DOE/NASA/12726-23

NASA TM-83469

NASA-TM-83469

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NASA Redox Storage System

Development Project

Calendar Year 1982

National Aeronautics and Space Administration

Lewis Research Center

October 1983

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LANGU:V RESEARCH CENTER

LIBRARY, NASA

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Prepared for

U.S. DEPARTMENT OF ENERGY

Conservation and Renewable Energy

Division of Energy Storage Systems
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DOE/NASA/12726-23

NASATM-83469

NASA RedoxStorageSystem

• DevelopmentProject

Calendar Year 1982

National Aeronautics and Space Administration

Lewis Research Center

Cleveland, Ohio 44135

October 1983

Work performed for

U.S. DEPARTMENTOF ENERGY

Conservation and Renewable Energy

Division of Energy Storage Systems

Washington, D.C. 20545

Under Interagency Agreement DE-AI04-80AL12726

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NASAREDOXSTORAGESYSTEMDEVELOPMENTPROJECT

Calendar Year 1982

Lewis Research Center

National Aeronautics and Space Administaration

Cleveland, Ohio 44135

SUMMARY

The calendar year 1982 effort grew from the discovery during the previous

year that the iron/chromium Redox system must operate in the temperature range

55° to 65° C in order to avoid kinetic difficulties with the chromium redox

reaction. Membraneand electrode studies which were then underway at 25° C,

being performed by lonics, Inc., and Giner, Inc., respectively, have been completed, and the results became the starting points for proceeding to elevatedtemperature operation. At NASALewis single-cell evaluation of the mixedreactant mode of operation at elevated temperature has lead to significant advances in cell power density and in energy efficiency.

The effort of lonics, Inc., to develop Redox cell membranes focused for

most of the year on ways to reduce resistivity. Several approaches were taken.

The first was merely to incorporate thinner substrates than the usual calendered woven modacrylic. Generally, successful resistivity reductions by this

approach were accompanied by an unacceptable worsening of selectivity. The

other path taken was to develop heterogeneous, two-layer structures consisting

of a sturdy, fabric-supported, low-resistivity, polymer "board" bonded to a

thin film of highly selective ion exchange material to provide selectivity.

Various polymer chemistries were used for both layers. Even the best of these

structures was only marginally superior to the standard CDIL.

Analytical studies were carried out at NASALewis on the sensitivity of

projected system cost and efficiency to membraneresistivity and current density._ These studies indicated that current densities in the range 50 to 75

mA/cm_ are required to keep stack costs reasonable. However, to obtain acceptable energy efficiencies a_ such current densities, membraneresistivities at

least as low as 1.5 ohm-cm_ are required. The 2.6-ohm-cm_ resistivity of the

standard CDIL membraneat 25° C was unsatisfactorywith regard to resistivity.

Analytical studies of the relationship between system maintenance requirements and membraneselectivity showed that the optimum CDIL membrane, with a

room temperature selectivity of about 20 _g Fe/h-cm_-M, was also marginal

with regard to reactant refurbishment requirements. To prevent cross-mixing

from proceeedin9 beyond some moderate amount, say i0 percent, batch treatment

of the reactants would be required at intervals so frequent as to be too

costly. Thus, the present best membranefor 25° C operation with separate

reactants appeared to be marginal with respect to both resistivity and selec-

• tivity, and modifications to improve either property seemed always to be

detrimental to the other. On top of th_s, selectivity measurements at Lewis

on developmental membranes from lonics, Inc., and other sources showed that

raising the temperature to 65° C, while reducing resistivities by a factor of

2 or 3, resulted in up to a tenfold worsening of selectivities.

These studies and associated experimental work combined to show, with the
present generation of membranes at least, that controlling cross-diffusion of

the reactants while operating efficiently at or near 65° C was not feasible.

No option was left but to determine whether acceptable cell performance could

be achieved at 65° C using reactants that were totally mixed from the start.

The results attained in this mixed-reactant, elevated-temperature mode of operation were very encouraging. The first cell tested contained the standard

gold-lead catalyzed chromium electrode and a developmental cation membrane

(known to be very "open") with a low resistivity. Both reactant solutions in o

the fully discharged state were identical The resRlts were promising, with

the cell achieving high current density (64.5 mA/cm_), essentially complete

utilization of the reactants, low hydrogen evolution rate, and high energy efficiency. A singular advantage to operating with mixed reactants is that if

any partitioning of reactants or any osmotic transfer of solvent should occur

across the membrane, it can be countered simply by discharging the cell, _eblending the reactant solutions, and then redistributing them. The only penalties associated with the mixed-reactant mode of operation at elevated temperature are a loss of about 50 to 70 mV in open-circuit voltage, a decrease

in coulombic efficiency, and an increase in reactant cost. This cost increase

for reactants is balanced against a reduction in stack cost because of the

ability to operate at high current densities and a further reduction due to the

use of much less expensive membranes. Also, with regard to reactant cost, it

has been shown by studies completed during the year that these costs should not

preclude operation with mixed reactants.

At Lewis studies were carried out to understand why some chromium electrodes, after being cycled in working cells for some months, start to evolve

hydrogen at increasing rates. Of particular concern, in light of the switch to

mixed-reactant operation, was the possibility that reactant cross-mixing per se

was the culprit. Tests of this hypothesis indicated that exposure of the goldlead catalyzed chromium electrode to iron in the ferric (+3) oxidation state

could, indeed, lead to increased hydrogren evolution. However, the occurrence

of the ferric ion in the chromium reactant system requires a relatively unusual

sequence of events to occur, and this sequence is easily avoided. Iron in the

ferrous (+2) state was shown to have no detrimental effect on chromium electrode performance.

Another cause of the hydrogen evolution was the contamination of the

reactant system with trace amounts of platinum from the hydrogen electrode of

the rebalance cells typically used in conjunction with working cells. Apparently, the platinum was dissolving and migrating first into the iron stream

and ultimately into the chromium reactant system, where it was reduced to the

metal. Significantly, these causes of increased hydrogen evolution were

related to operational occurrences that can be easily avoided in practice.

Other electrode work carried on at Lewis involved the in-cell evaluation

of alterntive chromium electrode catalysts for elevated temperature operation.

These catalysts included a planar structure given enhanced surface area by

ion-etching, lead (alone), silver-lead, and bismuth-lead. The ion-etched

electrode, even with lead added, was too active in the evolution of hydrogen.

Charging proceeded well with lead as the catalyst on the standard carbon felt

substrates, but the discharge was irreversible. The silver-lead combination

performed poorly for both charge and discharge. However, the bismuth-lead

catalyst showed excellent properties. At 65- C it is active for the chromium

redox reactions; it is reversible and stable; and it has a high hydrogen over-
potential. This catalyst also has given evidence of being far more "forgiving"

than the standard gold-lead formulation when anomalies do occur.

The bulk of the recent electrode work at Giner, Inc., involved the goldlead catalyst combination at 25° C, evaluated by cyclic voltammetry. One part

of the effort was to evaluate various changes in the standard method of cleaning and applying gold to felt substrates. In general, these changes did not

result in the production of electrodes with better performance. By using

electron microscopy, it was ascertained that performance improved as gold particle size decreased; but a correlation could not be made between the method

of catalysis and the resultant gold particle size.

With regard to operation with mixed reactants, Giner, Inc., found that

adding ferrous chloride to the chromium reactant enhanced electrode performance, while the addition of chromic chloride to the iron reactant inhibited it.

Because of the low reactant concentrations used, however, the significance of

these results with regard to actual Redox cell operation is not yet known.

The result of the 1982 Redox technology development effort has been to increase the level of confidence in the viability of the iron-chromium system

concept. Although testing has not yet been extensive, initial results indicate

that operation with mixed reactants at elevated temperatures has many advantages and may be the breakthrough needed for the Redox system to fulfill its

early promise as an economical, long life storage device.

INTRODUCTION

During calendar year 1982, the focus of the Lewis Redox Project was on

technology issues. Very little was done in the systems areas such as scale-up,

applications analysis, component development, etc. Instead, most of the effort

involved single-cell experimentation, with the goal of building on the technical developments and advances of the previous several years. During those

years, many of the difficulties being encountered during operation of ironchromium Redox cells were shown to be directly related to the chemistry of the

chromic ion reactions (ref. i). The chromic ion (Cr_), which is the discharged form of the chromium reactant, exists as two distinct species, only one

of which is electrochemically active. The equilibrium concentrations of these

species at room temperature are about equal, and the rate of equilibrium is

slow. These characteristics caused the operational problems with the ironchromium Redox cells. Fortunately, it was later shown that raising the chromium reactant temperature to 55° to 65° C shifts and accelerates the equilibrium to favor the electrochemically active chromic ion species. This permits

charging at higher rates with lower applied voltages and the full utilization

of the reactant theoretical capacity. It was with these new understandings

that the 1982 work was begun. The aim was to exploit the technological logic

as it unfolded under the imperative of elevated temperature operation.

ELECTRODEDEVELOPMENT

NASALewis

Because it is necessary to catalyze the chromium electrode, the question

of catalyst poisoning or degradation due to the presence of iron cations in the

3
chromiumreactantsolutionwas of concern. As will be discussedin detail in

followingsections,to date no membrane has been developedthat has a suitably

low resistanceyet is able to perfectlyseparatethe respectivereactantmetal

cations. Initialevaluationof this potentialproblem involvedthe incremental

additionof ferrous chlorideto the chromiumsolutionof a working cell and

measuringthe rate of hydrogenevolutionat the gold-leadcatalyzedchromium

electrodeduringnormal cycling. As the FeCl2 concentrationin the chromium

reactantwas increasedfrom 1 to 40 percent of the chromiumsalt concentration,

the rate of hydrogenevolutionincreasedcommensurately. This conflictedwith

cyclic voltammetrystudiesbeing carried out under the contractwith Giner,

Inc., which showed virtuallyno effect of FeCl2 additionsto the chromium

reactantsolution. Therefore,the test was repeatedwith fresh reactantsand

unused cell components,but this time the ferrous chloridesolutionwas rebalanced (ref. 2) before additionto the chrbmiumsolutionto assure that no ferric ion was present. (Rebalancinginvolvesthe use of an electrochemicalcell

to reduce ferric ion (Fe+3) to the ferrous ion, using hydrogenas the reducing agent.) Over the next 3 months, the cell was continuouslycycled to increasinglydeeper depths of discharge(from 80 percent to 95 percent DOD) with

40 percent FeCl2 in the chromiumreactant. During this time the change in

the hydrogenevolutionrate was from i to about 3 percentof charge capacity.

At the end of this 3-month period,the cell was allowed to get out of balance

and then to sit short-circuitedwhile fully discharged. This in effect created

an i_on-ironconcentrationcell which caused the generationof ferric ions

(Fe+3) in the chromiumreactant. On subsequentcycling,the hydr_ evolution rate from the chromiumelectrodewas about four times greater than before,

being about 12 percent of charge capacity. Attempts to restore it to previous

performancewere unsuccessful. Thus, it was the ferric ion that attackedthe

gold-leadcatalystof the chromiumelectrode,causing an irreversiblechange

that somehow negated the high hydrogenoverpotentialof the lead componentof

the catalyst. All of this agrees with the earlier observationthat the exposure of a fully catalyzed,used chromiumelectrodeto air results in greatly

increasedhydrogenevolutionwhen the electrodeis reused in a cell. This

seems to indicatethat exposureof a catalyzedchromiumelectrodeto an oxidizing environmentcan bring about undesirableand irreversiblechanges.

These resultswere quite encouragingbecause they indicatedthat the previously noted tendencyfor long-termincreasesin the hydrogenevolutionrate

were not associatedwith the electrodeper se or with any aspect of the ironchromiumRedox cell concept. Rather, the problem is an operationaldifficulty

which results from allowinga rather specializedsequenceof events to occur.

This sequence,once recognized,is easily avoided.

All the above results and observationswere corroboratedin other cells;

and operationalprocedureswere establishedto preventoxidationof the chromium electrodecatalyst. Nonetheless,subsequentinstancesof increasinggas

evolutionfrom a working cell occurred. Finally,the componentsof a cell that

had failed in this mode were analyzedby X-ray techniques. Surprisingly,platinum was discoveredin the membrane taken from the cell. Platinum is a splendid catalyst for the reductionof water by chromous ions, thus generatinghydrogen. Ironically,the source of the platinumwas the hydrogenelectrodeof

the rebalancecell, the purpose of which was to consume any evolved hydrogen.

A sketch of a rebalancecell is presentedin figure 1. Typically,a shortcircuitedrebalancecell was used with each working cell on test, to keep the

reactantschemicallybalanced. This was accomplishedby using the evolved

hydrogen,which was collectedin the chromiumreservoir(or with hydrogen

4
supplied from an external source), to electrochemically reduce the excess

ferric ions. The iron reactant solution exiting the working cell, therefore,

flowed through the rebalance cell before returning to the iron reservoir.

Apparently, the platinum catalyst on the hydrogen electrode of the rebalance

cell was going into solution, diffusing through the rebalance cell membrane

into the flowing iron stream, and then migrating through the working cell

system until encountering chromous ions in, or at the surface of, the working

cell membrane. Here, the platinum ions were reduced to the metal which then

began to catalyze the chemical reaction between chromous ion and water to

yield hydrogen.

Later analyses of components of other cells that have undergone increased

hydrogen evolution rates while being tested in conjunction with rebalance cells

have also shown the presence of platinum in the membranes. Again, as was the

case with the indication that exposure to the Fe+3 ion could cause increased

hydrogen evolution, this is a satisfying result: It shows that another major

cause of increased hydrogen evolution is merely operational and not inherent

to the gold-lead catalyst or the system concept.

Of course, since there will always be some small rate of hydrogen evolution due to electrochemical reduction during the charging process, plus other

causes of system imbalance, the use of a rebalance cell will remain necessary.

Therefore, a means to avoid or alleviate the platinum contamination problem

must be developed. Several possible approaches, each promising, will be evaluated in 1983. In the meantime gas traps now are being used to collect and

measure any hydrogen produced from working cells on test.

During 1982 a variety of alternative catalyst approaches for the chromium

electrodes were examined. Studies were carried out both at room temperature

and at 65° C, as the project began to shift its emphasis toward elevatedtemperature operation. These studies were carried out according to the conventional concept of keeping the reactants separated by a highly selective

membrane and also according to the new, mixed-reactant concept of operation.

This latter concept will be discussed in some detail in later sections of this

report. In the mixed reactant mode of operation both reactant solutions, when

fully discharged, are identical. Typically, the composition would be 1.0 M

FeCI2, 1.0 M CrCI3, and 2.0 N HCI.

Concerning the chromium electrode catalysis, the first question addressed

was whether, at elevated temperature, any gold catalyst was indeed required.

It was hoped that at 65° C the carbon felt electrode substrate would have sufficient intrinsic electrochemical activity for the chromium Redox reactions.

Therefore, a cell was assembled with a bare carbon felt chromium electrode and

cycled at 65° C. Unmixed reactants were used, and the standard quantity of

saturated PbCI2 solution was added to the chromium solution so that during

charging a thin lead deposit would lessen the tendency toward hydrogen evolution. The cell did charge with very low hydrogen generation. However, polarization curves at 50 percent state-of-charge (SOC) (fig. 2) revealed irreversible behavior for both charge and discharge.

Next, a cell with the standard gold-lead catalyst was cycled intermittently for a total of 104 cycles at temperatures to 65° C, with unmixed reactants, to evaluate the effect of elevated temperature on this catalyst composition. Although the resistivity of the RAI Research Corporation developmental

membrane had increased significantly, polarization curves at the end of the
- Membrane

F Electrolytereservoir

I I

Polyethylenegasket-_ I ,-Hydrogenelectrode

f, 1i (platinumcatalyst)

__.____linlet ;

Iron 'nlet-'/I _ I ,1 _-f _ I!i .... "_HoYdlr_g en

Plasticendplate _' _ J _ ,-Graph te felt L I

• I I I

Graphitecurrent collectora _ L-Hydrogendiffuser

I

Iron electrode--,

Figure1. - Ferricchloride-hydrogenrebalancecellschematic.

1.6 --

I

20 40 60 80 100 120

Current density,mA/cm2

Fi9ure2. - Chargeanddischarge_larization curvesfor cell

with lead-catalyzedchromiumelectrode.Temperature,65o C;

unmixedreactants;50percentSOC.
test period showed that electrode performance remained reversible (fig. 3).

However, hydrogen evolution during operation at 65° C was consistently in the

neighborhood of 6 percent of charge capacity, compared with 1 to 3 percent at

room temperature. Generally, for all cells operated at 65° C, with the goldleaJ catalyst, the higher temperature did not cause any deterioration in polarization performance or gassing rate during the respective test periods. However, the actual magnitude of the hydrogen evolution rate did vary from cell

to cell, ranging from less than I percent to the aforesaid 6 percent. In a

large enough system generating significant quantities of hydrogen, the rebalance stack(s), instead of simply being short-circuited, could be used as a

power source for small loads such as pumps. In this situation the evolution

of hydrogen would not be a net coulombic loss mechanism. Even in the absence

of this advantage, it is possible that a gassing rate of 6 percent would be

acceptable in specific applications. Nonetheless, it was obviously worthwhile

to explore other possible chromium electrode catalyst systems.

Another attempt was made to take advantage of the elevated operating temperature so that catalysis of the chromium electrode would not be required. A

planar carbon surface was ion-etched to greatly increase its surface area.

Tested as a chromium electrode in a working cell at room temperature, the ionetched plate, unfortunately, showed far more activit_ for hydrogen evolution

than for the chromium Redox reactions. Heated to 55- C, the performance was

even worse. The addition of bismuth and lead salts to the chromium reactant

solution in order to depress hydrogen evolution was beneficial, but the net improvement left much to be desired (fig. 4). Probably, the attempt to deposit

a material such as lead on this surface structure resulted in an uneven distribution, with high spots being plated while recessed regions were not. Also,

the "fuzzy" character of the ion-etched surface undoubtedly leads to the growth

of an appreciable mass-transport boundary layer which inhibits performance.

An analog of the gold-lead catalyst, using silver instead of the gold, was

next evaluated. The rationale for this selection was that silver, having a

lower reduction potential than gold would be more easily stripped and redeposited, making possible a restoration of performance after deterioration, as evidenced by increasing hydrogen evolution rates. At 65° C with mixed reactants,

the initial hydrogen rate from a cell using this chromium electrode catalyst

was low. However, the electrode was irreversible for the chromium redox reactions and showed severalsteps in the current-voltage curves during constant

load discharges.

The final chromium electrode catalyst system evaluated in 1982 was

bismuth-lead. The rationale for this selection was pretty much the same as for

the silver-lead system. The choice was supported by evidence from cyclic voltammetry evaluations carried out at Giner, Inc., that bismuth was active for the

chromium redox reactions (ref. 3). This metal also is known for its hydrogen

reduction overpotential. The brief period of testing of this catalyst system

at the end of 1982 gave extremely encouraging results (ref. 4). Used in a

working cell at 65° C with mixed reactants, hydrogen evolution was very low,

electrode performance was reversible, and cycling between 95 and 0 percent SOC

revealed no catalyst instability. Cell voltage versus time (equivalent to SOC)

for a charge-discharge cycle at 64.6 mA/cm_ is shown in figure 5. Results

of a similar test with a cell using the standard gold-lead chromium electrode

catalyst are shown in figure 6 for comparison. There is much to be said about

these two tests, and they will be discussed further in the later section concerning the mixed-reactant concept. For now, the significant point is that the
1.6 --

1.2

> 1.0

o

o

.6

.4

2 I I I I I I I

10 20 30 40 50 60 70

Currentdensity, mA/cm2

Figure3.- Chargeanddischargepolarizationcurvesafter 104cyclesfor

cell with standardgold-leadcatalystfor chromiumelectrode.Temperature, 65o C; unmixedreactants;50percentSOC.

1.6 --

/ / Catalyst Temper1.4

1.2 _ ature,

oC

0 None 55

A Bismuth-lead 65

> 1.0

o

*,j • 8

.6

.4 --

2 I I I I ! I

20 40 60 80 100 120

Current density, mA/cm2

Figure4 - Chargeanddischargepolarizationcurvesfor a cell

with anion-etchedcarbonplatefor thechromiumelectrode.

Unmixedreactants;50percentSOC.
1.4

Charge

1.2

.8 Discharge

I I

"60 .2 .4 .6 .8 1.0 ' 1.2

Time,h

Figure5. - Cellvoltageduringa constant-currentcycle. Current

density,54.5mA/cm2:temperature650C: mixedreactants_

bismuth-leadchromiumelectrodecatalyst:0to 87percentSOC.

Cellresistivity 1.74_-cm2; membraneresistivity, 0.34£2-cm2"

energyefficiency,73.1percent: coulombicefficiency,94.6percent; voltageefficiency,71.2percent: hydrogenevolution,0.2

percentof chargecapacity.

1,2 --

1.0

>

o .8

.6

.4 I ! I I ! I I

0 .2 .4 .6 .8 1.0 1.2

Time,h

Figure6.-Celvoltage l duringaconstant-current cycle.Current 2 o

density,64.5mAIcm; temperature,65 C; mixedreactants;

gold-leadchromiumelectrodecatalyst; 0to 87percentSOC,Cell _

resistivity, 1.16£2-cm%membraneresisitvity,0.36Q-cm::

energyefficiency,72.3percent; coulombicefficiency,92.2percent: voltageefficiency,18.4percent; hydrogenevolution,

o.9percentof chargecapacity.
bismuth-leadelectrodecatalystshowed electrochemicalactivityfor the chromium redox reactionsthat was fully the equal of the gold-leadcatalyst. Furthermore,the tendencyfor hydrogenevolutionwas very low.

The in-houseelectrodedevelopmenteffort of 1982 producedthe very encouragingevidencethat a chromiumelectrodecatalysthas been producedwhich

meets requirementsfor elevatedtemperatureoperation. It is electrochemically

active, stable, and reversibleand has a high hydrogenovervoltage. Furthermore, the bismuth componentis more easily applied than the gold componentof

the previousstandardcatalyst.

Giner, Inc.

During calendaryear 1982, under contractDEN3-262,Giner, Inc., has focussed On the gold-leadcatalyzedchromiumelectrodefor operationat room temperature (25° C). One goal of this effortwas to develop the techniquesnecessary to consistentlyproduce catalyzedelectrodeswith suitableelectrochemical

, performance. The analytical tool used in most instances was cyclic voltammetry

The elements of the electrode production sequence that were examined included

the formation of the felt substrate from its precursor (ref. 5) and the subsequent cleaning and gold catalyzation of the felt. The second aspect of the

contract effort was to evaluate the effect of certain operational characteristics on electrode performance, again by cyclic voltammetry. The characteristics studied were the cross-mixing of the reactants and the acidity level of

the chromium reactant solution. The study of the effect of elevated temperature is also a part of the contract effort but was not begun during 1982. In

all these tasks the performance measurements were carried out at room temperature, even though it seems quite certain that actual Redox system operation

will be in the 55° to 65° C range. This was a matter of convenience coupled

with the belief that the conclusions reached based on room temperature experiments would not be significantly different had the experiments been performed

at some other temperature.

The first task of the contract effort was to systematically evaluate the

effect on ultimate electrode performance of the temperature at which the Rayon

felt precursor was pyrolyzed to carbon or graphite. A series of felt samples

was prepared by Fiber Materials, Inc., at processing temperatures of 1250\,

1350° , 1500° , 1650° , 1800° , and 2300° C, and delivered to Giner, Inc. The samples were _leaned in potassium hydroxide and catalyzed with a gold loading of

12.5 _g/cm_ according to a standardized procedure (ref. 6). Cyclic voltammetry was then used to evaluate the electrode samples in terms of hydrogen

evolution and chromium activity. Analysis for hydrogen evolution indicated

decreasing hydrogen ion reduction with increasing felt processing temperature

(fig. 7).

In general, little change in hydrogen evolution occurs beyond a processing

temperature of 1800° C. The chromium redox reaction activity of the six felt

samples decreased with increasing processing temperature. Probably the most

relevant overall measure of relative electrochemical efficiency is the comparison of chromium redox activity and hydrogen evolution. The ratio of the

quantity of chromous ion oxidized to the quantity of hydrogen ion reduced per

voltammetry cycle for each process-temperature sample of felt is shown in figure 8. The 1650° and 1800° C felts are the most favorable on this basis

I0
Two alternative catalysis techniques for the deposition of gold on the

felt electrode samples were compared with the standard technique used at Lewis.

One technique simply used a larger volume of a more dilute alcoholic gold chloride solution for soaking the felt. The second approach involved two separate

soaks and extra furnace drying steps. Results obtained by cyclic voltammetry

for samples catalyzed by these three methods were compared. Hydrogen evolution

on the electrodes prepared according to the Lewis standard technique was lower

than for either of the other methods. The ratio of chromium redox activity to

hydrogen reduction was also better for the standard technique.

Electron microscopy was used to determine gold particle size on the electrodes prepared by the three catalysis approaches (ref. 5). Cyclic voltammetry

was then used to see if there was a correlation between the gold distribution

and the electrochemcal performance of the electrodes. Felt samples having

smaller gold particle sizes exhibited high chromium activity and high hydrogen

evolution rates. Indications are that the gold deposition process may occur

by adsorption from a colloidal state induced by contact of the gold solution

with the carbon felt. In general, the Lewis standard catalysis method gave the

best electrochemical performance, which seems to correspond to an intermediate

gold particle size.

Although variations in results were observed, the two lowest processtemperature felts (1250 ° and 1350° C) are consistently less suitable for the

chrominum electrode application. Thus, the carbon felt substrate still seems

to be a contributor to variabiliy in results by influencing the gold deposition

process.

The effect of ferrous chloride (FeCI2) on the electrochemical performance of a gold-catalyzed electrode in a chromic ion solution was determined.

The FeCl2 concentration of the chromium solution was incrementally increased

from 0 to 0.5 molar. The results (fig. 9) show that the chromic ion reduction

reaction was shifted to slightly more negative potentials and that the total

quantity of chromium reacted increased as the Feel 2 concentration increased.

In addition, hydrogen evolution decreased with increasing FeCI2 concentration.

Thus, it seems that the presence of FeCI2 in the chromium reactant solution

has a beneficial effect, perhaps due to a shifting of the chromic ion equilibrium to favor the electrochemically active monochloropentaaquo species.

The effect of chromic chloride (CrCIR) on the iron redox reactions was

observed in a similar way. (See fig. 10._ The incremental additions of CrCI 3

increasingly depressed the level of the iron redox reactions, From these unexpected results it is apparent that the presence of CrCI3 interferes with

the iron redox reactions, perhaps by increasing the chlorlde complexes of the

ferric ion.

i

MEMBRANEDEVELOPMENT

Ionics,Inc.

The 1982 contracteffort at Ionics,Inc., was establishedbefore the decision by Lewis to concentrateon cell operationat a temperatureof 65° C. The

1982 work was, therefore,an extensionof the previouscontracteffort (ref. 7)

and focused primarilyon improvingmembraneresistivityat 25° C.

11
600--

0

-- 0

0 0

0

I i i ! I I

0 1200 1400 1600 1800 2000 2200 2400

Carbonfelt processingtemperature,°C

Figure7. - Effectof processingtemperatureonsubsequenthydrogenevolution

froma gold-lead-catalyzed, carbon-feltelectrode(evaluatedbycyclicvoltammetry).

$ o=.°g o o

=_'_ 0 0

•,6o_ o

o o

_- I I I I I I I

0 1200 1400 1600 1800 2000 2200 2400

Carbonfelt processingtemperature,°C

Figure8. - Effectof processingtemperatureonsubsequentactivityfor chromousion oxidationandhydrogen-ionreductionbyagold-lead-catalyzed, carbon-felt

electrode(evaluatedbycyclicvoltammetry).

80 240F

60 160

"8 40 c

80

2O

E E

0 _ 0 L- L_

FeCI2

-20 concen- CrCI3

,., tration, _ -80 concenmM 8 tration,

_ -40 --= mM

,.3 0 ,3

50 -160 "_500

-60 ----I00 --250

- --250 50

NoCr+3

-ao t I -240 I I

-1000 -800 -600 -400 -200 0 0 400 800 120n

Potential,mV(vsSCE) Potential,mV(vsSCE)

Figure9. - Effectof FeCI2additionsonchro- Figure10.- Effectof CrCI3additions

mium redoxreactions. Gold-leadcatalyst; on iron redoxreactions. [FeCI2] -

ICrC131- 50mM. 50mM.

12
Several approacheswere evaluatedfor the reductionof membrane resistivity. The one that receivedmajor emphasis involvedheterogeneousstructures

consistingof two bonded layers: one a highly porous, fabric-supportedlowresistance"board" to provide strengthand the other a much thinner,tighter,

and more highly charged film to provide selectivity. The boards were given a

porous characterby a nonpolymerizable(NP) content rangingfrom 50 to 70 percent in the resin composition. The resin used in these boards containedchloride, amine, or oxirane functionalgroups to provide compatibilitywith films

of various chemistries. The resistivitiesof the boards and the various boardfilm compositeswere measured in 0.1HCI in the standardway. Although some of

• the compositeshad resistivitiesas low as 3.4 ohm-cm_, ther_ was not a great

improvementover the standardCDIL value of about 4.1 ohm-cm_.

Another approachtowardresistivityreductionthat was evaluatedwas the

use of thinner membranesubstrate,includingmaterialsother than the standard

calenderedwoven modacrylic. Table I presentsfor these membranesresistivities measured by Ionics, Inc., and 24° and 55° C selectivitiesmeasuredat

Lewis. Some of these membraneswill be evaluatedat Lewis in working cells

operatingat 65° C.

Toward the end of 1982, Ionics, Inc., began to develop and deliver to

Lewis anionic and cationicmembranesfor in-cellevaluationat elevatedtemperature in the mixed reactantmode of operation,which will be discussedin a

later section. Ionics,Inc., has also initiatedstabilitytests on likelymembrane candidates,soakingthem 1000 h at 65° C in 1M FeCl3 acidifiedto 1N

with HCI.

NASALewis

By the beginning of 1982 no ion exchange membranehad been developed

that offered a really attractive combination of low resistivity and good

selectivity - the ability to prevent cross-diffusion of the reactant metal

cations. The difficulty, of course, is that these two properties are opposed

to one another. Anything done to decrease resistivity, for example, tends to

cause a deterioration in selectivity. Therefore, the best combination of

properties that was arrived at represented a compromise between conflicting

ideals. Furthermore, all the indications were, at least for the present generation of membranes, that the learning curve had gone flat. That is to say

that only marginal improvements were being made in either of the characteristics, and these improvements were always accompanied by unacceptable changes

in the other characteristics.

To get a better measure of the existing situation, a set of sensitivity

studies was carried out. The effects of membraneresistivity on calculated

system efficiency and cost, with current density as a parameter, were determined first. The results are pre_ented in figures 11 and 12. The range of

resistivities (1.5 to 2._5 ohm-cm_) is considered in the light of the existing

best value of 2.6 ohm-cmL, which results from a trade-off between membraneresistivity and selectivity. For low current densities, efficiency is not very

sensitive to changes in resistivity. However, the penalty in system cost for

operating at these low current densities is significant. (These system cost

projections include the factory cost for hardware and the sales price for reactants, both of which are based on assumed mass production.) One conclusion to

be derived from these results is that from a cost standpoint, current densities

13
TABLE I.- RESISTIVITIESAND SELECTIVITIESAT 24° and 55° C, FOR IONICS,INC.,

MEMBRANESWITH CDIL RESINON VARIOUSSUBSTRATES

Substrate Thick Non- Resistivity Selectivity, Temness, pol_,nerizable in O.l _g Fe/h-cm2-M peracm content, N CHI, ture,

percent ohm-cm2 °C

Woven modacrylic 0.041 30 4.1 24 24

35 3.7 60 24

40 3.7 I07 24

201 55

Fine woven 0.038 30 3.9 19 24

modacrylic 35 3.7 65 24

40 3.6 148 24

40 224 55

Nonwoven 0.32 30 2.7 59 24

modacrylic 30 |39 55

GoldenAramid 0.013 30 3.2 31 24

(on Kevlar) 30 91 55

35 3.4 81 24

35 159 55

40 3.4 129 25

40 221 55

CDIL-AA5-LC 0.052 27.5 4.2 20 24

(standard) 27.5 95 55

14
Currentdensity, 95 -- Currentdensity,

•75 -- mA/cm2 mA/cm2

"---'-"--'-'- 25 50

50 85 --

"_ .70 ,=

. 75--

_ 65- too

I I I 6o I I I

"6_._50 1.75 2.00 2.25 1.50 1.75 2.00 2.25

Membraneresistivity, ohm-cm2 Membraneresistivity, ohm-cm2

Figure11.- Effectof membraneresistivityonRedoxsystemenergy Figure12.- Effectof membraneresistivityonRedoxsystemprojectefficiency, edcost.

E,_, 2O--

,-EC c:_ mE

_ i0

i==

I I I I 1

0 24 48 72 96 120

Membraneselectivity,pg/h-cm2-M

Figure13. - Effectof membraneselectivityon reactantreatmentinterval

neededto limit cross-mixingto 10percent. Systemcapacity,5.0MWh;

reactantvolume,260kl; batchvolume,15kl.

15
as high as 75 mA/cm2 must be attained. For acceptable efficiences at such #urrent densities, cell resistivities must approach or be less than 1.5 ohm-cm_.

The compromise value of ~20 _g/h-cm2-M (ref. 8) for the other important

membrane characteristic, selectivity, results in a gradual diffusional loss of

each reactant into the other reactant system. If it is desired to prevent

such cross-mixing from proceeding beyond some limit, two options are available:

either begin operation with a great enough excess of reactants so that over the

life of the system the decrease in the respective concentrations does not exceed the limit; or institute a batch treatment mode of operation whereby portions of contaminated reactants are periodically replaced by fresh quantities,

and the contaminated quantities are then reprocessed. An estimate of the time

interval between such batch treatments, as a function of the membrane selectivity and based on a simple diffusion model, is presented in figure 13. For the

existing best compromise value of membraneselectivity of about 20 _gFe+3/

h-cm_-M, the refurbishment interval for the specific example presented is

about 9 days. In addition to being aesthetically less than pleasing, such a

batch treatment mode of operation represents a cost burden which will become

unacceptable as the time interval between refurbishments decreases.

Another characteristic of charged membranes in an electrochemical environment is the tendency of the electrolyte solvent to be transported preferentially in one direction. This net transfer results from the complex interactions between several system properties and is not easily analyzed. In principle, however, it should be possible to adjust the two reactant concentrations

so that an isotonic situation would exist. This was accomplished several times

in the laboratory, with cells being cycled for as long as 8 months with no net

solvent transfer. However, this experience was not universal; success may

depend on such things as the cycling regimen, the total reactant volume, the

volume-to-area ratio, or other unknown parameters. For whatever the cause,

when solvent transfer does begin, it often does not lead, as would be expected,

to an isotonic situation and a cessation of transfer. In certain dramatic instances, one of the reactant systems was osmotically pumped nearly dry. As was

the case with the cross-diffusion of reactant ions, the avoidance of this situation would require some mode of electrolyte refurbishment and an attendant

cost penalty.

All of the foregoing is based on experience with, and the known properties

of membranes at room temperature. In light of the apparent necessity for operation in the temperature range of 55 to 65° C to avoid difficulties charging

the chromic ion, it became important to determine the effect of elevated temperatures on the critical membrane characteristics of resistivity and selectivity. To this end, membraneswere acquired from various sources, primarily

lonics, Inc., RAI, and several Japanese producers, and characterized at Lewis.

Selectivity was focused on because previous studies had shown (ref. 9_ that,

for the lonics CDIL membrane, at least, a temperature increase from 25 to

65° C results in a resistivity reduction of 50 to 65 percent. It soon became

obvious the temperature effect on selectivity was not as felicitous. Selectivity was determined by exposing each membranesample to a 1 molar concentration gradient of ferric ion for 24 h, then measuring the quantity of ferric ion

transferred. In table II the effect of temperature on selectivity over the

range 25° to 80° C is presented for the standard lonics membrane, a similar but

"tighter" sample from lonics, plus samples from the Electrotechnical Laboratory in Japan and from RAI. It can be seen that the selectivity in all cases

is made dramatically worse by raising the temperature. Similar results were

16
obtainedfor a groupof anionexchangemembranessuppliedby RAI,shownin

tableIll.

Based on the previouslydiscussedsensitivitystudies,from which it can

be inferredthat a selectivityof 20 _g/h-cm2-Mis, at best, marginalfor a

realisticsystem, it follows that for membranespresentlyavailableraising the

temperaturehas a disastrouseffect. If diffusionis allowed to proceed unhindered at elevatedtemperatures,total cross-mixingof reactantswould occur

very rapidly. Batch-typerefurbishments,on the other hand, would have to

occur so frequentlyas to be economicallyunfeasible.

All of the above considerationsassociatedwith the effect of temperature

on membranecharacteristicsled finally to a seriousconsiderationof operation

with premixedreactantsolutionsof identicalcompositionin the discharged

state, from the very outset. This mode of operationseemedpotentiallyto

offer some very attractiveoptions,and its evaluationwas begun during the

final quarter of 1982.

OPERATIONWITHMIXEDREACTANTS

As was discussed in the preceeding sections, results in the areas of electrode and membrane development have made necessary a re-evaluation of the basic

Redox system concept. Two significant aspects of that basic concept were the

operation at temperatures in the 20° to 25° C range and the use of separators

able to prevent significant cross-diffusion of the reactant metal ions. However, operation of iron-chromium Redox cells at normal ambient temperatures

resulted in severe difficulties when attempting to charge the chromium half

cells. It later was learned that by raising the operating temperature to 55°

to 65° C these charging difficulties were eliminated (ref. 10). The next step,

then, was to determine the effect of such temperatures on critical membrane

properties. Measurements showed that a temperature increase from 25° to 65° C

could reduce the resistivity by a factor of two or three but that the companion

effect on selectivity was to make it worse, in some cases, by an order of magnitude. Such poor selectivity (i.e., such a high cross-diffusion rate) would

require, in order to maintain some semblance of separation between the reactants, an electrolyte treatment procedure that would be unacceptably expensive.

All of this led to two options. The first was to attempt to develop an

electrode catalyst that can alleviate at ambient temperatures the chromium

reactant charging problems and at the same time to continue to attempt to improve membraneselectivity and resistivity, also at normal ambient temperatures. This approach is considered to have a low probability of success, but

is being pursued, with regard to the electrode catalyst, at Case Western Reserve University and, with regard to membranes, at lonics, Inc.

The second possible approach, the one actively being evaluated at Lewis,

is to finesse the whole cross-diffusion problem by mixing the reactants beforehand. With this approach both reactant solutions, when the cell or system is

fully discharged, are identical in composition, typically 1M FeCl 2, 1 M CrCl 3,

and 1 N HCI. Whencharged, the CrCI3 in the positive (iron) reactant solution

behaves as a nonparticipating species, as does the FeCI2 in the negative (chromium) reactant solution.

The advantages accruing to this mixed-reactant mode of operation are

17
TABLE II. - THE EFFECTOF TEMPERATUREON MEMBARANESELECTIVITY

Membrane Selectivity,_g Fe/h-cm2-M,at25° C 45° C 65° C 80° C

CDIL-AAS-LC(27.5NP) (standard) 23.1 83.4 168 501

CDIL-A5-LC(20 NP) 5.5 42.4 96 212

ElectrotechnicalInstitutesample 77.0 184.3 293 659

RAI I133-I18-5 26.4 27.8 67 158

TABLE Ill. - EFFECTOF TEMPERATUREON RAI ANION EXCHANGE

MEMBRANESELECTIVITY

Membrane Selectivity,_g Fe/h-cm2-M,at -

25° C 50° C 65° C

1133-287-1 29 97 163

1133-287-3 20 62 104

1133-239-IB 19 84 153

I133-239-IC 23 84 167

I133-239-2A 68 187 310

1133-239-2C 94 274 455

I133-239-2B 92 254 482

IonicsCDIL-AA5-LC 23 --- 168

18
significant. By equalizingthe total ion concentrationof each reactanton

both sides of the separator,the tendencyfor diffusionaltransferto proceed

is greatly diminished. Because cross-mixingthen is much less a concern,membranes of a much looserstructure,perhaps even microporousseparators,can be

used. The purpose of these primarilyis to merely prevent or diminishconvective flow from one half-cellto the other. Such membranesoffer the added

benefit of a significantlylower resistivity,which permits operationat higher

current densities,and/or improvedround-tripvoltage efficiencies. Such membranes also can be expectedto be less expensivethan the highly selective

types. Operationwith mixed reactantsalso permits a simple techniqueto be

• employed if osmotic solventtransfershould occur or if there shouldbe a loss

of capacitydue to partitioningof reactants: merelydischargethe system and

reblend the reactantsolutions.

The above advantagesfor mixed-reactantoperationare not totallywithout

compensatingpenalties. Coulombicefficienciesare lower than those of cells

with membraneshaving better selectivitiesbecause it is easier for the charged

species of the two reactantsto come into contact and undergochemicaldischarge. Also, because of the dilutioneffect of the inert species on the activitiesof the respectivereactantspecies,p_us theothermodynamiceffect of

raising the cell or system temperaturefrom 25 to 65 C, the cell open-circuit

voltage at each state of charge is reduced from 50 to 70 mV. And finally,of

course, is the obvious fact that the mixed reactantmode of operationrequires

more reactantsand thereby costs more. However,it has been determinedthat

reactant costs probablywill be much less than had been previouslyassumed and

will not militateagainst the mixed-reactantconcept.

During the final quarterof 1982, experimentalevaluationof the mixed reactantmode of operationat 65° C was begun (ref.4). These tests were carried

out using the standard14.5 cm2 laboratorycells. The reactantswere heated as

they flowed toward the cell through Pyrex tubes wrapped with sheathedNichrome

heating wire. The membranegenerallyused was a cation type designatedML-21,

producedby RAI. This membranewas selectedbecause previousscreeninghad

shown it to have_ low resistivityand a very poor selectivity,on the order

of 1000 _gFe/h-cm_-M. Two types of chromiumelectrodecatalystwere evaluated: the standardgold-leadcompositionand the new bismuth-leadformulation.

The experimentalresultsfrom the brief period of testing at the end of

1982 were extremelyencouraging. Figure 5 shows the voltage _ersus time (or,

SOC) data for a completeconstant-currentcycle at 64.5 mA/cm_. The ability

to operate at this current density,especiallyduring the entire charge portion

of the cycle speaks eloquentlyof the advantageof operationat elevatedtemperaturewith a low-resistivitymembrane. If such a test were to be attempted

at ambienttemperatureusing a selectivemembrane,the chargingvoltagewould

have reached an unacceptablelevel before 40 percent of the theoreticalcapacity was reached. Even if chargingwere to be completedin the taper current

mode, the resultantvoltage efficiencyfor the cycle would have been on the

order of 60 percent,due to resistivelosses, insteadof the indicated77.2

percent.

For this test coulombicefficiencywas calculatedfrom measurementswith

an integratingampere-hourmeter. Energyefficiencywas obtainedby integrating the areas under the charge and dischargecurves and taking their ratio.

Voltage efficiencywas calculatedfrom these measuredvalues. A significant

point to be made concerningthese data is that althoughthe total cell

19
resistivitywas about 1.74 ohm-cm2, that of the membranealone was only about

0.34 ohm-cmL. The difference,of course, is the terminalcontribution. It

can, therefore,safelybe assumed that, in a multicell,bipolar stack having

highly conductivebipRlar plates, the averagecell resistivitywould be much

closer to 0.34 ohm-cm_ than to 1.74 ohm-cmL. From thesesorts of considerations it is possibleto project that completeRedox systemsmay be operated

with energy efficienciesas high as 75 percent.

A very similarset of data is presentedin figure 6. The main difference

between the two experimentsis that here the chromiumelectrodecatalystwas

bismuth-leadinsteadof gold-lead. The only differenceof any note between the

two sets of data is the reduced hydrogenevolution,0.2 percent insteadof 0.9

percent of charge capacity,for the bismuth-leadcomposition. The encouraging

thing here is that there seems to be no differenceat all in the polarization

characteristicsof the two types of catalyst.

Data for this second test cell provide insight into a system-relatedaspect of operatingwith mixed reactants. The cell was cycled automaticallyfor

70 cycles. Duringthis time there was a tendencyfor solvent to transfertoward the iron side of the systemto the extent that on three occasionsthe cell

was fully discharged,and the reactantsolutionswere reblendedand then divided equally to the respectivehalf-cellreservoirs. Polarizationcurves, taken

at 50 percent SOC severaltimes during this test period (fig. 14), show that

such reblendingof reactantsolutionshad no adverse effect on the activityof

the bismuth-leadelectrodecatalyst.

Thus, althoughthe amountof testingwas quite limited in 1982, the results obtainedfrom the experimentalevaluationof the mixed-reactantmode of

operationat elevatedtemperaturewere very satisfactory.

REACTANTCOSTS

As mentionedearlier the viabilityof the mixed-reactantmode of operation

is to some degreedependenton the cost penaltyfor the associatedincreased

reactantrequirements. During 1982, results became availablefrom two contract

studiesof the projectedcost for large-scaleproductonof the Redox reactants

CrCl3 FeCl2, and HCI. These analyseswere performedby CharlesRiver Associates iref. 11), a consultantfirm with broad experiencein hydrometallurgical

processes,and by the Allied ChemicalCo., a major producerof chromiumchemicals. Each organizationexaminedseveral process paths for the Redox reactants, startingwith either chromiteore (Cr/Fe= 1.58 mass ratio),ferrochrome

(Cr/Fe= 1.64),or the chemicalintermediate,sodium chromate.

The results from these two independentstudieswere a very pleasantsurprise. For the six processesexamined,the highest projectedcost was about

equal to what previouslyhad been consideredto be an optimisticestimate. The

remainingcost projectionsdecreasedto as low as 45 percent of the highest.

The two sets of cost projections,adjustedto a common basis, are shown in

table IV. The specificcosts in dollars per kilowatthour assume80 percent

utilizationof reactantsand an averagecell dischargevoltage of 0.9 V.

All of the processesbased on a startingmaterial of chromiteore or ferrochrome includeprovisionfor, at some point, separatingthe iron and chromium

chlorides. This would, of course, be an unnecessarystep, with an attendent

20
Cycle [HClI,

• 1.4 -- N

A 12 2

[] 28 3

O 70 3 (after3reblendings) /x

-_ 1.o

.(_ ! I i I ! I I

20 40 60 8n 100 120 140

Currentdensity, mA/cm2

Figure14.- Chargeanddischargepolarizationdatatakenperiodically

during a/0-cycletest. Temperature,65o C; mixedreactants;bismuthleadcatalystfor chromiumelectrode.

21
TABLEIV. - TOTALCOSTSFOR PRODUCTIONOF REOOXCHEMICALS

[A11costs In millionsof dollars;22,700metrictons per year contained

chromiumplus equivalentFeCI2 andHCI.]

(a) AllledChemicalCo. study

Chlorination Methanol Electrolytic Reductive

of prereducedreduction reductionof chlorination

chromiteore of sodium sodium of chromite

chromate chromate ore

Totalinvestment,milllonsof dollars

}72.00 }71.50 }lOg,50 $275.00

Annualcharges:

Raw materials $25.72 $33.56 $ 22.34 $ 31.20

Energy 1.72 ll.50 21.46 1.72

Laborand related 6.58 5.10 6.58 6.58

fixedcapitalcharges I0.I0 I0.80 17.10 40.00

Expense 2.80 2.70 2.70 3.00

Returnon investment 21.60 21.50 32.90 82.50

Total _68.50 }85.16 _I03.08 a_131.70

Totalreactantcost:

Per kg CrC13 $ .99 $ 1.23 $ 1.49 $ 1.91

Per kwh (unmixed

reactants) 8.13 10.10 12.23 15.68

Per kwh (mixed

reactants) 16.26 20.20 24.46 31.36

(b) CharlesRiverAssociatesStudy

Hydrometallurgicpalroduction

From From Reductive

chromite ferrochromechlorination

ore of chromite

ore

Totalinvestment,millionsof dollars

$69.34 _39.g0 $g5.33

Annualcharges:

Raw materials $18.56 _36.70 _21.2g

Energy II.50 1.00 5.83

Laborand related 2.65 1.60 3.62

fixedcapitalchargesl 10.42 4.70 14.44

Administrativeexpense 3.50 2.10 3.50

Returnon investment

(30 percent) 20.80 11.07 77.28

Total _67.63 }58.07 _77.28

Totalreactantcost:

Per kg CrCI3 $ .98 $ .84 $ 1.12

Per kwh (unmixed

reactants) 8.05 6.90 9.20

Per kwh (mixed

reactants) 16.10 13.79 18.39

alncludescreditfor sale of MgCI2 and AIC13,_33.3.

22
unnecessaryexpense, if the desiredfinal productwere to be mixedlreactants.

Therefore,thespecific costs for mixed reactantsfrom these particularprocesses can be expectedto be even less expensivethan shown.

Of the processesexamined,only those involvingthe reductivechlorination of chromiteore would require new or uncertaintechnology. The remaining

processesare well understoodand their associatedcost estimates,therefore,

should be quite reliable.

The cost estimatesfor Redox reactants,especiallyfor the mixed reactant

application,are certainlynot insignificant,ranging from _14 to _31 per kWh.

' However,the most likelyproductionroutes are the two hydrometallurgicalprocesses and the methanolreductionof sodiumchromateprocess. The specificcost

projectionsfor these are in the range of $14 to $20 per kWh for the mixed reactants. Such reactantcosts would not be expectedto be prohibitivefor the

Redox storage sytem.

CONCLUDINGREMARKS

Very significantachievementsduring 1982 have combinedto further enhance

the promise of the Redox systemas a viable candidatefor bulk electricalenergy storage applications.

The present generationof membranesfor Redox cells was shown to be unable

to adequatelykeep separatethe reactantspecieswhen cells are operatedaround

65° C, which is necessaryin order to achieveadequatechargingperformance.

This led to a seriousevaluationof the mixed reactantmode of operation,which

allows the use of nonselective,low-resistivitymembranes. The results of

preliminarytesting in this operatiqgmode were extremelygratifying. During

constant-currentcycles at 65 mA/cm_, energy efficienciesgreater than 70 percent were achieved. Concurrentstudies of reactantproductioncosts indicated

that the added expense for mixed reactantsis not prohibitive.

In the electrodearea a new bismuth-leadcatalysthas been developedthat

seems to have all the advantagesof the standardgold-leadcombination,plus a

higher hydrogenoverpotentialand a more forgivingnature. Also, with regard

to electrodes,a new understandingwas reached concerningthe causes of increased hydrogenevolutionfrom gold-leadcatalyzedchromiumelectrodesduring

prolongedoperation. These causes are related to operationalprocedures,are

not inherentto the electrodeor to the Redox concept per se, and can be

eliminated.

23
REFERENCES

1. Reid, M. A.; Johnson, D. A.: The Chemical and Electrochemical Behavior of

the Cr+3/Cr .2 Half Cell in the NASARedox Energy Storage System. DOE/NASA/

12726-17, NASATM-82913, 1982.

2. Thaller, L. H.: Recent Advances in Redox Flow Cell Storage Systems. DOE/

NASA/1002-79/4, NASATM-79186, 1979. •

3. Jalan, V.; Stark, H.; Giner, J.: Requirements for Optimization of Electrodes and Electrolyte for the Iron/Chromium Redox Flow Cell. DOE/NASA/

0097-80/1, NASACR-165218, Sept. 1981.

4. Hagedorn, N. H.: The NASARedox Project Status Summary. Presented at the

5th U.S. DOEBattery and Electrochemical Contractors' Conference, (Arlington, Va.), Dec. 7-9, 1982.

5. Jalan, V.; Morriseau, B.; Swette, L.: Optimization and Fabrication of

Porous Carbon Electrodes for Iron/Chromium Redox Flow Cells. DOE/NASA/

0198-1, NASACR-167921, July 1982.

6. Gahn, R. F.; et al.: Performance of Advanced Chromium Electrodes for the

NASARedox Energy Storage System. DOE/NASA/12726-15, NASATM-82724, Nov. 1981.

7. Hodgdon, R. B.; and Waite, W. A.: Anion Permselective Membrane. DOE/NASA/

0204-1, NASACR-167872, Apr. 1982.

8. Ling. J. S.; and Charleston, J.: Advances in MembraneTechnology for the

NASARedox Energy Storage System. DOE/NASA/12726-12, NASATM-82701, 1980.

9. Gahn, R. F.; Johnson, J. A.; and Ling, J. S.: Single Cell Testing of the

NASAFe/Cr Redox System at Elevated Temperatures. Presented at the Electrochemical Society Meeting (Detroit, Mich.), Oct. 17-22, 1982.

10. Hagedorn, N. H.; and Thaller, L. H.: Design Flexibility of Redox Flow

Systems. DOE/NASA/12726-16, NASATM-82854, 1982.

11. Ammann,P. R.; Loreth, M.; and Harvey, W. W.: Study to Establish Cost

Predictions for the Production of Redox Chemicals. DOE/NASA/0251-1, NASA

CR-167882, July 1982.

24
BIBLIOGRAPHY

Ammann,P. R.; Loreth, M.; and Harvey, W. W.: Study to Establish Cost Predictions for the Production of Redox Chemicals. DOE/NASA/0251-1, NASACR-167882,

July 1982.

Gahn, R. F.; Johnson, J. A.; and Ling, J. S.: Single Cell Testing of the NASA

Fe/Cr Redox System at Elevated Temperatures. Presented at ECSMeeting,

" (Detroit, Michigan), Oct. 17-22, 1982.

Hagedorn, N. H.; Hoberecht, M. A.; and Thaller, L. H.: NASARedox Cell Stack t

Shunt Current, Pumping Power, and Cell Performance Trade Offs. DOE/NASA/12726-

11, NASATM-82686, Feb. 1982.

Hagedorn, N. H.: The NASARedox Project Status Summary. Presented at the

Fifth U.S. DOEBattery and Electrochemical Contractors' Conference, (Arlington,

VA.), Dec. 7-9, 1982.

Hagedorn, N. H.; and Thaller, L. H.: Design Flexibility of Redox Flow Systems.

DOE/NASA/12726-16, NASATM-82854, 1982.

Hodgdon, R. B.; and Waite, W. A.: Anion Permselective Membrane. DOE/NASA/

0204-1, NASACR-167872, Apr. 1982.

Johnson, D. A.; and Reid, M. A.: Spectrophotometric and Electrochemical Studies of Chromium Species in Redox Solution. Presented at ECSMeeting (Detroit,

Mich.), Oct. 17-22, 1982.

NASARedox Storage System Development Project - Calendar Year 1980. DOE/NASA/

12726-18, NASATM-82940, Dec. 1982.

Reid, M. A.; and Johnson, D. A.: The Chemical and Electrochemical Behavior of

Cr(lll)/Cr(ll) Half-Cell in the NASARedox Energy Storage System. DOE/NASA/

12726-17, NASATM-82913, 1982.

Swette, L.; Jalan, V.; and Morriseau, B.: Optimization and Fabrication of

Porous Carbon Electrodes for Iron/Chromium Redox Flow Cells. DOE/NASA/0198-1,

NASACR-167921, July 1982.

25
1. Report No. 2. Government Accession No. 3. Recipient's Catalog No.

NASATM-83469

4. TitleandSubtitle 5. ReportDate

NASARedox Storage System Development Project October 1983

Calendar Year 1982 6: Performing Organization Code

776-72-41

7. Author(s) 8. Performing Organization Report No. "

E-1784

10. Work Unit No. "

9. PerformingOrganizationNameandAddress

National Aeronautics and Space Administration 11. Contract orGrant No.

Lewis Research Center

Cleveland, Ohio 44135

13. Type of Report and Period Covered

12. Sponsoring Agency Name and Address Techni cal Memorandum

U.S. Department of Energy

Division of Energy Storage Systems 14Sponsoring . Agency C'o'0_Rep " ort No.

Washington, D.C. 10545 DOE/NASA/12726-23

15. Supplementary Notes

Annual report. Prepared under Interagency Agreement DE-AIO4-80AL12726.

16. Abstract

Development was continued for iron-chromium battery operation at 650 C. Membranes that were adequate at 25o C were shown to be unacceptable at 650 C with

regard to selectivity. This led to the elevated-temperature, mixed-reactant

modeof operation, in which each reactant solution, when discharged, contains

both ferreus and chromic chlorides. This operating modeallows the use of very

low-resistivity membranes, resulting in high energy efficiencies at current densities. It also allows the use of very simple techniques to correct for solvent

or reactant transfer through cell membranes. Screening of candidate catalysts

for the chromium electrode led to the development of a bismuth-lead candidate

having several attractive characteristics. The project effort was supported by

the Division of Energy Storage Systems of the Office of Conservation and Renewable Energy of the U.S. Department of Energy under Interagency Agreement DE-AIO4-

80AL12726. Project supervision was provided by the Storage Batteries Division

of Sandia National Laboratories.

17. Key Words (Suggested by Authods)) 18. Distribution Statement

Electrochemistry Unclassified - unlimited

Redox STAR Category 44

Storage DOECategory UC-94cc

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