Countercation Manipulation Enables Selective Electrochemical CO ₂ ‐to‐Formate Conversion Over Bi‐Polyoxometalate‐Derived Catalysts

ABSTRACT Electrochemical conversion of CO 2 into formate (HCOO − ) represents a promising strategy for mitigating global warming while producing an industrially important chemical. Polyoxometalates (POMs), anionic molecular metal‐oxo clusters, offer unique opportunities as catalysts and catalyst precursors for the electrochemical CO 2 reduction reaction (CO 2 RR) owing to their structural diversity, compositional tunability, and flexibility in countercation selection. Despite these advantages, effective strategies for developing POM‐based CO 2 RR catalysts with high selectivity toward HCOO − remain limited. In this study, we demonstrate countercation manipulation as a powerful approach to achieving selective HCOO − production using a bismuth (Bi)‐incorporated POM, [Bi 4 O(γ‐SiW 10 O 36 ) 2 (OAc)] 7− ( Bi4 ). Under gas‐diffusion CO 2 RR conditions with a 2 M KHCO 3 aqueous electrolyte, the Ba 2+ salt of Bi4 immobilized on carbon (Ba‐ Bi4 /C) exhibited high HCOO − selectivity with a Faradaic efficiency of 84.3% at −0.79 V RHE , whereas the corresponding tetra‐ n ‐butylammonium (TBA + ) salt (TBA‐ Bi4 /C) exhibited substantially lower selectivity (26.6% at −0.79 V RHE ). Post‐reaction analyses revealed that both catalysts transformed into highly dispersed Bi species protected by WO x nanoaggregates. Control experiments and detailed characterizations indicated that the countercations, in conjunction with the POM‐derived catalyst structure, play a crucial role in governing CO 2 RR performance.