Hydrogen overpotential at nickel cathodes has been measured under very pure conditions in aqueous solutions of hydrogen chloride (0.001N minus; 1.0N) and in aqueous sodium hydroxide (0.001N minus; 0.2N). The measurements have been made in the current density range 10minus;8minus; 10minus;1amp/apparent cm2, and in the temperature range 0deg;mdash;50deg;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 ratehyphen;determining step associated with one act of the overhyphen;all hydrogen evolution reaction (i.e., the stoichiometric number mgr;). The application of mgr; has been extended to hydrogen overpotentials greater than about minus; 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 ratehyphen;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 observedpH 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 ratehyphen;determining discharge step can take place from a nickel surface substantially occupied with hydrogen atoms.
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