Hydrogen (H
2) 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 H
2 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 H
2 as fuels further developed for their production using renewable energy approaches since the implementation of the world’s first solar-powered H
2 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 H
2 in hydrogen fuel-cell systems is able to generate electricity with only water and heat as by-products
[2,3][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 CO
2 annually
[4,5][4][5]. Therefore, sustainable hydrogen generation should be sourced from renewable sources while reducing CO
2 emissions. Green hydrogen adopts H
2 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 H
2 [6,7,8][6][7][8]. Integrating electrolysers with renewable sources such as solar and wind power provides us with green and environmentally friendly H
2 sources with no toxic by-products. An alkaline water electrolyser is a developed technology that generates H
2 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 (H
2SO
4) 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][9][10][11][12][13].
2. Hydrogen Evolution Reaction
HER occurs as a cathode reaction within the water electrolyser system, where water is reduced to generate H
2 (2H
+ + 2e
− → H
2)
[27]. It is a two-electron transfer system with one catalytic step in general
[28,29,30][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/cm
2 [32,33][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.
Figure 3. (a) The Volmer, Heyrovsky and Tafel reaction mechanisms of electrochemical water splitting. (b) Electrocatalysis chamber with anodic and cathodic approaches. (c) Overall, HER and OER kinetics. (d) Various electrocatalysts for HER through volcanic plot.
Three possible systems for HER in an acidic environment:
Three possible systems for HER in an alkaline environment:
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 (H
3O
+) are reduced to vapor dihydrogen (H
2)
[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 H
2 (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, MoS
2 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/cm
2 is typically utilized to compare the activity of various catalysts
[41,42,43][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/cm
2 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 H
2 or O
2 to theoretically determined mass of H
2 or O
2 is known as faradaic efficiency
[56,57,58][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][63][64][65]. 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][64][66]. 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 1 summarizes the HER properties of several common MXenes studied for HER. Yet, the Ti-based MXenes, particularly the Ti
3C
2T
x or Ti
3C
2, 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 Mo
2CT
x MXenes are more catalytically active for HER than the most commonly used Ti-based MXenes
[152][67]. 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][68][69]. In terms of their durability, pristine MXenes such as Ti
3C
2T
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][70].
Table 21.
Summary of HER properties of commonly used MXenes under different structures and conditions.
MXene |
Structure |
Electrolyte |
Overpotential (mV) @ 10 mA/cm2 |
Tafel Slope (mV/dec) |
Ti3C2Tx [156,157][71][72] |
Multilayer |
1 M KOH |
>600 |
- |
Ti3C2Tx [158][73] |
Few layer |
1 M KOH |
>500 |
>100 |
Ti3C2Tx [152][67] |
Few-layer |
0.5 M H2SO4 |
609 |
124 |
Ti3C2Ox [159][74] |
Few-layer |
0.5 M H2SO4 |
190 |
60.7 |
Ti3C2(OH)x [159][74] |
Few-layer |
0.5 M H2SO4 |
217 |
88.5 |
Mo2CTx [152][67] |
Few-layer |
0.5 M H2SO4 |
283 |
82 |
Mo2CTx [160][75] |
Few-layer |
1 M KOH |
300 |
110 |
Mo2CTx [161][76] |
Multilayer |
1 M KOH |
280 |
118 |
The electronic structure of MXene plays a role in the intrinsic activity toward HER. Electronic properties are affected by a number of factors, including the ‘M’ element, surface terminations, layer thickness, effects of intercalation, and adding dopants. Pristine M
2X MXenes are primarily metallic. The presence of termination groups on the basal planes results in additional energy bands below the Fermi level that shift the MXene into a semiconductor, such as those of Ti
2CO
2. Ti
3C
2T
x can exhibit metallic properties where it was found that F-groups occupy the face-centred cubic adsorption site while O-groups have a partial occupation on the bridge sites and hexagonal close-packed sites
[162][77]. Further, electronic properties may vary between multilayer and few-layer Ti
3C
2T
x given that few-layers may offer a larger in-plane conductivity
[163,164][78][79]. Ti- and V-based MXenes also offer very high electron conductivity exceeding 1000 S/cm
[165][80]. High electron conductivity is desired for active HER . The electronic properties are adjusted by doping the MXene and introducing the HER-active materials. MXene also affects the electronic properties of the interacting material. Kong et al.
[166][81] found that the Ti site favours H adsorption in Ti
3C
2O
2 quantum dots (QDs). Graphene is able to form an interfacial interaction with the Ti
3C
2O
2 QDs that stabilizes the C—O configuration and shifts the d-band centre energy level by 0.4–0.5 eV in the Ti
3C
2O
2. The graphene-Ti
3C
2O
2 QDs with Gibbs free energy closer to zero value are more favourable towards HER. On the other hand, Ren et al.
[167][82] reported that H-adsorption occurs on the O-sites within the hybrid of MoS
2@Mo
2CO
2. The catalyst also exhibits metallic properties and Gibbs free energy for hydrogen adsorption ΔG
Hads closer to optimal. Interaction between the MoS
2 and the Mo
2CO
2 is also interfacial through charge transfer. The exchange of electrons between the two components is one of the drivers of improved HER. Single-atom catalysts (SACs) and doping on MXene have positive outcomes for HER. In the case of Ru SACs on Ti
3C
2T
x with N-doping, Ti—N and N—Ti—O bonds are formed after N-doping on the Ti
3C
2T
x. Ru SACs are attached in the form of pyrollic-N—Ru bonds. Interactions between MXene, Ru SACs and the N-groups result in a greater total density of state (TDOS) value indicating better electron conductivity. The partial density of state (PDOS) of Ru
SA-N-Ti
3C
2T
x near the Fermi level is attributed to the d-orbitals of Ti and Ru, where the Ru SACs brought about d-electron domination near the Fermi level that in turn benefits HER
[168][83]. For transition-metal SACs, Co SACs in V
2CT
x MXenes showed that electron transfer occurred between the Co SACs and V
2CT
x through —O— bonds that facilitate early-stage water dissociation. The d-band centre of Co@V
2CT
x is brought to an intermediate level and has high electron cloud distribution. d-band centre that is nearer the Fermi level is more favourable for adsorption/desorption of intermediates for both HER and OER
[169][84]. Therefore, the electronic structure of MXene would tailor the electron conductivity and intrinsic activity towards HER. Dopants and introducing HER-active materials potentially result in electron redistribution that may lead to more favourable HER binding energies.
Another factor is that the morphology of MXenes can be tailored into several different morphological structures. For example, Multilayer MXenes are intercalated and then delaminated to form nanosheets of few-layer MXenes. Further modifications can produce structures with multiple pores
[170][85] and unique structures such as crumpled
[171][86], rolled
[110][87], and spheres
[172][88]. Various morphology changes the overall surface area of MXene, the termination groups/active sites’ accessibility through the electrocatalytic active surface area (ECSA), electron conductivity, its ability to anchor the HER-active materials (Ni-based HER catalysts, single-atom catalysts, etc.) and their durability. This affects the overall HER activity of the MXene and MXene composites/hybrid catalysts. Several of these different morphologies of MXenes have been studied for HER.