Hydrogen Electrocatalysis on Single Crystals and on Nanostructured Electrodes

The pH-Induced Increase of the Rate Constant for HER at Au(111) in Acid Revealed by Combining Experiments and Kinetic Simulation

Origins of pH effects on the kinetics of electrocatalytic reactions involving the transfer of both protons and electrons, including the hydrogen evolution reaction (HER) considered in this study, are heatedly debated. By taking the HER at Au(111) in acid solutions of different pHs and ionic concentrations as the model systems, herein, we report how to derive the intrinsic kinetic parameters of such reactions and their pH dependence through the measurement of j-E curves and the corresponding kinetic simulation based on the Frumkin-Butler-Volmer theory and the modified Poisson-Nernst-Planck equation. Our study reveals the following: (i) the same set of kinetic parameters, such as the standard activation Gibbs free energy, charge transfer coefficient, and Gibbs adsorption energy for Had at Au(111), can simulate well all the j-E curves measured in solutions with different pH and temperatures; (ii) on the reversible hydrogen electrode scale, the intrinsic rate constant increases with the increase of pH, which is in contrast with the decrease of the HER current with the increase of pH; and (iii) the ratio of the rate constants for HER at Au(111) in x M HClO4 + (0.1 - x) M NaClO4 (pH ≤ 3) deduced before properly correcting the electric double layer (EDL) effects to the ones estimated with EDL correction is in the range of ca. 10 to 40, and even in a solution of x M HClO4 + (1 - x) M NaClO4 (pH ≤ 2) there is a difference of ca. 5× in the rate constants without and with EDL correction. The importance of proper correction of the EDL effects as well as several other important factors on unveiling the intrinsic pH-dependent reaction kinetics are discussed to help converge our analysis of pH effects in electrocatalysis.

The effects of polymorphism and lithium intercalation on the hydrogen evolution reaction for the basal planes of MoS2

The activity of Li-intercalated MoS2 phases for the hydrogen evolution reaction is investigated using density functional theory. The most stable semiconducting 2H phase, the metallic 1T' phase, and a polymorphous surface composed of alternating H and T' phases (1T″) are investigated. The local structure of the MoS2 surface is found to define its reactivity. In all cases, active sites for the hydrogen evolution process are restricted to T-like sulphur sites. Li-intercalation is found to promote hydrogen evolution reaction reactivity for the H phase whilst having little effect on the T phase. While improved compared to the non-intercalated phase, the Li-intercalated H phase MoS2 still has minimal activity for the hydrogen evolution reaction. The same effect of intercalation is also found for another transition metal dichalcogenide, MoSe2. The ability to improve reactivity in this way makes ion intercalation a promising space for designing new 2D catalysts for the hydrogen evolution reaction.

Catalysis of hydrogen evolution on Pt(111) by absorbed hydrogen

The activity of Pt(111) electrodes for the hydrogen evolution reaction (HER) in 0.5M H₂SO₄ solution is found to increase with continuous potential cycling in the HER potential region. In addition, the basic cyclic voltammograms obtained in 0.5M H₂SO₄ saturated with N₂ after HER show several characteristic changes: the current waves for hydrogen adsorption in the region of0.2 < E < 0.35 V and for sulfate adsorption at 0.35 < E < 0.5 V decrease and the current spike at 0.44 V for the phase transition of the sulfate adlayer gradually disappears. We suggest that these changes are caused by the absorption of a small amount of hydrogen in the subsurface layer and propose a mechanism by which this enhances hydrogen evolution.

Simulation of Metal-Supported Metal-Nanoislands: A Comparison of DFT Methods

We have evaluated various density functional theory (DFT) methods to simulate geometric, energetic, electronic, and hydrogen adsorption properties of metal-nanoparticles supported on metal surfaces. We used Pt and Pd nanoislands on Au(111) as model systems. The evaluated DFT methods include GGA (PW91, PBE, RPBE, revPBE, and PBESol), GGA with van der Waals (vdW) corrected (PBE-D3), GGA with optimized vdW functionals (revPBE-vdW), meta-GGA (SCAN and MS2), and the machine learning-based method BEEF-vdW. The results show that the various DFT methods yield similar geometric and electronic properties for Pt (or Pd) nanoislands on Au(111). The DFT methods also produce similar relative energetics for small Pt (or Pd) clusters with different conformations on Au(111). The results show that a triatomic cluster of Pt on Au(111) is more stable with a linear conformation. In contrast, a triatomic cluster of Pd is more stable with a triangular conformation. For clusters with four or more atoms, Pt and Pd clusters on Au(111) prefer non-linear conformation. We found that the various DFT methods yield different results only for the adsorption energy of hydrogen.

Electrochemical adsorption of hydrogen on mixed Pd2Pt nanostructures

In the present contribution we have focused on the electrochemical adsorption of a proton from the solution-the Volmer reaction-on a variety of systems based on bimetallic nanostructures-clusters and wires-of Pd and Pt deposited on a surface of Au(111). We have calculated the free energy surface for the electron transfer step by a combination of DFT calculations, MD simulations and the theory of electrocatalysis. We analyze in detail the interaction of the metal d band with the valence orbital of the hydrogen and its effect on the catalytic activity as well as several aspects that influence the electrode reactivity such as spatial arrangements of the nanostructures, the solvation shell and chemical factors. We found that the mixed Pd₂Pt wire interacts strongly with hydrogen, and retains an almost complete solvation shell, which is reflected in a substantially reduced activation energy for the Volmer step. Thus, Pd₂Pt wires on Au(111) are predicted to be efficient electrocatalysts for the reaction.

Mechanistic Insights about Electrochemical Proton-Coupled Electron Transfer Derived from a Vibrational Probe

Proton-coupled electron transfer (PCET) is a fundamental step in a wide range of electrochemical processes, including those of interest in energy conversion and storage. Despite its importance, several mechanistic details of such reactions remain unclear. Here, we have combined a proton donor (tertiary ammonium) with a vibrational Stark-shift probe (benzonitrile), to track the process from the entry of the reactants into the electrical double layer (EDL), to the PCET reaction associated with proton donation to the electrode, and the formation of products. We have used operando vibrational spectroscopy and periodic density functional theory under electrochemical bias to assign the reactant and product peaks and their Stark shifts. We have identified three main stages for the progress of the PCET reaction as a function of applied potential. First, we have determined the potential necessary for desolvation of the reactants and their entry into the polarizing environment of the EDL. Second, we have observed the appearance of product peaks prior to the onset of steady state electrochemical current, indicating formation of a stationary population of products that does not turn over. Finally, more negative of the onset potential, the electrode attracts additional reactants, displacing the stationary products and enabling steady state current. This work shows that the integration of a vibrational Stark-shift probe with a proton donor provides critical insight into the interplay between interfacial electrostatics and heterogeneous chemical reactions. Such insights cannot be obtained from electrochemical measurements alone.

Hydrogen Adsorption on Au-Supported Pt and Pd Nanoislands: A Computational Study of Hydrogen Coverage Effects

We have studied the dissociative adsorption of hydrogen under high coverage conditions of adsorbed hydrogen on Pd and Pt nanoislands supported on Au(111) using Density Functional Theory calculations. The results reveal that for Pd/Au(111), the free energy of hydrogen adsorption ΔG is close to 0 kJ/mol when the coverage of adsorbed hydrogen is near 1 ML, where the available catalytic sites are located at the edges of the Pd nanoislands. In the case of Pt/Au(111), ΔG ≈ 0 kJ/mol under a broad range of hydrogen coverage conditions, from 1 ML to 3 ML, depending on the size of the Pt nanoislands. This is the case because the available catalytic sites are located at both the steps and terraces of Pt nanoislands. These findings indicate that Au surfaces with Pd or Pt nanoislands offer catalytic sites with ΔG ≈ 0 for hydrogen reactions, one key factor for an ideal electrocatalyst for hydrogen reactions.

Nanoimpacts at Active and Partially Active Electrodes: Insights and Limitations

While the electrochemical nanoimpact technique has recently emerged as a method of studying single entities, it is limited by requirement of a catalytically active particle impacting an inert electrode. We show that an active particle-active electrode can provide mechanistic insight into electrochemical reactions. When an individual Pt electrocatalyst adsorbs to the surface of a partially active electrode, further reduction of electrode-produced species can proceed on the nanocatalyst. Current transients obtained during hydrogen evolution allow simultaneous measurement of the Pt catalyst over different length scales, size dependency suggests H atom intercalation as a catalytic deactivation mechanism. Although results show that outer-sphere redox probes are unproductive for particle characterization, the breadth of inner-sphere electrochemical reactions makes this a promising method for understanding the properties of catalytic nanomaterials, one at a time.

What is the trigger for the hydrogen evolution reaction? - towards electrocatalysis beyond the Sabatier principle

The mechanism of the hydrogen evolution reaction, although intensively studied for more than a century, remains a fundamental scientific challenge. Many important questions are still open, making it elusive to establish rational principles for electrocatalyst design. In this work, a comprehensive investigation was conducted to identify which dynamic phenomena at the electrified interface are prerequisite for the formation of molecular hydrogen. In fact, what we observe as an onset of the macroscopic faradaic current originates from dynamic structural changes in the double layer, which are entropic in nature. Based on careful analysis of the activation process, an electrocatalytic descriptor is introduced, evaluated and experimentally confirmed. The catalytic activity descriptor is named as the potential of minimum entropy. The experimentally verified catalytic descriptor reveals significant potential to yield innovative insights for the design of catalytically active materials and interfaces.

Leaching Mechanisms of Industrial Powders of Spent Nickel Metal Hydride Batteries in a Pilot-Scale Reactor

In view of a sustainable recycling process, the leaching mechanisms of nickel and rare-earth elements (REEs) contained within industrial samples of spent nickel metal hydride battery powders were investigated in HCl and H₂ SO₄ , under mild temperature (25-60 °C) and pH (3-5.5). First, in-depth characterization of the heterogeneous battery powder was carried out with powder XRD, SEM, electron probe microanalyzer wavelength-dispersive spectroscopy (EPMA-WDS) quantitative analyses of individual particles, and inductively coupled plasma optical emission spectrometry (ICP-OES) elemental analysis. An unusual result is the identification of particles that exhibit a core-shell structure, which is related to anode active mass aging mechanisms. Then, a leaching study in a 10 L pilot-scale reactor demonstrated the selective dissolution of REEs, with respect to nickel, at pH 3, which is attributed to 1) the kinetic inhibition of nickel metal dissolution, and 2) the specific core-shell structure of aged mischmetal particles. Furthermore, the use of H₂ SO₄ led to coprecipitation of lanthanide-alkali double sulfates and nickel salts.

An overview on Pd-based electrocatalysts for the hydrogen evolution reaction

The electrochemical hydrogen evolution reaction (HER) is a well-studied reaction which involves the reduction of protons for hydrogen production. Pd-based compounds are expected to have activity on par with or better than the expensive state-of-the-art Pt and can be considered as the future materials for the HER.

Unravelling the hydrogen absorption process in Pd overlayers on a Au(111) surface

Shedding light on the mechanism of hydrogen absorption occurring in nano-structured materials using the power of modern computational chemistry.

Hydrogen evolution reaction on palladium multilayers deposited on Au(111): a theoretical approach

We have investigated the electrocatalytic properties of multilayers of Pd epitaxially deposited on Au(111). In contrast to the numerous previous works in this area, we have focused on the kinetics of the electrochemical step for hydrogen adsorption (Volmer reaction) and determined its energies of activation. We have used a combination of density functional theory calculations and our own theory of electrocatalysis, which allows us to investigate the systems in an electrochemical environment. Contrary to our previous work with a submonolayer of Pd in Au(111), the activation barrier for the hydrogen adsorption process from proton is very low or almost zero for all bimetallic systems investigated. It is about 0.2 eV for pure Pd(111). In the case of two layers of Pd on Au(111) containing absorbed hydrogen in the subsurface, the adsorption free energy is less negative and the barrier lower than for the other investigated systems. This is in agreement with experimental data that shows a larger activity for hydrogen oxidation with hydride Pd systems.

Volcano plots in hydrogen electrocatalysis - uses and abuses

Sabatier's principle suggests, that for hydrogen evolution a plot of the rate constant versus the hydrogen adsorption energy should result in a volcano, and several such plots have been presented in the literature. A thorough examination of the data shows, that there is no volcano once the oxide-covered metals are left out. We examine the factors that govern the reaction rate in the light of our own theory and conclude, that Sabatier's principle is only one of several factors that determine the rate. With the exception of nickel and cobalt, the reaction rate does not decrease for highly exothermic hydrogen adsorption as predicted, because the reaction passes through more suitable intermediate states. The case of nickel is given special attention; since it is a 3d metal, its orbitals are compact and the overlap with hydrogen is too low to make it a good catalyst.

Tailoring the catalytic activity of electrodes with monolayer amounts of foreign metals

During the past decade, electrocatalysis has attracted significant attention primarily due to the increased interest in the development of new generations of devices for electrochemical energy conversion. This has resulted in a progress in both fundamental understanding of the complex electrocatalytic systems and in the development of efficient synthetic schemes to tailor the surface precisely at the atomic level. One of the viable concepts in electrocatalysis is to optimise the activity through the direct engineering of the properties of the topmost layers of the surface, where the reactions take place, with monolayer and sub-monolayer amounts of metals. This forms (bi)metallic systems where the electronic structure of the active sites is optimised using the interplay between the nature and position of the atoms of solute metals at the surface. In this review, we focus on recent theoretical and experimental achievements in designing efficient (bi)metallic electrocatalysts with selective positioning of foreign atoms to form a variety of active catalytic sites at the electrode surface. We summarize recent results published in the literature and outline challenges for computational and experimental electrocatalysis to engineer active and selective catalysts using atomic layers.

Enhancing the alkaline hydrogen evolution reaction activity through the bifunctionality of Ni(OH)2/metal catalysts

Active in alkaline environment: The activity of nickel, silver, and copper catalysts for the electrochemical transformation of water to molecular hydrogen in alkaline solutions was enhanced by modification of the metal surfaces by Ni(OH)(2) (see picture; I = current density and η = overpotential). The hydrogen evolution reaction rate on a Ni electrode modified by Ni(OH)(2) nanoclusters is about four times higher than on a bare Ni surface.