Polyelectrolyte-Coated Copper Hexacyanoferrate and Bismuth Oxychloride Electrodes for Efficient Salinity Gradient Energy Recovery in Capacitive Mixing

A mixing entropy battery (MEB) based on a pair of cationic and anionic electrodes is a novel method to extract salinity gradient (SG) energy. However, so far, only precious AgCl electrodes are reported as anionic electrodes in MEBs. Herein, it is demonstrated that bismuth oxychloride (BiOCl) can be an alternative anionic electrode for MEBs to harvest SG energy. In tests with 30 and 1 g L-1 NaCl solutions, the origin MEB with uncoated BiOCl anionic and copper hexacyanoferrate (CuHCF) cationic electrodes produces an energy density of 8 J m(-2) and an average power density of 59 mW m(-2). To boost the power output, the origin BiOCl and CuHCF electrodes are coated with polyelectrolytes. The energy density (12 J m(-2)) and average power density (87 mW m(-2)) of the coated MEB are nearly 50% higher than those of the origin MEB and are stable over 50 cycles, which is comparable and even higher than those with AgCl anionic electrodes. These results indicate that BiOCl can be a cheap alternative to the AgCl anionic electrode, and polyelectrolyte coatings can be a cheaper alternative to ion-exchange membranes in MEBs for efficient SG energy recovery.