GB1579747A - Galvanic cell - Google Patents

Description

(54) A GALVANIC CELL (71) We, C. ONRADTY NURNBLRG GMBH & CO. K.G., a company of the Federal Republic of Germany, of 8505 Rothenbach a.d. Pegnitz, Grunthal, Federal Republic of Germany and WOLF GANG PWTRULIA, a citizen of the Federal Rupublic of Germany, of 100 Berlin 19, Kastanienallee 15, Federal Republic of Genalany, do hereby declare the invention for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following Statement: The invention relates to a galvanic cell comprising a liquid cathode reactant.

Some known galvanic cells are of a kind made up of a cathode reactant which is silver chloride and a magnesium anode in sea water (Euler, J. Silbevchlorid-Fulleleme,lte Elektretechnische Zeitshrift Ausgabe B. 14, (1962) Pages 587 and 588). Silver chloride cells of the aforementioned kind have been found palticularly useful as power sources for incandescent bulbs, which are used e.g. with the cell to provide distress signals at sea or as signal sources in life-jackets.

In another known kind of cell, copper (I) chloride is used as the cathode reactant in combination, with a magnesium anode. Cells with zinc anodes are also known.

A feature of these cells is that the electrolyte does not enter the galvanic cell until operation starts. When such cells are used to provide distress signals at sea, the electrolyte consists of sea water. After it enters the cell, the reaction yielding electric energy continues until the cathode reactant is used up. When the magnesium dissolves, electrons are released and hydrogen is also evolved.

The operating voltage of cells using silver chloride and magnesium is about 1.3 - 1.35 V.

Since the electrolyte is not stored in the system, the energy content is relatively high and can reach values of 40 Wh/kg and energy densities of 70 Wh/Q are possible.

Magnesium is an extremely advantageous anode material in view of its availablity as a raw material and the absence of environmental pollution. There is also no difficulty in using sea water as the electrolyte, but the wide use of silver chloride (i.e. the chloride of a noble metal) for relatively large cells and batteries is made difficult by the critical raw material situation and the relatively high equipment costs. Consequently, the possibilities of other cathode reactants have been considered, thougl cells using silver chloride and magnesium are used for special applications even in large, very high-power units.

Atmosphere exygen is an obvious choice for the cathode reactant. Accordingly, galvanic cells have been developed from a porous carbor electrode and a zinc anode, wherein ammoniun chloride forms the electrolyte after adding tap water. Cells of this kind have a high energy content of 260-300 Wh/kg and a high energy density of 230 - 250 Wh/Q (without allowing for the addition of water) at an operating voltage of 1.05 - 1.2 V, but they can deliver only relatively weak currents.

In newly-developed magnesium and air cells the electrolyte is e.g. 3% to 10% sodium chloride solution using sea water, inland surface water or tap water. These cells are activated only on starting, by adding water. The anode metals are magnesium alloys, selected in accordance with the required current load. The atmospheric oxygen used as the cathode reactant is reacted at specially-developed catalyst electrodes comprising e.g. silver or nickel lattices and platinum or silver catalysts.

These catalyst electrodes are relatively expensive but have a long service life. Clearly the cathode reactant (atmospheric oxygen) is available in unlimited quantities.

Sodium chloride or similar salt solutions present no difficulties as electrolytes, aprt frorr undesirable metal corrosion.

Magnesium and air cells of this kind can yield operating voltages of 1.3 V with an energy content of 100 to 120 Wh/kg at a relatively low energy density. They are made suitable for repeated use by adding to the anode and the electrolyte. Efficient batteries of five magnesium and air cells have already been constructed. Such cells or batteries are very advantageous, with regard to the previously-mentioned raw-material and environmental considerations. The use of simple salt solutions as the electrolyte is safe, even when there is direct contact with the cells or batteries. This advantage is in contrast to known efficient electrochemical power sources using very acid or very alkaline electrolytes. and also applies to the other previously-discussed systems in which the electrolytes are aqueous salt solutions. These magnesium and air cells, however, have a serious disadvantage in that the catalyst electrodes for reacting the atmospheric oxygen are complicated and expensive, since they require noble-metal catalysts and a special manufacturing technique. In addition, when commercial magnesium alloys are used as the anode metal the rest potentials are relatively low i.e. - 1.65 to 1.7 V.

In order to ahviate these disadvantages.

pcroxydisulphates. also called persulphates, were tested for suitablity as a cathode reactant.

The research started from the fact that the redox potential for the reaction S2 0s - + 2c - > 2SO4 is +2,01 V relative to the normal hydrogen electrode, measured in alkaline solution. In spite of this vciy high positive redox potential and the Ii higli oxygen content, ordinary perslllpllates are relatively harniless More particularly. ammonium persulpIlate is manufactured in considerable quan t ties and frequently used in industry as an oxidiving agent. Potassium persuIpIl;lte and sodium persulphate are also used. It is also known to use persulphates in electrochemical power source (Botor I.A. and Wijnen M.D. Pliilips Energiepapier Technische Rnndschau, 28, (I ()(7 ) Page 201 . In this case it was proposed to use a mixture of potassium persulphate, carbon powder and paper fibres as the cathode material, the electrolyte being paper impreg- named with sodiuin cllI()ride and then moisten- cd. and the anode metal being niagnesiuni or zinc. The paper layers containing potassium persulpliate and sodium chloride can be con sot meted as foils and combined witli a discharge foil to fonii a sandwich.

This known system, however, difters from cells of tlie aforementioned kind in that it does not contain the cathode reactant in dissolved flirni and is thus not easy to empty and re till. Fort herniore its output is relatively low but its construction is fairly complicated particularly in relatively large units.

Finally, it is known to use higlinergy water-soluble salts, e.g. potassium permangan- ate, as the cathode reactant, but in such cases it is difficult to keep the solution away from the anode this can be done only by inserting diaphragms. which considerably increases the internal resistance of the cell. It has also been proposed (United States Patent Specification No. 2,636,851) Storm F.V. Elektrochemische Stromerzeugung", (1969) Page 56, to insert cation exchangers between the permanganate solution and the zinc anode in order to use dissolved cathode reactants.

The above noted disadvantages are over come according to the invention by a galvanic cell, comprising a negative electrode of a nonprecious metal and an aqueous solution of a peroxydisulphate as positive active substance in combination with highly porous carbon material, characterized in that the aqueous solution of a peroxydisulphate is simultaneously the electrolyte of the cell.

The galvanic cell of the invention, in which the cathode reaction is displaced into the discharge electrode, fulfils the requirements with regard to operating and manufacturing costs, compatibility with the environment and availability of the required raw materials. It is an advantageous feature of the invention, the use of highly porous carbon material, compacted to a given ektent, for the reaction and discharge cathode has given particularly good results in conjunction with carbon mou Ided bodies for derivation of current. The cathode can have any desired shape, i.e. plates, tubes, cylnders or the like. The pore size distribution and density of the entire cathode layer are important parameters. Very good results are obtained with a highly porous, flexible mat-like material.

It is advantageous to use anodes of magnesium alloys or iron alloys known per se advantageously tulle anodes are dimensioned so that after the dissolved reactants have been used up and removed, fresh reactant solution can be repeatedly introduced into the cell, thus appreciably increasing the service life, even during intermittent operation.

Particularly good results have been obtained with sod iii iii or lithium peroxydisulphate, which can be dissolved by using de-ionized water in the case of sodium or lithium peroxydisulphate. and also surface water, tap water or sea water in the case of sodium peroxydisul paste. Ammonium peroxydisulphate is less suitable, owing to the risk of ammonia being evolved in the alkaline region. Potassium peroxydisulphate is less soluble and give lower energy.

Advantageously the cell can be provided with a device for continuously supplying the reaction evolution, if discontinuous operation is not desired, in which case the spent solution can be continuously withdrawn from the cell.

A number of such galvanic cells can be combined in known manner in batteries, by being connected in series and in parallel.

The invention will now be described in greater detail, with reference to the embodiment given by way of example only and shown in the figures of the accompanying drawings, in which: Figure 1 is a longitudinal section through an empty box-shaped cell comprising a cover and a bottom part; Figure 2 is a cross-section through the bottom part of Figure 1 on section station 2-2 in Figure 1; Figure 3 is a cross-section of the cell in Figure 1, after the anode, cathode and carbon charge eliminators have been inserted; Figure 4 is a plan view of the cover of the box-shaped cell in Figure 1; Figure 5 is a longitudinal side view of the opened cell in Figure 1 showing the anode, and Figure 6 is a longitudinal side view of the opened cell in Figure 1, showing the cathode and carbon charge eliminators.

An important feature of the novel galvanic cell is that the aqueous persulphate solution used as the liquid cathode reactant also serves as the electrolyte solution. Usually it is advantageous to prepare a persulphate solution with pure, preferably completely de-ionized water, and pour it into the cell. Since the concentration of the persulphate in the aqueous solution should be high, sodium persulphate is used, since it has the highest solubility in water among commercial persulphates. Since lithium persulphate is even more soluble in water, it is also used. The operating voltages vary with the external resistance. It has been shown that useful cells, in which the working voltages of two volts decrease only slightly until the persulphate is used up, can be con- structed if the external resistance is in a certain ratio to the internal resistance.

The action of persulphates in aqeuous solution in an efficient cell containing no additional electrolyte, even when completely de-ionized water is used as the cathode reactant, can be explained as the result of a number of reaction steps. For example, dry sodium persulphate can be kept without decomposing, but in aqueous solution it decomposes to form hydrogen sulphate, evolving oxygen. The decomposition rate is dependent on temperature, i.e. persulphate radials can be destroyed by boiling, but at room temperature the decomposition is so slow that the efficiency of the cell is not affected.

It is assumed that the incipient decomposition leads to the starting reaction for dissolving a base anode, since HSO4 - ions are evolved.

This assumption is supported by the fact, for example, that the pH of a 1 m sodium persulphate solution in de-ionized water is about four. The corrosion of the anode proceeds with evolution of hydrogen. The corrosion in a persulphate solution is more vigorous than in a sodium sulphate solution. Owing to their high positive redox potential, persulphate anions have a strong tendency to absorb electrons and change to disulphate or suphate anions, the pore system of the cathode serving as the reaction space. The required electrons are rapidly supplied if a sufficiently base metal is used as the anode. Magnesium in the form of alloys, as used e.g. for cathode protection, is particularly suitable. For electrochemical reasons, it is impossible to prevent hydrogen being evolved at the same time as the magnesium is converted into magnesium cations. It is known to call such anodes "mixed anodes". If the cell are not too large, hydrogen can safely escape. In the case of larger cells the hydrogen can be collected and used. It can be reacted in known manner by catalytic combustion or by electrochemical oxidation, e.g. with an auxiliary electrode.

As a result of the evolution of hydrogen, magnesium hydroxide is produced; depending on the magnesium alloy and the current load, the magnesium is deposited at the anode or sinks to the bottom of the cell. Hydroxides of other alloy constituents such as zinc and maganese are likewise deposited. In combination with metal particles of anode material, these hydroxides form a sludge at the bottom which is removed with the spent electrolyte.

During operation, the resulting solution in the cell gradually approaches a pH value of eight and the evolution of hydrogen decreases.

When the cell delivers current, heat is produced by the action of the anode. The temperature rise is dependent on the current load and may be about 50"C.

If it is desired to reduce the amount of hydrogen evolved in the cells, less base anode metals can be used. Since, for the above-mentioned economic and raw-material considerations, the metals and alloys used must be available in very large quantities, the most likely substance apart from magnesium is iron.

Experiments with zinc have shown that this metal is usually attacked too violently by concentrated persulphate solutions, whereas aluminium is normally less suitable, owing to its lower activity.

If a galvanic dry cell having an anode of a conventional sheet steel ST 37 is used the rest potential is generally only 1.0 to 1.1 volts. When the cell is loaded with an external resistance designated for prolonged continuous operation of the cell, the operating voltage is 0.8 volt with a very advantageous discharge characteristic. When a relatively large sheetsteel anode is used, the cell can be repeatedly regenerated by replacing the spent sodium persulphate solution without any loss of efficiency. The spent reactant can be removed and the cell can be refilled continuously, using suitable devices, and the anodes can also be continuously replaced as used, thus considerably increasing the service life of such galvanic cells.

The best results are obtained with cells containing anodes made of magnesium alloys of the aforementioned kind. With regard to the concentration of sodium persulphate in water, 1 m solutions are preferred, although higher concentrations are possible, in the presence of solid phases if required. Higher concentrations given higher power, but there is a risk of forming crystalline deposits, particularly when the distance between the electrodes is small. Clearly, lower concentrations may also be used.

As shown in Figures I to 3, the cells can be constructed in the form of flat box-shaped elements 1, but round cells are also possible, thotigli not shown the housing is e.g. a poly ethyletie plastics vessel 3 measuring 160 x 140 x 40 mm closable by a cover 2 which, as shown in Figure 4, has recesses 8 in its top for two carbon discharge bars 7 for the cathode 6. In Figure 3 the bars are shown in section and are in conductive contact with the surface of cathode 6. Cover 2 has two other recesses 9 for two tongues 10 of the magnesiulll anode S which, is shown in Figure 5, project over tlic plastics vessel 3 and extend through recesses 9 in the cover. Cover 2 also has an aper ture 11 for filling the cell. In the operating state, aperture 11 remains open and allows the evolving hydrogen to escape and can also be used to remove spent solutions.

The cathode measles 120 x 130 x 5 mm and the two discharge bars 7 connected thereto each mcasure 50 x 200 x 5 mini. The anode measles 130 x 190 x 2.5 nini . 672 nil of a I m sodium peroxydisulpilate evolution is poured into vessel 3. The rest potential is 2.35 V. When tlie load is an outer resist atice RA of 2.5 ohms, the opelatillg voltage lea is I .73 volts tlic operat iiig current IA is 0.635 aiiip aiicl t the intcrnal resist a ice Rt is 0.976 ohm.

The box-shaped cell was operated continuously for 78 hours via an external resistance of 2.5 ohms during the day and 20 ohiiis during the night. Do ring operation at 20 ohms the operating voltage lJA was I 1.9 volts and t the operating ctirrciit IA was 0.092 amp. A total energy of 29.84 Wh was delivered, corres ponding to an energy content of 28.1 5 Wh/kg.

The calculation was based on the total weight of the cells. Since. however, in such a cell, the required water does not need to he added until just before operation. the energy content in practice is 69.55 Wh/kg. The temperature rose to 350C during the first hours. After 78 hours the power of the cell decreased considerably; consequently the spent solution was rciiinvcd and replaced by an equal quantity of fresh I m sodium persulphate. after which the efficiency was substantially the same as at the beginning.

The cell can he refilled with active solution until the magnesium anode has become too small. The cathode lasts for an unlimited period.

Consequently, depending on t the dimensions of the anode, the dry cell can be operated for a very long tinic, and without interruption if the reactant solution is continuously replaced. A new anode can be inserted when required.

Cells adapted to a heavier load and more frequent regeneration can be produced by reducing the distance between the cathode and anode, e.g. in round cells using a magnesium cylinder as the anode. Experiments have shown that a round cell of the aforementioned kind containing e.g. 500 ml of 1 m sodium persulphate solution, when operated with an external resistance of 1.2 ohms, delivers an operating voltage of 1.88 to 1.62 volts for six hours, with current intensities of 1.25 to 1.1 amps. A similar cell operated with 0.6 ohm, delivers 0.725 to 1.57 volts for 4 hours at currents of 2.35 to 2.2 amps.

A battery of two round cells connected in series, each containing 220 ml of I m sodium persulphate sol tit ion, delivers voltages of 4 to 3.6 volts and currents of 0.8 to 0.7 amp for 3 hours and 20 minutes when operated at 4.7 ohms.

The energy yield can be considerably increased by small admixtures of polar organic compounds such as alcohols having a chain length of C2 to Cs or sulphones such as dimethyl sulphoxide in concentrations up to 5%.

The energy yield is also dependent on the composition of the anode material, the size and spacing between the electrodes and the external load. An energy content up to approx. 50 Wh/ kg can be obtained if the weight of the added water is also taken into account. If the water is not taken into account, the energy content can be up to 160 Wh/kg.

The cells of the invention have important advantages over conventional known cells, such as a simple construction of the cathodes, the use of known metal alloys as anodes, and the possibility of economically manufacturing either small cells or large, high-powered cells and batteries. A high energy content can be obtained. particularly in view of the fact that the inserted water does not have to be stored in the cells. The cells can easily be regenerated by exchanging or rcnewing the reactant solution and anodes. If the anodes are suitably dimen sicined, tlie cells can be operated for long perriods simply be renewing the spent reactant solution, , so that continuous operation is possible if the cells are provided with suitable devices. The hydrogen evolved at the anode can be safely discharged form relatively small cells, whereas in large cells it can be destroyed or otherwise used. The reactant will keep for an unlimited period when dry, and is relatively harmless in spite of the high redox potential and the high oxygen content. The reactant solutions are only slightly acid or alkaline; they are neither poisonous nor corrosive and may therefore be safely carried about. The spent solutions are also substantially without adverse environmental effects. The high content of mineral salts relatively large quantities are discharged. Finally, the substances for making the cells are produced from raw materials avialable in practically unlimited quantities and tlie cathodes produced from carbon materials can be used for an unlimited period, so that in this respect also, the cells according to the invention are advantageous electrochemical power sources.

WIIAT WE CLAIM IS:- 1. A galvanic cell, comprising a negative electrode of a non-precious metal and an

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (9)

**WARNING** start of CLMS field may overlap end of DESC **. As shown in Figures I to 3, the cells can be constructed in the form of flat box-shaped elements 1, but round cells are also possible, thotigli not shown the housing is e.g. a poly ethyletie plastics vessel 3 measuring 160 x 140 x 40 mm closable by a cover 2 which, as shown in Figure 4, has recesses 8 in its top for two carbon discharge bars 7 for the cathode 6. In Figure 3 the bars are shown in section and are in conductive contact with the surface of cathode 6. Cover 2 has two other recesses 9 for two tongues 10 of the magnesiulll anode S which, is shown in Figure 5, project over tlic plastics vessel 3 and extend through recesses 9 in the cover. Cover 2 also has an aper ture 11 for filling the cell. In the operating state, aperture 11 remains open and allows the evolving hydrogen to escape and can also be used to remove spent solutions. The cathode measles 120 x 130 x 5 mm and the two discharge bars 7 connected thereto each mcasure 50 x 200 x 5 mini. The anode measles 130 x 190 x 2.5 nini . 672 nil of a I m sodium peroxydisulpilate evolution is poured into vessel 3. The rest potential is 2.35 V. When tlie load is an outer resist atice RA of 2.5 ohms, the opelatillg voltage lea is I .73 volts tlic operat iiig current IA is 0.635 aiiip aiicl t the intcrnal resist a ice Rt is 0.976 ohm. The box-shaped cell was operated continuously for 78 hours via an external resistance of 2.5 ohms during the day and 20 ohiiis during the night. Do ring operation at 20 ohms the operating voltage lJA was I 1.9 volts and t the operating ctirrciit IA was 0.092 amp. A total energy of 29.84 Wh was delivered, corres ponding to an energy content of 28.1 5 Wh/kg. The calculation was based on the total weight of the cells. Since. however, in such a cell, the required water does not need to he added until just before operation. the energy content in practice is 69.55 Wh/kg. The temperature rose to 350C during the first hours. After 78 hours the power of the cell decreased considerably; consequently the spent solution was rciiinvcd and replaced by an equal quantity of fresh I m sodium persulphate. after which the efficiency was substantially the same as at the beginning. The cell can he refilled with active solution until the magnesium anode has become too small. The cathode lasts for an unlimited period. Consequently, depending on t the dimensions of the anode, the dry cell can be operated for a very long tinic, and without interruption if the reactant solution is continuously replaced. A new anode can be inserted when required. Cells adapted to a heavier load and more frequent regeneration can be produced by reducing the distance between the cathode and anode, e.g. in round cells using a magnesium cylinder as the anode. Experiments have shown that a round cell of the aforementioned kind containing e.g. 500 ml of 1 m sodium persulphate solution, when operated with an external resistance of 1.2 ohms, delivers an operating voltage of 1.88 to 1.62 volts for six hours, with current intensities of 1.25 to 1.1 amps. A similar cell operated with 0.6 ohm, delivers 0.725 to 1.57 volts for 4 hours at currents of 2.35 to 2.2 amps. A battery of two round cells connected in series, each containing 220 ml of I m sodium persulphate sol tit ion, delivers voltages of 4 to 3.6 volts and currents of 0.8 to 0.7 amp for 3 hours and 20 minutes when operated at 4.7 ohms. The energy yield can be considerably increased by small admixtures of polar organic compounds such as alcohols having a chain length of C2 to Cs or sulphones such as dimethyl sulphoxide in concentrations up to 5%. The energy yield is also dependent on the composition of the anode material, the size and spacing between the electrodes and the external load. An energy content up to approx. 50 Wh/ kg can be obtained if the weight of the added water is also taken into account. If the water is not taken into account, the energy content can be up to 160 Wh/kg. The cells of the invention have important advantages over conventional known cells, such as a simple construction of the cathodes, the use of known metal alloys as anodes, and the possibility of economically manufacturing either small cells or large, high-powered cells and batteries. A high energy content can be obtained. particularly in view of the fact that the inserted water does not have to be stored in the cells. The cells can easily be regenerated by exchanging or rcnewing the reactant solution and anodes. If the anodes are suitably dimen sicined, tlie cells can be operated for long perriods simply be renewing the spent reactant solution, , so that continuous operation is possible if the cells are provided with suitable devices. The hydrogen evolved at the anode can be safely discharged form relatively small cells, whereas in large cells it can be destroyed or otherwise used. The reactant will keep for an unlimited period when dry, and is relatively harmless in spite of the high redox potential and the high oxygen content. The reactant solutions are only slightly acid or alkaline; they are neither poisonous nor corrosive and may therefore be safely carried about. The spent solutions are also substantially without adverse environmental effects. The high content of mineral salts relatively large quantities are discharged. Finally, the substances for making the cells are produced from raw materials avialable in practically unlimited quantities and tlie cathodes produced from carbon materials can be used for an unlimited period, so that in this respect also, the cells according to the invention are advantageous electrochemical power sources. WIIAT WE CLAIM IS:-

1. A galvanic cell, comprising a negative electrode of a non-precious metal and an

aqueous evolution of a peroxydisulphate as positive active substance in combination with highly porous carbon material, characterized in that the aqueous solution of a peroxydisulphate is simultaneously the electrolyte of the cell.

2. The galvanic cell according to claim 1, characterized in that the carbon material forms a reaction and derivation layer of a flexible, highly conductive, mat-like carbon body, which for the pupose of derivation of current can be combined with configured bodies of compressed carbon.

3. The galvanic cell according to claim 1 or 2, characterized in that the negitive electrode is dimensioned so that it can be used for several fillings of the cell with a solution of a peroxydisulphate.

4. The galvanic cell according to anyone of the claims I to 3, characterized in that as solution for a peroxydisulphate lithium peroxydisulphate within de-ionized water is used.

5. The galvanic cell according to anyone of the claims 1 to 3, characterized in that the peroxydisulphate is sodium peroxydisulphate.

6. The galvanic cell according to claim 4 or 5, characterized in that the concentration of the peroxydisulphates in water is from 0.1 molar to saturation.

7. The galvanic cell according to anyone of the claims 1 to 6, characterized in that alcohols having a chain length of C2 to C5 in concentration up to 5% are added to the evolution of a peroxydisulphate .

8. The galvanic cell according to anyone of the claims 1 to 6, characterized in that dimethyl supphoxide in concentrations up to 5% is added to the solution of a peroxydisulphate.

9. A galvanic cell constructed and arranged substantially as described and shown in the figures of the accompanying drawings.