Silicon photoanodes have been studied intensively for efficient photoelectrochemical (PEC)-mediated catalysis of the oxygen evolution reaction, but their performance and stability still need to be improved before they can be applied practically. Here, we demonstrate that the porous, nanocrystalline NiOx electrocatalyst integrated with a thermally-grown SiO2 protection layer results in unexpectedly high PEC performances as well as long-term stability in Si photoanodes. The porous structure of NiO(x )and its large active area in contact to the electrolyte improves electrocatalytic activity and greatly reduces the kinetic overpotential required for charge transfer. In addition, the redox-active surface contributed by nanocrystalline nature of NiOx results in the accumulation of charges during the PEC reaction, predominantly affecting the interface energetics of NiOx/n-Si junctions. This feature enables a high open circuit potential of similar to 0.6 V even without p-n junctions. Stoichiometric SiO2 affords strong electrochemical corrosion resistance during the PEC water oxidation in 1 M KOH electrolyte, in contrast to the wet chemical-grown, nonstoichiometric SiOx. The porous and nanocrystalline NiOx and stoichiometric SiO2 incorporated in the n-Si photoanode ensures continuous operation for 100 h at high performance. These results can help inform a new way to design a high performance Si photoanodes with long-term stability.