Comparative Evaluation of Catalytic Counter Electrodes for Co(III)/(II) Electron Shuttles in Regenerative Photoelectrochemical Cells

Different PEDOT-based counter electrodes, obtained by potentiostatic electropolymerization of 3,4-ethylenedioxythiophene (EDOT) monomer on fluorine tin oxide (FTO) surfaces, were compared to platinum- and gold-coated electrodes in order to evaluate the potential use of PEDOT counter electrodes in dye-sensitized photoelectrochemical cells (DSCs). In particular, a series of DSC devices utilizing Co(III)/(II) polypyridine redox mediators ([Co(bpy)3]3+/2+, [Co(phen)3]3+/2+, [Co(dtb)3]3+/2+; bpy = 2,2′-bipyridine; dtb = 4,4′ditert-butyl-2,2′-bipyridine; phen = 1,10-phenanthroline) having distinct electrochemical characteristics were evaluated. Electrochemical impedance spectroscopy (EIS) measurements reveal that [Co(bpy)3]3+/2+ and [Co(phen)3]3+/2+ mediators exhibit relatively fast heterogeneous electron transfer rates with porous PEDOT/ClO4 electrodes (k = 7 × 10−3 cm s−1 and 2.1 × 10−3 cm s−1, respectively) and correspondingly low charge transfer resistances (