Electrodes modified with cobalt hexacyanoferrate

263

J. Electroanal. Chem., 304 (1991) 263-269 Elsevier Sequoia S.A., Lausanne

Preliminary note

Electrodes modified with cobalt hexacyanoferrate James Joseph, H. Gomathi and G. Prabhakara Rao * Central Electrochemical (Received

12 December

Research institute,

Karaikudi-623006

[India]

1990; in revised form 19 February 1991)

INTRODUCTION

Electrodes modified with Prussian Blue (PB) have attracted much attention in recent years [l]. Their importance may be traced to the simultaneous presence of two sets of redox peaks representing one high-spin and one low-spin electronic transition of iron in the PB molecule [2,3] (separated by 0.650 V in KC1 supporting electrolyte), and has led to applications in electrochromic devices [3] (in multi-colour display devices), electrochemical power sources [4] (based on the differing potentials of the two redox transitions), and photocurrent response studies [5] (due to the stability of PB to the photocorrosion arising out of the inter~nvertibility of the redox states), and site-selective electrocatafysis [6] making use of either of the two redox centers as electrocatalytic sites. In addition to the above applications, PB, and various metal analogues afford systems with varying surface redox potentials without major chemical or structural changes in the redox centers. Thus for the first time direct verification of the Marcus theory of electron transfer has become possible at derivatised electrodes [7]. Considering the wide scope of electrochemical studies and their possible exploitation, attention is currently being paid to the prep~ation of a number of analogues of PB as revealed by the growing number of references on this subject [8,10-141. It is of interest that on careful examination of different reports on metal hexacyanoferrate (PB analogues) modified electrodes and their redox characteristics published so far, it is apparent that only a few have been found to be as successful as PB from the viewpoint of the varied applications listed above, perhaps due to the absence of the characteristic and reversible redox centers referred to in the case of PB. During the course of our studies on the preparation and the properties of the PB-modified electrodes and their analogues [5,8-lo] we noticed that the electrodes modified with the cobalt analogue of PB are exceptional - they exhibit excellent reversible redox centers very similar to those of PB, - so for the first time their preparation and characteristics are reported.

* To whom correspondence

002%0728/91/$03.50

should be addressed. 0 1991 - Elsevier Sequoia S.A.

264

The electrochemical experiments were performed with the use of a PAR 273 model digital potentiostat coupled to a Rikadenki recorder (RW 200). A three electrode cell assembly with a wax impregnated graphite (WIGE) working electrode with area 0.385 cm’, Pt counter electrode and 1 M calomel reference electrode were employed. Analytical grade chemicals were used for preparing solutions. The solutions were prepared from triply distilled water and deaerated by purging with N2. The modification of WIGE with cobalt hexacyanoferrate was achieved by cycling the electrode potential between 0.0 V and 1.0 V at 0.1 V s-i in a clear solution containing 0.5 mM K,Fe(CN), and 1 mM CoCl, in 0.5 M KCl. The film was found to have grown in thickness with each cycle, as revealed by the increasing charge under the deposition peak. In order to maintain constant coverage throughout the experiments, growth was restricted to 15 cycles. Modification of the electrodes was carried out by employing identical solution compositions for obtaining reproducible results. RESULTS AND DISCUSSION

Figure l(a) shows the cyclic voltammetric response of the CoHCF (cobalt hexacyanoferrate) modified WIGE. These CV’s were recorded by dipping the modified electrodes, prepared as described in the Experimental section, in 0.5 M NaCl after thorough washing. Corresponding peak potential data for the two sets of reversible redox peaks that are due to PB in KC1 are marked by arrows in the same figure for comparison. The reversible peak potentials for CoHCF aici, a2cZ, and a,c, occur at 0.30 V, 0.38 V, and 0.83 V, respectively. By comparing the redox potentials of free and complexed species of Co2+/Co3+ [15] we assigned the peak a,c, at 0.38 V to the Co”-, Co3’ transition. Similarly by comparing the redox potentials of low spin ferrocyanide-ferricyanide transition in PB occuring at 0.82 V with that in CoHCF, we assigned the redox peak a3cs at 0.83 V to the ferrocyanide-ferricyanide transition in CoHCF. There is an additional redox couple in CoHCF, viz., sic,, whose peak heights are relatively smaller, and which probably arise from a different form of CoHCF. It was found that the two sets of redox peaks a,c, and a,c, conform to the expected surface processes by their peak height dependence on potential scan rate and zero peak separation between anodic and cathodic peaks at low scan rates. Linear plots of i, vs. scan rate obtained for the peaks a2 and a3 are illustrated in Figure 2. The i, data could not be extracted quantitatively for a+, due to the low magnitude of currents involved. However the AEp (peak separation) also tends to zero mV for aici as the sweep rate approaches low values. It was also found that the reversible potentials for all the redox couples are shifted by approximately 60 mV s-’ for a decade change in ~ncentration of Na+ in solution, For example values of Ep found for a, (0_255 V), a, (0.335 V), and a3 (0.785 V) in 0.1 M NaCl are shifted to 0.310 V, 0.395 V, and 0.845 V, respectively in 1 M NaCl. It may be remarked that the modified CoHCF

265

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I

I

I

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OL

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E/V(vs.

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I

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1

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Fig. 1. (a) Cyclic voltammetric response of CoHCF modified WIGE in 0.5 M Nat3 Sweep rates (I) 0.005 Vs-’ (II) 0.020 V s-‘. The arrows 1 and 2 indicate positions of peak potentials for cathodic and anodic redox peaks of PB in 0.5 M KCI. (b). Cyclic voltammetric response of NiHCF modified WIGE in 0.5 M NaCl. Sweep rate 0.050 V s-l. (c) Cyclic voltammetric response of PB modified WIGE in 0.5 M NaCl. Sweep rate 0.050 V s-‘.

electrodes prepared using potassium ferricyanide and cobalt chloride in 0.5 M KC1 by the potential cycling procedure described in the Experimental section exhibit characteristics (Fig. 4) different from those described above in 0.5 M NaCl. These can be described as follows.

266

booa 1> - zoo-

0

0

001

0.08

012

" I v s-1

Fig. 2. Plot of peak currents (i,) vs. scan rates (u) for the peaks (1) a2 (0) and (2) a, (0).

Figure 3 gives the CV response recorded during the growth of CoHCF film on WIGE from solution containing the components CoCl,, ferricyanide and KC1 (see Experimental). Initially there are two sets of redox peaks but as the film grows, peaks in the positive sweep direction merge into one. The film formed during the above cycling, when washed thorou~ly with water and kept in pure supporting electrolyte of 0.5 M KCl, yields a very stable CV response (stable over several hundred cycles at scan rate 0.1 V s-l) that is represented in Fig. 4. At low scan rates anodic peak resolution occurs to give one peak at 0.50 V and another at 0.60 V.

1

lE/VIvs.NCEI Fig. 3. The growth cycle of CoHCF in a solution containing CoCI, (1.0 mM) and K,Fe(CN), Sweep rate 0.100 V s-‘.

(0.5 mM).

267

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I

I

I

0

04 E/V

I

I

0.8 hi.NCEl

Fig. 4. The cyclic voltammetric v s-1; (II) 0.100 v s-1.

’

response

of CoHCF-modified

WIGE in 0.5 M KCI. Sweep rates (I) 0.010

However at high scan rates the anodic peaks merged into one solely due to the quasi-reversibility of the couple at 0.5 V indicated by its large peak-to-peak separation of 100 and 240 mV at scan rates of 0.01 V s-r and 0.1 V s-r, respectively. One notices the distinctly different characteristics of redox peaks, their occurrence at different potentials and the lack of reversible character for the response in KC1 in contrast to the behaviour in 0.5 M NaCI described earlier. The response in 0.5 M LiCl was similar to that in KC1 except that the peak potentials were shifted to less positive values by 200 mV s-l (Fig. 5). The responses in ammonium chloride and rubidium chloride have also been investigated, but surface

I

-0.2

0 E/V(vs.NCEl

Fig. 5. The cyclic voltammetric s-1.

response

of CoHCF-modified

WIGE

in 0.5 M LiCI. Sweep rate 0.100 V

268

activity (surface voltammetric peaks) was lost after the electrode was cycled in 0.5 M NH,Cl or RbCl for a few minutes. The electrode failed to regain its activity in 0.5 M KC1 after cycling in ammonium chloride or rubidium chloride because of the solubility of the ammoniumor rubidium-substituted cobalt analogue. Similar observations have been reported for other analogues of PB as well as for PB itself as “supporting electrolyte effects” [2,9]. The interpretation of cation effects (on the assumption that the cation enters the host lattice of PB or its analogues) was made on the basis of the ionic size of cations vs. the diameter of the channels for Pb or its analogues with varied success [12]. The failure to conform to the above picture by a number of cations has prompted the search for an alternative mechanism for explanation of the cation effects, although no concrete suggestions are available [13,14]. Our results, involving the supporting electrolyte sodium chloride, tested in a number of cases including NiHCF and CoCHF, reveal the unique feature that the surface reaction involving ferrocyanide/ferricyanide is reversible, as can be readily seen from Fig. l(b) and l(a). This reversible behaviour is absent in other halide supporting electrolytes of rubidium, lithium, ammonium, etc. (PB also gives reversible peaks in KCl). The splitting of the peak for the ferrocyanide/ferricyanide center in the case of NiHCF at 0.35 V (Fig. l(b)) and at the ferrous/ferric or cobaltous/ cobaltic center (in PB at -0.10 V and in CoHCF at 0.30 V shown in Fig. l(c) and l(a) respectively) to two reversible peaks in the presence of sodium is conspicuous and indicates that more than one species of the analogue is present under the experimental conditions [ll]. The reversible characteristics associated with either sodium or potassium cation on accelerating the electron transfer of surface ferro-

f

0 E/V(vs,

2

1.0

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Fig. 6. The cyclic voltammetric response of CoHCF-modified WIGE (a) in 0.5 M NaCl; (b) in 0.5 M NaCl+O.OOl M KCl; (c) in 0.5 M NaCl+O.Ol M KCl; (d) in 0.5 M NaCl+O.l M KCl. Sweep rate 0.02 v s-1.

269

cyanide/ferricyanide reactions in NiHCF, CoHCF, and PB that we have observed are similar to Gerischers’s observation [16] on the influence of cations on the homogenous electron transfer rate of the above systems viz., ferrocyanide/ferricyanide in aqueous solutions. However the exact sequential order of cation influence is not exactly the same in the NiHCF and CoHCF systems. Perhaps the nature of the interaction between the different cations studied and the CoHCF surface film decides the response characteristics of the CV; the diffusion of the cations into the lattice of the modified film may also play a significant role. Further data from our experiments on the CV response of mixed systems of KC1 and NaCl supporting electrolytes are given in Fig. 6. CoHCF is seen to possess greater selectivity for K+ and so changes the reversible electron transfer promoted by Na+ gradually to an irreversible/quasi-reversible behaviour imparted by K+ involvement. Further work to substantiate the above and to explore the applications of CoHCF-modified electrodes is in progress. ACKNOWLEDGEMENT

The authors thank Professor S.K. Rangarajan, Director, C.E.C.R.1, Karaikudi, for his keen interest in this work. One of the authors (J.J.) thanks the C.S.I.R., India, for the award of a Senior Research Fellowship. REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

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