General Controlled Sulfidation toward Achieving Novel Nanosheet-Built Porous Square-FeCo2S4-Tube Arrays for High-Performance Asymmetric All-Solid-State Pseudocapacitors

A significant advance toward the design and fabrication of a novel hierarchical supercapacitor electrode consisting of FeCo2S4-tubes with well-defined square cross-section and intersecting nanosheets built porous shells on a 3D porous Ni backbone via controlled sulfidation is reported. This general method allows template-free synthesis of metal sulfides tubular structures with polygonal cross-sections and also fine control over the nanostructure leading to both maximized porosity and saturation sulfidation. New insights into concentration and time dependent sulfidation reaction kinetics are proposed. The FeCo2S4 electrode achieves a specific capacitance reaching 2411 F g(-1) at 5 mA cm(-2) and good rate capability, which are superior over those for nanotube arrays of other ternary transition metal sulfides. This is attributed to rich redox reactions, the highly porous but robust architecture as well as high electrical conductivity. Especially such porous shells effectively avoid "dead volume", thus improve the utilization ratio of the electrode material. Asymmetric solid-state device applying the FeCo2S4 as positive electrode and N-doped graphene hydrogel film as negative electrode has a high cell voltage of 1.6 V and thus delivers considerably higher energy density of 76.1 W h kg(-1) (at 755 W kg(-1)) than those reported for similar devices.