BACKGROUND OF THE INVENTION Membranes made of proton-conducting polymer electrolyte are increasingly used in electrochromic devices, batteries and fuel cells. The protons in such membranes are provided either by the addition of free acids such as sulphuric acid or orthophosphoric acid to a polymer, or by using polymers on which a sulphonic or phosphonic group has been grafted to the polymer backbone or side chain. Perfluorosulphonic acid polymers of the NafionW type are good examples. A well known macromolecular material is a complex of orthophosphoric acid-polyoxyethylene. The complex is conventionally prepared by dissolving both components in a compatible and inert solvent, such as acetonitrile, methanol, tetrahydrofuran, ethanol or mixtures thereof.

A major problem associated with the use of free acids in electrochromic devices is the chemical stability of the polymer in such a strongly acidic environment.

Corrosion and polymer degradation is frequently observed. In the field of glazings for buildings and cars, a yellowing phenomenon is observed overtime, probably caused by the formation of free radicals resulting from the degradation of the polymer. This drawback could be overcome by the addition of antioxidants, but because of compatibility

problems with the acidic medium and the solvents used for the preparation of the macromolecular material, few antioxidant solutions have been found.

Another problem associated with electrochromic devices is the presence of haze, which is equal to the diffused light proportion compared with the incident light.

When exceeding 1%, the eye starts to perceive deformation of images.

US 5,507,965 describes a protonic, conductive, macromolecular material comprising a solid complex of anhydrous orthophosphoric acid and polyoxyethylene.

The patent proposes the use of a triphenylphosphine as an antioxidant. The macromolecular material is obtained by dissolving anhydrous orthophosphoric acid in tetrahydrofuran, and adding this solution to polyoxyethylene, followed by the addition of acetonitrile to solubilize the polymer and form the complex. Triphenylphosphine is added in tetrahydrofuran since it is soluble therein. The resulting orthophosphoric acid- polyoxyethylene macromolecular complex is said to be useful as an electrolyte in electrochromic systems.

US 5,518,838 is directed to electrochemical cells such as batteries and capacitors, comprising a solid polymer electrolyte, which gives energy storage devices with very high power density. The solid polymer electrolyte includes polyoxyethylene, polyvinylalcohol, polyvinyl acetate, polyacrylamide etc. combined with sulphuric acid or phosphoric acid as a polymer binder.

US 4,844,591 concerns an electrochromic device comprising a polyvinyl acetate-orthophosphoric acid complex or a polyoxyethylene (POE)-orthophosphoric

acid complex. The POE-orthophosphoric acid complex is said to be prepared under rigorous anhydrous conditions.

SUMMARY OF THE INVENTION In accordance with the present invention, there is now provided an organoacid material obtained from the reaction of a strong acid with an organic compound, for use as a proton conductor in a polymer electrolyte. The organic compound replaces one or more hydrogen atoms on the acid, thus leading to an adduct that prevents the degradation of the polymer electrolyte. Preferred organic compounds include acetonitrile, acrylonitrile, a low molecular weight ether, a low molecular weight alcohol, or mixtures thereof. Preferred strong acids include orthophosphoric acid and sulphuric acid. Polymer electrolytes comprising the novel organoacid material are also part of the present invention.

DETAILED DESCRIPTION OF THE INVENTION The present invention is directed to an organoacid material suitable for use as a proton conductor in polymer electrolytes. Such polymer electrolytes are conventionally used in electrochromic devices, rechargeable batteries, fuel cells, etc. It has unexpectedly been found that the present organoacid material has the beneficial effect of preventing the degradation of the polymers while still providing excellent ionic conductivity, thus representing an alternative to orthophosphoric acid currently used for proton conduction in polymer electrolytes utilized in the above fields and in those requiring electrical energy production. Examples are microelectronics, generators or cells such as those for pacemakers, or for energy storage purposes.

The present organoacid material is obtained by reacting a strong acid with an organic compound, in order to replace at least one of the hydrogen atoms of the acid with an organic substituent. Examples of suitable organic compounds include acetonitrile, acrylonitrile, C,-C4 alcohol, low molecular weight ether and the like. Most preferred compounds are acetonitrile, diethyl ether and methanol. It has been found that tetrahydrofuran does not represent a suitable organic compound because it leads to side reactions such as colour and peroxide formation, which are likely to degrade the polymer electrolyte. Further, alcohols and ethers of higher molecular weights, i. e., containing 5 or more carbon atoms, tend to form coloured phosphates, which means that the compound is less stable, and will therefore cause undesirable polymer electrolyte degradation. These compounds are therefore less suitable, particularly in the field of electrochromic devices because of the change of color.

Example of strong acids suitable for the purposes of the present invention include orthophosphoric acid; sulphuric acid, perchloric acid and mixtures thereof.

Typically, the strong acid is reacted with the organic compound during one to several days under reflux, or slightly below the boiling point of the organic compound in a closed vessel. This has the effect of replacing at least one hydrogen atom, or more than one is that is the case, of the acid by an organic group from the organic compound.

Control of the number of hydrogen atoms replaced on the acid can be controlled by taking samples periodically and analyzing them by measuring the increase in mass of the formed adduct and by conventional spectroscopic methods such as Fourier transformed infrared (FTIR) or NMR spectroscopy.

The families of polymers used for the preparation of the electrolyte membranes are those currently used with orthophosphoric acid in the preparation of electrochromic devices, proton-conducting rechargeable batteries and fuel cells. They include, without being limited to, polyethers, polyamides, polyacrylates, polyvinylalcohols, polyvinylacetates, polyvinylpyridines, polyacrylamide, polyimides, polybenzimidazoles, polyvinylpyrolidone, polyaromatic pyrazoles, perfluoronated sulphonic acid polymers, polyarylenesulfone, and the sulphonic acid derivatives of trifluorostyrene, styrene/ethylene/butylene copolymers, fluorinated ethylene propylene polymer (FEP), polyvinylidene fluoride (PVDF), and mixtures thereof.

According to a most preferred embodiment of the present invention, orthophosphoric acid is mixed with the organic compound in a closed vessel, and heated to a few degrees below the boiling point of the organic compound. Alternately, the mixture can be heated under refluxing conditions at boiling point. In either process, heating is maintained for at least one, and generally, several hours or days, depending on the extent of reaction desired. The solid organophosphoric adduct is isolated by evaporating any unreacted organic compound, and drying under vacuum. The extent of the reaction of the organic compound with the orthophosphoric acid can be estimated 1) by looking at the increase in mass of the formed adduct and comparing with the starting amount of orthophosphoric acid; and 2) by looking at the increase in viscosity of the solution. This increase in viscosity has no effect on the optical, mechanical and electrochemical properties of the electrolyte films. It may be useful for the coating process, by allowing to reduce the organic compound content of the electrolyte solution and to increase the thickness of the electrolyte films. If viscosity is too high, however, the organic compound content may have to be increased again in order to get a solution possible to handle for coating. The duration of the reaction may therefore be chosen so as

to optimise the coating parameters, as a function of the polymer type, coating method used and electrolyte thickness desired. Typically, the thickness of the electrolyte can be from about 10 to about 300 pm.

As an example, when orthophosphoric acid is reacted with acetonitrile at 70°C during 1 hour or more, the increase in mass of the adduct corresponds to the addition of one acetonitrile molecule to one hydrogen of the acid, with the elimination of one molecule of water. The FTIR spectrum of the adduct shows the decrease of the P-OH groups at between 800 cm~'and 1000 cm~l, and the appearance of new absorption bands at 1500 cm''and 1700 ami', corresponding to N-O or N-P chemical bonds. As the reaction is allowed to proceed further during several hours or days, no more increase in mass of the formed adduct is observed, meaning that no additional hydrogen on the acid is replaced. However, a sharp increase in viscosity of the solution is observed, indicating that some type of self-polymerisation of the formed adduct is occurring. In this example, for coating purposes, the preferred duration of reaction is between 1 and 12 hours. The viscosity is much too high after 72 hours.

By using other types of organic compounds and/or higher temperatures, it is possible that other hydrogen atoms may be replaced. However, organophosphoric materials wherein one hydrogen only has been replaced have been found preferred in terms of ionic conductivity in the electrolyte and chemical resistance of the polymer towards degradation.

It should also be noted that slight coloration of the product might be observed depending on the strong acid-organic compound combination employed. For

electrochromic applications, the most suitable organoacids are obviously those producing no coloration.

With respect to the determination of the preferred molecular weight for the polymer material, anyone of ordinary skill in the art is well aware that higher molecular weight polymers are generally more structurally sound, while lower molecular weight polymers are less rigid, thus more flexible. Typically, a molecular weight varying from 100 000 to 5 000 000 is preferred. Higher molecular weight polymers have the advantage of containing less hydroxyl groups, which cause degradation of the polymer.

Further, the amount of organoacid material to be added to the polymer binder will vary depending on the end properties desired for the electrolyte. If the ratio of polymer binder to organoacid material is increased, the electrolyte films will be less electrochemically conducting, but also less amorphous, and therefore, more mechanically resistant, and vice-versa if the ratio is decreased.

The molar ratio of polymer to organoacid material in the electrolyte may vary from 0.2 to 10. A preferred range of from about 0.5 to about 1 has been found to provide optimal conductivity and mechanical properties.

Typically, fumed or pyrogenic silica products such as Aerosil@, are added in the electrolyte solutions to get dried electrolyte films more uniform, with less internal mechanical tensions and thus easier to manipulate. Fumed or pyrogenic silica products are generally ultrafine powders, i. e., of very low particle size, and are commonly used in

the coating industry for this purpose. They have no effect on the final optical, mechanical and electrochemical properties of the electrolyte films.

The following examples are provided to illustrate the present invention, and shall not be construed as limiting its scope.

Example 1 50 g of anhydrous orthophosphoric acid and 5 g of AerosilX are placed in 170 mL of stabilized tetrahydrofuran at room temperature, until the acid is dissolved.

Subsequently, 170 mL of acetonitrile and 48 g of polyoxyethylene of molecular weight 900 000 are added in medium. This solution is coated on a substrate of FEP film (-100 um) using a doctor blade, in order to prepare a film of polymer electrolyte (-300 um after drying). The polymer electrolyte film is hazy. After a few hours of storage, the film adheres strongly to the FEP film, and cannot be peeled off, indicating a partial degradation of the polymer into lower molecular weight compounds. After being left in ambient moist air, or after being treated one day at 90°C under nitrogen for complete drying, the electrolyte film loses its mechanical properties, indicating a total degradation of the polymer into a wax-like material.

Example 2 50 g of anhydrous orthophosphoric acid and 120 mL of acetonitrile are heated in a closed bottle for 24 hours at 70°C. The remaining unreacted acetonitrile is evaporated and the organophosphoric adduct dried under a vacuum. 32 g of the dried adduct is placed in 35 mL of tetrahydrofuran, then 29 g of POE and 3 g of AerosilX are added. A film of polymer electrolyte (-300 gm after drying) is prepared by coating the

solution of a FEP substrate in a conventional manner. The electrolyte film is perfectly clear, and can be peeled off from the substrate even after weeks of storage. It retains its mechanical strength after being left for extended periods of time in ambient moist air, or after being treated one day at 90°C under nitrogen, indicating that no significant polymer degradation has occurred.

Example 3 50 g of anhydrous orthophosphoric acid and 120 mL of stabilized tetrahydrofuran are heated at 70°C for 24 hours in a closed bottle. The solution darkens rapidly. The electrolyte films prepared as in Example 2 develop a dark yellow colour and lose rapidly their mechanical strength.

Example 4 The electrolyte film prepared in previous Example 2 is characterized for electrochromic applications. Its ionic conductivity is 10-5 Q~'cm~', the percentage of light transmission (TL%) is 80%, and the percentage of haze is 0. Such results are excellent for this particular application for the product.

A length of electrolyte film is placed between plates of electrochromic glass that have been coated with thin transparent electrodes of W03 in a conventional manner.

Electrochromic switching occurs for voltage differentials of around 2.5 V as expected.

After 48 hours of aging at 100°C, the percentage of haze is still very low.

Example 5

The electrolyte film prepared in previous Example 2 is placed between an anode consisting of a foil of an antimony-bismuth alloy, and a cathode consisting of a foil of nickel-molybdenum-chromium alloy, in order to obtain a rechargeable, proton- conducting, polymer electrolyte battery. The open cell voltage was about 0.4V.

As seen in the above examples, the organoacid material of the present invention provides an ionic conductivity comparable to that given by orthophosphoric acid, but has a much lower reactivity towards the polymer used in the electrolyte, thus preventing the degradation of the polymer overtime.

While the invention has been described in connection with specific embodiments thereof, it will be understood that it is capable of further modifications and this application is intended to cover any variations, uses or adaptations of the invention following, in general, the principles of the invention and including such departures from the present disclosure as come within known or customary practice within the art to which the invention pertains, and as may be applied to the essential features hereinbefore set forth, and as follows in the scope of the appended claims.