The Capacity of Passivated Iron Electrodes and the Band Structure of the Passive Layer

Abstract

Abstract The capacity of passivated iron electrodes was measured by the potentiostatic pulse method in dependence on the oxide layer thickness d and the electrode potential ϵ at ρH = 8.4. At constant d , the C (ϵ) dependence shows three characteristic ranges. With increasing potential, C decreases in the first range by a factor 2, in the second range it becomes constant, and finally C increases at high potentials. The results are discussed with respect to the band structure model of the semiconducting passive layer. In the first range a Schottky‐Mott type behaviour is found indicating donor concentrations N up to 10 20 cm −3 . The flat band potential ϵ FB = 0.1 V is obtained. In the second range, C is proportional to 1/ d. The capacity increase in the third range at high potentials is explained as a contribution from valence band charging at the oxide surface. The band gap E g = 1.6 eV follows. The electronic properties of oxide covered metal electrodes are discussed in relation to bulk semiconductor electrodes.