At a semiconducting oxide water interface, two double layers (space charge layer and electric double layer) can develop near the interface. The double layer potentials can vary against the voltage and pH because they depend on the density of surface charges that have both electronic and protonic components. While the surface protonic charge density is determined by the acid-base equilibrium between the surface and the solution, electronic charges can also develop owing to the presence of electronic surface states that serve as traps for electrons and holes. A further complication is that the protonation and reduction (or deprotonation and oxidation) can couple at certain surface sites through proton coupled electron transfer (PCET) reactions. This is believed to be the cause of the observed (non-)Nernstian relation between the changes in electrode potentials and the pH, an approximately linear relationship with varying slopes depending on the nature of oxide surfaces. With the aim of addressing this issue, we present a first principles study for computation of the PCET energies of the reductive hydrogenation of the aqueous TiO2 surface. Our calculation shows that excess electrons and protons don't interact chemically at the interface, suggesting that the coupling between them is mainly electrostatic in nature. Our previous finding, on the other hand, showed that the electronic holes can chemically couple with protons through the terminal water. Combining these two results, we conclude that TiO2 may show different dependences of the electrode potentials on the pH at the cathodic and anodic voltage conditions. (C) 2015 Elsevier Ltd. All rights reserved.