Nature Catalysis, Published online: 31 August 2026; doi:10.1038/s41929-026-01604-w The mechanism of cation enhancement in electrocatalytic CO2 reduction has been widely debated but spectroscopic evidence under operating conditions is lacking. Now operando X-ray and surface-enhanced infrared spectroscopy are combined with theory to reveal that desolvated Cs+ cations enable CO2 activation at substantially reduced overpotentials through partial covalent interactions.