Carbon aerogels density

Figure 23.17. Effective total thermal conductivity of a carbon aerogel (density p = 225 kg m , size of primary particles 100-150 nm, average pore size 570 nm, pyrolysis temperature 1,800 0) under different atmospheres and the corresponding sohd (red line) and radiative (green line) thermal conductivity as well as the sum of both (black line) as a function of temperature [44].

However, the performance of a fuel cell with these carbon aerogels as DLs was around a factor of six lower than the performance of commercial electrodes. This was due mainly to the fact that the authors did not use additional electrolyte when depositing the catalytically active layer, thus causing reduced ionic conductivity between the catalyst (Pt particles) and the membrane. In addition, the MEAs with carbon aerogels performed poorly at high current densities because the Pt particles used were 10 times larger than the ones normally used [20]. 

First results indicate a dependence of the surface capacitance of untreated carbon aerogels on their microstructure. Micro- and mesopores exhibit different storage capacitances (6.6 and 19.4 pF/cm in 1 M sulfuric acid, respectively).. An optimized thermal activation procedure of low density aerogels at 950°C in controlled CO2- atmosphere leads to an increase of the specific surface area and capacitance. On the other hand, the increase of the capacitive current after anodic oxidation in sulfuric acid is caused by electroactive surface groups, while the BET-surface area remains almost constant. 

In table 1 the densities and surface capacitances of various carbon aerogels are shown. The data reveal that the surface capacitance varies from one species to another.. According to Shi et al. the micro- and mesopore surface areas were separated and the total capacitance of the samples was split up in contibutions from both pore species [3]. The correlation can be expressed as... 

Density and surface capacitance of different carbon aerogels pyrolized at 1050°C. The surface capacitance was derived dividing the capacitance calcuted from the impedance data at 8.25 mHz by the BET surface area. 5-10% error must be assumed for the evaluated data. 

On the other hand, the mechanical properties of monolithic carbon gels are of importance when they are to be used as adsorbents and catalyst supports in fixed-bed reactors, since they must resist the weight of the bed and the stress produced by its vibrations or movements. A few smdies have been published on the mechanical properties of resorcinol-formaldehyde carbon gels under compression [7,36,37]. The compressive stress-strain curves of carbon aerogels are typical of brittle materials. The elastic modulus and compressive strength depend largely on the network connectivity and therefore on the bulk density, which in turn depends on the porosity, mainly the meso- and macroporosity. These mechanical properties show a power-law density dependence with an exponent close to 2, which is typical of open-cell foams. 

Finally, the nse of Pt-Rn catalysts snpported on a carbon aerogel as an anode for DMFC has been reported by Dn and co-workers [90]. The total metal loading was fixed to 20 wt%, and the Pt/Rn atomic ratio varied from 3 1 to 1 1. Metal particles were dispersed on the snpport nniformly, with a mean size of 3 nm. These anthors fonnd that with mnch less metal loading on the carbon aerogel, the membrane electrode assemblies had the same power density as that of commercial catalysts. This was attributed to the mesopore texture of the carbon aerogel, which facilitated methanol transportation in the electrode. 

Fig. 42. TEM image of a carbon aerogel with an apparent density of 0.4 g cm. Courtesy of Prof K. Kaneko of Chiba Univ.

Miller, J., B. Duim, T. Tran, and R. Pekala, Deposition of mthenium nanoparticles on carbon aerogels for high energy density supercapacitor electrodes. J. Electrochem. Soc. 1997,144(12), L309. 

Increasing gel pH Increases surface area and pore volumes of carbon aerogels Insignificant effect on surface area of carbon xerogels Increases the pore volume of carbon xerogels at high density of reactants Gelation time decreases... 

Fig. 2.7 SEM of carbon aerogels derived from polyurea aerogels made of Desmodur RE triisocyanate. Densities (inset) are those of the parent polyurea aerogels. Scale bar 5 p.m. Densities of the actual C samples (from left to right) top row, not measured (sample broke to pieces) 0.29 0.06 g cm 0.40 0.02 g cm lower row, 0.62 0.08 g cm 0.72 0.03 g cm 0.78 0.01 g cm ). Reprinted with the permission from Ref [36]. Copyright 2010 American Chemical Society...

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