Durable Polysulfones with Densely Sulfonated Segments for Highly Proton Conducting Membranes

University dissertation from Media-Tryck, Lund

Abstract: Fuel cells are considered as a promising alternative environmentally benign power source to the existing ones depending on petroleum oil. Especially, proton-exchange membrane fuel cells (PEMFCs) have been intensively developed during the last decade because of their wide application windows including various portable, stationary and transportation applications. The market has been and will be mainly driven by the automotive manufacturers that announced their commercialization of fuel cell vehicles in 2015. A key for success of the fuel cell market is the performance, durability and cost of the PEMFC components. Importantly, polymer membrane plays a key role at the heart of the system to decide much of the performance and durability of PEMFCs. State-of-the-art perfluorosulfonic acid (PFSA) membranes such as Nafion® developed by DuPont are today the most widely employed membrane material in PEMFC applications, because of their high performance and durability. However, PFSA membranes typically show some limitations for commercialization, and the main drawback are the high material cost and environmentally unfriendliness due to their perfluorinated structure. This has motivated an intensive research over the past decade for alternative membrane based on inexpensive and environmentally-friendly materials. The thesis work focused on extensive study of the preparation of hydrocarbon (HC) membranes with high performance and durability, based on the engineering polymers with high thermal/chemical/mechanical stabilities combined with low cost and environmental friendliness. The first part of the thesis gives a general description of fuel cells, a review of the current development of HC polymers and a brief summary of the the six research articles whose details are covered by the second part of the thesis. The important synthetic strategies applied in the thesis work are 1. the use of fully aromatic backbones based on polysulfones, 2. The incorporation of densely sulfonated units in the polymers, 3. The placement of the acidic groups in ortho-to-sulfone positions. The first point is to enhance the characteristic features of the HC polymers in relation to PFSAs. The second is to improve the proton conductivity and dimensional stability. In addition, the last is to give the most chemical stable acidic group among the HC polymers. In summary, some of the polymers obtained in the thesis gave quite comparable proton conductivities to that of PFSAs. One outstanding finding is that the polymers showed a higher proton conductivity than that of PFSAs at cold climate. The polymers were also found to possess a very high resistance towards the decomposition of the acidic groups. Finally, a new concept, microblock copolymers, was suggested as potential candidates for ion-exchange membrane applications towards commercialization because of the cost performance and straight-forward preparation, in addition to the performance and durability.

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