Chemistry: electrical current producing apparatus – product – and – Deferred action type – Responsive to addition of liquid
Patent
1993-12-03
1995-06-27
Kalafut, Stephen
Chemistry: electrical current producing apparatus, product, and
Deferred action type
Responsive to addition of liquid
429188, 429238, H01M 464
Patent
active
054278716
DESCRIPTION:
BRIEF SUMMARY
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 consumable anode have been known for a long time. One example is described in the International Patent Application No. WO 89/11165 (O. 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 degradation. Additionally, seawater batteries with magnesium anodes alloyed with aluminum, zinc or manganese pose no safety hazards, 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: with magnesium anodes are environmentally harmless.
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 or expanded metal as described in PCT/N090/00056 (O. Hasvold 5-1-1) have been used as cathodes in seawater cells.
The reduction of oxygen consists of the following steps: surface 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 together with hydrogen carbonate ions. The cathode reaction causes a pH increase at the cathode surface and if high enough, this will lead to precipitation of calcium and magnesium salts e.g.:
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 calcareous layers is detrimental as the cell power will be reduced. As seawater is nearly saturated with calcium carbonate, formed 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 layers 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 of 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 cells 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
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Garshol Tor
Hasvold Oistein
Forsvarets Forskningsinstitutt
Kalafut Stephen
Nuzzolillo M.
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