Co-Doping Effects of Iron and Copper into Carbon Nanotubes on Oxygen Reduction Reaction Activity

Abstract

Polymer electrolyte fuel cells (PEFCs), which can efficiently convert hydrogen into electricity without using fossil fuels, are a promising energy conversion device. In cathodes of PEFCs, oxygen reduction reaction (ORR) occurs and its low reaction rate limits the performance of PEFCs. Therefore, the electrocatalysts for the ORR are necessary. Currently, platinum-based electrocatalysts for the ORR are mainly used because platinum shows the highest electrocatalytic activity in the metals. Since platinum metal is expensive and rare, to make the fuel cells more widely used, there is a need to develop non-platinum-based electrocatalysts 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 and difficulty in isolation and purification. In this study, we developed iron-copper co-doped carbon electrocatalysts (Fe/Cu/CNT) synthesized from the mixtures of Fe and/or Cu complexes and carbon nanotubes in heat treatment inspired by the CcO’s active center. 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 electrocatalyst was compared with two electrocatalysts: iron-doped carbon electrocatalyst (Fe/CNT) and copper-doped carbon electrocatalyst (Cu/CNT). The Fe/Cu/CNT catalyst showed more positive onset potential for the ORR than Fe/CNT and Cu/CNT, indicating that Fe/Cu/CNT is more active and the coexistence of iron and copper in CNTs contributes to the improvement of ORR activity. References Kato et al., Chem. Lett., 45, 1213–1215 (2016).Kato et al., Appl. Energy Mater., 1, 2358–2364 (2018).Chung et al., Science, 357, 479–484 (2017).Yasuda et al., Adv. Funct. Mater., 26, 738–744 (2016).Sakamoto et al., PNAS, 108, 12271–12276 (2011).Kjaergaard et al., Inorg. Chem. 49, 3567–3572 (2010).