Mixed solvent exfoliated transition metal oxides nanosheets based flexible solid state supercapacitor devices endowed with high energy density

Abstract

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.