Anion-exchange membrane fuel cells face two challenges: performance and durability. Addressing the first, we demonstrate high performance with both O2 and CO2-free air supplies, even when using a Ag/C cathode. This was enabled by the development of a radiation-grafted anion-exchange membrane that was less than 30 μm thick when hydrated.

This article provides an up-to-date perspective on the use of anion-exchange membranes in fuel cells, electrolysers, redox flow batteries, reverse electrodialysis cells, and bioelectrochemical systems (e.g. microbial fuel cells). The aim is to highlight key concepts, misconceptions, the current state-of-the-art, technological and scientific limitations, and the future challenges (research priorities) related to the use of anion-exchange membranes in these energy technologies. All the references that the authors deemed relevant, and were available on the web by the manuscript submission date (30th April 2014), are included.

A novel alkaline exchange ionomer (AEI) was prepared from the radiation-grafting of vinylbenzyl chloride (VBC) onto poly(ethylene-co-tetrafluoroethylene) [ETFE] powders with powder particle sizes of less than 100 μm diameter. Quaternisation of the VBC grafted ETFE powders with trimethylamine resulted in AEIs that were chemically the same as the ETFE-based radiation-grafted alkaline anion exchange membranes (AAEM) that had been previously developed for use in low temperature alkaline polymer electrolyte fuel cells (APEFC). The integration of the AEI powders into the catalyst layers (CL) of both electrodes resulted in a H2/O2 fuel cell peak power density of 240 mW cm−2 at 50 °C (compared to 180 mW cm−2 with a benchmark membrane electrode assembly containing identical components apart from the use of a previous generation AEI). This result is promising considering the wholly un-optimised nature of the AEI inclusion into the catalyst layers.

Recent reports that the reaction between Nafion sulfonyl fluoride precursor and the cyclic diamine 1,4-dimethylpiperazine yields stable anion-exchange membranes appear to be premature. On aqueous work up, membranes with high cation-exchange capacities and zero anion-exchange capacities are produced.

Radiation-grafted anion-exchange membranes (AEM) containing pendent benzyltrimethylammonium, 1-benzyl-3-methylimidazolium and 1-benzyl-2,3-dimethylimidazolium functional head-groups were synthesised with ion-exchange capacities in the range 1.7–1.9 meq g−1. The ionic conductivities of the AEMs were also comparable (24.5 ± 1.8 mS cm−1 at 50 °C). The alkali stability (in aqueous potassium hydroxide (1 mol dm−3) at 60 °C) of the 1-benzyl-2,3-dimethylimidazolium head-groups was superior to the 1-benzyl-3-methylimidazolium but inferior to the benzyltrimethylammonium benchmark head-groups. Radiation-grafted AEMs containing pendent 1-benzyl-2,3-dimethylimidazolium head-groups are not suitable for application in electrochemical devices containing highly alkaline environments.

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−3) 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−3). 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.

A simple aqueous-processable alkaline ionomer (amenable to scale-up) has been developed for enhancing electrode/electrolyte interfaces in clean energy devices (e.g. alkaline polymer electrolyte membrane fuel cells). The water uptake of the alkaline ionomer is tuneable allowing its use as a tool for fundamental studies into these interfaces.

Alkaline anion-exchange membranes (AAEMs) are being developed for metal-cation-free solid alkaline fuel cells. Reduced solvent uptakes were observed after immersion in methanol, ethanol and ethylene glycol relative to a Nafion®-115 proton-exchange membrane (PEM); this translated directly into lower alcohol permeabilities. Alkaline polymer electrolytes showed lowered degrees of swelling (membrane thickness), when immersed in methanol and ethanol, relative to Nafion-115. The open circuit voltages, VOCV, of the corresponding direct alcohol fuel cells were superior to acid equivalents with membranes of identical fully hydrated thicknessess; this is indicative of a combination of reduced alcohol permeabilities and changed electrokinetics on PtRu anode catalysts at high pH. VOCV values for the AAEM-DAFCs were higher with ethanol than with methanol (consequent on lower permeability to ethanol), but were lower with ethylene glycol. Promisingly, and contrary to Nafion equivalents, peak power densities were not reduced when C2 alcohols (CC bond containing) replaced methanol.

A novel alkaline polymer has been developed as an interfacial material for use in the preparation of metal-cation-free alkaline membrane electrode assemblies (MEAs) for all-solid-state alkaline fuel cells (AFCs) with long-term performance stability.

Recent reports that the reaction between Nafion sulfonyl fluoride precursor and the cyclic diamine 1,4-dimethylpiperazine yields stable anion-exchange membranes appear to be premature. On aqueous work up, membranes with high cation-exchange capacities and zero anion-exchange capacities are produced.

A novel alkaline exchange ionomer (AEI) was prepared from the radiation-grafting of vinylbenzyl chloride (VBC) onto poly(ethylene-co-tetrafluoroethylene) [ETFE] powders with powder particle sizes of less than 100 μm diameter. Quaternisation of the VBC grafted ETFE powders with trimethylamine resulted in AEIs that were chemically the same as the ETFE-based radiation-grafted alkaline anion exchange membranes (AAEM) that had been previously developed for use in low temperature alkaline polymer electrolyte fuel cells (APEFC). The integration of the AEI powders into the catalyst layers (CL) of both electrodes resulted in a H2/O2 fuel cell peak power density of 240 mW cm−2 at 50 °C (compared to 180 mW cm−2 with a benchmark membrane electrode assembly containing identical components apart from the use of a previous generation AEI). This result is promising considering the wholly un-optimised nature of the AEI inclusion into the catalyst layers.