ba-1969-0090.ch006

6 The Operation Mechanism of a Porous Hydrogen Electrode with a Nickel Catalyst R. C H . B U R S H T E I N , A . G . P S H E N I C H N I K O V , and F . Z. SABIROV

Downloaded by UNIV OF LEEDS on June 18, 2016 | http://pubs.acs.org Publication Date: June 1, 1969 | doi: 10.1021/ba-1969-0090.ch006

Institute of Electrochemistry, Academy of Sciences of the USSR, Moscow, USSR

The performance of the hydrogen electrode containing Ni skeleton catalyst prepared from the Ni-Al-Ti alloy was investigated in several ways: structure effect, composition and surface area of the active layer, thickness of the opera tion, and protective layers. The obtained results are ex plained in connection with our former concept concerning the presence of gas which filled the active layer macropores and the electrolyte which filed the micropores.

T t follows from our previous investigations of the operation mechanism of porous gas electrodes (3, 4, 5, 8,9) that a porous electrode can be represented by a model shown in Figure 1. In Bacon s two-layer electrodes the fine pore layer practically does not participate i n the electrochemical process. Electrochemical processes occur only i n the layer of the elec trode, which consists of two kinds of pores: macropores and micropores. Micropores are filled with electrolyte and macropores with gas. Electro chemical reaction occurs on the walls of macropores covered with a thin electrolyte film. The thickness of the electrolyte film determined from the data for a half-immersed nickel electrode is 0.5-1μ (11). W e used such a model of the porous electrode i n the preparation of active electrodes for a hydrogen-oxygen cell. According to Justi, active metalloceramic hydrogen electrodes can be prepared using nickel skeleton catalyst (7). W e have developed a method of preparing hydrogen electrodes using a skeleton nickel catalyst, obtained from an alloy containing 5 0 % N i , 48% A l , and 2 % T i (10). It is known that such a catalyst has larger catalytic activity and stability than the usual Raney catalyst. The influence of the heating temperature in hydrogen upon the specific surface of a catalyst containing a T i addition was investigated. It was shown that the specific surface of 70

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

6.

B U R S H T E I N

E T

A L .

Porous Hydrogen Electrode

71


a powder, measured by the B E T method, remained practically un changed upon heating i n hydrogen up to 800°C. and was equal to 80-90 sq. meters/gram.

Figure 1. Porous gas electrode model Electrodes were prepared as follows: the alloy, previously ground in a vibrating mill, and the powder obtained was freed of aluminum by treating with 5N K O H . Such catalyst is highly pyrophoric, but when mixed in the wet state with nickel carbonyl, it loses this property, and such a mixture can be stored dry for a long time. The loss of pyrophoric properties by such catalysts when they are mixed with nickel carbonyl is accounted for by the slow diffusion of oxygen through the water film and the formation of a thin oxide film on the catalyst surface. A n investi gation of the electrodes with varying structures activated by the skeleton catalyst carried out by the method similar to that used i n Reference 4. In designing electrodes of large size of essential importance is the electrodes strength. T o ensure better strength a new design of electrodes with an internal gas feed was developed (6). Such an electrode is shown schematically i n Figure 2. The electrode consists of a shell made of readily sintering nickel carbonyl ( edge ( 1 ) and fine pore layer (2) with the active layer (3) inside (a mixture of skeleton catalyst and nickel carbonyl). Gas is supplied through side tubes (4). In the case of the electrodes having a 120-mm.-inches diameter strength-


72

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C E L L

SYSTEMS

II

ening and gas-feed paths (5) are used. Strength is ensured by the electrode edge and gas-feed paths, which sinter well with the fine pore layers. Such electrodes were pressed i n a special die i n one operation. Gas-feed paths can be of different forms. The electrodes operate both sides. Their operating surface is 165 c m . (both sides). W h e n assembling the cell the electrodes are placed into a tank with electrolyte and con nected in parallel with respect to gas and current.


2

Figure 2.

Scheme of an electrode with internal gas supply

The dependence of the current I,a upon the potential for a threelayer electrode with a = 0.25 at different temperatures is given i n Figure 4. Figure 4 shows the dependence of log I ( at φ = 80 mv. ) upon the reciprocal temperature. The apparent activation energy of the process calculated from the slope of the straight line is ^ 5 kcal./mole. Tests on a large number of electrodes have shown good reproduci bility of the results. The electrodes operated steadily i n a hydrogenoxygen cell for 12 months. F o r electrodes with high electrochemical activity the ohmic losses in the fine pore layer are large and therefore with a decrease i n its thick ness the current density increases. The experimental curve of the current density dependence upon the thickness of the fine pore layer is shown i n Figure 5.


E T A L .



B U R S H T E I N

Figure 4.

Dependence of lg I on l/T

φ = 0.08 volts; 7N KOH



74

F U E L

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SYSTEMS

II

Dependence of I upon the fine pore layer thickness

Figure 5.

(1) Φ = 0.025 volts; (2) φ = 0.08 volts 7N'KOH; 92°C; S = 165 cm. 2

e

Of considerable interest is the dependence of the electrochemical activity of the electrode upon the skeleton catalyst content i n the active mixture. As is evident from Table I, with an increasing ratio of the catalyst weight to that of the mixture (a) the current density rises up Table I. a Ima./cm. (φ = 0.08 volts) 2

0

0.10

0.25

0.30

0.40

0.50

0.75

1.00

50

108

140

151

168

185

136

108

to a = 0.5. W i t h further increase i n α(α = 0.75 and a = 1.00) the current density drops. As is shown by the structure measurements, it is dependent on the charge ratio of the macro- to micropores. Of con siderable interest i n designing the three-layer electrodes is the depend ence of the current density at a constant potential upon the active layer thickness. This dependence is shown i n Figure 6. It is evident from F i g ure 6 that at a thickness less than 1.5 mm. the current density decreases considerably. N o w let us consider how the concepts of the porous electrode opera tion developed i n (3, 5, 8, 9) can be applied to the electrode activated with a skeleton catalyst.


6.

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75


A L .

The Ι-φ dependences of the electrodes i n question being linear at small polarization (see Figure 3), the electrode resistance R can be considered to be determined by the relation: (1)

R = φ/I = r + r a

f

where r is the active layer resistance, r the fine pore layer resistance. The fine pore layer resistance is expressed by the equation: a

f

r = f

p

· e 8/v 2

(2)

f

where ρ = the electrolyte specific resistance, € = the sinuosity coefficient, v = total cross section of the pores in the fine pore layer per unit of visible surface, equal to the fine pore layer porosity. In most cases c = V 3 (I, 2).


f

τ ma 1cm*

800

100

J

I

I

1 2 Figure 6.

3

I

^

I

5

6

I

A.mm

Dependence of current density upon the active hyer thickness 7N KOH; 92°C.

N o w let us consider the dependence of the reaction resistance upon the electrode activity.

r

a

According to (3), the dependence of the current density upon the gas electrode parameters is of the form: ί=(1Α)

νΐ/ρ-ν^Φ'Φ

(3)

where Ρ is the total perimeter of the pores free of the electrolyte, Φ = the


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C E L L

total cross section of the pores filled with the electrolyte, λ =

S Y S T E M S

(-^—\

II

' x,

where i is the local current density, χ = the ratio of the true to the visible surface. L e t us assume that to the first approximation χ is pro portional to the specific surface of the active mixture S . It is evident that i n the mixture of the catalyst with nickel carbonyl. a

S =


a

'S

a

+ (1 — ) ' S

c a t

a

(4)

carb>

The specific surface measurements using the B E T method have shown the specific surface of the catalyst S . = 80 sq. meters/gram, the specific surface of the nickel carbonyl powder S . = 0.55 sq. meter/ gram. cat

carb

According to Equations 3 and 4, for the electrodes with the same structure we obtain: (fc/fa) = ( S / S a

carb

.)^ = {«(S ./S cat

carb

. - 1) + I}*/*

(

5

)

where r and r are the reaction resistances of the electrodes prepared from nickel carbonyl alone and from the active mixture, respectively. Substituting the values of S . and S ., we obtain for the powdered catalyst used by us: c

a

cat

= (144« + 1)™

(r /r ) c

carl)

a

(6)

Substituting r = φ/l, obtained from Equation 3 into Equation 5 and using Equation 2, we find for the total electrode resistance R the expression: c

J, -

^[( oat./S b.) " I ] + 1 } (1Α)(λ,/ ) / (ΡΦ) / S

car

Ρ

1

2

1

+ ?± g v

1/2

2

(

7

)

f

where A = (di/d λ for a smooth N i . c

c

Assuming for a 7N K O H at 90°C. = 0.715 ohm cm. (7) and taking the value of A from Reference 9, we obtain: P

c

(1/c) (K/p)

1/2

= 0.024 ohm." cm."^ . 1

2

It follows from the direct measurements of the fine pore layer porosity that Of = 0.5. Substituting the values of ρ and e , we obtain for the coefficient before 8:pe /v = 4.3 ohm cm. Taking Equation 7 into con sideration, we obtain from Reference 8 for the active nickel hydrogen electrode: 2

2

f


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77


A L .

where (ΡΦ)

41.5 (144α + 1 )

1/2

(8') 1/2

(8")

r = 4.38 f

The relation ( ? Φ ) = /(ΔΡ) obtained from the pores distribution curve of the electrode with the structure investigated by us, and α = 0.25 is shown i n Figure 7b. The fine pore layer thickness δ = 0.071 cm. In accordance with the determination of R ( I , 2) and Equation 8, for an electrode with the operating surface S = 165 c m . at φ = 0.08 volt, the current is equal to: 1 / 2

2


r

165 · 0.08

(8"') 4 L 5 (ΡΦ)-!/ + 4.3 · 0.071 6.1 Figure 7 shows the experimental (Curve 1) and the calculated (Curve 2) data of the current strength upon the pressure difference between gas and electrolyte. A t large values of ΔΡ the calculation and experimental results can be considered to agree fairly well. A t small ΔΡ the worst discrepancy appears to be caused by the small gas perme ability owing to some of the gas-supplying canals. 1=^-· R

φ=ζ

2

Ψ

α 3D

SO α

JO ι

CM-W

ι

I

M

6 /

1

m,

m

1

ι

1

200

600

,

0.50

075

801

, AP.MMH9

w

{ψ

(a)l = i (AT); S = 165 cm. ; = 0.08 volts; 0.7N 2

e

2

92°C. 1 — calculation and 2 = experiment (b) (τφΥ" = UaF)


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Using Equations 1 and 8 it is possible to calculate the dependence of the current I on an electrode upon the fine pore layer thickness. The results of the calculation and the comparison with the experimental data of Figure 5 are presented i n Table II. Table II. J, α Δ Ρ = 600 mm. H

φ = 0.025 volts


cm.

ohm. cm.

2

0 0.01 0.05 0.071 0.10

R ohm. cm.

calc.

0.32 0.36 0.53 0.63 0.74

12.9 11.2 7.8 6.6 5.6

2

0 0.042 0.212 0.31 0.425

exp.

φ = 0.08 volts exp.

calc. 41 37 25 21 17

—

9.0 7.5 6.0

—

30 22 17

It is clear from Table II that the agreement between the calculated and experimental values is quite satisfactory. According to Equation 8', r decreases and hence the current i n creases with increasing skeleton catalyst content ( a ) i n the active mixture. a

The experimental results for the electrodes with identical structure but varying catalyst content (a) are given i n Table I. The values of R and r calculated from the data i n Table II are listed i n Table III. Equation 6 relates the values obtained from the electrochemical and structure data for the electrodes with identical structures. The right hand and the left hand sides of Equation 6 are given i n Table III ( columns 4 and 5 ) . The comparison of the corresponding values shows that up to a = 0.5 the experimental and calculated data agree up to ± 2 0 % . The discrepancy between the data at a = 0.5 is caused by, as stated above, the change i n the electrode structure. a

Table III. a

R ohm. cm.

ohm. cm.

8 /8 -ar6

0 0.10 0.25 0.30 0.40 0.50 0.75 1.00

1.6 0.74 0.57 0.53 0.476 0.433 0.588 0.74

1.29 0.43 0.26 0.22 0.166 0.123 0.278 0.43

1 3.92 6.10 6.65 7.65 8.55 10.4 12.0

2

2

a

L AP = 600 mm.

f

1.0 3.0 5.0 6.9 7.8 10.5 4.65 3.0

A

1.3 0.4 0.25 0.2 0.15 0.1


cr

mm. 4.0 1.3 0.8 0.65 0.5 0.30

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A L .

As has been shown in (8, 9), the characteristic thickness is deter mined by Equation 9. (9)

Ε={1/ι)(Φ/ '?\)ν* Ρ

Using the obvious relation λ = X · S /S for the electrodes with identical structure α

c

a

L /L =(S /S y^ c

a

. and Equation 5, we obtain (10)

= r /r

carh

c

a

where L and L are characteristic lengths of the nonactive and active electrodes, respectively. According to References 8 and 9, at the thickness I < 1.6L the electrode can be considered to be of infinite thickness. For such elec trodes the current density is determined by Equation 3. Using Equations 3 and 10 and taking into consideration Equation 2, we obtain: c


a

carl)

a

(11)

Ε = ν -Φ/{

ba-1969-0090.ch006

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