High‐Current‐Density Acidic CO 2 Electroreduction to Formic Acid Enabled by Multiscale Microenvironment Regulation Over Defect‐Rich Sn/SnO x

ABSTRACT Acidic CO 2 electrolysis to formic acid circumvents carbonate formation and enables direct product recovery, yet is hindered by severe competition between formate‐producing *OCHO and hydrogen‐producing *H intermediates. Here, we develop a hierarchical site‐solvation‐transport regulation strategy to direct this competition toward formic acid. Mixed, undercoordinated Sn/SnO x nanoclusters provide catalytic environments associated with the *OCHO pathway, while K + ‐dependent interfacial solvation is associated with reduced hydrogen evolution reaction (HER) competition and enhanced formic acid selectivity. A Janus asymmetric‐wettability electrode further stabilizes CO 2 mass transport and prevents electrolyte flooding, sustaining a favorable cathodic microenvironment. This integrated strategy achieves ∼85% formic acid Faradaic efficiency in acidic media with an initial 18 h stability window at 400 mA cm −2 . This work establishes a multiscale strategy for coupling intermediate selectivity with mass‐transport management in acidic CO 2 electrolysis.