Microporous/mesoporous cobalt hexacyanoferrate nanocubes for long-cycle life asymmetric supercapacitors

Abstract

Metal hexacyanoferrates with both microporosity and mesoporosity are highly desirable for supercapacitors because efficient transport pathways of metal ions can be provided by mesopores. Here, we have successfully synthesized microporous/mesoporous cobalt hexacyanoferrate (CoHCF) nanocube by tuning the molar ratios of reactants. The CoHCF nanocube is characterized by X-ray diffraction spectroscopy, Fourier transform infrared spectroscopy, scanning electron microscopy, transmission electron microscopy, N2 adsorption/desorption and electrochemical techniques. The results show that the K3[Fe(CN)6]/CoCl2 molar ratios have great influence on texture, morphology and electrochemical performance. The CoHCF nanocube with the average size of around 250 nm has a Brunauer–Emmett–Teller specific surface area of 572 m2 g−1. And the material possesses a specific capacitance as high as 441 F g−1 at 1 A g−1 in 1 M Na2SO4 electrolyte. In addition, an asymmetric supercapacitor is assembled by using the CoHCF nanocube as the positive electrode and activated carbon as the negative electrode. The device exhibits good supercapacitor performance with a specific capacitance of 61.8 F g−1 (1 A g−1) and an energy density of 12.9 Wh kg−1 (632 W kg−1). Long-term cycling stability tests demonstrate that the capacity retains 91% after 6000 charge–discharge cycles.