On the Electrochemistry of Porous Zinc Electrodes in Alkaline Solutions

J r l U R N A L ElF T H E E L E C T R E ] I : : H E M I E I A L S r l B I E T Y

s 1 6 3 S C I E N C E

--AND T s , ~ SEPTEMBER

1972

On the Electrochemistry of Porous Zinc Electrodes in Alkaline Solutions

Z. Nagy *'~ and J. O'M. Bockris *~ Electrochemistry Laboratory, University of Pennsylvania, Philadelphia, Pennsylvania 19104

ABSTRACT

Porous zinc electrodes were discharged galvanostat ical ly in aqueous potassium hydroxide solutions. The morphology of the zinc oxide film formed w a s investigated with a scanning electron microscope. The cur ren t dis t r ibut ion in the porous electrode, and its dependence on current density, was determined b.v microslicing the electrode after discharge and chemical analysis. The oxide film had a porous, "carpet-l ike" structure, consisting of long needle crystals with occasional sidearms. The formation of this type of film can be explained by a dissolution-precipitat ion mechanism. The cur ren t distr ibution in the porous electrode, and its dependence on current density could also be explained, based on a model of oxide film consisitng of a thin, high resistance compact film, beneath the porous oxide.

The behavior of porous electrodes is an impor tant topic in electrochemistry since many practical electrode reactions (par t icular ly those of energy conversion and storage) take place in porous matrices. The basic question is the distr ibution of current (and potent ial) in the porous body. The theory of porous electrodes has evolved to a considerable sophistication dur ing the last decade. While the mathematics is com- plicated, analytical solutions have been obtained for a number of cases taking into account the IR drops in the electrolyte and the metal matrix, the diffusional processes of reactants and products, the kinetics of the charge t ransfer reaction, and the wet t ing character- istics of the metal by the electrolyte. A recent sum- mary of these theories is available (1). None of these early theories have, however, t aken into consideration the s t ructural changes taking place in the porous mat r ix dur ing the electrode process (e.g., the anodic dissolution of a metal and the formation of insoluble reaction products) which are very common in electrochemical energy storage systems. Two recent theoret i- cal papers began work in this area. The first one (2) treats the effects of s t ructural changes due to the anodic dissolution of the matr ix without product precipitation. The other (3) considers slightly soluble reactants and products on an inactive conducting matrix, and treats the s t ructural effects as changes in mass t ransport parameters of the active materials. Both of these t reatments result in differential equations solvable only numerical ly.

The exper imental de terminat ion of current dis t r ibu- t ion can be carried out in three ways: (i) the measurement of potential distr ibution by using numerous probes along the electrode, as was carried out with MnO2 electrodes (4), (ii) the direct measurement of the distr ibution by using sectioned electrode, and mea-

* Elect rochemical Society Act ive Member . 1 Presen t address : D iamond Shamrock Chemical Company, Pa ines-

v~lle, Ohio 44077. P resen t address : F l inders Univers i ty , Adelaide, Austral ia .

K e y words : porous electrode, Zn /ZnO, oxide film morphology, cur ren t dis t r ibut ion, effect of cu r ren t density.

suring the current separately to each section as was reported for MnO2 (5), and the Pb/PbSO4 system (6) and, (iii) sectioning the electrode after the exper iment and analysis of the products as was recently carried out with cadmium electrode (7). The first two methods allow repeated cycling of the electrode while carrying out in situ measurements, and the determinat ion of t ime dependence of the distr ibution is also possible, but the technique itself may interfere with the normal operation of the porous electrode thereby distort ing the results. The third method is free of this objection, bu t it is l imited to one observation per electrode, and is applicable only for processes with solid reaction products.

The zinc-zinc oxide alkal ine electrode is an impor- tant system for which cur ren t dis t r ibut ion studies have not been reported as yet. This is surprising considering the fact that it is one of the more promising electrodes for high energy and power density energy storers (8, 9). Oxide film formation and passivation have been investigated by many workers on flat zinc electrodes in alkaline solutions. Recent reviews a r e available (10, 11). There is a controversy in the l i tera- ture whether the passive film is formed directly on the surface or by a dissolution-precipitat ion mechanism (11-19). Recent results (11, 17-19) indicate, however, that there are two kinds of film formed dur ing anod- ization of zinc in an alkaline solution. One, a precipi- tated film, appears before passivation occurs; while the second film, forming beneath it, is the cause of passivation. Correspondingly, the prevail ing hydrodynamic conditions have been shown to have a strong influence on the formation of the films, and that the quiescent conditions inside a porous electrode wil l favor the pre- cipitated oxide. Further , it has been shown by cathodic reduct ion studies (20), that the direct, solid-state, re - duction of zinc oxide is energetically unfavorable compared to the dissolution-zincate reduction mechanism. Numerous reports also appeared (21-29) on the anodic behavior and passivation of porous zinc electrode in alkaline solutions, as a funct ion of current density,

1129

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1130 J. Electrochem. Soc.: E L E C T R O C H E M I C A L S C I E N C E A N D T E C H N O L O G Y

Fig. 1. Schematic of the experimental setup. I, Teflon body; 2, porous zinc electrode; 3, current contact; 4, glass cover; 5, channel to counterelectrode; 6, Teflon Luggin.

temperature , solution composition, and porosity of the electrode, but in none has the question of current dis- t r ibut ion wi th in the electrode been studied.

Exper imental Porous zinc electrodes were discharged galvano-

statically in alkal ine zincate electrolytes. The mor - phology of zinc oxide formed was examined with a scanning electron microscope. The penetra t ion of current, and its dependence on current density, were de- t e rmined at the end of the process by microslicing the electrode and chemical analysis. All tests were carr ied out at room temperature .

The ce l l - -The basic concept of the cell is i l lustrated in Fig. 1. The porous zinc pel let was pressfitted into the solution channel of the Teflon cell body, and the as- sembly covered t ight ly wi th a glass sheet. Current contact to the porous e]ectrode was made using a 0.0075 cm thick zinc foil. The solution channel on the other side of the porous electrode led to a countere lec t rode chamber, the counterelectrode was a p la t inum wire spiral immersed in KOH-ZnO slurry. The Teflon tub- ing Luggin led to a reference chamber holding a calo- mel electrode. Details of the cell design are given else- where (30).

Preparation of the porous electrodes.--The test electrodes were prepared from 99.9999% pure zinc powder (United Mineral and Chemicals Corporat ion) wi th a part icle size less than 0.015 cm (--100 mesh) . The powder was compressed into a 0.3 cm diameter, 0.1 cm thick pellet in a suitable mold, using a pressure of 8400 psi. The porosity of the electrodes was determined, f rom their calculated density, as 30%.

The electrolyte solutions.--High pur i ty potassium hydroxide-z inca te solutions were prepared with a technique described before (31). For all but one exper i - ment, a solution composition of 2M KOH and 0.1M zincate was used. In one case, the solution was 8M KOH containing 0.5M zincate.

Experimental procedure.~After assembling the cell, it was blown free of air wi th purified nitrogen, and the solution was introduced by applying vacuum on the outlet side of the channel. Fi l l ing of the electrode could be observed with a microscope through the glass cell cover: if bubbles were observed the evacuat ion and filling was repeated. A constant cur ren t was ap- plied, using an El ron galvanostat , in the range of 4-100

September 1972

m A / c m 2 apparent current densi ty (calculated on the geometr ic area) for a t ime durat ion of 2-40 hr. The electrode potential was measured wi th a Beckman digital pH meter . At the end of the test, the cell was opened, and the porous electrode t ransfer red into ethyl alcohol. It was washed several t imes with fresh alcohol to remove all water and potassium hydroxide. Finally, it was dried in a vacuum desiccator, where it was stored for fur ther examination.

Observation of oxide morphology.--Some electrodes were examined with a scanning electron microscope (JSM-U3, JOELCO, Japan) to determine the shape of the oxide part icles formed during the discharge. Only the top surface (facing the glass cover in the cell) was examined.

Determination o] current penetration.--Some electrodes were microsliced under a microscope, equipped with a cal ibrated scale, using stainless steel blades. First a 1 mm wide section was cut out longi tudinal ly (in the direction of current flow), f rom the middle of the electrode to avoid the influence of edge effects. This 3 X 1 X 1 mm piece was then fur ther cut into six, 0.5 mm wide, parts. The ZnO and Zn content of each part was then determined as follows. The oxide was dissolved in a solution of 1M NH4OH and 1M NH4C1. This solution dissolves the oxide but attacks the zinc only negligibly during the 5 min contact t ime (32). The solution was then filtered and analyzed for zinc wi th an EDTA t i t rat ion (33). The remaining zinc was dissolved in hydrochloric acid and the solution analyzed for zinc as above.

Results Morphology of the oxide f i lm.- -Scanning electron

microscopic invest igat ion was carried out on electrodes discharged at 4 and 40 m A / c m 2 current densities wi th a charge range of 4 to 18 mA-hr . In the inter ior of the electrode, a ve ry porous "carpe t - l ike" oxide film, consisting of long needle crystals, was found on the zinc grains. Examples are shown in Fig. 2. The appearance of the film was the same eve rywhere in the inter ior of the electrode, independent ly of distance from the solution, current density, and depth of discharge, wi th in the l imits of the experiments . On areas less densely populated with growth, occasionally side arms were observed on the needles (Fig. 3). At the edge of the electrode, toward the electrolyte, the film was more compact as shown in Fig. 4. An exper iment was carried out using an 8M KOH solution, and produced similar results (Fig. 5).

Current penetration into the porous electrode.~This exper iment was carried out at five current densities ranging f rom 5 to 80 m A / c m 2. The durat ion of discharge was also varied so as to keep the total charge used constant, at a value of 25% over -a l l conversion of the porous electrode. F r o m the chemical analysis of the sliced electrodes, the average conversion of zinc to zinc oxide was calculated for each section. The results are plotted in Fig. 6-10, as a function of distance f rom the electrolyte. The measured overpotent ials are given in Table I.

Discussion The morphology of the oxide/~Zrn.~The dendri t ic ap-

pearance of the oxide, wi th all the needles point ing towards the solution, suggests the fol lowing mechanism for its formation. The dissolving zinc supersaturates

Table I. Overpotentials after one minute of polarization

C u r r e n t d e n s i t y O v e r p o t e n t i a l (mA/cm'-') (mY)

5 4 10 10 "JO 14 40 23 80 39



VoI. 119, No. 9 POROUS ZINC ELECTRODES 1131

Fig. 2. Examples of SEM pic- tures of oxide film. A, B: 40 mA/ cm ~, 25% discharge; C, D, E: 4 mA/cm ~, 25% discharge; F: 4 mA/cm 2, 44% discharge. Mark represents 3~ for A, C and F; and I/~ for B, D, and E.

the solution in zincate [the stabil i ty of highly supersaturated zincate solutions is known (21)] unt i l a concentrat ion is reached when oxide nucleat ion suddenly begins. This will happen at the surface of the metal where the zincate concentrat ion is the highest. Once nucleat ion occurred, crystall ization will proceed very fast from the supersaturated solution and a concen- t ra t ion gradient is set up, creating a diffusion layer. If the radius of curva ture of some of the nuclei is small enough, to allow spherical diffusion to take over, conditions for dendrit ic growth are fulfilled (34, 35). Ap- parently, due to the s imultaneous nucleat ion of a large number of crystals, many nuclei wil l have the required small radius of curvature resul t ing in a dense, carpet- like, appearance of the film. The observation of occasional side branches (Fig. 3) supplies strong evidence for this mechanism. This mechanism is analogous to the dendrit ic solidification of a cylindrical mass of mol ten meta l (36). Figure 2E affords a side view of the film which peeled off the zinc grain.~

P e e l i n g w a s o b s e r v e d o n l y occas iona l ly . F r o m t h e p r e s e n t d a t a i t cou ld n o t be d e c i d e d w h e t h e r t h i s o c c u r r e d d u r i n g t h e e l e c t r o d e p rocess , o r a f t e r w a r d s d u r i n g t h e v a c u u m d r y i n g .

Current distribution.--There is strong evidence to in - dicate that the reaction in a porous zinc electrode proceeds through the soluble zincate intermediate. Anodic studies on p lanar zinc electrodes have shown, that under hydrodynamic conditions most resembling that of the porous electrode, the oxide film was formed by a dissolut ion-precipi tat ion mechanism (I1, 17-19). The morphology of the zinc oxide film (see above) also supports this mechanism. Further , cathodic studies (20) have proven that the direct (solid-state) reduction of ZnO does not contr ibute in measurable amount to the over-al l reaction. The first step in the discharge mechanism of a porous zinc electrode is, therefore, the electrochemical formation of zincate, a s tudy of which reaction was reported earl ier (31). The exchange current density of the reaction being high (80 m A / c m 2 for the solution concentrat ion used in these exper iments) , the activation overpotential in the present case can be considered to be in the l inear current densi ty-overpotent ial range for all, except the highest, cur rent density used, and even for the highest one it can be used as a good approximation (cf. also Table I). This consider- ably simplifies the problem. The sophisticated, and



1132 J. Electrochem. Soc.: E L E C T R O C H E M I C A L SCIENCE AND T E C H N O L O G Y

mathemat ical ly complex, theories which use the But- ler-Volmer equation (or the high overpotential approximation) in the description of the porous electrode behavior [cf. Bockris and Sr inivasan (1)], can be now reduced to their simplest form, taking the l inear relat ion between current and potential. This simple theory was considered earlier by Euler and Nonnenmacher (37) (without considering any struc- tural changes dur ing the reaction), and their equations will be used as a basis of approach for the present problem.

The porous electrode is considered as a ne twork of series and parallel resistors as shown in Fig. 11. The current flowing through the metallic phase is i l ( x ) , and that in the solution phase, ie(x) . Curren t t ransfer between the two phases ( through the surface resistance l/K) results in faradaic reaction, the value of which is expressed as: dil/dX, at any point x along the porous electrode. The current densities, and the poten- tials (El in the metallic, and E2 in the solution phase) are functions of distance (x) obeying the following relat ions

Combining Eq. ential equation

dE~ 1 - - - - i l ( x )

dx ~ra

dEs 1 - - - - i2(x)

d x asol

di,

dm

dh

dm

[1] 4

[1] through [4]

[2] ~

- E1 ( x ) ] [3]

[4] dx

gives the final differ-

ddEI ( 1 1 ) d2E1 �9 + , = 0 [5 ]

d x 4 a m O'so I d x 2,

The following boundary conditions are re levant to the present problem

i l ( x : 0) = 0; i l ( x = L) : i

i2(x = O) = i; E2(x = O) = 0

where L is the over-al l length of the porous electrode and i (= i l , at x -- L) is the total cur rent density. The solution for the dis tr ibut ion of the faradaic current (di l /dy) as a funct ion of fractional distance (y = x / L )

is given by

dil i �9 ~rsol �9 v

dy (asoi + am) sinh v

[ am c ~ 1 7 6

where /

~sol + am v = L " ~ / K , [7]

0"sol a m

Note, that in accord with the l inear i-~l restriction the surface resistance (l/K) was considered constant, in - dependent of potential. In the general sense the surface resistance can include not only the faradaic resistance but also the electrical resistance of any film formed on the surface as shown schematically in Fig. 12 for a single pore.

~ T h e m e t a l l i c a n d s o l u t i o n e o n d u c t i v i t i e s (crm, crsol) u s e d in Eq. [1] a n d [2] a r e n o t e q u a l to t h e c o r r e s p o n d i n g b u l k v a l u e s . T h e y r e p r e s e n t t h e r e s p e c t i v e c o n t r i b u t i o n s of t h e m e t a l l i c a n d s o l u t i o n p h a s e s to t h e o v e r - a l l c o n d u c t i v i t y of t h e s o l u t i o n f i l led p o r o u s e l e c t r o d e . F o r a n e l e c t r o d e h a v i n g p % p o r o s i t y t h e i r v a l u e s can be e s t i m a t e d , as a f i r s t a p p r o x i m a t i o n , as

i00 - p ffta ~ (O"m bulk) ~ , a n d

100

P 0"aol ~ (Gsol bulk)

I 0 0

S e p t e m b e r 19 72

Fig. 3. Appearance of sidearms. 40 mA/cm 2, 28% discharge. Mark represents 3~ for A, and 1~ for B.

In the derivat ion of Eq. [6] asol , am, and ~ were taken as constants, independent of distance wi th in the electrode and time of discharge. In the case of an electrode where s t ructural changes occur dur ing the process, these assumptions have to be re-examined.

For the present electrode process the values of asol, am, and K are known for the beginning of the process. 5 If these are subst i tuted in Eq. [6] it can be shown that the current will drop off very quickly 'with distance, reaching 10% of the over-al l cur rent value at y = 0.11. This prediction, that practically all the faradaic current will go to a layer only one tenth of the electrode's depth, is not in agreement with the exper imental data (Fig. 6-10) showing much greater penetration. The s tructural changes, dissolution of zinc, and precipitat ion of ZnO from the saturated zincate solution will have to be taken into account.

The dissolution of zinc will increase the resistance of the metallic phase (1/am). However, since the experiments were l imited to 25% conversion, the effect of this decrease in Cm will be negligible on the current distribution, considering that the specific conductivi ty of the metal is six orders of magni tude greater than that of the solution.

The precipitat ion of ZnO wil l have two effects: (i) it will plug up the pores, decreasing asol, and (ii) it will cover the surface of zinc, decreasing ~. The first effect (decrease of asol) wil l not cause the current to

O'sol = 0.32 • 0.3; ~n~ = 1.7 X 10 ~ X 0.7, w h e r e t h e v a l u e s a r e c o r r e c t e d f o r 30% po ros i t y . K = 575 x 6.15, w h e r e 575 is t h e a r e a in c m ~ of one cm3 of p o r o u s e l e c t r o d e ( c a l c u l a t e d f o r a 100~ a v e r a g e p a r t i c l e d i a m e t e r ) , a n d 6.15 is t h e r e c i p r o c a l of t h e f a r a d a i c r e - s i s t ance .



Vol. 119, No. 9 P O R O U S ZINC E L E C T R O D E S 1133

Fig. 4. Film at the edge of electrode. 40 mA/cm 2. Discharge: 25% for A, and 44% for B. Mark represents 3~.

penet ra te more deeply into the electrode. On the con- trary, it can be seen, considering Fig. 11, that a decrease in asot wi l l cause the current to transfer, f rom the solution to the high conduct ivi ty metal l ic phase, closer to x ---- 0. This can be shown also numerical ly, using Eq. [6]. Taking a decreased r (corresponding to a porosity of 15% only) the 10% current level will now be reached at y _-- 0.08. It can be calculated that during the discharge of the present electrodes, the average porosity decreases from 30 to 20%. 6 Therefore, this effect wil l change the cur ren t distr ibut ion only slightly and in the direction to make it less in agreement wi th the exper imenta l results.

There remains only the possibility that 1/~ repre - sents not only the faradaic resistance, but, in fact over - whelmingly, a film caused resistance arising f rom the oxide formation. Immedia te ly after the anodic current begins, during the free dissolution of the metal, K can be considered constant. Then the oxide precipi tat ion wil l suddenly decrease ~. The value of K wil l cont inu- ously decrease, af ter the precipitation, due to the thickening of the film. The t ime of precipi tat ion can be est imated as follows. The total vo lume of the electrolyte in the pores of the electrode is 2.1 • 10 -8 cm 8. The solubil i ty of ZnO in 2M KOH solution is 0.33M (38) and, considering that 0.1M zincate is present in the original solution, it can be calculated that a charge of 0.026 m A - h r is needed to saturate all the electrolyte

o T h i s w a s c a l c u l a t e d f o r 25% d i s c h a r g e t a k i n g i n t o c o n s i d e r a t i o n t h e d e n s i t y d i f f e r e n c e b e t w e e n Z n a n d Z n O (7.14 a n d 5.6, r e s p e c - t i v e l y ) .

Fig. 5. Film formed in 8M KOH. 100 mA/cm ~, 31% discharge. Mark represents 10~ for A, and 3~ for B.

4 0

t,-

0 .--

a~

~2o

o

X

30

I0

0

X

^" [--1 X X

I I .I .2 .3

Electrode Depth (cm)

Fig. 6. Per cent conversion as function of distance. 5 mA/cm ~. x: values calculated with ~ ~ 0.6.



1134

4 0

30

O ==

�9 " 20 0 fj

10

J. E l e c t r o c h e m . Soc . : E L E C T R O C H E M I C A L SCIENCE AND T E C H N O L O G Y

X

X

I x -lx

I I 0 .1 .2 .3

E l e c t r o d e D e p t h ( c m )

Fig. 7. Per cent conversion as a function of distance. 10 mA/cm 2. x: values calculated with • ~ 1.25.

3 0 X

C 0 ID

t - -

2o

4 0

10

0

X

X

X

I .I .2

Electrode Depth (cm)

.3

Fig. 8. Per cent conversion as a function of distance. 20 mA/cm 2. x: values calculated with K ~- 2.0.

within the pores. This charge is negligible as compared to the total charge of 8 m A - h r used in these experiments, and therefore, it can be considered that film precipi tat ion will occur everywhere in the electrode, instantaneously, at the beginning of discharge. 7 The

A c t u a l l y , a t t h e b e g i n n i n g , p r a c t i c a l l y a l l t h e c u r r e n t w i l l go to t h e f i r s t one t e n t h s of t h e e l e c t r o d e d e p t h (as w a s s h o w n e a r l i e r u s i n g Eq. [6] w i t h t h e i n i t i a l v a l u e s of c o n d u c t a n c e s ) , c a u s i n g p r e - c i p i t a t i o n in t h i s a r e a a n d s h i f t i n g t h e c u r r e n t to t h e n e x t l a y e r i n the e l e c t r o d e u n t i l t h e s o l u t i o n b e c o m e s s a t u r a t e d t h e r e , a n d so on. T h i s p rocess s w e e p s ac ross t h e e l e c t r o d e in a t i m e d u r a t i o n n e g l i - g i b l e as c o m p a r e d to t h e t o t a l t i m e of t h e e x p e r i m e n t .

~176 I 4o __ 3 0 -

r

.o 1,.

8 z o -

I 0 -

_

S e p t e m b e r 1972

X

X

X

X X

I .I .2

E l e c t r o d e D e p t h (cm)

.3

film then will thicken in time, causing K to decrease further. As a first approximation, this change in t ime will be smoothed out, and an average K wil l be considered in the following way.

An at tempt was made to fit Eq. [6] to the experi- menta l data (Fig. 6-10) with a suitable selection of the value of ~.s Reasonable agreement was obtained with the K's indicated. These values, and their cur ren t dependence can then be examined to determine whether they are physically realistic, and whether their dependence on current densi ty can be theoret i- cally explained.

To arr ive at an expected approximate dependence of on current density, the following extension of the

present model for the operation of the porous electrode is used. The film is formed by precipitation from a saturated solution formed by galvanostatic dissolution of the metal, under conditions, when the reaction products can be removed only by diffusion. Under these conditions, the t ime of precipi tat ion is related to the current density as

i tl/2 ~ const [8]

The resistance of the film, on the other hand, wil l be proport ional to its thickness, which depends on the total charge represented by the film

1/~ ~- const �9 i �9 t [9]

Therefore, the dependence of "K," the surface conductivity, on current can be approximated from [8] and [9] as

K ~ const �9 i [10]

predict ing a l inear relation, which in fact was observed (Fig. 13).

s I n t h e s e c a l c u l a t i o n s , am w a s t a k e n as 1.7 • 10~ • 0.7 a n d ~sol as 0.32 • 0.2. A d e c r e a s e d s o l u t i o n c o n d u c t i v i t y w a s u s e d to a c c o u n t fo r t h e p l u g g i n g of p o r e s by Z n O .

Fig. 9. Per cent conversion as o function of distance. 40 mA/cm 2. x: values calculated with K ~ 3.0.



VoL 119, No. 9 P O R O U S ZINC E L E C T R O D E S 1135

60

X

4 0

C 0

t=. G)

30 0 U

20

I0

•

X

I I .1 .2

Electrode Depth (cm) .3

Fig. 10. Per cent conversion as a function of distance. 80 mA/cm ~. x: values calculated with ~ -- 5.5.

-i2

P o r o u s Electrode Solution

X=O X L Fig. I I. Schematic representation of the porous electrode

Metal Contact

The actual values of %" can also be compared to resistances calculated for different model of the film. The surface resistance ( l /x ) is a sum of the terms: the faradaic resistance, the resistance of the porous zinc oxide film, and the resistance of any continuous film beneath the porous film. Assuming a surface area of 575 cm2/cm ~ (calculated for an average part icle size of 100~) the surface resistance values giving the best fits to the exper imenta l data range be tween 100 and 1000 ohms cm 2. Such a high value cannot be explained ei ther by the decrease of apparent io due to decrease of free area, or by the resistance of the porous film based on the observed film structures. It seems, therefore, that the assumption of the existence of a "com-

i / / / / / / / / / / / / / M

SOLUTION /

~ . ~ ~ ~ r eFsl s~a nce

K///, o,ooo,c L ~ ~ e sistance

Fig. 12. Schematic representation of a single pore

6 I

O

5 _ 0 0

I

I0 20 30 40 50 60 70 80 i [ m A / c m z ]

Fig. 13. Dependence of film conductivity on current density

4

K 3

2

0

pact" film undernea th the "porous" one is necessary to expla in the data. This is in agreement wi th the du- plex films recent ly found on planar electrodes (11, 17-19). Since the specific resistance of zinc oxide can vary f rom 10 -~ to 10 l~ ohm cm (39, 40) depending on stoichiometry of the oxide and on impurities, a ve ry thin film could account for the observed resistances.

The resistance of the film wil l increase as the film thickens. While this wil l make the current distr ibution more uniform, it wil l also increase the IR losses in the electrode. This accounts for the decrease of the useful vol tage of the ba t te ry as the discharge proceeds. This model, therefore, suggests a possible way to enhance the uni formi ty of current distr ibut ion in the porous electrode, namely the increase of oxide film resistance (possibly by suitable al loying of the zinc). This will, however , increase the IR losses, and the two effects wil l have to be optimized. On cathodic charging, the high resistance, compact film is expected to be reduced



1136 J. Electrochem. Soc.: E L E C T R O C H E M I C A L SCIE N CE A N D T E C H N O L O G Y September 1972

fast, t he reby a l te r ing the to ta l resis tance of the film and making the cur ren t d is t r ibut ion less uni form (as compared to tha t dur ing d ischarge) . On repea ted cycling, this difference of the anodic and cathodic cur ren t d is t r ibut ions m a y cause phys ica l dis tor t ion of the e lectrode s tructure.

The presen t resul ts can be compared with the cur ren t pene t ra t ion observat ion of Bre i te r (28). In that study, processes occurr ing in a porous zinc e lec t rode were in- ves t igated using potent ia l sweep technique. I t was con- c luded that only a smal l por t ion of the in ter ior of the electrode was active. Considering tha t the sweeps las ted a few minutes only, the resu l t is in agreement wi th the present model, since the film format ion m a y not have pene t ra ted across the whole e lect rode dur ing the exper iments .

Acknowledgments Our thanks are due to Mr. R. Whi te for his help and

advice dur ing the electronmicroscopic work. F inanc ia l assistance by the Nat ional Science Founda -

tion (Grant No. NSF-GK-16550) , and the A.R.P.A. P ro - g r am in Mater ia ls Science (Univers i ty of Pennsy l - vania) is g ra te fu l ly acknowledged.

Manuscr ip t submi t ted Dec. 7, 1971; revised manuscr ipt rece ived Apr i l 12, 1972.

Any discussion of this p a p e r will appear in a Discus- sion Section to be publ ished in the June 1973 JOURNAL.

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