Electrochemical flexible supercapacitor based on manganese dioxide-titanium nitride nanotube hybrid

The manganese dioxide-titanium nitride (MnO2-TiN) nanotube hybrid has been designed to improve its electroactivity and conductivity for a supercapacitor application. Titanium nitride (TiN) nanotube array was prepared by an anodization process of titanium in ethylene glycol (EG) solution containing ammonium fluoride, subsequent calcination process in an air atmosphere, and final nitridation process in an ammonia atmosphere. Electroactive MnO2 was then loaded into well-aligned TiN nanotubes to form MnO2-TiN nanotube hybrid with well-designed shell layer of MnO2 through a controlled cyclic voltammetry electrodeposition process, which was directly supported on ultra thin and supple titanium foil to establish a bendable electrode. The morphology and microstructure of MnO2-TiN nanotube hybrid were characterized by scanning electron microscopy and X-ray diffraction. The electrochemical capacitance was investigated by cyclic voltammetry and galvanostatic charge/discharge measurements. The villiform MnO2 thin shell layer was fully covered on TiN nanotube walls to form a coaxial heterogeneous structure. Superior electrical conductivity and accessible nanochannels of TiN contributed to a high capacitance performance of MnO2. The specific capacitance of MnO2-TiN was determined to be 853.3 F g−1 (or 213.2 mF cm−2) at a current density of 1.0 A g−1 (or 0.25 mA cm−2). All-solid-state flexible supercapacitor was constructed using two symmetric film electrodes of MnO2-TiN nanotube hybrid and a polyvinyl alcohol gel electrolyte of KOH-KI-EG. The volume specific capacitance, energy and power density were determined to be 4.01 F cm−3, 1.81 mW h cm−3 and 32.8 mW cm−3 at a high potential window of 1.8 V and a current density of 2.0 mA cm−2. Such MnO2-TiN nanotube hybrid electrode material exhibited effective energy storage in flexible supercapacitor application.