Removal of Na+ and Ca2+ with Prussian blue analogue electrodes for brackish water desalination

Desalination of brackish water sources is critical to addressing the growing global freshwater demand. One promising approach is electrically driven desalination using intercalation electrodes. While intercalation electrodes have been widely researched for energy storage applications, only a small subset of those materials is suitable for desalination. Here we report the synthesis, characterization, and in-device testing of three Prussian blue analogue intercalation compounds: copper, manganese, and zinc hexacyanoferrate with formulas K_xM[Fe(CN)₆]_z·nH₂O (M = Cu, Mn, Zn). The desalination performance for each of these materials against carbon electrodes is reported for Na⁺ intercalation and for Ca²⁺ intercalation using 1000 ppm NaCl and 1000 ppm CaCl₂ feed solutions, respectively. While the copper and manganese analogs showed promising performance for Na+ and Ca²⁺ intercalation, the zinc compound was unstable and underwent rapid dissolution. Manganese hexacyanoferrate showed the best desalination performance in terms of salt removal capacities and salt removal rates with NaCl while copper hexacyanoferrate performed the best with CaCl₂. The manganese analog proved to be the most stable intercalation material, retaining 83% and 72% of its salt removal capacity after 280 cycles in NaCl and CaCl₂ feed solutions respectively.