A new class of hydrogen storage material (HSM), the swelling clay minerals, is introduced by the investigation of laponite, a representative smectite. Simple ion exchange allows for a diverse range of charged species to be studied as possible adsorption sites for H2 within the laponite interlayer, while a sub-monolayer of water pillars the interlayers apart by 2.85 Å, close to the kinetic diameter of H2. Neutron diffraction shows that the 001 peak, representing the clay d-spacing, is directly affected by the introduction of H2 or D2, confirming intercalation into the interlayers. Volumetric adsorption isotherms and neutron scattering show that laponites with 3 wt% H2O rapidly physisorb 0.5-1 wt% H2 at 77 K and 80 bar, with low binding enthalpies (3.40-8.74 kJ mol-1) and consequently low room temperature uptake (0.1 wt% at 100 bar). The higher structural density of clays results in lower H2 densities than MOFs and activated carbons, however some cation-exchanged forms, such as Mg and Cs, show promise for improvement having capacities of 22.8 g H2 per litre at 77K, 80 bar, intermediate between AX-21 and IRMOF-20. At low coverage, INS spectra reveal up to five adsorption sites with low rotational energy barriers (0.7-4.8 kJ mol-1), persisting up to at least 50 K. Analysis of quasielastic neutron scattering (QENS) spectra for Ca-laponite expanded with 3 wt% H2O reveals two populations of interlayer H2: one immobile up to 100 K and localised to the Ca2+ cations, while the other diffuses by jump diffusion at a rate of 1.93 0.23 Å2 ps-1 at 80 K, 60% slower than in the bulk (Dbulk = 4.90 0.84 Å2 ps-1). Arrhenius analysis gives activation energies of 188 28 K for the calcium and 120 32 K for the sodium form, comparable to the range for activated carbons. The adsorbate phase density of H2 in laponite interlayers at 40 K is 67.08 kg m-3, close to the bulk liquid density of 70.6 kg m-3. Jump lengths of 3.2 0.4 Å for Ca-laponite measured by QENS at 40 K are similar to the H2-H2 nearest neighbour distance in condensed H2 (3.79 Å). Thus data from a variety of techniques provides a coherent model for the structure and behaviour of H2 in laponite. The experimental achievement of a two-dimensional film of liquidlike H2 confined within the interlayers up to 40 K is of great interest for the field of superfluidics, since it may be possible to supercool liquid hydrogen confined in laponite interlayers below the predicted Bose-Einstein condensation temperature at 1 K.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:639706 |
| Date | January 2015 |
| Creators | Edge, J. S. |
| Publisher | University College London (University of London) |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://discovery.ucl.ac.uk/1462102/ |
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