The computational results for VO-TiO2 are supported by the experimental in situ EPR spectroelectrochemical and electrochemical impedance spectroscopy (EIS) results in Figure and Figure for a defected TiO2 surface. An EPR signal for a Ti 3+ polaron state exhibits more drastic increase on the VO-TiO2 than observed on pristine TiO2 at a given electrode potential. The introduction of oxygen vacancies decreases the chargetransfer resistance of TiO2 and improves conductivity, which can be attributed to the facilitated polaronic charge hopping between Ti 4+ and Ti 3+ . In situ X-ray absorption near edge structure spectra (XANES) at the Ti K-edge on the VO-TiO2 sample shows a small shift towards lower energies from open-circuit potential (OCP) condition to U= -2.0 VSHE(inset in Figure ), indicating a lower Ti valence at the more negative potentials. Furthermore, the Ti 3+ polaron formation is accompanied by substantial lattice distortion and average atom displacements from 2.02 Å to 2.11 Å, evidenced by Fourier transform-extended X-ray absorption fine structure (FT-EXAFS) and CIP-DFT calculation results (see Figure ).