The Mechanism of the Cathodic Hydrogen Evolution Reaction

Some of the outstanding problems of concept and mechanism in the field of cathodic hydrogen evolution kinetics are discussed and clarified. A full derivation and correlation of kinetic equations which assume no mechanism reveals expressions for several parameters which take values specific to one or more mechanisms. The use of statistical methods of treatment of data proves indispensable in estimating the values of these parameters. A further method of ascertaining the mechanism of the hydrogen evolution reaction is to examine the kinetics of the individual reaction paths, so that the expected values of parameters common to all paths may be deduced. A number of mechanisms important in acid and alkaline solutions are thus treated, and are shown to be distinguishable experimentally. Using already published data, the actual conditions under which various reaction paths take place at mercury, silver, nickel, and smooth platinum cathodes are calculated. It is not only possible to compare these deduced data with observation, and thereby verify the occurrence of a particular reaction path, but also to demonstrate the impossibility of some mechanisms in specific cases. The recent advances which the foregoing methods have made possible are discussed in relation to data which have recently become available.