Proton exchange membranes (PEMs) play a critical role in many electrochemical devices that could solve the shortcomings of current energy storage and conversion systems. Hydrocarbon-based PEMs are an attractive alternative for replacing the state-of-the-art perfluorosulfonic acid PEMs; however, synthetic routes are generally limited to sulfonation of aromatic units (pre- or postpolymerization functionalization). Here we disclose a facile and scalable one-pot synthetic method of converting an alkyl halide functionality to a sulfonate in polymer systems. With this method, sulfonated hydrocarbon PEMs can be conveniently prepared from a precursor polymer of anion exchange membranes which have recently experienced significant advances. Polyphenylene type PEMs (BPSA and mTPSA in this report) were generated in one-pot SN2 reaction of bromoalkyl side chains of polymers followed by oxidation. These PEMs showed excellent proton conductivity with BPSA showing 250 mS/cm in water at 80 °C, nearly 1.5 times higher than that of Nafion 212. Furthermore, the separation of the sulfonic acid group from the rigid backbone with a flexible alkyl chain mitigates excessive water uptake and in-plane swelling ratio of the polymer, despite having a high ion exchange capacity of 2.6 mequiv/g. Oxidative stability was also shown to be superior for hydrocarbon-based PEMs with negligible changes in mass, NMR, and proton conductivity.
Proton exchange membranes (PEMs) are a crucial component to many electrochemical energy conversion devices, including low temperature hydrogen fuel cells and water electrolyzers. Perfluorosulfonic acid-based PEMs have overwhelmingly been the material of choice for these applications however, they are not ideal due to high cost, low glass transition temperature, and lack of synthetic diversity leading to unoptimized material performance. Hydrocarbon PEMs on the other hand have been explored as alternatives to perfluorosulfonic acid materials, although in general have lower proton conductivity, excessive water sorption and poor oxidative stability. Despite a large body of works focusing on hydrocarbon PEMs, most are based on aryl ether containing polymers such as poly(aryl ether sulfone). Recently, the PEM research community has shown that wholly aromatic polymer systems exhibit improved oxidative stability and restrict water uptake, allowing for higher ion exchange capacities. The synthetic routes of these polymer systems, however, are limited to sulfonation of aromatic unit through hash sulfonating conditions leading to ill-defined polymers or through complicated multistep synthetics routes, often requiring transition metal catalysts. Herein, we report widely applicable synthetic methods and exceptional properties of hydrocarbon based PEMs without any aryl ether linkages. To achieve this PEMs were synthesized from materials bearing a haloalkyl side chains. Since, anion exchange membranes (AEM) are commonly synthesized from haloalkyl bearing polymers and the polymer property requirements of PEMs and AEMs are similar, (i.e. no aryl ether linkages, high IEC, dimensional stability, ect.) this ushers in a new generation of stable hydrocarbon PEM materials from the vast library of stable AEMs. Furthermore, the synthetic procedures are based on simple SN2 and oxidation chemistry requiring no metal catalysts or harsh reaction condition. The resulting polymers show excellent conductivity and restricted water uptake at higher temperatures due to improved phase separation stemming from the attaching sulfonate group to a tethered side chain of polymer. Additionally, in highly oxidative Fenton’s reagent test, no discernable changes to the polymer performance of structure was observed.
The irreversible environmental impacts caused by energy generation from fossil fuels calls for a paradigm shift in the energy conversion and storage. Among many alternative energy technologies hydrogen based electrochemical devices gain significance because of its high efficiency and environmentally benign byproduct formation. Ion exchange polymers play a key role in electrochemical devices as solid-state electrolytes conducting ions between the electrodes while acting as a physical barrier to reactants and electrons. Electrochemical reactions carried out under high pH medium in fuel cells and electrolyzers using anion exchange membranes (AEMs) have gained significant traction recently, due to the prospect of using non-platinum group metal electrocatalyst for the redox reactions. While AEMs remain a crucial part of electrochemical devices such as alkaline exchange membrane fuel cells and alkaline electrolyzers, the absence of a suitable membrane is an impediment for the widespread usage of these technologies. Recently our group synthesized several anion exchange polymers via super acid catalyzed Friedel-Crafts polycondensation. These AEMs, BPN1-100, m-TPN1-100 and p-TPN1-100 showed excellent stability in alkaline medium due to the absence of labile aryl-ether bonds in the polymer backbone. Additionally, they also exhibit good ionic conductivities and mechanical properties. In order to further understand the effects of polymer backbone morphology in AEM properties another polymer FMN1-100 was synthesized. In this study, properties of the AEMs BPN1-100, m-TPN1-100, p-TPN1-100, and FMN1-100 (Figure 1) were evaluated and compared for the purpose of anion exchange polymer electrolytes. Furthermore, FMN2-60 and FMN2-50 containing dication side chains were synthesized and compared against FMN1-100 containing a monocation side chain. The effect of the backbone architecture and side chain cations on the AEM properties such as, ionic conductivity, water uptake ratio, mechanical properties, morphology, and alkaline stability were investigated to broaden the knowledge of polymer design for AEMs. Figure 1
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