Abstract
Direct methanol fuel cells (DMFCs) are attracting much more attention as a power source in portable electronic devices due to their advantages such as environment-friendly, high power density, easy handling of fuel, and low operating temperature. The core of a DMFC is the membrane electrode assembly that in general comprises a thin flat proton exchange membrane electrolyte with catalyst layers (in general, carbon-based material supported platinum (Pt) or platinum-ruthenium (Pt-Ru) nanoparticles with a mixture of proton conducting ionomer) bonded to both sides. The catalyst layers are conventionally pasted on the electron-conducting carbon materials such as carbon papers or carbon cloths. In order for a DMFC to operate more efficiently, the nature of catalysts with a smaller particle size and better dispersed characteristics is readily considered. One way to enhancing the dispersion of the catalysts is to construct catalyst supports with nanostructure of relatively high surface areas to volume (or mass ratio). Carbon nanotubes (CNTs) that can bear a high and accessible surface area through a deliberately engineered process have been shown a promising candidate supporting material as the catalyst layer of a low temperature fuel cell. In addition to the preparation of a more effective carbon supporting material, a novl method of depositing well-dispersed nano-sized Pt or Pt-Ru catalyst particles is also essential. Carbon monoxide poisoning on Pt or Pt-Ru catalysts surface during methanol oxidation is an issue of equal important to be addressed in pursuing the high performance catalyst layer for DMFC applications. Besides, another challenge for the commercialization of DMFC is to reduce the precious metal loading of the catalyst layers for cost-reduction without jeopardizing the fuel cell efficiency.In this study, for the simultaneous accessibility of catalysts to the fuels, the electron-conducting diffusion layer and the proton-conducting electrolyte, we employed a novel method to directly grow the CNTs on carbon cloths as the electron-conducting and supporting materials of the catalysts. Then, an improved electrodeposition technique to deposit Pt and Pt-Ru nanoparticles on the surfaces of these CNTs was adopted. Through an extensive effort on the parametric study of the electrodeposition process, including temperature, deposition period, deposition sequence of Pt and Ru, and molar ratio between the electrolyte and the metal precursor salts, a better dispersed nano-sized Pt and Pt-Ru catalysts electrodeposited on the surfaces of the CNTs was achieved when the ethylene glycol (EG) contained deposition electrolyte was adopted. The methanol oxidation efficiency of Pt and Pt-Ru/CNT electrodes analyzed by cyclic voltammetry (CV) revealed that the methanol oxidation efficiency of the Pt-Ru/CNT electrodeposited in ethylene glycol containing H2SO4 aqueous solutions was better than that of commercialized Pt-Ru/C electrode. The structural and composition characteristics of Pt/CNT and Pt-Ru/CNT electrodes were also analyzed by scanning electron microscopy, transmission electron microscopy, X-ray diffractometer, X-ray adsorption near-edge spectroscopy, and inductively coupled plasma mass spectroscopy. Furthermore, the power density of a DMFC using Pt-Ru/CNT as the anode was ~65% greater than that of another DMFC with a commercial Pt-Ru/C anode, clearly indicating a significantly improved catalytic activity of the new Pt-Ru/CNT electrode.