Nanoporous phloroglucinol-formaldehyde carbon aerogels for electrochemical use

Phloroglucinol-Formaldehyde (PF) organic aerogels were prepared from alcoholic sol-gel polycondensation of phloroglucinol with formaldehyde using KOH as base catalyst and followed by supercritical drying with carbon dioxide. Subsequent pyrolysis of PF organic aerogel under He flow produced carbon aerogels. Textural properties of PF organic and carbon aerogels were obtained by nitrogen adsorption-desorption, and their specific capacitances were measured by cyclic voltammetry. The resultant PF carbon aerogels were mostly mesoporous material with high surface area. The nanoporous structure and electrochemical behavior of PF carbon aerogels could be controlled by the molar ratio of phloroglucinol to catalyst (P/C) and carbonization conditions. PF carbon aerogels exhibited the highest surface area in excess of 1,200 m²/g and specific capacitance up to 250 F/g in comparison to other carbons.     

PREPARATION OF ORGANIC MESOPOROUS GEL BY SUPERCRITICAL/FREEZE DRYING

ABSTRACT

Resorcinol-formaldehyde (RF) aerogels were synthesized by sol-gel polycondensation of resorcinol with formaldehyde in a slightly basic aqueous solution and supercritical drying with carbon dioxide. The control of mesoporous structure of the aerogels was studied by changing the amounts of resorcinol, formaldehyde, distilled water, and sodium carbonate (basic catalyst) used in the polycondensation. As a result of characterization by nitrogen adsorption, the mesopore radius of the RF aerogel was controlled in the range of 2.5 – 9.2 nm. After the hydrogels were immersed in excess of t-butanol, RF cryogels were prepared by freeze drying. The cryogels prepared were mesoporous materials with high surface areas > 500 m²/g and large mesopore volumes > 0.8 cm³/g. Although the surface areas and mesopore volumes of RF cryogels were smaller than those of RF aerogels, the cryogels were useful precursors of mesoporous carbons.     

Porosity and surface area of monolithic carbon aerogels prepared using alkaline carbonates and organic acids as polymerization catalysts

Carbon aerogels were prepared by polymerization of a resorcinol-formaldehyde mixture using different polymerization catalysts such as: sodium or potassium carbonates, oxalic acid or para-toluenesulfonic acid. The carbon aerogel obtained with this last acid was further CO₂-activated to 8.5% and 22% burn-off. All samples were characterized by N₂ and CO₂ adsorption at −196 and 0 °C, respectively, and by mercury porosimetry, scanning electron microscopy, and thermogravimetric analysis. Samples prepared using Na₂CO₃ were denser than those prepared using K₂CO₃. In addition, the density of samples prepared under acidic conditions was greater than that of samples prepared using alkaline carbonates as catalysts. Most of the carbon aerogels prepared were mesoporous with narrow pore size distributions. Results obtained showed that the nature of the acid used in the preparation of these aerogels only affected the gelation process. Finally, it is noteworthy that CO₂ activation of the carbon aerogel prepared with para-toluenesulfonic acid as catalyst only increased and widened the microporosity and had virtually no effect on the mesoporosity.     

Ultralight conducting polymer/carbon nanotube composite aerogels

By embedding carbon nanotubes into poly(3,4-ethylenedioxythiophene)–poly(styrenesulfonate) (PEDOT–PSS) supermolecular hydrogels in the presence of a very small amount of polyvinyl alcohol (PVA), we have presented the fabrication of ultralight conducting polymer/carbon nanotube composite aerogels with the apparent density of 0.04–0.07 g/cm³ made by supercritical CO₂ drying of as-made composite hydrogel precursors. The carbon nanotubes employed here are directly applicable to pristine (MWCNTs) or acid treated (c-MWCNTs) multi-wall nanotubes. Infra-red spectroscopy is used to confirm that PVA used for stabilizing nanotubes during the synthesis of hydrogel precursors has been completely removed by solvent exchange before supercritical CO₂ drying. The morphology and textural properties of the resultant composite aerogels are investigated by scanning electron microscopy, nitrogen adsorption/desorption, and X-ray powder diffraction tests. The thermal stability, together with electrical conductivities, of the resulting composite aerogels is revealed by the thermal gravitational analysis as well as conductivity tests. The results show that embedding of either MWCNTs or c-MWCNTs into PEDOT–PSS aerogel matrix can significantly enhance the specific surface areas (280–400 m²/g), the thermal stability and electrical conductivities (1.2–6.9 × 10⁻² S/cm) of the resulting composite aerogels.     

Surface characteristics and electrochemical capacitances of carbon aerogels obtained from resorcinol and pyrocatechol using boric and oxalic acids as polymerization catalysts

Carbon aerogels were prepared by carbonizing (at 500–1500 °C) organic aerogels obtained from the polymerization reaction of resorcinol and/or pyrocatechol with formaldehyde using boric and oxalic acids as polymerization catalysts. Prepared samples were characterized by different techniques to ascertain their composition, surface chemistry, morphology, and surface physics, determining their electrochemical capacitances in acidic medium. The use of pyrocatechol yielded carbon aerogels that were micro–mesoporous, showing Type IV N₂ adsorption isotherms with Type H2 hysteresis cycles. The volume and size of mesopores depended on the acid catalyst used and the temperature at which the carbon aerogel was obtained. Conversely, the sample prepared with resorcinol and boric acid as catalyst was micro–macroporous and that obtained with a resorcinol–pyrocatechol mixture was micro–mesoporous but with large mesopores. Most of the boric acid used was lost during the exchange of water with acetone in the organic hydrogels before their supercritical CO₂ drying. Carbon aerogels obtained at 900 °C and using boric acid as polymerization catalyst showed a capacitance between 17 and 24 μF/cm². Boron influenced the capacitance because it increased the oxygen content. Sample synthesized using pyrocatechol, formaldehyde, and oxalic acid and heat-treated at 900 °C had the highest capacitance, 34 μF/cm².     

Preparation and performance of cobalt-doped carbon aerogel for supercapacitor

Carbon aerogels were prepared by polycondensation of resorcinol with formaldehyde in ambient conditions. The effect of resorcinol-to-catalyst ratio (R/C ratio) on volume shrinkage, BET surface area, and electrochemical property was investigated by changing R/C ratio from 50 to 2000. Carbon aerogel prepared at R/C ratio of 500 showed less than 2% of volume shrinkage and the highest BET surface area (706 m²/g). Specific capacitance of carbon aerogel prepared at R/C ratio of 500 was found to be 81 F/g in 1M H₂SO₄ electrolyte. Cobalt-doped carbon aerogels were then prepared by an impregnation method with a variation of cobalt content, and their performance was investigated. Among the samples prepared, 7 wt% cobalt-doped carbon aerogel showed the highest capacitance (100 F/g) and the most stable cyclability. The enhanced capacitance of cobalt-doped carbon aerogel was attributed to the faradaic redox reactions of cobalt oxide.     

Monolithic zinc oxide aerogel with the building block of nanoscale crystalline particle

In the present paper, cost-effective zinc chloride has been utilized to synthesize zinc-based aerogel using the epoxide addition sol–gel process. After supercritical drying of wet gels with CO₂, zinc-based aerogels have been prepared. With the subsequently thermal treatment in selected atmospheres and designed heating program, zinc oxide aerogel of high porosity, large specific area (125 m² g^?1), narrow particle size distribution and excellent hexagonal structure was obtained. The microstructures and compositions of zinc oxide aerogels obtained in different atmospheres were characterized. The results showed that decreasing the ratio between oxygen and nitrogen contributed to the maintenance of monolithic aerogels and desirable microstructure. FTIR, XRD, and TG were applied to analyze the chemical and physical changes of the thermal treatment process. Those analyses are able to provide a reference for preparing other metal oxide aerogels. It should be a promising method to design new metal oxide aerogels with attractive nanoarchitectures.     

Enhanced thermal shrinkage behavior of phenolic-derived carbon aerogel-reinforced by HNTs with superior compressive strength performance

A novel carbon/m-HNTs composite aerogel was synthesized by introducing the modified halloysite nanotubes (m-HNTs) into phenolic (PR) aerogels through chemical grafting, followed with carbonization treatment. In order to explore the best proportion of HNTs to phenolic, the micromorphology of PR/m-HNTs were investigated by SEM before carbonization, confirming 10 wt% of m-HNTs is most beneficial to the porous network of aerogels. The interaction between PR and HNTs was studied by FTIR spectra, and microstructure evolution of the target product-carbon/m-HNTs composite aerogel were illustrated by SEM and TEM techniques. SEM patterns indicated that the carbon/m-HNTs aerogels maintain a stable porous structure at 1000 °C (carbonization temperature), while a ~20 nm carbon layer was formed around m-HNTs generating an integral unit through TEM analysis. Specific surface area and pore size distribution of composite aerogels were analyzed based on mercury intrusion porosimetry and N₂ adsorption–desorption method, the obtained results stayed around 500 m²g^?1 and 1.00 cm³g^?1 (pore volume) without significant discrepancy, compared with pure aerogel, showing the uniformity of pore size. The weight loss rate (26.76%) decreased greatly compared with pure aerogel, at the same time, the best volumetric shrinkage rate was only 30.83%, contributed by the existence of HNTs supporting the neighbor structure to avoid over-shrinking. The highest compressive strength reached to 4.43 MPa, while the data of pure aerogel was only 1.52 MPa, demonstrating the excellent mechanical property of carbon/m-HNTs aerogels.     

Porosity of Mixed Iron Oxide-Chromia Alumina Aerogels

Mixed iron oxide-chromia-alumina aerogels were prepared by mixing a solution of iron and chromium acetylacetonates in methyl alcohol with a solution of aluminum tri-sec-butoxide in 2-butanol followed by hydrolysis and supercritical drying. The resultant aerogels were highly porous, weak monoliths that easily broke into fine porous particles. Addition of 8% Fe or 2% Cr to the aluminum precursor solution increased the surface area of the resultant alumina aerogel from 400 m²/g to 500 and 600 m²/g respectively. These materials have significant pore volume as judged by the amount of N₂ adsorbed at liquid nitrogen temperatures (range of 400 to 1,100 cm³ N₂/g at P/P₀ = 0.9). Post treatments of oxidation or reduction also increased the porosity of the aerogels to values as high as 2,500 cm³ N₂/g. It then appears that small percentages of foreign cations (Fe, Cr) can increase significantly the porosity of a material like an alumina aerogel.     

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     

有机-无机杂化柔性硅气凝胶的制备与表征

以甲基三甲氧基硅烷（MTMS）和四乙氧基硅烷（TEOS）为混合硅源、甲醇为溶剂,通过酸碱两步催化溶胶-凝胶法制备湿凝胶,经超临界流体干燥得到块状二氧化硅气凝胶。用扫描电镜、氮气吸附脱附测试以及热重分析等手段对气凝胶的微观形貌、比表面积、孔径分布、弯曲性、压缩性、热稳定性等进行研究,结果表明：MTMS/TEOS比例会影响气凝胶的微观结构、弯曲和压缩性以及热稳定性,以MTMS/TEOS=8/1制得的气凝胶密度为0.11 g·cm^-3、孔隙率为94.2%、比表面积为693.3 m²·g^-1、最大弯曲角可达92°、最大压缩比例可达41.2%、压缩回弹率为100%。     

Large size and low density SiOC aerogel monolith prepared from triethoxyvinylsilane/tetraethoxysilane

Crack-free silicon oxycarbide (SiOC) aerogel monolith was fabricated by pyrolysis of precursor aerogel prepared from triethoxyvinylsilane/tetraethoxysilane (VTES/TEOS) using sol-gel process and ambient drying. Effects of different precursors, the amount of base catalyst (NH₄OH) and the heating rate during pyrolysis on the properties such as monolithicity, bulk density, surface area and pore size distribution of aerogels were investigated. The results show that the crack-free SiOC aerogel can be easily obtained from VTES/TEOS as compared to that of methyltriethoxysilanes/tetraethoxysilane (MTES/TEOS) and phenyltriethoxysilanes/tetraethoxysilane (PhTES/TEOS) precursors. The influence of heating rate during pyrolysis process on shrinkage rate, ceramic yield and surface area of the SiOC aerogels could be ignored, while the variation in the amount of NH₄OH exerted a strong impact on the properties of SiOC aerogels. Increasing the amount of NH₄OH resulted in the decrease of bulk density and surface area of SiOC aerogels from 0.335 g/cm³ and 488 m²/g to 0.265 g/cm³ and 365 m²/g. The resultant SiOC aerogels exhibit high compressive strength (1.45–3.17 MPa). ²⁹Si MAS NMR spectra revealed the retention of Si-C bond in the SiOC aerogels after pyrolysis at 1000 °C. The present work demonstrates VTES/TEOS is a promising co-precursors to easily and low cost synthesize large size SiOC aerogel monolith.     

In situ production of spherical aerogel microparticles

Due to their high surface area, low density, open pore structure and excellent insulation properties aerogels are intensively investigated since the past decades for a diverse range of applications. The current methods of silica aerogel production by supercritical extraction produce monolithic aerogels, where the sol is aged in molds and dried by extraction with supercritical CO₂. Aerogels in the form of spherical microparticles would be beneficial for many applications, for instance, drug delivery for respiratory route; or as insulating materials. However, because of aerogel's mechanical properties, it is difficult, rather impossible, to obtain spherical microparticles by milling or crushing of the monolithic aerogels. This work presents a new method to produce biocompatible spherical aerogel microparticles using an emulsion technique (in situ production) followed by supercritical extraction of the resulted dispersion (gel-oil). Water in oil emulsion was produced by mixing the sol (dispersed phase) with a vegetable oil (continuous phase) followed by the gelation of the dispersed phase. The size distribution of the final gel particles was found to be influenced by agitation, surfactant concentration and sol:oil volume ratios. The gel-oil dispersion was subsequently extracted with supercritical CO₂, Silica aerogel spherical microparticles with a surface area of 1100 m²g⁻¹, pore volume of 3.5 cm³/g and different mean particle diameters ranging from 200 μm to a few millimeters were produced using the presented method.     

Effect of reduction temperature on the preparation of zero-valent iron aerogels for trichloroethylene dechlorination

Zero-valent iron (ZVI) aerogels have been synthesized by sol-gel method and supercritical CO₂ drying, followed by H₂ reduction in the temperature range of 350–500 °C. When applied to trichloroethylene (TCE) dechlorination, the ZVI aerogel reduced at 370 °C showed the highest performance in the conditions employed in this study. Thus, the effect of reduction temperature in preparing ZVI aerogels has been investigated by several characterizations such as BET, XRD, TPR, and TEM analyses. As the reduction temperature decreased from 500 to 350 °C, the BET surface area of the resulting aerogels increased from 6 to 30 m²/g, whereas their Fe⁰ content decreased up to 64%. It was also found that H₂ reduction at low temperatures such as 350 and 370 °C leads to the formation of ZVI aerogel particles consisting of both Fe⁰ and FeO _x in the particle cores with a different amount ratio, where FeO _x is a mixture of maghemite and magnetite. It is, therefore, suggested that reduction at 370 °C for ZVI aerogel preparation yielded particles homogeneously composed of Fe⁰ and FeO _x in the amount ratio of 87/13, resulting in high TCE dechlorination rate. On the other hand, when Pd- and Ni-ZVI aerogels were prepared via cogellation and then applied for TCE dechlorination, we also observed a similar effect of reduction temperature. However, the reduction at 350 or 370 °C produced Pd- or Ni-ZVI aerogel particles in which Fe⁰ and Fe₃O₄ co-exist homogeneously. Since both Fe⁰ and Fe₃O₄ are advantageous in TCE dechlorination, the activities of Pd- and Ni-ZVI aerogels reduced at 350 °C were comparable to those of both aerogels reduced at 370 °C, although the former aerogels have less Fe⁰ content.     

Preparation and structural evolution of SiOC preceramic aerogel during high-temperature treatment

Silicon oxycarbide (SiOC) ceramic aerogels in mesopores range have been fabricated by pyrolyzing polycarbosilane aerogels in nitrogen (N₂) atmosphere. The reactants, poly(methylhydrosiloxane) and 2, 4, 6, 8-tetramethyl-2, 4, 6, 8-tetravinylcyclotetrasiloxane have been heated in the presence of hydrochloroplatinic acid. As-prepared SiOC preceramic aerogel has specific surface area of 299 m²/g at room temperature, and decomposes during pyrolysis. Structural evolution of the aerogels as a function of heat-treatment temperature has been investigated by Fourier transform infrared spectrophotometer, X-ray diffraction analysis, transmission electron microscopy and X-ray photoelectron spectroscopy. Results indicate that tetrahedral Si–O–C network underwent four structural changes during thermal treatment from room temperature to 1600 °C.     

In-situ synthesis and textural evolution of the novel carbonaceous SiC/mullite aerogel via polymer-derived ceramics route

A novel carbonaceous SiC/mullite composite aerogel is derived from catechol-formaldehyde/silica/alumina hybrid aerogel (CF/SiO₂/AlOOH) via polymer-derived ceramics route (PDCR). The effects of the reactants concentrations on the physicochemical properties of the carbonaceous SiO₂/Al₂O₃ aerogel and SiC/mullite aerogel are investigated. The mechanism of the textural and structural evolution for the novel carbonaceous SiC/mullite is further discussed based on the experimental results. Smaller reactants concentration is favorable to formation of mullite. Reactants concentration of 25% is selected as the optimal condition in considering of the mullite formation and bulk densities of the preceramic aerogels. Spherical large silica particles are also produced during heat treatment, and amorphous silica is remained after this reaction. With further heat treatment at 1400 °C, silicon carbide and mullite coexist in the aerogel matrix. The mullite addition decreases the temperature of SiC formation, when compared with the conventional methods. However, after heat treatment at 1450 °C, the amount of mullite begins to decrease due to the further reaction between carbon and mullite, forming more silicon carbide and alumina. The carbonaceous SiC/mullite can be transferred to SiC/mullite binary aerogel after carbon combustion under air atmosphere. The carbonaceous SiC/mullite has a composition of SiC (31%), mullite (19.1%), SiO₂ (14.4%), and carbon (35%). It also possesses a 6.531 nm average pore diameter, high surface area (69.61 m²/g), and BJH desorption pore volume (0.1744 cm³/g). The oxidation resistance of the carbonaceous SiC/mullite is improved for 85 °C when compared with the carbon based aerogel.     

Zirconia aerogels: Effect of acid used in the sol-gel process on morphological properties

Zirconia aerogels have been prepared from butanolic zirconium(IV) tetra-n-butoxide diluted in ethanol via stoichiometric hydrolysis with water in ethanol. Nitric acid or acetic acid were used to modify the sol-gel process. After calcination in air at 573 K, the aerogel prepared with nitric acid possesses a specific surface area of 240 m² · g^–1 and a unimodal pore size distribution with a maximum at ca. 24 nm, whereas the use of acetic acid results in an aerogel with specific surface area of 228 m² · g^–1 and bimodal pore size distribution with maxima at 3 and 65 nm. The crystalline fractions of both aerogels are predominantly tetragonal with a small contribution of monoclinic ZrO₂.     

Diffusion of gases in metal containing carbon aerogels

Carbon aerogels containing Fe, Ni, Cu or no metal were prepared by carbonisation of polymer aerogels synthesised from 2,4-dihydroxybenzoic acid and formaldehyde and modified by CVD of benzene. Uptakes and diffusion coefficients of CO₂, CH₄, N₂ and O₂ were measured and the results compared with those obtained using a commercial carbon molecular sieve. The results indicated that the diffusion of light gas molecules in carbon aerogels cannot be interpreted solely on the basis of micropore diffusion, but that the very high mesopore volumes of the aerogel monoliths exert a strong influence on the kinetics of diffusion in these materials. The mesoporosity is decreased when the % solids used during synthesis of the polymer precursor increases and this resulted in kinetic behaviour which was more similar to that predicted by Fickian or LDF models. Increasing % solids was also accompanied by generally slower diffusion rates and generally lower uptakes. The single gas uptakes and diffusion coefficients could be altered by varying the % solids used during synthesis of the polymer precursor, by introducing different metals into the polymer hydrogel by ion exchange, or by CVD of benzene on the carbon 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.     

Polar and dispersion interactions at carbon surfaces: further development of the XPS-based model

Enthalpy of immersion (ΔH_i) in water has been measured for a series of ozone oxidised non-porous carbon blacks and, as in our previous studies been found to correlate directly with the total surface oxygen level [O]_T measured by X-ray photoelectron spectroscopy. An equation that allows calculation of either parameter from the other is given and shown to describe behaviour for a wide range of carbon black surfaces which contain ozone-generated or native oxygen functional groups. Using this approach, the surface polarity and the relative hydrophilic character of such surfaces can be predicted. A molar enthalpy for the polar interaction between water and surface oxygen atoms of 17 kJ mol⁻¹ is obtained by assuming a 1:1 co-ordination between water molecules and carbon surface oxygen atoms. The data lead to a predicted value of 37.5 mJ m⁻² for the immersion of oxygen-free carbon black external surface into water. This equates to a value of 2.5 kJ mol⁻¹ for the non-specific dispersion interaction between water and an oxygen-free carbon black surface when a molecular area of 10.5×10⁻²⁰ m² for water is assumed. The same carbon black when oxidised using nitric acid gives a different enthalpy of immersion to the ozone-treated and native oxide materials, this is attributed to differing chemistry of the two surface types, this aspect is discussed. The nitric acid treated carbons do, however, give the same value as the ozonated and native oxide carbons (37.5 mJ m⁻²) for the immersion of an oxygen-free carbon surface into water. A correlation between the point of zero charge (pH_PZC) of the carbons and ΔH_i or [O]_T is also presented. The results from these measurements show extremely good agreement with data from other groups who have used TPD to assess surface oxygen concentration. This gives a firm basis for confident prediction of the thermodynamic properties of carbon surfaces from single measurement techniques.