Electroanalytical Chemistry and lnterfacial Electrochemistry, 48 (1973) 55~1 (~) Elsevier Sequoia S.A., Lausanne - Printed i.n The Netherlands
DIFFERENTIAL CAPACITANCE OF POLYCRYSTALLINE ZINC PART I. EFFECT OF pH AND AMMONIUM IONS
L. M. B A U G H and J. A. LEE
The Ever Ready Co. (G.B.) Ltd., Central Laboratories, St. Ann's Road, London NI5 3TJ (England) (Received 9th June 1973)
The difficulties inherent in a determination of the characteristic double layer capacity of solid electrodes are numerous 1. If only those metals of high hydrogen overvoltage are considered then in the absence of any appreciable faradaic processes, two factors in particular are important. The first of these is the presence of oxide or hydroxide films on the electrode surface and the second is the exact state of electrode surface heteroteneity which depends on its crystallinity, lattice defects, grain structure and preparation artefacts. Both of these factors are important when considering the double layer capacity of polycrystalline zinc. The differential capacitance of the zinc electrode has been examined by several workers both in relation to the adsorption of ionic and neutral compounds 2-6 and to the determination of its potential of zero charge v-9. In common with some other metals of high hydrogen overvoltage it has been shown that single crystal 3"5'6 and specially prepared polycrystalline zinc electrodes 5"6 give almost "mercury-like" differential capacity curves particularly when immersed in electrolytes containing surface active organic compounds. However, the measured capacity has also been shown to be critically dependent upon the solution pH. Hampson et al. demonstrated 1o that in neutral solutions of NaC10 4 interaction of O H - ions with the electrode modified the differential capacity profile to such an extent that over the ideally polarizable potential range the true double layer capacity could not be distinguished. It was later shown 11 that O H - ion interaction could be effectively suppressed by lowering the pH of the solution to pH 3.4 and a similar result has been reported by Batrakov 4. The present communication, based on a comparative study, deals with some additional aspects of O H - ion interaction with a polycrystalline zinc electrode and is concerned with the first factor in the determination of the characteristic double layer capacity of solid electrodes, viz. film formation. In a subsequent paper ~6 the importance of the second factor, viz. electrode surface heterogeneity, will be examined. Polycrystalline electrodes are used in both the investigations because of their relevance to battery technology and because their differential capacity characteristics are somewhat less understood than those of single crystals.
L.M. BAUGH, J. A. LEE
Impedance bridoe A Marconi Universal Bridge Type TF.2700 in conjunction with a Levell R.C. Oscillator type T6 200 DM was used to measure a series resistance and capacitance analogue for the zinc-solution interface. The d.c. polarising circuit consisted of a 6-V accumulator connected across 1 kf~ and 100 f2 variable series resistances. A Brookdeal Phase Sensitive Detector type 401A was used with the impedance bridge as a null detector to increase the low frequency sensitivity. The current flowing in the cell was obtained by measuring the potential drop across a 10 kf~ resistor in the polarizing circuit using a potentiometer.
Cell and electrode assemblies A conventional electrochemical cell containing a Luggin capillary was used in the capacity measurements. The reference electrode was a 0.10 N calomel electrode which was used in contact with the working solutions. The counter electrode consisted of a hollow platinum cylinder of large surface area compared to that of the working electrode. The working electrode consisted of a 100 cm length of 99.999% pure zinc wire of 1 mm diameter which had been forced into a cylindrical P T F E sheath of tapered bore which protected all but the working tip of the wire from contact with the solutions.
Water, salts and solutions All salts were of the AnalaR grade and were recrystallized twice from three times distilled water obtained from a still which contained K M n O 4 in its second stage. Tetrabutylammonium perchlorate was recrystallized by dissolving in acetone and precipitating with water after which it was dried at 323 K under vacuum. Variations in pH were made by the addition of small quantities of HC104, HC1, N H 4 O H or N a O H to the solutions.
Electrode pretreatment In order to facilitate a comparison of data obtained using different electrodes it was thought desirable to reproduce the working surface area of the electrodes as accurately as possible and therefore a diamond polishing technique was employed. The working electrode surfaces were firstly ground flat using silicon carbide abrasive paper of grit size 600. The electrodes were subsequently polished using 6 #m, 1 #m and ¼ #m diamond paste producing a mirror-like scratch free surface when viewed under a microscope. Only those electrodes conforming to a satisfactory microscopic examination were retained for use. After polishing, the electrodes were washed in a stream of carbon tetrachloride and finally dried in a stream of nitrogen. The light chemical etching procedure, which is advocated by some authors as a final stage in electrode pretreatment, proved to have no influence on the measured capacity values reported in this investigation and was therefore omitted.
Experimental procedure During preliminary measurements the reproducibility of data obtaine d using different electrodes in the same electrolyte was found to be fairly poor. This was
CAPACITANCE OF POLYCRYSTALLINE Zn. I
attributed to variations in the surface roughness and crystallinity of different polycrystalline electrodes. In order to eliminate this problem and to obtain an absolute comparison between data obtained with different solutions it was necessary to use a single electrode in any experiment and to transfer it from one reference solution to a second working solution. Deviations between the two sets of data could then be attributed entirely to solution effects and not to variations in electrode surface roughness or crystallinity. In the experiments to be reported only two solutions were compared in this way (after which the electrode was repolished) although, in principle, the number could be extended. Two identical cells were utilized and filled with the two solutions. The reference solution was 0.1 M NaC10 4 at pH 5.8. The general success of this comparative procedure was demonstrated by transferring an electrode between two identical solutions whereupon the capacity curve was reproduced almost exactly. All measurements were made at 298 K. RESULTS AND DISCUSSION
Figure 1 shows the capacity curves for a polycrystalline zinc electrode in 0.10 M NaC104 at pH 3.4 and pH 5.8 at a bridge frequency of 1000 Hz. The pseudocapacity peak which occurs in the latter solution at - 1 . 4 1 V has been attributed to either the formation/removal of a hydroxide or oxide film 4 or to
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50 z.O 30 30
I _,29 _&
-4153 -,159 -,165
Fig. 1, Differential capacity curves at 1 kHz for a polycrystalline zinc electrode in aq. decinormal solns. of NaC104 (O) pH 5.8, ( × ) pH 3.4. Fig. 2. Differential capacity curves at 1 kHz for a polycrystalline zinc electrode in aq. decinormal solns, at pH 5,8. ( 0 ) NaC104, ( Q ) NH4C104, ( x ) NH4C1.
L.M. BAUGH.J. A. LEE
the adsorption of ad-atoms which are stabilised by O H - ions 12. In the more acid solution this peak is completely removed indicating that it is not due to the adsorption/desorption of organic impurities since this process would be largely independent of the solution pH. Qualitatively these results confirm those of Hampson et al. 1°'11 who also used electrolyte solutions purified over activated charcoal. However, these authors showed that acidification of the solutions to pH 3.4 caused a significant increase in the capacity of the plateau minimum, an observation which has not been substantiated here but which may be due to the use of higher perchlorate concentrations by Hampson et al. In the present work the capacity of the plateau and the location and size of the pseudocapacity peak were found to be almost pH independent in the range 4.5-8.6. Figure 2 shows the capacity curves for decinormal solutions of NH4C10 4 and NH4C1 compared with the curve for NaC10 4. All the curves refer to pH values 5.3 4 pH ~<5.8. It can be seen that the pseudocapacity peak is eliminated in solutions containing the ammonium ion despite the fact that the bulk pH is favourable for strong O H - ion interaction with the electrode and that the curves for the ammonium salts depend little upon the nature of the anion present. It might therefore be concluded that in the experimental potential range (-1.11 V ~>E ~>- 1.65 V) specific adsorption of the C1- ion is negligible or the effect of the adsorption of this ion on the capacity curve is screened by strong O H - ion adsorption which is thought4 to cause the anodic part of the curves to rise steeply at -1.26 V. However, the charge on the zinc surface is negative at potentials 8 less than about - 0.96 V and if a strict analogy between the properties of the zinc and mercury 13 electrodes is assumed, specific adsorption of C1- is unlikely. In order to investigate further the dependence of the capacity curves on the presence of the ammonium ion it was decided to compare the curves for decinormal solutions of NH4C10 4, and NaC104 at pH 3.4, when the results would not be influenced by a surface hydroxide film. Figure 3 shows both the capacity and the current profiles. The capacity curves have the same shape although the curve for the ammonium salt is displaced to slightly higher capacity values and the cathodic capacity rise is less steep. However, substantial differences occur in the current profiles and larger currents flow in the ammonium salt solution even though the bulk pH of both solutions is identical. The increase in the hydrogen evolution current is due to the reduction of NH~ ions 14. NH~-+e ~ NHa+½H 2 This process is probably responsible for the absence of a pseudocapacity peak in neutral solutions of ammonium salts, (cf. Fig. 2) since it is likely that surface acidity at the zinc electrode is sufficiently increased to inhibit O H - ion interaction in these solutions. At higher pH values, however, the presence of ammonium ions is not sufficient to eliminate this process and a pseudocapacitance reappears. This is shown in Fig. 4 at pH 8.6, where it is seen that the peak potential for 0.10 M NH4CIO 4 is shifted by about 0.1 V positive of that for 0.10 M NaC10 4. Figure 5 shows a comparison between the capacity curves for 0.10 M NaCIO4 at pH 5.8 and 0.10 M NaOH (pH 13). As expected a pseudocapacity peak occurs in the pure hydroxide solution, although shifted by about 0.1 V negative of that for the almost neutral NaC10 4 solution. This observation may reflect the more stable
CAPACITANCE OF POLYCRYSTALLINE Zn. I
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/~( _O_ _o. _O_ .o- 4~-- ~ .
/.0 I iI
i ~ _117 ......
I ~ 135
Fig. 3. Differential capacity curves at 1 kHz and current potential curves for a polycrystalline zinc electrode in aq. decinormal solns, at pH 3.4. ( 0 ) NaCIO4, ( x ) NH4CIO4; ( - - ) capacity, ( ...... ) current. Fig. 4. Differential capacity curves at 1 kHz for a polycrystalline zinc electrode in aq. decinormal solns, at pH 8,6. ( 0 ) NaCIO4, ( x ) NH4CIO4. 20(
1~20 -1~26 -1~32 -1'38
-; . . . .
. . . . . .
-126 . .
132 . .
. -14/~ .
-1 / . . . . . . .
Fig. 5. Differential capacity curves at l kHz and current potential curves for a polycrystalline zinc electrode in aq. decinormal solns. ( 0 ) NaCIO4 at pH 5.8, ( x ) N a O H at pH 13, ( - - ) capacity, ( . . . . . . ) current. Fig. 6. Differential capacity curves at 1 kHz for a polycrystalline zinc electrode in aq. solns. ( t ) 0.10 M NH4CIO4, ( x ) 0.113 M NH4CIO,~ + 10 -3 M Bu4NCIO 4.
L.M. BAUGH,J. A. LEE
Fig. 7. Dispersion of differentialcapacitancefor a polycrystallinezinc electrodein aq. 0.10 M NH4C104. nature of the film present in the former solution and the current in the region of the pseudocapacitance exhibits an active/passive transition. The presence of a fairly protective hydroxide film on the zinc surface in 0.10 M N a O H has also been inferred from corrosion work is where the corrosion rate passes through a sharp minimum at pH 13. In order to demonstrate that structural double layer properties can be adduced from differential capacity measurements using polycrystalline zinc prepared in the manner described in this investigation, Fig. 6 shows the effect of tetrabutylammonium (TBA) ions (10-3 M) on the capacity curve for 0.10 M NH4C104. In the presence of the specifically adsorbing TBA cations the capacity is lowered from 16 pF c m - z to 8 /~F cm -2. Although it was not possible to observe an accurate d'esorption potential for these ions, due to the rather extensive hydrogen evolution at the more negative potentials, a value close to - 1.68 V is probably a good approximation. This is the potential which is asymptotically approached by the capacity curve for the tetrabutylammonium perchlorate solution at the far cathodic side. This value is also very close to that obtained by Frumkin e t al. 5 for the desorption of the same concentration of TBA cations on single crystal and polycrystalline zinc from bromide solutions .The absence of a desorption peak in Fig. 6 is due to the slow rate of adsorption of TBA cations on solid electrodes and the non-equilibrium state of the adsorbed layer at the bridge frequencies used in this investigation, viz. 103 Hz. At much lower frequencies (65 Hz) a characteristic peak occurs in the capacitance profile 5. In Fig. 6 it is also seen that the capacity at the minimum in the base solution is about 16 #F cm -2. However, that this is somewhat accidental is shown in Fig. 7 which gives the frequency dispersion of the capacitance at the minimum for an electrode in 0.1 M NH4C104. The extent of this dispersion is very similar to that reported by Chuan-sin and Iofa 3 for polycrystalline zinc in 1 M KI. The capacity at the minimum also tended to vary from one electrode to another which explains why the values at 103 Hz in Figs. 6 and 7 are different since the data were obtained using two different electrodes. (Figs. 1-5 were obtained using the same electrode.)
CAPACITANCE OF POLYCRYSTALLINE Zn. I
The authors wish to thank the Directors of the Ever Ready Company (G.B.) Ltd. for permission to publish this work. SUMMARY
Differential capacity measurements have been made using a polished polycrystalline zinc electrode in aqueous solutions of NaC104 and NH4CIO4 at various pH's. In solutions of NaC104 at pH values > 3.4 strong O H - ion interaction with the electrode is confirmed. In NH4CIO4 solutions this interaction is not appreciable until the pH is raised to about 8.6. This is thought to be due tO the weakly acidic nature of the ammonium ion, which produces an increase in the local acidity at the zinc surface. To a first approximation the differential capacitance of the zinc electrode in neutral solutions of ammonium salts can be identified with the double layer capacity, since the characteristics of the electrode in the additional presence of tetrabutylammonium cations are very similar to those observed for zinc single crystals. This paper therefore re-emphasises the importance of solution pH in the determination of the double layer capacity of polished polycrystalline zinc. REFERENCES 1 B. B. Damaskin, O. A. Petrii and V. V. Batrakov, Adsorption of Organic Compounds on Electrodes, Plenum, New York, 1971, p. 177. 2 V. Ya. Bartenev, Elektrokhimiya, 6 (1970) 1197. 3 Tza Chuan-sin and Z. A. Iofa, Dokl. Akad. Nauk. SSSR, 131 (1960) 137. 4 B. V. V. Batrakov and A. I. Sidnin, Elektrokhimiya 8 (1972) 122. 5 A. N. Frumkin, V. V. Batrakov and A. I. Sidnin, J. Electroanal. Chem., 39 (1972) 225. 6 B. V. V. Batrakov and A. 1. Sidnin, Elektrokhimiya, 8 (1972) 743. 7 V. L. Kheifets and B. S. Krasikov, Dokl. Akad. Nauk. SSSR, 109 (1956) 586. 8 B. S. Krasikov and V. V. Syseova, Proc. Acad. Sci. USSR Phys. Chem. Sect., 114 (1957) 363. 9 V. L. Kheifets and B. S. Krasikov, Zh. Fiz. Khim., 31 (1957) 1992. 10 N. A. Hampson, D. S. Brown, J. P. G. Farr, D. Larkin and C. Lewis, J. Electroanal. Chem., 17 (1968) 421. 11 N. A. Hampson, P. Caswell and D. Larkin, J. Electroanal. Chem., 20 (1969) 335. 12 J. P. G. Farr and N. A. Hampson, J. Electroanal. Chem., 13 (1967) 433. 13 D. C. Grahame and R. Parsons, J. Amer. Chem. Soc., 83 (1961) 1291. 14 I. K. Marshakov, Ya. A. Ugai, V. I. Vigdorovich and M. I. Anokhina, Elektrokhimiya, 1 (1965) 1374. 15 B. E. Roecheli, G. L. Cox and W, B. Littreal, Metals Alloys, 3 (1932) 73. 16 L. M. Baugh and J. A. Lee, J. Electroanal. Chem., 48 (1973) 63.