A flexible solid-state supercapacitor based on iron sulfide (FeS2) nanospheres supported on carbon-paper is fabricated, which exhibits excellent electrochemical performance such as, high capacitance of 484 F g(-1) at a scan rate of 5 m Vs(-1), good rate capability, and excellent cycling stability (95.7% after 5000 cycles). The supercapacitor achieves high energy density of 44 Wh kg(-1) at power density of 175 W kg(-1) with high coulombic efficiency (97%). Three charged supercapacitors connected in series can power 12 green-color light-emitting-diodes (LED, 3.0 V, 20 mA) for 5.5 minutes. To understand the detailed electrochemistry, we have carried out both experimental and theoretical investigations. The pseudocapacitive characteristics of the FeS2 nanospheres are systematically investigated by a single electrode in aqueous electrolyte. According to our structural analysis, the FeS2 nanospheres have orthorhombic structure, where Fe atoms are surrounded by 6 S atoms to form a FeS6 octahedron. These octahedrons are connected to form a network structure, which provide tunnels (2.55 x 4.77 angstrom). With all the evidence, we believe that the FeS2 nanospheres could be a promising material for supercapacitor electrodes. (C) 2016 Elsevier Ltd. All rights reserved.