Structure Sensitivity of Electrochemical Reactions from First Principles: Applications to Nitrogen and Water Cycles

Abstract

Nitrogen cycle electrochemistry is an emerging area of interest in the electrocatalysis community, with applications ranging from removal of nitrates from wastewater streams to the development of fundamental understanding of NO electrochemistry. In spite of a significant amount of fundamental research for this chemistry on single crystal surfaces, however, the mechanistic details of the reactions are not fully known, and even such basic information as the nature of the rate-limiting step is not understood. In this work, we apply some of the innovations in theoretical electrochemistry that have emerged over the past 5-10 years to the study of NO and nitrate reduction on transition metal surfaces. We begin with a simple Density Functional Theory analysis of direct NO reduction in acid on Pt(111), Pt(100), and Pt(211) surfaces, and we identify potential- and rate-determining steps on each surface, using extensive ab-initio molecular dynamics simulations to estimate the impact of the aqueous environment on surface thermodynamics and kinetics (Ang. Chem. Int. Ed. 2015). We next introduce a novel approach that couples DFT calculations with rigorous kinetic Monte Carlo (kMC) simulations to understand, in unprecedented detail, the effect of higher adsorbed NO coverages on these chemistries, and we demonstrate that these coverage-dependent effects are essential for describing experimentally measured polarization curves (ACS Catalysis 2017). We subsequently generalize our analysis by developing universal Brønsted-Evans-Polanyi relationships that rigorously relate barriers for proton-coupled electron transfer to surface species to the corresponding thermodynamics across a large space of different transition metals and potentials. These relationships, combined with the potential- and coverage-dependent energetics determined on platinum, provide a basis to extrapolate our mechanistic analyses to other transition metals, including PtSn and related alloys. We employ these techniques to propose design strategies to tune selectivity of NO electroreduction to desired products, including N2 and hydroxylamine, on these different transition metals and alloys. If time permits, we will briefly describe some additional work on the development of structural and catalytic reactivity models at the interface between thin hydroxyoxide films and precious metal platinum substrates. We will examine the kinetics of the hydrogen evolution reaction as a function of the structure and oxidation state of these interfaces and will demonstrate that the bifunctional nature of the interfaces plays a key role in their electrocatalytic properties.