Using spin trapping electron spin resonance for determining the degradation mechanism of membranes used in fuel cells

Abstract

Fuel cells (FCs) convert the chemical energy from the reaction of H₂ with O₂ to electrical energy, and have become an alternative clean energy source for automotive, portable and stationary applications. FC operation is possible when the membrane located between the electrodes maintains its integrity in the oxidising FC environment. Spin trapping electron spin resonance (ESR) has been used for determining the degradation mechanism of the perfluorinated membranes used in FCs. The study of low molecular weight model compounds confirmed two possible degradation mechanisms in membranes: initiated at the backbone and at the side chain. In situ experiments in a FC inserted in the resonator of an ESR spectrometer offered the ability to monitor radical processes in a FC. The presence of the radicals was determined by addition of 5,5-dimethy-l-pyrroline N-oxide (DMPO) as a spin trap. Taken together, the in situ results pointed to crossover processes, reactions at the catalyst surface, and the involvement of H^• atoms in attack on the membrane; these processes cannot be visualised in ex situ studies. Therefore different membrane degradation mechanisms in the two types of experiments can be expected. The stability of the DMPO/OH adduct was increased by complexation with cavitands such as β-cyclodextrins and cucurbiturils.

Keywords:

• Electron spin resonance (ESR) spectroscopy
• spin trapping ESR
• in situ fuel cell
• membrane electrode assembly (MEA)
• gas crossover

Acknowledgements

This study was supported by the Polymers Program of the National Science Foundation, by the General Motors Electrochemical Energy Research Lab, and by Ford Motor Co.