Polyhydroxyalkylations for alkali-stable cationic polymers as hydroxide exchange membranes

Abstract: To alleviate worldwide dependence on fossil fuels for energy production and considerably reduce CO2 emissions, green energy sources and sustainable energy solutions are in high demand. Fuel cells are environmental-friendly electrochemical devices which may use clean fuels, e.g., green hydrogen gas, to generate electricity. Meanwhile, water electrolyzers provide an efficient strategy to produce green hydrogen without emitting greenhouse gases. Research on fuel cells and electrolyzers, especially on the component level, is currently progressing rapidly. Applying a solid electrolyte instead of liquid one potentially simplifies the cell operation and has motivated intensive research on suitable ion-conductive polymer electrolytes. Ion exchange membranes are fabricated from polymers functionalized with ionic groups. To date, devices employing proton exchange membranes (PEMs) are already commercialized, or on the brink of broad commercialization, but are rather expensive because of the requirement to use noble metal catalysts under acidic conditions. Shifting to alkaline operating conditions allows the potential use of non-platinum-group catalysts, e.g., silver or nickel, making the alkaline devices cost-effective alternatives. Still, these devices cannot fully compete with the PEM-based ones yet because the components such as the required hydroxide exchange membrane (HEM) do not yet have the necessary performance and durability.At present, two major issues with the HEMs are their insufficient lifetime and conductivity, which closely affect the life span and power output of the devices. The aim of the present thesis work was to prepare HEMs with high chemical stability and hydroxide conductivity. Using superacid-mediated polyhydroxyalkylation reactions, several series of different aromatic polymers functionalized with alkali-stable cations were synthesized and studied. HEMs were prepared from the polymers, and investigated in terms of water uptake, morphology, ion conductivity, thermal stability, and with a special emphasis on the alkaline stability. Changes of the HEMs’ molecular structure from chemical degradation after severe alkaline treatments were carefully studied and the degradation mechanisms were elucidated.Overall, most of the studied HEMs reached high hydroxide conductivities, approaching 180 mS cm−1 at 80 °C. To enhance the high alkaline stability of the HEMs, different strategies to vary the polymer/cation arrangements were employed. The combined results especially showed that attaching the cations to a rigid polymer backbone via flexible spacer units is highly beneficial to protect the HEM from hydroxide ion attack.