An Optocatalytic Model for Semiconductor–Catalyst Water-Splitting Photoelectrodes Based on In Situ Optical Measurements on Operational Catalysts

The optical properties of electrocatalysts are important for photoelectrochemical water splitting because colored catalysts on the surface of semiconductor photoelectrodes parasitically absorb photons and lower the system efficiency. We present a model that describes the coupling of colored oxygen evolution reaction (OER) electrocatalyst thin films with semiconductor photoelectrodes. We use this model to define an "optocatalytic" efficiency (Φo-c) based on experimental optical and electrokinetic data collected in basic solution. Because transition-metal oxides, hydroxides, and oxyhydroxides often exhibit electrochromism, in situ spectroelectrochemistry is used to quantify the optical absorption of active NiOx, CoOx, NiCoOx, Ni0.9Fe0.1Ox, and IrOx catalyst films at OER potentials. For the highest-activity Ni0.9Fe0.1Ox catalyst, Φo-c is maximized (0.64) for a thickness of ∼0.4 nm (∼2 monolayers). This work quantitatively shows that ultrathin catalyst films are appropriate to optimize the performance of water-splitting photoelectrodes and thus assists in the design and study of efficient photoelectrochemical water-splitting devices.