XII Iberian Meeting of Electrochemistry XVI Meeting of the ...

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XII Iberian Meeting of Electrochemistry & XVI Meeting of the Portuguese Electrochemical Society PD 04 Electrochemical characterization of polypyrrole-based conducting fabrics in different pH media Javier Molina, Javier Fernández, Ana Isabel del Río, José Bonastre, Francisco Cases Departamento de Ingeniería Textil y Papelera, EPS de Alcoy, Universidad Politécnica de Valencia, Plaza Ferrándiz y Carbonell s/n, 03801 Alcoy, Spain jamopue@doctor.upv.es Conducting fabrics of polyester covered with polypyrrole have been electrochemically characterized at different pHs (1, 7 and 13). The stability of the conducting fabrics in different pHs is fundamental in views of its application in electronic textiles or electrocatalysis. In the study, two counter ions have been employed: AQSA (organic) and PW 12 O 3- 40 (inorganic). The electrochemical techniques that have been employed for the electrochemical characterization have been cyclic voltammetry (CV) and scanning electrochemical microscopy (SECM). It has been demonstrated the influence of the scan rate in the form of the voltammograms obtained. High scan rates (50 mV·s -1 ) did not allow the observation of the redox processes and a resistive response was obtained. On the other hand, low scan rates (1mV·s -1 ) allowed the apparition of the redox peaks. The charge transfer is produced across the polymer chains since polyester is an insulating material. Lower pHs produced a higher and more characteristic electrochemical response due to the influence of protonation on the polypyrrole activity [1]. SECM allows the possibility of studying zonal electroactivity at its open circuit potential. Approach curves were obtained by SECM for the samples of PPy/AQSA and 3- PPy/PW 12 O 40 treated at different pHs. Positive feedback was obtained for all the conducting fabrics samples, so the fabrics were electroactive. The degree of positive feedback varied on the nature and the treatment of the polypyrrole layer. An increase in the pH of the solution caused the loss of part of the counter ions as XPS measurements have revealed and the layer electroactivity was affected. The organic counter ion was more stable than the inorganic one at pH 13. This is due to a reaction of decomposition that suffers the PW 12 O 3- 40 at pH>8.3 [2]. The PW 12 O 3- 40 decomposed into PO 3- 4 and WO 2- 4 [2]. Acknowledgments: Authors thank to the Spanish Ministerio de Ciencia y Tecnología and European Union Funds (F EDER) (contract CTM2007-66570-C02-02) and Universidad Politécnica de Valencia (Programa de apoyo a la investigación y desarrollo de la UPV (PAID-05-08)) for the financial support. J. Molina and A.I. del Río are grateful (Generalitat Valenciana) and the Spanish Ministerio de Ciencia y Tecnología respectively for the FPI fellowship. References [1] Krzysztof M. Electroanalysis, 2006, 18, 1537. [2] Zu Z.; Tain R.; Rhodes C. Canadian Journal of Chemistry, 2003, 81, 1044. September, 811, 2010. ISEL - Lisbon 86
XII Iberian Meeting of Electrochemistry & XVI Meeting of the Portuguese Electrochemical Society PD 05 Influence of Molybdenum on electronic structure of passive films on stainless steels M.A. Catarino, 1 M.I. Godinho, 1 L. Freire, 2 M. da Cunha Belo, 2 A.M.P. Simões, 2 M.J. Ferreira, 1 M.F. Montemor 2 1 ISEL, Área Departamental de Engenharia Química, Av. Conselheiro Emídio Navarro, 1, 1959-007, Lisboa, Portugal 2 ICEMS Instituto Superior Técnico, Universidade Técnica de Lisboa, Av. Rovisco Pais, 1049-001, Lisboa, Portugal acatarino@deq.isel.ipl.pt The study concerns the passive state of AISI type 304 and 316 stainless steels created in aqueous solutions of different pH, in the alkaline range (13-9), at room temperature without and with chloride additions. Experimental work is carried out by capacitance measurements using the Mott-Schottky method. The objective is to establish the influence of molybdenum which is a very important alloying element in the case of the 316 stainless steel. It has been demonstrated that molybdenum decreases the doping density of the constitutive oxides of the passive films formed in the neutral borate/boric solutions ( pH 9) [1]. Now, the results obtained with the solutions under study show that the capacitance behaviour of both stainless steels can be related to the existence of a diphasic passive films formed by an inner chromium rich oxide layer of p-type semi-conductivity and an outer iron rich oxide layer of n-type. From the analysis of the slopes of the Mott- Schottky plots, it can be concluded that the doping densities are not so high for films formed on 316 stainless steel, this means that molybdenum increases the thickness of the space charge layer at film-solution interface and probably also modifies the depletion or accumulation of charges at metal-film interface. Further, molybdenum can affect the electrical potential barrier situated inside the passive film where the transition from p-type to n-type conduction occurs, like the other two interfaces. This special junction (n-p) is a source of structural defects and influence migration processes. The electronic band structure model proposed to explain electron distribution considers that the passive film is very highly doped but can be described by the Boltzmann statistics. The Fermi level is placed in the Urbach tail where localized energy states could intervene in the electronic transfer of charges. Acknowledgments: Project PTDC/ECM/69132/2006. References [1] Hakiki, N.E.; Da Cunha Belo, M.; Simões, A.M.P.; Ferreira, M.G.S. J. Electrochem. Soc., 1998, 145, 3821. September, 811, 2010. ISEL - Lisbon 87
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