Nonprecious metal's graphene‐supported electrocatalysts for hydrogen evolution reaction: Fundamentals to applications

Carbon EnergyVolume 2, Issue 1 p. 99-121 REVIEWOpen Access Nonprecious metal's graphene-supported electrocatalysts for hydrogen evolution reaction: Fundamentals to applications Asad Ali, Asad Ali Collaborative Innovation Center of Sustainable Energy Materials, Guangxi Key Laboratory of Electrochemical Energy Materials, Guangxi University, Nanning, China State Key Laboratory of Processing for Non-ferrous Metal and Featured Materials, Guangxi University, Nanning, ChinaSearch for more papers by this authorPei Kang Shen, Corresponding Author Pei Kang Shen pkshen@gxu.edu.cn orcid.org/0000-0001-6244-5978 Collaborative Innovation Center of Sustainable Energy Materials, Guangxi Key Laboratory of Electrochemical Energy Materials, Guangxi University, Nanning, China State Key Laboratory of Processing for Non-ferrous Metal and Featured Materials, Guangxi University, Nanning, China Correspondence Pei Kang Shen, Collaborative Innovation Center of Sustainable Energy Materials, Guangxi Key Laboratory of Electrochemical Energy Materials, Guangxi University, 530004 Nanning, China. Email: pkshen@gxu.edu.cnSearch for more papers by this author Asad Ali, Asad Ali Collaborative Innovation Center of Sustainable Energy Materials, Guangxi Key Laboratory of Electrochemical Energy Materials, Guangxi University, Nanning, China State Key Laboratory of Processing for Non-ferrous Metal and Featured Materials, Guangxi University, Nanning, ChinaSearch for more papers by this authorPei Kang Shen, Corresponding Author Pei Kang Shen pkshen@gxu.edu.cn orcid.org/0000-0001-6244-5978 Collaborative Innovation Center of Sustainable Energy Materials, Guangxi Key Laboratory of Electrochemical Energy Materials, Guangxi University, Nanning, China State Key Laboratory of Processing for Non-ferrous Metal and Featured Materials, Guangxi University, Nanning, China Correspondence Pei Kang Shen, Collaborative Innovation Center of Sustainable Energy Materials, Guangxi Key Laboratory of Electrochemical Energy Materials, Guangxi University, 530004 Nanning, China. Email: pkshen@gxu.edu.cnSearch for more papers by this author First published: 29 December 2019 https://doi.org/10.1002/cey2.26Citations: 102AboutSectionsPDF ToolsExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat Abstract Sustainable production of hydrogen is a hopeful requirement of a strategic future economy and development. Water splitting driven by electricity is a favorable pathway for renewable hydrogen production. This critical review highlighted recent efforts toward the development of the nanoscale synthesis of nonprecious metal's graphene-supported electrocatalysts and their electrocatalytic features for remarkable hydrogen evolution reaction (HER). Different essential nonprecious metal's graphene-supported electrocatalysts, including metal carbides, sulfides, phosphides, selenides, oxides, and nitrides are reviewed. In the exploration, attention is given to the strategies of activity enhancement, the synthetic approach, and the composition/structure electrocatalytic-performance relationship of these HER electrocatalysts. We are hopeful that this review confers a new momentum to the rational design of remarkable performance nonprecious metal's graphene-supported electrocatalysts and comprehensive guide for researchers to utilize the subject catalysts in regular water splitting. 1 INTRODUCTION In recent years, renewable energy technology is the basic demand for future strategic development and the economy to solve the environmental energy crisis and pollution.1 The excessive utilization of fossil fuels generates too much CO2, which entails searching for renewable energy sources.2 Hydrogen has the highest energy density (146 kJ g−1),3 which has proven its potential with global energy security, eco-friendly features, and as an ideal energy career toward sustainable energy.4-6 Furthermore, H2 is also used in petroleum refining and the production of ammonia for fertilizer, making them a promising chemical feedstock in the developed era. Presently, the steam synthesis from fossil resources utilized both CO2 and H2, which are the prime source for large-scale H2 production. This production strategy aggravates the consumption of fossil fuels and as well as involves in CO2 emission. Therefore, searching a green, renewable, and efficient approach for wide-ranging H2 production is urgently demanded.7, 8 In recent years, electrochemical splitting of water has been broadly used to produce ultrapure hydrogen with zero carbon emission and can be considered parallel to the electrical energy generated from wind, solar, and other sustainable sources.9 However, the practical implications of water are limited due to the large overpotential of cathodic hydrogen evolution reaction (HER). Nanostructural electrocatalyst and supporting materials are very crucial to decide the activity and cost of the overall electrochemical process.10 For large-scale electrocatalytic H2 production, the electrocatalysts must possess (a) superior conductivity, (b) highly active electrocatalytic sites, (c) capability to withstand high current density, (d) strong acid stability, (e) low fabrication cost for commercialization, and eventually (f) earth abundant.11 Generally, rare earth noble metal Platinum (Pt) has been considered a principal catalyst for HER.3, 12 Unfortunately, the commercial implications of the precious metals are impeded by scarcity and high cost.13 A crucial approach is developed to replace the high-priced metal-based electrocatalyst with nonprecious metals.14 Transition metals (i.e. cobalt, iron, molybdenum, nickle, and tungsten, etc) based electrocatalysts have attracted substantial attention as their promising electrocatalytic activity, such as nonprecious metal sulfides,15 carbides,16 selenides,17 phosphides,18 nitrides,19 and oxides20 would globally promote for potential green energy sustainability. Hence, inspired by these multiple challenges, the study for highly efficient, economical, earth abundant, and long-term durability has been made of substantial interest toward earth-abundant metals electrocatalysts for HER.21-23 In addition, innumerable kinds of electrocatalyst supports have been designed in the last decades, for example, graphene, nanocarbon materials, carbon black, and polymers, and so forth owing to their abundant reserves and promising electrolytic performance.24-26 Among the various supporting materials, graphene is one of the most attractive support used for electrocatalysts. Graphene is a novel individual in the carbon family discovered by Novoselov et al27 in 2004 has opened a new chapter in the development of material science. Significantly, graphene is a two-dimensional (2D) hexagonal-packed carbon atoms nanosheet and fascinating characteristics have been exhibited, including superior electrical conductivity (5-6.4 × 106 S/m),28 remarkable mechanical strength, and huge theoretical surface area (2630 m2 g−1).29, 30 Graphene composed of sp2 carbon atom bonded three sigma (σ) bonds with adjacent carbon atoms.31 Graphene-based nanostructural materials including graphene as electroactive component and as well as functionalized support toward HER has produced significant interest.32-35 Among the various electrocatalysts, it is anticipated that nonprecious metal's graphene-supported electrocatalysts are increasingly finding water electrolytic applications and much research attention within catalysis due to its intrinsic properties.36 Substantial development has been made in the progress of nonprecious metal's graphene-supported electrocatalysts, for example, MoS2 QDs/rGO,37 WOxNWs/N-rGO,38 NiPS3@defective graphene,39 Fe2P@rGO,40 and CoO@Co/N-rGO,41 etc for HER. Over the last decades, as an emerging energy zone, several reviews on HER are available7, 42-45 but to the best of our knowledge, a comprehensive review on the nonprecious metal's graphene-supported electrocatalysts for HER is still not published. Considering the recent progress in the subject field, the current review is organized to highlight recent developments of the nonprecious metal's graphene-supported electrocatalysts toward HER (Figure 1). For instance, in section 2, we discuss a detailed survey for the mechanism of HER on nonprecious metal's graphene-supported electrocatalysts. In section 3, electrochemical evaluating parameters for HER are discussed. Our focus is on the applications of nonprecious metal's graphene-supported electrocatalysts toward HER in section 4. In the following section, we further explored the HER performances of nonprecious metal's graphene-supported electrocatalysts, including sulfides, carbides, selenides, phosphides, nitrides, and oxides. In addition, our review will be focused on the morphological synthesis of electrocatalyst and critical aspects of working mechanisms that have been designed to produce novel nonprecious metal's graphene-supported electrocatalysts. Finally, in section 5, we contribute some comprehension into the current progress of nonprecious metal's graphene-supported electrocatalysts along with personal viewpoints in future research trends and critical challenges for this emerging topic. We expect that the current review can be useful in a sustainable energy community and provides a comprehensive guide for researchers to push forward the nonprecious metal's graphene-supported electrocatalysts toward HER. Figure 1Open in figure viewerPowerPoint Schematic illustration of nonprecious metal's graphene-supported electrocatalysts for hydrogen evolution reaction (HER) 2 BASIC ELECTROCHEMISTRY OF HER The electrochemical HER is a cathodic half reaction that could be via the Volmer-Heyrovsky pathway or Volmer-Tafel pathway in both acidic and alkaline solutions shown in Figure 2.46 Volmer reaction is the first common step, in which electron couple with proton accumulated on the surface of the catalyst to give an intermediate adsorbed hydrogen (H*) atom. Furthermore, Volmer reaction is consequently accompanied by either amalgamation of an adsorbed H* atom with an electron and a proton (Volmer-Heyrovsky pathway) or combination of two H* atoms (Volmer-Tafel pathway). Figure 2Open in figure viewerPowerPoint Hydrogen evolution reaction (HER) pathway in acidic and alkaline media. (Reproduced with permission: Copyright 2019, Elsevier Ltd.46) In acidic solution: H + + e − + * → H * ( Volmer ) (1) H * + H + + e − → H 2 ( Heyrovsky ) (2) Or 2 H * → H 2 ( Tafel ) (3) In alkaline solution: H 2 O + e − → OH − + H * ( Volmer ) (4) H * + H 2 O + e − → OH − + H 2 ( Heyrovsky ) (5) Or 2 H * → H 2 ( Tafel ) (6)Where * denotes active sites of nonprecious metal-based graphene-supported catalyst. In both pathways, that is, Volmer-Heyrovsky and Volmer-Tafel the formation of H* intermediate is involved. The adsorption ability of hydrogen is the most significant factor considered during designing electrocatalysts. 3 ELECTROCHEMICAL EVALUATING PARAMETERS FOR HER To interpret the electrocatalytic performance of given HER electrocatalysts, there are key evaluating parameters that need to be calculated carefully. They mainly include onset potential/overpotential, electrochemical impedance spectroscopy (EIS) analysis, Tafel plot and exchange current density, Faradic efficiency, turnover frequency (TOF), Gibs free energy, and stability. 3.1 Onset potential/Overpotential The onset potential is the applied potential with the perceptible cathodic current. Although there is no clear way to calculate the onset potential, it is usually determined at a current density from 0.5 mA cm−2 or 1.0 mA cm−2. The supplementary potential beyond the thermodynamic demand to run a water splitting at a precise rate is called overpotential (η). Furthermore, according to the electrolysis equation, which consists three parts ( E electrolysis = E reversible + Δ E irreversible + IR ) ; ∆Eirreversible is called overpotential for HER, Ereversible expresses the theoretical disintegration voltage, and IR represents the drop voltage occurred via contact point, wires, and electrolyte. Usually, the overpotential is calculated at a current density of 10 mA cm−2 are widely used to evaluate the performances of the electrocatalysts regarded a standard reference by many researchers.47, 48 Ideal electrocatalysts are those which can give low overpotential at higher current density. To determine the precise overpotential, the scan rate of polarization curves should be low such as 2 to 5 mV s−1 to reduce the capacitive current. Currently, almost potential is compared with the reference hydrogen electrode (RHE). The common references are saturated calomel electrode (SCE) and Ag/AgCl electrode. According to the following calculations, the potential of common references should be converted potential vs RHE for unified standard evaluation. Conversion formula in case of Ag/AgCl, E ( vs . ⁢ RHE ) = E ( vs . Ag / AgCl , saturated KCl ) + E Ag / AgCl , saturated KCl 0 + 0.0592 * pH , (7) where E0Ag/AgCl, saturated KCl = 0.197 V; And conversion formula in case of SCE, E ( vs RHE ) = E ( vs SCE ) + E SCE 0 + 0.0592 * pH . (8) 3.2 Tafel plot and exchange current density Tafel plot represents the reliance of steady-state current density on a variation of overpotentials (η). Basically, the η is logarithmically associated with the current density and the linear portion of the Tafel plot is placed to the Tafel equation (η = a + b log j), the Tafel slope (b) can be obtained. Tafel equation is very important, we can get two parameters, that is, Tafel slope (b) and exchange current density (j0). Tafel slope is basically associated with the electrocatalytic procedure of the catalyst reaction and exchange current density is acquired when overpotential is supposed to be zero, explains the intrinsic electrocatalytic performance of the catalyst under equilibrium environment. Notably, ideal electrocatalysts are those having small Tafel slope and high exchange current density, for example, the smaller Tafel slope (b = 90 mV dec−1) of Ni3FeN/r-GO-20 is declared as an effective electron transportation for HER compared with other catalysts, which are Ni3Fe/r-GO-20 (b = 109 mV dec−1), Ni3N/r-GO-20 (b = 111 mV dec−1), Fe2N/r-GO-20 (b = 120 mV dec−1), and Ni3FeN (b = 123 mV dec−1) (Figure 3A).49 The Tafel slope of Fe3W3C nanorods (NRs)/rGO is 50 mV dec−1 (Figure 3B), which suggests that the hydrogen evolution Fe3W3C NRs/rGO probably proceed via Volmer-Heyrovsky reaction mechanism (H3O+ + e− → Habs + H2O; H3O+ + e− + Habs → H2 + H2O), in which the discharge reaction is fast and H2 is evolved by a rate-determining ion + atom reaction. The Tafel slope of WC NRs/rGO is 101 mV dec−1 (Figure 3B), which suggests that the discharge reaction is slow.16 The Tafel slopes of MoS2/N-rGO prepared from 160°C to 220°C are 57.7, 41.3, 70.4, and 126.6 mV dec−1, respectively (Figure 3C). Accordingly, the observed Tafel slope value of MoS2/N-rGO-180 catalyst is the smallest except for the commercial 20% Pt/C catalyst (30 mV dec−1), suggesting that the Volmer-Heyrovsky reaction mechanism dominates in the HER process and the electrochemical desorption is the rate-determining step.50 Furthermore, exchange current density (j0) is a significant parameter to assess the HER activity of an electrocatalyst. For example, the J0 value (1.24 mA cm−2) of Nickle Vanadate (NV)/N-rGO-2 is very close to the commercial Pt/C catalyst (1.62 mA cm−2) compared with other electrocatalysts NV (0.05 mA cm−2), NV/N-rGO1 (0.86 mA cm−2), and NV/N-rGO3 (1.04 mA cm−2) (Figure 3D).51 Figure 3Open in figure viewerPowerPoint (A) Tafel plots of Ni3FeN, Fe2N/r-GO-20, Ni3N/r-GO-20, Ni3Fe/r-GO-20, and Ni3FeN/r-GO-20. (Reproduced with permission: Copyright 2017, American Chemical Society.49). (B) Tafel plots of WC NRs/rGO, Fe3W3C NRs/rGO, and Pt/C. (Reproduced with permission: Copyright 2019, Elsevier Ltd.16). (C) Corresponding Tafel plots of MoS2/N-rGO-T prepared at different temperatures and commercial 20% Pt/C catalyst. Catalyst loading is 0.14 mg cm−2. Electrolyte is N2-saturated 0.5 M H2SO4. Scan rate is 5 mV s−1. (Reproduced with permission: Copyright 2016, WILEY, Weinheim.50). (D) Exchange current density (j0) of NV/N-rGO3, NV/N-rGO2, NV/N-rGO1, NV, and Pt/C electrocatalysts. (Reproduced with permission: Copyright 2019, The Royal Society of Chemistry.51) 3.3 Electrochemical impedance spectroscopy To acquire more comprehension of the HER catalytic performance of electrocatalyst, EIS is a significant method to characterize electrode kinetics and interface reactions in HER. The charge-transfer resistance (Rct) is associated with the interface electrocatalytic kinetics of the electrode, which can be achieved from the diameter of semicircles in the low-frequency zone. If the Rct value is low, so obviously the rate of reaction would be faster. EIS analysis is performed in the range of frequency from 0.01 to 105 Hz. Furthermore, before the EIS analysis, the voltage of an open circuit should be tested and consequently performed under HER voltage at 10 mA cm−2. For example, to know the mechanism of HER activity for MoS2 nanosheets loaded on vertical graphene interconnected to conductive carbon cloth (MoS2NS/VG/CC), the Rct of various electrocatalysts was analyzed. The Nyquist plots obtained from EIS show an extraordinary decrease in Rct of MoS2NS/VG/CC with the lowest value of 5.0 Ω compared with VG/CC (85 Ω), MoS2NS/CC (50 Ω), and CC (103 Ω) (Figure 4A). The significant reduction of Rct value shows faster electron transfer between the active edges of MoS2 and the graphene surface, which can be attributed to the increased active edges of MDNS and well-improved conductivity of graphene. Overall, the remarkable HER kinetics of MoS2NS/VG/CC attributed to low-ohmic resistance contact between VG and MoS2NS.52 As shown in Figure 4B, the semicircle is considered as the charge-transfer resistance (Rct) of H+ reduction at the electrode-electrolyte interface. Obviously, Iron phosphide nanorods supported by vertically aligned graphene nanosheets/carbon cloth (FePNRs/VAGNs/CC) exhibits a smaller Rct with a value of only 3 Ω than FePNRs/CC (11 Ω), indicating a more efficient electron transport at the FePNRs/VAGNs/CC-electrolyte interface.53 Figure 4Open in figure viewerPowerPoint (A) Nyquist plots of MoS2NS/CC, MoS2NS/VG/CC, VG/CC, and CC. (Reproduced with permission: Copyright 2015, Elsevier Ltd.52). (B) The Nyquist plots of FePNRs/CC and FePNRs/VAGNs/CC; the inset is the equivalent circuit used to fit the impedance spectra, in which the Rs, Rct, and C represent the electrolyte resistance, electron-transfer resistance, and the chemical capacitance, respectively. (Reproduced with permission: Copyright 2017, The Royal Society of Chemistry.53) 3.4 Turnover frequency To date, the evaluation of electrocatalytic performance mostly depends on the differentiation of overpotential at similar current density. Nevertheless, various techniques are consistent with dissimilar loading mass of electrocatalysts, making it inconvenient to evaluate the electrocatalytic activity. To solve this complication, TOF is prudently used as a significant parameter to assess the electrocatalytic performance. The electrocatalyst can convert the number of reactants to a required yield per electrocatalytic site per unit time and show that the intrinsic performance of each electrocatalytic site is called TOF. The higher TOF value exhibits remarkable performance. Practically, it is almost difficult to have every atom, in which nanoparticles are equally accessible and catalytically active. However, when identical nanomaterials are compared, such results could still be useful and relevant. There is a simple method used to approximate TOF values based on the two steps. First, the calculation of active sites (n), which is ascertained by CVs data, carried out at −0.2 to +0.6 V vs RHE in 1.0 M phosphate buffer solution (pH = 7) with a sweep rate of 20 or 50 mV s−1. Consequently, the volumetric charges, that is, anodic and cathodic are described during one single blank assessment that is enumerated. Considering the one-electron system for both oxidation and reduction, the upper limit of active sites can be calculated by the following equation: n = Q / 2 F . (9) Second, calculation of TOF (s−1) values with the help of the following equation: TOF = 1 / 2 Fn , (10)where F is the Faraday constant (96 485 C mol−1), n is the number active sites (mol), Q is the volumetric charge, and I is the current (A) during linear sweep voltammetry. The factor 1/2 in the equation illustrates that two electrons are necessary to generate one molecule of hydrogen from two hydrogen ions. TOF of hydrogen molecules generated per second (s−1) for each active site was calculated to assess the intrinsic performance of different electrocatalysts. The TOF can be measured by Jaramillo's method. For example, the MoS2QDs/rGO electrocatalyst exhibits the highest TOF of 1.39 seconds−1, which indicates a remarkable intrinsic HER performance.37 The TOF values of Fe2P-nano dendrite (ND) loaded on fluffy graphene (FGr) is 0.09 and 0.34 seconds−1 at an overpotential of 100 and 150 mV, respectively. The higher TOF value of Fe2P-ND/FGr hybrid exhibits remarkable performance compared with Fe2P/Gr and Fe2P electrocatalysts shown in Figure 5A.54 The calculated TOF, which is usually used to determine the active sites of the catalyst confirms a better catalytic ability for MoS2/N-rGO0.5 catalyst (TOF = 0.89 seconds−1 at an overpotential of 240 mV) when compared with the MoS2/rGO (TOF = 0.51 seconds−1) and pure MoS2 (TOF = 0.29 seconds−1) at the same overpotential (Figure 5B). Therefore, the vertically aligned nanosized structure of MoS2, nitrogen incorporation of rGO, and three-dimensional (3D) network structure have led to excellent HER performances of the MoS2/NrGO0.5 composite.55 Figure 5Open in figure viewerPowerPoint (A) Turnover frequency (TOF) values of the as-prepared Fe2P-ND/FGr, Fe2P/Gr, and Fe2P at different overpotentials. (Reproduced with permission: Copyright 2016, The Royal Society of Chemistry.54). (B) Turnover frequency (TOF) of various catalysts in a 0.5 M H2SO4 solution at a scan rat of 50 mV s−1. (Reproduced with permission: Copyright 2017, Elsevier Ltd.55) 3.5 Faradic efficiency The efficiency by which the electrons transfer by an external circuit to participate in HER is called Faradic efficiency (FE). FE is the ratio of experimentally quantified H2 to theoretically calculated H2. Practical production of H2 can be measured by gas chromatography or water gas displacement method and the theoretical production of hydrogen can be calculated by potentiostatic or galvanostatic electrolysis by integration. FE plays a key role in the observation of complete water splitting. For example, the FE of the experimentally measured and theoretically calculated amount of hydrogen for Mo2N-Mo2C/holy reduced graphene (H-rGr) is closed to 100% in both alkaline (1.0 M KOH) and acidic (0.5 M H2SO4) solutions (Figure 6A,B).56 Figure 6Open in figure viewerPowerPoint The theoretical and experimental amount of hydrogen vs time for Mo2N-Mo2C/H-rGr in (A) 0.5 M H2SO4 solution and (B) 1.0 M KOH solution of −200 mV for 60 minutes. (Reproduced with permission: Copyright 2018, WILEY-VCH Verlag GmbH & Co. KGaA.56) 3.6 Gibbs free energy The adsorption or desorption of hydrogen competes on the surface of electrocatalyst, which can be described by applying the Gibbs free energy (∆GH*) calculations. The ∆GH* for hydrogen adsorption on the electrocatalyst surface determines the HER kinetics. The calculated ∆GH* for hydrogen can be applied as a critical descriptor to evaluate the catalytic HER performance of the electrocatalysts. For example, the ∆GH* value of the nitrogen site of the interface of N-Mo-C is the smallest (0.046 eV), which is very small compared with single Mo2C (−0.405 eV) and Mo2N (−0.449) systems (Figure 7A).56 Furthermore, Figure 7B exhibits the ∆G on various electrocatalysts for HER. The adsorption of H* of bare Co reveals to be too vigorous while the naked graphene nanosheets were too weak, indicating their low performances toward HER. Although, after fabricating Co on graphene, the ∆G was tuned. The variation of ∆G of the N-doped graphene declared that the N-doping also beneficially promote HER performance. Moreover, the G value of the HER for nanohybrid of Co clusters, N-doping, and graphene (Co@N-C) was almost close to zero, indicating the best performance for HER.57 Figure 7Open in figure viewerPowerPoint (A) Diagram of calculated Gibbs free energy (∆GH*) for HER based on the different catalysts. (Reproduced with permission: Copyright 2018, WILEY, Weinheim.56). (B) The calculated Gibbs free energy (∆GH*) for HER on Co@N-C, Co@C, N-C, C, and Co. (Reproduced with permission: Copyright 2018, WILEY, Weinheim.57) According to the Sabatier principle,58 the overall rate of reaction for HER reaches maximum if the ∆GH* value is near to zero, represented as “volcano plot” for different electrocatalysts surfaces. Based on this principle, ideal electrocatalysts should to form a bond with an adsorbed hydrogen atom (Hads) that is the optimum for easy charge transfer and break readily to liberate as a hydrogen gas. On the other side, if the interaction between electrocatalyst and hydrogen is too strong, then the desorption reaction (Heyrovsky/Tafel) will be limited and if the interaction is too weak, it definitely creates an obstacle for Volmer reaction.59 3.7 Stability Stability is another significant parameter to assess the HER activity of electrocatalyst. Remarkable morphological and electrolytic durability for HER electrocatalysts is of crucial significance and needs to be explored because that has practically potential implications and mostly operates in strongly reductive electrolytes, that is, pH 0 to 14. There are three methods used for characterizing the electrolytic durability of an electrocatalyst toward HER, namely, chronopotentiometry (current-time curve) or chronoamperometry (potential time curve) and CV test. The durability is assessed by comparing the polarization curves before and after the continuous cycles (usually from 500 to 1000 cycles). If the final polarization curve almost overlays with an initial one or slightly increases (less than 10%) in the overpotential compared with the initial polarization curves, it argues excellent stability. For example, to evaluate the stability of the Co@N-CNTs@rGO hybrid electrocatalyst, the initial and final LSV curves (Figure 8A) clearly shows that the LSV curves of 1st and 1000th were almost overlapped.57 Figure 8Open in figure viewerPowerPoint (A) The comparison of the initial and final LSV curve. Inset in (B): FESEM images of the Co/N-CNTs/rGO after current-time analysis. (Reproduced with permission: Copyright 2018, WILEY, Weinheim.57) Chronopotentiometry or chronoamperometry analysis is tested for a specific duration with a given applied potential or current density. Among the researcher's chronopotentiometry (i-t curves) is a well-known test and the potential is normally 10 mA cm−2. Generally, 10 hours is considered as comparison standard but depends on the researcher evaluation strategy and electrocatalyst's nature. For example, the stability of Co/N-CNTs/rGO hybrid was also assessed for long term at higher current density. The Co@N-CNTs@rGO hybrid maintains a current density of 20 mA cm−2 for 100 hours with a negligible decline, as shown in Figure 8B. The characteristics of the overall structure of the Co/N-CNTs/rGO hybrid was still well-retained (Figure 8B).57 4 NON-PRECIOUS METAL'S GRAPHENE-SUPPORTED ELECTROCATALYSTS Over the last decades, promising development has been achieved for the synthesis of nonprecious metal's graphene-supported catalysts as substitutes to precious metal nanohybrids for effective HER (Table 1 list a detailed comparison of their catalytic activities). The current review paper mainly focused on earth-abundant nonprecious metal sulfides, carbides, selenides, phosphides, nitrides, and oxides, hybridized with graphene-based nanostructural materials including graphene as electroactive component and as well as functionalized support toward the HER has produced significant interest. Nonprecious metal nanosheets provide a relatively huge number of exposed active sites but poor electron transport and intrinsic conductivity greatly limit their use in catalytic applications. Furthermore, the introduction of graphene greatly promotes the texture structure and electrical conductivity of the electrocatalytic system.60 Owing to the vigorous interaction between metal ions and graphene, the size of synthesized metal nanoparticles is smaller, as well as excellent dispersion compared with bulk metals results in a large number of active sites. Moreover, the synergetic effect of active metal and graphene further promotes excellent charge transfer and increase the performance accordingly. Table 1. HER performance summary of nonprecious metal's graphene-supported electrocatalysts Electrocatalyst Electrolyte Onset potential/Overpotential Tafel slope (mV dec−1) Stability Ref. WS2/graphene/Ni foam 0.5 M H2SO4 −119 mV (vs. RHE) at 10 mA cm−2 43 8 hour