Nickel-cobalt oxide nanocages derived from cobalt-organic frameworks as electrode materials for electrochemical energy storage with redox electrolyte

Nickel-cobalt (Ni-Co) oxide nanocages were synthesized by etching the cobalt-based metal-organic framework (Co-MOF) with nickel nitrate in an isopropyl alcohol solution. The dodecahedral Co-MOF (300 nm in size) was composed of micropores, while the Ni-Co oxide nanocages gained additional mesopores and macropores. Both Co-MOF and Ni-Co oxide electrodes in KOH behaved more like the nondiffusion-controlled pseudocapacitor through the surface adsorption/desorption with a concomitant faradaic charge-transfer reaction. In the presence of ferrocyanide/ferricyanide redox electrolyte, both nondiffusion- and diffusion-controlled currents contributed equally to the current response of Ni-Co oxide electrode at lower scan rates. Nondiffusion-limited processes governed the charge-storage behavior of Ni-Co oxide electrode at faster scan rates. The Ni-Co oxide electrode with redox electrolyte showed high specific capacities of 810 and 496 C g−1 at the charge/discharge current densities of 10 and 50 A g−1, respectively, much greater than that electrode without redox electrolyte (384 and 348 C g−1). Ni-Co oxide electrode with highly porous nanocages could provide large electrolyte/electrode interface to store more charges and short pathways to expedite the transport of electron and ion, showing a noticeable increase in its electrochemical performance compared with Co-MOF electrode.