The thermodynamics of thin films (< 200 nm) of plasma-polymerized vinylferrocene (PPVF) deposited on various substrates have been examined in aqueous and non-aqueous electrolytes by controlled potential coulometry. The Nerst plots (E vs log[cFc+/cFc0]) of these data give slopes in the range of 120-150 mV/decade. Scanning electron microscopy of thicker films (> 400 nm) shows that significant physical damage can occur during electrolysis. These and related observations suggest that the super-Nersntian behavior may be related to stress induced by the forced incorporation of the anion of the supporting electrolyte, as is required to maintain electroneutrality within the films. A mechanical/electrochemical model is employed to explain the origin of the excess free energy required to convert ferrocene sites to ferrocenium sites within the PPVF films.
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The authors wish to thank R.P. Buck, P. Carr, T. Lodge, S. Prager, V. Shah and hf. Tirrell for many valuable discussions held during the course of these investigations. The financial support of the Sloan Foundation (JFE), the Army Research Office (grant No. DAAG-29-82-K-0063) and the Office of Naval Research (grant No. NO001480-C-0244) are gratefully acknowledged.