Capacitive mixing for the extraction of energy from salinity differences: survey of experimental results and electrochemical models.

The "capacitive mixing" (CAPMIX) technique is an emerging technology aimed at the extraction of energy from salinity differences, e.g. between sea and river waters. CAPMIX benefits from the voltage rise that takes place between two electrodes dipped in a saline solution when its salt concentration is changed. Several kinds of electrodes have been proposed so far: activated carbon materials (Brogioli, 2009), membrane-based ion-selective electrodes (Sales et al., 2010), and battery electrodes (Biesheuvel and van der Wal, 2010). The power production mainly depends on two properties of each single electrode: the amplitude of the potential rise upon salinity change, and the potential in the high-salinity solution. The various electrode materials that have been used returned different values of the two parameters, and hence to different power productions. In this paper, we apply electrokinetic and electrochemical models to qualitatively explain the experimentally observed behaviors of various materials under different experimental conditions. The analysis allows to devise techniques for tailoring new materials, particularly suited for the CAPMIX technique.