Intrinsic Origin of Superior Catalytic Properties of Tungsten-based Catalysts in Dye-sensitized Solar Cells

To clarify the intrinsic factor of the superior catalytic activity of tungsten (W)-based catalysts in DSSCs, W-based compounds, including W18O49, W2N, WC, and WO3, were prepared as CEs and tested in I-/I-3(-) redox shuttle. A systematic investigation of the catalytic properties and stability, as well as first-principle density functional theory (DFT) calculations, was performed for these W-based catalysts in DSSCs. The DSSCs assembled with the I-/I-3(-) redox shuttle, based on W18O49, W2N, WC, and WO3 CEs, exhibited power conversion efficiencies (PCEs) of 6.69%, 5.97% 5.20%, and 4.69%, respectively; these values were equal to or better than those of DSSCs with Pt CEs (6.52%) under the same conditions. The superior electrocatalytic activities of W-based compounds are determined by their electronic structure. Electronic density of states (DOS) revealed that the band gaps of W18O49, W2N, WC, and Pt were zero, whereas that of WO3 was 1.7 eV. Wider band gaps do not facilitate electron conduction as easily as Pt. In addition, the W/O ratio (1: 2.72) in W18O49 was greater than that in WO3 (1:3), and the W/N ratio (2: 1) in W2N was greater than that in WC (1:1). Higher ratios of W/(O or N) favored the electrocatalytic activity of the CE materials in the reduction of I-3(-) in DSSCs. This is the intrinsic reason why W-based compounds exhibit different catalytic properties in DSSCs under the same conditions. This work sheds light on the origin of W-based catalysts, and explains the differences in the catalytic performances of W-based compounds for this work and several published works. (C) 2017 Elsevier Ltd. All rights reserved.