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NASA Technical Memorandum 83371 NASA-TM-83371 19830022279

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! Activation Parameters of FlowThrough Battery Separators

Ratan L. BlokhraLewis Research CenterCleveland, Ohio

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ACTIVATIONPARAMETERSOF FLOWTHROUGHBATTERYSEPARATORS

Ratan L. Blokhra*

National Aeronautics and Space AdministrationLewis Research CenterCleveland, Ohio 44135

SUMMARY

Studies of the hydrodynamic flow of water and 45 percent potassiumhydroxide (KOH) solution through a microporous and an ion-exchange separatorare described. The permeability values are interpreted in terms of a pseudo-activation process. The enthalpy of activation, AH*, and the entropy ofactivation, AS*, have been estimated from Eyring's rate equation.

I

INTRODUCTION

The performance of most electrochemical devices has been largely attri-buted to the proper functioning of the separators. During operation of thesedevices, the electrolyte components move from one electrode compartment toanother through the separators. This flow depends on the selective charac-teristics of the separator arising from porosity, the hydrophilic or hydro-phobic character, and the charge density of the separator.

The hydrodynamic flow of a fluid through a porous medium can be eitherviscous flow or diffusional flow or a combination of the two as characterizedby irreversible thermodynamics. If there is a gradient of chemical potentialacross the separator, the overall flow will be dominated by the diffusionalflow, and if there is a pressure difference (developed due to evolution ofgases at the electrodes or to electro-osmosis), the flow will be viscous flow.Further, during charging and discharging of the battery, _emperature fluctua-tions occur, and these flows vary exponentially (ref. i) with temperature.The dependence of these flows on temperature is characterized in terms of theactivation energy (ref. 2). The present study determined activationparameters for the hydrodynamic flow of water and 45 percent KOHsolutionthrough a microporous separator (viz, fuel cell grade asbestos (FCGA) having 5percent butyl latex rubber (EBL) as binder supplied by Quinn-T. Co., NewHampshire) and an ion-exchange separator (viz, P2193, 40/60 supplied by RAIResearch Corporation, NewYork).

EXPERIMENTALPROCEDURE

Materials

Certified 45 percent KOHsolution (Fisher Scientific) and deionized water(conductivity ~ 106 ohm-1 cm-1) were used.

*Professor, Himachal Pradesh University, Simla, India, and NRC-NASAResearchAssociate.

Apparatus

The apparatus consisted of two half-cells made of Plexiglass. Each half-cell had a volume of about 10 milliliters and O.635-centimeter stainless steelfittings for connections to a pressure head. A glass capillary of 1 millimeterinternal diameter was used for determining the rate of flow. The half-cellsare clamped against the separator. Whenstudying the FCGA+ 5 percent EBLseparator, pressure was applied by raising or lowering the pressure chamber.In the case of the P2193, 40/60 separator, pressure was applied with nitrogengas. The studies with 45 percent KOHsolution were performed in a nitrogenatmosphere, and contact with the ambient atmosphere was avoided by using guardtubes and intermediate chambers of KOHpellets.

The separator was fixed between the two half-cells with neoprene rubbergaskets on both sides, and RTVI02 cement was used for sealing. Becauseleakage was the major problem when studies were performed with 45 percent KOHsolution, 3.0 psi pressure of nitrogen gas was applied on the inlet side ofthe cell placed in the water bath. Absence of evolution of gas bubbles in thewater surrounding the cell waschosen as the criterion for proper sealing. Astainless steel screen was used as the support for the ion-exchange membraneand a polyethylene screen for the microporous separator. All experiments wereperformed in a water bath where temperatures could be controlled to within• 0.05 ° C. The capillary height readings were recorded with a travelingmicroscope having a resolution of 0.001 centimeter. In the case of themicroporous separator, the flow was fast, and the time required for 5 centi-meters of flow in the capillary was recorded with a stopwatch having aresolution of 0.02 second.

RESULTSAND DISCUSSION

The dissipation function _ (ref. 3) for the transport process of liquidsthrough a membrane under the influence of a pressure difference AP and con-centration difference AC can be written as

: Jv.aP + JD.AX (I)

where Jv is the volume flow, JD is the diffusional flow, AP is thepressure difference, and AT is the difference in osmotic pressure across themembrane and is equal to RTAC. The phenomenological equations (refs. 3 and 4)relating the flows and forces given in equation (1) are

Jv : Lp*AP + LpD.RTAC (2)

JD = LDp'AP + LD*RTAC (3)

For such a system, Onsager's reciprocal relation (refs. 5 and 6) is

kpD = kDp (4)

where _ and LD are the mechanical coefficient of filtration (commonlycalled e permeability coefficient), and the diffusion coefficient,respectively.

Let us considerthe experimentin whichtheconcentrationof the soluteis the sameon bothsidesof the membrane,so that AT, if any, is zero. Now,if a pressuredifferenceis maintainedacrossthe membrane,thereexistsavolumeflow Jv. Whenthe concentrationis the sameon bothsidesof themembrane,the volumeflow Jv can be givenas

Jv = Lp AP (5)

The values of Lp are estimatedfrom the Jv and AP values. The valuesof Jv, AP, and Lp for water and 45 percentKOH solutionthroughtheseparatorsat differenttemperaturesare given in tables I and II. The plotsof Jv as a functionof AP for permeationof water throughFCGA + 5percent EBL are shown in figures 1 to 4. The Lp values of microporousseparatorshave an uncertaintyof 3.5 percentand ion-exchangeseparatorsanuncertaintyof 1.2 percent.

Tables I and II show that the permeabilityof the microporousseparatorLp is five orders of magnitudelarger than that of the ion-exchangesepa-rator. This is not unexpectedbecause the microporousseparatorhas largerpores. The permeabilityis equivalentto the fluidityof the liquid,and theviscosityis the reciprocalof the fluidity (ref. 2). In general, it is morecommon to refer to permeability,the tendencyto flow, rather than viscosity,the resistanceto flow.

In the case of the microporousseparator,the permeabilityis largerwithwater, and this may be explainedas follows: In the absenceof electrolyte,the thicknessof the electricaldouble layer in the pores is small enough notto affect the effectivepore diameter. As the electrolyteconcentrationincreases,the thicknessof the double layer increases,and the effectiveporesize decreases,thus decreasingthe permeabilityof 45 percentKOH solution.In the case of the ion-exchangeseparator,the permeabilityis smallerwithKOH solutionthan with water. This may be attributedto the swellingcharacteristicsof the separator. In the absenceof electrolytethe polymericion-exchangemembranemay be highly swollen. As the electrolyteconcentrationincreases,the membranemay shrink,thus making the effectivepore sizesmallerand decreasingthe permeability.

The compatibilityof equation (5) with Poissuille'slaw requires (refs. 7and 8) that

i=n4

Lp = _ _ Yii=l 8_ (6)

where y representsthe radii of the ith capillary,n is the numberofcapillariesor pores in the separator,n is the viscosityof the permeant,and £ is the thicknessof the separator. The replacementof a singlecapillaryor pore by a porous separatoris not expected to change the basicform of the equation (ref. 8). The variationof the viscosity n of a liquidwith temperaturecan be expressedas an activationprocess:

E /RTn = Ae n (7)

where A and En are constants,En being the activationenergy (ref. 1)per mole for the flow; R is the gas constant;and T is the temperatureinKelvin. Substitutionof equation (7) into (6) and taking the logarithmyield

3

En (8)log Lp = K - R-T

wherei =n 4

K = log _ 81A - constanti=1

When log Lp is plotted as a function of I/T, a straight line is obtained.This is shown in figures 5 to 8. The values of En estimated from theplots are given in _able III.

Identifying E_ with the enthalpy of activation for viscous flowAH* and using the Eyrin9 rate equation result in

n :_--exp exP\RT j (9)

where n is viscosityof the permeant,N is Avogadro'snumber,h isPlanck'sconstant,V is the molar volume of the permeant,and AS* is theentropyof activation. The entropy of activation AS* of flow throughtheseparatorwas estimatedby using n values for water and 45 percent KOH andsolutionfrom the literature(ref. 8), using values for the constants N andR, and taking V as 18.0 and 12.5 for water and KOH solution,respectively.These values of AS* are given with AH* in table IV.

The negative value of the entropy of activation indicates (ref. 10) thatthe water molecules were confined to states of high order as they weretransformed across the microporous separator. This may be attributed to thehydrophilic nature of the separator. Water enters into electrostatic inter-action with the chemical components of the walls of the pores of the separator,and consequently a state of high order is attained. With the KOHsolution,the smaller magnitude of AS* may be attributed to the competition betweenthe water molecules bonding with the separator material and the solution ionsdue to hydration. The positive value of AS* for the ion-exchange separatorsuggests the absence of electrostatic interactions as the permeant moleculesare transferred across the separator.

CONCLUDINGREMARKS

The study shows that there is a large difference in the activation param-eters for the transport of liquids through microporous and ion-exchangeseparators. The enthalpy of activation and entropy of activation of transportare useful properties for characterizing different battery separators.

REFERENCES

1. Glasston, S.; Laidler, K. J.; Eyring, H.: Theory of Rate Processes.McGrawHill, 1941.

2. Barrer, R. M.: Diffusion in and Through Solids. Cambridge UniversityPress (London), 1951.

3. Katchalsky, A.; and Curran, P. F.: Non-equilibrium Thermodynamics inBio-physics. Harvard University Press (Cambridge), 1967.

4. Lakshminarayanaiah, N.: Transport Phenomena in Membranes. AcademicPress, 1969.

5. Onsager, L.: Reciprocal Relations in Irreversible Processes I. Phys.Rev., vol. 37, Feb. 1931, pp. 405-426.

6. Onsager, L.: Reciprocal Relations in Irreversible Processes II. Phys.Rev., vol. 38, Dec. 1931, pp. 2265-2279.

7. Blokhra, R. L.; Parmar, M. L.; and Sharma, V. P.: Non-equilibrium Thermo-dynamic Studies of Electro-Kinetic Effects-X Ethyleneglycol and WaterMixtures. Colloid and Interface Science, vol. IV, M. Kerker, ed.,Academic Press, 1976, pp. 259-280.

8. Blokhra, R. L.; and Kohli, S.: Flow Through Porous Media. Part III. J.Electroanal. Chem. Interfacial Electrochem., vol. 124, 1981, pp. 285-295.

9. Weast, Ro C., ed.: Handbook of Chemistry and Physics. 57tho edo, CRCPress, 1976.

i0. Reid, C. E.; and Kuppers, J. R.: Physical Characteristics of OsmoticMembranes of Organic Polymers. J. Appl. Polym. Sci., vol. 2, 1959pp. 264-272.

TABLE I. - PERMEABILITYDATAFORWATERAND45 PERCENTKOHSOLUTIONACROSSMICROPOROUSSEPARATOR

(a) Permeant, water

Temperature, Pressure Volume flow, Permeability°C difference, Jv, coefficient,

AP, Lp,cm of water cm3 sec-I

cm3 sec-1 arm-1

25 4.00 3.00x10 -23.50 2.82 7.443.25 2.333.00 2.20

40 4.00 5.00x10 -23.50 4.68 13.023.00 3.822.50 2.87

45 3.75 6.03xi0 -23.50 5.56 16.543.00 4.752.25 3.65

50 4.00 6.68xi0 -23.50 5.80 16.933.00 4.852.50 3.90

(b) Permeant, 45 percent KOHsolution; pressure,2 centimeter height of solution

Temperature, Volume flow, Permeability°C Jv, coefficient,

Lp,cm3 sec-1

cm3 sec-1 atm-I(a)

24 2.60x10 -4 0.13530 3.33 .17235 4.18 .21640 5.33 .275

aValues represent averages of six or morereadings. Flow rate was estimated from timerequired for liquid meniscus to flow a fixeddistance under 2 cm of KOHsolution pressure.

TABLE II. - PERMEABILITYDATAFORWATERAND45 PERCENTKOHSOLUTIONACROSSION-EXCHANGESEPARATOR

(a) Permeant, water; pressure, 3.0 psi of nitrogen gas

Temperature, Volume flow, Permeability°C Jv, coefficient,

Lp,cm3 sec-1

cm3 sec-1 atm-1

22 1.04x10 -7 0.51 x10-630 3.49 1.7140 6.01 2.9450 8.94 4.38

(b) Permeant, 45 percent KOHsolution

Temperature, Volume flow, Permeability°C Jv, coefficient,

Lp,cm3 sec-i

cm3 sec-1 atm-I

20 0.79xi0 -7 0.39xi0 -630 1.39 .6840 4.06 1.9950 8.11 3.97

TABLE III. - ACTIVATIONENERGYOF FLOWTHROUGHSEPARATORS

Separator Permeant Activation energy,

kcal mo1-1

Microporous Water 1.59,0.08KOHsolution 3.39±0.10

lon-exchange Water 7.78*0.05KOHsolution 6.47*0.05

TABLE IV. - ACTIVATIONPARAMETERSOF FLOWTHROUGHSEPARATORS

Separator Permeant Enthaply, AH*, Entropy, AS*,

kcal mo1-1 cal deg-1 mo1-1

Microporous Water 1.59-0.08 -11.77-0.60KOHsolution 3.3930.10 - 6.77_0.20

lon-exchange Water 7.78*0.05 9.35*0.08KOHsolution 6.47±0.05 3.75±0.09

8

3x10"2

0 0

vJ

%

-_ 1o

I0 1 2 3 4 5

Pressuredifference,Ap, cmof water

Figure1. - Volumeflowasfunctionof pressuredifferencefor microporousseparatorat25°C.

4"_1°-2 oi

% o

-_ 2.4

1.2

I I II 2 3 4 5

Pressuredifference,AP, cmofwater

Figure2.-Volumeflowasfunctionofpressuredifferenceformicroporousseparatorat40°C.

m

_=2/o-- I I I I i0 I 2 3 4 5

Pressuredifference,AP, cmof water

Figure3. - Volumeflowasfunctionof pressuredifferencefor microporousseparatorat450C.

8x10-2

6

u

4

o

2

I I I I I0 I 2 3 4 5

Pressuredifference,AP, cmofwater

Figure4.-Volumeflowasfunctionofpressuredifferenceformicroporousseparatoratso°c.

1o6 m

1.2 m

.8--

.4 I I I I ! I306 314 322 330 338 346 354x10"5

liT, K-1

Figure5. - Arrhenius plotfor flowof waterthroughmicroporousseparator.

O

.60

.68

.84

,92 I I I I320 325 330 335 340x10"5

l/I, K°1

• Figure6. - Arrhenius plot for flowof 45percentKOHsolutionthrough microporousseparator.

6.0

6.5 I I I I I300 310 320 330 340 350x10"5

l/l, K-1

Figure7. - Arrhenius plotfor flowof waterthrough ion-exchangeseparator.

5 m

_°- 6

7 I I ! ! I300 310 320 330 340 350x10"5

liT, K-1

Figure8. - Arrhenius plot for flowof 45percentKOHsolutionthrough ion-exchangeseparator.

1. ReportNo. '2. GovernmentAccessionNo, 3. Recipient'sCatalogNo.NASATM-83371

4. Title and Subtitle 5. Report Date

May 1983ACTIVATION PARAMETERSOF FLOWTHROUGHBATTERYSEPARATORS 6. Performing Organization Code

506-55-527. Author(s) 8. PerformingOrganizationReportNo.

Ratan L. Blokhra E-163710. Work Unit No.

9. Performing Organlzatlon Name and Address11. Contract or Grant No. \

National Aeronautics and Space AdministrationLewis Research CenterC1eve1and, 0hi o 44135 13.TypeofReport and Period Covered

12.SponsoringAgencyNameandAddressTechnical Memorandum

National Aeronautics and Space AdministrationWashington, D.C. 20546 14.SponsoringAgencyCode

15.SupplementaryNotes

Ratan L. Blokhra, Himachal Pradesh University, Simla, India, and NRC-NASAResearch Associate.

16. Abstract

Studies of the hydrodynamic flow of water and 45 percent potassium hydroxide(KOH) solution through a microporous and an ion-exchange separator are described.The permeability values are interpreted in terms of a pseudoactivation process.The enthalphy of activation AH* and the entropy of activation AS* have beenestimated from Eyring's rate equation.

17. Key Words (Suggested by Author(s)) 18. Distribution Statement

Membranes; Separators; Activation energy Unclassified - unlimitedparameters STARCategory 25

19. Security Classlf. (of this report) 20. Security Classit. (of this page) 21. No. of pages 22. Price"

Unclassified Unclassified

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