Prussian blue as positive electrode material for aqueous sodium-ion capacitor with excellent performance

High-quality Prussian blue (PB), whose crystals have a small number of vacancies, is synthesized by utilizing Na₄Fe(CN)₆ as the only precursor. It is demonstrated that the crystals have an important influence on electrochemical performance. The PB electrode exhibits a specific capacity as high as 107 mA h g^-1 at a current density of 0.5 A g^-1 without apparent capacity loss after 1100 cycles in 0.5 mol L^-1 Na₂SO₄ aqueous solution due to fewer vacancies. Furthermore, a sodium-ion capacitor is also fabricated by combining the PB as a positive electrode and activated carbon as a negative electrode. It can operate at a cell voltage as high as 1.8 V with an energy density of 30 W h kg^-1. The capacitor displays not only high energy density but also excellent cycling stability because the as-prepared PB has outstanding ion storage capability and excellent structural stability. These results show that the PB is a promising candidate for aqueous sodium-ion capacitors.