Chemical and mechanical properties of redox polymer-modified electrodes: Part I. A mechanical/electrochemical model

Abstract

A model is developed to describe the coupling of the mechanical and electrochemical thermodynamics of redox polymer film conversion for systems in which there is a net change in the charge per redox site associated with this redox conversion. The electrostatically forced intrusion of compensating charge, in the form of ions of the supporting electrolyte, into the polymer film is predicted to result in non-Nernstian behavior because of the finite void volume within the film and the finite molar volume of the charge-compensating ion. The physical consequences of this phenomenon are the forced swelling of the polymer film, a process which in the presence of crosslinks (either covalent or ionic) requires energy input in excess of that needed to carry out the simple electron transfer to/from the redox sites. This energy is consumed in doing work against the elastic framework of the crosslinked polymer matrix, and when couched in terms of an elastic deformation, may be considered reversible in the thermodynamic sense. The model developed here treats only the simplest of such mechanical coupling in that reversible, isotropic stress-strain is proposed to account for the non-Nernstian thermodynamics of these redox polymer films.