As the frequency domain of each semi-circle gives a first indication of the underlying photophysical process, we marked the semi-circles with arrows based on the frequencies, and detailed information about the fitting will be described in the following sections. The information of the frequency domain is more clearly shown in the corresponding Bode plots in Figure in the Supporting Information. When the applied potential moves from 0.55 VRHE to 0.4 VRHE, the semi-circle near 50 -100 Hz ("mid-frequency," represented by the sky-blue arrow) gradually decreases and finally disappears near the onset potential (Figure ). After the midfrequency semi-circle has nearly vanished, the lowest frequency semi-circle (represented by the red arrow) begins to rapidly decrease (Figure ) followed by the onset of photocurrent. After the photocurrent onset, two new semi-circles are observed (Figure , the green and gray arrow) while the sky-blue and red arrows have disappeared. The frequency domain of the green one (~1 kHz) is distinguishable from the others. Although the gray arrow (1 ~ 10 Hz) has a similar frequency domain to the red one (0.1 ~1 Hz), it is assigned to a new process as we presume both semicircles represent different electrochemical processes (vide infra). In addition, the highest frequency semi-circle (the black arrow) remained nearly constant over all potential regions. As mentioned above, of paramount importance for exploiting EIS data is a model that accurately describes the underlying photophysical and electrochemical processes. EC models for the multi-layered photocathodes have hardly been elucidated, although there are some studies reporting such data without any verification of the models. Here, we propose and verify simple EC models for fitting and interpreting the EIS data of TiO2-coated Sb2Se3 photocathodes, as shown in Figure . For better fitting, a constant phase element (CPE) was used instead of an ideal capacitor. The real values of the capacitance (C) can be calculated from the values of the CPE (Q) and the resistance (R) using the following formula: C = ((QR) 1/n )/R.