Three-dimensional porous carbon aerogels from sodium carboxymethyl cellulose/poly(vinyl alcohol) composite for high-performance supercapacitors

Three-dimensional interconnected porous carbon aerogels were successfully prepared by the pyrolysis of sodium carboxymethyl cellulose/poly(vinyl alcohol) composite using borax as a cross-linking agent. After sol–gel, freeze-drying, carbonization and KOH-activation treatments, the as-obtained carbon aerogels exhibited high specific surface areas (SSAs) and excellent electrochemical performance. The optimal sample showed a high SSA of up to 601.4 m²/g with a total pore volume of 0.38 cm³/g. The aCA-3 sample exhibited an excellent capacitance of 182.8 F/g at a current density of 0.5 A/g, within a potential window of ??1.0 to 0.0 V in a 6 M KOH electrolyte solution. In addition, the aCA-3 electrode showed excellent cyclability with 96.2% capacitance retention efficiency at 2 A/g current density after 2000 cycles of charge/discharge. These easily prepared, well-structured and high-performance carbon aerogels have significant potential applications in advanced electronic devices and energy storage systems.     

Fabrication and characterization of the monolithic hydrophobic alumina aerogels

The monolithic hydrophobic mesoporous alumina aerogels were successfully synthesized by acid–base sol–gel polymerization of aluminium chloride hexahydrate (AlCl₃·6H₂O) in deionized water/alcohol solution (v/v = 3:7). To minimize shrinkage during drying, alumina hydrogels were aged in tetraethylorthosilicate (TEOS)/acetonitrile solution, and modified using trimethylchlorosilane (TMCS)/acetonitrile solution. Properties of the final product were examined by contact angle measurement, FTIR, FESEM, TEM and BET analyses. Surface modification was confirmed by FTIR spectroscopy. It was found that hydrophobic property of the alumina aerogels was affected by the contents of TMCS. When the molar ratio of TMCS to AlCl₃·6H₂O is 0.35, hydrophobic alumina aerogels shows lower bulk density (0.453 g/cm³) and higher surface area (495 m²/g) than those of unmodified alumina aerogels (0.933 g/cm³, 413 m²/g).     

Synthesis and characterization of monolithic carbon/silicon carbide composite aerogels

Resorcinol–formaldehyde/silica composite (RF/SiO₂) aerogels were synthesized using sol–gel process followed by supercritical CO₂ drying. Monolithic carbon/silicon carbide composite (C/SiC) aerogels were formed from RF/SiO₂ aerogels after carbothermal reduction. X-ray diffraction and transmission electron microscopy demonstrate that β-SiC was obtained after carbothermal reduction. Scanning electron microscopy and nitrogen adsorption/desorption reveal that the as-prepared C/SiC aerogels are typical mesoporous materials. The pore structural properties were measured by nitrogen adsorption/desorption analysis. The resulting C/SiC aerogels possess a BET surface area of 564 m²/g, a porosity of 95.1 % and a pore volume of 2.59 cm³/g. The mass fraction of SiC in C/SiC aerogels is 31 %.     

Ultralight,high-surface-area,multifunctional graphene-based aerogels from self-assembly of graphene oxide and resol

The self-assembly between graphene oxide sheets and resol-type phenolic prepolymers was investigated as a method to form three-dimensional porous carbon objects with high surface areas and low densities. After freeze-drying and subsequent pyrolysis of the assembled hydrogels, ultralight graphene/carbon composite aerogels with high surface areas and porosity, good conductivity, and well-defined bulk shape were obtained. By adjusting the amount of graphene oxide and resol in the precursor mixture, aerogels with a density as low as 3.2 mg/cm³ or a surface area as high as 1019 m²/g could be prepared. It is proposed that resol molecules are first adsorbed on the surface of graphene oxide sheets, and then the surface-coated sheets are crosslinked by the polymerization of resol prepolymers. The absorption performance was evaluated for the aerogel with the lowest density. Due to the high porosity, the aerogel displayed fast absorption rates for organic solvents as well as high absorption efficiencies. The high conductivity of the aerogels permits good performance as binderless monolithic electrodes for supercapacitors.     

Effect of the thermal treatment on microstructure and physical properties of low-density and high transparency silica aerogels via acetonitrile supercritical drying

In this paper, we reported the experimental results about the effect of the thermal treatment on microstructure and physical properties of low-density and high transparent silica aerogels. From our results, with tetramethyl orthosilicate as precursor and via acetonitrile supercritical drying process, silica aerogel monolith was obtained possessing the properties as low-density (0.018 g/cm³), high surface area (923 m²/g), high optical transparency (87.9 %, 800 nm). It should be noted that high transparency of silica aerogel can be maintained up to 600 °C (91.5 %, 800 nm). The mechanical properties of silica aerogel decreased with increasing heat treated temperature to 600 °C, and silica aerogels still maintained crack-free monoliths completely and possessed high homogeneous density even after 600 °C thermal treatment. Furthermore, thermal conductivity of the monoliths at desired temperatures was analyzed by the transient plane heat source method. When the temperature flowed from 25 to 600 °C, thermal conductivity coefficients of silica aerogels changed from 0.021 to 0.065 W (m K)^?1, revealed an excellent heat insulation effect in high-temperature area. Currently, the specific process developed for low-density aerogels affected by thermal treatment has not been reported in previous literature.     

Facile preparation of ultralight polymer-derived SiOCN ceramic aerogels with hierarchical pore structure

SiOCN ceramic aerogels with lightweight, high surface areas, and macro-meso pores have been synthesized by a facile method combining freeze-drying technique and polymer-derived ceramic route. The wet gels are synthesized via the hydrosilylation reaction between polysilazane and divinylbenzene with cyclohexane as solvent. The solvent is then removed by a freeze-drying process to form pre-ceramic aerogels. The SiOCN ceramic aerogels are finally obtained by pyrolyzing the pre-ceramic aerogels at 1000°C in ultrahigh purity N₂. The thermogravimetric and mass spectrometry system (TG/DSC-MS) is used to investigate the polymer-to-ceramic conversion process during pyrolysis. The phase composition, structure, and morphology of the SiOCN ceramic aerogels are investigated by XRD, FT-IR, XPS, and SEM. The results show that SiOCN ceramic aerogels are composed of amorphous matrix phase and “free carbon” phase. The SiOCN aerogels possess three-dimensional (3D) network porous structure with low density (0.19 g/cm³), high specific surface area (134 m²/g), large pore volume (0.49 cm³/g), and hierarchical pore structures of both macro and meso pores. The formation mechanism and evolution process of SiOCN ceramic aerogels are discussed.     

Magnetic mesoporous iron oxide/silica composite aerogels with high adsorption ability for organic pollutant removal

In this paper, a kind of magnetic mesoporous iron oxide/silica composite aerogels with high adsorption ability is prepared by ambient pressure drying method. The results indicate that the obtained magnetic aerogels with Fe/Si (molar ratio) >0.91 % have higher specific surface area with 310.8–411.0 m² g^?1 and pore volume with 0.85–1.12 cm³ g^?1. The adsorption test indicates that the obtained magnetic aerogels showed prominent adsorption capability with the adsorption rate for Rhodamine B in aqueous solution could attain to 95.8 % within 80 min. Moreover, all the composite aerogels exhibited good magnetic properties and could be easily separated from the water after adsorption.     

Superelastic,fatigue resistant and heat insulated carbon nanofiber aerogels for piezoresistive stress sensors

Ultralight flexible carbon aerogels have been applied in many fields but are always hampered due to their weak mechanical stability, large energy dissipation, and complex preparation methods. Here, a novel polyimide-derived carbon nanofiber aerogel with high fatigue resistance and excellent flexibility is prepared by using the designed “fiber gluing” to construct the elastic continuous fibrous network. The as-prepared carbon nanofiber aerogels exhibit ultralow density of 6.6 mg cm⁻³, excellent fire-resistant performance when exposing to alcohol flames (650 °C), and robust elastic resilience even under 55% compressive strain. In particular, the carbon nanofiber aerogels show ultralow plastic deformation (only 1.3%) and low energy dissipation (<0.22) after 1000 cyclic loading-unloading tests at 55% compressive strain. Benefiting from their excellent flexibility and robust mechanical stability, the as-prepared carbon nanofiber aerogel are a promising candidate in the application of wearable devices and piezoresistive stress sensors.     

Synthesis and characterization of a xonotlite fibers–silica aerogel composite by ambient pressure drying

Xonotlite fibers (XFs) reinforced silica aerogel composites were prepared by a sol–gel method under ambient pressure drying. XFs were synthesized through a dynamic hydrothermal route and had a noodle-like structure with length of 5–10 μm and average diameter of 150–200 nm. The microstructure analysis showed that XFs were inlaid in silica aerogel matrix by physical combination which contributed to restrict the volume shrinkage of alcogels and maintain the integrality aerogels during drying process. The physical, naonporous and thermal properties of the as prepared aerogel composites were investigated and discussed in detail. The new aerogel composites possessed porous nanostructure, which exhibited typical properties of 0.126 g/cm³ density, 4.132 cm³/g pore volume, and thermal conductivity of 0.0285 W/(m K). The results indicated that the introduced XFs didn’t significantly alter the porosity, hydrophobicity or thermal conductivity of aerogel matrix. It was also found that the aerogel composites had much more outstanding porosity than that of pure aerogel upon calcinations at 800 °C. This study fabricated XFs–silica aerogel composites and explored a new way for silica aerogels to endure and remain monolithic under ambient pressure drying.     

Rapid fabrication of low density melamine–formaldehyde aerogels

Low density melamine–formaldehyde (MF) aerogels were fabricated rapidly by adding the alkali to low concentration MF sol precursor before the formation of cross-linking. The gelation of the MF sol with the precursor concentration as low as 4% could be accomplished by the modified method. The gelation time for MF wet gels could be also reduced to even 5 h. After solvent exchanging and CO₂ supercritical drying, the MF areogel with the lowest density of about 55 mg/cm³ could be prepared. The samples were also characterized by scanning electron microscopy, transmittance electron microscope, Fourier transform infrared spectroscopy and nitrogen adsorption–desorption isotherms. The results indicated that the addition of the alkali to the MF sol precursor not only sped up the gelation process without changing the reaction mechanism, but also accomplished the low density MF aerogel.     

Review od ADVANCES IN DRYING,Vol. 3,

RF hydrogels were synthesized by the sol-gel polycondensation of resorcinol with formaldehyde and RF cryogels were prepared by freeze drying of the hydrogels with t-butanol. The cryogels were characterized by nitrogen adsorption, density measurements, and scanning electron microscope. Their porous properties were compared with those of the aerogels prepared by supercritical drying with carbon dioxide. RF cryogels were mesoporous materials with large mesopore volumes >5.8× 10^?4m³/kg. Although surface areas and mesopore volumes of the cryogels were smaller than those of the aerogels, the cryogels were useful precursors of mesoporous carbons. Aerogel-like carbons (carbon cryogels) were obtained by pyrolyzing RF cryogels in an inert atmosphere. The carbon cryogels were mesoporous materials with high surface areas >8.0× 10⁵m²/kg and large mesopore volumes >5.5× 10^?4m³/kg. When pyrolyzed, micropores were formed inside the cryogels more easily than inside the aerogels.     

Diamond cylindrical anodes for electrochemical treatment of persistent compounds in aqueous solution

Boron-doped diamond (BDD) films were deposited onto either silicon or niobium cylindrical substrates with areas up to 35 cm² for electrochemical applications. BDD electrodes were characterised in terms of their material and electrochemical properties by scanning electron microscopy, Raman spectroscopy and linear sweep voltammetry. These characterisation techniques indicated conductive polycrystalline BDD with low quantities of non-diamond carbon impurities. Electrochemical oxidations of pharmaceutical compounds were performed using these cylindrical electrodes and monitored by UV/Vis spectroscopy, chemical oxygen demand and total organic carbon. Mixtures of chlortetracycline, oxytetracycline and diclofenac were electrolyzed on a 9.42 cm² (? = 6 mm, h = 50 mm) cylindrical Si/BDD anode using a current density of 8.2 mA cm^?2. Ibuprofen was electrolyzed on an 18.0 cm² (? = 10 mm, h = 60 mm) cylindrical Nb/BDD anode using a current density of 25 mA cm^?2. Cylindrical-shape diamond electrodes present several advantages with respect to conventional plate-shape BDD electrodes such as handling, sealing and cell assembly. The obtained results show that BDD cylindrical anodes are promising for electrochemical wastewater treatment.     

Thermal conductivity of low density carbon aerogels

Carbon aerogels with densities ranging from 0.182 to 0.052?g/cm³, pore sizes ranging from 88 to 227?nm, and particle diameters ranging from 20 to 13?nm were prepared. Thermal conductivity measurements by laser flash method indicate that the lowest thermal conductivity can be obtained at a density of 0.066?g/cm³, in the temperature range from 100 to 300?°C in air. The lowest thermal conductivity is 0.0263?W/m?K at 200?°C. The characteristic density, at which the lowest thermal conductivity can be obtained, is temperature dependent. At a higher temperature, a higher density carbon aerogel will be more efficient in the reduction of the total conductivity by reducing the radiative conductivity.     

Preparation of mesoporous carbon by freeze drying

Resorcinol–formaldehyde (RF) cryogels were synthesized by sol-gel polycondensation of resorcinol with formaldehyde and freeze drying with t-butanol. The cryogels were characterized by nitrogen adsorption and density measurements. Their porous properties were compared with those of RF aerogels prepared by supercritical drying with carbon dioxide. RF cryogels were mesoporous materials with large mesopore volumes >0.58 cm³/g. Although surface areas and mesopore volumes of the cryogels were smaller than those of the aerogels, the cryogels were useful precursors of mesoporous carbons. Carbon cryogels were obtained by pyrolyzing RF cryogels in an inert atmosphere. Carbon cryogels were mesoporous materials with high surface areas >800 m²/g and large mesopore volumes >0.55 cm³/g. When pyrolyzed, micropores are formed inside the cryogels more easily than inside the aerogels.     

Ultralight and hydrophobic nanofibrillated cellulose aerogels from coconut shell with ultrastrong adsorption properties

Ultralight aerogels based on nanofibrillated cellulose (NFC) isolated from coconut shell were successfully prepared via a mild fast method, which included chemical pretreatment, ultrasonic isolation, solvent exchange, and tert‐butanol freeze drying. The as‐prepared NFC aerogels with complex three‐dimensional fibrillar networks had a low bulk density of 0.84 mg/cm³ (specific surface area = 9.1 m²/g and pore volume = 0.025 cm³/g), maintained a cellulose I crystal structure, and showed more superior thermal stability than the coconut shell raw materials. After the hydrophobic modification by methyl trichlorosilane (MTCS), the NFC aerogels exhibited high water repellency properties, an ultrastrong oil‐adsorption capacity (542 times that of the original dry weight of diesel oil), and superior oil–water separation performance. Moreover, the absorption capabilities of the MTCS‐treated NFC aerogels were as high as 296?669 times their own weights for various organic solvents and oil. Thus, this class of high‐performance adsorbing materials might be useful for dealing with chemical leaks and oil spills. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 42037.     

A novel aerogels/porous Si3N4 ceramics composite with high strength and improved thermal insulation property

A novel ZrO₂-SiO₂ aerogels/porous Si₃N₄ ceramics composite with high strength, low density, good dielectric properties and low thermal conductivity was synthesized by filling ZrO₂-SiO₂ aerogels into the porous Si₃N₄ ceramics through vacuum sol-impregnating. The effects of aerogels on the microstructure and properties of composite were discussed. The results show that aerogels could form a mesoporous structure and significantly decrease the thermal conductivity from 9.8 to 7.3 W m^?1 K^?1. Meanwhile, the density, mechanical and dielectric properties of the porous Si₃N₄ ceramics could not be affected after introducing ZrO₂-SiO₂ aerogels. The composite exhibits high porosity (62.6%), high flexural strength (53.86 MPa) and low dielectric constant (2.86). The ZrO₂-SiO₂ aerogels/porous Si₃N₄ ceramics composite shows great potential as radome materials applied in the fields of aerospace.     

Organic and carbon aerogels from the NaOH‐catalyzed polycondensation of resorcinol–furfural and supercritical drying in ethanol

Organic aerogels and related carbon aerogels were prepared from the NaOH‐catalyzed polycondensation of resorcinol–furfural (RF) and supercritical drying in ethanol. The effect of the preparation conditions, including the RF concentration, molar ratio of resorcinol (R) to NaOH, and molar ratio of R to furfural, on the gelation time and bulk density was studied. The chemical structure of the organic aerogel was revealed by IR spectroscopy. The pyrolysis process of the organic aerogel was investigated by thermogravimetric analysis. According to characterizations of transmission electron microscopy and nitrogen adsorption, the organic and carbon aerogels we obtained had a three‐dimensional network that consisted of around 30‐nm particles, which defined numerous mesopores of less than 30 nm. As a result, the aerogels had high Brunauer–Emmett–Teller surface areas (698–753 m²/g) and large mesopore volumes (1.09–1.64 cm³/g). X‐ray diffraction characterization indicated that the carbon aerogel was more crystalline than activated carbon but less activated than graphite. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 96: 1429–1435, 2005     

Effects of Solvent Exchange Period and Heat Treatment on Physical and Chemical Properties of Rice Husk Derived Silica Aerogels

In this study, hydrophobic silica aerogels were synthesized from rice husk ash-derived sodium silicate through sol-gel processing, solvent exchange, surface modification and ambient pressure drying. By volume, 10% of trimethylchlorosilane (TMCS) in 90% of n-hexane was used as a hydrophobic solution in the surface modification process. The physical and chemical properties of silica aerogels were characterized by density and porosity measurements, scanning electron microscopy (SEM), Fourier transforms infrared (FTIR) spectroscopy, Brunauer–Emmett–Teller theory (BET) and dynamic scanning calorimetry (DSC). The hydrogels prepared were in the form of 2.5 ± 0.5 mm beads and then converted into alcogels through solvent exchange with ethanol for repetition of 3, 6 and 9 days. It is found that the optimal quality of silica aerogels with the BET surface area as high as 668.82 m²/g was obtained from the alcogels of the solvent exchange period of 9 days. Depending on the size of the gel’s block, a longer solvent exchange period will ensure adequate removal of pore water. Post heat treatment on silica aerogels obtained from the 9 days of solvent exchange at 200, 300 and 400 °C for 2 h results in slight decreased of aerogel’s density from 0.048 g/cm³ to 0.039 g/cm³ and the hydrophobicity of the aerogels is decreased above 380 °C as confirmed by DSC analysis.

     

Synthesis of highly flexible silica aerogels by photoacids generation

Highly flexible silica aerogels were fabricated by using 2-(3-(triethoxysilyl)propylcarbamoyloxy) ethyl acrylate (TESEA), methyltriethoxysilane (MTES), and polydimethylsiloxane (PDMS) as co-precursors through photopolymerization and sol–gel process, then followed by ambient pressure drying. Attributing to photolysis of the photoacids generators, the radical and Brönsted acid (HSbF₆) were generated simultaneously under irradiation. The radical initiated the photopolymerization of acrylate which induced the gelation and fixed the shape of the gel quickly, the acid allowed the sol–gel process occurring gradually. Due to the enhancement of the polymer, the strength of aerogels was improved. The aerogels had low density (~?0.098 g/cm³), high porosity (~?94.8%), high flexibility and high elastic recovery. Moreover, the silica aerogels have hydrophobic and oleophilic properties, which endows them with an ability of oil water separation.     

Comparative studies on the physical properties of TEOS,TMOS and Na<Subscript>2</Subscript>SiO<Subscript>3</Subscript> based silica aerogels by ambient pressure drying method

In order to compare the various precursors of silica aerogels, three different precursors namely TMOS, TEOS and Na₂SiO₃ were studied in this paper. The property differences of the aerogels caused by the three precursors were discussed in terms of reaction process, gelation time, pore size distributions, thermal conductivity, SEM, hydrophobicity and thermal stability. It has been found that the gelation time of the silica gel is strongly dependent on the type of precursor used. During the surface modification process, organic groups were attached to the wet gel skeletons transforming the hydrophilic to the hydrophobic which were characterized by Fourier Transform Infrared spectroscopy (FTIR). It has been found that the contact angle of the Na₂SiO₃ and TMOS precursor based aerogels with water have the higher contact angle of 149° and whereas Na₂SiO₃ precursor based aerogel has the lower contact angle of 130°. The thermal conductivities of the Na₂SiO₃ and TMOS based aerogels have been found to be lower (0.025 and 0.030 W m^?1 K^?1, respectively) compared to the TEOS based (0.050 W m^?1 K^?1) aerogels. The pore sizes obtained from the N₂ adsorption measurements varied from 40 to 180, 70 to 190, and 90 to 200 nm for the TEOS, TMOS and Na₂SiO₃ precursor based aerogels, respectively. The scanning electron microscopy studies of the aerogels indicated that the Na₂SiO₃ and TMOS based aerogels show narrow and uniform pores while the particles of SiO₂ network are very small. On the other hand, TEOS aerogel show non-uniform pores such that the numbers of smaller size pores are less compared to the pores of larger size while the SiO₂ particles of the network are larger as compared to both Na₂SiO₃ and TMOS aerogels. Hence, the surface are of the aerogels prepared using TEOS precursor has been found to be the lowest (~620 m² g^?1) compared to the Na₂SiO₃ (~868 m² g^?1) and TMOS (~764 m² g^?1) aerogels.