How cations determine the interfacial potential profile: Relevance for the CO₂ reduction reaction

The strong effect of the electrolyte cation on the activity and selectivity of the CO₂ reduction reaction (CO₂RR) can only be understood and controlled if the cation's effect on the interfacial potential distribution is known. Using CO (the key intermediate in the CO₂RR) adsorbed on Pt as a probe molecule, and combining IR spectroscopy, capacitance measurements and ab initio molecular dynamics, we show that the cation size determines the location of the outer Helmholtz plane, whereby smaller cations increase not just the polarisation but, most importantly, the polarizability of adsorbed CO (CO_ad) and the accumulation of electronic density on the oxygen atom of CO_ad. This strongly affects its adsorption energy, the degree of hydrogen bonding of interfacial water to CO_ad and the degree of polarisation of water molecules in the cation's solvation shell, all of which can deeply affect the subsequent steps of the CO₂RR.