The results manifest the unique power of the EIS technique in analyzing multi-layered photocathodes under operational conditions. Multi-layered Sb2Se3 photocathodes were fabricated with an Sb2Se3 thin film obtained by sulfurization of an electrodeposited Sb layer, an n-type TiO2 overlayer (deposited by atomic layer deposition) and a photoelectrodeposited RuOx catalyst, as shown in Figure . Under simulated 1-sun illumination, the Sb2Se3 photocathode reveals a photocurrent density of 15 mA cm -2 at 0 V versus the reversible hydrogen electrode (VRHE) and an onset potential near 0.3 VRHE (Figure , Supporting Information), which are typical values for Sb2Se3 photocathodes with the same device configuration. For the EIS measurements shown in Figure , we used reduced light intensity (approximately 10 % sun) to avoid undesirable interference at high photocurrents (bubble formation and clinging at the photocathode/electrolyte interface). Figure shows the steady-state photocurrent density of the Sb2Se3 photocathode during the EIS measurement while the Nyquist plots at different applied potentials are shown in Figure .