Potential dependence of the CO2RR rate on Au in bicarbonate electrolytes with various alkali and organic cations was determined via the rotating ring disk electrode (RRDE) experiments following reported procedure (Methods, Fig. , Supplementary Note 1) . 𝑗 !" exhibits a strong cation-dependence, following the sequence of Li < Na < K < Cs and tetrapropylammonium (TPA) < tetraethylammonium (TEA) < tetramethylammonium (TMA) for alkali and organic quaternary ammonium cations, respectively (Fig. &), which are consistent with a previous report . It is established that the first electron transfer step is the RDS of CO2RR on Au . A significant kinetic isotope effect (KIE = 2~4) was observed when comparing the rates in reaction conducted in H2O and D2O (Fig. ), demonstrating that the RDS is a PCET process, as in: