Energy from Salinity Gradient

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Blue energy, i.e. the Gibbs energy from mixing seawater and river water, is a promising source of clean energy. Pressure-retarded osmosis (PRO) and reverse electrodialysis (RED) are by far the most successful methods for extracting such energy, although their performance is limited by challenging issues associated with the required membranes. Researchers at the University of Hawai’i have developed a method for direct electricity generation, in which carbon electrodes with asymmetric distributions of nanopores are used to generate electrical power from salinity gradients. The researchers demonstrated that two electrodes made with carbon, e.g. carbon nanotubes, respond differently to a salinity gradient if their pore distributions are dramatically different. Such different responses lead to a discrepancy in the capacity change of the electrical double layers (EDLs) that spontaneously establish at the solid-electrolyte interface, which can create a significant potential difference between the electrodes, and thus, generate electrical power. When the electrodes are successively immersed in solutions of various concentrations, a concentration gradient is created between the solution captured in the pore and that in the ambient. The Gibbs free energy from mixing of the two solutions can be harvested through the potential difference generated. Using NaCl solutions and electrodes fabricated with commercially available carbon materials, the researchers observed an energy density of 69 mJ/g and a power density of 1.720 kW/m3, higher than that of current membrane-based methods (1.1 kW/m3). In the seawater stage, the power density reached 7.4 kW/m3 21.