The electrolytic behaviour of thin films. Part I.—Hydrogen

Abstract

Few investigations have been made on the electrolytic behaviour of very thin metallic films. Whilst the work of Oberbeck and of Pring reveals the fact that a deposited layer of metal but a few atoms thick will produce an electrode possessing all the electromotive properties of the massive metal, yet information is lacking on the alteration of the electromotive force as these layers are built up. It might be anticipated that the behaviour of the electrode during the deposition of the first few layers would lead to interesting results, giving some insight into the mechanism of electrode processes and the range of action of forces of adhesion. In view of this it was considered a matter of some interest to investigate how far it might be possible to obtain data on the electrode potential and the rate of solution of the deposited atoms during the building up of the first atomic layer. The problem of metal ion deposition from aqueous solutions is complicated by the presence of other ions, such as the hydrogen ion, which can deposit simultaneously with the metal and affect the potential. For this reason the deposition of the hydrogen ion was first studied. It is well known that, in general, in order to bring about the continuous deposition of hydrogen ions at a metallic cathode, the potential must be maintained at a value considerably more negative than that of a reversible hydrogen electrode in the same electrolyte. The view most generally accepted is that this overpotential is due to an accumulation of electromotively active material on the electrode, and it has been suggested by various workers that it may consist of metallic hydrides, hydrogen atoms or negative hydrogen ions. With the exception of a paper by Knobel, little work has been done in determining the actual quantity of hydrogen accumulated on the cathode during the establishment of overpotential. Knobel, making the assumptions that the material was atomic hydrogen, that the relation between the solution pressure of the hydrogen P and the surface concentration of atoms C H is given by the relation P = k C H m , and that the potential is related to the pressure by the Nernst expression, calculated this quantity from the rate of growth of overpotential and found that for most metals it was considerably less than an atomic layer.