Ordered Mesoporous Tungsten Suboxide Counter Electrode for Highly Efficient Iodine-Free Electrolyte-Based Dye-Sensitized Solar Cells

A disulfide/thiolate (T-2/T-) redox-couple electrolyte, which is a promising iodine-free electrolyte owing to its transparent and noncorrosive properties, requires alternative counter-electrode materials because conventional Pt shows poor catalytic activity in such an electrolyte. Herein, ordered mesoporous tungsten suboxide (m-WO3-x), synthesized by using KIT-6 silica as a hard template followed by a partial reduction, is used as a catalyst for a counter electrode in T-2/T- -electrolyte-based dye-sensitized solar cells (DSCs). The mesoporous tungsten suboxide, which possesses interconnected pores of 4 and 20 nm, provides a large surface area and efficient electrolyte penetration into the m-WO3-x pores. In addition to the advantages conferred by the mesoporous structure, partial reduction of tungsten oxide creates oxygen vacancies that can function as active catalytic sites, which causes a high electrical conductivity because of intervalence charge transfer between the W5+ and W6+ ions. m-WO3-x shows a superior photovoltaic performance (79% improvement in the power conversion efficiency) over Pt in the T-2/T- electrolyte. The superior catalytic activity of m-WO3-x is investigated by using cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and Tafel polarization curve analysis.