Mechanism Of Hydrogen Evolution Reaction Research Articles

Enhanced hydrogen evolution from the face-sharing [RuO6] octahedral motif

Abstract Hampered by the ambiguous mechanism of hydrogen evolution reaction (HER) in basic media, the exploration of highly efficient catalytically active sites for alkaline HER is of significance. Herein, a metal oxide Sr4Ru2O9 engineering a face-sharing [RuO6] octahedra motif was synthesized through the solid-state method, and served as HER electrocatalyst. Benefited from the Ru-Ru metallic bonding crossing the common plane, the H* adsorption and reaction energy barriers were optimized. Sr4Ru2O9 only required an ultra-small overpotential (ƞ10) of 28 mV at a current density of 10 mA cm−2 for HER in 1.0 M KOH with an exceptional stability (180 hours), outperforming the commercial Pt/C (ƞ10 = 38 mV). These findings suggest a fresh insight in designing novel active sites for electrocatalysis.

Reaction coordinate mapping of hydrogen evolution mechanism on Mg3N2 monolayer

In this work, we have envisaged the hydrogen evolution reaction (HER) mechanism on Mg3N2 monolayer based on electronic structure calculations within the framework of density functional theory (DFT) formalism. The semiconducting nature of Mg3N2 monolayer motivates us to investigate the HER mechanism on this sheet. We have constructed the reaction coordinate associated with HER mechanism after determining the hydrogen adsorption energy on Mg3N2 monolayer, while investigating all possible adsorption sites. After obtaining the adsorption energy, we subsequently obtain the adsorption free energy while adding zero point energy difference (ΔZPE) and entropic contribution (TΔS). We have not only confined our investigations to a single hydrogen, but have thoroughly observed the adsorption phenomena for increasing number of hydrogen atoms on the surface. We have determined the projected density of states (DOS) in order to find the elemental contribution in the valence band and conduction band regime for all the considered cases. We have also compared the work function value among all the cases, which quantifies the amount of energy required for taking an electron out of the surface. The charge transfer mechanism is also being investigated in order to correlate with the HER mechanism with amount of charge transfer. This is the first attempt on this material to the best of our knowledge, where theoretical investigation has been done to mapping the reaction coordinate of HER mechanism with the associated charge transfer process and the work function values, not only for single hydrogen adsorption, but also for increasing number of adsorbed hydrogen.

Trends in Alkaline Hydrogen Evolution Activity on Cobalt Phosphide Electrocatalysts Doped with Transition Metals

Summary Understanding the mechanism of the alkaline hydrogen evolution reaction (HER) and finding a design principle to enhance the catalytic performance of electrocatalysts are of great significance in paving the way of alkaline water electrolysis. Here, taking cobalt phosphide (CoP) as a model material, we establish the alkaline HER activity trends as a function of a fundamental property, the proportion of the unoccupied 3d orbital (Pun). By doping CoP with a range of 3d and 4d transition metals with different Pun, we show that the intrinsic alkaline HER kinetics increases with Pun. DFT calculations indicate that the transition metals with higher Pun not only act as the oxophilic sites for enhanced water activation but also modulate the electronic structure of CoP to endow an optimized H adsorption (ΔG∗H). Moreover, by further comparing the correlation between water adsorption and dissociation or ΔG∗H and experimental alkaline HER activities, we find that water dissociation is the rate-determining step for alkaline HER.

Monitoring Hydrogen Evolution Reaction Intermediates of Transition Metal Dichalcogenides via Operando Raman Spectroscopy

AbstractA deeper understanding of the water‐splitting hydrogen evolution reaction (HER) mechanism during photocatalytic processes is crucial for the rational design of efficient photocatalysts. In particular, the HER mechanism promoted by multielement hybrid structures remains extremely challenging and elusive. Herein, an in situ photoelectrochemical/Raman measurement system is employed to monitor the HER mechanism of hybrid nanostructures under realistic working conditions via operando Raman spectra and linear‐sweep voltammetry curves. As a proof of concept, tunable composition transition metal dichalcogenides MoS2xSe2(1−x) nanosheets are used as a model photocatalyst to unveil the corresponding photocatalytic mechanism. The spectroscopic studies reveal that hydrogen atoms can be adsorbed to active sulfur and selenium atoms via intermediate species formed during the photocatalytic process. More importantly, the studies demonstrate that an exponential relationship exists between the number of reactive electrons and the Raman intensity of intermediate species, which can serve as a guideline to directly evaluate the HER performance in photocatalysts by comparing the Raman intensities of the intermediate species. As a simple, intuitive, and general analytical method, the designed operando Raman measurement approach provides a new tool for elucidating catalytic reaction mechanisms in a realistic and complex environment; and strategically improving H2 production performance of multielement photocatalysts.

Does power ultrasound (26 kHz) affect the hydrogen evolution reaction (HER) on Pt polycrystalline electrode in a mild acidic electrolyte?

In this study, we investigated the effects of power ultrasound (26 kHz, up to ∼75 W/cm2, up to 100% acoustic amplitude, ultrasonic horn) on the hydrogen evolution reaction (HER) on a platinum (Pt) polycrystalline disc electrode in 0.5 M H2SO4 by cyclic and linear sweep voltammetry at 298 K. We also studied the formation of molecular hydrogen (H2) bubbles on a Pt wire in the absence and presence of power ultrasound using ultra-fast camera imaging. It was found that ultrasound significantly increases currents towards the HER i.e. a ∼250% increase in current density was achieved at maximum ultrasonic power. The potential at a current density of −10 mA/cm2 under silent conditions was found to be −46 mV and decreased to −27 mV at 100% acoustic amplitude i.e. a ΔE shift of ∼+20 mV, indicating the influence of ultrasound on improving the HER activity. A nearly 100% increase in the exchange current density (jo) and a 30% decrease in the Tafel slope (b) at maximum ultrasonic power, was observed in the low overpotential region, although in the high overpotential region, the Tafel slopes (b) were not significantly affected when compared to silent conditions. In our conditions, ultrasound did not greatly affect the “real” surface area (Ar) and roughness factor (R) i.e. the microscopic surface area available for electron transfer. Overall, it was found that ultrasound did not dramatically change the mechanism of HER but instead, increased currents at the Pt surface area through effective hydrogen bubble removal.

Interface Modulation of MoS2 /Metal Oxide Heterostructures for Efficient Hydrogen Evolution Electrocatalysis.

Developing efficient earth-abundant MoS2 based hydrogen evolution reaction (HER) electrocatalysts is important but challenging due to the sluggish kinetics in alkaline media. Herein, a strategy to fabricate a high-performance MoS2 based HER electrocatalyst by modulating interface electronic structure via metal oxides is developed. All the heterostructure catalysts present significant improvement of HER electrocatalytic activities, demonstrating a positive role of metal oxides decoration in promoting the rate-limited water dissociation step for the HER mechanism in alkaline media. The as-obtained MoS2 /Ni2 O3 H catalyst exhibits a low overpotential of 84 mV at 10 mA cm-2 and small charge-transfer resistance of 1.5 Ω in 1 m KOH solution. The current density (217 mA cm-2 ) at the overpotential of 200 mV is about 2 and 24 times higher than that of commercial Pt/C and bare MoS2 , respectively. Additionally, these MoS2 /metal oxides heterostructure catalysts show outstanding long-term stability under a harsh chronopotentiometry test. Theoretical calculations reveal the varied sensitivity of 3d-band in different transition oxides, in which Ni-3d of Ni2 O3 H is evidently activated to achieve fast electron transfer for HER as the electron-depletion center. Both electronic properties and energetic reaction trends confirm the high electroactivity of MoS2 /Ni2 O3 H in the adsorption and dissociation of H2 O for highly efficient HER in alkaline media.

Unveiling a Photoinduced Hydrogen Evolution Reaction Mechanism via the Concerted Formation of Uranyl Peroxide.

We present a density functional theory study for the photochemical water oxidation reaction promoted by uranyl nitrate upon sunlight radiation. First, we explored the most stable uranyl complex in the absence of light. The reaction in a dark environmen proceeds through the condensation of uranyl monomers to form dimeric hydroxo-bridged species, which is the first step toward a hydrogen evolution reaction (HER). We found a triplet-state-driven mechanism that leads to the formation of uranyl peroxide and hydrogen gas. To describe in detail this reaction path, we characterized the singlet and triplet low-lying states of the dimeric hydroxo-bridged species, including minima, transition states, minimal energy crossing points, and adiabatic energies. Our computational results provide mechanistic insights that are in good agreement with the experimental data available.

Transition Metal Phosphide-Based Materials for Efficient Electrochemical Hydrogen Evolution: A Critical Review.

As hydrogen has been increasingly considered as promising sustainable energy supply, electrochemical overall water splitting driven by highly efficient non-noble metal electrocatalysts has aroused extensive attention. Transition metal phosphides (TMPs) have demonstrated remarkable electrocatalytic performance, including high activity and robust durability towards hydrogen evolution reaction (HER) in acidic and alkaline as well as neutral electrolytes. In this Review, up-to-date progress of TMP-based HER electrocatalysts is summarized. Various synthesis strategies of TMPs based on selected phosphorus sources are presented, and the reaction mechanisms of HER as well as the contribution of phosphorus in the TMPs to HER activity are briefly discussed. The multiscale approaches for promoting the activity and stability of TMP-based catalysts are discussed with respect to intrinsic electronic structure, hybrids, microstructure, and working electrode interface. Some crucial issues and future perspectives of TMPs are pointed out. These modulated approaches and challenges are also instructive for constructing other high-activity energy-related electrocatalysts.

On the importance of cyanide in diiron bridging carbyne complexes, unconventional [FeFe]-hydrogenase mimics without dithiolate: An electrochemical and DFT investigation

A large number of catalysts for hydrogen evolution reaction (HER) that are related (structurally and/or functionally) to the active site of [FeFe]-hydrogenases (H-cluster) has been proposed in the past few decades. Very recently, a novel recipe for HER catalysts has been added to the list. This consists in the combination of a diiron core, a ligand that can act as an intramolecular weak base to support protonation (both features that are common to the H-cluster), and a peculiar carbyne ligand that bridges the two metal centers. For instance, complex [Fe2{μ-CNMe2}(μ-CO)(CO)(CN)Cp2] (4) proved to be catalytically active towards HER. The novelty related to this family of biomimicry is not only structural, but also mechanistic. Indeed, an unprecedented (in the context of hydrogenase mimicry) ligand-based mechanism of HER, which avoids protonation at metal, has been dissected by DFT. In order to increase catalytic activity of this “non dithiolate based” system, a systematic search of ligand modification is therefore desirable. In this regard, herein we present results of replacing CN− (the proton shuttle in 4) by other ligands such as CNCH2Ph (5), PPh3 (6) or azide (7). The redox behavior of these compounds has been tested, in the absence and presence of a proton source. CV experiments combined with DFT calculations have revealed and rationalized critical points entailed by modifications of stereo-electronic features of 4. However, understanding the key factors underlying the absence of catalytic activity of these new compounds can be beneficial for the design of future improved biomimicry related to HER. As an example, properly tailored hydrocarbyl complexes replacing dithiolates at the Fe2 core might allow recovering the fruitful electronic properties of cyanides, when coordinated to metal centers.

In-Operando Insights on the Hydrogen Evolution Reaction Activity and Stability of Non-Noble Metal Electrocatalysts for Water Electrolysis

Market penetration of proton exchange membrane water electrolysers (PEMWE) as energy conversion technologies in a renewable energy sector would imply, given the 2.3$/Kg H2 production cost aimed by the DoE,1 a drastic decrease in total noble metal electrocatalyst loading. For the state-of-the-art hydrogen evolution reaction (HER) catalyst, platinum, recent targets suggest this limit to be 0.05mgPt/cm2.2 In some applications, like direct photoelectrochemical water splitting, application of any amount of noble metal as a co-catalyst cannot be tolerated at all. Thus, given increasing scarcity of platinum, alternative materials based on earth-abundant non-noble metals represent a significant interest as promising HER catalysts. Among them, transition metal dichalcogenides such as molybdenum disulfide (MoS2) have provided excellent performances, particularly in their cluster-based allotropic structure [Mo3S13]2-.3 Whilst the role of sulfide atoms in the Mo-terminated edges as HER active sites in crystalline MoS2 is well-established, the HER mechanism of [Mo3S13]-based materials is still under debate. Preceded by an electrochemical activation step, operando spectroscopic studies ascribed the active sites to the electrochemically-cleaved S2 2-,4 but other reports have suggested undercoordinated Mo sites to be responsible for the HER activity.5 To date, no study has evaluated the in operando activity and stability of [Mo3S13]-based materials, shown to be intimately dependent on the electrochemical conditioning and electrolyte pH.6 This work devotes to the simultaneous detection of Mo and S species under HER operating conditions in pristine and anchored [Mo3S13]2- clusters to nitrogen-doped carbon nanotubes, by use of our dedicated on-line inductively-coupled plasma mass spectrometry (on-line ICP-MS). Our findings demonstrate, contrary to previous understanding, that the electrochemical activation step is mostly related to Mo dissolution rather than S loss, which is one order of magnitude lower. After such activation, Mo dissolution is significantly mitigated, whereas S dissolution is still observed, with magnitudes dependent on the anchoring material. The overarching picture obtained by such study enabled to shed light on the HER activation mechanism, and demonstrate that the stabilities of these materials are comparable to some Ir-based catalysts. The aforementioned results provide further guidelines to implement non-noble HER metal electrocatalysts in PEMWE.

Into the “secret” double layer: Alkali cation mediates the hydrogen evolution reaction in basic medium

The hydrogen evolution reaction (HER) – as an essential half reaction in water electrolysis and chlor-alkali process has been well studied in acidic electrolyte, but much less has been known in basic medium. In this study, by combining kinetic modeling and electrochemical measurements, we show that hydrated alkali cation clusters can adsorb on the surface of HER catalyst in basic electrolyte. The bound H2O molecules in the “clusters” can thus be in-situ activated through the hydration effect, which dissociate on the catalyst surface as the reactant of HER. The effective concentration and hydration energy of alkali cation can influence the H2O dissociation rate, and hence the kinetics of HER. Our work demonstrates a new understanding of the HER mechanism in basic reaction electrolyte.

Surface Composition Dependent Ligand Effect in Tuning the Activity of Nickel-Copper Bimetallic Electrocatalysts toward Hydrogen Evolution in Alkaline.

Exploring efficient and low-cost electrocatalysts for hydrogen evolution reaction (HER) in alkaline media is critical for developing anion exchange membrane electrolyzers. The key to a rational catalyst design is understanding the descriptors that govern the alkaline HER activity. Unfortunately, the principles that govern the alkaline HER performance remain unclear and are still under debate. By studying the alkaline HER at a series of NiCu bimetallic surfaces, where the electronic structure is modulated by the ligand effect, we demonstrate that alkaline HER activity can be correlated with either the calculated or the experimental-measured d band center (an indicator of hydrogen binding energy) via a volcano-type relationship. Such correlation indicates the descriptor role of the d band center, and this hypothesis is further supported by the evidence that combining Ni and Cu produces a variety of adsorption sites, which possess near-optimal hydrogen binding energy. Our finding broadens the applicability of d band theory to activity prediction of metal electrocatalysts and may offer an insightful understanding of alkaline HER mechanism.

Combinatorial Design and Computational Screening of Two-Dimensional Transition Metal Trichalcogenide Monolayers: Toward Efficient Catalysts for Hydrogen Evolution Reaction.

Recent experiments showed that some layered ternary transition metal trichalcogenide compounds are efficient catalysts for the hydrogen evolution reaction (HER). Motivated by these, we have combinatorially designed and computationally screened, through an efficient, automated approach based on density functional theory, single layers of such compounds, including those not reported in widely used crystal structure database like the International Crystal Structure Database (ICSD), for their efficiency as HER catalysts. On the basis of our theoretical prediction of overpotentials determined from the reaction coordinate mapping corresponding to the HER mechanism, 13 of these compounds are found to be promising catalysts, out of which three are suggested to be as efficient as platinum, the best known HER catalyst to date.

Homolytic versus Heterolytic Hydrogen Evolution Reaction Steered by a Steric Effect.

Several H-H bond forming pathways have been proposed for the hydrogen evolution reaction (HER). Revealing these HER mechanisms is of fundamental importance for the rational design of catalysts and is also extremely challenging. Now, an unparalleled example of switching between homolytic and heterolytic HER mechanisms is reported. Three nickel(II) porphyrins were designed and synthesized with distinct steric effects by introducing bulky amido moieties to ortho- or para-positions of the meso-phenyl groups. These porphyrins exhibited different catalytic HER behaviors. For these Ni porphyrins, although their 1e-reduced forms are active to reduce trifluoroacetic acid, the resulting Ni hydrides (depending on the steric effects of porphyrin rings) have different pathways to make H2 . Understanding HER processes, especially controllable switching between homolytic and heterolytic H-H bond formation pathways through molecular engineering, is unprecedented in electrocatalysis.

Homolytic versus Heterolytic Hydrogen Evolution Reaction Steered by a Steric Effect

AbstractSeveral H−H bond forming pathways have been proposed for the hydrogen evolution reaction (HER). Revealing these HER mechanisms is of fundamental importance for the rational design of catalysts and is also extremely challenging. Now, an unparalleled example of switching between homolytic and heterolytic HER mechanisms is reported. Three nickel(II) porphyrins were designed and synthesized with distinct steric effects by introducing bulky amido moieties to ortho‐ or para‐positions of the meso‐phenyl groups. These porphyrins exhibited different catalytic HER behaviors. For these Ni porphyrins, although their 1e‐reduced forms are active to reduce trifluoroacetic acid, the resulting Ni hydrides (depending on the steric effects of porphyrin rings) have different pathways to make H2. Understanding HER processes, especially controllable switching between homolytic and heterolytic H−H bond formation pathways through molecular engineering, is unprecedented in electrocatalysis.

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.

An effective hybrid electrocatalyst for the alkaline HER: Highly dispersed Pt sites immobilized by a functionalized NiRu-hydroxide

Developing a highly active and robust catalyst for the hydrogen evolution reaction (HER) using the minimal amount of Pt is a continuing challenge. Herein, we demonstrate a functionalized NiRu-hydroxide (NiRu-OH) catalyst that acts as a capable candidate to immobilize highly dispersed single Pt atoms. We synthesize this catalyst by means of Ru leaching from a NiRu-layer double hydroxide (NiRu-LDH) under oxygen evolution reaction (OER) conditions. The hybrid catalyst shows extremely high HER activity, with an overpotential of 38 mV to drive a typical current density of 10 mA cmgeo−2 in alkaline media. We ascribe the excellent catalytic activity to the synergistic effect of the highly dispersed single Pt atoms and NiRu-OH in a dual-site alkaline HER mechanism. This work forms the basis for the exploration of matrices capable of trapping single atoms for various applications.

Pt/Mo2C heteronanosheets for superior hydrogen evolution reaction

Abstract One of the most primary challenges to achieve large-scale hydrogen generation from water electrolysis is the sluggish kinetics and noble metal dependence of cathodic hydrogen evolution reaction (HER). By considering the excellent water dissociation catalytic activity of Mo2C, abundant Pt/Mo2C interfaces were facilely engineered via galvanic replacement (gr) by using Mo/Mo2C nanosheets as self-sacrificed templates to alter the alkaline HER mechanism on Pt based catalyst. The rational designed interface-rich gr-Pt/Mo2C catalyst exhibited excellent activity with the overpotential to drive 10 mA/cm2 current density decreased by 18.5 mV compared with the commercial Pt/C catalyst. 34.3 mV/dec Tafel slope confirms the Volmer-Tafel HER route on gr-Pt/Mo2C in alkaline condition. Platinum utilization is calculated to be improved by 9.7 times by considered the low Pt loading in the gr-Pt/Mo2C catalyst. With its satisfied stability, the scalable gr-Pt/Mo2C catalyst shows promising application potential in industrial electrolysis systems.

The role of strain induced band modulation of WS2-GeC heterostructure for the hydrogen evolution

To devise widespread and economical materials to produce hydrogen is fundamental for turning hydrogen into an ideal clean energy source. Many research findings have proved that using 2D structured transition metal dichalcogenides (TMDs) as electro-catalysts is a feasible way to prompt hydrogen evolution reaction (HER) activity. This work exhibits a theoretical research on pristine WS2/GeC and twisted WS2-GeC composites with comprehensive first-principles calculations to lay a theoretical framework on the band re-alignment and kinetic barriers for H2 evolution. The results reveal that the WS2-GeC is a typical well-defined type-II heterostructure, which can effectively separate carriers. A further electronic phase transition from semiconducting to metallic can be obtained by applying the external strain, which originates from the internal energy-level shift. The calculated barrier of Tafel mechanism react with water for H2 evolution is only 0.8 eV when unaxial strain is applied on HS-3. This work not only proposes a detailed HER mechanism of heterostructure, but reveals a new possibility for application in nanomaterials since strain engineering is a flexible and feasible approach.

Synthetic Metallodithiolato Ligands as Pendant Bases in [FeIFeI], [FeI[Fe(NO)]II], and [(μ-H)FeIIFeII] Complexes.

The development of ligands with specific stereo- and electrochemical requirements that are necessary for catalyst design challenges synthetic chemists in academia and industry. The crucial aza-dithiolate linker in the active site of [FeFe]-H2ase has inspired the development of synthetic analogues that utilize ligands which serve as conventional σ donors with pendant base features for H+ binding and delivery. Several MN2S2 complexes (M = Ni2+, [Fe(NO)]2+, [Co(NO)]2+, etc.) utilize these cis-dithiolates to bind low valent metals and also demonstrate the useful property of hemilability, i.e., alternate between bi- and monodentate ligation. Herein, synthetic efforts have led to the isolation and characterization of three heterotrimetallics that employ metallodithiolato ligand binding to di-iron scaffolds in three redox levels, (μ-pdt)[Fe(CO)3]2, (μ-pdt)[Fe(CO)3][(Fe(NO))II(IMe)(CO)]+, and (μ-pdt)(μ-H)[FeII(CO)2(PMe3)]2+ to generate (μ-pdt)[(FeI(CO)3][FeI(CO)2·NiN2S2] (1), (μ-pdt)[FeI(CO)3][(Fe(NO))II(IMe)(CO)]+ (2), and (μ-pdt)(μ-H)[FeII(CO)2(PMe3)][FeII(CO)(PMe3)·NiN2S2]+ (3) complexes (pdt = 1,3-propanedithiolate, IMe = 1,3-dimethylimidazole-2-ylidene, NiN2S2 = [N,N'-bis(2-mercaptidoethyl)-1,4-diazacycloheptane] nickel(II)). These complexes display efficient metallodithiolato binding to the di-iron scaffold with one thiolate-S, which allows the free unbound thiolate to potentially serve as a built-in pendant base to direct proton binding, promoting a possible Fe-H-···+H-S coupling mechanism for the electrocatalytic hydrogen evolution reaction (HER) in the presence of acids. Ligand substitution studies on 1 indicate an associative/dissociative type reaction mechanism for the replacement of the NiN2S2 ligand, providing insight into the Fe-S bond strength.