chapter  21
Proton-Conducting Membranes for Fuel Cells
Pages 48

Polymer electrolyte-based fuel cells are emerging as attractive energy conversion systems suitable for use in many industrial applications, starting from a few milliwatts for portables to several kilowatts for stationary and automotive applications. The ability of polymer electrolyte membrane fuel cells (PEMFCs) to offer high chemical to electrical fuel efciency and almost zero emissions in comparison to today’s prevailing technology based on internal combustion engines (ICEs) makes them an indispensable option as environmental concerns rise.1-6

Although the basic principles of fuel cells have been known for at least a century, the introduction of solid polymer electrolyte membranes a few decades ago revolutionized fuel cell technology. Initially, poly(styrenesulfonic acid) (PSSA) and sulfonated phenol-formaldehyde membranes were used, but the useful service life of these materials was limited because of their tendency to degrade in fuel cell operating conditions.7,8 A critical breakthrough was achieved with the introduction of Naon®, a peruorinated polymer with side chains terminating in sulfonic acid moieties, which was invented in the 1960s for the chlor-alkali industry at DuPont. This material and its close peruorosulfonic acid (PFSA) relatives are currently the state of the art in PEMFCs. PFSA-based membranes have good proton conductivity, high chemical and mechanical stability, high tear resistance, and very low gas permeability in fuel cell operating conditions.9,10

But some problems associated with PFSA-based membranes have precluded large-scale market adoption of fuel cells. Their relatively high cost, limits to the range of temperature over which they can be reliably used (the upper limit is considered to be somewhat above 100°C, because the glass transition temperature is around 120°C; at higher temperatures >100°C, membranes have low water content and thus low proton conductivity), faster oxidative degradation and faster deterioration in mechanical properties at elevated temperatures, and a stringent requirement for external humidication of reactant gases under these conditions make the fuel cell balance of a plant more complicated. Additionally, for liquidphase direct methanol fuel cells (DMFCs), the PFSA membrane is permeable to methanol and water, whose presence on the cathode side seriously degrades the DMFC performance.