Stable Solar-Driven Water Oxidation to O_2(g) by Ni-Oxide-Coated Silicon Photoanodes

Abstract

Semiconductors with small band gaps (<2 eV) must be stabilized against corrosion or passivation in aqueous electrolytes before such materials can be used as photoelectrodes to directly produce fuels from sunlight. In addition, incorporation of electrocatalysts on the surface of photoelectrodes is required for efficient oxidation of H_2O to O_2(g) and reduction of H_2O or H_2O and CO_2 to fuels. We report herein the stabilization of np^+-Si(100) and n-Si(111) photoanodes for over 1200 h of continuous light-driven evolution of O_2(g) in 1.0 M KOH(aq) by an earth-abundant, optically transparent, electrocatalytic, stable, conducting nickel oxide layer. Under simulated solar illumination and with optimized index-matching for proper antireflection, NiO_x-coated np+-Si(100) photoanodes produced photocurrent-onset potentials of −180 ± 20 mV referenced to the equilibrium potential for evolution of O_2(g), photocurrent densities of 29 ± 1.8 mA cm^(–2) at the equilibrium potential for evolution of O_2(g), and a solar-to-O_2(g) conversion figure-of-merit of 2.1%.