Fe-regulated δ-MnO2 nanosheet assembly on carbon nanofiber under acidic condition for high performance supercapacitor and capacitive deionization

Highlights: • The δ-MnO₂@Fe-CNFs with various morphology were fabricated by varying pH and Fe ratio. • The δ-MnO₂@Fe-CNF-5% at pH = 2 shows tubular structure with uniform MnO₂ nanosheet assembly. • The δ-MnO₂@Fe-CNF-5% has excellent supercapacitor performance. • The δ-MnO₂@Fe-CNF-5% achieves a high salt adsorption of 20 mg/g in CDI device. • The δ-MnO₂@Fe-CNF-5% has high stability, superior to δ-MnO₂@CNF. Birnessite-type MnO₂ (δ-MnO₂) nanosheet assembly with various structures were achieved by redox reaction of KMnO₄ under acidic condition with Fe regulation on Fe-doping carbon nanofibers (Fe-CNFs)_. The Fe-CNFs with various Fe contents were conveniently obtained by carbonization of the electrospun ferric acetylacetonate-polyacrylonitrile (AAI-PAN) fiber with various AAI ratios. X-ray diffraction and transmission electron microscopy demonstrated the formation of δ-MnO₂ on the Fe-CNFs. pHs of KMnO₄ solution and Fe content in the fiber affected the morphology of δ-MnO₂. At pH = 2, the uniform δ-MnO₂ nanosheet assembly was transferred into a tubular structure by adjusting Fe content in the Fe-CNFs template (AAI = 5%, mass ratio). The obtained δ-MnO₂@Fe-CNF-5% exhibited the highest aspect ratio, large surface area and best charge-transfer behavior. The δ-MnO₂@Fe-CNF-5% electrode delivered a specific capacitance of 210 F/g (0.3 A/g) and a superior cycling stability with 94% capacitance retention in 4500 cycles. The δ-MnO₂@Fe-CNF-5%, as a negative electrode presented an excellent performance both in supercapacitor with high energy density (20 Wh/kg) and in capacitive deionization cell with the salt adsorption capacity of 20 mg/g. The unique nanostructure and excellent electrochemical performance render the δ-MnO₂@Fe-CNF-5% composite as a much promising material for charge storage and deionization applications.