CuHCF as an electrode material in an aqueous dual-ion A1³+/K+ion battery

Copper-hexacyanoferrate (CuHCF) is capable of 28000 cycles in 1 mol dm-3 AlCl3 in a 3-electrode cell, while performanceimproves with the addition of K+. However, CuHCF synthesised by co-precipitation is in a partially oxidised state, resulting in aneffective state-of-charge (SoC) of 90%. Chemical reduction is possible by soaking electrodes in 10 mmol dm-3 Na2S2O3 to producean electrode at 0% SoC, while chemical oxidation in 100 mmol dm-3 KMnO4 is possible in order to produce electrodes at 100%SoC. Despite this, a full cell constisting of a TiO2 negative electrode, CuHCF positive electrode and 1 mol dm-3 AlCl3/1 mol dm-3KCl, allows ca. 1100 cycles due to the premature degradation of CuHCF. This is due to the low coulombic efficiency (CE) of TiO2,which results in consistent overcharging of CuHCF and the likely loss of Fe3 to the electrolyte