Invited: Developing Alkaline Anion Exchanging Membrane Fuel Cells
Non-precious metal development was focused on producing high performance catalysts for oxygen reduction reaction (ORR) at low cost. Catalysts prepared with high temperature heat treatment of the transition metals embedded in nitrogen containing polymer matrix supported on carbon have demonstrated comparable mass activity to that of Pt-based catalysts, in addition to their superior performance stability and contamination tolerance under the alkaline conditions.
Some key properties of an anion exchanging polymer electrolyte membrane (AEPEM) measured include ion-exchanging capacity, water uptake, anion conductivity, electro-osmotic drag coefficient, methanol permeation rate, and CO2 permeation rate. These properties are compared to the proton exchanging membrane to illustrate the limitations of current AEPEM.
With current available AEPEM and ionomer materials, fuel cell electrode optimization was conducted by varying the ionomer content in the catalyst layer. Compared to Nafion ionomer used for PEMFCs, the current ionomer for the AEMFCs was made from hydrocarbon polymer, which has a lower ionic conductivity, a higher water uptake and a lower polymer density. All these properties limit the fuel cell electrode performance in term of catalyst layer ionic conductivity, catalyst utilization, and reactant gas mass transport. The optimized fuel cell performance has demonstrated to achieve a power density at 300 mW/cm2, which is still much lower than that achieved by PEMFCs. The performance in AEMFCs limitation could be overcome by developing a new generation ionomers with improved properties.