Kelsey Hatzell, Vanderbilt University, New York, NY, Marm Dixit, Vanderbilt, Nashville, TN, Daniel Moreno, Mechanical Engineering, Georgia Institute of Technology, Atlanta and Marta Hatzell, Woodruff School of Mechanical Engineering, Georgia Tech, Atlanta, GA
Capacitive deionization (CDI) based on flowable electrodes was introduced to address scalablility and salt-removal capacity issues in traditional CDI systems. Instead of using static film electrodes, the active material is suspended in a liquid. Typically this technology uses the flow architecture adopted from the flow battery community in order to: (1) decrease materials costs, (2) decouple salt removal capacity and treated water volume, (3) achieve continuous deionization and energy recovery, and (4) extends the desalination time. FCDI systems have three flow channels, two of which serve to flow the active material (activated carbon) which removes or electroadsorbs ions (energy storage), and a center channel where the feedwater is directed. The center channel is separated from the flowable electrodes by an anion exchange membrane and a cation exchange membrane. These membranes serve to improve the efficiency of salt removal, by avoiding a 'ping-pong' or co-ion repulsion effect during the discharge process. Tanks contain the active material in a 'suspension' form and the size of the tank dictates the desalination/energy storage period. During operation, the flow electrode flows down the current collector and becomes charged when it enters the charging cell. To maintain electroneutrality, ions of opposite charge are attracted from the feedwater channel and adsorb on the surface of the active material.
Fundamentally, flowable electrodes are examples of complex fluids that can display a reconfigurable microstructure or active material rearrangement during flow. This deformation in the underlying active material arrangement can cause a decrease in the electron percolation and limit material utilization (salt removal capacity or energy storage capacity). Thus, the flow properties of the suspension electrode is important for minimizing pumping requirements and promoting energy efficient charge storage and transfer mechanisms. Depending on the mass loading, the electrode can be unflowable (40 wt%), gel-like (20 wt%) or liquid-like (10 wt%). These forms can take on vastly different mechanical (rheological) behaviors and ion removal capacities . Oscillating rheological experiments can probe the 'microstructure break-down' but cannot provide direct insights into the qualitative nature of the active material arrangement. Herein, we discuss advances in imaging of these biphasic materials systems using ultra-fast techniques enabled by synchrotron facilites.
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