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ChemComm COMMUNICATION View Article Online View Journal | View Issue Cite this: Chem. Commun., 2014, 50, 2416 Received 20th September 2013, Accepted 7th December 2013 DOI: 10.1039/c3cc47214k www.rsc.org/chemcomm The zeolite-T membrane was discovered to have high proton permselec- tivity against vanadium ions and exhibit low electrical resistance in acidic electrolyte solutions because of its enormous proton concentration and small thickness. The zeolite membrane was demonstrated to be an efficient ion exchange membrane in vanadium redox flow batteries. Redox-flow batteries (RFBs) are promising for cost-effective energy storage in solar and wind power systems and large-scale electric grids.1 To date, most RFBs have used proton-selective, perfluori- nated or non-perfluorinated polymeric ion exchange membranes (IEMs). However, in the extremely acidic and oxidizing RFB electro- lyte solutions, the polymeric IEMs experience undesirable metal ion crossover and the non-perfluorinated IEMs also suffer from chemical instability that limit the cell efficiency and lifetime.2 Here we report a purely inorganic, molecular sieve zeolite membrane for highly proton-selective ion transport and demonstrate its applica- tion as an alternative IEM in the all-vanadium RFB (VRFB). Zeolites are microporous aluminosilicate crystals containing ordered pores of uniform sizes ranging from 0.3 to over 1 nm depending on the crystallographic structure. The zeolite frameworks are formed by interlinked [SiO4] and [AlO4] tetrahedrons in which large numbers of exchangeable extraframework cation charge com- pensators exist at the [AlO4] sites. The zeolite pore size and surface chemistry can be fine-tuned by isomorphous elemental substitution and ion exchange.3 Recently, zeolite membranes were demonstrated for water purification from salt solutions by the size-exclusion (steric) effect because metal ions form hydration shells that make them too large to enter the zeolitic pores of sub-nanometer sizes.4 Unlike metal ions, protons exist as polyatomic H3O+ (hydronium) ions in water without definable hydration shells. The H3O+ is thus much smaller than the hydrated multivalent metal ions involved in a Department of Chemical Engineering, University of Cincinnati, Cincinnati, OH 45221, USA. E-mail: Junhang.dong@uc.edu; Fax: +1-513-556-3743; Tel: +1-513-556-3992 b State Key Laboratory of Material-Oriented Chemical Engineering, Nanjing University of Technology, Nanjing 210009, China † Electronic supplementary information (ESI) available: Procedures and data of material synthesis, characterization, and analyses. See DOI: 10.1039/c3cc47214k RFB systems5 and effective separation of protons from metal ions may be achieved by zeolite membranes with appropriate pore sizes. However, ionic zeolite membranes have never been studied for selective ion transport and proton conduction in the literature. In this work, the zeolite-T membrane was synthesized and studied as a proton exchange membrane in the acidic electrolyte solutions for the VRFB. The zeolite-T membrane has an erionite and offretite inter-grown structure.6 Its effective zeolitic pore open- ings are the 0.36 nm 0.51 nm elliptical windows of the erionite structure. Zeolite-T materials with Si/Al ratios of 3–4 contain large amounts of extraframework alkaline metal ions and are known to be acid-resistant.7 The zeolite-T membrane used here was grown on a 2 mm thick, 2.5 cm diameter porous a-alumina disc with an average pore size of B0.1 mm and a porosity of B30%. The top layer of the zeolite was B5 mm-thick based on the SEM image in Fig. 1. The energy dispersive X-ray spectroscopic (EDS) survey of the as-synthesized membrane revealed that the atomic ratios of Si:Al, (Na + K):Al, and Na:K in the zeolite-T crystals were 3.5 0.05, 1.1 0.05, and 0.41 0.03, respectively. A zeolite ion exchange membrane for redox flow batteries† Zhi Xu,a Ioannis Michos,a Xuerui Wang,b Ruidong Yang,a Xuehong Gub and Junhang Dong*a 2416 | Chem. Commun., 2014, 50, 2416--2419 This journal is © The Royal Society of Chemistry 2014 SEM image of the a-alumina supported zeolite-T membrane surface with the inset showing the membrane cross-section. Fig. 1 Published on 09 December 2013. Downloaded by Nanjing University of Technology on 18/04/2014 06:15:35.PDF Image | zeolite ion exchange membrane for redox
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