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.