Alkaline exchange membrane (AEM) fuel cells are an alternative to more conventional proton exchange membrane (PEM) fuel cells and electrolyzers. In recent years, anion conducting polymer membranes have achieved stability and ionic conductivity competitive with Nafion. Lightly cross-linked AEMs based on vinyl addition polymerized norbornene have record-high hydroxide conductivity (212 mS/cm) with <1% performance degradation over 1,200 hours in 1 M NaOH at 80 °C. Composite versions of these AEMs have also been used to construct membrane electrode assemblies for alkaline fuel cells with a peak power density of 3.4 W/cm², 70% higher than the previous record for this type of system to date.

The presence of carbon dioxide in air, or carbonate in the electrolyzer water feed impacts device performance because of the high pH of the membrane. Carbon dioxide in the fuel cell air stream poses a practical limitation for large scale deployment of alkaline fuel cells. AEM fuel cell tests are often operated with oxygen or CO₂-free air to avoid the complications of carbonate formation, which inherently lowers conductivity and the overall fuel cell performance. The focus of this talk is on the transport of hydroxide, bicarbonate and carbonate in poly(norbornene) membranes. The transport properties of CO₂ have been characterized and relationships between the diffusivity and solubility of CO_2­ in the polymer membrane in parallel with the in situ flux of carbonate ions through the AEM have been studied.

Financial support provided by the ARPA-E IONICS program for G.H. and M.M. is gratefully acknowledged.