Annealing-Free Synthesis of K-doped Mixed-Phase TiO2 Nanofibers on Ti Foil for Electrochemical Supercapacitor

- An annealing-free and KOH assisted strategy was presented to prepare K-doped mixed phase TiO2 nanofibers on Ti foil.
- The phase and K-dopant percentage of nanofiber was considerably dependent on the concentration of KOH.
- The percent of Ti3+ interstitials and −OH functionalization were also dependent on the KOH concentration.
- The optimized nanofiber based electrode exhibits high specific capacitance and cycling stability.

This research demonstrated the annealing-free synthesis of K-doped mixed-phase TiO2 (anatase and rutile, AR) nanofibers (K-TNF) on Ti foil at 150 °C assisted by KOH(aq.) for electrochemical supercapacitors (ESCs) applications. The aggregated network and the average diameter of K-TNF was slightly decreased with the increase of KOH(aq.) concentrations from 4 to 6 M, while the amount of K-doping, Ti3+ interstitials, and OH functional groups was substantially increased. The TiO2 phase was entirely rutile at 4 M KOH(aq.), while it was a mixed AR phase at 5 and 6 M. All the K-TNF modified Ti electrodes (K-TNF/Ti) exhibited quasi-rectangular shaped cyclic voltammograms (CVs) in a wide potential range and the specific capacitance (Cs) for the optimal electrode with mixed AR phase TiO2 was ca.102.12 and 97.30 mF/cm2, obtained from the CV (scan rate, 5 mV/s) and charge-discharge (CD, current density, 50 μA/cm2) measurements, respectively. The higher Cs for the optimal K-TNF/Ti electrode can be ascribed to the synergistic effect of mixed AR phase, high percentage of K-doping (ca.17.20%), and Ti3+ interstitials (ca.16.20%), respectively. The directional electron transport through the 1D channel as well as the OH functional groups on the K-TNF surface is also contribute to enhance Cs. The K-TNF/Ti electrode is covered excellent stability with the Cs retention of ca. 91% and a very small change of internal series resistance (Rs) and charge transfer resistance (Rct) at the electrodeǀelectrolyte interface after 2500-CD cycles.

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