Influence of Activated Carbon Film Thickness on the Electrode Performance in Capacitive Deionization

Problems associated with water-scarcity, drought and water pollution are already a global problem and are expected to grow worse in the coming decades, even in regions currently considered to be rich with respect to water resources. That is the case of Brazil, where the drought in the northeast and southeast regions has become a great challenge. In order to address this situation different measures such as developing “greener” methods of water desalination that are energy and cost efficient while minimizing the use of chemicals should be investigated.

Capacitive Deionization (CDI) is a promising technology that can be used to remove ions based on the concept of electric double layer (EDL). When a low potential is applied to a CDI cell, the ions are attracted and stored in the EDL formed at the electrode/solution interface. When the removal cycle is complete, the electrodes are mostly saturated with anions and cations. Electrode regeneration is subsequently conducted by switching the voltage or by shorting the system letting, in that way, the adsorbed ions come back to the bulk.

Different carbon materials and composites have been investigated as electrodes for CDI. A good candidate for CDI electrode must have a high specific surface area, high conductivity and wettability, and high capacity to remove ions. Since activated carbon (AC) fits all of these characteristics and also presents a low price it has been considered as one the best candidates for being used as electrosorption material.

Is this work it was evaluated the effect on CDI performance of the AC films thickness deposited on graphite substrate. Electrodes characterization was conducted by measuring physical properties such as surface resistivity or wettability. In addition, cyclic voltammetry and electrochemical impedance spectroscopy experiments were performed in order to determine the capacitance and the different components of the resistance in the system. Moreover, desalination experiments were conducted in a batch operational mode (Figure 1). The electrode performance was analyzed then in terms of desalination rate, regeneration kinetics, capacitance, coulombic efficiency and specific energetic consumption. 

As expected, the thicker the electrode the higher the total amount of ions removed (R). However, the study of the electrosorption capacity normalized by mass of electrode material (q) showed that the increase of the electrode thickness led to lower values of q. This result indicated that part of AC electrode material was not being effectively used for removing ions from the solution. On the other hand, it was observed that the variation of the electrode thickness did not have any impact on the specific energy consumption (in kJ per gram of electrode). Thusly, depending on what is aimed and considering the low cost of the AC electrode, the thicker electrode might represent an interesting alternative to thin film electrodes due its higher capacity of storing charge. As the main disadvantage of the thickest electrodes, it was found that desorption kinetics became slower when the electrode thickness increases, thus making the electrosorption/desorption cycles longer. These results suggest that the electrode thickness must be optimized in order to maximize the electrosorption capacity and the desorption kinetics.

Figure 1. Schematic representation of the CDI system.