Ti3+ Induced Brown TiO2 Nanotubes for High Performance Sodium-Ion Hybrid Capacitors

Sodium-ion hybrid capacitors are attracting great attention and are emerging as promising energy storage devices, with their remarkable footsteps in energy and power densities. However, the development of efficient electrode materials that result in a minimum trade-off between their energy and power densities, and that allow long-term cycling stability, still remains a challenge for realizing their full potential as an alternate energy storage system for commercial applications. Herein, for the first time, we study the sodium-ion intercalation pseudocapacitance behavior of brown TiO2 nanotubes for their application as an efficient anode material for Na-ion hybrid capacitors. We synthesized semicrystalline and crystalline anatase brown TiO2 nanotubes, aggregated in a flowerlike morphology, through a hydrothermal route, and performed detailed electrochemical studies. The kinetic studies reveal that semicrystalline brown TiO2 exhibits a Na-ion intercalation pseudocapacitive behavior with 57% of capacitive storage at 1.0 mV s(-1), whereas crystalline brown TiO2 is more faradaic in nature. Further, hybrid Na-ion capacitors are fabricated with brown TiO2 materials as an anode and activated carbon as a cathode, and the fabricated device showed an excellent electrochemical performance with a high energy density of similar to 68 Wh kg(-1) and a high power density of similar to 12.5 kW kg(-1) and with a good cycling stability up to 10 000 cycles with similar to 80% capacitive retention. The obtained results represent a promising approach toward developing efficient electrodes for hybrid Na-ion capacitors.