Leveraging Direct Pyrolysis for the Synthesis of 10 nm Monodispersed FeO/FeC 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 FeO@carbon nanofibers supercapacitors (FeO@C). However, our study systematically challenges this notion by demonstrating that pre-oxidation facilitates the formation of polydispersed and large FeO nanoparticles (FeO@C) through "external" Fe Kirkendall diffusion from carbon, resulting in subpar electrochemical properties. To address this, direct pyrolysis of Fe-salt/polymer Nf is proposed, promoting "internal" Fe Kirkendall diffusion within carbon and providing substantial physical confinement, leading to the formation of monodispersed and small FeO nanoparticles (FeO@C). In 1 M HSO, FeO@C demonstrates ~2.60× and 1.26× faster SO diffusivity, and electron transfer kinetics, respectively, compared to FeO@C, with a correspondingly ~1.50× greater effective surface area. Consequently, FeO@C exhibits a specific capacity of 161.92 mAhg, ~2× higher than FeO@C, with a rate capability ~19 % greater. Moreover, FeO@C retains 94 % of its capacitance after 5000 GCD cycles, delivering an energy density of 26.68 Whkg in a FeO@C//FeO@C device, rivaling state-of-the-art FeO/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@C materials.