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

In modern society, the consumption of fossil fuels has been increasing drastically, leading to significant emissions of carbon
dioxide (CO2), air pollution, global warming, and political and economic imbalances. This has increased the interest in
renewable and sustainable energy sources such as wind- and hydropower, solar energy, and in energy conversion by fuel cell
technology, and water electrolyses. Fuel cell technology is considered attractive because it can be applied not only in stationary
applications such as power generation systems, but also in automotive applications. The fuel cell converts chemical energy into
electricity 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 as
flexibility in the fuel. Anion exchange membrane (AEM) is the core component in this fuel cell because it is responsible for the
hydroxide 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 counter
ions. During long-term operation of the fuel cell, the AEM is prone to be attacked, resulting in degradation of the ion
conductivity 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 be
synthesized 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 were
based 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-mediated
polyhydroxyalkylation reactions and atom transfer radical polymerizations (ATRPs) were employed to synthesize polymer
backbones with unique architectures. The introduction of the cationic quaternary ammonium (QA) groups was achieved by
Menshutkin 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 at
which the QA group is attached to the polymer backbone, have been investigated with respect to the properties mentioned
above.