Facile synthesis of cryogenically controlled cobalt-nickel-based Prussian blue analogues for high-performance symmetric supercapacitors

Cobalt hexacyanoferrate (CoHCF) with high energy density as a potentially useful electrode for energy storage systems still suffers from inferior rate capability and low cycling performance due to high-spin nitrogen-coordinated Co ions. While nickel hexacyanoferrate (NiHCF) with low energy density can exhibit great cycling stability due to the electrochemical inertness of Ni. Herein, we have developed a series of cobalt nickel hexacyanoferrate (CoNiHCF) via a facile cryogenically controlled co-precipitation method. We explored the Co/ Ni molar ratio and synthesis temperature on the effects of the specific capacitance in detail. As supercapacitor cathode, the Co0.7Ni0.3HCF with the Co/Ni molar ratio of 0.7:0.3 synthesized at 0 degrees C can exhibit a high specific capacitance of 406 F g-1 at 1 A center dot g- 1 with capacitance retention of 81.89 % after 5000 cycles. The exceptional electrochemical performance is ascribed to the upgraded conductivity resulting from the coexistence of bimetallic components, the low particle size caused by the moderate introduction of Ni, the low interstitial water content, and the synergistic effect between Co and Ni. Moreover, the symmetric supercapacitor (SSC) of Co0.7Ni0.3HCF//Co0.7Ni0.3HCF can provide a high energy density of 72.96 W h kg-1 and 40.96 W h kg-1 at specific powers of 2876 W kg-1 and 28,577 W kg-1, respectively with a capacitance retention of 74.83 % after 4000 cycles.