Enhancement of Bifunctional Activity of the Hybrid Catalyst of Hollow-Net Structure Co3O4 and Carbon Nanotubes

Abstract

We demonstrate the noble metal-free hybrid catalysts of cobalt oxide and carbon nanotubes (CNTs) as a low-cost, active, and stable bifunctional electrocatalyst for both the oxygen reduction (ORR) and evolution reaction (OER) of unitized regenerative fuel cells. A bifunctional catalyst is synthesized hydrothermally by coating hollow-net structure Co3O4 onto various lengths of CNTs. For further improvements of the bifunctional performance of hybrid catalysts, an oxidation approach is adopted to introduce sufficient oxygen-containing groups onto the surface of CNTs in a KMnO4 solution. The chemically treated Co3O4/CNTs catalysts supply 10 mA cm−2 at an overpotential (ηover) of 411 mV with strong OER activity and exhibit comparable OER performance with the highly active Ir/C catalyst (ηover: 410 mV). For the ORR activity, the Co3O4/CNTs catalysts show on-set and half-wave potentials around 0.84 V and 0.66 V, respectively. The Co3O4/ox-CNTs has an oxygen electrode activity of 1.044 V and compares favorably to the other transition metal-based nanostructures and precious metal electrocatalysts. Furthermore, the hybrid Co3O4/CNTs catalysts show excellent durability for long periods of OER and ORR conditions (by the potential cycling). This excellent bifunctional performance is attributed to the enhanced incorporation of conducting CNT frameworks between neighboring Co3O4 nanoparticles during chemical functionalization.