The extensive deployment of the electrocatalytic CO 2 reduction reaction (CO 2 RR) is presently limited by the utilization of alkaline/neutral electrolytes in which carbonate formation severely reduces the carbon efficiency and electrolysis stability. By contrast, the CO 2 RR in a strong acid electrolyte can overcome these shortcomings, yet the hydrogen evolution reaction (HER) greatly outcompetes the CO 2 RR in acidic media. Herein, CO 2 reduction to HCOOH, a significant chemical intermediate in many industrial processes, was realized in strong acid (pH ≤ 1) through introducing K + cations into the electrolyte. The K + -assisted acidic CO 2 RR accordingly manufactured HCOOH with a high Faradaic efficiency of 92.2% @−1.23 V RHE and a commercially relevant current density of −237.1 mA cm −2 . More importantly, a high single-pass carbon efficiency of 27.4% for HCOOH production was demonstrated in acid, which exceeded the value obtained in the alkaline CO 2 RR. Further mechanistic studies demonstrated that K + can engineer the local microenvironment over the Bi catalyst surface by reducing the proton coverage to suppress the competing HER and creating local interaction to stabilize the *OCOH intermediate, which ultimately promotes high-efficiency CO 2 conversion to HCOOH in strong acidic media.

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.

Advancing the development of electrocatalytic CO2 reduction reaction (CO2RR) to address the environmental issues caused by excessive consumption of fossil fuels requires rational design of remarkable electrocatalysts, where the identification of active sites and further understanding of structure–performance relationship are the bases. However, the notable dynamic evolution often appears on the catalysts, with typical examples of Cu‐based catalysts, under operating conditions, causing great difficulty in identifying the real active sites and further understanding the correlations between structure and catalytic property. In this context, understanding the dynamic evolution process of catalytically active sites during CO2RR is of particular importance, which inspires to organize the present review. Herein, the fundamental principles of dynamic evolution in CO2RR including thermodynamics and kinetics aspects, followed by the introduction of operando techniques employed to probe the evolution under operating conditions are first highlighted. The dynamic evolution behaviors, involving atomic rearrangement and change in chemical state, on typical catalysts are further discussed, with emphasis on the correlations between evolution behaviors and catalytic properties (activity, selectivity, and stability). The emerging CO2 pulsed electrolysis technique that behaves promise to manipulate the dynamic evolution and future opportunities are finally discussed.

Gold–copper (Au‐Cu) Janus nanostructures (Au‐Cu Janus NSs) are successfully prepared using N‐oleyl‐1,3‐propanediamine as capping agent and Cu(acac)2 as the precursor in a typical seeded growth strategy. By preferably depositing Cu atoms on one side of concave cubic Au seeds, the Cu part gradually grows larger as more Cu precursors are added, making the size tuning feasible in the range of 74–156 nm. When employed as an electrocatalyst for electrochemical CO2 reduction (CO2RR), the Au‐Cu Janus NSs display superior performance to Au@Cu core‐shell NSs and Cu NPs in terms of C2+ products selectivity (67%) and C2+ partial current density (−0.29 A cm–2). Combined experimental verification and theoretical simulations reveal that CO spillover from Au sites to the nearby Cu counterparts would enhance CO coverage and thus promote C–C coupling, highlighting the unique structural advantages of the Au‐Cu Janus NSs toward deep reduction of CO2. The current work provides a facile strategy to fabricate tandem catalyst with a Janus structure and validates its structural advantages toward CO2RR, which are of critical importance for the rational design of efficient CO2RR catalyst.