Electrochemical behavior of chromium(III)-hexacyanoferrate film modified electrodes: Voltammetric and electrochemical impedance studies

Journal of Electroanalytical Chemistry, 370 (1994) 95-102

95

Electrochemical behavior of chromium( III) -hexacyanoferrate modified electrodes: voltammetric and electrochemical impedance studies Zhiqiang Gao

film

l

Laboratory of Analytical Chemistry, A&o Akademi Universily, SF-20500,

Turku-A&o (Finland)

(Received 27 January 1993; in revised form 6 September 1993)

Abstract Chromium(III)-hexacyanoferrate (CrHCF) films on wax impregnated graphite electrodes have been studied by voltammetry and electrochemical impedance spectroscopy. When a CrHCF film is transferred from HCI to another electrolyte solution a “break-in” effect is observed during continuous potential cycling past the formal potential of the film. Cyclic voltammetry shows a single electron transfer reaction, and the voltammetric behavior of the electrode can be interpreted in terms of the insertion electrode concept. The ion effect of the supporting electrolyte indicates that the ion permeability of the film is mainly determined by the radius of the hydrated cation. Electrochemical impedance results suggest that the electrode reaction of the film is mainly controlled by the transport of counter-ions. The electrochemical impedance spectra of the original film and the “broken-in” film reveal that the large limiting current of the original film in HCI solution is caused by the catalytic evolution of oxygen, rather than by a capacitive current.

1. Introduction

The transition metal hexacyanometallates are an important class of insoluble mixed-valence polynuclear compounds which have the general formula M!,JMi’ (CN),) where M’ and Mu are transition metals with different formal oxidation numbers, and m and II represent stoichiometric numbers. One of the bestknown examples of these compounds is Prussian blue (PB). The structure of PB was first discussed by Keggin and Miles in 1936 [l], and the physical and chemical properties of this group of compounds have been studied extensively [2,3]. Investigations of the electrochemical behavior of electrodes coated with thin films of these compounds have been reported recently following Neff’s pioneering work on PB-modified electrodes [4]. The studies have shown that these compounds can be used as electrode-coating materials for electrochromic displays [5], electrocatalysis [6], ion selective electrodes [7,8] and solid-state batteries [91. To date,

l

Present address: Department of Materials and Interfaces, mann Institute of Science, Rehovot 76100, Israel.

0022-0728/94/$7.00 SSDZ 0022-0728(93)03208-7

Weiz-

cyclic voltammetry and other electrochemical techniques involving large potential perturbations have been the favorite tools for the investigation of these modified electrodes [lo] because the same technique can be used in both electrodeposition and preliminary characterization of the films. There appears to have been little investigation of the electrochemical impedance of these modified electrodes. The electrochemical impedance spectra of copper hexacyanoferrate film modified electrodes were studied by Siperko and Kuwana 1111 and Engel and Grabner [12]. We have carried out an electrochemical impedance study of the properties of cobalt hexacyanoferrate film modified electrodes 1131,with the aim of investigating the kinetic properties and the influence of the electrolyte on charge transfer and diffusion processes. The electrochemistry of chromium(III)-hexacyanoferrate (CrHCF) film modified electrodes has been studied in some detail, particularly with regard to problems related to film deposition [14,15]. CrHCF film modified electrodes show distinct electrochemical responses in different electrolyte solutions which require further investigation. In addition, the mechanism of counter-ion transport responsible for the electrode re0 1994 - Elsevier Sequoia. All rights reserved

96

2. Gao / Electrochemical behavior of CrU-hexacyanoferrate

action is not well understood. It has been suggested that the electrochemical reaction is a simultaneous injection/ extraction of an ion and an electron to/ from the film. In contrast with organic polymers, CrHCF and most other inorganic films possess a regular lattice structure. Evidently, the diffusion pathways in these films should be quite different from those in organic polymer films. Furthermore, with respect to the chemical modification of electrodes, inorganic films have some unique properties and advantages over organic polymers such as stability and specificity. Since successful practical application depends on a thorough understanding of the underlying electrode reaction, it is necessary to perform a detailed investigation of the electrochemical process in these films. In the present work cyclic voltammetry and electrochemical impedance were used to study the electrode reaction of CrHCF films. 2. Experimental 2.1. Materials

and apparatus

All chemicals were of certified analytical grade (Merck) and used as received. Solutions were prepared with distilled deionized water from a Millipore system. A wax-impregnated graphite electrode (WIGE) of diameter 6 mm was polished to a mirror finish on polishing paper and then transparent paper. The electrode was then rinsed with water and 1: 1 nitric acid. Thereafter it was thoroughly cleaned in an ultrasonic bath with water and alcohol. The cyclic voltammetric experiments were performed with a PAR 174A polarographic analyzer (PAR, USA) and the voltammograms were recorded on a type 2900 A3 Bryans x-y recorder. The electrochemical impedance experiments were carried out using an HP9816 computer-controlled S-5720B frequency response analyzer and a model 2000 potentiostat-galvanostat with a GP-IB interface (NF Circuit Design Block Co. Ltd., Japan). The frequency range used in this work was from 50 mHz to 100 kHz and the electrochemical impedance was measured at five discrete frequencies per decade. The sinusoidal voltage signal had a typical amplitude of 5 mV. The experimental data were analyzed using complex non-linear least-squares (CNLS) fitting to a model represented by an equivalent electrical circuit composed of resistors, capacitors and distributed circuit elements [16]. The CNLS program used in this work was the LEVM program developed by Macdonald [161. All electrochemical measurements were conducted using a conventional three-electrode system. The electrodes were in a triangular arrangement spaced about 1 cm apart. The CrHCF modified electrode was used as the working electrode, with a

film modified electrodes

platinum foil counter-electrode and a saturated calomel reference electrode (SCE). All potentials reported in this work are referred to the SCE. 2.2. Procedures CrHCF film modified electrodes were prepared by electrodeposition as described previously [14]. Following this, a “break-in” process was carried out. The original electrode was dipped in 0.1 M NaCl solution and cycled in the potential range 0.0-1.0 V until a steady-state cyclic voltammogram was obtained. The electrode was then transferred to the electrolyte solution of interest for further studies. The procedure for the electrochemical impedance measurements was as follows. The modified electrode was equilibrated in the electrolyte solution for 5 min at a given potential, after which a sinusoidal modulation of 5 mV was added to the dc bias voltage and the impedance was measured over the range of 50 mHz to 100 kHz. The electrode potential was then changed and the procedure was repeated. After each impedance measurement a cyclic voltammogram was recorded to assess any loss of electroactive species from the film. 3. Results and discussion 3.1. Cyclic voltammetry

of CrHCF films on WIGEs

Gradual growth of a CrHCF film on the WIGE can clearly be seen in preparative cyclic voltammetry. Typical electrodeposition of a CrHCF film on a WIGE is shown in Fig. 1. Although the CrHCF film could be formed in an aged solution, the deposition procedure was generally carried out in a fresh solution using a carefully cleaned WIGE. Both ferricyanide and C&II) are electroactive in the potential range studied. Therefore in the first negative scan the ferricyanide reduction peak appeared at ca. 0.3 V and the cathodic wave of Cr(II1) appeared at ca. - 1.5 V, where chromium was deposited onto the electrode surface. On the reverse scan the corresponding ferrocyanide oxidation peak was significantly smaller than the other current peak at a more positive potential (0.7 V) which was due to the oxidation of deposited CrHCF film. As can be seen in Fig. 1, two current peaks are formed and their peak currents increased on further potential cycling between 1.0 and - 1.7 V. However, the voltammetric responses of Fe(CN)i-/4in the solution became significantly lower. Obviously, a surface-immobilized electroactive film was formed during potential cycling, which resulted in the change in the electrode properties leading to the change in the electrode kinetics of Fe(CN)i-/4-. The deposited film was characterized by Fourier transform IR (FTIR) spectroscopy [14,15] which confirmed that it was composed of CrHCF. The

Z. Gao / Electrochemical behavior of CrW-htxacyanoferrate

97

film modifEd electrodes

actual mechanism of CrHCF film deposition is not yet clear, but it was found that the pre-deposition of chromium was necessary to ensure deposition of the film on the electrode surface. If the potential was reversed far away from the cathodic peak of Cr(III), for example to - 1.0 V, no deposition took place even after numerous cycles. Figure 2 shows the cyclic voltammograms of the “break-in” process in 1 M NaCl solution. On the first positive scan an extended anodic current region without a peak structure can be seen followed by an anodic peak which is associated with oxidation of the film. This current may be due to the catalytic evolution of oxygen by the film or to the capacitive current observed in organic conducting polymers [17]. A cathodic peak is observed on the reverse scan. As discussed later, the electrochemical impedance spectra of the original and the broken-in films show that the large

0.20

0.95

E/V(vs.

1.7 SCE)

Fig. 2. “Break-in” process for a CrHCF film (1.0 km) in 1.0 M NaCl solution at a potential scan rate of 100 mV s-l: cycle 1; --cycle 5; .-‘-. cycle 10.

1.0

-0.8 E/V(us.

-2.6

SCE)

Fig. 1. Formation of a CrHCF film on a WIGE substrate during preparative cyclic voltammetry: solution, 0.01 M CrCI, + 0.01 M K,[Fe(CN),)]+ 1.0 M HCI; potential scan rate, 100 mV s-l.

anodic current can be attributed to the catalytic evolution of oxygen by the original film on the electrode surface. Upon continuous potential cycling, the anodic charge Q, decreases whereas the cathodic charge Q, increases and there is a simultaneous negative shift of the peak potential, which indicates a reorganization process. Eventually the shape of the cyclic voltammogram remains unchanged upon further potential cycling. The ratio of the anodic to cathodic charge also changed during this treatment: in the first cycle Q,/Q, = 1.30, in the 10th cycle Q,/Q, = 1.1 and in the 20th cycle QJQ, = 1.0. This indicates the inhibiting effect of the “broken-in” film on the catalytic reaction of oxygen evolution. If the electrode is transferred to a fresh solution of 1.0 M HCI after “break-in” has occurred, the large anodic current observed for new films is not regained, indicating an irreversible process. Examination of the “break-in” process suggests that the cation in the electrolyte plays a role in the reorganization and the oxidation-reduction of the film. When the original modified electrode was scanned in 1 M HCl solution, no “break-in” was observed even after hundreds of cycles. The “break-in” process is probably due to the substitution of a proton by a cation in the film, such as potassium or sodium, accompanied by a

2. Gao / Electrochemical behavior of Cr(III)-hexacyanoferrate

98

r

film modifwd electrodes d’*/(mVsV

120

cl t

n

50

l-4

1 0

L 0.75

0.0

E/V(vs.

km.

SCE) pm) in NaCl 1.0 M NaCl;

gradual change in film structure. This ion exchange can be described by the following equations: ,, =

[Cr(Fe(CN),

I],, +ne-

+ nH+ [HCr(Fe(CN)6)]n-m[KCr(Fe(CN)6)],

n + ne++ mK++

PPww

[KCr(Fe(C%)]n e

= (n - m)H+

[Cr(Fe(CN),)], + nK+

+ ne-

(llmln)

(1)

When the broken-in film modified electrode was placed in a fresh electrolyte solution, the cyclic voltammograms shown in Fig. 3 were obtained. Only a single redox couple, which was due to the Fe(CN)z-/ Fe(CN)z- redox process in the film, was observed in the electrolyte used. This result agrees with the literature data [18]. As shown in Fig. 4, the peak currents for thin films were proportional to the potential scan rate up to 100 mV s-l, as expected for a surface process [19]. At low scan rates the anodic and cathodic peaks were almost symmetrical about the zero current axis and their potentials were almost independent of scan TABLE Cation

1. The effect of the cation Stokes radius/pm

on E,,

125 184 238

Film thickness,

and E,

of CrHCF

film modified

electrodes

-% per decade

46 + 5 57 f I 70 f 8 1.0 km; potential

rate. However, a gradual increase in peak-to-peak potential separation was observed as the film thickness was increased. For thicker films the peak currents were no longer proportional to the potential scan rate. It appeared that the peak current was proportional to the square root of the potential scan rate, which is characteristic of a diffusion-controlled process [193. The effect of ions on the electrochemical behavior of the film was investigated by changing the cation in the electrolyte solution. The peak potentials of the redox couple are given in Table 1 as a function of the concentration of different cations in the electrolyte solution. Proportionality was obtained over the whole range of concentration for the cathodic peak and above ca. 0.05 M in the case of the anodic peak. At low concentrations the plots of anodic and cathodic peak potentials diverge, which gives the appearance of asymmetry in the cyclic voltammogram (Fig. 3). The slope of the plot in Table 1 increased with increasing radius of the hydrated cation. It is likely that the trend is a function of hydrated cation size, which suggests that the cations enter the film in the hydrated form, as has been observed in studies of PB and its analogs [lo]. E,, are preand Epa, rather than E, = (E,, + E,)/2), sented in Table 1 because of the asymmetry of the cyclic voltammogram. Such a problem does not exist in the case of ideal systems, where the half-width of the

E,, Slope/mv

K+ Na+ Li+

I,

100

50

Fig. 4. The dependence of peak currents on the potential scan rate in 1.0 M NaCl solution for film thicknesses of (a) 5.0 km and (b) 0.2

1.5

Fig. 3. Cyclic voltammograms of CrHCF film (1.0 solution at a potential scan rate of 100 mV s-l: --0.1 M NaCI: .-.-. 0.01 M NaCI.

[HCr(Fe(CN),)]

.-J’-x

scan rate,

Linear 1.0-0.08 1.0-0.05 1 .o-0.05

100 mV s-‘,

range/m01

I- ’

Slope/mV 90* 6 115+ 6 130 f 10

per decade

Linear

range molI-l

1.0-0.001 1.0-0.001 1.0-0.001

Z. Gao / Electrochemical behavior of Cr(IlI)-hexacyanoferte

current peak is 90.6 mV n-l (n represents the electron transfer number) and the peak-to-peak potential separation is zero [20]. In the case of CrHCF film, there is a barrier to cation ejection at high electrolyte concentrations as a result of the high affinity of the film for cations, which is a common feature of these compounds [21,22]. The cation concentration in the film was practically constant at different solution concentrations, which was confirmed by the constant redox charge observed at all concentrations. Therefore it is more difficult to remove the cation from the film at higher electrolyte concentrations than at very low concentrations where the anodic peak potential is independent of the electrolyte concentration. It was found that there was no difference in the electrochemical behavior of the modified electrode with Cl-, Br-, NO; and ClO; as anions and Na+ as the cation. As discussed previously [14], the redox process of the film is related to the uptake/release of ions by the electrolyte solution according to the following equation: M”+[Cr(Fe(CN)6)]

ne

nCr( Fe( CN)6) + M”++ne-

(2) where the cation is present to satisfy the requirement of charge neutrality [lo]. Unlike other PB analogs, the CrHCF film was very stable in electrolyte solutions containing alkali ions, alkaline metal ions and protons. The peak currents increased, and the peak potentials shifted positively with increasing electrolyte concentration. However, the total electrical charge was almost constant at different electrolyte concentrations, which agreed well with the insertion electrode concept. The slight decrease in peak current may be due to an increase in charge transfer resistance. This hypothesis is supported by electrochemical impedance measurements of the film where it was found that the charge transfer resistance increased with decreasing electrolyte concentration. At a constant concentration, the peak potential and the waveform changed when the cation in the solution was changed. For K+, the peak potentials became more positive, the peak currents were higher and the waves were narrower compared with those in other cation solutions. For Mg2+ broadened waves with more negative peak potentials were observed. These changes were interpreted in terms of the radii of the hydrated cations and the channel size of the CrHCF films. Like other mixed-valence compounds, CrHCF has a face-centered cubic lattice. It is zeolitic with a channel of width ca. 350 pm and acts as a molecular sieve. The Stokes ionic radii of the hydrated cations K+, Naf, Li+, Ca2+ and Mg2+ are 125 pm, 184 pm, 238 pm, 310 pm and 347 pm respectively [23]. Since cation insertion

film modified electrodes

99

occurs during reduction, it is obvious that the smallest hydrated cation (i.e. K+) is inserted most easily and that this happens at the most positive potential. Conversely, the largest cation (Mg2’) is the most difficult to insert into the film and this only occurs at the most negative potentials. The increase in the half-width of the current peak and the simultaneous decrease in peak current with increasing radius of the hydrated cation can also be explained in terms of the insertion electrode concept. The reason for this is the deviation from ideal insertion by the cation. In the theory of insertion electrodes [20], this deviation is accounted for by incorporating a concentration-dependent interaction term into the relationship for the chemical potential p: p=Ui+U’x+RT

ln(x/l-x)

(3)

where Ui is the energy per mole of a lattice site, x is the mole fraction, U’ is the interaction energy and the other symbols have their usual meanings. Here the interaction energy U’ affects the half-width of the current peak. For a repulsive interaction (U’ > 0) between the inserted cation and the CrHCF host lattice the half-width of the current peak becomes larger than the theoretical values without interaction, whereas for an attractive interaction (U’ < 0) the half-width becomes smaller. The half-width of the current peak for CrHCF films was always larger than that of a one-electron transfer reaction without interaction [20], indicating the existence of a repulsive interaction between the inserted cation and the host lattice. The interaction becomes less repulsive with decreasing cation size. This result is in good agreement with the concept described above, i.e. the size of the cation relative to the size of the zeolitic channel controls the cation insertion process. The effect of the electrolyte concentration on the voltammetric behavior was studied by changing the concentration of the sodium chloride solution. The voltammograms obtained are presented in Fig. 3. It can be seen that the cathodic peak undergoes a more dramatic shift with concentration than the anodic peak. The peak current decreased with decreasing electrolyte concentration. All these changes were reversible. The asymmetry in the shape of the cyclic voltammogram at low electrolyte concentration appears to be related to the difficulty in inserting the cation into the film since the cathodic peak shifts to a greater extent than the anodic peak. 3.2. Electrochemical impedance film modified electrodes

spectroscopy

of CrHCF

Figure 5 presents complex-plane impedance plots for the modified electrode at 0.60 V. In the high

Z. Gao / Electrochemical behavior of Cr(III)-hexacyanoferrate

100 400 -

.-

film modifEd electrodes

CPE

0.1

%J--(<+-~ .

.

Fig. 6. The equivalent circuit used for fitting the experimental data: 0.4

R,, ohmic drop of the system; R,,, charge transfer resistance of the

.l:..,,

film; CPE, constant phase element; Zo, general diffusion impedance.

c .

.

0

‘

Hz, the impedance is controlled by diffusion in the solution and thus the response represents mass transport in the solution. The response assumes linear behavior with a frequency-independent phase angle. Finally, at extremely low frequencies (< 0.1 Hz) the impedance approaches a purely capacitive response to account for the limiting charge saturation effect [25]. It is widely accepted that a simple electrochemical interface can be represented satisfactorily by a Randles equivalent circuit [251. When thin porous film electrodes are used, such as the CrHCF films studied here, the classical Randles circuit is modified by replacing the double-layer capacitance and the Warburg impedance by a constant-phase element (CPE) and a diffusion element Z, respectively (Fig. 6). The CPE is used to describe the impedance response of a porous electrode, which was be expressed as follows [16]:

300

F k

.??/a

4-

631

I .

I

:

15.Sk

ZcpE= l/[ Fig. 5. Electrochemical impedance plots for a CrHCF film (2.5 urn) in 1.0 M KC1 solution. The electrode potential was 0.60 V and the frequencies are in hertz.

frequency range, the expected response is a depressed semicircle, whose time constant is dominated by the double-layer capacitance and the charge transfer resistance [24]. The intercept with the real axis Z’ at high frequency provides the value of the uncompensated solution resistance, i.e. the ohmic drop in the system. In the medium frequency region i.e. from 5k Hz to 5

TW’]

(4)

where T is a real frequency-independent constant, i = J<- 1) and o/rad s-l is the frequency. The exponent cp determines the phase angle. In the special case of cp= 1, the CPE acts as a capacitor with T equal to the capacitance. Z, is known as the general diffusion element, and represents homogeneous diffusion in solution with a general boundary condition [161 including both finite-length and infinite diffusion processes. R, and R,,represent the ohmic drop and charge transfer resistance of the system respectively. The experimental results obtained for CrHCF film electrodes can be

TABLE 2. Impedance data of CrHCF film modified electrodes Film thickness/pm

W-Q

0.25 0.50 1.00 1.50 2.50

8.0 8.0 8.0 8.0 8.0

R,,/fl 0.24 0.29 0.29 0.32 0.49

j. a/A cm-*

T/p9

0.38 0.31 0.31 0.28 0.18

6.2 5.9 6.1 6.1 6.2

R-’

C,/mF

lo4 /co b/cm s-’

lo9 D/cm* s-l

2.8 4.6 15.5 25.1 49.4

5.0 4.6 4.6 4.1 2.7

9.5 12 10 7.5 8.5

Film thickness, 1.0 urn; electrolyte, 1.0 M KCl; electrode potential kept at E,. a j, = RT/(nFR,,A). b /co = jo/(nFC) [19] is the heterogeneous

assuming a face-centered

rate constant; c is the concentration cubic lattice with a cell constant of 1.03 nm [11,13].

of electroactive

species in the film, which is calculated by

2. Gao / Electrochemical behavior of Cr(III)-hexacyanoferrate

01

En,-150 11,”

I

I

I

Fig. 7. The dependence of impedance potential in 1.0 M KCI: film thickness,

0.0 En, + 150 n,V

E,,,

parameters 2.5 p.m.

on the electrode

discussed on the basis of the proposed modified Randles equivalent circuit, taking into account the following factors revealed by the voltammetric studies: (1) the film is porous; (2) the kinetics of the electrode process (eqn. (2)) may be controlled by the diffusion of chargecompensating ions in the solution; (3) the films are sufficiently thin to induce substantial charge saturation effects. With these considerations, the circuit parameters of the CrHCF electrode were estimated from the experimental impedance data as described previously [13] and were used as the first estimates in fitting to the model proposed in Fig. 6 by means of the CNLS program 1161.According to the CNLS program, if the parameters PDSV and PDRMS describing the overall goodness of fit are in the range 10-2-10-3, the fitting results are reasonable. Values higher than 0.2 indicate a poor fit [16]. In the case of CrHCF film modified electrodes, these parameters were found to be less than 0.02. The impedance parameters obtained are presented in Table 2 and Fig. 7 as functions of the film thickness and the electrode potential respectively. The ohmic drop R, of the system comprises both the solution and the film resistance. It was found that the value of R, did not depend on the potential or the film thickness (Table 2). This is obviously because the solution resistance is associated with the electrolyte between the film modified electrode and the reference electrode, and the good conductivity of these films [lO,ll]. The double-layer capacitance cannot be deduced directly from the fitting data because of the presence

film modified electrodes

101

of the CPE. Instead, T is used to describe the doublelayer characteristics 1261. Because 40 is in the range from 0.82 to 0.90 the value of T does not correspond to the real capacitance, but it allows a good comparison between different experiments as shown in Table 2. The values of T were almost constant for all the films and they were not much higher than typical values of the electrode-electrolyte double-layer capacitance. According to the classic double-layer model, the double-layer capacitance is only associated with the phase boundary of the electrode which is not affected by the potential and the film thickness. As shown in Fig. 7, R,, decreased significantly with increasing electrode potential, reaching a minimum at the value of E, corresponding to the largest current density j, and the highest heterogeneous rate constant k” of the film. A slight increase in R,, was observed when the electrode potential was increased further. This behavior can be explained by considering the change in the ratio of Fe(CN$-/Fe(CN)zredox centers in the film as the electrode potential is varied. Since this ratio will deviate significantly from unity when the electrode potential is far from E,, it is expected that reduction on the negative side of E,, where the concentration of Fe(CN)iis small, and conversely oxidation on the positive side of E, will result in very small current densities on both sides of E,. The current density was largest at the value of E, where the concentrations of Fe(CN>i- and Fe(CN)zwere equal. This current density corresponded to that at the top of the current peak. Therefore the potential dependence of R,, should be the reciprocal of the current density of the film, symmetrical about E, and with a minimum at E,. This ideal pattern was not obtained for CrHCF films, possibly because of the presence of a parallel faradaic process, most probably the evolution of oxygen at potentials more positive than E,, leading to an increase in current density in that potential range. A slight increase in R,, with increasing film thickness was observed (Table 2). The origin of this increase is unknown. The diffusion coefficient D of this system can be calculated from the fitting data of the impedance spectra using the following equation [16]: D =1*/r

(5) where T is the effective diffusion time constant and 1 is the film thickness. It was found that the value of D was independent of CrHCF film thickness, suggesting that the charge compensation during the redox process of the film is only associated with the ion transport process across the film-solution interface. However, a decrease in D was observed with increasing radius of the hydrated cation. This indicates that the transport

Z. Gao / Electrochemical behauior of Cr(IIl)-hexacyanoferrate

102

of charge-compensating cations plays an important role in the overall electrode reaction. As can be seen in Fig. 7 the dependence of C,, which is directly related to the charge capacity of the film is the inverse of that of R,,but is similar to the potential dependence of the current density observed with cyclic voltammograms. A maximum value was obtained at E,. This behavior agrees well with the theory of insertion electrodes [20], which has been used to interpret the cyclic voltammetric behavior of these electrodes. Thus the Fe(CNji-/ Fe(CNjz- redox centers can be considered as the elements of a capacitor, which is fully charged at E, where the concentration ratio of Fe(CNji-/Fe(CNjzis unity. According to this argument, a linear relationship between C, and the film thickness is expected for an electrode of fixed area (Table 2j, because the number of redox centers is directly proportional to the total amount of CrHCF on the electrode surface. The electrochemical impedance responses were also studied with respect to the cations in the electrolyte solution. Both the charge transfer and the diffusion processes increased with increasing radius of the hydrated cation, indicating the importance of charge compensation in the electrode. Because of the existence of the “break-in” process for new films, special attention was paid to the electrochemical impedance responses of new and broken-in films in 1.0 M HCl solution. The experimental results are given in Table 3. The impedance data for the two films are similar, particularly the values of Cr. Similar C, values suggest that the large anodic current observed in the new film is not capacitive. It must be attributed to the presence of a parallel faradaic process on the positive side of the anodic current peak which is catalyzed by the original CrHCF film. This process is probably the evolution of oxygen from water because other components in the solution are not expected to be electroactive in that potential range. As shown in Table 3, a smaller charge transfer resistance was obtained with the original CrHCF film, which confirms that the decrease in R,,is due to the existence of a faradaic process at more positive potentials than E,. The stability of the modified electrodes is such that, after all experiments (over 50 h of measurementsj, the decrease in the peak current was less than 10%. This permitted us to perform all tests with a single elecTABLE 3. Comparison of new and broken-in CrHCF film modified electrodes in 1.0 M HCl solution R,/0 New film 8.0 Broken-in film 8.0

R,,/R

j,,/A cm-’

T/us*

0.18 0.29

0.43 0.31

6.2 6.1

R-t

Film thickness, 1.0 pm; electrode potential kept at E,.

C,/mF 15.3 15.5

film modifKd electrodes

trode. Obviously the errors caused by the differences between individual electrodes were avoided. CrHCF films should be a strong candidate for further practical applications because of their extremely high stability. 4. Conclusion CrHCF film coated electrodes were studied in detail by cyclic voltammetry and electrochemical impedance spectroscopy. The electrochemical behavior of the film was mainly controlled by the transport of charge-compensating ions in solution. The overall electrochemical response of the modified electrode could be satisfactorily interpreted by the insertion electrode model. References 1 J.F. Keggin and F.D. Miles, Nature (London) 137 (1936) 577. 2 H.E. Williams, Cyanogen Compounds (2nd ed), Arnold, London, 1948. 3 D.F. Shriver, Structures and Binding, Springer, Berlin, 1966. 4 V.D. Neff, J. Electrochem. Sot. 125 (1978) 886. 5 K. Itaya, K. Shibayama, H. Akahoshi and S. Toshima, J. Appl. Phys. 53 (1982) 804. 6 B.F.Y.Y. Hin and CR. Lowe, Anal. Chem. 59 (1987) 2111. 7 Z. Gao, X. Zhou, G. Wang, P. Li and Z. Zhao, Anal. Chim. Acta 244 (1991) 39. 8 D. Engel and E.W. Grabner, Ber. Bunsenges. Phys. Chem. 89 (1985) 982. 9 V.D. Neff, J. Electrochem. Sot. 132 (1985) 1382. 10 K. Itaya, I. Uchida and V.D. Neff, Act. Chem. Res. 19 (1986) 162. 11 L.M. Siperko and T. Kuwana, J. Electrochem. Sot. 130 (1983) 396. 12 D. Engel and E.W. Grabner, Z. Phys. Chem. 160 (1988) 151. 13 Z. Gao, J. Bobacka and A. Ivaska, Electrochim. Acta 38 (1993) 379. 14 Z. Gao, Ph. D. Dissertation, Wuhan University, Wuhan, China, 1990. 15 M. Jiang, X. Zhou and Z. Zhao, J. ElectroanaI. Chem. 287 (1990) 389. 16 J.R. Macdonald, LEVM, University of North Carolina, Chapel Hill, NC, 1989. 17 J. Tanguy, N. Mermilliod and M. Hoclet, J. Electrochem. Sot. 134 (1987) 795. 18 W.M. Latimer, Oxidation Potentials, Prentice-Hall, New York, 1952. 19 A.J. Bard and L.R. Faulkner, Electrochemical Methods, Wiley, New York, 1980. 20 J.O’M. Bockris and B. Conway (Eds.), Modem Aspects of Electrochemistry, Vol. 15, Plenum,.New York, 1983, p. 235. 21 M.T. Ganzerli-Valentini, V. Pisani, S. Meloni and V. Maxia, J. Inorg. Nucl. Chem. 37 (1975) 1269. 22 S. Kawamura, S. Shibata and K. Kurotaki, Anal. Chim. Acta 81 (1976) 91. 23 A.L. Howath, Handbook of Aqueous Electrolyte Solutions, EllisHorwood, Chichester, 1986. 24 T.R. Jow and L.W. Shacklette, J. Electrochem. Sot. 135 (1988) 541. 25 C. Ho, I.D. Raistrick and R.A. Huggins, J. Electrochem. Sot., 127 (1980) 343. 26 D.C. Grahame, J. Electrochem. Sot. 99 (1952) 370C.