Electronic Structure and Reaction Mechanism on Nitrogen-doped Carbon Electrode Catalysts and Design of Catalyst Based on the Mechanism

Nitrogen-doped carbons are plausible candidates for alternative catalysts of platinum-based catalysts for oxygen reduction reaction (ORR). Although nitrogen-doped carbons are highly active in alkaline media, the activities critically decrease under acidic conditions in fuel cells. To overcome the problem, we have investigated the reaction mechanism based on the electronic structures by using model catalysts. On the active site of pyridinic nitrogen (pyri-N), we have reported that O2＋pyri-NH＋＋e－→O2a＋pyri-NH governs the ORR activities, where the adsorbed O2 make a bond with an electron doped into π* orbital of the carbons. In the elementary step, the hydration of pyri-NH＋ decreases the redox potential and resultantly the ORR activity. Therefore, we designed a nitrogen-doped graphene catalysts with higher hydrophobicity to prevent the hydration. To compensate the decreased conductivity, we introduced a proton-conductive particles. Thus, the compatible hydrophobicity and proton conductivity achieved the highest activity as a metal-free ORR catalyst.