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Hydrogen adsorption by alkali metal graphite intercalation compounds

Citation

Purewal, Justin (2010) Hydrogen adsorption by alkali metal graphite intercalation compounds. Dissertation (Ph.D.), California Institute of Technology. http://resolver.caltech.edu/CaltechTHESIS:03042010-183429960

Abstract

Adsorption occurs whenever a solid surface is exposed to a gas or liquid, and is characterized by an increase in fluid density near the interface. Adsorbents have drawn attention in the current effort to engineer materials that store hydrogen at high densities within moderate temperature and pressure regimes. Carbon adsorbents are a logical choice as a storage material due to their low costs and large surface areas. Unfortunately, carbon adsorbents suffer from a low binding enthalpy for H_2 (about 5 kJ mol^(-1)), well below the 15 to 18 kJ mol^(-1) that is considered optimal for hydrogen storage systems. Binding interactions can be increased by the following methods: (1) adjusting the graphite interplanar separation with a pillared structure, and (2) introducing dopant species that interact with H_2 molecules by strong electrostatic forces. Graphite intercalation compounds are a class of materials that contain both pillared structures and chemical dopants, making them an excellent model system for studying the fundamentals of hydrogen adsorption in nanostructured carbons. Pressure-composition-temperature diagrams of the MC_(24)(H_2)_x graphite intercalation compounds were measured for M = (K, Rb, Cs). Adsorption enthalpies were measured as a function of H_2 concentration. Notably, CsC_(24) had an average adsorption enthalpy of 14.9 kJ mol^(-1), nearly three times larger than that of pristine graphite. The adsorption enthalpies were found to be positively correlated with the alkali metal size. Adsorption capacities were negatively correlated with the size of the alkali metal. The rate of adsorption is reduced at large H_2 compositions, due to the effects of site-blocking and correlation on the H_2 diffusion. The strong binding interaction and pronounced molecular-sieving behavior of KC_(24) is likely to obstruct the translational diffusion of adsorbed H_2 molecules. In this work, the diffusion of H_2 adsorbed in KC_(24) was studied by quasielastic neutron scattering measurements and molecular dynamics simulations. As predicted, the rate of diffusion in KC_(24) is over an order of magnitude slower than in other carbon adsorbents (e.g. carbon nanotubes, nanohorns and carbon blacks). It is similar in magnitude to the rate of H_2 diffusion in zeolites with molecular-sized cavities. This suggests that H_2 diffusion in adsorbents is influenced very strongly by the pore geometry. The H_2 diffusion process in KC_(24) contains at least two distinct jump frequencies. Bound states of adsorbed H_2 in KC_(24) were investigated by inelastic neutron scattering measurements and density functional theory calculations. Spectral peaks in the neutron energy loss range of 5 meV to 45 meV were observed for the first time. These peaks were interpreted as single- and multi-excitation transitions of the H_2 phonon and rotational modes. The rotational barrier for H_2 molecules is many times larger in KC_(24) than in other carbon adsorbents, apparently due to the confinement of the molecules between closely-spaced graphitic layers. Evidence was found for the existence of at least three H_2 sorption sites in KC_(24), each with a distinctive rotational barrier.

Item Type:Thesis (Dissertation (Ph.D.))
Subject Keywords:hydrogen; graphite intercalation compound; neutron scattering; quasielastic; adsorption; physisorption; kc24
Degree Grantor:California Institute of Technology
Division:Engineering and Applied Science
Major Option:Materials Science
Thesis Availability:Public (worldwide access)
Research Advisor(s):
  • Fultz, Brent T. (advisor)
  • Ahn, Channing C. (co-advisor)
Thesis Committee:
  • Fultz, Brent T. (chair)
  • Ahn, Channing C.
  • Atwater, Harry Albert
  • Johnson, William Lewis
  • Rossman, George Robert
Defense Date:9 February 2010
Funders:
Funding AgencyGrant Number
DOE EEREDE-FC36-05GO15079
Projects:Hydrogen Sorption Center of Excellence
Record Number:CaltechTHESIS:03042010-183429960
Persistent URL:http://resolver.caltech.edu/CaltechTHESIS:03042010-183429960
Default Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:5574
Collection:CaltechTHESIS
Deposited By: Justin Purewal
Deposited On:19 Mar 2010 16:50
Last Modified:17 Apr 2013 22:00

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