A dual porosity carbon support for direct methanol fuel cells

Abstract

Carbons with well defined porous structures are synthesized using different types of mesoporous silcas as templates. The silica templates include SBA-15, SBA-16, FDU-12, and M41S, leading to carbon structures with different pore size and distribution corresponding to the templates. These carbons are used as supports for platinum-ruthenium nanoparticles and tested for activity towards methanol oxidation in a fuel cell electrode. The mixed metal nanoparticles are synthesized by the ethylene glycol method with good control of size and composition. The carbon structures as well as the silica parents were characterized by transmission electron microscopy (TEM), X-ray diffraction pattern (XRD) and nitrogen sorption analyses. Performance of the electrocatalysts is determined by polarization curves and AC impedance measurement. Attempts are made to correlate catalyst performance to the structural parameters of the carbon including pore size, pore distribution, surface area, and pore volume. It is shown that a single structural parameter such as surface area or pore volume cannot determine performance. The arrangement of the porous network at different length scales collectively determine performance especially at higher current densities when ionic and mass transport become important. A dual porosity structure synthesized with M41S silica of controlled size shows the best performance. This carbon has a meso-cellular foam like carbon structure composed of 30 to 50 nm porous cells with internal hexagonally arranged channels of 3 nm diameter. The smaller pores favor anchoring of PtRu nanoparticles whereas the larger channels of 13 nm between cells favor transport. The dual porosity network structure had a high surface area (> 1000m2/g) and large pore volume (> 1.2 mL/g). When loaded with Pt-Ru nanoparticles, the PtRu/carbon became a good electrocatalyst and gave higher activity for methanol oxidation compared to a commercial catalyst.