Capacitive energy extraction based on double layer expansion (CDLE) is a renewable method of harvesting energy from the salinity difference between seawater and freshwater. It is based on the change in properties of the electric double layer (EDL) formed at the electrode surface when the concentration of the solution is changed. Many theoretical models have been developed to describe the structural and thermodynamic properties of the EDL at equilibrium, e.g., the Gouy– Chapman–Stern (GCS), Modified Poisson–Boltzmann–Stern (MPBS), modified Donnan (mD) and improved modified Donnan (i‐mD) models. To evaluate the applicability of these models, especially the rationality and the physical interpretation of the parameters that were used in these models, a series of single‐pass and full‐cycle experiments were performed. The experimental results were compared with the numerical simulations of different EDL models. The analysis suggested that, with optimized parameters, all the EDL models we examined can well explain the equilibrium charge–voltage relation of the single‐pass experiment. The GCS and MPBS models involve, how-ever, the use of physically unreasonable parameter values. By comparison, the i‐mD model is the most recommended one because of its accuracy in the results and the meaning of the parameters. Nonetheless, the i‐mD model alone failed to simulate the energy production of the full‐cycle CDLE experiments. Future research regarding the i‐mD model is required to understand the process of the CDLE technique better.

Capacitive mixing (CapMix) is a renewable method of extracting energy from the salinity difference between seawater and freshwater. In this study, we systematically investigate the system behavior and performance of the CapMix system under four operational modes namely, capacitive energy extraction based on double layer expansion (CDLE), capacitive energy extraction based on the Donnan potential (CDP), and CDP with additional charging of constant voltage (CDP-CV) and constant current (CDPCC). The results indicate that the application of additional charging in the CDP technique can break the limits of the Donnan potential and significantly improve the system's performance. Accordingly, in terms of energy production and average power density, CDP-CC and CDP-CV are the two superior operational modes, followed by CDP and CDLE. In addition, our results reveal that CDP-CC is determined by the accumulated charge and applied current. CDLE is dependent on the applied voltage, while CDPCV is not sensitive to the applied voltage. Increasing the external load can considerably increase the energy production of both CDLE and CDP. In summary, the findings in this study provide practical information for the optimization and application of CapMix technologies.

Capacitive mixing (CapMix), one of the most recent techniques that extract salinity gradient energy from the salt difference between seawater and freshwater, has attracted more attention in recent years. However, one big challenge that remains to solve in the traditional CapMix is to produce energy in a continuous way instead of an intermittent way. Here we achieve effective continuous energy production by using a two-cell flow electrode CapMix (F-CapMix) system. The performance of the F-CapMix is investigated under different experimental parameters that influence the power production of the system. Results show that the two-cell F-CapMix system is capable of continuously producing energy for long-term operation. Generally, the power density is dependent on the activated carbon loading, amounts of carbon black, feedwater flow rate, flow electrode flow rate, and the connected external resistance. Increasing the carbon loading and carbon black amounts is beneficial for improving power production. A slower flow rate of flow electrode and feedwater could improve the system’s performance. The external resistance should be matched to the internal resistance of the cell to achieve maximum power production. These results indicate the potential of F-CapMix and provide directions for its further optimizatio

The generation of the electricity from the salinity difference energy between the seawater and freshwater through Capacitive Mixing (CapMix) systems with solid electrodes was limited by the intermittent energy production. In this study, a single-cell CapMix system with flow-electrode (F-CapMix) was examined to produce electricity continuously from a simulated seawater and freshwater. The performance of the presented F-CapMix system is dependent on the activated carbon loading, amounts of carbon black, and the connected external resistance. Results suggest that performance can be enhanced by increasing the activated carbon loading and carbon black amounts because of the decrease on the system’s internal resistance. Furthermore, to achieve the maximum power density of the system, the external resistance should be matched to the internal resistance. The maximum power density of the presented   single-cell F-CapMix system was 74.3 mW/m2, which was comparable to the previous CapMix and F-CapMix systems. These results indicate the potential of F-CapMix and provide developing directions for its further optimization.