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Advanced perfluorinated anion exchange membrane polymers and their issues in electrochemical conversion devices
Divekar, Ashutosh G.
Divekar, Ashutosh G.
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2020
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Abstract
After decades of dedicated efforts in research and development of the polymer electrolyte membranes for electrochemical conversion devices, the technology is nearing their large-scale commercialization. Improvements like utilization of thin mechanically supported membranes (< 15 μm), advanced Pt catalysts with enhanced activity have made the proton-exchange membrane class of polymers very attractive for vehicular fuel cell and other electrochemical conversion applications. However, with the increasing energy demand for the rapidly growing population high performing commercial devices with non-precious catalysts need further attention. Anion exchange membrane polymers perfectly fit this description due to its compatibility with the cheaper electrochemical catalysts. In this work, the potential of novel perfluorinated anion exchange membranes primarily for low-temperature fuel cell applications were tested. Three iterations of polymer membranes with a PTFE backbone were electrochemically, physiochemically, and morphologically characterized to conclude that the six-carbon alkyl spacer chain is the most promising candidate with a high ionic (OH-) conductivity. Ex-situ characterization of this class of polymers was performed to understand the interaction of the hydroxyl charge carrier with the atmospheric CO2 in the ambient air as in a commercial fuel cell device, ambient air is used as an oxidant. It was concluded that the CO2 not only interacts with the ionic domains of the polymer but also hampers the crystallinity of the backbone which could potentially lead to mechanical failures while operating for longer durations. From the knowledge gained from this study, a standard fuel cell device was tested to report the highest air-fed anion exchange membrane fuel cell performance to date (446 mW cm-2). For the first time, the segmented fuel cell hardware was used to understand the spatial differences in the anion exchange membrane fuel cell performance due to the variation in humidification, fuel or oxidant starvation and the durability issues. Over several days of operation, it was found that the cell degrades primarily in the feed inlet section due to difference in the hydration or water accumulation over the channel length. FTIR analysis was performed to prove that the chemical functionality of the membrane changes due to the fuel cell operation. The catalyst-ionomer interface was investigated using polymer dispersion spin-coated on model Si and Ag substrates. From the grazing incident x-ray scattering study, phenomenon like parallel polymer chain alignment with respect to the surface at a higher ionomer thickness and their variation with hydration and type of substrate was investigated. With increasing thickness, the film formation undergoes two transition regimes: formation of crystalline polymer domains followed by intra-molecular alignment of CF2 units within the polymer chain. It was also found that the silver surface is interacting strongly with the polymer. From the knowledge gained, it is recommended to design the catalyst inks with lower ionomer content so that the parallel alignment of the polymer chains is limited to mitigate the mass-transport limitations. This work can serve as a guide to design higher performing catalyst inks, optimal conditions of water management, and ambient air operation to produce higher-performing fuel cell devices. However, learnings can also be applied to other electrochemical devices like water-splitting, electro-dialysis, CO2 reduction, and lead-air batteries. Understanding of the CO2 limitations from this work could also help in designing CO2 sequestration devices containing anion conducting ionic liquids.
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