Functionalized Graphene Sheets in Dye-Sensitized Solar Cell Counter Electrodes

Abstract

The use of thermally exfoliated graphite oxide, commonly referred to as functionalized graphene sheets (FGSs), was investigated as a catalytic counter electrode material in dye-sensitized solar cells to substitute for platinum nanoparticles traditionally used in devices. A catalyst's activity depends both on the material's intrinsic activity as well as on its surface area accessible for reaction. Thus, this work aimed i) to determine the intrinsic activity of FGSs with various chemical compositions and structures, and ii) to create high surface area networks of FGSs to use as catalytic electrodes in dye-sensitized solar cells.

Monolayers of FGSs were fabricated and electrochemically tested to determine the intrinsic catalytic activity for a common dye-sensitized solar cell redox mediator, cobalt bipyridine. It was found that lattice defect rich, oxygen-site poor FGSs catalyze the reduction of the cobalt complex as well as platinum does, exhibiting a rate constant of ~ 6 × 10<super>-3</super> cm/s. This rate is an order of magnitude faster than exhibited with oxygen-site rich graphene oxide, and over two orders of magnitude faster than found with the basal plane of graphite (as a surrogate for pristine graphene).

FGSs are less catalytic towards the iodide/triiodide redox mediator, thus larger surface areas must be used for effective catalysis. In this work, conductive, high surface area networks of FGSs were produced by first tape casting surfactant-stabilized aqueous suspensions of FGSs and then thermolyzing the surfactant materials. Iodide/triiodide mediated dye-sensitized solar cells using these FGS electrodes exhibited power conversion efficiencies within 10% of devices using platinum nanoparticles. Furthermore, to interpret the catalytic activity of FGSs towards the reduction of triiodide, a new electrochemical impedance spectroscopy equivalent circuit was proposed that matches the observed spectra features to the appropriate phenomena.

Lastly, improved catalytic performance was achieved through better control of electrode morphology. By using ethyl cellulose as a sacrificial binder, and partially thermolyzing it, electrodes were created which exhibited lower effective charge transfer resistance (< 1 Ohm*cm<super>2</super>) than the traditional platinum electrodes for the iodide/triiodide, the cobalt bipyridine, and a sulfur-based redox couple. Dye-sensitized solar cells using these FGS electrodes had power conversion efficiencies equal to or greater than those using platinum nanoparticles with each of the three major redox mediators.