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[en] Composite materials of molybdenum carbide and porous carbon were synthesised from molybdenum carbide using high temperature chlorination method with different applied chlorination times in order to use them as catalyst supports and electrode materials in various devices. X-ray diffraction, Raman spectroscopy, low temperature N2 sorption and high resolution transmission electron microscopy methods were used to characterise the structure of synthesised materials. The microporous-mesoporous material particles consist of β-Mo2C surrounded by porous amorphous carbon material. The partially chlorinated Mo2C particles were not surrounded by a graphitised shell. The specific surface area of the powders increased from 180 m2 g−1 up to 2020 m2 g−1 with increasing chlorination depth, i.e. decreasing carbon content in the particles. The stability and electrochemical behaviour of the synthesised composite materials were studied in 0.5 M H2SO4 solution using cyclic voltammetry and electrochemical impedance spectroscopy. The Mo2C phase in the studied composite materials was not stable in the acidic solution and dissolution of Mo2C was observed. The electrochemically treated working materials from which the Mo2C phase had been electrochemically dissolved had very good stability. The gravimetric capacitance increased with the increase of specific surface area and reached values up to 140 F g−1.
[en] Highlights: • MnO2-SiO2 composite film is prepared by potentiodynamical deposition. • Hierarchical porous MnO2 films is obtained after the etching of SiO2. • The obtained MnO2 film electrode exhibit high specific capacitance. - Abstract: We report a novel silica co-electrodeposition route to prepare nanostructured MnO2 films. Firstly, MnO2-SiO2 composite film was fabricated on a stainless steel substrate by potentiodynamical deposition, i.e. cyclic deposition, and then the SiO2 template was removed by simple immersion in concentrated alkaline solution, leading to the formation of a porous MnO2 (po-MnO2) matrix. The structure and morphology of the obtained films were characterized using Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The electrochemical properties of the po-MnO2 film were evaluated by cyclic voltammetry (CV), galvanostatic charge–discharge (GCD) and electrochemical impedance spectroscopy (EIS). Results showed that this porous MnO2 derived from the MnO2-SiO2 composite film exhibits good electrochemical performance for potential use as a supercapacitor material.
[en] Highlights: • Permselectivity of two cation-exchange membranes between cations was tested. • Weak bases increased the permselectivity Na+ to H+ more than 100 times. • Methanol and ethanol as co-solvent in aqueous solutions changed the permselectivity. • Ethanol had a larger influence on the permselectivity than methanol - Abstract: In view of the separation and purification of salt streams, the permselectivity of ion-exchange membranes between different counter-ions is of interest. This paper investigates the permselectivity of two cation-exchange membranes (Neosepta CMS and Neosepta CMX). The separation between either sodium/calcium, sodium/protons or calcium/protons was studied and the influence of operating parameters was tested. It was found that a change in concentration ratio did not have a significant effect on the permselectivity. Furthermore, the monovalent selective membrane enables the preferential removal of sodium in an excess amount of calcium. The separation of sodium or calcium from protons is hampered because of the high mobility of protons in aqueous solutions. In the presence of weak bases, such as acetate and formate, the permselectivity (Na+ relative to H+) is increased more than 100 times. As a solvent either water or a solvent mixture of water with methanol or ethanol was used. Generally, the permselectivity changed according to the variation in ion mobility in the solvent mixture. The permselectivity between sodium and protons was enhanced, while the permselectivity between sodium and calcium was decreased. Hereby, the more nonpolar co-solvent, ethanol, has a larger influence than methanol.
[en] We present a new hybrid material composed of molybdenum (IV) oxide (MoO2) shell on highly conducting silver nanowire (Ag NW) core in the network form for the realization of coaxial Ag NW/MoO2 nanocomposite supercapacitor electrodes. Ag NWs were simply spray coated onto glass substrates to form conductive networks and conformal MoO2 layer was electrodeposited onto the Ag NW network to create binder-free coaxial supercapacitor electrodes. Combination of Ag NWs and pseudocapacitive MoO2 generated an enhanced electrochemical energy storage capacity and a specific capacitance of 500.7 F/g was obtained at a current density of 0.25 A/g. Fabricated supercapacitor electrodes showed excellent capacity retention after 5000 cycles. The methods and the design investigated herein open a wide range of opportunities for nanowire based coaxial supercapacitors.
[en] Secondary LiFePO4/C microspheres (LFP) are synthesized with different carbon sources by the spray drying process. The carbon sources effect on the structures, morphologies, and 3D conductivity of the secondary structure are systematically investigated. LFP samples prepared with polyethylene glycol (PEG) and beta-cyclodextrin (β-CD) as mixing carbon sources possesses the loose structure with higher specific surface area, showing the best rate capability, cycling stability and low-temperature discharge characteristic. Additionally, the differences of 3.3 V plateau performance at room temperature and 2.85 V plateau performance at −20 °C are investigated. It could be observed that the electronic and ionic conductivities are reduced gradually with the decrease of the discharge cut-off voltage, while the electronic conductivities are greater than ionic conductivities for the four LFP samples, indicating that the ionic transport is more difficult and the electrochemical reaction is more and more difficult with the increase of Li-ion intercalation. Li-ion diffusion coefficients at the cut-off voltage of 3.30 V under room temperature and at the cut-off voltage of 2.85 V under −20 °C are both the highest for the LFP sample synthesized with PEG and β-CD, further indicating that PEG and β-CD as mixing carbon sources can decrease the charge transfer resistance and promote the 3D electronic/ionic conductivities and Li-ion diffusion coefficients in the secondary structure, thus greatly improve the rate capability, cycling stability and low-temperature capacity of LFP cathode.
[en] Al@C/expanded graphite was successfully synthesized through a facile method including ultrasonic and heat treatment. In the well-designed three dimensional structure, expanded graphite works as a conductive matrix to support Al particles coated by carbon. The structure integrates the advantages of carbon coating and expanded graphite as conductive support, preserving the electrode activity and integrity and improve the electrochemical performances. Al@C/expanded graphite delivers 657.4 mAh g−1 at 50 mA g−1 and 75.9% capacity retention after 50 cycles. The preliminary results demonstrate that Al@C/expanded graphite is a potential candidate for anode material of lithium ion batteries.
[en] Highlights: • A combination strategy of bulk Zr doping and PPy surface coating is proposed. • Enhanced electrochemical performances are reached using combination strategy. • Improved electrochemical property is ascribed to stable structure and better Li+ ion and electron diffusivity. - Abstract: The combination of zirconium doping and surface conductive polypyrrole coating is successfully demonstrated to enhance the high voltage electrochemical performances of LiNi0.5Co0.2Mn0.3O2 (NCM) cathode materials. The lattice parameters calculated from the X-ray diffraction (XRD) patterns by Rietveld refinement reveal that the cation mixing degree is restrained and the lithium slabs spacing is broadened after zirconium substitution (NCMZ). A compact and continuous polypyrrole thin film is satisfactorily coated onto the surface of zirconium-doped (NCMZ/PPy) sample, which is confirmed by fourier transform infrared spectrometry (FT-IR), scanning electron microscopy (SEM) and transmission electron microscope (TEM). Though the initial discharge capacity of NCMZ/PPy sample is slightly lower than pristine one, the cycling stability is dramatically improved. Notably, NCMZ/PPy sample shows capacity retention of 89.83% at 1C after 100 repeated cycles, while that of pristine one remains only 69.36%. When the electrodes cycled at 2C for 200 cycles, the capacity retention of NCMZ/PPy still reaches up to 78.83%, and that of pristine one is only 56.79%. The presence of polypyrrole network acts as a capsule shell, which can protect the active substance from erosion caused by the HF attacking. As a result, the cycling stability and rate performance of co-modified sample are both better than those of bare LiNi0.5Co0.2Mn0.3O2 sample and zirconium doped materials.
[en] A polycrystalline boron-doped diamond (BDD) electrolyte solution-gate field effect transistor (SGFET) for use as a pH sensor was developed. The polycrystalline diamond films with a boron-doped layer possessed semiconducting properties that were comparable to hydrogen-terminated non-doped diamond. The hydrogen-terminated BDD surface was successfully transferred to a partially oxygen-terminated surface by ozone exposure, and its SGFET current–voltage (I–V) characteristics were evaluated with bias voltages within the potential window of diamond. The drain-source current(Ids)–drain-source voltage(Vds) characteristics showed pinch-off and saturation. In addition, they stably operated in electrolyte solutions with pH values from 2 to 12. The transfer characteristics exhibited a pH sensitivity of approximately 30 mV/pH, which is comparable with the pH sensitivity of the conventional oxygen-terminated non-doped SGFET and the single-crystal BDD SGFET investigated in our previous work. Furthermore, the BDD SGFET exhibited improved long-term stability, and the coefficient of variation (CV) of Ids for 10 months was up to 10%.
[en] Highlights: • Dissolved O_2 molecules in H_2O-EG system react with carbon atoms on GO sheets. • EG in mixed solvents reduces holey GO into holey graphene sheets. • Holey graphene sheets assemble into aerogel at higher GO concentration. • The aerogel built with holey graphene show improved ion transport property. - Abstract: A green yet effective solution-based route for scalable preparation of holey graphene was developed by solvothermal reaction of graphene oxides (GO) in water-ethylene glycol system (H_2O-EG). The O_2 molecules dissolved in H_2O-EG system was found to serve as a moderate oxidant which can oxidize partial carbon atoms on GO sheets, leaving behind nanopores on graphene sheets. Importantly, by increasing the GO concentration to 2.0 mg mL"−"1, these holey graphene sheets can assemble into macroscopic aerogel (HGA). Benefiting from the in-plane nanopores, HGA showed much improved surface area as compared with the graphene aerogel built with non-holey graphene sheets (GA). The electrochemical characterization results showed that holey graphene sheets in HGA contributed to a higher specific capacitance, an improved rate capability and faster ion kinetic when serving as binder-free supercapacitor electrode.
[en] Highlights: • A new hyperbranched multi-arm star polymer was successfully synthesized. • The star polymer electrolyte has good thermal stability and forming-film property. • The ion conductivity electrolyte can reach 8.3 × 10"−"5 S cm"−"1 at room temperature. • The star polymer electrolyte has wide electrochemical windows of 4.7 V. - Abstract: A new hyperbranched multi-arm star polymer with hyperbranched polystyrene (HBPS) as core and polymethyl methacrylate-block-poly(ethylene glycol) methyl ether methacrylate(PMMA-b-PPEGMA) as arms was firstly synthesized by atom transfer radical polymerization. The obtained hyperbranched multi-arm star polymer (HBPS-(PMMA-b-PPEGMA)_x) exhibited good thermal stability with a thermal decomposition temperature of 372 °C. The transparent, free-standing, flexible polymer electrolyte film of the blending of HBPS-(PMMA-b-PPEGMA)_x and lithium bis(trifluoromethanesulfonyl) imide (LiTFSI) was successfully fabricated by a solution casting method. The ionic conductivity of the hyperbranched star polymer electrolyte with a molar ratio of [EO]/[Li] of 30 could reach 8.3 × 10"−"5 S cm"−"1 at 30 °C (with the content of PPEGMA of 83.7%), and 2.0 × 10"−"4 S cm"−"1 at 80 °C (with the content of PPEGMA of 51.6%). The effect of the concentration of lithium salts on ionic conductivity was also investigated. The obtained all-solid-state polymer electrolyte possessed a wide electrochemical stability window of 4.7 V (vs. Li"+/Li), and a lithium-ion transference number (t_L_i"+) up to 0.31. The interfacial impedance of the fabricated LiÔöépolymer electrolyteÔöéLi symmetric cell based on hyperbranched star multi-arm polymer electrolyte exhibited good interfacial compatibility between all-solid-state polymer electrolyte and electrodes. The excellent properties of the hyperbranched star polymer electrolyte made it attractive as solid-state polymer electrolyte for lithium-ion batteries.