U.S. patent application number 12/590136 was filed with the patent office on 2010-02-25 for multilayer electrolyte membrane.
This patent application is currently assigned to BASF Fuel Cell GmbH. Invention is credited to Joachim Kiefer, Oemer Uensal.
Application Number | 20100047669 12/590136 |
Document ID | / |
Family ID | 29271567 |
Filed Date | 2010-02-25 |
United States Patent
Application |
20100047669 |
Kind Code |
A1 |
Uensal; Oemer ; et
al. |
February 25, 2010 |
Multilayer electrolyte membrane
Abstract
The present invention relates to a proton-conducting multilayer
electrolyte membrane with a barrier layer, a process for producing
it and a fuel cell containing such a membrane.
Inventors: |
Uensal; Oemer; (Mainz,
DE) ; Kiefer; Joachim; (Losheim am See, DE) |
Correspondence
Address: |
HAMILTON, BROOK, SMITH & REYNOLDS, P.C.
530 VIRGINIA ROAD, P.O. BOX 9133
CONCORD
MA
01742-9133
US
|
Assignee: |
BASF Fuel Cell GmbH
Frankfurt
DE
|
Family ID: |
29271567 |
Appl. No.: |
12/590136 |
Filed: |
November 3, 2009 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10512264 |
Dec 8, 2004 |
7625652 |
|
|
PCT/EP03/04117 |
Apr 22, 2003 |
|
|
|
12590136 |
|
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Current U.S.
Class: |
429/530 |
Current CPC
Class: |
Y02E 60/50 20130101;
H01M 8/1048 20130101; Y02P 70/50 20151101; H01M 8/103 20130101;
H01M 8/0293 20130101; H01M 8/1027 20130101; H01M 8/1053 20130101;
H01M 8/1025 20130101 |
Class at
Publication: |
429/40 |
International
Class: |
H01M 4/00 20060101
H01M004/00 |
Foreign Application Data
Date |
Code |
Application Number |
Apr 25, 2002 |
DE |
DE 102 18 367.8 |
Apr 25, 2002 |
DE |
DE 102 18 368.6 |
Claims
1. A membrane-electrode unit comprising: a) at least one multilayer
electrolyte membrane that includes: i) a sheet-like material doped
with one or more mineral acids, and ii) at least one barrier layer
which covers at least one of the two surfaces of the sheet-like
material; and b) two electrodes, the multilayer electrolyte
membrane arranged between the two electrodes.
2. The membrane-electrode unit of claim 1, characterized in that
the barrier layer applied on a cathode side is thicker than the
barrier layer located on an anode side.
3. An electrode coated with a layer of a cation-exchange material,
wherein the thickness of the layer is from 10 to 30 m.
4. The electrode of claim 3, characterized in that the
cation-exchange material has at least one of the physical
properties selected from the group consisting of a cation-exchange
capacity of less than 0.9 meq/g, an area swelling in water at
80.degree. C. of less than 20%, and a conductivity of less than
0.06 S/cm at 80.degree. C. in a moistened state.
5. An electrode coated with a layer of a cation-exchange material,
wherein the thickness of the layer is less than 10 m.
6. The electrode of claim 5, characterized in that the
cation-exchange material has at least one of the physical
properties selected from the group consisting of a cation-exchange
capacity of less than 0.9 meq/g, an area swelling in water at
80.degree. C. of less than 20%, and a conductivity of less than
0.06 S/cm at 80.degree. C. in a moistened state.
7. A membrane-electrode unit comprising: a) an electrolyte membrane
doped with mineral acids, and b) at least one electrode coated with
a layer of a cation-exchange material, wherein the thickness of the
layer is from 10 to 30 m.
8. The membrane-electrode unit of claim 7, characterized in that
the cation-exchange material has at least one of the physical
properties selected from the group consisting of a cation-exchange
capacity of less than 0.9 meq/g, an area swelling in water at
80.degree. C. of less than 20%, and a conductivity of less than
0.06 S/cm at 80.degree. C. in a moistened state.
9. A membrane-electrode unit comprising: a) an electrolyte membrane
doped with mineral acids, and b) at least one electrode coated with
a layer of a cation-exchange material, wherein the thickness of the
layer is less than 10 m.
10. The membrane-electrode unit of claim 9, characterized in that
the cation-exchange material has at least one of the physical
properties selected from the group consisting of a cation-exchange
capacity of less than 0.9 meq/g, an area swelling in water at
80.degree. C. of less than 20%, and a conductivity of less than
0.06 S/cm at 80.degree. C. in a moistened state.
11. A fuel cell system comprising a plurality of different or
similar membrane-electrode units of which at least one contains a
multilayer electrolyte membrane that includes a sheet-like material
doped with one or more mineral acids and at least one barrier layer
which covers at least one of the two surfaces of the sheet-like
material.
Description
RELATED APPLICATIONS
[0001] This application is a divisional of U.S. application Ser.
No. 10/512,264, having a 371(c) date of Dec. 8, 2004, which is a
U.S. National Stage of International Application No.
PCT/EP03/04117, filed Apr. 22, 2003, published in German, and
claims priority under 35 U.S.C. .sctn.119 or 365 to Germany
Application Nos. 102 18 368.6 and 102 18 367.8, both filed on Apr.
25, 2002.
DESCRIPTION
[0002] The present invention relates to a proton-conducting
multilayer electrolyte membrane, a process for producing it and a
fuel cell containing such a membrane. A fuel cell usually comprises
an electrolyte and two electrodes separated by the electrolyte. In
the case of a fuel cell, a fuel such as hydrogen gas is supplied to
one of the two electrodes and an oxidant such as oxygen gas is
supplied to the other electrode and chemical energy from the
oxidation of the fuel is in this way converted into electric
energy. The electrolyte is permeable to hydrogen ions, i.e.
protons, but not to reactive gases such as the hydrogen gas and the
oxygen gas.
[0003] A fuel cell generally has a plurality of single cells known
as MEUs (membrane-electrode units) which each comprise an
electrolyte and two electrodes separated by the electrolyte.
[0004] Electrolytes employed for the fuel cell are solids such as
polymer electrolyte membranes or liquids such as phosphoric acid.
In recent times, polymer electrolyte membranes have attracted
attention as electrolytes for fuel cells. An in-principle
distinction can be made between 2 categories of polymer
membranes.
[0005] The first category comprises cation-exchange membranes
composed of a polymer framework containing covalently bound acid
groups, preferably sulphonic acid groups. The sulphonic acid group
is converted into an anion by release of a hydrogen ion and
therefore conducts protons. The mobility of the proton and thus the
proton conductivity is directly related to the water content. If
the membrane dries out, e.g. as a result of high temperature, the
conductivity of the membrane and consequently the power of the fuel
cell decreases drastically. The operating temperature of fuel cells
containing such cation-exchange membranes is thus limited to the
boiling point of water. For this reason, perfluorosulphonic acid
polymers, for example, are used as materials for polymer
electrolyte membranes. The perfluorosulphonic acid polymer (e.g.
Nafion) generally has a perfluorohydrocarbon framework, e.g. a
copolymer of tetrafluoroethylene and trifluorovinyl, and a side
chain which is bound thereto and bears a sulphonic acid group, e.g.
a side chain having a sulphonic acid group bound to a
perfluoroalkylene group. Moistening of the fuels is an important
industrial requirement for the use of polymer electrolyte membrane
fuel cells (PEMFCs) in which conventional, sulphonated membranes
such as Nafion are used.
[0006] A second category which has been developed comprises polymer
electrolyte membranes composed of complexes of basic polymers and
strong acids. Thus, WO 96/13872 and the corresponding U.S. Pat. No.
5,525,436 describe a process for preparing a proton-conductive
polymer electrolyte membrane, in which a basic polymer such as
polybenzimidazole is treated with a strong acid such as phosphoric
acid, sulphuric acid, etc.
[0007] A fuel cell in which such a polymer electrolyte membrane is
used has the advantage that it can be operated without moistening
and at temperatures of 100.degree. C. or above.
[0008] In J. Electrochem. Soc., volume 142, No. 7, 1995, pp.
L121-L123, doping of a polybenzimidazole in phosphoric acid is
described.
[0009] In the case of the basic polymer membranes known from the
prior art, the mineral acid used for achieving the required proton
conductivity (usually concentrated phosphoric acid) is either used
after shaping or, as an alternative, the basic polymer membrane is
produced directly from polyphosphoric acid as in the German patent
applications No. 10117686.4, No. 10144815.5 and No. 10117687.2.
Here, the polymer serves as support for the electrolyte consisting
of the highly concentrated phosphoric acid or polyphosphoric acid.
The polymer membrane here fulfils further essential functions, in
particular it has to have a high mechanical stability and serve as
separator for the two fuels mentioned at the outset.
[0010] An important advantage of such a membrane doped with
phosphoric acid is the fact that this system can be operated at
temperatures above 100.degree. C. without moistening of the fuels
which would otherwise be necessary. This is due to the ability of
phosphoric acid to be able to transport protons without additional
water by means of the Grotthus mechanism (K.-D. Kreuer, Chem.
Mater. 1996, 8, 610-641).
[0011] The ability to operate the fuel cell system at temperatures
above 100.degree. C. results in further advantages for the system.
Firstly, the sensitivity of the Pt catalyst to gas impurities, in
particular CO, is greatly reduced. CO is formed as by-product in
the reforming of the hydrogen-rich gas from carbon-containing
compounds, e.g. natural gas, methanol or petroleum spirit, or as
intermediate in the direct oxidation of methanol. The CO content of
the fuel typically has to be less than 100 ppm at temperatures of
<100.degree. C. However, at temperatures in the range
150-200.degree. C., 10 000 ppm or more of CO can also be tolerated
(N. J. Bjerrum et. al. Journal of Applied Electrochemistry, 2001,
31, 773-779). This leads to substantial simplification of the
upstream reforming process and thus to cost reductions for the
overall fuel cell system.
[0012] A great advantage of fuel cells is the fact that in the
electrochemical reaction the energy of the fuel is converted
directly into electric energy and heat. Water is formed as reaction
product at the cathode. Heat is thus produced as by-product of the
electrochemical reaction. In the case of applications in which only
the electric power is utilized for driving electric motors, e.g.
for automobile applications, the heat has to be removed to avoid
overheating of the system. Additional, energy-consuming equipment
is therefore necessary for cooling, and this further reduces the
overall electrical efficiency of the fuel cell. In the case of
stationary applications such as for central or decentralized
generation of electric power and heat, the heat can be utilized
efficiently by means of existing technologies, e.g. heat
exchangers. To increase the efficiency, high temperatures are
desirable. If the operating temperature is above 100.degree. C. and
the temperature difference between ambient temperature and the
operating temperature is large, it becomes possible to cool the
fuel cell system more efficiently or to use small cooling areas and
dispense with additional equipment compared to fuel cells which,
owing to moistening of the membrane, have to be operated at below
100.degree. C.
[0013] Besides these advantages, such a system has two critical
disadvantages. Thus, phosphoric acid is present as an electrolyte
which is not bound permanently by ionic interactions to the basic
polymer and can be washed out by water. Water is, as described
above, formed at the cathode in the electrochemical reaction. If
the operating temperature is above 100.degree. C., the water is
mostly removed as vapour through the gas diffusion electrode and
the acid loss is very small. However, if the operating temperature
drops below 100.degree. C., e.g. on starting up and shutting down
the cell or in part-load operation when a high current yield is
sought, the water formed condenses and can lead to increased
leaching of the electrolyte, viz. highly concentrated phosphoric
acid.
[0014] In the above-described mode of operation of the fuel cell,
this can lead to a continual decrease in the conductivity and cell
power, which can reduce the life of the fuel cell. A further
disadvantage of fuel cells in which phosphoric acid functions as
electrolyte is inhibition of the reduction reaction at the cathode,
resulting in a high overvoltage. This leads to a low equilibrium
rest potential and a relatively low power.
[0015] Furthermore, the known membranes doped with phosphoric acid
cannot be used in the direct methanol fuel cell (DMFC). However,
such cells are of particular interest since a methanol/water
mixture is used as fuel. If a known membrane based on phosphoric
acid is used, the fuel cell fails after quite a short time.
[0016] It is therefore an object of the present invention to
provide a polymer electrolyte membrane in which the leaching of the
mineral acid is reduced or prevented and which additionally has a
reduced overvoltage, in particular at the cathode. In particular,
the operating temperature should be able to be in the extended
range from <0.degree. C. to 200.degree. C.
[0017] A further object of the present invention was to provide a
membrane which even in operation has a low permeability to a wide
variety of fuels, for example hydrogen or methanol, and also
displays a low oxygen permeability.
BRIEF DESCRIPTION OF THE FIGURES
[0018] FIG. 1 is a schematic structure of the measurement apparatus
employed to measure the barrier action of the cation-exchange
membranes doped with phosphoric acid.
[0019] FIG. 2 is a plot of the values of pH as a function of time
for different cation-exchange membranes.
[0020] FIG. 3 is a plot of the values of pH as a function of time
for different cation-exchange membranes, corrected for the
blank.
[0021] FIG. 4 is a bar plot of the amounts of acid which passed
through the barrier layer and which was retained by the barrier
layers of different cation-exchange membranes.
[0022] FIG. 5 is a plot showing the values of pH of a volume of
water as a function of time, demonstrating effectiveness of a
barrier layer in an experimental setup shown in FIG. 1.
DETAILED DESCRIPTION
[0023] The object of the invention is achieved by a multilayer
membrane system comprising a polymer electrolyte membrane which is
doped with mineral acid and is coated on at least one side with a
barrier layer for the mineral acid. In this configuration, the
membrane doped with mineral acid performs the essential functions
as separator for the fuels and the provision of mechanical
stability. The barrier layer is intended to prevent the loss of
mineral acid and to reduce the overvoltage at the cathode.
[0024] A polymer electrolyte membrane according to the invention
has a very low methanol permeability and is particularly suitable
for use in a DMFC. Long-term operation of a fuel cell using many
fuels such as hydrogen, natural gas, petroleum spirit, methanol or
biomass is thus possible. Here, the membranes allow a particularly
high activity of these fuels. As a result of the high temperatures,
the methanol oxidation can occur with high activity.
[0025] The present invention accordingly provides a multilayer
electrolyte membrane comprising [0026] A. a sheet-like material
doped with one or more mineral acids, and [0027] B. at least one
barrier layer which covers at least one of the two surfaces of the
material specified under A.
[0028] In the case of the sheet-like materials A, use is made of
basic polymers, mixtures of basic polymers with other polymers or
chemically inert supports, preferably ceramic materials, in
particular silicon carbides (SiC) as are described in U.S. Pat. No.
4,017,664 and U.S. Pat. No. 4,695,518. These materials are capable
of transporting protons by the Grofthus mechanism.
[0029] A thermally stable and chemically inert support which is
filled with phosphoric acid to achieve proton conductivity can be
used as sheet-like material. Possible support materials are, for
example, ceramic materials such as silicon carbide SiC (U.S. Pat.
No. 4,017,664 and U.S. Pat. No. 4,695,518) or inorganic glasses.
This support can, for example, be in the form of a woven fabric or
a nonwoven. Furthermore, the support can also be made up of porous
materials.
[0030] As chemically inert support, it is also possible to use
porous organic polymers having an open pore structure. The open
pore volume is in this case more than 30%, preferably more than 50%
and very particularly preferably more than 70%. The glass
transition temperature of the organic base polymer of such a
membrane is higher than the operating temperature of the fuel cell
and is preferably at least 150.degree. C., more preferably at least
160.degree. C. and very particularly preferably at least
180.degree. C. Such membranes are used as separation membranes for
ultrafiltration, gas separation, pervaporation, nanofiltration,
microfiltration or haemodialysis.
[0031] Methods of producing such membranes are described in H. P.
Hentze, M. Antonietti "Porous polymers and resins" in F. Schuth
"Handbook of Porous Solids" pp. 1964-2013.
[0032] It is also possible to produce organic foams as chemically
inert supports. These foams can be produced by gases such as
CO.sub.2 being liberated in the synthesis of the organic polymer or
volatile liquids being used. Methods of producing organic foams are
described in D. Klempner, K. C. Frisch "Handbook of Polymeric Foams
and Foam Technology" and F. A. Shutov Advances in Polymer Science
Volume 73/74, 1985, pages 63-123. Supercritical CO.sub.2 can also
be used as pore former.
[0033] A particularly advantageous support is a phase separation
membrane composed of polybenzimidazole, which can be produced as
described in U.S. Pat. No. 4,693,824 or U.S. Pat. No. 4,666,996 or
U.S. Pat. No. 5,091,087. The chemical stability of these membranes
towards phosphoric acid or polyphosphoric acid can be further
improved by crosslinking by means of the method described in U.S.
Pat. No. 4,634,530. Furthermore, it is possible to use expanded
polymer films such as expanded Teflon as support materials. Methods
of producing proton-conducting membranes by filling such an
expanded perfluorinated membrane are described in U.S. Pat. No.
5,547,551.
[0034] Likewise, high-porosity thermosets which have been prepared
by chemically induced phase separation can likewise be used as
support materials. In this process, a highly volatile solvent is
added to a mixture of a plurality of monomers capable of
crosslinking. This solvent becomes insoluble during crosslinking
and a heterogeneous polymer is formed. Evaporation of the solvent
produces a chemically inert, porous thermoset which can
subsequently be impregnated with phosphoric acid or polyphosphoric
acid.
[0035] A particularly useful support can be produced from inorganic
materials, for example glass or materials which comprise at least
one compound of a metal, a semimetal or a mixed metal or phosphorus
with at least one element of main groups 3 to 7. The material
particularly preferably comprises at least one oxide of the
elements Zr, Ti, Al or Si. The support can consist of an
electrically insulating material, e.g. minerals, glasses, plastics,
ceramics or natural materials. The support preferably comprises
specific woven fabrics, nonwovens or porous materials composed of
high-temperature-resistant and highly acid-resistant fused silica
or glass. The glass preferably comprises at least one compound from
the group consisting of SiO.sub.2, Al.sub.2O.sub.3 or MgO. In a
further variant, the support comprises woven fabrics, nonwovens or
porous materials composed of Al.sub.2O.sub.3, ZrO.sub.2, TiO.sub.2,
Si.sub.3N.sub.4 or SiC ceramic. To keep the total resistance of the
electrolyte membrane low, this support preferably has a very high
porosity but also a low thickness of less than 1000 .mu.m,
preferably less than 500 .mu.m and very particularly preferably
less than 200 .mu.m. Preference is given to using supports which
comprise woven fibres of glass or fused silica, with the woven
fabrics preferably being composed of 11-tex yarns with 5-50 warp
threads or weft threads and preferably 20-28 warp threads and 28-36
weft threads. Particular preference is given to 5.5-tex yarns with
10-50 warp threads or weft threads and preferably 20-28 warp
threads and 28-36 weft threads.
[0036] As indicated above, supports comprising woven fabrics,
nonwovens or porous materials can be used. Porous materials based
on, in particular, organic or inorganic foams are known.
[0037] Preferred supports are permeable to mineral acids without a
barrier layer. This property can be confirmed by the experiment on
barrier action presented in the examples. According to a particular
aspect of the present invention, at least 5% of a mineral acid
present in the sheet-like structure is liberated within 1 hour if
the sheet-like material is exposed to a large excess of water (an
at least 100-fold amount, based on the weight of the sheet) having
a temperature of 80.degree. C.
[0038] Depending on the field of application, the sheet-like
structure A) can be stable to high temperatures. Stable to high
temperatures means that the support is stable at a temperature of
at least 150.degree. C., preferably at least 200.degree. C. and
particularly preferably at least 250.degree. C. Stable means that
the significant properties of the support are retained. Thus, no
change in the mechanical properties or in the chemical composition
occurs on exposure of the sheet-like material for at least 1
hour.
[0039] In general, the support is chemically inert. Chemically
inert means that a sheet-like material doped with a mineral acid is
chemically stable. Chemically stable means that the material is not
decomposed by the acid. Thus, the material after 100 hours displays
at least 95% of the mechanical properties displayed by the material
at the beginning of the measurement. This applies, for example, to
the modulus of elasticity and the microhardness.
[0040] As basic polymer membrane doped with mineral acid, it is
possible to use virtually all known polymer membranes in which the
protons are transported without additional water, e.g. by means of
the Grotthus mechanism.
[0041] A basic polymer for the purposes of the present invention is
a basic polymer having at least one nitrogen atom in a repeating
unit.
[0042] The repeating unit in the basic polymer preferably contains
an aromatic ring having at least one nitrogen atom. The aromatic
ring is preferably a five- or six-membered ring which has from one
to three nitrogen atoms and can be fused with another ring, in
particular another aromatic ring.
[0043] Polymers based on polyazole generally comprise recurring
azole units of the general formula (I) and/or (II) and/or (III)
and/or (IV) and/or (V) and/or (VI) and/or (VII) and/or (VIII)
and/or (IX) and/or (X) and/or (XI) and/or (XII) and/or (XIII)
and/or (XIV) and/or (XV) and/or (XVI) and/or (XVI) and/or (XVII)
and/or (XVIII) and/or (XIX) and/or (XX) and/or (XXI) and/or
(XXII)
##STR00001## ##STR00002## ##STR00003##
where [0044] the radicals Ar are identical or different and are
each a tetravalent aromatic or heteroaromatic group which may be
monocyclic or polycyclic, [0045] the radicals Ar.sup.1 are
identical or different and are each a divalent aromatic or
heteroaromatic group which may be monocyclic or polycyclic, [0046]
the radicals Ar.sup.2 are identical or different and are each a
divalent or trivalent aromatic or heteroaromatic group which may be
monocyclic or polycyclic, [0047] the radicals Ar.sup.3 are
identical or different and are each a trivalent aromatic or
heteroaromatic group which may be monocyclic or polycyclic, [0048]
the radicals Ar.sup.4 are identical or different and are each a
trivalent aromatic or heteroaromatic group which may be monocyclic
or polycyclic, [0049] the radicals Ar.sup.5 are identical or
different and are each a tetravalent aromatic or heteroaromatic
group which may be monocyclic or polycyclic, [0050] the radicals
Ar.sup.6 are identical or different and are each a divalent
aromatic or heteroaromatic group which may be monocyclic or
polycyclic, [0051] the radicals Ar.sup.7 are identical or different
and are each a divalent aromatic or heteroaromatic group which may
be monocyclic or polycyclic, [0052] the radicals Ar.sup.8 are
identical or different and are each a trivalent aromatic or
heteroaromatic group which may be monocyclic or polycyclic, [0053]
the radicals Ar.sup.9 are identical or different and are each a
divalent or trivalent or tetravalent aromatic or heteroaromatic
group which may be monocyclic or polycyclic, [0054] the radicals
Ar.sup.10 are identical or different and are each a divalent or
trivalent aromatic or heteroaromatic group which may be monocyclic
or polycyclic, [0055] the radicals Ar.sup.11 are identical or
different and are each a divalent aromatic or heteroaromatic group
which may be monocyclic or polycyclic, [0056] the radicals X are
identical or different and are each oxygen, sulphur or an amino
group which bears a hydrogen atom, a group containing 1-20 carbon
atoms, preferably a branched or unbranched alkyl or alkoxy group,
or an aryl group as further radical, [0057] the radicals R are
identical or different and are each hydrogen, an alkyl group or an
aromatic group and [0058] n, m are each an integer greater than or
equal to 10, preferably greater than or equal to 100.
[0059] Aromatic or heteroaromatic groups which are preferred
according to the invention are derived from benzene, naphthalene,
biphenyl, diphenyl ether, diphenylmethane, diphenyldimethylmethane,
bisphenone, diphenyl sulphone, thiophene, furan, pyrrole, thiazole,
oxazole, imidazole, isothiazole, isoxazole, pyrazole,
1,3,4-oxadiazole, 2,5-diphenyl-1,3,4-oxadiazole, 1,3,4-thiadiazole,
1,3,4-triazole, 2,5-diphenyl-1,3,4-triazole,
1,2,5-triphenyl-1,3,4-triazole, 1,2,4-oxadiazole,
1,2,4-thiadiazole, 1,2,4-triazole, 1,2,3-triazole,
1,2,3,4-tetrazole, benzo[b]thiophene, benzo[b]furan, indole,
benzo[c]thiophene, benzo[c]furan, isoindole, benzoxazole,
benzothiazole, benzimidazole, benzisoxazole, benzisothiazole,
benzopyrazole, benzothiadiazole, benzotriazole, dibenzofuran,
dibenzothiophene, carbazole, pyridine, bipyridine, pyrazine,
pyrazole, pyrimidine, pyridazine, 1,3,5-triazine, 1,2,4-triazine,
1,2,4,5-triazine, tetrazine, quinoline, isoquinoline, quinoxaline,
quinazoline, cinnoline, 1,8-naphthyridine, 1,5-naphthyridine,
1,6-naphthyridine, 1,7-naphthyridine, phthalazine,
pyridopyrimidine, purine, pteridine or quinolizine, 4H-quinolizine,
diphenyl ether, anthracene, benzopyrrole, benzooxathiadiazole,
benzooxadiazole, benzopyridine, benzopyrazine, benzopyrazidine,
benzopyrimidine, benzotriazine, indolizine, pyridopyridine,
imidazopyrimidine, pyrazinopyrimidine, carbazole, aciridine,
phenazine, benzoquinoline, phenoxazine, phenothiazine, acridizine,
benzopteridine, phenanthroline and phenanthrene, which may also be
substituted.
[0060] Here, Ar.sup.1, Ar.sup.4, Ar.sup.6, Ar.sup.7, Ar.sup.8,
Ar.sup.9, Ar.sup.10, Ar.sup.11 can have any substitution pattern;
in the case of phenylene, for example, Ar.sup.1, Ar.sup.4,
Ar.sup.6, Ar.sup.7, Ar.sup.8, Ar.sup.9, Ar.sup.10, Ar.sup.11 can be
ortho-, meta- or para-phenylene. Particularly preferred groups are
derived from benzene and biphenyls, which may also be
substituted.
[0061] Preferred alkyl groups are short-chain alkyl groups having
from 1 to 4 carbon atoms, e.g. methyl, ethyl, n- or i-propyl and
t-butyl groups.
[0062] Preferred aromatic groups are phenyl or naphthyl groups. The
alkyl groups and the aromatic groups may be substituted.
[0063] Preferred substituents are halogen atoms such as fluorine,
amino groups, hydroxy groups or short-chain alkyl groups such as
methyl or ethyl groups.
[0064] Preference is given to polyazoles which comprise recurring
units of the formula (I) and in which the radicals X are identical
within a recurring unit.
[0065] The polyazoles can in principle also be made up of different
recurring units which differ, for example, in their radical X.
However, the polyazole preferably has only identical radicals X in
a recurring unit.
[0066] Further preferred polyazole polymers are polyimidazoles,
polybenzothiazoles, polybenzoxazoles, polyoxadiazoles,
polyquinoxalines, polythiadiazoles, poly(pyridines),
poly(pyrimidines) and poly(tetrazapyrenes).
[0067] In a further embodiment of the present invention, the
polymer comprising recurring azole units is a copolymer or a blend
comprising at least two units of the formulae (I) to (XXII) which
differ from one another. The polymers can be in the form of block
copolymers (diblock, triblock), random copolymers, periodic
copolymers and/or alternating polymers. In a particularly preferred
embodiment of the present invention, the polymer comprising
recurring azole units is a polyazole made up only of units of the
formula (I) and/or (II). The number of recurring azole units in the
polymer is preferably greater than or equal to 10. Particularly
preferred polymers contain at least 100 recurring azole units.
[0068] For the purposes of the present invention, polymers
comprising recurring benzimidazole units are preferred. Some
examples of extremely advantageous polymers comprising recurring
benzimidazole units have the following formulae:
##STR00004## ##STR00005##
where n and m are each an integer greater than or equal to 10,
preferably greater than or equal to 100.
[0069] Further preferred polyazole polymers are polyimidazoles,
polybenzimidazole ether ketone, polybenzothiazoles,
polybenzoxazoles, polytriazoles, polyoxadiazoles, polythiadiazoles,
polypyrazoles, polyquinoxalines, poly(pyridines), poly(pyrimidines)
and poly(tetrazapyrenes).
[0070] Preferred polyazoles have a high molecular weight. This
applies in particular to the polybenzimidazoles. Measured as
intrinsic viscosity, it is in the range from 0.3 to 10 dl/g,
preferably from 1 to 5 dl/g.
[0071] Particular preference is given to Celazole from Celanese.
The properties of the polymer film and polymer membrane can be
improved by sieving the starting polymer, as described in the
German patent application No. 10129458.1.
[0072] The polymer film based on basic polymers which is used for
doping can contain further additions of fillers and/or auxiliaries.
In addition, the polymer film can be modified in further ways, for
example by crosslinking as in the German patent application No.
10110752.8 or in WO 00/44816. In a preferred embodiment, the
polymer film comprising a basic polymer and at least one blend
component which is used for doping additionally contains a
crosslinker as described in the German patent application No.
10140147.7. An important advantage of such a system is the fact
that higher degrees of doping and thus higher conductivities
combined with satisfactory mechanical stability of the membrane can
be achieved.
[0073] Apart from the abovementioned basic polymers, it is also
possible to use a blend of one or more basic polymers with a
further polymer. The blend component essentially has the task of
improving the mechanical properties and reducing the material
costs. A preferred blend component is polyether sulphone as
described in the German patent application No. 10052242.4.
[0074] Preferred polymers which can be used as blend component
include, inter alia, polyolefins such as poly(chloroprene),
polyacetylene, polyphenylene, poly(p-xylylene), polyarylmethylene,
polyarmethylene, polystyrene, polymethylstyrene, polyvinyl alcohol,
polyvinyl acetate, polyvinyl ether, polyvinylamine,
poly(N-vinylacetamide), polyvinylimidazole, polyvinylcarbazole,
polyvinylpyrrolidone, polyvinylpyridine, polyvinyl chloride,
polyvinylidene chloride, polytetrafluoroethylene,
polyhexafluoropropylene, copolymers of PTFE with
hexafluoropropylene, with perfluoropropyl vinyl ether, with
trifluoronitrosomethane, with sulphonyl fluoride vinyl ether, with
carbalkoxyperfluoroalkoxy vinyl ether, polychlorotrifluoroethylene,
polyvinyl fluoride, polyvinylidene fluoride, polyacrolein,
polyacrylamide, polyacrylonitrile, polycyanoacrylates,
polymethacrylimide, cycloolefinic copolymers, in particular those
of norbornene;
polymers having C--O bonds in the main chain, for example
polyacetal, polyoxymethylene, polyether, polypropylene oxide,
polyepichlorohydrin, polytetrahydrofuran, polyphenylene oxide,
polyether ketone, polyesters, in particular polyhydroxyacetic acid,
polyethylene terephthalate, polybutylene terephthalate,
polyhydroxybenzoate, polyhydroxypropionic acid, polypivalolactone,
polycaprolactone, polymalonic acid, polycarbonate; polymers having
C--S bonds in the main chain, for example polysulphide ethers,
polyphenylene sulphide, polyether sulphone; polymers having C--N
bonds in the main chain, for example polyimines, polyisocyanides,
polyetherimine, polyaniline, polyamides, polyhydrazides,
polyurethanes, polyimides, polyazoles, polyazines;
liquid-crystalline polymers, in particular Vectra and also
inorganic polymers, for example polysilanes, polycarbosilanes,
polysiloxanes, polysilicic acid, polysilicates, silicones,
polyphosphazenes and polythiazyl.
[0075] For use in fuel cells having a long-term use temperature
above 100.degree. C., preference is given to blend polymers which
have a glass transition temperature or Vicat softening temperature
VST/A/50 of at least 100.degree. C., preferably at least
150.degree. C. and very particularly preferably at least
180.degree. C.
[0076] Preference is here given to polysulphones having a Vicat
softening temperature VST/A/50 of from 180.degree. C. to
230.degree. C.
[0077] Preferred polymers include polysulphones, in particular
polysulphone having an aromatic in the main chain. According to a
particular aspect of the present invention, preferred polysulphones
and polyether sulphones have a melt volume rate MVR 300/21.6 of
less than or equal to 40 cm.sup.3/10 min, in particular less than
or equal to 30 cm.sup.3/10 min and particularly preferably less
than or equal to 20 cm.sup.3/10 min, measured in accordance with
ISO 1133.
[0078] To improve the use properties further, the sheet-like
material can contain fillers, in particular proton-conducting
fillers.
[0079] Nonlimiting examples of proton-conducting fillers are [0080]
sulphates such as: CsHSO.sub.4, Fe(SO.sub.4).sub.2,
(NH.sub.4).sub.3H(SO.sub.4).sub.2, LiHSO.sub.4, NaHSO.sub.4,
KHSO.sub.4, RbSO.sub.4, LiN.sub.2H.sub.5SO.sub.4,
NH.sub.4HSO.sub.4, [0081] phosphates such as
Zr.sub.3(PO.sub.4).sub.4, Zr(HPO.sub.4).sub.2,
HZr.sub.2(PO.sub.4).sub.3, UO.sub.2PO.sub.4.3H.sub.2O,
H.sub.8UO.sub.2PO.sub.4, Ce(HPO.sub.4).sub.2, Ti(HPO.sub.4).sub.2,
KH.sub.2PO.sub.4, NaH.sub.2PO.sub.4, LiH.sub.2PO.sub.4,
NH.sub.4H.sub.2PO.sub.4, CsH.sub.2PO.sub.4, CaHPO.sub.4,
MgHPO.sub.4, HSbP.sub.2O.sub.8, HSb.sub.3P.sub.2O.sub.14,
H.sub.5Sb.sub.5P.sub.2O.sub.20, [0082] polyacids such as
H.sub.3PW.sub.12O.sub.40.nH.sub.2O (n=21-29),
H.sub.3SiW.sub.12O.sub.40.nH.sub.2O (n=21-29), H.sub.xWO.sub.3,
HSbWO.sub.6, H.sub.3PMo.sub.12O.sub.40, H.sub.2Sb.sub.4O.sub.11,
HTaWO.sub.6, HNbO.sub.3, HTiNbO.sub.5, HTiTaO.sub.5, HSbTeO.sub.6,
H.sub.5Ti.sub.4O.sub.9, HSbO.sub.3, H.sub.2MoO.sub.4 [0083]
selenites and arsenides such as (NH.sub.4).sub.3H(SeO.sub.4).sub.2,
UO.sub.2AsO.sub.4, (NH.sub.4).sub.3H(SeO.sub.4).sub.2,
KH.sub.2AsO.sub.4, Cs.sub.3H(SeO.sub.4).sub.2,
Rb.sub.3H(SeO.sub.4).sub.2, [0084] oxides such as Al.sub.2O.sub.3,
Sb.sub.2O.sub.5, ThO.sub.2, SnO.sub.2, ZrO.sub.2, MoO.sub.3 [0085]
silicates such as zeolites, zeolites(NH.sub.4+), sheet silicates,
framework silicates, H-natrolites, H-mordenites,
NH.sub.4-analcines, NH.sub.4-sodalites, NH.sub.4-gallates,
H-montmorillonites [0086] acids such as HClO.sub.4, SbF.sub.5
[0087] fillers such as carbides, in particular SiC,
Si.sub.3N.sub.4, fibres, in particular glass fibres, glass powders
and/or polymer fibres, preferably ones based on polyazoles.
[0088] These additives can be present in customary amounts in the
proton-conducting polymer membrane, but the positive properties
such as high conductivity, long life and high mechanical stability
of the membrane should not be impaired too much by addition of
excessively large amounts of additives. In general, the membrane
comprises not more than 80% by weight, preferably not more than 50%
by weight and particularly preferably not more than 20% by weight,
of additives.
[0089] To produce the polymer film, the polymer constituents are
firstly dissolved or suspended as described in the above-cited
patent applications, for example DE No. 10110752.8 or WO 00/44816,
and subsequently used for producing the polymer films. Furthermore,
the polymer films as described in DE No. 10052237.8 can be produced
continuously.
[0090] As an alternative, film formation can be carried out by the
process described in the Japanese patent application No. Hei
10-125560.
[0091] Here, the solution is poured into a cylinder having a
cylindrical interior surface and the cylinder is subsequently set
into rotation. At the same time, the solvent is allowed to
evaporate by means of the centrifugal force caused by the rotation,
so that a cylindrical polymer film of largely uniform thickness is
formed on the interior surface of the cylinder. The basic polymer
having a uniform matrix can be formed by this process.
[0092] This process described in the Japanese patent application
Hei 10-125560 is likewise incorporated by reference into the
present description.
[0093] The solvent is subsequently removed. This can be achieved by
methods known to those skilled in the art, for example by
drying.
[0094] The film of basic polymer or polymer blend is subsequently
impregnated or doped with a strong acid, preferably a mineral acid,
with the film as described in the German patent application No.
10109829.4 being able to be treated beforehand. This variant is
advantageous in order to rule out interactions of the residual
solvent with the barrier layer.
[0095] For this purpose, the film of basic polymer or polymer blend
is dipped into a strong acid so that the film is impregnated with
the strong acid and becomes a proton-conducting membrane. For this
purpose, the basic polymer is usually dipped into a highly
concentrated strong acid having a temperature of at least
35.degree. C. for a period of from a number of minutes to a number
of hours.
[0096] As strong acid, use is made of mineral acid, in particular
phosphoric acid and/or sulphuric acid.
[0097] For the purposes of the present description, the term
"phosphoric acid" refers to polyphosphoric acid
(H.sub.n+2P.sub.nO.sub.3n+1 (n>1) usually has an assay
calculated as P.sub.2O.sub.5 (acidimetric) of at least 83%,
phosphonic acid (H.sub.3PO.sub.3), orthophosphoric acid
(H.sub.3PO.sub.4), pyrophosphoric acid (H.sub.4P.sub.2O.sub.7),
triphosphoric acid (H.sub.5P.sub.3O.sub.10) and metaphosphoric
acid. The phosphoric acid, in particular orthophosphoric acid,
preferably has a concentration of at least 80 percent by weight,
particularly preferably a concentration of at least 85 percent by
weight, more preferably a concentration of at least 87 percent by
weight and very particularly preferably a concentration of at least
89 percent by weight. The reason for this is that as the
concentration of the strong acid increases, the basic polymer can
be impregnated with a greater number of molecules of strong
acid.
[0098] The polymer electrolyte membrane obtained, namely the
complex of the basic polymer and the strong acid, is
proton-conducting. After doping, the degree of doping expressed as
mole of acid per repeating unit should be greater than 6,
preferably greater than 8 and very particularly preferably greater
than 9.
[0099] In place of polymer membranes based on basic polymers which
have been produced by means of classical methods, it is also
possible to use polyazole-containing polymer membranes as described
in the German patent applications No. 10117686.4, 10144815.5,
10117687.2. Such polymer electrolyte membranes provided with at
least one barrier layer are likewise subject-matter of the present
invention.
[0100] Accordingly, sheet-like materials according to the invention
can be obtained by a process comprising the steps [0101] i)
preparation of a mixture comprising [0102] polyphosphoric acid,
[0103] at least one polyazole and/or at least one compound which
are/is suitable for forming polyazoles under the action of heat as
described in step ii), [0104] ii) heating of the mixture obtainable
as described in step i) under inert gas to temperatures of up to
400.degree. C., [0105] iii) application of a layer to a support
using the mixture as described in step i) and/or ii), [0106] iv)
treatment of the membrane formed in step iii).
[0107] For this purpose, one or more compounds which are suitable
for forming polyazoles under the action of heat as described in
step ii) can be added to the mixture from step i). Mixtures
comprising one or more aromatic and/or heteroaromatic tetramino
compounds and one or more aromatic and/or heteroaromatic carboxylic
acids or derivatives thereof which have at least two acid groups
per carboxylic acid monomer are suitable for this purpose.
Furthermore, it is possible to use one or more aromatic and/or
heteroaromatic diaminocarboxylic acids for preparing
polyazoles.
[0108] Suitable aromatic and heteroaromatic tetramino compounds
include, inter alia, 3,3',4,4'-tetraminobiphenyl,
2,3,5,6-tetraminopyridine, 1,2,4,5-tetraminobenzole,
bis(3,4,diaminodiphenyl) sulphone, bis(3,4,-diaminodiphenyl ether,
3,3',4,4'-tetraminobenzophenone, 3,3',4,4'-tetraminodiphenylmethane
and 3,3',4,4'-tetraminodiphenyldimethylmethane and also salts
thereof, in particular monohydrochloride, dihydrochloride,
trihydrochloride and tetrahydrochloride derivatives thereof. Among
these, particular preference is given to
3,3',4,4'-tetraminobiphenyl, 2,3,5,6-tetraminopyridine and
1,2,4,5-tetraminobenzole.
[0109] Furthermore, the mixture A) can comprise aromatic and/or
heteroaromatic carboxylic acids. These are dicarboxylic acids and
tricarboxylic acids and tetracarboxylic acids or their esters or
their anhydrides or their acid halides, in particular their acid
halides and/or acid bromides. The aromatic dicarboxylic acids are
preferably isophthalic acid, terephthalic acid, phthalic acid,
5-hydroxyisophthalic acid, 4-hydroxyisophthalic acid,
2-hydroxyterephthalic acid, 5-aminoisophthalic acid,
5-N,N-dimethylaminoisophthalic acid, 5-N,N-diethylaminoisophthalic
acid, 2,5-dihydroxyterephthalic acid, 2,6-dihydroxyisophthalic
acid, 4,6-dihydroxyisophthalic acid, 2,3-dihydroxyphthalic acid,
2,4-dihydroxyphthalic acid, 3,4-dihydroxyphthalic acid,
3-fluorophthalic acid, 5-fluoroisophthalic acid,
2-fluoroterephthalic acid, tetrafluorophthalic acid,
tetrafluoroisophthalic acid, tetrafluoroterephthalic acid,
1,4-naphthalenedicarboxylic acid, 1,5-naphthalenedicarboxylic acid,
2,6-naphthalenedicarboxylic acid, 2,7-naphthalenedicarboxylic acid,
diphenic acid, 1,8-dihydroxy-naphthalene-3,6-dicarboxylic acid,
(diphenyl ether)-4,4'-dicarboxylic acid,
benzophenone-4,4'-dicarboxylic acid, (diphenyl
sulphone)-4,4'-dicarboxylic acid, biphenyl-4,4'-dicarboxylic acid,
4-trifluoromethylphthalic acid,
2,2-bis(4-carboxyphenyl)hexafluoropropane,
4,4'-stilbenedicarboxylic acid, 4-carboxycinnamic acid, or their
C1-C20-alkyl esters or C5-C12-aryl esters or their acid anhydrides
or their acid chlorides.
[0110] The heteroaromatic carboxylic acids are heteroaromatic
dicarboxylic acids and tricarboxylic acids and tetracarboxylic
acids or their esters or their anhydrides. For the purposes of the
present invention, heteroaromatic carboxylic acids are aromatic
systems which contain at least one nitrogen, oxygen, sulphur or
phosphorus atom in the aromatic. Preference is given to
pyridine-2,5-dicarboxylic acid, pyridine-3,5-dicarboxylic acid,
pyridine-2,6-dicarboxylic acid, pyridin-2,4-dicarboxylic acid,
4-phenyl-2,5-pyridinedicarboxylic acid, 3,5-pyrazoledicarboxylic
acid, 2,6-pyrimidinedicarboxylic acid, 2,5-pyrazinedicarboxylic
acid, 2,4,6-pyridinetricarboxylic acid,
benzimidazole-5,6-dicarboxylic acid, and also their C1-C20-alkyl
esters or C5-C12-aryl esters, or their acid anhydrides or their
acid chlorides.
[0111] Furthermore, the mixture i) can also contain aromatic and
heteroaromatic diaminocarboxylic acids. These include, inter alia,
diaminobenzoic acid, 4-phenoxycarbonylphenyl 3,'4'-diaminophenyl
ether and their monohydrochloride and dihydrochloride
derivatives.
[0112] The mixture prepared in step i) preferably comprises at
least 0.5% by weight, in particular from 1 to 30% by weight and
particularly preferably from 2 to 15% by weight, of monomers for
preparing polyazoles.
[0113] According to a further aspect of the present invention, the
mixture prepared in step A) comprises compounds which are suitable
for forming polyazoles under the action of heat as described in
step B), with these compounds being obtainable by reacting one or
more aromatic and/or heteroaromatic tetramino compounds with one or
more aromatic and/or heteroaromatic carboxylic acids or derivatives
thereof which contain at least two acid groups per carboxylic acid
monomer, or by reaction of one or more aromatic and/or
heteroaromatic diaminocarboxylic acids in the melt at temperatures
of up to 400.degree. C., in particular up to 350.degree. C.,
preferably up to 280.degree. C. The compounds used for preparing
these prepolymers have been described above.
[0114] Furthermore, polyazoles can be prepared using monomers which
contain covalently bound acid groups. These include, inter alia,
aromatic and heteroaromatic dicarboxylic acids or derivatives
thereof which have at least one phosphonic acid group, for example
2,5-dicarboxyphenylphosphonic acid, 2,3-dicarboxyphenylphosphonic
acid, 3,4-dicarboxyphenylphosphonic acid and
3,5-dicarboxyphenylphosphonic acid; aromatic and heteroaromatic
dicarboxylic acids and derivatives thereof which contain at least
one sulphonic acid group, in particular
2,5-dicarboxyphenylsulphonic acid, 2,3-dicarboxyphenylsulphonic
acid, 3,4-dicarboxyphenylsulphonic acid and
3,5-dicarboxyphenylsulphonic acid; aromatic and heteroaromatic
diaminocarboxylic acids containing at least one phosphonic acid
group, for example 2,3-diamino-5-carboxyphenylphosphonic acid,
2,3-diamino-6-carboxyphenylphosphonic acid and
3,4-diamino-6-carboxyphenylphosphonic acid; aromatic and
heteroaromatic diaminocarboxylic acids containing at least one
sulphonic acid group, for example
2,3-diamino-5-carboxyphenylsulphonic acid,
2,3-diamino-6-carboxyphenylsulphonic acid and
3,4-diamino-6-carboxyphenylsulphonic acid.
[0115] A polyazole membrane produced by the process described above
can contain the optional components described above. These include,
in particular, blend polymers and fillers. Blend polymers can,
inter alia, be dissolved, dispersed or suspended in the mixture
obtained as described in step i) and/or step ii). Here, the weight
ratio of polyazole to polymer (B) is preferably in the range from
0.1 to 50, more preferably from 0.2 to 20, particularly preferably
from 1 to 10, without this implying a restriction. If the polyazole
is formed only in step ii), the weight ratio can be calculated from
the weight of monomers for forming the polyazole, with the
compounds liberated in the condensation, for example water, being
taken into account.
[0116] To improve the use properties further, fillers, in
particular proton-conducting fillers, and additional acids can
additionally be added to the membrane. The addition can, for
example, be effected in step i), step ii) and/or step iii).
Furthermore, these additives can, if they are in liquid form, also
be added after the polymerization according to step iv). These
additives have been described above.
[0117] The polyphosphoric acid used in step i) is a commercial
polyphosphoric acid as is obtainable, for example, from Riedel-de
Haen. Polyphosphoric acids H.sub.n+2P.sub.nO.sub.3n+1 (n>1)
usually have an assay calculated as P.sub.2O.sub.5 (acidimetric) of
at least 83%. Instead of a solution of the monomers, a
dispersion/suspension can also be produced.
[0118] In step ii) the mixture obtained in step i) is heated to a
temperature of up to 400.degree. C., in particular 350.degree. C.,
preferably up to 280.degree. C., in particular from 100.degree. C.
to 250.degree. C. and particularly preferably in the range from
200.degree. C. to 250.degree. C. This is carried out using an inert
gas, for example nitrogen or a noble gas such as neon, argon.
[0119] The mixture prepared in step i) and/or step ii) can
additionally contain organic solvents. These can have a positive
influence on the processability. Thus, for example, the rheology of
the solution can be improved so that it can be extruded or applied
by doctor blade coating more easily.
[0120] The formation of the sheet-like structure in step iii) is
carried out by means of measures known per se (casting, spraying,
doctor blade coating, extrusion) which are known from the prior art
relating to polymer film production. Suitable supports are all
supports which may be regarded as inert under the conditions. These
supports include, in particular, films composed of polyethylene
terephthalate (PET), polytetrafluoroethylene (PTFE),
polyhexafluoropropylene, copolymers of PTFE with
hexafluoropropylene, polyimides, polyphenylene sulphides (PPS) and
polypropylene (PP). Furthermore, the membrane can also be formed
directly on the electrode provided with a barrier layer.
[0121] The thickness of the sheet-like structure produced in step
iii) is preferably from 10 to 4000 .mu.m, more preferably from 15
to 3500 .mu.m, in particular from 20 to 3000 .mu.m, particularly
preferably from 30 to 1500 .mu.m and very particularly preferably
from 50 to 1200 .mu.m.
[0122] The treatment of the membrane in step iv) is carried out, in
particular, at temperatures in the range from 0.degree. C. to
150.degree. C., preferably at temperatures of from 10.degree. C. to
120.degree. C., in particular from room temperature (20.degree. C.)
to 90.degree. C., in the presence of moisture or water and/or water
vapour. The treatment is preferably carried out under atmospheric
pressure, but can also be carried out at superatmospheric pressure.
It is important that the treatment occurs in the presence of
sufficient moisture, with the polyphosphoric acid present
contributing to strengthening of the membrane by partial hydrolysis
to form low molecular weight polyphosphoric acid and/or phosphoric
acid.
[0123] The partial hydrolysis of polyphosphoric acid in step iv)
leads to strengthening of the membrane and to a decrease in the
layer thickness and formation of a membrane. The strengthened
membrane generally has a thickness in the range from 15 to 3000
.mu.m, preferably from 20 to 2000 .mu.m, in particular from 20 to
1500 .mu.m.
[0124] The upper limit to the temperature of the treatment in step
iv) is generally 150.degree. C. If moisture acts for an extremely
short time, for example in the case of superheated steam, this
steam can also be hotter than 150.degree. C. The upper limit to the
temperature is critically dependent on the duration of the
treatment.
[0125] The partial hydrolysis (step iv) can also be carried out in
temperature- and humidity-controlled chambers in which the
hydrolysis can be controlled in a targeted manner under a defined
action of moisture. Here, the humidity can be set in a targeted
manner by means of the temperature or saturation of the gases, for
example, coming into contact with the membrane, e.g. air, nitrogen,
carbon dioxide or other suitable gases, or water vapour. The
treatment time is dependent on the parameters selected above.
Furthermore, the treatment time is dependent on the thickness of
the membrane.
[0126] In general, the treatment time is from a few seconds to
minutes, for example under the action of superheated steam, or up
to a number of full days, for example in air at room temperature
and low relative atmospheric humidity. The treatment time is
preferably in the range from 10 seconds to 300 hours, in particular
from 1 minute to 200 hours.
[0127] If the partial hydrolysis is carried out at room temperature
(20.degree. C.) using ambient air having a relative atmospheric
humidity of 40-80%, the treatment time is from 1 to 200 hours.
[0128] The membrane obtained in step iv) can be made
self-supporting, i.e. it can be detached from the support without
damage and subsequently processed further directly, if
appropriate.
[0129] The treatment in step iv) leads to hardening of the coating.
If the membrane is formed directly on the electrode, the treatment
in step D) is continued until the coating has a hardness sufficient
to be able to be pressed to form a membrane-electrode unit. A
sufficient hardness is ensured when a membrane treated in this way
is self-supporting.
[0130] However, a lower hardness is sufficient in many cases. The
hardness determined in accordance with DIN 50539 (microhardness
measurement) is generally at least 1 mN/mm.sup.2, preferably at
least 5 mN/mm.sup.2 and very particularly preferably at least 50
mN/mm.sup.2, without this implying a restriction.
[0131] The concentration and amount of phosphoric acid and thus the
conductivity of the polymer membrane of the invention can be
adjusted via the degree of hydrolysis, i.e. the time, temperature
and ambient humidity. According to the invention, the concentration
of phosphoric acid is reported as mole of acid per mole of
repeating unit of polymer. For the purposes of the present
invention, a concentration (mole of phosphoric acid per mole of
repeating units of the formula (III), i.e. polybenzimidazole) of
from 10 to 80, in particular from 12 to 60, is preferred. Such high
degrees of doping (concentrations) can be obtained only with great
difficulty or not at all by doping of polyazoles with commercially
available orthophosphoric acid.
[0132] The thickness of the barrier layer of a multilayer polymer
electrolyte membrane according to the invention is generally not
critical as long as this layer has a sufficient barrier action
against mineral acids. The barrier action can be determined via the
amount of mineral acid which can be leached out by means of water.
According to a particular aspect of the present invention, not more
than 10%, preferably not more than 5%, of the mineral acid goes
over into the aqueous phase during a period of one hour. These
values are based on the weight of mineral acid or the weight of the
sheet-like material doped with the mineral acid, with the area
which is in contact with water being in each case employed for
calculating the value.
[0133] In a particular embodiment of the present invention, the
thickness of the barrier layer is less than 10 .mu.m, preferably
from 1 to 8 .mu.m and particularly preferably from 2 to 6 .mu.m.
Such barrier layers have the advantage of a relatively low
resistance.
[0134] In a further embodiment of the present invention, the
thickness of the barrier layer is at least 10 .mu.m and is
preferably in the range from 10 .mu.m to 30 .mu.m. Such barrier
layers advantageously have a particularly high barrier action and
also a high stability.
[0135] The thickness of the barrier layer can be measured by means
of scanning electron microscopy (SEM). Here, the thickness of the
barrier layer is the mean of the thickness obtained via the ratio
of area to length of the barrier layer.
[0136] The barrier layer according to the invention is preferably a
cation-exchange material. This cation-exchange material allows
protons but not anions such as phosphate anions to be transported.
To improve adhesion, block copolymers comprising components of the
polymerelectrolyte membrane and the cation-exchange membrane can
also be used at the interface between polymer electrolyte membrane
and cation-exchange material.
[0137] This barrier layer can be joined (laminated) in the form of
a separate film, preferably self-supporting, to the doped polymer
membrane or the doped polymer blend membrane. Furthermore, the
barrier layer can be formed by applying a layer to the doped
membrane and/or the electrode. For this purpose, it is possible,
for example, to apply a mixture comprising cation-exchange material
or a precursor material to the membrane and/or the electrode.
Suitable processes include, inter alia, casting, spraying, doctor
blade coating and/or extrusion.
[0138] The barrier layer can also have a gradient. Thus, for
example, the concentration of acid groups can be varied. Such
gradients can be measured, for example, by means of
energy-dispersive X-ray scattering (EDX), location-resolved Raman
spectroscopy and location-resolved infrared spectroscopy.
[0139] In a variant of the present invention, if the
cation-exchange material is present in the form of a
self-supporting film, this can also be incorporated as a separate
film in an MEU between the doped polymer electrolyte membrane and
the catalyst layer or the electrode (also on both sides).
[0140] It has been found that it is advantageous for the barrier
layer to be located on the cathode side of the polymer electrolyte
membrane, since the overvoltage is significantly reduced. However,
apart from this embodiment, the barrier layer can also be applied
on both sides. As indicated above, the cation-exchange material is
not subject to any significant restriction. Preference is given to
materials whose cation-exchange capacity is less than 0.9 meq/g, in
particular less than 0.8 meq/g. The cation-exchange capacity is,
according to a particular aspect of the present invention, at least
0.1 meq/g, in particular 0.2 meq/g, without this implying a
restriction. Preference is given to materials whose area swelling
in water at 80.degree. C. is less than 20%, in particular less than
10%. Preference is given to materials whose conductivity at
80.degree. C. in the moistened state is less than 0.06 S/cm, in
particular less than 0.05 S/cm.
[0141] To measure the IEC, the sulphonic acid groups are converted
into the free acid. For this purpose, the polymer is treated in a
known manner with acid, with excess acid being removed by washing.
The sulphonated polymer is firstly treated for 2 hours in boiling
water. Excess water is subsequently dabbed off and the sample is
dried at 160.degree. C. and p<1 mbar in a vacuum drying oven for
15 hours. The dry weight of the membrane is then determined. The
polymer which has been dried in this way is then dissolved in DMSO
at 80.degree. C. for 1 hour. The solution is subsequently titrated
with 0.1 M NaOH. The ion-exchange capacity (IEC) is then calculated
from the consumption of acid to the equivalence point and the dry
weight.
[0142] At a high current density and temperatures above 100.degree.
C., moistening of this thin layer is effected by the product water
produced at the cathode. When hydrogen-rich reformer gas is used,
the moisture present in the reformer gas is sufficient to moisten
the barrier layer. Thus, the system requires no additional
moistening at temperatures above 100.degree. C. and high electric
power. However, it may sometimes be necessary to moisten the fuels
additionally on start-up or at low temperatures or at low current
densities. The barrier layer applied on the cathode side is
preferably thicker than the barrier layer located on the anode
side.
[0143] The barrier layer preferably comprises a cation-exchange
material. Here, it is in principle possible to use all
cation-exchange materials which can be processed to form membranes.
These are preferably organic polymers having covalently bound acid
groups. Particularly suitable acid groups include, inter alia,
carboxylic acid, sulphonic acid and phosphonic acid groups, with
polymers containing sulphonic acid groups being particularly
preferred. Methods of sulphonating polymers are described in F.
Kucera et. al. Polymer Engineering and Science 1988, Vol. 38, No 5,
783-792.
[0144] The cation-exchange materials preferably used as barrier
layers can generally not be used alone as cation-exchange membranes
in fuel cells, since their proton conductivity and swelling is too
low and mechanical stability cannot be ensured because of the low
thickness. However, the cation-exchange membranes described in the
prior art have been developed with high ion-exchange capacity, high
swelling, high proton conductivity and sufficient thickness to
achieve sole use as polymer electrolyte membranes in MEUs. The most
important types of cation-exchange membranes which have achieved
commercial importance for use in fuel cells are described
below.
[0145] The most important representative is the perfluorosulphonic
acid polymer Nafion.RTM. (U.S. Pat. No. 3,692,569). This polymer
can be brought into solution as described in U.S. Pat. No.
4,453,991 and then used as ionomer. Cation-exchange membranes are
also obtained by filling a porous support material with such an
ionomer. As support material, preference is given to expanded
Teflon (U.S. Pat. No. 5,635,041).
[0146] A further perfluorinated cation-exchange membrane can be
produced as described in U.S. Pat. No. 5,422,411 by
copolymerization of trifluorostyrene and sulphonyl-modified
trifluorostyrene. Composite membranes comprising a porous support
material, in particular expanded Teflon, filled with ionomers
consisting of such sulphonyl-modified trifluorostyrene copolymers
are described in U.S. Pat. No. 5,834,523.
[0147] U.S. Pat. No. 6,110,616 describes copolymers of butadiene
and styrene and their subsequent sulphonation to produce
cation-exchange membranes for fuel cells.
[0148] A further class of partially fluorinated cation-exchange
membranes can be produced by radiation grafting and subsequent
sulphonation. Here, a grafting reaction, preferably using styrene,
is carried out on a previously irradiated polymer film, as
described in EP-A-667983 or DE-A-19844645. In a subsequent
sulphonation reaction, the side chains are then sulphonated.
Crosslinking can also be carried out simultaneously with grafting
and the mechanical properties can be altered in this way.
[0149] Apart from the above membranes, a further class of
nonfluorinated membranes obtained by sulphonation of
high-temperature-stable thermoplastics has been developed. Thus,
membranes composed of sulphonated polyether ketones (DE-A-4219077,
WO 96/01177), sulphonated polysulphone (J. Membr. Sci. 83 (1993) p.
211) or sulphonated polyphenylene sulphide (DE-A-19527435) are
known.
[0150] Ionomers prepared from sulphonated polyether ketones are
described in WO 00/15691. Furthermore, acid-base blend membranes
produced by mixing sulphonated polymers and basic polymers as
described in DE-A-19817374 or WO 01/18894 are known.
[0151] To set the ion-exchange capacity for optimal acid retention,
a cation-exchange membrane known from the prior art can be mixed
with a polymer bearing no acid groups or only a small amount of
acid groups. Suitable polymers have been described above as blend
components, with high-temperature-stable polymers being
particularly preferred. The preparation and properties of
cation-exchange membranes comprising sulphonated PEK and a)
polysulphones (DE-A-4422158), b) aromatic polyamides
(DE-A-424-45264) or c) polybenzimidazole (DE-A-19851498) have been
described. As an alternative, the sulphonation conditions can be
chosen so that a low degree of sulphonation results
(DE-A-19959289).
[0152] Apart from the cation-exchange membranes mentioned in the
prior art which are based on organic polymers, the cation-exchange
material can also be made of organic-inorganic composite materials.
Such composite materials are preferably prepared by means of the
sol-gel process. As starting compounds, use is made of mixtures of
metal alkoxides, in particular siloxanes. These mixtures have a
high purity of the starting materials and a low viscosity. These
liquid precursor mixtures can be applied to a substrate by means of
known technologies, for example spraying or spin coating, to give
very thin and uniformly covering layers. Hydrolysis and
condensation of the precursor mixtures then enables solid films to
be produced on the surface. To obtain proton conductivity, the
organic radicals of the alkoxides contain acid-containing groups,
in particular sulphonic acid groups.
[0153] The precursor mixtures can likewise contain functional
organic groups which effect crosslinking of the layer formed and
thus a further reduction in the permeability to the mineral acid
and the fuels. Crosslinking can be carried out after layer
formation either thermally or by irradiation (electron beam, UV,
IR, NIR) or by means of an initiator.
[0154] The production of such a composite material is described,
for example in Electrochimica Acta volume 37, year 1992, pages
1615-1618. Furthermore, such composite materials are known from G.
W. Scherer, C. J. Brinker, Sol-Gel-Science, Academic Press, Boston,
1990.
[0155] One group of preferred compounds can be represented by the
formula (A)
(RO).sub.y(R.sup.1).sub.zM-X.sub.a (A)
where y is 1, 2 or 3, preferably 3, z is 0 or 1, preferably 0 and a
is 1 or 2, preferably 1, and R and R.sup.1 are each, independently
of one another, hydrogen, a linear or branched alkyl, alkenyl,
cycloalkyl or cycloalkenyl radical having from 1 to 20, preferably
from 1 to 8, carbon atoms, or an aromatic or heteroaromatic group
having from 5 to 20 carbon atoms, M is an element selected from
among Si, Zr, Ti, preferably Si, and the radicals X are each,
independently of one another, a linear or branched alkylene or
cycloalkylene group having from 1 to 20, preferably from 1 to 8,
carbon atoms or an aromatic or heteroaromatic group having from 5
to 20 carbon atoms and bearing at least one sulphonic acid or
phosphonic acid.
[0156] The radicals R, R.sup.1 and X can have further substituents,
in particular halogens such as fluorine atoms. The group X is
preferably a radical of one of the formulae Ph-SO.sub.3H,
C.sub.nH.sub.2n--SO.sub.3H, C.sub.nF.sub.2n--SO.sub.3H dar, where
Ph is phenyl and n is an integer from 1 to 20. The group R is
preferably a radical of the formula C.sub.nH.sub.2n+1, where n is
from 1 to 3.
[0157] Preferred compounds are, in particular, hydroxysilyl acids,
which are known per se and are described, for example, in DE 100 61
920, EP 0 771 589, EP 0 765 897 and EP 0 582 879.
[0158] Preferred hydroxysilyl acids can be represented by the
formula B or C
[(RO).sub.y(R.sup.2).sub.zSi--{R.sup.1--SO.sub.3.sup.-}.sub.a].sub.xM.su-
p.x+ (B)
[(RO).sub.y(R.sup.2).sub.zSi--{R.sup.1--O.sub.b--P(O.sub.cR.sub.3O.sub.2-
.sup.-}.sub.a].sub.xM.sup.x+ (C)
where M is H.sup.+, NH.sub.4.sup.+ or a metal cation having a
valence x of from 1 to 4, and y=1 to 3, z=0 to 2 and a=1 to 3, with
the proviso that y+z=4-a, b and c are 0 or 1, R and R.sup.2 are
identical or different and are each methyl, ethyl, propyl, butyl
radicals or H and R.sup.3 is M or a methyl, ethyl, propyl, butyl
radical, and R.sup.1 is a linear or branched alkyl or alkylene
group having from 1 to 12 carbon atoms, a cycloalkyl group having
from 5 to 8 carbon atoms or a unit of one of the general
formulae
##STR00006##
where n and m are each a number from 0 to 6.
[0159] Preferred hydroxysilyl acids or precursors (derivatives)
thereof are trihydroxysilylethylsulphonic acid,
trihydroxysilylphenylsulphonic acid, trihydroxysilylpropylsulphonic
acid, trihydroxysilylpropylmethylphosphonic acid and
dihydroxysilylpropylsulphonic diacid or salts thereof.
[0160] The structure of cation-exchange material can be set
precisely by appropriate choice of trihydroxysilyl acid (network
former), dihydroxysilyl acid (chain former) and monohydroxysilyl
acid (chain end) and by addition of further sol formers. Suitable
sol formers are, for example, the hydrolyzed precursors of
SiO.sub.2, Al.sub.2O.sub.3, P.sub.2O.sub.5, TiO.sub.2 or ZrO.sub.2.
Preferred compounds include, inter alia, tetramethoxysilane,
tetraethoxysilane, triethoxyvinylsilane, trimethoxyvinylsilane,
triethoxypropenylsilane and trimethoxypropenylsilane.
[0161] As substrates for the deposition of the barrier layer, it is
possible to use either a film of the basic polymer, a polymer
electrolyte membrane doped with mineral acid or an electrode coated
with noble metal catalyst.
[0162] In one variant of the invention, the barrier layer is
deposited on an electrode.
[0163] According to a particular aspect of the present invention,
the material from which the barrier layer is produced is chemically
compatible with the sheet-like material doped with at least one
mineral acid, so that good adhesion of the barrier layer to the
sheet-like material is achieved. Accordingly, when a polyazole film
is used, particular preference is given to using organic
cation-exchange polymers to which the above-mentioned polyazoles
have good adhesion. Such polymers include, in particular,
sulphonated polysulphones, polyether ketones and other polymers
which have aromatic groups in the main chain. When inorganic
materials are used, good adhesion to the organic or inorganic
supports can be achieved by choice of appropriate functional
groups.
[0164] When inorganic sheet-like materials are used, preference is
accordingly given to using the abovementioned inorganic layers
which can be obtained, for example, by hydrolysis of hydroxysilyl
acids.
[0165] The multilayer electrolyte membranes of the invention
display, taking into account the barrier layer, excellent
conductivity and performance.
[0166] The proton conductivity of preferred multilayer electrolyte
membranes at temperatures of 120.degree. C. is preferably at least
0.1 S/cm, in particular at least 0.11 S/cm, particularly preferably
at least 0.12 S/cm. This conductivity is also achieved at
temperatures of 80.degree. C. A membrane according to the invention
can be moistened at low temperatures. For this purpose, for
example, the compound used as energy source, for example hydrogen,
can be provided with a proportion of water. However, the water
formed by the reaction is in many cases sufficient to achieve
moistening.
[0167] The specific conductivity is measured by means of impedance
spectroscopy in a 4-pole arrangement in the potentiostatic mode
using platinum electrodes (wire, 0.25 mm diameter). The distance
between the current-collecting electrodes is 2 cm. The spectrum
obtained is evaluated using a simple model consisting of a parallel
arrangement of an ohmic resistance and a capacitor. The specimen
cross section of the membrane doped with phosphoric acid is
measured immediately before mounting of the specimen. To measure
the temperature dependence, the measurement cell is brought to the
desired temperature in an oven and the temperature is regulated by
means of a Pt-100 resistance thermometer positioned in the
immediate vicinity of the specimen. After the temperature has been
reached, the specimen is maintained at this temperature for 10
minutes before the start of the measurement.
[0168] The polymer membrane of the invention displays improved
materials properties compared to the previously known doped polymer
membranes. Owing to the low methanol permeability, the multilayer
membranes can be used, in particular, in direct methanol fuel
cells.
[0169] The crossover current density in a liquid direct methanol
fuel cell operated at 90.degree. C. using 0.5 M methanol solution
is preferably less than 100 mA/cm.sup.2, in particular less than 70
mA/cm.sup.2, particularly preferably less than 50 mA/cm.sup.2 and
very particularly preferably less than 10 mA/cm.sup.2. The
crossover current density in a gaseous direct methanol fuel cell
operated at 160.degree. C. using 2 M methanol solution is
preferably less than 100 mA/cm.sup.2, in particular less than 50
mA/cm.sup.2, very particularly preferably less than 10
mA/cm.sup.2.
[0170] To determine the crossover current density, the amount of
carbon dioxide liberated at the cathode is measured by means of a
CO.sub.2 sensor. The crossover current density is calculated from
the measured value of the amount of CO.sub.2, as described by P.
Zelenay, S. C. Thomas, S. Gottesfeld in S. Gottesfeld, T F. Fuller
"Proton Conducting Membrane Fuel Cells II" ECS Proc. Vol. 98-27 pp.
300-308.
[0171] The invention further provides for the preferred use of the
multilayer electrolyte membrane of the invention or the coated
electrode in a membrane-electrode unit (MEU) for a fuel cell.
[0172] The MEU comprises at least one multilayer electrolyte
membrane according to the invention and two electrodes between
which the multilayer electrolyte membrane is located in a
sandwich-like arrangement.
[0173] The electrodes each have a catalytically active layer and a
gas diffusion layer for bringing a reaction gas to the
catalytically active layer. The gas diffusion layer is porous so
that reactive gas can pass through it.
[0174] The multilayer electrolyte membrane of the invention can be
used as electrolyte membrane in electrochemical processes. In
addition, it is possible to produce the electrolyte membrane or an
intermediate structure for an MEU with one or both catalytically
active layers. Furthermore, the MEU can also be produced by fixing
the gas diffusion layer to the intermediate structure.
[0175] The present invention further provides a fuel cell system
comprising a plurality of different MEUs of which at least one
contains a multilayer membrane according to the invention. A
membrane-electrode unit according to the invention displays a
surprisingly high power density. In a particular embodiment,
preferred membrane-electrode units produce a current density of at
least 0.1 A/cm.sup.2, preferably 0.2 A/cm.sup.2, particularly
preferably 0.3 A/cm.sup.2. This current density is measured in
operation using pure hydrogen at the anode and air (about 20% by
volume of oxygen, about 80% by volume of nitrogen) at the cathode
at atmospheric pressure (1013 mbar absolute, with open cell outlet)
and a cell voltage of 0.6 V. Here, particularly high temperatures
in the range 150-200.degree. C., preferably 160-180.degree. C., in
particular 170.degree. C., can be used.
[0176] The power densities mentioned above can also be achieved at
a low stoichiometry of the fuel gases on both sides. According to a
particular aspect of the present invention, the stoichiometry is
less than or equal to 2, preferably less than or equal to 1.5, very
particularly preferably less than or equal to 1.2.
Examples 1 to 6
Production of Cation-Exchange Membranes
[0177] To produce cation-exchange membranes, the following stock
solutions were prepared.
[0178] a) 10 wt % of PES (Ultrason E 7020 P) in NMP
[0179] b) 17 wt % of sPEK (degree of sulphonation: 50.3%) in
NMP
[0180] The solutions were mixed in the ratios indicated in Table 1
and applied by means of a doctor blade coater (50 .mu.m). The films
were subsequently dried in an oven at 120.degree. C. for 11 hours.
The thickness of the films produced is 20-25 .mu.m.
[0181] The polymers used for producing the membrane are shown in
Table 1.
TABLE-US-00001 TABLE 1 PES sPEK [% by weight] [% by weight] Example
1 0 100 Example 2 20 80 Example 3 30 70 Example 4 40 60 Example 5
50 50 Example 6 60 40
[0182] The specific conductivity is measured by means of impedance
spectroscopy in a 4-pole arrangement in the potentiostatic mode
using platinum electrodes (wire, 0.25 mm diameter). The distance
between the current-collecting electrodes is 2 cm. The spectrum
obtained is evaluated using a simple model consisting of a parallel
arrangement of an ohmic resistance and a capacitor. The specimen
cross section of the sulphonated PEK membranes and sulphonated PEK
blend membranes is measured after swelling in water at 80.degree.
C. for 1 hour prior to mounting of the specimen. To measure the
temperature dependence and for moistening, the measurement cell is
rinsed with heated water. Before commencement of the experiment,
the cell is maintained at 80.degree. C. for 30 minutes and the
conductivity measurement is then commenced. Cooling is carried out
at 1 K/min. Before the start of each new measurement, the desired
temperature is then maintained for 10 minutes.
[0183] Table 2 shows the results of the conductivity measurements
on sulphonated PEK membranes and sulphonated PEK blend
membranes.
TABLE-US-00002 TABLE 2 Conductivity values of sulphonated PEK
membranes and sulphonated PEK blend membranes (proportion of PES
blend component in percent by weight) for use as barrier layer for
phosphoric acid T (.degree. C.) Ex. 1 Ex. 2 Ex. 3 Ex. 4 Ex. 5 Ex. 6
80 0.196 0.160 0.150 0.149 0.046 0.035 70 0.181 0.148 0.139 0.137
0.042 0.031 60 0.164 0.136 0.125 0.125 0.037 0.028 50 0.150 0.124
0.113 0.112 0.032 0.025 40 0.133 0.110 0.099 0.098 0.027 0.022 30
0.116 0.096 0.086 0.085 0.023 0.018 22 0.105 0.086 0.077 0.074
0.020 0.016
[0184] The conductivity and barrier action of the cation-exchange
membrane for phosphoric acid depend strongly on the content of acid
groups expressed by the ion-exchange capacity (IEC).
[0185] To measure the IEC, the sulphonated polymer or the
sulphonated blend membrane is firstly treated in boiling water for
2 hours. Excess water is subsequently dabbed off and the specimen
is dried at 160.degree. C. in a vacuum drying oven at p<1 mbar
for 15 hours. The dry weight of the membrane is then determined.
The polymer which has been dried in this way is then dissolved in
DMSO at 80.degree. C. for 1 hour. The solution is subsequently
titrated with 0.1 M NaOH. The ion-exchange capacity (IEC) is then
calculated from the consumption of the acid to the equivalence
point and the dry weight.
[0186] To determine the swelling behaviour, the sulphonated
membranes or blend membranes are swollen at 80.degree. C. for 2
hours and the increase in area is determined.
[0187] Table 3 shows the ion-exchange capacity of a sulphonated PEK
membrane (0% of PES) and blend membranes of sulphonated PEK and
various contents of PES.
TABLE-US-00003 TABLE 3 Ion-exchange capacity and swelling at
80.degree. C. of a sulphonated PEK membrane (0% of PES) and blend
membranes of sulphonated PEK and various contents of PES T =
80.degree. C. IEC (meq/g) Swelling (%) Example 1 2.06 156 Example 2
1.71 124.6 Example 3 1.34 61.6 Example 4 1.03 41.7 Example 5 0.8
8.6 Example 6 0.59 2
[0188] To measure the barrier action of the cation-exchange
membranes for the example of membranes doped with phosphoric acid,
the following procedure is employed:
[0189] A cation-exchange membrane having a diameter of 7 cm in the
dry state is firstly stamped out. This membrane is subsequently
dipped into 300 ml of water and the pH change is measured as a
function of time. In the case of these membranes, the pH can, owing
to the material selected, decrease because of residues of free acid
from the sulphonation reaction. Since each membrane has a different
content of acid groups, this blank has to be measured for each
individual membrane.
[0190] Such a membrane is subsequently clamped into the measurement
apparatus again and an acid-doped membrane is placed on top. To
carry out doping, a PBI film having an initial thickness of 50
.mu.m is placed in 85% phosphoric acid for at least 72 hours at
room temperature. A piece of this acid-doped membrane having a
diameter of 3 cm is stamped out and immediately laid on the
cation-exchange membrane. The sandwich produced in this way is then
placed in a glass beaker filled with 300 ml of water and the pH
change is measured over 15 hours at room temperature (20.degree.
C.). A schematic structure of the measurement apparatus is shown in
FIG. 1. The result obtained in this way is shown graphically in
FIG. 2.
[0191] The negative values in FIG. 3 after correction of the blank
can be explained by the loss of acid from the cation-exchange
membrane (blank) itself being greater than the passage of
phosphoric acid through the cation-exchange membrane.
[0192] In FIG. 4, the measurement of the amount of acid which has
passed through the barrier layer and that which has been retained
by the barrier layer is demonstrated beyond doubt. The results
demonstrate that the use of cation-exchange materials as barrier
layer leads to a surprisingly clear reduction in the liberation of
mineral acid.
[0193] It can, surprisingly, be seen from the results obtained that
preferred cation-exchange membranes according to the invention in
the moistened state at 80.degree. C. display a conductivity of
<0.06 S/cm, in particular <0.05 S/cm.
[0194] Preferred cation-exchange membranes according to the
invention have an IEC value of less than 0.9 meq/g. The swelling of
preferred cation-exchange membranes is less than 20% at 80.degree.
C.
[0195] It has surprisingly been found that the use of the membrane
of the invention, i.e. the membrane provided with a barrier layer,
having an ion-exchange capacity of less than 0.9 meq/g and a
swelling in water of less than 10% at 80.degree. C. leads to a
particularly significant reduction in the passage of phosphoric
acid and the acid concentration does not go above 0.0005 mol/l over
a period of 15 hours.
Example 7
Production of an Ultrathin Cation-Exchange Membrane as Barrier
Layer on the Membrane Surface
[0196] Production of PBI Film:
[0197] A 50 .mu.m thick film of a 15% strength by weight
polybenzimidazole (PBI) solution in DMAc was spread by means of a
doctor blade and dried at 120.degree. C. in an oven for 12
hours.
[0198] Preparation of the Spray Solution:
[0199] A 10% strength by weight solution of PES (Ultrason E 7020)
and sPEK (degree of sulphonation: 50.3%) in DMAc was prepared, with
the weight ratio of PES to sPEK being 60:40.
[0200] Coating:
[0201] To apply the coating, a glass plate was placed on a hotplate
and heated to 150.degree. C. After this temperature had been
reached, the PBI film was laid on the glass plate. As soon as the
film had drawn flat onto the glass plate, a metal template was
placed on top. The spray solution was sprayed onto the film surface
a number of times by means of an airbrush. The solvent was
evaporated after each spraying step. The metal template was then
taken off and the sprayed region was cut out. The thickness of the
coating was 4-5 .mu.m.
[0202] The coated polyazole film is clamped in place with the
coated side uppermost as shown in FIG. 1 and then dipped into a
glass beaker filled with 100 ml of water. In this configuration,
the underside is in contact with water while 0.5 ml of phosphoric
acid is applied to the opposite side.
[0203] The change in the pH was observed over a period of 50 hours.
For comparison, a polyazole film without a barrier layer was
subjected to the same test.
[0204] The results obtained are shown in FIG. 5, and the
effectiveness of the thin barrier layer can clearly be seen.
* * * * *