Oxygen vacancy enriched Na+ intercalated MnO2 for high-performance MXene (Ti3C2Tx)-based flexible supercapacitor and electrocatalysis

The increased interest in smart and portable electronic gadgets has led to the development of flexible and wearable energy storage systems. Herein, the oxygen vacancy-enriched Na-MnO2-x is synthesized using a simple, scalable, and inexpensive electrodeposition method. The oxygen vacancy enrichment effectively enhances the conductivity and reaction kinetics of the Na-MnO2 electrode. The Na-MnO2-x film electrode reveals an excellent specific capacitance of 395 F g-1 at the scan rate of 5 mV s- 1 with high capacitance retention of 85.9 % after 10,000 cycles at a current density of 5 A g-1. To verify the practicability, three asymmetric supercapacitors (ASCs) (Mn3O4//Ti3C2Tx, Na-MnO2//Ti3C2Tx, and Na-MnO2-x//Ti3C2Tx) are fabricated and their respective performances are contrasted. The Na-MnO2-x//Ti3C2Tx ASC reveals a maximum energy density of 25 Wh kg -1 at the power density of 1000 W kg -1, along with excellent capacitance retention of 98.8 % after 10,000 cycles. In addition, to validate the suitability of Na-MnO2-x electrode for flexible energy storage, the flexible Na-MnO2-x// Ti3C2Tx ASC is fabricated that operates in the potential window of 2 V in PVA: Na2SO4 polymer gel electrolyte and delivers a high volumetric energy density of 510.3 mWh cm -3 at a power density of 40,483 mW cm -3. Moreover, the electrocatalytic activity of Na-MnO2-x thin films reveals an overpotential of 439.7 and 381.2 mV to drive a current density of 10 mA cm -2 corresponding to HER and OER, respectively. Therefore, the electrodeposited, oxygen vacancy-enriched Na-MnO2-x film electrode has great potential to be used for both flexible energy storage and electrocatalysis.