▎ 摘　　要

For the future of fuel cells, investigations on high activity, stable, and low-cost oxygen-reduction reaction (ORR) catalysts are critical. Transition metals have attracted particular research interest in pursuit of such efficient and sustainable ORR electrocatalysts. In this work, very tiny Co nanoparticles were assembled on 3D graphene (3DG) aerogels uniformly with the help of ascorbic acid (AA) under a facile hydrothermal process. After further thermal annealing in argon, a few carbon layers are formed and wrapped on the Co nanoparticles. The synthesized materials were primarily investigated for ORR, and the experimental results indicate that the as-prepared Co/3DG shows an obviously positive onset potential (-0.01 V) and a much higher limiting current density (5.75 mA/cm(2)) than Co/3DG prepared without using AA (-0.1 V, 4.6 mA/cm(2)). Compared with commercial 20%Pt/C, Co/3DG prepared by using AA also displayed excellent durability and superb methanol tolerance performance, which suggests that the special structure of cobalt nanoparticles wrapped with a few carbon layers and its porous 3DG aerogel support can prevent its expansion and dissolution, improve its stability largely, and provide a new way for searching excellent ORR electrocatalysts.