Finally, we experimentally revealed that the local Cs + and H2O content at the Cu layer strongly affect selectivity. While a relatively low local Cs + content demonstrated high faradaic efficiencies towards CO, higher local Cs + content tuned the selectivity towards HER and C2+ products. Alongside previously proposed cation effects, we proposed that an increased HER at higher electrolyte concentrations is due to the correspondingly high H2O content at both the catalyst layer and in the macro-porous layer.