A new class of stable poly(ethylene-co-tetrafluoroethylene)-based alkaline anion-exchange membrane (AAEM) with enhanced tensile strength has been synthesized in response to the poor mechanical properties of previously reported poly(tetrafluoroethylene-co-hexafluoropropylene) radiation-grafted AAEMs; this type of AAEM exhibits significant through-plane conductivities (up to 0.034 ( 0.004 S cm -1 at 50 °C in water: conductivities that match requirements for application in fuel cells). The methanol permeabilities of this new AAEM class were found to be substantially reduced relative to Nafion-115 proton-exchange membranes; this offers the prospect that thin, low-resistance membranes may be used in direct methanol alkaline fuel cells with reduced methanol crossover. The fuel cell power performances obtained in a H 2 /O 2 single fuel cell at 50 °C with this AAEM is now within 1 order of magnitude of state-ofthe-art Nafion-based fuel cells. It is evident that the alkaline ionomers are not the primary performance limiters of alkaline membrane fuel cells; performances are currently limited by the electrode architectures that have been optimized for use in PEM fuel cells but not alkaline fuel cells. The need for electrodes and catalyst structures that have been specifically tailored for use in AAEM-containing fuel cells is highlighted.

Radiation-grafted alkaline anion-exchange membranes (AAEM) containing pendent groups with either benzyltrimethylammonium (BTM) or benzylmethylimidazolium (BMI) functionality were successfully synthesised from the same base membrane and with identical ion-exchange capacities. The conductivity of the new BMI-AAEM is comparable to the BTM-benchmark AAEM. The fuel cell performance obtained with the BMI-AAEM was, however, significantly poorer due to in situ AAEM degradation. FT-Raman spectroscopic studies on the stability of the two head-groups at 60 C in aqueous potassium hydroxide (1 mol dm À3) indicates that the BMI-group is intrinsically less chemically stable in strongly alkaline conditions compared to the BTM-benchmark head-group. However, the stabilities of both head-groups are improved when treated at 60 C in lower pH aqueous carbonate and bicarbonate solutions (1 mol dm À3). Contrary to a portion of the prior literature, there appears to be no real advantage in using anion-exchange polymer electrolytes containing pendent imidazolium groups in highly alkaline systems.

Despite over a century of study and decades of intensive research, few fuel cell products have appeared on the market. [1] The major inhibitor to mass commercialisation is cost. [2] H 2 /air alkaline fuel cells (AFCs) containing KOH(aq) electrolyte promise the lowest cost devices, [3,4] with the ability to use non-Pt catalysts. The fundamental problem with AFCs is that the KOH(aq) electrolyte reacts with CO 2 (cathode air supply) to form carbonate species, which lowers cell performance and lifetime (formation of carbonate precipitates in electrodes and reduction of OH -concentration in electrolyte). [4,5] However, the carbonate content of a aqueous-electrolyte-free (metal-cation-free) alkaline anion-exchange membrane (AAEM), that was pre-exchanged to the CO 3 2-form, decreased when operated in H 2 /air and methanol/air fuel cells. This remarkable result is contrary to prior wisdom; AAEMs inherently prevent carbonate performance losses when incorporated into AFCs. This experiment was made possible only by the recent breakthrough development of an alkaline interface ionomer, [6] which allows fabrication of membrane electrode assemblies that do not require incorporation of metal hydroxides species to perform well. [7] The widely perceived advantages of alkaline membrane direct alcohol fuel cells (AMDAFC) are the potential use of relatively inexpensive and abundant non-Pt electrocatalysts (as with the H 2 /air AFCs), [8][9][10] reduced alcohol crossover, [11,12] and enhanced electro-oxidation of high energy density alcoholic fuels. [13] However, metal hydroxides have traditionally been used as an additive, either in the electrode architectures [7] or in the aqueous alcohol supplies [11] due to the previous lack of an alkaline analogue to the perfluorosulfonic acid dispersions, [14] required for high-performance membrane electrode assemblies (MEAs) for proton-exchange membrane fuel cells (PEMFCs). Concerns persist about the effect of carbonate formation with such AMDAFCs. [15] The hypothesis that was tested in this study is that the tendency to form CO 3 2-can be reduced on the elimination of M n+ from AAEM-based solid alkaline fuel cells (SAFCs); precipitates of metal carbonates [4] cannot then form (the counter -N + R 3 cations are covalently bound to the polymer electrolyte analogous to the -SO 3 -counter anions in PEMs). The data presented in Table 1 compare the ex situ properties of the AAEM in CO 3 2-and OH -forms; the properties do not vary substantially. Importantly, the through-membrane conductivities at 30°C in a static relative humidity (RH) = 100% atmosphere were similar. The ionic performance of AAEMs would not be seriously compromised even on substantial formation of carbonate.Peak power densities of 37.9 ± 1.4 mW cm -2 were obtained in H 2 /air fuel cell tests with the AAEM MEAs in CO 3 2-form ( Figure 1); this was higher than the 32.9 ± 1.6 mW cm -2 obtained with the OH -benchmarks. The respective in situ cell resistances of 1.5 ± 0.2 Ω cm 2 and 1.7 ± 0.2 Ω cm 2 showed that there was only a small in...

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High performance benzyltrimethylammonium-type alkaline anion-exchange membranes (AEM), for application in electrochemical devices such as anion-exchange membrane fuel cells (AEMFC), were prepared by the radiation grafting (RG) of vinylbenzyl chloride (VBC) onto 25 μm thick poly(ethylene-co-tetrafluoroethylene) (ETFE) films followed by amination with trimethylamine. Reductions in electron-beam absorbed dose and amount of expensive, potentially hazardous VBC were achieved by using water as a diluent (reduced to 30 – 40 kGy absorbed dose and 5%vol VBC) instead of the prior-art method that used organic propan-2-ol diluent (required 70 kGy dose and 20%vol VBC monomer). Furthermore, the water from the aqueous grafting mixture was easily separated from residual monomer (after cooling) and was reused for a further grafting reaction: the resulting AEM exhibited an ion-exchange capacity of 2.1 mmol g-1 (cf. 2.1 mmol g-1 for the AEM made using fresh grafting mixture). The lower irradiation doses resulted in mechanically stronger RG-AEMs compared to the reference RG-AEM synthesised using the prior-art method. A further positive off-shoot of the optimisation process was the discovery that using water as a diluent resulted in an enhanced (i.e. more uniform) distribution of VBC grafts as proven by Raman microscopy and corroborated using EDX analysis: this led to enhancement in the Cl- anion-conductivities (up to 68 mS cm-1 at 80°C for the optimised fully hydrated RG-AEMs vs. 48 mS cm-1 for the prior-art RG-AEM reference). A down-selected RG-AEM of ion-exchange capacity = 2.0 mmol g-1, that was synthesised using the new greener protocol with 30 kGy electron-beam absorbed dose, led to an exceptional beginning-of-life H2/O2 AEMFC peak power density of 1.16 W cm−2 at 60°C in a benchmark test using industrial standard Pt-based electrocatalysts and unpressurised gas supplies: this was higher than the 0.91 W cm-1 obtained with the reference RG-AEM (IEC = 1.8 mmol g-1) synthesised using the prior-art protocol