Supported mixed metal nanoparticles and PFA-Nafion nanocomposite membrane for low temperature fuel cells

Abstract

Performance of low temperature fuel cells depends critically on the nanostructures of the material components in the electrodes and membranes. Some studies are reported here for 1) mixed metal nanoparticles supported on mesoporous carbon and 2) modification of nanopores of Nafion via in-situ polymerization of furfuryl alcohol. The anodic oxidation of small organic molecules such as alcohols in a low temperature fuel cell requires platinum based mixed metal electrocatalysts such as platinum-ruthenium. A number of techniques are now available for synthesizing mixed metal nanoparticles [1-3]. The challenge is to control both the size and composition of the mixed nanoparticles. The electrocatalysts are normally supported on activated carbon, such as Vulcan XC 72. A further challenge is to control the structure and properties of the support material. A number of novel carbon materials with well-defined nanostructures have been reported in the literature recently. We report here the synthesis of platinum and platinum-ruthenium nanoparticles supported on ordered mesoporous carbon CMK-3 [4]. The CMK-3 is synthesized from a template material SBA-15, an ordered mesoporous silica with uniform pores of a few nanometers in diameter. The mesoporous carbon supported Pt and Pt-Ru nanoparticles are characterized by HRTEM (Fig. 2) and EDX. The supported electrocatalysts are evaluated for electrochemical reduction of oxygen and electrocatalytic oxidation of methanol. A Pt-CMK-3 catalyst synthesized outperformed the commercial catalyst for oxygen reduction in a large area gas diffusion electrode. The Pt-Ru-CMK-3 catalyst did not perform as well, likely due to transport limitation in the long and narrow mesopores. Methanol crossing over from anode to cathode has been a major barrier to the development of direct methanol fuel cells. Studies have been made to modify the polymer electrolyte membrane, Nafion. A novel approach is reported here using in-situ polymerisation of furfuryl alchol [5,6]. The monomer is hydrophilic and penetrates the nanopores of Nafion. Upon polymerisation catalysed by sulphuric acid, the polymer becomes hydrophobic. This increase in hydrophobicity increases the barrier to methanol. Improvement of the membrane performance in a methanol fuel cell is reported here. The methanol permeability and proton conductivity are measured as a function of methanol concentration. The performance of a membrane-electrolyte assembly at room temperature shows marked increase in current and power compared to the unmodified membrane (Fig. 5).