(Invited) Effects of Nano-Structure on the Kinetics of the Oxygen Reduction over N-Doped Carbon

Abstract

Nitrogen-doped carbon catalysts for the electrochemical oxygen reduction reaction (ORR) have great potential to substitute precious metal alloy catalysts for the electrochemical synthesis of H2O2 based on a fuel cell setup. Consequently, obtaining a kinetic understanding of nitrogen-doped carbon catalysts in acidic media is essential. Our research group has established a mathematically and experimentally modified RRDE approach to calculate the corresponding kinetic rate constants, which significantly reduced the calculation noise by avoiding a rotation speed study. Furthermore, the overestimation of the 4-e reduction process, which is related to the (2+2)-e pathway (k 2 + k 3) in the catalyst layer matrix, was corrected by studying the effect of the catalyst loading density. The established method was successfully applied to the ORR over Fe/N/C and N/C catalysts in acidic and alkaline media (Table 1). The results indicate that Fe/N/C catalysts follow mixed reduction pathways of 4-e and 2-e reductions in acidic media, and a pure 4-e reduction pathway in alkaline media, whereas the N/C catalyst exhibits a pure 2-e reduction pathway in acidic media, and mixed reduction pathways of 4-e and 2-e reductions in basic media. Among the catalysts mentioned above, the N/C catalyst is the most promising for the electrochemical synthesis. To minimizing the effect of k 3, introducing mesoporous structure would be beneficial, because the H2O2 produced by k 2 can be smoothly eliminated without the further reduction to H2O by k 3 (Figure 1). In this context, we have synthesized a N-doped mesoporous carbon by pyrolyzing a N and C source in a mesoporous silica, KIT-6, as a hard-template. This synthesis resulted in a mesoporous carbon with a gyroid structure. The electrochemical study has revealed that this catalyst exhibits a quite high selectivity over 90 % to the 2-e reduction in acidic media. Figure 1