(417j) Proton Conductivity of Multi-Acid Ionomer Side Chains Under Confinement

Excessive carbon dioxide (CO₂) emission (75 Gt/yr in United States) in the atmosphere by road transportation elevates the greenhouse gas effect and poses a significant threat to the environment. Fuel cell is an energy conversion device which does not produce any CO₂ while producing electricity. Therefore, fuel cell is considered as a green technology. Proton exchange membrane fuel cell (PEMFC) uses hydrogen (H₂) as fuel to produce electricity and offers high energy density (9.17 MJ/L). Despite the high energy density, the high interfacial resistance at ionomer-catalyst interfaces of the electrodes results in a low power density (~40 W/kg) of PEMFC, which is not competitive to batteries (~120 W/kg). Ionomer layer at catalyst interface is below 30nm thick and at this thickness range, ion conductivity is often sacrificed due to poor water-polymer mobility. The poor interfacial ion conductivity in the thin ionomer layer leads to poor power density of fuel cell. There are several research efforts to design new ionomers for fuel cell application. However, very little is known about the impact of ionomer structure on the properties of ionomer films when those are several tens of nm thick. In this work, we studied some ionomers designed by 3M Inc. (825EW, PFIA) alongside current-state-of-the-art ionomer Nafion as a function of film thickness and relative humidity. We studied the local hydration environment and mechanical properties using fluorescence spectroscopy, quartz crystal microbalance and nanointendation. The results revealed that bis(sulfonyl)imide group conducting polymer, PFIA has the highest proton conductivity and the lowest hydration number in thin films while Nafion still shows the highest mechanical property. These works help to correlate mechanical property and ion conductivity with the structure of ionomers and will guide further ionomer development.