Resorcinol-formaldehyde Xerogels Research Articles

Nitrogen doping in carbon xerogels via ammonia pyrolysis: A case study

Nitrogen-doped carbon xerogels (CXNs) were carbonized across a range of temperatures (400 °C–800 °C) using sol-gel derived resorcinol-formaldehyde xerogel to elucidate their structural evolution during carbonization in ammonia. The investigation extensively probed chemical and structural changes in CXNs at varying carbonization temperatures, employing analytical methods such as X-ray photoelectron spectroscopy, infrared spectroscopy, and energy-dispersive X-ray spectroscopy. These findings were thoroughly compared with the local structure derived from total X-ray scattering and Raman spectroscopy. The results showed a transformation in the local ordering and chemical structure of CXNs, characterized by a reduction in oxidation sites, expansion of C sp2 network structures, leading to the growth of aromatic domains, and the incorporation of nitrogen into the carbon backbone structure. Pyrolysis in ammonia notably influenced the textural structure, resulting in a partial loss of mesopores and an increased micropore volume.

Characteristics of Power Supercapacitor with Electrodes Made of Composite Carbon Nanopaper Based on Carbon Nanotubes and Resorcinol–Formaldehyde Xerogel

Characteristics of Power Supercapacitor with Electrodes Made of Composite Carbon Nanopaper Based on Carbon Nanotubes and Resorcinol–Formaldehyde Xerogel

CARBON NANOPAPER PRODUCED FROM CARBON NANOTUBES/RESORCINOL-FORMALDEHYDE XEROGEL NANOCOMPOSITE FOR ELECTROCHEMICAL SUPERCAPASITORS

The nanocomposite of resorcinol-formaldehyde xerogel (RF-xerogel) and carbon nanotubes after carbonation at 800 °С was obtained in the form of composite carbon nanopaper (CCNP) with a thickness of 100-300 microns, a density from 0.1 g/cm2 to 0.5 g/cm2 and an electronic conductivity of more than 10 S/cm. According to the low temperature nitrogen adsorption data, the microporous structure of the nanopaper is formed by carbonized RF-xerogel, and the mesoporous structure is formed by the nanotube framework. The specific surface area of the nanopaper calculated by the method of nonlocal density function theory (NLDFT) exceeds 600 m2/g. The main contribution to the specific surface area of CCNP is made by pores with a width of ~ 0.7 nm, therefore, electrodes for a supercapacitor made of such paper are quite effective only in aqueous solutions of H2SO4 and KOH with small sizes of solvated ions. A technique for nanopaper activation with potassium hydroxide has been developed for the use of CCNP with organic electrolytes. The maximum specific surface area (NLDFT method) of activated CCNP reaches 1182 m2/g with a loss of carbon xerogel mass of ~ 25%. At the same time, the pore surface area of more than 1 nm width increases from 350 m2/g in CCNP to 685 m2/g in activated CCNP. Nanopaper has mechanical strength, pretty cheap and convenient for use in supercapacitors.

Effect of the Porous Structure on the Electrochemical Characteristics of Supercapacitor with Nanocomposite Electrodes Based on Carbon Nanotubes and Resorcinol–Formaldehyde Xerogel

Effect of the Porous Structure on the Electrochemical Characteristics of Supercapacitor with Nanocomposite Electrodes Based on Carbon Nanotubes and Resorcinol–Formaldehyde Xerogel

Synthesis of composite membranes from polyacrylonitrile/carbon resorcinol/formaldehyde xerogels: gamma effect study, characterization and ultrafiltration of salted oily wastewater

Abstract The subsequent activated carbons of resorcinol-formaldehyde xerogels are synthesized and exposed to wide range of gamma irradiation dose. Xerogels and their subsequent activated carbons are characterized by diverse techniques; FTIR, Raman, porosity analysis, SEM, EDX and AFM. The composite membranes are fabricated from polyacrylonitrile and activated carbon xerogels as composite membranes to be utilized in ultrafiltration process of salted oily wastewater. The soybean oil is exploited as organic feeding solution. The results declared that values of flux and rejection reach 157 (L m−2 h−1) and 99.8 (%), respectively. Overall, the best performing composite membrane is conducted by maximizing pure water flux. The optimally synthesized membrane performs well for purification of salted oily wastewater, and a significant increment in permeate flux is obtained with soybean oil rejection is at ∼99.8% and with maximum flux is at 32 (L m−2 h−1). Further, the composite membranes showed good promise for ultrafiltration of salted oily wastewater.

Hydrophobization of Monolithic Resorcinol-Formaldehyde Xerogels by Means of Silylation.

In materials research, the control of wettability is important for many applications. Since they are typically based on phenolics, organic aerogels, and xerogels are intrinsically hydrophilic in nature, and examples of the chemical functionalization of such gels are scarce and often limited to powders. This study reports on the silylation of monolithic resorcinol-formaldehyde (RF) xerogels using solutions of silyl chlorides and triflates, respectively, in combination with an amine base. The resulting gels are structurally characterized by means of elemental analysis, X-ray photoelectron spectroscopy, pycnometry, sorption analysis, and scanning electron microscopy with electron-dispersive X-ray spectroscopy. The wetting behavior of the silylated gels was studied by the determination of the contact angle to water after exposure of the gels to ambient air. Additionally, the uptake of liquid water and aqueous acids and bases was investigated. As a result, processes for the functionalization of RF xerogels with sterically demanding silyl moieties have been established. Although the analyses indicate that silylation occurred to a rather small extent, highly hydrophobic gels resulted which retained the wetting behavior over the course of several months with contact angles of >130°. Monoliths bearing sterically demanding silyl groups showed higher stability towards aqueous acid than trimethylsilylated RF gels.

Carbon-based nanohybrid fabricated in-situ and boosted the adsorption of anionic reactive yellow dye

The purpose of this research is to report a unique manipulation of producing carbon nanotubes/carbon xerogel (CNTs/CX) hybrid loaded with bimetallic catalysts of Fe–Ni via one-step chemical vapor deposition (CVD) of camphor. Active bimetallic catalysts were formed during carbonization of resorcinol–formaldehyde xerogel at 800 °C to form carbon xerogel (CX); meanwhile, the carbon gas was librated from camphor at 220 °C for 45 min. CX and CNTs/CX samples were analyzed using measurements of transmission electron microscope (TEM), X-ray diffraction tool (XRD) and Fourier transform infrared spectroscopy (FTIR). Further, the liquid-phase adsorption of a reactive yellow 160 (RY160) dye on CX and CNTs/CX samples and thermodynamic studies were investigated. TEM and XRD results revealed the formation of carbon nodules in CX combined with bundles of CNTs having outer diameters ranged from 40 to 80 nm in CNTs/CX. Adsorption of RY160 dye was highly relied on pH, temperature, initial dye concentration and contact time. Through kinetic modeling, pseudo-second-order kinetic model expressed closely the experimental data of RY160 adsorption. Equilibrium adsorption studies declared that the Freundlich model is the better with adsorption of RY160 on CNTs/CX due to it has heterogeneous surface character resulting from combination of CX and CNTs. Langmuir adsorption capacity (qL, mg/g) values exhibited that CNTs/CX sample have a superior adsorption of RY160 dye which were reached to 167 mg/g than that by CX (qL = 125 mg/g). Thereof, the produced CX and CNTs/CX samples present higher removal capacity and can be employed successfully for RY160 dye removal from a textile wastewater.

The Role of Cations in Resorcinol–Formaldehyde Gel Textural Characteristics

The production of resorcinol–formaldehyde xerogels has yielded insight into the gelation processes underpinning their structures. In this work, the role of the cation species from the catalyst is probed by studying the simultaneous addition of sodium carbonate and calcium carbonate to a resorcinol–formaldehyde mixture. Twenty-eight xerogels were prepared by varying the solids content, catalyst concentration, and catalyst composition, and each was analysed for its textural properties, including the surface area and average pore diameter. The results indicate that the role of the cation is linked to the stabilisation of the clusters formed within the system, and that the Group II catalyst causes the salting out of the oligomers, resulting in fewer, larger clusters, hence, an increase in pore size and a broadening of the pore size distribution. The results provide insight into how these systems can be further controlled to create tailored porous materials for a range of applications.

Fe-carbon hybrid composite interlayer for improved electrochemical performance of Li-S battery

The major bottlenecks in achieving the full potential of Lithium-sulfur (Li-S) batteries are the insulating nature of sulfur and Li-polysulfide (Li-PS) shuttling which lead to their poor rate performance and cyclic stability. In this work, we solve these critical issues using a unique Fe-carbon hybrid composite interlayer prepared by the catalytic graphitization of resorcinol formaldehyde (RF) xerogel. The physicochemical characterizations of the interlayer material reveal that it consists of iron-nanoparticles encapsulated by graphitic carbon within the amorphous RF xerogel derived hard carbon matrix (Fe-G-HC-IL). The Li-S battery, with Fe-G-HC-IL as a functional separator and activated candle soot-sulfur composite cathode, demonstrates a significantly improved electrochemical performance than without the interlayer. The Li-S battery with Fe-G-HC-IL shows the capacity of 1057 mAh g−1 at 1 C-rate with a decay rate of 0.055% per cycle and excellent rate capability. Further, density functional theory (DFT) calculation indicates that the Fe embedded carbon structure present in interlayer composite contributes significantly to the improvement in the conductivity and minimization of Li-PS shuttling. Additionally, this Fe-carbon hybrid structure electro catalyzes the conversion of Li-PS to desired end products of the redox reactions which further enhance the electrochemical performance of Li–S batteries.

Mesoporous Carbon Xerogels Adsorbents for Adsorption of Cadmium and p-Nitrophenol Pollutants: Kinetic and Equilibrium Studies

Mesoporous carbon xerogels were prepared by carbonization of resorcinol-formaldehyde xerogel in presence of tannic acid as a soft-template at 700oC for 30, 60 and 90 min, respectively. Morphology, textural, surface functional surface groups and thermal analyses were measured. Their efficiency in liquid-adsorption of cadmium ions and PNP in a batch mode was carried out. Effect of some key factors such as pH, carbon dose, and contact time of cadmium ions and PNP as well as kinetic adsorption studies were performed. It was found that the obtained carbon xerogels of mesoporous structure have a moderate total surface area reached to 268 m2/g with various surface functional groups are effective sites for adsorbing cadmium ions and PNP from aqueous solutions. The adsorption behavior of both adsorbates on the prepared carbon samples was best described by the pseudo-second-order kinetic and Langmuir adsorption models. The maximum adsorption capacities of cadmium ions and PNP were 99 and 133 mg/g, respectively; onto the prepared carbon xerogel at 700oC and 60 min. As a result of these observations, the produced mesoporous carbon samples are very effective adsorbents for extracting considerable amounts of cadmium ions and PNP from aqueous solutions.

Ultrahigh rate and high-performance lithium-sulfur batteries with resorcinol-formaldehyde xerogel derived highly porous carbon matrix as sulfur cathode host

Lithium-Sulfur Battery (LSB) is looked upon as a promising energy storage device to be used in high end applications due to their high energy metrics. However, the successful commercialization of LSBs is hauled by limitations of low sulfur utilization, insulating sulfur, and poor cyclic stability. Restricting polysulfide shuttling and improving electrochemical kinetics are important in developing high-performance LSBs. Here, we have resolved these critical problems associated with LSBs even at extreme operating current conditions using highly porous N-doped resorcinol–formaldehyde (RF) carbon xerogel as sulfur host. We developed a RF xerogel-derived highly porous N-doped carbon matrix by two-step carbonization-activation strategy for anchoring polysulfides and accelerating their redox transformation simultaneously. The high amount of N-doping (10%) and graphitic domains, amid the porous amorphous carbon, possess active sites to entrap polysulfides. Besides, they provide pathways for high electron transfer for their electro-catalysis. First principle calculations also establish that the enhanced performance is attributed to high pyrrolic nitrogen content in the porous carbon structure. As a result, the A-N-RFC/S cathode composite exhibits excellent cycling stability, maintaining a specific capacity of 631 mAh g−1 at 4 C even after 500 cycles, corresponding to an extremely low capacity decay of 0.0017% per cycle with coulombic efficiency of more than 95%. Additionally, the composite material displays astonishing rate performance exhibiting an outstanding discharge capacity of 359 mAh g−1 even at an ultra-high current rate 20 C. The synthesized material overcomes the two key parameters, cyclic stability and rate performance required for commercial LSBs by suppressing the detrimental polysulfide shuttling.

Effect of the porosity and microstructure on the mechanical properties of organic xerogels

The synthesis of resorcinol–formaldehyde (RF) xerogels is versatile enough to provide materials with custom pore size distributions in the meso-/macroporous range. Specifically, seven xerogels were synthesised by changing the pH of the same RF solution, from pH 3 to pH 6. The resulting materials presented meso-/macroporous size distributions with average pore sizes from < 5 to 510 nm, as determined by Hg intrusion and N2 adsorption. Most of the RF xerogels had very similar geometric densities, except for the gels obtained at the two highest pH values, which were more dense. Both flexural and uniaxial compression tests were carried out to determine the dependence of strength and stiffness on the porosity of the xerogels. The results followed a power-law relationship between the mechanical properties and the density of the materials. However, two series of RF xerogels were found to fit such law independently, with the gels obtained at the three most acidic pH values (pH 3–4) showing unexpectedly high compression moduli. Further characterisation of the xerogels microstructure revealed the existence of rod-like microstructures that would bear most loads during the compression tests. These microstructures would act as struts that, once broken, would cause the catastrophic failure (bursting) of the xerogel.

In situ graphitized hard carbon xerogel: A promising high-performance anode material for Li-ion batteries

Abstract

Nanopore effects on the combustion temperature of resorcinol-formaldehyde xerogels

Abstract The rationalization of thermal analysis data of porous substances is often complicated by the fact that it is not always possible to synthesise non-porous reference samples. Therefore, it is difficult to quantify the effect that the porosity has on the thermal behaviour. Here, we present detailed investigations about the thermal behaviour of a mesoporous model system consisting of a resorcinol-formaldehyde xerogel, synthesised via an acid-catalysed route. Samples were prepared with different pore sizes varied over the entire nanoscale range (from Thermogravimetric analysis of this model system revealed a pronounced increase in combustion temperature and decrease in combustion rate for samples with mean pore diameters smaller than 20 nm. Interestingly, this finding can be correlated with the Knudsen regime of the diffusion of oxygen.

Resorcinol-Formaldehyde Xerogel Derived Highly Porous Carbon Matrix: A Novel Approach for Suppressing the Shuttling Effect in High Rate Lithium-Sulfur Battery

Lithium-sulfur battery (LSB) is considered as the next generation battery with a remarkable theoretical specific capacity and energy density of sulfur electrodes as 1672 mA.h.g-1 and 2600 W.h.kg-1 respectively1. However, the bottle necks like insulating nature of sulfur, polysulfide shuttling and volume expansion of sulfur electrodes during cycling are detrimental to practically achieve such astounding electrochemical performance. Hence, the practical application of LSBs are hindered by low rate performance and capacity fading during cycling1-2. In order to address the above issues we have incorporated sulfur in a highly porous nitrogen doped carbon matrix derived from resorcinol-formaldehyde (RF) xerogel to synthesize a composite (a-N-RFC/S) cathode electrode for LSB. The high surface area and pore volume along with the nitrogen doping in RF-derived carbon xerogel imparts conductivity, accommodate volume expansion and restricts polysufide shuttling when tested electrochemically. As a result, the above synthesized composite a-N-RFC/S exhibits superior rate performance and stable cyclic performance.Nitrogen doped RF xerogel was prepared by sol-gel polycondensation reaction followed by sub-critical drying. The prepared xerogel was pyrolysed at 900 °C to obtain carbon xerogel which was subsequently activated with KOH under N2 atmosphere. The resultant a-N-RFC was subjected to melt diffusion for sulfur incorporation to yield a-N-RFC/S composite. The electrode was prepared by mixing composite, binder and carbon black in 75:10:15 ratio. As prepared electrode was used as working electrode and lithium foil as the counter and reference electrode in a coin-cell assembly.The N2 sorption studies reveal a high surface area of 1589 m2.g-1 was obtained for a-N-RFC due to the presence of micro as well as mesopores with a total pore volume of 0.96 cm3.g-1 after activation. Furthermore, the electrochemical performance of the prepared composite was carried out by subjecting it to rate capability studies at different current rates. The a-N-RFC/S composite delivered a reversible capacity of 1283, 814, 518 mAh.g-1 at 0.4, 4 and 10 C rate respectively with almost 100% coulombic efficiency. Even at current rate as high as 20 C, it delivered a specific reversible capacity of 497 mAh.g-1. Interestingly, when the current rate was swapped back, it maintained the capacity values suggesting an excellent capacity retention.Such remarkable electrochemical performance can be attributed to the high specific surface area of the host material which can accommodate the volume expansion of sulfur and confine the polysulfides in it during cycling. Moreover, nitrogen doping chemically absorb the higher order polysulfides and converts into Li2S1. Thus, polysufides are restrained in the host material by physical as well as chemical sorption leading to extraordinary performance. A detailed characterization of the prepared carbon host and composite electrode material along with their electrochemical performance will be presented during the conference meeting. 1. Chao Deng, Zhuowen Wang, Shengping Wang & Jingxian Yu, Inhibition of polysulfide diffusion in lithium–sulfur batteries: mechanism and improvement strategies. Mater. Chem. A, 7, (2019), 12381.2. Sang-Kyu Lee, Yun Jung Lee, & Yang-Kook Sun, Nanostructured lithium sulfide materials for lithium-sulfur batteries, Power Sources, 323 (2016) 174-188. Figure 1

Mimicking flower petals to fabricate self-cleaning and antireflective polymer surfaces

This work explored a facile way to fabricate superhydrophobic and antireflective polymer surfaces using Canna indica flower petals. A simple, inexpensive two-step soft-lithography technique was employed to replicate the surface structures of C. indica flower petals onto two different polymer surfaces namely, polydimethylsiloxane (PDMS) — a hydrophobic polymer and resorcinol–formaldehyde (RF) xerogel — a hydrophilic polymer. First, a negative replica was prepared in PDMS using the flower petal as a template. Second, the negative PDMS replica was later used as a stamp to prepare a positive replica in both PDMS and RF xerogel. These replicated structured surfaces greatly influenced the wettability properties. The contact angle of the replicated PDMS surfaces increased to the nearly superhydrophobic values of 145° (negative PDMS replica) and 144° (positive PDMS replica) compared with that of the plain surface, which is 101°. Interestingly, the contact angle of the replicated RF surface significantly increased to 151° (superhydrophobic) compared with that of the planar RF surface, which is hydrophilic (68°). Furthermore, the replicated polymer surfaces exhibited not only excellent superhydrophobic properties but also antireflective properties. These multifunctional surfaces with superhydrophobicity as well as an omnidirectional antireflective property may find a wide range of applications, such as solar energy harvesting, optical displays and marine applications.

Hydrophobic and heat-resistant poly(methylphenylsiloxane)-modified resorcinol–formaldehyde composite xerogel monoliths and the carbonized derivatives

Resorcinol–formaldehyde (RF) aerogels are widely used as thermal insulation materials. The present investigation of RF aerogels emphases on improving hydrophobic property and high-temperature residual yields. In this study, polysiloxane-modified RF composite xerogel monoliths were prepared by gelation of poly(methylphenylsiloxane) prepolymer with RF in cosolvent and ambient pressure drying. The influence of phenyl content on structure and thermal property of xerogels was investigated. The as-prepared RF composite monoliths were lightweight and hydrophobic with density lower than 0.300 g/cm3 and water contact angles approaching 130°. Morphology analyses indicated that polysiloxane grew and formed a continuous layer on RF porous skeleton with controlled reaction parameter. Thermogravimetry analyses (TGA) certified that the controlled incorporation of polysiloxane in composite xerogels enhanced the thermal degradation temperature at which the weight loss of RF xerogels reaches 5% (T5%) and increased the residual yields at 1000 °C. The carbonized derivatives were obtained for further enhancing the thermal stability by removal of small molecules during carbonization. Xerogel monoliths preserved hydrophobic property till 500 °C carbonization due to the thermal stability of poly(methylphenylsiloxane). The heat-resistant composite xerogels remained monolith structure after 800 °C treatment under nitrogen condition.

Influence of Chitosan Addition on Resorcinol–Formaldehyde Xerogel Structure

Gels are usually not environment-friendly due to their difficult biodegradability. Therefore, the addition of chitosan, even in small amounts, will make such gels biodegradable and thus can be useful in many applications that require environment-friendly materials. The addition of small quantities of chitosan to the reacting solution resorcinol–formaldehyde xerogel was investigated. Different hybrid resorcinol–formaldehyde–chitosan xerogels were characterized by different techniques, including Raman spectra, FTIR, XRD, TGA, SEM, surface area and porosity analyzer, and CHNS/O microanalyzer. It was seen that the addition of chitosan, even in a minor quantity, has a significant influence on the structural features of the resulting xerogels. The lattice order and crystallinity, chemical functions, thermal stability, morphology, elemental ratio, pore structure, and appearance were changed by adding chitosan into the xerogel structure.

Enhanced catalytic graphitization of resorcinol formaldehyde derived carbon xerogel to improve its anodic performance for lithium ion battery

Resorcinol-formaldehyde (RF)–derived carbon xerogels are hard carbons, which are not explored thoroughly as anodes in lithium-ion batteries as they exhibit poor crystallinity, conductivity and significant capacity fading during cycling. Graphitization of hard carbons, using metal catalysts at elevated temperatures, may be an effective method to enhance their crystallinity. In this study, RF xerogels were graphitized using an iron catalyst at a moderate temperature (1100 °C) in nitrogen atmosphere. Three different catalyst loadings (2, 5, and 10 wt. %) were used to study the extent of graphitization and thereby the effect on physiochemical properties of these RF-derived carbon xerogels. X-ray diffraction and Raman spectroscopy revealed an increase in the degree of graphitization and crystallinity, while high-resolution transmission electron micrographs showed an enhancement in the structural ordering of graphene layers with increase in catalyst loading. Interestingly, when tested electrochemically, the 5 wt.% catalyst loaded RF carbon xerogel was found to exhibit the best anodic performance as ever reported. A specific reversible capacity of 470 mA h/g at 0.2 C rate was retained even after 100 continuous charge/discharge cycles, with nearly 100% columbic efficiency. Furthermore, even at high C-rates (1C), a reasonably high reversible capacity of 348 mA h/g was maintained during cycling.

Investigation on the kinetic mechanism of the reduction of Fe2O3/CoO-decorated carbon xerogels: A non-isothermal study

The reduction of Fe2O3 and CoO oxides supported on carbon xerogels was studied to elucidate the effect of the nanoconfinement of the catalyst in carbon matrices. Resorcinol formaldehyde xerogels were synthesized, impregnated with iron and cobalt nitrates, and subsequently heated to obtain the oxides. The mechanism of oxide reduction to a metal with hydrogen was investigated by in-situ synchrotron X-ray diffraction in dynamic, non-isothermal conditions. Kinetic profiles of the reactions are obtained by plotting the diffraction intensities of selected Bragg peaks vs. temperature. The extracted Temperature-Programmed-Reduction (TPR) diagrams were best fitted by the Avrami-Erofeev and the n-order kinetic models for the iron and the cobalt oxide reduction reactions, respectively. The activation energies for the two-step reduction of Fe2O3 to FeO and then to Fe are 80 and 15.1 kJ mol−1. Such results may contribute to developing efficient and inexpensive catalysts based on non-noble metals like Fe, Co, via deposition of metal complexes on mesoporous supports.