(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.