Chloride-ion concentration flow cells for efficient salinity gradient energy recovery with bismuth oxychloride electrodes

Salinity gradient (SG) energy is a large, renewable, and clean energy source, which naturally exists between river water and seawater and can also be created by engineering processes. Here, bismuth oxychloride (BiOCl) electrodes was used for efficient SG energy recovery in a chloride-ion concentration flow cell (Cl-CFC) based on a new mechanism, that is, chloride ion (Cl−) intercalation/deintercalation. Compared to previous Cl-CFC with BiCl3 electrodes based on Cl− extraction/insertion (3.17 W m−2), the peak power density of the cell with BiOCl electrodes (4.36 W m−2) was higher under the same testing conditions when using synthetic river water (1 g L−1 NaCl) and seawater (30 g L−1 NaCl). Further investigations demonstrated that the relatively higher power density was attribute to the faster kinetics of Cl− intercalation/deintercalation at BiOCl electrodes than that of Cl− extraction/insertion at BiCl3 electrodes. In addition, three BiOCl electrodes with different carbon black ratios (CB, 10%, 20% and 30%) were also prepared and examined in this study. Compared to BiOCl/10%CB and BiOCl/30%CB electrodes, the lower charge transfer and ion diffusion resistances, larger ion storage capacities and higher cell open circuit voltages of the BiOCl/20%CB electrodes were all beneficial to its higher power output.