Substituent tuning of Cu coordination polymers enables carbon-efficient CO₂ electroreduction to multi-carbon products

CO₂ electroreduction is a potential pathway to achieve net-zero emissions in the chemical industry. Yet, CO₂ loss, resulting from (bi)carbonate formation, renders the process energy-intensive. Acidic environments can address the issue but at the expense of compromised product Faradaic efficiencies (FEs), particularly for multi-carbon (C₂₊) products, as rapid diffusion and migration of protons (H⁺) favors competing H₂ and CO production. Here, we present a strategy of tuning the 2-position substituent length on benzimidazole (BIM)-based copper (Cu) coordination polymer (CuCP) precatalyst - to enhance CO₂ reduction to C₂₊ products in acidic environments. Lengthening the substituent from H to nonyl enhances H⁺ diffusion retardation and decreases Cu-Cu coordination numbers (CNs), favoring further reduction of CO. This leads to a nearly 24× enhancement of selectivity towards CO hydrogenation and C-C coupling at 60 mA cm^-2. We report the highest C₂₊ product FE of more than 70% at 260 mA cm^-2 on pentyl-CuCP and demonstrate a CO₂-to-C₂₊ single-pass conversion (SPC) of ~54% at 180 mA cm^-2 using pentyl-CuCP in zero-gap electrolyzers.