Direct molecular dynamics and density-functional theoretical study of the electrochemical hydrogen oxidation reaction and underpotential deposition of H on Pt(111)

Laterally Resolved Free Energy Profiles and Vibrational Spectra of Chemisorbed H Atoms on Pt(111)

A scheme to compute laterally resolved free energy surfaces and spectral signatures of specifically adsorbed ions on electrode surfaces from their ab initio molecular dynamics (AIMD) trajectories is proposed. Considering H-covered Pt(111) electrodes, both in contact with water and vacuum and for various H coverages, we systematically explore the impact of explicit water and H-coverage on site occupancy, providing direct insight into the proportion of underpotential and overpotential deposited hydrogen adsorbates. Extending this approach further, we can obtain laterally resolved vibrational spectra of the Pt-H stretch modes. We discuss how the difference between the free energy basins of the on-top and fcc-hollow adsorption sites explains the features of the experimentally observed spectral fingerprints in this system. These fingerprints do not contain only information about the stable and metastable adsorption sites but also about intermediate short-lived adsorbate configurations. Our results also show that for these properties chemisorbed H2O acts as a spectator and does not qualitatively influence the relative stabilities of the adsorption sites and their spectral fingerprints.

Bimetallic ruthenium–nickel alloy nanostructure supported on nickel foam for efficient alkaline hydrogen evolution at large current density

A ruthenium–nickel alloy nanostructure exhibits a low overpotential and excellent activity stability for the hydrogen evolution reaction in 1 M KOH.

Hydrogen Coupling on Platinum Using Artificial Neural Network Potentials and DFT

To date, the understanding of reactions at solid-liquid interfaces has proven challenging, mainly because of the inaccessible nature of such systems to current experimental techniques with atomic resolution. This has meant that many important features, including free energy barriers and the atomistic structure of intermediates, remain unknown. To tackle these issues, we construct and utilize a high-dimensional neural network (HDNN) potential for the simulation of hydrogen evolution at the HCl(aq)/Pt(111) interface, taking into consideration the influence of adsorbate-adsorbate, adsorbate-solvent interactions, and ion solvation explicitly. Long time scale MD simulations reveal coadsorbed H_ad/H₂O_ad on the surface. The free energy profiles for the Tafel and Heyrovsky type hydrogen coupling are extracted using umbrella sampling. It is found that the preferential mechanism can change depending on the surface coverage, highlighting the dual mechanistic nature for HER on Pt(111). Our work demonstrates the importance of controlling the solvent-substrate interactions in developing catalysts beyond Pt.

Impact of Water Coadsorption on the Electrode Potential of H-Pt(1 1 1)-Liquid Water Interfaces

Density functional theory molecular dynamics simulations of H-covered Pt(111)-H_{2}O interfaces reveal that, in contrast to common understanding, H_{2}O coadsorption has a significant impact on the electrode potential of and plays a major role in determining the stability of H adsorbates under electrochemical conditions. Based on these insights, we explain the origin behind the experimentally observed upper limit of H coverage well below one monolayer and derive a chemically intuitive model for metal-water bonding that explains an unexpectedly large interaction between coadsorbed water and adsorbates.

Electrocatalytic nitrate reduction on rhodium sulfide compared to Pt and Rh in the presence of chloride

Chloride poisoning is a serious problem for the electrocatalytic reduction of aqueous nitrate (NO3−) and improved electrocatalysts are needed.

Mechanistic Insights into the Hydrogen Oxidation Reaction on PtNi Alloys in Alkaline Media: A First-Principles Investigation

The promising alkaline anion exchange membrane fuel cell suffers from sluggish kinetics of the hydrogen oxidation reaction (HOR). However, the puzzling HOR mechanism hinders the further development of highly active catalysts in alkaline media. In this work, we conducted detailed first-principles calculations to acquire a deep understanding of the alkaline HOR mechanism on PtNi bulk alloys [Pt₃Ni(111), Pt₂Ni₂(111), and PtNi₃(111)] and its surface alloy [PtNi_surf(111)]. The full free energy profiles suggest that the HOR on PtNi alloys proceeds via the Tafel-Volmer mechanism, that is, the direct decomposition of H₂ into two adsorbed H, followed by its reaction with OH^- in the electrolyte, as the rate-determining step, to form H₂O. Therefore, the HOR activity of PtNi alloys is solely impacted by the adsorption of hydrogen, rather than hydroxyl species, though the oxophilicity is also enhanced by alloying Pt with Ni. Thermodynamically, a moderate H adsorption free energy, ΔG_H* ≈ 0.414 eV, is calculated to be an optimal candidate for the HOR at pH = 13. Alloying Pt with Ni can elevate the d-band center (ε_d), push the value of ΔG_H* closer to 0.414 eV, and thus lower the free energy barrier (E_a) of the rate-determining Volmer reaction, leading to the highest HOR activity of PtNi₃(111) among all considered PtNi alloys. This situation is further confirmed by both the microkinetic model and the Tafel plot, where PtNi₃(111) exhibits the highest reaction rate (r = 9.42 × 10³ s^-1 site^-1) and the largest exchange current density (i₀ = 1.42 mA cm^-2) for HOR in alkaline media. This work provides a fundamental understanding of the HOR mechanism and theoretical guidance for rational design of electrocatalysts for HOR in alkaline media.

Strain and Low-Coordination Effects on Monolayer Nanoislands of Pd and Pt on Au(111): A Comparative Analysis Based on Density Functional Results

Recent experiments demonstrated that the catalytic centers for the hydrogen evolution reaction (HER) are different on Pd and Pt nanoislands on Au(111). Inspired by these experiments, we examined the geometric, energetic, electronic and hydrogen adsorption properties of monolayer model nanoislands of Pd and Pt supported on Au(111) with density functional theory calculations. Accordingly, Au-tensile strain effects can be nearly 50% larger on the geometric structure of nanoislands of Pd on Au(111) than their Pt analogs, resulting on different electronic properties for these nanoislands. Despite these differences between Pd and Pt nanoisland on Au(111), our computational modelling of the hydrogen adsorption suggests that the unique catalytic centers for the HER on Pd and Pt nanoislands supported on Au(111) derive from the existence of low-coordinated adsorption sites and the intrinsic properties of Pd and Pt, but not from Au-tensile strain effects.

The hydrogen evolution reaction: from material to interfacial descriptors

The production of sustainable hydrogen with water electrolyzers is envisaged as one of the most promising ways to match the continuously growing demand for renewable electricity storage. While so far regarded as fast when compared to the oxygen evolution reaction (OER), the hydrogen evolution reaction (HER) regained interest in the last few years owing to its poor kinetics in alkaline electrolytes. Indeed, this slow kinetics not only may hinder the foreseen development of the anionic exchange membrane water electrolyzer (AEMWE), but also raises fundamental questions regarding the parameters governing the reaction. In this perspective, we first briefly review the fundamentals of the HER, emphasizing how studies performed on model electrodes allowed for achieving a good understanding of its mechanism under acidic conditions. Then, we discuss how the use of physical descriptors capturing the sole properties of the catalyst is not sufficient to describe the HER kinetics under alkaline conditions, thus forcing the catalysis community to adopt a more complex picture taking into account the electrolyte structure at the electrochemical interface. This work also outlines new techniques, such as spectroscopies, molecular simulations, or chemical approaches that could be employed to tackle these new fundamental challenges, and potentially guide the future design of practical and cheap catalysts while also being useful to a wider community dealing with electrochemical energy storage devices using aqueous electrolytes.

Tuning the Catalytic Activity of Pd x Ni y (x + y = 6) Bimetallic Clusters for Hydrogen Dissociative Chemisorption and Desorption

Density functional theory was used to study dissociative chemisorption and desorption on Pd _x Ni _y (x + y = 6) bimetallic clusters. The H₂ dissociative chemisorption energies and the H desorption energies at full H saturation were computed. It was found that bimetallic clusters tend to have higher chemisorption energy than pure clusters, and the capacity of Pd₃Ni₃ and Pd₂Ni₄ clusters to adsorb H atoms is substantially higher than that of other clusters. The H desorption energies of Pd₃Ni₃ and Pd₂Ni₄ are also lower than that of the Pd₆ cluster and comparable to that of the Ni₆ cluster, indicating that it is easier to pull the H atom out of these bimetallic catalysts. This suggests that the catalytic efficiency for specific Pd _x Ni _y bimetallic clusters may be superior to bare Ni or Pd clusters and that it may be possible to tune bimetallic nanoparticles to obtain better catalytic performance.

High hydrogen evolution activity and suppressed H2O2 production on Pt-skin/PtFe alloy nanocatalysts for proton exchange membrane water electrolysis

We, for the first time, demonstrate high electrocatalytic activity for the hydrogen evolution reaction (HER) on PtFe alloy nanoparticles with stabilized Pt-skin layers supported on carbon black (PtxAL-PtFe/C), which allows the reduction of Pt loading to be lowered to 1/20 compared with a conventional Pt black cathode in proton exchange membrane water electrolysis (PEMWE). The area-specific HER activity of PtxAL-PtFe/C was found to be ca. 2 times higher than that of commercial Pt/C at 80 °C and -0.02 V vs. RHE. PtxAL-PtFe/C exhibited the additional important advantage of suppressed H2O2 production during the HER in the presence of O2, which inevitably diffuses from the anode in PEMWE. Both the excellent HER performance and low H2O2 production are attributed to the lower adsorption energies of atomic hydrogen on Pt-skin surfaces, as revealed by DFT calculations.

Weakened CO adsorption and enhanced structural integrity of a stabilized Pt skin/PtCo hydrogen oxidation catalyst analysed by in situ X-ray absorption spectroscopy

In situ X-ray absorption spectroscopy has afforded a detailed structural and electronic characterization of a newly developed stabilized Pt-skin/PtCo alloy nanoparticle catalyst for CO-tolerant H₂ oxidation.

Toward full simulation of the electrochemical oxygen reduction reaction on Pt using first-principles and kinetic calculations

First-principles molecular dynamics gave the kinetic and redox parameters of the oxygen reduction reaction in a fuel cell using a bias control scheme, and gave the current–voltage relationship.

Hydrocarbon adsorption in an aqueous environment: A computational study of alkyls on Cu(111)

Hydrocarbon chains are important intermediates in various aqueous-phase surface processes, such as CO2 electroreduction, aqueous Fischer-Tropsch synthesis, and aqueous phase reforming of biomass-derived molecules. Further, the interaction between water and adsorbed hydrocarbons represents a difficult case for modern computational methods. Here, we explore various methods for calculating the energetics of this interaction within the framework of density functional theory and explore trade-offs between the use of low water coverages, molecular dynamics approaches, and minima hopping for identification of low energy structures. An effective methodology for simulating low temperature processes is provided by using a unit cell in which the vacuum space is filled with water, employing the minima hopping algorithm to search for low-lying minima, and including dispersion (van der Waals) interactions. Using this methodology, we show that a high coverage of adsorbed alkyls is destabilized by the presence of water, while a low coverage of alkyls is stabilized. Solvation has a small effect on the energetics of hydrocarbon chain growth, generally decreasing its favorability at low temperatures. We studied higher temperatures by running molecular dynamics simulations starting at the minima found by the minima hopping algorithm and found that increased temperatures facilitate chain growth. The self-consistent continuum solvation method effectively describes the alkyl-water interaction and is in general agreement with the explicit solvation results in most cases, but care should be taken at high alkyl coverage.

A novel Pt-Co alloy hydrogen anode catalyst with superlative activity, CO-tolerance and robustness

PtCo nanoparticles, having two atomic layers of stabilized Pt skin, supported on carbon black (Pt2AL-PtCo/C), exhibited superlative mass activity for the CO-tolerant hydrogen oxidation reaction (HOR), together with high robustness with respect to air exposure, as a novel anode catalyst in reformate gas-based polymer electrolyte fuel cells. The high area-specific HOR activity and CO tolerance are consistent with DFT calculations.