Characterizing Mass Transport Effects for Photoelectrochemical Reactions in a Nanostructured, Light-Trapping Photoelectrode

The use of earth-abundant semiconductors for solar-to-fuels reactions is often limited by poor minority carrier diffusion characteristics of the material. The integration of nanostructures that trap light can improve photoelectrochemical yields by redirecting photocarrier generation to regions of the electrode near the semiconductor-electrolyte interface. However, the nanoscale features can cause unintended, deleterious mass transport limitations under steady state operation. Here, we explored the effects of mass transport limitations in a nanostructured, light-trapping photoelectrode structure based on an ultrathin TiO2 coating over a colloidal photonic glass. We showed that photocurrent enhancements identified in cyclic voltammetry (CV) measurements can be diminished in steady state, potentiostatic operation. CV experiments of a reversible ferricyanide-ferrocyanide couple can show the evolution of the local redox environment to compare mass transport effects in the photoelectrodes as a function of porosity. Additionally, we used finite-difference, time-domain simulations to characterize the conditions where the accumulation of the strongly absorbing ferricyanide products can generate parasitic absorption, a secondary efficiency loss route. This provides guidance for using CV as a method for comparing mass transport characteristics for nanostructured photoelectrodes.