An electrically rechargeable REDOX cell or battery system including one of more rebalancing cells is described. Each rebalancing cell is divided into two chambers by an ion permeable membrane. The first chamber is fed with gaseous hydrogen and a cathode fluid which is circulated through the cathode chamber of the REDOX cell is also passed through the second chamber of the rebalancing cell. Electrochemical reactions take place on the surface of insert electrodes in the first and second chambers to rebalance the electrochemical capacity of the anode and cathode fluids of the REDOX system

Thaller, L. H.

English

NASA Technical Reports Server

U.S. Patent 
HYDROGEN 
SOURCE 
Jun. 26, 1979 4,159,366 
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https://ntrs.nasa.gov/search.jsp?R=19790018303 2020-03-20T19:17:18+00:00Z
United States Patent [191 
Thaller 
11 11 4,159,366 
[45] Jun. 26, 1979 
[54] ELECTROCHEMICAL CELL FOR 
[75] Inventor: Lawrence H. Thaller, Strongsville, 
[73] Assignee: The United States ~f America as 
REBALANCING REDOX FLOW SYSTEM 
Ohio 
represented by the Administrator of 
the National Aeronautics and Space 
Administration, Washington, D.C. 
[21] Appl. No.: 914,260 
[22] Filed: Jun. 9,1978 
[51] Int. Cl .2  .............................................. HQ1M 8/04 
[52] U.S. Cl. ....................................... 429/15; 429/101 
[58] Field of Search ....................... 429/15, 14, 17, 19, 
429/21, 27, 29,46, 101, 107, 109 
1561 References Cited 
U.S. PATENT DOCUMENTS 
3,255,046 6/1966 Ghormley ........................ 429/107 X 
3,540,934 11/1970 Boeke .............................. 429/101 X 
3,551,207 12/1970 Herbst .................................... 429/14 
37 
4 
3,920,474 11/1975 Zito, Jr. et al. ........................ 429/15 
3,996,064 12/1976 Thaller ............................... 429/21 X 
Primary Examiner-Charles F. LeFevour 
Attorney, Agent, or Firm-Normal T. Musial; John R. 
Manning; James A. Mackin 
1571 ABSTRACT 
There is disclosed an electrically rechargeable REDOX 
cell or battery system including one or more rebalanc- 
ing cells. Each rebalancing cell is divided into two 
chambers by an ion permeable membrane. The first 
chamber is fed with gaseous hydrogen and a cathode 
fluid which is circulated through the cathode chamber 
of the REDOX cell is also passed through the second 
chamber of the rebalancing cell. Electrochemical reac- 
tions take place on the surface of inert electrodes in the 
first and second chambers to rebalance the electro- 
chemical capacity of the anode and cathode fluids of the 
REDOX system. 
15 Claims, 1 Drawing Figure 
25 
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1 
4,159,366 
ELECTROCHEMICAL CELL FOR REBALANCING 
REDOX FLOW SYSTEM 
ORIGIN OF THE INVENTION 
This invention was made by an employee of the Gov- 
ernment of the United States of America and may be 
manufactured or used by or for the Government with- 
out the payment of any royalties thereon or therefore. 
BACKGROUND OF THE INVENTION 
5 
10 
1. Field of the Invention 
This invention relates to electrical energy storage 
devices and is directed more particularly to an electri- 
cally rechargeable, reduction-oxidation (REDOX) type 15 
cell. 
2. Description of the Prior Art 
REDOX cells utilizing anode and cathode fluids sep- 
arated by an ion permeable membrane and employing 
REDOX couples as the electrochemical active materi- 2o 
als are generally well known. The electrochemical ca- 
pacity of these fluids is a function of the amount of 
active material in the solution and the oxidation state of 
the material. If the anode fluid and the cathode fluid 
contain the same number of equivalents of active mate- 
rial and both have the same degree of charge the 
REDOX system is said to be in balance. 
For a REDOX system using a chromous/ch;omic 
couple as the anode fluid and the ferrous/ferric couple 
as the cathode fluid, a balanced system has the same 
number of equivalents of chromous chloride in the 
anode fluid as there are equivalents of ferric chloride in 
the cathode fluid. The anode fluid is an aqueous solution 
of hydrochloric acid and chromium chloride while the 
cathode fluid is an aqueous solution of hydrochloric 
acid and iron chloride. 
During the operation of a REDOX system, electro- 
chemical imbalance can gradually build up if reactions 
other than the desired electrochemical REDOX reac- 
tions occur. For example, if a system when new had a 
capacity of 100 ampere hours and after a certain length 
of time had a capacity of only 90 ampere hours, it would 
be clear that an imbalance between the anode and cath- 
ode fluids had occurred. This imbalance is defined as a 
different state of charge in the anode and cathode fluids. 
An imbalance in a REDOX system can occur for a 
number of reasons including, but not limited to, the 
following: 
1. AIR INTRUSION INTO THE CATHODE 
FLUID 
If air leaks into the cathode fluid, oxygen in the air 
will chemically oxidize some of the ferrous ions to ferric 
ions. If this effect is allowed to continue over a long 
period of time, a significant amount of balance between 
the anode and cathode fluids will occur. The equation 
which describes the chemical oxidation of ferrous ions 
is as follows: 
2Fef2+ 10~+2H+--+2Fe+~+HzO. 
The net consequence of the effects of this reaction is 
that the iron solution (cathode fluid) will become fully 
charged prior to the chromium solution (anode fluid). 
The chromium solution becomes charged electrochemi- 
cally to the same extent as the iron solution is charged 
electrochemically. 
25 
30 
35 
40 
45 
50 
55 
60 
65 
2 
2. AIR OXIDATION OF THE ANODE FLUID 
If there is air intrusion into the chromium solution, 
the oxygen will chemically oxidize some of the chro- 
mous ions to chromic ions. This is equivalent to dis- 
charging the anode fluid while the cathode fluid is not 
being discharged. The chemical reaction is written as 
follows: 
2Cr+2 + 1 0 2  + 2H+-2Cr+3 + HzO. 
It is only during electrochemical discharge or charge 
that the chromium solution and the iron solution un- 
dergo a commensurate degree of oxidation or reduc- 
tion. 
3. THE CO-EVOLUTION OF H2 DURING 
RECHARGE. 
During the recharge of the anode fluid there are two 
possible electrochemical reactions that can take place. 
The electrochemical reduction of chromic ions to chro- 
mous ion is the predominant or major reaction as fol- 
lows: 
~r +3+ e--~r+z. 
The electrochemical reduction of hydrogen ions is pos- 
sible also but to a much lesset degree than chromium 
ion reduction as shown by the equation: 
4. CHEMICAL REDUCTION OF WATER BY 
CHROMOUS ION 
Chromous ion (Cr+2) is a strong reducing agent and 
can reduce the hydrogen ions contained in the anode 
fluid to hydrogen gas according to the equation: 
Cr+2+H+4r+3 +JHz. 
As in situation 3. there is a reduction in the amount of 
chromous ions without a commensurate reduction in 
the amount of ferric ions in the cathode fluid. The sys- 
tem imbalance in these instances would reveal them- 
selves as an early depletion of the chromous ions. 
All of the foregoing side reactions are minor but as 
their effects accumulate over the course of time, the 
capacity of the system as a whole will be reduced and 
limited by the capacity of the limiting reactant. 
A discussion of the electrode considerations and 
membrane considerations for REDOX cells is given in 
U.S. Pat. No. 3,996,064 which is incorporated herein by 
reference. 
OBJECTS ANI4 SUMMARY OF THE 
INVENTION 
It is an object of the invention to provide an im- 
proved REDOX cell system of high efficiency. 
It is another object of the invention to provide a 
REDOX cell system wherein an imbalance between the 
anode and cathode fluids is substantially prevented. 
Yet another object is to provide a REDOX cell sys- 
tem wherein gas produced by an undesirable side reac- 
tion is utilized to rebalance the system. 
Still another object of the invention is to provide a 
REDOX cell system having a rebalancing cell or an 
assembly of cells which provide electrical power output 
3 
while constantly balancing and equalizing the charge of 
the anode and cathode fluids. 
In summary, the invention provides a REDOX cell 
system (one or more cells) including one or more rebal- 
ancing cells which maintain an equal electrical charge 
in the anode and cathode fluids while at the same time 
supplying electrical power output. 
BRIEF DESCRIPTION OF THE DRAWINGS 
The single FIGURE is a schematic drawing of a 
REDOX cell system embodying the invention. 
DESCRIPTION OF A PREFERRED 
EMBODIMENT 
Referring now to the single FIGURE, there is shown 
a REDOX cell 10 divided into compartments 11 and 12 
by an ion conductive membrane 13. Electrodes 14 and 
15 of inert materials such as carbon, graphite, or the like 
are disposed in chambers 11 and 12, respectively, and 
are connected to electrical output terminals 16 and 17, 
respectively, to provide electrical potential thereon. 
The ion conductive membrane 13 is preferably an 
anion exchange membrane when the REDOX cell 10 is 
of the iron/chromium type. However, a cation ex- 
change membrane may be more suitable where different 
couples are used. A detailed discussion of membrane 
considerations for REDOX cells is set forth in U.S. Pat. 
No. 3,996,064. 
The REDOX cell 10 is preferably of the iron/- 
chromium type and will, therefore, have a chro- 
mous/chromic couple in chamber 11 and a ferridfer- 
rous couple in chamber 12. Accordingly, there is pro- 
vided an anode fluid source 18 containing an aqueous 
solution of HCl having dissolved therein chromium 
chloride. A pump 19 causes the anode fluid to flow 
through a circuit 20, through the chamber 11 and back 
to the anode fluid source 18 via a conduit 21. 
Cathode fluid having a ferridferrous couple for 
chamber 12 is provided from a cathode fluid source 22. 
The cathode fluid is an aqueous solution of HC1 having 
dissolved therein iron chloride. The cathode fluid flows 
through a conduit 23, a pump 24, the chamber 12 and a 
return conduit 25. 
The description thus far describes a conventional 
iron/chromium REDOX cell. As discussed previously, 
electrochemical imbalance can occur and will gradually 
increase thereby reducing the ampere hour capacity of 
the REDOX cell. 
To the end that imbalance of an iron/chromium 
REDOX cell will be substantially eliminated, there is 
provided in accordance with the invention a rebalanc- 
ing cell 26 divided into chambers 27 and 28 by an ion 
exchange membrane or other suitable membrane mate- 
rial 29. Inert electrodes 30 and 31 are disposed in com- 
partments 27 and 28, respectively, and are also con- 
nected to electrical terminals 32 and 33, respectively. 
As shown, electrode 30 is preferably in direct contact 
with the membrane 29 since it is a gaseous electrode. 
Cathode fluid from the cathode fluid source 22, which 
is the same cathode fluid provided to the chamber 12 of 60 1H2*H++e-. 
4,159,366 
4 
through valve 43 and is due to the side reactions of 
situations 3 or 4 above. Hydrogen gas can also be sup- 
plied by an independent hydrogen source 37 through a 
suitable throttle valve 38 and conduit 39. The hydrogen 
flows into chamber 27 where it is consumed electro- 
chemically. The impurities that build up in the hydro- 
gen may be released via conduit 40 and valve 41 into the 
atmosphere but are preferably directed into a purge gas 
collector 42. 
The hydrogen gas supplied to chamber 27 from 
anode fluid source 18 is obtained from a hydrogen space 
9 above the anode fluid. It is produced by the reduction 
of hydrogen ions during recharge of the REDOX cell 
and/or by chemical reduction of water by chromous 
ions. 
In chamber 27 of rebalancing cell 26, the hydrogen 
gas is electrochemically oxidized to form hydrogen ions 
which are transported through the membrane 29 to the 
cathode fluid. Ferric ions from the cathode fluid are 
electrochemically reduced to ferrous ions in the cath- 
ode chamber 28. While these rebalancing operations are 
occurring, electrical power can be drawn from termi- 
nals 32 and 33 of the rebalancing cell. . 
The valves 38 and 43 together with valve 41 may be 
adjusted to control hydrogen flow and pressure in 
anode chamber 27. Hydrogen from source 37 or from 
the hydrogen space 9 may be supplied severally or 
concurrently to chamber 27 by adjustment of valves 38 
and 43. 
In accordance with the rebalancing action of the 
rebalancing cell 26 wherein there is a transfer of ions 
between the cathode fluid of chamber 28 and the hydro- 
gen in chamber 27, a difference of electrical potential 
will be produced between the inert electrodes 30 and 
31. Advantageously, this difference of potential will 
provide energy for a suitable electrical load device 
connected to terminals 32 and 33 which, in turn, are 
connected to the electrodes 30 and 31, respectively. 
The terminals 32 and 33, however, can be connected 
together although the system will then have a some- 
what lower efficiency. It will be understood that one or 
more REDOX and/or rebalancing cells may be con- 
nected electrically in series or parallel. Also, the anode 
and cathode fluids may be directed in series or parallel 
through the cell chambers. 
As discussed previously, imbalance in a REDOX cell 
system occurs because of: 
1. Air intrusion into the cathode fluid; 
2. Air oxidation of the anode fluids; 
3. Co-evolution of hydrogen during recharge; 
4. Chemical reduction of water by chromous ions. 
Operation of the REDOX system incorporating the 
invention will now be explained. 
The electrochemical reactions that take place within 
the rebalance cell 26 or stack or rebalance cells are as 
follows: At the anode 30, hydrogen gas is oxidized to 
hydrogen ions, according to the equation: 
5 
10 
15 
20 
25 
30 
35 
40 
45 
50 
55 
_ _ _  . ~ 
By a desirable electroihemical process in the rebal- 
ancing cell 26, the cathode fluid is modified in a desired 65 
manner. Hydrogen gas is consumed in the anode cham- 
ber and ferric ions are reduced to ferrous ions. Hydro- 
Fe + e---tFe + * 
the overall equation is: 
gen gas is supplied to chamber 27 from fluid source 18 1H2+Fe+3+Fe+2+H+. 
5 
4,159,366 
If situations 1 and 2 are now examined in light of this, 
it can be seen that the imbalance situation can be over- 
come. 
If an external supply of hydrogen gas 9 and/or 37 is 
connected to the rebalance cell 26 or stack of rebalance 
cells, then as the rebalance cell is discharged, hydrogen 
is consumed as well as ferric ion. With the reduction in 
ferric ion content in the actual REDOX system, electro- 
chemical recharging can proceed and the remainder of 
the chromic ion that had hitherto remained uncharged 
can now be recharged and the system brought back into 
balance. This covers situation 1. where air intrusion in 
the hydrogen side necessitated the rebalance procedure. 
In situation 2 where air intrusion in the chromium 
side necessitated the rebalancing procedure, there is an 
excess of ferric ions present upon recharge and, thus, 
the same rebalance procedure can be used. For situa- 
tions 3 and 4 where hydrogen gas is generated in the , 
5 
10 
15 
anode fluid, this hydrogen can be conducted to the 
rebalance cell and there consumed as the cathode fluid 
is rebalanced. 
Advantageously, as these rebalance reactions are 
taking place, energy is available from these rebalancing 
cells at terminals 32 and 33. The voltage one can expect 25 
from such a device is that of an ironhydrogen cell. 
(0.77 volts per cell). 
It is to be noted that the sum of the side reactions that 
result from situation 1. 
30 
ZFC+~+ 102 + ZH+-2Fe+’+ Hi0 
and the rebalance reaction that is used to correct this 
situation: 
35 
2Fe+ i- Hz-r2F~+~+2H+ 
is: 
102+Hz-+H20. 40 
There will be a gradual buildup in the water content of 
the system. This, of course, should be negligible in the 
overall sense. Likewise, situation 2. when rebalanced 
results in gradual water buildup. Situations 3. and 4., on 45 
the other hand, do not result in any water buildups. 
It will be understood that changes and modifications 
to the abovedescribed REDOX system may be made 
without departing from the spirit and scope of the in- 
vention, as set forth in the claims appended hereto. 50 
What is claimed is: 
6 
a supply of hydrogen gas; and 
means for directing hydrogen gas from said supply of 
hydrogen gas to said anode compartment of said 
rebalancing cell to rebalance the charge of said 
anode and cathode fluids of said REDOX cell. 
2. The REDOX cell system of claim 1 wherein the 
inert electrode in the first compartment of the rebalanc- 
ing cell is in direct contact with the ion exchange mem- 
brane. 
3. The REDOX cell system of claim 1 and including 
means for removing nonreacted gas from the first com- 
partment of the rebalancing cell. 
4. The REDOX cell system of claim 3 and including 
means for collecting said nonreacted gas. 
5. The REDOX cell system of claim 1 wherein said 
anode fluid is an aqueous HC1 solution having chro- 
mium chloride dissolved therein; and 
said cathode fluid is an aqueous HC1 solution having 
6. The REDOX cell system of claim 1 wherein said 
supply of hydrogen is an independent source. 
7. The REDOX cell system of claim 1 wherein said 
supply of hydrogen gas is an anode fluid source. 
8. The REDOX cell system of claim 7 and including 
an independent source of hydrogen communicating 
with the anode chamber of the rebalancing cell. 
9. The REDOX cell system of claim 1 wherein said 
second ion exchange membrane passes only anions. 
10. A method of balancing the charge in the anode 
and cathode fluids of a REDOX cell of the type 
wherein hydrogen and chloride ions are exchanged 
through a first ion exchange membrane separating the 
fluids comprising the steps of: 
providing a rebalancing cell with a second ion ex- 
change membrane; 
passing said cathode fluid over one side of said sec- 
ond ion exchange membrane; 
disposing a first inert electrode adjacent said second 
ion exchange membrane in said cathode fluid; 
supplying hydrogen gas to the other side of said sec- 
ond ion exchange membrane; 
disposing a second inert electrode in contact with the 
other side of said second ion exchange membrane 
in said hydrogen gas; and 
electrically connecting an electrical device to be en- 
ergized between said first and second inert elec- 
trodes; 
whereby ions are exchanged through said second ion 
exchange membrane to rebalance the charge of 
said REDOX cell fluids. 
11. The method of claim 10 wherein said second inert 
iron chloride dissolved therein. 
1. In a REDOX cell system utilizing an anode fluid electrode is attached to said other side of said ion ex- 
and a cathode fluid wherein hydrogen ions and chloride change membrane. 
ions are exchanged through a first ion exchange mem- 12. The method of claim 10 wherein said anode fluid 
brane separating the fluids, the improvement compris- 55 is an aqueous HCl solution having chromium chloride 
ing: dissolved therein; and, 
said cathode fluid is an aqueous HCl solution having 
13. The method of claim 10 and including the step of 
an inert electrode disposed in each of said anode and 60 controlling the rate of hydrogen flow to said other side 
of the ~ ~ n d  membrane. 
14. The method of claim 10 and including the step of 
controlling the Pressure of hydrogen flowing to said 
other side of the second membrane. 
15. The method of claim 10 wherein said second 
membrane is an anion exchange membrane. 
a rebalancing cell having an anode compartment and 
a cathode compartment separated by a second ion 
exchange membrane; 
cathode compartments of said rebalancing cell and 
connected to respective electrical terminals to pro- 
vide electrical output to a load connected therebe- 
tween; 
means for directing a portion of said cathode fluid of 65 
said REDOX cell through wid cathode cornpart- 
ment of said rebalancing cell; 
iron chloride dissolved therein. 
* * * * *  


Electrochemical cell for rebalancing REDOX flow system

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