Electrochemical capacitors, also called supercapacitors, store energy using either ion adsorption (electrochemical double layer capacitors) or fast surface redox reactions (pseudo-capacitors). They can complement or replace batteries in electrical energy storage and harvesting applications, when high power delivery or uptake is needed. A notable improvement in performance has been achieved through recent advances in understanding charge storage mechanisms and the development of advanced nanostructured materials. The discovery that ion desolvation occurs in pores smaller than the solvated ions has led to higher capacitance for electrochemical double layer capacitors using carbon electrodes with subnanometre pores, and opened the door to designing high-energy density devices using a variety of electrolytes. Combination of pseudo-capacitive nanomaterials, including oxides, nitrides and polymers, with the latest generation of nanostructured lithium electrodes has brought the energy density of electrochemical capacitors closer to that of batteries. The use of carbon nanotubes has further advanced micro-electrochemical capacitors, enabling flexible and adaptable devices to be made. Mathematical modelling and simulation will be the key to success in designing tomorrow's high-energy and high-power devices.

https://hal.archives-ouvertes.fr/hal-02417326/document

Materials for electrochemical capacitors

In the present review, aerogels designate dried gels with a very high relative pore volume. These are versatile materials that are synthesized in a first step by low-temperature traditional sol-gel chemistry. However, while in the final step most wet gels are often dried by evaporation to produce so-called xerogels, aerogels are dried by other techniques, essentially supercritical drying. As a result, the dry samples keep the very unusual porous texture which they had in the wet stage. In general these dry solids have very low apparent densities, large specific surface areas, and in most cases they exhibit amorphous structures when examined by X-ray diffraction (XRD) methods. In addition, they are metastable from the point of view of their thermodynamic properties. Consequently, they often undertake a structural evolution by chemical transformation, when aged in a liquid medium and/or heat treated. As aerogels combine the properties of being highly divided solids with their metastable character, they can develop very attractive physical and chemical properties not achievable by other means of low temperature soft chemical synthesis. In other words, they form a new class of solids showing sophisticated potentialities for a range of applications. These applications as well as chemical and physical aspects of these materials were regularly detailed and discussed in a series of symposia on aerogels,1-5 the last of them being held in Albuquerque in 2000.6 Reviews were also regularly published, either on both xerogels and aerogels7 or more focused on the applications of aerogels.8-13 The particularly interesting properties of aerogels arise from the extraordinary flexibility of the solgel processing, coupled with original drying techniques. The wet chemistry is not basically different for making xerogels and aerogels. As this common basis has been extensively detailed in recent books,14 it does not need to be reviewed. Compared to traditional xerogels, the originality of aerogels comes from * To whom all correspondence should be addressed. † Institut de Recherches sur la Catalyse. ‡ Laboratoire d’Application de la Chimie à l’Environnement. 4243 Chem. Rev. 2002, 102, 4243−4265

https://is.muni.cz/el/1431/podzim2006/C7780/um/Read/2711172/aerogel_Pajonk_ChRev02_4243.pdf

Chemistry of Aerogels and Their Applications

The worldwide thirst for portable consumer electronics in the 1990s had an enormous impact on portable power. Lithium ion batteries, in which lithium ions shuttle between an insertion cathode (e.g., LiCoO2) and an insertion anode (e.g., carbon), emerged as the power source of choice for the highperformance rechargeable-battery market. The performance advantages were so significant that lithium ion batteries not only replaced Ni-Cd batteries but left the purported successor technology, nickel-metal hydride, in its wake.1 The thick metal plates of * To whom correspondence should be addressed. J.W.L: e-mail, jwlong@ccs.nrl.navy.mil; telephone, (+1)202-404-8697. B.D.: e-mail, bdunn@ucla.edu; telephone, (+1)310-825-1519. D.R.R.: e-mail, rolison@nrl.navy.mil; telephone, (+1)202-767-3617. H.S.W.: e-mail, white@chem.utah.edu; telephone, (+1)810-585-6256. † Naval Research Laboratory. ‡ UCLA. § University of Utah. 4463 Chem. Rev. 2004, 104, 4463−4492

Three-dimensional battery architectures.

A brief overview on the preparation and properties of resorcinol–formaldehyde organic 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 processing conditions play on the final properties are provided. Overall, resorcinol–formaldehyde organic gels undergo 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 properties 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 resorcinol–formaldehyde carbon gels are produced from the organic gels during a third stage, which is associated with carbonization or activation. Depending on the conditions, carbonization and activation both impact the structural and performance characteristics significantly.

Preparation and Properties of Resorcinol–Formaldehyde Organic and Carbon Gels

A preparation process of polyimide aerogels that composed of aromatic dianhydrides and aromatic diamines or a combined aromatic and aliphatic diamines is described. Also descried is a process to produce carbon aerogels derived from polyimide aerogel composed of a rigid aromatic diamine and an aromatic dianhydride. Finally, the processes to produce carbon aerogels or xerogel-aerogel hybrid, both of which impregnated with highly dispersed transition metal clusters, and metal carbide aerogels, deriving from the polyimide aerogels composed of a rigid aromatic diamine and an aromatic dianhydride, are described. The polyimide aerogels and the polyimide aerogel derivatives consist of interconnecting mesopores with average pore size at 10 to 30 nm and a mono-dispersed pore size distribution. The gel density could be as low as 0.008 g/cc and accessible surface area as high as 1300 m 2 /g.

Aerogel metallic compositions

Aqueous polycondensation of resorcinol with formaldehyde leads to organic gels the structure of which can be controlled by the reaction parameters. The amount of catalyst controls the size of the particles constituting the gel network. We could show that for very low catalyst concentrations in a high dilution of reactants, the particle growth can be further enhanced if the reaction temperature is kept low, and the gel time is prolonged. The resultant structure is largely affected by this treatment, producing particles with sizes of about 2 μm and average pore sizes of up to 7 μm. Due to these coarse structures it is possible to dry these RF gels subcritically with very little shrinkage. The structure has been studied by nitrogen sorption, small angle X-ray scattering and acoustic sound propagation. The change of the elastic modulus caused by pyrolysis at around 1000°C has been investigated. Mechanical properties of carbon aerogels are correlated with their electrical properties. The derived carbon aerogels have large specific surface areas, very little mesopore volume however, micron-sized macropores.

Carbon aerogels from dilute catalysis of resorcinol with formaldehyde

Resorcinol–formaldehyde (RF) gels with a very high molar ratio of resorcinol/catalyst (R/C), e.g., 1500 and a low relative molar mass content of resorcinol+formaldehyde (RF-ratio) of about 30% can be ambient pressure dried with minimal shrinkage. The resulting RF-aerogels exhibit average cluster and pore sizes >50 nm. Upon pyrolysis the RF-aerogels are transformed into carbon aerogels exhibiting the same morphology of the gel network but in addition showing a high amount of microporosity (<2 nm). Below an RF-ratio of thirty mass percent the average particle and pore size increase to several hundred nanometers. The macro- and microscopic properties of these aerogels were investigated by scanning electron microscopy, permeability measurements, small angle X-ray scattering and CO2 sorption.

Structure of carbon aerogels near the gelation limit of the resorcinol formaldehyde precursor

Structure and density of organic RF (resorcinol formaldehyde) aerogels and pyrolized RF aerogels (carbon aerogels) can be modified upon variation of the resorcinol to catalyst (RC) ratio and by varying the mass ratio of the reactants resorcinol and formaldehyde in the sol. Here, series of RF and carbon aerogels have been produced with RC 200, 500 and 1500, each covering a wide density range. All samples were investigated using small angle X-ray scattering (SAXS), N2 and CO2 adsorption. The creation of a huge amount of microporosity due to pyrolysis was observed and investigated by CO2 adsorption. SAXS scattering curves exhibit interesting structural changes. These investigation were accompanied by sound velocity measurements to obtain the elastic modulus.

Influence of monomer and catalyst concentration on RF and carbon aerogel structure

The thermal diffusivity, a, of various carbon aerogel samples with different [resorcinol]/[catalyst] ( R C ) ratios and bulk densities was determined from room temperature to about 700°C. The temperature dependence of a could be described by the same equation for all samples. The thermal diffusivity was found to vary linearly with density. Thermal conductivity values were calculated using the heat capacity, cp(T), and the sample density, ϱ: λ = aϱcp. Values between λ = 0.35 W m−1 K−1 (ϱ = 381 kg m−3) and λ = 2.75 W m−1 K−1 (ϱ = 984 kg m−3) at 700°C were derived. The thermal conductivity was found to be proportional to ϱ2.2. The sound velocities were determined from ultrasonic measurements and the microcrystallite size, Lα, using Raman scattering. A phonon mean free path, l, was calculated from the sound velocities. Both Lα (≈ 20 A) and l (≈ 16 A) were found to be independent of density.

V. Bock

Papers

Carbon aerogels from dilute catalysis of resorcinol with formaldehyde

Structure of carbon aerogels near the gelation limit of the resorcinol formaldehyde precursor

Influence of monomer and catalyst concentration on RF and carbon aerogel structure

Thermal properties of carbon aerogels