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.

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Anion-exchange membranes in electrochemical energy systems

Replacing fossil fuels with energy sources and carriers that are sustainable, environmentally benign, and affordable is amongst the most pressing challenges for future socio-economic development. To that goal, hydrogen is presumed to be the most promising energy carrier. Electrocatalytic water splitting, if driven by green electricity, would provide hydrogen with minimal CO2 footprint. The viability of water electrolysis still hinges on the availability of durable earth-abundant electrocatalyst materials and the overall process efficiency. This review spans from the fundamentals of electrocatalytically initiated water splitting to the very latest scientific findings from university and institutional research, also covering specifications and special features of the current industrial processes and those processes currently being tested in large-scale applications. Recently developed strategies are described for the optimisation and discovery of active and durable materials for electrodes that ever-increasingly harness first-principles calculations and machine learning. In addition, a technoeconomic analysis of water electrolysis is included that allows an assessment of the extent to which a large-scale implementation of water splitting can help to combat climate change. This review article is intended to cross-pollinate and strengthen efforts from fundamental understanding to technical implementation and to improve the ‘junctions’ between the field's physical chemists, materials scientists and engineers, as well as stimulate much-needed exchange among these groups on challenges encountered in the different domains.

Water electrolysis: from textbook knowledge to the latest scientific strategies and industrial developments

Highly base-stable cationic moieties are a critical component of anion exchange membranes (AEMs) in alkaline fuel cells (AFCs); however, the commonly employed organic cations have limited alkaline stability. To address this problem, we synthesized and characterized the stability of a series of imidazolium cations in 1, 2, or 5 M KOH/CD3OH at 80 °C, systematically evaluating the impact of substitution on chemical stability. The substituent identity at each position of the imidazolium ring has a dramatic effect on the overall cation stability. We report imidazolium cations that have the highest alkaline stabilities reported to date, >99% cation remaining after 30 days in 5 M KOH/CD3OH at 80 °C.

Imidazolium Cations with Exceptional Alkaline Stability: A Systematic Study of Structure–Stability Relationships

The alkaline stability of imidazolium salts and imidazolium-based alkaline anion-exchange membranes (AEMs) was investigated in this work. C2-substituted (with methyl, isopropyl or phenyl groups) imidazolium salts, 3-ethyl-1,2-dimethyl imidazolium bromine ([EDMIm][Br]), 3-ethyl-2-isopropyl-1-methylimidazolium bromine ([EIMIm][Br]), and 3-ethyl-1-methyl-2-phenyl- imidazolium bromine ([EMPhIm][Br]), were synthesized and characterized. The effect of the C2-substitution on the alkaline stability of imidazolium salts was investigated by 1H and 13C NMR spectroscopy. Compared with the C2-unsubstituted imidazolium salt, 3-ethyl-1-methylimidazolium bromine ([EMIm][Br]), the alkaline stability of C2-substituted imidazolium salts is significantly enhanced at elevated temperatures, probably due to the steric hindrance of the substituents, which protected the imidazolium cations against the hydroxide attack. Moreover, the higher LUMO energies may also improve the alkaline stability of imidazolium salts. The alkaline st...

Alkaline Stable C2-Substituted Imidazolium-Based Anion-Exchange Membranes

Understanding hydroxide solvation and transport in anion exchange membranes (AEMs) can provide important insight into the design principles of these new membranes. To accurately model hydroxide solvation and transport, we developed a new multiscale reactive molecular dynamics model for hydroxide in aqueous solution, which was then subsequently modified for an AEM material. With this model, we investigated the hydroxide solvation structure and transport mechanism in the membrane. We found that a relatively even separation of the rigid side chains produces a continuous overlapping region for hydroxide transport that is made up of the first hydration shell of the tethered cationic groups. Our results show that hydroxide has a significant preference for this overlapping region, transporting through it and between the AEM side chains with substantial contributions from both vehicular (standard diffusion) and Grotthuss (proton hopping) mechanisms. Comparison of the AEM with common proton exchange membranes (PEMs) showed that the excess charge is less delocalized in the AEM than the PEMs, which is correlated with a higher free energy barrier for proton transfer reactions. The vehicular mechanism also contributes considerably more than the Grotthuss mechanism for hydroxide transport in the AEM, while our previous studies of PEM systems showed a larger contribution from the Grotthuss mechanism than the vehicular mechanism for proton transport. The activation energy barrier for hydroxide diffusion in the AEM is greater than that for proton diffusion in PEMs, implying a more significant enhancement of ion transport in the AEM at elevated temperatures.

Hydroxide Solvation and Transport in Anion Exchange Membranes

Block copolymers of polystyrene-b-poly(vinyl benzyl trimethylammonium tetrafluoroborate) (PS-b-[PVBTMA][BF4]) were synthesized by sequential monomer addition using atom transfer radical polymerization. Membranes of the block copolymers were prepared by drop casting from dimethylformamide. Initial evaluation of the microphase separation in these PS-b-[PVBTMA][BF4] materials via SAXS revealed the formation of spherical, cylindrical, and lamellar morphologies. Block copolymers of polystyrene-b-poly(vinyl benzyl trimethylammonium hydroxide) (PS-b-[PVBTMA][OH]) were prepared as polymeric alkaline anion exchange membranes materials by ion exchange from PS-b-[PVBTMA][BF4] with hydroxide in order to investigate the relationship between morphology and ionic conductivity. Studies of humidity [relative humidity (RH)]-dependent conductivity at 80 °C showed that the conductivity increases with increasing humidity. Moreover, the investigation of the temperature-dependent conductivity at RH = 50, 70, and 90% showed a significant effect of grain boundaries in the membranes against the formation of continuous conductive channels, which is an important requirement for achieving high ion conductivity. © 2012 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2013, 51, 1751–1760, 2013

Synthesis and structure-conductivity relationship of polystyrene-block-poly(vinyl benzyl trimethylammonium) for alkaline anion exchange membrane fuel cells

Crosslinked anion exchange membranes with connected cations

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Synthesis and structure-conductivity relationship of polystyrene-block-poly(vinyl benzyl trimethylammonium) for alkaline anion exchange membrane fuel cells

Crosslinked anion exchange membranes with connected cations