Ion conductive matrixes and their use

Details Ion conductive matrixes and their use Ion conductive matrixes and their use

2000-10-18

2004-11-02

Bell, Bruce F. (Department: 1746)

Chemistry: electrical current producing apparatus, product, and

With pressure equalizing means for liquid immersion operation

C429S006000, C429S304000, C429S309000, C429S324000, C429S330000, C424S486000, C424S487000

Reexamination Certificate

active

06811911

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to ion conductive matrixes, membranes and electrodes, their manufacture and use. In particular, the present invention is concerned with membranes comprising composite polymeric films and composite polymers.
BACKGROUND OF THE INVENTION
Ion conducting membranes (to be referred to hereinafter as “ICM”) are to be found in many electrochemical cells, among which: fuel cells, electrolyzers, electrochromic cells, batteries, electrochemical sensors and others. In some cases, polymer electrolyte is used such as Nafion. However, Nafion based fuel cells suffer from two major disadvantages. The first is that Nafion is a very expensive material and the second, its characteristic to dry out during the fuel cell operation due to water dragging by the protons conducted.
U.S. Pat. No. 5,456,600 teaches the use of a polymeric membrane for making an lithium-ion rechargable battery cell. The membrane disclosed is a combination of a poly(vinylidene fluoride) copolymer matrix and a compatible organic solvent plasticizer which maintains a homogenous composition in the form of a flexible, self-supporting film.
U.S. Pat. No. 5,643,689 discloses a non-liquid proton conductor membrane which comprises a matrix polymer dissolvable in a solvent and an acidic multimer dissolvable in that solvent. In accordance with the disclosure of this publication, when the membrane is contacted with a second solvent, it swells to allow an improved electrical contact between the cathode and anode plates in which the membrane is interposed.
Another type of polymer membrane is described in U.S. Pat. No. 5,425,865. The membranes described comprise a porous matrix of a crosslinked polymeric material, and a second polymeric material that partially blocks the pores of that matrix.
Another type of cell is the phosphoric acid fuel cell (PAFC), wherein acid is absorbed in a porous matrix made of e.g. silicon carbide powder and metal phosphate or metal oxide or metal salt. Typically, such a cell operates at elevated temperatures, about 180° to 200° C. U.S. Pat. No. 4,623,415 describes a membrane comprising a porous matrix for retaining phosphoric acid electrolyte where the matrix consists of a substance which is unreactive with phosphoric acid and has electron insulating properties.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a novel, low cost and highly conductive ion conducting matrix, i.e. a matrix in which ions derived from acids, bases or salts are mobile.
It is another object of the present invention to provide novel, low cost and highly conductive ion conducting membranes and electrodes.
Another object of the present invention is to provide electrochemical cells comprising the matrixes of the invention.
It is yet a further object of the invention to provide methods for preparing such membranes and electrodes.
Still, it is the object of the present invention to provide uses for the ion conducting membranes and electrodes invention.
Other objects of the invention will become apparent as the description of the invention proceeds.
Thus, the present invention provides by the first of its aspects an ion conducting matrix comprising:
(i) 5% to 60% by volume of an inorganic powder having a good aqueous electrolyte absorption capacity;
(ii) 5% to 50% by volume of a polymeric binder that is chemically compatible with an aqueous electrolyte; and
(iii) 10 to 90% by volume of an aqueous electrolyte,
wherein the inorganic powder comprises essentially sub-micron particles, preferably from about 5 to about 150 nm in size. The matrix of the present invention may optionally comprise between about 0.1% to about 25% of a non-volatile liquid lubricant that is chemically compatible with all the components in the matrix.
In accordance with a preferred embodiment of the present invention, the inorganic powder is characterized in that it has a surface area of at least 10 m
2
/g, and possesses a good absorption capability for the aqueous electrolyte.
According to another aspect of the invention, there is a provided a membrane being a film made of the matrix of the invention.
According to a further aspect of the invention, there is provided a composite electrode comprising 10 to 70% by volume of the matrix of the invention and the balance is made essentially by an electrode material which is a material known in the art per se as a suitable material in the manufacturing of electrodes, eg. carbon, graphite, air, oxygen, H
2
, methanol electrodes, Zn, Cd, Ni, Pb, Fe, Cu or their alloys, metal oxide electrodes, e.g. RuO
2
, WO
x
, MnO
2
, NiOOH, AgO, Ag
2
O and the like.
In the case that the matrix of the invention is used as an ion conducting matrix in a composite electrode, the inorganic powder may be electronically conductive.
Preferably, the inorganic powder of the matrix of the present invention is a member selected from the group consisting of SiO
2
, ZrO
2
, B
2
O
3
, TiO
2
, Al
2
O
3
and the like.
The polymeric binder used in the matrix of the present invention is a material which is chemically compatible with an aqueous electrolyte used, i.e. non-soluble in that electrolyte, and is a member selected from the group consisiting of: polyvinilyden fluoride (PVDF), PVDF-hexafluoropropylene (PVDHFP), poly(tetrafluoroethylene) (PTFE), poly(methylmethacrylate) (PMMA), polysulfone amide, poly(acrylamide), polyvinyl chloride (PVC), poly(acrylonitrile), polyvinyl flouride and any combination thereof.
The aqueous electrolyte of the present invention consists of an aqueous soluble compound selected from a salt, a base or mixtures thereof. Examples of aqueous soluble salts are alkali metal salts, alkali earth metal salt, R
4
NX where R is hydrogen or an organic radical and X is an anion derived from an inorganic acid, NH
4
Cl, ZnCl
2
and any combinations thereof.
Examples of aqueous soluble bases for use in the present invention are R
4
NOH where R is hydrogen or an organic radical alkali or alkali earth base compounds and any combinations thereof.
In the case that the matrix of the invention is used as a proton conducting matrix it is referred to hereinafter as “TCM”. The acid according to the present invention, which may be also a mixture of acids, may be a pure acid or an acid dissolved in water or in another suitable non-aqueous solvent that is known per se in the art. Acids suitable according to the present invention are: CF
3
(CF
2
)
n
SO
3
H, HO
3
S(CF
2
)
n
SO
3
H wherein n is an integer having a value of 0 to 9, sulfuric acid, HCl, HBr, phosphoric acid, HNO
3
and the like. Preferred acids are CF
3
(CF
2
)
n
SO
3
H or HO
3
S
3
S (CF
2
)
n
SO
3
H where n is equal to 0, 1, 2, 3 or 4. These preferred acids can be used either in their pure form or as aqueous solutions having a molar concentration of 10 to 99%, and preferably a molar concentration of 25% to 99%.
The ICM of the present invention has the general appearance of a plastic film having good mechanical properties. It can typically be bent to about 180° with no substantial fractures occurring, and it can be prepared in thickness being in the range of from about 10 to about 1000 microns or more. Due to its stability and good ionic conductivity, it can be used at a large temperature range of from sub-zero to about 150° C.
According to a preferred embodiment of the invention, where the matrix is in the preparation of a membrane, the inorganic powder comprised in the matrix is a very fine, electronically non-conductive powder having a particle size of preferably less than 150 nm. According to this embodiment, the ICM pores in which the aqueous electrolyte is absorbed are very small, and their characteristic dimension is essentially smaller than 50 nm.
The absorption capacity or the retention capability of the membrane for the acid or the aqueous electrolyte used depends on several parameters, among which are the composition and the type of the inorganic powder, the polymeric binder and the type of the dissolved acid or electrolyte. The combination of these parameters should be optimized in order to tailor the produ

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