We used CIP-DFT 6 to model potential-dependent polaron formation on the most stable anatase TiO2 (101) surface. All calculations are spin-polarized, utilizing the DFT+U approach with a self-consistently computed Ud (Ti) of 4.5 eV in the TiO2 system. TiO2 (101) exhibits both 5-and 6-fold coordinated Ti (Ti5c and Ti6c) sites along with 2-fold coordinated bridging oxygen (Obr) and 3-fold coordinated oxygen (O3c) sites (see Figure ). CIP-DFT calculations on an ideal, uncharged TiO2 (101) surface correspond to the conditions of potential of zero charge (PZC), achieved at potential of 1.0 VSHE. A projected density of states (PDOS) plot in Figure shows that Ti atoms have a spin-paired electron configuration as the atomic magnetic moments (AMM) of Ti are zero and as the DOS does not display spin-dependency. The vacant 3d orbitals of Ti atoms dominate the conduction band. These features indicate that Ti cations of a neutral TiO2 (101) surface are in the quadrivalent state (Ti 4+ -3d 0 ) and that no electron polarons exist. Ti 3+ polaron formation and its electrode potential dependency are demonstrated in Figures . Figure (orange line) shows that at 0 V vs SHE (VSHE) the TiO2 (101) surface carries a negative charge of 0.29 e -while the PDOS plot (Figure ) displays that the Ef moves up in energy and is located at the bottom of conduction band. As no spin polarization is seen in the PDOS, the excess charges are not localized any particular atom and thus no electron polarons are formed in Ti-3d orbitals. Further decreasing electrode potential raises Ef leading to the accumulation of electrons at the surface. The PDOSs plots (Figures ) at -1.0 VSHE and -2.0 VSHE show that the initially unoccupied Ti-3d orbitals of a Ti6c surface atom are shifted below Ef . This shift is accompanied by the splitting of a single DOS peak into two peaks (Figure ). In particular, the DOS peak emerging below the Fermi level at U = -1.0 VSHE is due to the localization of excess electrons (0.72 e -) and a considerable AMM of 0.74 μB on a single Ti atom (Figure and): this indicates the formation of a localized electron polaron state at U = -1.0 VSHE. Figure further demonstrates that the presence of these states depends sensitively on the electrode potential. The application of a highly reducing potential, U = -2.0 VSHE, leads to further electron accumulation (1.09 e -) and shifts the initially unoccupied Ti-3d orbitals completely below the Fermi level. Since the AMM of the Ti cations is ~1.0 μB, we conclude that the Ti 4+ is fully reduced to Ti 3+ with a clear polaronic state (Figure ). We addressed the stability and dynamics of the Ti 3+ polarons as a function of the electrode potential through CIP-DFT molecular dynamics (MD) simulations. The CIP-DFT-MD results in Figure demonstrates that the potential-dependent Ti 3+ polaron forms on the TiO2 (101) surface at U = -2.0 VSHE, but hops from a Ti6c site to a Ti5c surface site. This thermal polaron hopping is accompanied by a small energy penalty (see the energy difference (ΔE) between Ti5c 3+ and Ti6c 3+ in Figure ). By monitoring the AMM of Ti5c 3+ as a function of time we see that Ti5c 3+ is stable and exhibits only minor fluctuations, see Figure black arrows. At more reducing potential of U = -3.0 VSHE, a Ti6c (4-δ)+ bipolaron state hops to Ti5c (4-δ)+ surface atom sites, see Figure . The large oscillations in AMM values shown in Figure suggests that a bipolaron state may be unstable at room temperature.