GALVANIC SEAWATER CELL

Abstract: This invention relates to galvanic seawater cells and batteries and in particular to cathodes which are suitable for use in galvanic cells that use an oxidant dissolved in the electrolyte as depolarizer. An example of such cells are seawater cells which use the oxygen dissolved in the seawater as oxidant. The cell has an inert electrode which consists of a number of conducting fibers (1, 3, 14) connected to a conducting body (2, 4, 15). The fibers (1, 3, 14) have different orientations relatively to each other and to the body (2, 4, 15). In a preferred embodiment the electrode body (2, 4) consists of two or more wires which are twisted together to constitute an electrode stem while clamping the fibers (1, 3) in fixed positions between the wires, as in a laboratory bottle brush.

Description: Galvanic Seawater Cell The present invention relates to galvanic seawater cells and batteries and in particular to cathodes which are suitable for use in galvanic cells that use an oxidant dissolved in the electrolyte as depolarizer. An example of such cells are seawater cells which use the oxygen dissolved in the seawater as oxidant. Low power seawater cells with inert cathodes which use the oxygen dissolved in the seawater as the oxidant and a consum¬ able anode have been known for a long time. One example is described in the International Patent Application No. WO 89/11165 (0. Hasvold 3). These cells have been successfully operated in the sea for more than two years. They use an inert metal cathode (titanium or stainless steel) which is coated with a catalyst that catalyzes the reduction of oxygen. In batteries, the cells are connected in parallel, and the cell voltage is converted to a useful value by a DC/DC converter. The consumable anode can be any electronegative metal or alloy made of e.g. lithium, magnesium, aluminum or zinc. Seawater batteries are attractive as they have a very high energy density which compares favorably with the best batteries presently available. As these batteries use the sea both as electrolyte and oxidizer, they can be stored in a dry condition for a very long time without any notable degrada¬ tion. Additionally, seawater batteries with magnesium anodes alloyed with aluminum, zinc or manganese pose no safety haz¬ ards, as they are neither flammable nor do they contain any elements which are considered toxic to the environment. The chemical reactions in a typical seawater battery with magnesium anodes are: 2 g = 2 Mg ++ + 4 e~ (anode reaction) O2 + 2 H2O + 4e~ = 4 OH- (cathode reaction) 2 Mg + 2 H 2 0 + 0 2 = 2 Mg++ + 40H- (cell reaction) As magnesium ions are present in the seawater already, seawa¬ ter batteries with magnesium anodes are environmentally harm- 1 ess. The cathodes are not influenced by the cell reactions, therefore a discharged battery can be recharged mechanically by inserting new magnesium anodes. If deteriorated by biofouling or calcareous deposits on the cathodes, cathode cleaning may be necessary. Whereas the energy content of the battery is limited by the amount of anode material and the cell voltage under load the power from practical batteries is determined by the rate of oxygen reduction. This rate is limited by the cathode surface area available and by the transport of oxygen to cathode surface. In previous designs, loosely packed metal wool coated with a catalyst as in the mentioned WO 89/11165 expanded metal as described in PCT/N090/00056 (0 Hasvold 5-1 1) have been used as cathodes in seawater cells. The reduction of oxygen consists of the following steps: 1 The transport of oxygen by convection and by diffusion to the electrode surface 2 The electrochemical reaction at the electrode surfac 3 The transport of the reaction products away from the electrode surface The more efficient the transport steps are, the more oxygen i available for the cathode reaction and the lower is the pH increase at the catalyst cathode surface. The importance of minimizing the pH increase at the surface is caused by the need to avoid calcareous deposits in and on the cathode surface. Seawater contains magnesium and calcium ions togethe with hydrogen carbonate ions. The cathode reaction causes a p increase at the cathode surface and if high enough, this will lead to precipitation of calcium and magnesium salts e.g.: Ca ++ + HCO3- + 0H ~ = CaC0 3 + H 2° and Mg ++ + 2 0H~ = Mg(0H) 2 These reactions are considered beneficial in cathodic protection of metal structures in seawater against corrosion as the calcareous layer decreases the current necessary to protect the structure. In seawater cells, formation of calcar eous layers is detrimental as the cell power will be reduced. As seawater is nearly saturated with calcium carbonate, forme deposits do not redissolve. To ensure a high surface area of the cathode, the inert metal can be in the form of wool sandwiched between two laye of metal net or expanded metal, as mentioned above. The packing of the wool is loose to ensure a low resistance to convection (flow through) and thus to provide an free flow o fresh, oxygen rich seawater through the cathode structure. Additionally, the cell must have an open structure to allow free access of fresh seawater and to get rid of the reaction products formed. If such a cell is used close to the water surface under conditions of strong wave action, the wool structure may be mechanically destroyed unless a very stiff quality is used. Also, close to the surface, biofouling (for example of algae barnacles and mussels) w ll be a problem which increases wit time unless copper or a copper alloy is used as cathode. The corrosion of copper will normally leak sufficient amount of copper ions to function as an excellent antifouling as long the cathode potential is not lowered too much from the free corrosion potential of the metal. As the corrosion potential of copper is low compared to the potential of catalyzed cathodes under load, the cell voltage of copper based seawat cells is roughly not more than 1.0 V compared to 1.6 V for cells using catalyzed stainless steel. Copper is, however, a ve r soft metal and copper-wool cathodes do not have sufficient strength to be used under rough conditions. Metal plate cathodes may be used, but unless the diffusion layer i disrupted by for example perforation of the plates, thereby reducing the characteristic length in the direction of the water flow, the maximum current density they can support, is 1 ow. One parameter which is used to describe material transpo is the limiting current density. This is the current density where the concentration gradient of the electroacti e reactan has its maximum, i. e. when the reactant concentration is zer at the electrode surface. It is well known from textbooks in electrochemistry that the limiting current density increases with increasing reactant concentration, electrolyte velocity and decreasing size of the electrode. For laminar flow over a flat plate, the mass transport limiting current density for plates is according to a textbook by J Yetter: "Electrochemical Kinetics", published by Aca¬ demic Press, New York 1967, proportional to the reactant concentration and the square root of the flow velocity and inversely proportional to the square root of the length of the plate in the direction of flow. Thus it is obvious that the extension of the electrode in the direction of flow should be as small as technically possi¬ ble. Further, the resistance to flow should be low to allow a high flow velocity within the cathode. This can be achieved with cathodes made from expanded metal or metal net. The expanded metal sheet(s) may be coiled into a stiff structure as described in the international application PCT/N090/00056. If the cathode consists of wires which are so thin that their thickness is comparable to the thickness of the diffusion layer, cylindrical diffusion will further increase the mass transport limiting current density and thus the maximum cur¬ rent density the cathode can support. The problem with cathodes made from thin wires or fibers is their lack of mechanical strength and stiffness. In the form of wool, the fibers will tend to stick together and even if the structure is initially loose with a low resistance to flow through the structure, hydrodynamic forces from wave action and sea currents together with the gravity force will break up the structure and if placed in a metal net basket, the fibers will end up compressed in the bottom of the basket. The object of the present invention is to provide a galvanic cell which has an improved cathode structure as compared to known cathodes. The main features of the invention are defined in the claims. The invention takes advantage of the high specific surface area and the high limiting current density of thin fibers. The fibers are separated from each other and surrounded by free flowing seawater. (If the fibers stick together, as in a woven cloth or a math, only the outer surfaces will be supplied with fresh oxygen containing seawater). At the same time, good electrical contact is maintained. Thus the ideal geometry of the fibers is considered to be similar to that of a common laboratory bottle brush. Above mentioned and other features and objects of the present invention will clearly appear from the following detailed description of embodiments of the invention taken i conjunction with the drawings, where Figure 1 shows the principles of a cathode fiber structure, Figutas 2 and 3 show an example of a cathode 'bottle brush' structure, Figure 4 shows a top view of a cell having a 'brush' cathode, Figure 5 and 6 show performance curves for the Figure 4 eel 1 , Figure 7 shows a top view of a prototype cell usable for sonobuoys, Figure 8 shows a compartment cell structure, and Figures 9, 10 and 11 show alternative cathode embodiments Figures 1, 2 and 3 show electrodes having a bottle brush structure. Here the fibers 1 and 3 are radially spread out which gives fresh seawater free access to the whole fiber surface. Current collection from the fibers is done through a electrode body or stem 2 or 4 which is made from metal wire. The metal wire in the stem may be made of any conducting allo which is stable at the potentials of interest. Typical materi als are titanium, stainless steel, silver or copper. The stem 2,4 consists of two or more wires which are twisted together to constitute an electrode stem while clamping the fibers 1,3 in fixed positions between the wires. The conducting wires 2,4 are twisted in such a manner that they form a double helix, while clamping the fibers 1,3 as in a laboratory bottle brush. The wires 2,4 may be twisted in a SZ fashion, i e one part of the stem is twisted on one direction, another part(s) in the other direction. The fibers may also be made from any conducting material which is stable in seawater, for example copper or silver alloys (for use where biofouling is a problem), catalyzed stainless steel, titanium and/or carbon, in any combination. Of particular interest are carbon fibers as carbon is a well known catalyst for the reduction of oxygen. This catalytic activity can be further increased by surface modification as described in the book "The electrochemistry of carbon", edi¬ tors: Sarangpani S, Akridge J R, and Schumm B. The Electrochemical Society Inc. (1984). Carbon fibers, including graphite fibers, have a high modulus of elasticity which make even thin fibers fairly stiff. This stiffness allows brush cathodes to keep their shape in the moving seawater. A carbon fiber cathode according to Figures 1, 2 and 3 wa made from a yarn consisting of 3000 fibers, each with a diameter of 7000 nm. The stem was made from stainless steel wire. After the production of the 'brush', the 'brush' was treated with a solvent to remove the sizing of the yarn fibers and heat treated in air to separate the fibers from each other and to improve the catalytic activity of the fiber surface. Each cathode had a diameter of 30 mm and a height of 150 mm. In a test cell, the cathode brush 5 was surrounded by four magnesium anodes 6, as shown in Figure 4, which shows a top view of the cell. The performance of the cell (Cell Voltage ( V) versus Current (mA)) is shown in Figure 5 which was measured in a location with strong sea currents and high seawater salinity. Even so, the performance is considered exceptionally good compared to cells made with catalyzed stainless steel wool cathodes of similar size. Figure 6 shows the performance of the cell (Cell Voltage ( V) versus time (h)) at constant load- in a location with lower salinity and less current. The load was 250 mA. The dips in the cell voltage are caused by periods with nearly stagnant water. Figure 7 shows the top view of a typical electrode configuration for a seawater cell which may be usable for sonobuoys. The cell consists of twelve anodes 7 and nine cath¬ ode elements 8. The height of the electrodes are 150 mm and the cell can be confined in a cylinder with 150 mm diameter. The cell capacity (when the anode diameter has been reduced by 50%) is 600 Ah and a typical cell load is 2.7A or less corresponding to a discharge time of nine days or more. Increased power and capacity can be achieved simply by increasing the length of the electrodes. The cathode length o 150 mm is just convenient for prototype production, but cathodes of any length may be produced. The electrodes were all connected in the top and the connections covered with a polymer. Other cell geometries may also be realized with bottle brush cathodes, as for example a cylindrical cell somewhat like the inverse of Figure 4, having a central anode rod and number of parallel cathode brush elements arranged in an annulus around the anode Bottle brush cathodes may also be used in cells which are intended for vehicle propulsion. In this case, the direction of flow s known as either the pressure differences caused by the vehicles motion through the sea or dedicated seawater pumps are used to move the seawater through the cells. Also cell types as shown in Figure 7 may be used for traction, but as solid products of the cell reaction may collect in the cathodes, a cell as shown in Figure 8 may be preferred. In this cell , the seawater passes through a row of cathodes 9 before it enters the anode compartment 10. In this figure, th anodes 11 are in the form of parallel rods, but other shapes which offers low resistance to the flow such as perforated magnesium plates (grates) may also be used. If a higher power is needed than possible with the natural concentration of oxygen in seawater (approximately 0.3 mol/m-*), the oxidant concentration can be increased through the addition of a suitable oxidant such as hydrogen peroxide, oxygen or chlorin to the seawater. In order to reduce the size and cost of the DC/DC con- verter usually used in connection with the cells, seawater cells for traction are serially connected. Leakage current between cells is kept at an acceptable level through the use of inlet 12 and outlet 13 ducts for the seawater. The resistance of the ducts increases with duct length and decreases with the cross section of the duct. The cell has an enclosure which is open to the ocean in two opposing ends leading the seawater past the cathode at substantially right angles to the cell structure with forced convection. 'Bottle brush' cathodes are believed to be the best choi among 'brush' cathodes as their radial geometry makes the whole fiber surface easily accessible. With thin fibers, the fibers are moved by the moving water which makes their hydrodynamic resistance under conditions of strong current low, this is also the reason why they do not easily clog: large particles just slip through the cathode in contrast to flow through felt cathodes which filters the seawater. Last but not least, bottle brush cathodes are easy to produce. Other 'brush' geometries may also be used for cathodes in seawater cells. Examples are shown in Figures 9, 10 and 11. The 'brush' has the fibers 14 inserted into holes in a conducting structure 15. Although inferior to bottle brush cathodes of comparable size, these can deliver more current than a perforated plate of the same size. In general, the inert cathode (electrode) of the galvanic cell may consist of a number of conducting fibers 1,3,14 connected to a conducting body 2,4,15 in such a manner that a substantial part of the fibers are oriented in directions substantially different from the plane of the body at the connection between the fiber and the body. The fibers 1,3,14 may have different orientations relatively to each other and to the body 2,4,15. The fibers may have orientations which are within plus/minus 45 degrees of the normal vector to the plane or to the main axis of the body. Preferably each fiber should be oriented approximately perpendicular or normal to the surface of the body. Whereas usually the fibers will have the same length and diameter, a substantial part or some of the fibers 1,3,14 may have different lengths and/or different di ameters.

Claims: Patent claims 1. Galvanic cell including at least one inert electrode, c h a r a c t e r i z e d n t h a t the electrode consists of a number of conducting fibers (1,3,14) connected to a conducting body (2,4,15) in such a manner that a substantial part of the fibers are oriented in directions substantially different from the plane of the body at the connection between the fiber and the body. 2. Cell according to claim 1, c h a r a c t e r i z e d i n t h a t the fibers (1,3,14) have different orientations relatively to each other and to the body (2,4,15). 3. Cell according to claim 1 or 2, c h a r a c t e r i z e d n t h a t the fibers have orientations which are within plus/minus 45 degrees of the normal vector to the plane or to the main axis of the body. 4. Cell according to claim 1, 2 or 3, c h a r a c t e ¬ r i z e d i n t h a t each fiber is oriented approximately perpendicular or normal to the surface of the body. 5. Cell according to claim 1, 2, 3 or 4, c h a r a c t e - r i z e d i n t h a t a substantial part or some of the fibers (1,3,14) have different lengths and/or different diameters . 6. Cell according to claim 1, c h a r a c t e r i z e d i n t h a t the electrode consists of fibers (1,3,14) of carbon, copper, copper alloys, silver, stainless steel and/or titanium or any combination as to materials and numbers, connected to a metal body (2,4,15). 7. Cell according to claim 1, c h a r a c t e r i z e d i n t h a t the conducting body (2,4,15) consists of stainless steel, copper or a copper alloy , silver or titanium. 8. Cell according to claim 7, c h a r a c t e r i z e d i n t h a t the electrode body (2,4) consists of two or more wires which are twisted together to constitute an electrode stem while clamping the fibers (1,3) in fixed positions between the wires as in a laboratory bottle brush. 9. Cell according to claim 7, c h a r a c t e r i z e d i n t h a t the wires (2,4) are twisted in a SZ fashion, i e one part of the stem is twisted on one direction, another part(s) in the other direction. 10. Cell according to claim 1, c h a r a c t e r i z e d n t h a t it is provided with a number of such electrodes (8,9) which together constitute a cathode of the cell. 11. Cell according to claim 1, c h a r a c t e r i z e d i n t h a t the cell (Fig 4, Fig 7) is open to the sea and has one or more metal anode(s) (6,7) interleaved with one or more cathode elements (5,8) and which uses oxygen dissolved in the seawater as oxidant. 12. Cell according to claim 10, having one or more metal anodes and which uses seawater as electrolyte and oxygen dissolved in the seawater as oxidant, c h a r a c t e ¬ r i z e d i n t h a t both electrodes (cathode elements (8) and anode elements (7)) are arranged in one or more parallel rows and that the anode and cathode rows are alternating, forming a complete cell structure (Fig 7). 13. Cell according to claim 12, c h a r a c t e r i z e d i n t h a t the complete cell structure (Fig 7) is confined within a compartment having one electrolyte inlet channel and one electrolyte outlet channel so that the electrolyte may flow through the compartment. 14. Cell according to claim 12 or 13 , having a flow-through cell structure, c h a r a c t e r i z e d i n t h a t the electrolyte (seawater) is made to flow first through the ca- thode element(s) and then through the anode element(s) (Fig 8). 15. Cell according to claim 11, 12, 13 or 14, c h a r a c t e r i z e d i n t h a t the cathode (9) and anode (11) electrodes are arranged in at least two separate rows within a compartment (10). 16. Cell according to any of the claims 11-15, c h a r a c t e r i z e d i n t h a t the cell has an enclosure which is open to the ocean in two opposing ends leading the seawater past the cathode at substantially right angles to the cell structure (Fig 8) with forced convection. 1 * Cell according to claim 1, c h a r a c t e r i z e d i n t h a t the electrode body consists of a conducting matrix (Fig 9,10,11) into holes or slots of which the electrically conducting fibers (14) are inserted. AMENDED CLAIMS [received by the International Bureau on 29 June 1992 (29.06.92); original claims 1,8,11 and 12 amended; new claim 18 added; other claims unchanged (3 pages)] Patent claiβs (Three replacement sheets) 1. Galvanic seawater cell using seawater as electrolyte and oxygen, hydrogenperoxi de or chlorine dissolved in the seawater as oxidant and having at least one inert electrode consisting at least of a number of thin, el ectrochemical ly active fibers, c h a r a c t e i z e d n t h a t the fibers (1,3,14) are connected to a current conductive body (2,4,15) in such a manner that a substantial part of the fibers are extending freely from the surface of a conducting body without contact with each other and other parts of the cell. 2. Cell according to claim 1, c h a r a c t e r i z e d i n t h a t the fibers (1,3,14) have different orientations relatively to each other and to the body (2,4,15). 3. Cell according to claim 1 or 2, c h a r a c t e r z e d i n t h a t the fibers have orientations which are within plus/minus 45 degrees of the normal vector to the plane or to the main axis of the body. 4. Cell according to claim 1, 2 or 3, c h a r a c t e ¬ z e d i n t h a t each fiber is oriented approximately perpendicular or normal to the surface of the body. 5. Cell according to claim 1 , 2, 3 or 4, c h a r a c t e - r z e d i n t h a t a substantial part or some of the fibers (1,3,14) have different lengths and/or different diameters. 6. Cell according to claim 1, c h a r a c t e r z e d i n t a t the electrode consists of fibers (1,3,14) of carbon, copper, copper alloys, silver, stainless steel and/or titanium or any combination as to materials and numbers, connected to a metal body (2,4,15). 7. Cell according to claim 1, c h a r a c t e i z e d i n t a the conducting body (2,4,15) consists of stainless steel , copper or a copper alloy , silver or titanium. 8. Cell according to claim 7, c h a r a c t e z e d i n t a t the electrode body (2,4) consists of two or more wires which in a way known per se are twisted together to constitute an electrode stem while clamDing the fibers '1,3) in fixed positions between the wires as in a laboratory bottle brush. \1 9. Cell according to claim 7, c h a r a c t e r i z e d i n t h a t the wires (2,4) are twisted in a SI fashion, i e one part of the stem is twisted on one direction, another part(s) in the other direction. 10. Cell according to claim 1, c h a r a c t e r i z e d i n t h a t it is provided with a number of such electrodes (8,9) which together constitute a cathode of the cell. 11. Cell according to claim 1, c h a r a c t e r ! ' z e d i n t h a t the cell (Fig 4, Fig 7) which is open to the seawater has one or more metal anode(s) (6,7) interleaved with one or more cathode elements (5,8). 12. Cell according to claim 10, having one or more metal anodes, c h a r a c t e r i z e d i n t h a t both electrodes (cathode elements (8) and anode elements (7)) are arranged in one or more parallel rows and that the anode and cathode rows are alternating, forming a complete cell structure (Fig 7). 13. Cell according to claim 12, c h a r a c t e r i z e d i n t h a t the complete cell structure (Fig 7) is confined within a compartment having one electrolyte inlet channel and one electrolyte outlet channel so that the electrolyte may flow through the compartment. 14. Cell according to claim 12 or 13 , having a flow-through cell structure, c h a r a c t e r i z e d i n t h a t the electrolyte (seawater) is made to flow first through the ca¬ thode element(s) and then through the anode element(s) (Fi g 8). 15. Cell according to claim 11, 12, 13 or 14, c h a r a c t e r i z e d i n t h a t the cathode (9) and anode (11) electrodes are arranged in at least two separate rows within a compartment (10). 16. Cell according to any of the claims 11-15, c h a r a c t e r i z e d i n t h a t the cell has an enclosure which is open to the ocean in two opposing ends leading the seawater past the cathode at substantially right angles to the cell structure (Fig 8) with forced convection. 17. Cell according to claim 1, c h a r a c t e r i z e d i n t h a t the electrode body consists of a conducting matrix (Fig 9,10,11) into holes or slots of which the electrically conducting fibers (14) are inserted. 18. Cell according to claim 1, c h a r a c t e r i z e d n t h a t the fibers (1,3,14) are so thin and stiff that they bend elastically under hydrodynamic stress and show visible oscillations under conditions of normal water flow through the eel 1. STATEMENTUNDERARTICLE 19 The cited document, US,A, 3674564 (A. E. Ketler Jr. ) teaches a structure of coarse fibers to allow for free circulation of seawater within the cathode (col 4 line 23-33. Electrical contact to the fibers is assured by mechanical compression or by metal 1 urgical ly bonding the fibers. In both cases, a rigid (stiff) cathode structure is obtained. In contrast, the present invention as defined in the amended main claim, teaches use of extremely fine fibers which are able to bend under hydrodynamic stress, thereby improving the transport of oxygen to the surface and at the same time allowing particles (which would otherwise clog and degrade the cell) to slip through the structure. Ketler does not teach use of cathodes having movable fibers. Cathodes made from fibers in the form of felt, wool or scrap contained in a container (basket) made from expanded metal, metal net or equivalent, have little resemblance with the present invention. The cited document US, A, 1,447,657, (Paul Gouin et al ) describes different current collectors for use in alkaline storage batteries, including a description of a current collector in the shape of a laboratory bottle brush. The current collector is to be embedded in an active material (a solid phase) to make one (or both) electrode(s) in an electrochemical cell. Although geometrically similar, it should be recognized that the present invention describe the use of a cathode where the movable fibers form the el ectrochemically active electrode surface and are surrounded by a liquid phase (seawater). This is in contrast to the function in Gouin's battery system where nickel- or iron- oxide form the active electrode. The purpose of the brush structure in Gouin's cell is to ensure low ohmic resistance to the active material. End of statement

Inventors: GARSHOL TOR, HASVOLD OEISTEIN, GARSHOL, TOR, HASVOLD, OEISTEIN
Applicants: FORSVARETS FORSKNINGS, FORSVARETS FORSKNINGSINSTITUTT
Application year: 1992
Main classification: H01M6
IPC codes: H01M4, H01M6