Effect of additional charging and current density on the performance of Capacitive energy extraction based on Donnan Potential

Abstract

The difference in the salt concentrations of river and seawater implies that wherever they mix, energy could be extracted from the salinity gradient. This is a renewable and clean means of generating energy that makes use of a natural process. Capacitive energy extraction based on the Donnan potential (CDP) is a promising technique for extracting this energy. We herein describe our investigation of the effect of additional charging on extraction behaviour using a forced current density. The study was conducted in a flow-through cell, using capacitive electrodes and ion exchange membranes. It is shown that increasing the accumulated charge in the system could be beneficial in terms of energy extraction. Furthermore, the addition of charge improved the power densities achieved. By charging at higher current densities and discharging at lower current densities, the performance of the system may be improved. The highest average power density achieved in this study was 0.205 ± 0.006 W m⁻² (1.26 ± 0.75 mW g⁻¹). This was obtained using a charge of 6 C (4.62 C g⁻¹), with a controlled constant current of 50 mA (38.5 mA g⁻¹ or 6.24 A m⁻²). Three main limiting factors to the performance of CDP were identified, namely (i) the voltage drop over time, caused by the self-discharge of the cell and the non-ideal behaviour of the membranes, (ii) the duration of the switching times and (iii) the loss over the internal resistance. Of these, the internal resistance was identified as being the most important parameter to be minimized in order to further improve the performances of CDP systems.