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+ (FEC2+ ) 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.