(Invited) Photocatalytic Water Splitting: Particulate Model and Reactor Engineering

Abstract

Particulate photocatalysts offer a materials platform that potentially converts light energy to chemical energy at scale, water splitting being an immediate application. However, the state-of-the art solar-to-hydrogen conversion efficiency by particles is <5%, whereas photoelectrochemical cells have achieved 20%. In a holistic view, the conversion process for photocatalysts dispersed in a reactor involve coupled photophysical and electrocatalytic processes: light absorption, carrier generation, charge separation, charge transfer across the photocatalyst/co-catalyst/liquid interface, as well as mass transport of reactants and products. First, we extend the Gerischer/Lewis kinetic expression to semiconductor/liquid junctions that co-evolve hydrogen and oxygen. Because of the locally non-equilibrium environment, we carefully designed steady-state experiments to validate a mutually-dependent relationship of kinetics and energetics. On a single particle level, co-catalysts induce spatially-varying asymmetric energetics, i.e. band bending vs. flat bands; these asymmetric energetics drive electrons and holes to separate laterally along the liquid interface and accumulate at their respective sites; and it was found that redox selectivity defines local energetics thus influencing charge separation efficiency. Assembled by photocatalysts, a particulate panel or suspension produces hydrogen and oxygen which transport under pressure gradient. We will show the systematic measurement of product diffusion rates. Finally, we apply the multi-scale particulate model to implement a novel coating-stabilized water-splitting panel that is efficient and safe, delivering pressurized hydrogen under sunlight.