Membranes for direct ethanol fuel cells
Fraunhofer Institute for Interfacial Engineering and Biotechnology
High efficiency, a long operating time thanks to energy-rich fuels and easy refilling make fuel cells the technology of the future for the powering of electrical appliances. Due to its high energy density and low toxicity, ethanol is the ideal fuel to give fuel cells mass-market appeal. Direct ethanol fuel cells (DEFCs) can convert ethanol electrocatalytically directly at the electrode (Fig. 1). A total of six Fraunhofer institutes are working together on developing this technology at both component and system levels. The role of the IGB is the development of innovative membranes for DEFCs.
Composite membranes for DEFCs
A significant challenge in developing a membrane for DEFCs is to avoid the loss of ethanol through the membrane. Together with protons, ethanol is transported through the membrane from the anode to the cathode (cross-over). This creates a mix-potential and reduces efficiency and performance. To minimize cross-over, we are developing composite membranes consisting of a polymer (sulfonated polyether-etherketone, sPEEK) and inorganic components (silica nanoparticles) (Fig. 2). These act as a barrier against ethanol loss while maintaining proton conductivity. Subsequent cross-linking of the silica particles enhances membrane stability.
- Fig. 3: Ethanol permeability coefficients determined by L-L diffusion.
- Fig. 4: DEFC demonstrator developed in the Fraunhofer DEFC joint project.
- Fig. 5: Performance of the DEFC with a total area of 40 cm2.
If tetraethoxysilane (TEOS) is added as a second inorganic component, the stability of the membrane further increases [1-3]. Hydrolysis and condensation of TEOS result in cross-linking of the silica particles, which reduces membrane swelling. Consequently, ethanol permeation through the composite membrane is lowered (Fig. 3). Using such membranes, membrane electrode assemblies (MEAs) and fuel cell stacks were prepared and characterized by our project partners. Our prototype for a direct ethanol fuel cell (Fig. 4) was presented at the “f-cell” forum . This DEFC produced a performance of 6.3 mW/cm2 (Fig. 5), while individual MEAs exhibited performances of over 9 mW/cm2.
Applications and perspectives
Sustainable and renewable raw materialsare of increasing importance, particularly due to their climate neutrality. Ethanol is already obtained on an industrial scale from sources such as sugar cane and has many applications as a liquid fuel. With the realization of DEFCs, the enormous investments associated with the wide-scale supply of hydrogen would become unnecessary, as the existing distribution network for liquid fuels could be used to a very large degree.
 K.S. Roelofs, A. Kampa, T. Hirth, T. Schiestel, The Behavior of Sulfonated Poly(Ether Ether Ketone) in Ethanol-Water Systems, J. Appl. Polym. Sci., 111(6), 2009, 2998.
 K.S. Roelofs, T. Hirth, T. Schiestel, Sulfonated Poly(Ether Ether Ketone) Based Silica Nanocomposite Membranes for Direct Ethanol Fuel Cells, J. Membr. Sci., 346(1), 2010, 215.
 C. Cremers, F. Jung, B. Kintzel, K.S. Roelofs, T. Schiestel, J. Tübke, Development of Direct Ethanol Fuel Cell Membrane Electrode Assemblies Using Sulfonated Polyetheretherketone Mixed-Matrix Membranes, ECS Trans., 25 (1), 2009, 1685.
 f-cell, 29.-30. September 2008, Stuttgart