Tip-welded ferric-cobalt sulfide hollow nanoneedles on highly conductive carbon fibers for advanced asymmetric supercapacitors

The ever-increasing concerns about global environmental problems and depletion of fossil resources have stimulated intensive research on low-cost and high-performance energy storage materials. Here, we introduce the two-step sulfidation of hierarchical ferric-cobalt sulfide hollow nanoneedles on electrical conductive carbon fiber paper (CFP) for high-performance supercapacitor electrodes. Under the growth regulation of macroporous CFP substrate, prepared free-standing composite paper with tip-welded ferric-cobalt sulfide hollow nanoneedles can offer substantial exposed electroactive sites for Faradaic redox reaction and effectively buffer the drastic volume change during long-term charge-discharge, and also provide fast charge transfer paths through the carbon skeleton for efficient energy storage. Due to these unique features, obtained composite paper electrode delivers a higher specific capacitance (2282 F g−1 at current density of 1 A g−1) than those of the previously reported MCo2S4 based electrodes. Asymmetric supercapacitor assembled using the composite paper as the binder-free positive electrode also shows a high energy density of 86.8 Wh kg−1 at a power density of 800 W kg−1, and an impressive 82.3% capacitance retention after 5000 cycles, demonstrating the great potential of this low-cost and free-standing electrodes towards high-performance energy storage.