Carbon Cryogel Research Articles

Effect of pore morphology on the electrochemical properties of electric double layer carbon cryogel supercapacitors

In this study, a group of carbon cryogels have been synthesized using resorcinol formaldehyde as precursors, and altered via catalysis and activation, to obtain varied nanostructures and pore size distributions. To understand the relation between structure and electrochemical properties, an alternate approach to de Levi’s cylindrical pore, transmission line method was utilized. Using electrochemical impedance spectroscopy, the capacitor can be studied as a dielectric system composed of a porous electrode and the electrolyte (tetraethylammonium tetrafluoroborate in propylene carbonate). The complex capacitance and power are used to study the behavior of the system below the relaxation frequency f0 (φ=−45°). Therefore, the relaxation of the capacitor system at the low frequency range, f<f0, may be used as a measure of pore/electrolyte interaction. The approach proposed here also allows for a direct experimental characterization of the capacitance and power at low frequencies, where small pores are likely to affect the diffusion kinetics and dynamics of the electrolyte molecules. The results suggest a correlation between the occurrence of small micropores and that of high power losses that are related to the resistive element at the low frequency range. Moreover, the impact of the micropore structure on the supercapacitor’s performance is apparent in its capacitance and energy as well. In addition to the complex power and capacitance, other measurements including nitrogen physisorption, cyclic voltammetry, galvanic cycling, and x-ray Raman scattering were used to characterize the samples and support these results.

Control of the porous structure and the surface property of resorcinol – Acetaldehyde carbon cryogels

Mesoporous carbon material enables material transfer more easily than that of microporous carbon material such as activated carbon and has a high porosity and effective surface area. The application for the separation material, the absorbent, the catalyst support and the electrode medium of electric double layer capacitor are expected for the mesoporous material. This study attempted to prepare the mesoporous carbon material. Resorcinol (R) as a raw material and acetaldehyde (A) as a cross-linking agent were used to synthesize the RA hydrogels via the sol-gel polycondensation in a slightly basic aqueous solution. The RA hydrogels were freeze-dried and subsequently pyrolyzed at 1073 K for 30 min (at a hearting rate of 10 K/min) to obtain the RA carbon cryogels. This study attempted to control the surface properties of RA carbon cryogels by varying the constituent ratio of precursor solution and pH of RA aqueous solution at the acid treatment to promote the formation of secondary particle of RA hydro gels. The pore structure of RA carbon cryogels depended on the constituent ratio of precursors and pH of RA solution, probably due to the variation of the subsequent gel particle size. The carbon gels prepared in this study were mesoporous materials with a modal diameter of 20 nm.

Improvement of mesoporosity of carbon cryogels by acid treatment of hydrogels

Wattle tannin–furfural (TFu) gels are synthesized by the sol–gel polycondensation of wattle tannin with furfural by using sodium hydroxide as a catalyst and cured at 363 K. After cured, the TFu hydrogels are treated in hydrochloric acid (HCl) solutions with various variables as follows: aging temperature, HCl concentration and pH. TFu cryogels aged are then freeze-dried and pyrolyzed under an inert atmosphere to obtain TFu carbon cryogels. The TFu and carbon cryogels were characterized by N 2 adsorption and scanning electron microscope. The high HCl concentration yields the increase of mesopore volume almost twice especially at high temperature and the mesopore size distribution can be greatly developed sharper and narrower when the high concentration of HCl or/and the high temperature are used. Moreover, this method is the versatile approach for developing the mesopore structure of the low and high porous TFu hydrogels and has no serious problem of weight loss after treatment.

Control and behaviors of resorcinol-acetaldehyde carbon cryogel porous structures as electrodes for electric double-layer capacitors

The porous structures of the mesoporous carbon materials, which are known to be effective energy storing systems, were manipulated and their behaviors as electric double-layer capacitor electrodes were evaluated. Resorcinol (R) as the raw material and acetaldehyde (A) as the cross-linking agent were used to synthesize the RA hydrogels via sol-gel polycondensation in a slightly basic aqueous solution. The RA hydrogels were freeze-dried and subsequently pyrolyzed at 1073 K for 30 min (heating rate, 10 K/min) to obtain the RA carbon cryogels. The obtained samples were confirmed to have a high mesoporous carbon content, with the primary pore size diameter distribution of 20 to 40 nm. Due to a high ratio of ion adsorption and desorption capacitances, the samples were effective electrode materials capable of high-speed electrical charge and discharge for use as electrode materials of electric double layer capacitors.

Temperature dependence of the kinetics of oxygen reduction on carbon-supported pt nanoparticles

The temperature dependence of oxygen reduction reaction (ORR) was studied on highly dispersed Pt nanoparticles supported on a carbon cryo-gel. The specific surface area of the support was 517 m2 g-1, the Pt particles diameter was about 2.7 nm and the loading of the catalyst was 20 wt.%. The kinetics of the ORR at the Pt/C electrode was examined in 0.50 mol dm-3 HClO4 solution in the temperature range from 274 to 318 K. At all temperatures, two distinct E-log j regions were observed; at low current densities with a slope of -2.3RT/F and at high current densities with a slope of -2.3?2RT/F. In order to confirm the mechanism of oxygen reduction previously suggested at a polycrystalline Pt and a Pt/Ebonex nanostructured electrode, the apparent enthalpies of activation at selected potentials vs. the reversible hydrogen electrode were calculated in both current density regions. Although ?H ?a,1 > ?H ?a,h , it was a,1 a, h found that the enthalpies of activation at the zero Galvani potential difference were the same and hence it could be concluded that the rate-determining step of the ORR was the same in both current density regions. The synthesized Pt/C catalyst showed a small enhancement in the catalytic activity for ORR in comparison to the polycrystalline Pt, but no change in the mechanism of the reaction.

Control of the porous structure and the surface property of resorcinol-acetaldehyde carbon cryogels

Control of the porous structure and the surface property of resorcinol-acetaldehyde carbon cryogels

Effect of Pore Size on Dehydrogenation Temperature of Carbon Cryogel-Ammoniaborane Nanocomposites

AbstractThis study reports the effects of pore size of porous carbon scaffold on the dehydrogenation of ammoniaborane in the coherent carbon- ammoniaborane nanocomposites. Porous carbon scaffold is obtained from resorcinol formaldehyde derived carbon cryogels. The nanocomposites are made by simple soaking porous carbon scaffold in ammonia borane solution. Nitrogen sorption analysis and differential scanning calorimetry are used to investigate the structure and dehydrogenation of the nanocomposites. The results reveal that dehydrogenation temperature decreases in nanocomposites as compared to neat ammonia borane, and is lower in nanocomoposites with smaller pore sizes. These findings can be used to tune the dehydrogenation temperature to meet specific hydrogen storage applications. Also, dehydrogenation kinetics of nanocomposites is enhanced as compared to neat ammonia borane.

Preparation and characterization of carbon cryogel (CC) and CC–SiO composite as anode material for lithium-ion battery

Carbon cryogel (CC) has been prepared through sol–gel polycondensation of resorcinol (R) with formaldehyde (F) followed by freeze-drying and carbonization in this work. The characteristics and lithium-ion insertion–extraction property have been investigated for the first time. Based on the results that CC has very excellent cycling stability but very limited lithium-ion charge–discharge capacity, SiO is employed to synthesize CC–SiO composite by high energy mechanical ball-milling to improve the applicability. The results showed that, CC–SiO is composed of active carbon, graphite, SiO and dispersed Si crystal, while CC is composed of active carbon and graphite. CC–SiO has smaller and much more uniform particles than CC. SiO can greatly improve discharge capacity of CC with an acceptable sacrifice of cycling stability, and the charge–discharge capacity of CC–SiO comes mainly from lithium insertion–extraction in Si–SiO in the sample. CC–SiO has excellent high-rate discharge ability and is promising anode material of lithium-ion battery for use of high power density purpose.

Spectroscopic Studies of Dehydrogenation of Ammonia Borane in Carbon Cryogel

Ammonia borane (AB) is of great interest for storing hydrogen, an important issue in the growing field of hydrogen technology. The reaction pathways leading to the thermal decomposition of solid-state AB incorporated in carbon cryogels (CC) have been studied by spectroscopic methods. The time-dependent thermal decomposition was followed by in situ 11B nuclear magnetic resonance (NMR) and showed a significant increase in hydrogen release kinetics for AB in CC compared to neat AB. Both 11B NMR and Fourier transform infrared spectroscopy show a new reaction product, formed in the thermal decomposition of AB in CC scaffold (CC-AB) that is assigned to reactions with surface oxygen groups. The results indicate that incorporation of AB in CC enhances kinetics because of the reactions with residual surface-bound oxygen functional groups. The formation of new products with surface -O-B bonds is consistent with the greater reaction exothermicity observed when hydrogen is released from CC-AB materials. Scanning electron microscopy shows different morphology of AB in CC-AB nanocomposite as compared to neat AB.

Control of mesoporous properties of carbon cryogels prepared from wattle tannin and furfural

Wattle tannin–furfural (TFu) carbon cryogels are synthesized by sol–gel polycondensation of wattle tannin with furfural by using sodium hydroxide (NaOH) as a catalyst, dried by freeze-drying technique and then pyrolyzed under inert atmosphere, respectively. The amounts of wattle tannin (T), furfural (Fu), NaOH (C) and distilled water (W) are changed for preparing the mesoporous TFu carbon cryogels. The mole ratio of tannin to catalyst T/C plays a crucial role for the synthesis of TFu organic and carbon cryogels. The results suggest that the T/C ratio should be above 0.25 but <1.0 to prepare the mesoporous and homogeneous cryogels. Although TFu carbon cryogels have the broad mesopore size distribution, the mesoporous structure is controllable by the synthesis conditions. The carbon cryogels possess the mesopore volume less than 0.56 cm3/g and the BET surface area less than 600 m2/g. Moreover, the ratio of catalyst to water C/W can be used to prepare the homogeneous and mesoporous carbon cryogels, and to control the mesopore radius of carbon cryogels in the range of 1.6–9.6 nm.

Evaluation of carbon cryogels used as cathodes for non-flowing zinc–bromine storage cells

Monolithic megaloporous carbon cryogels were examined for their potential applications as cathodic electrodes in secondary zinc–bromine cells. This work investigates the possibility of using their particular macroporous texture as microscopic bromine tanks in a zinc/bromine battery. The electrochemical behaviour of a cell based upon such a Br 2 electrode was studied and discussed in terms of energy yields, energy storage capability and cycle life. Good storages (over 20 Wh kg −1) could be obtained during the first 2 h of cell charging for currents between 10 and 20 mA g −1. The energy yield remains almost constant during a fairly large number of cycles, basically for weak charges (e.g. 25 C g −1). Our findings show that the good cyclability of the cathodic electrode is a consequence of the liquid state of the active bromine phase.

Preparation and characterization of monodisperse carbon cryogel microspheres

Monodisperse carbon cryogel (MCC) microspheres were synthesized through the membrane emulsification of a resorcinol-formaldehyde (RF) aqueous solution in an organic solvent using a shirasu porous glass (SPG) membrane with a controlled porous structure, followed by freeze-drying and pyrolysis in an inert atmosphere. The conditions of membrane emulsification, i.e., the pore size of the SPG membrane used and the curing temperature of the emulsion after the emulsification, were examined in terms of the effects on both the particle size distribution and the porous properties of the microspheres. As a result, independent control of the particle size and mesoporosity in the MCC microspheres was found to be possible. The mean diameter of the microspheres could be controlled by changing the pore size of the SPG membrane used for the emulsification, and the mesoporosity of the microspheres could be improved by increasing the curing temperature of the emulsion after emulsification.

Coherent Carbon Cryogel−Ammonia Borane Nanocomposites for H2 Storage

Coherent carbon cryogel-ammonia borane (C-AB) nanocomposites were synthesized, and improved H2 storage properties are reported. Porous carbon cryogels were impregnated with AB in tetrahydrofuran solution at 25 degrees C under argon; 30% of the carbon cryogel pore volume was filled to produce a 24 wt % C-AB nanocomposite. Nitrogen sorption isotherms, X-ray diffraction, differential scanning calorimetry, differential thermal/thermal gravimetric analyses, mass spectrometry, and 11B NMR were used to characterize the coherent C-AB nanocomposites. Findings include a merged two-step hydrogen release reaction with an appreciable reduction in the dehydrogenation temperature to <90 degrees C as well as the suppression of borazine release. The possible nanosize effects on the H2 storage properties are discussed.

Determination of dye concentration in water using mesoporous particle coated QCR sensor

A quartz crystal resonator sensor coated with mesoporous carbon cryogel particles is developed to utilize for the determination of dye concentration in water. The sensor module is designed for a flow measurement system to make an in-line application available. The preparation details of the carbon cryogel particles and the coated sensor are explained, and the detection performance of the developed module is examined with reactive dyes, Black 5 and Red E and phenol. The experimental results indicate that the measurement sensitivity of phenol is the highest, that of Black 5 is the next and that of Red E is the lowest. The sensitivity difference is explained with the pore size of particles and the molecular size of measured substances, and the possible utilization of the proposed measurement system for the determination of dye concentration in water is discussed.

Modified Carbon Cryogel-Ammonia Borane Nanocomposites for Hydrogen Storage

AbstractThis paper reports the synthesis and characterization of coherent Boron/Nitrogen –doped –carbon cryogels- ammonia borane nanocomposites for hydrogen storage. Resorcinol formaldehyde derived doped carbon cryogels (CC) were obtained via chemical modification.CC- ammonia-borane nanocomposites were made by incorporation of ammonia borane (AB), in CCs. Nitrogen sorption analysis, scanning electron microscopy, and X-ray photoelectron spectroscopy, are used to investigate the structure and morphology of the modified CCs. Differential scanning calorimetry is used to study the dehydrogenation of coherent doped-CC-AB nanocomposites. Modified CCs show higher mesoporosity, and more homogeneous porous structure compared to undoped CCs. Also, dehydrogenation kinetics of nanocomposites is enhanced as compared to neat AB. Possible nanoscale and catalytic effects of nanocomposites in improved dehydrogenation kinetics are discussed.

Characterization of a surface modified carbon cryogel and a carbon supported pt catalyst

A carbon cryogel, synthesized by carbonization of a resorcinol/formaldehyde cryogel and oxidized in nitric acid, was used as catalyst support for Pt nanoparticles. The Pt/C catalyst was prepared by a modified polyol synthesis method in an ethylene glycol (EG) solution. Characterization by nitrogen adsorption showed that the carbon cryogel support and the Pt/C catalyst were mesoporous materials with high specific surface areas (Sbet&gt;400 m2 g-1) and large mesoporous volumes. X-Ray diffraction of the catalyst demonstrated the successful reduction of the Pt precursor to metallic form. TEM Images of the Pt/C catalyst and Pt particle size distribution showed that the mean Pt particle size was about 3.3 nm. Cyclic voltammetry (CV) experiments at various scan rates (from 2 to 200 mV s-1) were performed in 0.5 mol dm-3 HClO4 solution. The large capacitance of the oxidized carbon cryogel electrode, which arises from a combination of the double-layer capacitance and pseudocapacitance, associated with the participation of surface redox-type reactions was demonstrated. For the oxidized carbon cryogel, the total specific capacitance determined by 1/C vs. v0.5 extrapolation method was found to be 386 F g-1. The hydrogen oxidation reaction at the investigated Pt/C catalyst proceeded as an electrochemically reversible, two-electron direct discharge reaction. .

Oxygen reduction at platinum nanoparticles supported on carbon cryogel in alkaline solution

The oxygen reduction reaction was investigated in 0.1 M NaOH solution, on a porous coated electrode formed of Pt particles supported on carbon cryogel. The Pt/C catalyst was characterized by the X-ray diffraction (XRD), transmission electron microscopy (TEM) and cyclic voltammetry techniques. The results demonstrated a successful reduction of Pt to metallic form and homogenous Pt particle size distribution with a mean particle size of about 2.7 nm. The ORR kinetics was investigated by linear sweep polarization at a rotating disc electrode. The results showed the existence of two E - log j regions, usually referred to polycrystalline Pt in acid and alkaline solution. At low current densities (lcd), the Tafel slope was found to be close to -2.3RT/F, while at high current densities (hcd) it was found to be close to -2?2.3RT/F. It is proposed that the main path in the ORR mechanism on Pt particles was the direct four-electron process, with the transfer of the first electron as the rate determining step. If the activities are expressed through the specific current densities, a small enhancement of the catalytic activity for Pt/C was observed compared to that of polycrystalline Pt. The effect of the Pt particle size on the electrocatalysis of oxygen reduction was ascribed to the predominant (111) facets of the platinum crystallites. .

Effect of Pore Morphology on the Electrochemical Properties of Electric Double Layer Carbon Cryogel Supercapacitors

ABSTRACTIn this study a group of resorcinol-formaldehyde carbon cryogels (CC) have been processed chemically, via catalysis and activation, to obtain varied nanostructures and pore size distributions. To understand the relation between structure and electrochemical properties the capacitor can be studied as a dielectric system composed of a porous electrode and the electrolyte (Tetraethylammonium tetrafluoroborate in propylene carbonate). Using Electrochemical impedance spectroscopy (EIS) the complex capacitance and power are used to study the behavior of the system below the relaxation frequency fo (φ = −45°). Therefore, the relaxation of the capacitor system at the low frequency range, f &lt; fo, may be used as a measure of pore/electrolyte interaction. The approach here proposed also allows for a direct experimental characterization of the capacitance and power at low frequencies where small pores are likely to affect the diffusion dynamics of the electrolyte molecules. The results suggest a correlation between the occurrence of small micropores and that of high power losses that are related to the resistive element produced at the low frequency range. Moreover, the impact of the micropore structure upon the supercapacitor's performance is apparent in its capacitance and energy as well. In addition to the complex power and capacitance other measurements including BET Nitrogen sorption, cyclic voltammetry, galvanic cycling and X-Ray Raman Scattering were used to characterize the samples and support these results.

Specificity of the UPD of H to the structure of highly dispersed Pt on carbon support

Home-made carbon cryogel synthesized by sol–gel polycondensation and freeze-drying is used as support for preparation of highly dispersed Pt catalyst that is made by a modified polyol synthesis method in an ethylene glycol (EG) solution. Specific surface area of carbon support and Pt/C catalyst is determined from nitrogen adsorption isotherm. The adsorption isotherm demonstrates a significant mesoporosity of carbon support. Specific surface area of the carbon support, calculated by the BET equation, is found to be 573 m 2 g - 1 . X-ray diffraction (XRD) results demonstrate a successful reduction of the Pt precursor to its metallic form, and transmission electron microscopy (TEM) images show very uniform Pt particle size distribution with mean particle size of about 2.7 ± 0.7 nm of the catalyst. Potentiodynamic studies of the underpotential deposition of hydrogen ( H upd ) on Pt/C electrode in 0.5 mol dm - 3 HClO 4 aqueous solution in the temperature range from 274 to 318 K are made, and thermodynamic state functions for the hydrogen adsorption are determined. The experimental results are analyzed assuming linear variation of the Gibbs energy of adsorption versus θ H upd on the basis of the surface heterogeneity. The increase of Δ G H upd θ with the surface coverage indicates the repulsive interactions between H upd adatoms. From the temperature dependence of the Gibbs energy of adsorption, the enthalpy and the entropy of adsorption are calculated. The values of these functions are determined to be Δ H H upd θ = 0 = - 5.6 kJ mol - 1 and Δ S H upd θ = 0 = 69.1 J mol - 1 K - 1 . The value of Δ H H upd θ allows determinations of the bond energy between electrode surface and H upd that is found to be E Pt – H = 223 kJ mol - 1 for θ = 0 .

Synthesis of ultrafine carbon cryogel microspheres using a homogenizer

AbstractUltrafine carbon cryogel (UCC) microspheres of diameter &lt;5 μm with desirable mesopores were successfully synthesized using a homogenizer during the inverse emulsion polymerization of a resorcinol–formaldehyde (RF) aqueous solution in an organic solvent as a continuous phase (CP) containing nonionic surfactant, SPAN80 as an emulsifier, followed by solvent exchange, freeze‐drying, and pyrolysis in an Ar atmosphere. The curing time of RF solution before emulsification is a significant factor in controlling the size of the microspheres without changing their porosity. Both the mesoporosity and the size of the microspheres decreased on increasing SPAN80 and RF solution concentration in an inverse emulsion, which showed the possibility of controlling the mesoporosity and the size of the UCC microspheres. Furthermore, hexane could also be used as the continuous phase in the emulsion polymerization. The mean diameter of the UCC microspheres increased and their mesopore volume decreased with increasing pyrolysis temperature. © 2006 American Institute of Chemical Engineers AIChE J, 2007