The design and development of anion exchange membranes for hydrogen fuel cells

Hydrogen fuel cells are an essential component of hydrogen economy, which have been advocated as a major path towards the decarbonization of the energy sector. Within the two main types of low-temperature hydrogen fuel cells, anion-exchange membrane fuel cells (AEMFCs) are potentially more cost-effective and more scalable than proton-exchange membrane fuel cells (PEMFCs). This is because the former may use platinum-group metal (PGM)-free catalysts, more affordable bipolar plates, inexpensive hydrocarbon-based membranes and ionomers. However, AEMFCs are still at an early stage of development and one main obstacle is the lack of suitable anion exchange membranes (AEMs). There are three scientific challenges associated with the design of AEMs: poor conductivity, insufficient chemical stability and the trade-off between conductivity and dimensional stability. Only few reported AEMs have sufficient conductivity and chemical stability and they are all subjected to the trade-off limitation. Therefore, developing new AEMs with all the desirable properties is of great importance. Herein, this dissertation will present my PhD research results in design, preparation, characterization and application of AEMs for AEMFCs.Chapter 1 outlines the motivation of this dissertation. Current scientific challenges in AEM development and our methodologies are explained. Chapter 2 conducts a comprehensive review of the latest researches on AEMs.Chapter 3 introduces a facile synthetic strategy to branched poly(aryl piperidinium)s (PAP) AEMs. The optimized b-PTP-2.5 membrane simultaneously exhibits high OH- conductivity (> 145 mS cm-1 at 80 oC) and dimensional stability, as well as excellent chemical stability during both ex situ and in situ tests. The membrane is flexible and easy to process. The present AEMFC based on b-PTP-2.5 membrane has one of the topmost peak power density (PPD) of 2.3 W cm-2 and can fully operate over 500 h. To the best of our knowledge, this was the first time an AEMFC based on a PAP-type AEM exhibits in situ durability longer than 500 h. This work demonstrates branched poly(terphenyl piperidinium) AEMs as promising membranes for applications in AEMFCs. The strategy of branching might be applicable to the development of other types of AEMs.Chapter 4 explores a fluorination strategy to create a phase-separate morphological structure in PAP AEMs. The highly hydrophobic perfluoroalkyl side chains augment phase separation to construct interconnected hydrophilic channels for OH- transport. As a result, these fluorinated PAP (FPAP) AEMs simultaneously possess high conductivity (> 150 mS cm-1 at 80 oC), high dimensional stability (swelling ratio < 20% at 80 oC), excellent mechanical properties (tensile strength > 80 MPa and elongation at break > 40%) and chemical stability (> 2000 h in 3M KOH at 80 oC). AEMFC with a non-precious Co-Mn spinel cathode using the present FPAP AEMs achieves an outstanding peak power density of 1.31 W cm-2. More significantly, it could operate stably over 500 h at 0.6 V and 0.2 A cm-2. This is the first reported AEMFCs with a PGM-free cathode that exhibits durability over 500 h. The fluorinated poly(aryl piperidinium)s might also be promising anion exchange ionomers (AEIs) for applications in AEMFCs.Chapter 5 summarizes our research works and gives a future outlook of AEMs developments.