A series of polyoxometalate (POM)-based metal–organic framework composites, XW₁₁Co@ZIF-8 (X = Si, P, Co), were prepared by encapsulating single cobalt-substituted Keggin XW₁₁Co polyoxoanions into the cavities of ZIF-8 at ambient temperature using a synthetic encapsulation strategy. This study investigated the impact of the central atoms in POMs on the electrochemical reduction of carbon dioxide (eCO₂RR). The results of the electrochemical test illustrated a significant enhancement in the current densities and eCO₂RR performance of the composites compared with pristine ZIF-8. Specifically, CoW₁₁Co@ZIF-8 exhibited 72.4% CO selectivity at a potential of −1.10 V vs. reversible hydrogen electrode (RHE), which was 1.5 times higher than that of ZIF-8 under the same potential. SiW₁₁Co@ZIF-8 and PW₁₁Co@ZIF-8 also exhibited improved electrocatalytic performance, though less pronounced, with faradaic efficiency for CO production (FE_CO) values of 63.6% and 57.0%, respectively. The central atom in XW₁₁Co significantly affects the catalytic performance of the composites. The [Co^IIIW₁₁O₃₉Co^II(H₂O)]^7– species carries more negative charges, enabling the provision of additional electrons to the active sites. Consequently, CoW₁₁Co@ZIF-8 exhibited the highest negative charge and charge transfer efficiency, the largest electrochemically active surface area, and the highest eCO₂RR activity.