Ion-containing block copolymers hold promise as next-generation proton exchange membranes in hydrogen and methanol fuel cells. These materials’ self-assembled ordered nanostructures facilitate proton transport over a wide range of conditions, a requirement for robust fuel cell performance. In this perspective, we will present an overview of the morphology and transport properties of ion-containing block copolymers that have been studied to gain insight into the fundamental behavior of these materials and, in some cases, are targeted toward applications in fuel cells and other electrochemical devices. We will discuss the challenges associated with predicting and obtaining well-ordered morphologies in block copolymers with high ion content, particularly those with chemistries that can withstand the chemical and mechanical stresses of the fuel cell, such as aromatic backbone block copolymers. New opportunities for ion-containing block copolymers in alkaline membrane fuel cells will also be reviewed.

A molecular-level understanding of dynamics in imidazolium-based ionomers with different counterions and side chain lengths was investigated using X-ray scattering, oscillatory shear, and dielectric relaxation spectroscopy (DRS). Variations of the counterion size and side chain length lead to changes in glass transition temperature (T g ), extent of ionic aggregation, and dielectric constant, with consequences for ion transport. A physical model of electrode polarization is used to determine the number density of simultaneously conducting ions and their mobility. Imidazolium-based ionomers with larger counterion and longer side chain have lower T g , resulting in higher ionic conductivity and mobility. The ionic mobility is coupled to ion motions that are directly measured as a second segmental process in DRS, as these are observed to share the same Vogel temperature. Time−temperature superposition (tTS) was applied to create linear viscoelasticity master curves and to investigate the delay in chain motion related to ionic associations. tTS works well for these materials, and the terminal relaxation time increases with decreasing side chain length and smaller counterion size. X-ray scattering confirms the extent of ionic aggregation and helps to rationalize the observed dielectric constants. Larger counterions or longer side chains diminish ionic aggregation, and their ionomers have higher dielectric constants, which agree reasonably with the Onsager prediction at all temperatures studied. Smaller counterions or shorter side chains promote ionic aggregation, and their ionomers have lower dielectric constants, which are directly reflected in the lower content of simultaneously conducting ions.

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The direct methanol fuel cell (DMFC) has the potential to replace lithium-ion rechargeable batteries in portable electronic devices, but currently experiences significant power density and efficiency losses due to high methanol crossover through polymer electrolyte membranes (PEMs). Numerous publications document the synthesis and characterization of new PEMs for the DMFC. This article reviews this research, transport phenomena in PEMs, and experimental techniques used to evaluate new PEMs for the DMFC. Although many PEMs do not show significant improvements over Nafion 1 , the benchmark PEM in DMFCs, experimental results show that several new PEMs exhibit lower methanol crossover at similar proton conductivities and/or higher DMFC power densities. These results and recommendations for future research are discussed.

We thoroughly investigate and quantify the chemical stability of an imidazolium-based alkaline anion exchange polymerized ionic liquid (PIL), poly(1-[(2-methacryloyloxy)ethyl]-3-butylimidazolium hydroxide) (poly(MEBIm-OH), over a broad range of humidities, temperatures, and alkaline concentrations using the combined techniques of electrochemical impedance spectroscopy and nuclear magnetic resonance spectroscopy. High chemical stability was observed under dry conditions (10% RH) at 30 °C, humid and saturated conditions up to 80 °C, and even in mild alkaline conditions ([KOH] < 1 M) at 25 °C. Degradation was only observed under more vigorous conditions: dry conditions (10% RH) at 80 °C or at higher alkaline concentrations ([KOH] > 1 M). Under these conditions, we suggest an imidazolium ring-opening mechanism as the primary degradation pathway, based on a detailed analysis of the 1 H NMR spectra. Similar to poly(MEBIm-OH), other alkaline anion (carbonate (CO 3 2À) and bicarbonate (HCO 3 À )) exchange PILs were also synthesized in this study via salt metathesis of the PIL precursor, poly(1-[(2-methacryloyloxy)ethyl]-3-butylimidazolium bromide) (poly(MEBIm-Br)). The thermal and ion conductive properties of each PIL in this study were characterized. The ionic conductivity of the hydroxide conducting PIL, poly(MEBIm-OH), was the highest of these PILs investigated at 9.6 mS cm À1 at 90% RH and 30 °C with an Arrhenius activation energy of 17.1 kJ mol À1 at 90% RH.