Does power ultrasound affect hydrocarbon Ionomers?

Area of Science:

• Polymer Science
• Electrochemistry
• Materials Science

Background:

• Hydrocarbon-based ionomers are crucial for fuel cell membranes and catalyst layers.
• Understanding the impact of processing techniques like ultrasonication on ionomer properties is essential for optimizing fuel cell performance.
• Current catalyst ink preparation methods may involve ultrasonication, necessitating an evaluation of its effects.

Purpose of the Study:

• To investigate the effects of low-frequency, high-power ultrasound on the properties of cation and anion exchange hydrocarbon-based ionomers.
• To determine how ultrasonication influences ionomer viscosity, molecular weight, and structural integrity.
• To assess the impact of pre-ultrasonicated ionomers on the performance of membrane electrode assemblies (MEAs) in fuel cells.

Main Methods:

• Studied sulfonated phenylated polyphenylene (sPPB-H+) and hexamethyl-p-terphenyl poly(benzimidazolium) (HMT-PMBI) ionomers.
• Applied ultrasonication using a bath and probe sonicator at 26 and 42 kHz with acoustic power ranging from 2.1 to 10.6 W.
• Analyzed changes in viscosity and molecular weight, and employed spectroscopic methods to evaluate polymer structure. Prepared catalyst layers and MEAs for electrochemical testing.

Main Results:

• Power ultrasound reduced the viscosity of both ionomer solutions.
• The molecular weight of sPPB-H+ decreased with increasing irradiation time.
• Spectroscopic analysis showed no significant structural changes except at very high doses, indicating potential free-radical degradation. Ultrasonication in an ice bath exacerbated changes, while carbon black presence reduced them.
• MEAs fabricated with ultrasonicated ionomers exhibited no significant difference in electrochemical performance compared to controls.

Conclusions:

• Ultrasonication significantly alters ionomer solution viscosity and molecular weight, with effects modulated by temperature and additives like carbon black.
• Despite structural and property modifications, the electrochemical performance of fuel cells is tolerant to ionomers subjected to ultrasonication during catalyst ink preparation.
• These findings suggest that ultrasonication can be a viable processing step in catalyst ink preparation without compromising fuel cell efficiency.