Speaker
Description
The oxygen reduction reaction (ORR) is a key electrocatalytic process for developing sustainable energy technologies. And yet, many aspects of the underlying reaction mechanism are still poorly understood at the molecular level. Especially at weak-binding electrode surfaces such as gold (Au), product selectivity and even the ability to bind aqueous O2 species as a first mechanistic step remain unclear. Resolving these questions requires going beyond simplified thermodynamic models of proton-coupled electron transfers that have been commonly used in computational electrocatalysis so far. Such models rely on the computational hydrogen electrode (CHE) as a referencing technique that includes the effects of applied potential and electrolyte pH at the level of thermodynamic reservoirs for electrons and protons, respectively. This elegant approximation thus reduces the task of creating an energy reaction diagram to a series of computationally efficient calculations of charge-neutral intermediates in a periodic simulation cell. What it precludes by construction, however, is any account of capacitive charging that characterizes the solid/liquid interface under realistic operating conditions.
Here, we go beyond the CHE approximation to investigate O2 adsorption at an electrified Au(111)/water interface. We perform molecular dynamics simulations based on periodic density-functional theory (DFT) and include the interfacial (local) electric field by enforcing an effective surface charge through re-parametrized PAW potentials on hydrogen atoms of explicitly modeled water. This allows for elucidating structural interfacial properties (such as water rigidity or orientation) as a function of surface charge, and shows that the latter can significantly alter the O2 binding energy. We specifically find adsorption is enhanced by up to ca. 0.8 eV under reducing conditions that are consistent with experimental ORR activity. This is accompanied by formation of a true chemisorbed state, according to free-energy profiles that we generate from umbrella sampling simulations. Our results thus overall suggest O2 adsorption as an electrochemical, rather than purely chemical, first step of the mechanism and show that electric field effects in corresponding DFT models cannot be neglected. The resulting dependence on (absolute) electrostatic potential may further explain the superior activity measured experimentally for this catalyst in alkaline vs. acidic media. Finally, from a methodological point of view, our dynamical data provide us with a unique opportunity to compare different levels of treating the electrochemical interface: explicit ab initio molecular dynamics, implicit solvation models, and application of a sawtooth-potential electric field in vacuum.
| Abstract Number (department-wise) | TH 14 |
|---|---|
| Department | TH (Reuter) |
