We perform the PEC tests in a homemade three-electrode reaction cell connected to an Autolab PGSTAT 204 electrochemical workstation (Metrohm, Switzerland). We use Ag/AgCl containing saturated KCl solution as the reference electrode and the Pt wire as the counter electrode. We immobilize the Si photoanodes between the Cu plate contact and the Teflon ring of the homemade reaction cell, exposing the photoanodes with a constant round-shape area of 1 cm 2 for illumination. We carry out all the PEC measurements under 1 sun illumination (AM 1.5G, 100 mW cm -2 ) with the solar simulator (Sciencetech SLB300A, Canada) calibrated by the AMPRORE SOLAR-100 power meter and THORLABS PM200 power meter at the position of photoanodes and using 1 M KOH as the electrolyte. Before J-V tests, we performed five cycles of CV tests without illumination from -1.0 to 1.8 V vs. RHE to activate the photoanodes (scan rate of 0.1 mV/s). After the activation, we collect the liner-sweep voltammetry (LSV) data with the Autolab electrochemical workstation from -1.0 to 1.8 V vs. RHE at a scan rate of 0.05 mV/s under illumination. We measure the electrochemical impedance spectroscopy (EIS) on the Autolab electrochemical workstation over the frequency range from 150 kHz to 0.1 Hz at constant potential (current density of working electrodes at 5 mA cm -2 ) under one sun illumination. We extract the equivalent series resistance from the Nyquist plots for the iR drop compensation of the LSV curves . The measured potentials of PEC tests are against the Ag/AgCl reference electrode, and we convert all the recorded potentials to reversible hydrogen electrode (RHE) potentials using the Nernst equation: