Note: Descriptions are shown in the official language in which they were submitted.
~S33~3
The invention relates to a galyanic process and to an
anode for carrying the process into effect. ~ore particularly,
the invention relates to an improvement of the operation of a
~alvanic primary cell by the elimination of operation-inhibiting
or passivating phase interfaces at the electrodes. The invention
also provides a special anode for carrying the process into
ef~ect.
Problems are often associated with galvanic elements or
cells containing an electrode of metal, since the reaction pro-
ducts being formed in the electrode process tend to be preci-
pitated or remain on the electrode, for instance as metal oxides
or metal hydroxides, whereby the electrode is passivated and
the ef~ectivity of the cell is diminished.
The problem is a very pronounced one in metal/air or
metal~oxygen cells in which the anode is of iron and the electro-
lyte is an alkali solution, such as an NaOH or XOH solution,
because the iron ions formed at the anode are precipitated or
remain on the anode in the form of a coating which heavily re-
duces or precludes the continued function of the anode.
An issue of great interest therefore is to prevent in some
way the formation of the above mentioned coating on the elec-
trode to thereby ensure a steady and effective function of the
electrode.
With regard to the mechanism of the formation of the passi-
vating coating on the electrode, it is known,for some simple
cases, such as with anodes of zinc and cadmium in alkaline
RB/pm
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electrolyte, that the metal at the electrode reaction is dis~
solYed prior to the precipitation of the reaction-inhibiting
coating. Intermediate forms arising at the anode and being of
the type Zn(OH~+ and Cd(OH~ , respectively, can continue reac-
ting with OH according to
Zn(OH) ~ OH ~ Zn(OH)2
and Cd(OH) + OH ~ Cd(OH)2, respectively,
and the inhibition of the reaction occurs only when the solubi-
lity product for the respective metal hydroxide has been exceeded.
The intermediate forms can also yield a passivating oxide
according to
Zn(OH) ~ ZnO + H
and Cd(OH)+ ~ CdO + H+, respectively.
When the solubility is moderately exceeded, that is, at low
supersaturation, the rate of precipitation increases as the
supersa.turation increases. At a high supersaturation the rate
of nucleation is high compared to thë rate of the growth
of the nuclei, and the precipita~ion is inhibited. This is
very customary with hydroxides having a low solubility product.
Any method which at low supersaturation leads to a decrease of
the concentration of such intermediary hydroxide complex forms,
thus involves a lesser risk of precipitation on the electrode
surace. At a high supersaturation, however, a decrease of the
concentration involves that the rate of precipitation increases.
In the case of zink or cadmium the formation of a reaction-
inhibiting precipitation on the anode is delayed by vigorous
agitation, and at continuous supply of flowing fresh electrolyte
~ L~533Z3
an inhibition is entirely prevented.
For most other metals, including iron, this simple process
is not sufficient for an elimination of reaction inhibitions
which are due to the formation of such coatings. Besides, the
passivation usually arises already before the galvanic cell is
put into operation. The di~ference may be due to the respective
solubility products being much lower, or to the fact that the
reaction mechanisms in the dissolution and passi~ation of the
~ .
metal in question are different from those for zinc and cadmium.
As a matter of fact, passivating coatings on electrodes of iron
or of other metals, which give passivating coatings of a type
~i corresponding to that for iron cannot, however, be eliminated
in the same simple manner as has been described above for zinc
and cadmium, and, as far as is known, no simple and effective
solution has been found to this problem before the present in-
~'; vention came into being.
In addition to the problem of passivating coatings discussed
above, a further problem is associated with metal/air or
metaljoxygen cells, which is that of supplying air or oxygen to
the cathode. In conventional electrodes of this type, air is
supplied, usually under pressure, to the cathode where the oxygen
ls reduced in the presence of a catalyst under formation of
hydroxide ions in the electrolyte. The effectivity of the re-
duction of the oxygen at the cathode, that is,the cathode reac-
tion, is limited at high current density by the rate at which
the oxygen is able to diffuse into the active surface of the
cathode, that is, by the transport of the oxygen. The oxygen
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transport and the cathode reaction are inhibited by the oxygen
being present in the gaseous phase, while the electrolyte is
liquid and the electrode solid. To improve the electrode reac-
tion of the cathode it is desirable to make the oxvgen transport
more effective by an increased flow of oxygen, such as an
increased supply rate of the oxygen or an increased concentration
of the oxygen supplied.
A further special problem inherent in oxygen electrodes for
metal/air cells is their complicated and critical construction
which to an essential degree is due to the fact that the reduc-
tion of the oxygen at the cathode reaction is a three-phase
reaction (gaseous oxygen, liquid electroly~e and solid electrode),
and that the three-phase interface must be steadily maintained.
To this end, the conventional oxygen cathodes are made porous,
the pores having precisely selected dimensions, and moreover,
the surfaces of the electrodes are made hydrophobic at least on
the gaseous side and/or a counter-pressure is placed on the
gaseous side. It goes without saying that such oxygen cathodes
are difficult and expensive in manufacture and that it would
imply a considerable simplification and improvement if the
oxygen could be supplied in concentration, dissolved in a li~uid,
in which case it would not be necessary to maintain a three-phase
interface, and moreover, other problems would be eliminated,
such as the nitrogen present in the air and tending to poison
the catalyst material in conventional oxygen cathodes.
It will clearly appear from the above outline that there
are essential problems associated both with passivating coatings
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~5~3~
on iron type electrodes and with the oxygen supply to oxygen
electrodes. Both of these problems ste~ from the passivating or
inhibiting phase interfaces at the e]ectrodes.
The object of the present invention therefore is to eliminate
such passivating or inhibiting phase interfaces, which, to put
it briefly, is realized by complexing, with the aid of complex-
ing agents, the substance which gives rise to the passivating
or inhibiting phase interfaces, and dissolving said substance.
More specifically, the invention provides a galvanic process
in the operation of a galvanic primary cell which comnrises
an electrolyte and two electrodes consisting of an anode and
a cathode which are interconnected via an outer circuit for
tapping electric energy, phase interfaces occurring at at least
one of the electrodes which interfaces inhibit the operation of
the electrode. The invention comprises contacting the electrode
when the phase interfaces are formed by an operation-inhibiting
electrode coating which includes an electrode product formed
during operation, with a first complexing agent which, in opera-
tion, is able at least partially to dissolve such an inhibiting
electrode coating, and contacting the electrode when the phase
interfaces are produced bY heterogenous supply of active ma-
terial which is consumed at the electrode, with a second complex-
ing agenk which, in operation, is able to reversibly dissolve
such active material.
These and further characteristic features of the process
will appear from the following and from the appended claims.
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The invention also relates to the proyision of an anode for
carrying the above process into effect. This anode comprises an
electrically conductive porous electrode body with a frontand
a back, the front being intended to contact the electrolyte in
an electrolyte chamber, while the back is provided wi.th active
material which is consumed at the anode process and whlch is adhe-
rently held to the electrode body by magnetic forces. Apart from
the advantages associated with magnetic adhesion of the active
anode ma~erial to the electrode ~ody, it is of particular advan-
tage to arrange the active anode material on the back of the
. electrode body, since this will eliminate the risk of short
eircuiting which must be taken into account when the active
materi.al is arranged at the front, especially when the anode and
the eathode are spaced but a small distance apart.
Aceording to specifieally preferred embodiments, the active
anode material is iron and the pores of the electrieal body are
of sueh a dimension as to prevent the passage of the aetive anode
material but allow the passage of eleetrolyte and complexed ions
of the aetive anode material.
As indicated above, in practising the inventi.on, a first
eomplexing agent is supplied to the anode to complex the reaetion
product formed in the eleetrode process, whereby a passivating ~.
eoating is prevented from forming on the anode. Likewise, ..
a seeond eomplexing agent for the aetive material at the eathode :. .
(the oxygen) is supplied at the eathode, for instanee in a metal/
air or metal/oxygen eell. More speeifieally, the invention imp- ~
lies with regard to an oxygen cathode that ~he oxygen is eomplexed ~ .
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i33~3
and brought into soluble form by means of a suitable complexing
agent which selectively dissolves oxygen from the air~ and that
the oxygen is supplied to the cathode in the li~uid phase, dis-
solved in the complexing agent or in a li~uid composition con-
taining the complexing agent. At the cathode the oxygen is
again released ~rom the complexing agent and undergoes the ordi-
nary electrode reactions. By causing the liquid containing the
dissolved oxygen to flow about or preferably through the cathode,
the "diffusion" or transport of the oxygen to t:he active surface
of the cathode can be heavily increased compared to conventional
oxygen cathodes where the only oxygen transport takes place by
diffusion and no actual flow of oxygen occurs. Said liauid
flow in the inventive process will also facilitate the transport
of OH ions from the cathode.
Moreover, the invention permits utilizing a simpler cathode
aonstruction than hitherto, since there are no longer three
different phases but only two phases at the cathode (liquid phase,
solid phase). The distance from the phase (the air) having the
active material (the oxygen) to the actlve surface of the elec-
trode will be less critical.
Finally, the invention also provides the advantage that
poisonous substances, such as nitrogen, can be eliminated by the
complexing agent dissolving only the oxygen.
Even though the invention will be described and exemplified
hereinafter, for greater simplicity, with reference to particu-
larly preferred active materials, that is, iron and oxygen, it
will be realized that the invention is not restricted to these
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33Z3
very materials, but that other suitable active materials arealso useful. The complexing agent must of course be adapted to
the active material used in each particular case, and such `
useful complexing agents belng known, those skilled in the art
will know which ones to use, without necessitating any compre-
hensive enumeration of such agents.
; As mentioned in the foregoing, a preferred active material
is iron which may wholly or partly form the anode. The term
"active material" is here meant to imply that iron is the
material that is consumed at the electrode process. Thus, ~he
anode can be for instance an electrode hody of ano~her conductive
material, such as an other metal, which is coated with the
active material. The electrode hody does not take part in the
electrode reaction proper but only serves to support the ac-
tive material, that is, the iron, and to carry away by its elec-
tric conductivity the electrons partaking in the electrode reac-
tion. At the actual electrode process the iron is transformed ;~
-into iron ions which according to the invention are complexed -~
with a complexing agent suitable for iron, preferably ethylene
- diamine tetraacetic acid (EDTA). As examples of other suitable
complexing agents mention may be made of cyanide tCN ),
thiocyanate (SCN ), citrate, nitrilotriacetate, ammonia,
ethylene diamine tetrapropionic acid or diethvlene triamine
pentaacetic acid.
In a particularly pre~erred emhodiment, the anode is part
o~ a metal/air or metalfoxygen cell which apart from the anode
comprises an electrolyte chamber containing an electrolyte, and
z3
a cathode or oxygen electrode which is electrically conductive
and porous and is supplied with air or oxygen on its side oppo-
site to the electrolyte chamber~ As examples of suitable com-
plexiny agents for the a~tive material at the cathode mention
may be made of mono- or polynuclear aromatics with two or more
electron-donating atoms in the rings or in functional groups
bound to the rings,such as hydroxyquinolines, multivalent phenols,
amino phenols, or their oxidation ~roducts. Amona multivalent
phenols mention may be made especially of hydroquinone, pyro-
catechol t pyrogallol or their oxidation ~roducts. Non-aromatics
which form reversible complexes with oxygen are also useful.
For a better understanding, the invention will be described
for purposes of illustration rather than limitation with refe-
rence to such a metal/air cell. In the drawings:
Fig. 1 schematically shows the metal/air cell;
Fig. 2 shows the voltage as a function of the current densi-
ty; and
Fig. 3 shows the current density as a function of the liquor
concentration at different contents of complexing agent.
The metal/air cell shown consists of an anode 1 and a
cathode 2 which are interconnected by an electric line 3 having
a resistor 4 for tapping of electric energy formed in said cell.
The electrodes are spaced apart so as to form an electrolyte t
chamber 5 between them.
Upon supply of complexing agent only to the anode the
oxygen electrode was a conventional oxygen electrode which was
formed by a porous plate of sintered nickel and silver. The
~S33:~3
anode consisted of an electrode body formed bv a porous silver
plate 6 which on its face remote fxom the electrolyte chamber
carried iron powder 7 which constituted the active material of the
electrode. The iron powder was held to the electrode magnetically
by an electro magnet (not shown~ disposed on the back of the
oxygen electrode. The dimension of the pores of the silver plate
is here critical only to the extent that they must not be so
large as to let ~he iron powder freely pass. Use was made of a
4 molar potassium hydroxide solution as an electrolyte.
~ lith the device described, an experiment was first made
without the use of complexing agent, that is, not in accordance
with the present invention. The electrolyte 5 was filled with
electrolyte solution,and fresh electrolyte solution in the form
of 4 molar potassium hydroxide solution was continuously supplied
to the electrolyte chamber by feed of that face of the anode
which was turned away from the electrolyte chamber, as indicated
by the arrow 8 in Fig. 1. At the same time the corresponding
amount of electrolyte solution was removed from the electrolyte
chamber. The fresh electrolyte solution first passed through the
layer of iron powder and then through the porous anode, where-
upon it was introduced into the electrolyte chamber. At the same
time the oxygen electrode connected to the anode was supplied
with air, as shown by the arrow 9, whereby hydroxide ions were
formed at the oxygen electrode. Because o~ the alkaline en-
vironment the iron ions produced hydroxide on the anode and no
satisfactory ~unction of the cell was obtained.
: . -. . - ,. , ; , . . . . . .. . ...
3 ~S;~3;~:3
.. . .
Then an experiment was made in accordance with the present
invention, proceedin~ in the same way as has been described above
but with the difference that the fresh electrolyte solution
supplied to the electrolyte chamber was first admixed with
50 g EDTA per liter solution. The electrolyte solution took up
the iron ions formed at the electrode reaction in the form of
a complex with EDTA so that they were not precipitated on the
metal anode. The complexed iron ions instead passed unimpededly
through the porous anode and were carried into the electrolyte
chamber, and only in said chamber was the iron precipitated in
the orm of iron oxide hydrate. As this precipitation was ~reely
present in the solution and was not precipitated on the anode
it could readily be removed with the electrolyte from the
electrolyte chamber. It was also established that the precipi-
tated iron oxide hydrate was ferrimagnetic, for which reason it
could readily be removed magnetically from the electrolyte so-
lution. The results obtained in making the experiment according
to the invention are indicated in Table 1.
Table 1 t
Temperature Current density Cell voltage
( C) ~ (mA/cm ) ~ ~(mV?
300 0
100 500
As an alternative, the above anode can be replaced by
other types o~ anodes, in which case the above electrode body,
that is, the porous silver plate, is a perforated or porous plate
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~ S33~3
of an other material which is electrically conductive and mag-
netic ! for instance a sintered plate of magnetic iron oxide and
barium oxide provided with an electrically conductive layer.
This will eliminate the above mentioned separate electromagnet,
and the iron powder is held to the electrode body by its self-
magnetism. In the embodiment described above the active material
of the anode, that is, the iron powder, is disposed on the face
of the electrode remote from the electrolyte chamber. This is
not, however, necessary since the iron powder can also be dis-
posed on the face of the electrode turned towards the electro-
lyte chamber Moreover, the a~ove described device consists of
a single cell, but for practical purposes the device is pre-
ferable equipped with double-acting electrodes, that is, the
anode shown has a corresponding electrode body also on the other
side of the iron powder, said electrode body facing a further
electrolyte chamber. In a corresponding manner, the oxygen elec-
trode is equipped with a further porous ~late of sintered nickel
and silver, which faces a third electrolyte chamber. In this
way, a battery of cells can be built up in a simple manner.
In such a battery air is supplied to the oxygen electrodes in
the space between the porous sintered plates of nickel and silver,
while electrolyte solution and complexing agent are supplied to
the iron powder in the space between the porous electrode bodies.
According as the active material, that is, the iron powder, is
spent, fresh iron powder can be supp~ied to the electrode bodies
and magnetically held thereto. A specific system for this Pur-
pose is exhaustively described in U.S. Patent No. 3,811,952,
13
~L~53323
to which reference is here made. It is also realized -that it is
not necessary continuously to furnish the cell with electrolyte
solution, complexing agen-t and~or active material, since the
cell can also be in the form of a primary battery in which the
metal electrode has been filled beforehand with iron powder
and a sufficient amount of complexing agent to complex the iron
ions formed by the iron powder.
In a further experiment the effect of the liquor concentra-
tion and the complexing agent concentration was studied in a
metal~air cell according to the above, which comprised an anode
of a porous sintered silver plate with a perforated magnet of
iron oxide and barium oxide, iron powder being held by magnetic
forces to the face of the anode remote from the e~ectrolyte
chamber. The cathode was a sintered plate of nickel and silver.
To the face of the cathode remote from the electrolyte chamber
air was supplied at a pressure of 0.1 to 0.5 atm. The anode
and the cathode each had an area of 2 cm2. The electrolyte
was XOH with a varying addition of EDTA as complexing agent,
and the electrolyte was allowed to pass through the cell at a
flow of about 20 ml per min. The cell was placed in a heating
box for holding the temperature constant at 50C. The current-
~oltage curve of the anode was measured against a reference
electrode (calomel electrode) in the cell. Fig. 2 shows the
current-voltage curve of the cell at different concentrations
of complexing agent (EDTA). It will appear from the Figure
that the current-voltage curve upon addition of an increasing
concentration of EDTA was first improved in order to reach an
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:
optimum value at about 5 g EDTA per 100 ml electrolyte (5 M KOH~.
Then the current density of the cell sinks at a certain voltage
upon further addition of EDTA, so that at 50 g EDTA per 100 ml
electrolyte t~e value of the current density at a certain voltage
is lower than at an addition of 2 g EDTA per 100 ml electrolyteO
It is obvious that under these conditions there is an optimum
with regard to the concentration EDTA at about 5-10 g EDTA per
100 ml electrolyte.
The corresponding conditions are also evident from Fig. 3
which shows the current density as a function of the electrolyte
concentration at different concentrations of complexing agent
. (EDTA). The lowermost curve shows the conditions without any
addition of complexing agent. Then the curves show that an
increasing concentration of ED~ gives an increasing current
density up to a concentration of about 5-10 g EDTA per 100 ml
electrolyte. At a further increase of EDTA concentration the
current density again sinks so that relatively low current densi-
ty values are obtained at a concentration of 50 g EDT~ per 100
ml electrolyte. They are nevertheless higher than the values
obtained without any addition of complexing a~ent.
It further appears from Fig. 3 that the current density
shows a relatively strong dependency on the electrolyte concen-
tration4 This dependency is not as pronounced at a lack of
complexing agent, whereas the current density at an optimum
addition of complexing agent, that is, about 5-10 g per 100 ml,
varies so much as about 125 mA~cm2 at varying electrolyte con-
centrations. It will appear from the curves that the optimum
:
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., , , ... : . . , ~ . :
- . ~ : : : , : : . :: . ~
~533Z~
value of the electrolyte concentration lies at abaut 5 moles ICOH
per liter.
Finally,an experiment was also made to investigate the effect
of a supply of complexing agent to the oxygen electrode. In this
experiment use was made of the a~ove device and the electrolyte
was 5 M ROH to which had been added 5 g EDTA per 100 ml electro~
lyte. Instead of supplying air under pressure to the oxygen
electrode at the face thereof remote from the electrolyte cham-
ber, the above mentioned electrolyte was supplied to it. This
electrolyte had been mixed with pyrogallol complexed with oxygen.
The oxygen was supplied complexed in the liquid phase, and not
in the gaseous phase. A satisfactory function of the cell was
established. The best result was obtained with use of a porous
cathode where the oxidized electrolyte was allowed to pass
through the electrode~
Pyrocatechol was also tested as a complexing agent for
oxygen and with the use of a graphite cathode gave a higher
short circuiting current than pyrogallol. Hydro~uinone and
ascorbic acid took an intermediate position.
In the above mentioned embodiment where iron is used as
active ma~erial for the anode the iron complexed in the electrode
process was precipitated in the electrolyte in the form of
magnetic; iron oxide hydrate which can be separated from the
electrolyte and recovered in a specific way which will be de-
scribed hereinbelow.
As shown in Fig. 1, electrolyte is withdrawn from the cell
to a container 10. The electrolyte contains magnetic iron oxide
~ 16
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~i3323
;
hydrate which has been precipitated from the complexed iron. When
the electrolyte solution enters the container la the magnetic
iron oxide hydrate is separated by means of the magnets 11.
Should the precipitation of iron oxide hydrate be incomplete the
solution in the container is grafted with crystals of magnetic
iron oxide, preferably under agitation of the solution. ~Jhen
all iron oxide material in the solution has ~een separated,
the purified solution is withdrawn from the container and can~
if desired, be returned to the metal/air cell illustrated in
Fig. 1. It should be mentioned that at the separation in the
container 10 the gradually increasing layer of iron oxide hydrate
will act as a filter for the impure electrolyte which is supplied
to the container, whereby the cleaning of the electrolyte wil]
be more effective.
17
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... ~, . . ,, . .. . . .- . . , - :
-, . , - . . . . :.
:: ':., ' ~ .' . '.. . . . " . , . .. ' ' ' ' '', '' ,, ! , . . . .
