February, 1953 issue of Plating
A Theory of Hydrogen Overvoltage
and Its Relation to the Electrodeposition of Metals
George Dubpernell, United Chromium
Inc., Waterbury, CT and
Roberta Dubpernell, Formerly of the Naugatuck Chemical Division, United States
Rubber Company, Naugatuck, CT
While the Haring Cell has been widely used lor about twenty-five years, it seems
to have been employed entirely in studies of metal deposition, especially copper,
nickel, and silver. Ferguson describes a special Haring Cell arrangement for
studies of polarization both with and without metal deposition, but the authors
are not familiar with any published reports on polarization or hydrogen overvoltage
Stareck and Taft give results
on plain KCN solution between platinum electrodes, but do not deduct the IR
drop measured in the center compartment, and do not report the IR drop measurements.
Using Ag CN and KCN they observed a black silver deposit at the cathode at high
current densities, which was most stable in alkaline solutions, and which evolved
hydrogen gas and turned white upon decomposition. They assumed this to be a
sil-ver hydride containing negative hydrogen, and noted that it had strong reducing
properties. On the basis of the views in this paper, this black deposit would
more simply be regarded as an electronegative metal alloy of silver, and this
would explain its reducing properties without the need to assume the existence
of negative hydrogen or nascent hydrogen.
The term nascent hydrogen
has been used frequently for over one hundred years to describe reactions not
attributable to hydrogen as it is ordinarily known, and yet there does not seem
to be closer approach to an understanding of the matter than formerly. The use
of the term was questioned by Reed and others, and a review is given in Gmelin.
Many workers refer also to such things as atomic hydrogen, overvoltage
compounds, overvoltage hydrides, active material,
and so on, without justifying their use of these terms.
The presence of atomic
hydrogen at the cathode was first suggested as a pure speculation by Ostwald,
who also suggested the rate of combination of the atoms as a possible mechanism
for the explanation of overvoltage. This type of explanation has been taken
up by many more recent workers with enthusiasm, even though it was not mentioned
in a later paper by Ostwald on Electrolysis and Catalysis, and the
concept of the secondary formation of hydrogen at the cathode after primary
deposition of an electronegative metal, was expounded at some length.
The properties of atomic
hydrogen are well reviewed by Glockler and Lind, and in Gmelin. Langmuir studied
it in some detail, but stated that he felt that it was a mistake to try to account
for overvoltage by means of infinitesimal concentrations of atomic hydrogen,
and again that slowness of progress in studies of overvoltage and passivity
. . . has probably been largely caused by an undue emphasis on what may
be called the thermo-dynamic viewpoint in electrochemistry. He established
that atomic hydrogen is only stable in appreciable quantities at temperatures
in the range of 2,000° C to 3,000° C or above. It may be stable for
a short time at room temperature in a high vacuum in glass tubes, but recombines
instantly to form molecular hydrogen in the presence of any oxygen or water.
It has some reducing power, but nothing comparable to cathodic reduction, or
that of metallic sodium.
Another reason for assuming
the existence of atomic hydrogen at cathodes is the diffusion of hydrogen sometimes
noted through thin metal foils, and the high- pressures sometimes built up inside
of sealed metal tubes treated cathodically, or inside of crevices in metal so
as to cause blistering. Ferguson and Dubpernell have shown that hydrogen does
not diffuse through even very thin foils of platinum or palladium, and that
the phenomena are due to porosity of the metal, and capillary and surface conductivity
effects. These vie-s have not yet received very wide acceptance, but make any
assumption of the existence of atomic hydrogen at room temperature unnecessary.
A recent paper by Fischer
and Heiling is accepted as demonstrating the diffusion of atomic hydrogen or
cathode polarization from the outside into the inside of a thin, annealed carbonyl
iron beaker in N H2SO4. However neither the thickness nor the exact method of
production of the iron beakers used is given, and apparently they were not tested
for porosity. There was probably a direct solution connection between the outside
and the inside, since the cathode polarization would increase on the inside
as much as 0.3 volt or more in a few seconds following an increase on the outside.
Likewise, every detail of polarization on the outside was reproduced on the
inside, but with lower values.
The controversy which followed
Le Blanc and Ostwalds suggestion of the primary rather than secondary
formation of hydrogen at the cathode, is reviewed in some detail by Sack. Arrhenius
reported the results of numerous experiments in which he found a substantial
time required to get hydrogen evolution on mercury cathodes from neutral salt
solutions, and which he explained as due to the primary deposition - of the
alkali metal before hydrogen evolution occurred. Le Blanc replied at some length,
and felt that hydrogen evolution occurred immediately and directly, but went
unobserved for one reason or another.
Nernst also investigated
the matter, and reached the conclusion that while primary hydrogen evolution
is possible, in practice it is mostly secondary. A student of Nernsts,
Glaser, published a more detailed study in which it was concluded that while
theoretically hydrogen may deposit primarily, in actuality, at moderate current
densities it is mainly formed secondarily.
Bancroft first favored the
primary deposition of sodium and secondary formation of hydrogen. In discussing
this 1905 paper, C. J. Reed favored the direct deposition of the more
easily reducible hydrogen, while J. W. Richards emphasized that sodium
deposits primarily on mercury, with no hydrogen evolution. In 191643 and 192044
Bancroft favored the assumption of the existence of atomic hydrogen at the cathode
to explain overvoltage, but in 1929 emphasized the direct deposition of sodium,
and the possibility of reproducing any cathode reaction chemically with sodium.
Fichter and co-workers appear
to have followed the suggestion of Bancroft, in connection with organic reductions.
On page eight of his book, Fichter refers to a number of his publications and
states that it has been shown in numerous examples that an alloy of pure lead
and sodium gives exactly the same reduction effect as a lead cathode in acid
solution. Similarly, Parker and Swann found that reduction of sugars in acid
solutions at zinc and lead cathodes only took place if an alkaline film could
be maintained at the cathode surface, and that, if the catholyte was free of
sodium ions, no reduction was obtained with an acid concentration above 0.1N.
Many phenomena become more
understandable in the light of the theory of cathode polarization here presented.
The change in surface tension of the mercury in the Lippman capillary electrometer
is probably partially associated with the deposition of a minute amount of sodium,
as pointed out by Freundlich. Edison discovered in 1874 in connection with the
development of telephone and telegraph equipment that friction was decreased
on a cathodically polarized metal surface and increased on the anode. The phenomenon
was studied by a number of workers including Arons, but no satisfactory explanation
was found. The effect might be due to a lubricating action of caustic soda formed
at the cathode, on the surface alloyed with alkali metal. The electrical
lubrication of equipment for handling clay by making the surfaces cathodic
might involve a similar explanation, although this lubricating effect is usually
ascribed to the production of a layer of water at the cathode by electroendosmosis.
In polarography, using the
dropping mercury electrode, the limiting current or diffusion
current which gives the concentration of the substance being determined,
is stabilized and made more reproducible by working in the presence of an indifferent
salt or supporting electrolyte, generally 0.1N KCl. This eliminates
what is called the migration current and leaves the diffusion current
as a more measurable quantity. It also tends to eliminate hydrogen waves
and has other desirable effects. When the alkali or alkaline earth metals are
being determined, a different supporting electrolyte is used such as tetramethylammonium
hydroxide. The observations in this field should be more consistently understandable
on the basis of the concept of hydrogen overvoltage proposed in the present
Potentials frequently seem
to involve adsorbed or otherwise held sodium or other alkali metal ions. The
glass electrode is a sodium electrode to a considerable extent according to
Dolea. MacGillavry, et al found little effect in acid solution, but substantial
negative potentials on nickel immersed in alkaline solutions.
The salt effect in connection
with hydrogen overvoltage was investigated by de Bethune, who observed many
irregularities using a mercury cathode with various concentrations of hydrochloric
acid and potassium chloride. When the concentration of acid was low and that
of potassium chloride high, he obtained potassium deposition with 100 per cent
current efficiency and overvoltages around 1.? volts or higher. By working carefully
at low current densities, and with more acid and less salt, he considered that
the mercury cathode functions as a hydrogen electrode, and measured overvoltages
up to about 1.2 volts. It is felt that it is much more reasonable to regard
the cathode potentials as due entirely to potassium rather than hydrogen, and
then proceed to ascertain if any of the polarization at low current densities
may be ascribed to hydrogen or not. The slight increase in overvoltage observed
in the more dilute solutions of acid is probably due to the relatively greater
effect of alkali dissolved from the glass, and future work should include the
avoidance of glass apparatus.
The importance of avoiding
the use of glass in some electrochemical experiments appears to be generally
overlooked. Much of modern electrochemistry could be said to be the electrochemistry
of glass containers. Lavoisier showed in 1770 that water only left a residue
if evaporated in glass containers, and that if the water was distilled and evaporated
in metal vessels it was pure and evaporated with no residue.
Sir Humphry Davys showed
by using gold containers that no alkali is formed at the cathode when electrolyzing
water, and that the alkali is ordinarily due to the glass containers used. By
working in a hydrogen atmosphere with the gold containers he was able to show
that pure water is decomposed into hydrogen and oxygen alone, and DO alkalinity
is formed at the cathode, or acidity in the anode compartment. It would
seem essential for modern electrochemists to be familiar with Davys work,
and to repeat and extend his experiments.
RELATION TO THE ELECTRODEPOSITION
OF METALS AND THE STRUCTURE OF DEPOSITS
The electrodeposition of metals from complex salts such as cyanides is sometimes
considered to involve the primary deposition of the alkali metal and secondary
formation of the heavy metal coating. Hedges & showed that the electrodeposition
of metals from cyanide solutions takes place through a secondary reaction, with
periodic hydrogen evolution when the cathode becomes more saturated with codeposited
alkali metal. Mercury would not deposit from 0.1N mercuric chloride until 0.1N
potassium chloride was added. Periodic codeposition was also observed with mercury
and sodium, barium, strontium, calcium, and aluminum. Tin deposition from
caustic soda solution gave poor, slow periods lasting 15 minutes. This is of
the same order of magnitude as the periods of 20 minutes which Snavely calculated
to correspond to the laminations in certain chromium deposits.
Fink suggested that the
primary deposition of sodium and secondary deposition of metals gave fine grained
deposits. - In the discussion which followed, W. Blum and A. L. Ferguson protested
that no sodium deposition was possible under usual conditions, without, however,
considering the possibility of depolarization by the metal of the cathode (alloy
formation). The ease of depositing sodium on mercury at low potentials was emphasized
by Evans, in connection with the explanation of tungsten deposition from
aqueous solutions by the formation of an alloy. In the same discussion, the
senior author suggested that periodic gassing in many plating processes, as
in bright cadmium plating, is caused by the charging of the deposit with an
alkali metal, giving increased polarization, until finally hydrogen is evolved.
The agitation due to the hydrogen evolution brings up fresh solution, decreases
the polarization, and the cycle sets in all over again.
It is suggested that something
like this is involved in the formation of banded, laminated, striated, or layered
deposits frequently observed, especially in connection with the production of
bright plates. Thus Meyer and Meyer and Phillips found that bright cyanide copper
deposits produced by alloying with a small amount of lead or cadmium had a laminated
or banded structure when examined in cross section. They also determined that
periodic fluctuations of cathode polarization occurred which corresponded to
the thickness of the bands in the deposits. They reviewed a number of previous
observations of banded or striated deposits of iron, cobalt, nickel, and copper.
It seems noteworthy that
cadmium and lead are also brighteners for nickel baths, and are in addition
the highest cathodic overvoltage metals, aside from mercury. It is not meant
to suggest that bright copper or nickel deposits containing cadmium or lead
also contain noteworthy amounts of alkali or other electronegative metals. These
electronegative metals probably participate in the formation of the deposits
and determine the cathode polarization and pH of the cathode film, without,
however, remaining in the deposit in appreciable quantities. Nevertheless, the
reducing power and ability to later give off hydrogen sometimes noted in freshly
made deposits, is probably an indication of the presence of some residual alkali
metal. The banded structure observed may be due to varying grain size, or varying
quantities of inclusions associated with varying pH of the cathode film.
Many studies of the banded
structure of nickel deposits have been published but the cause of the phenomenon
does not seem to be understood. A current paper gives an excellent review. In
his paper on the mechanism of bright plating, Henricks discussed periodic deposition
at some length. The present paper merely adds to all this the fact that the
periodic deposition of an alkali or similar metal is probably also involved,
and in fact is probably a controlling factor.
The authors wish to express their appreciation to United Chromium, Inc., for
encouragement and assistance with many phases of the investigation, and particularly
to the engineering department for help in the design of equipment and preparation
of the paper for publication. They also wish to acknowledge their substantial
indebtedness to Dr. Paul Doigan and others at the University of Connecticut
where the experimental work was done. Flame photometer measurements were made
under the supervision of J. Petrocelli and G. Tatoian at the Patent Button Company,
in collaboration with M. Orient and P. Mazzamaro of United Chromium, Inc.