Cavity structured S-NiO with improved energy density for aqueous asymmetric hybrid supercapacitors by CDA mechanism

Abstract

Recently, hybrid supercapacitors have gained high recognition due to their improved energy density performance without affecting their power density. In this work, sulfur-doped nickel oxide (S-NiO) honeycomb nanostructure electrodes were prepared via a cost-effective chemical bath method through a Capping, Doping and Annealing (CDA) mechanism for Aqueous Asymmetric Hybrid Supercapacitors (AAHSc). The structural, elemental, morphological and electrochemical analysis of S-NiO as a positive electrode is explored in detail. After annealing, an amorphous to crystalline transformation of NiO was observed with a cubic structure owing to the honeycomb like morphology for optimum 1.42 atomic % sulfur doping. The CDA mechanism played a beneficial role in a synergistic process to prepare the S-NiO electrode with enhanced electrochemical properties for supercapacitor performance, exhibiting 71% capacitive current contribution. This optimized S-NiO electrode exhibited a remarkable specific capacitance (791.67 F g⁻¹ at a scan rate of 20 mV s⁻¹) in 1 M KOH aqueous electrolyte. Graphite used as a negative electrode in the AAHSc device configuration (S-NiO//KOH//Graphite) showed a significant specific capacitance of 77.69 F g⁻¹ at 5 mA cm⁻² current density in a wide operating potential window of 1.8 V. The AAHSc device delivered a high energy density of 34.96 W h kg⁻¹ at a power density performance of 2567 W kg⁻¹ and 19.36 W h kg⁻¹ even at a high-power density of 7180 W kg⁻¹ for 5 mA cm⁻². More remarkably, 91% capacity retention with 81% coulombic efficiency was observed after 6,000 stability cycles at 5 mA cm⁻². The current work signifies the great potential of the S-NiO//KOH//graphite device in promoting and emphasising the development of a low-cost new approach towards hybrid energy storage materials.