O,N-Codoped 3D graphene hollow sphere derived from metal–organic frameworks as oxygen reduction reaction electrocatalysts for Zn-air batteries

Abstract

Although Pt-based oxygen reduction reaction (ORR) catalysts have excellent performance, they are expensive and suffer from poor durability. It is necessary to explore carbon-based ORR electrocatalysts with low cost, high specific surface area, large porosity, and strong chemical stability. Herein, we have synthesized a zinc-based metal–organic framework precursor (Zn-BTC) using a simple solvothermal method. Then, carbonization and N doping have been carried out by means of high-temperature pyrolysis, ultimately affording metal-free 3D hollow spherical O and N dual-doped graphene framework composites (O,N-graphene) with an average diameter of about 4 μm and specific surface area as high as 1801.4 m² g⁻¹. O,N-Graphene has superior ORR electrocatalytic activity with an onset potential E_onset = 1.01 V vs. RHE and a half-wave potential E_1/2 = 0.842 V vs. RHE, which are comparable with commercial 20 wt% Pt/C with a 4-electron reduction process. The O,N-graphene catalyst shows better durability and methanol tolerance at a lower cost than commercial 20 wt% Pt/C. The peak power density of O,N-graphene as the cathode of a traditional Zn-air battery is 152.8 mW cm⁻², which is higher than that of a commercial 20 wt% Pt/C battery (119.8 mW cm⁻²). Our findings indicate that synergy among the 3D hollow structure, large specific surface area, highly conductive graphene framework, and pyridine N and graphite N defects left in O,N-graphene accelerated O₂ diffusion and increased catalytically active sites, thereby affording superior ORR and improved Zn-air battery performance under alkaline conditions.