Quaternized fluorene-based hydroxide exchange membranes and polymers: design, synthesis, and characterization

Abstract: In modern society, the consumption of fossil fuels has been increasing drastically, leading to significant emissions of carbondioxide (CO2), air pollution, global warming, and political and economic imbalances. This has increased the interest inrenewable and sustainable energy sources such as wind- and hydropower, solar energy, and in energy conversion by fuel celltechnology, and water electrolyses. Fuel cell technology is considered attractive because it can be applied not only in stationaryapplications such as power generation systems, but also in automotive applications. The fuel cell converts chemical energy intoelectricity with only water as a by-product. Anion exchange membrane fuel cells (AEMFCs) operate under basic conditions.They are undoubtedly considered the next generation fuel cell technology devices, due to their distinct advantages, for example,possibility to use non-noble metals as a catalysts for electrochemical reactions, faster oxygen reduction kinetics as well asflexibility in the fuel. Anion exchange membrane (AEM) is the core component in this fuel cell because it is responsible for thehydroxide transportation from cathode to anode electrolyte and it has a direct impact on the fuel cell performance and durability.The AEM consist of a solid polymer backbone, cationic groups tethered covalently to it, and hydroxide ions (OH–) as counterions. During long-term operation of the fuel cell, the AEM is prone to be attacked, resulting in degradation of the ionconductivity and efficiency of the cell. Therefore, the requirements for AEMs to be considered are high ion conductivity,excellent alkaline stability, and low cost. To reach these targets, novel and different polymer architectures have been to besynthesized and investigated.In the current work, ether-free polymer backbone structures functionalized with N-heterocyclic ammonium groups (NHAs)were synthesized and characterized as candidate membranes for fuel cell applications. Polymer backbone architectures werebased on fluorene units, which were tethered with mono- and spirocyclic quaternary ammonium groups via an alkyl spacer.Different synthetic methods, including alkylations and Suzuki coupling, were used for the monomer synthesis. Acid-mediatedpolyhydroxyalkylation reactions and atom transfer radical polymerizations (ATRPs) were employed to synthesize polymerbackbones with unique architectures. The introduction of the cationic quaternary ammonium (QA) groups was achieved byMenshutkin reactions. The membranes were characterized with regard to hydroxide conductivity, water uptake, morphology,and thermal and alkaline stability. The effects of both the polymer backbone structure and the QA structure, and the position atwhich the QA group is attached to the polymer backbone, have been investigated with respect to the properties mentionedabove.