Development of new membrane materials for methanol fuel cells

Abstract

Development of new membrane materials for direct methanol fuel cells Direct methanol fuel cells (DMFCs) can convert the chemical energy of a fuel directly into electrical energy with high efficiency and low emission of pollutants. DMFCs can be used as the power sources to portable electronic devices like laptop computers, cellular phones and, to a less degree, vehicular applications. In order to compete with Li-ion batteries for portable applications higher power densities must be achieved. For that reason DMFCs should be operated at high methanol concentrations. The proton exchange membrane (PEM) is the key component of the DMFC. Currently Perfluorosulfonated ionomer (PFSI) membranes, like DuPont’s Nafion® and Asahi Chemical’s Aciplex®, are used as a PEM in DMFCs due to their excellent proton conductivity, mechanical strength and thermal and chemical stability. However, their high methanol cross over, especially at high methanol concentrations and, high cost (US$700/m2) due to the expensive fluorination step, severely limit their commercialization in fuel cells. Therefore there is a strong need to develop new polymeric membranes. To achieve this, various strategies were employed: • Impregnation of conductive polymer into a porous support to obtain composite membranes with low methanol crossover and long term stability • Incorporation of inorganic fillers into the bulk phase of conductive polymer to increase the proton conductivity and at the same time decrease the methanol cross-over • Surface micro-structuring of the conductive polymer by hot embossing to increase the effective catalytic surface area without increasing the geometric one. Impregnated membranes have high dimensional stability and low methanol cross-over leading to a higher DMFC performance at low and high methanol concentrations. Membranes with inorganic fillers have lower methanol permeability and higher proton conductivity than the pure membranes. The membrane with the highest zeolite content (5wt.%) has the best performance; namely high power density and stable performance in time with low fluctuations. Micro-structured membranes have less water content and methanol flux and at the same time similar resistances in comparison to the flat membranes. Owing to these properties, micro-structured membranes performed the best in the DMFC system.