ELECTROCHEMICAL QUARTZ-CRYSTAL MICROBALANCE STUDY OF TETRACYANOQUINODIMETHANE CONDUCTING SALT ELECTRODES

Electrochemical quartz crystal microgravimetry (EQCM) has been employed to study tetracyanoquinodimethane (TCNQ) redox chemistry in concentrated aqueous electrolytes. Surface films of the TCNQ were generated by oxidation of solid crystallite films of 9-aminoacridinium (TCNQ)2 on gold EQCM substrates in contact with 1 M Ca(OAC)2 or 1 M KOAc. After redox cycling between the neutral TCNQ and TCNQ-/(TCNQ2)2- states, the EQCM frequency transients were consistent with permselective ion motion for the cathodic insertion and anodic expulsion of CaOAc+ and K(H2O)4+ into and out of the solid-state lattice. The initial reduction step after electrogeneration of the TCNQ surface state gave anomalously large frequency transients indicative of extensive hydration or surface structural change. Chronoamperometric EQCM experiments at low overpotentials gave insight into the coupling of electron and ion motion during the TCNQ redox process. The anodic frequency transients indicated a two-step process: (i) insertion of anions of the supporting electrolyte in order to maintain charge neutrality; (ii) followed by slow diffusional loss of neutral electrolyte from the TCNQ solid-state lattice. This process gave rise to significant frequency transients in the EQCM cyclic voltammetry experiments at potentials where there was essentially no faradaic reaction taking place.