Self-Templating Synthesis of Cobalt Hexacyanoferrate Hollow Structures with Superior Performance for Na-Ion Hybrid Supercapacitors

Prussian blue (PB) and its analogues (PBA), especially with hollow structures, have attracted growing attention from the researchers of energy storage field. Herein, we have developed a facile self-templating method to synthesize hollow-structured cobalt hexacyanoferrate (CoHCF) with controllable morphologies by using water-soluble precursors as templates. The method is versatile and can be extended to synthesize various PB/PBA hollow structures with tunable composition and morphology. Profiting from the unique hollow structure, the CoHCF hollow prisms manifest exceptional electrochemical performance in the Na₂SO₄ aqueous electrolyte, including a high specific capacitance (284 F g^-1 at 1 A g^-1), a high rate capability, and an excellent cycling stability (92% retention after 5000 cycles). A hybrid supercapacitor device assembled with the CoHCF hollow prisms and activated carbon shows a high specific density of 47 W h kg^-1 at a specific power of 1000 W kg^-1 and stable cycling performance.