Hierarchically porous Co-N-C catalysts from bimetallic Zn/Co zeolitic imidazolate frameworks for selective CO₂...

The conversion of CO₂ to CO using electrochemistry requires the use of earth abundant catalysts with high selectivity, rapid reaction rate, and durability. In this work, hierarchically porous Co-N-C catalysts were synthesized via co-pyrolysis of a bimetallic zeolitic imidazolate framework (Zn/Co) and the dicyandiamide (DCD) followed by acid etching. By systematically adjusting the pyrolysis temperature (800 °C-1,000 °C) and the Co feed ratio (5-20 mol%), volcano-type activity relationships are found, which are controlled by the interplay of the three factors graphitization, nitrogen retention, and the density of Co-N_x sites. The optimized catalyst CoNC-900 achieves a Faradaic efficiency of CO of 94.9% ± 1.0% at -0.75 V vs. RHE in CO₂-saturated 0.5 M KHCO₃ with a CO partial current density of 16.0 mA cm^-2, a lower bound turnover frequency of ≥0.33 s^-1 and ≈11% current decay FE_CO retained above 92% over 24 h. The observed selectivity can be rationalized, as the density functional theory calculations indicate that the barrier of the rate-determining step, namely the formation of COOH is lowered on the Co-N₄ moiety (0.42 eV, whereas ≥1.79 eV on the metal-free nitrogen sites). A transferable design rule is developed for CO₂ electrolysis that relates bimetallic MOF precursor chemistry, hierarchical porosity, and M-N_x site engineering.