Rf Aerogel Review

7/27/2019 Rf Aerogel Review

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Preparation and Properties of ResorcinolFormaldehyde Organic and Carbon Gels**

By Shaheen A. Al-Muhtaseb and James A. Ritter*

1. Introduction

Resorcinolformaldehyde (RF) solgels have been receiv-ing considerable attention in the literature over the past de-cade or so.[163] Pekala and co-workers[10,23,26,49] appear to havebeen the first to synthesize RF organic solgels according to ahydrolysiscondensation reaction mechanism that is analo-gous to the solgel synthesis of inorganic oxides.[6466] Sincethese initial studies, numerous articles have appeared in theliterature that describe not only the various synthesis and pro-cessing conditions that can be used to produce organic and car-bon aerogels and xerogels, but also how these conditions affectthe final structure of these gels. These and other articles have

also reported on the uniqueness of the physical, chemical andelectrochemical properties of the organic aerogels and xero-gels. Hence, much of the literature has been concerned withtrying to understand how the synthesis and processing condi-

tions affect the final nanostructure of the RF gel and then torelate it to the macroscopic performance of the material.The particularly important and useful properties include high

porosities (>80%), surface areas (4001200 m2 g1), and porevolumes, the magnitudes of which depend markedly on the syn-thesis and processing conditions. As an example, it has been re-ported that certain RF organic aerogels exhibit extremely lowthermal conductivities, which was attributed to its ultra-porousstructure.[5] As another example, a number of studies haveshown how the nanoporous structure of RF carbon aerogels andxerogels controls the performance of the material as an elec-trode in electrochemical double-layer capacitors (EDLCs).[30]

Manyother studies havealso been donethat link the nanostruc-

ture of an RF gel to a particular macroscopic property.Therefore, the objective of this Review article is to give an

up-to-date and comprehensive overview of the growing litera-ture on RF organic and carbon aerogels and xerogels. Thisreview should serve as a starting point for those interested inthese types of materials, and it should provide insight on howto adjust the synthesis and processing conditions to tailor thenanostructure of an RF organic or carbon gel for a specificapplication. In Section 2, the synthesis and processing condi-tions and corresponding properties are reviewed. The signifi-cant trends are summarized in Tables 1 to 3. In Section 3, abrief summary of the trends is given; and to gain an apprecia-

Adv. Mater. 2003, 15, No. 2, January 16 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 0935-9648/03/0201-0101 $ 17.50+.50/0 101

A brief overview on the preparation and properties of resorcinolformaldehyde or-

ganic and carbon gels reveals very interesting features about their structural and

performance characteristics. The resulting nanostructure was very sensitive to the

various synthesis and processing conditions. This leads to a remarkable potential for

designing and tailoring these materials to fit specific applications. Based on step-by-

step comparisons of the published studies, approximate generalizations on the specific roles the synthesis and pro-

cessing conditions play on the final properties are provided. Overall, resorcinolformaldehyde organic gels under-

go two main stages during synthesis. The first stage is associated with the preparation of the sol mixture, and the

subsequent gelation and curing of the gel. The second stage is associated with the drying of the wet gel. The most

important factors that affect the properties of the organic gel during the first stage are the catalyst concentration,

the initial gel pH, and the concentration of the solids in the sol. The most important factors that affect the proper-

ties of the organic gel during the second stage are the drying procedure (e.g., super- or subcritical drying), and the

difference between the surface tensions of the solvent before and after drying. The corresponding resorcinolform-

aldehyde carbon gels are produced from the organic gels during a third stage, which is associated with carboniza-

tion or activation. Depending on the conditions, carbonization and activation both impact the structural and

performance characteristics significantly.

[*] Prof. J. A. Ritter, Dr. S. A. Al-MuhtasebDepartment of Chemical EngineeringSwearingen Engineering CenterUniversity of South CarolinaColumbia, SC 29208 (USA)E-mail: [email protected],

[**] Support by the U.S. Army Research Office under Grant No. DAAH04-96-1-0421 is greatly appreciated.


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tion of the vast range of possible conditions, a list of the syn-thesis and processing conditions utilized in each study is givenin Table 4. Due to space limitations, the authors haverestricted this Review to unmodified RF organic and carbongels; for example, this article does not include an in-depth re-view on the doping of RF gels with metals or metal oxides;

readers interested in this topic are referred elsewhere.[2,6771]

2. Synthesis, Processing, and Properties

RF carbon solgels have been produced with different vari-ations of essentially the same recipe. This recipe can be sum-marized as follows. First, resorcinol (R) and formaldehyde (F)are mixed at the appropriate molar ratio in the presence of abasic[10] (or, in very few cases, an acidic)[9] catalyst. Then thesolution is heated in a closed container to a predeterminedtemperature for a sufficient period of time to form a stable

crosslinked gel. The gel may then be washed (or not) with asuitable organic solvent to exchange the aqueous solvent.Next, the wet gel is dried either supercritically with carbondioxide or subcritically in air or nitrogen, which produces anorganic aerogel or xerogel, respectively. To produce a carbonaerogel or xerogel, the dried gel is carbonized usually in nitro-gen to form the highly porous carbon network.[10] The driedgel may also be activated, e.g., with CO2, either during or fol-lowing carbonization. Other activation methods may also beused, but few have been tried with RF carbon aerogels andxerogels. The effects of the different processing conditions on

the properties of RF gels are summarized in Tables 1 to 3.The following sections address the most noteworthy variationsof the synthesis and processing procedures, along with theresulting impacts from each variation on the properties of thecorresponding gels.

2.1. Materials and Initial Solution Recipes

Resorcinol (1,3-dihydroxybenzene, C6H4(OH)2) is a pheno-lic tri-functional compound, which is capable of adding for-maldehyde (HCHO) in the 2-, 4-, and/or 6- positions in itsaromatic ring. Varying the concentration ratios of the differ-ent reactants has a profound affect on the resulting propertiesof the gels, as summarized in Table 1. The stoichiometric R/Fmolar ratio of 1:2 is the most commonly used ratio in the lit-erature.[10] Nevertheless, using excess F results in a dilutioneffect, which increases the particle size near the gelation lim-

it.

[33]

Another major factor that can result in this dilutioneffect is the reduction of the density of the reactants byincreasing the amount of solvent. The solvent can be eitherdistilled (and preferably, deionized) water (W) or an organicsolvent (e.g., acetone, methanol, ethanol, n-propanol, or iso-propanol).[9,17] The final gels produced with water as the sol-vent are named hydrogels or aquagels, and those producedwith organic solvents are called lyogels (including the alco-gels, which are based on alcoholic solvents).

Overall, the density of the reactants in the initial solutionhas a considerable effect on the final density of the RF gel. [33]

102 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 0935-9648/03/0201-0102 $ 17.50+.50/0 Adv. Mater. 2003, 15, No. 2, January 16

S. A. Al-Muhtaseb, J. A. Ritter/ResorcinolFormaldehyde Organic and Carbon Gels

James A. Ritter received his Ph.D. in Chemical Engineering from the University of Buffalo in

1989. After spending four years with the Westinghouse Savannah River Technology Center, in

1993 he joined the faculty in the Department of Chemical Engineering at the University of South

Carolina. His major research interests lie in three areas: nanostructured materials for hydrogen

storage, magnetic nanoparticles and supports, electrodes, and adsorbents; magnetic field-en-

hanced processes for separations, targeted drug delivery, and manipulation of matter at the nano-

scale; and cyclic adsorption processes for gas separation and purification, and hydrogen storage.

Shaheen A. Al-Muhtaseb received his Ph.D. in Chemical Engineering from the University of

South Carolina in 2001. He is currently a Postdoctoral Research Associate with Professor Harry

J. Ploehn. His major research interests include the design of nanostructured materials for separa-

tion and energy transport and storage applications, characterization of nanostructured materials,

adsorption processes and vapor-adsorbed phase equilibria, extraction processes and liquidliq-

uid equilibria, absorption processes and vaporliquid equilibria, equations of state and adsorp-

tion isotherm models, and prediction of derived thermodynamic properties for multiphase sys-

tems.


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Higher reactant densities result in a denser formation of theRF crosslinked clusters. The RF solution density also has a

more pronounced effect on the properties of RF carbon (and,hypothetically, organic) xerogels than aerogels.[3] Increasingthe density of the reactants in the initial solution causes a de-crease in the surface area of RF carbon xerogels, and either adecrease in their total pore volume at low pH or vise versa.[3]

It also decreases the electrochemical double layer capacitanceof RF carbon xerogels (especially at low pH) and aerogels.[3]

Sodium carbonate (Na2CO3) is the most commonly used al-kaline catalyst (C) for the polymerization reaction of R withF. This catalyst activates a small portion of R to act as sites forthe growth of the monomer particles.[46] However, when theintention is to incorporate certain transition metals (e.g., Pt,Pd, or Ag) in the final structure of the RF carbon gels, varioussalts of these metals (e.g., [Pt(NH3)4]Cl2, PdCl2, or AgOOCCH3) are used as the catalyst.

[2] The addition of these metalsincreases the meso- and macropore volumes (and results inthe highest possible total pore volumes in the case of smallconcentrations of Pt).[2] In addition to the alkaline catalysts,dilute acidic catalyst solutions (e.g., HClO4

[9,39] or HNO3[38])

can be used. In the case of low RF concentrations, this pro-duces small, smooth, fractal aggregates of gel particles[9] withwide pore size distributions (PSDs).[9,38] In contrast, whenusing an acidic catalyst solution with high RF concentrations,the fractal aggregates are no longer observed and very narrowPSDs (67 nm) are obtained.[9] An example on these effects

of the catalyst type and ratio is shown in Figure 1. [38] In somecases, the use of an acidic catalyst resulted in a reduction in

the gelation time,[39] which may indicate a change in the poly-merization mechanism. These findings are summarized inTable 1.

In the case of Na2CO3, a molar R/C ratio ranging between50 and 300 is typical; but, in some cases ratios as high as 1500are used, in which case crosslinked gel microspheres areformed.[19,20] Overall, the final structure and properties of thepolymerized gels are mostly determined by the relativeamount of C in the sol.[4,22,38] Low R/C ratios results in smallpolymer particles (~35 nm) that are interconnected withlarge necks (giving the gel a fibrous appearance); [22,47] thisproduces higher density gels.[45] In contrast, high R/C ratiosresults in large polymer particles (16 to 200 nm in diameter)that are connected by narrow necks in a string-of-pearlsfashion.[22,23,33,38,47] These two types of gels are commonly de-scribed as polymeric and colloidal RF gels, respective-ly.[47] Overall, the polymeric RF carbon aerogels have a smallparticle size, high surface area, high compressive modulus(high mechanical strength), and exhibit substantial shrinkageduring supercritical drying,[47] whereas the opposite behaviorand characteristics are exhibited by the colloidal RF carbonaerogels.[47]

The surface area of RF organic aerogels, polymerized withan alkaline catalyst, increases slightly with an increase in thealready low R/C ratio, and then it decreases continuously with



Table 1. Stage 1: Preparation of initial solution, gelation, and curing: Effects of various factors on the resulting prop-erties.

Factor Effect

Decreasing reactant concentrations(equivalent to reducing R/F, R/W,or R/C ratios)

Smaller particles and pore sizesLess compaction (less voids) of gel structureIncrease surface areas of xerogels

Either reduce or increase pore volumes of xerogels, depending on pHIncrease electrochemical capacitanceEither increases or decreases lithium ion charge and discharge capacities,Depending on pyrolysis temperature and gel pH

Acidic catalyst solutions At low RF concentrations: small, smooth, fractal aggregates of particleswith wide PSDsAt high RF concentrations: no fractal aggregates, very narrow PSDsmay reduce gelation time

Alkaline catalyst solutions High concentrations: polymeric gels (small polymer particlesinterconnected with large necks, high surface areas, high mechanicalstrengths), reduces gelation timeLow concentrations: colloidal gels (large particles interconnected withnarrow necks, low surface areas, low mechanical strengths)

Increasing gel pH Increase surface areas and pore volumes of carbon aerogelsIncrease electrochemical capacitance of carbon aerogelsInsignificant effect on surface area of carbon xerogelsIncrease pore volume of carbon xerogels at high density of reactantsEither increase or decrease electrochemical capacitance of carbon xerogels,depending on concentration of reactantsEither increases or decreases lithium-ion charge and discharge capacities,depending on pyrolysis temperature and reactants concentrations

Gelation and curing Required for improving the crosslinking of polymer particles


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a further increase in the R/C molar ratio; hence, a maximumsurface area results at an R/C of~50 for the particular condi-tions.[4] Moreover, the peak radius of the PSD increases whenincreasing the R/C ratio or decreasing the C/W ratio. [34] Amonodispersed pore structure is obtained with either very lowR/C or very high R/W ratios, whereas an increase in the R/Cratio or a decrease in the R/W ratio produces polydispersedpore structures.[4] Moreover, the size of the gel particles canbe promoted to the micrometer scale when using high R/C ra-tios, low concentrations of reactants (low R/W ratio), or,equivalently, low C/W ratio as shown in Figure 2, [34] and lowgelation temperatures, which necessarily increases the timefor gelation.[19] These particles are sometimes referred to asfoams or microcellular materials.[19] Moreover, the electro-chemical double-layer capacitance increases when increasingthe density of the gel[41,47] or reducing the R/C ratio.[41,43]

Dilute acids (e.g., HNO3 or HCl) or bases (e.g., NH4OH)are typically used as buffers to control the pH of the initial so-

lution. The initial gelation pH has profound effects on the fi-nal properties of RF carbon aerogels[3] and xerogels.[25] Thereactants tend to precipitate at very low solution pHs,[49] whilethe polymerization-condensation reaction is hindered at veryhigh pHs.[25] Therefore, typical pH values are in the approxi-mate range of 5.4 to 7.6.[3,10,25,49] In general, the surface area ofRF carbon xerogels has a weak dependence on the initial so-lution pH in the acidic range,[3,25] but at a pH higher than 7

the surface area diminishes completely.[26] However, the porevolume of the RF carbon xerogels can increase when increas-ing the pH, but only at high reactants densities in the initialsolution.[3] The surface areas and pore volumes of RF carbonaerogels also increase significantly when increasing the pH.[3]

The electrochemical double-layer capacitance of RF carbonaerogels is expected to increase when increasing the pH, andthat of the RF carbon xerogel to either increase when increas-ing the pH at high reactants densities or vise versa. [3] Whenthese RF carbon gels are examined for use as the anode inlithium-ion batteries, increasing the gel pH has generally de-creased the reversible discharge capacity of the lithium ions,especially from RF carbon aerogels.[57]

2.2. Gelation and Curing

The main factor in the gelation step is the catalyzed, en-dothermic, polycondensation polymerization reaction of theprecursors under controlled conditions to form the polymerstructure known as the aquagel or alcogel when using wateror alcohol, respectively, as the solvent. Overall, the structureand properties of the RF organic and carbon gels dependstrongly on this reaction and the conditions at which it pro-ceeds. A summary of the effects of the gelation and curing



Fig. 1. Scanning electron microscopy (SEM) images of RF carbon aerogelssynthesized with a) low and b) high resorcinol to alkaline catalyst ratios andc) low resorcinol to acidic catalyst ratio [38]. (Reproduced with permission from

J. Non-Cryst. Solids, Elsevier, 2001.)

Pore size is small

(a)

First stage of gelation Formation of inter-connected structure

After gelation

Pore size is large

(b)

First stage of gelation Formation of inter-connected Structure

After gelation

Monomer (substituted resorcinol)

Catalyst

Reacted resorcinol and formaldehyde

Fig. 2. Model of gelation progress with a) high and b) low C/W ratios [34]. (Re-produced with permission from J. Non-Cryst. Solids, Elsevier, 2001.)


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conditions on the final properties of the gels is given in Ta-ble 1. The major reactions between R and F include an addi-tion reaction to form hydroxymethyl derivatives (CH2OH),and then a condensation reaction of the hydroxymethylderivatives to form methylene (CH2)- and methylene ether(CH2OCH2)-bridged compounds,

[25,26,47] as illustrated in

Figures 3 and 4. The alkaline C is important for the initial for-mation of the R anions during the addition reaction. These Ranions are much more active than the uncharged R towardsthe addition of F to form the hydroxymethyl derivatives,which are essential for the subsequent condensation reactions.This multi-step mechanism results in highly crosslinked clus-ters (7 to 10 nm in diameter)[48] of the polymer. After thecompletion of this step, the colloidal particles begin to aggre-gate and assemble into a stiff, interconnected structure locallyresembling a string of pearls that fills the original volume ofthe aqueous solution.[46]

To prepare for gelation (polymerization), typically R and F

are mixed with the polymerization C and the solvent (see theprevious section for guidelines on the appropriate propor-tions), and stirred for a short period (between 5[34] and30 min[37]) to form a homogeneous mixture that is commonlycalled the sol. The addition of any additives to the gels, such ascarbon cloths for reinforcing the final gels,[30] is usually doneduring this initial stage. However, the addition of such addi-tives influences the rate of gelation catalytically, causing a neg-ative effect because of the deposition of the RF sol on the wallsof the narrow, hydrophilic, solid boundaries. This depositiondilutes the solution and results in an effect similar to the one

discussed previously.[33] It is also recommended (but not neces-sary) to prepare the sol mixture in an inert atmosphere (e.g., ina glove box with a N2 atmosphere) to avoid the possibility ofcontaminating the gel with components in air such as CO2,which may change the pH of the solution. Finally, the contain-ers (molds) containing the reaction products are sealed before

being removed from the environmentally controlled chamber,or before being heated, to minimize solvent evaporation. Heat-ing is required to provide the required energy or the polymer-ization reaction. However, the heating requirement can belowered significantly (from ~80 to ~40 C) if acetone is used asthe solvent instead of water.[9] It is even possible to carry outthe polymerization reaction with water as the solvent at tem-peratures as low as 30 C,[30] but with far longer gelation timescompared to using an elevated temperature, such as 80 C.

Typically, gelation may occur by the second day if the initialsolution pH is higher than 7.0, and in several hours if the pHis less than 6.8.[25] However, the gelation process can occur

more rapidly with low R/C ratios, high reactants densities orhigh temperatures.[5] The initial cluster formation and particlegrowth reactions take only about one hour as indicated by theinitial increase and then leveling-off of the dynamic viscosityof the solution.[46] The sample at this stage constitutes a colloi-dal solution of the monomer particles (clusters). Nevertheless,the actual covalent crosslinking of these particles, which leadsto the stiffening of the gel, may start to take place only aftermany hours and then progress very slowly. [46] However, thisreaction can be hastened remarkably by lowering the pH atthis intermediate stage to compensate for the depletion of



H+,

OH

OH

CH2+

CH2+

+

OH

OH

CH2OH

CH2OH

OH

OH

CH2OH

CH2OH

OH

OH

CH2OH

OH

CH2OH

CH2

CH2

HO

O

CH2

OH

CH2OH

CH2OH

OH

OH

OH

CH2

CH2

OH

CH2

O

CH2

OH

H+, OH

O

CH2

OH

OH

OH

CH2OHHO CH2

HO

CH2

OH

CH2

CH2

OH

CH2OH

OH

O

CH2

HO

2. Condensation Reaction

OH

OH

Na2CO3

OH

O

C

O

H H

+ 2

OH

OH

CH2OH

CH2OH

1. Addition Reaction

Fig. 3. Molecular presentation of the polymeriza-tion mechanism of resorcinol with formaldehyde[25]. (Reproduced with permission from Carbon,Elsevier, 1997.)


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protons;[46] this facilitates the condensation reaction as illus-

trated in Figure 3. Nevertheless, if the pH is lowered too earlyduring the particle growth stage, this may result in an incom-plete aggregation reaction,[46] and also require much moreacid to complete the reaction.[46] This effect is possibly due tohindrance of the R anion formation during the addition reac-tion, which limits the extent of RF cluster formation.

A temperature program is typically applied during the gela-tion period, where the sample is kept at around room temper-ature for about 24 h, and then heated gradually (again over24 h) until it reaches the final temperature (~90C).[32] Never-theless, the direct placement of the sealed sample vial in anoven at the desired gelation temperature also provides satis-factory results.[3] The curing step is important because it al-

lows for the previously formed polymer clusters (particles) tocrosslink, which forms the final solid shape of the gel. Thisstep takes about a week at elevated temperatures (80 to90 C). In fact, prolonged curing times may be necessary tomake sure that the crosslinking reactions are sufficiently com-pleted to prevent swelling during the subsequent stage of ex-changing the aqueous solution with an organic solution.[46]

If it is desired to obtain the gel in a certain monolithicshape, it is important that the colloidal solution be pouredinto a mold (which can be made of glass, metal, or plastic) ofthe desired shape before the curing step. Also, it is importantto consider that the characteristics of the surface of the mold(especially when small molds are used), as well as any surfaces(e.g., fibers) that are added to the gel, may result in noticeablevariations in the final structure of the RF gel. The main fac-tors dictating these effects are associated with the hydropho-bicity of the solid surface and the comparative ratio of themean distance between two adjacent surfaces (i.e., volume/surface ratio) to the characteristic catalytic penetration depth(~1 to 50 lm).[33] In some cases, skin portions of the gel canform around the hydrophilic surfaces (e.g., glass).[33] A gener-al presentation of these effects of the solid surface and surfacedimensions is shown in Figure 5.[33]

Upon removal of the crosslinked (cured) gels from the con-tainers, they can optionally be placed in a dilute acid (e.g., a

5 to 15% acetic acid solution)[27,40] to increase the crosslinkingdensity by promoting the progress of the condensation reac-tion of the hydroxymethyl groups. This step is commonlyreferred to as aging of the gel sample and, surprisingly, itoccurs with no shrinkage of the sample. [40] This aging stepresults in a significant increase in the double layer capacitanceof RF carbon aerogels;[41] it also causes a slight decrease inthe surface area.[41] This step also increases the mechanicalstrength of the gel when carried out in an acidic medium asopposed to water (pH 7.0);[40] and when the gel is cured in anacidic medium, the significant changes to the mechanical

properties of the gel take place mostly during the first week.Overall, this aging step may be a very useful addition to thesynthesis procedure, especially when it comes to handling thefragile aerogel monoliths without breaking them.

2.3. Solvent Exchange

Once the final crosslinked gel is formed, it becomes neces-sary to remove the aqueous solvent that is possibly used asthe reaction medium. The different methods used to removethe solvent have dramatic effects on the properties of the RForganic gels, as outlined in Table 2. Typically, the aqueous sol-



CH2OH

OH

OH

OH

OH

CH2OH

CH2OH

OH

OH

CH2OH

HOCH2

OH

CH2OH

CH2OHHO

Fig. 4. Cluster growth of resorcinol-formaldehyde monomers [46]. (Reproduced with permission from Lawrence Livermore National Labora-tory, 1997.)

S/V 0(hydrophilic)

S/V -1

(hydrophilic)S/V

-1

(hydrophilic)S/V

-1

(hydrophobic)

generalized surface aerogel deposition aerogel particles

Fig. 5. Effects of solid surface and the comparative ratio of the mean distancesbetween adjacent surfaces (i.e., volume (V)/surface (S) ratio) to the characteris-tic catalytic penetration depth (k) on the gel structure and skin formation. [33](Reproduced with permission from Carbon, Elsevier, 2001.)


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vent is replaced with an organic one (e.g., methanol, acetone,isopropanol, or amyl acetate)[10,49] through a repetitive wash-ing procedure. The solvent can also be heated during thesewashing steps to accelerate its rate of diffusion and henceexchange.[49] The removal of water becomes essential in the

case of supercritical drying with CO2 because of the insolubil-ity between CO2 and water. Overall, no shrinkage of the RFgel samples is noticed after solvent exchange;[40] therefore, thechoice of the new solvent depends on either its evaporativeproperties or its mutual solubility with water and CO2 foraerogels. It is not clear whether this solvent exchange step isneeded for the production of RF xerogels; however, the effectof not using it is unknown, but probably noticeable. This stepmay be beneficial before the subcritical drying of RF xerogelsbecause it reduces the required time and temperaturerequired for evaporation, and it may reduce the surface ten-sion on the pore walls of the wet gel thereby minimizingshrinkage.

2.4. Drying Conditions

2.4.1. Subcritical Drying

Conventional evaporation of the solvent at atmosphericconditions may cause drastic changes in the surface tensionof the solvent upon the formation of the vaporliquid inter-face. This huge difference between the surface tensions ofthe coexisting vapor and liquid phases results in dramaticmechanical stresses that lead to the collapse of the pore

structure. This results in a densepolymer called a xerogel. How-ever, a comparison of the pore sizedistributions of RF xerogels showsthat the shrinkage is most pro-nounced for pore sizes >10 .[53]

Also, when a material is dried withhot air, the shrinkage generally de-pends on both the drying rate andthe thickness of the sample, wherethe shrinkage increases with an in-crease in the thickness of the sam-ple or an increase in the rate ofdrying.[32] Subsequently, the gel canbe dried subcritically (sometimesat atmospheric pressure) withoutmajor changes to its structurewhen it is mechanically strong

enough to withstand capillary pres-sures.[43] This includes the casewhen the gel is produced with highR/C ratios[43] (with particle sizes of100 nm or more)[20] or when thegel is reinforced by curing withinindividual fiber sheets.[33] Accord-ingly, air-drying is quicker, simpler

and less expensive than the supercritical or subcritical CO2extraction and drying procedures.[33]

2.4.2. Supercritical Drying

Sometimes it is desirable to retain the formed skeletonstructure of the wet gel through the subsequent stages of pro-cessing. Therefore, the previously exchanged (i.e., CO2-solu-ble) solvent can be further exchanged with another solvent oflower surface tension (e.g., CO2) by slowly bleeding the airfrom a supercritical drying chamber while filling it with liquidCO2. Then the liquid CO2 (Tc =31C, Pc = 7.4 MPa) is shiftedto its supercritical state condition (e.g., ~45C and ~11 MPa)without going through the vaporliquid interface, which mini-mizes the mechanical stresses against the walls of the pores. Itis then held at the supercritical condition for four hours ormore.[3,45] The gels dried supercritically are named aerogels,

and they are known to have outstanding characteristics (e.g.,higher surface areas, total pore volumes and sometimes elec-trochemical double-layer capacitances) and even higher lithi-um-ion charge and discharge capacities and efficiencies com-pared to carbon xerogels.[57] However, they are highlysensitive to the synthesis conditions for being tailored withinvery broad ranges of properties.[3] Hybrid RF xerogelaero-gels can also be formed by a partial evaporation followed bysupercritical drying.[11] Although, ideally it should not occur,some shrinkage of the gel also occurs during supercritical dry-ing; however, this shrinkage is minimal for gels with largepore and particle sizes and vice versa.[19] As another example,



Table 2. Stage 2: Solvent-exchange and drying: Effects of various factors on the resulting properties

Factor Effect

Solvent exchange Necessary for supercritical drying with CO2 or freeze-dryingFacilitates replacement with drying mediaReduction of surface tensions upon subcritical evaporation

Subcritical drying Production of dried dense polymers called xerogelsCauses significant shrinkage of especially wide poresEffects can be insignificant if gels were synthesized with highmechanical strengthIncreases lithium-ion charge and discharge capacities

Supercritical drying with CO2 Production of dried light polymers called aerogelsInsignificant shrinkage of pore structureHigh surface areas, pore volumes and, sometimes, electrochemicalcapacitancesRequires high pressures, long times for exchanging solvent with CO2

Supercritical drying with acetone Like supercritical drying with CO2, but with lower pressuresEliminates necessity for exchanging solvent with CO2, shortensprocessing time significantly

Requires high temperatures to shift acetone to supercritical conditionsMay cause partial thermal decomposition of dried gels

Freeze-drying Production of dried light polymers called cryogelsbased on sublimation of frozen solventsCryogels mostly mesoporousDensity of solvents must be invariant with freezing


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RF aerogels polymerized with acetone as the solvent and anacidic C do not fully preserve their structures during super-critical drying, possibly due to the polymer gel shrinkingslightly during the exchange of acetone with CO2.

[9] Overall,the main disadvantages of using supercritical CO2 drying arethe high pressures and long times (3 to 4 days) required for

solvent exchange and drying.[27]

To partially circumvent this problem, an alternative proce-dure to supercritical CO2 drying is the supercritical dryingwith an organic solvent that is initially used to exchange theaqueous solution in the aquagel (e.g., acetone,[27,51]

Tc =235C, Pc = 4.7 MPa). This eliminates the necessity of ex-changing the organic solvent with CO2. The correspondingprocess time is shortened significantly.[51] However, althoughthe gel textures obtained after supercritical drying with ace-tone and CO2 are similar, the shrinkage and density of the RFaerogels from the supercritical drying with acetone are largerthan those with CO2.

[27] The shrinkage ratio is also very sensi-

tive to the rate of depressurization, where a very high depres-surization rate results in very large shrinkage ratios.[51] Thesedeleterious effects of supercritical acetone drying can be mod-erated significantly by applying N2 at a pressure of~5 MPaprior to heating the chamber to the supercritical temperaturefor acetone.[27] The colors of the organic aerogels preparedthrough the supercritical drying with acetone are also darkerthan those with CO2 (and sometimes black).

[51] This is attrib-uted to the possibility of partial decomposition (or pyrolysis)and condensation of the gel and solvation of the solvent at thehigh temperatures required for shifting acetone to its super-critical state.[27]

2.4.3. Freeze-Drying

Cryogels are produced when the liquid solvent is removedby freeze-drying. With this method, the solvent is frozen andthen removed by sublimation thus avoiding the formation of avaporliquid interface.[24,28] Although the freeze-drying meth-od is not generally expected to exhibit vaporliquid interfaces,it is still believed that this method results in some shrinkage ofthe gel.[32] Therefore, freeze-drying of smaller samples is morepractical in producing mesoporous cryogels.[32] Overall, RFcarbon cryogels are mostly mesoporous with surface areas>800 m2 g1 and pore volumes >0.55 cm3 g1.[24,28] However,

the surface areas and mesopore volumes of the RF carboncryogels smaller than those of RF carbon aerogels, but micro-pore formation is easier upon pyrolysis.[24]

Before freeze-drying the wet gel, it is very important toexchange the aqueous solution with an alternative liquid (e.g.,tert-butanol) that does not exhibit considerable changes indensity upon freezing.[24,28] Otherwise, the expansion of theaqueous solution upon freezing not only may result in the de-struction of the gel structure, but it may also result in verylarge pores (macropores) due to ice crystal growth.[31] Rinsingthe aquagel twice with tert-butanol, for example, is sufficientfor exchanging the aqueous solution.[32] Moreover, the density

change of the cryogels freeze-dried with tert-butanol is muchsmaller than that with water,[34] and it causes the formation ofmore mesopores.[32] The reactant concentrations and time ofcuring also have significant effects on the micro- and meso-porosities of the cryogels.[31]

2.5. Pyrolysis (Carbonization)

The use of a pyrolysis (carbonization) step transforms theorganic gel into a relatively pure carbon structure by remov-ing any remaining oxide and hydrogen groups at an elevatedtemperature. These carbonized RF structures are often calledcarbon gels. Pyrolysis of RF gels is most commonly carriedout in a tube furnace under a constant, moderate(~200 cm3 min1) flow of inert gas, such as N2, argon or He atroom temperature for ~ one hour (to replace all the air in thefurnace),[34] and eventually at a fixed temperature ranging

from 600 to 2100C.

[11]

The desired pyrolysis temperature isgenerally approached gradually using temperature program-ming. Interestingly, the electrical conductivity of the sampleincreases significantly during this transformation, as indicatedby the common appearance of a broadband, semi-metal-likeinfrared absorbance.[42]

Variations in the pyrolysis conditions cause significantchanges in the properties of RF carbon gels. A summary ofthese changes is shown in Table 3. Higher pyrolysis tempera-tures tend to reduce the surface areas of both RF carbon aero-gels and xerogels; they also reduce the electrochemical doublelayer capacitance of RF carbon gels.[3,36,47] However, the mi-nor decrease in the surface area with increasing pyrolysis tem-

perature is limited to temperatures above ~600 C, whereasincreasing the pyrolysis temperatures while under 600 C in-creases the surface area.[36] However, RF carbon gels may notbe electrically conductive unless carbonized above 750 C.[54]

The pyrolysis temperature also has significant effects on thechargedischarge behavior of lithium ions from RF carbonaerogels and xerogels; but the effect also depends on the gelpH and the concentration of reactants.[57]

The specific volume (reciprocal density) of RF carbon aero-gels also decreases with an increase in the pyrolysis tempera-ture up to ~800[36] or 900C;[47] it is unaffected at higher tem-peratures. Figure 6, for example, shows that RF carbonxerogels carbonized at 1200C are significantly denser than

those carbonized at 600C.[37] The electrochemical doublelayer capacitance also exhibits a maximum between 800 and900C.[48] The specific volumes of RF carbon aerogels are alsoalways higher than these of RF organic aerogels.[36] In agree-ment with the last trend, the pore volumes of RF carbon xero-gels decrease slightly with an increase in the pyrolysis temper-ature,[37] and the corresponding skeletal density also increasesand levels off at just below the skeletal density of graphiteafter 1050 C.[37] This indicates that these particular RF car-bon xerogels have very few closed pores within the skeletalstructure. Moreover, the weight loss of RF carbon xerogelsdue to pyrolysis is practically limited to the period before




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reaching ~750C, after which it is almost constant at ~50 %

which indicates the initiation of structural changes within thecarbon gel without weight loss.[37] Overall, pyrolysis producesalmost the same PSD for RF carbon aerogels,[4,53] except atvery low R/C ratios.[4] In the latter case, pyrolysis causes thepore size distribution to spread further towards the microporeregion.[4]

The temperature required for complete graphitization ofRF gels may exceed 2000C;[42] nevertheless, aerogels pyro-lyzed at ~1050 C contain separated graphitic structures.[42]

Partial graphitic character has also been reported by othersbased on XRD patterns.[25] The endothermic pyrolysis reac-tions are most active during the programmed temperature

rises.[42] The first endothermic reac-tion, which occurs during the firstperiod of heating from room tem-perature to ~250 C, is attributed tothe release of adsorbed water (asindicated by the disappearance of

the OH-group infrared bands); thiscorresponds to a mass loss of~3 %.The other endothermic reactions,which occur during the second peri-od of heating, are attributed to therelease of organic compounds.[42]

However, very slight endothermicand exothermic reactions may also occur after these periodsduring the constant temperature carbonization step.[42]

The pyrolysis step reduces the number of macropores (pos-sibly due to shrinkage) and increases the number of micro-pores and mesopores, which leads to an increase in the surface

area

[4]

of RF carbon aerogels,

[2]

especially at low pyrolysistemperatures (in accordance with the previously discussedtrends). This effect is a result of the burnout of the organicgroups, which leads to the creation of new pores or voids inthe gel. Overall, shrinkage and mass reduction of 20 and50 %, respectively, can be expected due to pyrolysis of subcrit-ically dried RF xerogels.[20] Nevertheless, unlike the pore sizesand pore volumes, which both decrease upon pyrolysis, thesurface area of the carbon gel may increase after pyrolysisespecially with dilute sols, i.e., when low C/W (or, alterna-tively, R/W) ratios, are used.[34] Shrinkage and mass reductiondue to pyrolysis decrease with increasing R/C ratio.[19] More-over, in addition to the effect on the gelation rate and also

pore structure of carbon xerogels, the addition of organicfibers to the gel solution may also minimize shrinkage duringpyrolysis.[7]

2.6. Activation

RF organic aerogels and xerogels can also be activated,either subsequent to or during pyrolysis, with gases such asair, steam, or CO2. In fact, any activation method applied toactivated carbons can in principal be applied to RF gels. Asan example, thermal activation of RF gels is typically carriedout in a flow of air, steam, or CO2 (or dilute CO2 in N2)

[37]

at 750 to 1000C (or at the pyrolysis temperature) for 17 h.[37,60] After the activation step, pure N2 may be passedthrough the sample for about two hours to replace the CO2,and allow the sample time to gradually cool to room tempera-ture.[37] Overall, increasing the activation time with CO2 in-creases the pore volume and the peak pore widths signifi-cantly, especially in the narrow pore size (micropore) range[37]

and, to a lesser extent, in the mesopore range. [60] As an exam-ple, activated RF carbon aerogels with a surface area of2600 m2 g1 can be obtained after activation with CO2 forseven hours.[60] However, the electrochemical double layercapacitance of RF carbon aerogels may exhibit a maximum



Table 3. Stage 3: Pyrolysis and activation: effects of various factors on the resulting properties.

Factor Effect

Increasing pyrolysis temperature Reduces oxygen contentReduces surface area of carbon aerogels and xerogelsReduces pore volumes of carbon aerogels and xerogelsIncreases macropore size distributions

Increases micropore size distributions when very low R/C ratios are usedIncreases electrochemical capacitance up to ~850 C, thereafter reduces iteither increases or decreases lithium ion charge and discharge capacities,depending on gel pH and reactants concentrations

Increasing thermal activation time Increases pore widths, volumes and surface areasIncreases electrochemical capacitance up to ~3 h, thereafter reduces it

Fig. 6. Transmission electron microscopy images of RF carbon xerogels carbo-nized at A) 600C, and B) 1200C [37]. (Reproduced with permission fromCarbon, Elsevier, 2000.)


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after ~3 h of activation with CO2.[48] A summary of the effects

of the activation time on the properties of RF carbon gels isgiven in Table 3.

Activation with steam is also more effective than with CO2because it causes a more significant increase in the micro- andmesoporosities.[2] A three-hour activation of an RF carbon

xerogel with CO2 increases the total mass loss from 50 to75 %. This additional weight loss increases the cumulativepore volume and surface area markedly from 1.5 cm3 g1 and600 m2 g1 to 2.5 cm3 g1 and 1600 m2 g1, respectively.[37] Ther-mal activation with air also results in a comparable weight lossthat depends on the activation time and temperature.[43] Over-all, the thermal activation step with CO2 results in a 66% in-crease in the electrochemical double-layer capacitance.[41] Onthe other hand, the size of the RF carbon xerogel particles ishardly affected by activation with CO2;

[37] this suggests thatthe effect of activation and removal of surface functionalgroups and carbon, which creates more pores and surface

area, takes place mainly within the RF carbon xerogel parti-cles.[37]

Alternatively, RF gels can be activated chemically by plac-ing the carbon gel in an acidic solution (e.g., 60% HNO3)

[43]

for two days.[43] Regardless of the R/C ratio, this step in-creases the mass of the carbon gel by 10 % and the oxygencontent from 4 to 14%.[43] This indicates the attachment ofoxide functional groups to the surface of the carbon gel. Con-sequently, the chemically activated carbon gel exhibits a sig-nificant increase in both the electrochemical double-layercapacitance and the capacity for adsorbing CO2, with only aslight change of the surface area in comparison to thermalactivation.[43]

Electrochemical activation can also be performed by plac-ing the RF carbon gel in an electrolyte solution (e.g., 1 MH2SO4)

[50] and subjecting it to repetitive oxidationreductioncycles. Electrochemical activation results in an overall in-crease in the electrochemical double layer capacitance. Thisincrease is mostly due to the effect of the reduction steps ofthe cycles; therefore, it may be beneficial to use reductiontimes that are longer than those of oxidation. On the otherhand, the surface areas of the samples are relatively unaf-fected by the electrochemical activation procedure. This resultmay suggest the formation of surface functional groups duringelectrochemical activation that give rise to a pseudo-capaci-tance.[54]

2.7. Additional Processing Techniques

The reinforcement of RF gels with organic fibers (by addingthem to the gel solution) changes the gelation rate and porestructure of the xerogel, reduces the fragility of the final prod-uct (which makes it more suitable for different applications)and minimizes the shrinkage of the gel due to pyrolysis.[7]

Nevertheless, only selective fibers can be used for this pur-pose[33] such as Al2O3 (Saffil). Glass fibers, for example, areimpractical to employ in such sheets because of their low

melting points (especially during pyrolysis) and their reactiv-ity with carbon.[33]

Reinforcement of RF gels with fibers (e.g., carbon fibers)has several advantages for use in electrochemical capaci-tors[30,48,58,59] or capacitive deionization.[48] The fibers providehigh in-plane electrical conductivity while also improving the

mechanical strength and flexibility of the carbon gel. More-over, since the fibers prohibit linear shrinkage of the gelsupon pyrolysis, cracks form in the corresponding carbon gels,which gives an enhanced accessibility for the electrolyte solu-tion in the carbon gel. The skin formation which resultswhen the RF solgel deposits on the narrow, hydrophilic, solidsurface of the mold, also fosters the use of such materials inproton exchange membrane (PEM) fuel-cell electrodes. Thecoarse internal structure guarantees the fast release of waterin the cathode and the skin serves as a diffusion layer.[33]

Moreover, this skin allows for the finer deposition of metalparticles (e.g., Pt) on the surface of RF gels.[33]

RF carbon aerogel microspheres can also be produced bymixing the initial aqueous reactant solution with an organicsolvent (such as cyclohexane or a mineral oil) containing asurfactant (such as SPAN80)[28] and continuously stirring at anelevated temperature (e.g., 60 C)[28] during the whole periodof gelation and curing,[14,22] which lasts ~5 to 10 h.[28] Gelationcan also be conducted at room temperature, but the resultingorganic RF gel microspheres become very dense, almost non-porous, and exhibit very small pores on the surface (just abovethe molecular diameter of CO2 but less than that of N2).

[28]

These microspheres are made by dispersing the aqueous gelsolution into the heated, surfactant-containing, organic sol-vent, or mineral oil just after reaching the gel point (i.e., after

partial polymerization),[22]

but also just before the solutionloses its fluidity.[28] This approach is commonly referred to asinverse emulsion polymerization. Continuous agitation isrequired during the slow addition of the aqueous gel solutionto form a dispersed colloidal solution of spherical droplets,where the size of the droplet depends on the rate of agitationand the amount of surfactant.[22] The surface structure of theRF gel microspheres depends strongly on the temperature ofthe emulsion, and potentially on other factors such as the con-centration of the surfactant.[28]

At the end of this process, non-sticky gel microspheres areproduced with diameters ranging from micrometers to milli-meters, depending on the emulsification procedure.[14,22]

These gel microspheres can then be dried to form either RForganic aerogel[22] or cryogel[28] microspheres, and carbonizedto form the corresponding carbon microspheres. It is notewor-thy that no one has made RF carbon xerogel microspheres.Another unique feature of the RF carbon cryogel micro-spheres is that the largely mesoporous internal structure maybe encased in an ultramicroporous layer by varying the pyrol-ysis temperature, as shown in Figure 7, which imparts molecu-lar sieving properties to the RF carbon gel.[28]

Another noteworthy technique for producing high pore vol-ume RF carbon aerogels with plenty of mesopores utilizes sili-ca-particle templates.[6163] When using 3040 wt.-% surfac-




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tant-stabilized silica sols with an average particle size of 8 nmprior to the addition of the RF sol components, the resulting

RF carbon gels exhibit a very narrow PSD with an averagepore size of~10 nm.[61,62] Increasing the silica-to-RF ratio in-creases the pore volume significantly, but it also widens thepores and broadens the corresponding PSDs.[62] Overall, max-imum surface areas and pore volumes are obtained whenusing an initial solution pH of 8,[63] wherein the correspondingcarbon matrix around the template is essentially nonpor-ous,[63] in agreement with results reported elsewhere.[25] Stabi-lizing the silica-sol template with a surfactant is essential forobtaining such narrow PSDs.[61] These templated RF carbongels can be used as adsorbents for bulky pollutants such asdyes and humic substances.[63]

3. Summary

This article presents a brief overview on the fascinating andremarkably flexible properties of RF carbon and organic gelsand how these properties are related to the synthesis and pro-cessing conditions. Since the properties are uniquely relatedto the nanostructure, they can be easily tailored by rigidlycontrolling the conditions. However, slight variations in theconditions may cause drastic variations in the structural char-acteristics and hence properties. Therefore, the effects of thedifferent conditions must be understood before attempting to

tailor a material to a specific application. This review articleshould assist in this endeavor with the understanding that theparticular properties and trends reported in a particular studymay be difficult to generalize because they are very much de-pendent on the specific conditions utilized. For this reason, alist of all the experimental conditions from each study is pro-

vided in Table 4. Unfortunately, some of the studies neglectedto provide all the important details; nevertheless, generaliza-tions are offered based on a careful analysis of the reportedproperties and trends. Be aware, however, that the generaliza-tion must not be taken for granted.

Overall, the most significant variations of the synthesis andprocessing conditions can be classified into three stages. Thesestages are 1) the preparation of the sol mixture, and subse-quent gelation and curing, 2) drying of the wet gel, and 3) car-bonization or activation of the dry gel. The effects of these fac-tors are summarized below and in Tables 1 to 3. In the firststage, the most critical factors that determine the final charac-

teristics of the RF carbon and organic gels are the catalyst con-centration (with respect to the reactants), the gel pH, and theconcentration of solids in the sol. Factors that lead to adilution effect of the sol (e.g., using excess of one reactant orreducing the concentration of solids) increase the particle size,reduce the density of the final polymer remarkably, increasethe surface area of xerogels, and either increase or decreasethe pore volume of xerogels, depending on the pH. Increasingthe R/C ratio increases the particle size and produces a morefibrous structure; it also has a significant effect on the surfacearea, which may exhibit a maximum depending on the R/Cratio and other conditions. Increasing the R/C ratio also resultsin an equivalent effect of increasing the concentration of solids

in the sol. Increasing the gel pH generally increases the surfacearea, pore volume, and electrochemical double layer capaci-tance of RF carbon aerogels, and it either increases or de-creases these properties for RF carbon xerogels, depending onthe concentration of solids in the solution. The proper controlof the gelation and curing conditions is essential for complet-ing the polymerization reactions and associated crosslinking ofthe polymerized particles. The impact of these factors is mostapparent on the resulting mechanical properties of the gels.

The main purpose of the second stage is to remove thesolvent with minimal alteration of the polymeric structure.Depending on the solvent and the intended drying medium,the solvent may be exchanged with a more compatible solvent

that is easier to evaporate, with less surface tension, and morecompatible with other drying media such as supercritical CO2.Subcritical drying of the solvent produces RF organic xero-gels, which experience significant shrinkage. Alternatively, thesolvent can be replaced with one that can be easily transferredto its supercritical conditions (e.g., CO2). Gels dried supercri-tically (known as RF organic aerogels) exhibit the lowest pos-sible shrinkage. However, supercritical drying is a high-pres-sure process and it is a relatively long process. Alternativemethods to supercritical drying with CO2, which still minimizeshrinkage, include supercritical drying with substitute solventsthat require lower pressures and the freeze-drying of solvents.



Fig. 7. SEM images of a) microspheres, b) surface, and c) cross-section of RFcarbon cryogel microspheres pyrolyzed at 1300 C (1573 K) [28]. (Reproducedwith permission from Carbon, Elsevier, 2002.)


12/14



Table 4. Summary of the synthesis and processing conditions for RF organic and carbon gels [a].

Ref. R/F[mol/mol]

Catalyst (C) R/C[mol/mol]

Conc. pH Drying [b] Pyrolysis Activation Remarks

[2] 1:11:21:3

Na2CO3Pt(NH3)4]Cl2

PdCl2

AgOOCCH3

200:1800:1

13 % ? sup. CO2 1000 C 900 Cstm, 25 min

CO2 2 h

conc. R/W [mol-%]chosen C refers to

transition metal

[3, 53, 57] 1:2 Na2CO3 50:1 5%20%

5.57.0

sup. CO2/ambient

800 C1050 C

NA conc. [total wt.-%]

[4] 0.25-1:1 Na2CO3 25750:1 0.050.25:1 ? sup. CO2 950 C NA conc. [R/W]

[7] ? ? 1500:1 30 wt.-% ? ambient 1000 C NA gels cast in thin sheets(0.20.5 mm)

[9] 1:2 Na2CO3HclO4

50:1200:1

15 or 20 wt.-%5 or 20 wt.-%

? sup. CO2 ? NA solvent = water withalkaline C, acetone with

acidic Cconc. [R/W] ??

[10] 1:2 Na2CO3 200410:1 25% 6.57.4 sup. CO2 6001200 C NA conc. [units unspecified]

[11] ? Na2CO3 50400:1 ? ? sup. CO2/ambient

6002100 C NA hybrid aerogel/ xerogel

[14] 1:2 Na2CO3 >50:1 3070% ? sup. CO2 NA NA conc. [w/v]

[16] ? Na2CO3Li2CO3K2CO3

? ? ? sup./sub.CO2, air

1050 C NA thin slices of gel havereduced shrinkage

[17] ? ? ? ? ? sup. solvent NA NA used organic solvents

[19] 1:2 Na2CO3 1000:11500:1

? ? sub. acetone 1050 C NA

[21] 1:2 Na2CO3 1500:1 2530% ? ambient 1050 C NA thin slices of gel havereduced shrinkageconc. [mass/mass]

[22] 1:2 Na2CO3 50:1200:1300:1


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The pyrolysis (or carbonization) step, i.e., the third stage,transforms the dried gels into relatively pure carbon struc-

tures through the thermal decomposition and removal ofoxide and hydrogen groups, which depends on the pyrolysistemperature. The transformed carbon structures are referredto as RF carbon gels. Overall, increasing the pyrolysis temper-ature decreases the surface area of carbon gels (or increases itin the case of low concentrations of solids); but they arealways higher than those of RF organic gels. Similarly, thepore volume of RF carbon gels generally decreases when in-creasing the pyrolysis temperature, possibly due to decreasesin the macropore region. Activation of the carbon gels can beperformed in conjunction with or after pyrolysis of the RF gelwith gases such as air, steam, or CO2 at elevated temperatures(in the same range as those of pyrolysis). Overall, the activa-tion of carbon gels enhances the electrochemical propertiessignificantly and is very effective at increasing the pore vol-ume, pore width (especially in the narrow pore range) andsurface area.

In conclusion, RF carbon gels are receiving considerable at-tention towards their use in many commercial applicationssuch as adsorbent materials,[28] electrodes for capacitive de-ionization of aqueous solutions,[56] ion exchange resins,[55]

electrochemical double layer capacitors and supercapaci-tors,[56] gas diffusion electrodes in PEM fuel cells,[7,33] and an-odes in rechargeable lithium ion batteries.[59,72,73] This largevariety of potential and existing commercial applications is

largely due to the tunable properties of RF carbon gels, espe-cially the surface area, pore volume, and pore size distribu-

tion. The tunable properties of RF carbon gels are easily re-lated to the synthesis and processing conditions, whichproduce a wide spectrum of nanostructured materials withunique properties. It is anticipated that many new and excitingapplications will arise for RF organic and carbon gels as morefundamental research is done to expose their unique and tun-able characteristics.

Received: June 15, 2002Final version: November 15, 2002

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Table 4. continued

Ref. R/F[mol/mol]

Catalyst (C) R/C[mol/mol]

Conc. pH Drying [b] Pyrolysis Activation Remarks

[41] 1:2 Na2CO3 50:1200:1

25%40%

sup. CO2 10502500 C Chem.,HNO3

inverse emulsionpolymerization

conc. [w/v]

[43] ? ? 2001500:1 30, 50% ? sub. acetone 1050 C Chem.,HNO3conc. in mass ratio

[45] 1:2 Na2CO3 50400:1 >10% sup. CO2 1050 C NA conc. [units unspecified]

[46] 1:2-3 ? 100200:1 ? NA NA NA

[47] 1:2 Na2CO3 50300 7% ? sup. CO2 1050 C NA conc. [units unspecified]

[48] 1:2 Na2CO3 200:1 40, 70% ? sup. CO2 6001100 C CO2,1050 C

conc. [w/v]

[49] 1:2 Na2CO3 100210:1 25% 6.57.4 sup. CO2 NA NA conc. [units unspecified]

[50] 1:2 Na2CO3 ? ? ? sub. acetone 800, 1050 C CO2,950 C, 2 h/

electrochem.

[51] ? Na2CO3 ? ? ? sup. acetone ?

[52] ? NaOH ? 10% ? sup. CO2 ? conc. [w/v]solvent: ethanol,

n-propanol,n-butanol, or n-pentanol

[54] 1:2 Na2CO3 50:1 5% ? ambient 6001200 C 1050 C,CO2, 3 h

conc. [w/v]

[60] ? Na2CO3 200:1 ? ? sup. CO2 1050 C 900 C,CO2, 17 h

[61] 1:2 Na2CO3 500:7 35% 7.3 ? 850 C conc. [mol-% (silica-free)]

[62] 1:2 ? ? ? 8 ? 850 C[63] 1:2 ? ? ? 4.39.6 ? 850 C

[a] ?: Conditions are not given explicitly.[b] sup: supercritical; sub: subcritical.


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