Porous (NH4)Fe3(SO4)2(OH)6 microparticles derived from iron-based zeolite imidazole frameworks as negative electrode material for supercapacitors

Abstract

Iron-based compounds with high theoretical capacities have attracted great interest because they can cover the shortage of negative electrode materials in asymmetric supercapacitors. Herein, porous ammonium ihleite microparticles (p-AIMs, (NH4)Fe3(SO4)2(OH)6) have been successfully fabricated by using iron-based zeolite imidazole frameworks (Fe-ZIFs) as in-situ sacrificial templates. The specific capacity of the initial p-AIMs can reach 331 F g−1 at a current density of 20 A g−1 in a three-electrode system. As the number of cycles increases, the ammonium ihleite gradually transformed into FeOOH/Fe3O4 biphasic mixture, and the specific capacitance is elevated to 600 F g−1 at 20 A g−1. Besides, when they are assembled into an asymmetric supercapacitor (ASC) with nickel‑cobalt-based metal-organic frameworks (NiCo-MOFs), an ultra-wide potential window of 1.7 V and a specific capacitance of 312 F g−1 at 3.1 A g−1 are obtained. After 5000 cycles, the capacitance retention of the ASC is 84.6 %, indicating its good cycle stability. These excellent electrochemical performances of the p-AIMs are mainly ascribed to their porous nanostructure and the generation of the Fe3O4/FeOOH biphasic compounds during the cycle process. Moreover, an ultra-high specific capacitance of the p-AIMs can greatly reduce the total mass of active materials in the ASC and improve the overall specific capacitance of the ASC. Thus, this work provides a new direction for the development of negative electrode materials for supercapacitors.