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 degrees 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 (C-s) for the optimal electrode with mixed AR phase TiO2 was ca.102.12 and 97.30 mF/cm(2), obtained from the CV (scan rate, 5 mV/s) and charge-discharge (CD, current density, 50 mu A/cm(2)) measurements, respectively. The higher C-s 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 C-s. The K-TNF/Ti electrode is covered excellent stability with the C-s retention of ca. 91% and a very small change of internal series resistance (R-s) and charge transfer resistance (R-ct) at the electrode|electrolyte interface after 2500-CD cycles. (C) 2017 Elsevier Ltd. All rights reserved.