High-Current-Density Acidic CO2 Electroreduction to Formic Acid Enabled by Multiscale Microenvironment Regulation Over Defect-Rich Sn/SnOx.

Acidic CO2 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/SnOx 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 CO2 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 CO2 electrolysis.