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

With the development of renewable energy and electrified transportation, electrochemical energy storage will be more important in the future than it has ever been in the past. Although lithium-ion batteries (LIBs) are traditionally considered to be the most likeliest candidate thanks to their relatively long cycle life and high energy efficiency, their limited energy density as well as cost are still causing a bottleneck for their long-term application. Alternatively, metal–air batteries have been proposed as a very promising large-scale electricity storage technology with the replacement of the intercalation reaction mechanism by the catalytic redox reaction of a light weight metal–oxygen couple. Generally, based on the electrolyte, these metal–air batteries can be divided into aqueous and nonaqueous systems, corresponding to two typical batteries of Zn–air and Li–air, respectively. The prominent feature of both batteries are their extremely high theoretical energy density, especially for nonaqueous Li–air batteries, which far exceeds the best that can be achieved with LIBs. In this review, we focus on the major obstacle of sluggish kinetics of the cathode in both batteries, and summarize the fundamentals and recent advances related to the oxygen catalyst materials. According to the electrolyte, the aqueous and nonaqueous electrocatalytic mechanisms of the oxygen reduction and evolution reactions are discussed. Subsequently, seven groups of oxygen catalysts, which have played catalytic roles in both systems, are selectively reviewed, including transition metal oxides (single-metal oxides and mixed-metal oxides), functional carbon materials (nanostructured carbons and doped carbons), metal oxide–nanocarbon hybrid materials, metal–nitrogen complexes (non-pyrolyzed and pyrolyzed), transition metal nitrides, conductive polymers, and precious metals (alloys). Nonaqueous systems have the advantages of energy density and rechargeability over aqueous systems and have gradually become the research focus of metal–air batteries. However, there are considerable challenges beyond catalysts from aqueous to nonaqueous electrolytes, which are also discussed in this review. Finally, several future research directions are proposed based on the results achieved in this field, with emphasis on nonaqueous Li–air batteries.

Oxygen electrocatalysts in metal–air batteries: from aqueous to nonaqueous electrolytes

Within the past 20 years or so, there has occurred an explosion of interest in the magnetic behavior of pyrochlore oxides of the type $A_{2}^{3+}$$B_{2}^{4+}$O$_{7}$ where $A$ is a rare-earth ion and $B$ is usually a transition metal. Both the $A$ and $B$ sites form a network of corner-sharing tetrahedra which is the quintessential framework for a geometrically frustrated magnet. In these systems the natural tendency to form long range ordered ground states in accord with the Third Law is frustrated, resulting in some novel short range ordered alternatives such as spin glasses, spin ices and spin liquids and much new physics. This article attempts to review the myriad of properties found in pyrochlore oxides, mainly from a materials perspective, but with an appropriate theoretical context.

/pdf/magnetic-pyrochlore-oxides-3nd6sywsbi.pdf

Magnetic Pyrochlore Oxides

We present here a critical review of several technologically important electrocatalytic systems operating in alkaline electrolytes. These include the oxygen reduction reaction (ORR) occurring on catalysts containing Pt, Pd, Ir, Ru, or Ag, the methanol oxidation reaction (MOR) occurring on Pt-containing catalysts, and the ethanol oxidation reaction (EOR) occurring on Ni–Co–Fe alloy catalysts. Each of these catalytic systems is relevant to alkaline fuel cell (AFC) technology, while the ORR systems are also relevant to chlor-alkali electrolysis and metal-air batteries. The use of alkaline media presents advantages both in electrocatalytic activity and in materials stability and corrosion. Therefore, prospects for the continued development of alkaline electrocatalytic systems, including alkaline fuel cells, seem very promising.

Electrocatalysis of oxygen reduction and small alcohol oxidation in alkaline media

Reduction/Evolution Catalysts for Low-Temperature Electrochemical Devices Dengjie Chen,†,⊥,∇ Chi Chen,†,⊥ Zarah Medina Baiyee,† Zongping Shao,‡,§ and Francesco Ciucci*,†,∥ †Department of Mechanical and Aerospace Engineering, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, China ‡State Key Laboratory of Materials-Oriented Chemical Engineering, College of Chemistry & Chemical Engineering, Nanjing Tech University, No. 5 Xin Mofan Road, Nanjing 210009, China Department of Chemical Engineering, Curtin University, Perth, Western Australia 6845, Australia Department of Chemical and Biomolecular Engineering, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, China

Nonstoichiometric Oxides as Low-Cost and Highly-Efficient Oxygen Reduction/Evolution Catalysts for Low-Temperature Electrochemical Devices

https://iopscience.iop.org/article/10.1149/1.2120111/pdf

Oxygen Electrocatalysis on Some Oxide Pyrochlores

Chemical studies of polythionic acid stress-corrosion cracking

Process of producing finely divided metals and alloys

Olefins higher than 3-C were converted to ketones using aqueous chloropalladate ion as oxidant. The chloride ion content of the solution was kept at very low levels relative to normal usage to increase significantly the reaction rates of these higher olefins. The reduced palladium was reoxidized electrochemically using the ferric/ferrous couple as the transfer agent. Using cyclopentene as a model compound the ionic strength, the acidity, the concentration of palladium, the means of extracting product and the means of adding olefin were all adjusted as to achieve about 70% current selectivity with minimal palladium inventory. There were indications of even better behaviour with several open chain ketones.

Preliminary examination of an electrochemical process for converting olefins to ketones

1,098,926. Fuel cells. ESSO RESEARCH & ENGINEERING CO. April 22. 1965. No. 17028/65. Heading H1B. In a fuel cell utilizing a liquid fuel immiscible with the liquid electrolyte an organic surface active agent is added to the electrolyte to increase the size of the fuel-electrolyte interface. Lists of anionic, cationic and nonionic surfaceactive agents are given in the body of the Specification. In Fig. 1 the cell is divided by an ion permeable or exchange member into an anolyte zone 6 containing the fuel electrode 2 and a catholyte zone 5 containing the cathode 3 to the rear of which nitric acid oxidant is supplied. Anolyte compartment 6 contains a fuel, e.g. heptane, floating on an electrolyte, e.g. H 2 SO 4 containing a surface-active agent. In Fig. 3 (not shown) the electrolyte containing the surface-active agent is on the side of a porous impregnated carbon electrode opposite from the fuel. H 2 SO 4 . H 3 PO 4 , NaOH and KOH may be used as electrolyte. Platinum black coated on a glass frit may be used as the fuel electrode. Fuels include aliphatic saturated and unsaturated hydrocarbons such as hexane, hexene, heptane, heptene, octane and octene and hydrocarbon mixtures such as kerosene, gasoline and diesel fuel.

Hugh H Horowitz

Papers

Oxygen Electrocatalysis on Some Oxide Pyrochlores

Chemical studies of polythionic acid stress-corrosion cracking

Process of producing finely divided metals and alloys

Preliminary examination of an electrochemical process for converting olefins to ketones

Method for operating fuel cell with liquid fuel