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 (<1 Ω) that are competitive with results obtained for Au and Pt coated electrodes. The bulkier [Co(dtb)3]3+/2+ mediator exhibits a more complex surface dependent electrochemistry yielding somewhat faster electron transfer kinetics on gold (3.8 × 10−4 cm s−1) relative to PEDOT/ClO4 (1.7 × 10−4 cm s−1) coated counter electrodes. Nevertheless, this work establishes that porous PEDOT/ClO4 counter electrodes, because in part of their enhanced electroactive surface area and reduced charge transfer resistance, possess sufficient electrocatalytic properties when paired with cobalt-based redox mediators to make PEDOT-based counter electrodes attractive for use in DSC applications.