We evaluated PEC water oxidation through a comparative analysis of Cu2O photoanodes with and without Fe 2+ treatment using linear sweep voltammetry (LSV). LSV measurements were performed after prior CV cycling to confirm activation of each co-catalyst (i.e., nickel and nickel-iron oxyhydroxides) (Supplementary Fig. ). After the activation process, nickel oxyhydroxides showed negligible changes (Supplementary Fig. ), whereas nickel-iron oxyhydroxides exhibited improved photocurrent and fill factor following continuous CV cycling (Supplementary Fig. ). Accordingly, all subsequent PEC measurements were conducted after first activating the nickel-iron oxyhydroxides (denoted as NiFe). In LSV measurements, the Fe 2+ -treated electrode showed enhanced onset potential and photocurrent density in PEC water oxidation compared to the untreated electrode. Specifically, the onset potential was improved from 1.04 V to 0.74 V vs. RHE (Supplementary Fig. ), and the photocurrent density at 1.23 V increased from 5.04 mA cm -2 to 8.65 mA cm -2 (Fig. and Supplementary Fig. ). For photocurrent measurements, more than 18 Cu2O photoanodes were tested to confirm statistical relevance, and an average photocurrent density of 7.64 mA cm -2 was obtained (Supplementary Fig. ). The half-cell applied bias photonto-current efficiency (ABPE) was 1.72% at 0.91 V vs. RHE (Fig. ). The incident photon-tocurrent efficiency (IPCE) showed a steep rise from 640 nm, reaching about 90% at 520 nm and then falling linearly to about 40 % for the shorter wavelengths, a phenomenon that was observed previously . The integrated photocurrent matched with the photocurrent density measured in the LSV (Fig. ). Furthermore, chronoamperometry (CA) measurement demonstrated that the fabricated Cu2O photoanode was able to perform stable water oxidation at 1.23 V vs. RHE for 30 h (Fig. ), with a Faradaic efficiency ranging of ~ 85 to 100% over one and a half hours (Fig. ). The apparent lower Faradaic efficiency at early time points is due to the time required for the evolved gas to fill and reach a steady-state within the reactor and the tubing to the gas chromatograph (more details in Methods).