Exploring the Effects of Saturated and Transition-Metal-Substituted Dawson Doping on Electrocatalytic CO 2 Reduction Activity in P 2 W 18 @PCN-222 and P 2 W 17 M@PCN-222 (M = Co, Mn, Ni) Composites

To exploit the electron-sponge properties of polyoxometalates (POMs) and elucidate the mechanistic impact of doping with saturated versus metal-substituted Dawson-type POMs on electrocatalytic CO 2 reduction, P 2 W 18 @PCN-222 and P 2 W 17 M@PCN-222 (M = Co, Mn, Ni) composites were prepared using an impregnation method. Electrochemical analyses, in situ infrared spectroscopy, and DFT calculations reveal that substituted P 2 W 17 M clusters lower the free energy barriers for *COOH and *CO formation, promoting CO evolution. Specifically, P 2 W 17 Co acts as an electron donor, facilitating the directional transfer of electrons from the POM to the porphyrin active centers of the PCN-222. Consequently, P 2 W 17 Co@PCN-222 exhibits superior CO 2 reduction reaction (CO 2 RR) performance, achieving a Faradaic efficiency for CO (FE CO ) of 82% and a partial current density ( j CO ) of 3.4 mA cm –2 at −0.80 V vs RHE, an 11.9-fold and 34.1-fold enhancement over pristine PCN-222 (FE CO = 6.9%, j CO = 0.1 mA cm –2 ), respectively. In contrast, the saturated P 2 W 18 cluster failed to establish an effective electron transfer pathway with the porphyrin centers and did not reduce the energy barrier associated with the rate-determining step (*COOH formation), thus conferring no promotional effect on the CO 2 RR.