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ChemComm Communication Fig. 5 Cyclic charge–discharge curves for zeolite IEM and Nafion 117. EDS test due to possible substrate interference. A fresh zeolite IEM was tested for 80 cycles of charge–discharge over 20 days including two interruptions for solution rebalancing and recharging. A com- parison of ZC and ZV between the zeolite IEM and Nafion 117 was made for the first 30 cycles (Fig. 5). The zeolite IEM exhibited a stable ZC higher than that of Nafion 117; the Nafion IEM had higher ZV in the beginning but became the same as that of the zeolite IEM after 10 cycles due to their similar resistances. In summary, the zeolite-T membrane was found to have high proton permselectivity over vanadium ions due to its sub-nanometer pore size and low electrical resistance in the acidic VRFB electrolyte solutions because of its enormous proton concentration and film thinness. The zeolite-T membrane was demonstrated to be an effective IEM for VRFB operation with high power density, good efficiency, and short term stability. Zeolite membranes can be a potentially useful new class of inorganic IEMs for RFBs. However their long term stability is yet to be investigated. Significant opportunities exist for improving the zeolite IEMs by optimizing the pore size, structure and chemical composition (e.g., Si/Al ratio) as well as developing high-performing substrates. This work was supported by the U.S. National Science Founda- tion under grant numbers CBET-1263860 and CBET-0854203. Notes and references 1 (a) B. Dunn, H. Kamath and J.-M. Tarascon, Science, 2011, 334, 928; (b) M. Skyllas-Kazacos, M. H. Chakrabarti, S. A. Hajimolana, F. S. Mjalli and M. Saleem, J. Electrochem. Soc., 2011, 158, R55. 2 (a) X. Li, H. Zhang, Z. Mai, H. Zhang and I. Vankelecom, Energy Environ. Sci, 2011, 4, 1147; (b) M. Vijayakumar, M. S. Bhuvaneswari, P. Nachimuthu, B. Schwenzer, S. Kim, Z. Yang, J. Liu, G. L. Graff, S. Thevuthasan and J. Hu, J. Membr. Sci., 2011, 366, 325; (c) T. Mohammadi and M. Skyllas-Kazacos, J. Appl. Electrochem., 1997, 27, 153; (d) H. Dai, H. Zhang, H. Zhong, X. Li, S. Xiao and Z. Mai, Int. J. Hydrogen Energy, 2010, 35, 4209; (e) S. Kim, T. B. Tighe, B. Schwenzer, J. Yan, J. Zhang, J. Liu, Z. Yang and M. A. Hickner, J. Appl. Electrochem., 2011, 41, 1201. 3 (a) Z. Wang, J. Yu and R. Xu, Chem. Soc. Rev., 2012, 41, 1729; (b) V. Valtchev, G. Majano, S. Mintova and J. P ́erez-Ram ́ırez, Chem. Soc. Rev., 2013, 42, 263. 4 (a) J. Lin and S. Murad, Mol. Phys., 2001, 99, 1175; (b) L. Li, J. Dong, T. M. Nenoff and R. Lee, J. Membr. Sci., 2004, 243, 401; (c) L. Li, J. Dong and T. M. Nenoff, Sep. Purif. Technol., 2007, 53, 42; (d) K. P. Lee, T. C. Arnot and D. Mattia, J. Membr. Sci., 2011, 370, 1. 5 J. E. Huheey, E. A. Keiter and R. L. Keiter, Inorganic Chemistry, Harper Collins, New York, USA, 1993. 6 J. M. Bennett and J. A. Gard, Nature, 1967, 214, 1005. 7 R. L. Gorring, J. Catal., 1973, 31, 13; Y. Cui, H. Kita and K.-I. Okamoto, J. Membr. Sci., 2004, 236, 17. 8 K. Schmidt-Rohr and Q. Chen, Nat. Mater., 2007, 7, 75. View Article Online Fig. 4 Tests of the zeolite-T membrane equipped VRFB: (top) polarization and power density and (bottom) charge–discharge curves. sulfate solution in 2 M H2SO4, respectively. The VRFB exhibited a maximum power density of 404 mW cm2 at an output voltage of 0.952 V and a current density of 424 mA cm2 on the basis of active zeolite membrane area (Fig. 4, top). This power density is comparable to those of the VRFBs using polymeric IEMs.11 Fig. 4 (bottom) shows the charge–discharge curves at a current density of 33 mA cm2 for both the zeolite and Nafion 117 IEMs. After being fully charged, the VRFB with zeolite IEM exhibited an open circuit voltage (OCV) of B1.6 V and the negative and positive electrolyte solutions displayed characteristic colors of pure V2+ (purple) and V5+ (VO2+, yellow) solutions, respectively, indicating that a nearly 100% state of charge (SOC) was obtained. The zeolite IEM achieved higher Coulombic efficiency (ZC = (idischtdisch)/(ichtch)), slightly lower voltage efficiency (ZV = (Vdisch/Vch)), and slightly higher energy efficiency (ZE = ZVZC) as compared to the Nafion 117 IEM. At a current density of 33 mA cm2, the overall energy efficiencies of the VRFBs equipped with zeolite IEM and Nafion 117 were 61.2% and 58.5%, respectively. These efficiencies are lower than those of the recently reported polymeric IEM-based VRFBs12 because of the large resistances of the substrate, flow gaps, and additional interfaces involved the cell of this work (Fig. 3). The stability of zeolite-T was also studied by immersing the particles in a solution of 1 M V5+ + 0.5 M H2SO4 for 30 days and in 2 M H2SO4 solution for two weeks, respectively. After the treatments, the particles remained in the zeolite-T crystal phase as confirmed by the XRD patterns and there were no appreciable changes in particle shape and size according to the SEM observations. The EDS analysis indicated a small increase of the Si/Al ratio from B3.1 to B3.4 in 0.5 M H2SO4 due to slow dealumination in dilute acid. Zeolite dealumination was faster in 2 M H2SO4 but the Si : Al ratio tended to stabilize in just 2 days and reached B11 in 10 days. This suggests that Al content in the membrane might be overestimated by the 2418 | Chem. Commun., 2014, 50, 2416--2419 This journal is © The Royal Society of Chemistry 2014 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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