Performance characteristics of zinc hexacyanoferrate/Prussian blue and copper hexacyanoferrate/Prussian blue solid state secondary cells

Solid state secondary cells were constructed with Prussian blue (PB), i.e. iron (III) hexacyanoferrate (II) as the active material for cathodes and zinc (II) hexacyanoferrate (III) (Znhcf) and copper (II) hexacyanoferrate (III) (Cuhcf) as the active materials for anodes. The metal hexacyanoferrates were mixed with graphite powder, potassium chloride and dilute hydrochloric acid to form a thick paste. A Nafion membrane was used as separator transporting protons only. Charge–discharge studies were carried out using galvanostatic method and double step chronopotentiometric method. The cell voltage at half-discharge for Znhcf/PB cell varied from 0.6 to 0.8 V depending on the discharge rate. For the Cuhcf/PB cell, the cell voltage at half-discharge was 0.87 V for a precycled cell and it dropped to 0.75 V after 100 cycles. The practical capacity of the Znhcf/PB cell (12 A h/kg) was higher than that of the Cuhcf/PB cell (5 A h/kg), however, the recycling efficiency of the latter was far better than that of the former. Open-circuit voltage decay and recovery transients were recorded for both cells and analysed.