An Optical Study of the Iron Electrode in Alkaline Electrolyte

Ellipsometric measurements made as an iron electrode is cycled galvanostatically in unstirred 0.05 M NaOH show that a low density outer layer builds up progressively over an inner compact oxide layer. The inner layer reaches a maximum thickness of approximately 38Å at oxygen evolution and is removed by cathodic reduction, whereas the outer layer thickens by over 100Å on each cycle. The outer layer is made up of partially dissolved reaction products which are alternately oxidized and reduced on each cycle, but there is no evidence that the inner layer (of Fe 3 O 4 ) undergoes solid‐phase oxidation or reduction under galvanostatic cycling. Under anodic conditions, the inner layer acts as a barrier layer for field‐limited ionic transport, and when it is reduced on a cathodic cycle it provides the iron which enables the outer layer to grow on the subsequent anodic cycle.