced 12 530 TON in 13 h and 2160 000 TON over 4 months after 40 times addition of FA.¹³⁵ In neat conditions, Milstein's Ru hydride and Casado's cationic Rh complex also catalyze dehydrogenation, along with Ir systems. Ru catalysts are durable in FA at 95 °C, yielding H₂ and CO₂ with a TON of 1701 150.^126,136

Huang et al. grafted a fibrous silica nanosphere with a P,N,P-ligated Ir trihydride. This catalyst can dehydrogenate neat FA at 540 000 TON with a TOF of 13 290 h⁻¹. The immobilized catalyst exhibits a greater activity than that of the homogeneous catalyst, although both require HCO₂Cs (12.5 mol%).¹³⁷ They also demonstrated Ir picolinamide complex immobilization on three-dimensional fibrous modified silica in a solvent-free reaction medium without volatile bases. This catalyst achieved a very high TOF of 40 000 h⁻¹, which is comparable to that of its homogeneous counterpart.¹³⁷

All these catalysts illustrate that leveraging the relatively high acidity of pure FA can be advantageous when integrating a basic ancillary ligand into the catalyst design.^87,94,95

Hydrogen storage and release

However, FA dehydrogenation inevitably produces an equimolar amount of CO₂ alongside H₂. With increasing global concern over climate change, the release of CO₂ during FA dehydrogenation is considered a major limitation to its practical application as a H₂ carrier.¹³⁸ Addressing this issue is essential not only for H₂ production from FA but also for the broader advancement of synthetic fuels and hydrogen-based energy carriers.

CO₂ capture and utilisation (CCU) are crucial but technically challenging.^139–141 Post-combustion capture using amine-based solutions such as monoethanolamine (MEA) or diethanolamine (DEA) remains the most common method, with 85–90% efficiency,¹⁴² though high energy costs for regeneration limits its economic viability.^143,144 In Europe, CO₂ capture costs are included in the levelized cost of electricity (LCOE), influencing system feasibility. Widespread adoption of such policies could encourage innovation in site-specific capture and CO₂-to-fuel conversion, supporting a circular carbon economy. Advancing decarbonisation requires both improved capture efficiency and technologies for converting CO₂ into sustainable fuels and feedstocks.^144,145

Developing mild-condition catalysts that convert CO₂ to FA is big challenge. The objective is to achieve concurrent CO₂ hydrogenation and dehydrogenation of FA in a single reactor by modifying reaction parameters such as temperature, pressure, solvent, or pH. Dehydrogenation of formate to bicarbonate is more suitable for the reversible H₂ production and storage of H₂ than entropy-driven FA dehydrogenation to CO₂.⁹⁶ This advancement presents the potential for using the formate–bicarbonate system as a H₂ carrier and storage solution. Ideally, the introduced CO₂ remains in solution following each dehydrogenation step, thereby integrating carbon capture with H₂ storage and release cycles and eliminating the need for additional CO₂ charging steps. The selected examples are shown in Table 3.

Table 3 Examples for the reversible transformation of CO₂ and FA system

Catalyst	Hydrogenation T/p	Dehydrogenation temperature (°C)	Additives	TONHydrogenation/TONDehydrogenation	TOF (h^−1)Hydrogen/TOF (h^−1)Dehydrogen	Solvent
Ru-1 ^147	80 °C/100 bar	80	CsHCO3		23/10	H2O
Ru-2 ^148	65 °C/40 bar	90	DBU	310 000	65 000/93 100	DMF
Ru-3 ^149	25–80 °C/1–30 bar	>80	Non	7739/51 761	1000/500	BMIM OAc
Mn-1 ^146	115 °C/80 bar	90	Lysine	230 000/29 000	Non	H2O/THF
[^1432] + Ph2P(CH2)4Ph2P ^150	Rt/40 bar	rt	Et3N	312/Non	375/Non	Acetone
Ir-1 ^151	25–80 °C/1–50 bar	90	KHCO3	79 000/165 000	54 000/228 000	H2O
			H2SO4
Ir-2 ^152	60 °C/0.1 bar	25	K2CO3	—/—	36 at pH 9/2100 at pH2.8	H2O
Ir-3 ^153	50 °C/10 bar	70	NaHCO3	7750/10 000	650/6720	H2O

---

In an early study utilising pressure-resistant NMR tubes, Laurenczy et al. demonstrated that aqueous solutions of cesium formate and cesium bicarbonate, in the presence of Ru-1 (Fig. 8) and excess triphenylphosphine m-trisulfonic acid (mTPPTS) ligands, function as an effective H₂ storage-release system under mild conditions (80 °C) without the need for additional reagents. The system achieved TOFs of 23 h⁻¹ for H₂ storage and 10 h⁻¹ for H₂ release and could be recycled up 5 times in NMR tube without any loss of activity or solvent exchange. However, H₂ storage requires pressurisation at 100 bar, while H₂ release proceeds under ambient pressure.⁹⁷ By simply varying the temperature from 65 to 90 °C and the pressure from 5, 40, and 1 bar, Pidko et al. demonstrated 10 cycles of H₂ storage and release in DMF/DBU mixtures using a Milstein Ru-2 catalyst, demonstrating the reversible hydrogenation of CO₂ to formate.⁹⁸ Nielsen et al. reveal the application of Ru-3 pincer complexes in ionic liquids for the additive-free, reversible hydrogenation of CO₂ to FA as well as the dehydrogenation of FA under exceptionally mild conditions. The identical catalytic system operates effectively for a minimum of 13 cycles of hydrogenation and dehydrogenation without requiring sacrificial additives, demonstrating significant compatibility with continuous-flow conditions.⁹⁹ Sponholz, Junge, and Beller et al. developed a triazine-based Mn-1 complex for the reversible CO₂ hydrogenation and FA dehydrogenation, utilising lysine as a stoichiometric additive.¹⁴⁶

Fig. 8 Examples of reversible systems for the interconversion of CO₂/FA.

A notable characteristic of catalysts active in aqueous solutions is their dependence on pH. The pH controls the rate-limiting steps of protonation and deprotonation processes, affecting reversibility. Hull et al.,¹⁵¹ Fukuzumi et al.¹⁵² and Himeda et al.¹⁵³ independently demonstrated reversible H₂ storage under mild conditions by altering the pH of the reaction mixture, utilizing iridium complexes Ir-1, Ir-2 and Ir-3 as catalysts, respectively. H₂ can be efficiently stored and released on demand using the pH-dependent system, which is essential for applications. Moreover, continuous operational cycles may lead to a buffered pH within the system owing to the buildup of alkaline byproducts. One additional strategy involves the utilisation of organic solvents that possess exceptional CO₂ capture capabilities. A biphasic solvent system was utilized to evaluate the feasibility of recycling the catalyst and the organic solvent. Despite their intriguing characteristics, none of the documented methods have efficient and reversible CO₂/FA catalytic systems that work under mild conditions, without sacrificial additives and non-ideal solvents. The catalysts used often require inert conditions and degassed solutions, making the proposed methods more difficult to implement. Dilute aqueous solutions reduce volumetric energy storage capacity to an unsuitable level for practical use.

From the catalyst's viewpoint, transition metal-based pincer complexes have dominated the catalytic hydrogenation of captured CO₂ in recent years. Their reactivity and air/oxygen intolerance make them unsuitable. The development of complexes that dissolve in water and are inert to air, especially those based on non-noble metals, could reduce the need for organic solvents and a safe, inert environment. There is a need for heterogeneous catalytic systems to simplify their implementation in large-scale renewable energy systems. However, many heterogeneous catalysts have lower TON and TOF than homogeneous catalysts. Only a few non-noble metal-based heterogeneous catalysts have been developed for H₂ storage.

Methanol, readily synthesized from CO₂ or formic acid/formate hydrogenation, may also serve as a H₂ carrier in a H₂ battery. This system may exhibit greater energy efficiency owing to its elevated H₂ content. Recently, Beller et al. demonstrated methylformate as a H₂ source using an Ru-based pincer catalyst. They optimized the Ru-complex for methyl formate dehydrogenation with no detectable CO and high activity. At optimal conditions the catalyst shows that the maximum TOF is >44 000 h⁻¹ and TON > 100 000.¹⁵⁴ Methyl formate will inspire the development of renewable energy materials in future.

Implementation of formic acid in energy storage systems

An important step toward the development and practical application of a carbon-neutral H₂ storage and release system is the systematic investigation of the interconversion of metal bicarbonates and formates through hydrogenation and dehydrogenation. Despite their advantages such as low cost, less toxicity, and ease of handling, these salts have received relatively little attention for H₂ storage and release system, primarily because dehydrogenation from basic formate salts proceeds much slower than FA dehydrogenation. In contrast, FA dehydrogenation in acidic aqueous solutions is considerably more efficient, but it liberates CO₂, creating a significant hurdle for CO₂-based H₂ storage.¹⁵⁵ Consequently, introducing CO₂ into such systems requires additional CO₂, often supplied under high-pressure, to compensate for the CO₂ release during dehydrogenation.³²

Beller et al.¹⁴⁶ (Fig. 9a) demonstrated a molecularly defined Mn complex, along with naturally occurring Lys, enabling direct CO₂ hydrogenation to formate via Lys's CO₂ capture effect with a 93% yield and a 2000 000 of total TON. The system also produces H₂ from FA with Lys at a 99% yield and 600 000 of total TON.

Fig. 9 Development of a practical framework for a bicarbonate-based energy system. Reproduced from ref. 146 and 156 with permission from Springer Nature, copyright 2023, and 2024, and from ref. 157 with permission from John Wiley and Sons, copyright 2025.

Another example from the same group (Fig. 9b) showed a reversible H₂ storage and release system using an Fe-pincer complex as a catalyst. High selectivity (>99.9%) was achieved in the quantitative generation of CO-free H₂ from formamides. This system operates at 90 °C, utilising waste heat from sources such as proton-exchange membrane fuel cells. It achieves over 70% H₂ evolution efficiency and 99% H₂ selectivity across 10 charge–discharge cycles, preventing carbon emissions.¹⁵⁶

A recent study (Fig. 9c) describes a resilient, carbon-neutral, formate-based H₂ storage and release system composed solely of HCOOK, KHCO₃, water, an organic solvent, and trace amounts of a Ru complex.¹²⁵ Kawanami et al.¹⁵⁷ tested FA as a H₂ carrier using formate salts from CO₂ reduction under supercritical fluid conditions to store and produce H₂ (Fig. 9d). Cp*Ir homogeneous catalysts bearing amino substitutes converted CO₂ into formate salts in 2 h at 50 °C, with a TOF of 10 240 h⁻¹ and TON of 20 480. A cation exchange resin generated FA from the format salt solution, and over 98% of FA was dehydrogenated to regenerate H₂, with over 90% H₂ recovery.

These findings demonstrate that formate salts can effectively store and regenerate H₂ for energy applications. They highlight the efficacy of this method for efficient H₂ storage, transportation, and regeneration, providing a sustainable framework for a H₂ energy system based on FA.

Conclusions and future prospects

The conversion of FA to H₂ requires either heterogeneous or homogeneous catalysts. Extensive studies on active metals, supports and other various factors influencing FA dehydrogenation have provided valuable insights.^110,123,127

Pd remains one of the most effective active metals for FA-to-hydrogen catalysis; however, its high cost limits the economic viability of Pd-only systems. Doping or alloying Pd with other metals can enhance H₂ production, catalyst stability, and catalyst recyclability. The catalytic activity is strongly influenced by the average metal particle size, with smaller particles typically improving both catalyst dispersion and interactions with reactants at the active sites. Further research is urgently needed to clarify the interactions between supports and active metals, to elucidate nanoparticle nucleation and growth mechanisms, and to explore related challenges. In this regard, the systematic design of high-efficiency catalysts using non-novel metals will be a promising future direction.

Homogeneous catalysts also hold significant promise, producing H₂ with CO levels remaining below 10 ppm throughout the procedure. For large-scale applications, it is crucial to consider the performance of these catalysts over extended charge–discharge cycles. Non-noble metal-based homogenous catalysts offer an efficient way to combine CO₂ valorisation with the dehydrogenation of FA. These findings encourage further research into practical applications and lay the foundation for a carbon-neutral chemical H₂ storage and release system using safe catalysts and non-toxic additives.

Thanks to innovations in AI, machine learning methods have achieved notable progress across specialised fields, providing a strong driving force in research and development.^15,158–160 In catalysis, machine learning has accelerated advances in catalytic chemistry. When combined with DFT, feature engineering can connect intrinsic catalyst properties to catalytic efficiency, speeding up catalyst design. Moreover, these algorithms propose new material design strategies, expanding the range of potential materials and facilitating the discovery of high-performance catalytic nanomaterials for FA-to-hydrogen production.

Reversible H₂ charging and discharging cycles involving CO₂ and hydrogen–bicarbonate systems present exciting possibilities for H₂ storage. Beyond developing suitable dehydrogenation/hydrogenation catalysts, pH regulation is a crucial factor in both half-reactions. Emerging solutions for base recycling make practical and reversible H₂ storage increasingly feasible. Recent accomplishments in these areas support a smooth transition from laboratory-scale research to practical applications.

Author contributions

D. C. Santra: conceptualization, and writing – original draft, review & editing; H. Kawanami: conceptualization, funding acquisition, project administration, supervision, validation, writing – original draft, review & editing.

Conflicts of interest

There are no conflicts to declare.

Data availability

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

Acknowledgements

The authors also acknowledge the financial support by the Canon Foundation, Grants-in-Aid for Scientific Research (KAKENHI) (Grant Number 24K01619) from the Japan Society for the Promotion of Science (JSPS).

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