Note: Descriptions are shown in the official language in which they were submitted.
PCT/L1S93/00547
W0 94/17222
4
r
MEMBRANE-ELECTRODE STRUCTURE FOR ELECTROCHEMICAL CELLS
The present invention relates to an improved membrane-electrode
structure for use in an ion exchange membrane electrolytic cell. More
particularly, the invention is concerned with the use of an
intermediate layer for the membrane-electrode structure of chlor-
S alkali electrolyzers to reduce the amount of hydrogen in chlorine.
It is known to attain an electrolysis by a so called solid
polymer electrolyte (SPE) type electrolysis of an alkali metal'
chloride wherein a cation exchange membrane of a fluorinated polymer
is bonded with a gas-liquid permeable catalytic anode on one surface
and/or a gas-liquid permeable catalytic cathode on the other surface
of the membrane.
This prior art electrolytic method is remarkably advantageous as
an electrolysis at a lower cell voltage because the electric
resistance caused by the electrolyte and the electric resistance
caused by bubbles of hydrogen gas and chlorine gas generated in the
electrolysis can effectively be decreased. This has been considered
to be difficult to attain in the electrolysis with cells of other
configurations.
A high percentage of hydrogen in chlorine poses problems in
chlorine liquefaction processes. Extra steps are required to prevent
the formation of dangerous gas mixtures. The hydrogen problem is a
severe drawback if a cost effective method to reduce hydrogen
percentage cannot be identified.
The anode and/or the cathode in the prior art electrolytic cell
are bonded on the surface of the ion exchange membrane so as to be
partially embedded. The gas and the electrolyte solution are readily
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~.~4~~
permeated so as to remove, from the electrode, the gas formed by
the
electrolysis at the electrode layer contactinct the membrane. That
is,
there are few gas bubbles adhering to the membrane after they are
r
formed. Such a porous electrode is usually made of a thin porous
layer which is formed by uniformly mixing particles which act as
an
anode or a cathode with a binder. It has been found that when an
'
electrolytic cell having an ion exchange membrane bonded directly
to
the electrode is used, the anode in the electrolytic cell is brought
into contact with hydroxyl ions which migrate back from the cathode
compartment, and accordingly, both chlorine resistance and alkaline
resistance for anode material are required for this prior art method
and an expensive material must be used. When the electrode layer
is
directly bonded to the ion exchange membrane, a gas is formed by
the
electrode reaction between an electrode and membrane and certain
deformation phenomenon of the ion exchange membrane causes the
characteristics of the membrane to deteriorate. In such an
electrolytic cell, the current collector for the electric supply
to
the electrode layer which is bonded to the ion exchange membrane,
should closely contact the electrode layer. When a firm contact
is not
obtained, the cell voltage may be increased. Therefore, the cell
structure for securely contacting the current collector with the
electrode layer according to the prior art is disadvantageously
complicated.
Additionally, in chlor-alkali electrolyzers where the cathode is
directly bound to the membrane there is permeation of hydrogen
through
the membrane which enters the anolyte compartment and mixes with
the
chlorine. High percentages of hydrogen are then found in the chlorine
so as to cause problems in the liquefaction process.
Prior means for reducing the hydrogen percentage includes 1) the
use
of a platinum black layer on the anode side of the membrane, 2)
the
use of a layer (for example Ag) less electroactive than the electrode
layer itself between the membrane and the electrode, 3) the use
of
thickened membranes, and 4) the use of a membrane with a lower
permeation rate for hydrogen permeation. These methods have proved
to
be expensive and ineffective.
Perfluoro membranes which are used as membranes for electrolysis
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PCT/US93/00547
", WO 94/17222
reactions usually have fairly low water contents. As compared
with
conventional ion exchangers with same amount of water contents,
the
conductivity of the perfluoro membranes are abnormally high.
This is
because of base se aration existin in the
p p g perfluoro ionic membranes.
The phase separation greatly reduces the tortuosity for sodium
ion
diffusion. The hydrogen diffusion path is the aqueous ionic
region
and the amorphous fluorocarbon region. Therefore, the tortuorsity
experienced by the hydrogen molecules is also low for the
phase-
segregated fluorocarbon membranes as compared with conventional
hydrocarbon ionic membranes.
The phase-segregation characteristics of the fluorocarbon
membranes provides the high migration rates for sodium ions,
thus
relatively lower ionic resistivity is also the cause for
the high
hydrogen diffusion rates and the resulting high percentage
of hydrogen
in chlorine. Moreover,~the high permeation rate of hydrogen
is even
more enhanced by the high solubility of hydrogen in the fluorocarbon
membranes because of the hydrophobic interaction between
hydrogen
molecules and the fluorocarbon chains. Therefore, reducing
hydrogen
permeation rates by increasing the thickness of the membranes
or
modifying the structure of the membranes would not be very
effective
because the sodium migration rate would be reduced as one
tries to
reduce the hydrogen diffusion rate; and the tortuosity effect
is
difficult to introduce because of the phase separation.
A retardation layer is defined as a layer between the electrode
layer and the membrane to retard hydrogen permeation. Any
kind of
layer can have a certain effect to retard hydrogen permeation
as long
as it is (1) inactive for electrolytic hydrogen generation,
and (2)
flooded. The latter requirement is also important for low
resistance
(that is, lower voltage and good performance). With these
considerations a layer of a blend of inert solid particles
(usually
inorganic) and binders (usually organic) would serve the
purpose best.
,. The need for a binder is obvious: the binder can (1) bind
the
components in the barrier layer together and also (2) provide
the
necessary adhesion between the retardation layer and the
electrode
layer and that between the retardation layer and the membrane.
The
function of the solid particles is also two fold: (1) providing
the
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WO 94/17222 PCT/US93I00547
physical strength to the barrier layer so that there is very limited
interpenetration between different layers during fabrication, and (2)
forming an agglomerate with the binder.
i
The reason that the retardation layer is better than the
membrane itself in retarding hydrogen permeation is because (1) it
allows hydroxide and sodium ions to migrate at a faster rate so
relatively small voltage penalty has to be paid. On the other hand,
in the membrane, sodium ion diffusion is slowned down by the coulombic
interaction exerted by the sulfonate or carboxylate groups. The
situation is even worse when the membrane is immersed in strong
caustic solution as in the clor-alkali membrane. This is particularly
severe for the carboxylic membranes. Ion pairing between sodium ions
and carboxylate groups and hydroxide ions is believed to be the cause
for the very slow diffusion rate when membrane dehydration occurs
under this condition. The solubility of hydrogen is much lower in
caustic solution than in the membrane, so the permeation rate (the
product of diffusion coefficient and solubility) of hydrogen can be
reduced by a larger factor compared with that of the sodium and
hydroxyl ions. By introducing the blend of inorganic particles and
binder and with the necessary morphology, the resistance of the
caustic solution is increased. The ratio of the resistivity of the
porous medium saturated with electrolyte, Rp, to the bulk resistivity
of the same electrolyte solution, Rb is commonly called "formation
resistivity factor",
F = Rp/Rb = X/O.
This equation describes the relationship between F and "electric
tortuosity~~, X, and 0, the porosity. X is different from hydraulic
tortuosity which takes into account the fact the effective path length
experienced the diffusing species is increased by the presence of
impermeable blocking materials. On the other hand, X also takes into
account the special effects due to convergent-divergent nature of the
capillaries, called constrictedness, besides the hydraulic tortuosity. ,
Since conductivity is proportional to diffusion rate of the
ionic species, formation resistivity factor is also related to
diffusion rate in the porous medium, Dp, and the diffusion rate in the
bulk electrolyte, Db, by the following equation:
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64693-5401 .__
F'= Rp/Rb =- Db/Dp
The present invention provides an improved membrane-electrode
structure for use in electrochemical cells which comprises a
retardation layer between an ion exchange membrane and the cathode.
The retardation layer comprises a blend of 5 to 80 percent by weight
of inorganic solid particles with 95 to 20 percent by weight of a
thermoplastic polymers binder having a melting point of 230°F to
590°F
(110 to 232°C).
As described above, the function of the retardation layer is to
provide porosity and tortuosity so as to impede hydrogen diffusion.
That is, when a retardation layer with a porosity in the range of S
percent to 90 percent is prepared, it is preferable to have a porosity
in the range of 20 percent to 60 percent, preferably, in the range of
30 percent to 50 percent. Advantageously, the tortuosity/porosity
ratio is in the range of 2-500, preferably in the range of 5-100, and
more preferably in the range of 10-50.
The inorganic solid particles comprise one or more of the
borides, carbides and nitrides of metals of Groups IIIH, IVA, IV H, VB
and, VI B of the Periodic Table (CAS version). Typical examples of suitable
materials include SiC, YC, VC, TiC, BC, TiB, HfB, HV2, NbB2 MOB2, W2H,
VN, S13NQ, Zr02, NbN, BN and T1B. Preferably, silicon carbide is
used.
The binder which is used in the invention comprises novel ion
exchange polymers which can be used alone or blended with nonionic
thermoplastic binders. These polymers are copolymers of the following
monomer I with monomer II.
Monomer I is represented by the general formula:
CF2~CZZ' (I)
where;
Z and Z' are independently selected from the group consisting of
-H, -C1, -F, or -CF3.
Monomer II consists of one or more monomers selected from
compounds represented by the general formula;
Y-(CF2)a-(CFRf)b-(CFRf)c-O-[CF(CF2X)-CF2-O]n-CF=CF2 (II)
where;
Y is -S02Z
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Z is -i, -Br, -C1, -F, -OR, or -NR1R2;
R is a.branched or linear alkyl radical having from 1 to 1(1
carbon atoms or an aryl radical;
R1 and R2 are independently selected from the group consisting
of -ti, a branched or linear alkyl radical having from 1 to 10 carbon
atoms or an aryl radical;
a is 0-6;
b is 0-6;
c is o or 1;
provided a+b+c is not equal to O;
X is -C1, -Hr, -F, or mixtures thereof when n>1;
n is o to 6; and
Rf and Rf are independently selected from the group consisting
of -F, -C1, perfluoroalkyl radicals having from 1 to 10 carbon atoms
and fluorochloroalkyl radicals having from 1 to 10 carbon atoms.
The ionic binders can be mixed with a nonionic binder such as a
TM
fluorinated hydrocarbon, that is Teflon.
In accordance with the present invention, at least one of the
electrodes, preferably, the cathode, is bonded to an ion exchange
membrane through a retardation layer for use in an electrolytic cell,
particularly a chlor-alkali cell, so as to retard the diffusion of
hydrogen.
The composition for preparing the retardation layer is
preferably in the form of a suspension of agglomerate of,.p~rticles
5 having a diameter of 0.1 to 10 micrometers, preferably 1 to 4 micrometers.
The
suspension can be formed with an organic solvent which can be easily
removed by evaporation such as halogenated hydrocarbons, alkanols,
~TM
ether, and the Like. Preferable is Freon.
The inorganic particles are admixed with the particles of the
binder so as to comprise 5 to 80 percent by weight of total particles.
A higher percentage of inorganic particles results in poor adhesion of
the electrode layer to the retardation layer. The mixture of the
inorganic particles and the binder are then suspended in an organic
solvent and applied on an ion exchange membrane. The suspension can
be applied by spraying, brushing, screen-printing, and,the likeso as
to distribute the inorganic particles substantially throughout the
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WO 94117222 PCT/US93/00547
retardation.
After the organic solvent is evaporated, the composition is
preferably heat pressed by a roller or press at 80 to 220C
under a
pressure of 0.01 to 150 kg/cm2 to bond the layer to the membrane.
An
electrode layer can. then be heat pressed on the barrier layer
under
the same conditions.
The retardation layer is 0.3 to 3.0 mils (0.0076 to 0.0762
mm)
in thickness, preferably 0.4 to 1.0 mils (0.0102 to 0.0254
mm).
The canon exchange membrane on which the porous non-electrode
layer is formed, can be made of a polymer having cation exchange
groups such as carboxylic acid groups, sulfonic acid groups,
phosphoric acid groups and phenolic hydroxy groups. Suitable
polymers
include copolymers of a vinyl monomer such as tetrafluoroethylene
and
chlorotrifluoroethylene, and a perfluorovinyl monomer having
an ion-
exchange group, such as a sulfonic acid group, carboxylic acid
group
and phosphoric acid group or a reactive group which can be
converted
into the ion-exchange group. It is also possible to use a membrane
of
a polymer of trifluoroethylene in which ion exchange groups,
such as
sulfonic acid groups, are introduced or a polymer of styrene-divinyl
benzene in which sulfonic acid groups are introduced.
The cation exchange membrane is preferably made of a polymer
having the following units:
(M} (CFs-CXX') (M mole percent)
(N) (CFz-CX) (N mole percent)
Y-A
wherein X represents fluorine, chlorine or hydrogen atom or -CF3; X'
represents X or CF3(CHa)m; m represents an integer of 1 to 5.
The typical examples of Y have the structures bonding A to
fluorocarbon group such as
( CFa )x. -O( CFz )x. ( O-CFz-CF )y,
Z
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WO 94/17222 PCT/US93/00547
-CFa ( O-CFz-CF )
Z
( O-CFz-CF )x (O-CFz-CF )y and '
Z Rf
-O-CFz ( CF-O-CFz )x ( CFz )}, (CFz-O-CF )Z
Z Rf
x, y and z respectively represent an integer of 1 to 10; Z and Rf
represent -F or a Cl-C10 perfluoroalkyl group; and A represents -COOM
or SOgM, or a functional group which is convertible into -COOM or -
S03M by, hydrolysis or neutralization, such as -CN, -COF, -COOR1, -
SOzF and -CONRzR3 or -SOzNRaR3, and M represents hydrogen or an alkali
metal atom and R1 represents a C1-C10 alkyl group.
It is preferable to use a cation exchange membrane having an ion
exchange group content of 0.5 to 4.0 miliequivalence/gram dry polymer,
especially 0.8 to 2.0 miliequivalence/gram dry polymer, which is made
of said copolymer.
In the cation exchange membrane of a copolymer having the units
(M) and (N), the ratio of the units (N) is preferably in a range of 1
to 40 mol percent preferably 3 to 25 mol percent.
The cation exchange membrane used in this invention is not
limited to one made of only one kind of the polymer. It is possible
to use a laminated membrane made of two kinds of the polymers having
lower ion exchange capacity in the cathode side, for example, having a
weak acidic ion exchange group such as carboxylic acid group in the
cathode side and a strong acidic ion exchange group, such as sulfonic
acid group, in the anode side.
The cation exchange membrane used in the present invention can
be fabricated by blending a polyolefin, such as polyethylene,
polypropylene, preferably a fluorinated polymer, such as
polytetrafluoroethylene, and a copolymer of ethylene and
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tetrafluoroethylene.
The electrode used in the present invention has a lower over-
voltage than that of the material of the porous non-electrode layer
bonded to the ion exchange membrane. Thus the anode has a lower
S chlorine over-voltage than that of the porous layer at the anode side
and the cathode has a lower hydrogen over-voltage than that of the
barrier layer at the cathode side in the case of the electrolysis of
alkali metal chloride. The material of the electrode used depends on
the material of the retardation layer bonded to the membrane.
The anode is usually made of a platinum group metal or alloy, a
conductive platinum group metal oxide or a conductive reduced oxide
thereof.
The cathode is usually a platinum group metal or alloy, a
conductive platinum group metal oxide or an iron group metal or alloy
or silver.
The platinum group metal are Pt, Rh.Ru, Pd, Ir. The cathode is
TM
iron, cobalt, nickel, Raney nickel, stabilized Raney nickel, stainless
steel, a stainless steel treated by etching with a base.
The preferred catholic materials for use with the retardation
layer of the present invention are Ag and Ruo2.
Also, the polymers comprising the binders of the present
invention desirably have an equivalent weight within a certain desired
range namely, 550 to 1200. It is possible to tailor the polymer
preparation steps in a way to produce a polymer having an equivalent
weight within the desired range. Equivalent weight is a function of
the relative concentration of the reactants in the polymerization
reaction.
The polymers comprising the binders of the present invention
desirably have a melt viscosity within a certain desired range. It is
possible to tailor the polymer preparation steps in a way to produce a
polymer having a melt viscosity within the desired range. The melt
viscosity is based upon the concentration of the initiator and by the
temperature of the reaction.
The polymer obtained by one of the above process is then
hydrolyzed in an appropriate basic solution to convert the nonionic
thermoplastic form of the polymer to the ionic functional form which
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WO 94/17222 PCTJUS93/00~
will have ion transport properties. The hydrolysis step is .
particularly important in the process because during the hydrolysis
step the nonfunctional polymer film is heated and reacted as shown
below during which process, the polymer is softened and swollen with
moisture in a controlled manner. Incomplete hydrolysis leaves
covalentently bonded functional groups whose lack of mobile ions lead
to insulating regions within the binder. The density of the
hydrolysis solution is preferably between 1.26 and 1.28 grams per ml
at ambient temperature. The hydrolysis process requires two moles of
NaOH for each mole of the functional group in the polymer, as shown in
the following equation:
-CF~SOzZ + 2NaOH -> -CFsS03Na + NaZ + Hs0
where Z is -I, -Br, -C1, -F, -OR, or -NRlRz;
R is a branched or linear alkyl radical having from 1 to 10 carbon
. 15 atoms or an aryl radical;
R1 and Rz are independently selected from the group consisting of --H,
a branched or linear alkyl radical having from 1 to 10 carbon atoms or
an aryl radical, preferably phenyl or a lower alkyl substituted
phenyl.
For hydrolysis, the copolymers are placed in the hydrolysis bath
at room temperature, with inert, mesh materials holding the copolymers
in the liquid, making sure that there are no trapped bubbles around
the films. The bath is then heated to 60°C to 90°C and then held
at
that temperature for a minimum of four hours to insure complete
hydrolysis and expansion to the correct level.
After the hydrolysis heating step, the bath is allowed to cool
to room temperature and the polymers are then removed from the bath
and rinsed with high purity deionized water, then placed in a
deionized water bath to leach out residual ionic substances.
A pore former which can be leached out after fabrication may be
used in forming the retardation layer. It is advantageous that the
pores formed by the pore former are interconnected and extended from
the membrane-electrode interface to the electrode-catholyte interface.
The present invention will be further illustrated by certain
examples and references which are provided for purposes of
illustration only and are not intended to limit the present invention.
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WO 94/17222 PCT/US93/00547
E~~
Preparation of Binder
This example shows the preparation of a sulfonic fluoropolymer
having an equivalent weight of 794 and a low shear melt viscosity of
50,000 poise (dyne sec-cm-2) at
250°C and 4.25 sec-1 and a 100°C water absorption of 50 percent.
A 132 liter glass-lined reactor equipped; with an anchor
agitator, H-baffle, a platinum resistance temperature device, and a
temperature control jacket was charged with 527 grams of ammonium
perfluorooctanoate, 398.4 grams of NazHP047Hz0, 328.8 grams NaHzP04Hz0
and 210.8 grams of (NH4)zSzOg. The reactor was then evacuated down to
0.0 atmosphere, as measured on the electronic pressure readout, and
then an inert gas (nitrogen) was added to pressure up the reactor to a
pressure of 448 kPa. This was done a total of 4 times, then the
reactor was evacuated one more time. 99 liters of deoxygenated,
deionized water was added, the agitator was started and heat was
applied to the jacket. An agitator was set to 250 revolutions per
minute (rpm) and then 15 ml of a terminating agent such as isopropyl
alcohol was added, followed by 16.65 kg of 2-fluorosulfonyl
perfluoroethyl vinyl ether was added. When the temperature reached 50°
C, tetrafluoroethylene (TFE) gas was fed to the reactor at a rate of
from 0.5 to 0.567 kg per minute, until a pressure of 1060 kPa is
reached over a period of 17 minutes. The feed was continued until a
total of 18.18 kg. of TFE had been added to the reactor. At that
time, the feed was stopped and then nitrogen was blown through the gas
phase portion of the system and ambient temperature water was added to
the reactor jacket. The materials react to form a latex. The latex
was transferred to a larger vessel for separation and stripping of
residual monomer. After the contents were allowed to settle, a bottom
dump valve was opened to allow separate phase monomer to be drained
away. The vessel was then heated and a vacuum was applied to remove
any further monomer components. After this, a brine system circulates
20°C brine through cooling coils in the vessel to freeze the latex,
causing coagulation into large polymer agglomerates. After freezing
was completed, the latex was allowed to thaw with slight warming (room
temperature water) and the latex was transferred into a centrifuge
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64693-5401
where it was. filtered and washed repeatedly with deionized water.. The
latex polymer cake was then dried overnight in a rotary cone dryer
under vacuum (969 Pa) at 110°C. The water content of the polymer was
tested by Karl Fischer reagent and found to be 140 ppm. The isolated
polymer was weighed and found to be 23.18 kg. The equivalent Weight
of the above_polymer was determined to be 794.
The binder can be prepared in either thermoplastic form or ionic
form. To prepare it in the thermoplastic form, the dried polymer was
dispersed in a suitable solvent and attrited to a fine dispersion.
To prepare it in ionic form the polymer was then hydrolyzed in
an approximately 25 weight percent Na0)i solution. The density of the
hydrolysis solution was between 1.26 and 1.28 grams per ml at ambient
temperature. The hydrolysis process consumed two moles of NaOH for
each mole of the functional group in the polymer, as shown in the
following equation:
-CF~SOzF + 2Na0H -> -CFzS03Na + NaF + Hi0
The polymers were placed in the hydrolysis bath at room
temperature, with inert, mesh materials holding the polymers the
liquid-making sure_that there were no trapped bubbles. The bath was
then heated to 60°C to 90°C and then held at that temperature
for a
minimum of four hours to insure complete hydrolysis and expansion to
the correct level.
After the hydrolysis heating step, the bath was allowed to cool
to room temperature and the polymers were then removed from the bath
and rinsed with high purity deionizsd water, then placed in a
deionized water bath to leach out residual ionic substances.
A suspension of particles SiC and the copolymer of Example 1 was
xM.
formed in Freon at a ratio of 70:30. The resultant layer had a
tortuosity/porosity ratio in the range of 5-50. The composition was
sprayed onto a high performance sulfonic carboxylic bilayer ion
exchange membrane of Dow. After the solvent was evaporated the
composition was hot pressed at 475°F (246~C) and 0.5-100 PSI to form a
layer 0.4 mil (0.0102 rr~n) in thickness. An electrode layer of
Ag/Ru02/ binder (76 percent: l0 percent:8 percent) of 1 mil (0.0254 rmn)~.
in thickness was then sprayed on top of the barrier layer. The entire
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WO 94/17222 PCT/US93/005
unit was then heated at 400°F 1204°C) and pressed together at
0.5-100
Ps i .
The electrode/retardation layer/membrane was then treated to get
the final form for electrolysis. This could involve electrolysis in
an appropriate solution to hydrolyze the membrane and/or the binder if
either of them is in the thermoplastic form.
The resulting structure can be used as the membrane for a chlor-
alkali electrolyzer.
t
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