The oxygen reduction reaction (ORR) is a key reaction in polymer electrolyte fuel cells and metal-air batteries. In these electrochemical systems, platinum group metals (PGMs) have been widely used as ORR electrocatalysts. Because of material costs and scarcity of platinum group metals, non-PGM electrocatalysts are an ideal alternative for mass production with low material costs. Many non-PGM electrocatalysts have been intensively studied such as pyrolyzed Fe-, N-doped carbon (Fe-N-C) catalysts. However, many non-PGM electrocatalysts including Fe-N-C still suffer from product selectivity: the production of H2O2 as the byproduct. In this work, we synthesized an ORR electrocatalyst of Cu-, Fe-and N-doped carbon nanotubes, (Cu,Fe)-N-CNT. This heterobimetallic catalyst showed the selective 4ereduction of O2 to H2O with ca. 99%. Kinetic analysis of the electrocatalytic ORR and hydrogen peroxide reduction reaction (HPRR) in acidic media revealed that (Cu,Fe)-N-CNT showed two orders of magnitude higher rate constants for the direct 4ereduction of O2 to H2O than those for the 2ereduction of O2 to H2O2 whereas a monometallic Fe-N-CNT showed the same order of magnitude, indicating that the heterometallic cooperativity gave the drastic impact on the ORR kinetics. Our findings would open up possibilities to develop non-PGM ORR electrocatalysts with heterobimetallic active sites for the selective ORR. KEYWORDS. Non-PGM, oxygen reduction reaction, electrocatalysis, polymer electrolyte fuel cell, oxygen reduction kinetics, bio-inspired approach, heterobimetallic active sites.

Polymer electrolyte fuel cells (PEFCs), which can efficiently convert hydrogen into electricity without using fossil fuels, are a promising energy conversion device. At the cathodes of PEFCs, oxygen reduction reaction (ORR) occurs via 4-electrons' and 4-protons' transfer. Due to its low reaction rate limiting the performance of PEFCs, the electrocatalysts are necessary and the platinum-based electrocatalysts for ORR are mainly used because of its highest electrocatalystic activity in the metals. Since platinum metal is expensive and rare, there is a need to develop non-platinum-based electricatalysts based on iron and/or copper with high ORR activity1,2,3,4). A cytochrome c oxidase (CcO), which is a metalloenzyme and contains iron and copper metal ions, catalyzes the ORR efficiently to produce ATP in mitochondria5) and its activity is higher than that of artificial platinum-based electrocatalysts6). Metalloenzymes are not practical because of their stability, size, and difficulty to isolate and purify. In this study, iron and copper co-doped carbon electrocatalysts, synthesized from the mixtures of Fe and/or Cu complexes and carbon nanotubes in heat treatment inspired by the CcO’s active center, were developed. Synthetic conditions of Fe : Cu metal ratios and heating temperatures were optimized. The ORR activity was evaluated by hydrodynamic voltammetry using a rotating ring disk electrode in 0.05 M sulfuric acid aqueous solution saturated with oxygen. The activity of Fe/Cu/CNT catalyst was compared with two catalysts: iron-doped carbon carbon catalyst (Fe/CNT) and copper-doped carbon catalyst (Cu/CNT). The Fe/Cu/CNT catalyst showed more positive onset potential for the oxygen reduction in acidic media than Fe/CNT and Cu/CNT, indicating that Fe/Cu/CNT is more active, and we figure out that the coexistence of iron and copper in CNTs contributes to the improvement of ORR activity. References: 1. Kato et al., Chem. Lett. 45, 1213–1215 (2016). 2. Kato et al., ACS Appl. Energy Mater. 1, 2358–2364 (2018). 3. Chung et al., Science 357, 479–484 (2017). 4. Yasuda et al., Adv. Funct. Mater. 26, 738–744 (2016). 5. Sakamoto et al., PNAS 108, 12271–12276 (2011). 6. Kjaergaard et al., Inorg. Chem. 49, 3567–3572 (2010).

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. References [1] M. Kato, I. Yagi, e-J. Surf. Sci. Nanotechnol., 18, 81 (2020). [2] M. Kato, T. Murotani, I. Yagi, Chem. Lett., 45, 1213 (2016). [3] 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). [4] M. Kato, N. Fujibayashi, D. Abe, N. Matsubara, S. Yasuda, I. Yagi, ACS Catal., 11, 2356 (2021).