CA1324812C

CA1324812C - Salt water cell

Info

Publication number: CA1324812C
Application number: CA000598366A
Authority: CA
Inventors: õISTEIN HASVOLD
Original assignee: Alcatel STK AS
Current assignee: Equinor ASA
Priority date: 1988-05-02
Filing date: 1989-05-01
Publication date: 1993-11-30
Anticipated expiration: 2010-11-30
Also published as: DK261790A; BR8907415A; EP0415957A1; NO881914L; EP0415957B1; NO881914D0; AU632749B2; DK261790D0; JPH04500428A; NO164324C; WO1989011165A1; DE68909785D1; JPH0821391B2; AU3546989A; NO164324B

Abstract

Abstract The invention relates to primary galvanic cells (batteries) for use with salt water electrolytes, such as sea water. The cell has a rod like anode made preferably of a magnesium alloy, and a cathode consisting of a stainless steel substrate coated with a catalyst for the reduction of oxygen.
Examples of catalysts are cobalt spinel, cobalt nickel spinel and active carbon. The cell can be constructed with the cathode coaxially arranged around a rod like anode. The catalysts, which are the basis of this invention, have the ability to stimulate the reduction of oxygen that is dissolved in salt water. This stimulation of oxygen reduction will lead to an increased voltage of the galvanic cell at a given current load and in addition contribute to an increased loadability when the catalytic active biofilm is lacking.

Description

~` 1324812 The present invention relates to primary galvanic cells (batteries) for use with salt water electrolytes, such as sea water. The present invention is a substantial improvement compared to prior art devices, as ~ill be ex-plained in the following.
One prior art device is described in US Patent 3.401.063.
` Here a sea ~ater battery, consisting of an annular basket, containing metal wool, serving as the cathode of the cell, : and a cylindrical metal anode, positioned in the central cavity, is described, having as its main feature the abilitytn generate electrical energy for time length of several - years. Another important feature of this electrochemical ~_ cell is the ability to generate the electr~cal energy ` economically, both in terms of cost per kilo~atthours and in ~atthours per kilograms. ~ith magnesium as anode material and ~steel ~ool~ as cathode material, it produces long-term output ~oltages in the 0.35 to 0.7 Y bracket.
The main limitations with respect to output voltage of the prior art cell described ln the US Patent 3.401.063 is the corrosion of the cathode material. In addition to limiting ; the output voltage, corrosion on the Usteel wool" may lead to disintegration of the cathode material during the life time of the cell or when stored in humid air.
In order to avoid corrosion on the cathode material, the cell cathode must be polari2ed continuously, i. e. the cell ~ ~ust del~ver a certain minimum output current to cathodi-: cally protect the steel from corrosion, irrespec~ive of the po~er demdnd from the user. This mode of operation ~ncreases the probability of premature degradation of the cell due to calcareous deposition on the cathode material, and decreases the practical energy output of the cell.
By carefu!ly selecting the quality of the cathode material it is possible to avoid the main limitations of the sea ~ater battery described above. These limitations can be omitted by us~ng stainless steel cathodes, as mentioned in Russian Patent no 559307. This improved prior art device is not susceptible to corrosion neither in storage condition nor in operation. In said Russian Patent the output cell voltage .
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` ~ i 5 approximately 0.9 Y.
However by carefully selecting the qual~ty of the sta~n-less steel cathode material and by optimising cell con-struction this improved pr)or art device should be potentially able to deli~er output voltages up to ca 1.8 V. This would probably improve the energy density of the cell by a factor of between two and three.
One example of a stainless steel quality that fulfills the ` requirements, i. e. a stainless steel quality that ~s electro-; 10 chemically passive with respect to anodic corrosion or has a very low level of corrosion during operation of the sea water battery, is a quali~y that is normally denoted AISI 316 by the American Iron and Steel Institute. However any other stainless steel ~uality, with the term stainless steel defined as ferrous material that contain mor~ than 10 wtS chromium and at - least 50 ~tS iron. can, in principle, give ~mpro~ements compared to the device described in the US Patent 3.401.063.
These i~proved prior art de~ices, however, when submerged ` in natural sea ~ater, sho~ an inferior performance during an initial period after exposure to the sea water. After this initial period, the device shows an improvement in ~ performance ~hich is due to the formation of a biologic film `" on the surface of the stainless steel cathode, a film which is catalytically active ~ith respect to reduction of oxygen.
The duration of this initial period will typically be in ; the order of one week to one month, depending on several '; factors, the most important being cathode current density and ~ater flo~ veloclty. In this lnitial period the galvanic cell ~ill have inferior behaviour w~th respect to loadability and cell voltage compared to the same cell after the formation of the biologic active f~lm.
- The cell voltage in this ~nitial per~od is very dependant ~; on the current density on the cathode. At very low cathode loads, for ~nstance 3 mA per m2 cathode area, the cell voltage is in the order of 1.2 V, increasing to approxi-mately 1.7 V after the initial period. At moderate loads of 50 mA/m2 the cell voltage is in the order of 0.7 V during the initial period, but very much dependant on water flow ., .
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` 1 3248 1 2 velocity, while the cell voltage after one month exposure 1s ~ 1.4 to 1.65 Y depending on cell design and water flow velocity. At higher loads the initial cell voltage 1s even lower than 0.7 V. At the same time the durat~on of the initial period increases with increasing load, and ~f the load is too high, the cell will not reach this higher level of performance.
The inferior performance in the initial period of the above specified improved prior art devices, when used in natural sea ~ater~ may have as a consequence that the device cannot give sufficient energy to the equipment which ~t is supposed to power in this period. Another consequence may be that for uses ~ith duration which is shorter than this initial period, the device must be ~ncreased in weight and volume to compensate for the inferior performance to such an extent ; that it is not practical to use due to the weight, volume or cost constra~ns of the system.
~ hen used in crdinary salt water, meaning water conta~ning sodium chloride or potassium chloride with the amount of other const~tuents not defined, or in artificial sea water, for ~nstance according to DIN S0010, the formation of an catalytic active biofilm will not occur. This meuns that ~ith the use of this type of electrolyte the pr~or art galvanic cell will have a poor performance even after exposure to the electrol~te for a time longer than the above mentioned ln~tial period~ `
The present inventlon has as its main obJect,`as compared to the prior art dev~ces described above, to prov~de a device having the ab~lity to deliver nom~nal power from the time of exposure ln nearly any kind of salt water. The ob~ect is also to prov~de a substantlal improvement in performance of galvanic cell~s having stainless steel cathodes dur~ng the ~nitial period of operation.
Another object of the invention is to enable ~he galvanic cell to be used in ordinary salt water, or in artificial sea water, and to improve the performance of the cell to such an extent in this environment that it is comparable to the same cell used ~n natural sea water after the in~tial period.

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--` 1 3248 1 .2 Another object of the invention, when used wlth natural sea water, is to i~prove the reliability of the galvanlc cell, both in the initial period and afterwards. As mentioned above the reason for iDprovement after the initial perlod is the formation of a biofilm on the stainless steel surface. This film contains certain living organisms, the presence of which results in the ability to catalyze the reduction of oxygen in sea water. These ~ organis~s are susceptible to destruction if the lmmediate environment is changed, for instance by a sudden chanqe in load from zero to a high cathode current density, or if toxic material is brought in contact with the cell. The destruction of these organis~s results in te~porary reduced catalytic aativlty of the cathode. The ob~ect of t~e present inventlon is to a large extent to reduce t~e dependence of these living organisms, thereby - increaslng the relia~ility of the cell.
- Still anot~er object of the present invention is to enable t~e sa~e galvanic cell to be used several times, and each time of short or lonq duration. Between operations the cell should be capable of being removed fro~ sea water and stored for a non-specified length of time. When submerged in sea water after a temporary storage, t~e cell should lmnediately regain lts Dode of operation.
~ ccording to a broad aspect, the lnventlon provlde~ prlmary galvanlc cell comprising a salt water electrolyte, a metal anode, and a cathode consisting of a stainless steel substrate coated -- with a catalyst layer containing a cobalt oxide or cobalt spinel, for reduction of oxygen in the electrolyte.
Thl~ catalyst has the features which are described below.
Firstly, the catalysts, which are the basis of this invention, have the abillty to stinulate the reduction of oxygen t~at i8 dlssolved in salt water. Thls stimulatlon of oxygen reductlon wlll lead to an increased voltage of the galvanic cell at a given current load and in addltion contrlbute to an lncreased loadability when the catalytic actlve biofilm ls lacking.
Secondly, t~e catalysts are chemlcally and electro-chemically stable in the environment and under the conditions .. , ~

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13248~2 :`
- ~hich are experienced during the operat~on of the galvan~c cell.
Thirdly, the catalysts are mechan~cally stable and are ~ell attached to the substrate, in such a manner that they are not susceptible to shedding, blistering or other modes of detachment between the catalyst layer and the substrate during normal operation or handling of the cell.
Furthermore, the catalysts do not contain any precious metals, nor do they involve costly production processes, which significantly contributes to an increase in total ; cathode cost.
Furthermore, the catalysts do not contain tox~c mater~al, ; ~~' or produce such material, which may reduce the ability of the cathode to form the above described catalytic active biofilm on the cathode surface. ~his means that the catalysts which are the basis of this invention, do not lead to any : degradation of performance during operation after the initial period.
The introduction of these catalysts makes the operation of the galvanic cell in principle independent on the formation of catalytic active biofilm, although a slight decrease in performance is observed if the biofilm is removed from the surface or if the cell is operated in artificial sea ~ater.
The preferred substrate, on which the catalysts are ` applied, ~ust have a ~ery high degree of corrosion resistance, since the electrochemical potentials experienced by the cathode are very high~ ~orrosion of the substrate would lead to lower cell voltage due to higher local cathode current density, since the total cathode current density is the sum of current density due to external load and the corrosion current density. One type of stainless steel alloy that has sho~n sufficient corrosion reslstance to be suitable as a substrate for the present catalysts contain approximately 178 Cr, 128 Ui and 2.58 Mo, and are commonly denoted AISI
316. Other molybdenum containing stainless steel alloys will also be suitable for this purpose.
The preferred substrate may be present in either filamen-5~

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132~812 tary form, such as stainless steel wool, or ln sheets or plate~ in various forms or shapes.
The catalyst may be Dade by applylny a solution of cobalt nitrate diluted in alcohol, for lnstance ln l~opropyl alcohol, to t~e stainless steel substrate, and with a subsequent heat treatment of between 270 and 600C in oxidizlng atmosphere for a time length of 15 ~inutes to 24 hours.
- Alternat~vely a ~lxture of cobalt nitrate and nickel nitrate may be used instead of cobalt nltrate alone. The mixture 0 i8 dlluted in alcohol and heat treated as specified above. Tbe molar ratio betueen nicXel nitrate and cobalt nitrate should not exceed lsl if the good catalytic activlty shall be maintained .
Also other types of metallic elements other than nickel may partly substitute ~obalt to form a spinel structure that is more catalytic actlve to reduce oxygen dissolved in salt water than the bare stainless steel surface. Exa~ples of such elements that in co~bination Nith cobalt may create spinel ~tructures, are iron and alu~iniu~. Also other elements than those mentioned above ~ay to a certain extent be added to the cobalt spinel structure and still maintain the preferable properties of this lnventlon.
In order to further clarify the invention and the lnprove~ents that follo~ from the inventlon a~ compared to prior art devlces, reference will be made to the following figures.

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Figure 1 illustrates an example of use of a galvanlc cell, containing a stainless steel cathode without the use of any pre~ade catalyst, together with a magnesium anode and using a natural sea water electrolyte. The figure show~s the cell voltage versus ti~e at a constant current output, Figure 2 shows the results from two similar experiments as in Figure 1, but using cathodes with a cobalt containing catalyst applied to the surface of the stainless steel cathode substrate, Figure 3 shows the results from a similar experiment as in Figures 1 and 2, but with a catalyst contalning active carbon applied to the stainless steel cubstrate~ and Fiqures ~ to 9 lllustrate three possible cell deslgns, using the cathode catalysts ~hlch are the ba6is of thls lnvention.
To further illustrate the invention, references are made ; to the followlng exa~ples-Exa~ple ls A galvanlc cell, consisting of a cylindrical magnesium anodo and a stainless steel cathode, which conslsted of elght f 20 parallelly arranged square plates, each with a circular hole in the center of the platesS to glve space to the central anode. The plates had an indlvldual spaclng of 15 ~m, and each plate was 0.5 ~ ti~es 0.5 m and 1 mm in thlckness. The central hole had a dlaoeter of 0.2 ~. The cathode plates ` ' ~
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" :-- 1324812 were mechanically secured by using four parallel stainless steel rods, which were welded to each corner of the ptates and extending through all of the plates. At the end of these rods two end plates of an electrical insulating material were placed in order to secure the anode. The anode had a diameter of 0.14 m and a length of approximately 0.2 m.
Proper electrical connections to the metal anode and the cathode were made.
~: The anode material consisted of approx1mately 6~ aluminium and 3X zinc as the main constituents, apart from magnesium.
The cathode consisted of a stainless steel alloy of type 254 SM0 and was manufactured by the Swedish company Avesta AB.
This alloy contains approximately 20 ~ Cr, 18 S Ni and 6X Mo as its main constituents, apart ~from Fe. No further - l~ pretreatment was done to the stainless steel cathodes in this experiment.
~ he described galvanic cell was submerged in natural sea water in such a manner that the parallelly arranged cathode plates were orientated vertically and axis of the cylindri-cal anode orientated horizontally in the water. ~he sea " water had a temperature in the range of 8-12 C and the flow rate ~as in the order of l cm per second. ~he cell was put on a constant current load of 50 mA per m2 cathode area and the corresponding cell voltage was periodically registered.
Figure l shows the cell voltage during the first 90 days of operation. As will be seen from the figure, during the first 20 days the cell volt~ge ~s low, 1 e approximately 0.7 V
at the spec~fic load of 50 mA/m2. After 20 da~s the cell voltage lncreases to a plateau of approximately 1.4 V and 20 days later a new plateau ~s reached at approximately 1.7 Y.
~he ~ncrease in cell voltage which is observed after 20 days ~s due to the active biological film which is formed on the surface of the cathode, as discussed above in this descrip-tion. ~he reason for this two-step increase ln cel-l voltage ~s not known.
Dur~ng the first 20 days this cell is delivering less than half the amount of power compared to the power output after 40 days of operation at the specified cathode current :` ~
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density. In addition the loadability in this per~od is substantially lcwer as compared to after the initial period.
Example 2;
A galvanic cell of similar construction as in Example 1 ~as made, but differing from that cell by letting the cathode plates go through a pretreatment to increase the catalytic activity with respect to reduction of oxygen dissolved in the sea water during the ~nitial period of cell operation.
The pretreatment consisted of applying a thin layer of cobalt nitrate diluted in isopropyl alcohol after thoroughly : cleaning of the cathode surface. The concentration of cobalt ~ in this ~ixture ~as 0.2 moles per litre. After dryin~
the cathode plates were placed in a furnace at a temperature of approximately 400 C and kept there for one hour. The atmosphere in the furnace during the heat treatment was air.
The cell was placed under similar experimental conditions as in Example 1. Figure 2 shows the cell voltage during the first 40 days of operation at a specific cathode constant ; 20 current load of 50 mA/m2. As can be seen the cell voltage is initially approximately 1.5 V, as compared to 0.7 V for the ` prior art device shown in Example 1. After one week of operation the cell voltage increased from 1.5 V to 1.77 V.
~he cell voltage rematned at this high cell voltage for a period of seven ~onth ~hen the experiment was terminated.
The lncrease in cell voltage after the first week of operation shows that the catalyst which is the bas~s of this : ~nvent~on do not ~nhibit the formation of the active biofilm ~`~ on the cathode surface. In fact the duration of the in~tial ` 30 period seems to have been shortened by introduction of the : applied catalyst.
Also the loadability during the initial period is greatly increased by the lntroduction of the cobalt spinel type of catalyst. Furthermore, if the above described biofilm should be destroyed, only a slight decrease in performance would be observed, corresponding to a cell voltage reduction from approximately 1.7 V to 1.5 Y.
Furthermore, the power output from the galvanic cell in , ;-.'""' ~

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` 1324812 the initial period is increased by a factor of approx~mately , two by introducing the cobalt sp~nel catalyst.
Example 3:
Another experiment was performed by uslng a galvanic cell 5 of identical construction and cathode pretreatment as in Example 2. Except for a specific cathode current load of 100 mA/m2 the experimental conditions were ~dentical as in the example mentioned above.
As can be seen in Figure 2 the cell voltage shows a 10 similar behaviour as in the previous experiment. As can be ` expected the cell voltage is somewhat lower throughout th~s experi~ent because of the higher current load. Uowever even at ~_ this high cathode current load of 100 mA/m2 the cell vo1tage ~s approx~mately 1.45 Y during the initial period, which 15 is between two and three times higher than what should be expected from a comparable prior art device. This illustrates the increase in performance, both in terms of cell voltage, loadability, power output and reliability during the initial period, which is a direct result of 20 the present invention.
; Example 4:
Three stainless steel plates of type 254 SM0 were tested ~n water containing 0.5~olar sodium chloride. The first plate was catalyzed according to the pretreatment spectfied in ~` 25 Examples 2 and 3. The second plate was catalyzed by applying a thin layer of a mixture of cobalt nitrate and nickel ~ nitrate diluted ~n lsopropyl alcohol, and with a subsequent 2 heat treatment at 400 C for one hour. The molar ratio between the cobalt nltrate and the n~ckel nitrate was 2:1.
30 The third plate was an uncatalyzed plate.
The plates were tested in an electrochem~cal cell w~th a magneslum counter electrode. The electrolyte was c~rculated through the cell using a peristaltic pump. The electrolyte ` temperature was approximately 20 C. The exposed area of the 35 cathode plates were 3.5 cm2. The plates were tested potentiostatically. lhe corresponding cell currents were registered.
Table 1 shows the cell voltage and the corresponding cell . ~

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currents of the three different etectrochemical cells. The measurement was, made 20 hours after exposure to the electrolyte. The cell with the cobalt spinel cathode catalyst is denoted Cell~l. the cell w~th the cobalt-n~ckel S spinel cathode catalyst is denoted Cel1#2, and the cell with no catalyst is called Cell~3.

- . Cell ~ Cell#l . Cel1~2 . Cel1*3 . voltage .Cell current . Cell current. Cell current .
10 . IY) (mA) (mA) . (mA) .
. 1.25 . 5.2 . 14.0 . 0 1~
.
Table 1 The performance of three galvanic cells using cobalt spinel, cobalt-nickel spinel and no catalyst, respectively.
'' As can be seen from Table 1 the cell current after 20 hours of exposure to the salt water shows higher values for both the cells with catalyzed cathodes, as compared to the cell with no catalyst.
' Example 5:
In this example a sea water galvanic cell of identical cell construction as in Example 2 and 3, but with a cathode catalyst containing active carbon as the active component, ~as used.
The catalyst ln this experiment was made by applying a ~' thin layer of a mixture of active carbon and a polyvinyl chloride binder on a thoroughly cleaned stainless steel substrate. The mixture consisted of one part Iby weight) - A active carbon,of type Norit SX Ultra, manufactured by Norit ~` ~^ Activated Carbons, Holland and two parts of a polyvinyl ;~ chloride binder of type Tangit. To this mixture methylene dichloride was added to give the mixture a viscosity suitable for application of the mixture on to the stainless steel substrate. In this experiment this amounted to ten parts of methylene dichloride ~by weight) for each part of the ,~Jc~
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~324812 .
~ carbon-polyvinyl chloride mixture.
- After the application of the catalyst material on to the substrate, the material was allowed to solidify by evaporation of the volatile ingredients at a temperature of 60C.
The cell was submerged in natural sea water and exposed to the same experimental conditions as in Example 3, i. e. 100 mA/m2 specific cathode current load.
~ Figure 3 shows the cell voltage as a function of operation time of the galvanic cell, analogue to what was shown in Figure 2. This galvanic cell shows similar behaviour as in the two previous examples with a premade catalyst layer on the cathode surface. However, a slightly lower performance as compared to the cobalt spinel catalyst, is observed, although this may be due to a less optimised pretreatment for the active carbon catalyst~
In Figures ~ and 5 are schematically illustrated one embodiment of the invention. Figure 5 shows a cut through lines Y-Y in Figure 4, whereas Figure 4 shows a cut through lines IY-I~ in Figure 5~ A cathode 1 consisting of stainless steel ~ool ~ith a catalyst as specified above, is confined between two sta~nless steel grids 2 and 3 and a coaxially arranged anode 4 is supported within the cathode 1 by insula-tion ~eans 5 and 6. ~he cell output is taken from a cable 7 `~ 25 ~hich vla conneetor units 8 and 9 are connected to the cathode ` 1 and anode 4. At 10 and 11 are ~ndicated suspension means by wh~ch the cell may be installed vertically in the sea water.
~ The use of the ~nvention is not limited to one specific ` cell deslgn, for ~nstance the use of catalyzed stainless steel ~ool as cathode material, as exemplified above. There is a large range of possible ways of assembling a sea water cell, ~ by using the cataly2ed cathode material which is the basis of `~ this invention, to give the improvements compared to prior art devices, as stated above.
To lllustrate this, one alternative cell arrangement is schematically illustrated ~n Figures 6 and 7. Figure 7 shows a cut through lines VII-VII in Figure 6, whereas Figure 6 shows partly a cut through lines YI-VI in Figure 7. Here, the cell .~;
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consists of a central cylindrical metal anode 12 which may be a magnesium alloy, an aluminium alloy, a zinc alloy or a lith~um alloy, ~l e. any metal in the galvan1c series that is negative in respect to the stainless steel.
The cathode 13 in Figures 6 and 7 consists of numerous ; parallelly arranged plates 14 of a stainless steel material and with a catalytic active layer with the character~stics `~ stated above. In the center of each plate, ~hich may have a rectangular or circular shape, there ~s an aperture 15 giv~ng space for the ~etal anode 12. Each plate is attached to two or more rods 16, preferably in the same material as the cathode plates 14, in such a manner that good electrical contacts are provided bet~een the plates 14 and the rods 16. Electrical connection of the cathode is provided by contact means 17. At 1~ the ends of the anode/cathode assembly, there are two end plates 18 and 19, made from a material th~t is electrically insulating, and that ~ill enable mechanical security of the anode/cathode assembly. In one end of the anode, pro~isions 20 are made to allow current collection from the anode 12.
; 20 Mounting ~eans 21 and 22 ~re indicated.
~ In Figures 8 and 9 are illustrated an alternative way of ~~ making a salt ~ater cell having a plate cathode 29. Figure 8 ; sho~s a cut through lines VIII-VIII in Figure 9, whereas Flgure 9 shows a cut through the lines IX-IX in Figure 8 (the rod 35 ~s ho~e~er omitted in Figure 9). A number of catalyzed stalnless steel plates or sheets 30 are mounted in a radial ~;` fashion relatively to a rod ltke anode 31. The cathode plates 30 are mounted between two end plates 32 and 33 and the components are assembled by rods 34 and 35. Supporting arrangements and electrical connections are not shown.
~i It must be emphasl~ed that the above mentioned designs of the salt water cell should onty be taken as examples of three . possible arrangement of the constituents of the cell, and is merely a ~ay of ~llustrating the invention at handr rather than specifying the limitation of the invention. ~hile stainless steel wool or plates is considered the preferred catalyst substrate material for the cathode and magnes~um alloys the preferred anode material, other combinations of .

``-``~ 1324812 :
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~^ metals may be used to improve the performance of the galvanic cell as compared to prior art devices. It is ~- therefore understood that certain modifications, alter-nations and substitutions ~ay be made without departing from the scope of the present invention. The cathode plates -. can for instance be perforated metal plates.

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Claims

1. Primary galvanic cell comprising a salt water electrolyte, a metal anode, and a cathode consisting of a stainless steel substrate coated with a catalyst layer containing a cobalt oxide or cobalt spinel, for reduction of oxygen in the electrolyte.

2. Cell according to claim 1, wherein the catalyst is applied by coating the cathode substrate with a solution containing cobalt nitrate followed by heat treatment of the cathode substrate at a temperature of between 270 and 600°C.

3. Cell according to claim 1, wherein the catalyst 15 a cobalt nickel spinel.

4. Cell according to claim 3, wherein the molar ratio between cobalt nitrate and nickel nitrate contained in the cobalt nickel spinel is more than 1.

5. Cell according to claim 3, wherein the catalyst is applied by coating the cathode substrate with a solution containing a mixture of cobalt nitrate and nickel nitrate followed by heat treatment of the cathode substrate at a temperature of between 270 and 600°C.

6. Cell according to any one of claims 1 through 5, wherein the anode is rod-like and the cathode consists of filamentary catalyzed material coaxially arranged around the rod-like anode.

7. Cell according to any one of claims 1 through 5, wherein the cathode consists of a number of parallelly arranged catalyzed stainless steel plate or sheets.

8. Cell according to any one of claims 1 through 5, wherein the anode is rod-like and the cathode consists of a number of catalyzed stainless steel plates extending radially relatively to the rod-like anode.

9. Cell according to claim 1, wherein said salt water electrolyte is sea water.

10. Cell according to claim 1, wherein said metal anode consists of a magnesium alloy.

11. Cell according to claim 1, wherein the anode is rod-like and the cathode consists of catalyzed stainless steel wool coaxially arranged around the rod-like anode.