Thermally Converted CoO Nanoparticles Embedded into N-Doped Carbon Layers as Highly Efficient Bifunctional Electrocatalysts for Oxygen Reduction and Oxygen Evolution Reactions

Abstract

Hybrid materials that consist of transition-metal oxides and heteroatom-doped carbon materials have been researched recently as promising bifunctional electrocatalysts for both oxygen-reduction reaction (ORR) and oxygen-evolution reaction (OER) in alkaline media. Herein, CoO nanoparticles embedded into N-doped carbon layers were synthesized by a thermal conversion process of polypyrrole-coated Co3O4 nanoparticles supported on a carbon layer in Ar atmosphere at 900 degrees C. During the process, the initial Co3O4 phase was transformed to the CoO phase along with the thermal carbonization of the polypyrrole layer to the N-doped carbon layer. Owing to the oxidative combustion induced by the O species released from the Co3O4 nanoparticles, the N-doped carbon layer could contain pores around the CoO nanoparticles. Alkaline electrolytes could penetrate the N-doped carbon layer toward the CoO nanoparticles through the pores. The nanocomposites with the well-assembled CoO nanoparticles and porous N-doped carbon layer could exhibit superior catalytic activity for ORR and OER. In addition, the N-doped carbon layers effectively prevent the degradation of the catalyst by protecting the CoO nanoparticles from aggregation during the electrocatalytic processes. The hybrid material of CoO and N-doped carbon showing highly active and durable catalytic characteristics for ORR and OER is a promising electrocatalyst in fuel cells, metal-air batteries, and water-splitting systems and could be used instead of precious metals such as Pt, Ru, and Ir.