U.S. patent application number 10/481170 was filed with the patent office on 2004-12-30 for polyazole-based polymer films.
Invention is credited to Baurmeister, Jochen, Jordt, Frauke, Kiefer, Joachim, Uensal, Oemer.
Application Number | 20040262227 10/481170 |
Document ID | / |
Family ID | 7688642 |
Filed Date | 2004-12-30 |
United States Patent
Application |
20040262227 |
Kind Code |
A1 |
Kiefer, Joachim ; et
al. |
December 30, 2004 |
Polyazole-based polymer films
Abstract
The present invention relates to polymer films and a polymer
membrane having an improved mechanical property profile produced
therefrom, to a process for producing them and to their use. The
polymer films, polymer membranes and separation membranes of the
invention are produced from selected polymer raw materials and have
excellent chemical, thermal and mechanical properties as are
required for use as polymer electrolyte membranes (PEMs) in PEM
fuel cells or in apparatuses for the filtration and/or separation
of gases and/or liquids or for reverse osmosis.
Inventors: |
Kiefer, Joachim; (Losheim am
See, DE) ; Uensal, Oemer; (Mainz, DE) ;
Baurmeister, Jochen; (Eppstein, DE) ; Jordt,
Frauke; (Eppstein, DE) |
Correspondence
Address: |
HAMILTON, BROOK, SMITH & REYNOLDS, P.C.
530 VIRGINIA ROAD
P.O. BOX 9133
CONCORD
MA
01742-9133
US
|
Family ID: |
7688642 |
Appl. No.: |
10/481170 |
Filed: |
August 5, 2004 |
PCT Filed: |
June 19, 2002 |
PCT NO: |
PCT/EP02/06773 |
Current U.S.
Class: |
210/650 ;
210/500.21; 210/640 |
Current CPC
Class: |
C08J 5/2256 20130101;
C08J 5/18 20130101; C08J 2379/04 20130101 |
Class at
Publication: |
210/650 ;
210/640; 210/500.21 |
International
Class: |
B01D 061/00; B01D
071/00 |
Foreign Application Data
Date |
Code |
Application Number |
Jun 19, 2001 |
DE |
10129458.1 |
Claims
1. A polymer film based on polyazoles which is obtainable by a
process comprising steps A) dissolving the polyazole polymer in
polar, aprotic organic solvent, B) casting a polymer film using the
solution obtained from Step A), C) drying the film formed in step
B) until it is self-supporting, wherein a polyazole polymer powder
having a particle size in the range from 300 .mu.m to 1500 .mu. is
used in step A).
2. A polymer film as claimed in claim 1, wherein a polyazole
polymer powder having a particle size in the range from 300 .mu.m
to 1250 .mu.m, in particular from 300 .mu.m to 1000 .mu.m,
particularly preferably from 500 .mu.m to 1000 .mu.m, is used.
3. A polymer film as claimed in claim 1, wherein the
polyazole-based polymer used is a polymer comprising recurring
azole units of the formula (I) and/or (II) 3where Ar are identical
or different and are each a tetravalent aromatic or heteroaromatic
group which can be monocyclic or polycyclic, Ar.sup.1 are identical
or different and are each a divalent aromatic or heteroaromatic
group which can be monocyclic or polycyclic, Ar.sup.2 are identical
or different and are each a trivalent aromatic or heteroaromatic
group which can be monocyclic or polycyclic, X are identical or
different and are each oxygen, sulfur or an amino group bearing a
hydrogen atom, a group having 1-20 carbon atoms, preferably a
branched or unbranched alkyl or alkoxy group, or an aryl group as
further radical.
4. A polymer film as claimed in claim 3, wherein the
polyazole-based polymer used is a polymer comprising recurring
benzimidazole units of the formula (III) 4wherein n is an integer
greater than or equal to 10, preferably greater than or equal to
100.
5. A polymer film as claimed in claim 1, wherein drying of the film
in step C) is carried out at temperatures in the range from room
temperature to 300.degree. C.
6. A polymer film as claimed in claim 1, wherein drying of the film
in step C) is carried out for a period of from 10 seconds to 24
hours.
7. A doped polymer membrane based on polyazoles which is obtainable
by a process comprising the steps A) dissolving the polyazole
polymer in a polar, aprotic organic solvent, B) casting a polymer
film using the solution obtained from step A), C) drying the film
formed in step B) until it is self-supporting, D) doping the
polymer film obtained in step C) with a dopant, wherein a polyazole
polymer powder having a particle size in the range from 300 .mu.m
to 1500 .mu.m is used in step A).
8. A polymer membrane as claimed in claim 7, wherein a polyazole
polymer powder having a particle size in the range from 300 .mu.m
to 1250 .mu.m, in particular from 300 .mu.m to 1000 .mu.m,
particularly preferably from 500 .mu.m to 1000 .mu.m, is used.
9. A polymer membrane as claimed in claim 7, wherein the
polyazole-based polymer used is a polymer comprising recurring
azole units of the formula (I) and/or (II) 5where Ar are identical
or different and are each a tetravalent aromatic or heteroaromatic
group which can be monocyclic or polycyclic, Ar.sup.1 are identical
or different and are each a divalent aromatic or heteroaromatic
group which can be monocyclic or polycyclic, Ar.sup.2 are identical
or different and are each a trivalent aromatic or heteroaromatic
group which can be monocyclic or polycyclic, X are identical or
different and are each oxygen, sulfur or an amino group bearing a
hydrogen atom, a group having 1-20 carbon atoms, preferably a
branched or unbranched alkyl or alkoxy group, or an aryl group as
further radical.
10. A polymer membrane as claimed in claim 7, wherein the
polyazole-based polymer used is a polymer comprising recurring
benzimidazole units of the formula (III) 6where n is an integer
greater than or equal to 10, preferably greater than or equal to
100.
11. A polymer membrane as claimed in claim 7, wherein drying of the
film in step C) is carried out at temperatures in the range from
room temperature to 300.degree. C.
12. A polymer membrane as claimed in claim 7, wherein drying of the
film in step C) is carried out for a period of from 10 seconds to
24 hours.
13. A polymer membrane as claimed in claim 7, wherein doping is
carried out for a period of from 5 minutes to 96 hours.
14. A polymer membrane as claimed in claim 7, wherein the degree of
doping is from 3 to 15 mol of acid per mol of repeating units of
the polymer.
15. A polymer membrane as claimed in claim 7, wherein the dopant
used is sulfuric acid or phosphoric acid.
16. A membrane-electrode unit comprising at least one polymer
membrane as claimed in claim 7 and at least one electrode.
17. A polymer electrolyte fuel cell comprising at least one
membrane-electrode unit as claimed in claim 16.
18. A polyazole-based separation membrane obtainable by a process
comprising the steps A) dissolving the polyazole polymer in a
polar, aprotic organic solvent, B) casting a polymer film using the
solution obtained from step A), C) dipping this film into a
precipitation bath wherein a polyazole polymer powder having a
particle size in the range from 300 .mu.m to 1500 .mu.m is used in
step A).
19. A separation membrane as claimed in claim 18, wherein a
polyazole polymer powder having a particle size in the range from
300 .mu.m to 1250 .mu.m, in particular from 300 .mu.m to 1000
.mu.m, particularly preferably from 500 .mu.m to 1000 .mu.m, is
used.
20. The use of a separation membrane as claimed in claim 19 for the
filtration and/or separation of gases and/or liquids or in reverse
osmosis.
21. An apparatus for the filtration and/or separation of gases
and/or liquids comprising at least one separation membrane as
claimed in claim 18.
22. An apparatus for carrying out reverse osmosis comprising at
least one separation membrane as claimed in claim 18.
Description
[0001] The present invention relates to polymer films and a polymer
membrane having an improved mechanical property profile produced
therefrom, to a process for producing them and to their use.
[0002] Owing to its excellent chemical, thermal and mechanical
properties, the acid-doped polymer membrane described below can be
used in a wide variety of applications and is suitable, in
particular, as polymer electrolyte membrane (PEM) in PEM fuel
cells.
[0003] Acid-doped polyazole membranes for use in PEM fuel cells are
known. The basic polyazole membranes are doped with concentrated
phosphoric acid or sulfuric acid and act as proton conductors and
separators in polymer electrolyte membrane fuel cells (PEM fuel
cells).
[0004] For this application, electrodes coated with catalyst are
applied to both sides of the acid-doped polyazole membranes to form
a membrane-electrode unit (MEE). A plurality of such membrane
electrode units are then connected in series together with bipolar
plates and form the fuel cell stack.
[0005] As a result of the series construction, the cell voltage and
power of the stack depends on the number of membrane-electrode
units. Furthermore, failure of a single one of these
membrane-electrode units results in a break in the circuit and thus
failure of the entire fuel cell. For this reason, extraordinarily
high quality demands are made on the mechanical stability of all
components. The thin, usually <100 .mu.m thick polymer membrane
in particular is frequently regarded as the weakest link in this
chain. The membrane performs two essential tasks. Firstly, it has
to have a high proton conductivity in order to be able to conduct
the protons formed in the oxidation of a hydrogen-rich fuel at the
anode to the cathode. There, reduction with oxygen, preferably from
air, then takes place with formation of water. Secondly, the
membrane functions as a separator and should have a very low
permeability to the fuels present. In particular, when hydrogen and
oxygen are used, mixing of the two gases has to be prevented. For
this reason, the polymer membrane should not fail in operation,
even at high temperatures. The mechanical stability of the thin
(usually <0.2 mm) polymer film is reduced by the doping with
acid to generate a high proton conductivity. To be able to
withstand the stressing of the cells at operating temperatures of
>100.degree. C. over the long term, extremely resistant polymers
have to be used.
[0006] Due to the excellent properties of the polyazole polymer,
polymer electrolyte membranes based on polyazoles, converted into
membrane-electrode units (MEE), can be used in fuel cells at
long-term operating temperatures above 100.degree. C., in
particular above 120.degree. C. This high long-term operating
temperature allows the activity of the catalysts based on noble
metals which are present in the membrane-electrode unit (MEE) to be
increased. Particularly when using reformer products produced from
hydrocarbons, significant amounts of carbon monoxide are present in
the reformer gas and these usually have to be removed by means of a
costly gas work-up or gas purification. The ability to increase the
operating temperature enables significantly higher concentrations
of CO impurities to be tolerated over the long term.
[0007] The use of polymer electrolyte membranes based on polyazole
polymers allows, firstly, the costly gas work-up or gas
purification to be partly omitted and, secondly, the amount of
catalyst in the membrane-electrode unit to be reduced. They are
indispensable prerequisites for wide use of PEM fuel cells, since
otherwise the costs of a PEM fuel cells system are too high.
[0008] The acid-doped polyazole-based polymer membranes known
hitherto display a favorable property profile. However, owing to
the applications sought for PEM fuel cells, especially in the
automobile and stationary sector, these need to be improved
overall.
[0009] Thus, the polyazole-based polymer membranes known hitherto
display mechanical properties after doping with acid which are
still unsatisfactory for the above application. This mechanical
instability is shown by a low modulus of elasticity, a low ultimate
tensile strength and a low fracture toughness.
[0010] It is an object of the present invention to provide
acid-doped polymer membranes based on polyazoles which have,
firstly, improved mechanical properties and, secondly, retain the
advantages of polymer membranes based on polyazoles and allow an
operating temperature above 100.degree. C. without additional
humidification of the fuel gas.
[0011] We have now found that polyazole-based polymer films which
display a significantly improved mechanical stability after doping
with an acid can be obtained when selected polyazole raw materials
are used for producing the polymer film.
[0012] The present invention accordingly provides a polymer film
based on polyazoles which is obtainable by a process comprising
steps
[0013] A) dissolving the polyazole polymer in a polar, aprotic
organic solvent,
[0014] B) casting a polymer film using the solution obtained from
step A),
[0015] C) drying the film formed in step B) until it is
self-supporting, wherein a polyazole polymer powder having a
particle size in the range from 300 .mu.m to 1500 .mu.m is used in
step A).
[0016] The polyazole polymer powder having a particle size in the
range from 300 .mu.m to 1500 .mu.m which is used according to the
invention is obtained by sieving a commercially available polyazole
polymer. Polyazole polymers, for example those based on
polybenzimidazoles, are commercially available products and are
sold under the name Celazole.RTM..
[0017] The commercially available polyazole (Celazole, PBI polymer)
is separated into different particle size fractions by sieving.
Sieving avoids a complex fractionation as described, for example,
in Mat. Res. Soc. Symp. Proc. 548 (1999), pages 313-323.
[0018] It has surprisingly been found that the small particles
obtained by sieving give a low fracture toughness. This is
surprising because small particles have a high ratio of surface
area, SA, to volume, V. However, in a process for the
polycondensation of polyazoles as employed, for example, for PBI,
the degree of polymerization should increase with an increasing
SA/V ratio. Targeted selection of the fractions obtained on sieving
enables the mechanical properties to be improved significantly.
[0019] In a preferred embodiment of the invention, polyazole
polymer powders having a particle size in the range from 300 .mu.m
to 1250 .mu.m, in particular from 300 .mu.m to 1000 .mu.m,
particularly preferably from 500 .mu.m to 1000 .mu.m, are used.
[0020] The preparation of polymer solutions based on polyazoles as
in step A) has been comprehensively described in the prior art.
Thus, EP-A-0816415 describes a process for dissolving polymers
based on polyazoles using N,N-dimethylacetamide as polar, aprotic
solvent at temperatures above 260.degree. C. A substantially milder
process for preparing solutions based on polyazoles is disclosed in
the German patent application 10052237.8.
[0021] As polymers based on polyazoles, preference is given to
using polymers comprising recurring azole units of the formula (I)
and/or (II) 1
[0022] where
[0023] Ar are identical or different and are each a tetravalent
aromatic or heteroaromatic group which can be monocyclic or
polycyclic,
[0024] Ar.sup.1 are identical or different and are each a divalent
aromatic or heteroaromatic group which can be monocyclic or
polycyclic,
[0025] Ar.sup.2 are identical or different and are each a trivalent
aromatic or heteroaromatic group which can be monocyclic or
polycyclic,
[0026] X are identical or different and are each oxygen, sulfur or
an amino group bearing a hydrogen atom, a group having 1-20 carbon
atoms, preferably a branched or unbranched alkyl or alkoxy group,
or an aryl group as further radical.
[0027] Preferred aromatic or heteroaromatics groups are derived
from benzene, naphthalene, biphenyl, diphenyl ether,
diphenylmethane, diphenyldimethylrnethane, bisphenone, diphenyl
sulfone, quinoline, pyridine, bipyridine, anthracene and
phenanthrene, each of which may also be substituted.
[0028] Ar.sup.1 can have any substitution pattern; in the case of
phenylene, for example, Ar.sup.1 can be ortho-, meta- or
para-phenylene. Particularly preferred groups are derived from
benzene and biphenylene, each of which may also be substituted.
[0029] Preferred alkyl groups are short-chain alkyl groups having
from 1 to 4 carbon atoms, e.g. methyl, ethyl, n-propyl or isopropyl
and t-butyl groups.
[0030] Preferred aromatic groups are phenyl or naphthyl groups. The
alkyl groups and the aromatic groups may be substituted.
[0031] Preferred substituents are halogen atoms such as fluorine,
amino groups or short-chain alkyl groups such as methyl or
ethyl.
[0032] If polyazoles having recurring units of the formula (I) are
used for the purposes of the present invention, the radicals X
within a recurring unit should be identical.
[0033] The polyazoles used according to the invention can in
principle also have differing recurring units which, for example,
differ in their radical X. However, there are preferably only
identical radicals X in a recurring unit.
[0034] In a preferred embodiment of the present invention, the
polymer comprising recurring azole units is a copolymer comprising
at least two units of the formula (I) and/or (II) which differ from
one another.
[0035] In a particularly preferred embodiment of the present
invention, the polymer comprising recurring azole units is a
polyazole containing only units of the formula (I) and/or (II).
[0036] The number of recurring azole units in the polymer is
preferably greater than or equal to 10. Particularly preferred
polymers have at least 100 recurring azole units.
[0037] For the purposes of the present invention, preference is
given to using polymers comprising recurring benzimidazole units.
An example of an extremely advantageous polymer comprising
recurring benzimidazole units corresponds to the formula (Ill):
2
[0038] where n is an integer greater than or equal to 10,
preferably greater than or equal to 100.
[0039] The casting of a polymer film (step B) from a polymer
solution prepared according to step A) is carried out by methods
known per se from the prior art.
[0040] The drying of the film in step C) is carried out at
temperatures ranging from room temperature to 300.degree. C. Drying
is carried out under atmospheric pressure or reduced pressure. The
drying time depends on the thickness of the film and is in the
range from 10 seconds to 24 hours. The film dried as per step C) is
subsequently self-supporting and can be processed further. Drying
is carried out by drying methods customary in the film
industry.
[0041] As a result of the drying carried out in step C), the polar,
aprotic organic solvent is very largely removed. The residual
content of polar, aprotic organic solvent is usually 10-23%.
[0042] A further reduction in the residual solvent content to below
2% by weight can be achieved by increasing the drying temperature
and drying time, but this significantly prolongs the subsequent
doping of the film, for example with phosphoric acid. A residual
solvent content of 5-15% is thus advantageous to reduce the doping
time.
[0043] In one variant, drying can also be combined with a washing
step. A particularly mild process for after-treatment and removal
of the residual solvent is disclosed in the German patent
application 10109829.4.
[0044] The polymer films of the invention display a surprisingly
high mechanical stability, as shown by a high modulus of elasticity
combined with a high tensile strength, a high elongation at break
and a high fracture toughness.
[0045] The polymer films of the invention display, at a modulus of
elasticity of at least 2870 MPa, a fracture toughness of greater
than 2300 kJ/m.sup.2, preferably greater than 2320 kJ/m.sup.2, and
an elongation at break of at least 44%.
[0046] The present invention further provides dense or porous
polyazole-based separation membranes obtainable by a process
comprising the steps
[0047] A) dissolving the polyazole polymer in a polar, aprotic
organic solvent,
[0048] B) casting a polymer film using the solution obtained from
step A),
[0049] C) dipping this film into a precipitation bath wherein a
polyazole polymer powder having a particle size in the range from
300 .mu.m to 1500 .mu.m is used in step A).
[0050] In these separation membranes too, preference is given to
using polyazole polymer powders having a particle size in the range
from 300 .mu.m to 1250 .mu.m, in particular from 300 .mu.m to 1000
.mu.m, particularly preferably from 500 .mu.m to 1000 .mu.m.
[0051] The preferred polymer structures of the formulae (I) and
(II) are also preferred for these separation membranes.
[0052] Further information on separation membranes based on
polyazoles may be found in the specialist literature, in particular
the patents WO 98/14505; U.S. Pat. Nos. 4,693,815; 4,693,824;
375,262; 3,737,042; 4,512,894; 448,687; 3,841,492. The disclosure
of the abovementioned references in respect of the structure and
production of separation membranes is hereby incorporated by
reference as part of the present disclosure. In particular, such
separation membranes can be produced in the form of flat films or
as hollow fiber membranes.
[0053] Depending on the desired specification of the separation
membrane, the polymer film formed can be dried after step B) before
it is introduced into the precipitation bath (step C). Drying
allows better handling of the polymer film. In addition, the
morphology of the membrane can be adjusted by drying. To enable the
polymer film to be handled more readily, the film can be formed on
a support in step B). The polymer film formed, which is generally
not yet self-supporting, is subsequently introduced into the
precipitation bath. In this way it is possible to produce, for
example, asymmetric structures.
[0054] Apart from the known advantages of separation membranes
based on polyazoles, for example high thermal stability and
resistance to chemicals, the separation membranes of the invention
have improved mechanical properties as a result of a higher
molecular weight which lead to increased long-term stability and a
longer life and also an improved separation performance.
[0055] Such separation membranes can be produced as dense polymer
films, porous hollow fiber membranes or as porous, open-celled
polymer films, if desired with a compact surplus layer, by dipping
into the precipitation bath. The precipitation bath comprises one
or more nonsolvents for the polyazole and, if desired, one or more
solvents. Nonlimiting examples of nonsolvents for polyazoles are
water, acetone, glycols, alcohols, preferably methanol or benzyl
alcohol, and also other liquids which are not soluble in water.
Nonlimiting examples of solvents for polyazoles are DMAc, NMP, DMF,
DMSO and strong acids such as sulfuric acid, methanesulfonic acid
or trifluoroacetic acid.
[0056] To produce a porous membrane, the polymer solution from step
A) can likewise comprise a nonsolvent or pore formers such as
glycerol. In the precipitation in step C), solvent exchange occurs
and leads to formation of known porous structures. Different
morphologies of the separation membranes can thus be produced by
choice of the composition of the precipitant. For separation
applications, the following structures are preferred: i) symmetric,
porous structure, ii) asymmetric porous structure with a polymer
seal close to a membrane surface. Scanning electron micrographs of
such particularly suitable structures of a polybenzimidazole
membrane are disclosed in Journal of Membrane Science, Volume 20,
1984, pages 147-66.
[0057] Such phase inversion membranes and structures are known to
those skilled in the art. Membranes having a symmetric porous
structure are employed as separation or filtration membranes for
filtration of air and gases or the microfiltration or
ultrafiltration of liquids. Membranes having an asymmetric porous
structure can be used in a variety of reverse osmosis applications,
in particular desalination of water, dialysis or purification of
gases.
[0058] A particularly advantageous application is the separation of
hydrogen and carbon dioxide from gas mixtures in combination with a
porous metallic support. Alternative technologies for separating
off CO.sub.2 require, owing to the low thermal stability of the
polymer membrane, cooling of the gas to 150.degree. C., which
reduces the efficiency. The polyazole-based separation membranes of
the invention can be operated continuously at a temperature up to
400.degree. C. and thus lead to an increase in the yield and a
reduction in the costs.
[0059] The polymer films of the invention can be made
proton-conducting by appropriate doping.
[0060] Accordingly, the present invention further provides a doped
polymer membrane based on polyazoles which is obtainable by a
process comprising the steps
[0061] A) dissolving the polyazole polymer in a polar, aprotic
organic solvent,
[0062] B) casting a polymer film using the solution obtained from
step A),
[0063] C) drying the film formed in step B) until it is
self-supporting,
[0064] D) doping the polymer film obtained in step C) with a
dopant, wherein a polyazole polymer powder having a particle size
in the range from 300 .mu.m to 1500 .mu.m is used in step A).
[0065] In a preferred embodiment of the invention, polyazole
polymer powders having a particle size in the range from 300 .mu.m
to 1250 .mu.m, in particular from 300 .mu.m to 1000 .mu.m,
particularly preferably from 500 .mu.m to 1000 .mu.m, are used.
[0066] The preferred polymer structures of the formulae (I) and
(II) are also preferred for this doped polymer membrane.
[0067] In step D), the doping of the polymer film obtained in step
C) is carried out. For this purpose, the film is wetted with a
dopant or is placed in the latter. Dopants used for the polymer
membranes of the invention are acids, preferably all known Lewis
and Bronsted acids, in particular inorganic Lewis and Bronsted
acids. Apart from the acids just mentioned, it is also possible to
use polyacids, in particular isopolyacids and heteropolyacids, and
mixtures of various acids. For the purposes of the present
invention, heteropolyacids are inorganic polyacids which have at
least two different central atoms and are formed as partial mixed
anhydrides from weak, polybasic oxo acids of a metal (preferably
Cr, Mo, V, W) and a nonmetal (preferably As, I, P, Se, Si, Te).
They include, inter alia, 12-molybdophosphoric acid and
12-tungstophosphoric acid.
[0068] The polymer film used for the doping step D) can also be a
separation membrane comprising the polyazole according to the
invention. Owing to the increased porosity, this leads, as
described in WO 98/14505, to a reduction in the doping time,
increased acid loading and a further improved conductivity.
[0069] According to the invention, particularly preferred dopants
are sulfuric acid and phosphoric acid. A very particularly
preferred dopant is phosphoric acid (H.sub.3PO.sub.4).
[0070] The polymer membranes of the invention are doped. For the
purposes of the present invention, doped polymer membranes are
polymer membranes which as a result of the presence of dopants
display increased proton conductivity compared to the undoped
polymer membranes.
[0071] Methods of producing doped polymer membranes are known. In a
preferred embodiment of the present invention, they are obtained by
wetting a film of the polymer concerned with concentrated acid,
preferably highly concentrated phosphoric acid, for an appropriate
time, preferably 5 minutes-96 hours, particularly preferably 1-72
hours, at temperatures in the range from room temperature to
100.degree. C. and atmospheric or superatmospheric pressure.
[0072] The conductivity of the polymer membrane of the invention
can be influenced via the degree of doping. The conductivity
increases with increasing concentration of dopant until a maximum
value has been reached. According to the invention, the degree of
doping is reported as mol of acid per mol of repeating units of the
polymer. For the purposes of the present invention, a degree of
doping of from 3 to 15, in particular from 6 to 12, is
preferred.
[0073] The polymer membrane of the invention has improved materials
properties compared to the previously known doped polymer membranes
based on commercially available polyazoles. In particular, they
have very good mechanical properties.
[0074] Particularly when using polyazole polymer powders having a
particle size in the range from 500 .mu.m to 1000 .mu.m, the
acid-doped polymer membranes display a significantly improved
elongation at break of at least 40%, preferably from 40 to 65%.
[0075] Possible applications for the doped polymer membranes of the
invention include, inter alia, use in fuel cells, in electrolysis,
in capacitors and in battery systems. Owing to their property
profile, the doped polymer membranes are preferably used in fuel
cells.
[0076] The present invention also provides a membrane-electrode
unit comprising at least one polymer membrane according to the
invention. Further information on membrane-electrode units may be
found in the specialist literature, in particular the patents U.S.
Pat. Nos. 4,191,618, 4,212,714 and 4,333,805. The disclosure of the
abovementioned references [U.S. Pat. Nos. 4,191,618, 4,212,714 and
4,333,805] in respect of the structure and production of
membrane-electrode units is hereby incorporated by reference into
the present description.
[0077] The invention is illustrated below by means of examples and
comparative examples, without the invention being restricted to
these examples.
EXAMPLES:
[0078] A commercial polymer (Celazole, PBI polymer) in the form of
a powder is separated into various fractions by means of a stack of
sieves. The results of the sieve analysis are shown in table 1. The
sieve fractions obtained in this way are dried individually. As
soon as the water content of a sieve fraction is <0.1%, a
solution is prepared therefrom by mixing with dimethylacetamide
using a method described in the prior art. It is found that
particles having a size of >1500 .mu.m cannot be brought
completely into solution. For this reason, particles having a size
of >1500 .mu.m should not be used for the preparation of
solutions.
[0079] Each solution prepared using a separate sieve fraction is
then used to produce a film by conventional industrial casting
processes or by a manual doctor blade technique. The films produced
in this way are doped by dipping into 85% H.sub.3PO.sub.4 at room
temperature for 72 hours.
[0080] Results:
1TABLE 1 Results of the sieve analysis of a commercial PBI polymer
Sieve fraction Proportion (percent by weight) <200 .mu.m 6
200-300 .mu.m 15 300-500 .mu.m 43 500-750 .mu.m 15 750-1000 .mu.m
11 1000-1500 .mu.m 7 >1500 .mu.m 3
[0081] Mechanical properties of commercial PBI films
[0082] To determine the mechanical properties, test specimens of
type 1B in accordance with ISO 527-3 are stamped from the films and
examined by means of a uniaxial tensile test using a Zwick
universal testing machine model S100. The deformation rate is 5
mm/min and the test temperature is set to 160.degree. C. so as to
correspond to temperatures typical for use in fuel cells. At least
5 tensile tests are carried out on each sample composition and the
statistical mean is determined. Examples of tensile test curves of
film produced from individual sieve fractions are shown in FIG. 1.
The data obtained in this way are summarized in table 2.
[0083] It is found that the fracture toughness depends strongly on
the sieve fraction. In particular, high fracture toughness are
achieved for films which have been produced using sieve fractions
in the range 300-1000 .mu.m.
2TABLE 2 Results of the tensile tests on films produced from
various sieve fractions Polymer fraction Unsieved polymer <200
.mu.m 300-500 .mu.m 500-750 .mu.m 750-1000 .mu.m 1000-1500 .mu.m
Number of 6 5 5 5 5 6 measurements Modulus of 2850 2910 2870 2875
2661 2780 elasticity [MPa] Tensile 147 139 141 148 149 123 strength
[MPa] Elongation at 42 20 44 49 61 30 break [%] Fracture 2268 892
2320 2528 2910 1281 toughness [kJ/m.sup.2]
[0084] Mechanical properties of acid-doped membranes
[0085] After doping with acid, strip specimens having a width of 15
mm and a length of 120 mm are produced and tested in a tensile test
at T=100.degree. C. at an elongation rate of 50 mm/min. Examples of
tensile test curves as shown in FIG. 3 and the results of the
analysis are summarized in table 3.
[0086] The specimens obtained using the sieve fractions <200
.mu.m and 200-300 .mu.m are very unstable mechanically and rupture
at very low stresses. Only unsatisfactory mechanical properties
were able to be achieved using these fine fractions. As observed in
the case of the films, the membranes, too, display the best
mechanical properties in the case of materials produced from
powders in the range 300-1000 .mu.m. Surprisingly, it is likewise
found that the use of particles >1000 .mu.m leads to a worsening
of the mechanical properties.
3TABLE 3 Results of the tensile tests on acid-doped PBI membranes
produced from various sieve fractions Polymer fraction Unsieved
polymer <200 .mu.m.sup.1 200-300 .mu.m.sup.1 300-500 .mu.m
500-750 .mu.m 750-1000 .mu.m 1000-1500 .mu.m Number of 5 3 5 5 5 5
5 measurements Modulus of 8.2 5 5.1 3 5.2 4.2 5.3 elasticity [MPa]
Tensile 1.2 <0.5 0.2 0.9 1.2 1.6 0.5 strength [MPa] Elongation
at 26 <5 5 40 48 65 11 break [%] Fracture 21.6 <1 0.6 19.4
20.5 56 3.1 toughness [kJ/m.sup.2] .sup.1A number of specimens
rupture at even smaller forces and cannot be measured
[0087] 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 collector 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 via
a Pt-100 resistance thermometer positioned in the immediate
vicinity of the specimen.
[0088] After the temperature has been reached, the specimen is
maintained at this temperature for 10 minutes prior to commencement
of the measurement.
[0089] It is surprisingly found that, particularly in the
temperature range >100.degree. C., membranes produced using the
sieve fractions have higher conductivities than a membrane produced
using the unsieved polymer.
4TABLE 3 Specific conductivity (S/cm) of PBI membranes which have
been produced from various sieve fractions and doped with
phosphoric acid Unsieved T (.degree. C.) polymer <200 .mu.m
200-300 .mu.m 300-500 .mu.m 500-750 .mu.m 750-1000 .mu.m 1000-1500
.mu.m 25 0.053 0.073 0.051 0.049 0.050 0.048 0.037 40 0.066 0.069
0.062 0.064 0.050 0.054 60 0.040 0.052 0.059 0.060 0.061 0.042
0.051 80 0.043 0.058 0.062 0.056 0.057 0.051 0.053 100 0.062 0.077
0.084 0.069 0.070 0.071 0.068 120 0.077 0.089 0.103 0.091 0.089
0.091 0.088 140 0.075 0.090 0.109 0.097 0.091 0.092 0.091 160 0.073
0.089 0.105 0.099 0.085 0.090 0.090
Example 2 (according to the invention)
[0090] The fines (<300 .mu.m) and the coarse material (>1250
.mu.m) are separated off from a commercial PBI polymer (Celazole)
by sieving. This polymer is then dried and a solution is prepared.
A film is produced from the solution using conventional methods.
The film is subsequently doped in 85% phosphoric acid for 72 hours
so as to produce a membrane.
[0091] As shown by the comparison in FIG. 3, it is found that the
mechanical properties of such a membrane are virtually identical to
those of the best membrane from example 1. A maximum conductivity
of 0.09 S/cm is likewise measured at a temperature of 120.degree.
C.
* * * * *