Tuning the energy storage dynamics of electrospun Fe-based carbon nanofiber: Supercapacitor to supercapattery devices

The quest for engineering a prominent negative electrode material for energy storage applications has been gaining significant attraction. Among all the potential materials, the iron-based materials (like-Fe2O3, Fe3O4, and FeOOH) are considered to be important options due to their high theoretical capacitance (3625 F/g, 2299 F/g, and 2606 F/g), variable oxidation states, natural abundance, low cost, and high over potential for H2 evolution. However, low electronic conductivity and large volume expansion results in their poor rate performance and limited cycling stability. Among several processes for structural modification, electrospinning tends to offer multi-faceted advantages with industrial scalability and ease of fabrication. In this work, we report a study which focuses on the understanding of electrochemical energy storage mechanism of the electrospun carbon nanofiber (CNF) embedded with iron-based materials through surface tuning. We utilized two oxidative transformations that include in-situ electrochemical transformation, and ex-situ (post) thermal oxidative transformation for converting surface embedded iron/iron carbide into iron-oxide, distributed over CNFs. The detailed electrochemical studies revealed that the sample developed through the in-situ process exhibited supercapacitive property with specific capacitance of 328 F/g at 1 A/g, while the sample developed through the ex-situ process displayed supercapattery property having specific capacity of 640C/g at 2 A/g. The aqueous supercapacitor device possessed specific capacitance of 52 F/g at 1 A/g with a maximum power density of 18.76 kW/kg at the energy density of 5.21 Wh/kg, whereas the aqueous supercapattery device exhibited specific capacity of 189C/g at 1 A/g and achieved maximum energy density of 39.37 Wh/kg at power density of 750 W/kg. Furthermore, we have successfully fabricated and systematically studied a high-voltage (7.8 V) symmetric supercapattery device that could endure 35,000 cycles, retaining 83.3 % of its initial performance at 0.33 A/g, without a need of conductive carbons and binder additives.