Sustained-Release Method for the Directed Synthesis of ZIF-Derived Ultrafine Co-N-C ORR Catalysts with Embedded Co Quantum Dots

M-N-C catalysts with optimized local and external structures offer great potential for replacing expensive and labile Pt-based catalysts for the oxygen reduction reaction (ORR) in fuel cells. Herein, we report a novel and facile strategy of synthesizing ultrafine ZIF-derived Co-N-C catalysts by precisely controlling the crystallization rate of ZIFs. The employment of meta-soluble Co-doped basic zinc acetate (Co-BZA), which shows a sustained-release effect in solvents, allows for the control of the solubility of Co-BZA in solvents. Detailed investigations suggest that the solubility of Co-BZA in the solvent is the key for governing the grain size of the resulting Zn/Co bimetallic ZIFs. Therefore, the self-assembly process between ligands and metal ions can be regulated by tuning the composition of mixed solvents, thus enabling rational tuning of the grain size of the resulting ZIFs. One-step pyrolysis of the ultrafine Zn/Co bimetallic ZIF precursor leads to Co and N co-doped carbon with an ultrafine grain size (termed UF Co-N-C). The Co centers that are uniformly distributed in the carbon matrix possess a quantum-dot-level grain size. Furthermore, this type of carbon nanohybrid exhibits a hierarchical pore structure, as well as a high surface area. When used as an ORR catalyst, the UF Co-N-C catalyst possesses high ORR activity (with an E_1/2 of 0.9 V) that can rival 20 wt % commercial Pt/C (with an E_1/2 of 0.835 V) in alkaline media. Notably, this catalyst also displays strong ORR performance similar to that of Pt/C in acidic media. The superior durability and methanol tolerance in both alkaline and acidic media for UF Co-N-C compared to Pt/C illustrate its great potential in replacing commercial Pt/C catalysts. The outstanding ORR performance of UF Co-N-C could be attributed to the simultaneous optimization of both external structures and active sites, demonstrating the effectiveness of this strategy in constructing ORR catalysts with controlled structures and desired functionalities.