Carbide Derived Carbon
1. Carbide-Derived Carbons (CDCs)
CDC is produced by extraction of metals from metal carbides. The method permits synthesis of almost all known carbon structures including amorphous and nanocrystalline graphitic carbon, graphite ribbons, carbon nanotubes, carbon onions, nanodiamond, and ordered graphite. In addition, CDC synthesis allows formation of highly porous carbon materials with good mechanical properties. Microstructure, pore size, pore shape, and surface termination of nanoporous CDC can be precisely controlled by changing the process parameters and the composition and structure of the initial carbide precursor. As such, the process allows optimization of nanoporous CDC for various applications.
Figure 1. (a) TEM micrographs showing the transformation of crystalline SiC to amorphous CDC. (b) Tuning the Pore Size in Ti3SiC2 - CDC. (c) Dependences of the radius of gyration (Rg) and pore size on chlorination temperature[ Y. Gogotsi et al. Nature Materials 2003, 2,591].
Tuning the carbon structure and pore size with high accuracy by using different starting carbides and chlorination temperatures allows rational design of carbon materials with enhanced performance for the variety of applications.
Supercapacitors are electrochemical energy storage devices which share characteristics common to both capacitors and batteries. They are, essentially, capacitors which store charge at the electrical double-layer present on electrodes when they are charged in solution. In order to maximize capacitance, high surface area materials, most commonly activated carbons, are used in the electrodes. Because of the need to maintain the electrodes in an electrolyte solution, construction of supercapacitors is very similar to that for batteries. In general, supercapacitors are a compromise for energy vs power as compared to batteries and conventional capacitors. In other words they offer higher power than batteries but lower than conventional capacitors, and higher energy than conventional capacitors but lower than batteries.
Figure 2. High Resolution TEM images of various forms of carbon produced during chlorination of carbides: amorphous carbon (a), turbostratic graphite (b), fullerene-like carbon (c), nanodiamond (d), carbon onion (e), graphite ribbons (f), carbon nanotubes (g), barrel-like particles (h), and ordered graphite (i). [G. Yushin et al., in Nanomaterials Handbook, ed. by Y. Gogotsi, 2005, CRC Press, 237-280]
Figure 3. (a) Schematic of an electrochemical double-layer capacitor (b) Plot of specific capacitance normalized by BET SSA for the carbons with identical.
It should be noted that while batteries rely on a chemical charge-transfer (Faradaic) mechanism to store energy, supercapacitors rely on a purely physical mechanism. Because of this, supercapacitors offer much greater cycle life than batteries (100,000's of cycles as opposed to hundreds). They also offer much-improved temperature stability compared to batteries. However, as mentioned above, they still suffer from relatively low energy desnsity. The global market for supercapacitors is expected to reach $ 1 billion by the end of 2009. Current applications of supercapacitors includememory backup, mobile electronics, starting conventional engines and acceleration boost for hybrids, power for electric vehicles, and backup power supplies. CDC, as produced by our group, offers sub-nanometer pore size control by varying precursor used and chlorination temperature. By tuning for the correct pore size, CDC produced by our group outperforms other porous carbon materials for supercapacitor applications. CDC affords the highest volumetric and gravimetric capacitance, and thus the highest energy density for the produced devices [J. Chmiola et al. Science 2006, 313,1760; Angew. Chem. Int. Ed. 2008, 47,3392].
Figure 4. Cyclic voltammograms (CVs) taken at a scan rate of 20 mVs-1 on samples synthesized at (a) TiC-CDC500 oC Cl2 (b) TiC-CDC 800 oC Cl2.
Additionally, a carbon layer consisting of graphenes or carbon nanotubes (CNT) can be produced by vacuum decomposition of a SiC wafer at high temperature. It is expected that carbon nanotubes layer grown on SiC wafer is able to improve the electrochemical properties of supercapacitor [Yury Gogotsi, et al., Nature Materials 7, 845 - 854 (2008) ].
Figure 5. Schematic for Electrochemical application of Carbon nanostructures made from a SiC wafer.
2.2. Hydrogen Storage
The success of any future hydrogen economy depends, in large part, on our ability to develop inexpensive materials with sufficient hydrogen-storage capacity. Cryo-adsorption is considered a promising method of enhancing gravimetric and volumetric H2 storage for the future transportation needs. We showed previously that significant hydrogen storage capacities are obtained by opening small pores by purifying in hydrogen to remove Cl2 molecules trapped in nanopores during synthesis [Y. Gogotsi et al. J. Am. Chem. Soc. 2005, 127, 160006], leading to 3.0 wt% at 1 atm pressure and 77 K . Post-treatment of CDC samples, such as activation or surface modification, will continue to be crucial in ongoing efforts to meet criteria for applications.
Nanoporous carbons with tunable pore size and specific surface area up to 3000 m2/g available for hydrogen storage have demonstrated a gravimetric hydrogen storage density of 4.7 wt% at elevated pressure obtained either with TiC chlorinated at 600°C activated under CO2 or low temperature synthesized CDCs (400 or 500 °C) activated under KOH. While small pores (1 nm or below) are efficient for hydrogen sorption, mesopores do not contribute much to storage of hydrogen under these conditions. A higher SSA and larger pore volume increase the hydrogen uptake for a given pore size. Our findings dispel the popular myth that hydrogen physisorption is directly proportional to SSA, and provide guidance for optimal design of carbon materials for high-pressure hydrogen storage at cryogenic temperatures
Figure 6. (a) 77K Sieverts isotherms for TiC-CDC chlorinated at 600°C and CO2-activated for various times at different temperatures. (b) "Chahine" plot of 77 K peak excess capacity (~ 40 bar) vs. BET SSA for a variety of CDCs and activated carbons (ACs).
2.3 Methane Storage
The use of natural gas (methane) as an automotive fuel offers considerable advantages, including reduced emission, lower maintenance, and most importantly lower fuel cost relative to gasoline. Moreover, the world-wide reserves of natural gas considerably exceed the oil reserves and thus natural gas may offer a solution when oil wells run dry. However, in order to compete with gasoline now (and offer comparable driving distance for vehicles before re-fueling) advanced methane storage units with large volumetric and gravimetric methane storage capacity need to be developed. Storing natural gas in a compressed form has the disadvantage of the high cost of high pressure cylinders needed to provide the adequate methane storage capacity in a reasonably small volume. The use of inexpensive carbon adsorbent materials may allow storing considerable amounts of natural gas at relatively low pressures (<40 bar), making it more attractive for practical use. Similar to hydrogen-storage applications, CDC can be optimized to adsorb large quantities of methane. Through different activation approaches, we showed high uptake of the methane molecules, 18.5 wt% at 25 oC and 60 bar, entrapped in the pores of activated CDC materials. In volumetric methane uptake, CO2 activated CDC exhibits 145 v/v (81 % of the DOE target of 180 v/v at 25 oC and 35 bar), in which the packing density of completely dried carbon was applied for accurate volumetric conversion [S.-H. Yeon, et al., J. Power Sources (2009), doi:10.1016/j.jpowour.2009.02.019]. Thus, the activation of CDCs allows additional control of pore size, shape, and BET surface area for developing storage materials for various gases.
Figure 7. 25 oC methane excess adsorption isotherms for several TiC-CDCs.(a) CO2-activated for different times at 875 oC, (b) hydrogen annealed followed by KOH activation. Comparison of excess (c) gravimetric and (d) volumetric methane capacity for two CDCs, compared with data for a MOF, using the packing (dp), crystallographic (dc), and ideal or theoretical (dt) densities .
Porous carbons can be used for the purification of various bio-fluids, including the cleansing blood of inflammatory mediators in conditions such as sepsis or auto-immune diseases. Novel mesoporous carbon materials synthesized from ternary MAX-phase carbides can be optimized for efficient adsorption of large inflammatory proteins. Based on this work it appears that not only micropores (0.4-2 nm) but also mesopores (2-50 nm) can be tuned in a controlled way by extraction of metals from carbides, providing a mechanism for the optimization of adsorption systems for selective sorption of a large variety of biomolecules [Gleb Yushin, et al., Biomaterials 27 (2006) 5755-5762]. The synthesized carbons, having tunable pore size with a large volume of slit-shaped mesopores, outperformed all other materials or methods in terms of efficiency of TNF-a removal and the results are comparable only with highly specific antibody-antigen interactions.
Figure 8. (a) Adsorption of cytokines by porous carbons as a function of the surface area accessible to the cytokines. (A) TNF-a, (B) IL-1b, (C) IL-6, (D) IL-8. (b) Schematics of protein adsorption by porous carbons. (A) Surface adsorption in microporous carbon. Small pores do not allow proteins (shown inpink) to be adsorbed in the bulk of carbon particles (shown in blue). (B) Adsorption in the bulk of mesoroporous CDC. Large mesopores are capable to accommodate most of the proteins.