Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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This invention rela-tes to electrical energy storage
devices and particularly relates to electrically rechargeab~e
reduction-oxidation (REDOX) Elow cell systems.
A need exists for storing bulk quantities of elec-
trical power obtained from intermittent or random sources such
as wind-driven generators, and solar cells~
Pumped water storage systems wherein water from a
water storage pond at one level is directed to a water storage
pond at a lower level through a hydro-electric plant having a
water pumping capability has proven to be a viable method o~
energy storage. Unfortunately, such facilities are limited to
areas where the terrain is suitable for providing water sources
at different elevations.
A number of other methods have been considered in-
cluding the storage of compressed air in large reservoirs, fly-
wheels, capacitive storage, inductive storage and a number of
electrochemical schemes. Electrochemical storage batteries are
generally expensive.
Electrically rechargeable REDOX flow cell systems
are well known and have a very high overall energy efficiency
as compared to other systems. REDOX type cells also can be dis-
charged more completely than second~ry battery syste~s. Addi-
tionally, REDOX cells are inexpensive as compared to secondary
batteries and do not deteriorate as significantly when repeatedly
discharged and rec:harged.
United States Patent 3,996,064 is useful background
for the understan~ing of the present invention. In that patent
an electrically rechargeable REDOX flow cell is disclosed where-
in an electric potential is obtained between electrodes dis-
posed respectively in an anode fluid having a chromic/chromous
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couple and a cathode fluid having a ferrous/ferric couple. The
anode and cathode fluids are separated by an ion selective mem-
brane. The electrodes are both disclosed as comprised of porous
graphite products which are inert to electrochemical reaction
with the anode and cathode fluids while promoting the REDOX re-
action on their surfaces.
It is an object of the invention to provide a REDOX
cell which will deliver much greater current for any given elec-
trode surface area than prior art devices. It is another object
of the invention to provide a REDOX type cell which will deliver
an increasingly greater percentage of current when compared to
prior art REDOX cells as the cell approaches a discharged con-
dition.
In accordance with the invention, catalytic coatings
are provided on the surface of the anode electrode to enhance
the reduction and oxidation activity of the desired ions in the
fluid. In the preferred embodiment a thin metal layer of copper,
silver or gold and another layer of lead are deposited respect-
ively on the anode electrode. In the iron/chromium fluid system
of the preferred embodiment, the lead surface enhances the chrom-
ium reduction during the charging of the cell and produces a 60-
fold increase of reduction current density over an untreated
electrode.
When the REDOX cell is being discharged, the lead
also functions very well as a surface for the reversible electro-
chemical oxidation of the chromous ions to chromic ions, however,
the lead itself a:Lso undergoes electrochemical o~idation and is
deplated from the anode surface.
The thin metal layer of copper, silver or gold,
which becomes exposed to the anode fluid, provides an electro-
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chemically active surface for the rapid oxidation of the chromous
ions, and at the same time is less subject to being oxidized than
the lead. The copper produces a 50-fold increase in current
density and the gold and silver both produce about a 90-fold in-
crease over plain carbon electrodes. During the recharging
process, the lead is replated onto the layer of copper, silver
or gold and is again available to promote the reduction activity
of the chromic ions.
Embodiments of the invention will now b~ described
with reference to the accompanying drawings, in which:
Figure 1 is a cross-sectional view of the REDOX cell
embodying the invention showing the anode and cathode ~luid
supply systems schematically;
Figure 2 is a graph illustrating the increased
current density in a REDOX cell embodying the invention;
Figure 3 is a graph illustrating current density
versus potential for different electrode materials; and,
Figure 4 illustrates current density versus poten-
tial for a carbon electrode with a catalytic coating and utili~ing
desired ions in the fluid to plate out on the coated electrode.
Referring now to Figure 1, there is shown a REDOX
c~ll 10 comprising container 11, divided into compartments 12 and
13 by an ion conductive membrane 14. The graphite electrode 15
is disposed in the chamber 12 and connected to an output terminal
6 while a graphite electrode 7 is disposed in compartment 13 and
connected to an output terminal 18. Electrode 15 in the anode ~;
compartment 12 is coated with metal layers 16 and 17 to Da more
fully described hereinafter. ~ ~
As shown, a cathode fluid from a cathode fluid ~ -
source 19 is circulated by a pump 20 through compartment 13. ;
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Similarly, anode fluid from an anode fluid source 21 is circula-
ted by a pump 22 through the compartment 12.
In a preferred embodiment, the REDOX cell 10 utili-
zes an iron/chromium system wherein the cathode fluid contains
a ferrous/ferric couple while the anode fluid contains a chromic/
chromous couple. Both fluids are acidified solutions between 1
and 4 molars. The anode fluid preferably contains water and HCl
(agueous solution of HCl) having dissolved therein a chromium
chloride salt whereby cations in a reduced state are produced.
The cathode fluid likewise is water and HCl but has dissolved
therein an iron chloride salt whereby cations in an oxidiæed
state are produced. These fluids provide the desired couples in
each of the chambers 12 and 13. A more complete disc~ussion of
the couple, the fluid and electrode requirements and membrane
considerations is given in United States Patent 3,996,064.
While the REDOX cell herein has been described as
using an anode fluid having a chromous/chromic couple and a
cathode fluid having a ferrous/ferric couple, other couples may
be used as indicated in the aforementioned patent. For example,
titanium chloride may be used in the anode fluid to produce the
cations in the reduced state and vanadium chloride or manganese
chloride may be used in the cathode fluid to produce the cations
in the oxidized state.
In accordance with the present invention, it has
been found that a coating of lead on the inert electrode 15 sub-
stantially increases the current density of the electrode, and
consequently, the current available at terminals 6 and 18. This
results from the fact that chromic reduction has been found to
occur very rapidly on the lead surface. At the same time lead
0 is also representative of a class of non-noble metals that
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possess a very high hydrogen over voltage, which advantageously
results in a minimization of the rate of hydrogen evolution to
preserve the electrical balance of the fluids. Al-though lead
chloride is discussed ~erein, it is noted that cadmium chloride
may be substituted for or used in combination with lead
chloride. Cadmium chloride achieves the same results as lead
chloride in increasing catalytic activi-ty of a carbon electrode
for the reduction of chromic chloride with the simultaneous in-
hibition of hydrogen evolution.
10 On a lead surface which was prepared by the electro-
deposition of a very thin layer of lead onto a smooth carbon rod,
the reduction current was measured to be 12 mA/cm2 at an elec-
trode potential of -600 millivolts (mV) versus a standard calomel
electrode (SCE). Under like conditions, the reduction current
for an untreated electrode was only 0.2 mA/cm2. Thus, the lead
coating on a carbon rod provided a 60-fold increase of reduction
current.
Referring now to Figure 2, there is shown graphi-
cally the relative performance obtained over a wide range of
potentials of a solution that contained no lead ions and one
that contained 10 4 molars of lead ions.
The lead layer 17 may be coated onto the electrode
15 before it is disposed in chamber 12 to be contacted by the
circulating anode with fluid. The lead layer may also be ob-
tained by simply dissolving lead chloride in the anode fluid
before charging lhe REDOX cell and allowing the lead, which is
subject to anodic dissolution, to plate onto the electrode
structure during the charing mode of the chromous/chromic re-
action.
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As mentioned previously, each time the REDOX cell
is discharged, the lead is deplated from the anode by electro-
chemical oxidation and redissolved in the anode fluid. secause
the carbon and graphite material of the electrode structure does
not provide an active surface for the oxidation of the chromous
ions, a further aspect of the invention is the provision of metal
layer 16 as an electrochemically active surface between the lead
layer and the electrode surface for this purpose. Metal layer 16
may be selected from the group comprising copper, silver and gold
which have all been found to be electrochémically active surfaces
for the rapid electrochemical oxidation of chromous ions, and at
the same time, are less subject to being electrochemically oxi-
dized than lead. Figure 3 graphically illustrates the current
density versus potential for silver, copper and smooth carbon
surface. As shown, at an electrode potential of -550 mV versus
a saturated calomel electrode the rate of electrochemical oxi-
dation of a chromous ion is less than 0.1 mAfcm2. Under the
same conditions, the current produced by the silver surface is
about 9 mA/cm2 which is about a 90-fold increase. A copper sur-
face under the same conditions yields 5 mA/cm2 which is a 50-
fold increase over a smooth carbon surface.
Referring now to Figure 4, there is graphically
; shown the current density obtained for various electrode poten-
tials for the chromous/chromic couple where the anode electrode
is coated with gold and wherein lead chloride is dissolved in
the anode fluid to provide lead ions. With the gold coating on
the anode electrode it will be seen at negative 550 mV versus
the saturated calomel electrode the current for the electro
oxidation of chromous ion is 9 mA/cm2. This is a 90~fold increase
over a plain carbon elec-trode.
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The metal coating 16 of silver, gold or copper on
the inert electrode is very -thin and may be applied by various
procedures including electrodeposition, metal spraying, dipping
or the like. The amount of metal in the coating is on the order
or only a few molecular layers. A graphite felt was produced
with 25 micrograms of gold per cm2 of projected area. This pro-
vided suitable electrode performance during discharge. The lead
coating which goes over the silver, gold or copper coating on
the inert electrode, whether applied before the electrode is in-
lQ serted in the anode chamber or by deposition from the anode fluid,
can also be as thin as several monolayers, where a monolayer is
one molecule in thickness.
The gold, silver and copper catalysts which enhance
the oxidation of chromous ions during the dischar~e cycle may
also be dissolved in the anode solution as salts to provide in-
situ activation of the anode electrode. They can also be incor-
porated into the negative carbon electrode by saturating the
electrode with a salt solution of gold, silvex or copper followed
by heat treatment to dry it.
It will be understood that changes and modifications
may be made to the above described inventions by those skilled in ;~
the art without departing from the spirit and scope of the inven-
tion as set forth in the clalms appended hereto.
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