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

Carbon dioxide electroreduction to multicarbon (C2+) products in strongly acidic media offers a promising approach to mitigate carbon loss observed in alkaline and neutral electrolytes. However, achieving high C2+ selectivity at energy-efficient current densities remains challenging due to competing one-carbon (C1) product generation. Here, we report a proton-availability-promoted C2+ production in acidic CO2 electroreduction within a moderate current density regime. We demonstrated a remarkably enhanced C2+ production─C2+ faradaic efficiency increasing from 23.9 ± 2.7% to 48.1 ± 0.6% and C2+/C1 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 C2+ product formation.