Fuel and Water Management in Direct Methanol Fuel Cells

Abstract

This study has designed and setup experiments to measure the performance of MEAs made from Johnson Matthey’s gas diffusion electrodes (anode: 3 mg/cm2 Pt and 1.5 mg/cm2 Ru; cathode: 1.25 mg/cm2 Pt). The polarization curves of MEAs with fuel and water management layers, which are porous materials with different wettability and mass transfer resistance, have been experimental measured with dilute methanol solutions (3M or less). In addition, the water crossover, and methanol crossover, as well fuel efficiency, energy efficiency have been estimated from changes of weight and concentration of methanol solutions before and after constant current discharge experiments. Results show that adding a fuel management layer between the methanol channel and the MEA can significantly reduce the water and methanol crossover rate and increase the fuel and energy efficiency. The net water flux across the electrolyte membrane could be less than the water flux caused by electro-osmotic drag flux. In other words, the water flux by diffusion and convection could be from cathode to anode when a fuel management layer is applied. However, the fuel management layer also reduces the water flux from the channel to the MEA and increases the ohmic resistance of the electrolyte membrane. A water management layer between the MEA and cathode channel is recommended to reduce the water loss through the cathode, reduce the water and methanol and water crossover rate, and increase the fuel efficiency. This water management layer does not deteriorate the water flooding since the operating current of methanol fuel cells (<0.5 A/cm2) are typically much lower than that of proton exchange membrane. The limiting current with or without the water management layer keeps almost the same. In addition, if a fuel management layer is applied to reduce the methanol crossover, a thinner electrolyte membrane is recommended because it requires less water to be hydrated and it has relatively low ohmic resistance.