Capacitive behaviour of MnF₂ and CoF₂ submicro/nanoparticles synthesized via a mild ionic liquid-assisted route

Submicro-/nano-sized MnF₂ rods and hierarchical CoF₂ cuboids are respectively synthesized via a facile precipitation method assisted by ionic liquid under a mild condition. The as-prepared MF₂ (M = Mn, Co) submicro/nanoparticles exhibit impressive specific capacitance in 1.0 M KOH aqueous solution, especially at relatively high current densities, e.g. 91.2, 68.7 and 56.4 F g^-1 for MnF₂, and 81.7, 70.6 and 63.0 F g^-1 for CoF₂ at 5, 8 and 10 A g^-1, respectively. The mechanism of striking capacitance of MF₂ is clarified on the basis of analysing the cycled electrodes by different characterization techniques. Such remarkable capacitance is ascribed to the redox reactions between MF₂ and MOOH in aqueous alkaline electrolytes, which can not be obtained in aqueous neutral electrolytes. This study for the first time provides direct evidences on the pseudocapacitance mechanism of MF₂ in alkaline electrolytes and paves the way of application of transition metal fluorides as electrodes in supercapacitors.