Methods of Hydroxide-Ion Form Anion Exchange Membrane Characterization

Abstract

Alkaline fuel cells (AFCs) are of high interest because they utilize lower cost catalyst and separator materials relative to polymer electrolyte membrane fuel cells. AFCs require anion exchange membranes (AEMs) that have high alkaline durability and high hydroxide ion conductivity. Herein, numerous AEMs from top research groups in the field are evaluated and compared as hydroxide conducting solid state electrolytes. While the hydroxide-anion form of AEMs is the most relevant anion form for alkaline fuel cell and electrolysis applications, it remains difficult to evaluate hydroxide form membranes as they quickly convert to carbonate- and bicarbonate-anion forms upon exposure to air. NREL has developed protocols to allow for high throughput, parallel characterization of AEM materials in hydroxide-anion form. This work assesses six optimized membranes with varied backbone chemical structures: perfluoro (PF), poly(aryl ether sulfone) (PAES), polyethylene (PE), polyfluorene (PFN), polystyrene (PS), polyphenylene (PPN). Methods are established for evaluating true hydroxide-ion properties of these AEMs at varying temperature and relative humidity, including conductivity, durability, and water uptake. Standardized comparisons of the top performing hydroxide conducting AEMs of different backbone types provides valuable insight into optimal chemistries for achieving high conductivity and durability.