Countercation Manipulation Enables Selective Electrochemical CO2-to-Formate Conversion Over Bi-Polyoxometalate-Derived Catalysts
K. Kawakami, K. Yonesato, S. Akino, S. Kikkawa, S. Yamazoe, T. Kaneko, T. Uruga, F. Amano, Y. Honjo, T. Hatsui, K. Harano, K. Yamaguchi, K. Suzuki
ChemEurJ., in press.
Electrochemical conversion of CO2 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 CO2 reduction reaction (CO2RR) owing to their structural diversity, compositional tunability, and flexibility in countercation selection. Despite these advantages, effective strategies for developing POM-based CO2RR 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, [Bi4O(γ-SiW10O36)2(OAc)]7− (Bi4). Under gas-diffusion CO2RR conditions with a 2 M KHCO3 aqueous electrolyte, the Ba2+ salt of Bi4 immobilized on carbon (Ba-Bi4/C) exhibited high HCOO− selectivity with a Faradaic efficiency of 84.3% at −0.79 VRHE, whereas the corresponding tetra-n-butylammonium (TBA+) salt (TBA-Bi4/C) exhibited substantially lower selectivity (26.6% at −0.79 VRHE). Post-reaction analyses revealed that both catalysts transformed into highly dispersed Bi species protected by WOx nanoaggregates. Control experiments and detailed characterizations indicated that the countercations, in conjunction with the POM-derived catalyst structure, play a crucial role in governing CO2RR performance.