In this paper, a sequence of Zn-modified Co@N–C composites are designed and developed via the direct annealing of Zn–Co bimetallic zeolitic imidazolate framework materials with different Co/Zn ratios and at different pyrolysis temperatures. As proved by the various measurements made, the size of the Co particles in the composites could be successfully adjusted from 18 nm to 6 nm. In addition, with a decrease in particle size the Co particles become covered by carbon layers, and the specific surface area and number of Co–N–C bonds are successfully increased. As a result, the current density and CO faradaic efficiency (FE_(CO)%) in the electrochemical CO₂ reduction reaction gradually improved (from 1.8 to 4.2 mA cm⁻² for current density and from 25% to 60% for FE_(CO)%) with a decrease in the Co particle size. Meanwhile, the ratio of H₂/CO in the obtained syngas could be regulated in the range 5 : 1 to 1 : 1.5. The Zn–Co@N–C-25 composite displayed optimal activity, and there was no obvious decrease in catalytic performance, even after 40 h, implying excellent stability for the Zn–Co@N–C catalyst.