Cu2O catalysts were synthesized by a simple wet chemistry method according to the reference. 1 mL of 0.1M CuCl2 was added into 95.5 mL deionized (DI) water, then 2mL of 1M NaOH solution was added into the above solution under stirring. After 10 s, 0.87g SDS was added into the above mixture with vigorous stirring to make the powder solve into the solution. Then 3.5 mL of 0.2M NH2OH•HCl was injected into the above mixture and was shaken for 10 s. The mixture was centrifuged to obtain the precipitates after 12 h aging, and washed with water and ethanol three times, respectively. Finally, the precipitates were dried in a vacuum oven for 12h, and the powder was labeled as sample C. This was the primary catalyst used for electrocatalytic measurements For sample A to H, the synthesis steps are similar to sample C, except for different volumes of DI water and NH2OH•HCl. The volume of DI water and NH2OH•HCl for each sample were listed in Table .