Polymer hydrogel based quasi-solid-state sodium-ion supercapacitor with 2.5 V wide operating potential window and high energy density

The present work reports a quasi-solid state sodium ion-based asymmetric supercapacitor (ASC) device with nitrogen-doped reduced graphene oxide (NrGO) and boron-doped reduced graphene oxide (BrGO) electrodes. The choice of NrGO (BrGO) for the cathode (anode) is accomplished through a computational investigation of electrode quantum capacitances based on density functional theory (DFT). Nanocellulose is used as an electrode binder due to its biodegradability, enhanced wettability, good ion permeability, and low cost. The NaClO₄-PVA based hydrogel membrane is the separator-less quasi-solid state gel electrolyte in the fabricated device. The specific capacitance of the NrGO || BrGO ASC device is obtained as 251.2 F g⁻¹, at a current density of 1 A g⁻¹ with a wide potential window of 2.5 V which is better than the previous reports. This device offers a better performance in comparison to the rGO||rGO, NrGO||NrGO and BrGO||BrGO symmetric supercapacitor (SSC) devices due to the synergistic effect of non-faradic capacitance and pseudocapacitance. The ASC device shows an energy density of 54.5 Wh kg⁻¹ at a power density of 1.25 kW kg⁻¹ and significant capacitance retention of 84.8% after 10,000 cycles at a high current density of 15 A g⁻¹.