A substoichiometric tungsten oxide catalyst provides a sustainable and efficient counter electrode for dye-sensitized solar cells

Highlights: • Economical Pt-free tungsten oxide counter electrode for DSSCs is developed. • A facile annealing route was used to synthesize tungsten oxide. • Cell efficiency of 5.25% is achieved from WO_3-x DSSC approaching Pt DSSC (6.96%). • WO_3-x sample exhibits excellent catalytic activity and electrical conductivity. • W⁴⁺, W⁵⁺ and W⁶⁺ valence states in WO_3-x promote electrical conductivity. - Abstract: Development of Pt-free catalyst materials for the counter electrode (CE) in dye-sensitized solar cells (DSSCs) has been regarded as one of the crucial steps to improving energy conversion efficiency and cost effectiveness of DSSCs. In this work, low cost tungsten oxide (WO_3-x) counter electrodes, prepared by annealing tungsten metal sheets under an Ar and low O₂ atmosphere, exhibited high catalytic activity and energy conversion efficiency. The highest efficiency achieved here for DSSCs with WO_3-x counter electrodes, was 5.25%, obtained from a 500 °C annealed tungsten sheet. TEM and XPS analysis suggested the formation of sub-stoichiometric tungsten oxide layer (∼WO_2.6) with the presence of W⁶⁺, W⁵⁺ and W⁴⁺ oxidation states at the tungsten metal surface after the 500 °C annealing. Only W⁶⁺ and W⁵⁺ oxidation states were detected after a 600 °C annealing indicating the formation of a more stoichiometric tungsten oxide layer (∼WO_2.8) and resulting in a drop in efficiency of the DSSC. We suggest that mixed valence tungsten states account for the excellent catalytic activity and good electrical conductivity as evidenced by the highest cyclic voltammetry response of 0.76 mA/cm² and the lowest impedance value of 44.33 Ω, respectively