A N-doped NbOx nanoparticle electrocatalyst deposited on carbon black for oxygen reduction and evolution reactions in alkaline media

Abstract

There is growing interest in novel bifunctional electocatalysts for oxygen reduction (ORR) and evolution reactions (OER), for application in electrochemical energy conversion systems. While ultrafine oxides have been reported as efficient electrocatalysts for the ORR and OER, there are no reports on ultrafine oxides based on the group 4 or 5 metals as a bifunctional oxygen electrocatalyst. Herein, we present and demonstrate N-doped NbOx/CB nanoparticles as a highly active and stable, bifunctional oxygen electrocatalyst in alkaline media. The NbOx nanoparticles approximately 3 nm in size were successfully synthesized on carbon black (CB) by potentiostatic electrodeposition. The employment of a CB support with a large surface area resulted in a loading amount of 20 wt% Nb in the N-doped NbOx/CB and a uniform distribution of the oxide on CB, leading to an increase of the reaction site. The subsequent annealing in an NH3 flow decreased the concentration of oxygen and intercalated a small amount of nitrogen in the as-deposited NbOx particles, largely lowering the overpotentials of the ORR and OER. The surface Nb species on the N-doped NbOx particles were made of multivalent Nb4+ and Nb5+. As a result, the prepared N-doped NbOx/CB nanoparticles exhibited high catalytic activities for both the ORR and OER in a 0.1 M KOH solution. The potential difference, ΔE, indicating an index for the bifunctional oxygen electrocatalyst, was 1.13 V, which was comparable to those of the commercial 20 wt% Pt/CB and Ir/CB. In addition, the current densities obtained for the nanoparticles were significantly steady for both the long-term ORR and OER in alkaline media. Together these results demonstrate the N-doped NbOx/CB nanoparticles as potential bifunctional oxygen electrocatalysts.