Effect of Ammonium-Based Cations on CO2 Electroreduction

Electrolyte cations play a critical role in the electrochemical CO2 reduction reaction (CO2RR), as indicated by recent studies using alkali metal cations. Here, we show that nonmetal cations such as ammonium-based cations can have a profound effect on the CO2 adsorption characteristics and CO2RR activity that may supersede those of metal cations. Our ab initio calculations based on the grand canonical density functional theory find that both NH4+ and CH3NH3+ help bind CO2 to a Bi electrode more strongly than Na+ and track this difference to the electrostatic interaction between the cations and adsorbed *CO2, which depends on the characteristics of the charge distribution and hydration shell of the cations. Our experimental studies confirm the strong promotional effect of the nonmetal cations, particularly CH3NH3+, on the CO2RR and further reveal a significant impact of the cation identity and concentration on CO production activity but a minor one on formate. The cation-stabilized CO2 adsorption is essential for CO production but not necessary for formate production.