Electrolysis of alkali metal chloride in a cell having a nickel-molybdenum cathode

Disclosed herein is a cathode having an electroconductive substrate and a porous surface. The porous surface is characterized by containing a major portion of nickel and a hydrogen overvoltage reducing amount of molybdenum. The molybdenum may be present as elemental molybdenum, as an alloy with nickel, or as a molybdenum compound. Also disclosed is an electrolytic cell having an anode, a cathode, and a separator between the anode and cathode, where the cathode is characterized by a porous surface having a major portion of nickel and a hydrogen over voltage reducing amount of molybdenum, which molybdenum may be present as elemental molybdenum, and molybdenum alloy with nickel or a molybdenum compound. Further disclosed is a method of electrolyzing an alkali metal chloride brine by passing an electrical current from an anode to a cathode to evolve chlorine at the anode and hydroxyl ion at the cathode. The cathode is characterized by a porous surface containing a major portion of nickel and hydrogen overvoltage reducing amount of molybdenum. Also disclosed is a method of preparing a porous nickel electrode by flame spraying nickel bearing particles, leachable constituent bearing particles and molybdenum bearing particles onto a metal substrate and leaching out the leachable constituent whereby to form the porous surface.

DESCRIPTION OF THE INVENTION 
Alkali metal hydroxide and chlorine are commercially produced by 
electrolyzing an alkali metal chloride brine, for example an aqueous 
solution of sodium chloride or an aqueous solution of potassium chloride. 
The alkali metal chloride solution is fed into the anolyte compartment of 
an electrolytic cell, a voltage is imposed across the cell, chlorine is 
evolved at the anode, alkali metal hydroxide is evolved in the electrolyte 
in contact with the cathode, and hydrogen is evolved at the cathode. 
The overall anode reaction is: 
EQU Cl.sup.- .fwdarw.(1/2)Cl.sub.2 +e.sup.- ( 1) 
while the overall cathode reaction is: 
EQU H.sub.2 O+e.sup.- .fwdarw.(1/2)H.sub.2 +OH.sup.- ( 2) 
More precisely the cathode reaction is reported to be: 
EQU H.sub.2 O+e.sup.- .fwdarw.H.sub.ads +OH.sup.- ( 3) 
by which the monoatomic hydrogen is adsorbed onto the surface of the 
cathode. In alkaline media, the adsorbed hydrogen is reported to be 
desorbed from the cathode surface according to one of two processes: 
EQU 2H.sub.ads .fwdarw.H.sub.2, or (4) 
EQU H.sub.ads +H.sub.2 O+e.sup.- .fwdarw.H.sub.2 +OH.sup.- ( 5) 
The hydrogen desorption step, that is either reaction (4) or reaction (5) 
is reported to be the hydrogen overvoltage determining step. That is, it 
is the rate controlling step and its activation energy bears a 
relationship to the cathodic hydrogen overvoltage. The hydrogen evolution 
potential for the overall reaction (2) is on the order of about 1.5 to 1.6 
volts measured against a saturated calomel electrode (SCE) on iron in 
alkaline media. Approximately 0.4 to 0.5 volt represents the hydrogen 
overvoltage on iron while 1.11 volts is the equilibrium decomposition 
voltage. 
Iron, as used herein to characterize cathodes includes elemental iron such 
as carbon steels, and alloys of iron with manganese, phosphorus, cobalt, 
nickel, molybdenum, chromium, vanadium, palladium, titanium, zirconium, 
niobium, tantalum, tungsten, carbon and the like. 
As disclosed herein, it has been found that the hydrogen overvoltage may be 
reduced, for example, to from about 0.04 volt to about 0.20 volt by 
utilizing a cathode having a conductive substrate and a porous catalytic 
surface of nickel containing an effective amount of either molybdenum or 
an alkali-resistant molybdenum compound or both for example, elemenal 
molybdenum, an alloy of molybdenum and nickel, molybdenum carbide, 
molybdenum boride, molybenum nitride, molybdenum sulfide, or molybdenum 
oxide. 
According to a still further exemplification of this invention, it has been 
found that a particularly desirable electrolytic cell may be provided 
having an anode, a cathode, and permionic membrane therebetween to 
separate the anolyte compartment from the catholyte compartment, wherein 
the cathode is characterized by a conductive substrate, a porous catalytic 
surface of nickel, and an effective amount of molybdenum or a molybdenum 
compound in the porous nickel surface, where the molybdenum compound is as 
described above. 
According to a still further exemplification of this invention, it is 
possible to electrolyze alkali metal halide brines by feeding the alkali 
metal halide brine to the anolyte compartment, evolving the halogen at the 
anode, and hydroxyl ion at the cathode, where the cathode is characterized 
by a conductive substrate, with a porous catalytic surface of nickel on 
the substrate, which porous catalytic surface being further characterized 
by the presence of an effective amount of either molybdenum or an alkali 
metal hydroxide-resistant molybdenum compound as described above. 
According to a still further exemplification of the method of this 
invention, a cathode is prepared having an electro-conductive substrate 
with a porous nickel catalyst containing an effective amount of molybdenum 
compound therein by flame spraying nickel bearing particles, as alloys or 
as the separate elements, leachable constituent bearing particles, and 
molybdenum bearing particles as alloys or as the substrate elements, onto 
a metal substrate and leaching out the leachable constituent whereby to 
form a porous surface. 
By an effective amount of molybdenum or a molybdenum compound is meant an 
amount that is sufficient to either reduce the initial overvoltage of the 
porous nickel surface, or to maintain the low overvoltage of the porous 
nickel surface at a low value after extended periods of electrolysis, or 
to both reduce the initial overvoltage of the porous nickel surface and to 
maintain a low overvoltage over extended periods of electrolysis. 
DETAILED DESCRIPTION OF THE INVENTION 
As contemplated herein, the cathode comprises an electro-conductive 
substrate having porous nickel surface, which porous nickel surface 
contains an effective amount, i.e., an overvoltage reducing or overvoltage 
stabilizing amount of either molybdenum or an alkali-resistant molybdenum 
compound or both. 
The substrate is typically an iron substrate. As used herein, iron includes 
elemental iron, iron alloys, such as carbon steels, and alloys of iron 
with manganese, phosphorus, cobalt, nickel, chromium, molybdenum, 
vanadium, palladium, titanium, zirconium, niobium, tantalum, tungsten, 
carbon, and the like. However, the electro-conductive substrate may also 
be an electro-conductive metal such as aluminum, copper, lead, or the 
like, having a suitable alkali-resistant surface thereon. Alternatively, 
the substrate can be cobalt, nickel, molybdenum, tungsten, or other alkali 
resistant metal. According to one particularly preferred exemplification, 
the electroconductive substrate has a nickel surface thereon whereby to 
protect the substrate from attack by concentrated alkali metal hydroxide 
catholyte liquors. 
According to one particularly desirable exemplification of the invention, 
the substrate, especially an iron substrate, has a thin coating, for 
example, a coating of from about 20 to about 125 micrometers of nickel 
whereby to provide a barrier for corrosive attack of the substrate and to 
prevent undermining of the porous surface by the catholyte liquor. 
The substrate itself is macroscopically permeable to the electrolyte but 
microscopically impermeable thereto. That is, the substrate is permeable 
to the bulk flow of electrolyte through individual elements thereof such 
as between individual rods or wires or through perforations, but not to 
the flow of electrolyte into and through the individual elements thereof. 
The cathode itself may be a perforated sheet, a perforated plate, metal 
mesh, expanded metal mesh, metal rods, or the like. 
The catalytic surface has a Brunnauer-Emmett-Teller surface area of from 
about 1 to about 100 square meters per gram, and a porosity of the active 
surface of from about 0.5 to about 0.9. 
The surface itself is characterized by pores, fissures, peaks, and valleys. 
Generally, when examined under a scanning electron microscope, the surface 
appears as having been formed by partially molten or deformable particles 
impacted against the substrate which partially molten or deformable 
particles are thereafter leached. 
The porous catalytic surface has a hydrogen evolution voltage less than 
about 1.21 volts versus a saturated calomel electrode and 0.97 volt versus 
a normal hydrogen electrode at 200 Amperes per square foot in aqueous 
alkaline media. 
The surface comprises nickel and molybdenum. The nickel is generally above 
about 50% and less than about 95%, and generally from about 65 to about 90 
percent nickel, calculated as nickel metal, basis total weight of the 
porous active surface. 
The molybdenum is present in the porous catalytic surface in a hydrogen 
overvoltage lowering amount. This is above about 2.5%, preferably above 
about 5%, but below about 50%, and generally from about 10 to about 35 
weight percent, calculated as molybdenum metal, basis total nickel 
calculated as metal and molybdenum calculated as metal in the surface. 
Generally, the amount of molybdenum in the surface is high enough to have 
a hydrogen overvoltage lowering effect, but low enough to avoid the high 
overvoltage identified with porous surfaces that are mainly moldybdenum. 
While the mechanism of the hydrogen over voltage lowering effect of the 
molybdenum is not clearly understood, it is known that porous molybdenum 
alone is high in hydrogen overvoltage, but that a low hydrogen overvoltage 
over extended periods of electrolysis is observed when molybdenum is used 
in conjunction with porous nickel. The molybdenum is believed to 
depolarize or catalyze one step of the hydrogen evolution process. For 
this reason, the upper limit of the molybdenum is below the concentration 
at which the surface has the hydrogen overvoltage properties of 
molybdenum, i.e. below about 50 percent and generally below about 35 
percent. 
The molybdenum itself may be present as elemental molybdenum, that is as 
molybdenum having a formal valence of 0, as an alloy with nickel or as a 
alkali-resistant compound such as molybdenum carbide, molybdenum nitride, 
molybdenum boride, molybdenum sulfide, molybdenum phosphide, molybdenum 
oxide, or any molybdenum compound that is insoluble in concentrated alkali 
metal hydroxide. Preferably, the molybdenum is present as elemental 
molybdenum, a molybdenum alloy with nickel, or molybdenum carbide. 
One particularly outstanding cathode of this invention is one having a 
perforated iron plate substrate, a thin layer of electro deposited nickel 
about 20 to about 125 micrometers thick, and a porous surface of nickel 
and molybdenum containing about 82 weight percent nickel, and about 18 
weight percent molybdenum basis total nickel and molybdenum and having a 
porosity of about 0.7 and a thickness of about 75 to about 500 
micrometers. 
According to a further exemplication of the method of this invention, the 
cathode herein contemplated is prepared by depositing a film of nickel, 
molybdenum, and a leachable material, and thereafter leaching out the 
leachable material. 
The leachable material may be any metal or compound that can be 
co-deposited with nickel and molybdenum or with nickel compounds and 
molybdenum compounds and leached out by a strong acid or strong base 
without leaching out significant quantities of the nickel or molybdenum or 
causing significant deterioration or poisoning of the nickel or 
molybdenum. 
The film may be deposited by flame spraying particles of nickel, 
molybdenum, and leachable materials, or electrodeposition of nickel, 
molybdenum, and leachable material, or by codeposition of solid particles 
and an electrodeposited film which film attaches the particles to the 
substrate, or by chemical deposition for example, by hypophosphite 
deposition or by tetraborate deposition of nickel compounds, molybdenum 
compounds, and leachable materials, or even by deposition and thermal 
decomposition of organic compounds of nickel, molybdenum, and leachable 
materials, for example, deposition and thermal decomposition of 
alcoholates or resinates. 
According to one particularly desirable exemplification, of the method of 
preparing the electrode of this invention, fine particles for example on 
the order of about 0.5 to 70 micrometers in diameter, of nickel, 
molybdenum or a molybdenum compound, and leachable material are impacted 
against the substrate at a temperature high enough to cause some 
deformation of the particle and adherance of the particle to the electro 
conductive substrate. 
The leachable materials may be present in the particle with the nickel or 
may be separate particles. Typical leachable compounds include copper, 
zinc, gallium, aluminum, tin, silicon or the like. Especially preferred 
for flame spray deposition are nickel particles containing about 30 to 
about 70 percent nickel, balance aluminum, as Raney alloy. In the 
exemplification of the method of this invention, where Raney alloy is 
flame sprayed against the porous substrate, the temperature of the flame 
spray is about 2200 to about 3100 degrees Centigrade whereby to provide 
deformable particles which adhere strongly to the substrate. The 
temperatures herein contemplated may be provided by a flame spray of 
oxygen and acetylene or oxygen and hydrogen. 
The flame spray continues to build up individual coats, to a total 
thickness from about 10 to about 50 micrometers in order to obtain a total 
thickness from about 75 to about 500 micrometers. Thereafter, the surface 
is leached in alkali, such as 0.5 normal caustic soda or 1 normal caustic 
soda, in order to remove aluminum, and thereafter rinsed with water. It 
is, of course to be understood that some of the leachable material may 
remain in the porous electrode surface without deleterious effect. Thus, 
for example, where Raney nickel-aluminum alloy, and molybdenum are flame 
sprayed, the surface may contain nickel, molybdenum, and aluminum, after 
leaching. The resulting surface, may, for example, contain amorphous 
nickel, crystalline molybdenum, nickel-aluminum alloys, and traces of 
alumina. 
According to a particularly desirable method of this invention, the leached 
nickel-molybdenum bearing substrate is annealed at a temperature of above 
about 200.degree. C. and below temperatures dictated by the thermal 
expansion differentials of the substrate and porous surface, for example 
between about 200.degree. C. and 600.degree. C. in a suitable nonoxidizing 
atmosphere such as a hydrogen atmosphere, a nitrogen atmosphere, or an 
inert atmosphere such as an argon or helium atmosphere, whereby to provide 
a particularly desirable cathode. 
Thus, according to one particularly desirable exemplification of the method 
of preparing a cathode according to this invention, the flame spray powder 
is prepared by mixing 90 grams of 0.5 to 15 micrometer Raney 
nickel-aluminum alloy power with 10 grams of 2 to 4 micrometer molybdenum 
powder and 10 to 15 grams of a spraying aid such as an amide of a fatty 
acid. The powder is then mixed, heated, broken up, and screened to obtain 
a minus 60 plus 250 mesh per inch fraction. One inch by four and three 
quarter inch by 13 gauge steel perforated plate, which has previously been 
sandblasted and the perforations filled with a cement, is scraped with 
silicon carbide bar and then flame sprayed with an adherent material. 
Thereafter, 10 coats of the flame spray powder are applied by flame 
spraying with 45 volume percent oxygen 55 volume percent acetylene. The 
cathode surface is then cooled, and leached in 0.5 normal caustic followed 
by leaching in 1 normal caustic. The cathode may then be annealed at a 
temperature of 400.degree. in argon and subsequently utilized as a cathode 
in an electrolytic cell. 
According to a still further exemplification of the method of this 
invention, an electrolytic cell may be provided having an anode, and a 
cathode separated from the anode by a permionic membrane. The anode has a 
valve metal substrate with a suitable electroconductive, electrocatalytic 
surface thereon. By a valve metal is meant a material that forms an oxide 
when exposed to acidic liquors under anodic conditions, such as titanium, 
zirconium, hafnium, niobium, tantalum, or tungsten. By a suitable 
electroconductive surface is generally meant a surface having a chlorine 
evolution overvoltage of less than (0.1 volt) at a current density of 200 
Amperes per square foot. Such surfaces include the titanium 
dioxide-ruthenium dioxide surfaces where the titanium dioxide is present 
in the rutile form which is isostructural with the ruthenium dioxide 
material. 
The permionic membrane is typically a cation selective permionic membrane 
of the type described for example, in U.S. Pat. Nos. 3,718,627; 3,784,399; 
3,882,093; and 4,065,366 having a perfluoro-alkyl backbone with pendant 
acid groups such as sulfonic acid groups, carboxylic acid groups, 
phosphonic acid groups, phosphoric acid groups, precursors thereof, or 
compounds thereof. The electrolytic cell herein contemplated further 
includes a cathode having an electroconductive substrate such as an iron 
substrate with a porous surface on the substrate, the porous surface 
having a major portion of nickel and an effective amount of molybdenum. 
The molybdenum may be elemental molybdenum, molybdenum carbide, molybdenum 
boride, molybdenum nitride, molybdenum sulfide, molybdenum oxide, or an 
alloy of molybdenum and nickel. The porous surface generally contains from 
about 10 to about 35 weight percent molybdenum, the balance being 
essentially nickel, with trace amounts of the leachable component, e.g., 
aluminum, also being present. 
According to a still further exemplification of the method of this 
invention, alkali metal chloride brine for example, sodium chloride brine, 
containing about 320 to about 340 grams per liter of sodium chloride is 
fed to the anolyte compartment of the electrolytic cell. The anolyte 
liquor typically contains from about 125 to about 250 grams per liter of 
sodium chloride at a pH from about 2.5 to 4.5 and is separated from the 
alkaline catholyte liquor by permionic membrane. Electrical current passes 
from the anode to a cathode of the electrolytic cell whereby to evolve 
hydrogen at the cathode and hydroxyl ion in the catholyte liquor. The 
concentration of sodium hydroxide in the catholyte liquor is generally 
from about 15 to about 40 weight percent. The cathode herein contemplated, 
having an electroconductive substrate with a porous nickel-molybdenum 
surface thereon is utilized in the process of the invention.

The following examples are illustrative: 
EXAMPLE 1 
A cathode was prepared by flame spraying fine Raney Nickel-Aluminum alloy 
powder and fine molybdenum powder onto a perforated steel plate and 
leaching the flame sprayed surface with aqueous sodium hydroxide. 
The flame spray power was prepared by mixing 90 grams of 0.5-20 micrometer 
Harshaw Raney Nickel-Aluminum alloy powder with 10 grams of 2 to 4 
micrometer Cerac molybdenum powder, and twelve grams of Cerac "Spray Aid" 
ammonium stearate. The mixed powder was then heated to 110.degree. C., 
where the mix turned gummy, but solidified upon cooling. The resulting 
solid was broken up in a mortar and pestle and screened to recover a minus 
60 plus 250 mesh per inch fraction. 
The steel plate, measuring 13 guage by 1.0 inch by 43/4 inches, was 
sandblasted. The perforations were then filled with a cement containing 3 
parts of Dylon "C-10" refractory cement and 1 part of H.sub.3 BO.sub.3, 
and the perforated plate was abraded with a silicon carbide bar. 
Thereafter the plate was flame sprayed with one coat of Eutectic Corp. 
Xuperbond nickel-aluminum bond coat. 
Thereafter ten coats of the powder described above were applied by flame 
spraying with an oxygen-fuel mixture of 45 volume percent oxygen and 55 
volume percent acetylene. 
After cooling, the coating was leached in 0.5 normal NaOH for two hours at 
25.degree. C., then in 1.0 normal NaOH for fifteen minutes at 25.degree. 
C. The cathode was then rinsed in water, blotted with a paper towel, and 
allowed to dry in air. 
The cathode was then tested in an electrolytic cell where it was separated 
from the anode by a DuPont NAFION 715 perfluorcarbon-perfluorocarbon 
sulfonic acid microporous diaphragm spaced 23/8 inch (53 millimeters) from 
the cathode. 
Electrolysis was carried out for 145 days. The cathode potential on the 
front surface of the cathode was between 1.139 and 1.154 volts, and the 
cathode potential on the back surface of the cathode was between 1.177 
volts and 1.190 volts, at a current density of 200 amperes per square 
foot. 
EXAMPLE II 
A cathode was prepared by flame spraying coarse Raney nickel-aluminum alloy 
powder and molybdenum powder onto a perforated steel plate, and thereafter 
leaching the flame sprayed surface with aqueous sodium hydroxide. 
The flame spray powder was prepared by mixing 90 grams of 1-70 micrometer 
Ventron Raney nickel alloy, 10 grams of Cerac 2 to 4 micrometer molybdenum 
powder and 12 grams of Cerac "Spray Aid" ammonium stearate. The powder was 
then heated, broken up, and screened as described in Example 1, above, to 
obtain a minus 60 plus 250 mesh per inch fraction. 
A one inch by four and three-quarter inch by 13 guage steel perforated 
plate was sandblasted, the perforations filled with a cement of 3 parts of 
Dylon "C-10" refractory cement and one part of H.sub.3 BO.sub.3. The 
surface of the plate was then scrapped with a silicon carbide bard, and 
then flame sprayed with Eutectic Corp. Xuperbond nickel-aluminum bond 
coat. 
Thereafter ten coats of the powder described above were applied by flame 
spraying with an oxygen-fuel mixture of 45 volume percent oxygen and 55 
volume percent acetylene. After spraying the cathode was cooled, and 
leached in NaOH as described above. 
The cathode was then tested in an electrolytic cell where it was separated 
from the anode by a DuPont NAFION 715 microporous diaphragm spaced 25/8 
inch (63 millimeters) from the cathode. Electrolysis was carried out for 
95 days. The cathode potential on the front surface of the cathode was 
between 1.153 and 1.160 volts, and the cathode potential on the back 
surface of the cathode was between 1.179 and 1.189 volts at a current 
density of 200 amperes per square foot. 
EXAMPLE III 
A series of three cathodes were prepared to determine the effect of 
annealing on cathodic properties. 
The flame spray powder prepared in Example I above, was utilized in 
preparing all of the cathodes for the tests. 
Three perforated steel plates, each measuring four and three quarter inches 
by one inch by 13 guage were sandblasted, had their perforations filled, 
and had their surfaces scrapped with silicon carbide, and were precoated 
with Eutectic Corp "Xuperbond," as described in Example II, above. Ten 
coats of the flame spray powder were applied to each plate as described in 
Example I, above. Thereafter, the cathodes were leached in aqueous sodium 
hydroxide, rinsed with water, and blotted, as described in Example I, 
above. 
The cathodes were then annealed in a tube furnace having a gas source and a 
one and one half inch diameter by twelve inch long tubular heating 
element. The cathodes were individually annealed as shown in the Table, 
and thereafter utilized as cathodes. Each cathode was separated from an 
anode by a DuPont NAFION 715 diaphragm. The results obtained are shown in 
the Table. 
Table 
______________________________________ 
Annealed Cathodes 
______________________________________ 
Annealing Gas H.sub.2 H.sub.2 Ar 
Annealing Temperature 
200.degree. C. 
400.degree. C. 
400.degree. C. 
Annealing Time 40 hours 16 hours 16 hours 
Days of electrolysis 
35 71 71 
Cathode voltage, 
front surface 1.174-1.180 
1.171-1.75 
1.157-1.159 
Cathode voltage, 
back surface 1.196-1.212 
1.193-1.214 
1.179-1.195 
(at 200 amperes per square foot). 
______________________________________ 
EXAMPLE IV 
A cathode was prepared by flame spraying Raney nickel-aluminum alloy powder 
and molybdenum carbide powder onto a perforated steel plate, and leaching 
the flame sprayed steel surface with aqueous sodium hydroxide. 
The flame spray powder was prepared by mixing 40 grams 1-17 micrometer 
Ventron Raney nickel-aluminum alloy, 10 grams of Starck-Berlin 1 
micrometer molybdenum carbide alloy; and 6 grams of Cerac Spray-Aid 
ammonium stearate. The mixed powder was processed as described in Example 
I, above. 
A perforated steel plate measuring 43/4 inches by 1 inch by 13 guage was 
sandblasted, its perforations filled with cement as described in Example 1 
above, its surface scrapped with silicon carbide, as described in Example 
1, above, and then flame sprayed with Eutectic Corp. "Xuper-Ultrabond 
3500" nickel-aluminum bond coat. Thereafter, ten coats of the Raney 
nickel-molybdenum carbide powder mixture was flame sprayed onto the 
substrated with an oxygen-fuel mixture of 45 volume percent oxygen and 55 
volume percent acetylene. 
The surfaced cathode was cooled, leached with aqueous sodium hydroxide, 
rinsed with water, blotted, and dried as described in Example 1, above. 
The resulting cathode was then tested for 39 days in a laboratory cell, as 
described in Example 1, above. The cathode potential of the front surface 
was 1.148 volts and the cathode potential of the back surface was 
1.175-1.182 volts at a current density of 200 amperes per square foot. 
While the invention has been described with reference to certain 
exemplifications and embodiments thereof, the invention is not to be so 
limited except as in the claims appended hereto.