Figure and D show the Faradaic efficiencies for 0.01 M and 0.1 M CsHCO3. With a relatively low local Cs + content (~25 arb. u.) at the cathode catalyst layer, the dominant product at all current densities is CO (Figure ), while higher local Cs + content (~40 arb. u.) results in higher selectivity towards C2H4 and liquid products (Figure and Table ). These results show that a small difference in local cation concentration results in a large selectivity change for the Cu catalyst.