Hydrogen Evolution Reaction Rate Research Articles

The Sensitivity of Metal Oxide Electrocatalysis to Bulk Hydrogen Intercalation: Hydrogen Evolution on Tungsten Oxide.

Metal oxides are attracting increased attention as electrocatalysts owing to their affordability, tunability, and reactivity. However, these materials can undergo significant chemical changes under reaction conditions, presenting challenges for characterization and optimization. Herein, we combine experimental and computational methods to demonstrate that bulk hydrogen intercalation governs the activity of tungsten trioxide (WO3) toward the hydrogen evolution reaction (HER). In contrast to the focus on surface processes in heterogeneous catalysis, we demonstrate that bulk oxide modification is responsible for experimental HER activity. Density functional theory (DFT) calculations reveal that intercalation enables the HER by altering the acid-base character of surface sites and preventing site blocking by hydration. First-principles microkinetic modeling supports that the experimental HER rates can only be explained by intercalated HxWO3, whereas nonintercalated WO3 does not catalyze the HER. Overall, this work underscores the critical influence of hydrogen intercalation on aqueous cathodic electrocatalysis at metal oxides.

Molybdenum nitride as a metallic photoelectrocatalyst for hydrogen evolution reaction via introduction of electron traps to improve the separation efficiency of photogenerated carriers

Metallic photoelectrocatalysts possess a wide light absorption range and the fast hydrogen evolution reaction (HER) kinetics, which can be used as the next generation of catalysts towards photoelectrocataytic HER. In this work, molybdenum nitride has been fabricated via an in-suit growth method on metal molybdenum substance (Mo 3 N 2 /Mo foil). The metallic and optical property of Mo 3 N 2 was confirmed by the DFT calculations and experimental results from UV–visible absorption spectrum and valence X-ray photoelectron spectroscopy spectrum. Photocatalytic HER rate of Mo 3 N 2 reached to 158.78 μmol h −1 g −1 . Furthermore, Mo 3 N 2 –MoS 2 /Mo foil was prepared to improve photoelectrocatalytic performance. Herein, a suitable energy band alignment for Mo 3 N 2 –MoS 2 /Mo foil was proposed based on experiments and DFT calculations, and the formation of a heterojunction (Mo 3 N 2 –MoS 2 ) effectively suppressed the recombination of photo-generated carriers. The results of photoelectrocatalytic experiments suggested that the photocurrent density of Mo 3 N 2 –MoS 2 /foil was effectively enhanced about 1.5 times than that of simplex Mo 3 N 2 /Mo foil. The electrochemical experiments (LSV and EIS) indicated that the metallic nature of Mo 3 N 2 was also beneficial to electrocatalytic HER, and the overpotential of Mo 3 N 2 –MoS 2 /Mo foil at 10 mA cm −2 was −173 mV. This work provides a potential candidate for photoelectrocatalytic electrodes. • Metallic Mo 3 N 2 plays a dominant role in photoelectrocatalytic HER activity. • The metallic and optical nature were verified by experiments and DFT calculation. • Mo 3 N 2 –MoS 2 heterojunction was used to improve the photoelectrocatalytic property. • Schottky barrier effectively suppress the recombination of photoinduced carriers. • Mo 3 N 2 –MoS 2 /Mo foil electrode exhibits superior stability for HER.

Digitally controlled organic electrocatalyst for water electrolysis

Rapid growth of information processing and storing technology enforces the rapid relevancy reduction of predecessor technology. Electronic waste accumulation and energy scarcity in the future can be tackled through waste re-utilization. This study intends to develop a digitally controllable organic electrocatalyst based on CD-R. The developed catalyst hydrogen evolution reaction rate was evaluated on electrolysis test in saltwater solvent. The mechanistic view of the kinetics was built from the functional group, phase, and morphological characterization followed by surface charge analysis through digital imaging technique. The result shows that digital information alters electrocatalytic performance of CD shavings and undamaged CD. The half-loaded data CD shavings produced 449101 ppm H 2 which surpass fully-loaded CD shavings with 421879 ppm H 2 . All CD shavings final H 2 were higher than the undamaged CD final concentration. The size of the data loaded on CD determines the surface polarity pattern which enabling digital control. The ununiformed CD shaving kinetics is strongly depends on its size and shape which deviates the role of loaded data size. The strong interchanging surface polarity of fully-loaded CD is effective to interact with negative and positive water ion cluster. In conclusion, this study has enabled electrocatalyst kinetic control through digital information. • Digitally controllable electrocatalyst is possible to be designed. • Digital information alters the electrocatalytic performance. • CD electrocatalyst surface polarity is tunable. • Loaded data size determine the surface polarity series.

Two-dimensional/two-dimensional heterojunction-induced accelerated charge transfer for photocatalytic hydrogen evolution over Bi5O7Br/Ti3C2: Electronic directional transport

Regulating electron density at the active site by electronic directional transport from special channels is an effective strategy to accelerate the reaction rate in photocatalytic water splitting. Here, a novel two dimensional/two dimensional (2D/2D) Bi5O7Br/Ti3C2 heterojunction with special interfacial charge transfer channel was fabricated successfully via in-situ growth of Bi5O7Br on the surface of ultrathin Ti3C2 by using a convenient hydrolysis method. The electrostatic attraction between Bi3+ cations and electronegative Ti3C2 ensures the construction of 2D/2D heterojunction and a strong intimate interface contact between Ti3C2 and Bi5O7Br, which establishes an electronic transport channel, and shortens the charge transport distance, assuring excellent bulk-to-surface and interfacial charge transfer abilities. Meanwhile, X-ray photoemission spectroscopy (XPS) and density functional theory (DFT) calculation revealed that the local electron density at the Ti3C2 active sites is remarkably increased because of the transfer of interfacial electrons from Bi5O7Br to Ti3C2, which is a key factor for enhancing the photocatalytic performance. Thus, the resultant Bi5O7Br/Ti3C2 exhibits significant improvement on the performance of photocatalytic hydrogen evolution under visible light irradiation. The hydrogen evolution reaction rate obtained on the optimized Bi5O7Br/Ti3C2 composite is 1.97 times higher than that of pristine Bi5O7Br. This work provides a new protocol for the construction of 2D/2D heterojunction photocatalytic systems and regulating electron density by electronic directional transporting.

Efficient and long-term photocatalytic H2 evolution stability enabled by Cs2AgBiBr6/MoS2 in aqueous HBr solution

Lead-free Cs 2 AgBiBr 6 (CABB) double perovskite as a new-type photocatalytic material alternative to lead halide perovskites holds promise to implement the solar-H 2 conversion, but the interior recombination of photo-generated carriers and thus low photocatalytic hydrogen evolution reaction (HER) rate of CABB restrict its further industrial applications. Herein, we report the composite fabrication of MoS 2 /CABB heterostructure for high-efficiency and durable photocatalytic HER by anchoring non-noble MoS 2 onto CABB via a facile dissolution-recrystallization method. The optimized MoS 2 /CABB performs a visible-light HER rate of 87.5 μmol h −1 g −1 in aqueous HBr solution, ca. 20-fold compared to that of pure CABB (4.3 μmol h −1 g −1 ), and presents a discontinuous 500-h photocatalytic HER stability with no evident loss. The superb performance of MoS 2 /CABB can be ascribed to the kinetics-facilitated heterostructure consisting of stable CABB and MoS 2 . This work proposes a facile and versatile tactic to construct a low-cost Cs 2 AgBiBr 6 -based heterostructure for efficient and long-term photocatalytic HER. • Lead-free CABB/MoS 2 are prepared by a facile dissolution-recrystallization method. • CABB/MoS 2 achieves boosted charge transfer by the constructed heterostructure. • CABB/MoS 2 owns both high visible-light HER rates and long-term stability. • CABB/MoS 2 exhibits superb photocatalytic HER stability during 500-h cyclic test.

The potential of tantalum as an efficient electrocatalyst for green hydrogen production

Worldwide, discovering a low-cost, highly active and durable electrocatalyst for green hydrogen production is an attractive research field for future renewable energy. Herein, we study the suitability of utilizing metallic Ta as promising electrocatalytic material for hydrogen creation from acidic solutions. The morphological structure and crystal lattice system, as well as the chemical composition and oxidation states of Ta are examined using scanning electron microscopy, X-ray diffraction analysis, and X-ray photoelectron spectroscopy, respectively. Hydrogen evolution reaction (HER) on Ta is investigated using cathodic polarization and electrochemical impedance techniques as a function of electrolyte concentration, applied potential, and temperature. Various kinetic parameters distinguishing catalytic activity of the process on Ta including Tafel slope, exchange current density, cathodic transfer coefficient, and HER rates are determined. At −1500 mV cathodic potential in 0.5 M H2SO4 solution, the results confirm well that, to reach a similar HER rate of say 100 mA cm-2, the overpotential on Ta cathode is found equal -520 mV, which is lower than the value of -940 mV attained on the platinum cathode, indicating high electrocatalytic activity of the HER on Ta surface. The obtained results also demonstrated that, for the hydrogen production process the overpotential greatly diminishes and its current density significantly improves with increasing either the electrolyte concentration or its temperature. Additionally, Tafel slope analysis and impedance parameters establish that HER proceeds by the Volmer–Heyrovsky mechanism. The outcome of this study reveals that the valve metal Ta is a sustainable, efficient, and promising cathode for green hydrogen generation fuel from sulphuric acid electrolytes.

Understanding hydrogen evolution reaction in bicarbonate buffer

Buffers are often employed as electrolytes for their ability to resist to pH changes. Still, they are not just spectators in the electrocatalytic processes, rather they may take a primary role. Here, we attempt to understand the mechanism of the hydrogen evolution reaction (HER) in bicarbonate-containing electrolytes on two distinct materials, Au and Pt. Keeping the bulk pH constant, voltammetric measurements reveal a promotional effect on the HER rate in the presence of bicarbonate buffer, as well as a substantial change in the reaction mechanism with a Tafel Slope in bicarbonate resembling the one in acidic media (ca. 100 and 40 mVdec−1 on Au and Pt, respectively). Nevertheless, because of the increase in surface pH due to the concomitant water reduction (2H2O + 2e−→ H2 + 2 OH−), bicarbonate is consumed by acid-base reactions and the bicarbonate branch of HER becomes significant only in the presence of large bulk buffer concentrations. Bicarbonate and water reduction appear intrinsically different, as they exhibit opposite dependence on the cation identity and scale differently with the specific area of the electrode. Through microkinetic modelling, we exclude that HER in bicarbonate can be explained via the generation of a proton by solution acid-base reactions. Rather, we suggest a direct bicarbonate reduction pathway, which, due to the negatively charged reactant, is strongly dependent on the cation concentration.

In Situ Generated Amorphous Molybdenum Sulfide on Reduced Graphene Oxide Nanocomposite Catalyst for Hydrogen Evolution in a Biphasic Liquid System

AbstractIn situ deposition of catalysts are drawing attention to the liquid/liquid interfaces by using raw materials for the energy conversion reactions such as hydrogen evolution and oxygen reduction. Herein, in situ generation of amorphous molybdenum sulfide on reduced graphene oxide (rGO/MoSx) is investigated in the hydrogen evolution reaction (HER) by decamethylferrocene electron donor at the liquid/liquid interfaces by using (NH4)2MoS4 and graphene oxide precursors in the aqueous phase. rGO/MoSx catalyst shows better catalytic activity than the uncatalyzed reaction and free‐MoSx, which increase the HER rate 57‐ and 1.7‐fold, respectively. The enhanced catalytic activity of rGO/MoSx catalyst is related to the increased surface area, active sites and conductivity of rGO. The catalytic activity of rGO/MoSx are examined by four‐electrode voltammetry and also two‐phase reactions. The obtained rGO/MoSx catalyst are characterized in detail by structural and morphological techniques.

L-Cysteine capped Mo2C/Zn0.67Cd0.33S heterojunction with intimate covalent bonds enables efficient and stable H2-Releasing photocatalysis

ZnxCd1-xS is a prospective photocatalyst with wide visible-light absorption up to 500 nm, but its stability and activity are still facing huge challenges. Herein, L-Cysteine capped Mo2C (L-Mo2C) nanoplates were used to anchor Zn2+/Cd2+ to in-situ grow Zn0.67Cd0.33S nanomaterial for constructing a highly efficient and robust L-Mo2C/Zn0.67Cd0.33S heterojunction through intimate Mo-S covalent bonds for photocatalytic hydrogen evolution reaction (HER). This work not only suppresses the photo-corrosion of Zn0.67Cd0.33S nanomaterial by the construction of strong L-Mo2C/Zn0.67Cd0.33S Schottky heterojunction but also greatly improves the charge transfer efficiency by generating a strong internal electric field. As a result, the optimized L-Mo2C/Zn0.67Cd0.33S-5 heterojunction exhibits an enhanced HER rate of 34.66 mmol h−1 g−1 without an obvious decrease for continuous 24 h under visible irradiation, which is 4.8 times and 1.23 times than that of pristine Zn0.67Cd0.33S and uncapped Mo2C/Zn0.67Cd0.33S-5. This research highlights the role of Mo2C in photocatalytic HER, which provides a novel method to suppress the photo-corrosion of Zn0.67Cd0.33S-based composite.

Novel Ni–Cr-based alloys as hydrogen fuel sources through alkaline water electrolytes

The hydrogen evolution reaction (HER) from water splitting is particularly attractive for green and clean power sources, but it remains a significant challenging issue. For the successful application of hydrogen-based renewable energy such as fuel cells, highly active and cost-saving metallic electro-catalysts for H2 fuel production are vital. The goal of this study is to look into the effects of alloyed molybdenum, chromium, and iron on the electrocatalytic activity of Ni-based alloys in an alkaline electrolyte for HER. SEM with an energy dispersive spectroscopy (EDX) unit was used to determine the chemical composition of alloys. The electrocatalytic effectiveness of the explored cathodes on HER was evaluated in alkaline solutions employing open circuit potential measurements, linear polarization and electrochemical impedance spectroscopy (EIS). The impact of KOH concentrations on HER rate was also investigated. Tafel curves were used to calculate the HER's kinetic parameters, and the mechanisms of the HER were discovered. EIS observations at HER's cathodic potential have been explored and compared to a theoretical model. The Ni–Cr–Mo–Fe alloy has a low over-potential of 232 mV @ 10 mA cm−2 and a Tafel slope of 57.7 mV dec−1 in 1.0 M KOH media, resulting in an efficient HER. These findings indicate that the addition of Mo and Fe to Ni–Cr improves the catalytic efficiency of HER significantly. The Ni–Cr–Mo–Fe cathode is an economical and practical material for alkaline HER production.

Electrocatalysis under Cover: Enhanced Hydrogen Evolution via Defective Graphene-Covered Pt(111)

The production of hydrogen via water electrolysis using renewable electricity is a promising carbon-neutral technology. In this contribution, we report insights into the hydrogen evolution reaction (HER) in H2SO4 on Pt(111) and graphene-covered Pt(111), in addition to the electrochemical properties of graphene overlayers. As-prepared graphene overlayers are selectively permeable to H+ ions in the electrolyte, allowing H+ ions into the confined layer between graphene and Pt(111) while excluding SO42– and other anions. We demonstrate that defects in these as-prepared graphene overlayers can be generated from oxidation at high overpotentials or reduction from the production of H2 bubbles and postulate that HER occurs locally at only Pt(111) in the proximity of defects in graphene overlayers on as-prepared G/Pt(111) electrodes, and as defects in graphene increases, more of the Pt(111) surface becomes utilized for HER. Kinetically, the addition of defective graphene overlayers can increase the geometric HER rate by up to 200%, while Tafel slopes and [H+] reaction orders remain unchanged. These results shed kinetic insight into the nature of graphene overlayers and their effect on HER catalysis and also demonstrate the promise of confinement modifications in designing catalysts with properties closer to achieving optimum rates.

A facile and green large-scale fabrication of single atom catalysts for high photocatalytic H2 evolution activity

Atomically dispersed metal catalysts, which have maximized atom utilization efficiencies, have demonstrated high activities for the Hydrogen evolution reaction (HER). However, there is a lack of simple strategies for the preparation of Single-atom catalysts (SACs). Herein, we report a clean and robust electrochemical method for synthesizing a series of SACs. This methodology employs a Carbon nitride (CN) suspension in pure water as a bath solution and a pure metal target as an easily removable precursor instead of a strong ionic electrolyte. The simplicity of this system facilities the large-scale production and direct use of SACs without any subsequent processing. The mass loading of metal atoms is precisely adjusted using the applied electric field intensity and the processing time. We demonstrate the atomic dispersion and the coordination structure of metal atoms in the prepared SACs using aberration-corrected high-angle annular dark-field scanning transmission electron microscopy and X-ray absorption spectroscopy. The experimental results show that the Ag1/CN SAC exhibits a higher HER activity than that of Pt nanoparticles on CN, even at low mass loadings. For instance, based on metal loading, Ag1/CN exhibits a record-high photocatalytic HER rate of 1688.9 mmol h−1 g−1metal, much higher than that of Pt nanoparticles (328.5 mmol h−1 g−1metal), which confirms the super-high utilization efficiency of Ag atoms. Density functional theory calculations and experiments suggest that this high photocatalytic activity was attributable to a reduced H2 evolution barrier, good proton adsorption and H2 desorption properties. This work provides a novel approach for the synthesis of durable SACs with excellent photocatalytic activities.

Molybdenum sulfide cocatalyst activation upon photodeposition of cobalt for improved photocatalytic hydrogen production activity of ZnCdS

Molybdenum sulfide (MoSx) is a prevalent photocatalytic and electrocatalytic material, attributed to its highly active unsaturated sites. Despite that, its intrinsic activity is still under expectation for large-scale hydrogen evolution reaction (HER). Though the incorporation of Co-promotor could further promote MoSx activity, a tedious preparation is required due to the incompatible properties of Co and Mo. Herein, we report a facile one-step precipitation of cobalt-modified amorphous molybdenum sulfide (Co-MoSx) cocatalyst while focusing on its promotional effects to ZnCdS at λ > 420 nm region. Co-MoSx at different Co/Mo ratios were photochemically-anchored on ZnCdS support through an in-situ light-induced reduction at ambient conditions. In particular, ZnCdS/Co-MoSx (Co/Mo = 1/4, 1 mol%) was emerged as the best formulated photocatalyst, with HER rate and apparent quantum efficiency (AQE) of 551.48 μmol h−1 and 21.7% (420 nm), respectively. While reducing the overpotential for HER, Co-modification could also facilitate the separation of photogenerated electrons and holes for the enhanced HER. Therefore, the reported HER rate of ZnCdS/Co-MoSx is in superior to that of ZnCdS/MoSx and pristine ZnCdS, by 1.4 times and 23 times, respectively. Meanwhile, similar ascendency of Co-MoSx was also observed in its comparison against the benchmarked Pt, Au and Ag co-catalysts. Additionally, the recycling test evinced the satisfactory stability of ZnCdS/Co-MoSx catalyst, with a slightly reduced activity observed after 25 h consecutive HER, marking an average STH of 1.4% under AM 1.5G solar simulator. This work provides a facile way to synthesis Co-modified MoSx co-catalyst that rendered with excellent properties for efficient HER.

Carbon-mediated electron transfer channel between SnO2 QDs and g-C3N4 for enhanced photocatalytic H2 production

Graphitic carbon nitride (g-C3N4) is a promising material for photocatalytic water splitting but suffers from the self-agglomeration and fast recombination of photogenerated electron–hole pairs. Tin oxide (SnO2) has a high electron extraction ability and can play a key role in the charge separation and transfer dynamics of composites. Herein, we report a 0D/2D heterostructure of carbon-encapsulated SnO2 quantum dots (SnO2@C QDs) anchored on g-C3N4 nanosheets (SnO2@C/CN). The construction of interface between SnO2@C and g-C3N4 dramatically increases the surface area and the number of active sites for photocatalytic hydrogen evolution reaction (HER) and provides a driving force for efficient charge separation/transfer kinetics. The carbon layer encapsulating SnO2 QDs acts as a bridge that facilitates electron transfer from g-C3N4 to SnO2 QDs. The champion SnO2@C/CN achieves an exceptional HER rate of 2,544.3 μmol g−1 h−1 (with 3 wt% Pt) with an apparent quantum efficiency of 9.63 % (λ = 420 nm) and excellent photostability. A photoactivity enhancement mechanism is proposed based on the interfacial energy band alignment. This work provides insights into the designing of heterostructured photocatalysts of enhanced charge separation via interface engineering.

Surface strain engineered Ni-NiO for boosting hydrogen evolution reaction in alkaline media

Electrolysis of water in alkaline media to produce hydrogen is a green and promising energy storage technology, but the kinetically sluggish hydrogen evolution reaction (HER) in alkaline electrolytes limits its development, requiring cheap and highly efficient catalysts to increase the HER rate. The HER activity of transition metal-based catalysts is closely associated with the d-band structures. Herein, the surface strain of Ni and NiO was engineered to regulate the d-band centers and the HER activity. DFT calculations found that the Ni(111) supported NiO(111) shows expanded surface lattice compared to the unsupported NiO(111), and the NiO(100) supported Ni(100) shows compressed surface lattice compared to the unsupported Ni(100), leading to ∼2 eV upshift and downshift of d-band centers of the surface Ni atoms, respectively. Guided by the DFT calculation, the surface-oxidized Ni (so-Ni) and surface-reduced NiO (sr-NiO) catalysts were synthesized and shows expanded and compressed surface lattice respectively, leading to enhanced HER activity. The synthesized sr-NiO shows the highest HER activity with an overpotential of 164 mV to deliver a current density of 10 mA cm−2, which outperforms many of the recently reported Ni-based HER catalysts in alkaline electrolytes.

Sustainable synthesis of low-cost nitrogen-doped-carbon coated Co3W3C@g-C3N4 composite photocatalyst for efficient hydrogen evolution

The availability of low-cost noble-metal-free photocatalyst is vital for sustainable and large-scale hydrogen production. Herein, N-doped-carbon coated Co3W3C nanocrystallines (NC-Co3W3C), a novel cocatalyst, are anchored onto g-C3N4, forming a (NC-Co3W3C@g-C3N4) composite photocatalyst for application in photocatalytic hydrogen evolution reaction (HER). Under visible light irradiation, NC-Co3W3C@g-C3N4 photocatalyst shows a remarkable photocatalytic HER rate of 1332 μmol h−1 g−1, achieving 148 times higher than pristine g-C3N4 photocatalyst and 89.22% as high as Pt/g-C3N4 photocatalyst (the expensive noble-metal-based catalyst). The mechanism investigation via systematic experimental and theoretical analysis (Density Functional Theory (DFT) calculation) suggests that the high photocatalytic efficiency could be attributed to boosted light absorption and accelerated HER kinetic that is resulted from the enhanced conducitity, reinforced stability, and the suitable H adsorption free energy introduced by the cocatalyst. This work suggests the construction of highly efficient catalyst for photocatalytic hydrogen evolution is chemically and economically viable for the green hydrogen economy.

One-pot molten salt method for constructing CdS/C3N4 nanojunctions with highly enhanced photocatalytic performance for hydrogen evolution reaction

The construction of heterojunction photocatalysts for efficiently utilizing solar energy has attracted considerable attention to solve the energy crisis and reduce environmental pollution. In this study, we use the energy released from an easily-occurred exothermic chemical reaction to serve as the drive force to trigger the formation of CdS and C3N4 nanocomposites which are successfully fabricated with cadmium nitrate and thiourea without addition of any solvents and protection of inert gas at initial temperature, a little higher than the melting point of thiourea. The as-prepared CdS/C3N4 materials exhibit high efficiency for photocatalytic hydrogen evolution reaction (HER) with the HER rate as high as 15,866 μmol/(g∙hr) under visible light irradiation (λ > 420 nm), which is 89 and 9 times those of pristine C3N4 and CdS, respectively. Also, the apparent quantum efficiency (AQE) of CdS/C3N4–1:2–200–2 (CdS/C3N4–1:2–200–2 means the ratio of Cd to S is 1:2 and the reaction temperature is set at 200°C for two hours) reaches 3.25% at λ = 420 ± 15 nm. After irradiated for more than 24 hr, the HER efficiencies of CdS/C3N4 do not exhibit any attenuation. The DFT calculation suggests that the charge difference causes an internal electric field from C3N4 pointing to CdS, which can more effectively promote the transfer of photogenerated electrons from CdS to C3N4. Therefore, most HER should occur on C3N4 surface where photogenerated electrons accumulate, which largely protects CdS from photo-corrosion.

Correlation of Exchange Current Density j₀ and the Standard Potential of the Metal Electrodes E0 M : A Different View of the Hydrogen Evolution Reaction (HER)

Technologically, the hydrogen evolution reaction (HER) is the foundation of many electrochemical energy systems. Fundamentally, HER is important as 2H+ + 2e ⇌ H2 defines the standard reference potential E0 H = 0.000 V in thermodynamics. All evolutions in kinetic theory are critically tested against HER.In electrocatalysis, the HER mechanism is historically specified in three steps. The first step, Volmer, couples concerted adsorption and reduction of proton to form electrochemically adsorbed hydrogen.H+ + e ⇄ Hads (Volmer) (1)Once the first proton electroadsorbs, H2 forms by two parallel pathways. The Heyrovsky step proceeds as a second proton is electroadsorbed to the same metal atom and hydrogen gas desorbs from the common metal center.Hads + H+ + e ⇄ H2 (Heyrovsky) (2)In the Tafel step, electrochemically adsorbed hydrogen forms on an adjacent metal center and H2 desorbs.Hads + Hads ⇄ H2 (Tafel) (3)Commonly, HER electrode kinetics are evaluated according to the Volmer-Heyrovsky-Tafel scheme, where rate is measured as exchange current densities j₀. For HER at metal electrodes, log j₀ depends strongly on the electrode metal where log j₀ falls between -3 for Pt, Re, and Pd and -12 for Hg. Although metals are not explicit in the Volmer-Heyrovsky-Tafel scheme, j₀ is evaluated in view of Steps (1) to (3). Here, metal electrode properties are introduced as the standard potential E0 M of the electrode metal. For metal cation reduction to metal M⁰,Mz+ + ze ⇌ M⁰ E0 M log j₀ values are cataloged in a paper by Trasatti (Electroanalytical Chemistry and Interfacial Electrochemistry 1972, 39, 163-184), where he partitioned 31 metal electrodes into d and sp metals. For the transition metals, the d metals, log j₀ is well and linearly correlated with E0 M . For the sp metals, there is little to no correlation of log j₀ with E0 M .For the transition metals, several observations and a model sketch are noted. HER rates as log j₀ increase with more positive values of E0 M .For the initial step of the metal dependent HER process, it is sketched that electron(s) release from M0 to form Mz+ immediately at the electrode surface.For this initial step, formation of Mz+ immediately at the electrode solution interface yields log j₀ linearly dependent on E0 M .Within transition state or activated complex theory, a proposed transition state is formed of electron(s) shared between Mz+ and H+.Formation of the transition state [(1/z)Mz+⋯e⋯H+]‡ precedes the Volmer step, and where applicable, is a rate determining step.Overpotential η pinned by E0 M may characterize the counter intuitive observation that rate (log j₀) increases as E0 M is more positive.This sketch of HER yields a kinetic model where a component of the activation energy is set by F(E0 H -E0 M ). The sketch of an electrode dependent transition state may pose a general approach to characterizing electrocatalysis and interfacial electron transfers. The proposed transition state introduces physical properties of specific electrode materials into the rate expressions a priori.

Electrocatalysis Under Cover: Enhanced Hydrogen Evolution Reaction (HER) Via Defective Graphene Covered Pt(111)

The production of hydrogen through water electrolysis using renewable electricity is a promising carbon-neutral technology. We present insights into the hydrogen evolution reaction (HER) in aqueous H2SO4 (0.005 to 0.5 M) on Pt(111) and Graphene/Pt(111) under ambient temperatures and pressures. Graphene was grown on Pt(111) using chemical vapor deposition (CVD) of hydrocarbons [1]. Cyclic voltammograms (CV) and HER currents of Pt(111) in H2SO4 were quantitatively benchmarked to literature to ensure reproducibility and comparability of kinetic measurements [1-3]. CV sweeps (0.005 to 0.5 M, 23°C, 0.1 to 0.8 VRHE) indicate that graphene overlayers are selectively permeable to H+ ions in the electrolyte, allowing H+ ions into the confined layer between Pt(111) and graphene while excluding SO4 2- and other anions [2]. The extent of defects in graphene was quantified by integrating the anion adsorption region from CVs. The anion adsorption coverage on electrochemically pre-treated Graphene/Pt(111) (250 cycles between 0.1 to 1.2 VRHE in 0.005 M H2SO4) increases with HER reaction time while untreated Graphene/Pt(111) exhibits a slower increase. Though the defect generation rate is slower on untreated Graphene/Pt(111), a sudden order-of-magnitude increase in anion coverage concurrent with transformation of the Pt(111) to polycrystalline Pt is observed after ~10 hours of HER. In contrast, no sudden increase in the anion capacity is observed on electrochemically pre-treated Graphene/Pt(111) for up to 15 hours of HER, though a transition from Pt(111) to polycrystalline Pt is also observed after ~10 hours of HER.Initial HER currents on both pre-treated and un-treated Graphene/Pt(111) are slower than Pt(111) by 1 to 2 orders of magnitude (0.005 to 0.5 M, 23°C, -0.08 VRHE). HER currents on both pre-treated and un-treated Graphene/Pt(111) increase with HER reaction time to values equal to or higher than that of Pt(111), concurrent with increasing graphene defects as probed using CV and SEM. Unchanged HER Tafel slopes and [H+] reaction orders suggest the same HER mechanism between Pt(111) and Graphene/Pt(111). We postulate that untreated graphene is mass transfer limited and HER currents transition from mass transfer limited to kinetics limited with increasing graphene defects. Li et al. computes that a graphene overlayer would weaken the hydrogen binding energy to Pt(111), shift the reaction closer to the peak of the HER volcano plot, and thus increase HER currents [6,7]. Though our results agree with prediction, we cannot negate other possible explanations for the observed increase in HER rate. These results demonstrate the promise of overlayer and confinement modifications in designing catalysts with properties closer to achieving optimum currents.

Interface Engineering of a 2D-C3N4/NiFe-LDH Heterostructure for Highly Efficient Photocatalytic Hydrogen Evolution.

Photocatalytic water splitting offers an economic and sustainable pathway for producing hydrogen as a zero-emission fuel, but it still suffers from low efficiencies limited by visible-light absorption capacity and charge separation kinetics. Herein, we report an interface-engineered 2D-C3N4/NiFe layered double hydroxide (CN/LDH) heterostructure that shows highly enhanced photocatalytic hydrogen evolution reaction (HER) rate with excellent long-term stability. The morphology and band gap structure of NiFe-LDH are precisely regulated by employing NH4F as a structure-directing agent, which enables a fine interfacial tuning via coupling with 2D-C3N4. The formation of a type II interface in CN/LDH enlarges the active surface area and promotes the charge separation efficiency, leading to an HER rate of 3087 μmol g-1 h-1, which is 14 times higher than that of 2D-C3N4. This study highlights a rational interface engineering strategy for the formation of a heterostructure with a proper hole transport co-catalyst for designing effective water-splitting photocatalysts.