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@CDA). In 1 M H2SO4, FeOx@CDA demonstrates ~2.60× and 1.26× faster SO42− diffusivity, and electron transfer kinetics, respectively, compared to FeOx@CI−DA, with a correspondingly ~1.50× greater effective surface area. Consequently, FeOx@CDA exhibits a specific capacity of 161.92 mAhg−1, ~2× higher than FeOx@CI−DA, with a rate capability ~19 % greater. Moreover, FeOx@CDA retains 94 % of its capacitance after 5000 GCD cycles, delivering an energy density of 26.68 Whkg−1 in a FeOx@CDA//FeOx@CDA 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. The commonly accepted practice of pre–oxidizing electrospun Fe–salt/polymer nanofibers before pyrolysis to produce high–performing FeOx@CNF supercapacitors is challenged in this study. We demonstrate that this traditional approach is flawed due to the polydispersity and large size of FeOx resulting from pre–oxidation. Instead, direct pyrolysis of Fe–salt/polymer nanofibers yields monodispersed and small FeOx exhibiting state–of–the–art specific capacity, stability, and rate performance.