Note: Descriptions are shown in the official language in which they were submitted.
~194~9~l
IMPROVED ELECT~ODE COATING METHOD
BACKGROUND OF THE INVENTION
This invention generally relates to a method for
applying coating materials to an electrode substrate, generally
of a valve metal, wherein the coating compositions have tin
compounds which are later converted to thçlr oxide forms to
produce an electrode having significantly increased reproduci-
bility for method of manufacture, savings in manufacturing costs - -
due to the more complete utilization of the tin metal, and
reduced atmospheric pollution caused by the volatilization of
tin compounds during the coating process. ~ore particularly
the present disclosure~relates to a much improved method for
manufacturing an electrode having a valve metal substrate such
as titanium carrying an electrode coating composition having
tin as a component thereof by using a sulfate form of the tin
compound in the coating composition mixture.
Electrochemical methods of manufacture are becoming
- ever increasingly important to the chemical industry due to
their greater ecological acceptability9 potential for energy
conservation, and the resultant cost reductions possible.
Therefore, a great deal of research and development efforts
have been applied to electrochemical processes and the hardware
for these processes. One major element of the hardware aspect
is the electrode itself. The object has been to provide: an
electrode which will withstand the corrosive environment within
an electrolytic cell; an electrode having a minimum over-
potential for the desired electrochemical reaction; and an
electrode that can be manufactured with high quality control at
a cost within the range of commercial feasibility. Only a few
materials may effectively constitute an electrode especially
~as~s~
to be used as an anode because of the susceptibility of most
other substances to the intense corrosive conditions present
within the anode compartment of an electrolytic cell. Among
these materials are: graphite, nickel, lead, lead alloy,
platinum, or platinized titanium. Electrodes of this type
have limited applications because of the various disadvantages
such as: a lack of dimensional stability; high cost; high wear
rate; contamination of the electrolyte; contamination of a
cathode deposit; sensitivity to impurities; or high overpotentials
for the desired reaction. Overpotential refers to the excess
electrical potential over the theoretical potential at which
the desired reaction occurs at a given current density.
The history of electrodes is replete with examples of
attempts and proposals to overcome some of ~he problems assoc-
iated with the electrode in an electrolytic cell, none of which
have accomplished an optimi~ation of the desirable characteristics
for an electrode to be used in an electrolytic cell. The
problem is to find an electrode which will overcome many of the
undesirable characteristics listed above and additionally have
low overpotentials at higher current densities so as to conserve
energy. It is known for instance that platinum is an excellent
material for use in electrodes to be used as anodes in an electro-
winning process and satlsfies many of the above-mentioned criteria.
- However, platinum is expensive and hence has not been found
suitable for industrial use to date. Carbon and lead alloy
electrodes have been used commercially, but the carbon anode
wears fast which greatly pollutes the electrolyte, incraases
electrical resistance, and increases the half cell potential.
This higher half cell potential causes the electrolytic cell
to consume more electrical energy than is desirable. A dis-
advantage of the lead alloy anode is that the lead dissolves
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in the electrolyte producing a lead deposit on the cathode which
contaminates the desired deposit obtained. Also, PbO2 changes
to a Pb304 which is a poor conductor. Oxygen may penetrate
below this layer and flake off the film resulting in particles
becoming trapped in the deposited copper on a cathode. This
causes a degrading of the copper plating which is very undesir-
able.
It has been proposed that platinum or other precious
metals be applied to a titanium substrate to retain their
attractive electrical characteristics and further reduce the
manufacturing costs. However, even th~s limited use of precious
metals such as platinum which can cost in the range of $30.00
per square foot ($323.00 per square meter) of electrode surface
area is expensive and therefore not desirable for industrial use.
It has also been proposed ~hat the surfaces of titanium be
plated electrically with platinum to which another electrical
deposit either of lead dioxide or manganese dioxide~is applied.
The electrodes with the lead dioxide coating have the dis-
advantage of co~paratively high oxygen overpotentials and both
~types of coatings have high internal stresses when electrolytical-
ly deposited which are liable to be detached from the surface
during commercial usage, contaminating the electrolyte and the
produc- being deposited on the cathode surface. Thus, the
current density of such anodes is limited and handling of such
anodes must be done with extreme care. Another attempted
improvement has been to put a layer of manganese dioxide on the
surface of a titanium substrate which is relatively porous in
nature and building up a number of layers of the manganese
dioxide as to present an integral coating. This yields relative-
ly low overpotentials as long as the current density remains
.
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below 0.5 ampere per square inch (77.5 milliamperes per squarecentimeter) but as the current density is increased to near
1 ampere per square inch (155 milliamperes per square centimeter)
the overpotential required rises rather rapidly, resulting in a
considerable disadvantage at higher current densities.
More recently a number of coatings have employed the
use of titanium, ruthenium and tin dioxides, or tin and antimony
oxides upon which a top coating of either manganese or lead
oxide is pla~ed. These coatings have shown substantial promise
in the area of lowering overpotential and yielding good life-
times in the corrosive conditions within an electrolytic cell.
The ma;or drawback of these materials is that the methods of
applying especially the tin oxide materials have resulted in
volatilization of substantial amounts of the tin upon baking
the coating to the tin oxides. This is because the tin
compounds such as stannic chloride pentahydrate when baked
converts to a stannic hydroxide species and then to the stannic
oxides which are desired in the given electrode coating.
During this process much of the tin itself is volatilized into
the atmsophere instead of remaining in the coating. This occurs
at least partly because the stannic chlorides have boiling
- points in the area of 114 centigrade and since the transform-
ation of the tin compounds to their respective oxides occurs
at much higher temperatures, most of these materials are lost
into the atmosphere resulting in less than 50 percent utilization
of the tin material in the actual coating. This causes a
severe problem in the quality control of methods of manufacture
for electrodes of large sizes and large quantities. The
reproducability of a coating composition is nearly impossible
with the volatilization of the tin caused by the current
process for applying the coatings to the substrate materials.
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39~
Therefore, only theoretical tin calculations can be made causing
problems with regard to calculating possible lifetimes of a given
electrode. To date, the use of tin in coating compositions has
not met with the commercial success it should have because vola-
tilization of the tin causes a reproducability problem, increases
pollution which is under stringent standards currently, and in-
creases the cost of production of a given electrode due to the
loss of the tin.
SUMMARY OF TH~E INVENTION
It is therefore an object of the present invention to provide
a method of preparation for electrode coating compositions having
the desired quality control characteristics and a manufacturing
cost within the range of commercial feasibility.
Another object of the present invention is to provide a
method of preparation for electrode coating compositions which
will significantly reduce the volatilization of tin into the
atmosphere thus reducing atmospheric pollution problems associated
with this manufacturing process.
These and other objects of the present invention, together
with the advantages thereof over existing and prlor art forms
which will become apparent to those skilled in the art from the
detailed disclosure of the present invention as set forth herein-
below, are accomplished by the improvements herein described and
claimed.
Thus, in accordance with the present teachings, a method is
provided for the manufacture of an electrode which comprises the
steps of:
selecting a valve metal substrate from the group consisting
of aluminum, molybdenum, niobum, tantalum, titanium, tungsten,
zirconium, and alloys thereof;
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948~1
applying to at least a portion of the surface of the sub-
strate, a coating material consisting of an antimony compound and
a tin sulfate compound;
drying the coating material;
baking the coating in an oxidizing atmosphere to convert
the antimony and tin compounds to their respective oxide forms;
and
applying to the surface of the coating metal, a topcoating
of metal dioxide selected from the group consisting of manganese
and lead.
DESCRIPTION OF THE PREFERRED EMBODIMENT
The improved method for manufacture of electrode coating
compositions of the present invention may be used in the pre-
paration of any coating having as a substituent any tin compound
for which the volatilization problem of the tin exists presently.
In the past, electrode coating compositions employing the use of
tin have utilized thermally decomposable compounds of the chloride
form which have a lower boiling point and therefore a volatiliza-
tion problem. The present invention employs the use of a sulfate
form of the tin or the use of the chloride compounds along with
sulfuric acid to result in a sulfate form of the tin which has a
simple decomposition mechanism for formation of the oxide form in
the ultimate electrode coating composition and therefore drastic-
ally reduces the volatilization of the tin upon baking. The sub-
strate material for use in such an electrode coa~ing composition
can be any electroconductive metal having sufficient mechanical
strength to serve as a support for the coatings, typical metals
including: aluminum, molybdenum, niobium, tantalum, titanium,
tungsten, zirconium, nickel, steel, stainless steel and alloys
thereof. A preferred valve metal based on cost, availability,
electrical and chemical properties is titanium. There are a
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~48¢~
number of forms the titanium substrate may take in the
manufacture of an electrode, including for example: solid
sheet material; expanded metal mesh material with a large
percentage of open area; and porous titanium with a density
of 30 to 70 percent pure titanium which can be produced by
cold compacting titanium powder.
One type of coating the present invention may be
utilized in is cne where a substrate material such as described
above is coated with a semi-conductive intermediate coating of
tin and antimony oxides. These compositions are generally
mixtures of tin dioxide with minor amounts of antimony "dopent",
the latter being present in an amount between 0.1 and 30 weight
percent, calculated on the basis of total weight percent of
SnO2 and Sb203. The preferred amount of antimony trioxide in
most of these cases is between 3 and 15 weight percent. In the
past these intermediate coatings have generally used a stannic
chloride pentahydrate as one,of the materials of the mixture
to be painted or somehow applied to the substrate material.
The present invention utilizes a tin sulfate material or the
stannic chloride pentahydrate plus sulfuric acid to obtain the
sulfate form of the tin. The sulfate form has a simple decomposi-
tion mechanism at a temperature in the area of 320" centigrade
such that the temperatures of baking to transform these materials
into their respective oxides results in very little volatiliz-
ation of the tin into the atmosphere. This allows the semi-
conductive intermediate coating to be applied in a very few
applications versus the past methods of using several làyered
applications of the material to obtain a resultant tin weight
in the desired range. Over the top of this semi-conductive
intermediate coating may be applied a top coating of either
manganese or lead dioxides in order to produce electrodes of
good current ~fficiencies and good lifetimes.
There are many other examples of electrode coating
composi~ions utilizing tin compounds in their makeup to produce
a usa~le electrode coating compositions. Those skilled in the
art may desire to precoat the substrate material with numerous
other compositions before applying the tin sulfate containing
coating composition.
A second example is a single layer coating composition
having titanium, ruthenium and tin dioxides applied in a method
similar to that described above. This type of coating
composition is further described in the following patent
- U.S. Patent ~o. 3,855,092.
A third type of coatlng composition employing the use
of tin-containing compounds is a mixture of the oxides of tin,
antimony, a platinum group metal, and a valve metal. These
coating compositions are further described in the following
patent, -- U.S. Patent ~o.
3,875,043,
In order that those skilled in the art may more
~readily understand the present invention and certain preferred
aspects by which it may be utilized, the following specific
examples ar~ afforded.
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4~
EXAMPLE 1
A series of electrodes were prepared by coating the
substrate metal, in this case titanium, with a solution contain-
ing antimony trichloride~ ruthenium trichloride and various
compounds containing tin all in such amounts as to allow an
initial tin/ruthenium ratio to be calculated and compared to
an analysis of the final tin/ruthenium ratio. This shows the
amount of volatilization of tin that occurred in each instance.
The initial tin/ruthenium ratio was determined from weights of
the starting materials in the coating solution. Since the
ruthenium compound does absorb water to some extent to form
hydrates there is some inaccuracy to an amount of approximately
5 percent on thP calculation of the initial amount of ruthenium
in the ratio. After these various materials were applied to
the substrate material they were baked in an oxidizing atmosphere
at temperatures of 475 to 625 centigrade for periods of 5 to
10 minutes to transform the compounds into their respective
oxides. This process was repeated several times to achieve a
layer of desired weight gain. The amount of coating material
had no observed affect upon the resultant tin/ruthenium ratios.
Therefore any convenient welght of coating material could be
used. Once this was accomplished the final tin/ruthenium ratio
was determined by stripping the catalytic layer off of the
titanium substrate by means of molten salts which are later
dissolved in water to precipitate the metals and analyzing the
resulting solution by atomic absorption to establish a final
ratio of tin/ruthenium in the coating material. These ratios
along with the tin compounds used are reported in Table I
below.
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TABLE I
-
Sn Compound Used Initial Sn/Ru Final Sn/Ru
SnCl4 5H20 21.8 3.3
" 10.9 1.7
" 10.9 1.98
" 4.3 0.5
" 4.36 1.2
" 4.36 1.8
" 4.36 1.7
Sn(C4H9) 4 4.3 0.6
SnCl4 5H20/H2S04 5.7 6.4
" 7.6 6.7
" 7.6 7.5
" 7.6 7.7
: " 7.6 7.8
" 7.6 7.7
.
It can be seen that there is a tin volatilization
loss in the order of 10 to 1 when the stannic chloride penta-
hydrate was used versus a negligible loss of tin where the
:~ tin compound used is stannic chloride pentahydrate reacted
with sulfuric acid. In some cases the final ratio is e~en
higher than the initial ratio where the sulfate form is used.
It is felt that this is due to experimental error caused by
the ruthenium compound absorbing water and perhaps some
material loss during ~he stripping process.
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EXAMPLE 2
A second experiment to show the substantial increase
in the amount of tin retained in the coating was conducted. In
this case a known amount of the solution mixture according to
Example 1 using the various compounds containing tin was fired
in a crucible and the residue analyzed by atomic absorption.
The firing temperatures and cycles were similar to that employed
in Example 1. The results of this experiment in terms of
percentage of the given element remaining in the coating
material after such firing is reported in Table II below.
TABLE II
Sn Compound % Sn % Ru % Sb
Used Retained Retained Retained
SnCl4 5H20/H2S04 81 90 43
SnS0~ 94 95 61
SnCl4 5H20 9 97 23
SnCl4 refluxed 19 94 15
in amyl alcohol
From Table II, i~ can be seen that the use of a
sulfate form of the tin yields significantly higher percentages
of tin retention versus the usage of the chloride forms used
heretofore.
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EXAMPLE 3
A series of electrodes were prepared to evaluate
half cell potentials and lifetimes of these electrodes in
comparison with electrodes utilizing the chloride form compounds
in such larger amounts as to yield a resultant amount of tin
in the coating equal to that of the sulfate form compounds.
It was Eound that 25.1 grams of stannic chloride pentahydrate
yielded approximately the same amount of tin as a mixture
containing 5.48 grams stannic chloride pentahydrate reacted
with sulfuric acid. It can be seen that in this case that
approximately five times as much of the tin compound is necessary
when the sulfate form is not used in the coatings. It was also
found that when these two materials were applied in equal amounts
in terms of grams per square foot of ruthenium on the titanium
sample, the resultant electrodes gave approximately the same
half cell potentials and had lifetimes as reported in Table III
below.
TARLE III
.
Grams per Lifetime of Lifetime of
Square Foot Chloride form Sulfate form
Ru in hours in hours
.
~.1 17 14
0.2 5~ 68
0.3 79 108
Thus it can be seen that approximately five times as
much of the chloride form of the tin versus the sulfate form of
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tin is required to yield equal lifetimes from the resultant
electrodes. This means that a significant lesser amount of
the sulfate form tin compounds can be used therefore resulting
in a net manufacturing cost savings for a given electrode
lifetime. As can be seen from Table I the reproducibility of
the sulfate form tin compounds is significantly higher than
that for the chloride form compounds thereby lending itself
much more readily to a scale up of manufacturing process for
~ the electrode. Use of the sulfate form also will result in
significantly less tin being volatilized into the atmosphere
thus eliminating one pollution concern of the prior art process-
ing methods.
Thus it should be apparent from the foregoing
description of the preferred embodiment that the composition
herein described accomplishes the objects of the invention and
solves the problems attendant to the manufacture of electrode
coating compositions and electrodes for use in electrolytic
cells for electrochemical production.
