Many efforts are being made worldwide to replace fossil fuels with greener alternatives. Hydrogen (H2) is a promising option that is currently in the spotlight; it can be used to generate electricity in fuel cells with water generated as the only byproduct. However, the technology is not quite ready for commercialization because proton-exchange membrane fuel cells, the most widely studied type, suffer from high cost and stability issues.
In contrast, anion-exchange membrane (AEM) fuel cells use cheaper catalysts and can offer superior performance. In these cells, hydroxide ions (OH-) are circulated instead of protons through the use of a polymer electrolyte composed of a polymer backbone and ion-conducting groups. One way to improve the properties of such electrolytes is by crosslinking -- physically or chemically linking polymer units to each other through molecular side chains.
Although oxygen-containing crosslinkers improve the stability and ion conductivity of AEMs by virtue of their hydrophilicity, or affinity for water, the effects of crosslinker length, which defines the number of oxygen atoms, are not understood in detail.
To gain deeper insight into this issue, scientists at Incheon National University recently carried out a study where they prepared long AEM polymers with ammonium ion-conducting groups and bound these molecules together using ethylene oxide (EO)-containing crosslinkers of various lengths. Through a wide variety of experiments, they compared AEMs with different crosslinker lengths in terms of their mechanical and thermal properties, water retention capacity, OH- ion conductivity, morphology, and stability. Their findings are published in the Journal of Membrane Science, a top journal in the field of polymer science.
The experiments helped the scientists elucidate the mechanisms by which excessive crosslinker length can ultimately degrade the performance of AEMs. Professor Tae-Hyun Kim, who led the study, explains: "Though it was easy to predict that oxygen-containing crosslinkers would increase hydrophilicity and possibly lead to better ionic conductivity, our results reveal that an excessively large number of repeating oxygen units increases the crystallinity -- or degree of order -- of the resulting material. In turn, this actually reduces hydrophilicity and ultimately compromises many physicochemical properties of the AEM."
After establishing the optimal length for their crosslinker, the researchers prepared an AEM fuel cell and found that the resulting performance was markedly better than when using AEMs without oxygen-containing crosslinkers. Excited about the results, Professor Kim comments: "The main takeaway from our study is that adding molecules with high water affinity, such as ethylene oxide, to crosslinkers of optimal length is a valid strategy to improve the fundamental properties of AEMs and their performance in actual fuel cells."
Although there is still room for improvement before AEM fuel cells can be effectively used in practice and commercialized, this study takes our society a step further towards the popularization of next-generation ecofriendly energy sources.
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