Concentration Flow Cells Based on Chloride-Ion Extraction and Insertion with Metal Chloride Electrodes for Efficient Salinity Gradient Energy Harvest

Abstract

Salinity gradient (SG) is a natural and renewable energy source existing in estuaries, and can also be produced during various desalination and industrial processes. Here, a new method is proposed to efficiently recover SG energy based on chloride-ion (Cl–) extraction and insertion with metal chloride electrodes and the Donnan potential over a cation-exchange membrane in a concentration flow cell. Three different metal chloride electrodes (BiCl3, CoCl2, and VCl3) were investigated in the cell, and their properties after discharging in 30 g L–1 (seawater) and 1 g L–1 (river water) NaCl solutions were studied by cyclic voltammetry, electrochemical impedance spectroscopy, and X-ray photoelectron spectroscopy. The cell with BiCl3 electrodes yielded the largest power density (max. = 3.17 W m–2) compared to that of CoCl2 and VCl3 electrodes, which was higher than those of most previous technologies for SG energy recovery. Fast Cl– extraction and insertion processes were observed on BiCl3 electrodes due to small charge transfer resistance and Cl– diffusion resistance. BiCl3 was reduced to metal Bi as Cl– released from the electrode to river water, while metal Bi was oxidized to BiCl3 as Cl– inserted into the electrode from seawater.