Leveraging Direct Pyrolysis for the Synthesis of 10 nm Monodispersed Fe₃O₄/Fe₃C NPS@Carbon to Improve...

The prevailing practice advocates pre-oxidation of electrospun Fe-salt/polymer nanofibers (Fe-salt/polymer Nf) before pyrolysis as advantageous in the production of high-performance FeO_x@carbon nanofibers supercapacitors (FeO_x@C). However, our study systematically challenges this notion by demonstrating that pre-oxidation facilitates the formation of polydispersed and large FeO_x nanoparticles (FeO_x@C_I-DA) through "external" Fe³⁺ Kirkendall diffusion from carbon, resulting in subpar electrochemical properties. To address this, direct pyrolysis of Fe-salt/polymer Nf is proposed, promoting "internal" Fe³⁺ Kirkendall diffusion within carbon and providing substantial physical confinement, leading to the formation of monodispersed and small FeO_x nanoparticles (FeO_x@C_DA). In 1 M H₂SO₄, FeO_x@C_DA demonstrates ~2.60× and 1.26× faster SO₄ ^2- diffusivity, and electron transfer kinetics, respectively, compared to FeO_x@C_I-DA, with a correspondingly ~1.50× greater effective surface area. Consequently, FeO_x@C_DA exhibits a specific capacity of 161.92 mAhg^-1, ~2× higher than FeO_x@C_I-DA, with a rate capability ~19 % greater. Moreover, FeO_x@C_DA retains 94 % of its capacitance after 5000 GCD cycles, delivering an energy density of 26.68 Whkg^-1 in a FeO_x@C_DA//FeO_x@C_DA device, rivaling state-of-the-art FeO_x/carbon electrodes in less Fe-corrosive electrolytes. However, it is worth noting that the effectiveness of direct pyrolysis is contingent upon hydrated Fe-salt. These findings reveal a straightforward approach to enhancing the supercapacitance of FeO_x@C materials.