Corentin Trégouët
ESPCI Paris PSL, Laboratoire CBI MIE (Matériaux innovants pour l’énergie)
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How salt diffusion triggers electric currents within electro-chemical cells even in open circuit conditions
T. Derkenne, A. Colin, C. Tregouet
(IE, CBI, ESPCI Paris, Université PSL,CNRS, 75005 Paris, France)
The osmotic energy is the energy that can be collected from the environment thanks to osmotic effects between two water solutions of different salinities. These two water feeds can originate from a river and the ocean for instance, or from waste water. A promising method to extract energy is the reverse electrodialysis (RED). This method is based on the transport of ions from the high to the low concentration solutions through ion-selective membranes.
Such ion-exchange membranes are characterized by their selectivity to cations or anions. To measure this selectivity, a cell is used where membranes are placed between two electrolytes at different concentrations: the diffusion of the cation coupled to the blockage of the anions (or the opposite) by the membrane results in transmembrane voltage. This voltage is thus an indication of the membrane selectivity, which is a key indicator to estimate the power produced by a RED cell.
We show experimentally and theoretically that for membranes with a finite selectivity, the selectivity measurement suffers from strong concentration. However, varying the flow rates of the solutions enables us to finely characterize the ion transport through the membranes. Ultimately, this allows us to finely measure the membrane properties.
We also demonstrate that for low feeding flow-rate, concentration polarization is not homogeneous, and loops of currents appear through the membrane at the scale of the whole cell. Accordingly, both the measured selectivity and the maximum harvested power per unit area can depend on the feeding flow rate, but also on the size of the membrane, and must be interpreted with extra care.
These experiments and modeling allow us to understand how to avoid these detrimental effects. We edit rules on how to scale membrane-processes to adjust the size based on the chosen flow rates, and how to rationalize the design of segmented electrodes.
