Electrochemical CO2 reduction to multicarbon products faces challenges of unsatisfactory selectivity, productivity, and long-term stability. Herein, we demonstrate CO2 electroreduction in strongly acidic electrolyte (pH ≤ 1) on electrochemically reduced porous Cu nanosheets by combining the confinement effect and cation effect to synergistically modulate the local microenvironment. A Faradaic efficiency of 83.7 ± 1.4% and partial current density of 0.56 ± 0.02 A cm−2, single-pass carbon efficiency of 54.4%, and stable electrolysis of 30 h in a flow cell are demonstrated for multicarbon products in a strongly acidic aqueous electrolyte consisting of sulfuric acid and KCl with pH ≤ 1. Mechanistically, the accumulated species (e.g., K+ and OH−) on the Helmholtz plane account for the selectivity and activity toward multicarbon products by kinetically reducing the proton coverage and thermodynamically favoring the CO2 conversion. We find that the K+ cations facilitate C-C coupling through local interaction between K+ and the key intermediate *OCCO.

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The electrocatalytic nitrogen reduction reaction (NRR) for NH 3 synthesis is still far from being practical and competitive with the common Haber-Bosch process. The rational design of highly selective NRR electrocatalyst is therefore urgently needed, which requires a deep understanding of both the electrode-electrolyte interface and the mass transport of reactants. Here, we develop a theoretical framework that includes electric double layer (EDL), mass transport, and the NRR kinetics. This allows us to evaluate the roles of near-electrode environment and N 2 diffusion on the NRR selectivity and activity.The EDL, as the immediate reaction environment, remarkably impedes the diffusion of N 2 to the cathode surface at high electrode potentials, which explains experimental observations. This article also gives microscopic insights into the interplay between N 2 diffusion and reaction activity under the nano-confinement, providing theoretical guidance for future design of advanced NRR electrocatalytic systems.