Recent development in polyolefin-based anion exchange membrane for fuel cell application

Abstract

Anion exchange membrane (AEM) plays a critical role in many environmental and energy devices and processes, such as fuel cell, redox flow battery, and electrodialysis. In particular, when polymeric AEMs are exploited in alkaline fuel cells to replace liquid electrolytes, AEM fuel cells (AEMFC) have attracted worldwide attention because of several inherent advantages over proton exchange membrane fuel cells, e.g. the utilization of less expensive metallic catalysts and enhanced kinetics of oxygen reduction. Among different kinds of AEMs, polyolefin-based AEMs showed great potential for large-scale commercialization because of their excellent chemical stability, easy processability, and low cost. The strategies to synthesize polyolefin anion exchange membrane materials included direct (co)polymerization with functionalized monomers and post-modification of polyolefins. Generally, the ring-opening metathesis polymerization and Ziegler-Natta catalyst mediated polymerization techniques have been successfully employed to synthesized polyolefin AEMs, using the functional monomers including the α-olefin, norbornene, and cyclooctene. Therefore, the chemical structure and composition can be readily tuned to achieve the optimized properties of the resulting AEMs. However, the functional monomers that need complex synthetic procedure have greatly hampered its further commercialization. Radiation-grafting polymerization of vinylbenzyl chloride (VBC) on the polyolefin backbone has been confirmed as another effective method to produce AEMs. Both (partially or fully) fluorinated and hydrocarbon-based polyolefin have been grafted with poly(VBC) under high energy radiation (such as commercial electron-beam accelerators or 60Co γ-ray facilities), and subsequent quaternization allow the synthesis of AEMs with different ionic head-group. In order to produce high-performance AEMs, high irradiation doses were used to get high degree of grafting, which in turn results in a detrimental reduction in the mechanical properties of the resulting AEMs. Therefore, it is necessary to optimise the conditions of grafting reactions. The physicochemical properties of polyolefin-based AEMs are investigated in detail by the measurement of ion exchange capacity, water uptake, swelling ratio, and ionic conductivity, which are well corelated to their chemical structure and microstructure. Moreover, H2/O2 AEMFC assembled from polyolefin-based AEMs (especially for radiation-grafted AEMs) are tested under various conditions in terms of types of electrochemical catalysts, cell temperature, gas flow rate, and so on. Thus, this review summarized the recent developments of polyolefin-based AEMs from the synthetic methods to the properties of AEMs and their performance in H2/O2 fuel cell. The relationship between chemical structure, properties and morphology of polyolefin-based AEMs will be discussed. At last, we extend the discussion of AEMs to their performance in H2/O2 fuel cell, as dramatic improvements of AEM fuel cell performance is accomplished when using high- performance polyolefin-based AEMs.