Study of flow dynamics and gravity settling of slurry electrodes within an electrochemical cell using simultaneous current measurements and high-speed imaging*

We study flowable carbon slurries as an electrode component for capacitive deionization and semi-solid flow batteries. A carbon particle slurry in 0.5 M NaCl solution was flowed into two horizontal, co-flowing channels of an electrochemical cell. The two channels were arranged one above another, and separated by a cation-exchange membrane. We performed simultaneous high-speed imaging and electrochemical measurements to investigate slurry dynamics and gravity-based settling on charge transport. Optical access enabled imaging in a plane parallel to gravity. The cell has four independently addressable electrodes, such that each channel has an electrode at the top and bottom (streamwise-spanwise) walls. Current responses for 1 V bias were measured across electrode pairs as a function of slurry flow rate. Based on the image and current data, two regimes can be identified. The first is characterized by low flow rates wherein dense particle beds promote charge percolation (electron transfer), and the electrodes which contact these beds yield the highest current. The second regime is characterized by thin particle beds and well mixed particle flows, and the electrodes in closest proximity achieve a current higher than the other electrode pairs. These observations provide insights into the design of flow cells and show that gravitational settling can yield a steady-state current enhancement.