Exploring the role of sandwich-type polyoxometalates in {K 10 (PW 9 O 34 ) 2 M 4 (H 2 O) 2 }@PCN-222 (M = Mn, Ni, Zn) for electroreduction of CO 2 to CO.

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 CO 2 (ECR), we assemble sandwich-type POMs, K 10 [(PW 9 O 34 ) 2 M 4 (H 2 O) 2 ] (M = Mn, Ni, Zn, shortened as P 2 W 18 M 4 ), into the hexagonal channel of a porphyrin-based metal-organic framework (MOF) PCN-222 to form P 2 W 18 M 4 @PCN-222 composites. Their ECR behavior displays polyoxoanion-dependent activity. P 2 W 18 Mn 4 @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. P 2 W 18 Ni 4 @PCN-222 and P 2 W 18 Zn 4 @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 P 2 W 18 Mn 4 @PCN-222 and P 2 W 18 Ni 4 @PCN-222 is significantly reduced compared to that of PCN-222. It is different in P 2 W 18 Zn 4 @PCN-222. Frontier molecular orbitals electron distribution results hint at directional electron transfer from P 2 W 18 Mn 4 /P 2 W 18 Ni 4 to the porphyrin ring active center in PCN-222, promoting the electro-reduction of CO 2 activity. By contrast, P 2 W 18 Zn 4 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.