BACKGROUND TO THE INVENTION In alkaline electrolytic cells having an aluminum anode, electrons are provided by means of the equation:- Al-). Al + 3e' The trivalent aluminum species forms a series of soluble aluminum complexes such as Al02- (or its hydrated form Al (OH) 4-), which eventually reach a saturation level and produce a highly hydrated aluminum hydroxide Al (OH) 3. x H20 solid.

Maintaining the dissolution of aluminum species while avoiding the precipitation of poorly conducting solids on the aluminum anode surface remains a problem to be overcome. Furthermore, if the aluminum is too active, a chemical corrosion reaction may take place which produces no useful power, as given by the equation:- 2AI + 6H20 o 2AI (OH) 3 + 3H2 Aqueous aluminum air cells rely on active aluminum metal dissolution in an aqueous alkaline electrolyte which must be sufficiently aggressive to prevent the formation of a passive layer on the aluminum surface. The passive layer formation can be partially inhibited or prevented by using an electrolyte which is sufficiently alkaline, e. g. 4-8 M, or which is strongly saline. These solutions are effective up to the point where aluminum hydroxide begins to precipitate. Although the unwanted aluminum corrosion reaction can be reduced by the use of inhibitors, these do not benefit the power output of the cell.

It is well-known in the field that certain electrolyte additions can alter the

performance of the aluminum anode to prevent undue corrosion. For example U. S.

3,850,693 discloses that soluble tin and zinc additions to an alkaline electrolyte can inhibit aluminum anode corrosion. U. S. 3,880,671 discloses that aluminum anodes can be protected against corrosion in an alkaline electrolyte by addition of citrate plus soluble compounds of tin or lead.

Organic additives have also been used to control or reduce the corrosion rate of metals, such as zinc, under similar alkaline conditions. U. S. 4,777,100 discloses that organic phosphate reduces the corrosion reaction of zinc even better than mercury additions.

In heat transfer applications, the use of aluminum and inhibitors to prevent aluminum attack by the fluid are common. U. S. 4,514,154 discloses that an aqueous and/or alcohol solution containing an alkylene silane grafted polyether can be used with alkali metal hydroxides or amines as a corroson inhibitor for aluminum.

Furthermore since aluminum is a good conductor of heat and is used extensively in radiators and cooling coils, there are corrosion inhibitors for the heat transfer fluid. The heat transfer fluid does not have the aggressive alkaline components of an aluminum air battery and, thus, additional inhibitors can be used. U. S. 4,389,371 discloses that carbonates, borates, phosphates, silicates, salts of benzoic or toluic acids and heterocyclic nitrogen-containing compounds can be used in antifreeze alcoholic solutions to prevent corrosion of aluminum.

Despite the benefits of protecting aluminum metal from the respective electrolytes, these solution additives do not address the principal focus of an aluminum air cell, which is to produce electrical energy. For energy production, it is important to have the aluminum metal as active as possible. U. S. 4,942,100 discloses that soluble indium additions at the saturation level are advantageous for voltage and efficiency when an aluminum cell is run at current densities of less than 400 mA/cm2. Similarly the Journal of Power Sources Volume 22 No. 3/4 Mar./Apr. 1988, pp 261-267 discloses that indium and gallium additions to the electrolyte are advantageous for improving electrical performance.

One of the common failure modes for an aluminum air cell is due to lack of electrolyte capacity because during power production, the reaction product from aluminum dissolution combines with water and produces aluminum hydroxide species.

This reduces the amount of free water available for ionic conduction and for reaction with the aluminum anode. Thus, there has been considerable emphasis on producing crystalline aluminum hydroxide which has a lower water content than Al (OH) 3. x H20.

Aforesaid U. S. 4,942,100 discloses the addition of aluminum hydroxide seed crystals to the electrolyte to reduce the degree of supersaturation of dissolved aluminum and to encourage the formation of larger aluminum hydroxide crystals. Crystallization modifiers are taught in another aluminum application. The process of extracting alumina from bauxite ore, requires that impure alumina be dissolved in caustic solution and then reprecipitated as aluminum hydroxide. In this process, the formation of large crystals of aluminum hydroxide is an advantage because the product, aluminum hydroxide must be filtered and large crystals filter more easily. Thus, there is considerable literature reporting beneficial crystal-forming additives for use in alumina production. For example, U. S. 5,312,603 reports that polyglycerines aid in producing large crystals of'aluminum hydroxide in the Bayer process of alumina recovery from bauxite. U. S. 5,106,599 reports that polysaccharides can help to produce large crystals in the Bayer process. U. S. 4,737,352 reports that surfactants with an oil can produce larger crystal sizes during cooling of the hot caustic Bayer process liquor. The oils preferred were parafinic or naphthenic with a suitable surfactant. U. S. 4,608,237 reports that polymers can aid in the recovery of crystals of aluminum hydroxide from the Bayer process. In these reports, the objective has been to produce large crystals from the saturated hot caustic solution produced in the Bayer process. However, these references are silent as to whether these materials would be effective in an aluminum air cell which operates at lower temperatures and which requires low solution resistivity. For an electrolyte to be functional in an aluminum air cell, it must maintain a high conductivity, otherwise resistance in the electrolyte would cause voltage loss and temperature rise and ultimately premature cell failure due to corrosion of the aluminum and evaporation of the electrolyte which are accelerated at higher temperatures. Oils, polymers, polyglycerines and polysaccharides are not ionic species and would reduce the conductivity of the electrolyte and, thus, would not be expected to be desirable in an aluminum air cell.

In another field where fine particulate matter and sludges are encountered, the use of flocculating agents are often used. These flocculating agents allow the solids to

be separated from the liquid phase more quickly. The agents do not cause crystallization, as in the previous examples, but cause clumping of the highly hydrated solids so that settling and separation occur more easily. Of relevance is the disclosure of U. S. 3,575,868. In this patent, a starch and polyacrylic acid are used. In U. S.

4,330,409 a hydrolyzed starch is held to be more effective than the polyacrylamide flocculant previously cited. In this patent, the starch is prepared by contact with a metal salt, such as aluminum phosphate, before being used to destabilize a sludge suspension. This material is claimed to be effective in separating the suspensions found in bitumen tar sands mining operations. Its effectiveness is claimed to be further enhanced when the starch was additionally treated with an alcohol, acetone, yeast or lactic aid. U. S. 5,281,497 discloses that starch may also be added to a system to cause gel formation. In-an alkaline zinc cell, an epichlorhydrin modified starch has been used as a gel coating on the zinc anode to protect the anode from contaminants from the cathode, but which allows the hydrogen formed, as zinc corrodes, to escape. In an aluminum air cell, there is no need for protection from cathode reaction products and, thus, the use of a gel former does not appear to be useful. A flocculant with or without alcohol treatment would thus appear to be disadvantageous in an aluminum air cell because of the low conductivity of the materials as compared to the electrolyte of 4-8M caustic.

Aluminum anode alloying, such as described in U. S. 5,032,474 is known to be able to produce a hyperactive anode for brief periods, i. e. minutes, of very high current density operation.

There remains, however, a need for maintaining a high power activity output of an aluminum air cell over significant periods of time.

There is also a need to operably activate an aluminum anode to provide acceptably good current density over a time frame of hours which is clearly distinguished over the aforesaid prior art.

SUMMARY OF THE INVENTION It is an object of the present invention to provide an aluminum anode-gas diffusion alkaline cell having an improved high power activity output over significant

periods of time.

Accordingly, in one aspect the invention provides an improved aluminum-air diffusion cathode battery having an aluminum or aluminum/alloy anode; an air diffusion cathode; an aqueous electrolyte containing metal cations selected from the group consisting of alkali metal cations; the improvement wherein said electrolyte comprises 15-25% W/W sodium hydroxide and an organic dehydrative additive selected from the group consisting of (a) 1-15% w/w of a monosaccharide; (b) 1-15% w/w of starch; (c) 0.5-5% w/w of polyacrylamide; (d) 1-10% w/w Cl-C4 alcohol; and mixtures thereof.

The term"organic dehydrative additive"as used in this specification and claims is meant a compound, which effects a reduced water uptake by hydrated aluminum hydroxide species in gel form in suspension within the electrolytic solution, either by dehydration of an aluminum hydroxide species or by prevention of said uptake by said species. The aforesaid hydrated aluminum hydroxide species is formed as a product of the aluminum anodic reaction particularly when the cell is under either a high or low load, but also when under no load.

Preferably, the monosaccharide is selected from an aldopentose, ketopentose aldohexose and ketohexose. Most preferably the aldohexose is D-glucose.

Preferably, the Cl-C4 alcohol is selected from ethyl alcohol and propyl alcohol.

Prior art alkaline electrolytic cells having aluminum anodes form such hydrated aluminum hydroxide species as particulate matter in such form as to be deposited and coat the anode surface and/or remain in suspension to cause significant decrease in cell voltages for any given current.

Yet further, removal of water from the aqueous electrolytic solution to provide hydrated solids has a detrimental effect on the free water available for the electrochemical oxidation of the aluminum anode.

Highly hydrated aluminum hydroxide gel has also been found to have a detrimental effect on the cathode, in an aluminum anode-gas diffusion cathode electrolytic cell.

Thus, without being bound by theory, it is believed that the dehydrative additive effects change of the aluminum hydroxide from the amorphous form to a less water- containing crystalline form.

Surprisingly, in preferred embodiments according to the invention, synergistic energy output enhancement has been obtained, particularly, with combinations of inorganic and organic additives.

Further, the additives of use in the practice of the invention were found to provide power and energy enhancing abilities at ambient temperatures.

In a preferred form, the invention provides a battery as hereinbefore defined wherein the electrolyte further comprises 0.01-0.2 mole/I anion selected from stannate and indate.

In further preferred forms, the electrolyte comprises D-glucose in combination with polyacrylamide and/or ethyl alcohol.

In a further preferred form, the electrolyte comprises 5-20 % W/W sodium chloride.

Most preferably, the organic dehydrative additive is rectified starch.

Although the aqueous electrolyte may be prepared by the addition of the appropriate compounds, additives and the like in the form of, for sample, salts, solutions, suspension, and the like, we have found that a most satisfactory method of obtaining the Sn and In anions in the solution at the desired concentration is to electrochemically dissolve the respective metal as a metal anode.

BRIEF DESCRIPTION OF THE DRAWINGS In order that the invention may be better understood, preferred embodiments will not be described, by way of example only, with reference to the accompanying drawings wherein:- Fig. 1 shows comparative graphs of battery voltage discharge values over time to drop- off for an aluminum anode-air diffusion cathode battery having different electrolytic compositions.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS All examples used common aluminum alloy anodes (99.4% Al + 0.6% In) and air diffusion nickel mesh/polymer cathodes so that the direct effect of the electrolyte additives could be clearly seen. The electrolyte was in each case 4M NaOH (20% W/W) with various additives. The concentrations of the additives reported were from a series of tests conducted at room temperature and at a constant discharge rate of 2.5A.

An aluminum air cell such as used in Canadian Patent Application No. 2,301,470 filed 15 March 2000 entitled"Ecologically Clean Mechanically Rechargeable Air-Metal Current Source"was used. A plot of the effectiveness of the various electrolytes described below is given in Fig. 1. The numerical data of cell performances is shown in Table I.

Example 1- (Prior Art) The electrolyte contained 0.05 M tin as Sn4+ (sodium stannate). As described in the prior art, soluble tin additions can have a positive influence on electrical performance. During discharge, the steady state discharge voltage was at about 1.2 V and the current was sustained for 5 hours before cell failure. This electrolyte provided 12.5 Ah capacity and 15 Wh energy. The following comparative examples according to the invention show how this good performance of the prior art can be exceeded in the practice of the present invention.

Example 2 The electrolyte contained 2 wt% D-glucose. The initial voltage was slightly reduced (1. 1V) but the duration of discharge was considerable extended to 6.5 h. The longer run duration provided a higher capacity of 16.4 Ah and energy of 18 Wh.

Example 3 The electrolyte contained 15 wt% NaCI. Although sodium chloride is known to prevent aluminum passivation, the addition gave a steady state discharge voltage of 1.15V and a discharge time of 5.6 h. The capacity was 13.9 Ah and the energy was 16 Wh.

Example 4 The electrolyte contained 2 wt% ethanol. Surprisingly this solution gave a good discharge voltage of 1.18V and a discharge time of 5. 8h. The discharge capacity was

14.4 Ah and the energy was 17 Wh.

Example 5 The electrolyte contained 0.05 M In 2+ as sodium indate. The discharge voltage was one of the highest at 1.25 V and the discharge time was good at 5.9 h. Overall the discharge capacity was 14.4 Ah and the energy was 18 Wh.

Example 6 The electrolyte was 4.0M NaOH with the addition of all of the previous additions. The discharge voltage was the highest at 1. 4 V and the discharge time was almost the longest at 6.9h. The discharge capacity was 17 Ah and the energy was 24.0 Wl1.

Example 7 The electrolyte was 4.0M NaOH having 2% w/w D-glucose and 1% w/w polyacrylamide. The discharge voltage was 1.12V and the discharge time was the longest at 7 hr. The discharge capacity was 17.5 Ah and the energy was 19.7 Wh.

Example 8 The electrolyte was a 4.0M mixture of sodium hydroxide, potassium hydroxide and lithium hydroxide in the ratio 16: 3: 1. The discharge voltage was 1.33 and the discharge time 6.3 hr. The discharge capacity was 15.75 Ah and the battery energy 20.0 Wh.

Table 1 Example Additive 4M Voltage Time Rated Rated Battery NaOH (V) (Hours) Current Capacity Energy Electrolyte Discharge (Ah) (Wh) (A) 1 0. 05M Na2SnO4 1.2 5 2.5 12.5 15 (Prior Art) 2 2% w/w D-1. 1 6.5 2.5 16.4 18 glucose 3 15% w/w NaCI 1.15 5.6 2.5 13.9 16 4 2% w/w (ethyl 1.18 5.8 2.5 14.4 17 alcohol) 5 0. 05M NaIn02 1.25 5.9 2.5 14.4 18 6 1-5 cumulative 1.4 6.9 2.5 17 24 7 2% w/w D-1. 124 7 2.5 17.5 19.7 glucose+ 1% w/w polyacrylamide 8 80% 4 Mol/L 1.33 6.3 2.5 15.75 20.0 NaOH + 15% 4 Mol/L KOH + 5% 4 Mol/L LiOH

The results show that additives such as glucose or alcohols can match or provide better capacity and energy output than the prior art additions of tin or indium per se. The advantage that the organic additives give to the aluminum air battery according to the invention is believed, as aforesaid described, to result from reducing the amount of water tied up in the aluminum hydroxide product and in improving the ability for the aluminum anode to dissolve in the electrolyte. The synergistic effect of combining the additives is significant.

Although this disclosure has described and illustrated certain preferred embodiments of the invention, it is to be understood that the invention is not restricted to those particular embodiments. Rather, the invention includes all embodiments which are functional or mechanical equivalents of the specific embodiments and features that have been described and illustrated.