Aqueous Dispersible Graphene/Pt Nanohybrids by Green Chemistry: Application as Cathodes for Dye-Sensitized Solar Cells

Aqueous dispersible nanohybrids (NHBs) of graphene nanosheets (GNSs) and Pt nanoparticles (Pt-NPs) were synthesized through the one-pot reduction of their precursors using an environmentally benign chemical, vitamin C. The concurrent reduction of the precursors, which includes graphene oxide (GO) to GNS and H₂PtCl₆ to Pt⁰, was facile and efficient to yield GNS/Pt-NHBs in which face-centered cubic (fcc) crystalline Pt-NPs with average diameters of ∼5 nm were robustly attached on the surface of the GNSs. The conversion yield during Pt reduction was fairly high (∼90%) and the Pt content within the NHBs was easily controllable. The resulting stable aqueous colloidal dispersion of GNS/Pt-NHBs was successfully fabricated as thin films without using any binder by the electro-spray method at room temperature, and the fabricated samples were used as counter electrodes (CEs) for dye-sensitized solar cells (DSSCs). The electrocatalytic activity of the NHBs for I^–/I₃^– redox couples in conventional DSSCs was investigated using cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) analysis. Doping of GNSs with small amounts of Pt-NPs (<10 wt %) could dramatically enhance the redox kinetics. The enhanced electrocatalytic activity of the GNS/Pt-NHBs was reflected in the performance of the DSSCs. The power conversion efficiency of optimized DSSCs using the NHB-CEs was 8.91% (<i>V</i>_OC: 830 mV, <i>J</i>_SC: 15.56 mAcm^–2, and FF: 69%), which is comparable to that of devices using the state-of-the-art Pt-based CEs (8.85%)