Thermo-Stable Non-PFSA Cation-Conductive Fluoropolymers for Medium-Temperature PEMFCs

Medium-temperature PEMFCs operated at a temperature between 100 and 180 ^oC can be used as an alternative to phosphoric acid fuel cells—avoiding the leach and loss of phosphoric acid electrolytes from the cells during fuel cell operations.  At an elevated temperature, Pt catalysts are less sensitive to carbon monoxide poisoning—allowing the use of low-quality hydrogen fuels produced from reforming processes.  However, the thermal stability of perfluorosulfonic acid (PFSA, e.g., Nafion^TM) membranes and PFSA catalyst binders is the bottleneck problem for raising the operation temperature of PEMFCs above 100 ^oC.  It was reported that Nafion^TM membranes can only have a lifespan of several hundred hours under humidified conditions at 130 ^oC.

Recently, we designed, synthesized and tested a group of non-PFSA fluoropolymers as cation-conductive electrolytes via a novel C-C coupling reaction.  Heteroaromatic groups can be covalently attached to the side chains of a PFSA—with no acid or base labile sulfonyl groups.  The protonated form of such fluoropolymers can transport protons and can be fabricated into thin films.  An example of non-PFSA polymers examined in our laboratories is shown in the enclosed figure.  Many of non-PFSA fluoropolymer membranes showed ion conductivities, mechanic strengths, and electrochemical properties, compatible to Nafion^TM membranes, but with much improved thermal stabilities.  Some preliminary test results of a prototype membrane are shown in table 1:

Table 1.  Comparison of our prototype membrane vs. commercial N-117 membrane.

	Thickness (µm)	In-Plane Conductivitya (in de-ionized water, 25 oC)	IEC (meq/g)	Water uptake (%,25 oC)
N-117	183	77 mS/cm	0.9	17
Our Prototype	193	71 mS/cm	0.7	18

*Four-electrode AC impedance measurements.

Most remarkably, our prototype membrane showed much improved thermal stability against N-117.  Within three hours, a commercial N-117 membrane quickly lost its ion conductivity in a heated water bath of 150 ^oC in a pressure reactor, while our prototype membrane showed no sign of the reduction of its conductivity after five months.