Electrosorption at metal surfaces from first principles

Nicolas G Hörmann,Karsten Reuter,Nicola Marzari

doi:10.1038/s41524-020-00394-4

Abstract

Electrosorption of solvated species at metal electrodes is a most fundamental class of processes in interfacial electrochemistry. Here, we use its sensitive dependence on the electric double layer to assess the performance of ab initio thermodynamics approaches increasingly used for the first-principles description of electrocatalysis. We show analytically that computational hydrogen electrode calculations at zero net-charge can be understood as a first-order approximation to a fully grand canonical approach. Notably, higher-order terms in the applied potential caused by the charging of the double layer include contributions from adsorbate-induced changes in the work function and in the interfacial capacitance. These contributions are essential to yield prominent electrochemical phenomena such as non-Nernstian shifts of electrosorption peaks and non-integer electrosorption valencies. We illustrate this by calculating peak shifts for H on Pt electrodes and electrosorption valencies of halide ions on Ag electrodes, obtaining qualitative agreement with experimental data already when considering only second order terms. The results demonstrate the agreement between classical electrochemistry concepts and a first-principles fully grand canonical description of electrified interfaces and shed new light on the widespread computational hydrogen electrode approach.

Highlights

In recent years, calculations based on ab initio thermodynamics have increasingly contributed to unraveling key processes in interfacial electrochemistry; e.g., in batteries, fuel cells, and other electrocatalytic systems
We show by analysis of a generic second-order Taylor expansion of the interface energies with respect to surface charge[7,11,20,34,39,42,43,44,45] that the computational hydrogen electrode (CHE) approach can be understood as a first-order approximation to the fully grand-canonical (FGC) energetics, while to second order, double-layer charging is represented by changes in work function and interfacial capacitance
The central quantity in our ab initio thermodynamics approach to surface electrochemistry is the Gibbs excess energy of interface configuration α4,5,13, GαexcðT ; p; μs; μ~a; ΦEÞ 1⁄4 GαsurfðT ; p; Nαs ; Nαa ; Neabs;αÞ À Nαs μs ÀNαa μ~a þ Neabs;αeΦE: (1)

Summary

Introduction

Calculations based on ab initio thermodynamics have increasingly contributed to unraveling key processes in interfacial electrochemistry; e.g., in batteries, fuel cells, and other electrocatalytic systems. In the application to interfacial electrochemistry a key challenge to this general ab initio thermodynamics concept is the necessity to exchange electrons with a reservoir representing the electrode potential In principle, this requires to perform DFT calculations in various charge states. This requires to perform DFT calculations in various charge states This clashes with the common representation of the electrode as finite slab using periodic-boundary conditions, for which straightforward calculations can only be performed at zero total charge of the cell. For this reason, in most practical application, the early computational hydrogen electrode (CHE) approach[2,6,12] relies solely on the energetics of charge-neutral electrode calculations, in the absence of the electrochemical double layer. The dependence on the electrode potential is included in the analysis as an a posteriori shift of the electrochemical potential of the electrons taken from the reservoir, whose number is a priori fixed according to the charge-neutrality condition

Methods

Results

Conclusion