Synthesis of bimetallic Ni_xCo_1-xP hollow nanocages from metal-organic frameworks for high performance hybrid supercapacitors

Hollow nanostructures are favorable for electrode materials to enhance their energy storage performance due to their unique structural features. In this work, hollow bimetallic phosphide (Ni_xCo_1-xP) was fabricated via etching treatment of ZIF-67 and further phosphorization. The as-obtained Ni_xCo_1-xP composites display a high specific capacity of 548 C g⁻¹ at 1 A g⁻¹ in 2 M KOH aqueous solution, excellent rate capability (83.7%, 77.6%, 71.7% and 66.2% capacity retention at 10, 20, 30 and 40 A g⁻¹), and remarkable cycling stability (86% capacity retention at 7 A g⁻¹ after 3000 cycles). Furthermore, a hybrid supercapacitor, constructed by Ni_xCo_1-xP as anode and active carbon as cathode, exhibits excellent specific capacitance (115.8 F g⁻¹ at 1 A g⁻¹), high gravimetric energy/power density (31.52 Wh kg⁻¹ at 700 W kg⁻¹), and outstanding long-term cycling stability (98.3% capacitance retention even after 10000 cycles). The excellent electrochemical performance should be attributed to the good interfacial contact between electrode and electrolyte, unique hollow structure with suitable surface area and good electrical conductivity of Ni_xCo_1-xP. The results suggest a great potential of Ni_xCo_1-xP composites in electrochemical energy storage devices.