Proton-Tuned Surface Chemistry Promotes Multicarbon Formation in Acidic CO2 Electroreduction

Carbon dioxide electroreduction to multicarbon (C 2+ ) products in strongly acidic media offers a promising approach to mitigate carbon loss observed in alkaline and neutral electrolytes. However, achieving high C 2+ selectivity at energy-efficient current densities remains challenging due to competing one-carbon (C 1 ) product generation. Here, we report a proton-availability-promoted C 2+ production in acidic CO 2 electroreduction within a moderate current density regime. We demonstrated a remarkably enhanced C 2+ production─C 2+ faradaic efficiency increasing from 23.9 ± 2.7% to 48.1 ± 0.6% and C 2+ /C 1 ratio from 0.4 to 1.6─by increasing the proton concentration from pH 2 to pH 1 in the bulk acidic electrolyte. Our in situ Raman spectroscopy and simulation studies revealed that higher proton concentration enhances *CO coverage and favors a low-frequency *CO binding configuration on copper surface, thereby facilitating C–C coupling and promoting C 2+ product formation.