Catalytic open-circuit passivation by thin metal oxide films of p-Si anodes in aqueous alkaline electrolytes

Abstract

Ni and NiO_x-based protective thin films are shown to catalyze the oxidation of Si in the presence of O₂ in strongly alkaline KOH(aq) even in the absence of illumination. The O₂ in solution drove the open-circuit potential of the electrode to >0.4 V, which is positive of the Si passivation potential. The elevated electrochemical potential of the surface promoted formation of passive oxides on exposed Si regions of Si/Ni electrodes. Catalytic passivation of Si extended the durability of an np⁺-Si(100)/NiO_x photoanode to >400 h while operating under simulated day/night cycles. In contrast, electrodes without a Ni(O_x) layer and/or without O₂ in solution displayed direct etching of the Si and corrosion pitting during non-illuminated, simulated nighttime episodes of day/night cycling. The O₂-derived catalyzed passivation of Si using thin films can be generalized to multiple phases of NiO_x as well as to materials other than Ni. Relative to operation in aqueous alkaline conditions, decreasing the pH of the electrolyte decreased the dissolution rate of the protective oxide layer formed by the catalyzed passivation process, and consequently increased the durability of the photoanode, but yielded lower photoelectrode fill factors for water oxidation due to the relatively large kinetic overpotentials for the electrocatalyzed oxygen-evolution reaction at near-neutral pH.