Size effects of platinum nanoparticles on the electrocatalytic ability of the counter electrode in dye-sensitized solar cells

The size effects of the Pt nanoparticles (PtNPs) on the electrocatalytic ability for the reduction of triiodide ions (I3-) were studied and further applied to of the counter electrode (CE) of a dye-sensitized solar cell (DSSC). First, well dispersed carbon black-supported Pt nanoparticles (CB/PtNPs) with various sizes of the PtNPs (6-2nm) were synthesized by a CO-assisted method. The particle size and size distribution of the CB/PtNPs with various sizes of the PtNPs were verified by XRD and HR-TEM. Electrochemical surface areas (ECSA) of the PtNPs with various sizes were estimated by using CO anodic stripping voltammetry. Slurries of these catalysts were deposited as the catalytic films on ITO glasses to prepare CE for DSSCs. The size effect of the PtNPs on the electrocatalytic ability of the CEs on DSSC for the reduction of the I3- was studied from the perspectives of the photovoltaic performance of the cell. Cyclic voltammetry (CV) was performed to understand the reaction kinetics and electrocatalytic ability of the CEs containing the films of CB/PtNPs with various sizes of the PtNPs. The charge transfer resistances (Rct) for the reduction of I3- at the interfaces of the CB/PtNPs films with the electrolyte were investigated by electrochemical impedance spectroscopy (EIS) and Tafel polarization plots. Intrinsic heterogeneous rate constant (k0) and effective electroactive surface area (Ae) of the catalytic films were determined by using a rotating disk electrode (RDE). The density functional theory (DFT) simulation results suggested that the tri-iodine molecule (I3) had the stronger adsorption energy (Eads) among the PtNPs. Adsorption energies of I3- on the PtNPs were estimated through CO stripping voltammetry, and the corresponding binding energies were obtained through XPS. Apparent Activation energies (Ea) of the films of CB/PtNPs were calculated from Arrhenius plots. With an optimized size of PtNPs (4nm), the pertinent DSSC showed a solar-to-electricity conversion efficiency (η) of 9.32±0.08% (Pt usage: 44.4μgcm-2), which was found to be higher than that of the cell with sputtered Pt (s-Pt) on its CE (η=8.43%, Pt usage: 64.4μgcm-2). © 2015 Elsevier Ltd.