Cobalt pentlandite structured (Fe,Co,Ni)9S8: Fundamental insight and evaluation of hybrid supercapacitor

The preparation of metal sulfide electrodes using the synergistic effect between multi-metal elements and their application to the field of supercapacitors is a highly promising technology. For metal sulfides, which have been extensively studied, the introduction of new structures (cobalt pentlandite structure) can promote the improvement of metal sulfide systems and give them new vitality for better design, improvement and development of high-performance supercapacitor materials. In this study, we have designed a ternary metal sulfide (FeCoNi) 9 S 8 with a cobalt pentlandite structure in the form of nanoflowers, in which the three atoms of Fe, Co, and Ni occupy equal positions in the crystal structure, and the synergistic effect between the atoms can be well realized in the structure. The optimized FCNS-2 has shown a remarkably high-specific capacity (970 F g -1 at a current density of 1 A g -1 ) as well as favorable cycling stability (87.1% capacitance retention over 5000 cycles at 6 A g -1 ). Moreover, a button-type asymmetric supercapacitor (ASC)was fabricated utilising our designed electrode material together with an activated carbon (AC) electrode and showed a relatively high-energy density of 50 Wh kg -1 for a power density up to 1353 W kg -1 and excellent cycle stability. Also, two ASC can power a red LED for 22 min, which has shown its attractive and promising possibilities for practical energy storage devices. • We synthesized the cobalt pentlandite structured (FeCoNi) 9 S 8 originated from natural ores in the laboratory by relatively mild means, and found that the material of this structure has good potential for use in supercapacitor materials. • The structure–activity correlation of the optimized FCNS-2 with cobalt pentlandite structure in supercapacitor applications and the real active sites were carefully investigated by DFT calculations. • A button-type ASC for FCNS-2//AC devices show a superb energy density of 50 Wh g -1 for a power density up to 1535 W kg -1 and maintain a great capacity retention rate after continuous cycles. • Cobalt pentlandite structured metal sulfides will be an important addition to the family of polymetallic sulfide materials available for hybrid energy storage devices. From the perspective of improving energy density, research efforts to design electrode materials that can be used in supercapacitors based on mature transition metal sulfides are already so common and efficient but insufficient. So it is a nice approach to empower the traditional transition metal sulfides with new vitality by composite structures to enrich the family of metal sulfides. In this study, we synthesized a kind of cobalt pentlandite structured (Fe,Co,Ni) 9 S 8 originally existed only in natural ores by relatively mild means, and found that such structured materials have good potential for use in supercapacitors. The optimized (Fe,Co,Ni) 9 S 8 sample presents a remarkably high specific capacity of 135 mAh g -1 at 1 A g -1 as well as favorable cycling stability. Density functional theory simulations confirmed the metallic properties of such sample, and Co sites are more favorable for the adsorption of OH - . Moreover, a button-type hybrid supercapacitor was fabricated utilizing our designed electrode material, presenting a relatively high energy density of 50 Wh kg -1 at 1353 W kg -1 and good cyclic stability. Moreover, two such hybrid supercapacitors can power a red LED for 22 min, which has shown its attractive and promising possibilities for practical energy storage devices.