Tải bản đầy đủ - 0 (trang)
V. CONDUCTIVITY AND PERMEABILITY IN MEMBRANES

V. CONDUCTIVITY AND PERMEABILITY IN MEMBRANES

Tải bản đầy đủ - 0trang

480



Richard S. Yeo and Howard L. Yeager



in such applications is the membrane conductivity, because ohmic

losses due to membrane resistance can significantly increase energy

consumption of the electrolytic cell and energy loss of the fuel cell.

Extensive studies of the conductivities of conventional Nafion

membranes in various industrially important electrolytes have been

carried out in several laboratories in recent years. The conductivity

of the carboxylate membranes has been studied only in alkaline

electrolytes because of its primary application in the chlor-alkali

industry.

1. Conductivity in Pure Water—Solid Polymer Electrolyte Systems

In the dry state Nafion behaves like an insulator.63183 However,

the membrane becomes conductive when hydrated. Figure 19 shows

the conductivity of hydrated Nafion,183 along with that of several

polymers containing sulfonic acid groups,8116'215 as a function of

water content. The Nafion polymer becomes conductive when it is

exposed to the atmosphere and absorbs ~6H 2 O/SO^ of moisture.

It has been shown that membranes with this amount of hydration

have sufficient conductivity for use as a semisolid proton conductor

in WO3-based electrochromic displays.116 The water content of the

membrane further increases after immersion in water, resulting in

higher conductivity. This membrane conductivity is regarded as the

intrinsic conductivity because it stems from the strong acidity of

the materials [Section II.5(iv)]. The solid curve is calculated from

the Bruggemann equation216:

K=0.54Ke(l-Vp)15



(11)



where Vp is the volume fraction of polymer and Ke is considered

as the conductivity of sulfuric acid with concentration equal to that

of the sulfonic acid group of the membrane. Equation (11) implies

that K is a strong function of Vp. This is supported by the fact that the

conductivity of the E-form membrane is higher than that of N-form

samples, despite the fact that the latter has a higher value of Ke.lS3

Table 6 compares the conductivity of various polymers and

electrolytes containing sulfonic acid groups. The intrinsic conductivity of Nafion is high and is very similar to other polymers

containing sulfonic acid groups. The activation energy of proton

conduction of Nafion is low in comparison with other polymers,



Properties of Perfluorinated Ion-Exchange Membranes



481



O



o

'e 6



Q



O

O



2 <

Q:



GD



10



20



30



Figure 19. Conductivity of ion-containing polymers as a

function of water content: (O) Nafion; (

) calculated

from Eq. (11); (• • •) polystyrene sulfonic acid; (

)

poly-2-acrylamido-2-methylpropane

sulfonic

acid;

(— • —) polyethylene sulfonic acid. (Ref. 182; reprinted

by permission of the publisher, The Electrochemical

Society, Inc.)



presumably because the water in Nafion is not strongly hydrogen

bonded [Section II.5(iii)]. This high intrinsic conductivity, together

with the strong hydrophilicity [see Section II.5(ii)], makes Nafion

an excellent "electrolyte" for many electrochemical applications.

At present, this polymer is employed as the SPE in many

electrochemical cells.217 Besides, many new ideas of using this

polymer as electrolyte have been suggested. It has been shown that

Nafion has potential for use as a solid "superacid" catalyst for

many electroorganic syntheses67103"107 as well as a conductive polymer coating for electrodes.26'27'58'89126157

In SPE cells, electrocatalyst particles are bonded to the membrane surfaces. It is important to mention that good bonding



482



Richard S. Yeo and Howard L. Yeager



Table 6

Conductivity for Some Ion-Containing Polymers in Water0

K



Polymers



IEC b



(O" 1 cm" 1 )



Eac



References



Nafion

AMPS*

PESA*

PSSA /

PSAg



0.83





0.06

0.03

0.08

0.09

0.04



9.41

19.25

19.25



183

116

116

8

8



1.97

2.00







a



From Reference 182.

IEC is the ion-exchange capacity (meqg-polymer" 1 ).

c

Ea is the activation energy (kJ moP 1 ).

d

AMPS is poly-2-acrylamido-2-methylpropane sulfonic acid (water soluble).

e

PESA is polyethylene sulfonic acid (water soluble).

f

PSSA is polystyrene sulfonic acid.

g

PSA is phenol sulfonic acid.

b



between electrode and membrane is essential, because any water

film and gaseous products which exist between the electrode and

membrane surface can produce an extremely high contact resistance. Although deionized water is the only fluid circulated through

the SPE water electrolyzer, the environment that the electrodes

encounter is highly acidic. The sulfonic acid groups at the membrane

surfaces produce an acidity equal to 20 wt % sulfuric acid [Section

II.5(iv)]. Because of this acidic medium, acid-resistant noble metals

or their oxides are utilized as anode materials,156'218 while platinum

serves as the cathode material.79

The perfluorinated carboxylic acid membrane exhibits a higher

resistance than Nafion membranes in SPE water electrolyzers.219

This is primarily due to small membrane swelling and slight dissociation of the carboxylic acid group in water or acid electrolyte with

a pH < 2.178181

2. Conductivity in Acidic Electrolytes



The conductivities of Nafion in HC1 and HBr have been studied

extensively by Yeo et a/.174175 Results for E-form Nafion in HBr

are given in Fig. 20. There are several features noteworthy. First,

membrane conductivity is about one order of magnitude less than

the electrolyte conductivity due to the large volume fraction of



483



Properties of Perfluorinated Ion-Exchange Membranes

14 |



1 14



LL)



f>



uJ

- 2



10

20

30

40

HBr CONCENTRATION wt %



50



Figure 20. Membrane (

) and free-acid (

) conductivity as a function of HBr concentration: (•) H2O and (O)

HBr. (Ref. 175; reprinted by permission of the publisher,

The Electrochemical Society, Inc.)



nonconductive material in the membrane. Second, the membrane

has a maximum conductivity at lower acid concentration as opposed

to the conductivity maximum for the acids. This maximum conductivity occurs because the hydrogen ion concentration decreases at

low acid concentrations and the water content of the membrane

decreases with increasing acid concentrations. Another salient

feature is that, for electrolytes of less than 0.3 in molarity, the

membrane conductivity is higher than the electrolyte conductivity

because the intrinsic conductivity of the membrane becomes





18^



important.

If one considers that the mobility for diffusion is the same as

that for conductance, which is particularly true for the cases of

water and dilute solutions, then the diffusivity can be related to the



484



Richard S. Yeo and Howard L. Yeager



conductivity by the Nernst-Einstein equation220:

1000K RT

l



~



D



(12)



F2



where K is the equivalent conductance and C, is the concentration

of the mobile ions; R, T, and F have their usual meanings. The

conductivity of Nafion in varying HCl electrolytes has been reported

previously,174 while the diffusivity data was shown in Table 3.

Hence, the concentration of mobile ions in Nafion in equilibrium

with various HCl concentrations can be calculated from Eq. (12)

and the results are shown in Table 7. The concentration of fixed

ions calculated from Eq. (8), based on the ion-cluster model are

also included in Table 7 for comparison. It is clear that the transport

data support the ion-cluster morphology suggestion for Nafion

[Section II.3(ii)].

As expected, the membrane conductivity is improved at elevated temperatures174 due to the increase in electrolyte uptake and,

to a lesser degree, the increase of the electrolyte conductivity. An

improvement in the performance of HCl cells by the better membrane conductivity at elevated temperatures has been observed.221

This is an excellent example since the electrode kinetics of the

hydrogen and chlorine reactions are fast and the only "overpotential" is ohmic.

The conductivity of these perfluorinated membranes has been

found to be affected strongly by the history of treatment of the

membrane.174175 As shown in Table 8, the conductivity increases

in the order S-form
Comparison of Ionic Charge Concentrations Derived from

Transport and Uptake Measurements0

Electrolyte

concentration

0%

10%

20%

25%

a



HCl

HCl

HCl

HCl



From Reference 182.



Calculated from

Eq. (8)



Calculated from

Eq. (12)



5.21 mol dm" 3

7.13

9.40

11.21



8.08 mol dm" 3

12.05

11.52

12.76



Properties of Perfluorinated Ion-Exchange Membranes



485



Table 8

Effect of Pretreatment on Conductivity

of Nation Membrane in 24% HBr"



a

b



Sample



We



K



E-form

N-form

S-formb



25.9

17.8

11.4



0.098

0.076

0.035



From Reference 175.

Dried in vacuum at 140°C.



trolyte uptake is an important factor in determining membrane

conductivity and the voltaic efficiency of many membrane cells.

Yeo and Chin175 have observed that the membrane hysteresis

effect occurs when the change of electrolyte concentration is faster

than the change of concentration in the membrane, which is controlled by the diffusion of electrolyte in the membrane. The electrolyte

content in the membrane does not reach its equilibrium value under

this condition. The authors have thus suggested various methods

for controlling this hysteresis effect and the electrolyte content of

the membrane, so that a higher cell efficiency can be obtained.

3. Conductivity in Alkaline Electrolytes



Most studies of membrane conductivity in alkaline electrolytes have

been carried out in concentrated solutions.63'77'96133'142'146'176'222 The

conductivity of Nafion in concentrated alkaline solutions is generally one order of magnitude smaller than that in pure water or

in acid electrolytes, because the membranes absorb far less electrolyte when in neutralized form, as shown by Eq. (6).

The effect of NaOH concentration and temperature on the

swelling and conductivity of Nafion has been recently studied by

Men'shakova et al.96 The influence of NaOH concentration on the

membrane conductivity is rather similar to the case of acid electrolytes in that a conductivity maximum occurs. The authors found

that the conductivity maximum of the membrane is at —20% NaOH,

i.e., close to the concentration where one observes the conductivity

maximum of the free electrolyte.96 However, the conductivity

decrease following the maximum is more drastic in the membranes



486



Richard S. Yeo and Howard L. Yeager



than in solutions of the same concentration. The low conductivity

of concentrated solutions is ascribed to the less hydrated state of

the membrane. It has been pointed out that this is evidence for

stronger binding of the mobile ions by the matrix at lower water

contents in the matrix.

It has been recently reported142 that Nafion membranes show

an ohmic behavior in 5 M NaOH, while in 10 M NaOH solution

the specific conductance of the membranes increases with increasing

current density. It is suggested that the passage of high currents at

a severely dehydrated membrane may produce morphological

changes that alter the character of the ionic conduction paths in

the polymer. Hsu et al63 have observed that the membrane conductivity of Nafion in alkaline electrolyte exhibits ion percolation

behavior and can be described by

* = ic e (l- V P -V T ) 1 5



(13)



where VT is the threshold volume fraction of the aqueous phase

and Ke is the electrolyte conductivity. There is an (ionic) insulatorto-conductor transition in Nafion around VT. Based on the percolation theory, VT is 0.15 whereas an experimental value of 0.10 for

Nafion in NaOH electrolyte is observed. It is interesting to note

that Eq. (13) resembles the Bruggeman equation and Eq. (11) if

VT = 0.

The Flemion membrane behaves somewhat differently in that

the conductivity maximum appears at a concentration of less than

12 wt % NaOH.146 The membrane conductivity decreases with

increasing caustic concentration; this is ascribed to the decrease in

ionic mobility which is caused by the loss of water in the membrane.

The very low value of the conductivity and the high value of the

activation energy in a 40% caustic solution implies that there exists

a strong interaction between counterions and fixed ions in the

membrane. Figure 21 compares the conductivity of the sulfonate

and carboxylate membranes in NaOH. For the same IEC and

electrolyte concentration, the sulfonate membrane is more conductive than the carboxylate membranes,133146 primarily because sulfonate samples absorb more electrolyte. However, the carboxylate

membranes of high IEC exhibit higher conductivity than the sulfonate samples because the IEC of the carboxylate materials can be

higher than that of the sulfonate polymer in membrane forms.



Properties of Perfluorinated Ion-Exchange Membranes



487



ACID TYPE



I



08



10



12



I



I



J_



14



16



18



20



ION EXCHANGE CAPACITY, meq/g

Figure 21. Dependence of membrane conductivity on ionexchange capacity for perfluorosulfonate and carboxylate

membranes, in 35% NaOH, at 90°C. (Ref. 146; reprinted by

permission of the publisher, The Electrochemical Society, Inc.)



Yeo et al. have reported176 an analysis of the conductivity of

Nafion in different alkaline electrolytes, based on the correlation

of membrane conductivity with water content. The analysis reveals

that larger conductivities arise when the membranes are equilibrated

with NaOH solutions than with KOH solutions of equal molarity.

Also, it is shown that better conductivity can be realized with thinner

and lower-EW membranes. These effects have been proven in an

alkaline-water electrolyzer176 and in relation to the conductivity

Table 9

Effect of Pretreatment on Conductivity of Nafion

Membrane in 40% KOHa



a



Temperature

(°C)



Pressure

(atm)



(wt%)



( I T 1 cm" 1 )



100

150

150

175

200



1.0

4.9

35.2

35.2

703.0



28.2

38.8

86.4

116.2

370.0



1.15 x 10" 4

0.011

0.046

0.071

0.200



From Reference 222.



K



Richard S. Yeo and Howard L. Yeager



488



results of Bratin and Tomkiewicz.23 It has been reported142'222 that

the conductivity of Nafion membrane can be increased considerably

by soaking the membrane in water at temperatures above 100°C

and at elevated pressure, as shown in Table 9. The membrane is

fully swollen and the electrolyte uptake is high with such pretreatments. In general, membrane conductivity exhibits Arrhenius

behavior: the activation energy data for ion conduction of Nafion

in various electrolytes are summarized in Table 10, whereas in Table

11 data are compared for carboxylate and sulfonate membranes.

Table 10

Conductivity of Nafion Membrane of 1200 EW in Various

Electrolytes"

Electrolyte

concentration



Electrolyte

uptake



Conductivity

at 25°C

(xl0 3 ft" 1 cm" 1 )



H2O

H2O

H2O



7.8

17

30



22.8

52.2

68.0



9.46

9.46

9.37



183

183

183



12.9

11.9

10.5

8.8

7.8

6.8

4.8



58.8

62.5

46

23

14.3

6.8

3.9



10.33

11.59

10.75

11.59

16.95

13.47

15.69



174

174

174

174

174

174

174



3.93

5.3



12.07

17.74



49

49



6.0



1.7



25.02



82



5.7







77



12.8

12.1

9.6

8.4

8.1



6.0

9.3

9.7

6.0

1.0



15.43

14.86

13.08

15.43

17.79



96

96

96

96

96







10.0c

10.0c



142

142



5%

10%

15%

22%

26%

30%

37%



HC1

HC1

HC1

HC1

HC1

HC1

HC1



4.5% KC1

25% NaCl

5.4% KOH



7

10



0.4% NaOH

5%

10%

20%

30%

40%



NaOH

NaOH

NaOH

NaOH

NaOH



17% NaOH

30% NaOH

a

b

c



F



b



From Reference 182.

Ea is the activation energy (kJ moP 1 ).

For Nafion of 1150 EW and a temperature range of 100-180°C.



References



Properties of Perfluorinated Ion-Exchange Membranes



Table 11

Conductivity of Perfluorinated Ionomer Membranes in

35% NaOHfl

Ion-exchange

capacity

(meqg" 1 )



Conductivity

at 25°C



Sulfonate



0.82

0.91



1.6

21



38

40.6



Carboxylate



1.23

1.48

1.70

1.88



2.3

8.5

87

300



64

60

50

43.5



Membrane



a



Activation

energy

(kJ moP 1 )



From Reference 146.



4. Conductivity of Nafion in Protic Solvents



The membrane conductivity of Nafion in various protic solvents

has been measured in a recent study179 and the results are given in

Table 12. The ratio of the membrane conductivity (K) to the solvent

conductivity (KC) is listed in the last column of the table. A plot of

the conductivity ratio (K/K€) VS. the solubility parameter of the

solvent is shown in Fig. 22. The membrane conductivity is higher

than the solvent conductivity in all solvents except formamide,

Table 12

Conductivity of N-form Nafion in Protic Solvents"

Solvent

6



Water

Formamide

Glycerol

Ethylene clycol

Methanol

Ethanol

1-Propanol

2-Propanol

i-Amyl alcohol

a

b

c



From Reference 179.

E-form.

Units: (cal/cc) 1/2 .



Solubility

parameter c

23.4

19.2

16.5

14.6

14.5

12.7

11.9

11.5

10.0



K



(IT 1 cm"1)



((T 1 cm"1)



6



2



1.6

4.3

4.4

1.6

2.3

1.66

1.72

2.7

1.9



x 10"

x 10~4

x 10~6

x 10"6

x 10"5

x 10~6

x 10~6

x 10"6

x 10"6



6.8

1.6

7.5

1.1

6.8

1.94

1.88



x 10~

x 10"4

x 10"6

x 10~5

x 10"5

x 10~5

x 10~5



1.17 xlO" 56

2.9 x 10"



K/K€



4 xlO 4

0.4

1.7

6.9

2.2

11.7

10.9

4.3

1.5



490



Richard S. Yeo and Howard L. Yeager



6

10

14

18

22

SOLUBILITY PARAMETER OF SOLVENT,

(cal/ccV'a

Figure 22. Conductivity ratio ( K / Ke) of Nafion vs. solubility

parameter of solvents. (Ref. 179; reprinted by permission

of the publisher, The Electrochemical Society, Inc.)



presumably because of the high dissociation of the ionic groups in

these protic solvents and the appearance of the intrinsic conductivity. Also, the membrane exhibits an extraordinarily high conductivity in water. This is the reason why this material is used as an

"electrolyte" layer in SPE cells (see Section V.I).

5. Permeation of Molecular Species



The passage of a gas through a membrane takes place via a diffusion

mechanism. The gas dissolves in the membrane on the high-pressure

side, and desorbs out of the membrane on the low-pressure side.

The electrolyte uptake of the membrane plays an important role in

gas diffusion since it is primarily in the aqueous phase that the gas

dissolves. The coulombic loss in the cell is related to the membrane

diffusion current, which is determined by the permeation of the

gases and is given by

iD = nFDcjL



(14)



where D is the diffusion coefficient, c0 the concentration of the



Tài liệu bạn tìm kiếm đã sẵn sàng tải về

V. CONDUCTIVITY AND PERMEABILITY IN MEMBRANES

Tải bản đầy đủ ngay(0 tr)

×