Reduced Ti-Nb-O nanotube arrays with co-doping of Nb and Ti3+/Vo as a high-performance supercapacitor electrode for enhanced electrochemical energy storage

TiO2 nanotube arrays (NTAs) are excellent energy storage materials due to their chemical stability, high specific surface area and wide voltage window. However, poor electrochemical activity and electrical conductivity limit the application in supercapacitors. Herein, we develop a promising modification strategy for improving elec-trochemical performance of TiO2, through bulk-phase Nb-doping by in-situ anodization of Ti-Nb alloy and surface self-doping of Ti3+/oxygen vacancy (Vo) with one-step electrochemical reduction. Material character-izations indicate the successful formation of Nb5+ in the lattice, as well as Ti3+/Vo and hydroxyl are also introduced. Electrochemical measurements demonstrate that reduced co-doping system (denoted as R-Ti-Nb-O) yields a superior areal capacitance (19.56 mF cm-2 at 0.1 mA cm-2), which enhances by 3 orders of magnitude compared with pristine TiO2. Furthermore, R-Ti-Nb-O exhibits high energy density (1.33 mWh cm-2), superior power density (35 mW cm-2), outstanding rate capability (81.70%) and remarkable cycling stability (76.76% capacitance retention after 1000 cycles). Density functional theory (DFT) calculations further reveal that reduced co-doping system indeed significantly increases the carrier density, electrical conductivity and hydrophilicity. This work involving bulk-phase Nb-doping and surface oxygen defective engineering may help provide a feasible and effective strategy to improve areal capacitance of Ti-based nanostructures for enhanced electrochemical energy storage.