Transition metal dichalcogenides (TMDs) are emerging low‐dimensional materials with potential applications for electrochemical capacitors (EC). Here, physicochemical and electrochemical characterizations of carbon composites with two sulfides ReS2 and FeS2 are reported. To enhance conductivity, multiwalled carbon nanotubes (NTs) serve as a support for ReS2 while 3D graphene‐like network (3DG) is utilized for FeS2 deposition. Unique structure of carbon/TMDs composites allows a faradaic contribution of sulfides to be exploited. Capacitance values, charge/discharge efficiency, capacitance retention, charge propagation, cyclabilty, and voltage limits of EC with carbon/sulfide composites in aqueous neutral solutions (Li2SO4, Na2SO4) are analyzed. Special attention is devoted to energetic efficiency of capacitive charge/discharge processes. Structure‐to‐capacitance correlation for the composites with various TMDs loading is thoroughly emphasized. The more defected structure of layered NTs/ReS2 composite is responsible for the lower capacitor voltage (0.8 V) owing to quicker electrolyte decomposition. Additionally, the catalytic effect of Re for hydrogen evolution reaction plays a crucial role in EC voltage restriction. Contrary, the operating voltage of capacitor based on 3DG/FeS2 is able to be extended until 1.5 V in sodium sulfate electrolytic solution.

Transition metal dichalcogenides (TMDs) with a two-dimensional character are promising electrode materials for an electrochemical capacitor (EC) owing to their unique crystallographic structure, available specific surface area, and large variety of compounds. TMDs combine the capacitive and faradaic contribution in the electrochemical response. However, due to the fact that the TMDs have a strong catalytic effect of promoting hydrogen and oxygen evolution reaction (HER and OER), their usage in aqueous ECs is questioned. Our study shows a hydrothermal l-cysteine–assisted synthesis of two composites based on different carbon materials—multiwalled carbon nanotubes (NTs) and carbon black (Black Pearl-BP2000)—on which MoS2 nanolayers were deposited. The samples were subjected to physicochemical characterization such as X-ray diffraction and Raman spectroscopy which proved that the expected materials were obtained. Scanning electron microscopy coupled with electron dispersive spectroscopy (SEM/EDS) as well as transmission electron microscopy images confirmed vertical position of few-layered MoS2 structures deposited on the carbon supports. The synthetized samples were employed as electrode materials in symmetric ECs, and their electrochemical performance was evaluated and compared to their pure carbon supports. Among the composites, NTs/MoS2 demonstrated the best electrochemical metrics considering the conductivity and capacitance (150 Fg−1), whereas BP2000/MoS2 reached 110 Fg−1 at a current load of 0.2 Ag−1. The composites were also employed in a two-electrode cell equipped with an additional reference electrode to monitor the potential range of both electrodes during voltage extension. It has been shown that the active edge sites of MoS2 catalyze the hydrogen evolution, and this limits the EC operational voltage below 1 V. Additional tests with linear sweep voltammetry allowed to determine the operational working voltage for the cells with all materials. It has been proven that the MoS2/carbon composites possess limited operating voltage, that is, comparable to a pure MoS2 material.

The use of a pyrite‐like structure transition metal dichalcogenides (TMDs) such as NiS2 is not reported as frequently as layered structured TMDs in electrochemical capacitors. Our study shows a reduced graphene oxide (rGO) preparation and its further use as a substrate for the deposition of NiS2 during the hydrothermal reaction. The influence of pH for the GO solution on the structural and textural properties of the resultant rGO was explained in detail by using Raman spectroscopy, UV, X‐ray diffraction (XRD), scanning electron microscopy (SEM) and nitrogen sorption at 77 K. The electrochemical response of the rGO samples obtained from various pH solutions was studied and compared in symmetric two electrode Swagelok® cells. As a result, rGO pH 12 was chosen for further modification with NiS2. The average size of the crystallites, and electronic structure of the rGO/NiS2 composite have been determined. The effect of the NiS2 content in the composite on the electrochemical results was further investigated. It occurred that 5 wt % NiS2 in the composite was sufficient for a significant improvement of the capacitive response. Interestingly, an asymmetric cell with rGO/5 wt % NiS2 working as a positive electrode and rGO pH 12 as a negative electrode was able to reach a high capacitance of 165 F g−1 at 0.2 A g−1 with 92 % coulombic and 79 % energetic efficiency.

Electrochemical capacitors (ECs) are considered to be the very promising energy storage systems because they possess a high power density as well as a long life span. However, contrarily to the batteries, the energy density of ECs is rather moderate. It is very well known that the energy stored in ECs is mostly dependent on the specific surface area of the electrode materials, i.e., microporosity, but conductivity also plays an important role. Energy can be greatly increased by the utilization of redox reactions from electrolyte and/or electrode materials. For this work, two dimensional transition metal dichalcogenides (2D-TMDs) have been applied as electrode material components. TMDs are layered materials of the MX2 type where M is a transition metal (Mo, Re, W, Co or V) and X is a chalcogen (S, Se or Te). In their structure, one layer of M atoms is sandwiched between two layers of X atoms. Different salts such as sodium molybdenate, ammonium perrhenate, cobalt nitrate with thiourea or L-cysteine as a sulfur source were used as TMD precursors. Taking into account moderate conductivity of dichalcogenides, various mesoporous carbon materials such as carbon nanotubes, reduced graphene oxide rGO, 3D-graphene hydrogel, carbon black have been used by us for composite preparation. In some cases functionalization of multiwalled carbon nanotubes has been performed to improve adhesion in the composite. The amount of carbon in the C/TMD composites varied from 5-30%. Hydrothermal method have been successfully used for carbon/TMD production. The composites were characterized with the various textural/structural methods: scanning electron microscopy (SEM), X-ray diffraction (XRD) and Raman spectroscopy. After a detailed physico-chemical characterization, all composites served for electrode preparation. Mostly, 90wt% of active material, 5 wt% of conducting agent C65 and 5wt% of Teflon as binder were used as electrode components. Two-electrode and three-electrode configuration cells have been applied for capacitance measurements. Aqueous solutions (neutral and alkaline) served as electrolytes (1M lithium sulfate, 6M KOH). The capacitor voltage ranged from 1-2V. The electrochemical performance of EC cells were studied by cyclic voltammetry, galvanostatic charge/discharge and electrochemical impedance spectroscopy. The capacitance values, coulombic and energetic efficiency were affected by composite structure, its specific surface area as well as its conductivity and composition. Typically for faradaic reactions, the lower regime, the lower energetic efficiency was observed. High capacitance values (100 F/g – 300F/g) have been reached for C/MoS2. Selected carbon/TMD composites have been used as a positive electrode whereas a negative electrode was composed only from carbon materials. In such a way, high energy electrochemical capacitor has been constructed and investigated.

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