Hydrogen (H₂) as a fuel offers the pathway for a clean, abundant, renewable and efficient source of energy that is currently crucial in the worldwide effort to lower emissions of harmful pollutants. The implementation of H₂ in fuel cells came about around 219 years after its discovery, as can be seen in the timeline in Figure 1. The technology has evolved from its first use in the U.S. Apollo Space Program in the 1950s to the development of the first fuel-cell vehicle in the 2000s [1]. Throughout this course, the utilization of H₂ as fuels further developed for their production using renewable energy approaches since the implementation of the world’s first solar-powered H₂ production towards the development of the grid system for hydrogen generation in the 2020s and onwards. Hydrogen is an abundant element and non-toxic. Utilizing H₂ in hydrogen fuel-cell systems is able to generate electricity with only water and heat as by-products [2,3]. However, the majority (95%) of global hydrogen production is still based on non-renewable resources via steam methane reforming, as of 2020, emitting 830 tonnes of CO₂ annually [4,5]. Therefore, sustainable hydrogen generation should be sourced from renewable sources while reducing CO₂ emissions. Green hydrogen adopts H₂ production methods that do not rely on non-renewable sources such as fossil fuels. Water electrolysers are an emerging technology that utilizes photocatalytic or electrocatalytic water splitting to generate H₂ [6,7,8]. Integrating electrolysers with renewable sources such as solar and wind power provides us with green and environmentally friendly H₂ sources with no toxic by-products. An alkaline water electrolyser is a developed technology that generates H₂ by water electrolysis. However, highly concentrated potassium hydroxide (KOH) solution in alkaline electrolysers poses a great risk of component corrosion [9]. The proton exchange membrane water electrolyser (PEMWE) and anion exchange membrane water electrolyser (AEMWE) offer a more compact design that uses a lower-concentration electrolyte (0.5 M sulfuric acids (H₂SO₄) for PEMWE and 1 M KOH for AEMWE) and is able to overcome some limitations related to corrosion and carbonate formation in conventional electrolysers [9,10,11,12,13].

The electrochemical water-splitting system has two essential reactions: hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). The ideal thermodynamic potential to split water into its component hydrogen and oxygen is 1.23 V. In practice, the potential is much higher due to the mass, electrolyte and transport resistances, in addition to the slow HER and OER kinetics. Additional potential to achieve water splitting is known as overpotential [14]. HER is the cathode reaction in the water electrolyser where H₂ is generated. Active research is still ongoing in the field of HER. Moreover, increasing publications related to HER show that challenges and opportunities still exist to be tackled (Figure 2). The rate at which HER occurs varies between the acidic and alkaline electrolyte environments [15,16]. The H₂ production rate is determined from the activity of the HER electrocatalyst, where it is desired to have a high current density at the lowest possible potential. Therefore, the smaller HER overpotential of an electrocatalyst indicates its high activity. The benchmark HER electrocatalysts are based on noble or platinum-group metals (PGM), most commonly Pt/C, owing to their very low overpotential in acidic and alkaline conditions [17]. Therefore, to lower the cost of electrode fabrication, PGM-based catalyst loading must be minimized without losing the optimum HER activity.

Various electrocatalysts alternative to Pt/C has been extensively studied to identify the best lower-cost catalysts for HER [18]. Ni, Mo and Co-based materials are among the potential catalysts for HER. This includes alloys such as NiMo, transition-metal sulphides, chalcogenides and nitrides [19]. Single-atom catalysts (SACs) such as those of Pt and Ir SACs offer the advantage of utilizing ultra-low loadings of PGM-based catalysts [20]. However, nanoparticles/nanosheets of these catalysts, including SACs, are prone to aggregation, causing instability in their HER performance. Two-dimensional (2D) nanomaterials such as graphene and MXene are excellent supports to stabilize these catalysts, which in turn also enhances the HER. MXenes, belonging to a family of 2D transition metal carbides/nitrides, contain an abundance of the termination groups on their basal planes that enable them to anchor catalyst nanoparticles and SACs, facilitating the catalysts’ distribution [20,21,22,23]. These termination groups (–O, –F and –OH) also participate in HER as active sites. Their high conductivity helps lower the electron transfer resistance for enhancing intrinsic HER activity. The types of transition metal and whether it is carbide or nitride determine the electronic structure of MXene and its effectiveness towards HER alongside the termination groups [21]. Past reviews have highlighted the properties of different types of MXenes, the effect of termination groups, their synthesis and their roles as supports for catalysts for HER as well as other applications, including OER [21], photocatalytic water splitting [24], CO₂ reduction [24,25] and nitrogen reduction reaction [26]. While the MXene types and their atomic structure significantly affect catalytic activity, the Mxene’s morphology also contributes to the catalysts. Since its discovery in 2011, MXenes with different structures and morphologies, such as multilayer, few-layer and porous, have been successfully synthesized and investigated. Changing morphologies can determine how well the MXene participates in the HER by the ease of access to its active sites, its resistance to restacking, overall surface area, effectiveness in supporting HER-active phases and its durability. Hence, identifying the most suitable out of all the possible MXene morphologies can help cater to the improvement of the overall HER performance.

2. Hydrogen Evolution Reaction

HER occurs as a cathode reaction within the water electrolyser system, where water is reduced to generate H₂ (2H⁺ + 2e⁻ → H₂) [27]. It is a two-electron transfer system with one catalytic step in general [28,29,30]. Therefore, to achieve high kinetic effectiveness for electrochemical water splitting, an active electrocatalyst must be used to reduce the overpotential that drives the HER process [31]. The PGM-based electrocatalyst platinum (Pt) is a well-known HER catalyst that needs smaller overpotentials even at high reaction rates (especially in acidic solutions). Pt/C can typically exhibit overpotentials as low as 20 mV to around 80 mV at 10 mA/cm² [32,33]. However, the scarcity and high cost of Pt limit its technological usage, prompting the effort to minimize the loading of Pt in electrodes or replace it with lower-cost transition-metal-based electrocatalysts.

2.1. Mechanism of Electrochemical HER

The mechanism of HER is dependent on the driving environment, such as alkaline and acidic solutions [34], represented by Equations (1) and (2), respectively. Further, the literature shows that these equations are furthermore divided into various sub-steps [35]. It contains proposed HER kinetics in acidic and alkaline environments, as depicted in Figure 3. $$2{\mathrm{H}}_3\mathrm{O}+2{\mathrm{e}}^{-} \to {\mathrm{H}}_2\uparrow \left(\mathrm{Acidic}\right)$$ $$2{\mathrm{H}}_3\mathrm{O}+2{\mathrm{e}}^{-} \to {\mathrm{H}}_2\uparrow +2\mathrm{O}{\mathrm{H}}^{-}\left(\mathrm{Alkaline}\right)$$ Three possible systems for HER in an acidic environment: $${\mathrm{H}}_3{\mathrm{O}}^{+}+{\mathrm{e}}^{-}\to {\mathrm{H}}_{\mathrm{ads}{}_{}+{\mathrm{H}}_2\mathrm{O}\left(\mathrm{Volmer}\right)}$$ $${\mathrm{H}}_{\mathrm{ads}{}_{}+{\mathrm{H}}_3{\mathrm{O}}^{+}+{\mathrm{e}}^{-}\to {\mathrm{H}}_2+{\mathrm{H}}_2\mathrm{O}\left(\mathrm{Heyrovsky}\right)}$$ $${\mathrm{H}}_{\mathrm{ads}{}_{}+{\mathrm{H}}_{\mathrm{ads}{}_{} \to {\mathrm{H}}_2\left(\mathrm{Tafel}\right)}}$$ Three possible systems for HER in an alkaline environment: $$2{\mathrm{H}}_2\mathrm{O}+2{\mathrm{e}}^{-} \to 2{\mathrm{H}}_{\mathrm{ads}{}_{}+2\mathrm{O}{\mathrm{H}}^{-}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\left(\mathrm{Volmer}\right)}$$ $${\mathrm{H}}_{\mathrm{ads}{}_{}+{\mathrm{H}}_2\mathrm{O}+{\mathrm{e}}^{-} \to {\mathrm{H}}_2+\mathrm{O}{\mathrm{H}}^{-}\left(\mathrm{Heyrovsky}\right)}$$ $${\mathrm{H}}_{\mathrm{ads}{}_{}+{\mathrm{H}}_{\mathrm{ads}{}_{} \to {\mathrm{H}}_2\left(\mathrm{Tafel}\right)}}$$ where H_ads is a hydrogen atom adsorbed on an active site. Normally, one stage actively limits the electrochemical response and is known as the rate-determining step (rds) [36]. Hydrogen evolution energy is firmly reliant upon the terminal material, such as a mercury (Hg) electrode, which shows slow energy, while the HER on platinum is one of the quickest known electrocatalytic processes [37]. It is striking that the energy is dependent upon varieties of boundaries, such as the nature of the electrolyte or the crystalline nature and direction of the electrode (single-crystalline, polycrystalline, amorphous and so on) [38]. However, HER is faster in acidic than alkaline environments because of the water dissociation half-cell reaction at the HER (cathode) of electrolysers under an alkaline one [39]. When water electrolysis is performed in an acidic medium, hydronium ions (H₃O⁺) are reduced to vapor dihydrogen (H₂) [40]. Such sub-electrode reactions would take place at the voltage of such a reference hydrogen electrode from a thermodynamic perspective (RHE). Equation (3) describes the first stage of this reaction, which is the reduction of a proton on such an active site of the surface of the catalyst, proceeded either by the evolution of molecule H₂ (Equation (4)) or through the recombination of two adsorbed protons (Equation (5)). In addition, the overall Tafel, Heyrovsky and Volmer steps are depicted in Figure 3a. The whole electrocatalysis chamber, which demonstrates the HER and OER, is schematically presented in Figure 3b. Extensive efforts have been made to identify alternative catalysts to Pt/C that are electrocatalytically active in alkaline and acid conditions with exceptional long-term durability. Several studies have attempted to lower the loading of Pt on electrodes while maintaining the HER activity. Conversely, transition metal (TM)-based HER electrocatalysts such as nickel and molybdenum-based nanoparticles/nanosheets have shown excellent potential as lower-cost catalyst material. To date, the activity of TM-based catalysts is still inferior to Pt-based catalysts, yet some are found to have extended durability than the latter. TM-based nanoparticles/nanosheets can be susceptible to aggregation, negatively affecting the HER. Two-dimensional (2D) materials such as graphene, MoS₂ and MXene are actively studied as support for these nanoparticles/nanosheet electrocatalysts to minimize their aggregation as well as participate in the HER reaction to boost the activity. MXenes show the advantage of high electron-conducting properties and availability of HER-active sites, making them attractive for application in electrocatalytic HER.

2.2. Catalyst Activity toward Overpotential, Current Density and Tafel Slope

Because of fundamental activity, hurdles found on both the anode and the cathode are really what primarily causes the excessive potential, also known as overpotential (η), to exist. Therefore, assessing electrocatalysts’ activity and overpotential is a significant feature. The overpotential value associated with a current density of 10 mA/cm² is typically utilized to compare the activity of various catalysts [41,42,43]. The Tafel slope as well as exchange current, which are also additional parameters to assess activity through overpotential vs. reactive current connection, are expressed in the following equation: η = a + b log j, where j is the current density, and η is the overpotential (shown in Figure 3c). The linear connection refers to two notable kinetic parameters for the Tafel plot. The other is the exchange current density (j₀), which may be determined by extracting the current at zero overpotential. One is the Tafel slope (b). According to the kinetics of electron transport, the Tafel slope (b) is associated with the catalytic reaction mechanism [44]. The lower Tafel slope indicates that the electrocatalytic reaction kinetics is occurring more quickly and that the overpotential shift results in a significant increase in current density [45]. Under equilibrium conditions, overall basic electron transfer is described by the exchange current density [46]. Greater charge transfer rates and a lower response barrier are correlated with increased exchange current density.

2.3. Catalyst Activity for Current–Time Curve

Stability is a key factor in determining if a catalyst has the potential to be used in experimental water-splitting cells [47]. There are two approaches to determining stability. One of those is by using chronoamperometry (I-t curve) and chronopotentiometry (E-t curve), which measure both occurrences with time under a constant potential or the potential variation with time under a fixed current [48]. The higher the stability of the catalyst, the faster the tested current or potential is the same. People frequently set a current density of greater than 10 mA/cm² for at least 10 h of testing in order to compare results with those of other research groups [35]. Another method is cyclic voltammetry (CV), which determines current by cycling the potential and often requires more than 5000 cycles at a scan rate (such as 50 mV/s) [49]. The chronopotentiometric technique used linear sweep voltammetry (LSV) to investigate an overpotential change before and after the durability test. The electrocatalyst with the lowest potential change is considered to be the desirable electrocatalyst [50].

2.4. Efficiency toward Turnover Frequency (TOF) and Faradaic Efficiency

Turnover frequency (TOF) is an important parameter for describing the kinetics rate of catalytic sites, which indicates significant activity of the metal catalysts [51]. In addition, the TOF generally shows how several reactants may be transformed into the required product per active site per unit of time [52]. Furthermore, calculating the total TOF value for more heterogeneous electrocatalysts for catalytic sites at every electrode is often an estimation [53]. Furthermore, while being an imperfect method, TOF is a crucial tool for comparing the catalytic activity of diverse catalysts, particularly within a comparable system or under similar conditions [54]. Its faradaic efficiency is a quantitative technique for defining the effectiveness of transferring electrons from an external circuit toward the surface of the electrode for such an electrochemical reaction [55]. The ratio of experimentally examined amount of H₂ or O₂ to theoretically determined mass of H₂ or O₂ is known as faradaic efficiency [56,57,58]. The theoretical values can be estimated using chronoamperometry or chronopotentiometric analysis. On the other hand, the experimental values can be obtained by measuring the gas generation using the water-gas displacement technique or gas chromatography [59]. The focus of research and development affects the study of electrocatalyst stability, activity and efficiency. Furthermore, in accordance with the specific concentration for efficiency, analysis, structural characterization and process determination, the current studies of reaction, efficiency and stability may be gathered in three areas for the synthesis and production of an electrocatalyst [60]. Assessment of the current/potential-time curve, on the other hand, provides information for assessing the stability of the electrocatalyst, which is helpful for practical applications [61]. Finally, estimating overpotential, Tafel slope, exchange current density, faradaic efficiency and turnover frequency are the primary parameters for assessing electrocatalytic kinetics [62]. Notably, coupling these electrochemical approaches to spectroscopic and microscopic levels provides the structural properties required to design a robust and active electrocatalyst.

3. MXenes as Emerging Materials for HER

MXene is a 2D nanomaterial based on transition-metal carbide or nitride, having the general formula of M_n+1X_nT_x, where M = transition metal, X = C and/or N and T_x = surface termination groups such as F, O, OH and Cl. The n number varies from 1 to 4 [148,149,150]. MXene is fabricated from the etching of MAX phases, where their general formula is M_n+1AX_n. ‘A’ is the group 13–16 elements (i.e., Al, Ga), where n varies from 1 to 3. During etching, the ‘A’ layer is removed as the metallic bonding of the M-A bonds is weaker than the ionic/covalently bonded M-X bonds [149,151]. MXene has been extensively studied for the application of electrocatalytic HER as well as OER for water splitting. Past reviews have highlighted the clear potential of different types of MXenes based on Ti, Mo and V. Table 2 summarizes the HER properties of several common MXenes studied for HER. Yet, the Ti-based MXenes, particularly the Ti₃C₂T_x or Ti₃C₂, showed the majority. Termination groups are crucial for MXene’s role in HER and as support. This is owing to their characteristics, including large surface area, metallic properties, high electron conductivity and the presence of hydrophilic termination groups [21]. Gibbs free energy for hydrogen adsorption (ΔG_Hads) and intrinsic HER activity highly depend on ‘M’ transition metal and the surface termination groups. For instance, Mo-based Mo₂CT_x MXenes are more catalytically active for HER than the most commonly used Ti-based MXenes [152]. O-termination groups also benefit HER. It has been shown that O-groups facilitate the desorption of H from the MXene surface, bringing the ΔG_Hads closer to optimum (zero). HER activity is limited if the F-termination coverage is high [153,154]. In terms of their durability, pristine MXenes such as Ti₃C₂T_x are prone to oxidation within a short term (~12 days) when exposed to oxygen in the water. Modifications such as doping will potentially minimize oxidation to extend the MXene’s durability [155].