where p dipole slab-ads(|water) is the induced surface dipole at the deprotonated surface 130 with cation adsorption, and p dipole slab|water is that for the stoichiometric NiOOH (0001) surface. In both cases, the slab forms an interface with a water layer. d is the width of the double layer, i.e., the distance between electrode surface and electrolyte which lies in the range of 2.5 and 3.5 Å; here, we apply the value of d=3 Å consistent with the previous study .