Does power ultrasound affect hydrocarbon Ionomers?

Abstract

The effect of low-frequency high-power ultrasound on hydrocarbon-based ionomers, cation exchange sulfonated phenylated polyphenylene (sPPB-H+) and anion exchange hexamethyl-p-terphenyl poly(benzimidazolium) (HMT-PMBI), was studied. Ionomer solutions were subjected to ultrasonication at fixed ultrasonic frequencies (f = 26 and 42 kHz) and acoustic power (Pacous = 2.1 – 10.6 W) in a laboratory-grade ultrasonication bath, and a probe ultrasonicator; both commonly employed in catalyst ink preparation in research laboratory scale. Power ultrasound reduced the polymer solution viscosity of both hydrocarbon-based ionomers. The molecular weight of sPPB-H+ decreased with irradiation time. Changes in viscosity and molecular weight were exacerbated when ultrasonicated in an ice bath; but reduced when the solutions contained carbon black, as typically used in Pt/C-based catalyst inks. Spectroscopic analyses revealed no measurable changes in polymer structure upon ultrasonication, except for very high doses, where evidence for free-radical induced degradation was observed. Ionomers subjected to ultrasound were used to prepare catalyst layers and membrane electrode assemblies (MEA)s. Despite the changes in the ionomer described above, no significant differences in electrochemical performance were found between MEAs prepared with ionomers pre-subjected to ultrasound and those that were not, suggesting that fuel cell performance is tolerant to ionomers subjected to ultrasound.