Characterizing the structural overpotentials induced by bubble evolution during water electrolysis on spatially-distributed electrocatalyst-semiconductor interfaces.

by Schichtl, Zebulon; Coridan, Robert

The spatial distribution of an electrocatalyst layer at a semiconductor-liq. junction (SLJ) can have mixed effects on the photoelectrochem. energy conversion efficiency of the junction. A layer of discrete electrocatalytic metal particles can preserve the photovoltage-producing energetics of the SLJ and reduce parasitic light reflection or absorption vs. a continuous metal layer. However, the electrochem. active area on a discrete surface is smaller than on a continuous surface, and the addnl. overpotential required to achieve the same area-normalized c.d. reduces the efficiency of the electrode. For gas-evolving reactions like splitting water to H2 and O2, the gas-solid-electrolyte three-phase boundary formed by a growing bubble can block discrete catalytic areas, further reducing catalytic activity and adding an effective 'structural overpotential'. Here, we investigate the relationship between the organization of electrocatalysts on a semiconductor interface and the energy conversion efficiency of that interface for the gas-evolving hydrogen evolution reaction in aq. systems. We describe an empirical model for bubble evolution, coalescence, and detachment on electrocatalyst-semiconductor interfaces based on high-speed x-ray imaging expts. With this model, we are able to est. the collective effects of the surface structure on the rate of gas evolution, and elucidate strategies for engineering interfaces to maximize the efficiency of the interface through control of the three-phase boundary. Finally, we outline the consequences and benefits the organization of electrocatalysts can have on photoelectrochem. device design.