Manganese hexacyanoferrate/MnO2 composite nanostructures as a cathode material for supercapacitors

A composite of manganese hexacyanoferrate (MnHCF) coated by an amorphous manganese dioxide layer was synthesized by a facile co-precipitation method and a further step called "deep electro-oxidation". The structure and components of the resulting MnHCF/MnO2 composites were characterized by scanning electron microscopy, transmission electron microscopy, X-ray diffraction and X-ray photoelectron spectroscopy. Electrochemical testing showed a capacitance of 225.6 F g(-1) at a sweep rate of 5 mV s(-1) within a voltage range of 1.3 V and a high energy density of 74.5 W h kg(-1) at a current density of 0.5 A g(-1) during galvanostatic charge/discharge cycles, which is superior to most cathode materials, including some reported graphene/MnO2 nanocomposites. It is confirmed that the two components, manganese hexacyanoferrate and manganese dioxide, lead to an integrated electrochemical behavior and a capacitor with enhanced performance. The electrochemical testing and corresponding XPS analysis also demonstrated that the manganese coordinated by cyanide groups via nitrogen atoms in MnHCF did not get involved in the charge storage process during potential cycles.