Recently, metal-organic framework (MOF) materials have been studied as cathode materials for Li‒O2 batteries due to their high catalytic activity, uniform structure, and simple preparation process. However, further optimization of materials and structures should be undertaken to improve their electrochemical performance for the practical application.1-3 Here we report a uniform MOF-derived multiple-capsule heterostructure, nitrogen-doped cobalt@graphene, as the cathode catalysts for Li‒O2 cells. This N-Co@graphene multiple-nanocapsule structure improves the electrical properties and makes the high/uniform electroactive zones possible. In addition, the rational design of flexible foam-based electrodes possessing colander-like porous structures facilitates the oxygen diffusion, catalytic reaction, and deposition of discharge products. The electrode thus exhibits much improved electrochemical performance associated with unique morphologies of electrochemically grown lithium peroxides.
Figs. 1a-c display the microstructure and morphology of N-Co@graphene nanocomposite. The composite has a lotus shower-like shape, where the crystalline Co nanoparticles, sized of 10‒20 nm, display highly ordered lattice fringes, being wrapped by few-layer graphenes, thus forming core-shell nanostructures. Note that there are few layers of specific lattice fringes with 0.25 nm lattice spacing existing on the surface of Co particles, which are indexed to be cobalt nitride that are generated from the nitrogen dopant into surficial lattices of Co crystalline. Fig. 1d show its EELS profile, which confirms the presence of C, N, and Co elements in the composite. Figs. 1e, f show its electrochemical performance in Li‒O2 cells. The composite electrodes exhibit reduced charge-discharge overpotential and good cycling stability. This finding is mainly attributed to the improved electrical properties, electrocatalytic activity, and structural stability of N-Co@graphene nanostructures.
Fig. 1 Structural and electrochemical characterization of MOF-derived N-Co@graphene catalysts. (a-c) TEM images, and (d) EELS profile of N-Co@graphene nanocomposite. (e,f) Voltage profiles versus selected cycles of N-Co@graphene composite based on carbon fiber (e) and nickel foam (f) electrodes.
1. Li, Q.; Xu, P.; Gao, W.; Ma, S.; Zhang, G.; Cao, R.; Cho, J.; Wang, H. L.; Wu, G. Adv. Mater. 2014, 26, 1378-1386.
2. Wu, D.; Guo, Z.; Yin, X.; Pang, Q.; Tu, B.; Zhang, L.; Wang, Y. G.; Li, Q. Adv. Mater. 2014, 26, 3258-3262.
3. Tan, G.; Chong, L.; Amine, R.; Lu, J.; Liu, C.; Yuan, Y.; Wen, J.; He, K.; Bi, X.; Guo, Y.; Wang, H.-H.; Shahbazian-Yassar, R.; Hallaj, S. A.; Miller, D. J.; Liu, D; Amine, K. Nano Lett. 2017, DOI: 10.1021/acs.nanolett.7b00207.