Nanocomposites based on hierarchical porous carbon fiber@vanadium nitride nanoparticles as supercapacitor electrodes

In this study, a hybrid electrode material for supercapacitors based on hierarchical porous carbon fiber@vanadium nitride nanoparticles is fabricated using the method of phase-separation mediated by the PAA-b-PAN-b-PAA tri-block copolymer. In the phase-separation procedure, the ionic block copolymer self-assembled on the surface of carbon nanofibers, and is used to adsorb NH₄VO₃. Thermal treatment at controlled temperatures under an NH₃ : N₂ atmosphere led to the formation of vanadium nitride nanoparticles that are distributed uniformly on the nanofiber surface. By changing the PAN to PAA-b-PAN-b-PAA ratio in the casting solution, a maximum specific capacitance of 240.5 F g^-1 is achieved at the current density of 0.5 A g^-1 with good rate capability at a capacitance retention of 72.1% at 5.0 A g^-1 in an aqueous electrolyte of 6 mol L^-1 KOH within the potential range of -1.10 to 0 V (rN/A = 1.5/1.0). Moreover, an asymmetric supercapacitor is assembled by using the hierarchical porous carbon fiber@vanadium nitride as the negative electrode and Ni(OH)₂ as the positive electrode. Remarkably, at the power density of 400 W kg^-1, the supercapacitor device delivers a better energy density of 39.3 W h kg^-1. It also shows excellent electrochemical stability, and thus might be used as a promising energy-storage device.