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

Abstract Electrocatalytic CO 2 reduction reaction (CO 2 RR) to multi‐carbon products (C 2+ ) 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 C 2+ 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 CO 2 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 CO 2 RR in 3 M KCl at pH=1. As a result, a Faradaic efficiency of 90.69±2.15 % for C 2+ (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 CO 2 flow rate of 7 sccm.