Controlled-Potential Simulation of Elementary Electrochemical Reactions: Proton Discharge on Metal Surfaces

Abstract

The simulation of electrochemical reaction dynamics from first principles remains challenging, since over the course of an elementary step, an electron is either consumed or produced by the electrode. For example, the hydrogen evolution reaction begins with a simple proton discharge to a metal surface, but with conventional electronic structure methods, the simulated potential, which is manifested as the metal’s workfunction, varies over the course of the simulation as the electron is consumed in the new metal–hydrogen bond. Here, we present a simple approach to allow the direct control of the electrochemical potential via charging of the electrode surface. This is achieved by changing the total number of electrons in the self-consistent cycle, while enforcing charge neutrality through the introduction of a jellium counter charge dispersed in an implicit solvent region above the slab. We observe that the excess electrons localize selectively at the metal’s reactive surface and that the metal workfunction responds nearly linearly to the variation in electronic count. This linear response allows for control of the potential in simulations with a minimal computational penalty compared to standard electronic structure calculations. This scheme can be straightforwardly implemented with common electronic structure calculators (density functional theory in the present work), and we find this method to be compatible with the commonly used computational hydrogen electrode model, which we expect will make it useful in the construction of potential-dependent free-energy diagrams in electrochemistry. We apply this approach to the proton-deposition (Volmer) step on both Au(111) and Pt(111) surfaces and show that we can reliably control the simulated electrode potential and thus assess the potential dependence of the initial, transition, and final states. Our method allows us to directly assess the location along the reaction pathway with the greatest amount of charge transfer, which we find to correspond well with the reaction barrier, indicating this reaction is a concerted proton–electron transfer. Interestingly, we show that the Pt electrode has not only a more favorable equilibrium energy with adsorbed hydrogen but also a lower intrinsic barrier under thermoneutral conditions.

A Challenge to the G ∼ 0 Interpretation of Hydrogen Evolution

“…This interfacial water geometry has been shown to be the most stable geometry for protonated, that is, acidic water bilayers, at the potentials of interest for HER. 47,[53][54][55] The reaction pathways are all performed at constant potential within a tolerance of 0.01 V, and the relevant energy comparison is therefore the energy at constant potential (often referred to as Ω [38][39][40][41][44][45][46][47][48][49]56 ),…”

Electrochemical proton-coupled electron transfer of an anthracene-based azo dye

“…Over the past decade, PCET at electrochemical interfaces has gained attention due to the signicant role electrochemical PCET plays in energy conversion reactions and catalysis. [6][7][8][9][10][11][12][13][14][15] Experimental, theoretical, and computational work has shown non-innocence of protons at electrochemical interfaces. [14][15][16][17][18][19][20][21][22][23][24][25] Indeed, many chemical reactions, including reactions involving homogeneous catalysis and heterogeneous catalysis at electrode surfaces show a clear dependence on the pH of the solution.…”

Electrode polarization effects on interfacial kinetics of ionic liquid at graphite surface: An extended lagrangian‐based constant potential molecular dynamics simulation study

“…Toward the understanding, computational approaches as typified by electronic structure calculations and molecular dynamics (MD) simulations have become a promising way that allows us to gain valuable insights into electrochemical interfaces at the atomic level. Among them, first‐principles calculations play a central role for studies of electrode/electrolyte interfaces . The calculations are able to deal inherently with chemical reaction processes occurring at an interface, leading to the prediction of redox products and the elucidation of a reaction mechanism.…”

“…Among them, first-principles calculations play a central role for studies of electrode/electrolyte interfaces. [4][5][6][7][8][9][10][11] The calculations are able to deal inherently with chemical reaction processes occurring at an interface, leading to the prediction of redox products and the elucidation of a reaction mechanism. They are also able to describe complicated interactions, like electronic polarization, charge transfer, and Pauli repulsion, between a conductive electrode and polar electrolyte molecules without any empirical parameters.…”