The electrocatalytic HER performance of TiO2 and VO-TiO2 electrodes was studied in a 0.1 M H2SO4 electrolyte. The linear sweep voltammetry (LSV) experiments reveal that a VO-TiO2 nanoarray electrode requires an overpotential of -0.35 VSHE to achieve a HER current density of 20 mA cm -2 (Figure ). In contrast, the pristine TiO2 demands a substantially more reducing potential, -0.70 VSHE, to attain the same current density. At a higher overpotential of -1.5 VSHE, the VO-TiO2 nanoarray electrode reaches a current density of 260 mA cm -2 , which is significantly higher than the value measured on pristine TiO2 nanoarray (37 mA cm -2 ). The low HER activity of the pristine TiO2 is attributed to the absence of polaronic states as evidenced by the computational results as well as the in situ UV-Vis and EPR spectra discussed earlier.