Electrochemical impedance spectroscopy of dodecylsulphate doped polypyrrole films in the dark and under illumination

Journal of the Brazilian Chemical Society

De Paoli, Marco-Aurélio¹  Matencio, Tullio¹  Martini, Milena¹  Alonso-Vante, Nicolás¹  Universidade Estadual de Campinas, Campinas, Brazil¹  Universidade Federal de Minas Gerais, Belo Horizonte, Brazil¹  Universite de Poitiers, Poitiers, France¹  Universidade de São Paulo, São Paulo, Brazil¹ 

关键词: conducting polymer;    photoelectrochemistry;    eletrochemical impedance spectroscopy;   

DOI  :  10.1590/S0103-50532000000100010

学科分类：化学（综合）

来源: SciELO

【 摘 要 】

Transport and transference processes of electronic and ionic charges of conducting polymers in contact with electrolytes are important for their applications in electrochemistry and photoelectrochemistry. Impedance spectroscopy permits to separate inductive, kinetic, diffusive and charge saturation processes, that take place within the material and at the active interfaces of the system. Different characteristic relaxation times and their dependence upon an a.c. frequency of small amplitude applied to the system are the keys for a better comprehension of the phenomena involved in the synthesis of the films and the functioning and ageing of electro- and photoelectrochemical cells built with them.Double layers with different electrical charges, such the Helmholtz double layer and the Gouy-Chapman region, and a space charge region are formed in a semiconductor electrolyte interface under appropriate conditions. At each of them a differential capacitance can be associated. When a space charge region with finite length, or depletion layer, is present in the semiconductor material, it determines the general system behaviour, and can be studied in details with the aid of EIS. Electrochemical impedance measurements for the determination of the flat band potential in photoelectrochemical systems based on polyaniline1, poly(3-methylthiophene)2 and polypyrrole3 films were already performed in our laboratory.Many works are presented in the literature in which the electrochemical properties of polypyrrole and other conducting polymers synthesised in different conditions are studied by EIS. Bard and cols.4 have applied the technique for the first time with polypyrrole, PPy. The authors have shown that PPy films were porous to the electrolyte but electron transfer reactions can also occur at the polymer surface. The decreasing of admittance in potentials more negative than - 0.7 V vs SCE have confirmed the insulating nature of reduced PPy. The authors have used the model of a single pore to discuss the results obtained with impedance measurements around +0.1 V vs SCE, neglecting the mass transfer effect.Using the transmission line model presented by Rubinstein5, Albery and cols.6 have fitted the mobilities of the charge carriers in the polymer and of the counter-ions in the pores containing solution. Each line could have its own resistance. The results have shown that the two resistances were equal, indicating that the motion of the charge carriers in the polymer backbone and of the ions in the solution are coupled. The capacitance, distributed between the two lines is described by Feldberg7. The electron propagation may take place through molecular exchange reactions involving conformational changes of the polymer chains. But when the interchain transport is too fast, and the interchain hopping is the rate controlling process, this model cannot satisfactorily describe the experimental impedance data.Waller and Compton8 studied PPy films (0.025 ~ 0.15 mm thick) electrosynthesised on Pt electrodes in the presence of BF4- in acetonitrile, simultaneously by Electron Spin Resonance and EIS. They found that not only bipolarons but also polarons are involved in the conductivity of PPy. The complex plane plots of the films were dependent upon their doping degree. For applied potentials more negative than 0.0 V vs SCE two depressed semicircles were observed and they were attributed to the double layer at the PPy | Electrolyte interface (low frequency) and to the bulk PPy (high frequency). A charge saturation region (at low frequency), a depressed semicircle (at high frequency) and a straight line (45°) at intermediary frequencies were observed at potentials more positive than 0.0 V vs SCE. This line can be interpreted in terms of a reversible reaction involving the diffusion of charge-compensating counter-ions or of porous electrode. The depressed semicircles can be assigned to the roughness of the polymer surface.By controlling the microscopic structure of polypyrrole, with chains growing perpendicularly to the substrate, Naoi and Osaka9 observed that the doping process is favoured in comparison to a film synthesised normally without structure control. The total impedance in the former

【 授权许可】

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