Extraction of Salinity-Gradient Energy by a Hybrid Capacitive-Mixing System.

Salinity-gradient energy (SGE) is a renewable energy source available wherever two solutions with different salinity mix. Capacitive-mixing (Capmix) is a technology that directly extracts the SG potential through the movements of ions in high- and low-concentration solutions. However, the energy-harvesting performance of Capmix needs further improvement. Herein, a hybrid Capmix that consists of a battery and capacitive electrodes is proposed. In this system, sodium ions and anions are captured/released by the metal oxide and carbon electrodes, respectively. The hybrid Capmix extracted an energy density that was approximately three times higher (130 J m ) and exhibited a notable power output (97 mW m ) compared to the previous Capmix using ion-exchange membranes. Furthermore, the hybrid system operated successfully with real river water and seawater. These results suggest that the hybrid Capmix could be a viable option to harvest energy from salinity gradients.