Oxygen Reduction at a Porous Silver Electrode - Advances in

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Oxygen Reduction at a Porous Silver Electrode

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on March 16, 2016 | http://pubs.acs.org Publication Date: June 1, 1969 | doi: 10.1021/ba-1969-0090.ch007

M . R. T A R A S E V I C H , R. C H . B U R S H T E I N , and Y U . A. C H I S M A D Z H E V Institute of Electrochemistry, Academy of Sciences of the USSR, Moscow, U.S.S.R. To elucidate the mechanism of current generation at a porous silver electrode some electrochemical and structure measurement were performed on various types of electrodes differing in their pore radii distributions. According to porometry, the electrodes had macropores formed by the particles of the pore forming agent and micropores between silver particles. A correct concept of the operation mechanism of an actual gas-diffusion oxygen electrode can be obtained on the basis of the structure data showing the presence of two kinds of pores: those filled with electrolyte and those filled with gas. The comparison of the theory with experiments has shown, that in the case of a porous silver electrode the current generation occurs in the "liquid" and the "gas" pores.

C o m e earlier studies (2, 3) carried out at our laboratory postulated the ^ presence i n an actual porous electrode of macro- and micropores forming an intersecting system inside it. It was shown on the basis of electrochemical and structure measurements that i n the case of hydrogen ionization on a nickel electrode, current generation occurs on the walls of macropores covered with the electrolyte film near the micropores filled with the electrolyte. It follows (4,5) that i n certain cases an electrochemical reaction can occur i n micropores near their intersections with gas pores. The object of the present study was to investigate the mechanism of oxygen electro-reduction at porous silver electrodes with different struc tures. It was of interest to determine the relation between different mechanisms of current generation, depending on the structure parameters, pressure differences, and polarization. 81

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

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Table I.


No. 1 2 3 4 5 6 7 8 9 10 11 Experimental

Size NH HCO ,

NHfiCOs, %

k

0 5 10 15 20 25 30 40 20 20 20

cm. cm.

2

s

44-61 44-61 44-61 44-61 44-61 44-61 44-61 270-580 104-140 61-104

go>%

3

28,800 26,000 20,200 17,300 14,200 13,100 11,500 7,500 14,300 15,000 16,000

50 50 65 70 75 77 81 87 75 74 74

Procedure

The experiments were carried out using metal-ceramic two layer electrodes, 24 mm. i n diameter, consisting of a 0.5 mm. protective layer, a 3.0 mm. operating layer, and a compact edge. It was established by preliminary experiments that by increasing the thickness of the operating part of the electrode over 3 mm., its electrochemical activity remained constant. The protective layer and the compact edge were made of powdered nickel carbonyl, and the operating layer was made of a mixture of powdered silver and ammonium bicarbonate. B y varying the amount and dispersion of N H H C 0 , it was possible to prepare electrodes with different structures. The investigation of the electrode structure was carried out using the method of driving away the alkali of the pores, which permitted determining the pores from 1.5 to 200 μ. Total porosity ( g ) was deter mined by the weight method; the total true surface ( S ) was measured by the B E T method. Electrochemical measurements were performed i n IN K O H at 90 °C. In all experiments, except the measurements of the dependence on the oxygen partial pressure, the pressure over the electro lyte was atmospheric. A l l the potentials refer to the hydrogen electrode in the same solution. 4

3

0

0

Discussion of the

Results

In Table I the results obtained on the electrodes with different porous structures are compared. It is seen from this data that by increasing the ammonium bicarbonate content (Structures 1-8), g is increased and S is decreased. 0


0

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83

ΔΡ = 0, 8 atm.

i X 10 > a cm.

β, %

80 1110 1050 1180 870 815 890 480 750 710

1.5 6.9 7.8 13.2 13.7 14.3 21.2 8.9 8.5 7.9

2

2>


r

μ

3

2 4.4 8.0 10.4 13.0 17.0 17.4 18.0 11.2 12.0

3.0 26 32 45 51 52 63 46 37 39

7

0

cm. S, cm.

2

0.5 1.0 1.2 3.0 3.1 1.9 3.0 1.5 1.2 1.2

Figure 1 and 2 show the integral and differential curves of the pore radii distribution for a number of electrode structures. In all cases the volume, g , of larger macropores, which have been freed of the electrolyte at Ρ = 1 atm., is markedly less than the corresponding values of g . This is because of the presence of micropores with the radius < 2 μ i n the electrodes. A n increase i n the ammonium bicarbonate content leads to an increase i n the mean radius, r , and porosity, g , corresponding to macropores, and to a decrease i n the porosity, g i ( g i = go — g ) , corre sponding to the micropores. W i t h increasing degree of dispersion of NH4HCO3, the volume ratio of micro- to macropores remains approxi mately constant. 2

0

2

2

2

The surface area per unit volume of macropores ( S ) freed of the electrolyte was determined from the expression: 2

(1) assuming that the pores have a cylindrical cross section over all their length. It should be pointed out, however, that the values of S thus calculated are somewhat too high. This is evidenced by the presence of hysteresis between the ascending and the descending branches of the g — AP curves (Figure 1), which appears to be caused by the pores with a varying cross section. 2

2

The electrochemical investigations have shown the number of elec trons (n) participating i n the reaction of oxygen reduction on a porous silver electrode to be four, and the value of the current is linearly de pendent on the square root of the oxygen partial pressure ( P . ). As w i l l 0 2


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be evident from subsequent theoretical treatment, this indicated that oxygen ionization is a first order reaction, the slow step of this reaction is the result of adding the first electron to the oxygen molecule ( I , 6).

_j

10

l 4

;

r

,/

j_

2 A

Figure 1. Dependence of the volume of the pores freed of electrolyte on ΔΡ for electrodes with dif ferent structures

Figure 2. Differential curves of pore radii distri bution for different electrode structures


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Figure 3 shows the dependences of the current on the pressure dif ference between the gas and the electrolyte ( Δ Ρ ) i n the range 0.1-1 atm. reduced to oxygen partial pressure of 1 atm. It can be seen that i n some cases (Structures 2, 3, and 9) the current increases with increasing ΔΡ and i n others (Structures 7 and 8) the increase of ΔΡ leads to a decrease i n the electrochemical activity.

0 I—Sr ι 0.2

1 0.4

1 0.6

1 0.8

L 1.0

Δ atm. Figure 3. Dependence of the electrochemical^ activity of electrodes with different structures on Ρ at φ = 0.88 ν and Ρ = 1 atm.; 7N KOH, 90°C. θ 2

A l l the pores i n the electrode are assumed to have a cylindrical cross section and at a given pressure difference the pores filled with liquid, ("liquid" pores) to have the mean radius r and those filled with gas ("gas" pores )to have the mean radius r . The pores with the radii i\ and r correspond to the porosités g i and g and surface areas Si and S . The following characteristics of the porous medium are also important: the mean number of "gas" pores per 1 c m . of an arbitrary cross section (ra), the mean length of a " l i q u i d " pore (1), the mean number of inter sections of " l i q u i d " and "gas" pores per unit volume of the electrode (N ). These quantities are interrelated by the equations: x

2

2

2

2

2

12

g — 7Tr m

(2)

S. = 2nr m

(3)

2

2

2

2

2

S = 2irr · N 1

N

1

12

=

gl

1 2

l

· g2

27rr r 1

2

2


(4) (5)

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i=—V=

II

(β)


In the derivation of the equations describing the polarization charac teristics of the electrode, a rapid convection was assumed to occur i n the pores. Therefore, the binarity of the electrolyte was not taken into con sideration. The oxygen concentration i n the gas medium was also assumed to be constant along the thickness of the electrode. Let us consider two limiting cases: 1. Generation occurs only i n the pores filled with electrolyte. It can be shown that the ohmic losses i n the " l i q u i d " pore are small and the potential constant. The gas concentration distribution i n the electrolyte C = C / C is determined by the equation: 0

d C _ C * - e-* 2

-1

e

Όψ

17—

where 1^ = j / ^ ^ ?

0

(7)

= 0

~~

d

v

1

is the characteristic diffusion length and φ =

Q f l

(φ = polarization). The current generation by one pore is determined by the expression: I = Trr^nFDCo

^ dy

(8)

and the polarization distribution φ i n the electrode can be found from the equation: -^p-=

, ψ|χ = ο — ψοι»Φ|χ = ο ο ^ "

W

where L, = j /

RTx

(*g2) gi vi

1/2

is the characteristic length (χ = the specific conductivity of the electro lyte) and = a

1 _2Qrg2) l S g

2

1 / 4

l/ v S i V FDC

(10) ogl

Solving Equation 9 and taking into consideration that the current gen erated i n 1 c m . of the electrode is equal to the product I · N , it is possible to determine the electrochemical activity of an electrode of infinite length i n which the current is generated only by the pores filled with electrolyte: 3


12

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(11) where φ ι is the polarization corrected for the ohmic losses in the pro tective layer. Two limiting regimes are possible depending on the value of a: (a) αβ 01/2 < < 1 (all "liquid pores generate current i n accordance with the kinetic regime). Then th(ae* ) = α and as it follows from Equation 9 0


φ

φ / 2

/2

h

u

=

J/32RTX

G

A

I

O

V

C

H

I

O

I

_

1

(

1

2

)

(b) a > 1.6 (all " l i q u i d " pores generate current in accordance with the internal diffusion regime). The th(ae/ ) = 1 and 2

The analysis of Equation 13 shows that by increasing ΔΡ, a diminishes, since S increases faster than \/ S decreases. Hence, the current gen eration i n accordance with the internal diffusion mechanism should prevail at small pressure differences. 2. The current generation occurs only on the surface of "gas" pores under the electrolyte film. Since the exchange current of oxygen ioniza tion on silver is relatively small ( i — 10" a/cm. ) , when the electrode polarization is not too high the process rate is determined by electro chemical kinetics and the expression for the form: 2

x

6

0

2

5

(14)

j = 2ni sh4> 0

The φ distribution in the electrode is described by the equation: ^-Jr =

^ΓΊΓ'

φ|χ = ο =

|οι> |χ = χ

0

(15)

where

is the characteristic length. By solving Equation 15, it is possible to find the electrochemical activity of an electrode of infinite length, generating current according to the film mechanism: i = 2

g l

S i Vch£ 2

0

0 1

-1


(16)

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3. O f most interest is the general case, when the current generation occurs both i n " l i q u i d " and "gas" pores. O n the basis of the foregoing, it can be shown that the polarization curve is of the form:


y V FDC„

tn

gl

0 1

*/

[β*/* (1 - --·*) th («e*/*)]d φ + 4S t (eft ψ - 1) β

2

0

01

(17) Expressions 11 and 16 can be obtained from Expression 17 as special cases. It should be pointed out that i =^= ii + i . This is because of the nonlinearity of the equations for φ and lack of equal access to the internal surface of the gas electrode. The contribution of the current generation according to the film mechanism can be estimated for the relation: 2

β

4S i (c/ιφο! - 1) 2

=

Γ"

^/FDCjo^^p

0

[e** (1 - e--'*) th («e*/*)]d φ + 4S i (ch φ - 1) 2

0

01

(18) It is clear from Equation 18 that the contribution of the film mechanism increases with increasing i , since the part of current generated b y the film is proportionally to i and the current generated by the " l i q u i d " pores to Λ / i . 0

0

0

In order to compare the calculated results with the experimental data, the i — ΔΡ dependences for different mechanism of current generation were obtained. W e used the following values of the constants: D = 3 Χ 10" c m . V s e c , C = 8 X 10~ mole/cm. ( 6 ) , χ = 1.1 ohm" cm." . The effective value of the exchange current was determined from the value of the current at a certain pressure difference. F o r illustration i n Figure 4 are given the results for the electrodes with Structure 4. In the case of the internal kinetic regime (Curve 4 ) , a decrease and, in the case of the internal diffusion (Curve 2) and the film (Curve 3) mechanisms, an increase i n the activity are observed with ΔΡ increasing from 0.1 to 1.0 atm. The experimental i — ΔΡ dependence (Curve 5) has a maximum in the region of ΔΡ — 0.5 atm. Theory is i n agreement with experiment only when the calculation was performed using the general Equation 17 (Curve 1). The contribution of the current generated according to the film mechanism increases with increasing ΔΡ ( Curve 1' ). Figure 5 shows the i — ΔΡ dependences for different electrode structures calculated b y means of Equation 17 and Table I—the corresponding values of i . The comparison of these data with the results given i n Figure 3 shows that the 5

0

8

3

1

0


1


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1

1

I

0

0.2

0.4

I

0.6

I

0.8

L

1.0

ΔΡ,αϊπι.

Figure 4. Dependence of the current on ΔΡ, calculated by means of Equation 17—Curve I, Equation 13— Curve 2, Equation 16—Curve 3, Equation 12—Curve 4 and experimentally obtained for Structure 4—Curve 5. Curve 1 '—contribution of the current generated accord ing to the film mechanism theory is in fair agreement with experiments. The discrepancies ob served i n some cases are owing to the fact that the characteristics of the actual porous structure were not taken into account with sufficient accuracy. The mean value of the exchange current is 1 — 2x Χ 10" a/cm. . This value seems to be too low, however, owing to the values of S used i n the calculations being too high. A very close similarity is observed also i n the case of the i — φ curves measured experimentally and calculated by means of Equation 17 (Figure 6). It is also interesting to note that i n accordance with the theoretical conclusions, the contribution of the current generated by the film increases with decreasing surface area and porosity of " l i q u i d " pores (see Table I) or with increasing polarization (Figure 6 and Curves 4' and 10'). The results of the present study show that a correct concept of the operation mechanism of an actual gas-diffusion oxygen electrode can be operated on the basis of the structure data showing the presence of two 7

2

2


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90

Figure 6. Experimental (solid lines) and calculated (dashed lines) i-φ curves for Structures 4 and 10. Curves 4' and 10'—contribution of the current gen erated according to the film mechanism; 7N KOH, 90° C.


7.

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kinds of pores: those filled with electrolyte and those filled with gas. Sometimes it is necessary to take into consideration the possibility of current generation according to different mechanisms, the relation be tween these being determined by electrochemical and structure parameters. RECEIVED

November 20, 1967.

Symbols g gi g 5 51 5 r fi r m I Νi,2

= = = = = = = = = = = =

0

2

0

2

2

Total porosity, cm. /cm. Porosity of micropores, cm. /cm. Porosity of macropores, cm. /cm. Total true surface, cm. /cm. Surface area of micropores, cm. /cm. Surface area of macropores, cm. /cm. Radius of pores, μ Mean radius of " l i q u i d " pores, μ Mean radius of "gas" pores, μ Mean number of "gas" pores per 1 c m . Mean length of " l i q u i d " pores, μ Mean number of intersection of "liquid and "gas" pores per 1 c m . of the electrode = Pressure difference between the gas and electrolyte, atm. = Gas concentration i n the electrolyte, mole/cm. = Diffusivity, cm. /sec. = Faraday constant = Gas constant = Specific conductivity ( Ω · c m . ) = Exchange current, a/cm. = Polarization, mv. = F φ/2 RT = Polarization, corrected for the ohmic losses i n the protective layer = Current generation by one " l i q u i d " pore = Current density on the surface of "gas" pore = Current density (or electrochemical activity) of " l i q u i d " pores only = Current density of "gas" pores = Total current density of electrode, i = i\ + %2 = \/\ parameter = Diffusion length = Characteristic length for the " l i q u i d " pores mechanism — Characteristic length for the "gas" pores mechanism 3

3

3

3

3

2

3

3

2

3

2

3

2

3

ΔΡ C D F R χ 1 φ φ φοι 0

0

/ / 11 1 i

2

a

lg Li L

2

3

2

-1

2

g9

Literature Cited (1) Beer, S. Z., Sandler, J. L.,J.Electrochem. Soc. 112, 1133 (1965). (2) Burshtein, R. Ch., Pshenichnikov, A. G., Shumilova, Ν. Α., Dokl. Akad. Nauk SSSR 143, 168 (1962).


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(3) Burshtein, R. Ch., Markin, V. S., Pshenichnikov, A. G., Chismadzhev, Yu. Α., Chirkov, Yu. G., Electrochim. Acta 9, 773 (1964). (4) Edward, Α., Grens, H., Ind. Eng. Chem. Fundamenta 5, 542 (1966). (5) Markin, V. S., Chernenko, Α. Α., Chismadzhev, Yu. Α., Chirkov, Yu. G., "Fuel Cell," V. S. Bagotskii, Yu. B.Vasil'ev,Eds., Consultants Bureau, New York, 1966. (6) Shumilova, Ν. Α., Zhutaeva, G. V., M-R. Tarasevich, Electrochim. Acta 11, 967 (1966).


Oxygen Reduction at a Porous Silver Electrode - Advances in

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