The Behavior of Silver Cathodes in Solid Electrolyte Fuel Cells

21 The Behavior of Silver Cathodes in Solid Electrolyte Fuel Cells H . T A N N E N B E R G E R and H . SIEGERT

Downloaded by UNIV OF LEEDS on June 18, 2016 | http://pubs.acs.org Publication Date: June 1, 1969 | doi: 10.1021/ba-1969-0090.ch021

Battelle Institute, Geneva Research Center, Switzerland Characteristics of cells of the type Ag(O )/(ZrO ) (Yb O ) /Ag(O ) 2

2 0.9

2

2

3 0.1

with varying electrolyte thickness were measured. Extrapolating the total internal resistance of the cells to zero electrolyte thickness, a residual resistance R is obtained which is equivalent to the polarization of the silver electrodes. Using a current interruptor technique, the slow and the rapid voltage drop across the cells were measured. The nonohmic resistance of the cells, calculated from the slow potential drop, was smaller than R . Based on the experimental results and on analysis of oxygen diffusion through solid silver cathodes, an explanation for the ohmic part of R is given: the electrodes are active only on discrete spots. By simulation in an electrolytic tank, the dependence of R on the dimensions of and the distance between the active spots was determined. 0

0

0

0

THuel cells with solid zirconia electrolytes that work at 750° to 1000 °C. are promising systems to convert the chemical energy of cheap fossil fuels into electrical energy (1, 11). Expensive platinum catalyst can be avoided because of the high working temperatures. Silver as the cathode material and nickel as the anode material have proved to be successful up to 900°C. (7) (Figure 1). If the electrolyte i n these fuel cells has a thickness of more than 1 mm., the electrolyte resistance predominates i n the total internal resistance of the cell. To increase the power output, the thickness of the electrolyte should be reduced. In so doing, polariza tion becomes important. This paper deals with the polarization of silver electrodes. A tenta tive explanation for the observed phenomena w i l l be given. 281 Baker; Fuel Cell Systems-II Advances in Chemistry; American Chemical Society: Washington, DC, 1969.

282

F U E L C E L L SYSTEMS

II

1.5

1.0

0.5


CO- H 20%

50

^ > 3

CO^

100

150

+

l%H 0 2

200

250

i (ma/cm ) 2

Figure 1.

Performance of solid electrolyte cell, Ag/(ZrO ) JYb O ) /Ni Τ = 800 C, electrolyte thickness: 1.5 mm. 2 0

2

3 0A

e

Polarization Phenomena in High Temperature Solid Electrolyte Fuel Cells. A fuel cell with an oxygen-ion conductor as the solid electrolyte creates a voltage between its electrodes because of the difference of the oxygen partial pressure on both electrodes. This voltage is expressed b y the Nernst equation: RT nF

c

l n P 0 2

p

A

02

If a current flows through a solid electrolyte fuel cell, the voltage between the terminals w i l l drop because of the internal resistance of the cell. The internal resistance, which limits the power output of the fuel cell, can be divided into the following parts: ( 1 ) Polarization of the cathode (2) Ohmic resistance of the electrolyte (3) Polarization of the anode The electrode polarization thus defined contains all sorts of losses i n the electrodes, such as the electronic resistance, diffusion resistance of the gases, activation polarization, and so on. The voltage at the terminals of the fuel cell under load can thus be expressed b y : V

_

V =

E - V 0

d c

Terminal voltage

Baker; Fuel Cell Systems-II Advances in Chemistry; American Chemical Society: Washington, DC, 1969.

21.

TANNENBERGER AND siEGERT

= Open circuit voltage = Polarization of the cathode = Thickness of the electrolyte = Apparent cross section for the current flow through the electro lyte = Specific resistivity of the electrolyte = L o a d current = Polarization of the anode

E V d s

0 c


ρ i V

283

Silver Cathodes

a

The mechanism of the electrode polarizations is far from being understood. It is even quite difficult to determine unequivocally the voltage losses owing to polarization from the total internal resistance by experiment. By using current interrupting techniques, a rapid and then slow voltage decay is observed by several authors (5, 9). Whereas the slow or non-ohmic voltage drop is undoubtedly a polarization phenomenon, it is not quite clear which part of the rapid voltage drop is caused by the ohmic resistance of the electrolyte and the electrodes on one hand or to some rapid polarization phenomenon like "electrode-electrolyte con tact resistance" on the other hand. We tried to obtain further information about polarization phe nomena of silver electrodes by investigating cells of the type Ag ( 0 ) / ( Z r 0 ) . ( Y b 0 ) o. i/Ag ( 0 ) 2

2

0

9

2

3

2

with varying electrolyte thickness, which permits the separation of the electrolyte resistance from the polarization of the electrodes more readily. E q u a l oxygen pressure on both sides of the cell was chosen to facilitate the experimental set-up. Experimental Figure 2 shows the experimental set-up schematically. The cell to be studied was made of a wafer of the electrolyte of — 30 mm. diameter and about 3 mm. thickness, held between two tubes made of steel. In the center of the electrolyte wafer there was a depression, at the bottom of it was placed the one electrode, the other electrode being on the opposite side of the wafer. The surface of the electrodes was about 1 c m . B y varying the depth of the hole, the thickness of the electrolyte between the two electrodes could be adjusted to the desired value. 2

This configuration of the sample allowed its position to be fixed i n the oven and current collectors made of silver to be pressed against the electrodes by springs. Thermocouples were placed on both sides of the electrolyte wafer. A n oxygen stream of 100 cc./min. was directed against both electrodes.


284

F U E L C E L L SYSTEMS I I

Oven


Thermocouples

Electrolyte wafer

Figure 2.

Expenmental set-up of the solid electrolyte cell, Ag(O )/(ZrO ) JYb O ) ^/Ag(O ) 2

2 0

2

3 0

JTEKTRONIX

2

OSCILLOSCOPE

TYPE 503

o . POTENTIOSTAT KROHN-HITE UHR - T 361 S

it

0.4ms

ΛΛ/ΗΙ R 10 ms

0-

SAMPLE

mA RELAY STRUTHERS-DUNN TYPE MICC-C5- 6.3 AC

Figure 3.

Electrical measuring circuit

The electrical circuit, shown i n Figure 3, allowed measurement of the current-voltage characteristics of the cell. A current interruptor tech nique was used to measure the ohmic and the non-ohmic part of the total cell resistance. T o do this, a short time interruption relay (type StruthersD u n n M I C C - C 5 - 6 . 3 a.c. ) was placed i n the current circuit, which could break the current periodically for 0.4 msec, at a frequency of 100 c.p.s. The voltage-time curve was observed on an oscilloscope Tektronix type 503, and could be easily photographed. Steady-state characteristics were measured b y short circuiting the relay. The rapid part of the voltage decay after interrupting the current


21.

TANNENBERGER A N D

285

Silver Cathodes

siEGERT

could be measured directly on the oscilloscope, whereas the slow part of the voltage decay was found as the difference between the steady-state voltage and the rapid voltage decay. As electrolyte, cubic zirconium dioxide of the composition 90 mole % Z r 0

and

10 mole % Y b 0 2


2

3

was employed. The resistivity as a function of temperature of this compo sition (2, 8,10) is shown i n Figure 4. The electrolyte wafer was prepared by sintering a compact of the mixed oxide powders at 1900°C. for three hours. The apparent density was about 9 2 % of the theoretical density and the wafers were gas tight. The surface was prepared b y rubbing with S i C paper, then rinsed with distilled water and heated at 1000°C. for a .short time. Silver electrodes were prepared by painting two layers of a silver paste, Degussa Leitsilber 200, followed by a heat treatment up to 850°C. Zr0 -Yb 0 2

2

3

/

90-10

Ρ ( i l cm) 2 ΙΟ

2

5

2 ΙΟ 800°

Figure 4.

750°

700°

650° C

Resistivity of the electrolyte (ZrO ) (Yb O ) tion of temperature 2 09

2

3 01

as a func

Results Figures 5 and 6 show typical characteristics of the cells. They are straight lines passing through the origin of the coordinates as they should, because the partial pressure on the two electrodes is the same: Po

c

2

= Po

A

2

= 1

a t m

-


286


II


795°C

Figure 5.

Characteristics of the cell: Ag(O )/(ZrO ) JYb O ) /Ag(O ) Electrolyte thickness: 3.65 mm. 2

2 0

2

s 0tl

S!

Volt A 0.8-

634° C 7I9°C

0.60.4 H

8II°C 0.2120

100 I

__l

i/cm

80 I

60 I

40

20

L

Ί 20

2

1 40

1 60

1 80

Γ 100

ma/cm π *~ 120

2

0.2 -0.4 -0.6 -0.8 " Volt

Figure 6.

Characteristics of the cell: Ag(O) /(ZrO ) (Yb O ) . /Ag(O ) Electrolyte thickness: 0.38 mm. 2

2 09

2

3 0 1

2

A t current densities higher than about 200 ma./cm. the characteris tics being to flatten out. This phenomenon, also observed b y other workers, may be explained by the heating of the electrolyte owing to the current flow ( 9 ) . This hypothesis is supported by the fact that the current 2


21.

TANNENBERGER A N D

siEGERT

287

Silver Cathodes

density, above which the internal resistance of the cell decreases, depends on the thickness of the electrolyte and the nominal operating temperature. O n some of the cells the rapid and the slow part of the voltage drop were measured by the current interruption technique. Figure 7 shows the rapid and the slow voltage drop as a function of the current density across a cell having an electrolyte thickness of 0.3 mm. Figure 8 shows the voltage-time relationship for a periodical interruption of a constant current photographed on the oscilloscope.


0.5

0

Volt

0.4 sic w voltage drop 0.3

total νvoltage drop

0.2

0.1

ra pid voltage drop

-

50

150 ma / c m

100

2

200

Figure 7. Characteristics of the cell: Ag(air)/(ZrO ) (Yb O ) /Ag(air) Electrolyte thickness: 0.30 mm. 2 09

2

3 01

The importance of the slow voltage drop relative to the total voltage drop across a cell decreased strongly with increased electrolyte thickness. It is interesting to note that the initial polarization at low current densities, observed i n the cell with a nickel anode (Figure 1), does not occur when silver is used at the anode, with p = 1 - This indicates that the initial polarization is due to the nickel anode working i n a H atmosphere. We restrict further discussions to the linear part of the characteristics. Figure 9 shows the total resistance per c m . of cells having different electrolyte thicknesses, as a function of temperature. It is remarkable that the temperature dependence of the total resistance of the cell is appre ciably the same as that of the resistivity of the electrolyte, independently of the thickness of the electrolyte that varies by a factor of ten. Figure 10 shows a cross-plot of Figure 9—i.e., the total resistance of the cell as a function of the thickness of the electrolyte at three tempera0 2

A

a t m

2

2



288


Figure 8.

II

Voltage-time relationship for a periodical interruption of the current for 0.4 msec.

tures. As can be seen, a residual resistance R remains if the thickness of the electrolyte is extrapolated to zero. The temperature dependence of R is nearly the same as that of the resistivity of the electrolyte. Q

Q

The non ohmic resistance, calculated from the slow voltage drop, was always smaller than the residual resistance R . 0

Discussion The essential result of our measurement is that we were able to separate the electrolyte resistance of the cells from their total internal resistance. The observed residual resistance R represents the total polari zation of both electrodes. The resistance calculated from the slow voltage drop across the cell is smaller than the residual resistance R . The temperature dependence of the residual resistance Ro equals ap proximately that of the resistivity of the electrolyte. 0

0

In the following, a tentative interpretation of the residual resistance Ro is given. Activation polarization, involving losses associated with adsorption or surface reactions, which yield rapid voltage drops when the current is interrupted, seems to us improbable with silver electrodes at the high temperatures used.


21.

TANNENBERGER AND siEGERT ΙΟΟι

800°C-

289

Silver Cathodes

725°C

650°C

d= 3.65 mm

d= 1.10 mm


d = 0.38mm

0.90

0.95

1.00

1.05

•1/TxlO

1.10

Figure 9. Total internal cell resistance of three cells with different elec trolyte thicknesses as a function of temperature Τ = 650 °C

50 R

(Ω

cm2)

40

30

Τ = 725 °C

20

10

Τ = 800°C

*

4

d(mm)

Figure 10. Total internal cell resistance as a function of elec trolyte thickness d at three temperatures


3

290

F U E L

C E L L

SYSTEMS

II

If we attempt to ascribe the residual resistance R to a concentration polarization due to the diffusion of oxygen through the silver electrode, we obtain the following results: Q

As concentration polarization at the cathode is higher than at the anode, we consider only the cathodic case. The potential drop owing to cathodic concentration polarization is: *7 = - | ^ l n

(1-j/h)


where R Γ F ;'i

= = = =

Gas constant Absolute temperature Faraday constant Limiting current density.

The limiting current density can be expressed by _

4Fc D 0

where c = D = d = 0

Solubility of oxygen i n silver at p = 1 atm. Diffusion coefficient of oxygen i n silver Thickness of the silver layer. 02

Figure 11 shows the potential drop η as a function of current density with the limiting current density as unity. The characteristics of our cells are practically linear and never showed a tendency to limiting currents i n the measured current range. If, nevertheless, we assume that the polarization is due to the diffusion of oxygen i n silver, and the limiting current density is much higher than the measured current density range, therefore the logarithmic behavior of the concentration polarization may approximate to the observed linear behavior, theory predicts much smaller values for the polarization—e.g., η = 13 mv.

for 7 = 0.25 ft at 800°C. than the observed polarization. Furthermore, the limiting current density calculated from the known values of c and D (3) at 800°C. is only 850 ma./cm. for a silver elec trode having a thickness of 1 μ. The thickness of the silver electrodes in our cells, as determined by microscopical examination and electric 0

2


21.

TANNENBERGER A N D SIEGERT

291

Silver Cathodes

resistance measurements, is about 5 μ. W e must therefore conclude that the oxygen transport to the electrode-electrolyte interface does not take place by diffusion through a solid silver layer, but probably through pores in the silver electrodes. V


(mv)

0

0.5

1.0

j/j

Figure 11. Concentration polarization as a function of current density in units of the limiting current density ) t

This leads to the assumption that the silver electrodes are active only on discrete spots of small size, distant one from one another. In this case the effective cross section for the current flow through the electrolyte w i l l be reduced i n a certain region under the electrode-electrolyte inter face and cause a residual resistance as is shown schematically in Figure 12. This residual resistance may be interpreted as the observed R . This view finds a strong support i n the fact that the observed temperature dependence of Ro was approximately the same as that of the resistivity of the electrolyte as should be the case, since Ro is due to a pure geometric effect i n the electrolyte. 0

In any case, where discontinuous electrodes are placed on a solid electrolyte, a residual resistance w i l l occur owing to the reduced effective cross section for the current flow through the electrolyte. The "electrode-


292


II

electrolyte contact resistance" yielding a rapid voltage drop when the current is interrupted, observed by some workers, may rather be due to this effect. Relation Between the Residual Resistance Ro and the Geometrical Configuration of Active Spots. In the following, a quantitative estima tion of the relation between, on one hand, the dimensions of and the distance between active spots, and, on the other hand, the resulting residual resistance Ro w i l l be given with the aid of simulating the element in an electrolytic tank.

H R (£îcm )


2

electrolyte thickness

Figure 12. Total internal cell resistance as a function of electrolyte thickness for a cell having electrodes being active only on isolated spots p = Electrolyte resistivity Seff = Effective electrode area per cm. (area of the active spots) 2

Various models of discontinuous electrode geometry have already been proposed (4, 6), and may be considered (Figure 13). It is evident that the residual resistance Ro for the different models, b and ρ kept constant, w i l l be different. R w i l l be biggest for the model A , becoming smaller for models Β and C . W e simulated the models A and Β i n an electrolytic tank. Figures 14 and 15 shows the simulation for the band model B. The resistance of one "element" dependent on its thickness d was measured i n the electrolytic tank. Figure 16 shows the resistance of this "element" as a function of its thickness expressed by the parameter 0


21.


Ρ

Ρ

Model

Figure 13.

293

Silver Cathodes

TANNENBERGER A N D s i E G E R T

C

Different geometric models of electrodes

for a conductivity: ρ = 1 Ω cm. and for a bath depth B: B =

l

cm.

It is important to note that the resistance form of

can be expressed i n the

for values of θ β>1


294


II

Active strips


Electrolyte Simulated element

Current lines

Figure 14. Model for the simuhtion of the electrode L = length, Β = width, d = thickness of the electrolyte, ρ = distance be tween "active strips," b = width of the "active _P_

2

Current lines

Figure 15.

Simulated single "element"

independently of b/p. This means that the current flow is homogenous above a distance d, where: d>

Ρ

Figure 17 gives the residual resistance JR as a function of b/p. 0


TANNENBERGER A N D SIEGERT

Silver Cathodes


21.

Figure 17.

Residual resistance R as a function of — (model B) Ρ 0


295

296


II

The resistance R ' of an "element" with resistivity ρ and width Β


is then given by

For the ohmic resistance R of an entire cell as illustrated in Figure 14 with Β = W i d t h of electrode L = Length of electrode d = Thickness of electrolyte ρ = Resistivity of electrolyte we obtain: _

R

Ρ . Ρ .™

d

+

In this formula, the term Ρ . Ρ .» _ LB 2 ^°- ° R

K

is identical with the residual resistance R which is obtained by extrapo lating the electrolyte thickness to zero (Figure 12). The same calculation holds for the model A , which we simulated i n a three dimensional electrolytic tank. The ohmic resistance R of an entire cell of length L, width B, thick ness d, and resistivity ρ is 0

R

=

TB' '*° P

+

I>-TB

ψο as a function of b/p, obtained from the measurements on the three-dimensional electrolytic tank, is shown i n Figure 18. Again, the term

is identical with the residual resistance Ro. For convenience, the residual resistance R may be expressed as the resistance of an additional electrolyte layer of thickness d . In our models A and B, d is then: 0

0

0

(Α)

ά =

(B)

d =

0

0

ρ-ψ

0

V-^


21.

297

Silver Cathodes

TANNENBERGER A N D s i E G E R T

and we obtain: R = -£- (do B

+ d)


for both models.

Figure 18.

Residual resistance P as a function of 0

— (model A) Ρ It should be noted that from the knowledge of d , it is not possible to determine ρ and b (from JR or ψ ), but only pairs of values ρ and b. Figure 19 shows, for both models, the interdependence of ρ and b for different d . If we interpret the results obtained on silver electrodes (Figure 10) at 8 0 0 ° C , we obtain the following relationship: suppose L = Β = 1 cm., we can first calculate ρ of the electrolyte from the slope of the straight line i n the diagram of total resistance vs. electrolyte thickness. This yields Q

0

0

0


298

F U E L

C E L L

SYSTEMS

II

ρ = 28 Ω cm., which is i n good agreement with the value of resistivity measured inde pendently. Suppose for the residual resistance of the silver cathode due to these geometrical effects R = 0.4 Ω cm. , or d = 140 2

o

0

μ

and therefore Ro = 0.4 = J L ·|.^

(model B)

ο


or ° =

R

0

A

=

L B

P

$° (

m o d e l A

)'

W e can then calculate the relationship between the parameters b and p, which is represented i n Table I. 1000

d = 300 0

SU

μ

do =

\00μ

do =

30 μ .

Ρ

Μ

100

10 μ d = d = 300/χ

10

0

0

A

]

147

25

2.8

0.34

b[>]

45

(I)

1.7

51 Χ 10"

1.9 Χ 10"

1

5.0 X 10"-

6.8 Χ 10"

1.5 Χ 10"

1

1.7 Χ 10"

1.7 Χ 10"

2

4

3

9.0 Χ 10:

4

4

W e attribute this ohmic part of the polarization, which is often called "electrode-electrolyte resistance," to a pure geometric effect: the silver electrodes are active only on discrete spots. The ohmic residual resistance, or polarization, is then due to the loss of effective cross section for the current flow through the electrolyte. This view is supported by the analysis of the diffusion of oxygen through solid silver electrodes and by the fact that the observed tempera ture dependence of the residual resistance R is approximately the same as that of the resistivity of the electrolyte. It can generally be stated that for both electrodes of solid electrolyte fuel cells, where the transport of the reactants cannot be assumed by diffusion through the electrode material and therefore takes place through pores only, the electrode reactions always occur on discrete spots. As a consequence, an additional ohmic voltage loss w i l l invariably be observed. This is particularly true for cathodes made of materials that do not dissolve and diffuse oxygen sufficiently, such as platinum, certain oxides, or even solid silver. The picture changes completely with molten silver cathodes through which oxygen diffusion is rapid enough and where, therefore, the cathodic reaction can occur homogeneously on the whole electrode-electrolyte interface. The residual resistance of the described silver electrodes is equivalent to the resistance of an electrolyte layer of a thickness of about 100 to 200 μ. This is negligibly small for solid electrolyte fuel cells having electro0


300


II

lyte thicknesses of more than 1 mm., but becomes significant for cells with thin electrolytes—e.g., 100 μ or less. As a general conclusion it can be stated that for porous electrodes i n solid electrolyte fuel cells, relatively large voltage losses are caused by the discontinuous structure of the electrodes. These losses should be minimized by diminishing the mean distance between the active spots.


Literature Cited (1) Archer, D. H., Elikan, L., Zahradnik, R. L., Proc. Symp. Hydrocarbon Air Fuel Cells 9, No. 3, Part 1 (1965). (2) Dixon, J. M., LaGrange, L. D., Merten, U., Miller, C. F., Porter II, J. T., J. Electrochem. Soc. 110, 276 (1963). (3) Eichenauer, W., Müller, G., Z. Metallkunde 53, 321 (1962). (4) Eisenberg, M., Fick, L., Proc. Symp. Jet Fuels 6, No. 1 (1961). (5) Filjaev, A. T., Palguev, C. F., Karpatchev, C. V., Akademia Nauk SSSR, Uralski Filial, Trudi instituta electrochimii, 2, 199 (1961). (6) Gorin, E., Recht, H. L., "Fuel Cells," W. Mitchell, Ed., p. 193, Academic Press, New York, 1963. (7) Schachner, H., Tannenberger, H., Compt. Rend.III,49 (1965). (8) Strickler, D. W., Carlson, W. G.,J.Am. Ceramic Soc. 48, 286 (1965). (9) Sverdrup, E . F., Archer, D. H., Ailes, J. J., Glaser, A. D., Proc. Symp. Hydrocarbon Air Fuel Cells 9, No. 3, part 2 (1965). (10) Tannenberger, H., Schachner, H., Kovacs, P., Compt. Rend. III, 19 (1965). (11) White, D. W., Compt. Rend.III,10 (1965). RECEIVED November 21, 1967.


The Behavior of Silver Cathodes in Solid Electrolyte Fuel Cells

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