(364f) Transport of Vanadium and Vanadyl Ions Across Zeolite Membranes: A Molecular Dynamics Study

Effective energy storage is one of the limitations of emerging renewable energy technologies. Solar and  wind energy generation methods have proven to be capable of producing levels of energy that exceed immediate needs and the storage of this excess energy often presents a problem. Redox flow batteries (RFBs) have become an attractive form of 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. These 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 selectivity to improve the performance of RFBs and make their use more feasible on a larger scale. A molecular dynamics study of three types of these membranes (BEC, CFI, DON) and the ions present in the vanadium-RFB has been undertaken to determine their transport behavior. The hydration of the vanadium [V²⁺] and vanadyl [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 CFI and DON zeolite framework types and the dynamics/properties of this transport were studied. It was found that a relatively large pore (>7Å) was necessary 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.