Electrochemical studies of heterogeneous reduction of tetracyanoquinodimethane in polymer electrolytes using ac impedance at an ultramicroelectrode

Electrochemical studies of the TCNQ(0)/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 TCNQ(0)/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(-)/TCNQ(2-). Diffusion coefficients of both TCNQ and TCNQ(-) are equal, D=1.0x10(-6) cm(2) 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.(1) The different end groups significantly influence the viscosity and hence k(s).