Fabrication of Assembled FeS 2 Nanosheet and Application for High- Performance Supercapacitor Electrodes Farkhod Azimov, Jinseok Lee, Subin Park, and Hyun Min Jung* Cite This: ACS Appl. Mater. Interfaces 2023, 15, 26967-26976 Read Online ACCESS Metrics & More Article Recommendations * sı Supporting Information ABSTRACT: To overcome the low-energy-density limitation of supercapacitors, we aimed to achieve a material with a high specifc capacitance by manipulating the nanostructure of FeS 2 , which comprises the most abundant and afordable elements. In this study, nanosheet-assembled FeS 2 (NSA-FeS 2 ) was fabricated using a novel method. Sub-micron droplets of sulfur particles stabilized with polyvinylpyrrolidone were formed in silicone oil medium, and Fe(CO) 5 was absorbed and reacted on the surface to form core- shell particles, ES/[Fe], with a sulfur core and an iron-containing outer shell. The high temperature treatment of ES/[Fe] produced NSA-FeS 2 , in which pyrite FeS 2 nanosheets grew and were partially interconnected. In a three-electrode system, the as-prepared NSA-FeS 2 and NSA-FeS 2 /polyaniline (PANI) composites exhibited specifc capacitances of 763 and 976 Fg −1 , respectively, at a current density of 0.5 Ag −1 , with corresponding capacitance retentions of 93 and 96% after 3000 charge−discharge cycles. The capacitance retention of the NSA-FeS 2 /PANI composites was 49% when the current density was increased from 0.5 to 5 Ag −1 . Notably, the obtained specifc capacitances exhibited the highest values in pure FeS 2 and FeS 2 -based composites, indicating the signifcant potential for the utilization of iron sulfde in pseudocapacitive electrode materials. KEYWORDS: iron sulfide electrode, pyrite, nanosheet-assembled, supercapacitor, high capacitance 1. INTRODUCTION Supercapacitors (SCs), which have high power densities and rapid charging/discharging characteristics, have been used in electric vehicles and mobile electric devices to complement batteries. However, the low energy density of SCs is a signifcant limitation; thus, electrode materials with high capacitance are constantly being developed to overcome this obstacle. 1−4 WO 3 , TiO 2 , NiO, RuO 2 , MnO 2 , and Fe 2 O 3 have been used to develop pseudocapacitive electrode materials with a capacitance ten or more times greater than the electrical double-layer capacitance of carbon-based electrodes. 5−10 With the exception of RuO 2 , the generally low electrical conductivity of metal oxides results in lower capacitances than the theoretical capacitances of metal oxides, which is a fundamental disadvantage. 11,12 Recently investigated transition metal di-chalcogenide pseudocapacitive materials have drawn the attention of researchers owing to their semiconducting range. Furthermore, owing to their multivalent redox states, metal di-chalcogenides exhibit a relatively high specifc capacitance. Various transition-metal dichalcogenides, such as MoS 2 , CuS, CoS 2 , WS 2 , and NiS, have been studied and evaluated as SC electrode materials over the last decade. 13−18 Iron-based materials have always been of interest among various metal sulfdes and oxides because iron is one of the most abundant and economically and environmentally preferred metals. However, despite signifcant research on nanostructures, the retention capacity of iron oxides has not exceeded 200 Fg −1 ; thus, despite the advantages of iron, iron- based materials have not received signifcant attention. Iron sulfdes exhibit higher capacitances than the oxides because of the narrow bandgap (0.95 eV), conductivities in the semi- conductor range and multivalent redox activities of the former, indicating their potential as low-cost, stable, and environ- mentally friendly high-capacitance electrode materials. Owing to their high practical application potential due to these characteristics, iron sulfde electrode materials with various structures and composites have been intensively studied. 19−22 Research on FeS 2 as an electrode material can be divided into two categories: evaluation of the performance of FeS 2 alone, and that of FeS 2 combined or composited with a nanostructured support. The capacitance of pure pyrite (FeS 2 ) was evaluated using nanoparticles produced by the solvother- mal technique, yielding a capacitance of 260 Fg −1 and an energy density of 46.8 Wh kg −1 . 23 Based on the fndings of similar studies, this is thought to be the typical capacity level of pure Received: March 14, 2023 Accepted: May 12, 2023 Published: May 24, 2023 Research Article www.acsami.org © 2023 American Chemical Society 26967 https://doi.org/10.1021/acsami.3c03707 ACS Appl. Mater. Interfaces 2023, 15, 26967−26976 Downloaded via KUMOH NATL INST OF TECHNOLOGY on July 6, 2023 at 08:30:19 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.