Carbon aerogels - promising materials for fuel cell applications

Carbon aerogels, first introduced by Richard Pekala in 1989, are three-dimensional, open porous solid materials produced via carbonization of organic aerogels based on e.g. resorcinol-formaldehyde, phenol-formaldehyde or melamine-formaldehyde polymers. The synthesis parameters of organic aerogels such as pH-value, the amount of catalyst, the molar ratios of educts, temperature influence the formation of the microstructure of aerogels significantly. The microstructure, in turn, is reflected in properties of aerogels. An increase in pH during stirring leads to the formation of small particles. Differences in the molar ratios of the reactants affect the connectivity between the particles and have a corresponding direct effect on the electrical properties of the aerogel. The final structure of carbon aerogel depends very critically on carbonization process, e.g. temperature, duration, gases. Unique properties of carbon aerogels such as well-controlled porosity and pore size, large specific surface area about 500-2000 m²/g, high electrical conductivity, and low envelope density make them promising material for application in adsorption, catalysis, supercapacitors, fuel cells or as a cathode host in metal-sulfur cells. Their remarkable electrical conductivity is one of the key factors for electrochemical applications. The open-pore network with adjustable microstructure offers a high level of freedom in material design to specifically adapt the carbon matrix to the requirements of the electrode. Within the presentation we will report on the properties of carbon aerogel materials and dependences of the synthesis and carbonization routines

Electrical conductivity of monolithic and powdered carbon aerogels and their composites

Carbon aerogels are three-dimensional, open-porous, amorphous materials introduced by R. Pekala 1989. Starting from organic aerogels, carbon aerogels exhibit unique properties such as well-controlled pore size distribution, high porosity, large specific surface area, high electrical conductivity, and low envelope density. This make them promising material for applications in adsorption, catalysis, supercapacitors or as a sulfur hosting material in cathodes of metal-sulfur battery cells. One of the key factors for electrochemical applications is the electrical conductivity. For amorphous carbon materials it is related to their electronic structure, the size of graphitic lattices or graphitic character, heteroatoms and so-called bulk electrical conductivity. In most electrochemical applications, the carbon materials are used as powders for e.g. electrode materials. Therefore, the measurement of the electrical conductivity of powder materials is of great importance. For powders, conductivity consists of: 1) the conductivity of individual grains and 2) the conductivity of the powder. The conductivity of individual grains depends only on monolithic conductivity of the material. In contrast, the conductivity of powder depends on several factors e.g. the shape of grains, their packing, compressibility, and the contact between the grains. Measurements of the electrical resistivity of powders are usually performed on the bed of grains under pressure. Within this presentation, we will report on our recent studies showing the correlation between structural, physical, mechanical and electrical properties of pure and activated carbon aerogels, as well as aerogel-composites. For this purpose, the influence of applied force, compressibility of aerogels and composites, and particle shape were investigated using the four-pin method to measure electrical conductivity. Monoliths and powders were measured at room temperature, and for powders the resistivity was determined in the force range from 1 to 20 kN. For structural and physical characterization nitrogen sorption, scanning electron microscopy, and pycnometry were used

Insights into the Micromechanics of Organic Aerogels Based on Experimental and Modeling Results

While the characteristics of the macroscopic mechanical behavior of organic aerogels are well known, the mechanisms responsible for the substructural evolution of their networks under mechanical deformation are not fully understood. Herein, organic aerogels from the aqueous sol−gel polymerization of resorcinol with formaldehyde are first prepared. Specifically, the resorcinol to water (R:W) molar ratio is varied for obtaining diverse highly open‐cellular porous structures with mean pore sizes ranging between 30 and 50 nm. The corresponding network structures are then characterized and exhibit different morphological and mechanical properties. Furthermore, a micromechanical constitutive model based on the pore‐wall kinematics is proposed. While the arrays of particles forming the pore walls are moderately connected, the pore walls are considered to behave as solid beams under mechanical deformation. Moreover, the damage mechanisms in the pore walls that result in the network collapse are defined. All model parameters are shown to be physically derived, and their sensitivity to the macroscopic network behavior is analyzed. The model predictions are shown to be in good agreement with the experimental stress−strain data of the different aerogels

Reduction of shrinkage and brittleness for resorcinol-formaldehyde aerogels by means of a pH-controlled sol–gel process

Aerogels have attracted remarkable attention as porous low-density superinsulating material. However, they are typically brittle and tend to shrink during preparation or work-up preventing their use as a composite material. In that context, we have developed a sol-gel-process towards resorcinol-formaldehyde aerogels that includes a precise pH-adjustment at an early stage of the polymerization. As a result, less brittle analogues of Pekala aerogels were obtained that resemble Pekala gels in many aspects (skeletal density, porosity, thermal properties). However, significant differences were found in terms of inner surface area and compressive modulus, which are smaller for lower pH-values. Since pH-adjustment leads to minimal gel shrinkage, this sol-gel-process may be useful for the development of aerogel composites

Combining aerogels with honeycombs – a new stiff and flexible superinsulation

Saving energy is the most important issue in the 21st century. New high qualitative thermal insulation materials are of critical importance to energy-efficient building design, transportation and aircraft industry. We propose to combine aramid honeycombs with aerogels to manufacture such new types of advanced insulation materials. Aramid honeycombs produced from aramid fibers by the expansion method possess extremely high stiffness-to-weight ratio and are heat-resisting up to 550°C. Aerogels are nanostructured highly open-porous solid materials synthesized by sol-gel process, mostly dried with supercritical carbon dioxide. They possess densities about 0.05-0.1 g/cm3, surface area in the range of 500-1000 m2/g, low thermal conductivity 0.01-0.03 W/mK and high sound absorption. Due to these excellent properties aerogels are brilliant insulation materials. The filling of honeycomb cells with aerogels can considerably reduce the thermal conductivity due to elimination of air convection and radiation effects. We investigated aramid honeycomb-cores with various cell sizes filled with two types of organic resorcinol-formaldehyde (RF) aerogel: a rubber-like flexible one [1] to ensure a certain elasticity of the composite which is important for some applications and a hard, brittle one produced according to Pekala synthesis route[2]. The resulting composite materials exhibit a combination of both component properties. On the one hand the aerogel decreases the thermal conductivity of the composite, on the other hand the honeycomb increases its stiffness. Thus we could produce a resilient, light insulating material with a good mechanical load capacity. The poster presents the fabrication route, the materials properties and discusses the results

Flexible Carbon Aerogels

Carbon aerogels are highly porous materials with a large inner surface area. Due to their high electrical conductivity they are excellent electrode materials in supercapacitors. Their brittleness, however, imposes certain limitations in terms of applicability. In that context, novel carbon aerogels with varying degree of flexibility have been developed. These highly porous, light aerogels are characterized by a high surface area and possess pore structures in the micrometer range, allowing for a reversible deformation of the aerogel network. A high ratio of pore size to particle size was found to be crucial for high flexibility. For dynamic microstructural analysis, compression tests were performed in-situ within a scanning electron microscope allowing us to directly visualize the microstructural flexibility of an aerogel. The flexible carbon aerogels were found to withstand between 15% and 30% of uniaxial compression in a reversible fashion. These findings might stimulate further research and new application fields directed towards flexible supercapacitors and batteries

Flexibilisation of Resorcinol-Formaldehyde Aerogels

Organic resorcinol - formaldehyde (RF) aerogels are usually hard and brittle with Young’s-moduli in the range of 1-2 MPa, strengths about 100 kPa and densities in the range of 0.2 to 0.4 g/cm3. We observed that special conditions of aerogel synthesis lead surprisingly to flexible aerogels. They can be bent easily (see figure 1), similar to the well-known flexible silica aerogels. They have a low density, a very low elastic modulus of around 70 kPa and are elastically deformable by more than 40% in an almost reversible manner. The flexible RF aerogel are prepared with resorcinol (R) to water (W) molar ratio 0.008, the R/F molar ratio is fixed at 1:2. The molar ratio R/C is 50:1. The pH is adjusted in the range 5.4-5.6. After one week in the oven at 80°C the gel is washed in acetone and not dried supercritically but in an oven at 80°C. The flexible RF-aerogels show densities around 0.060 g/cm3; a measurable shrinkage is not observed. Their thermal conductivity is ≈ 0.046 W/(mK). The SEM picture in Figure 2 shows the continuous network of particles with sizes of around 1 µm and pores of about 10 µm. The effects of various sol-gel parameters on the flexibility, such as R/W, R/C molar ratios, pH of initial solution, gelation temperature, have been investigated. The aerogels of different densities, microstructures and particle size were obtained by varying the R/W molar ratio. It has been observed that the pH plays a very important role in synthesis of flexible resorcinol-formaldehyde aerogels. The differences between hard and flexible RF aerogels were studied by recording 13C NMR spectra and TGA-FT-IR curves

Flexible carbon aerogels

Carbon aerogels are highly porous materials with a huge inner surface area. Due to their high electric conductivity they are excellent electrode materials in supercapacitors. Their brittleness, however, imposes certain limitations in terms of applicability. In that context, novel carbon aerogels with varying degree of flexibility have been developed. These highly porous, light aerogels are characterized by a high surface area and possess pore structures in the micrometer range allowing a reversible deformation of the aerogel network. A high ratio of pore size to particle size was found to be crucial for high flexibility. For dynamic microstructural analysis, compression tests were performed in-situ within a scanning electron microscope allowing us to directly visualize the microstructural flexibility of an aerogel. The flexible carbon aerogels were found to withstand between 15% and 30% of uniaxial compression in a reversible fashion. These findings might stimulate further research directed towards flexible supercapacitors and batteries

Flexible RF und carbon aerogels

Organic resorcinol - formaldehyde (RF) aerogels dried under ambient conditions with a large resorcinol (R) to catalyst (C) ratio are usually hard and brittle with strength about 100 kPa and Young’s-moduli in the range of 1-2 MPa and densities in the range of 0.2 to 0.4 g/cm3. We observe that special conditions of aerogel preparation yield surprisingly flexible aerogels. They can be bent easily, similar to the well-known flexible silica aerogels. They have a low density about 0.06 g/cm3, a very low elastic modulus of around 70 kPa and are elastically deformable by more than 40% in an almost reversible manner. The thermal conductivity of these aerogels is around 0.046 W/(mK). The flexible RF-aerogel shows a continuous network of particles with sizes of around 1 µm and pores of about 10 µm. Carbonization of the flexible RF aerogels leads to flexible carbon aerogel pieces. This paper discusses the effect of resorcinol-water (R/W) molar ratio on the mechanical properties, pore structure and bulk density of flexible RF aerogels