Chemical and Ionic Conductivity Degradation of Ultra-Thin Ionomer Film by X-ray Beam Exposure

Abstract

X-ray based techniques have been adopted as a diagnostic tool for both ex-situ and in-situ material characterization techniques in PEMFC. Possible damage of Nafion due to X-ray irradiation is a concern especially for thin films. To investigate the X-ray irradiation effect on ultra-thin Nafion film structure and properties, supported 10 nm films were prepared on SiO2/Si substrate with inter-digitated gold arrays (IDA) and exposed to X-ray radiation at three X-ray power sources: 300, 200 and 100 W for 45 mins. Proton conductivity of each film was measured by electrochemical impedance spectroscopy before and after X-ray exposure. We determine that X-ray radiation damages the polymer by preferential removal of sulfur species from the film that results in a clear loss of proton conductivity. Damage by 300, 200, and 100 W radiation over 45 minutes removes 78, 74 and 54% of the sulfur present, respectively, yielding a loss of 99, 97 and 95% in the proton conductivity of the film. It is suggested that Nafion degradation takes place by not only the removal of the sulfonic group, but also by the removal of a fraction of fluorine consistent with cleavage at the ether group of its side chain.