Resorcinol-formaldehyde Sol Research Articles

Mechanical and thermal insulation properties of carbon fibre-reinforced carbon aerogel composites

ABSTRACT Carbon fibre-reinforced carbon aerogel composites (C/CAs) were prepared by using polyacrylonitrile (PAN) and phenolic (PH) carbon precursor fibres as reinforcements, respectively, then impregnated with resorcinol-formaldehyde sol, aging, solvent exchanging, CO2 supercritical drying and carbonization at 1000°C. The physical properties, structural morphology and mechanical and thermal insulation behaviour of the two kinds of composites were further compared. Pore structure analysis shows that the PH-based C/CAs have a larger specific surface area (438.92 m2·g−1) and richer mesoporous volume than the PAN-based C/CAs. Mechanical behaviour of the C/CAs has been changed, but the stronger enhancement effects belong to PAN-based C/CAs (compressive strength in xy and z-directions are 1.63±0.10 MPa and 1.22±0.19 MPa, bending strength is 3.62±0.50 MPa). However, the PH-based C/CA composites have relatively low thermal conductivities, ranging from 0.027 to 0.164 W·m−1·K−1 between 25 and 1800°C.

A stabilization synthesis strategy for atomically dispersed metal-N4 electrocatalysts via aerogel confinement and ammonia pyrolyzing

Catalytically active metals that are atomically dispersed on supports exhibit the highest atom utilization and most cost-effective pathways for electrocatalyst design. However, the high-throughput scalable production of inexpensive, efficient, and durable atomically dispersed electrocatalysts remains challenging. Herein, a hierarchical porous carbon aerogel loaded with atomically dispersed metal-N4 (metal-N-C) was synthesized via the NH3 pyrolysis of a metal-doped polymer aerogel. This novel synthetic strategy requires the tailoring of various materials, such as metal sol, resorcinol formaldehyde sol, hydrogel, and metal-N-C. The synthetic applicability of this strategy was demonstrated via the facile synthesis of Co-N-C, Ni-N-C, and Fe-N-C. Notably, Fe-N-C exhibited a half-wave potential of approximately 0.933 V vs. reversible hydrogen electrode and lost approximately 4 mV after 5000 cycles of accelerated aging test in a 0.1 mol/L KOH solution for the oxygen reduction reaction. In a solid-state zinc-air battery, Fe-N-C exhibited a maximum power density of 167 m W cm−2, an energy density of 956 W h Kg−1, and long-term stability over 120 h, which significantly exceeds that of commercial Pt/C. The high activity and durability of Fe-N-C is attributed to the double Fe-N4 active center, where the synergistic effect of the neighboring Fe-N4 promotes oxygen dissociation and produces less H2O2. The developed strategy provides an aerogel-based solution for fabricating inexpensive, efficient, and durable atomically dispersed electrocatalysts with potential for high-throughput scalable production and expands the understanding of the synthesis of atomically dispersed electrocatalysts.

Effect of Process Conditions on the Properties of Resorcinol-Formaldehyde Aerogel Microparticles Produced via Emulsion-Gelation Method.

Organic aerogels in the form of powder, microgranules and microsized particles receive considerable attention due to their easy fabrication, low process time and costs compared to their monolithic form. Here, we developed resorcinol-formaldehyde (RF) aerogel microparticles by using an emulsion-gelation method. The main objective of this study is to investigate the influence of curing time, stirring rate, RF sol:oil ratio and initial pH of the sol in order to control the size and properties of the microparticles produced. The emulsion-gelation of RF sol prepared with sodium carbonate catalyst in an oil phase at 60 °C was explored. RF microparticles were washed with ethanol to remove the oil phase followed by supercritical and ambient pressure drying. The properties of the dried RF microparticles were analyzed using FT-IR, N2 adsorption isotherm, gas pycnometry, wide angle X-ray scattering and scanning electron microscope. RF microparticles with high surface area up to 543 m2/g and large pore volume of 1.75 cm3/g with particle sizes ranging from 50–425 µm were obtained.

Anisotropic Magnetism in Gradient Porous Carbon Composite Aerogels

Porosity is of high importance for functional materials, as it allows for high surface areas and the accessibility of materials. While the fundamental interplay between different pore sizes and functionalities is quite well understood, few studies on gradually changing properties in a material exist. To date, only a few examples of such materials have been synthesized successfully. Herein, we present a facile method for synthesizing macroscopic carbon aerogels with locally changing pore sizes and functionalities. We used ultracentrifugation to fractionate differently functionalized and sized polystyrene nanoparticles. The assembly into gradient templates was conducted in a resorcinol–formaldehyde (RF) sol, which acted as a liquid phase and carbon precursor. We show that the modification of nanoparticles and a sol–gel precursor is a powerful tool for introducing dopants (sulfur and phosphorous) and metal nanoparticles (e.g., Ni) into gradient porous carbons formed during the carbonization of the RF sol. Understanding the underlying interactions between particles and precursors will lead to a plethora of possibilities in the material design of complex functionally graded materials. We showed this by exchanging parts of the template with magnetite–polystyrene composites as templating nanoparticles. This led to the incorporation of magnetite nanoparticles in the formed gradient porous carbon aerogels. Finally, gradually increasing concentrations of magnetite were obtained, ultimately leading to macroscopic carbon aerogels with locally changing magnetic properties, while the graded porosity was maintained.

Carbon Xerogel Nanostructures with Integrated Bi and Fe Components for Hydrogen Peroxide and Heavy Metal Detection.

Multifunctional Bi- and Fe-modified carbon xerogel composites (CXBiFe), with different Fe concentrations, were obtained by a resorcinol–formaldehyde sol–gel method, followed by drying in ambient conditions and pyrolysis treatment. The morphological and structural characterization performed by X-ray diffraction (XRD), Raman spectroscopy, N2 adsorption/desorption porosimetry, scanning electron microscopy (SEM) and scanning/transmission electron microscopy (STEM) analyses, indicates the formation of carbon-based nanocomposites with integrated Bi and Fe oxide nanoparticles. At higher Fe concentrations, Bi-Fe-O interactions lead to the formation of hybrid nanostructures and off-stoichiometric Bi2Fe4O9 mullite-like structures together with an excess of iron oxide nanoparticles. To examine the effect of the Fe content on the electrochemical performance of the CXBiFe composites, the obtained powders were initially dispersed in a chitosan solution and applied on the surface of glassy carbon electrodes. Then, the multifunctional character of the CXBiFe systems is assessed by involving the obtained modified electrodes for the detection of different analytes, such as biomarkers (hydrogen peroxide) and heavy metal ions (i.e., Pb2+). The achieved results indicate a drop in the detection limit for H2O2 as Fe content increases. Even though the current results suggest that the surface modifications of the Bi phase with Fe and O impurities lower Pb2+ detection efficiencies, Pb2+ sensing well below the admitted concentrations for drinkable water is also noticed.

Creating Directionality in Nanoporous Carbon Materials: Adjustable Combinations of Structural and Chemical Gradients

AbstractThe properties of porous materials benefit from hierarchical porosity. A less noted element of hierarchy is the occurrence of directionality in functional gradient materials. A sharp boundary is replaced by a transition from one feature to the next. The number of cases known for porous materials with either structural or chemical gradients is small. A method capable of generating combinations of structural and chemical gradients in one material does not exist. Such a method is presented with a focus on silver and nitrogen containing carbon materials because of the potential of this system for electrocatalytic CO2 reduction. A structural gradient results from controlled separation using ultracentrifugation of a binary mixture of template particles in a resorcinol–formaldehyde (RF) sol as carbon precursor. A new level of complexity can be reached, if the surfaces of the template particles are chemically modified. Although the template is removed during carbonization, the modification (Ag, N) becomes integrated into the material. Understanding how modified and unmodified large and small particles sediment in the RF sol enables almost infinite variability of combinations: chemically graded but structurally homogeneous materials and vice versa. Ultimately, a material containing one structural gradient and two chemical gradients with opposing directions is introduced.

N-doped porous carbon film electrodes for electrochemical capacitor, made by electrospray of sol precursors

Heteroatom doping in carbon structure enhances surface wettability, electrical conductivity, and electron-donor/acceptor properties that in turn, improves the electrochemical performance of carbon electrodes. In this article, in situ grown carbon film electrodes are obtained through electrospray of polymeric sol on carbon paper, followed by curing, and carbonization. The nitrogen functionality in these binder-free carbon film electrodes is successfully incorporated from melamine addition in resorcinol-formaldehyde sol. The film electrode shows a capacitance of 223 mF cm−2 (446 F g−1), and volumetric energy density of 15.2 mWh cm−3 at 1 mA cm−2 in a symmetric cell with 2 M KOH as electrolyte. Furthermore, these electrodes exhibit excellent cyclic stability even after 5000 charge-discharge cycles with almost complete capacitance retention at 5 mA cm−2. The outstanding electrochemical performance of carbon film electrodes are due to the combined effect of following steps: (i) incorporation of nitrogen and oxygen in right amount, (ii) retention of the desired pore size distribution, and fraction of micropores through systematic balancing of resorcinol to melamine ratio in precursor sol, (iii) disintegration of sol by electrospray prior to deposition on carbon paper, that in turn produces a porous three-dimensional interconnected network of nanoparticulate film, and (iv) lyophilization of wet gel layer that retains the pore spaces during solvent removal.

A novel carbon film electrode for supercapacitor by deposition of precursor sol on the current collector, followed by carbonization and activation in situ

This article describes the making of a carbon electrode, where the precursor sol is directly electrosprayed on the current collector. The gel was cured in situ, and the use of carbon fiber paper ensured sufficient adhesion for such processing. Prior to carbonization of resorcinol-formaldehyde (RF) gel, the solvent is removed by lyophilization, whereby the internal pore structure is least affected. The carbon pores are activated by heating the composite in a CO2 environment. The features of nanoporosity and the oxygen functionality in the film after CO2 activation are evident from bulk characterization, e.g., BET, FTIR, and XRD. The specific capacitance of such carbon film with aqueous KOH solution as electrolyte is found to be more than 500 F g−1 at current density of 1 A g−1. 99.12% of this capacitance is retained after 1000 charge-discharge cycles. The product quality is further analyzed with reference to the mode of solvent removal from the RF sol and the extent of CO2 exposure during activation.

Resorcinol Formaldehyde Based Carbon Fibers as Anode Material for Lithium Ion Batteries

We hereby introduces electrospun resorcinol-formaldehyde sol based carbon fibers as anode material for Li-ion batteries. To ensure the electrospinnability of resorcinol-formaldehyde sol, we blended it with poly-methyl methacrylate in 2:1 (v/v) ratio followed by controlled pyrolysis. As-prepared thin carbon fibers were then characterized by Raman & Infra-red spectroscopy and scanning electron microscope to examine the crystallinity and morphological changes during the carbonization. Later, we explored the electrochemical properties of these carbon fibers to test them as a potential anode material for lithium ion battery. We found a significant reversible capacity (624 mAh/g) with reasonable higher first cycle efficiency of 65%.

Resorcinol Formaldehyde Based Electrospun Carbon Fibers As High Capacity Anode Material for Lithium Ion Batteries

Introduction Resorcinol formaldehyde (RF) polymer derived carbon gels have been widely used as active electrode material in energy storage devices such as supercapacitors and lithium ion batteries due to their tunable surface area by varying drying conditions.1 In addition to the easily tunable surface area, morphology can also be modified by changing the synthesis parameters and methods.2,3 However, RF xerogel derived carbon nanofibers were not studied as anode material for Li-ion battery due to its non-electrospinnability. Recently, RF-based electrospun carbon nanofibers were prepared by blending it with stretchable Polymethyl methacrylate (PMMA).4 In this work, we have synthesized RF xerogel and PMMA blend based carbon nanofibers and used them as potential anode material for lithium ion battery. These RF xerogel derived electrospun carbon nanofibers show larger lithium ion intercalation capacity not only as compared to RF xerogel derived other carbon morphologies such as powder, interconnected carbon nanoparticles but also other carbon nanofibers based electrodes.5 , 6 Experimental RF sol was prepared by mixing resorcinol and formaldehyde (4:1 molar ratio) in acetone and HCl. Thus prepared RF sol was blended with templating PMMA polymer (1:2 Vol. ratio). A blend of this solution was used as precursor material for electrospinning. Electrospinning process parameters were tuned to yield long, continuous fibers. Controlled two step pyrolysis of these RF xerogel/PMMA electrospun fibers yielded carbon nanofibers. Results and discussions: RF-PMMA blend polymer fibers were electrospun directly on stainless steel wafer used as a substrate cum current collector. Average diameter of RF-PMMA electrospun RF-PMMA nanofibers as shown in Figure 1a was found to be 2.3 µm. These fibers were then pyrolyzed at 900 ⁰C to obtain carbon fibers with average diameter 1.4 µm. X-ray diffraction and Raman spectroscopy studies were done in order to confirm the glassy nature of as-derived carbon nanofibers. These RF-PMMA derived carbon nanofibers based electrode was then used as working electrode in Li (reference electrode) based half-cell. Galvanostatic charge discharge experiments were carried out at 37.2 mA/g current density. An initial reversible capacity of 620 mAh/g (Figure 1b) was observed for these carbon nanofibers which was significantly higher than reported values in literature for RF based powder, RF based graphitic powder and carbon nanoparticles. Further, cyclic stability and coulombic efficiency were found to be excellent. A detailed characterization of these carbon nanofibers and their electrochemical performance at different current densities will be presented during the conference.

On the synthesis and structure of resorcinol-formaldehyde polymeric networks – Precursors to 3D-carbon macroassemblies

With the new impetus towards the development of hierarchical graphene and CNT macro-assemblies for application in fields such as advanced energy storage, catalysis and electronics; there is much renewed interest in organic carbon-based sol–gel processes as a synthetically convenient and versatile means of forming three dimensional, covalently bonded organic/inorganic networks. Such matrices can act as highly effective precursors, scaffolds or molecular ‘glues’ for the assembly of a wide variety of functional carbon macro-assemblies. However, despite the utility and broad use of organic sol–gel processes – such as the ubiquitous resorcinol-formaldehyde (RF) reaction, there are details of the reaction chemistries of these important sol–gel processes that remain poorly understood at present. It is therefore both timely and necessary to examine these reactions in more detail using modern analytical techniques in order to gain a more rigorous understanding of the mechanisms by which these organic networks form. The goal of such studies is to obtain improved and rational control over the organic network structure, in order to better direct and tailor the architecture of the final inorganic carbon matrix. In this study we have investigated in detail, the mechanism of the organic sol–gel network forming reaction of resorcinol and formaldehyde from a structural and kinetic standpoint, by using a combination of real-time high field solution state nuclear magnetic resonance (NMR), low field NMR relaxometry and differential scanning calorimetry (DSC). These investigations have allowed us to track the network formation processes in real-time, gain both detailed structural information on the mechanisms of the RF sol–gel process and a quantitative assessment of the kinetics of the global network formation process. It has been shown that the mechanism, by which the RF organic network forms, proceeds via an initial exothermic step correlated to the formation of a free aromatic aldehyde. The network growth reaction then proceeds in a statistical manner following a first order Arrhenius type kinetic relationship – characteristic of a typical thermoset network poly-condensation process. And despite the relative complexity and ill-defined nature of the formaldehyde staring material, the final network structure is to a large extent, governed by the substitution pattern of the resorcinol molecule.

Effects of architecture on the electrochemistry of binder-free inverse opal carbons as Li–air cathodes in an ionic liquid-based electrolyte

In this work, different electrodes consisting of a layer of nanostructured binderless carbon supported on a stainless steel (SS) mesh have been developed and tested as cathodes for Li–air batteries. Inverse opal (IO) carbons were developed using poly(styrene-co-methacrylic acid) (PS-MAA) spheres of different sizes as templates via a resorcinol–formaldehyde sol–gel process. The resulting electrodes, which were mechanically stable and easy to manipulate, were electrochemically tested at both 25 and 60 °C by galvanostatic cycling in an ionic liquid-based electrolyte (0.3 M LiTFSI in PYR14TFSI). Different ratios of co-monomers used in the preparation of the template polymeric spheres to control their size significantly influenced the resulting surface area, pore volume and pore distribution in IO carbons of different macropore size. From the electrochemical characterisation, transverse trends in reversibility and rate capability were identified depending on the macropore size of the inverse opal carbon. Smaller pores favor a better charge–discharge reversibility. Large pores contribute to an improved rate capability and large capacity, which are likely due, respectively, to deeper oxygen diffusion into the electrode, and to larger pore bottlenecks.

Resorcinol–formaldehyde gel method to synthesize porous Ce0.8Sm0.2O1.9 nanoparticles

We report a novel resorcinol–formaldehyde sol–gel (RF) method to prepare samarium-doped ceria (SDC). The synthesized SDC powders exhibit cotton-like micro-morphology with nanosized pores. The structure is composed of interconnected nanoparticles with average particle size of 4.3nm and remarkably uniform size distribution. The nanoparticles are softly aggregated with Brunauer–Emmett–Teller (BET) surface area as high as 78m2/g after being calcined at 500°C.

Carbon tunnels formed in carbon/carbon composite cryogels

A carbon/carbon (C/C) composite cryogel was prepared by putting almost the maximum content of cotton fibers in vials and then filling them with resorcinol–formaldehyde (RF) sols, followed by solvent exchange with t-butanol, freeze drying and pyrolysis under inert atmosphere. The mesopore structure of C/C composite cryogels was observed by a scanning electron microscope (SEM) and their porous structures were characterized by nitrogen adsorption and mercury porosimetry. The SEM images confirm that the C/C composite cryogels, being heterogeneous carbon materials, possess ordinary carbon cryogels and carbon tunnels. Here, the tunnels are comprised of (i) carbon fibers, (ii) spaces between carbon fibers and nanoporous carbon spheres, (iii) nanoporous carbon spheres with surrounding nanocages, and (iv) dense ring structures. The formation model of carbon tunnels is also proposed. The porous properties of cryogels could be changed by the cotton fiber addition and the synthesis conditions. The results support that the synthesis condition of RF sol determines the porous properties of C/C composite cryogels.

Carbon Nanofoam-Based Cathodes for Li-O2 Batteries: Correlation of Pore-Solid Architecture and Electrochemical Performance

We investigated the effect of cathode pore-size distribution on the specific capacity of Li-O2 cells using binder-free, catalyst-free carbon nanofoam architectures prepared by infiltrating carbon fiber papers with a resorcinol-formaldehyde sol. Four sets of carbon nanofoam papers were synthesized with pore-size distributions ranging from mesopores (5-50 nm) to small macropores (50-200 nm). The specific capacity of Li-O2 cells tracked the average pore size of the pore network in the carbon nanofoam of the air cathode, rather than the specific surface area of the nanoscale carbon network. The macroporous nanofoams yield cell capacities of 400-500 mAhg-1, ~ 2x that of the mesoporous nanofoams (~200 mAhg-1), even though the macroporous foams had lower specific surface areas (~350-450 vs. ~500 m2g-1). For the four nanofoam architectures studied, the capacity is limited by pore occlusion arising from solid product electrogenerated near the O2-facing boundary and not the respective total pore volumes.

Net-Like ZnO/Carbon–Aerogel Composite as Anode Material for Ni-Zn Secondary Battery

Nanosized ZnO particles were prepared by a facile homogeneous precipitation method. Resorcinol-formaldehyde sol with addition of the ZnO particles was carbonized at 700 °C for 1.5 h to prepare ZnO/carbon-aerogel composite. TEM investigation shows that ZnO particles were em-bedded in 3-dimensional net-like carbon and covered uniformly by amorphous carbon coating. Compared with ZnO, the ZnO/carbon-aerogel composite showed better cycling stability, higher discharge capacity and lower charge-transfer resistance. The discharge capacity delivered by the ZnO/carbon-aerogel composite remained above 550 mAh g-1during 40 cycles with little capacity loss. The enhanced electrochemical performances were mainly ascribed to the effects of the net-like structure of amorphous carbon, which uniformly surrounded the ZnO particles to guarantee good contact and acted as a skeleton to alleviate the shape change and dendrite growth of zinc electrode.

Resorcinol-formaldehyde based carbon nanospheres by electrospraying

Carbon nanospheres were synthesized using sol-gel processing of organic and aqueous resorcinol formaldehyde (RF) sols combined with electrospraying technique. RF sol was electrosprayed to form nano-droplets which were collected on a Si wafer. After oven drying at 60°C for 12 h, RF nano-droplets were pyrolyzed at 900°C in an inert atmosphere to yield the carbon nanospheres. This study reports the optimization of various process parameters including needle diameter, applied electric potential and liquid flow rate in order to get spherical, mono-disperse particles. For the organic RF sol, the optimized parameters, needle diameter 0·241 mm, electric potential, 1·5 kV/cm and a flow rate of 0sd8 ml/h, enabled the synthesis of nearly monodispersed carbon nano-spheres with diameter of 30·2 ± 7·1 nm. With the same conditions, aqueous RF sol produced irregularly shaped nanoparticles with a smaller mean diameter and much higher variance (17·4 ± 8·0 nm). The surface properties were significantly influenced by the surface morphologies as demonstrated by the water contact angle (WCA) studies. The surface covered with the RF derived carbon nano-spheres was extremely hydrophilic (WCA 10·1°) as compared to a much weaker hydrophilicity of the RF derived carbon films (WCA 83·3°). The hydrophilic carbon nanospheres reported here may have potential applications as adsorbents and in controlled drug delivery, biosensors and carbon-based microelectromechanical systems (C-MEMS) including bio-MEMS.

Nanostructured SnS/carbon composite for supercapacitor

Net-like nanostructured SnS/carbon composite was prepared by heating mixture of SnS nanoparticles and resorcinol–formaldehyde sol at 650 °C. The morphology and structure of prepared SnS and SnS/carbon composite were studied by transmission electron microscopy (TEM) and X-ray diffraction (XRD). Electrochemical investigation indicated that SnS/carbon composite presented superior electrochemical performances than pristine SnS. SnS/carbon composite had better capacitive response in cyclic voltammetry and could deliver larger specific capacitance of 36.16 F/g in galvanostatical charge–discharge process. Net-like structure of SnS/carbon composite and good conductivity of carbon were considered to be responsible for its preferable electrochemical performances.

Synthesis of Ordered Mesoporous Carbons Using Resorcinol-Formaldehyde Sol as the Carbon Source and As-synthesized MCM-48 as the Template

Ordered mesoporous carbons were synthesized by using as-synthesized MCM-48 as the template and resorcinol(R)-formaldehyde sol as the carbon precursor.By changing the molar ratio of resorcinol(R) to template(w(R)/w(MCM-48)) and resorcinol(R) to water(w(R)/w(H2O)),mesoporous carbons with different porosities were produced.The obtained samples were characterized by N2 adsorption-desorption,X-ray diffraction,scanning electron microscopye,transmission electron microscopye and thermogravimetric analysis.It is found that the optimized w(R)/w(MCM-48) and w(R)/w(H2O) ratios are in the rang from 0.50 to 0.65 and from0.15 to 0.35,respectively,and the resulting ordered mesoporous carbons has BET specific surface areas greater than 1125m2/g,mean pore sizes from 2.2 to 2.4nm,and high thermal stability.

Net-like SnS/carbon nanocomposite film anode material for lithium ion batteries

SnS particles with sizes of 5.0–6.5 nm were prepared by a facile method. Resorcinol–formaldehyde sol with addition of the as-prepared SnS nanoparticles was spin-coated on a copper foil to prepare net-like SnS/C composite thin-film electrode for lithium ion batteries after carbonization at 650 °C. The SnS/C nanocomposite thin-film electrode showed preferable first coulombic efficiency and excellent cycling stability. The discharge and charge capacities were respectively 542.3 and 531.3 mAh/g after 40 cycles. The attractive electrochemical performances were mainly ascribed to the ultra fine particle, which showed no evident aggregation in high-resolution TEM image, and the effects of 3-dimensional net-like carbon structure, which uniformly surrounded the SnS nanoparticles to guarantee the contact, acted as a buffer matrix to alleviate the volume expansion of Li–Sn alloy and provided enough paths for electrolyte to reach SnS active material during discharge–charge process.