CONSPECTUS: Anion exchange membranes (AEMs) based on hydroxide-conducting polymers (HCPs) are a key component for anion-based electrochemical energy technology such as fuel cells, electrolyzers, and advanced batteries. Although these alkaline electrochemical applications offer a promising alternative to acidic proton exchange membrane electrochemical devices, access to alkaline-stable and high-performing polymer electrolyte materials has remained elusive until now. Despite vigorous research of AEM polymer design, literature examples of high-performance polymers with good alkaline stability at an elevated temperature are uncommon. Traditional aromatic polymers used in AEM applications contain a heteroatomic backbone linkage, such as an aryl ether bond, which is prone to degradation via nucleophilic attack by hydroxide ion. In this Account, we highlight some of the progress our group has made in the development of advanced HCPs for applications in AEMs and electrode ionomers. We propose that a synthetic polymer design with an all C−C bond backbone and a flexible chain-tethered quaternary ammonium group provides an effective solution to the problem of alkaline stability. Because of the critical demand for such a polymer system, we have established new synthetic strategies for polymer functionalization and polycondensation using an acid catalyst. The first approach is to graft a cationic tethered alkyl group to pre-existing, commercially available styrene-based block copolymers. The second approach is to synthesize high-molecular-weight aromatic backbone polymers using acid-catalyzed polycondensation of arene monomers and a functionalized trifluoromethyl ketone substrate. Both strategies involve a simple two-step reaction process and avoid the use of expensive metal-based catalysts and toxic chemicals, thereby making the synthetic processes easily scalable to large industrial quantities. Both polymer systems were found to have excellent alkaline stability, confirmed by the preservation of ion exchange capacity and ion conductivity of the membrane after an alkaline test under conditions of 1 M NaOH at 80−95 °C. In addition, the advantage of good solvent processability and convenient scalability of the reaction process generates considerable interest in these polymers as commercial standard AEM candidates. AEM fuel cell and electrolyzer tests of some of the developed polymer membranes showed excellent performance, suggesting that this new class of HCPs opens a new avenue to electrochemical devices with real-world applications.

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Quaternized polymers as electrode ionomeric binders enable fuel cell operation under high-pH or anhydrous conditions. Herein we report quaternized poly­(fluorene) ionomers with controlled hydrophobicity (contact angle of electrodes with the ionomers 109–164°) by changing the length of tethered fluoroalkyl chains. The anion-exchange membrane fuel cell employing the hydrophobic ionomer exhibits improved durability (voltage loss 0.41 mV h–1) through better water management. The high-temperature proton-exchange-membrane fuel cell using the ionomer shows superior H2/air performance (1.7 A cm–2 at 0.4 V). The finding in this study highlights the benefits of hydrophobic ionomers for emerging fuel cell applications.

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.

The reliance on antibiotics and antimicrobials to treat bacterial infectious diseases is threatened by the emergence of antibiotic resistance and multi-drug-resistant organisms, thus having the potential to greatly impact human health. Thus, the discovery and development of antimicrobials capable of acting on antibiotic-resistant bacteria is a major area of significance in scientific research. Herein, we present the development of a eumelanin-inspired antimicrobial capable of killing methicillin-resistant Staphylococcus aureus (MRSA). By ligating quaternary ammonium-functionalized "arms" to a eumelanin-inspired indole with intrinsic antimicrobial activity, an antimicrobial agent with enhanced activity was prepared. This resulting antimicrobial, EIPE-1, had a minimum inhibitory concentration of 16 μg/mL (17.1 μM) against a clinical isolate of MRSA obtained from an adult cystic fibrosis patient. The biocidal activity occurred within 30 min of exposure and resulted in changes to the bacterial cell surface as visualized with a scanning electron microscope. Taken together, these studies demonstrate that EIPE-1 is effective at killing MRSA.

Novel organic electrically conductive organic fibers (ECFs) have been fabricated using a facile, economical and scalable technique by staining nonconductive fibers (both natural and synthetic) with a conductive ink composed of two intrinsically conductive materials, i.e., single walled carbon nanotubes (SWCNTs) and regioregular poly­(3-hexylthiophene) (rr-P3HT). These organic ECFs exhibit low resistance of 0.50 kΩ cm–1 with a conductive ink composed of 0.8 mg/mL of SWCNTs and 1.6 mg/mL of P3HT while maintaining the mechanical properties of the original fibers. These organic ECFs were characterized by resistance measurements, Raman spectroscopy, scanning electron microscopy, transmission electron microscopy and stress–strain measurements. Finally, the recording properties of the organic ECFs were examined by both electromyography and electrocardiography in terms of the signal-to-noise ratio, which was found to be similar and/or exceeded the data obtained by standard metal electrodes.

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