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There is a critical need for higher performing proton exchange membranes for electrochemical energy conversion devices that would enable higher temperature and drier operating conditions to be utilized. A novel approach is to utilize multiacid side chains in a perfluorinated polymer, maintaining the mechanical properties of the material, while dramatically increasing the ion-exchange capacity; however, as we show in this paper, the more complex side chain gives rise to unexpected physical phenomena in the material. We have thoroughly investigated a doubly functionalized perfluorosulfonic imide acid side chain perfluorinated polymer (PFIA), the simplest of many possible multiacid side chains currently being developed. The material is compared to its simpler perfluorosulfonic acid (PFSA) analogue via a battery of characterization and modeling investigations. The doubly functional side chain profoundly influences the properties of the PFIA polymer as it gives rise to both inter- and intraside chain interactions. These affect the nature of thermal decomposition of the material but, more importantly, force the backbone of the polymer into an unusually highly ordered more crystalline configuration. Under water saturated conditions, the PFIA has the same proton conductivity as the PFSA material, indicating that the additional proton does not contribute to the ionic conductivity, but the PFIA shows higher proton conductivity at lower RH conditions owing to dynamic changes in its local molecular environment. A transition is observed between 30 and 60 °C, indicating an order/disorder transition that is not present in the PFSA analogue. The mechanism of proton transport in the PFIA is due to more delocalized protons and more flexible side chains with better-dispersed, smaller water clusters forming the hydrophilic domains than in the PFSA analogue.

Herein, we report the anion and water transport properties of an anion-exchange membrane (AEM) comprising a block copolymer of polyethylene and poly(vinylbenzyl trimethylammonium) (PE-b-PVBTMA) with an ion-exchange capacity (IEC) of 1.08 mequiv/g. The conductivity varied little among the anions CO32–, HCO3–, and F–, with a value of Ea≈ 20 kJ/mol and a maximum fluoride conductivity of 34 mS/cm at 90 °C and 95% relative humidity. The Br–conductivity showed a transition at 60 °C. Pulsed gradient stimulated spin echo nuclear magnetic resonance (PGSE NMR) experiments showed that water diffusion in this AEM is heterogeneous and is affected by the anion present, being fastest in the presence of F–. We determined the methanol self-diffusion in this membrane and observed that it is lower than that in Nafion 117, because of the lower water uptake. This article reports the first measurements of 13C-labeled bicarbonate self-diffusion in an AEM using PGSE NMR spectrometry, which was found to be significantly slower than F–self-diffusion. Back-calculation of the bicarbonate conductivity using the Nernst–Einstein equation gave a value that was significantly lower than the measured value, implying that bicarbonate transport involves OH–in the transport mechanism. Fourier transform infrared spectroscopy, PGSE NMR spectrometry, and small-angle X-ray scattering (SAXS) indicated the presence of different types of waters present in the membrane at different length scales. The SAXS data indicated that there is a water-rich region within the hydrophilic domains of the polymer that has a temperature dependence in intensity at 95% relative humidity (RH).

Anion exchange membranes (AEMs) contain cations that act as dipoles. The dipoles interact with each other and give rise to clustering in the AEM and this clustering can affect the ionic conductivity and mechanical properties of the polymer. Polymers designed to have dispersed cations can, when wet, have their cations cluster as the material undergoes an order-disorder transition at some temperature at which the system has enough energy to reorganize. Clustering is often unavoidable in AEMs and imposes a limit on the increase in conductivity that can be achieved by increasing IEC. Above a certain IEC AEMs do not show increased ionic conductivity only increased swelling on exposure to higher RH. Understanding these phenomena is essential to designing high performance AEMs. Here we propose an AEM based on a scalable random copolymer with rapidly deployed photo-crosslinking to lock the morphology in place to avoid cation clustering.

A chemically stable copolymer [poly(2,6 dimethyl 1,4 phenylene oxide)-b-poly(vinyl benzyl trimethyl ammonium)] with two ion exchange capacities, 3.2 and 2.9 meq g[?]1, was prepared as anion exchange membranes (AEM-3.2 and AEM-2.9). These materials showed high OH[?] conductivities of 138 mS.cm[?]1 and 106 mS.cm[?]1, for AEM-3.2 and AEM-2.9 respectively, at 60degC, and 95% RH. The OH[?] conductivity = 45 mS.cm[?]1 for AEM-3.2 at 60% RH and 60degC in the absence of CO2. Amongst the ions studied, only OH[?] is fully dissociated at high RH. The lower Ea = 10-13 kJ.mol[?]1 for OH[?] compared to F[?] [?] 20 kJ.mol[?]1 in conductivity measurements, and of H2O from self-diffusion coefficients suggests the presence of a Grotthuss hopping transport mechanism in OH[?] transport. PGSE-NMR of H2O and F[?] show that the membranes have low tortuosity, 1.8 and 1.2, and high water self-diffusion coefficients, 0.66 and 0.26 x 10[?]5 cm2.s[?]1, for AEM-3.2 and AEM-2.9 respectively. SAXS and TEM show that the membrane has several different sized water environments, ca. 62 nm, 20 nm, and 3.5 nm. The low water uptake, l = 9-12, reduced swelling, and high OH[?] conductivity, with no chemical degradation over two weeks, suggests that the membrane is a strong candidate for electrochemical applications.

We have studied anion exchange membrane/polycationic systems with different percentages of fluoride and chloride as counterions by molecular dynamics simulations. We also experimentally measured the self-diffusion constant of fluoride in a diblock copolymer that has the same hydrophilic block and found satisfactory agreement with simulations within a factor of 2. At 300 K, our simulations showed that the self-diffusion constant of fluoride increases by about 70% when fluoride content decreases from 100% to 40% (and 60% Cl), and it increases by about 140% when fluoride content decreases from 100% to 10%. Increasing % Cl also slightly decreases the attraction between fluoride and the cations. We hypothesize that the root cause of the enhancement in fluoride mobility is due to the larger size of the chloride ion, which more readily loses its water solvation shells because of a lower charge/radius squared (surface electric field). This in turn frees up more water for ion transport. We believe this effect is likely more general than just the fluoride/chloride case reported here.

Anion exchange membranes (AEMs) contain cations that act as dipoles. The dipoles interact with each other and give rise to clustering in the AEM and this clustering can affect the ionic conductivity and mechanical properties of the polymer. Polymers designed to have dispersed cations can, when wet, have their cations cluster as the material undergoes an order-disorder transition at some temperature at which the system has enough energy to reorganize. Clustering is often unavoidable in AEMs and imposes a limit on the increase in conductivity that can be achieved by increasing IEC. Above a certain IEC AEMs do not show increased ionic conductivity only increased swelling on exposure to higher RH. Understanding these phenomena is essential to designing high performance AEMs. Here we propose an AEM based on a scalable random copolymer with rapidly deployed photo-crosslinking to lock the morphology in place to avoid cation clustering.

A chemically stable copolymer [poly(2,6 dimethyl 1,4 phenylene oxide)-b-poly(vinyl benzyl trimethyl ammonium)] with two ion exchange capacities, 3.2 and 2.9 meq g−1, was prepared as anion exchange membranes (AEM-3.2 and AEM-2.9). These materials showed high OH−conductivities of 138 mS.cm−1and 106 mS.cm−1, for AEM-3.2 and AEM-2.9 respectively, at 60°C, and 95% RH. The OH−conductivity = 45 mS.cm−1for AEM-3.2 at 60% RH and 60°C in the absence of CO2. Amongst the ions studied, only OH−is fully dissociated at high RH. The lower Ea= 10–13 kJ.mol−1for OH−compared to F−∼ 20 kJ.mol−1in conductivity measurements, and of H2O from self-diffusion coefficients suggests the presence of a Grotthuss hopping transport mechanism in OH−transport. PGSE-NMR of H2O and F−show that the membranes have low tortuosity, 1.8 and 1.2, and high water self-diffusion coefficients, 0.66 and 0.26 × 10−5cm2.s−1, for AEM-3.2 and AEM-2.9 respectively. SAXS and TEM show that the membrane has several different sized water environments, ca. 62 nm, 20 nm, and 3.5 nm. The low water uptake, λ = 9–12, reduced swelling, and high OH−conductivity, with no chemical degradation over two weeks, suggests that the membrane is a strong candidate for electrochemical applications.

Anion Exchange membranes (AEMs) are being developed for a variety of applications. Among these are fuel cells and electrolyzers that require the AEM to conduct hydroxide anions. Measurement of the ionic conductivity of hydroxide is challenging because the hydroxide is a strong nucleophile and attacks polar groups in the AEM and also rapidly reacts with atmospheric CO2to form the less conductive carbonate and bicarbonate counter anions. We have developed a rigorous method to fully exchange AEMs into the hydroxide form while excluding CO2and remove all residual inorganic salt from the film prior to measurement. These measurements of the hydroxide ion conductivity coupled with self diffusion measurements of water and the similarly sized fluoride anion in these films are beginning to shed insight into how hydroxide anion transport can be maximized in state of the art AEMs.

Extensive transport and modeling studies have been performed on a series of polymers based on the benzyltrimethylamonium cations. We have created a realistic reactive model of solvated hydroxide that has interesting properties in the polymer matrix. In ordered di-block polymers we have strong theoretical evidence for a heterogeneous distribution of water across the channel. Fluoride is used as a non-reactive surrogate for hydroxide to increase the computational efficiency of transport calculations that can be performed. In addition to these studies we are discovering new base stable processable robust polymer backbones and cations. These bulky cations seem to dramatically affect the transport properties of the anions showing a VTF rather than an Arrhenius behavior with temperature.

Understanding ion transport in Anion Exchange Membranes (AEMs) has been a widely researched area. In this work we have utilized the method of 19F diffusion NMR to our advantage for this purpose. Fluoride ion and hydroxide ion have similar ionic sizes which makes it as an ideal replacement for the caustic anion. In particular we have studied the effect of temperature and increasing diffusion times (Δ) on self diffusion of fluoride ions. We have also made a unique effort at measuring bicarbonate and carbonate diffusion through AEMs here.