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 NbO_x/CB nanoparticles as a highly active and stable, bifunctional oxygen electrocatalyst in alkaline media. The NbO_x 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 NbO_x/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 NbO_x particles, largely lowering the overpotentials of the ORR and OER. The surface Nb species on the N-doped NbO_x particles were made of multivalent Nb⁴⁺ and Nb⁵⁺. As a result, the prepared N-doped NbO_x/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 NbO_x/CB nanoparticles as potential bifunctional oxygen electrocatalysts.