Advances in acidic electrocatalytic CO₂ reduction

The electrocatalytic CO2 reduction reaction (CO2RR) enables the efficient conversion of waste CO2 into valuable chemical products under mild operating conditions, presenting a promising pathway for sustainable CO2 utilization. The CO2RR is typically conducted in neutral or alkaline electrolytes, where the associated high pH value facilitates the adsorption of CO2 and inhibition of the competitive hydrogen evolution reaction (HER). However, in neutral and alkaline electrolytes, CO2 reacts with OH− to form carbonate ions, which results in a low single–pass carbon utilization efficiency(SPCE) and increases the costs associated with gas–liquid separation and electrolyte regeneration. Operating the CO2RR in acidic electrolytes offers a potential solution, which leads to a theoretical SPCE of 100% and considerably reduces operational costs. Nevertheless, the high concentration of H+ in acidic electrolytes promotes the HER, thereby resulting in low Faradaic efficiency(FE) for the CO2RR. This review systematically summarizes recent advances in suppression of the HER and enhancement of CO2RR performance under acidic conditions by focusing on catalyst design and interfacial microenvironment regulation.Catalyst design strategies refer to tuning the electronic structure and optimizing the morphological structure of catalysts. In particular, tuning the electronic structure is achieved by altering the composition and coordination environment of the catalysts. This review systematically summarizes four major catalyst categories: single-atom catalysts or dual-atom catalysts, alloy catalysts, molecular catalysts, and metal–organic framework/covalent organic framework catalysts. For each category, recent advancements in the regulation of the coordination environments of active sites to improve FE in the acidic CO2RR are discussed. Optimization of various structures, such as a shell-encapsulated structure and an interconnected hierarchically porous structure, can improve the associated mass transfer efficiency. The shell-encapsulated structure comprises catalytically active nanoparticles enclosed within a protective shell layer (e.g., carbon layer). The involved shells create a confinement effect that reduces proton accessibility to catalytically active sites and increases the local pH within the shell. The interconnected hierarchically porous structure features a network of micropores ( mesopores (2–50 nm), and macropores (>50 nm). These hierarchical pores facilitate rapid gas diffusion by minimizing transport resistance, thus enhancing the CO2R reaction performance.Interfacial microenvironment modulation strategies, including electrolyte engineering and electrode interfacial engineering, are aimed at establishing conditions favorable to the CO2RR. Electrolyte engineering emphasizes the role of cations, which can suppress the HER while promoting the CO2RR. The enhancement mechanisms are further attributed to tip effects in cation-associated phenomena. Electrode interfacial engineering strategies include the incorporation of hydrophobic groups and charged groups. Hydrophobic group modification regulates the localization and distribution of H2O molecules at the electrode interface, while charged group modification controls proton diffusion by leveraging electrostatic interactions with charged intermediates or dissolved metal cations. Finally, the review provides prospects for future research in the advancement of the acidic CO2RR by focusing on three aspects: (1) development of salt-free systems to effectively prevent salt deposition and flooding on the involved cathode surface, (2) integration of machine learning for the rational design of stable catalysts and elucidation of structural evolution mechanisms using advanced in situ characterization techniques, and (3) advancement of the scale of the CO2RR through device-level innovations, including the optimization of electrolyzer design and operating parameters, which together offer a critical development pathway toward industrial implementation.