One-pot synthesis of porous nickel cobalt sulphides: tuning the composition for superior pseudocapacitance

Abstract

Nickel cobalt sulphides with different stoichiometric nickel and cobalt contents have been synthesized and used as pseudocapacitive materials for supercapacitors. The as-employed polyol method is robust enough to use a one-pot synthesis of nickel cobalt sulphides with similar porous morphology and crystal structure at different nickel and cobalt ratios. The electrochemical performance of the nickel cobalt sulphides can be easily tuned by the varying the Ni and Co content. Owing to the combined contributions from both Ni and Co ions, the bimetallic Ni–Co sulphides show a superior pseudocapacitance compared to monometallic Co and Ni sulphides in terms of high specific capacitance, excellent rate capability and long cycle stability. In particular, the Ni_1.5Co_1.5S₄ sample shows the highest specific capacitance of 1093 F g⁻¹ at 1 A g⁻¹, superior rate capability of 69% capacitance retention after a 50-fold increase in current densities, and longer cycle stability with increased specific capacitance of 108% of capacitance retention after 2000 cycles. In addition, the Ni_1.5Co_1.5S₄ was also used to assemble an asymmetric supercapacitor with reduced graphene oxide and attains excellent capacitive performance with high specific capacitance (113 F g⁻¹ at 1 A g⁻¹), high energy density (37.6 W h kg⁻¹ at 775 W kg⁻¹) and high power density (23.25 kW kg⁻¹ at 17.7 W h kg⁻¹).