Membrane Separators for Batteries and Fuel Cells

Dear Colleagues,

Many different battery technologies have been developed, each replacing the last to some extent, but not completely. Application requirements dictate the creation of technologies. For example, lead acid batteries are almost exclusively used for automotive applications; they are not surpassed in terms of price, lifetime, and performance characteristics. Alkaline nickel hydroxide batteries possess a high discharge capacity, high charge acceptance, long cycle life, and could remain a dominant type of battery, according to the amount of sales attained a few years ago, but drawbacks related to the nicked hydroxide swelling are reducing their use. Despite their toxic components, large nicked cadmium cells (NiCd) are still sometimes used for high energy and fast charge rugged applications. Nickel metal hydride batteries (NiMH) were developed in the 1970s and have a higher energy density than NiCd. The NiMH battery had its own issues and, so, not too many years after its introduction, its use has now largely been replaced with lithium ion batteries and other related lithium-based technologies, including the now widely-used lithium polymer batteries. Since the first primary lithium ion batteries (LIBs) became commercially available in 1991, LIBs have caught on quickly and become the main power sources on the consumer electronics market.

LIBs are characterized by high specific energy and high specific power, which are the advantages that most other electrochemical energy storage technologies cannot offer. In addition, some other advantages, such as high efficiency, long life cycle, and low self-discharge rate, make lithium ion batteries well-suited for applications such as energy storage grid and electric motors for the development of sustainable vehicles, such as hybrid vehicles (HEVs), plug-in hybrid vehicles (PHEVs), and full electric vehicles (EVs). Today, other battery technologies offer promising storage solutions, namely, next-generation high-energy rechargeable and all exotic types (Li-S, room temperature Na-S, Li-organic, Li-air, large zinc, redox flow, etc.). Among these, redox flow batteries, particularly the all-vanadium redox flow battery, are considered the best option to store electricity from medium- to large-scale applications, but major issues still exist, requiring further attention.

Like batteries, fuel cells produce electrical energy through an electrochemical process. Fuel cells also typically have a pair of electrodes and electrolytes, as well as structural supports. Unlike batteries, fuel cells are conversion devices that change some kind of chemical fuel into electricity. Fuel cells cannot directly store electrical energy, but they have a great deal of flexibility in fuels. Therefore, fuel generation and storage components must be employed, each with their own unique material requirements. Fuel cells also typically require an electrocatalytic material to promote energy conversion. Like batteries, fuel cells can be combined into stacks with an effectively unlimited size. While fuel cells have received considerable attention over the last 15–25 years, the history of hydrogen fuel cells dates back to 1838. It would take more than a century before polymers would be implemented as an electrolyte for proton transport (1955) and another 40 years after that before a real renaisssance would be sparked that would finally make polymer electrolyte membrane (PEM) fuel cells a conceivable means of electrochemical energy conversion. Among fuel cells, the proton exchange membrane fuel cell (PEMFC) is one of the promising renewable energy devices, owing to its high energy density, low operating temperature, zero emission, and low noise. However, there are still some barriers that inhibit the widespread application of PEMFCs, such as their high cost, low power density, and low durability. Recently, anion exchange membrane fuel cells (AEMFCs) have received a significant amount of interest due to their potential advantages over the currently-used PEMFCs, such as facile oxygen reduction reaction (ORR) kinetics due to the high pH environment, enabling the use of low-cost Pt-free catalysts, and lower fuel crossover, thanks to the use of hydrocarbon-based anion exchange membranes (AEMs). Extensive efforts to modify and improve the basic materials studied for application in ion exchange membrane fuel cells, namely the ion exchange membranes (perfluorosulfonic acid ionomers, polysulfones and phosphazenes, block copolymers and blends, anhydrous proton conducting membranes, and anion exchange membranes), as well as a proper address of the durability–cost factor of fuel cell commercialization are required for these devices to reach their full potential.

Batteries and fuel cells have five components: Two active elements (a cathode and an anode), a separator, an electrolyte medium for carrying ions between the reactants through the separator, and a container. The separator is a permeable membrane whose main function is to keep the two device electrodes apart to prevent electrical short circuits while also allowing the transport of ionic charge carriers that are needed to close the circuit during the passage of current in the electrochemical device. Separators are critical components in batteries and fuel cells because their structure and properties considerably affect the device performance, including energy and power densities, lifetime, safety, and cost. Very little work (relative to research of electrode materials and electrolytes) is directed toward characterizing and developing new, stable, safe, smart, and sustainable separators, and this Special Issue will be a perfect forum to bring together the latest results and innovations obtained by key laboratories presently engaged in advanced membrane separators for batteries and fuel cells. More specifically, authors from top laboratories are encouraged to submit their manuscripts showing how R&D efforts towards the characterization and development of their novel separators have advanced battery and fuel cell performance, lowered cost and enhanced reliability and, consequently, enabled the significant recent advancements in battery and fuel cell technologies.

Prof. Dr. César Augusto Correia de Sequeira
Guest Editor

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