(Invited) Oxygen Reduction Reactivity at Fe- and Cu-Codoped Carbon Nanostructures

Abstract

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).