Figure presents the Tafel slopes calculated from the polarization curves in Fig. . Photoanodes with 2 nm NiFe alloy layer show the fastest kinetics among all the samples by exhibiting the smallest Tafel slope of 34.9 mV/dec. By contrast, photoanodes with fewer NiFe (1 nm) exhibit a sluggish kinetic process with a Tafel slope of 76.5 mV/dec. These results confirm the importance of controlling the thickness of the NiFe layer to enable high-performance PEC water oxidation reaction.