Si photoanode protected by a metal modified ITO layer with ultrathin NiO_x for solar water oxidation

We report an ultrathin NiO_x catalyzed Si np⁺ junction photoanode for a stable and efficient solar driven oxygen evolution reaction (OER) in water. A stable semi-transparent ITO/Au/ITO hole conducting oxide layer, sandwiched between the OER catalyst and the Si photoanode, is used to protect the Si from corrosion in an alkaline working environment, enhance the hole transportation, and provide a pre-activation contact to the NiO_x catalyst. The NiO_x catalyzed Si photoanode generates a photocurrent of 1.98 mA cm^-2 at the equilibrium water oxidation potential (E _OER = 0.415 V vs. NHE in 1 M NaOH solution). A thermodynamic solar-to-oxygen conversion efficiency (SOCE) of 0.07% under 0.51-sun illumination is observed. The successful development of a low cost, highly efficient, and stable photoelectrochemical electrode based on earth abundant elements is essential for the realization of a large-scale practical solar fuel conversion.