J. MATER. CHEM., 1994, 4(5),771-772 771 MATERIALS CHEMISTRY COMMUNICATIONS Conducting Polymer-Clay Composites for Electrochemical Ap pl ica tions Peter W. Faguy," Wanli Ma," J. Alan Lowe," Wei-Ping Panb and Terri Brownb a Department of Chemistry, University of Louisville, Louisville, KY 40297, USA Department of Chemistry, Western Kentucky University, Bowling Green, KY 42707, USA Pyrrole can be polymerized within montmorillonite clays via chemical means utilizing Fe3+ and Cu2+ as the oxidizing species. The resultant composite has properties of both the conducting polymer and the host material. Vibrational spectroscopy, thermal analysis and conductivity data all indicate that polypyrrole is present in the interlayer region of the clays utilized. Electrochemically, the conducting polymer-clay composite shows promise for both sensor and electrolysis applications.A widely studied and potentially useful facet of heteronuclear aromatic ring systems is their ability to polymerize oxidatively, often forming conjugated electronically conducting poly- mers.1-2 Both electrochemical and chemical polymerizations can be carried out within porous or layered materials to realize a composite material with attributes of both the conducting polymer and the host material. Conducting poly- mers spontaneously polymerized inside aluminosilicates are a class of these composite materials which show much promise in electrochemical application^,^,^ but have yet to be investi- gated for electrocatalytic properties. A large body of work exists which details in situ polymerizations of pyrrole, thio- phene and aniline intercalated into clays,5 zeolites: layered metal oxide/halides: and various silicates.' These studies have focused on conductivity and structural information, very little electrochemistry has been performed. Our laboratory has recently begun to prepare and to characterize polypyrrole- montmorillonite clay composites.The research stems from an expectation that some of the electrochemical and catalytic properties of exchangeable clays can be coupled with the electrochemical and electronic properties of conducting polymers. The nature and extent of polymerization of pyrrole in transition-metal-exchanged montmorillonite clays are depen- dent on the particular transition metal and solvent used.For the present study, reactions were carried out in distilled water and with distilled pyrrole. The clay used was a commercially available montmorillonite catalyst support KSF (Aldrich). From preliminary X-ray powder patterns and TG results it appears that the clay is disordered, containing some non- montmorillonite clay-mineral phases. When there is Fe3+ or Cu2 present the initial reaction slurry changes from colour- + less to green, through blue and finally to a shiny black. Very similar observations were made for other intercalative poly- merization~.~-~Two-point conductivity measurements show that with intercalation of the conducting polymer, conductivit- ies increase by three or more orders of magnitude if no copper (Fe3+ only) is present in the clay and by a factor of ca.10 if more than 0.3% (m/m) copper is present. A possible expla- nation of this phenomenon is the strong interaction of Cu2+ with the polymer which localizes electronic charge and reduces conductivity. Thermal analysis and scanning electron microscopy provide indirect evidence that the conducting polymer is intercalated. The SEM shows, even for pyrrole: clay loadings of 3: 1, no detectable morphological changes in the clay particles. If the pyrrole was oxidizing on the particle surface it would be expected that polymeric dendrites would be evident. Two major observations can be made from the TG md DTA results: the initial dehydration of the disordered mont morillon- ite occurs at 105°C and is lowered by ca.14" with pyrrole treatment; and an endothermic transition in the CPCC samples at 580 "C is associated with the intercalated polypyr- role. The increased volatility of interlayer water may be evidence that the polypyrrole exists at the ion-e vchanged oxide surface in the interlayer region, displacing Rater. The fact that no pyrrole or low-molecular-weight oligimers are detected points to a thermally stable polymer. Similar thermal studies of pyrrole intercalated in FeOCl layered solids showed the same polymer ~tability.~ Infrared and Raman measure- ments of CPCC powder indicate that the polymeric material -1 00 200 500 V/mV vs. SCE Fig. 1 Cyclic voltammetry of carbon paste electrodes in 0.1 mol 1-' HC104 at 10 mV s-' with (-) and without (---) 5 mmol 1-' ascorbic acid: (a) carbon only, (b) carbon; CPCC (4:l) and (c) as in (b) but held at -0.1 V for 10 min prior to anodic sweep in the clay is structurally very similar to electrochemically synthesized polypyrrole.’ The potential for CPCC electrodes in sensor applications is shown in Fig.1. The onset of ascorbic acid oxidation on CPCClcarbon paste electrodes [Fig. 1 (b)] occurs ca. 50 mV before it does on carbon paste electrodes alone [Fig. l(a)]. This favourable shift in potential occurs despite diffusion overvoltages associated with adsorption processes. Such a polarization shifts the uncatalysed oxidation of dopamine’ more than 50mV positive, CPCC/carbon paste relative to carbon paste alone.From the Cu2+/Cuo couple in the clay [Fig. l(a) dotted line; ca. 220 mV] and the onset of ascorbic acid oxidation it would seem that Cu2+ sites in the CPCC act electrocatalytically. Pre-concentration via adsorption can lead to increased oxidation currents as is evident by comparing Fig. l(b) and l(c). This preliminary report demonstrates that conducting poly- mer-clay composites do possess many characteristics of both polypyrrole and the clay host. Their initial applicability to electrocatalytic oxidations has been indicated, with catalysis and adsorption both playing roles. The unique electronic, electroactive and adsorptive characteristics of these com-posites produce an electrode material which could be utilized J.MATER. CHEM., 1994, VOL. 4 in such electrochemical applications as energy storage and waste water remediation. The authors wish to acknowledge the National Science Foundation and the Commonwealth of Kentucky for financial support through the NSF-Kentucky EPSCoR Advanced Development Program (grant EHR-9108764). References 1 A. F. Diaz and J. Bargon, in Handbook of Conducting Polymers, ed. T. A. Skotheim, Marcel Dekker, New York, 1986,vol. 1, p. 81. 2 R. J. Waltman and J. Bargon, Can. J. Chem., 1986,64,76. 3 D. R. Rolison, R. J. Nowak, T. A. Welsh and C. G. Marsh, Talanta, 1991,38,27. 4 C. M. Castro-Acuna, F. F. Fan and A. J. Bard, J. Electroanal. Chem., 1987,234,347. 5 T. H. Chao and H. A. Erf, J. Catal., 1986,100,492. 6 P. Enzel and T. Bein, Synth. Met., 1993,55-57 1238. 7 M. G. Kanatzidis, L. M. Tonge, T. J. Marks, H. 0. Marcy and C. R. Kannewurf, J. Am. Chem. Soc., 1989,111,4138. 8 V. Mehrotra and E. P. Giannelis, Sol. State Comrnun., 1991,77, 155. 9 P. W. Faguy, W. Ma, J. A. Lowe and W. E. Brewer, Electrochim. Acta, submitted. Communication 4/014491; Received 1st March, 1994