We first discuss the effect of accounting for explicit water layers as well as the induced dipole on the stable surface structure of NiOOH(0001), followed by the discussion on the origin of this effect. Fig. (a) shows the change in the surface Gibbs free energy of adsorption, calculated in the gas phase, as a function of potential for a varying degree of deprotonation (Eq. 4) at a constant pH value of 13. At a given potential, the lowest value of ∆γ corresponds to the most stable surface state. As the potential increases, deprotonation becomes more favourable at NiOOH(0001). Fig. (b) shows the results computed in the gas phase but by accounting for the interaction term of induced surface dipole by deprotonation with the electrode potential (Eq. 8). It is seen that the surface dipole can significantly facilitate the deprotonation by decreasing the free energy of adsorption. This effect will be discussed later by computing ∆Φ as a function of the degree of deprotonation. Compared with the gas phase system in Fig. (a), surface water molecules tend to prevent deprotonation by increasing the change in the adsorption free energy. In fact, water molecules stabilize surface OH species by forming hydrogen bonds with them. This effect has been explained in more detail in our previous work on the structure of water at NiOOH(0001) . . This leads to a significantly polarized surface as the deprotonation degree increases, and as a result the surface dipole, with a direction towards the surface, increases. It should also be noted that a favourable H-down orientation of water at the surface further increases ∆p toward the surface, which is consistent with the previous study . Moreover, our calculated work function of 5.33 eV in the presence of water for the stoichiometric NiOOH surface agrees well with the value reported in an experimental study . In the case of cation adsorption, as reported in Table , the electron transfer is from To understand the cation size effect on the electronic charge distribution, we Moreover, the decrease was found to be larger for Cs + than Na + . In addition, the same trend was reported for K + /Rh(111) , Cs + /W(100) , and for Na + , K + , Cs + adsorption on TiO 2 (100) . A more recent experimental study reported that the local work function around an adsorbed K + decreases by 1.5 ± 0.3 eV on CuO thin films. Realizing this local effect is particularly important for oxide surfaces where adsorbed alkali metals invoke more localized charge transfer as compared with the metal surfaces .