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.