Acidic electrochemical CO2reduction reaction (CO2RR) can minimize carbonate formation and eliminate CO2crossover, thereby improving long-term stability and enhancing single-pass carbon efficiency (SPCE). However, the kinetically favored hydrogen evolution reaction (HER) is generally predominant under acidic conditions. This paper describes the confinement of a local alkaline environment for efficient CO2RR in a strongly acidic electrolyte through the manipulation of mass transfer processes in well-designed hollow-structured Ag@C electrocatalysts. A high faradaic efficiency of over 95% at a current density of 300 mA cm−2and an SPCE of 46.2% at a CO2flow rate of 2 standard cubic centimeters per minute are achieved in the acidic electrolyte, with enhanced stability compared to that under alkaline conditions. Computational modeling results reveal that the unique structure of Ag@C could regulate the diffusion process of OH−and H+, confining a high-pH local reaction environment for the promoted activity. This work presents a promising route to engineer the microenvironment through the regulation of mass transport that permits the CO2RR in acidic electrolytes with high performance.