Carbon Xerogels Research Articles

Synthesis of novel ZnO/carbon xerogel composites: Effect of carbon content and calcination temperature on their structural and photocatalytic properties

This paper reports the development of new ZnO/carbon xerogel composites (XZn w) for photocatalytic applications. The use of black wattle tannin as a precursor to the carbon xerogel aimed at reducing costs and environmental impacts. The composites were characterized by diffuse reflectance spectroscopy (DRS), BET surface area, scanning electron microscopy (FEG-SEM), X-ray photoelectron spectroscopy (XPS), energy dispersive spectroscopy (EDS), infrared spectroscopy (IR), and X-ray diffraction (XRD). The photocatalytic performance of the materials was evaluated in the decomposition process of methylene blue, a known toxic pollutant. The impacts of the catalyst dosage and calcination temperature on the photocatalytic process were also examined systematically. The X-ray profiles of the XZn w evidenced the existence of the hexagonal structure of the zinc oxide (wurtzite) in the composites. The XPS and XRD analyses confirmed the incorporation of carbon in the zinc oxide crystalline structure. The higher carbon content resulted in a larger surface area. All composites presented the ability to absorb radiation in less energetic wavelengths, contrary to pure zinc oxide that only absorbs radiation of wavelengths below 420 nm. The optimal dosage and calcination temperature were found to be 0.2 g L−1 and 300 °C. All the developed composites displayed significant photocatalytic activities in the decomposition of methylene blue under both visible and solar light. The composites had superior photocatalytic efficiency under visible light when compared to pure zinc oxide. The XZn 0.5 presented the best degradation efficiency under visible radiation. All materials presented similar photocatalytic responses under solar light, evidencing the synergy between the carbon xerogel and the zinc oxide. The photocatalytic mechanism was evaluated by trapping experiments to be mainly controlled by the electron vacancies that are generated during the photoexcitation of the composites.

The role of conductive additives on the performance of hybrid carbon xerogels as electrodes in aqueous supercapacitors

Three different hybrid carbon xerogels containing Graphene Oxide (AXGO), Micronized Graphite (AXMG) and Carbon Black (AXCB) were synthesized using an easy, fast and affordable method. These three additives were initially selected to improve the electrical conductivity of the pristine activated carbon xerogel (AX) thus expecting to improve its performance in aqueous supercapacitors. Capacitances of the corresponding devices were measured as a function of current density and results of the high and low charge transfer regime of the supercapacitors were discussed separately. In both regimes, the differences observed between the hybrid electrodes were analyzed on the basis of the concurrent influence of the micro and mesoporosity, surface chemistry and electrical conductivity of the materials. Accordingly, even though all the hybrid carbon xerogels showed higher electrical conductivities, only AXGO rendered a better performance than AX, showing the highest capacitances in the whole interval of intensities studied. Consequently, at 16 A g−1, the energy and power densities of the AXGO supercapacitors increased up to 16% and 97%, respectively, with respect to AX, and of 143% and 409%, respectively, with respect to a commercial activated carbon used as reference. The performance of AXCB and, especially AXMG was worse than AX supercapacitors due to a combination of inadequate pore size distributions and/or a poor surface chemistry. Finally, TEM analysis helped to understand the different way the three additives were affecting the nanostructure (and final properties) of the hybrid carbon xerogels.

Cutting the Green Waste. Structure‐Performance Relationship in Functionalized Carbon Xerogels for Hydrolysis of Cellobiose

AbstractThe hydrolysis of cellulose and hemicelluloses in the presence of solid acid catalysts is of great importance as an entry process for the production of biofuels and chemicals. In this work, the effect of carbon surface chemistry on the catalyst conversion and selectivity in hydrolysis of cellobiose to glucose was studied. Solid catalysts based on carbon xerogels (CXs) containing various functionalities incorporating heteroatoms (S, P, N, O), as confirmed by XPS, TPD, ICP and EA analyses, were successfully prepared. CXs containing 3.4 wt % of phosphorus showed the best catalytic performance, reaching 90 % conversion of cellobiose with 72 % selectivity to glucose, under oxidative atmosphere and in a short reaction time of only 4 h. The presence of phosphonates (−P−C−) was found to increase the selectivity to glucose up to unprecedented 87 %. On the contrary, carbonyls, quinones, pyrydinic, pyrrolic and sulfonic groups were found to promote glucose degradation, leading to a final low yield of this product. Additionally, the increased concentration of oxygen in the reaction atmosphere was shown to significantly weaken the β (1–4)‐glycosidic bond, increasing the final yield of glucose.

Effect of synthesis route in the photocatalytic, structural and morphological properties of carbon xerogel-Nb2O5 composites

This project aimed to evaluate the synthesis route influence in the structural, morphologic and photocatalytic properties of carbon xerogel/Nb2O5 composites. Infrared spectroscopy and X-ray diffractometry confirm the existence of niobium-based oxides in the composites, whereas the diffuse reflectance and energy dispersive spectroscopy prove the incorporation of the carbon xerogel. The XC-24Nb composite presented the best photocatalytical response, as it presented higher crystallinity, high surface area and superior carbon xerogel incorporation. The XC-24Nb carb was the least effective material, probably due to its considerably inferior surface area. The degradation of methylene blue is controlled by electrons and electron vacancies.

Novel microfluidic and analytical approaches for screening of engineered strains: Micro-sensor technologies for pH and organic acids detection

Textural properties of carbon xerogels and their applications depend on the path of synthesis. The variables to be considered are the starting reagents, relation and the catalysis medium used. Here we present a dataset for a series of carbon xerogels obtained by the polymerization of resorcinol-formaldehyde (R/F) at different resorcinol/Na2CO3 (C) ratios (R/C). To examine the effect R/C ratio on the morphology and surface chemistry of the xerogels, N2 isotherms at 77K, XRD, FTIR were measure as well as of immersion calorimetry measurements were taken in water and benzene; point zero charge (pzc) were also determined. These results are reported here.

How do the micropores of carbon xerogels influence their electrochemical behavior as anodes for lithium-ion batteries?

Abstract This work aims at shedding light on how the microporous texture of porous carbons influences their electrochemical behavior when used as anodes for Li-ion batteries. To this aim, a synthetic hard carbon (carbon xerogel, CX), prepared from a resorcinol-formaldehyde precursor gel, underwent several post-synthesis treatments in order to modulate its micropore to total pore volume ratio. The micropore volume was either expanded by physical activation or decreased using chemical vapor deposition (CVD) of a carbon layer. Several variables other than the micropore texture of the obtained carbons, which could influence their behavior as anode active materials for Li-ion batteries, such as the particle size or the electrode characteristics, were carefully controlled. The thickness of electrode coatings and the pore texture of the active material-binder composite were analyzed. It was shown that CX-binder composites resulting from water-based slurries preserve the microporosity of the starting materials. Detailed electrochemical characterization of the electrodes prepared with carbon xerogels displaying various defined micropore textures was performed. A clear linear dependency could be evidenced between the Li+ insertion and de-insertion in half-cell configuration with the increase of the volume of supermicropores (0.7–2 nm), demonstrating the effect of micropore enlargement by activation on the storage capacity, provided the maximum charge potential value is set at 3.0 V vs. Li+/Li.

Porous Properties of Carbon/Carbon Composite Xerogels

Carbon/carbon composite xerogels are prepared by a vacuum drying technique whereas a carbon cryogel is synthesized by a freeze drying technique to compare the effect of these drying methods at the selective synthesis condition. Resorcinol and formaldehyde are used to prepare a matrix phase and cotton fibers are acted as a disperse phase of the carbon/carbon composite xerogels. Here resorcinol and formaldehyde is utilized to synthesize the carbon cryogel only. The carbon/carbon composite xerogels and the carbon cryogel were analyzed by a nitrogen adsorption apparatus and a field emission scanning electron microscope. The results support that the vacuum drying can decrease the pore shrinkage despite of the gas-liquid interface. The porous properties of the carbon xerogel is quite equivalent to the porous properties of the carbon cryogel. When the porous properties of carbon xerogels are considered, their porous properties can be preserved at the high porosity until 0.15 g/g of the cotton fibers/resorcinol ratio. At 0.25 g/g of this ratio, the porous properties start decreasingly.

The effect of surfactants on carbon xerogel structure and CO2 capture

Porous carbon xerogels were successfully fabricated by carbonization of resorcinol-formaldehyde-phloroglucinol gels in the presence of surfactants. The effect of non-ionic, cationic, anionic and zwitterionic surfactants on the porous structure of carbon xerogels was investigated. Micropore structural carbon xerogels were obtained when a cationic surfactant was used. The result of surface tension test showed the non-ionic surfactant Tween 60 gave the lowest surface tension. Thermal conductivity experiments revealed the carbon xerogels in this work possessed low thermal conductivity and are suitable as thermal insulation materials. The pore size directly determined the thermal conductivity of carbon xerogels, and the thermal conductivity decreased with pore size. Among the obtained samples, the one using hexamethylenetetramine (HTM) exhibited the highest CO2 adsorption capacity (2.89 mmol g−1) and adsorption selectivity as a result of the high microporosity (88.0%). Therefore, HTM-CA was identified as a promising material for CO2 adsorption.

Change of self-discharge mechanism as a fast tool for estimating long-term stability of ionic liquid based supercapacitors

A systematic study is presented on self-discharge (SD) characteristics and leakage current levels of supercapacitors based on various carbon materials in ionic liquids: 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide (EMImTFSI) and 1-butyl-3-methylimidazolium tetrafluoroborate (BMImBF4). The higher SD rate occurring with partly mesoporous carbon xerogels, opposed to microporous activated carbon, suggests underlying faradaic induced processes rather than SD governed by charge redistribution. The passivation of some surface active sites of carbon xerogels by high-temperature di-hydrogen treatment resulted in reduced SD extent. The cell potential hold dependence of SD was studied for individual electrodes, showing changes of SD mechanism in EMImTFSI for both electrode polarities at cell potential higher than 2.8 V, against ∼3.4 V and >3.6 V for the negatively and positively charged electrodes in BMImBF4.A good correlation was established between the electrochemical stability limits determined by long-lasting floating tests and the limits suggested by changes in the SD mechanism and increase in measured leakage current. Hence, SD and leakage current analysis is a sensitive and much faster tool for determining the maximum cell potential applicable to supercapacitors than life time tests by floating, and should be more extensively applied in the future for such objective.

Insights on palladium decorated nitrogen-doped carbon xerogels for the hydrogen production from formic acid

N-doped carbon xerogels synthesized from melamine-resorcinol-formaldehyde aqueous gels were used as supports for the preparation of Pd-based catalysts for the formic acid dehydrogenation reaction. The effect of the pore texture and nitrogen content of the carbon xerogels on the catalytic activity for the hydrogen production was investigated. It was observed that an appropriate nitrogen doping in a well-defined micro-mesoporous carbon xerogel enables the preparation of Pd catalysts with appreciable catalytic enhancement towards the formic acid dehydrogenation as compared to the N-free counterpart, reaching an initial TOF value of 2014 h−1.

Effect of carbon precursor and initial pH on cobalt-doped carbon xerogel for oxygen reduction

In this study, cobalt-doped carbon xerogel (Co-CX) was synthesised via sol-gel polymerisation of phenolic compounds (i.e., resorcinol, phenol and m-cresol) and formaldehyde, and this polymerisation was catalysed by cobalt nitrate and followed by a carbonisation process. The effect of the initial pH value (5.5, 6.5 and 7.5) as well as the type of carbon precursors on the structural properties of Co-CX was investigated via field emission scanning electron microscope (FESEM), Brunauer-Emmett-Teller (BET) and X-ray diffractometry (XRD). The catalytic activity of Co-CX for the oxygen reduction reaction (ORR) in 0.1 M KOH was studied using a rotating ring-disk electrode (RRDE) technique. The structural properties and ORR activities were affected by different initial pH values as well as the type of carbon precursor. A carbon precursor consisting of resorcinol-formaldehyde with an initial pH value of 7.5 exhibited the best catalytic activity. The initial pH plays an important role in promoting micro/mesopores. The FESEM and BET results revealed that Co doping promotes the formation of additional pores. The RRDE result indicated that Co-CX exhibited good catalytic activity that tends to favour a four-electron pathway.

Carbon xerogels graphitized by microwave heating as anode materials in lithium-ion batteries

An easy method to prepare carbon xerogels with tailored porous properties and high degree of graphitization is reported. A pristine carbon xerogel was obtained by microwave-assisted synthesis, which was then transformed into a graphitic carbon also via microwave heating. Graphitized carbon materials were obtained by using different microwave power densities and processing times, these parameters enabling the control of both the microporosity and the degree of graphitization of the initial carbon xerogel. This simple method has been found to improve the electrical conductivity of the pristine carbon xerogel up to 90%. The prepared graphitized materials were also evaluated as anode materials in lithium-ion batteries, resulting in stable cycle performances with specific capacities 70% higher than that of raw carbon xerogels.

Carbon xerogel based electric double layer capacitors with polymer gel electrolytes – Improving the performance by adjusting the type of electrolyte and its processing

Polymer gel electrolytes for electric double layer capacitors (EDLC), also known as supercapacitors, represent a safe and low-cost alternative to liquid electrolytes. This study shows that EDLCs with polymer gel electrolytes may accomplish gravimetric capacitances comparable to or even higher than those of EDLCs with the same carbon electrodes and the respective ions in aqueous electrolyte. One of the most important parameters in this context is the effective electrode/electrolyte interface. This can be improved by using a suitable electrode and assembling method for the polymer electrolyte component. As polymer gel electrolytes we investigated polyvinyl alcohol (PVA) with either phosphoric or sulfuric acid as well as PVA and sulfuric acid with hydroxyethyl-cellulose as an additive in combination with porous carbon (xerogel) electrodes with different meso/macrostructures. The gravimetric capacitances determined for EDLCs with the sulfuric PVA electrolyte were considerable higher than those of EDLCs using phosphoric acid. Furthermore, the results of our study indicate a clear influence of the electrodes pore structure and the applied assembling method on the specific capacitance. The processing method providing the best contact between polymer electrolyte and electrode yields EDLCs with a gravimetric capacitance comparable to or even higher than that of EDLCs with liquid H2SO4. The experimental findings strongly suggest that micropores in the interconnected carbon backbone of the electrode - including such in regions that are not in direct contact with the polymer gel phase - are saturated with electrolyte and contribute to the capacitance.

Low-Temperature Selective Catalytic Reduction of NOx with Urea Supported on Carbon Xerogels

Low-temperature selective catalytic reduction (SCR) of NOx on urea-supported carbon xerogels is studied. The kinetics results show that the orders of reaction for NO and O2 are 1 and 0.5 for NO reduction, while the orders of reaction for NO2 and O2 are 1 and 0.03 for NO2 reduction, respectively. The apparent activation energies of NO and NO2 reduction are calculated to be −14.0 and 0.79 kJ/mol, respectively. Possible mechanisms are proposed, based on the hypothesis that there exist two kinds of active sites on the surface of carbon xerogel, including nonreductive and reductive ones. Spillover of the NO3 species generated on the nonreductive ones into the reductive ones which are subsequently oxidized by NO3 and the migration of NO3 species from nonreductive ones into the oxidized carbon sites are considered to be of great importance for the successive urea-SCR process and the formation of steady-state NOx removal period.

A novel catalyst of ceria-nanorods loaded on carbon xerogel for catalytic wet oxidation of methyl green dye

In this study, promising catalysts of ceria nanorods-supported on the porous carbon xerogels via impregnation route were prepared. Three different molar concentrations of ceria catalysts (0.15, 0.25 and 0.50 mol/L) loaded onto carbon xerogels and denoted as follows 0.15Ce/CX, 0.25Ce/CX and 0.50Ce/CX were studied in the catalytic oxidation of methyl green (MG) dye. Nanorods-like shape of the CeO2 supported on the carbon xerogel was determined using both scanning electron microscope (SEM) and transmission electron microscope (TEM). Other techniques such as FTIR, XRD and N2 adsorption–desorption were investigated. The catalytic activity of the obtained samples was examined under various variables such as ceria loading, initial concentrations of dye, H2O2 dose and temperature to optimize the operating conditions. Catalytic wet oxidation experiments revealed that 0.25Ce/CX catalyst has the highest catalytic activity toward degradation of MG dye. The presence of ceria generated a large content of reactive basic oxygen species at the surface of CX and thus enhanced the catalytic oxidation of the pristine CX support. The optimal parameters showing efficient catalytic oxidation toward MG dye found to be 0.25 mol/L cerium loading, 0.139 mol/L H2O2 and temperature of 40 °C at pH 6.0 where 100% degradation of MG dye was obtained within 30 min. The maximum removal of dye through adsorption process only was reached to 54 and 42% onto CX and 0.25Ce/CX at 120 min. Conclusively, the studied catalytic system assessed that there was a synergetic effect as a result of a combination of basic oxygen groups and redox properties of CeO2 at the surface of CX which is effective for a heterogeneous Fenton-catalysis in liquid-phase processes.

Deep eutectic solvents-assisted cost-effective synthesis of nitrogen-doped hierarchical porous carbon xerogels from phenol-formaldehyde by two-stage polymerization

The cost-effective synthesis of nitrogen-doped hierarchical porous carbons has been demonstrated. Phenol and formaldehyde based polymers were used as the carbon precursor, deep eutectic solvents (DES) synthesized from choline chloride (CCl) and urea (U) in a molar ratio of 1:2 served as solvent as well as structure-directing agent, and also, the source of nitrogen of the carbons. A designed two-stage synthesis was applied for the polymerization. Carbon xerogels were derived via ambient drying followed by thermal treatment at 800 °C using N2 flow. The obtained carbons exhibit a hierarchical porous structure with modest macropores and well-developed micropores. The homogeneous incorporation of nitrogen from the urea-type DES at the solution stage, in combination with the two-stage synthesis process, allowed an excellent nitrogen-doping efficiency (ca. 5–9% for samples aged at 50 °C and ca. 3% for that at 90 °C) in the carbons even after thermal treatment at 800 °C. The unique carbons with nitrogen-rich hierarchical porous architecture are highly desirable for CO2 capture at both 0 °C and 25 °C and ~1 bar.

Performance of carbon xerogel-graphene hybrids as electrodes in aqueous supercapacitors

A pristine carbon xerogel (AX) and two hybrid samples (AX-3% and AX-9%), with different graphene percentages (3 and 9 wt%), were synthesized using a fast and economical process. It was observed that graphene produces less shrinkage of the xerogel structure during synthesis. Moreover, the electrical conductivity of the materials increases linearly with the percentage of graphene added. Thus, AX-9% presents an electrical conductivity 135 and 321% greater than that of AX-3% and AX, respectively. As a result of the good pore size distribution and high electrical conductivity of AX-9%, when this material is used as electrode in supercapacitors, the resistance of the cell is reduced; therefore, better power densities are obtained. However, its capacitance values are the same as those of AX-3%. The performances of these materials as electrodes in supercapacitors were evaluated taking into account the influence of their porosity and electrical conductivity. Moreover, AX and AX-9% were subjected to mild oxidation with air to study the effect of air on the porosity, electrical conductivity, and performance of these treated samples as electrodes in supercapacitors.

Catalytically Graphitized Nanostructured Carbon Xerogels As High Performance Anode Material for Lithium Ion Battery

Carbon materials are most widely used anode materials for Lithium ion battery since their introduction in 1991. Among various types of carbonaceous materials, graphite is still being used widely as anode material in commercial Lithium ion batteries because of its reasonable capacity and good cyclic efficiency. The problem with long term availability and less purity of macro crystalline natural graphite which is more suitable for battery application led researchers to look into preparation of synthetic graphite, basically prepared by carbonizing various graphitizing carbons like petroleum coke, pitch coke etc.. Various other forms of carbons and their composites have been tested for Li ion intercalation. Though they have higher capacity, their poor cyclability and higher irreversible capacity, higher reduction potential restrict them as commercial Li ion battery anode. Other than carbon materials, Ti, Si and Sn based alloys and oxides have been reported in recent years with 3 to 5 times increase in their intercalation capacity compared to graphite but capacity fade is high due to pulverization. In this presentation, we will present the ways to capitalize the higher capacity of hard (non graphitizing) carbons but at the same time minimizing the irreversible capacity losses and thus maximizing the energy density by catalytic graphitizing them at much lower temperature (< 1800 0C) than as usual graphitization temperature (2800-30000C). Although it is reported that control of internal nanostructure is difficult during catalytic graphitization of various organic precursors, some of the recent studies show that for organic aerogels, one can tune porosity by controlling synthesis and processing conditions during the catalytic graphitization. However although highly porous aerogels shows higher initial capacity due to adsorption of lithium ions inside the pores but also have larger irreversible capacity due to increased overall resistance (impedance) by increased double layer capacitance of the active electrode material. In this work, we shall present preparation of RF xerogel derived graphitic carbon with controlled porosity and thus specific surface area by catalytic graphitization in which metal nanoparticles as additive will be mixed with RF sol. During graphitization, these embedded metal additives react with carbon and forms metal carbide before decomposing to form graphite which will act as nuclei for further graphitization. Intercalation capacity of carbon xerogels have been earlier reported as 145 mAh/g however catalytic graphitization of these xerogels not only enhances it significantly but also increase the electrical conductivity by ordering of graphite layers in disordered carbon xerogels, which are the important characteristics of good electrode material. Thus, these nanoporous graphitic carbon behaves as an excellent anode material because of their minimal double layer capacitance compared to aerogels and good inlayer conductivity because of graphitization.

Porous Hollow Ptni/C Electrocatalysts: Carbon Support Considerations to Meet Stability Requirements

Porous hollow PtNi/C nanoparticles (NPs), an highly active electrocatalyst for the oxygen reduction reaction (ORR)1, were successfully synthesized on different carbon supports. The latter plays an essential role in the NPs synthesis, providing a way to control their morphology and Ni content. The physico-chemical properties of the different carbon supports were crucial during accelerated stress tests simulating start-stop events at a PEMFC cathode (i.e. step potentials between 1.0 and 1.5 V vs. RHE) and were discussed by the means of identical-location transmission electron microscopy (IL-TEM) and electrochemistry (see. Figure 1). It was observed that electrocatalysts supported on graphene nanosheets and carbon xerogel suffer from NPs detachment and carbon complete corrosion while COsurf forms on carbon blacks supports (i.e. incomplete oxidation of the surface). The degradation of the carbon support resulted in mass activity and specific activity losses, as the result of the increase of the resistance of the catalytic layers (ΔR k). Figure 1

Microporous nanohybrids of carbon xerogels and multi-walled carbon nanotubes for removal of rhodamine B dye

In the current paper, microporous nanohybrids of carbon xerogel and multi-walled carbon nanotubes (CX/MWCNTs) with different loadings of MWCNTs (1, 2 and 4 g/L) were synthesized. Scanning electron microscope (SEM) and transmission electron microscope (TEM) assessed the decoration of microspheres of carbon particles in CX matrix with bundles of MWCNTs. Nitrogen adsorption at −196 °C revealed that the N2 uptake was enhanced when CX was decorated with MWCNTs and the specific surface area (SBET) increased from 192 to 643 m2/g. The resulting hybrids are mainly microporous materials. Rising the amount of MWCNTs from 1 to 4 g/L led to a decrease in the SBET from 643 to 515 m2/g (i.e. decreased approximately 20%). Fourier transform infrared spectra (FTIR) and Boehm's titration showed that both acidic and basic oxygen functional groups were equally distributed while carboxylic groups were increased considerably by increasing the amount of MWCNTs. The efficiency of adsorption and catalytic oxidation properties of CX/MWCNTs nanohybrids towards removal of rhodamine B (RB) was examined. Adsorption equilibrium data were analyzed using five different adsorption isotherms, i.e. Langmuir, Freundlich, Flory-Huggins, Halsey and Elovich models. The equilibrium adsorption results proved that adsorption of RB dye onto these nanohybrids can be well-fitted using Langmuir and Halsey isotherms. These nanohybrids exhibited considerable adsorption capacities that varied from 154 to 256 mg/g. The catalytic performance indicated that the investigated nanohybrids could eventually be used for the removal of RB dye in presence of H2O2 and visible light within 60 min, affirming that the CX/MWCNTs obtained are effective materials in liquid-phase applications for treating dye-contaminated industrial wastewater.