Na₂MoO₄-Incorporated Polymer Gel Electrolyte for High Energy Density Flexible Supercapacitor

The recent advancement in flexible/bendable displays such as phones, smart bands, laptops, and televisions demands flexible energy storage devices. Heteroatom-doped graphene electrodes are promising candidates for flexible energy-storage applications because of its dual nature of electrochemical double-layer capacitance and pseudocapacitance. The low energy density of supercapacitors limits their potential use and attempts are in progress to overcome this issue. Redox-additive electrolytes are presumed to be one of the elite tactics to augment the energy density of the supercapacitor device. Here, the sodium molybdate (Na2MoO4)-incorporated polymer gel electrolyte has been demonstrated to enhance the energy density of a flexible supercapacitor. The supercapacitor electrode with Na2MoO4/H2SO4 (714 F/g) acquired ∼2.4-fold greater specific capacitance than with H2SO4 (300 F/g) in a three-electrode configuration. Moreover, the energy density of symmetric supercapacitor with Na2MoO4/H2SO4 (23 Wh/kg) is ∼2-fold higher compared with H2SO4 electrolyte (13 Wh/kg). The fabricated flexible supercapacitor device with sodium molybdate/poly(vinyl alcohol)/sulfuric acid (Na2MoO4/PVA/H2SO4) gel electrolyte showed 3.4-fold higher energy density (E = 24 Wh/kg) compared to a device with PVA/H2SO4 gel electrolyte (E = 7 Wh/kg). The increase in energy density has been observed after the incorporation of Na2MoO4, which is due to the faradaic reaction between the MoO42− ions in the redox-additive and H+ ions in the H2SO4 electrolyte.