Aspects of the electrochemistry of lead in acid media

J Electroanal. Chem, 145 (1983) 339-353

339

Elsevier Sequoia S A., Lausanne - Printed m The Netherlands

A S P E C T S OF T H E E L E C T R O C H E M I S T R Y OF LEAD IN ACID MEDIA P A R T lI. ACTIVE D I S S O L U T I O N IN OXYGENATED AND DEOXYGENATED P E R C H L O R I C ACID S O L U T I O N S

L.M. BAUGH, K.L. BLADEN and F L TYE

Berec Group Ltd, Group Techmcal Centre, St Ann's Road, London N15 3TJ (England) (Received 14th June 1982)

ABSTRACT Steady-state current-potential, double-layer capacity and impedance measurements have been made on lead m 0 1 M and 5 M perchlortc acid solutions under mtrogen and oxygen saturated con&tlons It ts shown that the metal dissolution reaction is a quasi-reversible process. Evidence for a charge-transfer mechanusm mvolwng two consecuttve one-electron transfers ts gtven w~th Pb [1] as an mtermedLate. A large enhancement in the rate of &ssolut~on m the presence of oxygen is confirmed, although there is no evidence for a change in the reaction mechamsm It ~s suggested that these results wdl have consequences with regard to the operation of l e a d - a o d batteries since, in particular, passwat~on processes at the negatwe are hkely to affected

INTRODUCTION

In Part I [1] a reappraisal was made of the cathodic behaviour of lead in oxygenated and deoxygenated sulphuric acid solutions. It was concluded that an interaction between the metal and trace quantities of oxygen occurs to produce a surface oxide which is reducible only at extreme cathodic potentials ( - 1150 mV vs. SHE) and which re-forms at potentials more positive than this value. However, oxygen has been reported to affect the anodic as well as cathodic behaviour of lead. Fleming et al. [2] using linear sweep voltammetry concluded that in sulphuric acid solutions the presence of oxygen enhances the active dissolution process. In view of the importance of this possibility to a proper understanding of the mechanism of operation of the negative plate in lead-acid batteries, confirmation of the effect was considered necessary. In the present paper steady-state current-potential, impedance and double-layer capacity measurements have been used to characterise the electrode system over a wide potential range. In order to eliminate interference from passivation processes at high overpotentials, perchloric acid has been used as the electrolyte in place of H2SO 4.

0022-0728/83/0000-0000/$03.00

© 1983 Elsevaer Sequoia S.A

340 EXPERIMENTAL

Electrode preparatton, cell and electrode assembhes These have been discussed previously [1 ].

Soluttons All solutions were prepared from AristaR grade reagents with triple-distilled water. The working solutions were 0.10 M and 5.0 M HC104.

Reference electrodes and potenttal scale A hydrogen electrode was used directly in contact with the working solutions, i.e. without liquid junctions. Potentials were converted to the standard hydrogen scale by substracting 65 mV from the measured potential in the 0.10 M solutions and adding 102 mV to the potentials in the 5.0 M solutions.

lnstrumentatton This has been described previously [1]. Impedance measurements over an extended frequency range to very low values were made using automatic equipment which has also been described [3].

Procedure The general procedure for carrying out experiments, determination of steady-state currents, electrode impedance and double-layer capacitance has been described previously [ 1]. All experiments were conducted on electrodes rotating at 53.3 Hz in solutions at room temperature (25 + 2°C). RESULTS AND DISCUSSION

Steady-state current-potenttal and capacttance measurements Figures 1 and 2 show steady-state current-potential curves in oxygenated and deoxygenated 0.10 M and 5.0 M HC104 solutions respectively, over a broad potential range from - 4 0 0 to - 150 mV. In the deoxygenated solutions it can be seen that the extent of hydrogen evolution at cathodic potentials ( E < - 250 mV), although appreciable in the 5 M solution, is almost imperceptible in the 0.1 M solution. At more positive potentials (E>_ - 2 5 0 mV) a sharp rise in current occurs in both solutions, reflecting the onset of active dissolution. Saturation of the solutions with oxygen produces large increases in the cathodic currents as a result of oxygen

341

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Fig. 1 Steady-state current-potential curves for lead m 0.10 M HC104, ( × ) Deoxygenated soluuon; (O) oxygenated soluUon; ( - - - - - - ) curve for oxygenated solution when slufted along the current axas by an a m o u n t equal and opposite to the magnitude of the h n ~ t m g oxygen reducUon current. Electrode rotauon speed 53,3 Hz.

/

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FlmV -400 I

E

-350 I

-300 I

-250 L

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Fig 2 Steady-state current-potenUal curves for lead m 5 0 M HCIO 4. ( × ) Deoxygenated solution, (O) oxygenated solutmn; ( - - - - - - ) curve for oxygenated solution when sl~fted along the current axis by an a m o u n t equal and opposite to the m a g m t u d e of the hrmting oxygen reductmn current. Electrode rotatmn speed 53.3 Hz.

342

reduction. From the curves it is possible to determine limiting diffusion currents of 7.4 m A c m -2 and 5.0 m A cm -2 for the 0.1 M and 5 M solutions respectively, although the accuracy with which the latter can be defined is clearly affected by interference from the concomitant hydrogen evolution. The higher limiting value for the more dilute solution is presumably a consequence of its greater capacity to dissolve oxygen. At more positive potentials the current for the oxygenated solutions deviates significantly from the values for oxygen reduction. If the observed net current /net is simply the algebraic sum of that due to metal dissolution /dLss and hmiting oxygen reduction lo2, then ~no, =

;~,ss + ;o2

(1)

and substraction of the latter from the former should yield a current identical with

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Fig. 3. Steady-state current-potennal curves and double-layer capacity curves for lead m 0.10 M HC104. (©) Deoxygenated soluuon, ( × ) oxygenated solution, ( - - - - - ) current, ( ) capaoty Electrode rotation speed 53.3 Hz.

343 2.0 110

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,.co

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10

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I

I

-550

-500

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-450

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-400

-9 0 I

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I

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-350

-300

-250

-200

-150

Fig. 4. Steady-state current-potential curves and double-layercapacity curves for lead m 5.0 M HCIO4. (O) Deoxygenated solution, (×) oxygenated solution, ( - - - - - ) current; ( ) capacity. Electrode rotation speed 53.3 Hz. that observed in the absence of oxygen /net -- 10 2 =

/dlss

(2)

However, reference to Figs. 1 and 2 shows that this is clearly not the case with the deviation being more pronounced in the more dilute solution, 1.e. the solution having the higher oxygen concentration. Two theories can be invoked to explain these deviations. Firstly, in accordance with the original claim of Fleming et al. [2], it can be assumed that the presence of oxygen causes an increase in the active dissolution current. These authors suggested that oxygen complexes could be formed with the dissolving lead species thus reducing the activation energy for charge transfer. Secondly, in accordance with the hypothesis of Armstrong and Bladen [4], it can be

344

assumed that the oxygen reduction current is inhibited in the region where lead dissolves. These authors suggested that anion adsorption could be responsible. Further experimental evidence is required in order to distinguish between these possibilities. Figures 3 and 4 compare the steady-state current data with simultaneously determined double-layer capacity data. Over the range of cathodic potentials the capacity remains relatively constant for the deoxygenated solution with values in the range ca. 10-20/xF cm -2. These values are lower than expected and possible reasons have been discussed previously [1]. However, as the potential is made more positive the capacity rises steeply. This coincides with the potential where the metal lattice begins to dissolve appreciably. The steepness with which the capacity rises over a small potential range suggests that the change is due predominantly to an increase in surface roughness rather than specific anion adsorption. The similarity in the capacitance values also seem to support this view. When the solutions are saturated with oxygen the rise in capacity occurs at more negative potentials; this is particularly marked for the 0.1 M solution, demonstrating that the effect is proportional to oxygen concentration. Thus, a direct relationship exists between the changes in steady-state current and capacity which can be explained most simply by assuming that as the dissolution of lead commences an increase in surface roughness occurs. Further, the capacity data indicates that in the presence of oxygen the potential at

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-210

-200

-190

-180

-170

Fig. 5. Plots of log i vs. E (Nernst plots) for lead m HC104 solutions m the active dissolution regzon. (O) 0 10 M HC104; ( x ) 5.0 M HCIO4; ( ) deoxygenated solutions; ( - - - - - ) oxygenated solutions Electrode rotation speed 53.3 Hz.

345 which active dissolution proceeds is shifted to more negatwe values. These results suggest that the active dissolution of lead is enhanced in the presence of oxygen. Figure 5 shows semi-logarithmic plots of currents vs. potential m the active dissolution region for both oxygenated and deoxygenated solutions. In the case of the former a correction for the effect of the limiting oxygen current has been made according to eqn. (2). It can be seen that irrespective of acid concentration and oxygen content, a good linearity is achieved with slopes lying close to the value predicted for a diffusion-controlled process of 29.6 mV per decade. These "Nernstian" slopes are in agreement with the view that, in common with the situation in H2SO 4, the active dissolution of lead in HC104 proceeds reversibly, the net rate being dependent upon the rate of diffusion of dissolving species away from the electrode and their surface concentration being determined by the Nernst equation for the metal/metal ion process. The presence of oxygen has the effect of shifting the plots to higher current values, the extent of the shift being more pronounced the higher the oxygen concentration. Further, the linearity of the plots for the oxygenated solutions is evidence against the view that an inhibition of the oxygen reduction occurs since such an inhibition would be expected to be potential dependent resulting in deviations from linearity throughout the potential range. These results strongly suggest that oxygen increases the exchange current for the P b / P b ( I I ) reaction. Thus, at any given potential the net dissolution current will be higher in the presence of oxygen and the electrode can be said to be more reversible.

Impedance measurements Impedance measurements provide a very useful means of studying the present system: firstly, because the effects of any diffusion-limited processes like oxygen reduction are eliminated from the analysis as a result of the fact that their rates cannot be perturbed by the ac signal; and secondly, because they provide the possibility of studying the kinetics and mechanism of the fast P b / P b ( I I ) chargetransfer reaction. Figures 6 and 7 show impedance diagrams down to low frequencies over a representative potential range for deoxygenated and oxygenated 5 M HC104 solutions respectively, in the region of active dissolution. Spectra of a similar profile were obtained for the 0.1 M HC104 solutions. It can be seen that the diagrams take the form of a high-frequency semicircle followed by a shape having the general appearance of a Warburg impedance which relaxes to the resistive axis as the frequency decreases. The low-frequency shapes are in agreement with those expected from the steady-state current-potential data shown in Fig. 5, confirming the reversible nature of the dissolution process. However, the appearance of the charge-transfer semicircle at high frequencies and the relatively short frequency range over which the true Warburg impedance can be discerned reflects the fact that the lead electrode in HC104 is somewhat less reversible than it is in H2SO 4 where no characteristics of the charge-transfer process can be found even at the highest frequencies [4]. A similar observation has been made by Fleming and Harrison [5]. Thus, it seems likely that these differences are related to the fact that in H2SO 4

346

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Fig 6 Complex plane impedance diagrams for lead m deoxygenated 5.0 M HC104 in the actwe dissolution region over a representative range of potentials. Electrode rotation speed 53.3 Hz.

Pb/HSO£ or Pb/SO42- complexes are formed which may be adsorbed, thus lowering the activation energy for charge transfer, whereas in HC104 no such complexes occur. A similar rate acceleration has been observed for the dissolution of polycrystalhne zinc in complexing C1- media compared with non-complexing C104solutions [6,7]. The impedance curves represented in Figs. 6 and 7 also provide a direct confirmation of the role of oxygen in the dissolution process. Thus, at constant potential the charge-transfer resistance is smaller and therefore the rate of charge transfer is greater in the oxygenated solution. This results in a correspondingly

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Fig. 7. Complex plane impedance dlagrarns for lead m oxygenated 5 0 M HCIO 4 m the actwe dissolution regton over a representative range of potentxals. Electrode rotat,on speed 53.3 Hz

greater definition of the Warburg impedance in the latter which is sufficiently good for a more detailed analysis to be made. This was carried out using the relationships

[8] Z ' = Rot + oo:- ]/2

(3)

and Z " = 009 - 1 / 2 Jr" 2 o 2 C a l

(4)

where Z ' and Z " are the real and imaginary components of the impedance ( Z ' = R~ and Z " = 1/6oCs), Rct is the charge transfer resistance, Ca] the double layer capacity

348 50

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~o-1/2/10-2sl/2 F~g. 8. Real and imaginary components of the ~mpedance as a function of frequency for lead m oxygenated 5.0 M HC104 at a potential of - 194 inV. ( × ) Z', (©) Z". Electrode rotation speed 53 3 Hz.

and o the Warburg coefficient. Figure 8 shows a plot of Z' and Z " vs. w-1/2 at a representative potential according to eqns. (3) and (4). The plots are not quite parallel as required by the theory because the Warburg region of Fig. 7 is not an ideally straight line of slope 45 °. Nevertheless, values of o were determined from the slopes of plots of the form of those shown in Fig. 8 and these are plotted as a function of potential in Fig. 9. This reveals parallel hnear plots of slope 33 mV per decade and this value is close to the Nernstian value of 29.6 mV per decade. These results are in agreement with those expected for a reversible dissolution process alone and they confirm that the impedance characteristics are independent of the oxygen reduction reactions and therefore that the latter must proceed under complete diffusion hmitation throughout the potential range - 2 1 0 to - 1 7 8 mV. Further confirmation is provided by a determination of the Nernst slopes at each potential from the expression [9]

~E/~ log tss =

2.303lss(aE/atss ) = 2.303tssgac

(5)

where ~ss is the steady-state current and Rdc the slope of the steady-state current-potential curve, i.e. the zero frequency intercept on the resistive axis of the respective impedance spectrum. Figure 10 compares these slopes with those determined from the Warburg analysis and the steady-state current-potential data for oxygenated 5 M HC104. It can be seen that the slope determined from the Ra~ values lie predominantly between those from the Warburg and steady-state analyses. These impedance results, therefore, confirm independently the conclusion from the steady-state data that the oxygen reduction reaction cannot be inhibited in the potential region where lead dissolves. Further analysis of the impedance data concerned the high-frequency spectra. Charge transfer resistances R~t were determined at each potential for all solutions by

349

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Fig. 9 Plots of log o vs. E for lead in oxygenated 5.0 M HC10 4 m the actwe dlssoluUon region. (e) o from Z ' vS. to-1/2 plots; ( × ) o from Z " vs. to-1/2 p l o t s E l e c t r o d e r o t a t i o n s p e e d 53 3 Hz.

the method of graphical extrapolation of the partially defined semicircles to the resistive axes of the impedance diagrams. From these Rct values, "effective" exchange currents t~ were calculated by means of the expression [10]. l"'0 - - R T / 2 F R c t

(6)

It is necessary to obtain a physical understanding of the significance of i~ in order that a further analysis can be pursued. For a metal/metal ion electrode this exchange current can be simply represented as l 0' ---

i=,

(7)

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Fig 10 Plots of (OE/a l o g o vs. E (Nernst slopes) for lead m oxygenated 5.0 M HCIO 4 m the active dissolution region. ( × ) From zero frequency complex plane intercepts (R de), ( - - - - - - ) from analysis of Warburg impedances (Fig. 9), ( - - ) from measured steady-state current (Fig. 5). Electrode rotation speed 53 3 Hz.

where i is the partial metal dissolution current which increases exponentially and i the partial metal deposition current which decreases exponentially with potential and which is additionally dependent upon the concentration of metal ions in equilibrium with the electrode. Unlike the case of a conventional exchange current which pertains at a unique Nernst potential ( E a ) governed by the constant bulk concentration of the oxidised species, t~ is potential dependent. This is because for a highly reversible process the electrode is in a state of equilibrium over a range of potentials for which t~ >> ( i - t ) with the concentration-dependent part of t (and therefore i~) being determined by the surface concentration of the oxadised/diffusing species which in turn is related to the potential via the Nernst equation applied to the surface. It is this dependence of surface concentration upon potential, the former rising with increasing anodic overpotential, which is responsible for maintaining equilibrium conditions ( t - - t ) , since this increases t and thereby provides compensation for the direct effect of the overpotential which always decreases ~. Therefore, a range of increasing exchange currents can be obtained as the anodic potential increases. However, t is not dependent upon the equilibrium concentration of the oxidised species and for a solid metal electrode only depends upon potential. This means that there is only one "Tafel" line (In i vs. E ) for the metal dissolution process at a constant concentration of supporting electrolyte. Since the exchange current i~ represents points of intersection on this Tafel line with the Tafel lines for the metal deposition process (In t vs. E) then the potential dependence of i~ yields directly the Tafel line for the dissolution process. Figure 11 shows semi-logarithmic plots of i' o vs, E for all solutions. It should be noted that the exchange currents are apparent values with no corections for diffuse layer structure. If the dissolution

351 1.5

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t - 260

t -250

. -240 I

l -230

I

-220

L -210

I -200

I -190

-lt08

-1|07

E/mV Fig. 11. Plots of log z~ vs E (Tafel plots) from analysis of lugh-frequency complex plane Impedances (R CT) for lead m HC104 solutions m the active dissolution region (@) 0.10 M HCIO 4; ( × ) 5.0 M HCIO 4; ( ) deoxygenated solutions; ( . . . . ) oxygenated solutions. Exchange currents are apparent values. Electrode rotation speed 53.3 Hz

proceeds via a mechanism involving two consecutive one-electron charge-transfer reactions with Pb(I) species as intermediates Pb ~ Pb(I) + e

(8)

Pb(I) -=, Pb(II) + e

(9)

it can be shown that a Tafel slope of 40 mV per decade (2.3 × 2RT/3F) would be expected if the second (electron-transfer) step is rate determining [10]. In Fig. 11 the lines have been drawn with this slope and it can be seen that in general a reasonably good correlation exists between the experimental data and the predictions from this simple model. A similar mechanism has been suggested in the case of the dissolution of polycrystalline lead in H 2 S O 4 solutions [11], cadmium in alkaline solution [12] and zinc in acid [7] and alkaline [9] solutions. Evidence that the intermediate species are adsorbed has been obtained from impedance measurements in the case of zinc [7,9]. Nevertheless, it is interesting to note that the conclusions regarding the mechanism of dissolution of lead are at variance with those of Gioda et al. [13] who carried out a kinetic investigation of the Pb/Pb(II) exchange reaction using predominantly the potentiostatic polarisation technique. These authors concluded that a mechanism with a chemical step involving the formulation of dimers from monovalent intermediates is rate determining according to the scheme below. (Evidence in support of this idea has recently been claimed by Lazarides et al. [14].)

352 Pb ~

Pb(I) + e

(10)

2 Pb(I) ~

[Pb(I)] 2

(11)

2 Pb(II) + 2 e

(12)

[Pb(I)]2 ~

It was stated by Gioda et al. that this mechanism would require a Tafel slope of 30 mV per decade and a Stolchlometric number of 0.5 and that both were experimentally realised. However, confusion exists in this work with regard to the extent to which mass transfer effects were eliminated m the determination of these parameters. Thus, despite the use of a rotating disc electrode, no attempt was made to extrapolate out the effects of diffusion with the result that an observed " N e r n s t " slope of 30 mV per decade was confused with the true Tafel slope. Similarly, it was stated that the polarisation data close to the equilibrium potential was rotation speed dependent, yet despite this, values of (Rdc),~ 0 were used to evaluate the stoichiometric number without due regard for the rotation speed dependence of the former parameter. Thus, there must be some dispute concerning the mechanism of Gioda et al. pending a more thorough kinetic investigation. Irrespective of the mechanism of charge transfer it is clear from Fig. 11 that the effect of oxygen is to shaft the Tafel hnes to higher current densities. It can also be seen that the extent to which the increase occurs is greater for the more dilute HC10 4 solution--in agreement with the trend observed in the steady-state measurements (cf. Figs. 11 and 5 respectively). It is at present by no means clear by what mechanism the presence of oxygen enhances the dissolution of lead. It is possible that an interaction between adsorbed intermediates in both the oxygen reduction and metal dissolution reactions occurs which in some way catalyses the dissolution process. However, such a speculative suggestion must remain tentative pending further investigations. CONCLUSIONS The present investigation has demonstrated that lead dissolution in perchloric acid solutions is a reversible process, though somewhat less reversible than that in sulphuric acid. In common with many other metal dissolution reactxons there is evidence that the charge-transfer mechanism involves two consecutive one-electron transfers with monovalent Pb(I) intermediates which may be adsorbed. A large enhancement of the rate of dissolution in the presence of oxygen has been confirmed which increases with increase in oxygen concentration, although it is not possible to explain the effect in mechanistic terms at the present time. It is predicted that results will have consequences with regard to the operation of lead-acid batteries since it is likely that the energetics of passivation processes at the negative will be affected. An investigation of the dissolution and passivation characteristics of lead in oxygenated and deoxygenated sulphuric acid solutions will be reported in a subsequent communication.

353 ACKNOWLEDGEMENTS

This work was based on a collaborative project with The University of Newcastle upon Tyne and the authors would like to thank Dr. R.D. Armstrong for helpful discussions. REFERENCES 1 L M Baugh and K.L. Bladen, J. Electroanal. Chem., 145 (1983) 325. 2 A.N Flermng, J.A. Hamson and J. Thompson m D H. Colhns (Ed.), Power Sources, Vol. 5, Academic Press, London, 1975, p. 1. 3 R D. Armstrong and M F Bell and A.A. Metcalfe, J. Electroanal Chem., 77 (1977) 287 4 R D. Armstrong and K L. Bladen, J Appl Electrochem., 7 (1977) 345 5 A N Fletmng and J A Hamson, Electrochlm. Acta, 21 (1976) 905 6 Ya D. Zymner and I.A. Kravtsova, Electrokhlmlya, 11 (1975) 1219 7 L.M. Baugh, Electrochlm. Acta, 24 (1979) 657. 8 J H Sluyters, Rec. Trav. Cham., 79 (1960) 1092 9 R D. Armstrong and M F. Bell, J Electroanal. Chem., 55 (1974) 211 10 P Delahay, Double Layer and Electrode Kanetlcs, Intersoence, New York, 1965, p. 178 11 N A Hampson and J B. Lakeman, Power Sources, 4 (1979) 21. 12 R D Armstrong and G.D. West, J. Electroanal Chem., 30 (1971) 385. 13 A S. Gloda, M C. Glordano and VA. Macagno, J Electrochem Soc, 124 (1977) 1324. 14 C Lazandes, N A. Harnpson, G M. Bulman and C. Knowles, Paper No. 40, International Power Sources Symposium, Brighton, 1980.