Quantifying the Photoelectrochemically Active Surface Area for Oxygen Evolution Metal Oxide Photoelectrocatalysts

Abstract Nanostructuring is established as a photoelectrode performance enhancing route for solar-driven catalytic processes. However, it is often unclear to what extent the increase in photoelectrode physical surface area directly results in a commensurate increase in photoelectrochemically active surface area. While there are established methods for estimating the physical and electrochemically active surface areas, it is challenging to quantify the operando photoelectrochemically active surface area of nanostructured or nanoparticulate photoelectrodes. Herein, we demonstrate that photoinduced absorption (PIA) spectroelectrochemistry can be employed to quantify the operando photoelectrochemically active area of photoanodes for the oxygen evolution reaction (OER) by exploiting the strong dependence of OER turnover frequency on physical surface hole density. We investigate BiVO4, α-Fe2O3, and TiO2 photoanodes with contrasting morphologies – planar, nanoparticulate, and nanostructured. Our study combines operando measurements of TOF and geometric surface hole density to quantify the photoelectrochemically active surface area of each catalytic system for OER. For the nanostructured photoelectrodes, the photoelectrochemically active surface areas determined from our PIA analyses are shown to be in agreement with their physical surface area, consistent with their good OER performance. This contrasts with the low photoelectrochemically active surface areas compared to their high physical surface area determined by the nanoparticulate photoelectrodes, resulting in poor performance. We conclude by demonstrating how the photoelectrochemically active surface areas of nanoparticulate photoelectrodes increase when driving more kinetically facile reactions or by adding co-catalysts.