Understanding the performance of flow-electrodes for capacitive deionization through hydrodynamic voltammetry

Flow-electrode capacitive deionization is a promising approach to scale up this desalination technology, because of its ability to regenerate the electrodes outside the main cell, and thereby remove a higher amount of salt from the feed water using carbon slurries as flowable electrodes. Previous works have focused on improving the performance by changing the flow-electrode composition or by proposing different cell designs, using a wide range of volumetric flow rates for the electrode, as well as performing mathematical and experimental modelling considering the slurries as Newtonian fluids. In this work, hydrodynamic voltammetry is used to understand the electrochemical behaviour of the carbon slurries, to develop an optimization method that will give an improved electrochemical performance of the slurries. Results show that the apparent viscosity of the slurries must be considered to explain the behaviour of the slurry within the cell, as well as the effect of shear thinning on the migration of activated carbon particles inside the slurry. A dimensionless number is proposed to define a threshold that will ensure a capacitive behaviour of the flow-electrode.