Jan 17, 2012
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| Tunable sub-nm and supra-nm pores in KOH-activated carbon |
Activated carbons provide large surface areas (as high as 3000 m2/g) generated by a maze of nanoscale pores for high-performance adsorption. Applications include the storage of hydrogen and natural gas for advanced transportation (pressure-controlled adsorption/desorption of gas at supercritical temperature). Here, two distinct types of pores have attracted the interest of researchers: sub-nm (<1 nm) pores host deep potential wells – surface sites with high binding energies – which adsorb gas molecules as a high-density fluid; supra-nm (>1 nm) pores host lower binding energies, but offer space for multilayer adsorption. In addition, sub-nm pores favour high volumetric storage capacity; supra-nm pores favour high gravimetric storage capacity. Experts are looking for ways to optimize gas storage capacities by controlling the number of pores in each of the two classes.
In a recent study, published in the journal Nanotechnology, scientists at the University of Missouri, US, have demonstrated that such control is possible. Carbons chemically activated with potassium hydroxide (carbon oxidation; intercalation of metallic potassium into carbon lattice) gave bimodal pore-size distributions with a large, approximately constant number of sub-nm pores and variable number of supra-nm pores (1–5 nm, peaked around 1.5 nm). The control variables were the KOH:C mass ratio and activation temperature.
Tunable pore space
The team showed that supra-nm pores are absent when the KOH:C ratio and activation temperature are low, and increase rapidly in number with increasing KOH:C ratio and activation temperature. By appropriate choice of the variables, the volume in supra-nm pores can be varied anywhere from 0 to 1.0 cm3/g, while the volume in sub-nm remains approximately 0.6 cm3/g.
This tunable pore space will allow researchers to selectively optimize carbons for high volumetric or high gravimetric storage capacity, depending on the requirements in different vehicles for on-board storage of hydrogen and natural gas.
High volumetric capacity is important for designs where space is limited (passenger vehicles - light duty vehicles) and a high gravimetric capacity is key when vehicle weight must be kept to a minimum.
The research was conducted by members of the Alliance for Collaborative Research in Alternative Fuel Technology (ALL-CRAFT), a partnership of the University of Missouri and several other institutions, funded by the National Science Foundation, US Department of Energy, US Department of Defense, California Energy Commission, and Southern California Gas Company, to develop novel storage materials for natural gas and hydrogen for advanced next-generation clean vehicles.
Source: IOP Publishing
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