Platinum is the most common electrocatalyst used as a counter electrode (CE) in dye-sensitized solar cells (DSSCs). However, due to its high cost, Pt presents an obstacle to popularizing DSSCs in energy-harvesting applications. Therefore, effective utilization of Pt and good understanding of the role of its composites are critical issues for developing low-cost DSSCs with high efficiency. In this study, a graphene/Pt nanoparticles (GN/PtNPs) nanocomposite is synthesized as the catalyst for the CE of a DSSC. GN/PtNPs catalysts with various of PtNP loadings (10–60 wt %) are obtained by using a polyol reduction method, and are subsequently characterized by using X-ray diffraction, transmission electron microscopy, field-emission scanning electron microscopy, and energy-dispersive X-ray spectroscopy. A solar-to-electricity conversion efficiency (η) of 8.79 % is achieved for a DSSC with a GN/PtNPs CE containing 20 wt% PtNPs (GN/PtNPs-20 %); this η value is higher than those of the cells with CEs consisting of pristine GN (7.65 %) or sputtered Pt (s-Pt, 8.58 %). Electrochemical impedance spectroscopy, cyclic voltammetry, and Tafel polarization plots reveal that the higher η value of the cell with GN/PtNPs-20 % is due to the higher electrocatalytic ability of the CE for the reduction of triiodide ions (I3−) and the reduced charge-transfer resistance at the CE/electrolyte interface. The excellent electrocatalytic performance of GN/PtNPs-20 % is attributed essentially to its high intrinsic heterogeneous rate constant for the I3− reduction reaction and partly to its high electrochemical surface area, which are quantitatively calculated by means of a rotating disk electrode system and the Koutecký–Levich equation.
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