We measure PEC water oxidation performances of the as-prepared photoanodes in a standard setup composed of a three-electrode reaction cell filled with 1.0 M KOH electrolytes under air mass (AM) 1.5G one-sun illumination. In each measurement, the copper plate at the reaction cell's backside contacts the photoanode's metal contact layer to form a complete circuit. The input area exposed to solar illumination is 1.0 cm 2 for each photoanode. Figure reports the photocurrent density versus applied potential (J-E) of photoanodes with varying initial NiFe alloy film thickness from 1 nm to 8 nm. The photoanode with 2 nm NiFe shows the lowest onset potential of 1.06 V vs. RHE, defined at the current density of 10 mA cm -2 , compared with other thicknesses, and achieves a high saturated current density of 38.4 mA cm -2 . Thinner (1 nm) or thicker (5 nm and 8 nm) NiFe co-catalyst layer results in higher onset potential, indicating the key role of atomic thickness control in the NiFe layer to achieve optimal performances. The saturated current density for 5 nm and 8 nm devices decreases from 36.1 mA cm -2 to 31.0 mA cm -2 (Fig. ). This behavior originates from parasitic light absorption in the metallic NiFe layer, a phenomenon also observed in other PEC devices employing Pt and Ni catalysts . Cyclic voltammetry (CV) measurements under dark conditions report the reduction peak potential dependence on the NiFe layer thickness (Fig. ), in which the photoanodes with 2 nm NiFe layer shows the lowest reduction peak (Ni III /Ni II ) potential of 0.97 V vs. RHE (Fig. ). Moreover, the peak current density value increases from -0.435 mA cm -2 to -0.814 mA cm -2 with the NiFe layer thickness under dark conditions (Fig. ).