Composition and morphology transition of NF/MnP/NiCoP composite electrode induced by charge/discharge activation

Electrochemical activation of metal phosphides presents an important significance in fabricating self-activated supercapacitors. To explore the charge/discharge activation mechanism of phosphides, a self-supporting nickel foam (NF)/MnP/NiCoP composite is prepared, in which, NiCoP nanoneedles are depositing on porous MnP nanosheets. Combining in-situ Raman testing and ex-situ characterization techniques, the composition and morphology transitions of NF/MnP/NiCoP are observed at different charge/discharge cycles, and corresponding activation mechanism is put forward. In the KOH electrolyte, metal phosphides transform into Ni, Co, Mn-hydroxides rapidly, which is mostly completed in the first two cycles. With an increase of charge/discharge time, the microstructure of hydroxides tends to ordering. The flower-like microspheres observed at the 1270th cycle are considered as the optimum morphology, which delivers the highest specific capacitance of 1367F g(-1) at 5 A/g, 1.566 times higher than initial capacitance (873F g(-1)). After the maximum performance, the activated composite is similar to conventional Ni, Co, Mn-hydroxides. The specific capacitance gradually decays in the following cycling test, accompanied with the destruction of microstructure. When assembled in an asymmetric supercapacitor with NF/active carbon (AC) negative electrode, NF/MnP/NiCoP composite also exhibits a charge/discharge activation phenomenon. The maximum capacitance is obtained at 2000th cycle and the capacitance retention rate is 103.2 % for 5000 cycles.