Invited: Developing Alkaline Anion Exchanging Membrane Fuel Cells

To achieve sustainable development with limited natural resources and environmental concerns, fuel cells are expected to play an important role in providing clean power at high efficiency.  After past decades of intensive development, the proton conducting polymer electrolyte membrane fuel cells (PEMFCs) have made substantial advancement in cost, performance and stability.   As the results, some real commercial applications have been demonstrated around the world, such as in the combined heat and power generations, vehicle powers, stationary backup powers, and portable powers.  Still, the high cost, scarceness and performance stability of the highly dispersed platinum catalysts in the strong acid environment under dynamic fuel cell operation conditions remain as the major challenges for the PEMFCs to reach widespread mass applications.  To address these difficulties, alternative approach in developing alkaline anion exchanging membrane fuel cells (AEMFCs) has recently gained momentum.  In this presentation, we report our efforts in developing AEMFCs at Army Research Laboratory.  It covers our non-precious metal catalysts development, alkaline polymer electrolyte membrane characterization, fuel cell electrode optimization, and fuel cell tests.  The results of this study illustrate some of the key factors affecting the current AEMFCs performance, and the remaining challenges.

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.