Oxygen vacancy enriched Na⁺ intercalated MnO₂ for high-performance MXene (Ti₃C₂T_x)-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-MnO_2-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-MnO_2-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//Ti₃C₂T_x, Na-MnO₂//Ti₃C₂T_x, and Na-MnO_2-x//Ti₃C₂T_x) are fabricated and their respective performances are contrasted. The Na-MnO_2-x//Ti₃C₂T_x 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-MnO_2-x electrode for flexible energy storage, the flexible Na-MnO_2-x//Ti₃C₂T_x 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-MnO_2-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-MnO_2-x film electrode has great potential to be used for both flexible energy storage and electrocatalysis.