Local mean-field study of capillary condensation in silica aerogels

We apply local mean-field (i.e. density functional) theory to a lattice model of a fluid in contact with a dilute, disordered gel network. The gel structure is described by a diffusion-limited cluster aggregation model. We focus on the influence of porosity on both the hysteretic and the equilibrium behavior of the fluid as one varies the chemical potential at low temperature. We show that the shape of the hysteresis loop changes from smooth to rectangular as the porosity increases and that this change is associated to disorder-induced out-of-equilibrium phase transitions that differ on adsorption and on desorption. Our results provide insight in the behavior of $^4$He in silica aerogels.Comment: 19 figure

Polymer-stable magnesium nanocomposites prepared by laser ablation for efficient hydrogen storage

Hydrogen is a promising alternative energy carrier that can potentially facilitate the transition from fossil fuels to sources of clean energy because of its prominent advantages such as high energy density (142 MJ per kg), great variety of potential sources (for example water, biomass, organic matter), and low environmental impact (water is the sole combustion product). However, due to its light weight, the efficient storage of hydrogen is still an issue investigated intensely. Various solid media have been considered in that respect among which magnesium hydride stands out as a candidate offering distinct advantages. Recent theoretical work indicates that MgH2 becomes less thermodynamically stable as particle diameter decreases below 2 nm. Our DFT (density functional theory) modeling studies have shown that the smallest enthalpy change, corresponding to 2 unit-cell thickness (1.6 {\AA} Mg/3.0{\AA} MgH2) of the film, is 57.7 kJ/molMg. This enthalpy change is over 10 kJ per molMg smaller than that of the bulk. It is important to note that the range of enthalpy change for systems that are suitable for mobile storage applications is 15 to 24 kJ permolH at 298 K. The important key for the development of air/stable Mg/nanocrystals is the use of PMMA (polymethylmethacrylate) as an encapsulation agent. In our work we use laser ablation, a non-electrochemical method, for producing well dispersed nanoparticles without the presence of any long range aggregation. The observed improved hydrogenation characteristics of the polymer/stable Mg-nanoparticles are associated to the preparation procedure and in any case the polymer laser ablation is a new approach for the production of air/protected and inexpensive Mg/nanoparticles.Comment: Hydrogen Storage, Mg - Nanoparticles, Polymer Matrix Composites, Laser Ablation, to appear in International Journal of Hydrogen Energy, 201

High yield and high packing density porous carbon for unprecedented CO2 capture from the first attempt at activation of air-carbonized biomass

The first attempt at activation of air-carbonized carbon reveals unusual resistance to activation and unprecedentedly high yields (32–80 wt%) of high packing density (0.7–1.14 g cm−3) microporous carbon dominated by 5.5–7 Å pores, which are just right for CO2 uptake (up to 5.0 mmol g−1) at 1 bar and 25 °C. The high gravimetric uptake and packing density offer exceptional volumetric storage, and unprecedented performance for low pressure swing adsorption (PSA) with working capacity of 6–9 mmol g−1 for a pure CO2 stream (6 to 1 bar) and 3–4 mmol g−1 for flue gas (1.2 to 0.2 bar). The working capacity for vacuum swing adsorption (VSA) is attractive at 5.0–5.4 mmol g−1 under pure CO2 (1.5 to 0.05 bar), and 1.8–2.2 mmol g−1 for flue gas (0.3 to 0.01 bar). The pure CO2 volumetric working capacity breaks new ground at 246–309 g l−1 (PSA) and 179–233 g l−1 (VSA). For flue gas conditions, the working capacity is 120 to 160 g l−1 (PSA). The performance of the activated air-carbonized carbons is higher than the best carbons and benchmark zeolites or MOFs