(Invited) Chloride-Ion Concentration Flow Cells for Efficient Salinitygradient Energy Recovery

Abstract

Salinity gradient (SG) is a natural and renewable energy source, which naturally exists between river water and seawater and can also be created by engineering processes. Here, a novel chloride-ion (Cl−) concentration flow cell based on two symmetrical electrodes (BiCl3, CoCl2, VCl3 or BiOCl) separately by a cation-exchange membrane (CEM) was used as an efficient method to recover SG energy. Analysis from X-ray diffraction and X-ray photoelectron spectroscopy indicated that the concentration flow cell with metal chloride electrode (BiCl3, CoCl2 or VCl3) was based on Cl− extraction and insertion, and that of the BiOCl electrode was depended on Cl– intercalation and deintercalation. Besides, the Donna potential across the CEM was also employed for the SG energy harvest. The cell with BiOCl electrodes had the highest power density of 4.36 W m-2 with synthetic river water (1 g L-1 NaCl) and seawater (30 g L-1 NaCl), which was higher than those of most previous technologies for SG energy recovery. The higher power output could be attributed to the faster kinetics of Cl− intercalation/deintercalation on BiOCl electrodes with a layered structure compared to that of Cl- extraction/insertion on metal chloride electrodes. In addition, the results from cyclic voltammetry, electrochemical impedance spectroscopy and galvanostatic charge/discharge suggested that low charge transfer and ion diffusion resistances, and the high ion storage capacities of the BiOCl electrode also benefit its the higher power output.