Molten salt synthesis of porous carbon and its application in supercapacitors: A review

Carbon materials have taken an important role in supercapacitor applications due to their outstanding features of large surface area,low price,and stable physicochemical properties.Considerable research efforts have been devoted to the development of novel synthesis strategy for the preparation of porous carbon materials in recent years.In particular,molten salt strategy represents an emerging and promising method,whereby it has shown great potential in achieving tailored production of porous carbon.It has been proved that the molten salt-assisted production of carbon via the direct carbonization of carbonaceous precursors is an effective approach.Furthermore,with the incorporation of electrochemical technology,molten salt synthesis of porous carbon has become flexible and diversiform.Here,this review focuses on the mainstream molten salt synthesis strategies for the production of porous carbon materials,which includes direct molten salt carbonization process,capture and electrochemical conversion of CO_(2)to value-added carbon,electrochemical exfoliation of graphite to graphene-based materials,and electrochemical etching of carbides to new-type carbide-derived carbon materials.The reaction mechanisms and recent advances for these strategies are reviewed and discussed systematically.The morphological and structural properties and capacitive performances of the obtained carbon materials are summarized to reveal their appealing points for supercapacitor applications.Moreover,the opportunities and challenges of the molten salt synthesis strategy for the preparation of carbon materials are also discussed in this review to provide inspiration to the future researches.

Molten salt synthesis of α-MnO_(2)/Mn_(2)O_(3) nanocomposite as a high-performance cathode material for aqueous zinc-ion batteries

Thanks to low cost,high safety,and large energy density,aqueous zinc-ion batteries have attracted tremendous interest worldwide.However,it remains a challenge to develop high-performance cathode materials with an appropriate method that is easy to realize massive production.Herein,we use a molten salt method to synthesize nanostructured manganese oxides.The crystalline phases of the manganese oxides can be tuned by changing the amount of reduced graphene oxide added to the reactant mixture.It is found that the α-MnO_(2)/Mn_(2)O_(3) nanocomposite with the largest mass ratio of Mn_(2)O_(3) delivers the best electrochemical performances among all the products.And its rate capability and cyclability can be significantly improved by modifying the Zn anode with carbon black coating and nanocellulose binder.In this situation,the nanocomposite can deliver high discharging capacities of 322.1 and 213.6 mAh g^(-1) at 0.2 and 3 Ag^(-1),respectively.After 1000 cycles,it can retain 86.2% of the capacity at the 2 nd cycle.Thus,this nanocomposite holds great promise for practical applications.

Molten Salt Synthesis of Ba(Mg_(1/3)Nb_(2/3))O_3 Powder

The single-phase Ba(Mg_(1/3)Nb_(2/3))O_3(BMN) powder was successfully prepared by the KCl molten salt synthesis(MSS) method.The temperature for single-phase BMN powders by MSS was about 400℃ lower than that by the solid-phase method.The average particle size(APS) was about 0.91μm at 900℃ and increased with increasing synthesis temperature.Based on the APS,the activation energy for particle growth in the MSS,whose value was 64.1kJmol^(-1),was attained.The sinterability of the powder prepared by MSS method was better than that prepared by solid-phase method.

Molten salt synthesis, formation mechanism, and oxidation behavior of nanocrystalline HfB2 powders

Nanocrystalline Hf B2 powders were successfully synthesized by molten salt synthesis technique at 1373 K using B and Hf O2 as precursors within KCl/Na Cl molten salts.The results showed that the as-synthesized powders exhibited an irregular polyhedral morphology with the average particle size of 155 nm and possessed a single-crystalline structure.From a fundamental aspect,we demonstrated the molten-salt assisted formation mechanism that the molten salts could accelerate the diffusion rate of the reactants and improve the chemical reaction rate of the reactants in the system to induce the synthesis of the high-purity nanocrystalline powders.Thermogravimetric analysis showed that the oxidation of the as-synthesized Hf B2 powders at 773–1073 K in air was the weight gain process and the corresponding oxidation behavior followed parabolic kinetics governed by the diffusion of oxygen in the oxide layer.

Recent progress in molten salt synthesis of low-dimensional perovskite oxide nanostructures, structural characterization,properties, and functional applications: A review

Molten salt synthesis （MSS） method has advantages of the simplicity in the process equipment, versatile and large-scale synthesis, and friendly environment, which provides an excellent approach to synthesize high pure oxide powders with controllable compositions and morphologies. Among these oxides, perovskite oxides with a composition of ABO3 exhibit a broad spectrum of physical properties and functions （e.g. ferroelectric, piezoelectric, magnetic, photovoltaic and photocatalytic properties）. The downscaling of the spatial geometry of perovskite oxides into nanometers result in novel properties that are different from the bulk and film counterparts. Recent interest in nanoscience and nanotechnology has led to great efforts focusing on the synthesis of low-dimensional perovskite oxide nanostructures （PONs） to better understand their novel physical properties at nanoscale. Therefore, the low-dimensional PONs such as perovskite nanoparticles, nanowires, nanorods, nanotubes, nanofibers, nanobelts, and two dimensional oxide nanostructures, play an important role in developing the next generation of oxide electronics. In the past few years, much effort has been made on the synthesis of PONs by MSS method and their structural characterizations. The functional applications of PONs are also explored in the fields of storage memory, energy harvesting, and solar energy conversion. This review summarizes the recent progress in the synthesis of low-dimensional PONs by MSS method and its modified ways. Their structural char- acterization and physical properties are also scrutinized. The potential applications of low-dimensional PONs in different fields such as data memory and storage, energy harvesting, solar energy conversion, are highlighted. Perspectives concerning the future research trends and challenges of low-dimensional PONs are also outlined. ~ 2017 Published by Elsevier Ltd on behalf of The editorial office of Journal of Materials Science ＆ Technology.

Molten Salt-Shielded Synthesis(MS^(3))of MXenes in Air

MXenes are two-dimensional transition metal carbides and/or nitrides with unique physiochemical properties and have attracted extensive interest in numerous fields.However,current MXene synthesis methods are limited by hazardous synthesis conditions,high production costs,or difficulty in largescale production.Therefore,a general,safe,cost-effective,and scalable synthesis method for MXenes is crucial.Here,we report the fast synthesis of MXenes in the open air using a molten salt-shielded synthesis(MS^(3))method,which uses Lewis-acid salts as etchants and a low-melting-point eutectic salt mixture as the reaction medium and shield to prevent MXene oxidation at high temperatures.Carbide and nitride MXenes,including Ti_(3)C_(2)T_(x),Ti_(2)CT_(x),Ti_(3)CNT_(x),and Ti_(4)N_(3)T_(x),were successfully synthesized using the MS^(3) method.We also present the flexibility of the MS^(3) method by scaling the etching process to large batches of 20 and 60 g of Ti_(3)AlC_(2) MAX precursor in one pot.When used as negative electrodes,the prepared MS^(3)-MXenes delivered excellent electrochemical properties for high-rate Li-ion storage.

Molten salt synthesis of neodynium oxyfluoride in various fluoride media with different fluoride ion activities

Neodymium oxyfluoride has received much attention in the fields of anionic solid electrolytes.luminescent,catalytic and magnetic materials because of its structure combined advantages of rareearth cations with F^(-)and O_(2)^(-)anions.In this work,neodynium oxyfluoride was synthesized by the reaction between neodymium oxide and four fluoride media with different fluoride ion activities.The synthesis processes in molten LiF-CaF_(2)-NdF_(3),LiF-NdF_(3),NaF-CaF_(2)-NdF_(3)and NaF-KF-NdF_(3)are observed in situ by a confocal scanning laser microscope.The expansion of neodymium oxide particle is observed in the LiF-CaF_(2)-NdF_(3),LiF-NdF_(3),and NaF-KF-NdF_(3)melts,and the growth of needle crystals on neodymium oxide particle is clearly observed in molten NaF-CaF_(2)-NdF_(3).Based on scanning electron microscopy(SEM)-energy dispersive X-ray spectroscopy(EDS)and X-ray diffraction(XRD)analyses of products,neodynium oxyfluoride was successfully synthesized in the four fluoride media.The neodynium oxyfluoride generated in the LiF-CaF_(2)-NdF_(3),LiF-NdF_(3),and NaF-KF-NdF_(3)melts is a tetragonal structure.However,in molten NaF-CaF_(2)-NdF_(3),neodynium oxyfluoride with a rhombohedral structure is formed.It is suggested that the substitution of Na(Ⅰ)and Ca(Ⅱ)for Nd(Ⅲ)can transform NdOF from tetragonal structure to rhombohedral structure.The growth rate of needle crystals generated in molten NaF-CaF_(2)-NdF_(3)was calculated based on the result of a confocal scanning laser microscope,and it is found that the reaction kinetics of crystal formation is zero-order reaction.The effect of fluoride media on the structure and morphology of formed NdOF were evaluated by XRD,X-ray photoelectron spectroscopy(XPS)and SEM.The neodymium oxyfluoride prepared in the fluoride media with high fluoride ion activity has low binding energy of F 1 s.The ratio of adsorbed oxygen to lattice oxygen for neodymium oxyfluoride prepared in molten LiF-NdF_(3)is larger than those in the other three fluoride media,so it can have better catalytic performance.

Molten salt synthesis of Z-scheme CeO_(2)/C_(3)N_(4) photocatalysts with excellent properties for removal of organic pollutants:Characterization,kinetics and mechanisms

In this work,we firstly synthesized a CeO_(2)/C_(3)N_(4) photocatalyst with Z-scheme heterojunction by a facile LiC-KCI molten salt method.The synthesized catalyst has an excellent quality for removing organic pollution of dyes and antibiotics in wastewater.As an example,the CeCN-1:5 prepared with a mass ratio of Ce_(2)(CO_(3))_(3)·xH_(2)O:C_(3)H_(3)N_(6)=1:5 exhibits a methylene blue(MB)removal capacity of 100%within 90 min and tetracycline(TC)removal capacity of 94.6%.After 4 cycles,the CeCN-1:5 keeps a removal efficiency of nearly 100%in 150 min for MB and 85.7%for TC.The kinetics study reveals that the MB removal process with the CeCN-1:5 fits the modified Elovich model with strong adsorption while TC removal fits the first-order model.The large surface area(238 m^(2)/g)and negative zeta potential(-39.3 mV)of CeCN-1:5 contribute to superior adsorption capacity to MB.However,the adsorption of TC is restricted due to the positive surface/pore potential in acidic solution.CeCN-1:5has combined Z-scheme heterojunction and exhibits a low recombination rate of electrons(e^(-))/holes(h^(+))and the photo-generated active radicals of·OH/·O_(2)^(-)that promotes the photocatalytic performance.This novel CeO_(2)/C_(3)N_(4) photocatalyst with an excellent photocatalysis removal activity has an enormous potential for photocatalytic applications.

Low Temperature Molten Salt Synthesis of Perovskite-type ACeO3（A=Sr, Ba） in Eutectic NaCl-KCl

A molten salt method was developed for the synthesis of ACeO3（A=Sr, Ba） with perovskite structure at 750 ℃（SrCeO3） and 850 ℃（BaCeO3） in the eutectic NaCl-KCl. The synthetic temperature was much lower than that of the conventional method（generally〉1000 ℃）. The structure and morphology of the product were characterized by means of X-ray diffraction（XRD）, field emission scanning electron microscopy（FESEM） and transmission electron microscopy（TEM）. The results show that the synthesized octahedral SrCeO3 crystallizes in the orthorhombic system with the unit cell parameters of a=0.85855 nm, b=0.61523 nm, c=0.60059 nm, and the synthesized cuboid BaCeO3 crystallizes in the cubic system with the unit cell parameter of a=0.43962 nm. The result of X-ray photoelectron spectroscopy（XPS） analysis indicates that both Ce4＋ and Ce3＋ exist in the two structures, and the Ce4＋/Ce3＋ peak area ratios for SrCeO3 and BaCeO3 are 1.93 and 2.12, respectively. Meanwhile, the adsorbed/lattice oxygen ratios（1.87 for SrCeO3 and 3.04 for BaCeO3） indicate the existence of a lot of oxygen vacancies in the structures of SrCeO3 and BaCeO3, which indicates a far-reaching significance to study the corresponding physicochemistry performance.

Fabrication plate-like BaBi_(4)Ti_(4)O_(15) single-crystalline particles by the molten salt synthesis method

Plate-like single-crystalline BaBi_(4)Ti_(4)O_(15) particles were synthesized by the molten salt synthesis(MSS)method.The effects of sintering temperature,holding time,and NaCl-KCl molten salt content on the phase structure and morphology of plate-like BaBi_(4)-Ti_(4)O_(15) particles were investigated.The results show that plate-like BaBi_(4)Ti_(4)O_(15) particles can be synthesized when the sintering temperature is above 800°C.The size of particles increases with increasing sintering temperature and molten salt content.Largely anisotropic plate-like BaBi_(4)Ti_(4)O_(15) particles with diameter≥10μm and thickness of~0.3μm can be obtained under the optimum process parameters.The crystal structure of BaBi_(4)Ti_(4)O_(15) was determined as A21am by TEM,which should be attributed to the Bi3+and Ba2+diffusing into[TiO6]octahedrons.

Preparation and Magnetic Property of La_(0.7)Sr_(0.3)MnO_3 Nanorod by Combination Sol-Gel with Molten Salt

La0.7Sr0.3MnO3（LSMO） nanorods were synthesized by a method combining sol-gel with molten salts at 950 ℃ for 10 h, which employed KCl＋NaCl（mass ratio 4：1） as eutectic molten salts. The morphologies and magnetic properties of the resulting LSMO nanorods were investigated by X-ray diffraction（XRD）, scanning electron microscopy（SEM）, transmission electron microscopy（TEM）, and vibrating sample magnetometer（VSM） measurements. It was found that the obtained perovskite manganite LSMO was a uniform nanorod with a diameter of about 50 nm and a length of longer than 500 rim. The Curie temperature（To） of the LSMO nanorod used here was 262 K, much lower than that of bulky single crystal LSMO（360 K）. The low Curie temperature might be a result of the great disorder near the grain boundary, which could be observed clearly from the TEM picture.

Structures and magnetic anisotropy of β-Mn_2V_2O_7 crystals synthesized by the molten salt method

β-Mn2V2O7 crystals with strip shape are successfully prepared by the molten salt method in a closed crucible, and are characterized by x-ray diffraction （XRD）, scanning electron microscopy （SEM）, transmission electron microscopy （TEM）, selected area of electron diffraction （SAED） and high-resolution transmission electron microscopy （HRTEM）. The results indicate that the sample is of the β-Mn2V2O7 crystal with monoclinic symmetry, level natural cleavage facets and directional growth. Magnetic properties are measured by vibration sample magnetometry （VSM） at room temperature, and the magnetic hysteresis loop indicates that the β-Mn2V2O7 has anti-ferromagnetic properties with low coercive force and remnant magnetization. The magnetic measurement results in different directions exhibit that the β-Mn2V2O7 has magnetic anisotropy, which is due to the fact that the magnetic interaction energy of the β-Mn2V2O7 is lowest only when the electron configuration is in a certain direction.

Synthesis and Microstructure of Partly-oriented Bismuth Layer Structure Ferroelectrics Ceramics SrBi_4Ti_4O_(15)

SrBi4Ti4O15 powder was synthesized by conventional solid state synthesis （ CS ） and molten salt synthesis （ MSS ） . MSS method can synthesize plate-like SrBi4Ti4O15 at lower temperature （900℃） than CS method. Plate-like form becomes more distinct when the synthesis temperature increases. This would help cause the grain orientation of the ceramics after sintering. The sintered samples of MSS had grain orientation at （0,0, 10） plane. The degree of （0,0,10） grain orientation F was 62.1% . Hot pressing made （0,0,10） grain orientation more distinct （ F = 85.7% ）. The microstructures of the sintered samples were detected by SEM. Due to the grain orientation the density of samples fabricated by MSS was lower than that of prepared by CS.

In-situ Synthesis of Cr_(3)C_(2) Nanosheets by Carbon Reduction Route from Cr_(2)AlC

Carbon black and Cr_(2)AlC were used as raw materials to obtain a large number of Cr_(3)C_(2)nanosheets by means of the molten salt heat treatment at 1100℃for 1.5 hours.Results showed that carbon black can promote the decomposition of a large number of Cr_(2)AlC to form Cr_(3)C_(2)and Cr_(7)C_(3)nanoparticles at 1100℃in the absence of molten salt.Under a molten salt environment,carbon black can promote the complete decomposition of Cr_(2)AlC to form Cr_(3)C_(2)and Cr_(7)C_(3)nanosheets.The thickness of chromium carbide nanosheets is approximately 10-20 nm,and the length is approximately 100-200 nm.The addition of excess carbon black can promote the complete decomposition of Cr_(2)AlC into a material with Cr_(3)C_(2)as the main phase.

Synthesis of LiSm_(0.01)Mn_(1.99)O_4 by molten salt technique

Samarium substituted lithium manganese oxide powders were successfully prepared by molten-salt synthesis(MSS) using eutectic mixture of LiCl,SmCl3.6H2O and MnO2 salt at 700 °C.The synthesis was carried out in open atmosphere.The crystalline powders were characterized for their phase identification using X-ray diffraction(XRD) analysis.The physico-chemical properties of the samarium substituted lithium manganese oxide powders were investigated by thermal analysis(TGA/DTA),FT-IR spectroscopy,EDAX,electron par...

Facile synthesis of hollow Ti_(3)AlC_(2)microrods in molten salts via Kirkendall effect

The microstructure and morphology of Ti_(3)AlC_(2)powders not only affect the preparation of Ti_(3)C_(2) MXene but also have a great influence on their potential applications,such as microwave absorbers,alloy additives,or catalytic supports.However,the synthesis of Ti_(3)AlC_(2)powders with desired microstructure and morphology remains a challenge.Herein,hollow Ti_(3)AlC_(2)microrods were prepared for the first time in NaCl/KCl molten salts by using titanium,aluminum,and short carbon fibers as starting materials.It was found that the short carbon fibers not only performed as carbon source but also acted as sacrificial template.Furthermore,it was revealed that TiC and Ti2AlC were initially formed on the surface of carbon fibers.The subsequent reactions between the outer Ti,Al and the inner carbon were dominated by the Kirkendall effect which gave rise to the formation of a hollow structure.Based on this mechanism,hollow Ti_(3)AlC_(2)microspheres and a series of hollow TiC,Ti_(2)AlC,and V_(2)AlC powders were also successfully fabricated.This work provides a facile route to synthesize hollow MAX phases and may give enlightenment on preparing other hollow carbide powders via the Kirkendall effect in the molten salts.

Enhanced electrochemical performance in lithium ion batteries of a hollow spherical lithium-rich cathode material synthesized by a molten salt method

A high voltage layered Li1.2Ni0.16Co0.08Mn0.56O2 cathode material with a hollow spherical structure has been synthesized by molten-salt method in a NaCI flux. Characterization by X-ray diffraction and scanning electron microscopy confirmed its structure and proved that the as-prepared powder is constituted of small, homogenously sized hollow spheres （1-1.5 μm）. The material exhibited enhanced rate capability and high first cycle efficiency due to the good dispersion of secondary particles. Galvanostatic cycling at different temperatures （20, 40, and 60 ℃） and a current rate of 2 C （500 mA.g-1） showed no significant capacity fade.

Low Temperature Preparation of Carbide Coated Carbon for Refractory Castable Applications

Titanium carbide （ TiC ） coated graphite flakes （GF） and carbon black （CB） powders were prepared at relatively low temperatures （ 750 - 950 ℃ ） using a no- vel molten salt synthesis technique. The in-situ formed TiC coatings were homogeneous and crack-free and their thicknesses could be readily controlled/tailored by simply adjusting the Ti/C ratio. Compared to their uncoated counterparts, as prepared TiC coated GF and CB showed much improved water-wettability/dispersivity and rheological properties, and thus could be potentially used to prepare carbon-containing refractory castables.

Chrysanthemum-like high-entropy diboride nanoflowers: A new class of high-entropy nanomaterials

High-entropy nanomaterials have been arousing considerable interest in recent years due to their huge composition space,unique microstructure,and adjustable properties.Previous studies focused mainly on high-entropy nanoparticles,while other high-entropy nanomaterials were rarely reported.Herein,we reported a new class of high-entropy nanomaterials,namely(Tao2Nbo2Ti.2Wo.2Moo2)B2 high-entropy diboride(HEB-1)nanoflowers,for the first time.Formation possibility of HEB-1 was first theoretically analyzed from two aspects of lattice size difference and chemical reaction thermodynamics.We then successfully synthesized HEB-1 nanoflowers by a facile molten salt synthesis method at 1423 K.The as-synthesized HEB-1 nanoflowers showed an interesting chrysanthemum-like morphology assembled from numerous well-aligned nanorods with diameters of 20--30 nm and lengths of 100-200 nm.Meanwhile,these nanorods possessed a single-crystalline hexagonal structure of metal diborides and highly compositional uniformity from nanoscale to microscale.In addition,the formation of the as-synthesized HEB-I nanoflowers could be well interpreted by a classical surface-contolled crystal growth theory.This work not only enriches the categories of high-entropy nanomaterials but also opens up a new research field on high-entropy diboride nanomaterials.

Lithium storage properties of Ti3C_(2)Tx(Tx= F,Cl,Br) MXenes

In this work, Ti3C_(2)TxMXene with-F,-Cl and-Br surface terminations are synthesized and the effect of these halogen terminations on the lithium storage properties is investigated. A maximum Li+storage capacity of 189 m Ah/g is achieved with Ti3C_(2)BrxMXene much higher than Ti3C_(2)Clxand Ti3C_(2)Fxwith 138 m Ah/g and 123 m Ah/g, respectively. Density functional theory(DFT) calculation shows that the adsorption formation energy of halogen atoms on Ti atoms follows the trend of Ti-F > Ti-Cl > Ti-Br, leading to the same trend in the content of terminations on corresponding MXenes. In addition, inevitable exposure of MXene to oxygen causes competition between halogen and oxygen. Theoretical results show Ti3C_(2)BrxMXene has the highest Ti to O ratio and the lowest Ti to Br ratio, the high lithium affinity of O explains the maximum Li-ion storage capacity with Ti3C_(2)BrxMXene. This work shed light on the opportunity for achieving improved lithium storage properties of MXene electrodes by regulating the surface chemistry.