Facile Synthesis of Nanostructured Binary Ni-Cu Phosphides as Advanced Battery Materials for Asymmetric Electrochemical Supercapacitors

Transition-metal phosphides (TMPs) enjoy metalloid characteristics with good electrical conductivity, making them potential candidates for electrochemical supercapacitors. However, TMPs are difficult to synthesize by conventional methods, limiting their practical use in a plethora of applications. Herein, we demonstrate the successful fabrication of Ni-Cu binary phosphides (NCP) via a one-step, facile solvothermal method. More importantly, the correlation between the degree of phosphidation and the electrochemical behavior of the material is explored and discussed. The NCP electrode exhibited a battery-like behavior with an ultrahigh specific capacitance (C-s) of 1573 F g(-1) at 1 A g(-1). Upon use as a positive electrode, it showed superior performance in a hybrid supercapacitor device with bioderived activated carbon (BAC) as the negative electrode (NCP//BAC), providing a high energy density of 40.5 W h kg(-1) at 875 W kg(-1) with exceptional capacity retention after 10,000 cycles. These values are 4 times higher than that of commercial supercapacitors (10-12 W h kg(-1)), suggesting the unique supercapacitance performance of the NCP//BAC device compared to the phosphide-based devices reported so far.