Size-controlled Co/CoO heterogeneous nanoparticles confined in N-doped mesoporous carbon for efficient oxygen reduction in zinc-air batteries.

The size of metal compound particles and interface electronic structure of heterojunctions in a matrix greatly affect oxygen reduction performance in zinc-air batteries. However, it is still a big challenge to precisely control or regulate the size of these metal compound particles and the heterojunction structure. Herein, cobalt complexes with different-sized ligands are chosen as cobalt resources and adsorbed onto a mesoporous carbon, after which they are coated with polydopamine and calcined. Under the confinement effect of mesopores and the isolation effect of ligands and dopamine, the as-obtained Co/CoO heterogeneous nanoparticles are restricted to nano-size and uniformly dispersed in N-doped carbon (NC). The sizes of Co/CoO are estimated to be 39.7, 24.9 and 15.6 nm with increased CoO contents, corresponding to the adopted cobalt precursors of Co(OAc), Co(acac) and Co(acac), respectively. The smallest Co/CoO/NC-S shows excellent catalytic activity for oxygen reduction reaction, with a half-wave potential of 0.82 V vs. RHE and a limiting current density of 4.59 mA cm. When applied to the cathode of zinc-air battery, a high peak power density of 131.9 mW cm is achieved, which surpasses that of the battery powered by Pt/C. The excellent performance can be attributed to the formation of heterogeneous structures between Co and CoO, the smaller Co/CoO nanoparticles, and N-doped mesoporous carbon with effective charge/mass transport. This work provides an effective way to regulate the size and phase contents of heterogeneous particles in mesoporous carbon, which is highly valuable in electrocatalytic systems.