Atomic-Layer-Deposited TiO2-IrO X Nanoscale Thin-Film Electrocatalysts for Water and Chloride Oxidation: Influence of Local Phase Separation

TiO2-IrO X alloyswith a range of compositions synthesized by atomic layer deposition(ALD) were configured as anodes for water and chloride oxidation.The effects of the alloys' average composition on oxygen evolutionreaction (OER) and chloride evolution reaction (CER) activity wereinvestigated and correlated with their nanoscale structures and electricaltransport properties. A higher electronic conductivity and superiorelectrochemical performance were obtained for the TiO2-IrO X alloy films with 38% iridium in comparisonwith alloy films of either lower or higher IrO X content. This composition exhibits the lowest activation overpotentialsand Tafel slopes for both the OER and CER of the alloy compositionsinvestigated with values for the CER approaching those of IrO X (no TiO2 component present).The 38% Ir composition also exhibits the largest photovoltage, thatis, 650 mV, during water oxidation experiments on TiO2-IrO X alloy/n-silicon Schottky photoanodes. Thecomposition dependence of (photo)electrocatalyst properties is foundto correlate with observed trends for phase separation of the alloysinto locally TiO2- and IrO2-rich regions. Conductiveatomic force microscopy and chemical mapping using energy dispersiveX-ray spectroscopy in scanning transmission electron microscopy indicategreater phase separation for the 38% IrO X composition than for the others synthesized.