Mixed cobalt–zinc hexacyanoferrate hollow micro-cubes for a high-voltage hybrid supercapacitor

The behavior of Prussian Blue Analogs (PBAs) is determined by the external transition metal coordinated to the N-end of the hexacyanoferrate block. Combining transition metal ions allows obtaining a material with improved performance for energy-related applications. A mixed PBA was formed with cobalt hexacyanoferrate (CoHCF) and ZnHCF. The former has two redox processes and easy modulation of its morphology, while the latter exhibits fast kinetics during the charge transfer. The morphology of the solids was controlled by composition, shaping hollow structures at a high concentration of Co²⁺ in the PBA. Spectroscopic characterization displayed the change in the charge density at the N-end of the cyanide bridges caused by the polarizing power of Zn²⁺. XRD showed the formation of more than one crystalline phase. The presence of Co²⁺ provided larger specific capacity and stability during cycling, while Zn²⁺ afforded a higher potential and improved rate capability to the solid. The material with the best electrochemical behavior was employed to assemble a hybrid supercapacitor, reaching specific energy of 30 Wh kg⁻¹ at a power of 450 W kg⁻¹ and 7 Wh kg⁻¹ at 5.5 kW kg⁻¹. Additionally, the device retained 85% of its initial capacitance after 5000 galvanostatic charge–discharge cycles.