Danilczuk, Marek, Shulamith Schlick, and Mariana Pinteala
Perfluorinated ionomers such as Nafion, Aquivion and 3M membranes exhibit excellent mechanical, chemical, and thermal stability in both oxidative and reductive media, and their fuel cell (FC) performance is close to current DOE requirements for automotive applications. However, some problems remain, among them high cost, low oxygen reduction rate at the low pH, and high gas crossover, hydrogen to the cathode and oxygen to the anode. These drawbacks led to intense activity in the research and development of alternative, nonfluorinated, proton-exchange membranes (PEMs). Due to their properties and low cost, poly(arylene ether)-based sulfonated polymers have been considered as potential candidates for FC applications . We present a study of the chemical stability of sulfonated poly(ether ether ketone) (SPEEK), deduced by a comparison of ex situ experiments under Fenton conditions , and in situ experiments in a fuel cell inserted in the resonator of the electron spin resonance (ESR) spectrometer [3,4]. The formation of radicals was monitored by spin trapping ESR, with 5,5-dimethyl-1-pyrroline N-oxide (DMPO) as the spin trap. The magnetic parameters and the relative intensities of DMPO-adducts were determined by simulation of the ESR spectra. The major adducts detected in the in situ experiment were DMPO/OOH and DMPO/H . The generation of the DMPO/OOH adduct at the anode was explained by reaction of crossover oxygen with hydrogen atoms formed at the Pt catalyst; at the cathode this adduct can be generated by hydrogen crossover to the cathode, reaction at the Pt catalyst, and reaction with oxygen. No membrane-derived carbon-centered or oxygen-centered radical adducts were detected in the in situ FC. However, in SPEEK membranes directly exposed to hydroxyl radicals in the presence of the spin trap DMPO, the DMPO/Ph and DMPO/OPh adducts were detected , indicating poor chemical stability of SPEEK in the ex situ Fenton test; here Ph and OPh are, respectively, phenyl and phenoxyl radicals. The results presented in this study indicate that hydrocarbon membranes such as SPEEK demonstrate different chemical stabilities in the two tests.