Heterogenization of a Sandwich [(PW9O34)2Co4(H2O)2]10− in PCN-222/PCN-222(M): Exploring the Electron Transfer for Electrocatalytic CO2 Reduction

Abstract

In this study, we designed and prepared polyoxometalate@metal-organic framework (POM@MOF) composite catalysts through the anchoring of a sandwich POM [(PW₉O₃₄)₂Co₄(H₂O)₂]¹⁰⁻ (shortened as P₂W₁₈Co₄) to the hexagonal channel of the PCN-222 (metal-free) or PCN-222(M) (M=Fe, Co) frameworks. The composite materials were applied to the electrocatalytic reduction of CO₂ reaction (CO₂RR) to analyse the effect of incorporating P₂W₁₈Co₄ on catalytic activity. The P₂W₁₈Co₄@PCN-222 composite exhibited enhanced activity across a wide potential range (−0.60~−0.85 V vs. RHE) and an optimal FE_CO of 72 % at −0.75 V vs. RHE, which was more than double that of PCN-222 (FE_CO=33 %). The current density surpassed that of the PCN-222 precursors by over sixteen times at the same potential. In contrast, the P₂W₁₈Co₄@PCN-222(M) composite demonstrated decreased current density, minimal enhancement in CO₂RR activity, and a competing HER behaviour. Density functional theory calculations were conducted on simplified models of P₂W₁₈Co₄@H₂-TCPP and P₂W₁₈Co₄@M-TCPP to elucidate the divergent catalytic performances. The findings revealed that while both configurations exhibited the same rate-limiting step (formation of the *COOH intermediate), a significantly reduced reaction barrier was only observed in the P₂W₁₈Co₄@H₂-TCPP setup, explaining its substantial activity improvement.