Photoelectrochemical (PEC) water-splitting cells, a promising way to convert solar energy into storable hydrogen fuel, typically consist of semiconductor photoelectrode and electrocatalyst. The semiconductor-electrocatalyst interface affects the PEC performance of the electrode but little has been known to date. As photoanode material, silicon is beneficial for a broadband light absorption, but its thermodynamic open circuit potential (V_oc) limits a thermodynamic driving force to oxidize water. Integrating a superior electrocatalyst onto a Si photoanode is therefore essential for improving a catalytic activity as well as the energetics at the catalyst/semiconductor interface. We have found that a porous, electrolyte-permeable NiO_x thin film revealed a superior water oxidation behavior onto a SiO₂-grown n-Si photoanode, in comparison to a dense NiO_x film. In a porous NiO_x film, the built-in potentials and Fermi-levels of the NiO_x/Si junction were varied in-situ with oxidation. As a result, a thermodynamic open-circuit potential (V_oc), which is normally limited by the amount of photovoltage (V_ph), is observed to decouple from the value of V_ph. The V_oc of 550 mV was finally achieved in porous NiO_x/Si junctions presenting ~200 mV of V_ph.Photoelectrochemical (PEC) water-splitting cells, a promising way to convert solar energy into storable hydrogen fuel, typically consist of semiconductor photoelectrode and electrocatalyst. The semiconductor-electrocatalyst interface affects the PEC performance of the electrode but little has been known to date. As photoanode material, silicon is beneficial for a broadband light absorption, but its thermodynamic open circuit potential (V_oc) limits a thermodynamic driving force to oxidize water. Integrating a superior electrocatalyst onto a Si photoanode is therefore essential for improving a catalytic activity as well as the energetics at the catalyst/semiconductor interface. We have found that a porous, electrolyte-permeable NiO_x thin film revealed a superior water oxidation behavior onto a SiO₂-grown n-Si photoanode, in comparison to a dense NiO_x film. In a porous NiO_x film, the built-in potentials and Fermi-levels of the NiO_x/Si junction were varied in-situ with oxidation. As a result, a thermodynamic open-circuit potential (V_oc), which is normally limited by the amount of photovoltage (V_ph), is observed to decouple from the value of V_ph. The V_oc of 550 mV was finally achieved in porous NiO_x/Si junctions presenting ~200 mV of V_ph.