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 (WO3-x) counter electrodes, prepared by annealing tungsten metal sheets under an Ar and low O-2 atmosphere, exhibited high catalytic activity and energy conversion efficiency. The highest efficiency achieved here for DSSCs with WO3-x counter electrodes, was 5.25%, obtained from a 500 degrees C annealed tungsten sheet. TEM and XPS analysis suggested the formation of sub-stoichiometric tungsten oxide layer (similar to WO2.6) with the presence of W6+, W5+ and W4+ oxidation states at the tungsten metal surface after the 500 degrees C annealing. Only W6+ and W5+ oxidation states were detected after a 600 degrees C annealing indicating the formation of a more stoichiometric tungsten oxide layer (similar to WO2.8) 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/cm(2) and the lowest impedance value of 44.33 Omega, respectively. (C) 2014 Elsevier Ltd. All rights reserved.