J O U R N A L OF M A T E R I A L S S C I E N C E L E T T E R S 13 (1994) 602-606

Surface modification of Ni-epoxy electrode by potassium ferricyanide M. M. D.&VILA

Department of Physical Chemistry, University of Puebla, Puebla, Mexico A. ROIG, F. VICENTE*, E. MARTINEZ

Faculty of Chemistry, C/o Dr Moliner, 50, 46100 Burjassot-Valencia, Spain H. SCHOLL

Institute of Chemistry, Lodz University, Lodz, Poland

The Ni(OH)z/NiOOH electrode has been the subject of numerous investigations because of its use in the Ni-Cd cell system [1-3]. The oxidation of nickel hydroxide in basic solutions is typically written as Ni(OH)2 + O H - ~ NiOOH + H20 + ealthough it is well known that the reaction is actually more complex [4-10]. It appears to involve two phases related to reduced and oxidized forms of nickel hydroxide. The electronic properties of oxidized and reduced nickel hydroxide films have been studied previously by various techniques [11-20]. Electrochromic nickel hydroxide films undergo significant deterioration during coloration/bleaching cycles [21-22]. The investigations of the electrochromic properties of nickel hydroxide have revealed promising characteristics for smart window

applications including a high coloration efficiency throughout the visible region [23-28]. Extensive background literature is available on the mechanical and thermal properties of Ni-polymer composites [29-32]. Nevertheless, these technological applications depend largely on the structures and physicochemical characteristics of the nickel oxide layer on the electrode [33-35]. Therefore, the aim of this paper is to study the electrochemical behaviour and charge capacity of the composite Ni/epoxy resin with superficial granular structure. To that end, this type of system has been studied electrochemically by comparing them to the nickel system, the objective being the study of mouldable nickel/epoxy electrodes and their ferricyanide superficial modification. The voltammograms were obtained by a potentiostat-galvanostat PAR 363 and an E-612 Metrhom

Figure l (a) Scanning electron micrography of Ni-epoxy (85 wt%) surface on glass surface. (b) Ni electrochemicallydeposited on polished surfaceof Ni-epoxy(50 wt %) mouldedin Eppendorfpipes.

*Author to whomall correspondenceshouldbe addressed. 602

0261-8028 © 1994 Chapman & Hall

VA scanner with a Denshi Recorder F-35. Ag/ AgC1,KCI(3 M) and Hg/Hg2SO4,K2SO4(sat) reference electrodes were used in NaNO3 and KOH media respectively. IR spectra (KBr pellets) were recorded on an I R - F T Perkin-Elmer FT1750 spectrophotometer. X-ray powder diffraction patterns were obtained by means of a Kristalloflex 810 Siemens diffractometer using CuKo~ radiation. JCPDF cards were used as comparative standards. X-ray fluoresecence spectra were obtained from a Kristalloflex 805-SRS 2500 Siemens spectrometer using CrKo~radiation. A composite electrode was made from epoxy resin 7025 (SIER S.A.) with 10 wt% diethylene tetraamine (SIER S.A.) and nickel powder of grain size lesser than 10 mm (36). The nickel content was between 50 and 87 wt %. The contact across copper and the composite material was made with colloidal silver A-1208 (BIO-RAD). Ni deposits were made at 333 K by means of a Gravimetron EG-01 in a NiSO4 0.3 M, (NH4)2SO 4 4.6 x 10-3 M, NiC12 7.1 x 10.2 M and H3BO 3 0.5 M bath with a nickel anode. SEM observations were performed by a JEOL 25S microscope (Fig. i). K3Fe(CN)6 and NaNO3 and other substances except KOH were of analytical grade (Merck). Water was deionized. In NaNO3 0.1 M medium, the Ni/epoxy composite becomes passive (Fig. 2) by the low anodic scan (from 0 to 800 mV), due to the gradual formation of the corresponding oxide on the electrode surface. From the Tafel plot in the anodic wave base we obtained a slope of 44 mV -1. During the dipping of the Ni/epoxy composite into a solution of 1.6 × 10-3 M of KaFe(CN)6 and 0.1 M of NaNO3 in an open circuit for 30 min, an abrupt shift in the electrode potential (0.65 V) towards positive potentials (Fig. 3) was observed. This variation in potential is the result of the strong chemical interaction between the ferricyanide and the electrode surface. In the presence of nitrate ions the electrode is covered only with an oxide film. Conversely, when potassium ferricyanide is found in the medium, the variation of the electrode potential in time is not observed. When carrying out the scan in the anodic direction up to 0.8 V, we are able, by reversing the scan sense, to observe two peaks: 0.55 V and 0.35 V. These correspond to the Fe(II)/Fe(III) system. During repetitive scanning at 100mVs -~, an increase in the anodic and cathodic peak currents has been observed, as well as a slight shift of the peak potentials towards the anodic and cathodic potential ranges respectively (Fig. 4). The formation of the film is checked by means of voltammograms during the anodic scan. Peak potentials are in agreement with those of nickel electrode after the same treatment in the ferrocyonide medium (Fig. 5). After the potentiodynamic treatment, the electrode appeared to be coated with an orange film. The X-ray diffractogram shows the crystalline nature of this material, but in the comparison with diffractograms collected in the JCPDF database its identifica-

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Figure2 Successive scan voltammograms in NaNO3 0.1 M. Ni (85%)/epoxy electrode on glass. T = 2 9 8 K . Apparent area S = 0.65 _4-0.02 cm2; scan rate v = 50 mV.s-1; start potential Ei = 0 V.Ag/AgC1,KC1 (3 M) reference electrode. (a) Anodic limit potential E~ = 800 mV, (b) E~ = 1800 mV.

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tion has not been possible. In order to acquire complementary information about the metal constituents and functional groups presents in the material, further experiments using X-ray fluorescence (XRF) and IR(FT) spectroscopy techniques were performed. XRF data indicate the presence of Ni and Fe as sole metal components in the phase, although iron is the most abundant IR spectra exhibit bands at 3400-3200 cm -1 and 1620 cm -1, assignable to symmetric and asymmetric v-OH and &HOH vibrations of lattice water respectively. Bands appearing below 1500cm -1 cannot be assigned unambiguously. Furthermore, the spectra lack sharp bands at 3650 cm -~ (v-OH of hydroxyl ion) and 2100 cm -1 (vC = N). Neither characteristics bands of NO2- or NO3- groups are observed [37]. All data consist of 603

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Figure4 Film formation by successwe voltammograms in ferricyanide solution, v = 50 mV.s-1. T = 298 K. (a) Ni(85%)/epoxy resin, S = 0.65 _ 0.02 cm2. K3Fe(CN)6, 5.6 x 10_3 M, NaNO3 0.1 M. (b) Ni electrode. S = 1.0 + 0.02 cmL K3Fe(CN)6 2.4 x i0 -3 M, NaNO3 0.1 M. the formation of a hydrated iron oxide in which Fe is partially substituted by Ni. In basic K O H medium, the anodic and cathodic p e a k intensities of v o l t a m m o g r a m s increase in successive scans. The p e a k recorded around 0.15 V corresponds to the oxidation of the fi-Ni(OH)2 phase to fi-NiOOH. The cathodic p e a k is associated to the reduction of the /3-NiOOH phase. After 30 cycles, the v o l t a m m o g r a m remains constant (Fig. 6). Then, the ratio between the anodic and cathodic charge transferred is Qa/Qc = 0.952. The presence of potassium ferricyanide causes a new anodic p e a k 604

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Figure5 Voltammograms at different scan velocities obtained after anodic potentiodynamic treatment. T = 298 K, NaNO3 0.1 M. (a) Composite electrode, after anodic potentiodynamic treatment (Fig. 4a). (b) Ni electrode, after anodic treatment (Fig. 4b).

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around - 0 . 0 7 V. In K O H medium, the electrochemical treatment leads to a coating of the electrode with a blue material. The X-ray difffactogram presents two distinct p e a k sets (Fig. 7). One is the same as that which appears in the diffractogram of the orange material obtained in KNO3 medium. The remaining peaks allow us to point out the probable presence of dark blue Ni2(Fe(CN)6)xH20 [38]. This is consistent with the presence of the bands at 2097 cm -1 (v F e - C ) and 470 cm -1 (6 F e - C N ) in their I R spectrum [39]. On the other hand, X R F spectra show that on the electrode surface material only Ni and Fe were observed, in an approximately 1:1 molar ratio. The electrochemical behaviour of the material investigated is similar to that of a pure Ni electrode in the same experimental conditions. The different conductivities and superficial characteristic differences observed between both electrodes cause only a

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resented by the metallic nickel electrode undergoing the same processes. However, in the case of the composite electrode, we can obtain modified conductor films and mouldable electrodes, and their surfaces can be metallized by means of nickel deposits. Therefore, besides electrodes of variable form, we can obtain different morphologies of the electrode surface. The electrochemical behaviour of the Ni/epoxy ferricyanide modified electrode opens up the possibility of its use in electrocatalysis and etectrochromic design devices based in hexacyano compounds.

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Acknowledgement

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This work w,as supported by CICYT (MAT 464/90). DIGICYT, (SAB 88/90-92).

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References

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1.

S. U. FALK and A. J. SALWID, "Alkaline storage batteries" (Wiley and Sons, London, New York, 1969) p. 48. 2. J.P. HOARE, "The electrochemistry of oxygen" (Interscience, New YorE, 1968). 3. R. M, B E N D E R T and D. A. C O R R I G A N , J. Electrochem. Soc. 136 (1989) 1369.

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-0.5

~/

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Figure6 Voltammograms of Ni-epoxy electrodes in basic

5.

medium, (a) Successive scans. Ni(85%)/epoxy electrode, 0.1 M KOH. S = 0,65 + 0,01 cm 2. (b) Voltammograms of stabilized Ni electrode in 1 M KOH. S = 1.0 + 0.01, v = 3 mV.s -I. (c) Addition of potassium ferricyanide: ( - - ) stabilized Ni(85%)/epoxy electrode in K O H 0 , 1 M . ( - - - ) Addition of 2.4.10-3M K3Fe(CN) 6.

6. 7. 8. 9. t0.

little shift of the peak potentials, which is more obvious in the reverse direction of the reduction reaction than in the direct one of oxidation. The electrochemical behaviour is analogous to that rep-

11.

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Ni substrate = Ni2Fe(CN)s.YH20

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2e (degrees) Figure 7 R X diffractograms of a surface eIectrode treated by cyclic anodic polarization for 30 cycles between - 0 . 5 and 0.2 V in a 2.4.10- 3 M potassium ferricyanide and 0.1 M K O H aqueous solution.

605

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Received 12 March and accepted 6 July 1993