Note: Descriptions are shown in the official language in which they were submitted.
~ 8
--1--
This invention pertains ~o a method for
prepa~ing elec~rodes and to their use i~ electrolytic
cells, for e~ample, brine electrolysis cells.
There are three general types of electrolytic
cells used or the production of chlor-alkali: (1) the
mercu ~ c~ll, (2~ ~he diaphragm-cell, and (3) the
membr~n~ c~ll. Th~ operation of each of these cells is
discu~sed i~ Volum~ I o~ the ThirdrEdition of the
KIR~C-OTEIMER ENCYI::LOPEDIA OF C~qICAL T~ 3NOLOGY, page
~` ~0 799 ~t. s~q. Qther electroIytic cells which employ
electro.des for electrolysis of agueous solutions are
the so alled "chlorate cells" which do not use a
divider or separator between the ca~hodes and anodes.
In the mercuxy cell, the alkali metal values produc~d
by electrolyzing a~ alkali mekal salt form an amalgam
~Ith the mercury; the amalgam, when react~d wi~h water,
produces N~O~ and ~ree~ the mercury which can be
recovered and cycled back for fur~her use as a liguid
cathode.
I~ many chlor-alkali electrolytic processes a
b A ne solution (electrolyte) is ~lectrolyzed by passing
31,010-F
-2-
electric current therethrough in a cell ha~ing a diaphraym
or a membrane positioned between the cathode and the
anode. Chlorine is produced at the anode while sodi.um
hydroxide (NaOH~ and hydrogen (~2) are formed at ~he
ca~hode. Brine is fed continuously to the cells, while
C12, NaOH and H2 are continuously withdra~m from the
cells.
~ he minimum voltage re~uired to electrolyze
an electrol;yte into Cl2, NaO~ and H2 may be calculated
using the thermodynamic data. However, in commercial
practice, the theoretical amount of voltage is not
achievable and higher voltages must be used to overcome
the various resistances i~here~t in the various types
of cells. To increase the efficiency of the operation
of a ~iaphragm or a membrane cell one may attempt to
reduce the over~oltages of the electrodes, to reduce
~he electrical resis-tance of the diaphragm or membrane,
or re~uce the electrical resistanc~ o~ the brine b~ing
electrol~ze~. The inventio~ herein described results
in an el~c~rode.particularly useful as a cathode in the
electrolysi~ o brine; cathode overvoltage is substantially
r~duced, resulti~g in increased power efficiencies.
Because of the multi million-ton quantity of
alkali metal haIides and water electrolyzed each year,
even a reduction of as little as 0.05 volts in working
voltage tr~nsla~es; ~o vexy meaningful energy savings.
Consequently, the industry has sought means to reduce
the voltage requirement.
Throu~hout the development of chlor-alkali
technoloyy, various methods have-be~n developed ~o
reduce the~cell voltage. Some practitioners have
31.,010-F ~2-
1~4~V(3B
concentrated on reducing cell voltage by modifying the
physical design of the electrolytic cell, while others
have concentrated their e~foxts on reduciny the over-
voltage at the anode or the cathode. The present
~- 5 ~4~ ~ e pertains, in part, to a novel process to
make an electrode that is characterized by a signifi-
. e ~-
cantly ~ overvoltage and to the use of these electrodes
in electrolytic cells.
It has been disclosed that a~ electrode's
overvoltage is a ~unc~ion of the curren~ density and
i~s composi~ion (reference: PHYSICAL CHEMISTRY, 3rd
ed., W. J. Moore, Prentice ~all (1962), pp. 406-408),
where the current density refers ~o ~he amperage applied
per unit of true surface area of an elec~rode and
composition refers ~o the chemical and physical makeup
o the electrode. Therefore, a process that will
inc~ease a~ electrod~'s ~urface area should decrease
its overvoltage at a give~ apparent current density.
I~ is also~ desirable ~o use a compositian of matter
tha~ is a good electrocatalyst; this ~urther reduces
the overvoltage.
It i~ well known in the art to use plasma or
flame spraying ~o coat an electrode with an electro-
conductive metal~ In U.S. Patent No. 1,263,959 it was
2S tau~ht that anodes may be coated by spraying fine
nickel particles.onto an anode, wherein the particles
~re rendered molten and impacted on the iron substrate
by means of a blast.
Cathodes, also, have been coated with electro-
conductive metals.. In U.S. Patent No. 3,992,278,ca~hodes were coated by plasma spraying or ~lame spraying
31,010-F -3-
--4--
an admixture o~ particulate cobalt and particulate
zirconia. When these electrodes are used for the
electrolysis of water or an aqueous alkali metal halide
salt solution, they are said to give prolonged lowering
of hydrogen overvoltage.
Various metals and combinations of metals
have been used to coat electrodes by plasma or flame
spraying: U.S. Patent No. 3,630,770 -teaches ~he use of
lanthanum boride; U.S. Pate~t No. 3,649,355 teaches ~he
use of tungsten ~r tungsten alloy; U.S. Patent No.
3t788.968 teaches the use o~ ti~anium car~ide or titanium
nitride and at least one metal and/or metal oxide of
the platinum group and a second oxide coating which is
porous; U.S. Patent No. 3,945,907 teaches the use of
rhenium; and U.S. Patent N~. 3,974,058 teaches the use
of cobalt as ~ coating with an overcoat o~ ruthenium.
I~ i~, lik~ise, well known in the art to
make porous ele~trode coa~in~s by seIective leaching.
Coating an electrode with particulat~ nickel, ~hen
~i~terin~ the nickel as taught in U.S. Patent Nos.
2,928,783 and 2,969,315; electrodepositing an alloy
onto a substrate then leaching out one compon~nt of the
alloy as~ taught in U.SO Pat. No. 3r272,788, pressing or
cementing two or more components together or onto an
electrode subs~rate and then selec~ively leaching out
o~e or more of the coating components as illustrated by
U.S;. Patent Nos. 3,316,159; 3,326,725; 3,427,204;
3,713,891 a~d 3,802,878.
It is also disclosed in the art to combine
the steps of making electrodes by plasma- or ~lame-
sprayin~ followed by leaching. It is alco disclosed to
3},010-F ~4~
61)0~
--5--
Combine the steps of electroplating followed by leaching.
Examples of known methods are illustrated in the following
patents; U.S. Patent No. 3,219,730 (Institute of Gas
Technology~ dated November 23, 1965 teaches coating a
substrate wi~h a multiple oY~ide film coating then removing
the substrate by leaching, thus forming an electrode;
U.S. Patent No. 3,4~3,057 ~Carrier Corporation) dated
September 24, lg~8 teaches flame or plasma spraying a
Raney alloy onto a substrate followed by leaching aluminum
out of the alloy thus leaving a porous electrode; U.S.
Patent No. 3,492,720 (Badische Ani'in- & Soda-Fabrik
Aktiengesellschaft~ dated February 3, 1970 teaches plasma
spraying tungsten, titanium or alloys thereof along with
aluminum, thorium and zirconium oxides onto a substrate.
The substrate was su~sequently removed, leaving a porous
electrode.
U.S. Patent No. 3,497,425 (Imperial Metal Industries
~Kynoch) Ltd.~ dated February 24, 1970 teaches preparing
porous electrodes ~y coating the substrate with a relatively
insoluble metal followed by a coating of a more easily
dissolvable metal. The teaching requires heat treating to
cause inter-diffusion of the two coats, while optimum
conditions re~uire separate heat treatments for each coat.
The dissolvable metal is subsequently leached out, leaving
a porous electrode. U.S. Patent No. 3,618,136 (T. Fujita)
dated November 2, 1971 teaches forming porous electrodes
by coating a binary salt composition onto a substrate
and leaching a soluble component from the system. The
patent teaches that it is critical that the binary salt
mixture is an eutectic composition and that optimum
results are obtained when the same anions are used for
both the active and the inactive salts, e.g. silver
chloride -- sodium chloride.
31,010-F _5_
~2~i0V~
--6--
Canadian Patent No. 1,07~,915 (~ooker Chemical
& PlastiC Corp.~ teaches the preparation of porous
cathodes by applying to a substrate a coating of at
least one non-noble metal from the group of nickel,
cobalt, chromium, manganese and iron, alloyed with a
secondary, less noble, sacrificial metal. Specifically,
the sacrificial metal is chosen from the group of zinc,
aluminum, magnesium and tin. The sacrificial metal is
removed by leaching with a lye solution or an acid
solution.
U.S. Patent No. 4,279,709 discloses a method
for making electrodes including electrodes having reduced
ovexvoltage by applying an admixture of particulate
metal and a particulate inorganic compound pore-former
1~ and then leaching out the pore-former to form pores.
Electrodes of film-forming metal substrates,
especially titanium, coated with oxides of Group VIII
metals of the Periodic Table of The Elements have been
taug~t, especially conjointly with other metal oxides,
as being useful as anodes in electrolytic processes,
such as in brine electrolysis. Ruthenium oxides, platinum
oxides, and ~ther oxides of the "platinum
31,010-F -6-
~` .
--7--
metal series", in association with various other metal
oxides have received much acclaim as coatings for valve
metal substrates (esp. Ti) ~or use as anodes. Patents
relating to such anodes are, e.g. U.S. Patent Nos.
3,63~,498 and 3,711,3~5. These coatings may be applied
in several ways, for example, U.S. Patent No. 3,869,312
teaches ~hat platinum group metal oxides, combined with
film-forming metal oxides may be deposited on valve
metal substrates by applying a mixture of the.rmally-
-decomposable compound~ of platinum group metals and a
thermally-deco~posable organo compound o~ a film-forming
metal in an organic liquid vehicle which may also
optio~ally contain a r~ducing agent, to a support
member, drying the coa~ing by evaporation of ~he organic
vehicle, then heating the member in the range o~ 400-55GC
to ~orm metal oxides. Repeated coats are applied to
increase the ~hickness of ~he coating. Also an o~ercoa~ing
o a ilm-~orming metal oxide is applied. U.S. Paten~
3,632,498 teaches tha~ coati~gs o~ ~lnely divided
oxid`es of platinu~ group metal& and film-forming metals
may b~ produced by use of a plasma burner, by heating
substxa~es which have been coated with thermally-
-decomposable compounds of platinum group metals and
film-foDmi~g metals, b~ electrically depositing the
metals in a galvanic bath followed by heating in air to
form the oxid~, among others.
Some further patents relating to electrodes
having metal oxide surfaces are, e.g., U.S. Patent Nos.
3,616,445; 4,003,817; 4,072,585; 3-,g77,958; 4,061,549;
4,073~,873; an~ 4,142,005.
The use of platinum group metal oxides,
particularly ruthenium oxide, in active coatings for
31,010-~ -7-
3~`~
--8--
the evolution o~ hydrogen is also known (ref. ~elendres,
Carlos A., SPRI~G MEETING ELECTROCHEM. SOC., May 11~
1975). U~S. Paten~ No. 3,816,464 (Hodogaya Chemical Co.,
Ltd.~. dated November 17, 1~81refer to the use of a mixture
of platinum group metal oxide(s~ with another metal
oxide as active cathode coatings. U.S. Patent No. ~7
4,238,311 ~.Chlorine Engineers Corp., Ltd.) dated
December 9, 1980 teaches that a cathode coating consisting
of fine particles of platinum group metals and/or
platinum group metal oxides in nickel is useful as a
cathode coating.
In general, it is known by those skilled in
the art that the use of oxides of platinum group metals
as active catalysts for the evolution of hydrogen in
modern electrolytic chlor-alkali cells employing permionic
membranes is not useful because of extreme conditions
of NaOH concentration and temperature now possible,
wherein NaOH concentrations of 30 percent and temperatures
exceeding 95C are not uncommon. Oxide coatings prepared
according to the known art are found to decrepitate
with use and.fail by loss of adherence to the substrate,
accompanied presumably by substantial reduction, in
some cases, to base metals.
It is also well known to those practiced in
the art that catalytic coatings consisting of metals
with intrinsically low hydrogen overvoltage properties
are subject in actual practice to loss of catalytic
activity due to overplating with metallic contaminants,
such as iro,n or example, which are commonly present in
brine and water employed in the process of electrolysis.
Consequently, active coatings found useful by those
practiced in the art for evolution of hydrogen in
modern electrolytic membrane chlor-alkali cells are
31,010-F -8-
k
g
limited to the type characterized by high surface area,
or porous coatings, with compo~itions resistant to some
degree to chemical at~ack at these co~ditions, e.g.
nickel or various stainless steels.
In these cases, the ~ull effec~ of ~he catalytic
nature of intrinsically low hydrogen overvoltage catalysts
are not realized in practice, since, as is well known
to those practiced in the art, the performance of these
essentially high surface area coatings degrades in time
to a level characterized by the eguivalent coating of
the predominant metallic contaminant present in the
brine or water Qmployed i~ the electrolytic process,
usu211y Fe. Conse~uently, the Tafel slope charactexizing
the electroly~ic activity of the applied coating changes
to ess~ntially that of iron, with a resul~ing increase
in hydrogen overvQltage, especially at higher urrent
densities, 0.23 to 0.54 amp/cm~ ~1.5 to 3.5 amps/in2)
and above, as ar~ common in modern membrane chlox-alkali
c~llæ.. In con~rast, it i~ desirable to maintain the
intrinsically low overvoltage properties of khose
materials which are k~own- to be characterized by low
Tafel slopes, i.e~ pla~i~um group metal oxides,
particularly ru~henium oxide, during long-term operation
i~ membxa~e chlor-aIkali cells. It has now been
discovered, among other things, that active coatings of
o~ide~ of platinum group metals and secondary electro-
catalytic me~als when prepared according to the process
of the i~vention/ exhibit unexpected properties of low
hydro~en over~oltage, physical stability, and long~term
efficacy as cathodes in the el~ctxolysis of brine at
conditions of high NaO~ con~entrations, temperatures,
and process pressures. It has also been discovered
that the use of these electrodes in electrolytic
31,010-F -g-
` ~16~
-10-
process wherein chlorine and cau.stic soda are produced
at certain process conditions of temperature~ ~aOH
concentration, pressure, etc., results in reduced
energy requirements not otherwise attainable in
practice.
The invention particularly resides in a method
o~ making a low hydrogen overvoltage cathode which
comprises a nickel substrate having coated thereon an
electrocatalytically-active heterogeneous mixture of
nickel 02ide and a platinum group metal oxide, said
method comprising the steps of (a) depositing on a
nickel substrate, a coating solution containing a
nickel compound which is effective as a precursor for
nickel oxide, and at least one platinum group metal
compound which is effective as a precursor for a
platinum group metal oxide, said coating solution also
containing an etchant capable of etching the surface of
the substrate and/or any previously applied coating,
(b) heating to remove volatiles from the~so-coated
substrate to cause the metal values of the precursor
compounds and those etched from the substrate or
previous coating to be concentrated and recoated on the
substrate or previously applied coating9 an~ (c)
further heating, in the presence of oxygen, air or an
oxidizing agent, to a temperature sufficient to oxidize
the metal values, said steps (a), (b), and (c) being
performed a plurality of timesO
3o
The present invention also resides in a process
for the electrolysis of aqueous solutions of sodium
chloride in an electrolysis of aqueous solutions of
sodium chloride in an elec~rolytic cell comprising an
anolyte compartment and a catholyte compartment
separated by a cation exchange membrane to produce an
31,010-F -10-
h~
fi~3~8
-10a- 4693-3433
aqueous solution of sodium hydroxide in the catholyte
compartment, and chlorine in the anolyte compartment,
wherein the cathode of said proces~ is a low hydrogen
overvoltage cathode made by applying to an
electroconductive substrate a coating solution of
nickel oxide and platinum group metal oxide precursor
compound(s) and an etchant capable of etching the
surface of the substrate and/or any previously applied
coating, heating to remove volatiles from the so-coated
substrate to cause the metal values of the precursor
compounds and those etched from the substrate or
previous coating to be concentrated and recoated on the
substrate or previously applied coating, and further
heating, in the presence of oxygen, air or an oxidizing
agent~ to a temperature sufficient to oxidize the metal
values, thereby obtaining on said substrate an electro-
catalytically active heterogeneous coating o~ a mixture
of nickel oxide and at least one platinum group metal
oxide.
The present invention further resides in an
electrode for use in an eletrochemical cell comprising
a substrate having an electrocatalytically active
coating deposited thereon, said coating comprising a
heterogeneous mixture of metal oxides containing nickel
oxide and at least one oxide of a metal selected from
Ru, Rh, Pd, Os, Ir 7 and Pt.
The present invention further resides in a low
hydrogen overvoltage cathode for use in a chlor-alkali
electrolytic cell comprising a substrate having an
electrocatalytically active coating deposited thereon,
said coating comprising a heterogeneous mixture of
metal oxides containing at least one oxide o~ a metal
selected ~rom Ru, Rh, Pd, Os, Ir, and Pt, and nickel.
31,010-F -lOa-
, .
.
3L24~ 8
-lOb-
Figure 1 illustrates graphizally data from some
of the tests described hereinafter.
Electrodes comprising an electrically
conductive, or non-conductive substrate having a
coating of heterogeneous oxide mixtures of platinum
group metals and secondary electrocatalytic metals are
prepared by applying soluble metal compounds and an
etchant for the substrate, and, in cases of successive
coats, etching the metal oxides previously applied to
the substrate, thereby, it is believed, attacking and
solubilizing the least chemically resistant portions of
the coating, then, as the substrate is heated to
oxidize the metal values, concentrating and
redepositing the said metal
3o
31,010-F -lOb-
.
values on the subs~rate, and oxidizing them to produce
a substantially hard, stable mixture of heterogeneou~
oxides of the metal values.
The pre~errsd electrically-conductive sub-
strate may be any metal structure which re~ains itsphysical integrity during the preparation o~ the
electrode. Metal laminates may be usedt such as a
~errous metal coated with another metal, e.g., nickel
or a film-forming metal (also known as.valve metal).
The subs.~ra~e may be a ~errous metal, such as iron,
steel, stainless steel or other metal alloys wherein
the major component is iron. The ubstrate may also be
a non-ferrou~ metal, such as a film~forming metal or a
non-film-forming metal, e.g., Ni. Film-forming metals
are well known in these relevan~ ar~ as including,
notably, titanium, tantalum, zirconium, niobium,
tungsten and alloys o-~ these with each other and with
minor amounts of othe~ metals~ Non-conductive sub-
~trates ma~ ~ employed, especially if they are then
20 coated with a conductive layer o~to which the instant
metal oxides are deposited.
Th~ shape or c:onfiguration of the substrate
used in the present coating process may be a flat
sheet, curved surface~ con~oluted surface, punched
25 plat~, woven wire, ~xpanded metal sheet, rod, tube,
porous., no~-porous, sin~ered, filamentary, re~ular, or
irr.e~lar~ The p~esent novel coating process-is not.
depe~dent on having ~ substrate of a particular shape,
since the chemical and thermal steps in~olved are
applicable ~o virtually any shape which could be useful
as an electrode article. Many electrolytic cells
contain foraminous (mesh) sheets or flat plate sheets;
31,010-F -11-
-12-
-these are sometimes ben-t to form "pocket" electrodes
with substantially parallel sides in a spaced-apart
relationship~
The preferred substrate configura~ion com-
pris:es expanded mesh, punched plate, woven wire, sinteredmetal, plate, or sheet, with expanded mesh being one of
the most preferred of the porous substxates.
~ he preferred composition of the substrate
comprises nickel, iron, copper, steel, stainless steel,
or ferrous metal laminated with nickel, with nickel
bei~g especially preferred~. It will be understood that
these substrates, onto which the me~al oxide coatings
are to be- deposited, may themselves be supported or
xeinforced by an undexlying substrate or me~ber,
especial~ wh~r~ nickel, iron, or copper is carried by,
or ~r an. underlying substrate or member. The substrate,
o~o which th~ me~al oxide coating is ~o be deposi~ed,
ma~ itseL~ be an outer la~er o~ ~ laminate or coated
structure, and i~ may be, optio~ally, a non-conductive
substrate.onto which the metal oxide coating is deposited.
Th~ pla~inum metal serie~ comprises Ru, Rh,
Pd, Qs, Ir, and Pt.. Of these, the pre~erred metals are
platLnum and ruthe~ium, w~h xuthenium b~ing most
preferred. The soluble.platinum metal compound may be
~5. ~he halids, sulphate, nitrate or other soluble salt or
soluble compou~d of th~ metal and is preferably the
halide salt, such as RuCl3 hydrate, PtCl4 hydxate, and
the like.
The seconda~y electrocatalytic metal oxide
precursor o~ ~he present coating ma~ be at least one
31,010-F -12-
-13-
derived from a soluble compound of Ni, Co, Fe, Cu, W,
V, Mn, Mo, Nb, Ta, Ti, Zr, Cd, Cr, B, Sn, La, or si.
The preferred of these are Ni, Zr, and Ti, with Ni
being khe most preferred.
The solution of the present invention contains
at least one chemically active agen~ capable of etching
~he s~bstrate, and, in the case of second and later
coatings, etching and solubilizing the most chemically-
-susceptible portions o~ the 02ides previously formed,
while also, preferably as the ~emperature is elev~ted,
vaporizing, in many cases, from the heated mixture,
along with volatilized anions or negative-valence
radicals from the platinum metal oxide precursor and
th~ secondary electrocatalytic metal oxide precursor.
The preferred chemically active etchants comprise most
common acids, such as hydrochloric acid, sulphuric
acid, ~i~ric acid, p~osphoric acid; also hydrazine
hydrosulphate, and the lik~, with hydrochloric acid and
hydrazine~hydrosulphat~ being among the most preferred.
In general, ~he preferred method contemplated
in the present in~ention comprises applying to the
de~ired;substrate a solution containing at least one
platinum me~al series compound, at leas~ one electro-
catalytic me~al compound, and a chemical etchant,
preferably containing a volatile organic vehicle, such
as isopropanol, a~d allowing the volatile vehicle to
evaporate, leaving the etcha~t and the dissolved metal
values; the~ heatin~ ~he substrate to a temperature
sufficient to concentrate the metal values, also sub-
30 ~tantially driving out the volatilized etchant alongwith the anions or ne~ative-valence radicals released
from th~-metal oxide precursors, and heating the
31,01Q-F -13-
-14-
substrate in the presence of oxygen or air to a temper-
ature sufficient to ~he~nally oxidize and convert the
metals to metal oxides in-situ on the substrate. The
steps may be repeated a plurality of times in order to
attain the best full effect of the i~vention by increasi~g
the thic~ness of the coating. Furthermore there is, at
times, a benefit to be derived from laying do~m 2 or
more layers of the metal oxide precursors between each
thermal oxidation step.
In a particularly preferred embodiment an
electrode material is prepared by applying a hetero-
g.e~eou& metal oxide coating, said heterogeneous metal
o~id~ coatin~ comprising nickel o~ide and a platinum
group me~al oxide ~op~io~ally contai~ing a modifier
metal oxide, e.g., ZrO2), onto a nickel metal layer
(which.may be in ~he form-of a nickel layer on an
elec~roconducti~e substrate~ by the process which
compri~e~ (a) applyi~ ta said nickel metal layer a.
coating solutio~ compriæing a Dickel oxide precursor, a
platinum group metal oxide precursor, an optional
modifier metal o~ide precursor, and an etchant for
dissolving the mos-t soluble portions of the nickel
m~tal ~urface, (b) heating.to e~aporate volatile
porti:ons o~ the coating solution, thereby concentrating
and depo~iting ~he metal oxide precursors on the
~o-etched nickel metal surface, (c) heating i~ the
presence of air or oxygen at a temperature hetween
3Q0C to 600C for ~ time suficient to oxidize the
~- metals of the m0tal oxide precursors, and (d) cooling
~he so-prepared electrode material. Additional coati~gs
may be applied in similar manner so as to increase the
thickness o~ ~he so-produced heterogeneous metal oxide
coating on the nlckel metal surface, though the etchant
31,010-F -14-
~2~
-15-
for ~he second and later coating applications may
beneficially be the same as, or different from, the
etchant used in the initial coating application. There
is thus prepared an electrode material comprising a
nickel metal layer ha~ing tigh~ly adhered thereto a
heterogeneous metal oxide coating comprising nickel
o~ide and a platinum group metal oxide, op~ionally also
containing a modifier metal oxide. Preferably, the
platinum group metal oxide is ru~henium oxide. The
pre~erred optional modifier metal oxide is zirconium
oxide.. A~ economical form of the nickel metal layer is
that of a nickel layer on a less expensive electro-
conductiv~ substrate, such as steel or iron alloys.
Such electrode material is particularly useful as
cathodes in chlor-alkali cells.
Ordinarily-the temperatures at which thermal
oxidatlon of the metals:is achieved is somewhat depen-
den~ on the metaIs, but a temperature in the range o~
fr~m~ 3:00Q to 650C, more or less~, is generally e~fective.
It is; generally preferred ~hat the thermal oxidation be
per~ormed at a temperature in the range of from 350 to
550C.
The effect of the invention is to produce a
s~bstantially hard, adherent coating of heterogeneou6
oxides of th~ solubilized metals.
It is within ~he purvie~ of the present
i~ventive co~cept that the ~olubilization, reconcen-
tration, and in-situ deposition.of the solubilized
- metals, usin~ chemical etchi~g of the previously
deposited layers and/or substrate produces an intimate
mixture of oxides which are mutually s~abili2ing and
electrocatalytically compleme~tary.
31,010-F -15~
~16-
The following examples illustrate particular
embodiments, but the invention is not limited to the
particular embodiments illustrated.
Exam~le 1
A solu~ion was prepared which con~isted of
1 part RuCl3 3H20, 1 part NiC12 6~20, 3.3 parts
H2NNH2-H2SO4 (hydrazine hydrosulphate), 5 parts HzO,
and 28 parts isopropanol. The solution was prepared by
first mixing together all ingredients other than ~he
isopropan~l by stirring overnight, then adding the
isopropanol and continuing to s~ir for approximately 6
houxs.
A cathode was prepared which was constructed
of a 40% expanded mesh of nickel. The cathode was
first sa~dblasted, ~h~n etched in 1:1 ~Cl. I~ was
subse~ue~tIy ri~sed, dipped in isopropanol and air
dried~ T~ cathode was coated by dipping it into the
coa~ing solution, alIowing it to air dry, ~hen baking
it in a~ oYe~ at 375C for 20 minutes. In the same
maDner, a total of 6 coats were applied. The ca~hode
waæ immersed in a heated bath containing 35% NaOH at a
~empe*ature of 90C. A current was applied and potential
measurements were take~ using ~-standard Calomel Reference
Electrode (SCE) a~d a Luggi~ probe. The cathode potential
was measured at -1145 millivol~s vs. SCE at a current
density oi~ 2 amp5 per square inch (0.31 amps per cm2 ) .
The cathode was asæembled in a laboratory membrane
chlorine cell and operated a~ 90C, producing C12 at
the anod~ and E2 at the cathode, at 31-33% NaO~
concentration, operating at 0.31 ampjcm2 (2 amp/in
current density. The pvten~ial of the cathode was
monitore~ and averaged per week. The results are shown
in Table I.
31,01Q-F -16-
- ~Z~6~
-17-
.
Example 2
A solution was prepared which consisted of
1 part RuC13-3H20, 1 part Nicl2~6H2o~ and 3.3 parts
concentrated HCl. It was allowed to mix overnight.
Subseguently, 33 parts isoprop~nol were added and
mi~ing continued 2 hours. A cathode was prepared in
accordance with the procedure of Example 1. The cathode
was then coated in the same manner as Example 1 except
baki~g was carried out at a temperature of 495-500C.
Ten coats were applied. The cathode potential was
measured as in E~ample 1. The potential was -1135
millivolts vs. SCE. The cathode was assembled in a
laboratory cell containing a commercially a~ailable
NAFION* polymer ~*a tradeRame o~ E. I. duPont de Nemours)
15 membrane. The cell was operated a~ gOC, 31-33% NaOH,
and 0.31 amp/Cm2 ~2 amp/in2) curren~ density. The
pote~tial of the cathode was monitored and averaged per
week. The resul~s ar.e shown in Table I.
E~ampl ~3
A so:lution wa~ prepared which consisted of
1 par~ N~20~C1, 5 parts concentrated RCl, 2 parts 10%
PtClS~6E~0, 1 par~ NiCl2 6H20, and 1 par~ RuCl3 3H~O.
The solution was~ allowed to mix ~or 12 hours. Then
75 parts isopropanol were added and mixing continued
for 2 hours~ A cathode-was prepared according to
Example l. The cathode wa-q then coated in ~he same
manner as Example 1 except baking:was carried out at a
tempera.ture of 470-480C. Five coats were applied.
sixth cQa~ was applied and the alectrode was baked for
30 minutes at a temperature o~ 470-480C~ The potential
of the cathode was measured as in Example 1. The
pote~tial was ~1108 millivolts vs. SCE. The cathode
. was assembled in a laboratory membrane chlorine cell
31,010-F -17-
-18-
containing a commercialy availcible membrane, as in
Example 2. The cell was operated at 90C, 31-33% NaQ~,
and 0.31 amp/cm2 (2 amp/in2) current density. The
potential of the cathode was monitored and averaged per
week. The results are shown in Table I.
A solution was prepared which consisted of
3 parts RuC13 3H2O, 3 parts NiClz 6H2O, 1 part ZrCl~, 5
parts concentrated ~Cl, and 42 parts isopropanol. The
solution was allowed to mix 2 hours. The cathode was
then coated in ~he same manner as Example 1 except
baki~g was carried out at a temperature of 495-500C.
. Eight coats were applied. A ninth coa~ was appli~d and
the electrode was baked for 30 minutes at a temperature
o~ 470~480C. The po~ential of the cathode was measured
as in Example 1. The pote~tial was. 1146 milli~olts
vs. SCE. T~e~ca~h~d~ wa assembled in a laborato~y
membrane~chloriné c~l-1 contai~in~ a.commercially available
membrane, ~ in ~ample 2. The cell was operated at
90C, 31-33~ NaOX~ ~nd 0.31 amp/cm2 (2 amp/i~Z) current
de~sity. The poten~ial of th~ cathode was monitored
and a~araged per week. The results are shown in Table
I.
Example 5
A cathode was prepared as in the previous
e~amples, ~hen d~pped in a solution contai~iny 1 gxam
of tetraisopropyl titanate in 100 ml of isopropan~l.
Th~ cathode waa then baked at a temperature of
475-500C for 10 minutes. Thre~ co~ts were applied.
A solutio~ was prepared as in Example 2. The cathode
was dipped in the soluti~, air dried, and baked at a
temperatur~ of 475-500C. Six coats were applied.
31,010-F- -18-
3~4~
--19--
The potential of the cathode was measured as in khe
previous examples. The potential was -1154 millivolts
vs, SCE. The cathode was assembled in a laboratory
membranP chlorine cell containing a commercially
5 available membrane, as in Example 2. The cell was
operated a~ 90C, 31~33% NaOH, and 0.31 amp/cm2
( 2 amp/in2 ) current density. The potential of the
cathode was monitored and averaged per week. The
results axe shown in Table I and also in Figure l. .
Exam~le_6 (Comparative Example)
A 40% expanded mesh electrode of steel was
prepared, but no~ coated, and assembled as the cathode
in a laboratory cell as in ~xamples 2-5, using the same
type membrane. The potential of the cathode was
monitored and averaged per week. The results are shown
in Tabl~ I.
Exame~e 7 (Comparativ~ E~ample)
A 40% expanded mesh elec~rode of ~ickel was
prepared, but not coate~, and assembled as the cathode
in a laboratory cell as in E~amples 2-5, using the same
type membrane. The poten~ial of the cathode was
monitore& and a~eraged per week. The results are shown
in Table I and also in Figure 1.
31,010~F -19-
~24~
20-
TABLE I
NegatiYe volkage* averaged ~ach week for
No. oElectrodes No 1 thru 7
. .
Weeks Ex. 1 Ex. 2 Ex. 3 Ex. 4 Ex. 5 CEx. 6 CEx. 7
11.145 1.120 1.135 l.lZ0 1.140 1.~75 1.49~
21.1~0 1.1~0 1.150 1.130 1.130 1.460 1.475
31.150 1.125 1.160 1.150 1.110 1.455 1.470
41.155 1.130 1.150 1.155 1.080 1.455 1.470
51.155 1.130 1.150 1.150 1.070 1.465 1~475
61.150 1.130 1.~80 1.15~ 1.060 1.475 1.480
71.150 1.125 1.185 1.155 1.060 1.480 1.4g5
81.150 1.125 1~180 1.160 1.060 1.480 1.510
91.140 1.120 ~.160 1.155 1.070 1.480 1.510
lO1.130 1.110 1.185 1.160 1.080 1.475 1.510
111.115 1.11~ 1.190 1.170 1.080 1.480 1.515
121.1~0 1.1~0 1.190 1.165 1.080 1.490 1.520
13I.100 1.110 1.150 10165 1.080 1.485 1.520
~ l00 1.115 l.l90~ 1.1701.080 - - 1.520
I5l.Og5 1.120 I.190 1~170 1.090 - - 1;525
l*~.090 1.120 1~190 1.170 1.090 - - 1.530
171 ~85~ 0 1.190 1.170 1.09~
18I.080 1.120 1~.190 1.1651.100
lg1.0~0 l.llO 1.190 1.160 1.100 - - - -
1.080~ 1~110 1.190 -1.100 - - - -
21l.Q80 l.110 1.190
221.090 - - 1.190
23l.Q90 - ~l.l9Q-
24I.100 - l.lgO
251.100
26I.090
271.090 ~ - - - - - - - -
CEx - Comparative Example
* The voltages recorded in Table I were all measured in
the sam~ manner, using a Luggin probe, thus are relevant
31,010-F 20-
..
4~V~
-21-
to each other, though all are believed to be slighkly
lower than what one should expect to find from a theo-
retical calculation~ By thermodyn~nic calculations,
the actual absolute reversible voltage should be about
-1.093V for a cell at'90C, 31-33% NaOH, and at a
current density of 0.31 amp/inZ t2 ~np/in2).
Example 8
The cells o~ Examples 2-7 were operated at
90C, 31-33% NaOHr and 0.31 amp/cm2 (2 amp/inZ) current
de~sity while maintaini~g atmospheric pressures in the
anolyte an~ catholyte compartments of the cell. Sodium
chIoride brine and water were fed to the anolyte and
catholyte compartments, respecti~ely, in order to
main~ai~ anolyte concentrations i~ the range 180-200
grams per liter NaCl a~d 31-33% NaO~. Internal mi~ing
of the cells was accomplished by natural gas lift due
to evolution of hydrogen gas at the cathode and chlorine
gas at the anode. Data including mass and energy
balance& were~collected periodically over the period of
2Q operation o~ ~h~ celIs and energy requirements for the
~roductio~ o~ NaOH were caLcuIate~. The results are
show~ in Tabl~ 2.
TABLE 2
Electrode ~ Cathode KW~MT NaO~
2 coate~ 2208
3 coated 2221
4 coated 2229
coated 2259
6 steel 2497
7 nickel 2504
31,010-F -21-
-22-
Example 9
In a large scale tes~, two series of pressure
membrane chlorine cellq were con~tructed. The con-
struction and de~ign of the cells were identical except
that the series identified as Series 1 had a nickel-wall
catholyte compartment and nickel electrode~ installed in
the catholyte compartment of the cells, while the ~eries
identified as Series 2 have a steel-wall cathode
compartment and steel cathodes. The electrodes of
Series 1 were coated according to the process of the
invention, while those of Series 2 were uncoated. Both
series were erected with a commercially available cation
exchange membrane, as in Example 2. The two series were
operated simultaneously at 90C, 0.31 amp/cm2
~2 amp/in2) current denqity, and 31 to 33% sodium
hydroxide in the catholyte chamber. The series were
operated at pressures of 101,325 to 202,650 Pa (1 to 2
atmospheres) while recirculating the anolyte and the
catholyte through the cells using centrifugal pumps.
The ratio of the catholyte flow to the anolyte flow was
maintained at a value greater than 1. Energy and mass
balance data were collected and average performance data
were calculated over a period of 45 days. The results
clearly show that the energy savings attained with the
use of the electrodes of the present invention
(Series 1) averaged greater than a 5% reduction in
energy, compared with Series 2.
It is within the purview of the present
invention to employ the present novel electrodes at
temperatures encountered in cell~ operated at super-
atmospheric pressures, as well as at atmospheric or
subatmospheric pressures. The electrodes are especially
suitable for opera~ion in the elevated temperature
31,010-F -22-
-23-
range of from 85 to 105C. Pressures a~ around 101,325 Pa
(1 a~m.), more or less, are ordinarily used iTl chlor-alkali
cells, though pressures up to abouk 303,975 Pa (3 atm.)
or more may be used.
The electrodes of the present invention are
useful in cells wherein circulation within each electrolyte
compar~ment is created by ~he gas~lift (displacement)
actio~ of gaseous products produced therein, though in
some cells, such as in electrolyte series flow from
cell-to-cell, another pumping means may be provided to
supplement, or substi~ute for, the gas-lift action. We
fi~d it ad~isable, in some cases, to maintain the ratio
o~ ~he volume o~ catholyte pumped to that of -the a~olyte
volume pumped, at a ratio greater than unity.
The electrodes of this invention are useful
i~ chlor-alkali electro~ytic cells in which the anoly~e -
has, or i~. adju~ted:to haYe, a p~ in the range of from
l-to.5~ su~h as when an acid~, ~.g. ~Cl, is added to the
analyte.
31,010-F -23:-
