Transport of Vanadium and Oxovanadium Ions Across Zeolite Membranes: A Molecular Dynamics Study
KR Hinkle and CJ Jameson and S Murad, JOURNAL OF PHYSICAL CHEMISTRY C, 118, 23803-23810 (2014).
Redox flow batteries (RFBs) have become an attractive form of energy storage because of their safety, capacity, and small environmental footprint; however, this technology is not yet widely available due to inefficiencies in the ion-exchange membrane. The current technology widely utilizes polymeric membranes that have stability problems in the highly reactive environment of the RFB and tend to break down, shortening the life of the battery. Also, they present less than desirable selectivity for proton transport, which is crucial to the overall efficiency of the battery. It has been proposed that thin zeolite membranes will provide both the stability and the selectivity to improve the performance of RFBs and make their wide-scale application more feasible. A molecular dynamics study of six types of these membranes (ERI, LTA, MFI, BEC, CFI, DON) and the ions present in the vanadium-RFB has been undertaken to determine their transport behavior and investigate the molecular level requirements for their suitability for IEM applications. The hydration of the vanadium(II) V2+, vanadium(III) V3+, oxovanadium(IV) VO2+, and dioxovanadium(V) VO2+ ions plays a key part in ion transport and was examined in detail. Structures and dynamics of the hydration shells were investigated and found to agree with previously reported findings when available. Ion transport was observed with the BEC, CFI, and DON zeolite framework types and the dynamics/properties of this transport were studied. It was found that a relatively large pore (similar to 7 angstrom) was necessary for ion transport due to the strongly bound hydration shell that effectively increases the size of the ion. As the ions pass through the membrane, the shape and structure of their hydration shells remain unchanged. This verifies that the size of the hydrated ion complex is a key factor in zeolite membrane transport. The only ion transport observed through membranes with smaller pores (<5 angstrom) such as ERI, LTA, and MFI zeolite frameworks was that of the hydronium ion. Therefore, these membranes demonstrate the selective transport of hydronium ions over vanadium ions that is an essential requirement for IEMs in vanadium RFBs.
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