ARTICLES Hybrid Material Polypyrrole/[SiCr(H 2 O)W 11 O 39 ] 5- : Electrogeneration, Properties, and Stability under Cycling Shaoan Cheng, †,| Toribio Ferna ´ ndez-Otero,* ,†,‡ Eugenio Coronado,* ,§ Carlos J. Go ´ mez-Garcı ´a, § Eugenia Martı ´nez-Ferrero, § and Carlos Gime ´ nez-Saiz § Lab. de Electroquı ´mica, Fac. de Quı ´mica, UniVersidad del Paı ´s Vasco, P.O. Box 1072, 20080 San Sebastia ´ n, Spain, Departamento de Quı ´mica-Fı ´sica, UniVersidad Polite ´ cnica de Cartagena, Paseo de Alfonso XIII, 38, 30303, Cartagena (Murcia) Spain, and Instituto de Ciencia Molecular, UniVersidad de Valencia. C/ Dr. Moliner 50, 46100 Burjasot (Valencia) Spain ReceiVed: NoVember 29, 2001; In Final Form: February 11, 2002 Hybrid materials constituted by polypyrrole doped with the polyoxometalate (POM) anion [SiCr(H 2 O)W 11 O 39 ] 5- were electrogenerated from acetonitrile solutions. The electrochemical behavior of the hybrid films was studied by cyclic voltammetry in three different electrolytes formed by two anions and two cations, dissolved in acetonitrile. Following both, the film weight evolution and the solutions composition with cycling, together with an EDX study of the film showing the electrochemical evolution, have allowed the identification of the ions interchanged during the redox processes for the different studied electrolytes. Prevalent cation interchange, continuous substitution of the POM anion by the anion in solution, and partial electrodissolution of the hybrid film were identified as prevalent processes in the different electrolytes. Conducting measurements show that the films present a high room temperature conductivity that is thermally activated. Magnetic measurements confirmed the presence of the paramagnetic POM in the films and indicate that the number of pyrrole monomers per cationic charge is about 4.5. These results have been confirmed by elemental analysis. 1. Introduction Hybrid films made of polypyrrole (Ppy) (Figure 1a) and polyoxometalates (POMs) (Figure 1b) have been attracting much interest in electrochemistry and catalysis due to their easy generation and good electrocatalytic and electrochemical properties. 1-4 Keggin-type polyoxometalates inserted into a Ppy matrix confer these materials special electrocatalytic, 5-10 pho- toelectrochemical, 6,11 and electrochemical 3,12 properties that make them very useful as catalysts. 2,13 The most used POMs are the Keggin heteropolyanions of formula [X n+ M 12 O 40 ] (8-n)- (M ) W VI or Mo VI ) (Figure 1b). One of the advantages of this highly symmetric structure is the possibility of substitution of one or more of the constituents M(VI) ions by transition metals ions (addenda), like W(VI), Mo(VI), V(V), and Re(VII), and by many other of the first row, such as Cr(III), Mn(II), Fe(III), Co(III), Co(II), and Cu(II), although for these last cases the terminal oxo group is replaced by a water molecule. 14 In this way, the electrochemical character of the POMs can be widely changed. For example, in the reduction of [PW 11 VI Mo VI O 40 ] 3- and of [PW 11 VI V V O 40 ], 4- the added electron is accepted by the Mo and V atoms, respectively, forming [PW 11 VI Mo V O 40 ] 4- and [PW 11 VI V IV O 40 ]. 5- These sub- stituted POM complexes have been widely used as heteroge- neous catalysts. 15 Polymer doped with these substituted POM ions are much more interesting because they exhibit the redox properties inherent to both the POMs and the polymer moiety. 5,16 Bidan and co-workers reported the synthesis of poly(N- methylpyrrole) films doped with the iron-substituted heteropoly- tungstate [PFe III (H 2 O)W 11 O 39 ] 4- and its electrochemical prop- erties in solution in the presence of NO 2 - ions. 17 Polyaniline films doped with H 4 [PFe III (H 2 O)Mo 11 O 39 ] and H 4 [PMn II (H 2 O)- Mo 11 O 39 ] were described by Lapkowski et al. 13 Wang et al. described polyaniline films doped with [PV 2 Mo 10 O 40 ] 5- formed on carbon fiber microelectrodes and its utilization for the electrocatalytic reduction of ClO 3 - and BrO 3 - ions. 18 POM anions in polymer films can provide a secondary mechanism * Corresponding authors. E-mail: toribio.fotero@upct.es (T.F.-O.) and eugenio.coronado@uv.es (E.C.). † Universidad del Paı ´s Vasco. ‡ Universidad Polite ´cnica de Cartagena. § Universidad de Valencia. | Present address: Department of Chemistry, Zhejiang University, Hangzhou 310027, P.R. China. Figure 1. Polypyrrole (a) and the Keggin polyoxometalate [SiCr- (H2O)W11O39] 5- used in this work (b). The Cr 3+ ion is located in the shaded octahedron. 7585 J. Phys. Chem. B 2002, 106, 7585-7591 10.1021/jp014340y CCC: $22.00 © 2002 American Chemical Society Published on Web 07/16/2002