General approach to the interpretation of electrochemical quartz crystal microbalance data. 1. Cyclic voltammetry: Kinetic subtleties in the electrochemical doping of polybithiophene films

Electrochemical quartz crystal microbalance (EQCM) data can be used to demonstrate the existence of a global equilibrium during redox state changes in electroactive polymer films. In the absence of a global equilibrium, the rate-limiting process can be associated with the motion of electrons, ions, or neutral species. Criteria are developed for using EQCM data to distinguish between all these possibilities. The criteria necessitate consideration of charge, mass, and a new function, Φ (derived from EQCM data), as functions of potential and mass and Φ as functions of charge. A systematic procedure for applying these criteria to cyclic voltammetric studies is given. An illustration of these general principles is provided by their application to the simple case of polybithiophene doping in a single symmetrical electrolyte, tetraethylammonium tetrafluoroborate/CH₃CN. In this particular case, the approach to global equilibrium is governed by neutral species transfer.