<p>The acidic electroreduction of CO₂ into multicarbon (C₂₊) products is much attractive for the improved carbon utilization than alkaline or neutral electroreduction. How to improve the efficiency of C₂₊ products generation by acidic electroreduction of CO₂, is important, especially at high current density and in electrolyte with low K⁺ concentration. Herein, we propose a strategy of capping Cu surface with a polymeric ionic liquid (PIL) adlayer for boosting the acidic electrocatalytic CO₂ conversion to C₂₊ products at high current densities (ampere-level) and low K⁺ concentration. In the electrolyte with a relatively low K⁺ concentration (1.0 M), the Faradaic efficiency (FE) for C₂₊ products reaches 82.2% under a current density 1.0 A·cm⁻² in acidic environment (pH=1.8). Particularly, when the current density is as high as 1.5 A·cm⁻², the C₂₊ FE still keeps 75.8%. Experimental and theoretical studies reveal that the presence of PIL adlayer on Cu catalyst can well inhibit H⁺ diffusion to catalyst surface, enrich more K⁺ and facilitate C-C coupling reaction.</p>