Monolithic Carbon Aerogels Research Articles

Hidden Features: Characterizing Carbon Electrodes in Low Ionic Strength Electrolytes for Capacitive Desalination Applications

Capacitive desalination (CDI) is a promising technology for reducing the salinity of brackish water streams. CDI typically employs a nanoporous carbon supercapacitor architecture with the added ability to flow electrolyte through the cell for deionization. Unlike supercapacitors, though, CDI operates with low ionic-strength liquids (e.g., 20 mM NaCl). Characterization in these electrolytes unveils important features in the CV and EIS data that are not commonly observed in supercapacitor studies. Examples of such features are the emergence of a strong “charge-transfer” resistance at mid-to-high frequencies in the EIS spectrum, local minima in the CV curves that are related to a point of zero charge, and potential artifacts due to ion depletion. These features ultimately underlie the system capacitance, device resistance, and effective operational voltage window, which are all strongly related to desalination performance. Here we examine these features in flow-through electrode CDI devices (fteCDI) with activated hierarchical carbon aerogel monoliths (HCAMs) derived from carbonized and activated resorcinol-formaldehyde aerogel. Work at LLNL was performed under the auspices of the US DOE by LLNL under Contract DE-AC52-07NA27344.

Carbon Papers and Aerogels Based on Graphene Layers and Chitosan: Direct Preparation from High Surface Area Graphite.

In this work, carbon papers and aerogels based on graphene layers and chitosan were prepared. They were obtained by mixing chitosan (CS) and a high surface area nanosized graphite (HSAG) in water in the presence of acetic acid. HSAG/CS water dispersions were stable for months. High resolution transmission electron microscopy revealed the presence of few graphene layers in water suspensions. Casting or lyophilization of such suspensions led to the preparation of carbon paper and aerogel, respectively. In X-ray spectra of both aerogels and carbon paper, peaks due to regular stacks of graphene layers were not detected: graphene with unaltered sp2 structure was obtained directly from graphite without the use of any chemical reaction. The composites were demonstrated to be electrically conductive thanks to the graphene. Chitosan thus makes it possible to obtain monolithic carbon aerogels and flexible and free-standing graphene papers directly from a nanosized graphite by avoiding oxidation to graphite oxide and successive reduction. Strong interaction between polycationic chitosan and the aromatic substrate appears to be at the origin of the stability of HSAG/CS adducts. Cation-π interaction is hypothesized, also on the basis of X-ray photoelectron spectroscopy findings. This work paves the way for the easy large-scale preparation of carbon papers through a method that has a low environmental impact and is based on a biosourced polymer, graphene, and water.

Mechanically robust and highly compressible electrochemical supercapacitors from nitrogen-doped carbon aerogels

Macroscopic, free-standing and flexible three dimensional porous carbon aerogel holds great potential for supercapacitor design but is often hindered by their inherent hydrophobicity, structure-derived fatigue failure and weak elasticity. In this study, a strong and flexible nitrogen-doped carbon aerogel is prepared by direct pyrolysis of bamboo cellulose nanofibers /melamine/graphene oxide hybrid aerogel, which features the characteristics of being catalyst-free, cost-effective and of small environmental footprint. The obtained monolithic carbon aerogel is constructed by welding amorphous carbon nanofibers and graphene nanoplatelets together into a hierarchical carbon-graphene architecture, demonstrating outstanding microstructure-derived resilience and mechanical strength. It is capable to undergo at least 40% reversible compressive deformation and the maximum compressive strength reaches 29.3 kPa. Impressively, due to the unique carbon-graphene architecture together with the hydrophilicity resulted from nitrogen-doping, the carbon aerogel exhibits an excellent specific capacitance of 225 F/g at a current density of 0.25 A/g as well as high energy and power densities of 31.25 Wh kg−1 and 12.9 kW kg−1, respectively. Furthermore, after 100 compression-release cycles, the fully recovered carbon aerogel still maintains great capacitive performances, indicating its superior mechanical durability and electrochemical stability.

Template-Method Synthesis of High-Surface-Area Monolithic Carbon Aerogels and Their Applications for Hydrogen and Deuterium Adsorption

In this work, novel monolithic carbon aerogels obtained by using a polymer template method were characterized and evaluated for their applications in the hydrogen and deuterium adsorption capacity. The properties (i.e., surface area, pore size distribution, hydrogen and deuterium adsorption capacities, etc.) of the carbon aerogels were affected by the polymer templates. The results showed that the carbon aerogel with the molar ratio of polyacrylic acid (PAA) to zinc chloride (ZnCl2) being 0.75:40 was featured the highest surface area (1806 m2/g) and had the highest hydrogen adsorption capacity. Moreover, the deuterium adsorption capacity of the carbon aerogel was to be further elucidated.

Monolithic cobalt-doped carbon aerogel for efficient catalytic activation of peroxymonosulfate in water

As an emerging carbonaceous material, carbon aerogels (CAs) display a great potential in environmental cleanup. In this study, a macroscopic three-dimensional monolithic cobalt-doped carbon aerogel was developed by co-condensation of graphene oxide sheets and resorcinol-formaldehyde resin in the presence of cobalt ions, followed by lyophilization, carbonization and thermal treatment in air. Cobalt ions were introduced as a polymerization catalyst to bridge the organogel framework, and finally cobalt species were retained as both metallic cobalt and Co3O4, wrapped by graphitized carbon layers. The material obtained after a thermal treatment in air (CoCA-A) possesses larger BET specific surface area and pore volume, better hydrophilicity and lower leaching of cobalt ions than that without the post-treatment (CoCA). Despite of a lower loading of cobalt content and a larger mass transfer resistance than traditional powder catalysts, CoCA-A can efficiently eliminate organic contaminants by activation of peroxymonosulfate with a low activation energy. CoCA-A can float beneath the surface of aqueous solution and can be taken out completely without any changes in morphology. The monolith is promising to be developed into an alternative water purification technology due to the easily separable feature.

Synthesis, characterization and mechanism of formation of carbon aerogels incorporated with highly crystalline lanthanum oxychloride particles

Low-density carbon aerogels incorporated with high crystallization LaOCl particles: (a) photograph, (b) SEM, (c) TEM and (d) XRD.

Synthesis of monolithic porous carbon aerogels from sorghum straw and their application in rapid oil removal

Synthesis of monolithic porous carbon aerogels from sorghum straw and their application in rapid oil removal

Strong and machinable carbon aerogel monoliths with low thermal conductivity prepared via ambient pressure drying

Carbon aerogels are among the most attractive porous carbon materials currently, but their real-world applications are greatly limited by their high cost, complicated preparation process and low mechanical properties. Herein, we report a very facile route to prepare lightweight but mechanically strong carbon aerogel monoliths (CAMs), through a sol-gel polymerization of linear phenolic resin and hexamethylenetetramine (HMTA), followed by ambient pressure drying and carbonization. The good capability of linear phenolic resin with ethanol could induce the formation of large polymer particle and good particle connectivity, affording robust network to suppress the collapse during the ambient drying. The synthesis is scalable and flexible, permitting a facile tailor of density, porous structure and mechanical strength by adjusting the ratio of phenolic resin to HMTA and phenolic resin concentration. The obtained CAMs possess macroporous/microporous hierarchical structure with low density as low as 0.07 g cm−3, high mechanical strength of 0.9–5.0 MPa and low thermal conductivity (0.032–0.069 W m−l K−1). Further CO2 activation can greatly develop the microporosity without sacrificing the monolithic structure. Moreover, as-prepared CAMs can be fabricated in large sizes, as well as being post-machined into many shapes and sizes for potential applications.

On the correlation between the porous structure and the electrochemical response of powdered and monolithic carbon aerogels as electrodes for capacitive deionization

The combined effect of resorcinol/catalyst (100≤R/C≤800) and resorcinol/water (0.04≤R/W≤0.13) molar ratio on the textural and capacitive properties of carbon aerogels with potential application for capacitive deionization has been evaluated. Activated and pyrolyzed aerogels were synthesized by the sol-gel polymerization of resorcinol-formaldehyde mixtures and dried in supercritical conditions. Data show that high R/C and R/W molar ratios lead to materials with large pores in the mesopore range, whereas the surface area and micropore volumes remain somewhat the same. The activation of the aerogels increased the differences in the specific surface and micropore volumes due to the development of microporosity. This effect was more remarkable for the samples with low R/C whatever the R/W ratio, indicating that the carbon aerogel obtained using high amounts of catalyst are more prone to be activated. Regarding the electrochemical features of the aerogels, low capacitance values were measured in aerogels combining low R/W and high R/C and reciprocally low R/C and high R/W molar ratios, due to their higher resistance. Polarization resistances were found to be slightly higher for the pyrolyzed than for activated aerogels, and followed a decreasing trend with the mesoporosity, indicating the outstanding contribution of the mesoporous network to provide a good kinetic response. The desalting capacity of monolithic aerogels showed a simultaneous dependence with the surface area and the resistivity of the electrodes, pointing out the importance of performing electrochemical measurements in adequate cell configurations (i.e., desalting units) upon the intended application.

Mn-Containing N-Doped Monolithic Carbon Aerogels with Enhanced Macroporosity as Electrodes for Capacitive Deionization

We propose the use of siliceous diatomite as an antishrinkage additive for the synthesis of monolithic carbon aerogel electrodes with enhanced electrochemical response for capacitive deionization of NaCl solutions. Hybrid N- and Mn-doped carbon aerogels with improved electrical conductivity and wettability in the aqueous electrolyte are prepared by introducing modifications in the synthesis (carbon black and/or diatomite loading before the polymerization sol–gel polycondensation of the monomers, followed by manganese infiltration). The textural and morphological characterizations of the materials reveal that the presence of the additives does not impede the cross-linking of the monomers to form branched clusters and colloidal aggregates leading to a highly porous structure. Furthermore, the aerogel materials display an open macroporous network inherited by the diatomite, even after its lixiviation. As a result, the monolithic carbon aerogel electrodes present good electrochemical performance in terms of l...

Synthesis of Porous and Mechanically Compliant Carbon Aerogels Using Conductive and Structural Additives.

We report the synthesis of conductive and mechanically compliant monolithic carbon aerogels prepared by sol-gel polycondensation of melamine-resorcinol-formaldehyde (MRF) mixtures by incorporating diatomite and carbon black additives. The resulting aerogels composites displayed a well-developed porous structure, confirming that the polymerization of the precursors is not impeded in the presence of either additive. The aerogels retained the porous structure after etching off the siliceous additive, indicating adequate cross-linking of the MRF reactants. However, the presence of diatomite caused a significant fall in the pore volumes, accompanied by coarsening of the average pore size (predominance of large mesopores and macropores). The diatomite also prevented structural shrinkage and deformation of the as-prepared monoliths upon densification by carbonization, even after removal of the siliceous framework. The rigid pristine aerogels became more flexible upon incorporation of the diatomite, favoring implementation of binderless monolithic aerogel electrodes.

Highly Ordered Mesoporous Fe3O4@Carbon Embedded Composite: High Catalytic Activity, Wide pH Range and Stability for Heterogeneous Electro‐Fenton

AbstractHighly ordered mesoporous Fe3O4@carbon embedded composite was prepared and grafted onto monolithic carbon aerogel to form a composite cathode (Fe3O4@OMC/CA). The Fe3O4@OMC/CA was used as heterogeneous electro‐Fenton cathode for the degradation of dimethyl phthalate (DMP). 95 % removal and 65 % mineralization of DMP was achieved within 120 min. Highly dispersed Fe3O4 nanoparticles increased the number of catalytic active sites, and the OMC layer provided accessible pathway to the catalytic active sites and reduced the resistance of mass transport. The main oxidants were ⋅OH and the surface‐controlled heterogeneous catalysis was the critical step. The composite cathode remained electrocatalytic activity after five consecutive runs.

Effect on mass transference phenomena by textural change inside monolithic carbon aerogels

The effects on mass transference phenomena due textural changes of monolithic carbon aerogels were studied by hexane adsorption. The monolithic carbon aerogels were prepared after carbonization of the organic aerogels obtained by resorcinol–formaldehyde polymerization, using p-toluenesulfonic acid (acid-catalyst) and sodium carbonate catalysts (basic-catalyst). Internal texture was modified by CO2 activation. The characterization by gas adsorption showed that the monolithic carbon aerogels presents a bi-modal pore size distribution with presence of both microporous and mesoporous. It was shown that the activation process of monolithic carbon aerogels increases their micropore volume bigger than the other one acid-catalyst aerogel. The mesopores volume in the carbon aerogels plays an important role on mass transport mechanism. The samples with presence of significant mesopore volume present a lower height of mass transfer zone than others less mesopore volume; therefore better efficiency of adsorption in mass transfer zone in dynamic adsorption. The breakthrough curve methodology proposed in this work has allowed finding a relationship between the structural parameters and dynamic adsorption variables, which opens new approaches for measuring textural parameters of material.

N-doped monolithic carbon aerogel electrodes with optimized features for the electrosorption of ions

N-doped carbon aerogels were synthesized by the polycondensation of resorcinol–formaldehyde–melamine mixtures. The prepolymerization of the precursors was determinant for the preparation of meso/macroporous aerogels with varied nitrogen content, regardless the solution pH and M/R molar ratio. The effect was more pronounced at pH 6, as the hydrogels prepared by the one-step route displayed essentially a microporous character, as opposed to the mesoporous network after prepolymerization. The prepolymerized carbon aerogels presented large capacitance values due to the optimum balance between pore structure, improved wettability and low polarization resistance for the highly doped materials. Salt deionization capacity revealed the importance of the macropore structure in the monolithic configuration of the aerogels for a fast electrosorption of ions.

Effect of initial sol concentration on the microstructure and morphology of carbon aerogels

New carbon aerogels were prepared using Novolac sol as precursor. The elaboration process and the structural characterizations of these porous carbon materials are described in the present study. Series of monolithic carbon aerogels were prepared in 2-propanol by crosslinking Novolac with hexamethylene tetra amine catalyst via sol–gel polymerization in the solvent vapor-saturated atmosphere followed by pyrolysis. Low density materials (from 0.3 to 0.6 g/cm3) were obtained after pyrolysis. These newly carbon aerogels were characterized using mercury porosimetry, nitrogen adsorption and scanning electron microscopy. All prepared materials have shown a nanostructured solid network (specific surface areas between 554 and 635 m2/g) and all samples have pore volumes as large as 0.97 cm3/g. Influence of Novolac concentration in the initial sol on the carbon aerogel structure was investigated. A special attention was dedicated to the different microstructures and morphologies occurring after the elaboration process. In particular, changing solid network from colloidal to polymeric network was studied in terms of micro, meso and macro porosity. In parallel, mechanical properties were presented in terms of bulk modulus.

A novel method to synthesize monolithic carbon aerogels from polyacrylic acid by using CO as reducing agent

This was a fascinating study which introduced a brand new method to synthesize carbon aerogels from polyacrylic acid by using CO as a reducing agent, during which, with the help of metal salt, polyacrylic acid ensured the development of the aerogels׳ properties of huge surface area and low density. Required supercritical drying and thermal treatment were adopted to generate high surface area (1560m2/g). According to the results demonstrated by the X-ray photoelectron spectroscopy (XPS) and Energy-dispersive spectrometry (EDS), the sample was an inorganically porous material, with a density of approximately 20mg/cm3. In summary, the attractive properties of the carbon aerogel expounded the potential application possibility of their nanostructures on electrode.

Multifunctional structural energy storage composite supercapacitors.

This paper addresses the challenge of producing multifunctional composites that can simultaneously carry mechanical loads whilst storing (and delivering) electrical energy. The embodiment is a structural supercapacitor built around laminated structural carbon fibre (CF) fabrics. Each cell consists of two modified structural CF fabric electrodes, separated by a structural glass fibre fabric or polymer membrane, infused with a multifunctional polymeric electrolyte. Rather than using conventional activated carbon fibres, structural carbon fibres were treated to produce a mechanically robust, high surface area material, using a variety of methods, including direct etching, carbon nanotube sizing, and carbon nanotube in situ growth. One of the most promising approaches is to integrate a porous bicontinuous monolithic carbon aerogel (CAG) throughout the matrix. This nanostructured matrix both provides a dramatic increase in active surface area of the electrodes, and has the potential to address mechanical issues associated with matrix-dominated failures. The effect of the initial reaction mixture composition is assessed for both the CAG modified carbon fibre electrodes and resulting devices. A low temperature CAG modification of carbon fibres was evaluated using poly(3,4-ethylenedioxythiophene) (PEDOT) to enhance the electrochemical performance. For the multifunctional structural electrolyte, simple crosslinked gels have been replaced with bicontinuous structural epoxy-ionic liquid hybrids that offer a much better balance between the conflicting demands of rigidity and molecular motion. The formation of both aerogel precursors and the multifunctional electrolyte are described, including the influence of key components, and the defining characteristics of the products. Working structural supercapacitor composite prototypes have been produced and characterised electrochemically. The effect of introducing the necessary multifunctional resin on the mechanical properties has also been assessed. Larger scale demonstrators have been produced including a full size car boot/trunk lid.

Adsorption of Benzene and Phenolic Derivatives in Monolithic Carbon Aerogels

A monolithic carbon aerogel with a specific pore texture was used as benzene adsorbent. Adsorption was carried out under static conditions at 25 o C and at saturation, an adsorption capacity as high as 0.77 cm 3 .g �1 or 837 mg.g �1 was obtained. This work is devoted to the application of carbon aerogels for the removal of hydroquinone, catechol and resorcinol from aqueous solutions. These materials were tested and characterised regarding their morphology, particle size distribution, surface area and porous structure. Equilibrium tests were carried out at different adsorbate concentrations and the experimental data were correlated by means of Langmuir and Freundlich isotherms. The equilibrium data were well described by Langmuir and Freundlich in most cases. The maximum adsorption capacity of the Langmuir model was observed for the adsorption of benzene onto aerogel (242.33 mg/g), though the most promising results were obtained for toluene adsorption, owing to the greater adsorption energy involved. Comparing these results with other reported results, our carbon aerogel hydrophobics were found to exhibit a remarkable performance for the removal of benzene.

Rapid Fabrication of Monolithic Carbon Aerogels Based on Resorcinol-Formaldehyde by Ambient Pressure Drying

Rapid Fabrication of Monolithic Carbon Aerogels Based on Resorcinol-Formaldehyde by Ambient Pressure Drying

Capacitive desalination with flow-through electrodes

Capacitive desalination (CD) is a promising desalination technique as, relative to reverse osmosis (RO), it requires no membrane components, can operate at low (sub-osmotic) pressures, and can potentially utilize less energy for brackish water desalination. In a typical CD cell, the feed water flows through the separator layer between two electrically charged, nanoporous carbon electrodes. This architecture results in significant performance limitations, including an inability to easily (in a single charge) desalinate moderate brackish water feeds and slow, diffusion-limited desalination. We here describe an alternative architecture, where the feed flows directly through electrodes along the primary electric field direction, which we term flow-through electrode (FTE) capacitive desalination. Using macroscopic porous electrode theory, we show that FTE CD enables significant reductions in desalination time and can desalinate higher salinity feeds per charge. We then demonstrate these benefits using a custom-built FTE CD cell containing novel hierarchical carbon aerogel monoliths as an electrode material. The pore structure of our electrodes includes both micron-scale and sub-10 nm pores, allowing our electrodes to exhibit both low flow resistance and very high specific capacitance (>100 F g−1). Our cell demonstrates feed concentration reductions of up to 70 mM NaCl per charge and a mean sorption rate of nearly 1 mg NaCl per g aerogel per min, 4 to 10 times higher than that demonstrated by the typical CD cell architecture. We also show that, as predicted by our model, our cell desalinates the feed at the cell's RC timescale rather than the significantly longer diffusive timescale characteristic of typical CD cells.