Influence of Polymer Structure, Cation Type, and Substrate on Ionomer Thin Film Hydration Properties
Low carbon energy systems are undeniable solutions for addressing environmental and climate change issues facing the world. Hydrogen-fueled polymer electrolyte fuel cell (PEFC) are at the forefront of clean energy technology solutions since they are producing no particulate emissions and only water as a by-product at the point source (cars) and can improve air quality. Critical to their widespread adoption is the need for a reduction in the PEFC stack cost which is mainly attributed to the expensive Platinum (Pt) catalyst and development of new materials in catalyst layer. This work aims at investigating the effect of material (such as ionic polymer or ionomer type, substrate and contamination) and operational conditions (such as temperature and relative humidity, RH) on hydration behavior (water uptake and proton conduction) of ionomer thin film in catalyst layer, and then developing a correlation describing how those parameters effect hydration properties (water uptake and proton conductivity) of the ionomer thin film. The results of this study will enable the engineers to better optimize the performance of their produced PEFCs. In this work, we reported the water content and proton conductivity properties of thin-film ionomers (30 nm) at 80 °C over a wide range of relative humidity (0−90%) for seven different ionomers differing in the side-chain structure, including the number of protogenic groups, with the equivalent weight ranging from 620 to 1100 g/mol of sulfonic acid. The results show that the acid content or equivalent weight of the ionomer is the strongest determinant of both the swelling and the proton conductivity of ionomer films at a given relative humidity. The proton conductivity of low-equivalent-weight ionomers was higher than that of higher-equivalent-weight ionomers. Significantly higher values of both water content and proton conductivity are observed at 80 °C compared to those at 30 °C, implying that room temperature data are not reliable for estimating ionomer properties in the fuel cell catalyst layer. We also studied the impact of exchange of protons with cobalt ions on the humidity dependent (0−90% RH) hydration and conductivity of ∼30 nm thin ionomer films at a fuel cell-relevant temperature (80 °C). A significant suppression (up to 2 orders of magnitude at low RH) in ionic conductivity was observed for all ionomers upon exchange of protons with cobalt ions, evidently because the water content of the ionomer films decreases upon Co2+ exchange. The most interesting finding of the study is that a large variation in conductivity between the H+ form and Co2+ form of ionomer films at a given RH is significantly minimized when conductivity is correlated with the water content. Then we focused on how carbon and Pt substrate impact the 10 nm ionomer swelling rate under different RHs. It was found that films on Pt substrate, have higher swelling rate than those on carbon and SiO2 substrates. These results prove the evidence of better water network and higher proton conductivity in ionomers on Pt substrate, indicating that the interactions between ionomers and substrate affects internal structure of ionomers as well as the film surface especially in ultra thin films (< 10 nm). Moreover, water sorption studies on ionomer thin films shows that water absorption is slower than water desorption. It could indicate that the rate of water absorption controlled by the rate of interfacial transport and swelling while the desorption rate mainly controls by interfacial mass transport.
Polymer electrolyte fuel cell, Catalyst layer, Ionomer thin films, Hydration properties
Eskandari, H. (2023). Influence of polymer structure, cation type, and substrate on ionomer thin film hydration (Doctoral thesis, University of Calgary, Calgary, Canada). Retrieved from https://prism.ucalgary.ca.