(647c) Nanofiber Network Membranes for Fuel Cell Applications

New generation of proton conducting membranes for PEM fuel cell applications will combine the latest developments in both materials chemistry and membrane nanomorphology control. A critical challenge is to achieve high proton conductivity under low humidity fuel cell operating conditions. The most obvious way to enhance proton transport in sulfonic acid containing membrane polymers is to increase the sulfonation degree so as to maximize the number of protogenic carriers and the membrane's water retention capability, which are especially important for applications in hydrogen fuel cells. Unfortunately, this approach leads to problems with membrane dimensional/mechanical stability. Recent studies on multiblock sulfonic acid copolymers open up an interesting avenue of approach, but the use of block copolymers has limits imposed by the monomer/oligomer reactivity, block stoichiometry and the identification/availability of a good casting solvent.

An entirely new approach for fabricating fuel cell membranes has been developed by the present authors. The method can be universally applied to a wide range of ion/proton conducting materials. Briefly, a three-dimensional, interconnected network of proton-conducting polymer nanofibers is fabricated via electrospinning and then embedded in an inert/impermeable polymer matrix. The resulting nanomorphology is similar to that desired in a block copolymer material. The nanofiber network, occupying about 70% of the dry membrane volume, is composed of a high ion-exchange capacity (low equivalent weight) sulfonic acid polymer to ensure high water affinity and a high concentration of fixed charge acidic sites. The inert (hydrophobic) polymer matrix controls water swelling of the nanofibers and provides mechanical strength to the membrane. Unlike other fuel cell membranes, the role of the mechanical support is decoupled from that of the proton-conductor.

The talk will review the experimental details for nanofiber network membranes fabrication, where the nanofibers were composed of sulfonated poly(arylene ether sulfone) with/without molecular silica (sulfonated octaphenyl polyhedral oligomeric silsesquioxanes). The measured water swelling, proton conductivity, and thermal/mechanical properties of the resulting membranes will be discussed.