One step synthesis of Ni/Ni(OH)2 nano sheets (NSs) and their application in asymmetric supercapacitors

Abstract

Ni(OH)₂ is a useful electrode material for electrochemical capacitors, due to its high theoretical specific capacitance and low cost, but its application has been limited by poor electrical conductivity. Hence, we fabricated Ni(OH)₂ nano sheets (NSs) with nickel metal NPs via the hydrothermal partial reduction of Ni(II) salt by ethanol in basic medium. The significance of the basic medium (presence of KOH) and other reaction parameters and the mechanism for the formation of Ni/Ni(OH)₂ NSs are elaborated. The Ni/Ni(OH)₂ NSs have been used as a positive electrode in an asymmetric supercapacitor (ASC) with a larger voltage window using the activated carbon (AC) as a negative electrode, which resulted in high energy and power densities. By optimizing the mass ratio between AC and Ni/Ni(OH)₂ NSs in the fabrication of electrodes, we found a maximum specific capacitance (C_S) of 62 F g⁻¹ at 2 mA cm⁻² at a voltage of 1.65 V and observed the maximum energy and power densities of 23.45 W h kg⁻¹ and 4598 W kg⁻¹, respectively. The galvanostatic charge–discharge study shows high capacity retention up to 90%, even after 6000 consecutive cycles, which is a noteworthy achievement, considering the ASCs. Moreover, we believe that the presence of nickel metal in Ni/Ni(OH)₂ NSs helped to reduce the charge transfer resistance (R_CT), which resulted in better performance. These results certainly demonstrate that such Ni/Ni(OH)₂ NSs with Ni metal NPs are promising materials for the construction of next generation aqueous ASCs with higher specific capacitance. The synthesis procedure can be applied to other transition metals to synthesize their metal/metal hydroxide composites and enhance their conductive nature, instead of using conductive substrates.