Reducing impedance to ionic flux in capacitive deionization with Bi-tortuous activated carbon electrodes coated with asymmetrically charged polyelectrolytes

Capacitive deionization (CDI) with electric double layers is an electrochemical desalination technology in which porous carbon electrodes are polarized to reversibly store ions. 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 bitortuous 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 26.7–27.8 F g⁻¹), round-trip efficiency (52–71 to 71–80%), and charge efficiency (33–59 to 35–67%). Improvements in these key performance indicators also translated to enhanced 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.