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

Abstract

TiO2–IrOX alloys with 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 evolution reaction (OER) and chloride evolution reaction (CER) activity were investigated and correlated with their nanoscale structures and electrical transport properties. A higher electronic conductivity and superior electrochemical performance were obtained for the TiO2–IrOX alloy films with 38% iridium in comparison with alloy films of either lower or higher IrOX content. This composition exhibits the lowest activation overpotentials and Tafel slopes for both the OER and CER of the alloy compositions investigated with values for the CER approaching those of IrOX (no TiO2 component present). The 38% Ir composition also exhibits the largest photovoltage, that is, 650 mV, during water oxidation experiments on TiO2–IrOX alloy/n-silicon Schottky photoanodes. The composition dependence of (photo)electrocatalyst properties is found to correlate with observed trends for phase separation of the alloys into locally TiO2- and IrO2-rich regions. Conductive atomic force microscopy and chemical mapping using energy dispersive X-ray spectroscopy in scanning transmission electron microscopy indicate greater phase separation for the 38% IrOX composition than for the others synthesized.