Nitrogen-rich hollow carbon spheres decorated with FeCo/fluorine-rich carbon for high performance symmetric supercapacitors

Abstract

A novel approach to fluorine-rich carbon (FC) shell formation on an electron-rich metal surface based on the electronegativity concept is reported. Basically, highly electronegative elements are strongly attracted by low electronegative/electron-rich elements through dipole interaction, which leads to the formation of fluorine-rich carbon shells on metals at various fluorine quantities. Herein, nitrogen-rich hollow carbon spheres decorated with fluorine-rich carbon shell covered metals (FC@M/NHCS, M = Fe, Co, and FeCo) were synthesized by co-polymerization on SiO2, adsorption of metal precursors, and etching of the SiO2, followed by sintering. The fluorine content, quantified by XPS and SEM-EDS studies, decreased according to FeCo > Fe > Co in FC@M/NHCS. HAADF-STEM elemental mapping studies clearly confirmed fluorine-rich carbon shell formation on the metal surface. The influence of fluorine content order in the as-synthesized materials was reflected in their capacitance performances. FC@FeCo/NHCS electrode depicted the maximum specific capacitance of 302.0 F g-1 at 0.2 A g-1 in 6 M KOH medium, delivering excellent stability with no losses over 5000 cycles at 5 A g-1. The symmetric supercapacitor (SSC) devices operated at 1.5 V by delivering maximum device specific capacitance of 51.2 F g-1 at 0.2 A g-1. It exhibited 81.3% of capacitance retention at 10 A g-1 with the FC@FeCo/NHCS. The maximum energy density of 15.3 W h kg-1 at 0.2 A g-1 and the maximum power density of 5100 W kg-1 at 10 A g-1 were delivered by the FC@FeCo/NHCS device. This study provides an ideal way for synthesizing fluorine-rich carbon materials for high energy storage/conversion applications. © 2018 The Royal Society of Chemistry