Hydrogen diffusion in potassium intercalated graphite studied by quasielastic neutron scattering

The graphite intercalation compound KC24 adsorbs hydrogen gas at low temperatures up to a maximum stoichiometry of KC_(24)(H_2)_2, with a differential enthalpy of adsorption of approximately −9 kJ mol^(−1). The hydrogen molecules and potassium atoms form a two-dimensional condensed phase between the graphite layers. Steric barriers and strong adsorption potentials are expected to strongly hinder hydrogen diffusion within the host KC_24 structure. In this study, self-diffusion in a KC_(24)(H_2)_0.5 sample is measured experimentally by quasielastic neutron scattering and compared to values from molecular dynamics simulations. Self-diffusion coefficients are determined by fits of the experimental spectra to a honeycomb net diffusion model and found to agree well with the simulated values. The experimental H2 diffusion coefficients in KC_24 vary from 3.6 × 10^(−9) m^2 s^(−1) at 80 K to 8.5 × 10^(−9) m^2 s^(−1) at 110 K. The measured diffusivities are roughly an order of magnitude lower that those observed on carbon adsorbents, but compare well with the rate of hydrogen self-diffusion in molecular sieve zeolites

Measurements of Hydrogen Spillover in Platinum Doped Superactivated Carbon

Hydrogen uptake was measured for platinum doped superactivated carbon at 296 K where hydrogen spillover was expected to occur. High pressure adsorption measurements using a Sieverts apparatus did not show an increase in gravimetric storage capacity over the unmodified superactivated carbon. Measurements of small samples (~0.2 g) over long equilibration times, consistent with the reported procedure, showed significant scatter and were not well above instrument background. In larger samples (~3 g), the hydrogen uptake was significantly above background but did not show enhancement due to spillover; total uptake scaled with the available surface area of the superactivated carbon. Any hydrogen spillover sorption was thus below the detection limit of standard volumetric gas adsorption measurements. Due to the additional mass of the catalyst nanoparticles and decreased surface area in the platinum doped system, the net effect of spillover sorption is detrimental for gravimetric density of hydrogen

Thermophysical properties of MOF-5 powders

a b s t r a c t We present a comprehensive assessment of the thermophysical properties of an industrial, pilot-scale version of the prototype adsorbent, metal-organic framework 5 (MOF-5). These properties are essential ingredients in the design and modeling of MOF-5-based hydrogen adsorption systems, and may serve as a useful starting point for the development of other MOF-based systems for applications in catalysis, gas separations, and adsorption of other gasses or fluids. Characterized properties include: packing density, surface area, pore volume, particle size distribution, thermal conductivity, heat capacity, stability against hydrolysis, differential enthalpy of H 2 adsorption, and Dubinin-Astakhov isotherm parameters. Hydrogen adsorption/desorption isotherms were measured at six temperatures spanning the range 77-295 K, and at pressures of 0-100 bar

Improved Hydrogen Storage and Thermal Conductivity in High-Density MOF‑5 Composites

Porous adsorbents such as MOF-5 have low thermal conductivities which can limit the performance of adsorption-based hydrogen storage systems. To improve the thermal properties of these materials, we have prepared a series of high-density MOF-5 composites containing 0–10 wt % expanded natural graphite (ENG), which serves as a thermal conduction enhancer. The addition of 10 wt % ENG to MOF-5 and compaction to 0.5 g/cm³ was previously found to increase the thermal conductivity relative to neat MOF-5 of the same density by a factor of 5. In this study, detailed measurements of the hydrogen storage behavior of MOF-5/ENG composites between 77 and 295 K are reported. We find that MOF-5 pellets with 0 wt % ENG and a density of 0.5 g/cm³ have a total volumetric hydrogen storage density at 77 K and 100 bar that is 23% larger than powder MOF-5 and 41% larger than cryo-compressed hydrogen. The addition of 10% ENG to 0.5 g/cm³ MOF-5 pellets produces only a small decrease (6%) in the total volumetric hydrogen storage compared to neat MOF-5 pellets of equal density. The excess, absolute, total, and deliverable hydrogen storage amounts by the MOF-5 composites are compared for ENG additions of 0–10 wt % and pellet densities of 0.3–0.7 g/cm³. Three adsorption models (Unilan, Tóth, Dubinin–Astakhov) are compared for their effectiveness in describing hydrogen adsorption isotherms of MOF-5 and MOF-5/ENG composites. The Unilan model provides the most accurate description of the experimental data, requiring only five temperature-invariant parameters to accurately fit the data across a wide temperature range