Membrane Permeability and Water Management in Alkaline Membrane Fuel Cells

We address in this paper fundamentals of water management in a hydroxide exchange membrane fuel cell ( HEMFC ), operated in hydrogen/air mode.  In such cell, the water product can be removed to large degree  with the air flow. This would make it unnecessary to remove a significant part of the product water through the anode exhaust and, thereby, prevent underutilization of hydrogen fuel.   However, such water management approach requires to use inlet air RH significantly lower than 1 and, consequently, risks insufficient water content in the cell cathode, resulting in poor cathode performance.

We describe in this talk possible solutions for combining “benign” removal of part of the product water from the HEMFC anode exhaust, while removing the other part of the product water with the air stream. A key facilitator in achieving such optimized water management scheme is the HEMFC membrane. The higher the water permeability of the HEMFC membrane, the easier is the implementation of this optimized scheme and, what counts here is the permeability under cell operation conditions.

When the HEMFC is under current and the air inlet stream under-saturated, the water content may vary strongly along the thickness of the membrane which is  in contact with liquid water on the anode side and water vapor of RH<1 on the cathode side. It is the latter interface which remains a significant challenge regarding  its full description , not to mention proper remediation. In the last two years,  several reports have shown that, for PEMs, a “skin” of inferior transport characteristics develops at the ionomer/gas interface when the RH of the gas is significantly lower than 1.  In the case of the HEMFC, such membrane skin formed next to the cathode could be a critical barrier to water transport into the cathode, with significant deleterious effect on cell performance.

Measurements of the relevant membrane characteristics and the water management options in the HEMFC will be described in this talk.