The Si photocathodes were fabricated with commercially available Si wafers from Si-Mat (Boron doping, 1-30 ohm-cm, single side polished). Si wafers were cleaned by ultrasonication for 10min each in acetone, soapy water, water and isopropanol, followed by 10 min at 50 o C in H2O/H2O2/NH4OH (5:1:1) solution and 22 50 o C in H2O/H2O2/HCl (5:1:1) solution. The cleaned Si wafers were immersed in 2% HF solution at room temperature and then dried with N2 gas and immediately transferred to the ALD deposition. The deposition processes of the ALD-TiO2 layer and RuOx catalyst are the same as described above. The EIS data of the Si photocathode was obtained by measuring Nyquist plots under 10 % sun light in 1 M H2SO4 electrolyte with 0.1 mM Triton X. For EIS data of the Cu2O photocathodes, we re-fitted the previous data used in our previous publication. In the previous study, only the resistive components were considered without any description of capacitance values. In the present study, we re-fitted the data with the addition of the high-frequency R-C element (see Figure -S12 in the Supporting Information) and took the capacitances into account.