160 
CHEMISTRY: MacINNES AND ABLER 
HYDROGEN OVERVOLTAGE 
By Duncan A. MacInnes and Leon Abler 
Chemistry Department, University of Illinois 
Communicated by W. A. Noyes, March 29, 1919 
Hydrogen overvoltage may be defined as the difference of potential that 
exists between a reversible hydrogen electrode, and an electrode, in the same 
solution, at which hydrogen, H2, is being formed from hydrogen ions. A 
reversible hydrogen electrode is one at which the reaction 
2H+ + 2€ = H2 (1) 
(e = electron) has reached equiUbrium. This equilibrium is attained, and 
maintained during the passage of very small currents, only on electrodes 
covered with a dispersed ^ noble' metal, such as platinum. If stronger cur- 
rents are passed across such a metal-electrolyte boundary, or if other electrodes 
are used, the reaction does not take place under equilibrium conditions and an 
overvoltage appears. For instance, it requires an overvoltage of about 0.2 volt 
to liberate hydrogen gas from a polished platinum surface, and about 0.7 
volt from a lead surface. 
On attempting to determine the overvoltage of small electrodes of 'plat- 
inized' platinum the authors observed some interesting fluctuations in the 
measured voltage, a typical series of measurements being plotted in figure 1. 
Here ordinates represent overvoltage in millivolts and abscissae time in 
seconds. Bubbles were evolved at the points marked by small circles, there 
being one bubble to each fluctuation. At the low current densities used, the 
bubbles came off at a single point on the electrode, making it appear probable 
that the nucleus of the next bubble remained on the electrode after each bubble 
had separated. 
When Reaction 1 takes place it is probable that the greater part of the 
liberated hydrogen goes directly into solution. Unless carried away by dif- 
fusion, stirring, or other means, the concentration of dissolved hydrogen, in 
immediate contact with the electrode, will tend to rise and produce a super- 
saturated solution. It is this layer of dissolved hydrogen that is responsible 
for the overvoltage. However, if there is a nucleus of gaseous hydrogen on 
the electrode a portion of the liberated hydrogen will enter this gaseous 
phase. A gaseous nucleus will thus play a similar role to that of a small 
crystal in a supersaturated salt solution. It is evident that hydrogen gas in 
the form of small bubbles must have a larger energy content, per mol of gas, 
than the same volume of undispersed gas, as energy must be expended in 
overcoming the surface tension in the formation of the bubbles. Such 
bubbles will be more soluble (i.e., remain at equihbrium with more concen-. 
trated dissolved hydrogen) than the undispersed gas. This is analogous to 
