Exploring the role of sandwich-type polyoxometalates in {K₁₀(PW₉O₃₄)₂M₄(H₂O)₂}@PCN-222 (M = Mn, Ni, Zn) for electroreduction of CO₂to CO

Abstract

To overcome the drawbacks of high solubility and instability of polyoxometalates (POMs) in aqueous solution and to expand their application in the electrocatalytic reduction of CO₂ (ECR), we assemble sandwich-type POMs, K₁₀[(PW₉O₃₄)₂M₄(H₂O)₂] (M = Mn, Ni, Zn, shortened as P₂W₁₈M₄), into the hexagonal channel of a porphyrin-based metal-organic framework (MOF) PCN-222 to form P₂W₁₈M₄@PCN-222 composites. Their ECR behavior displays polyoxoanion-dependent activity. P₂W₁₈Mn₄@PCN-222 demonstrates a faradaic efficiency of 72.6% for the CO product (FE_CO), more than four times that of PCN-222 (FE_CO = 18.1%), and exhibits exceptional electrochemical stability over 36 h. P₂W₁₈Ni₄@PCN-222 and P₂W₁₈Zn₄@PCN-222 slightly increase (26.9%) and decrease (3.2%) in FE_CO, respectively. We combine the results with density functional theory (DFT) calculations to help understand the intrinsic reasons which reveals that the rate-determining step (RDS) reaction energy of P₂W₁₈Mn₄@PCN-222 and P₂W₁₈Ni₄@PCN-222 is significantly reduced compared to that of PCN-222. It is different in P₂W₁₈Zn₄@PCN-222. Frontier molecular orbitals electron distribution results hint at directional electron transfer from P₂W₁₈Mn₄/P₂W₁₈Ni₄ to the porphyrin ring active center in PCN-222, promoting the electro-reduction of CO₂ activity. By contrast, P₂W₁₈Zn₄ may accumulate electrons from PCN-222, thus facilitating the hydrogen evolution reaction (HER). This work reveals the critical role of sandwich-type POMs in manipulating the electron transfer pathway during the electrocatalytic process. Our findings would broaden the scope of POM applications in electrochemical carbon dioxide reduction.