Hydrogen Storage and Concepts: Focus on CNTs

Tram DangDaniel EstradaScott MaghyAndy Zelinski

What are CNTs?

• A rolled sheet of hexagonal carbon forming a seamless tube [1]

• Can be capped by fullerene hemispheres [1]• Can be single- or multi-walled [1]• Diameter and orientation can defined [2]:

[1] B. Bhushan, Springer handbook of nanotechnology, 2nd ed. (Springer, 2006), pp. 44-47.[2] D. Qian, G.J. Wagner, W.K. Liu, M.F. Yu, R.S. Ruoff, “Mechanics of carbon nanotubes,” Appl. Mech. Rev. 55 (6), 495 (2002).

CNT variations

[3] D. Qian, G.J. Wagner, W.K. Liu, M.F. Yu, R.S. Ruoff, “Mechanics of carbon nanotubes,” Appl. Mech. Rev. 55 (6), 495 (2002).[4] R. Andrews, D. Jacques, A.M. Rao, F. Derbyshire, D. Qian, X. Fan, E.C. Dickey, J. Chen, “Continuous production of aligned carbon nanotubes: a step closer to commercial realization,” Chem. Phys. Let. 303, 467-474 (1999).

[3] [4]

Why CNTs?

• Like graphite, has high surface area [5]• Curvature creates cavities of molecular

dimension [5]• Superposition of potential fields in cavities [5]

– Larger attractive force on hydrogen molecules

• Theoretical RT storage capacity [6]– Negligible physisorbtion– 7.7 wt% by chemisorption

[5] P. Barbaro, C. Bianchini, Catalysis for Sustainable Energy Production (Wiley-VCH, 2009) pp. 123-124.[6] J. Li, T. Furuta, H. Goto, T. Ohashi, Y. Fujiwara, S. Yip, “Theoretical evaluation of hydrogen storage capacity in pure carbon nanostructures,” Journ. Of Chem. Phys. 119 (4), 2376-2385 (2003).

Economics

• Purification can be more costly than actual CNT production [7]– Costs to remove non-nanotube carbon– Costs of catalyst material – No simple and routine method to measure purity.

• Purchasing CNTs [8]:• 1 Kg SWNT = $50,000• 1 Kg MWNT (8-15 nm diam) = $2,000

[7] J. Robertson, “Realistic application of CNTs,” Materials Today 7 (10), 46-52 (2004). [8] Cheaptubes.com: The Source for Low Carbon Nanotubes Prices, http://www.cheaptubes.com/carbon-nanotubes-prices.htm#Single_Walled_Nanotubes_Prices#ixzz0HDWX1APi&A http://www.cheaptubes.com/carbon-nanotubes-prices.htm#Single_Walled_Nanotubes_Prices (2009)

Chemisorption of SWNT

• Predicted as a feasible method to reach DOE goal of 6.5 wt.% for use in Fuel Cells.

• Density Functional Theory (DFT) widely used in studies to predict chemical environment

• Theoretical Upper Limit of 7.7 wt% corresponds to a 1 to 1 C-H ratio.

• Challenges still need to be overcome– Operating Temperature and Pressure– Kinetics: Need fast uptake/extraction!!

Chemisorption of SWNT• Stanford Group: Used XAS and XPS to see (and quantify) formation of C-H

bonds through electronic structure modification• XAS:

Demonstrates a decrease of intensity of π* from unsaturated C-C bonds in walls of SWNT and an increase in the σ* and C-H* resonance due to hydrogenation.

Chemisorption of SWNT• XPS:

Hydrogenation causes formation of “shoulder” in spectrum, which is attributed to hydrogenated Carbon atoms.

Based on intensity ratio of the two peaks and theoretical calculations, the amount of hydrogenated Carbon atoms was estimated to be 5.1 +/- 1.2 wt. % of SWNT. (close to 6.5%!)

Found to be stable at ambient; breaks up completely by 600º C reverseable process.

Theoretical values of the carbon 1s core-level chemical shifts between sp-2 and sp-3 due to C-H bond formation for SWNTs. Measured results correspond well with calculations!

Chemisorption of SWNT• XPS/XAS Conclusions:

– Hydrogenation leads to the breaking of C-C bonds and the formation of C-H bonds with a corresponding rehybridizaton of the Carbon atoms in the SWNT from sp² to sp³.

– DFT Calcs predicted variation of C-H bond length.• Tunability?

– Seems to be reversible• At 600 C, shoulder disappears. All H gone.

Chemisorption of SWNT Limitations

But, really need low temperature desorption• Possibly Use Metal Catalyst• Theoretical Calculations indicate larger radius

SWNT have weaker C-H bonds.

Kinetics Considerations

se TkE

B /1~

11310~SWNT of freq. Debye; s

So at room temperature, activation energy needs to be ~0.7eV.

-This could be a fundamental problem: H₂ binding energy ~4.7eV. H-H bond needs to be broken for Chemisorption to occur.

• For fuel cell applications, we want fast cycles.

Kinetics ConsiderationsIntrinsic metallicity of some CNTs can be influenced by hydrogenation!

•Experiments and calculations have indicated that even (3n,0) zigzag CNTs, DOS curves show gaps--indicative that band gaps have opened up.

•Indicates that at max coverage, the bonding environment has turned fully covalent Challenge!

•Activation energies tend to be greater in covalent environments

•Strong C-H bonds slow kinetics of H₂ recombination at room temperature.

• widely-used technique for determining the local geometric and/or electronic structure of matter• XAS data are obtained by tuning the photon energy using a crystalline monochromator to a range where

core electrons can be excited (0.1-100 keV photon energy). The "name" of the edge depends upon the core electron which is excited: the principal quantum numbers n=1, 2, and 3, correspond to the K-, L-, and M-edges, respectively. For instance, excitation of a 1s electron occurs at the K-edge, while excitation of a 2p electron occurs at an L-edge (Figure 1).

• X-ray photoelectron spectroscopy (XPS) is a quantitative spectroscopic technique that measures the elemental composition, empirical formula, chemical state and electronic state of the elements that exist within a material. XPS spectra are obtained by irradiating a material with a beam of X-rays while simultaneously measuring the kinetic energy (KE) and number of electrons that escape from the top 1 to 10 nm of the material being analyzed. XPS requires ultra high vacuum (UHV) conditions.

Initial and relaxed zigzag