Heyrovsky Reaction Research Articles

Electrocatalytic study of the hydrogen evolution reaction on MoS2/BP and MoSSe/BP in acidic media.

Molecular hydrogen (H2) production by the electrochemical hydrogen evolution reaction (HER) is being actively explored for non-precious metal-based electrocatalysts that are earth-abundant and low cost like MoS2. Although it is acid-stable, its applicability is limited by catalytically inactive basal planes, poor electrical transport and inefficient charge transfer at the interface. Therefore, the present work examines its bilayer van der Waals heterostructure (vdW HTS). The second constituent monolayer boron phosphide (BP) is advantageous as an electrode material owing to its chemical stability in both oxygen and water environments. Here, we have performed first-principles based calculations under the framework of density functional theory (DFT) for the HER in an electrochemical double layer model with the BP monolayer, MoS2/BP and MoSSe/BP vdW HTSs. The climbing image nudged elastic band method (CI-NEB) has been employed to determine the minimum energy pathways for Tafel and Heyrovsky reactions. The calculations reveal that the Tafel reaction shows no reaction barrier. Thereafter, for the Heyrovsky reaction, we obtained a low reaction barrier in the vdW HTSs as compared to that in the BP monolayer. Subsequently, we have observed no significant difference in the reaction profile of MoS2/BP and MoSSe/BP vdW HTSs in the case of 2 × 2 supercell configuration. However, in the case of 3 × 3 and 4 × 4 configurations, MoSSe/BP shows a feasible Heyrovsky reaction with no reaction barrier. The coverages with 1/4H+ concentration (conc.) deduced high coverage with low conc. and low coverage with high conc. to be apt for the HER via the Heyrovsky reaction path. Finally, on observing the activation barrier of the Heyrovsky pathway along with that of second H adsorption at the surface, the Heyrovsky path is expected to be favoured.

Кинетика реакции выделения водорода на LnNi2Ge2-электродах (Ln = лантанид) в щелочном растворе

The kinetics of hydrogen evolution reaction on the intermetallic compounds LnNi2Ge2 (Ln = Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm, Lu) in 1 M KOH solution were studied using polarization measurements and electrochemical impedance spectroscopy. The Tafel constants vary in the intervals: а = 0.475–0.52 V; b = 0.070–0.078 V. The cathodic process was shown to be the combination of the hydrogen evolution reaction and hydrogen absorption reaction; the reaction of H2 evolution proceeds through the Volmer – Heyrovsky mechanism with the rate-determining Heyrovsky reaction. The values of the rate constants of the elementary steps were estimated. The dependences of the rate constants on the atomic number of Ln pass through extremums, these are assumed to correspond to a maximal metal-hydrogen bond strength.

Insight into the mechanism of hydrogen permeation inhibition by cerium during electroplating: Enhanced kinetics of hydrogen desorption

In our previous work, we found that hydrogen permeation can be noticeably reduced during Ni–Cu electroplating by the addition of Ce salt to the plating solution. The mechanism of hydrogen permeation inhibition via Ce salt was further studied in the present work. Through the Iver–Pickering–Zamenzadeh (IPZ) model fitting of the kinetic of hydrogen evolution reaction, we found that the trace Ce salt that precipitated during electroplating could improve Tafel reaction kinetic parameters and reduce the strength of the Ni–H and Cu–H bonds due to its abundant d/f electrons and enough d/f orbitals. Meanwhile, Ce can provide electrons for the Heyrovsky reaction. These effects promoted surface electron migration and thus led to the desorption of adsorbed hydrogen atoms (Hads) and the decreased diffusion of Hads into the Ni–Cu coatings. The accuracy of the IPZ model fitting results was verified by hydrogen evolution rate experiments during the electroplating process. Hence, Ce salt can effectively inhibit hydrogen permeation and reduce the dehydrogenation annealing time, thereby showing great potential for energy saving and emission reduction in the electroplating industry.

Mechanistic insights into hydrogen evolution reaction on Ni2B(001) facet using first-principle calculations

In this paper, first-principle calculations based on density functional theory (DFT) were used to investigate the performance and mechanism of the hydrogen evolution reaction (HER) on the typical active (001) facet of the novel electrocatalyst Ni2B. There were two types of atomic distribution on the Ni2B (001) surface, namely the B-rich surface and the Ni-rich surface. The investigation of the reaction mechanism revealed that the Volmer-Heyrovsky mechanism was easier to be realized on this Ni2B (001) facet, and the Heyrovsky reaction was the reaction rate-determining step. The Gibbs free energy(ΔGH) on the B-rich surface was - 0.02 eV, which was closer to 0 eV than that on the Ni-rich surface of Ni2B (001). The HER reactivity on the Ni-rich surface was increased by Cr-doping (ΔGH = - 0.01 eV), which indicated that the introduction of other transition metal atoms might effectively increase the HER electrocatalysis activity of Ni2B (001) surface. This work paves a new avenue for exploring efficient and durable non-precious metal electrocatalysts for HER in acidic medium.

ML-guided design and screening of chalcogenide catalysts for hydrogen evolution reaction

Transition metal chalcogenides are regarded as the promising electrocatalysts for the hydrogen evolution reaction (HER). However, based on the larger chemical composition space of transition-metal single atom and chalcogenides, the design and screening of excellent HER electrocatalysts remain the challenges. Herein, a machine learning (ML) model was proposed to predict the HER performance of single-atom chalcogenide catalysts, and used to screen the excellent electrocatalysts by combining with density functional theory calculations. The results show that the ML model can predict the HER catalytic activity well. The band gap of support materials is identified as the most important descriptor of single-atom chalcogenide catalysts for HER. Sn@CoS and Ni@ZnS exhibit excellent catalytic activity towards HER, and even outperform the current most efficient Pt catalysts. The hydrogen adsorption free energies of Sn@CoS and Ni@ZnS are 0.04 eV and −0.05 eV, respectively. Both Heyrovsky and Tafel reaction mechanisms are responsible for the HER of Ni@ZnS catalyst. The HER of Sn@CoS catalyst is mainly controlled by the Heyrovsky mechanism. Sn@CoS and Ni@ZnS are considered as the promising electrocatalysts for the HER. This study can provide a competitive tool to predict the activity trends and to accelerate the catalyst design and screening for other catalytic reactions.

Surface Coverage as an Important Parameter for Predicting Selectivity Trends in Electrochemical CO2 Reduction.

The electrochemical CO2 reduction reaction (CO2RR) is important for a sustainable future. Key insights into the reaction pathways have been obtained by density functional theory (DFT) analysis, but so far, DFT has been unable to give an overall understanding of selectivity trends without important caveats. We show that an unconsidered parameter in DFT models of electrocatalysts—the surface coverage of reacting species—is crucial for understanding the CO2RR selectivities for different surfaces. Surface coverage is a parameter that must be assumed in most DFT studies of CO2RR electrocatalysts, but so far, only the coverage of nonreacting adsorbates has been treated. Explicitly treating the surface coverage of reacting adsorbates allows for an investigation that can more closely mimic operating conditions. Furthermore, and of more immediate importance, the use of surface coverage-dependent adsorption energies allows for the extraction of ratios of adsorption energies of CO2RR intermediates (COOHads and HCOOads) that are shown to be predictive of selectivity and are not susceptible to systematic errors. This approach allows for categorization of the selectivity of several monometallic catalysts (Pt, Pd, Au, Ag, Zn, Cu, Rh, W, Pb, Sn, In, Cd, and Tl), even problematic ones such as Ag or Zn, and does so by only considering the adsorption energies of known intermediates. The selectivity of the further reduction of COOHads can now be explained by a preference for Tafel or Heyrovsky reactions, recontextualizing the nature of selectivity of some catalysts. In summary, this work resolves differences between DFT and experimental studies of the CO2RR and underlines the importance of surface coverage.

Effect of tensile stress on the hydrogen adsorption of X70 pipeline steel

The effect of stress on the cathodic hydrogen evolution behavior of X70 pipeline steel was investigated by electrochemical tests, tensile tests, and microstructural characterization. The results indicated that the tensile stress enhanced the activity of hydrogen adsorption sites on the metal surface, which was considered as the dominating factor affecting generation, adsorption, and permeation of hydrogen atoms. The subsurface hydrogen atom concentrations quantified by Cyclic voltammetry (CV) tests and the data calculated by hydrogen permeation experiments showed a good correspondence. The results indicated that the tensile stress enhanced the adsorption of hydrogen atoms on the surface and an inhibitory effect on the Tafel and Heyrovsky reaction, thereby leading to the increase of the subsurface hydrogen atom concentration, enhance the hydrogen embrittlement susceptibility of the X70 steel material as demonstrated by plasticity loss in the tensile tests.

Magnetic FeM (M = Ag, Co, Cu, and Ni) nanocrystals as electrocatalysts for hydrogen evolution reaction

Iron-based magnetic nanocrystals (FeM@OAm, M = Ag, Co, Cu, and Ni; OAm = oleylamine) electrocatalysts have been successfully synthesized via oleylamine reduction of metal salts method. FeCo is arranged in a body-centered cubic (bcc) unit cell, while FeNi, FeAg, and FeCu are in a face-centered cubic (fcc) structure. All the samples have different morphologies with a diameter average size varying of 6.4 ± 1.0 to 21.7 ± 5.1 nm, depending on the bimetallic composition. The samples have ferrimagnetic behavior with low coercive field and high saturation magnetization values at room temperature. Furthermore, it has been shown that FeAg fits better in the solid solution category. Among heterogeneous electrocatalysts synthesized, FeCo nanocrystals show lower overpotential (−564 mV) in comparison to FeAg (−584 mV), FeNi (−666 mV), and FeCu (−591 mV). The Tafel plots showed that the hydrogen evolution reaction (HER) activity for electrocatalyst following a mixed of Volmer−Heyrovsky reaction mechanism, suggests that reaction Volmer is the determining step. In addition, all electrocatalysts showed stability in tests of continuous operations showing only low potential variation. The EIS study shows system characterized by two time-constant, both of them may related to the kinetics of the HER related to the charge transfer kinetics and the hydrogen adsorption. Thus, these materials are sustainable for electrochemical water splitting since showed acceptable electrocatalytic performance toward HER in alkaline media with excellent physical stability and abundance of active sites for HER.

Simulating a catalyst induced quantum dynamical phase transition of a Heyrovsky reaction with different models for the environment

Through an appropriate election of the molecular orbital basis, we show analytically that the molecular dissociation occurring in a Heyrovsky reaction can be interpreted as a quantum dynamical phase transition, i.e., an analytical discontinuity in the molecular energy spectrum induced by the catalyst. The metallic substrate plays the role of an environment that produces an energy uncertainty on the adatom. This broadening induces a critical behavior not possible in a quantum closed system. We use suitable approximations on symmetry, together with both Lanczos and canonical transformations, to give analytical estimates for the critical parameters of molecular dissociation. This occurs when the bonding to the surface is times the molecular bonding. This value is slightly weakened for less symmetric situations. However simple, this conclusion involves a high order perturbative solution of the molecule-catalyst system. This model is further simplified to discuss how an environment-induced critical phenomenon can be evaluated through an idealized perturbative tunneling microscopy set-up. In this case, the energy uncertainties in one or both atoms are either Lorentzian or Gaussian. The former results from the Fermi golden rule, i.e., a Markovian approximation. The Gaussian uncertainty, associated with non-Markovian decoherent processes, requires the introduction of a particular model of a spin bath. The partially coherent tunneling current is obtained from the generalized Landauer–Büttiker equations. The resonances observed in these transport parameters reflect, in many cases, the critical properties of the resonances in the molecular spectrum.

Кинетика реакции выделения водорода на CeM2Ge2-электродах (M = Fe, Co, Ni) в щелочных растворах

The kinetics of hydrogen evolution reaction on the intermetallic compounds CeM2Ge2 (M = Fe, Co, Ni) in 0.5–2.0 M KOH solutions were studied using polarization measurements and electrochemical impedance spectroscopy. For 1 M KOH, the Tafel constants vary in the intervals: а = 0.46–0.57 V; b = 0.082–0.096 V. The catalytic activity in the hydrogen evolution reaction increases in the sequence CeFe2Ge2 – CeNi2Ge2 – CeCo2Ge2. The cathodic process was shown to be the combination of the hydrogen evolution reaction and hydrogen absorption reaction; the reaction of H2 evolution proceeds through the Volmer – Heyrovsky mechanism with the rate-determining Heyrovsky reaction; the Langmuir isotherm holds for the adsorption of atomic hydrogen.

Restructuring electronic structure via W doped 1T MoS2 for enhancing hydrogen evolution reaction

The intrinsic electronic structure on the surface of 1T MoS2 makes the non-optimal bonding strength with hydrogen atoms, which is the key factor that the catalytic performance cannot be improved. However, modifying surface electronic structure by doped exotic atom is an effective approach to optimal the bonding strength between hydrogen atoms and 1T MoS2. Herein, this work reports W doped 1T MoS2 with different concentrations by hydrothermal method. The results indicate that W-1T MoS2-15 exhibits excellent catalytic activity for hydrogen evolution reaction (HER), the catalytic activity is better than that of 2H MoS2 and pristine 1T MoS2, the lowest overpotential and tafel slope in acid electrolyte is 292 mV and 55.7 mV dec−1, respectively. Theoretical calculation results show that W-1T MoS2-2/9 has relatively much lower barrier than pristine 1T MoS2 both in Volmer reaction and Heyrovsky reaction. Therefore, W-1T MoS2-2/9 has a faster reaction kinetics for HER. In addition, introducing W atoms can trigger spontaneous electrons redistribution on the basal plane, optimize energy level, and reduce the Gibbs free energy. This study confirmed that doping of atom with similar physical and chemical properties can optimize electronic structure of 1T MoS2 on the basal plane, and effectively improve its electrocatalytic activity.

The distribution effect of sulfur vacancy in 2H–MoS2 monolayer on its H2 generation mechanism from density functional theory

In this paper, the effects of the sulfur vacancy (VS) distribution in 2H–MoS2 monolayer on the H2 evolution mechanism and activity are researched by density functional theory (DFT) calculations. The calculation results reveal that the H2 generation on VS follows the Volmer-Heyrovsky mechanism with the Heyrovsky reaction as the rate determined step (RDS) with an energy barrier of 18.5 kcal/mol. When two VS are separated by one S atom, the H2 evolution on VS remains the Heyrovsky-step-determined Volmer-Heyrovsky mechanism and the RDS energy barrier increases to 20.0 kcal/mol. Removing adjacent S of VS causes that the Tafel-step-determined Volmer-Tafel mechanism works for HER and the barrier of RDS increases to 21.4 kcal/mol. Though the barriers of RDS ascend when the concentration of VS is enhanced by these two strategies, the Tafel step can take place at a lower potential and more VS are exposed. Therefore, two adjacent or next-near VS may obtain better H2 generation performance. Three adjacent or spaced out VS by one S may be less favorable for HER.

Geometric and electronic modulation of fcc NiCo alloy by Group-Ⅵ B metal doping to accelerate hydrogen evolution reaction in acidic and alkaline media

Bimetallic nickel–cobalt (Ni-Co) alloy has been regarded as an emerging class of material for the electrochemical hydrogen evolution reaction (HER). However, it operates at an overpotential significantly above the thermodynamic requirement. Here we present a facile one-step electrochemical reduction approach to incorporate fcc NiCo alloy with ⅥB group transition metals (M−NiCo, M = chromium (Cr), molybdenum (Mo), or tungsten (W)). This method enables to control the induction time of protrusion growth (τ) to produce hierarchical structure by imposing high current density (-0.6 A cm−2) and strong electric field on the cathode. Herein, W-NiCo holey nanotower arrays show tremendously enhanced HER activity compared with pristine NiCo alloy, which provides an overpotential of 109.2 mV (reduced by 58.7%) to achieve a current density of −10 mA cm−2, with a Tafel slope of 110.3 mV dec-1 in alkaline medium. The same law applies to the acid electrolyte with an overpotential of 78.5 mV (reduced by 66.1%) and low Tafel slop of 56.6 mV dec-1. Based on the experimental and theory analysis, We find W-doping can promote more active sites exposed, tune the electron configuration, moderate the gibbs free energy of the H intermediate, and accelerate the desorption of H2. It makes the rate-determining step change from Volmer to Heyrovsky reaction and finally surmount the limit of sluggish dynamics. The novel route provides an efficient doping control on Ni-based hierarchical alloys via 4f-orbital electronic and geometric structure engineering, which suggests an environmentally friendly method for energy-related process in practical applications.

Electrochemical study in acid aqueous solution and ex-situ X-ray photoelectron spectroscopy analysis of metallic rhenium surface

• Exchange current density approximately at 10 –6 Acm −2 and Tafel slope at 0.06 Vdec −1 . • Volmer–Heyrovsky is the mechanism of hydrogen evolution reaction on metallic rhenium. • Double layer capacitance is controlled by adsorption hydroxide and sulfate anions. • There is a formation of surface complex between Re(I) and sulfate. The electrochemical behavior of metallic rhenium in acid aqueous solution was investigated in this study. Ex-situ X-ray photoelectron spectroscopy (XPS) spectra were recorded to the rhenium electrode, after controlled cathodic potential electrolysis, which decrease the amount of oxide present on the surface. This electrolysis implies the lowest quantity of surface oxide reported, to the best of our knowledge, in literature. The electrochemical behavior of Re(0) in 0.5 M H 2 SO 4 displays a separation of the overall potential interval into hydrogen region (potential inferior than −0.11 V vs RHE), double layer region (-0.11 V to 0.43 V vs RHE) and oxygen region (potential superior than 0.43 V vs RHE). The hydrogen region displayed an exchange current density in the order of magnitude of 10 −6 Acm −2 , and the Tafel slope was approximately at 0.06 Vdec −1 , which suggest that the Volmer–Heyrovsky reaction is the mechanism of hydrogen evolution reaction. The capacitance (≈500 μFcm −2 ) in the double layer region is controlled by adsorption of sulfate and hydroxide anions. The formation of the surface complex ( R e 2 1 + S O 4 2 - ), was suggested by ex-situ XPS analysis, which is to the best of our knowledge reported for the first time. In the oxygen region, the Tafel slope of the anodic branch is 130 mVdec −1 and the current corrosion is in the order of magnitude of 10 −7 Acm −2 . Ex-situ XPS provides further evidence indicating that the f levels of Re (0) are Re4 f 7/2 at 40.0 eV and Re4 f 5/2 at 42.4 eV. The oxidation state of Re(I) displayed the Re4 f 7/2 level at 41.3 eV and the Re4 f 5/2 level at 43.6 eV.

Defects engineering promotes the electrochemical hydrogen evolution reaction property of phosphorene surface

Compared with Pt and MoS2, the pristine phosphorene surface is limited in the development of electrocatalysts for hydrogen evolution reaction (HER) due to the much lower HER activity. Herein, the HER properties of defective phosphorene are studied based on the first-principle calculations. Defect engineering can break the inertia of phosphorene surface, enhance the interactions between H∗ and adsorption sites, and promote the HER activity of phosphorene surface obviously, as a result of that the defective phosphorene structures SV-(55|66), DV-(5|8|5)-1, DV-(12|12), DV-(9|4|9), DV-(55|66)-1 and DV-(55|66)-2 present the better or equivalent HER activity compared to Pt (111). In particular, the monoatomic defect SV-(55|66) shows the far better HER activity than Pt (111), and its energy barrier for Tafel reaction in the H2 desorption process decreases by 47.2% and 17.1% by comparison with those of pristine phosphorene for Tafel and Heyrovsky reaction, respectively. This study can provide a novel way to promote the HER activity of phosphorene surface via defect engineering at atomic level and a theoretical basis for designing metal-free phosphorene based electrocatalysts with high catalytic activity and stability.

Improved hydrogen evolution activity by unique NiS2-MoS2 heterostructures with misfit lattices supported on poly(ionic liquid)s functionalized polypyrrole/graphene oxide nanosheets

Unique NiS2-MoS2 heterostructures with misfit lattices composed of cubic NiS2, metallic 1 T-MoS2 and semiconductive 2H-MoS2 have been immobilized on poly(1-vinyl-3-ethylimidazolium bromide) functionalized polypyrrole/graphene oxide via an ordinary hydrothermal procedure. The obtained NiS2-MoS2/PVEIB/PPy/GO nanosheets exhibited the excellent electrocatalytic performance for hydrogen evolution reaction (HER). PVEIB played an essential role in the fabrication of this inorganic/organic hierarchical lamellar micro-/nanostructures, which not only played as a good connector between transition metal dichalcogenides (TMDCs) heterostructures and PPy/GO, inducing the formation of metallic 1 T-MoS2, but also effectively stabilized more electrons and further facilitate the Heyrovsky reaction for the generation and release of H2. Though NiS2 possessed poor conductivity and induced partial 1 T-MoS2 transformed to 2H-MoS2 on PVEIB/PPy/GO, the preeminent HER activity was still gained, because NiS2 could effectively prevent the oxidation of MoS2 exposed in the atmosphere. Especially, the crystal phase transition of MoS2 from 1 T to 1 T’ phase in NiS2-MoS2/PVEIB/PPy/GO could be detected by XPS after long-term HER test. Therefore, poly(ionic liquid)s and another metal ion would induce the crystal structures of TMDCs and optimize their electrocatalytic performance, which might afford a new thought towards the reasonable design of electrocatalysts based on TMDCs for electrochemical water splitting.

Poly(3-aminobenzoic acid) Decorated with Cobalt Zeolitic Benzimidazolate Framework for Electrochemical Production of Clean Hydrogen.

A novel composite of poly(3-aminobenzoic acid) (PABA) and a cobalt zeolitic benzimidazolate framework (CoZIF) has been studied for the production of hydrogen through the hydrogen evolution reaction (HER). The structural characteristics and successful synthesis of PABA, CoZIF and the PABA/CoZIF composite were confirmed and investigated using different techniques. Probing-ray diffraction for phase analysis revealed that the composite showed a decrease and shift in peak intensities, confirming the incorporation of CoZIF on the PABA backbone via in situ polymerization, with an improvement in the crystalline phase of the polymer. The thermal stability of PABA was enhanced upon composite formation. Both scanning electron microscopy and transmission electron microscopy showed that the composite had a rough surface, owing to an interaction between the CoZIF and the external surface of the PABA. The electrochemical hydrogen evolution reaction (HER) performance of the synthesized samples was evaluated using cyclic voltammetry and Tafel analysis. The composite possessed a Tafel slope value of 156 mV/dec and an α of 0.38, suggesting that the Volmer reaction coupled with either the Heyrovsky or Tafel reaction as the rate determining step. The fabricated composite showed high thermal stability and excellent tolerance as well as high electroactivity towards the HER, showing it to be a promising non-noble electrocatalyst to replace Pt-based catalysts for hydrogen generation.

Galvanostatically Deposited Ptni Thin-Films As Electrocatalytists for the Hydrogen Evolution Reaction

Hydrogen renders high energy density without any direct carbon emissions, which makes it an ideal energy carrier [1]. PtNi nanomaterials exhibit high catalytic activity for hydrogen production through the hydrogen evolution reaction (HER), in some cases superior to bare Pt [2] and with lower cost due to the replacement with Ni. Efforts devoted to the synthesis of PtNi nanomaterials, most of them based on solvothermal methods [3,4], involve several steps and reaction times amounting to hours. Electrochemical deposition is a fast and simple method that allows high control over the morphology of the deposits. Previously, our group used single-pulse galvanostatic electrodeposition to generate Ni thin-layers on gold electrodes; the Ni deposits exhibited high catalytic activity for methanol electro-oxidation [5]. In this work we investigate the catalytic activity of galvanostatically generated PtNi deposits on gold towards HER in alkaline media.After mechanical polishing and electrochemical cleaning of a Au electrode, the electrodeposition was carried using a 1.5 mM K2[PtCl4] + 50 mM NiSO4 + 500 mM NaCl plating solution saturated with N2. Figure 1a shows the typical E – t transient response to a two seconds current pulse. Its stair-like shape hints the presence of at least four electrochemical reactions, identified as regions I to IV. Based on the Nernst potentials (horizontal dashed lines) and the processes reported for nickel by Salinas et al. [5], regions I and II can be assigned to Pt deposition, Ni deposition and hydrogen evolution from H+ reduction to region III, and hydrogen evolution form water reduction to region IV. The effect of the applied current density on the potential transients is shown in Fig. 1b, higher values lead to shorter durations of each stage and lower potential values. The presence of Ni and Pt was confirmed by cyclic voltammetry in 1 M KOH and 50 mM H2SO4, respectively, but no particles could be detected by SEM, which suggest that thin films were formed. Liu et al. [6] and Wang et al. [7] reported the electrodeposition of Pt and Ni thin films which were self-terminated by potential pulses negative enough to favor the formation and adsorption of species blocking further growth of the deposits. Since during the galvanostatic deposition the electrode reaches potential values negative to the self-termination threshold, it its likely that a thin layered PtNi films were formed.The activity of the PtNi deposits towards HER was evaluated by linear sweep voltammetry in 1 M KOH. The overpotentials required to attain a current of 10 mA cm-2 (η 10) were 64 mV for the deposit obtained at 8 mA cm-2, while the highest overpotential, 77 mV, was obtained at 2 mA cm-2. The η 10 of the PtNi/Au deposits falls within the range reported for similar materials: 39 mV[4] to 186 mV [8]. However, the specific current at 38 mV of the most active material obtained, 6390 A gPt -1, is far larger that the one of the most active material reported in the literature of 1740 A gPt -1 [4]. This may be attributed to an optimal Pt/Ni ratio and a thin-layered deposit, achieved by the complex potential trajectories generated by galvanostatic electrodeposition. Tafel plots (Fig. 1c) exhibited two linear regions, with values independent of applied current density of deposition: 35 mV dec−1 for the first slope and 100 mV dec−1 for the second one. This suggests that HER follows the same pathway on all the deposits and the Heyrovsky reaction is the limiting step [8].Figure1 a) Typical potential transient recorded during the galvanostatic deposition of PtNi. b) Potential transients recorded during a galvanostatic deposition of PtNi on a clean Au electrode at different j dep. c) Tafel plots for the deposits produced at different current densities.

Exploring single atom catalysts of transition-metal doped phosphorus carbide monolayer for HER: A first-principles study

• The single Ir atom doped α-PC has much smaller Gibbs free adsorption energy of H atom comparing to the reported Pt(111). • The Gibbs free adsorption energy of H atom is highly related to the d-band center and the period of the doped TM atom. • The Volmer–Heyrovsky step has smaller energy barrier than the Volmer-Tafel step on Ir-αPC. • The Ir-PC is physically and thermally stable according to phonon calculation and AIMD result. Hydrogen has been identified as one of the most promising sustainable and clean energy. Developing hydrogen evolution reaction (HER) catalyst with high activity is essential for satisfying the future requirements. Considering novel advantages of two-dimensional materials and high catalytic activity of atomic transition metal, in this study, using density functional theory calculation, the HER on single transition-metal (23 different TM atoms) doped phosphorus carbide monolayer (α-PC) has been investigated. The Volmer–Tafel and Volmer–Heyrovsky reaction mechanisms, and the stability of the most promising HER catalyst are also included. The results show that Ir-αPC with high physical and thermal stability has the most optimal value of Gibbs free adsorption energy for H atom. The relationship of d band center and the HER activity shows a volcano-like curve. The calculation of reaction energy barrier indicates that the Volmer-Heyrovsky step is more favorable than the Volmer-Tafel step.

Graphene Activation Explains the Enhanced Hydrogen Evolution on Graphene-Coated Molybdenum Carbide Electrocatalysts.

Molybdenum carbides (MoxC) have shown high catalytic activities toward hydrogen evolution reaction (HER) when coupled with graphene. Herein, we use density functional theory (DFT) calculations in conjunction with ab initio thermodynamics and electrochemical modeling on γ-MoC supported graphene to determine the origin of the enhanced HER activities. In addition to previous claims that graphene's main role is to prevent agglomeration of MoxC nanoparticles, we show that the interplay between γ-MoC coupling and graphene defect chemistry activates graphene for the HER. For γ-MoC supported graphene systems, the HER mechanism follows the Volmer-Heyrovsky pathway with the Heyrovsky reaction as the rate-determining step. To simulate the electrochemical linear sweep voltammetry at the device level, we develop a computational current model purely from the thermodynamic and kinetics descriptors obtained using DFT. This model shows that γ-MoC supported graphene with divacancies is optimum for HER with an exchange current density of ∼1 × 10-4 A/cm2 and Tafel slope of ∼50 mV/dec-1, which are in good agreement with experimental results.