Water-energy Nexus: Studies On Salinity Gradient Energy Harvest And Desalination

Abstract

Water and energy are fundamentally linked, and both are important for the development of human society. The demand for renewable energy and freshwater are two global challenges in the 21st century. Herein, a novel chloride-ion (Cl−) concentration flow cell (CFC) based on two symmetrical electrodes (BiCl3, CoCl2, VCl3, or BiOCl) separately by a cation-exchange membrane was used as an efficient method to recover salinity gradient (SG) energy. The CFC with metal chloride electrodes (BiCl3, CoCl2, and VCl3) was based on Cl− extraction/insertion, and that with the BiOCl electrodes was depended on Cl– intercalation/deintercalation. Similarly, the facile designed MnOx/biochar composite was also demonstrated as an efficient alternative for efficient SG energy recovery using CFC based on Na+ intercalation/deintercalation. The CFC with MnOx/biochar electrodes possessed the highest power density of 5.67 W m-2, which was higher than those of most previous methods for SG energy harvest. At the presence of other inorganic ions (K+, Mg2+, Ca2+, and SO42-) in feed solutions, both the open circuit voltage and power density were inhibited, indicating that pretreatment would be needed to obtain better power output when using natural feed waters. In addition, the BiOCl electrode was also proved to be an alternative and efficient anion-capturing/releasing electrode in capacitive mixing (CapMix) for SG energy recovery. The polyelectrolytes coated mixing entropy battery (MEB) were comparable and even higher than those with AgCl anionic electrodes. Besides, both the graphene oxide (GO) and carbon nanotube (CNT) were successfully introduced to the matrix of poly(acrylic acid-co-acrylamide) hydrogel for SG energy harvest depended on the swelling and shrinking under freshwater and seawater. The results suggested that GO/CNT hybrid hydrogel can be used for efficient SG energy harvest. Lastly, a membrane-free capacitive deionization (CDI) cell was constructed with MnO2 and polypyrrole grafted activated carbon (Ppy/AC) electrodes, which can reversibly and selectively intercalate Na+ (MnO2) and Cl- (Ppy/AC) ions because of the faradaic pseudocapacitive behaviours. Both the salt removal capacity and salt removal rate of the Ppy/AC-MnO2 cell were higher than reported conventional CDI, membrane-CDI and hybrid-CDI, suggesting the Ppy/AC-MnO2 CDI cell can be an efficient desalination method.