Breaking K⁺ Concentration Limit on Cu Nanoneedles for Acidic Electrocatalytic CO₂ Reduction to Multi-Carbon Products

Electrocatalytic CO₂ reduction reaction (CO₂ RR) to multi-carbon products (C₂₊ ) in acidic electrolyte is one of the most advanced routes for tackling our current climate and energy crisis. However, the competing hydrogen evolution reaction (HER) and the poor selectivity towards the valuable C₂₊ products are the major obstacles for the upscaling of these technologies. High local potassium ions (K⁺ ) concentration at the cathode's surface can inhibit proton-diffusion and accelerate the desirable carbon-carbon (C-C) coupling process. However, the solubility limit of potassium salts in bulk solution constrains the maximum achievable K⁺ concentration at the reaction sites and thus the overall acidic CO₂ RR performance of most electrocatalysts. In this work, we demonstrate that Cu nanoneedles induce ultrahigh local K⁺ concentrations (4.22 M) - thus breaking the K⁺ solubility limit (3.5 M) - which enables a highly efficient CO₂ RR in 3 M KCl at pH=1. As a result, a Faradaic efficiency of 90.69±2.15 % for C₂₊ (FE_C2+ ) can be achieved at 1400 mA.cm^-2 , simultaneous with a single pass carbon efficiency (SPCE) of 25.49±0.82 % at a CO₂ flow rate of 7 sccm.