Structure optimization of ZIF-12-derived Co-N-C for efficient oxygen reduction and oxygen evolution

The rational design and preparation of bifunctional electrocatalysts with high catalytic performance for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are extremely crucial for the development of fuel cells and regenerative metal-air cells. In this work, a simple compound-assisted pyrolyzing strategy is reported to synthesize Co-N-C (CoNC-xcompound-800) materials by high-temperature annealing of ZIF-12. Results show that the physicochemical properties of the obtained CoNC-xcompound-800 are dependent on the type of the assistant compound. With the introduction of organic compound (i.e., C3H6N6, C6H14O6, C16H32O2, CH3(CH2)(16)COOH, C6H5NO3), the specific surface area, the carbon defect content, the content of pyridinic-N and graphitic-N, and the Co-N-x concentration of the obtained CoNC-0.5compound(organic)-800 are greatly increased, which result in the improved activities of the 0.5compound(organic)-800 samples. Differently, with the addition of inorganic compound (i.e., Na3PO4, KCl, K2CO3), the carbon defect content, the content of pyridinic-N and graphitic-N, and the Co-N-x concentration of the obtained CoNC-0.5compound(inorganic)-800 are greatly decreased, which lead to reduced activities of the 0.5compound(inorganic)-800 samples. Among these materials, the CoNC-0.5C(6)H(14)O(6)- 800 shows satisfactory ORR performance with a half-wave potential of 0.89 V. Meanwhile, CoNC-0.5C(6)H(14)O(6)- 800 exhibits an optimal OER performance with an overpotential being only 425 mV at current density of 10 mA cm(-2). Our work provides a simple and universal method to adjust the physicochemical properties of the ZIF-12-derived Co-N-C materials. Besides, the results demonstrate that optimal pore structure, higher carbon defect content, higher content of pyridinic-N and graphitic-N, and higher Co-N-x concentration are of great importance for the improvement of the electrochemical performances of the Co-N-C catalysts.

Automated summary

The rational design and preparation of bifunctional electrocatalysts with high catalytic performance for both ORR and OER are crucial for the development of fuel cells and regenerative metal-air cells. In this work, a simple compound-assisted pyrolyzing strategy is reported to synthesize Co-N-C materials by high-temperature annealing of ZIF-12, and the results demonstrate the importance of optimal pore structure, higher carbon defect content, and higher Co-N-x concentration for improving the electrochemical performances of the Co-N-C catalysts.