Hydrogen Evolution Reaction Onset Potential Research Articles

Tuning Electrocatalytic Activities of Dealloyed Nanoporous Catalysts by Macroscopic Strain Engineering.

It is technically challenging to quantitatively apply strains to tune catalysis because most heterogeneous catalysts are nanoparticles, and lattice strains can only be applied indirectly via core-shell structures or crystal defects. Herein, we report quantitative relations between macroscopic strains and hydrogen evolution reaction (HER) activities of dealloyed nanoporous gold (NPG) by directly applying macroscopic strains upon bulk NPG. It was found that macroscopic compressive strains lead to a decrease, while macroscopic tensile strains improve the HER activity of NPG, which is in line with the d-band center model. The overpotential and onset potential of HER display approximately a linear relation with applied macroscopic strains, revealing an ∼2.9 meV decrease of the binding energy per 0.1% lattice strains from compressive to tensile. The methodology with the high strain sensitivity of electrocatalysis, developed in this study, paves a new way to investigate the insights of strain-dependent electrocatalysis with high precision.

Ce-Doped TiO2 Fabricated Glassy Carbon Electrode for Efficient Hydrogen Evolution Reaction in Acidic Medium.

The quest for sustainable and clean energy sources has intensified research on the Hydrogen Evolution Reaction (HER) in recent decades. In this study, we have presented a novel Ce-doped TiO2 catalyst synthesized through the sol-gel method, showcasing its potential as a superior electrocatalyst for HER in an acidic medium. Comprehensive characterization through X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Energy dispersive X-ray (EDX), and Raman spectroscopy confirms the successful formation of the catalyst. Electrocatalytic performance evaluation, including open circuit potential (OCP), electrochemical impedance spectroscopy (EIS), and Tafel analysis, demonstrates that GCE-5wt.%CeTiO2 outperforms bare GCE, as well as Ce and TiO2-based electrodes. Kinetic investigations reveal a Tafel slope of 105 mV dec-1, indicating the Volmer step as the rate-determining step. The onset potential for HER at GCE-5wt.%CeTiO2 is -0.16 V vs. RHE, close to the platinum electrode. Notably, the catalyst exhibits a low overpotential of 401 mV to achieve a current density of 10 mA cm-2 with an impressive 95 % Faradaic efficiency. Furthermore, the catalyst demonstrates outstanding durability, maintaining a negligible increase in overpotential during a 14-hour chronoamperometry test. These results have far-reaching implications for the development of cost-effective and efficient electrocatalysts for hydrogen production.

New approach to produce cubic-WC at low temperature for hydrogen evolution reaction

A novel catalyst, a nanostructured cubic-WC (c-WC) flat film, is introduced for the hydrogen evolution reaction (HER) using a specialized magnetron sputtering system. This method results in over 94% crystalline c-WC0.95 formation at room temperature, featuring high-density stacking faults and twin defects. The catalytic performance of the c-WC film for HER is investigated through theoretical simulations and experimental studies. Density-functional theory (DFT) suggests that the c-WC (100) surface exhibits faster reaction kinetics than c-WC (111) surface, approaching the efficiency of Pt (111) surfaces. Experimentally, the c-WC0.95 film exhibits a high electrochemical surface area and increased active sites due to surface defects. Differential electrochemical mass spectrometry (DEMS) confirms an onset potential for HER of −50.0 mV for c-WC0.95 films and establishes electrodesorption as the rate-determining step. Overall, the c-WC film emerges as a cost-effective and efficient catalyst for green hydrogen production, showcasing excellent reproducibility and electrochemical stability.

Hydrogen evolution mediated by sulfur vacancies and substitutional Mn in few-layered molybdenum disulfide

MoS2 is widely praised as a promising replacement for Pt as an electrocatalyst for the hydrogen evolution reaction (HER), but even today, it still suffers from low performance. This issue is tackled by using Mn3+ as a surface modifier to trigger sulfur vacancy formation and enhance electron transport in few-layered 2H MoS2. Only 10% of Mn is sufficient to transform the semiconductive MoS2 into an active HER electrocatalyst. The insertion of Mn reduces both HER onset potential and Tafel slope which allows reaching 100 mA/cm2 at an overpotential of 206 mV, ten times larger of what undoped MoS2 can achieve. The enhanced activity arises because Mn3+ introduces electronic states near the conduction band, promotes sulfur vacancies, and increases the hydrogen adsorption. In addition to its facile production and extended shelf-life, Mn–MoS2 exhibits an efficiency of 73% at 800 mA/cm2 and 2.0 V when used in proton exchange membrane water electrolyzers.

In Situ Mapping of Activity Distribution of V(II)/V(III) and Onset Potential Distribution of Hydrogen Evolution Side Reaction in Vanadium Flow Batteries.

Vanadium flow batteries (VFBs) face a challenge with the low reaction rates of the V(II)/V(III) redox couple, which limits the performance of VFBs. Additionally, the negative electrode in VFBs is often accompanied by the persistent hydrogen evolution reaction (HER), which is difficult to eliminate. Therefore, understanding the spatial distribution of activity on the negative electrode and the HER side reaction on the electrode surface is of critical importance. This study proposes a weak measurement imaging method to characterize the spatial distribution of surface activity and HER onset potential on the negative electrode in VFBs). This method enables the visualization and in situ detection of key parameters such as the absolute values of |ipa |, |ipc |, |∆E|, |ipc /ipa |, and the HER onset potential. By comparing three different types of graphite felts with varying activity levels, it validates the feasibility of this method. Furthermore, electrochemical stability tests are conducted to study the electrodes repeatability, uniformity, and durability. This method holds promise in guiding the design of electrodes with enhanced activity, good reversibility, minimized HER side reactions, and uniform distribution.

Unexpected Potential Dependent HER Behaviors of Cationic Phosphorus Doped WO3 Nanosheets

TMOs are generally considered as catalytically inert for hydrogen evolution reaction (HER).[1] This unique property has promoted a wide variety of applications of TMOs, such as aqueous-based lithium ionic batteries, electrochromic smart windows, electrocatalysts and/or catalyst supports. However, hydrogen evolution as an undesired side reaction is commonly observed when they are used in aqueous-based supercapacitors and battery applications.[2] The unexpected HER mechanism for intrinsic TMOs has not been studied until very recently by McKone and Mpourmpakis groups.[3] With careful theoretical calculations, they clearly demonstrated that intrinsic WO3 is not HER active. Bulk hydrogen (H) intercalation into WO3 is a prerequisite for its catalytic activity toward HER. Applying electrochemical potential to WO3 primarily sets the bulk H stoichiometry of thus formed hydrogen bronzes HxWO3. The level of intercalation (x), precisely to say, the density of the intercalated hydrogen in the WO3, in turn determines the formation of W-H*, the bonding strength of W-H*, and their consumption to H2 (HER). Significant HER is only seen at potentials negative enough when x > 0.5. Beyond this intercalation level, HER is no longer dictated by potential-dependent activation energies, instead, is controlled by the density of the hydrogen intercalation on the surface of WO3 catalysts. In line with this extensive theoretical study, Augustyn group very recently experimentally demonstrated the dependence of HER activity of WO3 materials on their electrochemical insertion characteristics.[4] By including crystal water between the WO3 sheets, proton intercalation is greatly facilitated likely with a Grotthuss mechanism. Accordingly, the HER kinetics is largely improved with a large positive shift of the HER on-set potential. On the other hand, by inclusion of molecular pillars in the WO3 lattice to physically block proton intercalation, the HER activity of WO3 was largely inhibited. Recently, we developed a facile approach to cationically dope WO3 nanosheets with red phosphorus. The proton interaction and the associated HER behavior as a function of electrochemical potentials were carefully studied with multiple electrochemical techniques. Interestingly, we found that proton interaction is largely suppressed even without including molecular pillars in the P-doped WO3 lattice. At lower intercalation levels before the so-called “potential independent regime”, the HER kinetics is largely decreased, as expected, compared to the non-doped WO3 control sample. While in the “potential independent regime”, the HER kinetics is largely enhanced. Furthermore, including alkaline metal ions into the electrolyte, the HER kinetics is further suppressed as demonstrated by largely negatively shifted onset potentials and the increased Tafel slopes. Interestingly, in the “potential independent regime”, the HER kinetics is further drastically enhanced. Extensive structural characterization and theoretical calculations are undergoing to reveal the mechanism behind these interesting tunable potential dependent - proton interaction and HER behavior, which will be reported in this work. Understanding how heteroatomic doping could modulate proton interaction and HER behaviors of TMOs at atomic levels would provide practical guidance for designing more efficient and selective electrochemical catalysts for various catalytic reduction reactions, such as carbon dioxide reductions and nitrogen/nitrate reductions to valuable compounds.[1] Zhang, S.; Zhang, X.; Rui, Y.; Wang, R.; Li, X., Recent advances in non-precious metal electrocatalysts for pH-universal hydrogen evolution reaction. Green Energy & Environment 2021, 6 (4), 458-478.[2] Wang, R.; Mitchell, J. B.; Gao, Q.; Tsai, W.-Y.; Boyd, S.; Pharr, M.; Balke, N.; Augustyn, V., Operando Atomic Force Microscopy Reveals Mechanics of Structural Water Driven Battery-to-Pseudocapacitor Transition. ACS Nano 2018, 12 (6), 6032-6039.[3] Miu, E. V.; McKone, J. R.; Mpourmpakis, G., The Sensitivity of Metal Oxide Electrocatalysis to Bulk Hydrogen Intercalation: Hydrogen Evolution on Tungsten Oxide. Journal of the American Chemical Society 2022, 144 (14), 6420-6433.[4] Spencer, M. A.; Fortunato, J.; Augustyn, V., Electrochemical proton insertion modulates the hydrogen evolution reaction on tungsten oxides. The Journal of Chemical Physics 2022, 156 (6), 064704. Figure 1

Manganese Oxide/Iron Carbide Encapsulated in Nitrogen and Boron Codoped Carbon Nanowire Networks as Accelerated Alkaline Hydrogen Evolution and Oxygen Reduction Bifunctional Electrocatalysts.

Along with the widespread applications of various energy storage and conversion devices, the prices of precious metal platinum (Pt) and transition-metal cobalt/nickel keep continuously growing. In the future, designing high-efficiency nonprecious-metal catalysts based on low-cost iron (Fe) and manganese (Mn) metals for hydrogen evolution reaction (HER) and oxygen reduction reaction (ORR) is fairly critical for commercial applications of hydrogen fuel cells. In this study, for the first time, we design novel three-dimensional (3D) hybrid networks consisting of manganese oxide (MnO)-modified, iron carbide (Fe3C)-embedded, and boron (B)/nitrogen (N) codoped hierarchically porous carbon nanofibers (denoted FeMn@BNPCFs). After optimizing the pyrolysis temperatures, the optimal FeMn@BNPCFs-900 catalyst displays the best HER and ORR catalytic activities in an alkaline solution. As expected, the HER onset potential (Eonset) and the potential at a current density of -10 mA cm-2 for FeMn@BNPCFs-900 in 1.0 M KOH are just 36 and 194 mV more negative than the state-of-the-art 20 wt % Pt/C catalyst with more superior stability. In particular, the FeMn@BNPCFs-900 catalyst shows excellent ORR catalytic activity with a more positive Eonset (0.946 V vs RHE), a more positive half-wave potential (E1/2 = 0.868 V vs RHE), better long-term stability, and higher methanol tolerance surpassing the commercial 20 wt % Pt/C (Eonset = 0.943 V vs RHE, E1/2 = 0.854 V vs RHE) and most previously reported precious-metal-free catalysts in 0.1 M KOH. The synergistic effects of 3D hierarchically macro-/mesoporous architectures, advanced charge transport capacity, abundant carbon defects/edges, abundant B (2.3 atom %) and N (4.9 atom %) dopants, uniformly dispersed Fe3C@BNC NPs, and MnO nanocrystallines are responsible for the excellent HER/ORR catalytic activities of the FeMn@BNPCFs-900 catalyst.

Electro-reduced graphene oxide nanosheets coupled with RuAu bimetallic nanoparticles for efficient hydrogen evolution electrocatalysis

• One-step electroreduction method was used for RuAu-RGO catalyst preparation. • Homogeneously anchored RuAu nanoparticle on RGO sheets show excellent HER activity. • Catalyst showed strong electronic coupling interaction between Ru, Au, and RGO. • In Zn-CO 2 cell system, catalyst showed low overpotential. Electrocatalytic Hydrogen Evolution Reaction (HER) in water electrolysis holds a great promise for sustainable hydrogen production but the preparation of an efficient and stable catalyst is remained challenging. Herein, a simple one-step simultaneous electro-reduction of graphene oxide, ruthenium chloride, and gold chloride precursors without involving any pre-/or post- mechanical, hydrothermal, or carbonization step, is reported to prepare homogeneously anchored ruthenium (Ru) and gold (Au) nanoparticles on reduced graphene oxide (RGO) as an efficient electrocatalyst for HER. The highly dispersed RuAu bimetallic nanoparticles on RGO sheets induce faster reaction kinetics with high turnover frequency, associating with strong electronic coupling interaction between RGO, Ru, and Au nanoparticles which improve the intrinsic HER activity of the catalyst as revealed by combined experimental and theoretical results. The as prepared RuAu-RGO catalyst demonstrates excellent HER activity with an overpotential of 56 mV at the current density of 10 mA cm −2 outperforming the monometallic Ru-RGO and Au-RGO catalysts in 1 M KOH solution. The RuAu-RGO catalyst exhibits utmost stability in the practical water splitting, far surpassing the benchmark Pt/C-IrO 2 system. Furthermore, the catalyst was employed as a cathode coupling with zinc (Zn) anode in an aqueous Zn-CO 2 system to generate hydrogen, it demonstrated positive shift in the onset potential of HER by 0.24 V than for HER in typical three-electrode testing configuration broadening its applicability.

Deep eutectic solvent assisted electrosynthesis of ruthenium nanoparticles on stainless steel mesh for electrocatalytic hydrogen evolution reaction

Deep eutectic solvents (DES) are considered as a green non-toxic electrolyte for the replacement of cyanide based toxic electrolytes towards the electrodeposition of noble metal and alloy nanoparticles on conducting surfaces. In this work, ruthenium nanoparticles (RuNPs) are electrochemically synthesized over cathodically treated stainless-steel mesh (CSS) by applying the cathodic current of −15 mA/cm2 using ruthenium chloride and deep eutectic mixture consists of choline chloride/urea as electrolyte. The resulting material is abbreviated as RuNPs@CSS and are characterized by surface analytical tools such as X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), field emission scanning elctron microscopy with energy dispersive spectrum (FE-SEM with EDS). Further, the electrocatalytic hydrogen evolution reaction (HER) activity of RuNPs@CSS is accessed and compared with state of art Pt electrode using open circuit potential (OCP), linear sweep voltammetry (LSV), Tafel plot and electrochemical impedance spectroscopy (EIS) measurements in 0.5 M H2SO4. The calculated HER's onset potential and over potential @ −10 mA/cm2 of RuNPs@CSS are about −0.0273 and −0.0657 V vs. RHE, which are very close to the bare Pt values. The EIS results suggested that, RuNPs@CSS possess excellent conductivity, which decrease the charge transfer resistance and enhances the HER. This study proved that the electrodeposited RuNPs@SS is better replacement for Pt based electrocatalysts towards acidic HER.

Intrinsic effects of thickness, surface chemistry and electroactive area on nanostructured MoS2 electrodes with superior stability for hydrogen evolution

Developing inexpensive, earth abundant materials for electrocatalytic and photoelectrochemical water splitting is critical to the decarburization of our energy system. Aerosol-assisted chemical vapor deposition (AACVD) is a potentially scalable, one-step synthesis technique that can produce large area, homogenous edge-aligned films of transition metal dichalcogenides including MoS2. Herein, we report a hydrogen evolution reaction (HER) study on such electrode films under acidic conditions. Using iR compensated near-steady state DC current-potential measurements and linear sweep voltammetry, the HER onset potential is recorded at −175 mV vs. RHE (current density 0.1 mA cm−2), while a current density of 10 mA cm−2 per nominal area is reached at an overpotential of −430 mV vs. RHE. A Tafel slope of 113 mV dec−1 suggests the existence of increased edge-active sites on the surface of 2H-MoS2. The electrodes offered high stability over 5000 cycles, which continued to improve with cycling, with the current density at −350 mV improved by a factor exceeding 20 between the 1st and 5000th cycle. X-ray photoelectron spectroscopy, contact angle measurements and electrochemical impedance spectroscopy were employed to provide thorough insights on the changes in the intrinsic properties of the material, which led to the observed HER activity enhancement with cycling. It is shown that a reduction in the surface oxide layer and increased electrowetting are the reasons for the improved performance. Electrodes of differing thicknesses were prepared and their performance was compared in association with the surface morphology. The best performing electrode had the lowest mass loading of MoS2, which is attributed to improved faradaic efficiency through the resistive 2H-MoS2 electrode.

Increasing the Hot‐Electron Driven Hydrogen Evolution Reaction Rate on a Metal‐Free Graphene Electrode

AbstractRecently, it has been shown that a semiconductor–insulator–graphene device can drive the hydrogen evolution reaction (HER) at the graphene surface with a reduced onset potential by injecting hot electrons into graphene. However, the catalytic properties of graphene are limited by the large hydrogen adsorption energy and lack of electrochemically active sites. To address these limitations, a n‐silicon/insulator/plasma etched graphene device is investigated, where a dry etch process is used to increase the number of active sites on the graphene by creating a greater number of active edge sites, increasing hydrogen adsorption at a given potential. This has been shown to improve the properties of devices with cold electrons. However, here it is shown that this approach can improve the HER rate with hot electrons. The electrons injected into the graphene from the silicon shift the onset potential of HER by as high as ≈0.8 V reaching a current density of 90 mA cm−2 at an overpotential of ‐0.5 V versus RHE. Furthermore, the comparison between device with pristine graphene shows a ≈2X improvement in current density at high overpotentials. This result shows that hot‐electron devices can be improved by modifying the catalytically active sites without metal catalysts.

Effect on the Ligand of Triruthenium Unit for Hydrogen Evolution Reaction

At present, various energy problems occur on the earth. In order to solve the problems utilization of hydrogen is a candidate. Because hydrogen is a clean and highly efficient energy that does not produce sulfur oxides and nitrogen oxides and is attracting attention as a substitute for primary energy such as petroleum and natural gas. In this study, we focused on the ruthenium cluster unit (1-mer) forming a cyclic structure having three ruthenium ions in a mixed-valence state. Here, we examined a catalytic function of hydrogen evolution reaction (HER) at the 1-mer modified highly oriented pyrolytic graphite (HOPG) electrode. In addition, we have studied effects of alcohol as ligands for 1-mer. Cyclic voltammetry (CV) and linear sweep voltammetry (LSV) were used for electrochemical measurements. HOPG was modified by drop casting of 0.5 μM 1-mer dichloromethane or methanol solution and used as a working electrode. A Ag/AgCl (KCl sat.) or a reversible hydrogen electrode (RHE) was used as a reference electrode, and a platinum plate was used as a counter electrode. LSV and CV were carried out in 0.1 M HClO4 in the presence of various alcohols such as methanol and ethanol. Figure 1 shows the LSVs of 0.1 M HClO4 at the 1-mer modified HOPG. The surface concentration of 1-mer was cocurated to be 18 nmol cm-2. At the 1-mer modified HOPG electrode, the starting potential of HER was slightly shifted to positive as compared with that of bare HOPG electrode. This means that the 1-mer modified HOPG has a little catalytic activity for HER. On the other hand, addition of alcohol to the system changes drastically the catalytic activity of the HER. Onset potential of HER was shifted to positive by the addition of alcohol. Comparing the current densities in the presence of each alcohol at –0.80 V, the HER current was influenced by the kinds of alcohol. Especially, when methanol was added to the system, the current density of 0.02 A cm-2 at –0.80 V. Although the reason why the catalytic current for HER depends on the type of alcohol is not clear, it may be considered that the THF ligands of 1-mer are substituted by the alcohol used. We have been are studying the effect of surface concentration and nanoscale structure of 1-mer, and the anion of the electrolyte. Figure 1

Co nanoparticles coupling induced high catalytic activity of nitrogen doped carbon towards hydrogen evolution reaction in acidic/alkaline solutions

The development of highly-efficient and noble-metal-free catalysts with low cost and high durability for hydrogen evolution reaction (HER) is of great importance to many energy-related technologies. This work reports that nitrogen-doped carbon coated Co nanoparticles encapsulated in bamboo-like nitrogen-doped carbon nanotubes (Co@NC/B-NCNTs), synthesized by a simple calcination of Co(NO3)2, melamine and P123, is a promising catalyst for HER in both acidic and alkaline solutions. In acidic solution, the Co@NC/B-NCNTs has the HER onset potential of ∼0 V and only needs an overpotential of 7 mV to drive the current density of 10 mA cm−2. Its HER catalytic activity is very close to that of Pt/C. In alkaline solution, its onset potential and overpotential to drive 10 mA cm−2 are 100 and 182 mV, respectively. Both the values are lower than those of many other catalysts reported. The DFT calculations show electronic coupling between Co nanoparticles and NC layer plays an important role in high catalytic activity of Co@NC/B-NCNTs. The Co nanoparticles can change the charge density distribution of the outer NC layer, affecting its properties for hydrogen adsorption and desorption. Additionally, the pyridinic N in the outer NC layer is calculated to be a promising active site for HER.

Potential Oscillation during Hydrogen Evolution Reaction: Hidden N-Shaped Negative Differential Resistance Due to Hydrogen Bubbles

Electrochemical oscillations are attractive phenomena from the viewpoint of dynamic self-organization of molecular systems. In general, an N-shaped negative differential resistance (N-NDR) plays a crucial role in the appearance of oscillations because it gives rise to an oscillatory instability [1]. Most of the oscillations can be classified into an N-NDR type or “hidden” N-NDR (HN-NDR) type oscillator. The former shows current oscillations under potential controlled conditions and hysteresis loops under current controlled conditions, whereas the latter shows not only current oscillations but also potential oscillations. Thus, the electrochemical system that shows potential oscillations falls into an HN-NDR type oscillator. However, it is not always true because there are different types of oscillators that show potential oscillations, namely, strictly potentiostatic type, S-shaped NDR (S-NDR) type, coupled N-NDR (CN-NDR) type, and electrochemical reactions and diffusion–convection (ERDC) type oscillators. Hydrogen evolution reaction (HER), which is the cathode reaction in water electrolysis, has been extensively studied using various kinds of electrodes because it is a fundamental electrochemical reaction and also because it is an important reaction for the production of H2. When the electrode potential is much negative than the onset potential of HER, a large number of hydrogen bubbles are formed on electrode surface. This often causes a disturbance in electrochemical reactions, leading to difficulties in electrochemical measurements, controls, and analysis. Thus, the HER has been mostly studied in a low overpotential region where no bubbles are formed or in an intermediate region where bubbles do not affect electrochemical measurements. We have reported [2] that the HER on Pt and Au in 0.10 M (M = mol/dm3) H2SO4 solution is anomalously affected by salts such as Na2SO4 and K2SO4 in a high overpotential region, e.g., more negative than -0.5 V vs. SHE, as shown in Figure 1a. The reduction current due to the HER became low (in absolute value) when Na2SO4 or K2SO4 was added to the solution. Interestingly, a potential oscillation, named HER oscillation, appeared under current controlled conditions (Figures 1b and 1c). As reported recently [3], the oscillation also appeared in the absence of the salts when the concentration of H2SO4 was as low as 0.03 M. The amount of hydrogen bubbles oscillated synchronously with the potential oscillation: the bubbles evolved more vigorously in the low potential region where the reduction of water occurred than in the high potential region where the reduction of H+ occurred. Therefore, we proposed that the bubble evolution played role in both the positive and negative feedback mechanisms, the combination of which caused an oscillatory instability. The proposed mechanism, however, cannot explain N-NDR characteristics though the oscillation seems to be an HN-NDR type oscillator because it appears as potential oscillations. In the present study, to clarify how the bubble evolution is related to N-NDR characteristics and an NDR-hiding factor, the bubble evolution is studied using Pt electrodes under atmospheric pressure (0.1 M Pa) conditions and also under high-pressure (up to 1 M Pa) conditions. Furthermore, the bubble behaviour is carefully observed by using a high-speed camera. In this presentation, we will show how hydrogen bubbles concurrently induce and hide the N-NDR characteristics. REFERENCES [1] M. Orlik, Self-Organization in Electrochemical Systems I, Springer, Berlin (2012). [2] Y. Mukouyama, M. Kikuchi, H. Okamoto, J. Electroanal. Chem., 617 (2008) 179. [3] Y. Mukouyama, R. Nakazato, T. Shiono, S. Nakanishi, H. Okamoto, J. Electroanal. Chem., 713 (2014) 39. FIGURE CAPTION Figure 1. Current (I) - potential (E) curves for a Pt electrode in 0.10 M H2SO4 solution with or without 0.05 M Na2SO4, measured (a) under potential controlled conditions and (b) under current controlled conditions. (c) Time course of E measured at I = -16 mA. Figure 1

MoS2-graphene-CuNi2S4 nanocomposite an efficient electrocatalyst for the hydrogen evolution reaction

We present a facile methodology for the synthesis of a novel 2D-MoS2, graphene and CuNi2S4 (MoS2-g-CuNi2S4) nanocomposite that displays highly efficient electrocatalytic activity towards the production of hydrogen. The intrinsic hydrogen evolution reaction (HER) activity of MoS2 nanosheets was significantly enhanced by increasing the affinity of the active edge sites towards H+ adsorption using transition metal (Cu and Ni2) dopants, whilst also increasing the edge sites exposure by anchoring them to a graphene framework. Detailed XPS analysis reveals a higher percentage of surface exposed S at 17.04%, of which 48.83% is metal bonded S (sulfide). The resultant MoS2-g-CuNi2S4 nanocomposites are immobilized upon screen-printed electrodes (SPEs) and exhibit a HER onset potential and Tafel slope value of – 0.05 V (vs. RHE) and 29.3 mV dec−1, respectively. These values are close to that of the polycrystalline Pt electrode (near zero potential (vs. RHE) and 21.0 mV dec−1, respectively) and enhanced over a bare/unmodified SPE (– 0.43 V (vs. RHE) and 149.1 mV dec−1, respectively). Given the efficient, HER activity displayed by the novel MoS2-g-CuNi2S4/SPE electrochemical platform and the comparatively low associated cost of production for this nanocomposite, it has potential to be a cost-effective alternative to Pt within electrolyser technologies.

Electrochemical study on hydrogen evolution and CO2 reduction on Pt electrode in acid solutions with different pH

Hydrogen evolution reaction (HER) is the major cathodic reaction which competes CO2 reduction reaction (CO2 RR) on Pt electrode. Molecular level understanding on how these two reactions interact with each other and what the key factors are of CO2 RR kinetics and selectivity will be of great help in optimizing electrolysers for CO2 reduction. In this work, we report our results of hydrogen evolution and CO2 reduction on Pt(111) and Pt film electrodes in CO2 saturated acid solution by cyclic voltammetry and infrared spectroscopy. In solution with pH>2, the major process is HER and the interfacial pH increases abruptly during HER; COad is the only adsorbed intermediate detected in CO2 reduction by infrared spectroscopy; the rate for COad formation increases with the coverage of UPD-H and reaches maximum at the onset potential for HER; the decrease of COad formation under HER is attributed to the available limited sites and the limited residence time for the reduction intermediate (Had), which is necessary for CO2 adsorption and reduction.

Insights on Tafel Constant in the Analysis of Hydrogen Evolution Reaction

Tafel-equation-based electrochemical analysis is widely employed in hydrogen evolution reaction (HER) to evaluate and characterize electrocatalysts. Tafel slope and exchange current density are the only two parameters that are invariably obtained and discussed with respect to Tafel equation in the literature. Herein, insights on Tafel constant in the analysis of HER are discussed, and its practical advantage is indicated. It is proposed that Tafel constant can be considered as the onset potential of HER. Tafel constant becomes the defining parameter between two electrocatalysts when other parameters such as Tafel slope or exchange current density become same. The significance of the same Tafel constants is illustrated using CoSe2 and NiSe2. Variation of the Tafel constant within a series of Co(1–x)NixSe2 is examined. The occurrence of the same Tafel constants between two electrocatalysts leads to one electrocatalyst becoming the better catalyst in the lower current regime, whereas the other exhibits highe...

Wafer-Scale Ultrathin, Single-Crystal Si and GaAs Photocathodes for Photoelectrochemical Hydrogen Production.

Crystalline Si and III-V compound semiconductors with appropriate band edge positions for the reduction of water have been widely utilized in photoelectrochemical (PEC) cells for the hydrogen evolution reaction (HER). However, the high cost of manufacturing those PEC cell photoabsorbers makes it difficult to achieve cost-effective hydrogen production. To overcome this issue, a new approach to fabricate a photoabsorber with low cost yet high performance for the HER is highly necessary. Here, we present a controlled fracture method, the so-called spalling process, to fabricate a cost-effective thin semiconductor applicable to the PEC HER. Using this method, a wafer-scale thin Si, whose thickness can be controlled from a few micrometers to sub-50 μm, was fabricated from a thick Si mother substrate without material loss. Pt nanoparticle-decorated 16 μm thick spalled Si with an np+ rear junction exhibited an HER onset potential of 332 mV (vs reversible hydrogen electrode (RHE)) and a photocurrent density of 20.1 mA cm-2 at 0 V (vs RHE), which are the best performances among previously reported planar-type thin Si-based photocathodes. Finally, we demonstrated that 20 μm thick GaAs could also be successfully fabricated by the spalling process, while exhibiting a PEC HER performance comparable to 350 μm thick bulk GaAs.

Gold Nanoparticles Incorporated in a Zinc‐Based Metal‐Organic Framework as Multifunctional Catalyst for the Oxygen Reduction and Hydrogen Evolution Reactions

AbstractSuccessful synthesis of gold nanoparticles incorporated in a zinc‐based metal‐organic framework (Au@Zn‐MOF) is reported in this paper. The synthesis of Au@Zn‐MOF is confirmed by UV‐Vis, FT‐IR and X‐ray photoelectron spectroscopy (XPS) studies. The Au@Zn‐MOF catalyst demonstrates electrocatalytic activity towards the oxygen reduction reaction (ORR) and the hydrogen evolution reaction (HER). The relative catalytic performance of Au@Zn‐MOF towards ORR and HER has been studied under acidic condition. ORR proceeds via a two electron and two proton mechanism with hydrogen peroxide as the end product, while HER follows the Volmer mechanism i. e., adsorption of H+ on the catalyst's active sites. Au@Zn‐MOF exhibits an ORR onset potential of 0.45 V (vs. RHE) with two different Tafel slopes, −93 and −103.6 mV in acidic solution. Further, an excellent catalytic activity is observed for HER with an onset potential of 0.02 V (vs. RHE) and a Tafel slope of 87 mV in N2 saturated 0.1 M HClO4 solution. However, in O2 saturated 0.1 M HClO4 solution, a HER onset potential of 0.04 V (vs. RHE) is observed with two different Tafel slopes, 610 and 220 mV. The value of Tafel slopes in the presence of O2 advocates the diminution of the ORR activity because of the HER. Thus, Tafel slope values of HER and ORR suggest that protons and O2 compete to reach the electrode surface for getting reduced.

Magnetron Sputter-Coated Nanoparticle MoS2 Supported on Nanocarbon: A Highly Efficient Electrocatalyst toward the Hydrogen Evolution Reaction.

The design and fabrication of inexpensive highly efficient electrocatalysts for the production of hydrogen via the hydrogen evolution reaction (HER) underpin a plethora of emerging clean energy technologies. Herein, we report the fabrication of highly efficient electrocatalysts for the HER based on magnetron-sputtered MoS2 onto a nanocarbon support, termed MoS2/C. Magnetron sputtering time is explored as a function of its physiochemical composition and HER performance; increased sputtering times give rise to materials with differing compositions, i.e., Mo4+ to Mo6+ and associated S anions (sulfide, elemental, and sulfate), and improved HER outputs. An optimized sputtering time of 45 min was used to fabricate the MoS2/C material. This gave rise to an optimal HER performance with regard to its HER onset potential, achievable current, and Tafel value, which were −0.44 (vs saturated calomel electrode (SCE)), −1.45 mV s–1, and 43 mV dec–1, respectively, which has the highest composition of Mo4+ and sulfide (MoS2). Electrochemical testing toward the HER via drop casting MoS2/C upon screen-printed electrodes (SPEs) to electrically wire the nanomaterial is found to be mass coverage dependent, where the current density increases up to a critical mass (ca. 50 μg cm–2), after which a plateau is observed. To allow for a translation of the bespoke fabricated MoS2/C from laboratory to new industrial applications, MoS2/C was incorporated into the bulk ink utilized in the fabrication of SPEs (denoted as MoS2/C-SPE), thus allowing for improved electrical wiring to the MoS2/C and resulting in the production of scalable and reproducible electrocatalytic platforms. The MoS2/C-SPEs displayed far greater HER catalysis with a 450 mV reduction in the HER onset potential and a 1.70 mA cm–2 increase in the achievable current density (recorded at −0.75 V (vs SCE)), compared to a bare/unmodified graphitic SPE. The approach of using magnetron sputtering to modify carbon with MoS2 facilitates the production of mass-producible, stable, and effective electrode materials for possible use in electrolyzers, which are cost competitive to Pt and mitigate the need to use time-consuming and low-yield exfoliation techniques typically used to fabricate pristine MoS2.