Electrochemical studies of heterogeneous reduction of tetracyanoquinodimethane in poly(ethylene oxide) electrolytes using ac impedance and cyclic voltammetry at an ultramicroelectrode

Abstract

Electrochemical studies of the TCNQ⁰/TCNQ⁻ couple have been carried out using ac impedance spectroscopy and cyclic voltammetry at platinum ultramicroelectrodes (UME). Liquid poly(ethylene oxide) (PEO) CH₃–O–(CH₂–CH₂–O)₄–CH₃ has been used as the solvent with different concentrations of the TCNQ⁰/TCNQ⁻ couple and LiClO₄ as the supporting electrolyte. On the basis of the ac impedance results at the UME it has been found that the double layer capacitance and standard heterogeneous rate constant are independent of the presence of electroactive species and supporting electrolyte concentrations, indicating that adsorption of electroactive species onto the electrode is not significant. The standard heterogeneous rate constant k_s was found to be 0.109 ± 0.005 cm s⁻¹. A similar value of k_s = 0.094 cm s⁻¹ was obtained for the second reduction step TCNQ⁻/TCNQ²⁻. Diffusion coefficients of both TCNQ and TCNQ⁻ are equal, D = 1.0 × 10⁻⁶ cm² s⁻¹ for a 0.5 M LiClO₄ solution. Higher diffusion coefficients are obtained in less concentrated supporting electrolyte. Comparison is made between these results and those reported previously for PEO-400 HO–(CH₃CH₂O)₈–OH. The different end groups significantly influence the viscosity and hence k_s.