Carboxylate-assisted ZIF-derived Co nanoclusters anchoring hierarchically porous carbon as high-efficient zinc-air batteries cathode catalysts

Abstract

Metal and nitrogen-doped carbon (M-N-C) catalysts are generally considered to be ideal substitutes for platinum-based noble metal oxygen reduction catalysts. Owing to the significant Fenton effect of the Fe element and the subsequent instability, the Fenton-free Co-based N–C catalyst should perform better in ORR. Pyrolysis of metal-organic frameworks (MOFs) is commonly used to make Co-based N–C catalysts. Still, most of the MOF-derived catalysts reported so far have a microporous structure, which results in hindered mass transfer and partial Co-Nx active sites that cannot function as catalysis. Herein, Co nanoclusters (NCs) with high loading (7.96 wt%) were prepared on hierarchically porous carbon substrates via a carboxylate-assisted strategy. The mesoporous structure of Co nanoclusters/nitrogen-doped hierarchically porous carbon (Co NCs/HPNC) can facilitate mass transport. The smaller size Co nanoclusters can increase the contact area between the carbon matrix and Co-Nx active sites to effectively expose more active centers and increase the density of Co-Nx active sites. A synergistic effect of the two can greatly improve ORR results. Surprisingly, the Co NCs/HPNC catalyst shows excellent ORR activity under alkaline conditions, and Co NCs/HPNC has a better half-wave potential (0.88 V vs. 0.84 V) compared with 20 wt% Pt/C. And it only loses 5% of its current density after 36,000 s of operation, which is significantly less than Pt/C (20%). Furthermore, Co NCs/HPNC battery have a greater open-circuit potential, power density, and specific capacity than Pt/C-based zinc-air batteries (1.51 V, 109.6 mW cm−2, 818.1 mAh gZn−1 vs. 1.46 V, 92.4 mW cm−2, 716.8 mAh gZn−1). This discovery paves the way for high-efficiency electrocatalysts with hierarchically porous architectures and a large number of metal loads to be developed.