Zhanyou Xu

Electrochemical CO₂ reduction reaction (CO₂RR) powered by renewable energy provides a promising route to CO₂ conversion and utilization. However, the widely used neutral/alkaline electrolyte consumes a large amount of CO₂ to produce (bi)carbonate byproducts, leading to significant challenges at the device level, thereby impeding the further deployment of this reaction. Conducting CO₂RR in acidic electrolytes offers a promising solution to address the “carbonate issue”; however, it presents inherent difficulties due to the competitive hydrogen evolution reaction, necessitating concerted efforts toward advanced catalyst and electrode designs to achieve high selectivity and activity. This review encompasses recent developments of acidic CO₂RR, from mechanism elucidation to catalyst design and device engineering. This review begins by discussing the mechanistic understanding of the reaction pathway, laying the foundation for catalyst design in acidic CO₂RR. Subsequently, an in‐depth analysis of recent advancements in acidic CO₂RR catalysts is provided, highlighting heterogeneous catalysts, surface immobilized molecular catalysts, and catalyst surface enhancement. Furthermore, the progress made in device‐level applications is summarized, aiming to develop high‐performance acidic CO₂RR systems. Finally, the existing challenges and future directions in the design of acidic CO₂RR catalysts are outlined, emphasizing the need for improved selectivity, activity, stability, and scalability.

Advancing the performance of the Cu‐catalyzed electrochemical CO₂ reduction reaction (CO₂RR) is crucial for its practical applications. Still, the wettable pristine Cu surface often suffers from low exposure to CO₂, reducing the Faradaic efficiencies (FEs) and current densities for multi‐carbon (C₂₊) products. Recent studies have proposed that increasing surface availability for CO₂ by cation‐exchange ionomers can enhance the C₂₊ product formation rates. However, due to the rapid formation and consumption of *CO, such promotion in reaction kinetics can shorten the residence of *CO whose adsorption determines C₂₊ selectivity, and thus the resulting C₂₊ FEs remain low. Herein, we discover that the electro‐kinetic retardation caused by the strong hydrophobicity of quaternary ammonium group‐functionalized polynorbornene ionomers can greatly prolong the *CO residence on Cu. This unconventional electro‐kinetic effect is demonstrated by the increased Tafel slopes and the decreased sensitivity of *CO coverage change to potentials. As a result, the strongly hydrophobic Cu electrodes exhibit C₂₊ Faradaic efficiencies of ≈90 % at a partial current density of 223 mA cm⁻², more than twice of bare or hydrophilic Cu surfaces.

Advancing the performance of the Cu‐catalyzed electrochemical CO₂ reduction reaction (CO₂RR) is crucial for its practical applications. Still, the wettable pristine Cu surface often suffers from low exposure to CO₂, reducing the Faradaic efficiencies (FEs) and current densities for multi‐carbon (C₂₊) products. Recent studies have proposed that increasing surface availability for CO₂ by cation‐exchange ionomers can enhance the C₂₊ product formation rates. However, due to the rapid formation and consumption of *CO, such promotion in reaction kinetics can shorten the residence of *CO whose adsorption determines C₂₊ selectivity, and thus the resulting C₂₊ FEs remain low. Herein, we discover that the electro‐kinetic retardation caused by the strong hydrophobicity of quaternary ammonium group‐functionalized polynorbornene ionomers can greatly prolong the *CO residence on Cu. This unconventional electro‐kinetic effect is demonstrated by the increased Tafel slopes and the decreased sensitivity of *CO coverage change to potentials. As a result, the strongly hydrophobic Cu electrodes exhibit C₂₊ Faradaic efficiencies of ≈90 % at a partial current density of 223 mA cm⁻², more than twice of bare or hydrophilic Cu surfaces.

Improving carbon utilization efficiency is the key to developing next‐generation electrolysers for CO₂ reduction reaction (CO₂RR). The current CO₂RR electrolyser relies on the use of neutral/alkaline media to suppress the competitive hydrogen production and improve the activity of CO₂RR. However, the produced carbonate/bicarbonate leads to severe carbon loss. Performing CO₂RR in acidic media can suppress the carbonate formation while hydrogen production is the major issue. Herein, we found that the activity of acidic CO₂RR can be well tuned through electrolyte optimization on Cu and Ag catalyst. DFT calculation suggests that this results from the change of local electronic structure on Cu by surface adsorbed alkali metal ions. Electrolytes with high content of K⁺ promote the overall CO₂RR activity, especially multi‐carbon production in acidic media. CH₄ is the dominant product in Na⁺ only electrolyte on Cu, with a Faradaic efficiency of 48 % at 220 mA cm⁻² in pH=2 solution.