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Introduction

Ultracapacitors may have the potential to become key components for energy storage in the industrial market with the rising push for environmental technology. There are several different approaches to creating ultracapacitors, as detailed here, and tunable nanaoporous carbon is a relatively new approach in research dedicated to improving such technology. Researched and developed primarily by Dr. Ranjan Dash, a Doctoral graduate of Drexel University, and Dr. Yury Gogotsi, a tenured professor at Drexel, this approach maximizes the surface area of carbon for ultracapacitor use. Dr. Dash and Dr. Gogotsi produce carbon materials known as Carbide-Derived Carbons (CDCs) through their startup company Y-Carbon from metallic carbides by chemically removing the metallic element, leaving a systematic array of pores on the nanometer level. While ultracapacitors charge quicker than batteries they typically do not hold their charge as long, though materials produced by Y-Carbon's researchers have yielded higher energy density, also known as volumetric uptake. According to research published by Dr. Dash, Dr. Gogotsi and a few other researchers they have been able to increase storage by 75% in some cases through adsorption treatment with hydrogen in the synthesis process as well as utilize almost 100% of the surface area exposed by the pores. 10 years' worth of research into the process has provided a technology that may come to be a contender in the renewable energy market as well as improve fuel storage, toxic gas handling, fuel cells, and water desalination and purification.

Concept

Carbon-based materials have been popular with researchers and developers of ultracapacitors and are useful for the process of physisorption. First CDCs are created by chlorinating the metallic carbides at high temperatures and then H₂ is adsorbed into the porous material. According to “Carbide-Derived Carbons: Effect of Pore Size on Hydrogen Uptake”, a paper published by Dr.’s Dash and Gogotsi, “In order to maximize the H₂ sorption at the desired temperature and pressure one needs not only to maximize the number of adsorption sites per unit mass and volume of the solid (which could indeed be proportional to SSA) but also tune the H₂-solid interaction energy that would allow more sorption sites to adsorb H₂ molecules”, where SSA stands for Specific Surface Area. This paper relates some work published by Suresh K. Bhatia and Alan L. Myers postulating that the ideal temperature for carbon materials to adsorb hydrogen is 15kJ/mol. Experiments were conducted on four CDCs: Titanium carbide (TiC), Zirconium carbide (ZrC), Silicon carbide (SiC), and Boron carbide (B₄C). These materials exhibited the best results for fine tuning the pore size distribution (PSD) and are inexpensive, giving much potential to commercialization. Research has also shown that there are several influences to the pore size of CDCs, including “the spatial distribution of carbon atoms in the precursor carbide, the synthesis temperature, the size of the chloride molecules, the presence of catalytic particles, and the effect of optional post-treatments, such as purification or activation” .

By varying the temperature at which the precursor carbides are chlorinated Y-Carbon is able to systematically control the size and distribution of pores across the surface of the resulting carbon material. Temperatures up to 600°C, considered low temperatures for chlorination, tend to produce common and uniform pore sizes on an object and increasing beyond that threshold creates larger size pores and broader distribution. Dr. Dash and Dr. Gogotsi believe that smaller pores are more efficient in H₂ sorption because there is a strong reaction with hydrogen molecules and state that, “The total interaction between the adsorbate molecule and a solid is greater if the molecule can interact with a larger number of surface atoms, as happens in small curved pores (Fig. 4a and b) or narrow slit pores”. This is because the carbide has a larger surface area for hydrogen adsorption. It is possible that there are other factors involved with the carbide structure that may affect or influence sorption properties such as “pore shape, degree of disorder, or internal surface chemistry”. Through research TiC-derived CDC (TiC-CDC) has shown the greatest potential storage capacity after testing TiC-CDCs synthesized at temperatures of 400° C, 600° C, 800° C, and 1000° C and compared to TiC-CDCs treated in hydrogen at temperatures of 400° C and 800° C .

According to a Doctoral dissertation submitted by Dr. Dash in 2006, CDCs have been proven to have a greater volumetric and gravimetric storage capacity compared to other carbon-based storage technologies such as single-walled carbon nanotubes (SWCNTs), multi-walled CNTs (MWCNTs), and metal-organic frameworks (MOFs) . Because TNPC works with gas for energy storage pressure may affect the capacity of the material. Dr. Dash writes, “Considering that only 30% of the total CDC pore volume accessible to Ar is currently used by H₂ at ambient pressure, there is a large potential for increasing capacity at elevated pressure” . The dissertation also states that an increase in the volume of pores with diameter greater than 2 nm causes a decrease in hydrogen adsorption, further demonstrating that a higher volume of small pores correlates with an increase in storage capacity .

Industrialization

With 12 patents pending Y-Carbon is attempting to commercialize their approach and enter the competitive renewable energy market. This move has been supported through company funding and grant money, with which the company has produced a working prototype for a defense contractor . CEO Dr. Ken Malone credits the Pennsylvania NanoMaterials Commercialization Center with providing Y-Carbon an impetus and drive to improve their business structure through a donation of $243, 835 . According to an article published in MIT’s Technological Review magazine’s 2009 TR35 awards Y-Carbon will be working with other companies to develop applications aside from hydrogen energy storage and ultracapacitor development, and that the first ultracapacitor products could be released in the next two and a half years .