Chemisorption processes of H and OH occur at a few percentage of surface sites in this potential region, see Figure ). These adsorption sites possess lower (higher) equilibrium potentials for OH (H) adsorption. The distribution of the adsorption equilibrium potential could be explained in two ways. On one hand, there may be structural defects occupying a few percentage of the solid surface. Adsorption of OH (H) occurs at lower (higher) potentials at these structural defects. On the other hand, interfacial water molecules have an energy distribution. Higher-energy water molecules can be decomposed and then adsorbed as OHad at lower potentials. In general, the electrochemical potentials of protons, water molecules, and adsorbed species at the metal interface are distributed, resulting in a distribution in equilibrium potentials of any electrochemical reaction involving these species.