Synthesis of valuable chemicals from CO 2 electroreduction in acidic media is highly desirable to overcome carbonation. However, suppressing the hydrogen evolution reaction in such proton-rich environments remains a considerable challenge. The current study demonstrates the use of a hollow fiber silver penetration electrode with hierarchical micro/nanostructures to enable CO 2 reduction to CO in strong acids via balanced coordination of CO 2 and K + /H + supplies. Correspondingly, a CO faradaic efficiency of 95% is achieved at a partial current density as high as 4.3 A/cm 2 in a pH = 1 solution of H 2 SO 4 and KCl, sustaining 200 h of continuous electrolysis at a current density of 2 A/cm 2 with over 85% single-pass conversion of CO 2 . The experimental results and density functional theory calculations suggest that the controllable CO 2 feeding induced by the hollow fiber penetration configuration primarily coordinate the CO 2 /H + balance on Ag active sites in strong acids, favoring CO 2 activation and key intermediate *COOH formation, resulting in enhanced CO formation.