Capacitive deionization performance of bi-tortuous activated carbon electrodes coated with asymmetrically charged polyelectrolytes

Capacitive deionization (CDI) is an electrochemical desalination technology in which porous carbon electrodes are polarized to reversibly store ions from a brackish water feed in electrical double layers in the electrodes. CDI is an attractive alternative to commonly used brackish water desalination technologies like reverse osmosis and thermal distillation due to its low energy consumption and potential for energy recovery. Electrode morphology and characteristics impact the desalination performance of CDI. Few of the CDI performance metrics like equivalent circuit resistance and charge efficiency also greatly impact CDI operation and performance. Planar composite CDI electrodes exhibit poor energetic performance due the resistance associated with salt depletion and tortuous diffusion in the macroporous structure. In this work, we investigate the impact of bi-tortuosity on desalination performance by etching macroporous patterns along the length of activated carbon porous electrodes in a flow-by CDI architecture. Capacitive electrodes were also coated with thin asymmetrically charged polyelectrolytes to improve ion-selectivity while maintaining the bi-tortuous (BT) macroporous channels. Under constant current operation, the equivalent circuit resistance in CDI cells operating with bi-tortuous electrodes was approximately 2.2 times less than a control cell with unpatterned electrodes, leading to significant increases in working capacitance (20 – 22 to 27 – 28 F g-1), round-trip efficiency (52 – 71 to 71 - 80%), charge efficiency (33 – 59 to 35 – 67%), which led to improvements in salt adsorption capacity, rate, and most importantly, the thermodynamic efficiency of salt separation (1.0 - 2.0 to 2.2 – 4.1 %). These findings demonstrate that the use of bi-tortuous electrodes is a novel approach of reducing impedance to ionic flux in CDI. In summary, the heightened performance of fb-CDI with coated BT electrodes elucidates that both ESR and are key indicators of energy efficiency desalination