Membrane Capacitive Deionization with Constant Current vs Constant Voltage Charging: Which Is Better?

Membrane capacitive deionization (MCDI) can be typically operated with constant voltage (CV) or constant current (CC) mode in the charging stage. While a series of previous studies have compared both charging modes to identify the better operating mode, neither their performance evaluation protocols were consistent, nor did their conclusions unanimously converge. This study presents a new framework to evaluate and compare MCDI performance, considering the kinetic efficiency, the energetic efficiency, and the intrinsic trade-off between the two. A key prerequisite for making rational comparison of performance between MCDI operations is that the operations being compared should all result in the same target adsorption. With this key prerequisite and the new evaluation framework based on the trade-off curve between kinetic and energetic efficiencies, our experimental assessment and theoretical analysis suggest that whether CC or CV charging is more efficient is strongly dependent on the target adsorption and, to a less extent, on the kinetic rate of charging. However, the advantage in energy or kinetic efficiency of one charging mode over that of the other is relatively small in all cases. Our study also reveals that, for a given MCDI system, there exist regimes of target adsorptions and kinetic rates that can only be achieved by either CC or CV charging, or even regimes that can be achieved by neither charging mode. In summary, this study revises our current understanding regarding the comparison of the two typical charging modes in MCDI, and introduces a new framework for comparing the performance of different MCDI and CDI operations.