Phase controlled Fe₂N@Fe₃O₄ core-shell nanoparticles hybridized with nitrogen-doped reduced graphene oxide for boosted charge transfer in asymmetric supercapacitor

Due to environmental concerns and requirements for sustainable development, there are increasing demands for electrode materials with high energy density in supercapacitors, utilizing earth-abundant materials. In that response, we designed a novel composite material by hybridizing core–shell nanoparticles (NPs), which were composed of iron oxide (Fe₃O₄) and its derivative iron nitride (Fe₂N) as plentiful elements in the earth crust or air, with nitrogen-doped reduced graphene oxide (NrGO). The electrode prepared with the Fe₂N@Fe₃O₄ NPs/NrGO demonstrated outstanding specific capacitance of 341.3 F g⁻¹ at 0.5 A g⁻¹, along with an improved rate capability approximately 4 times higher than that of pristine Fe₃O₄ NPs/GO. Also, the Fe₂N@Fe₃O₄ NPs/NrGO exhibited stable performance within a wide potential range of −1.05 to 0.15 V and excellent cycle stability of 91.2 % at 5 A g⁻¹ after 10,000 cycles. These exceptional characteristics might be attributed to the enhanced electrical conductivity and increased surface area of the material, achieved through the simultaneous formation of the NrGO from the GO and the phase-transformed Fe₂N from the pristine Fe₃O₄ in a monolithic nitridation process. In addition, the core–shell hybrid was used for an asymmetric supercapacitor (ASC) anode, and it exhibited a wide potential range of 1.65 V and a maximum energy density of 28.6 Wh kg⁻¹ at a power density of 825.0 W kg⁻¹. Moreover, a green LED was successfully powered by the serial connection of two ASCs. These results and demonstrations prove that our strategies for designing materials composition, hybrid heterostructure, and core–shell configuration are highly effective in improving energy density, making them promising and economical next-generation energy device materials utilizing earth-abundant elements (e.g. Fe, O, N).