However, a significant improvement in PEC performance was observed in an electrolyte containing 0.25 M Na2SO3 in 0.5 M KPi buffer solution (i.e., sulfite oxidation). The Cu2O photoanode showed an onset potential of 0.1 V vs. RHE and a photocurrent density of 10.7 mA cm -2 at 1.23 V vs. RHE (Fig. ). It is noteworthy here that the remarkably low onset potential of 0.1 V vs. RHE was achieved, although some amount of current doubling was observed during sulfite oxidation 50 (Supplementary Fig. ).