C. R. Leidner

Kinetics of electron transfer reactions of metal complexes at impermeable redox active polymeric films on electrode surfaces and charge transport within the polymer film

Abstract At rotated Pt disk electrodes coated with thin films of the redox polymer poly-[Ru(vbpy)3]2+, ruthenium and iron bipyridine complexes dissolved in acetonitrile can become oxidized by two pathways. The first is diffusion of the solute complex through the polymeric film to react at its normal potential Esub0, at the Pt/polymer interface. The second is a mediated electron transfer cross-reaction between the solute complex and poly-[Ru(vbpy) 3]2+ sites generated in the film at adequately positive potentials. The mediated reaction, as judged from the lack of variation of its rate kcrsΓ with the quantity of polymer mediator sites present in the multimolecular layer film, and from other evidence, is confined to the outer few (one?) monolayers of ruthenium polymer film sites. The mediated reaction becomes the dominant pathway for films with ΓT ∼2×10−9 mol/cm2 of ruthenium polymer sites, owing to the low permeability (measured independently) of the solutecomplexes into the film. The rate kcrsΓ could be measured when the solute complex oxidation potential is more positive than that of the redox film, and is too fast to measure when Esub0, is more negative than the redox film Ecal−0, New theory is presented and evaluated to describe the rising portions of the voitammetric waves for the nine solute complexes studied. The rate of charge transport through the poly-[Ru(vbpy)3]2+ film becomes controlling under certain conditions and can be thereby measured as well.

Bilayer electrodes: Theory and experiment for electron trapping reactions at the interface between two redox polymer films

Abstract Theory is presented for rates of oxidation state trapping (charge trapping) in the outer polymer film on electrodes coated with two, spatially-segregated redox polymer films (a bilayer). The linear potential sweep voltammetric response of the bilayer Pt/poly-[Ru(vbpy) 3 ] 3+ /poly-[Fe(vbpy) 3 ] 2+ in 0.1 M Et 4 NClO 4 /CH 3 CN is analyzed for two possible rate limiting steps: electron transfer across the interface between the two polymer films and charge transport (electron diffusion) within the inner film. The leading edge of the bilayer charge trapping current peak is best fit by electron diffusion theory, up to 80% of i peak . Accurate description of the concentration-potential relationship of Ru(III) states in the inner poly-[Ru(vbpy) 3 ] 2+/3+ film is crucial for quantitative comparison of the leading edge currents to theory. Currents near i peak are not controlled by inner film electron diffusion; the controlling process(es) there were not easily assignable. A redox conduction experiment with a Pt/poly-[Ru(bpy) 2 (vpy) 2 ] 2+ /poly-[Os(bpy) 2 (vpy) 2 ] 2+ /Au sandwich electrode additionally demonstrates electron diffusion limitation as opposed to interfacial electron transfer.

Charge trapping reactions in bilayer electrodes

Abstract The cyclic voltammetric peaks for charging trapping and untrapping reactions between the inner and outer redox polymer films of five bilayer electrodes are compared to a theory for control of the rate of charge trapping by electron diffusion rates in the inner polymer film. The five bilayer electrodes use various different redox polymer films (electropolymerized poly-pyridine complexes of Fe, Ru, and Os, and polyvinylferrocene) arranged in different orders. The currents on the rising edge of the bilayer trapping and untrapping peaks follow the electron diffusion theory up to ca. 80% of the peak current; currents thereafter are controlled by another process(es). The analysis yields values for the electron diffusion constants in the inner bilayer polymer films, which agree with one another for different bilayers having the same inner film polymer films and which also agree with independent determinations by other methods. Two of the bilayers are made from the same two polymers, arranged in different inner-outer order. These bilayers also illustrate the occurrence of a “leak reaction”, in which charge trapped in the outer film is discharged via a thermodynamically unfavorable electron transfer reaction with the inner polymer film.