An acrylate-polymer-based electrolyte membrane for alkaline fuel cell applications.

Alkaline fuel cells (AFCs) recently attracted renewed attention because of their potential to surpass proton exchange membrane fuel cells (PEMFCs). The long-existing issues of PEMFCs, including expensive noble-metal catalysts and polymer electrolytes, as well as CO poisoning and inferior temperature endurance, prevented them from being used in a broad range of applications. Contrarily, advantages of AFCs include fast kinetics in the reduction of the oxidizing agent and the possibility to use base-metal catalysts. However, a critical challenge for conventional AFCs is the use of aqueous alkaline electrolytes, which can react with CO2 from air to form carbonate salts (e.g. , K2CO3). As a result, the performance of the fuel cell would quickly deteriorate. To solve this problem, recent investigations focused on intrinsically OH -conducting alkaline polymer electrolyte (APE) materials to replace the alkaline electrolytes. By using APEs, the formation of carbonate salts can be prevented, which is attributable to the absence of metal ions. However, carbonate ions might still be formed through a reaction with CO2, which would result in a reduced OH conductivity. Application of APEs can also enable a compact design and eliminate corrosion from alkaline solutions. These advantages confirm that APE fuel cells (APEFCs) present a very promising energy conversion technology. Because APEs are a key component determining the ultimate performance, they should exhibit a high OH conductivity and superior mechanical properties, and in addition be of low cost. To date, the most common synthesis route for APEs is chloromethylation of polymers having a phenyl structured backbone, which is followed by quaternization. Many polymers have been used as precursors to synthesize APEs, including polysulfone, poly(arylene ether sulfone), polyetherketone, poly(ether imide), polyethersulfone cardo, poly(phthalazinon ether sulfone ketone), poly(dimethyl phenylene oxide), and poly(phenylene). Also, a recent study by Lin and co-workers reported high conductivity and mechanical strength for an alkaline polymer electrolyte based on a crosslinked ionic liquid. The phenyl backbones of the polymers have in common that they are all excellent engineering polymers exhibiting good mechanical properties because of rigid ring structures. However, this advantage can be seriously weakened by the chloromethylation–quaternization process, which converts the polymer from an ionic insulator into an ionomer and thus, from hydrophobic to hydrophilic. As a result of the hydrophilicity, the mechanical properties of the APEs in the humid working environment of a fuel cell can be very different from that of the precursors. Because their backbones consist of aromatic groups, these precursor polymers can be modified to exhibit extreme hydrophilicity through the chloromethylation–quaternization process. The resulting APE may have a very high anionic conductivity, but with very poor mechanical properties in humid environment. Therefore, an obvious shortcoming of the chloromethylation–quaternization process is the difficulty to control the degree of chloromethylation and quaternization precisely, thus making it difficult to balance conductivity and mechanical properties. Cost is also a concern, since the aforementioned APE precursors are highcost polymers because of the sophisticated synthesis process. In a previous study, we reported a novel APE made from poly (methyl methacrylate-co-butyl acrylate-co-vinylbenzyl chloride) (PMBV). This copolymer was synthesized using solution-free radical polymerization. Xu and co-workers also reported an independent study on APE made from a copolymer using similar polymerization methods. Although this copolymer exhibits a promising performance, our previous study encountered two problems: The three monomers, methyl methacrylate (MMA), butyl acrylate (BA), and 4-vinylbenzyl chloride (VBC), have different reactivity ratios so that they polymerize at different reaction rates. Because of the slow diffusion of propagating copolymer chains and the diluted monomer concentration in the polymerization solution, the monomers with lower reactivity ratios have a smaller possibility for complete conversion. Therefore, the copolymer composition did not match the designed monomer ratio. The second concern is that the molecular weight of the copolymer in our previous study was not as high as expected, which could considerably weaken the mechanical strength. To address these problems, we demonstrate a novel bottom-up synthesis of PMBV by using mini-emulsion polymerization for the first time. Unlike chloromethylation of existing polymers, we synthesized PMBV by using various monomers selected to meet the specifications for conductivity and mechanical strength. Specifically, VBC (15 mol%) contained the chloromethyl functional group, which could be quaternized and then successively ion-exchanged to obtain OH conductivity. Polymerized MMA exhibits a high rigidity and toughness. As a result, the MMA monomer (80 mol.%) was chosen to provide mechanical strength. The brittleness inherent to MMA and VBC was overcome by adding a small portion of BA (5 mol%), which conferred flexibility to the resulting APE. [a] Y. Luo, Dr. J. Guo, Prof. Dr. C. Wang Chemical and Biomolecular Engineering 2113 Chemical and Nuclear Engineering University of Maryland, College Park, MD 20742 (USA) Fax: (+1)301-314-9126 E-mail : [email protected] [b] Dr. D. Chu Sensors and Electron Device Directorate US Army Research Laboratory Adelphi, MD 20783 (USA) Supporting Information for this article is available on the WWW under http://dx.doi.org/10.1002/cssc.201100287.