Oxygen reduction reaction (ORR) is a key reaction in future energy generation devices such as polymer electrolyte fuel cells (PEFCs) and metal–air batteries. To drive the ORR in state-of-the-art PEFCs, platinum group metal (PGM) alloys have been used. Since platinum is rare and expensive, there is a need to develop highly active PGM-free ORR electrocatalysts for wide spread applications of PEFCs. In nature, catalytic active sites based on 3d transition metals are widely utilized as catalytic reaction centers in metalloenzymes: for example, laccases and cytochrome c oxidases are known to catalyze the ORR with almost no energy loss at the active sites of multinuclear metal complexes.^[1,2]

We present ORR electrocatalysts with multinuclear copper active sites incorporated in nitrogen-doped carbon nanosheets, which were inspired by the active site of laccases. Our copper-based electrocatalyst exhibited high ORR activity in alkaline aqueous solution. Physicochemical measurements including in situ X-ray absorption spectroscopy^[3] revealed that multinuclear copper sites were embedded in nitrogen-doped carbon nanosheets and worked as the catalytic active center.^[4] We also prepared a Fe–Cu-codoped carbon electrocatalyst inspired by the active site of cytochrome c oxidase.^[5] Optimization of synthetic conditions and doping ratios of Fe to Cu into carbon resulted in the improvement of the ORR activity in acidic media. The electrocatalyst codoped with Fe and Cu showed higher ORR activity than those with Fe or Cu. Our bioinspired approaches will offer guidance for developing non-PGM ORR electrocatalysts with high activity.

References

[1] N. Mano, V. Soukharev, A. Heller, J. Phys. Chem. B, 110, 11180 (2016).

[2] C. H. Kjaergaard, J. Rossmeisl, J. K. Nørskov, Inorg. Chem. 49, 3567 (2010).

[3] M. Kato, K. Kimijima, M. Shibata, H. Notsu, K. Ogino, K. Inokuma, N. Ohta, H. Uehara, Y. Uemura, N. Oyaizu, T. Ohba, S. Takakusagi, K. Asakura, I. Yagi, Phys. Chem. Chem. Phys., 17, 8638 (2015).

[4] M. Kato, M. Muto, N. Matsubara, Y. Uemura, Y. Wakisaka, T. Yoneuchi, D. Matsumura, T. Ishihara, T. Tokushima, S. Noro, S. Takakusagi, K. Asakura, I. Yagi, ACS Appl. Energy Mater., 1, 2358 (2018).

[5] M. Kato, T. Murotani, I. Yagi, Chem. Lett., 45, 1213 (2016).