Excellent electrochemical performance of graphene oxide based strontium sulfide nanorods for supercapacitor applications

The supercapattery designed with a binary composite of niobium silver sulfide (NbAg2S) and activated carbon for enhanced electrochemical performance

A supercapattery is a hybrid device that is a combination of a battery and a capacitor. Niobium sulfide (NbS), silver sulfide (Ag₂S), and niobium silver sulfide (NbAg₂S) were synthesized by a simple hydrothermal method. NbAg₂S (50/50 wt% ratio) had a specific capacity of 654 C g^-1, which was higher than the combined specific capacities of NbS (440 C g^-1) and Ag₂S (232 C g^-1), as determined by the electrochemical investigation of a three-cell assembly. Activated carbon and NbAg₂S were combined to develop the asymmetric device (NbAg₂S//AC). A maximum specific capacity of 142 C g^-1 was delivered by the supercapattery (NbAg₂S//AC). The supercapattery (NbAg₂S/AC) provided 43.06 W h kg^-1 energy density while retaining 750 W kg^-1 power density. The stability of the NbAg₂S//AC device was evaluated by subjecting it to 5000 cycles. After 5000 cycles, the (NbAg₂S/AC) device still had 93% of its initial capacity. This research indicates that merging NbS and Ag₂S (50/50 wt% ratio) may be the best choice for future energy storage technologies.

Alkaline-Earth Chalcogenide Nanocrystals: Solution-Phase Synthesis, Surface Chemistry, and Stability

Increasing demand for effective energy conversion materials and devices has renewed interest in semiconductors comprised of earth-abundant and biocompatible elements. Alkaline-earth sulfides doped with rare earth ions are versatile optical materials. However, relatively little is known about controlling the dimensionality, surface chemistry, and inherent optical properties of the undoped versions of alkaline-earth mono- and polychalcogenides. We describe the colloidal synthesis of alkaline-earth chalcogenide nanocrystals through the reaction of metal carboxylates with carbon disulfide or selenourea. Systematic exploration of the synthetic phase space allows us to tune particle sizes over a wide range using a mixture of commercially available carboxylate precursors. Solid-state NMR spectroscopy confirms the phase purity of the selenide compositions. Surface characterization reveals that bridging carboxylates and amines preferentially terminate the surface of the nanocrystals. While these materials are colloidally stable in the mother solution, the selenides are susceptible to oxidation over time, eventually degrading to selenium metal through polyselenide intermediates. As part of these investigations, we have developed the colloidal syntheses of barium di- and triselenides, two among few reported nanocrystalline alkaline-earth polychalcogenides. Electronic structure calculations reveal that both materials are indirect band gap semiconductors. The colloidal chemistry presented here may enable the synthesis of more complex, multinary chalcogenide materials containing alkaline-earth elements.