(Invited, Digital Presentation) Cooperative Effects of Fe and Cu Sites in N-Doped Carbon Nanotubes on Oxygen Reduction Activity and Selectivity

Abstract

The oxygen reduction reaction (ORR) is a key reaction in polymer electrolyte fuel cells (PEFCs) and metal–air batteries. Although platinum group metal (PGM)-based ORR electrocatalysts have been widely used in state-of-the-art PEFCs, PGMs are rare and expensive. For future wide-spread applications of PEFCs, non-PGM electrocatalysts with high ORR activity and product selectivity to H2O must be developed. Natural catalysts of metalloenzymes including laccases and cytochrome c oxidases are known to catalyze the ORR and therefore encourage us to develop metalloenzyme-inspired electrocatalysts based on non-PGMs for the ORR because these metalloenzymes use iron and/or copper ions as active sites [1-3].We develop non-PGM electrocatalysts of Cu- Fe- and N-doped carbon nanotubes, (Cu,Fe)-N-CNT, for the ORR, inspired by the heterometallic active site of cytochrome c oxidase [4]. The co-presence of Cu and Fe active sites increase the ORR activity and selectivity to H2O in acidic media, compared with monometallic Fe-N-CNT or Cu-N-CNT. Kinetic analysis revealed that the selective 4-electron reduction of O2 to H2O is dominant for (Cu,Fe)-N-CNT whereas the sequential (2+2)-electron reduction mainly proceeds for Fe-N-CNT or Cu-N-CNT, indicating that the ORR mechanism can be modulated by the co-presence of Cu and Fe active sites. We will discuss details on the ORR mechanistic insights based on spectroscopic data including in situ X-ray absorption spectroscopy and Mossbauer spectroscopy.