Leveraging Direct Pyrolysis for the Synthesis of 10 nm Monodispersed Fe₃O₄/Fe₃C NPS@Carbon to Improve SupercapacitANCE in Acidic Electrolyte

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 FeOx@carbon nanofibers supercapacitors (FeOx@C). However, our study systematically challenges this notion by demonstrating that pre-oxidation facilitates the formation of polydispersed and large FeOx nanoparticles (FeOx@CI-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 FeOx nanoparticles (FeOx@C-DA). In 1 M H2SO4, FeOx@C-DA demonstrates similar to 2.60x and 1.26x faster SO42- diffusivity, and electron transfer kinetics, respectively, compared to FeOx@CI-DA, with a correspondingly similar to 1.50x greater effective surface area. Consequently, FeOx@C-DA exhibits a specific capacity of 161.92 mAhg(-1), similar to 2x higher than FeOx@CI-DA, with a rate capability similar to 19 % greater. Moreover, FeOx@C-DA retains 94 % of its capacitance after 5000 GCD cycles, delivering an energy density of 26.68 Whkg(-1) in a FeOx@C-DA//FeOx@C-DA device, rivaling state-of-the-art FeOx/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 FeOx@C materials.