Flexible Fe3O4@Carbon Nanofibers Hierarchically Assembled with MnO2 Particles for High-Performance Supercapacitor Electrodes

Self-enhanced and efficient removal of As(III) from water using Fe-Cu-Mn composite oxide under visible-light irradiation: Synergistic oxidation and mechanisms

Here, we prepared a novel nanostructured Fe-Cu-Mn composite oxide (FCMO_x) adsorbent using an ultrasonic coprecipitation method. The maximum adsorption capacity of As(III) and As(V) reached 158.5 and 115.2 mg/g under neutral conditions, respectively. The effects of several environmental factors (coexisting ions, solution pH, etc.) on the removal of inorganic arsenic using FCMO_x were studied through batch experiments. The results showed that except for PO₄^3- and high initial pH, it was not significantly affected by ionic strength and other existing anions, implying a higher selectivity and adaptability. Combined with EPR, FTIR, and XPS analysis, we concluded that the Cu component and the reactive oxygen species (ROS) it generates played a decisive role in maintaining the stability of the redox cycle between Mn(IV)/Mn(III)/Mn(II) and enhancing the oxidation efficiency of As(III). Meanwhile, the adsorption mechanism of As(V) was mainly through the replacement of the FCMO_x surface -OH to form stable inner-sphere arsenic complexes, while the removal mechanism of As(III) may involve the process of synergistic oxidation and chemisorption coupling. Additionally, the effective removal of As from the simulated As-contaminated water and its satisfactory reuse performance make FCMO_x adsorbents favorable candidates for the removal of As-contaminated water in the future.

Synthesis of V-shaped MnO2 nanostructure and its composites with reduced graphene oxide for supercapacitor application

A unique V-shaped MnO₂ nanostructure is synthesized with a weak acid (acetic acid) using the microwave-assisted hydrothermal technique. To improve the performance of MnO₂ in supercapacitor applications, its composite was prepared with reduced graphene oxide (rGO), i.e., MnO₂/rGO, with different weight ratios of MnO₂ and rGO. The specific capacitance values of the as-synthesized V-shaped MnO₂ nanostructure and MnO₂/rGO nanocomposite were calculated to be 64.75 and 88.95 F g^-1 at a current density of 0.5 A g^-1, respectively, in 1 M Na₂SO₄ electrolyte. Furthermore, a two-electrode asymmetric supercapacitor device was fabricated using the MnO₂/rGO nanocomposite as a positive electrode and activated carbon as a negative electrode. The device has shown energy densities of 25.14 and 17.95 W h kg^-1 at 0.25 and 1 kW kg^-1 power densities, respectively. These values suggest that the MnO₂/rGO nanocomposite is a promising material for supercapacitor devices.

Investigation on the Mass Distribution and Chemical Compositions of Various Ionic Liquids-Extracted Coal Fragments and Their Effects on the Electrochemical Performance of Coal-Derived Carbon Nanofibers (CCNFs)

Coal-derived carbon nanofibers (CCNFs) have been recently found to be a promising and low-cost electrode material for high-performance supercapacitors. However, the knowledge gap still exists between holistic understanding of coal precursors derived from different solvents and resulting CCNFs’ properties, prohibiting further optimization of their electrochemical performance. In this paper, assisted by laser desorption/ionization (LDI) and gas chromatography–mass spectrometry (GC–MS) technologies, a systematic study was performed to holistically characterize mass distribution and chemical composition of coal precursors derived from various ionic liquids (ILs) as extractants. Sequentially, X-ray photoelectron spectroscopy (XPS) revealed that the differences in chemical properties of various coal products significantly affected the surface oxygen concentrations and certain species distributions on the CCNFs, which, in turn, determined the electrochemical performances of CCNFs as electrode materials. We report that the CCNF that was produced by an oxygen-rich coal fragment from C₆mimCl ionic liquid extraction showed the highest concentrations of quinone and ester groups on the surface. Consequentially, C₆mimCl-CCNF achieved the highest specific capacitance and lowest ion diffusion resistance. Finally, a symmetric carbon/carbon supercapacitor fabricated with such CCNF as electrode delivered an energy density of 21.1 Wh/kg at the power density of 0.6 kW/kg, which is comparable to commercial active carbon supercapacitors.

Enhanced heavy metal removal from an aqueous environment using an eco-friendly and sustainable adsorbent

Thiol-lignocellulose sodium bentonite (TLSB) nanocomposites can effectively remove heavy metals from aqueous solutions. TLSB was formed by using -SH group-modified lignocellulose as a raw material, which was intercalated into the interlayers of hierarchical sodium bentonite. Characterization of TLSB was then performed with BET, FTIR, XRD, TGA, PZC, SEM, and TEM analyses. The results indicated that thiol-lignocellulose molecules may have different influences on the physicochemical properties of sodium bentonite, and an intercalated-exfoliated structure was successfully formed. The TLSB nanocomposite was subsequently investigated to validate its adsorption and desorption capacities for the zinc subgroup ions Zn(II), Cd(II) and Hg(II). The optimum adsorption parameters were determined based on the TLSB nanocomposite dosage, concentration of zinc subgroup ions, solution pH, adsorption temperature and adsorption time. The results revealed that the maximum adsorption capacity onto TLSB was 357.29 mg/g for Zn(II), 458.32 mg/g for Cd(II) and 208.12 mg/g for Hg(II). The adsorption kinetics were explained by the pseudo-second-order model, and the adsorption isotherm conformed to the Langmuir model, implying that the dominant chemical adsorption mechanism on TLSB is monolayer coverage. Thermodynamic studies suggested that the adsorption is spontaneous and endothermic. Desorption and regeneration experiments revealed that TLSB could be desorbed with HCl to recover Zn(II) and Cd(II) and with HNO3 to recover Hg(II) after several consecutive adsorption/desorption cycles. The adsorption mechanism was investigated through FTIR, EDX and SEM, which demonstrated that the introduction of thiol groups improved the adsorption capacity. All of these results suggested that TLSB is an eco-friendly and sustainable adsorbent for the extraction of Zn(II), Cd(II) and Hg(II) ions in aqueous media.

Pulse electrodeposited manganese oxide on carbon fibers as electrodes for high capacity supercapacitors

Arrays of manganese dioxide (MnO₂), a pseudocapacitive material, have been deposited on the carbon fibers (CF) of a carbon woven fabric by electrodeposition from a solution containing manganese sulfate and sulfuric acid using galvanostatic square waves. The thickness of the MnO₂ was varied by increasing/decreasing the time of deposition, and the electrochemical performance of the MnO₂ has been analyzed. The CF serves as a substrate material with high surface area, good electrical conductivity and excellent mechanical strength. The electrochemical properties of the resultant electrode were examined by cyclic voltammogram (CV), galvanostatic charge/discharge, and electrochemical impedance spectroscopy in a three-electrode system. From the specific capacitance calculations obtained from CV and charge-discharge, a high specific capacitance of 769 F g^-1 @ 5 mV s^-1 (low weight electrode) has been achieved. The maximum area capacitance, estimated from the charge-discharge curves, was 790 mF cm^-2 @ 5 mV s^-1. The high-performance is attributed to the double layer capacitance of the CFs combined with the pseudocapacitive nature of MnO_2, the large surface area, and high degree of ordering of the ultrathin MnO₂.

Hydrophilic “bridge” tannins for stabilizing the metal selenides onto activated carbon for binder-free and ultralong-life asymmetric supercapacitors

Tannin-doped activated carbon by employing tannin as the structure coupling bridge between the two components could effectively tune the electronic structural states and result in strong coupling effects with NiCo₂Se₄.

3D Interconnected Binder-Free Electrospun MnO@C Nanofibers for Supercapacitor Devices

Rational design of binder-free materials with high cyclic stability and high conductivity is a great need for high performance supercapacitors. We demonstrate a facile one-step synthesis method of binder-free MnO@C nanofibers as electrodes for supercapacitor applications. The topology of the fabricated nanofibers was investigated using FESEM and HRTEM. The X-ray photoelectron spectroscopy (XPS) and the X-ray diffraction (XRD) analyses confirm the formation of the MnO structure. The electrospun MnO@C electrodes achieve high specific capacitance of 578 F/g at 1 A/g with an outstanding cycling performance. The electrodes also show 127% capacity increasing after 3000 cycles. An asymmetric supercapacitor composed of activated carbon as the negative electrode and MnO@C as the positive electrode shows an ultrahigh energy density of 35.5 Wh/kg with a power density of 1000 W/kg. The device shows a superior columbic efficiency, cycle life, and capacity retention.