Leveraging Direct Pyrolysis for the Synthesis of 10 nm Monodispersed Fe 3 O 4 /Fe 3 C 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 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" Fe 3+ Kirkendall diffusion from carbon, resulting in subpar electrochemical properties. To address this, direct pyrolysis of Fe-salt/polymer Nf is proposed, promoting "internal" Fe 3+ 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 H 2 SO 4 , FeO x @C DA demonstrates ~2.60× and 1.26× faster SO 4 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.