N-doped ZrO₂ nanoparticles embedded in a N-doped carbon matrix as a highly active and durable electrocatalyst for...

Fabricating highly efficient and robust oxygen reduction reaction (ORR) electrocatalysts is challenging but desirable for practical Zn-air batteries. As an early transition-metal oxide, zirconium dioxide (ZrO₂) has emerged as an interesting catalyst owing to its unique characteristics of high stability, anti-toxicity, good catalytic activity, and small oxygen adsorption enthalpies. However, its intrinsically poor electrical conductivity makes it difficult to serve as an ORR electrocatalyst. Herein, we report ultrafine N-doped ZrO₂ nanoparticles embedded in an N-doped porous carbon matrix as an ORR electrocatalyst (N-ZrO₂/NC). The N-ZrO₂/NC catalyst displays excellent activity and long-term durability with a half-wave potential (E_1/2) of 0.84 V and a selectivity for the four-electron reduction of oxygen in 0.1 M KOH. Upon employment in a Zn-air battery, N-ZrO₂/NC presented an intriguing power density of 185.9 mW cm^-2 and a high specific capacity of 797.9 mA h g_Zn ^-1, exceeding those of commercial Pt/C (122.1 mW cm^-2 and 782.5 mA h g_Zn ^-1). This excellent performance is mainly attributed to the ultrafine ZrO₂ nanoparticles, the conductive carbon substrate, and the modified electronic band structure of ZrO₂ after N-doping. Density functional theory calculations demonstrated that N-doping can reduce the band-gap of ZrO₂ from 3.96 eV to 3.33 eV through the hybridization of the p state of the N atom with the 2p state of the oxygen atom; this provides enhanced electrical conductivity and results in faster electron-transfer kinetics. This work provides a new approach for the design of other enhanced semiconductor and insulator materials.