Sukanta De

The pure bismuth oxyiodide (BiOI) nanoflower and BiOI/reduced graphene oxide (rGO) nano composites have been prepared via solvothermal route for supercapacitor application.Various mass ratios of BiOI with rGO were incorporated in this study. Additionally, the electrolyte gel was modified with graphene oxide (GO) to investigate the enhancement in supercapacitive charge storage properties. The structural and morphological properties of the pure BiOI nanoflower and BiOI/rGO hybrids were analysed using X-ray diffraction (XRD), Raman spectroscopy, scanning electron microscopy (SEM) with energy dispersive X-ray analysis (EDAX), Transmission Electron microscopy (TEM), N ₂ adsorption–desorption isotherms (BET) and X-ray photoelectron spectroscopy (XPS). The electrochemical performances of pure BiOI nanoflower and BiOI/rGO hybrids with different mass ratio were tested by cyclic voltammetry (CV), galvanostatic charge/discharge (GCD) and electrochemical impedance spectroscopy (EIS). The optimal BiOI/rGO hybrid electrode delivered specific capacitance 235 F/g at scan rate 5 mV/s in PVA/H ₂ SO ₄ gel electrolyte with energy density 8.1 Wh/kg and power density 0.146 kW/kg, also with PVA/H ₂ SO ₄ + GO gel electrolyte 288 F/g at 5 mV/s scan rate with energy density 10 Wh/kg at power density 0.18 kW/kg. The BiOI/rGO hybrid electrode maintained capacitive retention of 79% and 71% after 10,000 successive CV cycles at scan rate 100 mV/s in PVA/H ₂ SO ₄ gel and PVA/H ₂ SO ₄ + GO gel electrolytes respectively.

Bismuth oxychloride (BiOCl) nanoflowers and bismuth oxychloride/reduced graphene oxide (BiOCl/rGO) composites with different weight percentage were prepared by solvothermal method. BiOCl/rGO hybrids electrode based symmetric solid state supercapacitor was fabricated by sandwiching alkaline polyvinyl alcohol (PVA/KOH) gel electrolyte in presence and absence of potassium iodide (KI). It delivered maximum specific capacitance of 611 F g⁻¹ in presence of PVA/KOH+KI gel electrolytes, increasing by 8.75% compared to the PVA/KOH based supercapacitor. The energy and power density of the supercapacitor in presence of PVA/KOH+KI were reached 21.21 Wh kg⁻¹ and 1.37 kW kg⁻¹ respectively. The supercapacitor exhibits 90% capacitance retention even after 10 000 cycles.

Among different nanostructures, two dimensional nanosheets of transition metal oxides (TMOs) are promising pseudocapacitive materials for constructing thin flexible supercapacitors. High surface area and abundant electrochemical sites in 2D TMO nanosheets make them appropriate for high energy density supercapacitors. Here we report large scale production of TMO nanosheets by exfoliation in ethanol-water mixed solvent instead of using highly toxic organic solvents. After optimization, this method gives high concentration (0.42 mg ml⁻¹, 0.47 mg ml⁻¹ and 0.40 mg ml⁻¹ for MoO₃, MnO₂ and RuO₂ respectively) stable dispersions of TMOs nanosheets. The TMOs nanosheets/SWCNT composite based thin film electrodes in a three electrode system display a specific capacitance as high as 1205.08 F g⁻¹, 1168.69 F g⁻¹ and 1308.45 F g⁻¹ at 5 mV s⁻¹ scan rate for MoO₃/SWCNT, MnO₂/SWCNT, and RuO₂/SWCNT in Na₂SO₄ aqueous electrolyte. In contrast, the symmetric supercapacitors with the same composite thin films using a gel electrolyte exhibit a specific capacitance of 717 F g⁻¹ (MoO₃/SWCNT), 540 F g⁻¹ (MnO₂/SWCNT) and 676 F g⁻¹ (RuO₂/SWCNT) at the same scan rate and high energy density (24.89 W h kg⁻¹ at 1.61 kW kg⁻¹, 18.73 W h kg⁻¹ at 1.21 kW kg⁻¹ and 23.48 W h kg⁻¹ at 1.52 kW kg⁻¹). These solid state symmetric supercapacitors having high energy density demonstrate excellent stability without any changes in their specific capacitance values under different bending conditions.