The electrochemistry of porous zinc—III. Plain and polymer-bonded porous microelectrodes in perchlorate solutions

rhafa&.,dm Acm. Val . 30, Na.7, M947 %cn, 1955 hinted

m

Greet

aria.

(n

J013-4686/55 S3.W+e.0a 1 1955 . Pe,ramon Pros . Ltd .

THE ELECTROCHEMISTRY OF POROUS ZINC-Ill . PLAIN AND POLYMER-BONDED POROUS MICROELECTRODES IN PERCHLORATE SOLUTIONS P. F. MAHER, A. J . S . MCNFIL and N . A . HAMPSON Department of Chemistry, Loughborough University of Technology, Loughborough, Leicestershire LEI 13TU, U .K . (Received 25 September 1984 ; in revised form I I December 1984)

Abstract-The results of an impedance study of the electrochemistry of solid, plain porous and porous polymeric (incorporating polystyrene) zinc electrodes in t MNaCIO4 solution are presented . The transformation of the solid zinc electrode to the porous format results in great enhancement of reaction rules, but this is subject to limitations on removal of reaction products by mass transfer. Thus porous electrodes are prone to gas-filling by the HER, and to film formation and pore plugging at anodic overpotentials . The incorporation of moderate (10%) amounts of an electro-inactive polymer, such as polystyrene, have little effect on electrode activity . Larger amounts (50%) tun produce an anomalous electrode which appears perfectly porous, but which can be little more active than the solid electrode .

1 . INTRODUCTION

RAPRA') were made up from a water-free paste comprising zinc dust, zinc oxide, polystyrene (PS) solution in a 50 :50 tetrahydrofuran/methanol mixture. The presence of methanol has been shown to have marked effects upon the state of zinc particles in the paste and to be a necessary constituent of large format elctrodes[12] . Polystyrene was incorporated to the extent of 2, 10, and 50% of the dry mix weight . Solid zinc electrodes were abraded on 1200 grit carbide paper, and etched in 60 % HCl to remove the mechanically deformed surface layer[13] and reveal the underlying grain structure . Porous electrodes were fully reduced at 2 mA in I M aqueous NaC1O 4 solution (see[5]). The experimental cell was based on a longestablished design[14, 15] . with two working compartments separated by a glass frit, and a nitrogen lift side arm containing activated charcoal[15, 16] to clean both solution and cell walls. The reference electrode was a saturated sodium calomel type (E = 0.236 V vs nhe) in a side arm connected to the working electrode compartment by a partly closed PTFE tap. Glass frits fitted to the electrode compartments enabled the electrolyte solution (1 M Analar sodium perchlorate in triple distilled water) to be deperated with oxygen-free nitrogen before each experiment . The counter electrode was a large area (100 cm 2 ) platinum gauze. The impedance of the solid electrode-electrolyte interphase was first measured as a series combination of resistance (R,) and capacitance (Cr ) using a Schering bridge[17] . This proved inadequate for measuring the very low impedances of the porous electrodes . Measurements on these electrodes were made using either a Solartron 1250 frequency response analyser (FRA) operated by a Kemitron K3000 E microcomputer, or a manually operated Solartron 1172 FRA, in

There have been many studies made of the electrochemistry of zinc in the solid format, however, it is recognized[l] that the majority of practical zinc electrodes are not perfectly solid, but are porous to some degree and the more complex behaviour of zinc in this format ought to be considered. Hampson et al. have extended earlier galvanostatic passivation studies of solid zinc electrodes[2] to consider porous elee trodes[3, 4] . McNe it and Hampson[5] have recently studied the polarization behaviour of plain porous zinc. A number of French workers[-8] have more recently been applying the impedance technique to the study of solid zinc in a variety of electrolytes and are now[9] using that technique to characterise the porous structure of zinc particle electrodes . The use of a porous electrode allows the admixture of a variety of addition agents (see[ I] on zinc) to control electrode behaviour, eg gas evolution reactions, shape change . One important class of addition is polymeric materials, the prime example of which is PT FE, added to improve mechanical properties as well as cycling behaviour[l0, 11] . PTFE is incorporated in the porous electrode from a suspension, but a variety of polymers can be incorporated from the solution state. Some of these polymers can enhance the cycle life of these porous electrodes[12] . Here we report the results of an impedance study of the electrochemical behaviour of porous zinc with admixture of polystyrene (PS) in aqueous perchlorate solutions .

2. EXPERIMENTAL PROCEDURES The preparation of solid and plain porous zinc electrodes (projected area 0 .0707 ctrl°) is fully doscribed elsewherc[5] . Polymeric porous electrodes incorporating polystyrene (PSI, supplied by

Rubber and Plastics Research Association, Shawbury, Shrewsbury, Shropshire SY4 4NR . U .K . 947



948

P . F. MAaER . A . J. S . MCNEIL AND N . A. HAMPSUN

both cases using the 1186 Electrochemical Interface . Measurements of polarization and differential capacitance (at f = 1 kHz) were made on these systems by scanning the potential range and measuring current and imaginary impedance 2 minutes after setting each new potential . 3 . EXPERIMENTAL RESULTS

3.1 . The corrosion of solid zinc In decimolar perchlorate electrolytes zinc assumes a free corrosion potential of - - 1100 mV, where the dissolution of zinc is balanced by the generation of hydrogen (HER) . This can lead to significant reorganization of the electrode surface, which reveals itself in the impedance spectra . Figure 1 is the impedance spectrum for solid zinc soon after surface preparation by etching ; Fig . 2 represents the surface

80o

after 6 h free corrosion in the same electrolyte . These spectra can be interpreted in terms of a rough Randles circuit [18] in which both the charge transfer resistance (6) and the high frequency dihedral angle (¢) are increasing. These changes are consistent with a transition from the variety of active high index crystal planes produced by the fast etching treatment to a more homogeneous surface of stable low index planes produced by the slow corrosion process. Brown et al.[19] have also observed time dependence of C, of polycrystalline zinc in near neutral KCI solutions . Hendrikx et al .[20] observed that the activity of their zinc electrodes was a function of waiting time at the rest potential. As corrosion proceeds, there is a decrease in crystallographic heterogeneity as well as a decrease in geometric surface area . These lead to an increase in the electrode impedance, most conveniently shown by Fig . 3 for the series capacitance[21] . The cathodic shift of the rest potential (-50 mV within 10 min of immersion) suggests the absence of any oxide film[22], in support of the differential capacitance measurements . Although these changes in electrode condition were marked, they were also slow, and consistent results were obtained in experiments on solid zinc performed soon after stabilization of the rest potential . No surface treatment of porous electrodes was possible, of course, and experiments were commenced immediately after terminating cathodic reduction .

3 .2. Porous electrode structures 400

0 0

400

800

1200

1600 R /Oh .

Fig . 1 . Impedance spectrum for solid zinc at its free corrosion potential. E - - 1103 mV (see) in M/10 NaC10„ pH 3 .5, about 30 min after etching in 60% HCI . Figures give frequency in Hz.

600

400

g

0 400

coo

1200

1600 R /6h .

Fig . 2. Impedance spectrum of the same electrode as in Fig . l, but 6 h later .

The fully reduced porous electrodes were soaked in methanol to wash out the perchlorate electrolyte, broken to reveal a fracture surface, and coated with 10 nm of sputtered gold before examination in an ISI SS40 scanning electron microscope (SEM) . The plain zinc electrode (Fig . 4) possessed a complex structure comprising variously sized zinc granules embedded in a continuous thread-like phase. Electrodes made only from ZnO showed only the thread-like form of zinc . Thus it is likely that the granules are the zinc dust, and the thread-like phase derives from reduction of ZnO . The intervening space is geometrically complex and contains voids of a wide size range . The presence of 50% PS (Fig . 5) has produced a continuous 3dimensional net of polymer (somewhat deformed in fracture), within which are dispersed finely porous aggregates of zinc . The zinc surface area is much reduced in this way, and the fine polymer mesh not only renders the zinc phase less accessible, but is also prone to blockage by discharge reaction products . Furthermore, the polymer forms a skin over the external free surface of the electrode (Fig. 6). This is pierced by hales of various sizes . with zinc deposits growing through some of the larger ones . Figure 7 shows an edge view of this superficial skin, with the porous bulk structure (including a large facetted zinc granule) underneath . This skin will impose a further impediment to electrode activity, and again be prone to blockage by reaction products . Initially the polymer solution in the paste surrounds and fully wets the zinc and ZnO particles . On the visual evidence it seems that the PS solution has considerable mobility in the paste and collects at free surfaces to form a skin . The loss of the THF solvent would



The electrochemistry of porous zinc

111

949

00 .

u 50

40

20

0 10

160

lobo

f /Nz

10 .000

Fig . 3. Frequency dispersion of series capacitance for the solid zinc electrode represented in Figs I and 2, in the fresh (o) and corroded (x) conditions.

Fig . 4. The microstructure of the plain porous zinc electrode (bar = 10 pm).

introduce a considerable void fraction (the volume ratio of PS to its solution in THF was - 5), and create the rigid 3-D polymer mesh, seen in Fig . 4 . Within the

constraint of this mesh the ZnO reduces to zinc with a further volume reduction (1 :0.63) and aggregates into dense microporous clumps .



950

P. F. MAILER, A . J. S. MCNEIL AND N . A . HAMpsoN

Fig . 5 . The bulk microstructure of the Zn/PS50 electrode (scale as Fig . 4).

Fig 6 . The free surface of the Za/PS50 electrode (scale as Fig . 4) . 3 .3 . Polarization and differential capacitance measuremeats The solid zinc electrode is not completely polarizable at any point on the potential scale (Fig . 8), as was

also found by Caswell er al .[23] and by Baugh[24, 25] . The measured currents are larger than those reported by the former workers, but comparable with the measurements of Baugh[25], though smaller than the measurements of Baugh and Lee[24] for M/10



The electrochemistry of porous zinc-III

951

Fig . 7 . An edge section of the Zn/PS50 electrode, showing the porous structure underlying the polymer surface skin (scale as Fig . 4). 10 0 -1 030 -1

060/'_

1 100 -1140

-1180 -1 220 E/ mV

-10 l -20 -30 40

f

I

-50 -oo -70 -80 -90

Fig. 8. Polarization curve for solid zinc: 1 M NaCIO„ pH 3 .4 . NaCIO, . The anodic currents passed by the porous electrodes (Fig. 9) were generally much higher than for solid zinc, due to their large internal area- However, the

measurements could not be made in the normal way, by stepping the potential in the anodic direction because of interference by entrapped hydrogen generated at cathodic potentials . The curves shown in Fig. 9 were determined by sweeping from the rest potential of - 1100 mV, first in the cathodic direction, and then after stabilizing again at I = 0 for E = - 1100 mV, in the anodic direction . The cathodic HER could be selflimiting (eg for Zn/PSIO) because the gas generated drove the electrolyte solution out of the electrode pores. The incorporation of 50 % PS severely curtailed the generation of both anodic and cathodic current . Figure 10 shows the potential dependence of capacitance and resistance of the solid zinc electrode, as determined using the Schering bridge, and is in good agreement with earlier studies[23] . The discrepancy between these measurements and those of Baugh and Lee[24] may perhaps be due to the difference in real surface area between an etched electrode surface and a highly polished one. The minimum value of 4014F cm -2 for capacitance and the small change in resistance indicate the cleanliness of the electrode surface, as well as the importance of maintaining a low solution pH. Increasing the solution pH beyond - 4 caused electrode filming with a marked drop in C„ in agreement with Caswell et al.[23] . Measurements on porous electrodes (Figs I I and 12) were determined in the same way as the polarization curves . Now, in marked contrast to the solid electrode, the capacitance values could show little or no rise from the minimum in both the anodic and cathodic directions, before falling . Capacitance values for the 0 and 10% electrodes were very much higher than for solid zinc (x50 For 0% PS and x 7O for Zn,PSIO at -1100 mV) indicating the very large internal surface



952

P . F. MAHER, A . .1. S. MCNEIL AND N. A . HAMESON

1000-

aoo000

A 40o 200

0 900

-1000

i-1100

-1200

v

-1300 -11.00

-1500

-1600

E/mV

-200 0

c

-400 600

0

-000

c

_1000/ -12001 Fig . 9 . Polarization curves for porous zinc electrodes containing 0 % PS (o), 10 PS ( K)and 50o PS (C). o

4

7

1L6 114 6

147

740 30

s

13s

`

134 132 130

-~_ 020

-I 'C50' -=1100' -1140

-1

lee

-1 7.20

1260

e / ,„ vV

Fig . 10. Potential dependence of differential capacitance (o) and electrode resistance ( x ) for solid zinc in I M NaC1O 4 , pH 3.4 .

area of these porous electrodes . However, the increase was not nearly so large for the Zn/PS50 electrode (x 13 at - 1100 mV), suggesting a relatively small surface area for this electrode (Fig . 12) . What is also interesting is the way capacitance values have varied with potential . Unlike the solid electrode, there is no simple capacitance minimum at an intermediate potenriot, instead capacitance tends to fall in both anodic and cathodic directions. The electrodes were polarized from the rest potential first in the anodic direction, then cathodically, and in the case of the Zn/PS50 electrode, anodically again . The initial rise in C„ as with Zn/-, is likely to be pseudocapacitanec due to the an0dic dissolution . The subsequent fall in C, is probably due to the development of a type I prepassive oxide film[26] within the quiescent interior of the electrode (the oscillations in C, for the Znj-electrode are reminiscent of current oscillations in the polarization curve before the onset of passivatiorif 271), and this may well he materially assisted by a reduction in internal surface area due to dissolution . The result, on returning to the rest potential at the start of the cathodic sweep is a much reduced value of C, . In the cathodic sweep C, sharply increases, due to the HER pseudocapacitance, as well as to the reduction in the hydroxide film, but then may fall (Fig . 11) as the internal working surface is occluded by cathodically generated hydrogen. On returning to the rest potential

The electrochemistry of porous zinc-III

953

c rc 40

36 36 34 32 30

28 26 24

Fig . 11 . Potential dependence of differential capacitance (o) and electrode resistance (x) for Zn/PS 0 j swept from the rest potential of - 1100 mV first anodically, then cathodically.

the value of C, is somewhat restored to its original figure, but falls again in the second anodic sweep. (Fig. 12 for Zn/PS50). 3 .4. Impedance spectra 3 .4 .1 . Solid zinc . The impedance spectra recorded for solid zinc (Figs 13 to 15) are all consistent with the Randles equivalent circuit for a fairly smooth electrode. All show a clearly defined charge transfer semicircle at high frequencies, rising from the real axis at nearly 900 , and a long Warburg diffusion tail at low frequencies . This does not show the theoretically predicted[19] 45° dihedral, but is rather tending to return to the real axis, as the ac and do diffusion fields overlap, as has been discussed by Armstrong et aL[28] . Estimation of the double layer capacitance from the 2 spectra gave the values of 46,uFem - at - 1350 mV, and 60 pF cm - ' at -1093 mV, in fair agreement with the differential capacitance measurements . As the potential was made anodic the charge transfer resistance markedly dropped, and at - 1040 mV a clearly defined inductive loop, with a relaxation frequency of - 1 .2 Hz was observed (Fig. 15), indicative of the presence of adsorbed intermediate species in the dissolution reaction . The existence of such species has EL

30 ;7w

been widely reported[6-8, 25, 29, 30], for a variety of electrolyte solutions . It is not realistic to compare the present data with that for dissolution in OH - or Cl electrolytes, because of the specific adsorption of both these anions on zinc, whereas the C1O4 ion is very weakly adsorbed[25] . However, the spectrum in Fig . 15 matches one obtained by Baugh .25] for zinc dissolution in I M Na 2 SO4 , whose S04 - ion is also only weakly adsorbed. The adsorbed species in Fig . 15 is almost certainly Zn(I), which is widely accepted[25] as the mechanism of zinc dissolution in a variety of electrolyte solutions, including CIO, [31] . 3 .4.2 . Plain porous zinc . When the zinc electrode is constituted in a porous format the impedance spectra (Figs 16-19) are radically transformed . The measured impedance values are very much lower than for solid zinc, reflecting the contribution of the large internal area already revealed by SEM, and the form of the spectra are also changed in accordance with de Levies transmission line theory[18]. Thus the impedance locus recorded at - 1350 mV (Fig . 16) rises from the real axis at '-45° at the high frequency end and settles to a 25' dihedral at the low frequency end . This is an interesting correspondence, in view of the great structural complexity of the zinc electrodes compared with



954

P . F . MAHER, A . 7. S. MCNEIL AND N . A. HAMPSON

1000

SOD

0 500

1030

1500

2000 R /Ohm

Fig. 14 . Impedance spectrum for solid zinc at a rest potential of E = -1093 mV .

-900

-1600

-11X

-1200

-1330

-1.00 E / mV

Fig. 12 . As Fig. 11 but Zn/PS50; first anodie sweep--, second anodic sweep

R /ahm

Fig. 15 . Impedance spectrum for solid zinc at E _ - 1040 mV .

4

2

1

0

a

10

R /Ohmx1000

Fig . 13 . Impedance spectrum for solid zinc in I M NaCID4 at pH 3 .4 and E - - 1350 mV.

de Levie's simple physical model_ As a consequence, the centre of the charge transfer semicircle lies below the real axis and no reliable estimate of the charge transfer resistance can be made . The spectrum at the rest potential again shows perfectly porous behaviour, but now the presence of a film, suggested by the steepness of the locus (-7g') at low frequencies . This is consistent with the observation of reduced C, values at

anodic overpotentials, discussed earlier (Fig . I1) . As the potential is made more anodic the low frequency locus returns to the real axis partly because of the presence of adsorbed reaction intermediates, and partly because of the problems of mass transport within the pores. Porous zinc electrodes did not show the clear inductive loop characteristic of the solid (Fig . 15), probably because at any one potential the intermediate Zn(I) species were never present uniformly throughout the electrode, but only in some localized region . The nearest that Zn/- came to producing an inductive loop was at -960 mV (Fig . 18), where the locus, after displaying charge transfer and diffusion features, ends with the tangle of low frequency points close to the real axis. At a greater anodic overpotential (Fig . 19) the impedance locus hardly rises off the real axis at all, and we see zinc oxidizing largely to produce a solid film within the porous structure . rather than dissolving into solution . This increases the real impe-



The electrochemistry of porous zinc-Ill

E L

955

25

c

20

15

10

5

0 I5

20

25

30

35 R/Ohm

40

Fig . 16. Impedance spectrum for plain porous zinc, Zn/-, in I M NaC1O, at pH 3 .4 and E = - 1350 mV.

E L O U S

75

4

3

1

15

30

45

60 R'Ohm

75

Fig . 17. Impedance spectrum for Zn!- at E _ - 1100 mV .

dance of the electrode, diminishing the electrode current, and thus extends the low frequency locus along the real axis . McNeil and Hampson[5] studied the polarization behaviour of plain porous zinc and concluded, on the basis of the doubling of Tafcl slope, that the electrode behaved in a semi-infinite porous manner . Moreover, as the anodic overpotential was increased the ratio of

current delivered by the porous electrode to that delivered by the solid electrode decreased, due to limitations on long term mass transfer . These two conclusions, based on do polarization measurements, have been substantiated by the present ac impedance observations . One curious feature of the impedance spectra is their high frequency behaviour . Some spectra commenced



9 56

P. F. MAHER, A . J . S. MCNEIL Atm N. A. HAMPSON

on the real axis (eg Fig. 18) and others (not shown here) displayed a small inductive tail, but there were others (Figs 16, 17 and notably 19) which show the locus rising at the high frequency end . This phenomenon has also been observed in other porous zinc electrodes[32] and in porous PbO 2 electrodes[33] . Its significance is not yet clear . 3 .4.3. Polymeric porous electrodes . The behaviour of the 2 % and 10% electrodes were similar to that of the plain porous electrode . The low PS density would he expected to lead to a large volume fraction .

c

However the SEM examination makes it clear that the large volume contraction occurring with the loss of the THF solvent creates a considerable void fraction, so that the zinc electrode reaction is little impeded . The 10% PS electrodes displayed no new features, only a slightly lower general activity, revealed as slightly increased impedance values . The 50% PS electrode (Figs 20-24) showed more definite changes and of a rather contradictory nature, for they displayed clear porous electrode characteristics, but were very slow kinetically, sometimes being comparable more with the solid than with the

15

1

200

150

5 100



0

a

50

0

so

0

-5 20

25

30

35

100

190

40

200

250 R /Oh .

R/Oh .

Fig. 18. Impedance spectrum for Zn/ at E _ -960 mV .

Fig 20 . Impedance spectrum for Zn/PSSOin I MNaC]O,at pH 3 .4 and E = - 1420 mV .

8

4

0 o o r

2

0 o

0

20

22

0

-2

-4 8

~24

26

28 R/Chm

Fig . 19. Impedance spectrum for Zn/ at E _ - 900 mV .

30



The electrochemistry of porous zinc-Ill

E

957

700

O U } 600

500

400

300

200

I., 100 o 0 0

Too

200

300

400

500 R'Ohm

600

700

Fig . 21 . Impedance spectrum for Zn/PS50 at E _ -1100 mV .

E L O U 3

Fig . 22 . Impedance spectrum for Zn/PS50 at E plain porous zinc . The SEM micrographs show the electrode to be truly porous, but that the zinc is largely inaccessible. The differential capacitance measurements (Fig. 12) are consistent with a much smaller internal area for the PS50 electrode, and this is probably the cause of the decreased electrode activity, as at the cathodic and rest potentials (Figs-20 and 21) .

-1020 mV .

For this same reason the effect of internal filmingat the latter potential does not contribute such a large imaginary impedance at low frequencies as it does in the case of Zn,- . When the electrode is made anodic we see its sluggishness and self-contradictory behaviour. At E = - 1020 mV (Fig . 22), for example, the impedance locus



P . F. MAHER, A. J . S . McNEtI . ANT) N . A. HAMPSON

958 E L 0

150

100

50

0

-50 0

50

100

150 R/Ohm

200

Fig. 23 . Impedance spectrum for Zn/ PS50 at E _ - 900 mV; impedance measuremen is started immediately on setting the potential . 400 .

200

100_ o 0

-100 0

0

100

200

300

400 R'Ohm

Fig . 24 . As Fig . l9, but impedance measurements made after holding some time at charge already withdrawn . corresponds to a perfectly porous electrode, that is, the high frequency locus angle is 42' and - 23` for the low frequency, and as for Zn/-, there is again no sign of any adsorbed species. However, the size of the charge transfer semicircle, in so far as it can be estimated, is comparable to or even greater than for solid zinc . At -960 and -900 mV (Fig . 23) the impedance locus displayed a single charge transfer semicircle which

500 900 mV, with 1 C

returned rapidly to the real axis with no sign of any contribution from diffusion in solution (compare Zn/-at -960 mV, Fig. 1S) . When the electrode had been held at -900 mV, and -- I C charge withdrawn, the impedance locus (Fig. 24) was the same overall shape, but much larger, due to the blocking of the porous structure by reaction products, thus decreasing the accessibility of the remaining zinc within the PS-



The electrochemistry of porous zinc-Ill

U

a rv

Z,/ . h,ns

Fig. 25. High frequency impedance spectra for ; (i) solid zinc (o); (ii) plain porous zinc (x); and (iii) Zn/PS50 (U), all at - 1250 mV, the closest approach to a polarizable condition. The scales are in arbitrary units to aid comparison : abscissa; (i) a = 0, h = 15000 (ii) a = 20 .5, b = 23.5 0 and (iii) a = 22, b = 52 1k The ordinate is scaled from 0 to y to match the abscissa. Figures indicate ac frequency in Hz,

959

on the removal of reaction products by mass transfer . The porous zinc electrodes suffer internal gas filling at cathodic overpotentials, and film formation and port blocking at anodic overpotentials . In addition the porous format can have further consequences, for example in the high frequency capacitive tail in the impedance spectrum at large anodic overpotentials for Zn/- and Zn/PSSO, that are not yet understood . The incorporation of an electro-inactive material, such as PS, does not introduce any new electrochemical features, as would be expected . The presence ofa sufficient quantity of PS reduced electrode activity, so that the PS50 electrodes displayed fully porous characteristics, yet with reaction rates that were more characteristic of the solid zinc electrode. This behaviour is consistent with structural observations . It will be shown elsewhere[12] that small additions of a polymer such as PS can enhance the cycle life of the zinc electrode. What is the unusual feature of the results presented here is that. these modest polymer additions (10%) do not significantly reduce electrode activity .

Acknowledgement-We thank Dr M . C . Ball for assistance with electron microscopy. We are grateful for financial support from SERC (for AJ .S.M .) .

REFERENCES rich matrix . The spectrum in Fig . 24 represents a more stable electrode with less drift in current, and so does not show the extension of the low frequency tail along the real axis that is seen in Figs 19 and 23 . 3 .4.4. Afurt her comparison of electrodes. The zinc electrode is nowhere completely polarizable over the potential range, but most closely approaches this condition at -1250 mV where the slope of the current-potential curve is at its minimum . Figure 25 compares the high frequency impedance loci for the zinc electrode in 3 formats: (1) solid, (2) plain porous Zn/-, and (3) incorporating 50% PS. Changing the format from solid to porous has caused the locus angle to fall from -75' to 40"; incorporating 50% PS slightly increased this angle to 52° . It would seem that the 50 % PS electrode is still truly porous even though its structure is very different to that of Zn/-. This comparison is very similar to that made by de Levie (see Fig . 9 in[18]) for plane and porous platinum and nickel electrodes, and offers further support for the validity of the transmission line model as a description of the working porous electrode . The porous electrodes showed no sign of the transition from apparently porous to apparently solid electrode behaviour, as frequency decreased (with increase in penetration depth) seen with zinc particle electrodes by Cachet and Wiart[9].

4. CONCLUSIONS The transformation of the zinc electrode to the porous format greatly accelerates the charge transfer process, due to the creation of a large internal surface area, but this is subject in the long term to limitations

1 . N . A. Hampson and A . J. S . McNeil, Electrochemistry, S.p .R., Vol . 8 . Royal Soc. Chem ., London (1983) . 2 . N . A . Hampson and M . J . Tarbox, J , electroehem . Soc . 110, 95 (1983); N . A . Hampson, M . J. Tarbox, J . T. Lilley and J. P . G . Fan, Electrochem . Techno). 2, 309 (1964). 3 . R. N. Elsdale, N . A. Hampson, P. C . Jones and A. N . Strachan, J. app! . Electrochem. 1, 213 (1979). 4 . G. Coates, N . A . Hampson . A . Marshall and D . F. Porter, J . Appl . Electrochem . 4, 75 (1974). 5 . A . J . S . McNeil and N . A. Hampson, Surf. Tech . 19, 335 (1983). 6 . C . Cachet, U . Stroder and R. Wiart, Elenrorhirn. Asia 27, 903 (1982). 7 . C . Cachet and R. Wiart . J, etertroanal. Chem. 111, 235 (1980). 8 . C . Cachet, U. Strdder and R . Wiart, J . app! . Electrochem . 11, 613 (1981). 9. C . Cachet and R . Wiart, Electrohim . Acta 29,145 (1984). 10 l. Goodkin, Proc. 22nd Annual Power Sources Conf., p . 79 (1968). 11 . Y . Sato, M . Kanda, H . Niki, M . Ueno, K. Murata, T. Shirogami and T. Takamura, J . Power Sources 9, 147 (1983) 12. N . A. Hampson and A. J . S . McNeil, In preparation for publication . 13 . L . E . Samuels, Metaliographic Polishing by Mechanical Means. Pitman, London (1971) . 14 . l. P . G . Fan and N. A . Hampson, Trans . Faraday Soc . 62, 3493 (1966). 15 . N . A . Hampton nd D . Larkin, J_ eleclraanal . chem- interf. Electrochem . 18, 401 (1968). 16 . G . Barker, Modern Electroanalytical Methods (Edited by G. Charcot) Elsevier, Amsterdam (1958). 17 . J . a B . Randles, Trans- Faraday Sor . 50, 1246 (1954) . IS . R . de Levie, Ade_ electrochem. elevirochem. Eng . 6, 329 (1967). 19. D. S . Brown, J. P. C . Farr, N. A. Hampson, D. Larkin and C . Lewis, J . electroanal. chem. inter£ Electrochem . 17,421 (1968). 20. J . Hcndrikx, A . Van der Putten, W. Visscher and E .



960

P . F . MATER, A. J . S. MCNEIL AND N . A. HAMPSON

Rarendrechi. F,leclrochim, Act . 29, 81 (1984). 21 . R . Leek and N . A . Hampson, Surf. Technol. 7,151 (1978j. 22. U . R. Evans, An Introduction to Metallic Corrosion, p . 85. Arnold, London (1981). 23 . P. Caswell, N . A . Hampton and D . Larkin, J . electroanal . them. interf. Electrochem . 20, 335 (1969) . 24. L. M . Baugh and 1. A . Lee, J. elertroanal. them. interf. 48, 55 (1973) . 25 . L. M . Baugh, Etecrrochim . Aria 24, 657 (1979). 26 . Z. Nagy and J . O'M . Bockris, J . elecirochem. Sor . 119, 1129 (1972). 27 . M . C. H . McKubre and D . D. Macdonald, J. electrochem . Soc. 128, 524 (1981).

28 . R. D. Armstrong, M . F. Bell and A. A. Metcalfe, Electrochemistry, Vol . 6. Roy . Soc. Chem ., London (1978) . 29 . R. D . Armstrong and M . F . Bell, J, electroanal. them . interf. Elecrrochem. 55, 201 (1974). 30. M . Joussellin and R . Wart, Flerrrorhim. Acre 24, 891 (1979) . 31 . L. Gainer and K . F . Hamlet, FAertrochim Aria 15, 161 (1970) . 32 . P. C . Morgan, Ph .D. Thesis, Loughborough University of Technology ( 1983). 33 . J. A . Bialacki, Ph.D . Thesis, Loughborough University of Technology (1984) .