Note: Descriptions are shown in the official language in which they were submitted.
- 1 -
The present invention xelates to improved
cathodes for use in elec-trolytic cells.
This application is a division of Canadian
Patent Application Serial Number 365,389, filed
November 25, 1980.
The cathodes of this invention have
improved surface coatings on their active sides
which substantially lowers the hydrogen overvoltage
and results in a more efficient operation of the
electrolytic cell. The cathodes of the present
invention are particularly useful in the electrolysis
of aqueous solution~ of alkali metal halides to
produce alkali metal hydroxides and halogens, or
:in the electrolysis of aqueous solutions of alkali
metal halides to produce alkali metal halates, or
in water electrolysis to produce hydrogen.
In an electrochemical cell, large quantities
of electricity axe consumed to produce alkali me~al
hydroxides, halogens, hydrogen, and alkali me-tal
halates in electrochemical processes familiar to
those ski.lled in the art. With increased cost of
energy and fuel, -the savings of electricity, even
in relat.ively min~r amounts, is of great economic
advantage to the commercial operator of the ceLl.
Therefore, the ability to efEect savings in elec
tricity t:hrough cell oper~tion, cell design, or
im~)rovement in components, such as anodes and
cathodes, is of increasing significance~
In such electrolytic pro~esses3 hydrcgen is evolYed at the
cathode. and the overall reaction may be the~retically represented
~s:
~ 2~ ~ 2 e~ ~ H~ ~ 2 ~
However~ the cathode reac~ion actually produces monoatomic
hydrogen on -the cathode surface, and consecutive stages of re~ction
(13 can be represented as follows:
H O ~ e ~ H ~ OH
(2)
2 ff ~ 2
or as:
~2~ ~ e~ ~ H ~ OH
(3)
~ H2~ + e ~ + OH
The ~onatomic hydrogen generated as shown in reactions ~2)
or (3) is adsorbed on the surface of the cathode and desorbed as
hydrogen gas.
The Yoltage or potential that is rPquired in the operation
of an electrolytic cell includes the total of the decoMposition
voltage of ~he compound being electrolyzed, the voltage required
to overcome the resistance of the electrolyte9 and the ~oltage
required to overconle the resistance of the electrical connections
w~th~n the cell. In ~ddition, a potential, known as ~'overvoltage",
is ~lso required. ~he cathode overvoltage is the di~ference be-
tween the thermodynamic potent;ial 4f the hydrogen electrode ~a~
equilibrium) and the potential oF an electrode on which hydrogen
is evolved due to an impressed electr7c current. The cathode
overvoltage is related to such factors as the ~echanism of hy-
~5 drogen evolution and desorption, the current density, the tempe
rature and composition of the electrolyte. the cathode ~terial
and the surface area of the cathodeO
-- 3 -
In recent years, ;ncreas;ng attention has been directed to-
ward ;mprov;ng the hydrogen overYol~age characteristlcs of electro-
ly~ic cell cathodes~ In ~ddition to having a reduced hydr~gen
overvoltage, a oathode should also be constructed From materials
that are inexpensiYe, easy to fabricate~ mechanically stronga and
capable o~ withstanding the environmental conditions oF the electro-
lytic cell. Iron or steel fulfills many of these requlrelnents, and
has been the traditionzl material used comrnercially for cathode
fabrication 1n the chlor-alkali indus~ry. When a chlor-alkali cell
is by-passed. or in an open c;rcuit condition, ~he iron or steel
cathodes become prone to electrolyte att~ck and their useful life
is thereby significantly decreased.
Steel cathodes generally exhibit a cathode overvoltaye in the
range of from about 300 ~o about 500 millivolts under typical cell
operating conditions9 for example, at a temperature o-F about 100C.
and a current density of betw~en about 100 and about 200 ~illi-
amperes per square centimeter. Efforts to decrease the hydrogen
overvoltage of such cathodes have generally Focused on improving
the catalytic effect of the surface material or providing a larger
effectiYe surface area. In practice~ these efforts have frequently
been frustrated by cathodes or cathode coatings which have been
found to be either too expensive or which have only a l`;mited use-
ful life ln commercial operation.
Various co~tings have been sug~ested to improve the hydrogen
overYolta(Je characteristics of electroltyic cell cathodes in an
economically viable manner. A si~nificant number of the prior art
coatings have included nickel9 nr mixtures, allQys or intermetallic
compounds of nickel with ~arious other metalsO Frequently9 when
n~ckel is employed in admixture with another metal or compound~
the second metal or compound can be leached or extracted in a
solution of~ for example, sodium hydroxide~ to provide a high
surface area coatings9 such as Raney nickel coatings.
RepresentatiVe coatings of the prior art are dis losed in
U.S. Patent 3,2919714, issued December 13~ 19669 and U.S. Patent
3,350,2g4, issued October 31, 1967. These patents disclose _ter
alia cathode coatings comprising alloys of nickel-molybdenum or
__
nickel-molybdenum-tungsten electroplated on iron or steel sub-
strates. The electro-deposition of nickel-moly~denu~ alloys
utilizing a pyrophosphate bath is also discussed by Havey, Krohn,
and Hanneken in " The Electrodeposition of Nickel~Molybdenum
Alloys", Journal ~ Soc;et~, Yol. 110~ page
362, (1963).
Other attempts ha~e been made in the prior art to produce
coat;ngs of this general variety which offer an acceptable com-
promise ~etween coating life and low overvoltage characteristics.
U.S. Patent 4,105,532, issued August 8D 1978, and U.S. Patent
l~ ~,152,240, issued May 1~ 1979, are representative of these
attempts d~sclosing, respectively, alloys of nickel-Molybdenum-
vanadium and nickel-molybdenum using specially selected substrate
and intermedia~e coatings of copper and/or dendritic copper.
Simllar coatings are also disclosed in U.S. Patents 4,033,837 and
3,291D714.
The surface trea~nent of a Raney nickel electrode with a
cadmi~m n;trate solution for the purpose of reducing nydrogen
overvoltage has been investigated by Korovin, Kozlowa and Savel'eva
~n "Effect of the Trea~ent of Surface Raney Nickel with Cadmium
2~ Nitrate on the Cathodic Evolution of Hydrogen", 50viet Electro-
hemis~, Yol~ 14, page 1366 (1978). Although the initial
results of such a coating exhibit a good reduction in h~drogen
overvoltage, it has ~een found thak the overvoltage increases
rapidly to the original 1evel after a short period of opera~ion~
30 i.e. about ~ hours~ -
Even though many of the coatings described
above have been successful in reducing hydrogen
overvoltage, they have not proven entirely satisfac-
tory due to rapid deteriora-tion of the coating in
caustic environments, ultimately~leading to -the
separation of the coating from the substrate material.
It is thus a primary object of the present
invention ko provide cathodes suitable for use in
electrolytic c011s that are economical to prepare,
have reduced hyclrogen overvoltage charac-teristics,
and exhibit minimal deterioration after prolonged
operat:ion in electrolytic environments.
I~ere are d.isclosed cathodes for use in
electrolytic processes, and a method for producing
such cathodes. Such a cathode has at least part
of its surface portion formed ~rom a aodeposit of
a first metal selected from the group consisting of
.iron, cobalt, nickel, and mixtures thereof, a second
metal selected from the group consisting of cadmium,
mercury, lead, silver, thallium, bismuth, copper,
and mi~ture~ thereof, and a leachable third metal
or metal ox.ide, preferably se].ected from the group
consist.ing of molyhdenu~rl, manganese, titanium,
tunysten, vanadium,indillm, chromium, zinc . -their
2S ox.ides, and combinations thereof. Prefe~rably,
thi.s composition is applied as a coating to at least
a portion of a substrate material sui.tably selectPd
from cathode substrates known in the art such as,
for example, nickel, tltanium, or a ferrous metal,
such as iron or steel. The coatings are produced
by codepositing, preferably using an electroplating
bath or solution, a mixture of the three metals
~5
-- 6 --
or metal oxides on the substrate surfaceO If the
substrate is other than nickel, the substrate may
be coated with a thin intermediate layer of nickel
or alloys thereof, prior to depositing the active
cathode surface. The third metal or metal oxide i5
subsequently removed, suitahly by leaching using an
alkaline solution, such as an aqueous solution of an
alkali metal hydroxide to produce a cathode of the
invention. The leaching operation can be performed
prior to placing the cathod~in operation in an elec-
trolytic cell, or during actual operation in the cell
by virtue of the presence of an alkali metal
hydroxide in the electrolyte. Optionally, the cathodes
of the present invention can he heat treated either
before or a-fter at least partial leaching to improve
the performance even further. The preferred coating
of the present .invention comprises a codeposit of
nickel and cadmium.
The cathode of the invention comprises at
least an active surface portion formed fxorn a codeposit
of a first metal selected from the group consisting
of iron, cobalt, nickel, and mixtures thereof, and a
second metal selected from the group con.sisting of
cadmium, m~rcury, lead, silver, thalli.um, bismuth,
copper, and mixtures thereof. ~he first and second
meta:Ls are characteriæed as being substantia~ly
nonleachable, i.e. they are removed very slowl~, if
at al.l., by Leaching or extraction in an alkaline
solution. Cathodes can also be obtained with a
3Q third component namely a metal or metal oxide.preferably
selected from the group consisting of molybdenum,
manganese, titanium, tungsten, vanadium, indium,
~ 3~
chromium, zinc, their oxides, and combinations
thereof~
The third component is a leachable comp~nent,
i.e~ at least a substa~ial p~rtion of this component
is removable by leaching in an al~aline solution.
Hence, the proportions of the metals in the composition
can be initially expected to change during operation
in the cell, primarily due to the e~traction or
leaching of the third component. The leaching action
may be so extensive that virtually all of the third
component is removed from the codeposita Under such
circumstances, the absence of measurable amounts
of the third component does not have an adverse
effect on the performance of the cat'hode. In fact,
leaching actually improves the perforrnance of the
cathode by increasing the roughness and surface area
of the cathode surface.
The present invention i5 concerned with
cathodes having measurable quantities of only the
first and second metal components in the codeposit
after such leac'hing.
~ hus in accordance with the invention,
suitable cathodes can be formed frorll a codeposit
initially containing only the fir.st and second metal
components, provided that the surface of the cathode
has a rou~lness factor (defined as the ratio of the
measurable surEace area to the geometrical surface
area) 5uff'iciently hic~h enough to provide the
d~sired clecrease in hydrogen overvoltage~ An
3(~ acceptable surface roughness factor in the context
of this invention would be at least about 100, and
5~
preferably at least about l,000c Such cathodes can
be prepared, for example, using chemica~ vapor
deposition techniques, or by more conventional
techniques, such as thermal fusion of the metals
and subsequently etching the surface with a strong
mineral acid. In this particular embodiment, the
composition of the codeposit preferably contains from
about 0.5 to about 25 atomic percent, and most
preferably from about 1 to about 10 atomic percent,
of the second metal component.
In addition to the two metal components,
the composition may also include additional elements
or compounds due to the particular method utilized
for preparing the cathode. Such additional materials
may be present in amounts of up to about 50% based
on the total weight of the composition, and are
perfectly acceptable provided they do not adversely
afect the performance of the cathode.
The preferred metals of the present
invention are nickel,and cadmium, present in the range
of from about 0.5 to about 25 atomic percent, and
preferably 1 to about 10 atomic percent, of cadmium,
b~ased on the combined weight of nickel and caclmium,
the nickel comprising the halance of the codepo~it.
Such a cathode has been found to produce surprisingly
good results when utilized to electrolyze sodium
chloride. For example, hydrogen overvoltages in the
range of about 120 millivQlts of 150 ma/cm2 without
heat treatment, and 80 millivolts at 150 ma~cm2 after
heat treatment, are easily achievable using the cathode
surface of this invention w~en applied to a standaxd
ferrous substrate. These results can be even further
improved by the appropriate selection of substrate
material and cathode configuration, such as a
woven wire mesh, a foraminous sheet, or a perforated
and/or expanded me~al sheet. Furthermore, simulated
life testing of this cathode for a pe~iod of 90 days
in a 150 gr./liter caustic solution produces a
relatively constant cell voltage, indicating suitability
for long term operation in a cell.
Although the cathodes of the present
invention rnay be formed entirely from the compositions
described hereinabove, it is desirable, both from the
standpoint of mechanical durability and reduced
costs, to apply the codeposit in the form of a
coating to a suitable substrate material. The sub-
strate may be selected from any suitable materialhaving the requi.red electrical and mechanical pro-
per.ties, and the chemical resistance to the parti-
cular electrolytic solution in which it is to be used.
Generally conductive metals or alloys are useful,
~uch as ~errous metals (iron or steel), nickel,
copper, or vaLve metals such as tungsten, titanium,
-tantalum, niobium, vanadium, or alloys of these
metals, such as a titanium/palladium alloy containin~
0~2% pal.ladium. Because of their mechanical pro-
p~rties, eas0 of fabri.cation, and cost, ferrousme~tals, ~uch as iron or steel, are comrnonly used in
chloî-alkali cells. However, in chlorate cells wh~e
corrosion of the substrate material is a significant
problem, titanium or titanium alloys are preferred.
It may also be desirable to apply an intermediate
layer to the substrate material to protect the sub-
strate from corrosion in the electrolytic cell
- 10 -
environment. Suitable intermediate layers for this
purpose include nickel, nickel codeposited with
cadmium, and nickel codeposited with cadmium and f
bismuth.
The preferred method for applying the sur-
face coating to the substrate material is by electro-
deposition in a suitable electroplating solution or
bath. Although electrodeposition is a preferred
method of preparation primarily due to the favorable
economics of this particular proced~re, other methods
o~ applica-tion, such as vapor deposition, thermal
deposi.tion, plasma spraying or flame spraying are
also within the scope of this invention~
Prior to coating the substrate in the plating
bath, the substrate is preferably cleaned to insure
good a~lesion of th~ coating. Techniques for such
preparatory cleaning are conventional and well known
in the art. For example, vapor degreasing or sand
or grit blastinc; may be utilized, or the substrate
may be etched in an acidic solution or cathodically
cleaned in a caustic solution. If a substrate material
other than nickel is utilized in the present invention,
a plating of,nickë:L:~ suitably electrodeposited, may
be initially applied to the portion of the substrate
that is to be coated with the cathode surfac~.
After cleaning, the substrate can then be
directly immersed in a plating bath to simultaneously
cod~posit the components~ The basic electroplating
t~chnique which can be utilized in this invention is
3~ known in the prior art. For example, U.S. Patent
4,105,532, issued August 8, 1978, and Ha~ey, Krohn,
and Hannekin in "The Electrodeposition of ~ickel-
Molybdenum Alloys", Journal of_the Electroche_ical
, ~ol. 110, page 362 (1963), describe,
respectively~ typical sulfate and pyrophosphate
plating solutions. By way of illustration, a suitable
plating bath for codepositing a coating of nickel,
molybdenum and cadmium is described below:
Na2MoO~ 0002 M
~iC12 0.0~ ~
Cd(~03)~ 3.0 x 10 M
Na4P207 0~13 M
NaHC03 0.89 M
~2H4.H2S04 0~025 M
pH 7.5-9.0
Temperature 20~C.
Current Density 0~5 ASI
Plating Time 30 minutes
In general, the pH level of the plating
solution is significaht in the terms of the efficiency
o the platincJ operation. p~I levels in the range of
from about 7.5 to about 9.5 are preferred since a pH
of less than about 7.5 will tend to produce a
coating having a higher hydrogen overvoltaye, wh:ile
a pH of greater than about 9.5 will tencl to prec.ipitate
nickel hy~roxide which, being nonconducti.ve, will5 also increas~ the hyclroyen overvoltage.
Generally, other sources of nickel, molyb-
clenum, ~nd cadmium may be employed in -the plating bath
other than those speci-fically described above~ Other
soluble salts of the corresponding metals are acceptable.
Other complexing agents, such as citrates, other
5~
buffering agents and supporting electrolytes, and
other reducing agents may also be suitably utilized
in substitution for the corresponding ingredients
prescribed aboveO
The actual thickness of the coating will
depend, at least in part, on the duration of the
electroplating procedure. Coating thicknesses of
from about 2 to about 200 micr~ns are acceptable,
although thicknesses oE ~rom about 10 to about 50
microns are perhaps more useful. Coatings of less
than about 10 microns in thickness usually do not
have acceptable durability, and coatings of more than
50 microns usually do not produce any additional
operating advantages.
Although the concentrations and relative
proportions oE the various ingredients of the plating
bath are not critical, particularly good coating~
are produced when ~he concentration of the cadmium
ions in the bath is within the range of from about
1.5 x 10 4 M to ahout 6.0 x 10 4 M, and when the
relative proportion of molybdenum ions to nickel
ions in the bath is maintained at about 1~2. Such
coatings contain less than about 40 atomic percent
o molybdenum prior to leaching. It has also heen
2$ ~ound that small quantities of a soluble lead salt
when aclded to the plating bath advantageously improve
the efficiency oE the plating operation.
~le codeposit of the components may be in
the ~orm of a mixture, an alloy, or an inter-metallic
compound, depending on the particular conditions
utilized in preparing the codepositr
The term "codeposît", as used in the present speci-
fication and claims, includes any of the various
alloys, compourlds and inter~metallic phases of the
components and does not imply any particular method
or process of formulationO
After the coating has been deposited on
the substrate material, the third me~al component
of the coating, e.g. molybdenum, can then be removed.
q~is may be accomplished by immersing the coated
cathode in an alkaline solution to leach the molyb-
denum component. Typi~ally, a 2 to 20% by weight
aqueous solution of sodium or potassium hydroxide
for a period of about 2 - 100 hours, suitably at
about ambient temperature, can be utilized. If
stronger alkaline solutions are employed, or if the
alkaline solution is heated, for instance from 50C.
to 70C. shorter leaching periods are pos~ible.
Alternatively, the electroplated cathode can be placed
directly into service in an electrolytic cell,with
- the leaching or extraction being carried out in situ
in the cell by the electrolyte during cell operation~
Pa-~ticularly good coatings have been obtained
by heat treating the coating either before, during
or after removal of a portion of the molybdenum
com~)ollent. Generally, the heat treatment can be
carried out at temperatures of from about 100C, to
about 350C. or a period of from about 1/2 hour to
about 10 hours. The heat treatment is preferably
carried out in an atmosphere in which the coating is
inert, for example, argon, nitrogen, helium or neon
are applicable, although o~ygen-containing atmospheres,
~ s~
- 14 -
can be used for convenience~
It is particularly advantageous and con-
venient to heat treat the coated cathode con~urrently
with a polymer-reinforced diaphragm which has been
deposited on the cathode, In fact, it is perfectly
acceptable to perform the entire plating operation in
a diaphragm cell container using con~entional
dimensionally stable anodes. Under these conditions,
the heat treatment can be accomplished in about one
hour at about 275C.
I~e cathodes of the present invention have
applications in many types of electrolytic cells
and can function effectively in various electrolytes.
Cathodes haviny an assortment of configurations and
designs can be easily coated using the electroplatin~
technique of this invention, as will be unde~stood
by those skilled in the art.
~ he following examples further illustrate
and describe the various aspects ~f the invention,
but are not intended to limit it~ Various modifi-
cations can be made in the invention without departing
from the spirit and scope thexeof, a~s will be read:ily
appreciated hy those skilled in the art. Such modi-
fications and variations are considered to be within
th~ purview and scope of the appended claims.
Unless otherwise specified, temperatures in
the followin~ examples are in dec~rees centigrade, and
all parts and pexcentages are ~y weight. Hydrogen
overvoltages were measured using a reversible hydrogen
reference electrode.
~ 5
EXAMPLE 1
Two nickel plates were cleaned and i~mersed respectively in
two 267 millil;ter Hull cellsO The first Hull cell contained an
aqueous bath of 0.02 M Na2MoO4; 0.04 M NiCl~; 0.13 M Na4P~07;
0.89 M NaHC03; and 0.02~ M N2H4.H2S04. The second Hull cell
contained the same bath but also included 3.0 x 10 4 M Cd~N0332.
Both Hull cells were connected in series, and the pla~ing was
carried out at 20C. at a total curren~ of 4A for 30 minutes.
Two 2 x 2 cm. plated electrodes were cut out of each of the nickel
plates9 and were leached in 20X NaOH for 15 hours at 70C. The
electrodes were tested as hydrogen cathodes in a solution of 150
9.~1 NaOH ancl 170 g./l. NaCl at 95C. and a current densi~y of
300 ma/cm2. A hydrogen overvoltage of 184 mv. was recorded for
the control electrode plated in the first Hull cell withDut cad-
mium, and a hydrogen overvoltage,of 144 mv. was recorded for the
electrode plated in the Hull cell containing cadmium.
EXAMPLE 2
The procedure of Example 1 was repeated to codeposit nickel,
molybdenu~ ~nd cadmium on a nickel expanded mesh screen (50~ open~
at an average impressed current of 0.65 ~ or 30 minutes~ The
electrode was subsequently leached in 20% NaOH at 70C. for 15
hours, and heat treated at 275C. for 1 hour. The electrode~ was
tested as a hydrogen cathode ~ollowin9 the proceclure of Example
1, and a hydrogen overvoltage of 87 mv. was recorclecl.
EXAM~PIE 3
?~r' The procedure of Example 2 was repeated except that the
cadmium content of the bath was reduced to 1.5 x 10 4 M. The
electrode was again tested as a hydrogen cathode fo'llowing the
procedure of Example 2, and a hydrogen overvol-tage of 108 mY. ~las
recorded .
- 16 -
A comparison of the results illustrated in Examples 1-3
demonstrates ~he improvement in hydrogen overvoltage obtained by
the cathodes o~ the present invention as compared to the contrDl
cathode of the prior art. In particular, Example 1 demonstrates
that a 40 millivolt reduction in hydrogen overvo1tage is achieved
by the cathodes of the present invention. Further improvements
obtained by heat treating the cathodes ~re demonstra~ed in Examples
2 and 3 for varying concentrations o~ cadmium in the plating ba~h.
