Advanced membrane electrode assemblies for fuel and water management in direct methanol fuel cells

The direct methanol fuel cell (DMFC) is being developed as a mobile power source for portable electronic devices, such as laptop computers and cellular telephones. In these applications, where space is at a premium, DMFCs are seen as a good fit, due in large part to the high theoretical energy density of DMFCs, directly related to the liquid nature of methanol fuel. However, DMFCs suffer from a practical energy density far lower than the theoretical value. A significant factor leading to this discrepancy is the inability of DMFCs to directly and efficiently use concentrated fuel. Because methanol and water react on a one-to-one molar basis in the methanol oxidation reaction in the anode, and because three moles of water are produced by the oxygen reduction reaction in the cathode for every one mole consumed in the anode, fuel (methanol) and water management are intricately tied together. In the work presented, a 1D, two-phase computational model is used to first explain fundamentally this intricate coupling between fuel and water management, and specifically how it relates to proper membrane electrode assembly (MEA) design. Next, a theoretical explanation is given as to how the inclusion of a hydrophobic anode micro-porous layer (MPL) is effective in reducing water crossover to the cathode, which is a prerequisite for the use of more highly concentrated fuel. Following this, a novel MEA design is described, incorporating an anode transport barrier, which in conjunction with the anode MPL facilitates the direct and efficient use of concentrated methanol fuel. Finally, we show that the membrane selectivity, the ratio of the membrane's ionic conductivity to methanol permeability, traditionally used as a membrane figure of merit to predict performance, is not overly accurate. We demonstrate that a more inclusive figure of merit must also incorporate water transport characteristics of the membrane.