In case of Al 2 O 3 , quantum chemical calculations show that the value of the cation adsorption energy decreases exponentially (Figure ) as the ε HOMO value becomes lower and almost linearly when the ε LUMO value goes down (Figure ). The strongest binding to the surface is observed for alkali metal cations and H 3 O + and the weakest binding for EMI + . The distance from the electrode surface to the cation also follows the E ads trends, where z 0 becomes lower as the ε HOMO (Figure ) and ε LUMO (Figure ) values decrease. Only H 3 O + is slightly out of the general E ads (ε HOMO ) trend, which may be caused by the formation of strong adsorption bonds between the hydrogen of H 3 O + and the oxygen of the Al 2 O 3 surface.