Energy has been categorically classified into renewable and non-renewable forms, with approximately two-thirds of the world’s energy demand being fulfilled by non-renewable liquid and gaseous fossil energy resources (mostly petroleum and natural gas) [1]. The primary detrimental effects of these fossil energy sources are their emission of CO₂ and other harmful greenhouse gases, causing global warming and climate change. This damaging impact, along with the ever-increasing price of fossil energy and health and safety concerns, is pushing the international community to develop and look for other less harmful and preferable renewable energy sources to meet their daily demands ^[2][3][2,3], including solar, wind, hydroelectricity, tidal, geothermal, and electrochemical routes [2]. However, owing to their unpredictable nature, these renewable energy sources would only be feasible for use with an energy storage system ^[4][5][4,5].

Currently, among the feasible energy storage technologies, electrochemical energy storage has been considered the most effective and easily convertible. Since stored hydrogen can be readily transformed into electricity using fuel cell technology, the use of electricity derived from renewable energy to electrolyze water and produce hydrogen is one of the efficient electrochemical energy storage technologies, which has led to the development of effective water electrolysis technologies for hydrogen production in the gaseous form ^[6][7][6,7]. Also, the potential of hydrogen as a substitute for gasoline, diesel, and biofuels in the automotive and fuel cell industry holds promise. A recent study showed that various factors, such as energy expenses and process efficiency, significantly influence the potential for cost-effective hydrogen generation via electrolysis [6]. Currently, 48% of the total hydrogen in the world is produced from natural gas: 30% from oils, 18% from coal, and only 4% from electrolysis. Additionally, 95% of all hydrogen is produced via steam methane reforming, while the remaining hydrogen evolution processes (gasification of coal, hydrogen from biomass, and water electrolysis) constitute only ~5% ^{[8][9][10][11]}[8,9,10,11]. Water electrolysis has been proven to be the best method for hydrogen production, given its zero-carbon emission, and it only uses renewable H₂O, with the byproducts being pure hydrogen and oxygen. However, the hydrogen conversion efficiency for this process is lower (40~60%) than steam methane reforming (SMR), which is the most developed contemporary hydrogen production technology ^[2][12][2,12]. Depending on the operating temperature and the type of electrolytes and electrodes used, hydrogen evolution reactions can be further classified into three different electrolysis technologies: alkaline water electrolysis (40~100 °C), proton exchange membrane electrolysis (20~100 °C), and solid oxide electrolysis (500~1000 °C) technologies ^[12][13][14][12,13,14].

In parallel, a very high heating value and a very low mass density render the storage of hydrogen to be one of the major technical challenges. This storage technology mainly consists of three categories: physical storage (hydrogen is stored either in gaseous or liquid form at a very high pressure), physical adsorption (atomic or molecular hydrogen creates weak van der Waals bonds with specific porous materials and is adsorbed into the walls and inside the porous structure), and chemical absorption (atomic and molecular hydrogen is chemically absorbed into metal and chemical hydrides). However, these technologies are challenged for storing large quantities of hydrogen for a long period. Considering the significant potential of hydrogen as a future energy source and its notable prospects for energy systems, the imminent questions that arise include how we can develop a comprehensive way to generate hydrogen more efficiently and how we can store it effectively without compromising the production rate and energy losses during storage to increase the hydrogen economy. 

Electrocatalysts are necessary to accelerate hydrogen production using water electrolysis since the oxidation–reduction reaction of water that produces pure H₂ at the cathode and pure O₂ at the anode is typically sluggish. The two half-reactions that occur during the electrochemical splitting of water include the oxygen evolution reaction (OER) on the anode and hydrogen evolution reaction (HER) on the cathode; so, the overall electrolysis reaction can be expressed as 2H₂O→2H₂+O₂. This reaction happens under a standard cell voltage of 1.23 V operating in a standard thermodynamic condition of 1 atm pressure and 25 °C temperature [2]. While electrocatalysts help to increase the reaction kinetics of the OER and HER, they are not as widely used for alkaline water electrolysis as in the case of the proton exchange membrane and solid oxide electrolysis [2]. Several noble metal catalysts like Ru, Ir, and Pt have been used for their small overpotential, but because they are essentially expensive and also increase the hydrogen production cost, metal-based nanoparticles like Ag, Ru, RuO₂, and IrO₂ are gaining importance for their ability to dissociate water quickly with an enhanced electron transfer rate ^[15][16][17][19,20,21]. One such example is demonstrated by Li et al., where a very small overpotential of only 281 mV with a current density of 100 A/m² is achieved by coating a layer of discontinuous IrO₂ on the RuO₂ surface ^[18][22]. Irrespective of the high catalytic efficiency for the OER, the expensive noble metal compounds are being replaced by nickel-based oxides, hydroxides, double hydroxides, phosphides (NiO-based films, Fe-doped NiO_x, NiFe-layered double hydroxides, Fe-doped Ni₂P, FeNiP, etc.), Ni-based alloys (NiCo, NiMo, NiCoCr, NiCoMn, etc.), and cobalt and manganese-oxides (Co₃O₄, Fe-Co₃O₄, MnCo₂O₄, MnO₂, Ni with Mn₂O_3, etc.) owing to their low overpotentials and ability to dissociate water at higher current density ^{[19][20][21][22][23][24][25]}[23,24,25,26,27,28,29]. Generally, it is found that the ability to perform as an electrocatalyst for three common metals (Ni, Fe, and Co) follows the order Fe < Co < Ni ^[26][30]. Several research directions emerge, including the development of new and cost-effective production methods for metal-based catalysts having high electrocatalytic performances, the selection of new catalysts based on the fundamental mechanisms during the HER and OER, and the design of non-noble metal-based catalysts for the HER and OER in proton exchange membrane (PEM) electrolysis and solid oxide electrolysis cells (SOECs) [2]. As for alkaline water electrolysis, generating a larger amount of hydrogen production yield requires a higher net electricity input into the electrolyzer stack. This net demand in electricity for hydrogen production is determined by the electrochemistry of hydrolysis and the electrochemical model of the water dissociation process. Some of these models have been previously developed and are used to successfully predict the electrochemical behavior of the alkaline water electrolyzer stack under different pressure and temperature ranges ^[27][28][31,32]. The model developed by Ulleberg ^[27][31] has been improved by Sánchez et al. ^[28][32] to incorporate the polarization curve model to derive the final optimized voltage for each electrolyzer cell (V_cell):

Vrev is the reversible cell voltage = 1.23 V for water dissociation at standard conditions (25 °C and 1 atm pressure).

For hydrogen storage, typically, two contemporary approaches are considered: (a) physical storage of hydrogen as compressed gas and cryogenic liquid and (b) material-based or solid-state hydrogen storage. Currently, most of the hydrogen worldwide is being stored in compressed gaseous forms at a pressure of 350–700 bars as it is a simple technology with a fast filling-releasing rate. However, the major drawback is that the volumetric density of hydrogen does not rise with increasing pressure, posing a critical constraint on storage tank design ^[29][33]. To compensate for some of these challenges, cryogenic hydrogen storage has been incorporated, but the large energy consumption for hydrogen liquefication is the primary concern. Also, the storage efficiency is reduced with time due to continuous heat input into the storage tank, which leads to the evaporation of significant amounts of hydrogen ^[29][30][33,34]. In the case of material-based storage, its high efficiency is correlated to the large amount of hydrogen that can be stored at ambient conditions within a small volume, and this mainly constitutes two processes, viz., absorption and adsorption. In case of absorption, hydrogen atoms react and integrate into the lattice structure of Li, Mg, Na, Ti, and other similar metals (M) to form metal hydrides (MHxMHx) following the generalized chemical reaction, M(s)+x2H2(g)↔MHx(s)+QMs+x2H2g↔MHxs+Q, where Q refers to the heat of the formation of the hydride. On the other hand, hydrogen adsorption is a surface-level interaction happening at low pressure and occurs predominantly with a porous material having a large surface area-to-volume ratio (generally graphite, carbon nanotubes, boron nitride nanotubes, and C₆₀ buckyballs) because this increases the rate of hydrogen kinetics and reduces the binding energy ^[31][35]. However, the major disadvantage is the relatively low hydrogen storage capacity (in %wt) and a low gravimetric density of hydrogen.