Abstract
This study investigated the nanostructure and the stability of electrodes for applications in direct methanol fuel cells (DMFCs) and dye-sensitized solar cells (DSSCs) by modifying the surface and the interface of electrode materials. For improving the long-term methanol oxidation reaction (MOR) stability at DMFC anodes, three different amounts of Au (10%, 17%, and 26% to Pt in molar ratio) were deposited onto the surface of Ru-Pt core-shell nanocatalysts (Ru@Pt NCs) to suppress the surface oxidation of NCs. The results of amperometric test showed that 17% was the optimum Au deposition amounts for sustaining the long-term MOR stability. This 17% Au deposition sample (Ru@Pt@Au NCs 17%) could retain the MOR current density 2-fold higher than that of the Ru@Pt NCs after 2 h. The Au atoms were found to be intercalated at the low-coordinate sites on NC surface. The as-formed Au clusters (a few atomic layers) could suppress the surface oxidation on NCs by blocking the low-coordinate sites which could be easily oxidized otherwise. Moreover, these Au surface clusters could extract the valence electrons from Pt sub-layer atoms and preserve the negative charges at the clusters. Such negatively charged domains on NC surface could restrain the CO poisoning and hinder the electrical double layer (EDL) formation by repulsing the anionic ligands (e.g., O2-, OH-)/electronegative molecules (e.g., CO) and by suppressing the surface oxidation during MOR. This consequently improved the long-term MOR stability of NCs effectively. The comparisons between these NCs before and after amperometric tests indicated that the change in oxidation extent (decrease in Pt oxidized species) in Ru@Pt@Au NCs was lower than that in the Ru@Pt NCs. This implies that the Au surface clusters could stabilize the NC surface by preventing the surface Pt atoms from being oxidized during long-term MOR. These Ru@Pt@Au NCs were also employed as the cathodic catalysts for DSSCs. The results of cyclic voltammetric (CV) potential cycling showed that the catalytic current density and the long-term stability of Ru@Pt@Au NCs toward I3- reduction were higher than those of Ru@Pt and Pt NCs. Such enhancements were attributed to the electron relocation by Au surface clusters. The negatively charged Au domains on NC surface not only improved the electron transfer at catalyst/electrolyte interface but also hindered the iodine chemisorption on NC surface. To satisfy the efficient charge transfer at counter electrodes and thereby achieve the high photovoltaic performance of DSSCs, two structural modifications on the photoanode thin films by the sol-gel TiO2 (SG-TiO2) and the TiO2-coated Ag nanowires (AgNWs@TiO2) were implemented for improving the nanostructure and the interfacial properties of DSSC photoanodes. Both the TiO2 thin films modified with sol-gel based interconnecting network and the AgNWs/TiO2 composite films reduced the TiO2/dye/electrolyte interfacial charge transfer impedance, and thus enhanced the energy conversion efficiency of DSSCs by ~20% and ~13%, respectively. This study clarifies the morphology of NCs by Au surface modification and the corresponding impacts on the long-term MOR and I3- reduction reaction stability which offer the possibility for the development of chemically durable DMFCs and DSSCs.