Ferromagnetic self-assembled heterostructures of Co/Co3O4 embedded N-doped carbon network for outstanding energy storage performance under moderate magnetic fields

This study aims to create an efficient anode hybrid electrode material for the assembly of asymmetric supercapacitor (ASC) and to further enhance electrochemical performance under moderate external magnetic fields (MFs), addressing the challenges of low energy density in supercapacitor devices while maintaining cycle life and power density. The well-integrated hybrid heterostructures, which embed Co/Co₃O₄ nanoparticles in nitrogen (N)-doped carbon shells and sheets (Co@N–CNS), demonstrate exceptional physical and electrochemical properties. At a current density of 1.5 A g⁻¹, it exhibits a specific capacitance of 1579.8 F g⁻¹ in a half-cell configuration. Furthermore, the gravimetric capacitance increases to 2429 F g⁻¹ under a MF of 6 mT. The enhanced energy storage performance is attributed to the reduced charge transfer resistance (R_ct) and solution resistance (R_s) resulting from magnetoresistance and magnetic hydrodynamics (MHD) effects. The asymmetric supercapacitor exhibits an outstanding energy density of 221.4 W h kg⁻¹ and power density of 1.58 kW kg⁻¹ at 1.5 A g⁻¹ under a 6 mT MF strengths. It also demonstrates excellent cycling stability (96.2 %) after 10,000 cycles under the same field strength. These results outperform most cobalt-based hybrid electrode materials reported to date. This is the first investigation on a ferromagnetic hybrid electrode material for supercapacitors that demonstrates superior electrochemical performance under both zero and moderate MFs.