Charge Transport in Conducting Polymer Film Electrodes
G. Inzelt Eötvös Loránd University, Institute of Chemistry, Budapest, Pázmány Péter sétány 1/A, H-1117, Hungary E-mail: email@example.com Review Received: August 4, 2006 Accepted: December 13, 2006
The essential features of charge transport in conducting polymer film electrodes are discussed. Selected experimental results are presented, which shed light on the complex nature of the processes occurring in these systems. The problems of the theoretical elucidation and practical consequences are also emphasized. Key words: Conducting polymers, charge transport, film morphology, relaxation
The elucidation of the nature of charge transfer and charge transport processes in electrochemically active polymer films may be the most interesting theoretical problem of the field. It is also a question of great practical importance, because in the majority of their applications fast charge propagation through the film is needed.1–14 A polymer film electrode can be defined as an electrochemical system in which at least three phases are contacted successively in such a way that between a first-order conductor (usually a metal) and a second-order conductor (usually an electrolyte solution) is an electrochemically active polymer layer. The polymer layer is more or less stably attached to the metal, mainly by adsorption (adhesion). The fundamental observation that should be explained is that even rather thick polymer films in which most of the redox sites are as far from the metal surface as 100 – 10 000 nm may be electrochemically oxidized or reduced. According to the classical theory of simple electron-transfer reactions, the reactants get very close to the electrode surface, and then electrons can tunnel over the short distance (some nanometers) between the metal and the activated species in the solution phase. In the case of polymer-modified electrodes the active parts of the polymer cannot approach the metal surface, because polymer chains are trapped in a tangled network, and chain diffusion is usually much slower than the time scale of the transient electrochemical experiment. Therefore, the transport of electrons can be assumed to occur either via an electron exchange reaction (electron hopping) between neighboring redox sites if the segmental motions make it possible or delocalized electrons
can move through the conjugated systems (electronic conduction). The former mechanism is characteristic of redox polymers that contain covalently attached redox sites, either built in the chain or as pendant groups, or redox-active ions held by electrostatic binding. Polymers that possess electronic conduction are called conducting polymers. Electrochemical transformation – usually oxidation – of the non-conducting form of these polymers usually leads to a reorganization of the bonds of the macromolecule and the development of an extensively conjugated system. The electron hopping mechanism is likely to be operative between the chains (interchain conduction) and defects even in the case of conducting polymers. However, attention has to be paid not only to the electronic charging of the polymer film (i.e. to the electron exchange at the metal/polymer interface and the electron transport through the surface layer) since in order to preserve electroneutrality within the film ions will cross the film / solution interface. The motion of counterions (or less frequently that of the co-ions) may also be the rate-determining step. We may regard our film as a membrane or a swollen polyelectrolyte gel (i.e. the charged film contains solvent molecules and, depending on the conditions, co-ions, in addition to the counterions). As a consequence of the incorporation of ions and solvent molecules into the film, swelling or shrinkage of the polymer matrix takes place....