The electrical double layer of carbon and graphite electrodes

383

J. Electroanal. Chem., 260 (1989) 383-392 Elsevier Sequoia S.A., Lausanne - Printed

in The Netherlands

The electrical double layer of carbon and graphite electrodes Part V. Specific interactions

with simple ions

D. Golub, A. Soffer and Y. Oren Division of Chemistv, (Received

Nuclear Research Center-Negev,

P.O. Box 9001, Beer-Sheva,

84190 (Israel)

24 June 1987; in revised form 12 May 1988)

ABSTRACT The specific interaction of halide and alkali metal ions with carbon and graphite electrodes was investigated by monitoring the dimensional changes and the charging current during a linear potential scan. The potential of zero charge (pzc), as detected by a minimum in the length vs. potential curve, is shifted towards more negative values when a larger halide is the anion in a single electrolyte solution and as the solution concentration increases. This is interpreted in terms of increasing specific interactions. Alkali cations, on the other hand, do not exhibit any measurable effects of specific interaction. The differential capacity of both carbon and graphite increases due to specific interactions. However, the contribution to the capacity of the carbon is much larger than that for the graphite. This is interpreted on the basis of the much larger contribution of the edge planes in the carbon.

INTRODUCTION

Specific electroadsorption of ions has been investigated mostly with the traditional mercury electrode [l-3]. It was shown that when specific adsorption takes place, the pzc becomes a function of both the type and concentration of the ion adsorbed. As a general rule, anion specific adsorption shifts the pzc towards increasingly negative potentials, and when cations are specifically adsorbed, there is a shift towards positive potentials. When both anions and cations are specifically adsorbed, the shift direction of the pzc depends on the electrolyte concentration [4]. Considering specific adsorption on solid electrodes, perhaps the most studied system is that of halide ions on silver electrodes [5,6]. Data are also avaliable for the specific adsorption of ions on gold electrodes [7]. In a previous paper [8], evidence for the specific adsorption of large organic ions (p-toluene sulphonate and tetraethylammonium ion) on a graphite electrode was presented. No direct measurement of the shift of the pzc was conducted, but in 0022-0728/89/$03.50

0 1989 Elsevier Sequoia

S.A.

384

comparison with NaF adsorption, a significant increase in the double-layer capacitance was observed for these ions in the potential regions of opposite electrode and ion charge polarities. In more recent papers [9-111, it has been shown that changing the electrochemical potential of a high surface area carbon electrode results in dimensional changes which in turn are proportional to the corresponding variations in surface tension. This effect was shown to be powerful for the study of the electrical double layer of porous carbon and graphite electrodes. In the present paper, the specific adsorption of simple ions on carbon and graphite electrodes is discussed. Pzc shifts are measured by the above-mentioned method together with double-layer capacity variations resulting from the different nature of the interacting ion. EXPERIMENTAL

The electrochemical cell, the method of length variation measurements and data processing have been described in detail previously [lO,ll]. Carbon (FC-43 from Pure Carbon Co.) and graphite (Papyex from Le Carbone Lorraine) electrodes were used. Their properties are described elsewere [9-111. The experiments were performed with 0.04 to 4 M solutions of KF, KC1 and KBr and 0.8 M solutions of LiCl, NaCl and CaCI. All the solutions were prepared from Analytical Grade salts in lo6 D/cm water (obtained from a Millipore MilliQ unit). Solution replacement was done under open circuit conditions. The new solution was equilibrated with the electrode and then replaced several times by a fresh solution in order to remove the previous electrolyte completely. Cyclic voltammograms were taken at sweep rates of 0.1 and 1 mV/s. The slower sweep rate was applied in order to allow the electrodes (particularly the more porous and thus slowly responsive carbon) to be as close as possible to equilibrium conditions. Potential cycling was repeated until reproducible length vs. potential and current vs. potential curves were obtained. This procedure was important since it has been shown that due to kinetic effects at both carbon and graphite electrode interfaces, initial and irreversible length and charge increments take place whenever the electrode is cycled after prolonged relaxation at potentials close to the pzc [lo]. RESULTS AND DISCUSSION

The effect of ion type and concentration on the location of the pzc Following the analysis outlined previously [9], the length vs. potential curves were differentiated with respect to the potential in order to obtain values related to the double-layer charge 4, according to: q = k(dL/dE)p

(I) where k is a constant, L is the electrode length, E is the electrode potential and p is the chemical potential.

385

-cl .,4e

--me

208

0 E

400

/nV

Fig. 1. Length derivative with respect to the potential (0) KF; (+) KCl; (0) KBr. Sweep rate: 1 mV/s.

as a function of the potential

for FC-43 carbon.

In Fig. 1 the length derivative (LD) with respect to the potential is given as a function of the potential for the carbon electrode in different 4 M potassium halide solutions. The pzc of the electrode obviously falls at the intersection of the curves with the abscissa. In Table 1 the pzc values measured by this method for the two electrodes in the various solutions are summarized. Pzc values for mercury are given for comparison. As shown in the table, the expected trend of negative pzc shifts for larger halide ions observed for mercury occurs in the case of carbon and graphite electrodes as well. Furthermore, the pzc shifts for the carbon and graphite electrodes are as large as those for mercury. This may be considered as a qualitative indication that the halide specific interaction with carbon and graphite is as strong as with mercury. It has been shown previously [8-111 that hysteresis in charge-potential and length-potential curves occurs always when the potential of the carbon and graphite electrodes is cycled. These hystereses were independent of the electrode geometry,

TABLE

1

Pzc values of carbon, graphite and mercury electrodes in various salts Electrolyte

E,., /mV (vs. SCE) Ascending half cycle

Descending

half cycle

Carbon

Graphite

Carbon

Graphite

Mercury a

KF KC1 KBr

290 225 150

- 125 - 175 - 220

190 140 110

- 280 - 350 - 350

- 432 (0.1 M) -466 (0.1 M) - 534 (0.1 M)

LiCl KC1 CsCl

220 225 200

a The values in parentheses

115 140 140 are the related salt concentrations.

- 516 (1 M) - 517 (1 M)

386

electrolyte type and concentration, and were attributed to slow transfer of electrical charge to and from the interface. As a result, changes of the electrode surface variables are generally delayed. Therefore, the pzc for the ascending (positively scanned) curves is located at a more positive value than that of the descending (negatively scanned) curves, as shown in Table 1. However, for both the ascending and descending half cycles the above-mentioned trend of the pzc values is maintained. This indicates that this effect is not kinetic in nature, but originates rather from time-independent differences in interaction strengths. It is also apparent from Table 1 that the pzc of the graphite electrode is more negative than that of the carbon. This effect has been observed and discussed previously [10,12]. It is related to differences in surface polarization which are caused by the existence of edge plane surface sites on carbons. As these sites are much more chemically active as compared to the basal planes of graphite, they are more likely to carry chemically bound oxygen surface groups, which in turn shift the pzc of carbon electrodes to more positive values as compared to those of graphite electrodes. In contrast to the clear effect of the halide anions, the influence of the alkali metal cations on the pzc of the carbon electrode is less apparent. As shown in Table 1, the pzc does not shift towards more positive potentials upon changing the salt from Li to Cs chloride (as the Cs+ ion is expected to show a stronger tendency for specific adsorption [4]) but the results are somewhat dispersed around 200 and 140 mV for the ascending and descending half cycles, respectively. It should be noted that this behaviour is also characteristic of the mercury electrode, which exhibits a very weak dependence of both the pzc and the differential double-layer capacity on the alkali metal cations (Table 1 and ref. 4). Additional evidence for the occurrence of specific adsorption of simple anions can be gained by following the concentration dependence of the pzc. It can be shown [4] that when pure double-layer adsorption is considered (namely, no specific interactions take place), the pzc should show no dependence on the electrolyte activity. Any deviation from this behaviour [13] should be attributed to specific adsorption. The results shown in Figs. 2a and 2b, although dispersed, reveal the independence of the pzc on the concentration of KF solutions for both the carbon and graphite electrodes. These results are expected in view of the fact that the absence of specific adsorption of F- ion is well known and was also found for mercury [4]. The relatively large dispersion of the experimental points given in Figs. 2a and 2b is explained as follows: For the graphite electrode, this results from the fact that due to its small specific surface area the length variations are rather small and are in the range where the signal to noise ratio becomes a limiting factor. For the carbon electrode, it can be observed that only the pzc for the descending half cycle in the least concentrated solution deviates strongly from the other points, namely it is considerably less positive. As will be discussed in the subsequent section, the carbon electrode exhibits molecular sieving characteristics [14] for fluoride adsorption. This phenomenon becomes more pronounced as the concentration of the electrolyte

387

Cl , -0.1

CRT/F>ln

160 0

a/U

0.02

-270 -6.T

ln

alU

0

6.02

Fig. 2. Pzc for Fe-43 (a) and Papyex (b) in KF solutions half cycle; (0) descending half cycle.

as a function of (RT/F)

In 0. (0)

Ascending

decreases due to the much slower mobility of the anions in the narrow pores. This causes accumulation of positive charges on the electrode side which, in turn, brings about a shift in the pz.c towards more negative values. In comparison with the behavior in fluoride solutions, it is shown in Figs. 3a and 3b that there is a strong dependence of the pzc on the activity of KBr solutions for both the graphite and carbon electrodes at the descending and ascending half cycles. The pzc is shifted towards more negative values at higher concentrations due to the increasing surface excess of specifically adsorbed Br- ions which causes stronger surface polarization. The effect of ion type and concentration on the electrical double-layer

capacity

In Fig. 4 cyclic voltammograms for the carbon electrode taken with different alkali cations are given. The figure reveals the independence of the capacity of this electrode on the type of alkali ion used. The same behaviour was also observed for the graphite electrode. Considering the negligible specific interaction of large alkali cations mentioned above, this behavior is to be expected.

388

(b)

, -O.i

CRT/F)

III

a/V

8

0.02

Fig. 3. Pzc for FC-43 (a) and Papyex (b) in KBr solutions as a function of (RT/F) half cycle; (0) descending half cycle.

In o. (0)

Ascending

On the other hand, it is apparent from Fig. 5 that halide anions affect the double-layer capacity of carbon in the expected trend, namely the capacity is larger the more strongly adsorbed the ion. Moreover, it was found that, within experimen-

Fig. 4. Differential capacity as a function of the relative potential for FC-43. Sweep rate: 1 mV/s. (0) KCl; (0) CsCl; (X) LiCl.

389

20

-350

-150 (E-E

*J

nu

150

358

Fig. 5. Differential capacity as a function of the relative potential for FC-43. Sweep rate: 1 mV/s. (0) KCl; (0) KBr; (x) KF.

tal error, and unlike carbon, there is no dependence of the double-layer capacity of graphite on these ions. This is in contrast to the considerable shifts in the pzc which indicate clearly the existence of specific interaction. In view of the evidence on a limiting serial capacitance at the graphite side of basal plane interfaces (resulting from the charge-depleted graphitic layer [SJO]), it should be expected that specific adsorption of ions in the inner Helmholtz region will have only a minute effect on the total capacity of the graphite electrode. This effect is also valid for the basal plane portions of the carbon surface. On the other hand, the contribution of the edge planes (in which carbons are richer than graphites) is not subject to this limiting capacity and should therefore be more responsive to variations in the electrical double-layer capacities on the solution side. Therefore some increase in capacity may be expected for the larger halides if they are specifically adsorbed, as is experimentally observed (Fig. 5). In addition and in relevance to both types of electrode, it should be taken into account that due to the highly concentrated solutions used in these experiments, the solution side has a high capacity, thus reducing its contribution to the total capacity which therefore is less sensitive to the type of adsorbed ions. It should be noted at this point that capacity independence of graphite electrodes on univalent as well as on multivalent cations and anions was also observed in previous work [8]. Another approach to the interpretation of the dependence of the double-layer capacity of carbon on the anion type lies in the fact that the carbon electrode was cycled to slightly more positive potentials than the graphite electrode. This routine was adopted since the behaviour of the two electrodes had to be compared on a rational potential basis and the pzc of the carbon electrode was 400 mV more positive. At about 250 mV negative to the redox potential of the Br,/Br- couple, and 350 mV negative to that of the Cl,/Clcouple, strongly and partially irreversibly adsorbed dihalogen molecules [15,16] are very likely to be formed at the carbon electrode interface and to be reduced back upon cycling to more negative potentials. This obviously adds a pseudo-capacitance component to the electrode, as is in fact observed.

L

s

*

-100

a

I

I.

I

ice (E-Ep,B,)h”

.

200

I

Fig. 6. Cyclic voltammograms for FC-43 taken at different (+) 4 M. Sweep rate: 0.1 mV/s.

KF concentrations.

(0)

0.04 M; (0) 0.4 M;

Fig. 7. Cyclic vokunrnograms for FC-43 taken at different (+ ) 4 M. Sweep rate: 0.1 mV/s.

KBr concentrations.

(0)

0.04 M; (0) 0.4 M;

TABLE

2

Double-layer concentration Electrolyte

capacities

concentration

for

carbon

and

graphite

Carbon

at the

capacity

pzc

as a function

of the

Graphite

capacity

/M

/F g-’

/F g-’

KBr 0.04 0.4 4.0

9.6 11.2 13.4

0.50 0.62 0.73

7.8 9.3 10.7

0.51 0.58 0.68

KF 0.04 0.4 4.0

KF

and

KBr

391

@.8t

-1 LOGC

0 CONCEWTRGTIOWAl

Fig. 8. Capacity (KBr)/capacity FC-43 (0).

8.5 3

1

(KF) at the pzc as a function of In(concentration) for Papyex (0)

and

In Figs. 6 and 7, cyclic voltammograms for the carbon electrode taken at different concentrations of KF and KBr, respectively, are shown. Special attention should be paid to the non-symmetrical character of the voltammograms taken at low KF concentrations. Namely, the electrode capacity decreases remarkably upon applying positive potentials where adsorption of the fluoride ion occurs. This phenomenon was reported also in previous work [14] and is characteristic of carbons in which a considerable fraction of the surface area is carried by ultramicropores. The fact that it is not observed with KBr solutions implies that the average opening diameter D of the ultramicropores is in the range D,,- > D < D,- (where D,,- and D,- are taken for the hydrated ions). This range of pore sizes is responsible for the flow response of the electrode, which results in much smaller capacity values and, as discussed above, in abnormal shifts in the pzc. Capacity values at the pzc as calculated from the plots of Figs. 6 and 7 and also for the graphite electrode are summarized in Table 2. In Fig. 8 the capacity in KBr solutions relative to the capacity for KF is presented. It is clear from Table 2 that there is a significant increase in the capacity as the concentration of the solutions increases for both the carbon and graphite electrodes, as can be expected from the increasing contribution of the compact double layer. However, Fig. 8 reveals that the capacity ratio is slightly higher than unity (at the higher concentrations) for the graphite electrode and much higher for the carbon electrode. This behaviour is undoubtedly due to the increase in surface excess of the specifically adsorbed Brions as compared to the F- ions, and is consistent with the above discussion about the relative contribution of the interactions of these ions with the basal and edge planes. REFERENCES 1 D.C. Grahame, Chem. Rev., 41 (1947) 441. 2 D.C. Grahame and B.A. Soderberg, J. Chem. Phys., 22 (1954) 449. 3 R. Parsons, Proc. 2nd Int. Congress of Surface Activity, Vol. 3, Butterworths, London, 1957, p. 38.

392 4 5 6 7 8 9 10 11 12 13 14 15 16

P. Delahay, Double Layer and Electrode Kinetics, Interscience, New York, 1965. A. Bewick, K. Kunimatsu and B.S. Pons, Electrochim. Acta, 25 (1980) 465. T. Vitanov and A. Popov, Trans. Sot. Adv. Electrochem. Sci. Technol., 10 (1975) 1. A. Hamelin and J.P. Bellier, Surf. Sci., 78 (1978) 159. Y. Oren, H. Tobias and A. Soffer, J. EIectroanaI. Chem., 162 (1984) 87. Y. Oren, I. Glatt, A. Livnat, 0. Kafri and A. Soffer, J. Electroanal Chem., 187 (1985) 59. Y. Oren and A. Soffer, J. Electroanal. Chem., 206 (1986) 101. D. Golub, Y. Oren and A. Soffer, Carbon, 25 (1987) 109. H. Tobias and A. Soffer, J. Electroanal. Chem., 148 (1983) 221. D.C. Grahame, Annu. Rev. Phys. Chem., 6 (1955) 337. J. Koresh and A. Soffer, J. Electrochem. Sot., 124 (1977) 1379. B.R. Pm-i and R.C. Bansal, Carbon, 3 (1966) 533. H. Tobias and A. Soffer, Carbon, 23 (1985) 281.