DESCRIPTION "Electrolytic mixture in organic solvent for fuel cell"

[0001] Field of the invention

[0002] The present invention relates to an electrolytic mixture to be used in molten carbonate fuel cells. [0003] State of the art

[0004] Fuel cells are systems which allow converting the chemical energy of a fuel into electric energy by an electrochemical oxidative reaction of the fuel, without ntermediate conversions to thermal and mechanical energy. Therefore, they are capable of higher conversion yields compared to those of the conventional thermal machines . [0005] The fuel oxidation is achieved as a sum of two complementary semi-reactions, occurring concomitantly in environments which are contiguous but physically divided by a gas seal membrane.

[0006] In the molten carbonate fuel cells (MCFC) the membrane, called matrix, is a LiAlO ₂ thin sheet, having a thickness ranging between 0.5 and 1.5 mm, containing the electrolyte from which the ions which are consumed by a semi-reactions and produced by the other one originate. In this case, the electrolyte is an alkaline carbonate solution which melts at the cell operative temperature (about 650 ⁰ C) .

[0007] For the proper operation of the system, it is necessary that the matrix is completely filled, with electrolyte in order to act as a battery and allow the transport of the carbonate ion. Furthermore, in order to achieve a good performance, it is crucial that the electrodes are optimally wet by the electrolyte to keep the carbonate-gas-catalytic sites contact constant, thus allowing the reactions to proceed. [0008] Therefore, it is necessary to ensure the proper amounts, the proper positioning of the electrolyte inside the cell, and also some reserve in order to compensate for the expected consumption of the carbonates due to phenomena of corrosion, evaporation, and migration of the molten salts inside the stack (by stack is meant an array of cells arranged in series) .

[0009] All these problems have been coped with, during the years , in several manners .

[0010] One proposal to provide a suitable amount of electrolyte in the cell is cited in the patent EP 0 509 424 : it is proposed to manufacture a matrix tape with lithium aluminate particles mixed to carbonate powders and binders to be inserted between the electrodes . Upon the temperature rise during the stack turning on, the carbonates can melt "in situ" in the matrix and occupy the pores which have been set free by the binders, which

meanwhile have decomposed. Nonetheless, there is a limitation: the carbonate particles, unduly spacing the lithium aluminate particles out, result in too much large pores for a matrix which has to be able to hold by capillarity the carbonates therein in order to perform its function at the best.

[0011] In order to control the problem of the electrolyte consumption, U.S. patent No. 4,980,248 proposed to introduce a layer of a lithium-containing composite oxide in the area with an atmosphere of air. In the case of a change in the Li/K ratio, the compound would react with the environmental CO ₂ , thus releasing lithium carbonates among the other by-products, thus restoring the desired amount . [0012] A further idea is set forth in U.S. patent 4,538,348, which provides an anode impregnation which is achieved by positioning the powder carbonates on the finished porous component, and subjecting it to a thermal treatment which arrives to the melting temperature of the salts, so that they can enter the pores, -once liquid.

[0013] Nonetheless, this is a system which proves to be efficient only in the case of electrodes having a considerable thickness, because, on the contrary, these would not have the necessary capacity to hold a sufficient amount of carbonates so that the stack can

operate on a prolonged basis.

[0014] U.S. patent 5,468,573 provides for the electrolyte uploading on the current collectors in the form of a "paste" so that, upon a temperature rise, the carbonates melt and enter the matrix and electrodes.

[0015] This system allows providing a preset amount of reserve electrolyte. This paste is obtained by mixing the carbonate powders with glycerine to achieve a mixture having such a consistence and lubricity as to be capable of being laid on the current collectors without laterally leaking during the assembling of the stack or, more generally, of a cell.

[0016] Glycerine only serves as a carrier, not posing problems during the ticklish melting step, since it is removed upon heating at about 300 ⁰ C without leaving harmful residues .

[0017] A variation of U.S. patent 5,468,573 is cited in patent WO 03/073544, where it is explained how, in order to avoid using a flammable solution, such as glycerine, a slurry of the carbonate eutectic mixture is prepared by using water as the solution.

[0018] The electrolyte slurry is prepared with a mixer and coated into the channels of the bipolar separator plates. Therefore, in order to avoid the K ₂ CO ₃ solubilization, a controlled process of air drying is carried out. Only

once the carbonate mixture is well dried, the cell can be assembled.

[0019] The patent application WO 2005/096429 discloses the use of a mixture which is composed of glycerine and water in order to obviate the drawbacks occurring when using these two solvents separately.

[0020] The patent application highlights the importance of using a Li ₂ CO ₃ ZK ₂ CO ₃ carbonates eutectic mixture 62/38 composed only of the Li ₂ CO ₃ and LiKCO ₃ phases (salts that are much less water-soluble than K ₂ CO ₃ ) , so that the water percentage which is used in the mixture carrier does not solubilize the electrolyte.

[0021] Once the glycerine and water carrier solution is prepared, the electrolyte paste is obtained by adding the carbonate powder at room temperature; finally, the paste is coated on the current collector without further treatments .

[0022] This solution, even if it is undoubtedly very efficient, nonetheless has a drawback, in that it requires the use of a carbonate eutectic mixture comprising exclusively the two Li ₂ CO ₃ and LiKCO ₃ phases. [0023] In fact, if the mixture were "impure" and had K ₂ CO ₃ traces, this salt would be solubilized by the water in the carrier, thus forming compounds other than the carbonates in the electrolyte.

[0024] A further drawback of using the glycerine/water mixture has been observed during several conditioning tests, both on single cell and large-scale stacks. The glycerine in the carrier, during the first heating of the cell, has a difficult evacuation from the anode compartment, thus leading to the observation that the carbonates remain dirty with carbon residues (deriving from the glycerine itself) , therefore they can have a lower mobility inside the cell, they can wet less quickly the porous components, thus they can not completely fill the matrix in some cell areas where these residues tend to persist .

[0025] The problem underlying the present invention is to provide an electrolytic mixture comprising a carrier solution which can solve the drawbacks listed above. In particular, the carrier solution has to be usable with any carbonate eutectic mixtures and promote an improved filling of the fuel cell matrix pores. [0026] Such drawback is solved by an electrolytic mixture comprising a carrier solution as set forth in the annexed claims.

[0027] The detailed description of the invention which is reported herein below refers to the annexed Figures, in which: [0028] Fig. 1 shows the trend of the DSC analysis for the

electrolyte paste prepared with three different carrier solvents: glycerine/water mixture (of the prior art) , n- octanol, and 1,2-propanediol;

[0029] Fig. 2A-C shows the appearance of the current collectors after a start-up test interrupted at 530 ⁰ C; the electrolyte has been loaded with the 3 different solvents: Fig. 2A, Glycerine/Water; Fig. 2B, n-octanol;

Fig. 2C, 1, 2-propanediol .

[0030] DETAILED DESCRIPTION OF THE INVENTION [0031] The present invention relates to an electrolytic r mixture for fuel cells, preferably molten carbonate fuel cells, comprising a carbonate mixture and a carrier solution, comprising one or more organic solvents of formula (I) : Rl- (CH ₂ ) _n -R2

(D in which the- (CH ₂ ) _n - group is straight or branched; Rl and R2 are selected from the group consisting of -H or - OH, provided that Rl and R2 are never equal to -H at the same time; n is an integral ranging from 2 to 10; and in which the compounds of formula (I) have a boiling point less than or equal to 200 ⁰ C.

[0032] In a preferred aspect, the carbonates are present in the carbonate mixture in such stoichiometric ratios as to obtain the Li ₂ CO ₃ /K ₂ CO ₃ eutectic mixture in a 62/38 ratio.

[0033] In a further preferred aspect, the integral n ranges from 3 to 8.

[0034] In a further preferred aspect, the- (CH ₂ ) _n - group is straight . [0035] In a further aspect, the compounds of formula (I) have a boiling point of less than 200 ⁰ C.

[0036] The carrier solution must have a boiling point below or equal to 200 ⁰ C to ensure that, upon the electrolyte melting, the carrier has completely evaporated. [0037] Furthermore, among the solvents with a low boiling point, it is advisable to employ those with such risk phrases as not to be classified as irritating, corrosive, toxic, cancerogenic, and mutagenic substances, and having a low environmental impact. [0038] The electrolytic mixture of the invention preferably comprises from 50 to 90% by weight of electrolyte, and from 10% to 50% by weight of carrier solution. [0039] Preferably, the electrolytic mixture comprises from 70% to 80% by weight of electrolyte, and from 20% to 30% of carrier solution.

[0040] The carrier solvents which are preferred for the purposes of the invention are n-octanol (or n-octyl alcohol) and/or 1,2 propanediol (or propylene glycol).

[0041] The electrolytic mixture is prepared by mixing in a beaker, preferably at room temperature, the precise amount of the eutectic mixture 62/38 of Li ₂ CO ₃ /K ₂ CO ₃ carbonates and one or more organic solvents of formula (I) so as to have an electrolyte concentration ranging between 50% and 90%, preferably between 70 and 80%. [0042] The powder of eutectic mixture is gradually added by continuously stirring the paste which is being formed. [0043] At this point, the thus formed electrolyte paste is coated on the current collector at room temperature, so as to make the carbonate mixture enter all the component channels . [0044] Once the operation is completed, the collector is turned and placed on the electrode through an surface not coated by the electrolyte paste.

[0045] Preferably, the carrier solvent used to form the electrolyte paste is n-octanol and/or 1, 2-propanediol . [0046] Therefore, a further object of the invention is a current collector comprising the electrolytic mixture according to the invention.

[0047] Another object is a molten carbonate fuel cell comprising the above-mentioned current collector.

[0048] EXPERIMENTAL PART

[0049] In order to show the effectiveness of the two solvents under examination, thermal analyses have been carried out with the differential scanning calorimeter (DSC) to observe the behaviour of the carrier solvent inside the electrolyte paste. From Fig. 1, it is possible to notice that glycerine, although having a boiling point of 290 ⁰ C, interacts with the electrolyte powder and is completely removed within 450 ⁰ C (under the conditions of the DSC analysis, with a heating of 20 °C/min in air) , while : [0050] - n-octanol evaporates, without interacting with the carbonates, within 200 ⁰ C without leaving residues;

[0051] - 1,2-propanediol evaporates, within 250 ° C-300 ⁰ C without interacting with the carbonates and without leaving residues. [0052] After these preliminary analyses, single cell tests have been performed during which the exhausted gases have

been monitored in order to observe the carrier solvent emission:

[0053] - Glycerine/water: it has been observed the emission of glycerine and the combustion and/or decomposition products thereof (such as CO, CO ₂ , acetone, and water) ; [0054] - n-octanol: it has been observed the emission of the n-octanol molecule, confirming that this solvent does not burns, but evaporates, during the first heating of the cell; [0055] - 1, 2-propanediol : it has been observed the emission of the 1, 2-propanediol molecule, without combustion and/or decomposition products of the carrier solvent. [0056] Finally, single cell tests have been carried out which were interrupted at 530 ⁰ C, a temperature at which the carrier solvent should be completely removed, and the electrolyte loaded in the current collector should have been molten and migrated inside the porous components, leaving the collector clean (Fig. 2) : [0057]- Glycerine/water (Fig. 2. a): it is possible to notice how the current collector is covered by a black powdery layer which, through DSC analysis, shown to be an electrolyte eutectic mixture not yet migrated from the collector to the porous components with carbon residues deriving from the incomplete combustion of the glycerine; [0058]- n-octanol (Fig. 2.b): the current collector, after

the cell opening, shows to be completely clean and free from electrolyte powder and/or carbon residues left from the carrier solvent; [0059]- 1,2-propanediol (Fig. 2.c): the current collector appears as clean, with a slight white-grey coat of electrolyte powder. [0060] ADVANTAGES

[0061] The use of one or more organic solvents of formula (I) allows improving the removal of the carrier solvent from the electrolyte paste, therefore from the cell compared to the carrier solvents used in the prior art. [0062] Furthermore, the organic solvents of formula (I) do not solubilize the electrolyte carbonates (in any phases they are) and are not hazardous for the operators intended to the stack assembling.

[0063] The carrier solvents of the invention evaporate at temperatures below or equal to 200 ⁰ C and, differently from the glycerine of the mixture used in the prior art, do not interact with the carbonates, and are removed from the cell within 300 ⁰ C, well below the carbonate eutectic mixture melting point (490 ⁰ C) .

[0064] As a result, the electrolyte, after the carrier solvent removal, is free from carbon residues that may inhibit the mobility thereof and, once the melting temperature has been reached, is able to be melted and to

migrate inside the porous components of the cell, distributing according to the porosimetric distributions of the anode, cathode, and matrix. This results in an improvement of the cell performance.