Carbon catalysts derived from ZIF-8: Joule heating vs. furnace heating

Carbon catalysts synthesized using metal-organic frameworks are promising for various applications. However, their carbonization process is still poorly understood. Here, we systematically studied carbon catalysts derived from ZIF-8 using the standard furnace heating (FH) and a new Joule heating (JH) method. The JH method produced carbon catalysts with significantly higher catalytic activity for oxygen reduction reaction (ORR). Statistical analysis based on the correlation between different structural features and performance descriptors of the nitrogen-doped carbon catalysts identifies the carbon atoms bonded to graphitic-N as their ORR active sites. Surprisingly, the abundance of N species in JH-derived carbon catalysts is only half that of FH-derived carbon catalysts, indicating lower active site density. However, further evaluation revealed that the intrinsic activity of individual active sites in the JH-derived carbon catalysts with a higher graphitization degree is more than twice that of the FH-derived carbon catalysts. The enhanced intrinsic catalytic activity of these active sites outpaces the reduction in their numbers, resulting in overall better activity. Theoretical calculations indicate that the rate-determining step on these active sites is the conversion of OH∗ to H 2 O with a lower energy barrier. Our findings reveal the synergistic effect of heteroatom coordination environment and graphitization degree of carbon structures in determining the catalytic performance of carbon catalysts while featuring JH as a new approach to realize high-performance carbon catalysts through simultaneously regulating these properties.