Hydrogen overvoltage and the reversible hydrogen electrode

<jats:p> There are two fairly sharply contrasted methods by which hydrogen is liberated in electrolysis. At high overvoltage electrodes Tafel found that the current and electrode potential were related by <jats:italic>i</jats:italic> = <jats:italic>ke</jats:italic> <jats:sup> - <jats:italic>ε</jats:italic> V/2 <jats:italic>k</jats:italic> T </jats:sup> , a relation more recently confirmed by Bowden. These electrodes are not reversible and there is no evidence that the process H <jats:sub>2</jats:sub> → 2H <jats:sup>+</jats:sup> + 2ε can occur to any appreciable extent at potentials accessible to observation. Even at ordinary bright platinum, which has a fairly low overvoltage, Armstrong and Butler found that the ionization of hydrogen occurred to only a slight extent in the region between the reversible hydrogen potential and the potential at which oxygen begins to be formed. On the other hand, reversible hydrogen potentials have long been known at platinized platinum and similar electrodes, and it has been found that bright platinum and similar metals can be “activated” in various ways, whereby it is brought into a condition in which the reversible hydrogen potential can be realized. In this state the hydrogen overvoltage is low and at small displacements from the reversible value the potential varies linearly with the current, <jats:italic>i. e</jats:italic> ., V = V <jats:sub>0</jats:sub> — <jats:italic>ki</jats:italic> This relation can be accounted for on the assumption that there are two processes at the reversible electrode which are influenced exponentially by the potential, but in opposite directions. The theories of overvoltage which have been put forward have been concerned mainly with the behaviour of high overvoltage electrodes. In Gurney’s theory the potential determining process was the transfer of electrons from the metal to the hydrogen ions in the solution. As Gurney pointed out, this process is essentially irreversible, for the reverse transfer of electrons from either free hydrogen atoms or molecules cannot occur to an appreciable extent in the same potential region. Horiuti and Polanyi have suggested an alternative mechanism in which the primary process is the transference of hydrogen ions to adsorption positions at the surface of the metal, in the course of which neutralization occurs. This process may under some circumstances be reversible, but on the assump­tions made has a high activation energy of 20-30 k. cals. </jats:p>