Jeffrey P. Greeley

Advances in the field of electrocatalysis over the past several decades have been driven both by improvements in fundamental techniques for probing the solid–liquid electrochemical interface and by the technological imperative to develop enhanced low temperature electrocatalytic devices. In this review, we describe how a synergistic interaction between fundamental science and technological progress has resulted in both the emergence of greatly enhanced understanding of electrocatalytic systems and the development of practically improved electrocatalysts. Since it is not possible to summarize in detail all relevant developments in this broad field in such a brief space, we focus selectively on the early historical development of the field and on the use of trends-based analyses to describe the properties of electrocatalytic materials in terms of relatively simple catalytic properties, or descriptors. We begin by discussing aspects of the historical development of “reversible fuel cells” in acidic media, including topics relevant both to fuel cells, wherein hydrogen and oxygen are converted to water with concomitant production of electricity (electrons), and to electrolyzers, wherein electrons are used to initiate water splitting to yield hydrogen and oxygen. We then show how this development has stimulated the development of in situ and ex situ surface sensitive probes and spectroscopic methods capable of elucidating fundamental (atomic-/molecular-level) chemical and electronic properties of electrode–electrolyte interfaces. We further discuss how enhanced computational approaches, that can accurately calculate covalent bonding interactions in these systems, have contributed to the growth of a synergistic experimental/computational approach to electrochemical surface science that has resulted in a highly successful paradigm for the understanding of reactivity trends across a space of different metals, alloys, and metal oxides; this work, in turn, has spurred the development of alternative energy systems for efficient conversion and storage of chemical energy. We conclude with a discussion of some further needs for methodological developments and future research directions.

Surface steps caused by stacking faults or twin boundaries in copper nanoparticles are key to the activity of the methanol synthesis catalyst.

A combination of first principles Density Functional Theory calculations and thermochemical scaling relationships are employed to estimate the thermochemistry and kinetics of methanol decomposition on unsupported subnanometer metal clusters. The approach uses binding energies of various atomic and molecular species, determined on the pure metal clusters, to develop scaling relationships that are then further used to estimate the methanol decomposition thermodynamics for a series of pure and bimetallic clusters with four atoms per cluster. Additionally, activation energy barriers are estimated from Brønsted–Evans–Polanyi plots relating transition and final state energies on these clusters. The energetic results are combined with a simple, microkinetically-inspired rate expression to estimate reaction rates as a function of important catalytic descriptors, including the carbon and atomic oxygen binding energies to the clusters. Based on these analyses, several alloy clusters are identified as promising candidates for the methanol decomposition reaction.

Li transport characteristics are studied by means of density functional theory (DFT) and molecular dynamics (MD) simulations in order to investigate concentration effects on Li chemical diffusivity and conductivity in TiO₂ rutile. Our MD simulations predict one-dimensional diffusion of Li ions via jumps between the octahedral sites along the channels parallel to the c-axis. The diffusion barrier and diffusion coefficient (at room temperature) for the isolated Li, determined by means of DFT calculations, correspond to 60 meV and 9.1 × 10⁻⁶ cm² s⁻¹, respectively. Such a small barrier suggests rapid mass transport along the channels. MD simulations are performed to evaluate the concentration dependent diffusivity profiles. The changes in Li energetics and dynamics are studied as a function of Li content, which is varied primarily between 10% and 50%. In addition, we consider a couple of compositions over 50% although this is above the intercalation limit. Our results suggest that Li diffusivity is strongly dependent on the Li ∶ TiO₂ ratio, and it decreases with increasing Li concentration. For instance, at room temperature, we find Li diffusivity for high concentrations (50% Li) to be three orders of magnitude slower than that for lower concentrations (10% Li). Our analyses on the energetics and dynamics suggest that the changes in the diffusivities originate from successive increases in the barriers with increasing concentration. The decrease in diffusivity as a function of increasing Li content is attributed to the fact that additional Li ions successively block the energetically preferred vacant sites along the channels. Our analyses also show that increasing Li concentration enhances the Li–Li repulsion within the channels, and as a result, diffusion is hindered. We also compare concentration-dependent diffusivities for Li diffusion in anatase, rutile and amorphous TiO₂. Interestingly, we find differing concentration dependence of the diffusivity in these chemically identical but structurally non-equivalent TiO₂ polymorphs. Our study suggests that these differences result from intrinsic structural characteristics of TiO₂ polymorphs, which ultimately contribute to intercalation limit, diffusivity, ionic conductivity, and the electrochemical performance in energy storage applications.