N-coordinated dual metals for enhanced energy density and capacitive performance of supercapacitors

To meet the demands for high-performance electrical supercapacitors, electrode materials need to be developed with high specific capacity, high energy density, and long cycle life. In this study, a zeolitic imidazolate framework (ZIF-8) and a zinc‑cobalt-bimetallic metal-organic framework (ZnCo-BMOF) were prepared. Then, the Zn 2+ ions present in these compounds were replaced with Fe 3+ ions using the Kirkendall effect to develop nitrogen-doped carbons containing iron and/or cobalt for supercapacitor applications. The textural properties of ZIF-8 and ZnCo-BMOF were examined using transmission electron microscopy, nitrogen sorption test, thermogravimetric analysis, and X-ray diffraction. The nitrogen sorption test reveals that ZIF-8 and ZnCo-BMOF have specific surface areas of 1900–1970 m 2 ·g −1 . The replacement of Zn 2+ ions with Fe 3+ ions reduces the specific surface areas of ZIF-8 and ZnCo-BMOF to 700–960 m 2 ·g −1 , resulting in a less-ordered but denser structure, with microporous channels fully available for the transport of electrolytic ions during the formation of electrical double-layer capacitance. Among the synthesized materials, the Fe/Co dual metal-doped N-containing carbon material exhibits the highest specific capacitance of 144.4 F·g −1 at a current density of 0.1 A/g. It exhibits long-standing cyclic performance with 90.6% retention of capacitance after 10,000 galvanostatic charge/discharge cycles.