Water Structure and Dynamics at Hematite Electrodes

Abstract

Electrocatalysis at a transition metal oxide (TMO) electrode in aqueous electrolyte is controlled in part by the behavior of interfacial water. Atomically precise measurements of TMO-electrolyte interfaces under electrocatalytic working conditions are therefore prerequisite for developing accurate models of interfacial reactions. We present the most detailed measurements to date of electrolyte ordering at the reactive hematite (α-Fe2O3) (1-102) (r-cut) surface at open circuit conditions and under applied cathodic bias in 5 mM Na2SO­4 solution. Interface structures are measured in situ by synchrotron X-ray crystal truncation rod (CTR) scattering using a novel electrochemical mini-cell. CTR results are interpreted in light of first-principles molecular dynamics simulations. We find that the equilibrium structure of the r-cut hematite-electrolyte interface is defined by frequent exchange of water molecules between terminal aquo ligands and solution, resulting in a steady-state partial coverage of undercoordinated surface iron sites. Under cathodic bias, the surface is excessively protonated, strengthening the near-surface hydrogen-bonding network. The steady-state coverage of terminal aquo ligands subsequently increases, armoring the hematite electrode against reductive dissolution. The ordering and separation of water layers depend on the cathodic bias magnitude. Taken together, these results reveal the fine details of the interface structure which underlie the electrical double layer models used to describe macroscopic electrochemical measurements. The methods developed through this work open a new frontier for the in situ characterization of TMO-electrolyte interfaces during electrocatalytic reactions.