Proton conductors which are thermally stable over a wide...

Details Proton conductors which are thermally stable over a wide... Proton conductors which are thermally stable over a wide...

1999-07-02

2001-07-24

Ojcles, Nechslus (Department: 1751)

Compositions

Electrically conductive or emissive compositions

C252S062200, C252S062200, C429S006000, C204S296000

Reexamination Certificate

active

06264857

ABSTRACT:

The present invention relates to proton conductors which are thermally stable over a wide range and have high conductivities and also to their use in electrochemical cells, e.g. fuel cells, secondary batteries and electrochromic displays.
Proton conductors which are thermally stable above 100° C. are known from the literature. However, the known proton conductors have significant disadvantages.
Thus, proton-conducting mixtures of oxo acids or their salts (e.g. phosphoric acid, sulfuric acid, perchloric acid, etc. or their salts) and an amphoteric material consisting of water-free substances have usable proton conductivities only at temperatures of about 200° C. (Th. Dippel et al., Solid State Interionics, 1993, 61, 41; K. D. Kreuer et al., Chem. Mater., 1996, 8, 610-41). The power per unit weight (W/kg) and the power per unit volume (W/I) are unfavorably small compared to polymer electrolyte membranes (PEMs) because of the lower proton conductivity values. This limits the possible applications, in particular for use as proton-conducting membranes for fuel cells for mobile use.
Also known is the use of oxides, hydroxides and apatites as high-temperature proton conductors (Kreuer et al., Chem. Mater.; Vol. 8, No. 3, 1996, p. 615 ff). However, relatively good proton conductivities can be achieved with these materials only at temperatures above 500° C. In the intermediate and low temperature range they do not have a sufficient proton conductivity. The power per unit weight and the power per unit volume are, compared to PEMs, even lower than when using oxo acids and their salts (W. Dönitz, “Fuel Cells for Mobile Applications, Status, Requirements and Future Application Potential”, Proc. Of the 11th World Hydrogen Conference, Dechema, Stuttgart, 1996, p. 1623).
The previously known proton-conducting polymer electrolyte membranes (PEMs) have high proton conductivities in a low temperature range (≦100° C.) and when used in fuel cells make possible a rapid rise in power. Thus, for example, a Dow membrane comprising a perfluorinated polymer has a conductivity of 0.1-0.2S/cm at room temperature (G. A. Eisman, Journ. of Power Sources, Vol. 29, 1990, 389-398). The performance of proton-conducting membranes is significantly dependent on the ampholyte content and the acid content of the membrane. The known PEMs use water as ampholyte. This restricts the upper limit of the operating temperature to about 100° C. Above this limit, dehydration of the membrane occurs, resulting in a reduction in the performance of the membrane (proton conductivity and thus electric power output and also mechanical strength) (S. Gottesfeld et al., Polymer Electrolyte Fuel Cell Model, J. Electrochem. Soc., 1994, 141, L46-L50).
Although some of the proton conductors known from the prior art can be used at high temperatures (oxo acids, e.g. phosphoric acid, and also hydroxides, oxides and apatites), their power per unit weight and power per unit volume are both too small. At low temperatures, these systems do not have satisfactory proton conductivities.
Although polymer electrolyte membranes have good proton conductivities in the temperature range below 100° C., they are generally not very stable at temperatures above 100° C.
It is therefore an object of the invention to provide proton conductors which have good proton conductivities over a wide temperature range, have high chemical and electrochemical stability and possibly mechanical strength and are chemically resistant to attack by acid and bases. Furthermore, they should have a high power per unit volume and a high power per unit weight.
This object is achieved by the present invention by providing proton conductors which comprise 1-99% by weight, preferably from 10 to 90% by weight, in particular from 20 to 80% by weight, of an acid and 99-1% by weight, preferably from 90 to 10% by weight, in particular from 80 to 20% by weight, of a nonaqueous amphoteric material and are thermally stable in a temperature range up to 400° C., in particular from −50 to 300° C. The proton conductors of the invention have proton conductivities of ≧10
−5
S/cm, in particular ≧10
−3
S/cm, in this temperature range
The acid present in the proton conductor of the invention and also the amphoteric material can be of low molecular weight or high molecular weight. Likewise, it is possible to use mixtures of low molecular weight and polymeric acids or amphoteric materials.
In a preferred embodiment, the proton-conducting mixture comprises a low molecular weight amphoteric material and a high molecular weight or low molecular weight acid, which may, if desired, be present in a high molecular weight polymer (as support), or a low molecular weight or high molecular weight amphoteric material and a low molecular weight acid.
For the purposes of the present invention, high molecular weight acids are, in particular, acids having a molecular weight of >1000 g/mol, preferably >2000 g/mol. The polymeric, high molecular weight acids used according to the invention have ionically dissociable, covalently bound groups which act as Brønsted proton donors toward the amphoteric material. Particular preference is given to using functionalized polyarylenes, halogenated aliphatic polymers or functionalized copolymers comprising aromatic and aliphatic monomer units.
Preferred functional substituents are, for example, —SO
3
M, —PO
3
M
1 or 2
or —COOM, where M is H, Na, K, Li, NH
4
, Ag, Cu, Ca, Mg or Ba.
Examples of preferred high molecular weight acids are aromatic and aliphatic polymers, in particular perhalogenated, preferably perfluorinated, aliphatic polymers, and also polyether ketones, polyether sulfones, polyimides, polyphenylene sulfides, polyphenylene oxides and copolymers which comprise units from these polymers and are substituted by sulfonic acid groups (SO
3
M), phosphoric acid groups (PO
3
M
1 or 2
) or carboxylic acid groups (COOM).
In a preferred embodiment, the acid is present as a polymer or bound to the amphoteric material. In this way, escape of the acid and thus contamination of the environment with the corrosive acid are avoided.
The low molecular weight acids having a molecular weight of ≦1000 g/mol, preferably ≦500 g/mol, which are present in the proton conductors of the present invention have, like the polymeric acids, ionically dissociable, covalently bound groups which act as Brønsted proton donors toward the amphoteric material. Preference is here given to using organic aromatic compounds and also halogenated aliphatic or aromatic compounds having covalently bound functional groups such as —SO
3
M, —PO
3
M
2
, —COOM, —B(OM)
2
or —CF
2
SO
3
M, where M is as defined above. Particular preference is given to using organic aliphatic and aromatic sulfonic acids, e.g. p-toluenesulfonic acid, methylsulfonic acid or trifluoromethylsulfonic acid, and also aromatic and aliphatic carboxylic acids.
Also preferred are inorganic mineral acids such as sulfuric acid, phosphoric acid and perchloric acid.
It is possible to use either high molecular weight or low molecular weight amphoteric materials or mixtures of high molecular weight and low molecular weight amphoteric materials.
For the purposes of the present invention, high molecular weight amphoteric materials are, in particular, those which have a molecular weight of >1000 g/mol, preferably >2000 g/mol. As amphoteric materials, preference is given to using aliphatic, halogenated or unhalogenated polymers which have amphoteric groups in the side chain and aromatic polymers which have amphoteric structures in the main chain.
In particular, these are polymers or copolymers which have heteroaromatic or heterocyclic, in particular nitrogen-containing, structural units in the main or side chain.
Such structural units are, for example:
where X:O, S, NH R:CH
3
, C
2
H
5
, C
6
H
5
, nC
4
H
9
, tC
4
H
9
, C
6
H
4
—CH
3
, CF
3
Y:N, NR′
+
Preferred amphoteric groups are, for example, imidazole, benzimidazole, pyrazole, oxazole, carbazole, indole, isoindole, dihydrooxazole, isooxazole, thiazole

Affiliated with

Also associated with

Rating

Proton conductors which are thermally stable over a wide... does not yet have a rating. At this time, there are no reviews or comments for this patent.

If you have personal experience with Proton conductors which are thermally stable over a wide..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Proton conductors which are thermally stable over a wide... will most certainly appreciate the feedback.

Rate now

Comments { 0 }

Profile ID: LFUS-PAI-O-2559902