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.

Hydrogen overpotential at nickel cathodes has been measured under very pure conditions in aqueous solutions of hydrogen chloride (0.001N − 1.0N) and in aqueous sodium hydroxide (0.001N − 0.2N). The measurements have been made in the current density range 10−8 − 10−1 amp/apparent cm2, and in the temperature range 0°—50°C. Observations were also made of the buildup and rate of decay of overpotential and of the capacity of the electrode/electrolyte interface. Direct measurements were made of the number of acts of the rate-determining step associated with one act of the over-all hydrogen evolution reaction (i.e., the stoichiometric number μ). The application of μ has been extended to hydrogen overpotentials greater than about − 20 millivolts. The experimental data were treated statistically and show that the most probable mechanism of hydrogen overpotential at nickel cathodes is that of a rate-determining discharge step followed by a recombination of hydrogen atoms. The discharge probably takes place from hydroxonium ions in acid solution. In alkaline solution the observed pH effect on overpotential is best explained by assuming that the discharge occurs from water molecules whose activity depends on the electrode field. Consideration of the role of chemisorption in the hydrogen electrode process indicates that a rate-determining discharge step can take place from a nickel surface substantially occupied with hydrogen atoms.

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