We also prepared co-catalyst modified photoanodes (BiVO4/CoOOH and BiVO4/PEI/TiO2/CoOOH) to facilitate efficient water oxidation (Supplementary Fig. ). The BiVO4/PEI/TiO2/CoOOH photoanode exhibited a notable increase in photocurrent of 2.03 mA cm -2 (at 1.23 V vs. RHE), along with an onset potential of 0.28 V vs. RHE, estimated by extrapolating the linear rising region of the photocurrent to the x-axis (Fig. ). Subsequently, we evaluated the stability of the photoanodes by chronoamperometry (CA) in PEC water oxidation. While the bare BiVO4 and BiVO4/CoOOH photoanodes rapidly degraded within 5 h, the BiVO4/PEI/TiO2/CoOOH maintained its PEC activity for an extended duration of 120 h, achieving 96% Faradaic efficiency for oxygen evolution as measured by gas chromatography (GC) (Fig. and Supplementary Fig. and). The stability of BiVO4/PEI/TiO2/CoOOH was also enhanced even under alkaline conditions (pH 11), in which BiVO4 is inherently unstable (Supplementary Fig. ). Moreover, inductively coupled plasma-optical emission spectroscopy (ICP-OES) measurement demonstrated a negligible dissolution of Bi and V from BiVO4 to the electrolyte after the stability test of BiVO4/PEI/TiO2/CoOOH (Fig. ). The PEI/TiO2 layer therefore offers corrosion protection in addition to conductivity for holes.