Hydrogen is considered to be the preferred successor to gasoline due to its clean combustion, but its efficient and save storage remains the bottle-neck and one of the main challenges in hydrogen based technologies, especially those addressing mobile applications. Among different storage methods, reversible physical adsorption of molecular hydrogen in nanoporous materials is considered as the one of most attractive options. However, despite of more than 20 years of intensive experimental research no existing porous structures – from traditional activated carbons to porous polymer networks or metal organic frameworks - possess the required adsorbing properties.Therefore, we used computer simulations to explore the experimental options toward the most promising solutions. We showed that boron substituted graphene-based nanoporous structures may reach necessary storage performance if both key parameters defining the storage capacity of a sorbent (its specific surface and the energy of hydrogen adsorption) will be optimized. A potentially effective way to synthetize such optimized structures is arc-discharge procedure, successfully used in the past to synthetize fullerenes and nanotubes. We have assumed that the synthesis parameters can be modified to prepare graphene-based structures with a variety of substitution sites, shapes, sizes, and interconnections between graphene fragments.The first boron-substituted carbons prepared in this way show promising properties: they contain a variety of organized, graphene based structures decorated with boron nanoclusters, partially incorporated into graphene layers. The strongest adsorption occurs with the binding energy higher than 10 kJ/mol, and at least 10 % of adsorption sites adsorb hydrogen with the energy higher than 6.5 kJ/mol, significantly larger than in activated carbons (~4.5 kJ/mol). The specific surface of as-prepared samples is low (~ 200 m2/g). To increase it, both physical (heating in O2 reach atmosphere) and chemical (with KOH) activation are currently in progress.

Hydrogen storage is a key technology for the fuel applications. Hydrogen has the highest energy density of any fuel; however, its low ambient temperature density results in a low energy per unit volume. High density hydrogen storage is a challenge for stationary and portable applications and remains a significant challenge for transportation applications. Presently available storage options typically require large-volume systems that store hydrogen in gaseous form. On a mass basis, hydrogen has nearly three times the energy content of gasoline—120 MJ/kg for hydrogen versus 44 MJ/kg for gasoline. On a volume basis, however, the situation is reversed; liquid hydrogen has a density of 8 MJ/L whereas gasoline has a density of 32 MJ/L, as shown in the figure https://energy.gov/eere/fuelcells/hydrogen-storage) comparing energy densities of fuels based on lower heating values. Onboard hydrogen storage capacities of 5–13 kg hydrogen will be required to meet the driving range for the full range of light-duty vehicle platforms. One of alternative methods is storage by adsorption in porous materials.The numerical modeling of hydrogen adsorption has been a part of research with the goal to provide adequate hydrogen storage for onboard light-duty vehicle, material-handling equipment, and portable power. Here we discuss how this methodology has been used for the last 20 years and what is it contribution towards final solution. The discussion will be focused on three aspects. First, we will discuss why the existing high-surface porous materials are not promising hydrogen sorbents. Then, we will show how chemical modifications of the adsorbing surface may increase the binding energy between the hydrogen molecule and the surface. Finally, we will present the “universal” limits of hydrogen adsorption in porous structures and discuss the resulting problems and future research perspectives.

The search for sustainable automotive fuels has driven numerous studies of sorption-based hydrogen storage for hydrogen fuel cell vehicles. Storage by adsorption is fully reversible, achieves fast fill/discharge demands by simple pressurization/depressurization, and operates at much lower pressure than compressed hydrogen and at much less demanding temperatures than liquid hydrogen. In support of the DOE 2020 storage capacity target of 40 g hydrogen/L system, we have investigated the density of the adsorbed hydrogen films in a variety of porous carbons synthesized at the University of Missouri. The investigation decomposes storage into a high-density adsorbed film and low-density non-adsorbed gas and determines the fraction of pore volume occupied by the two phases.We find exceptionally dense H2 films at liquid nitrogen temperature, 77 K. Saturated film densities are 100-120 g/L across all samples at pressures as low as 35-70 bar. This is 1.4-1.7 times the density of liquid hydrogen at its normal boiling point, 71 g/L (20 K). Experimental film thicknesses are 0.30-0.32 nm, and fractions of total pore volume filled with high-density film are 0.25-0.53. Thus high storage capacities, well in excess of the DOE target and even in excess of liquid hydrogen, can be achieved at 77 K in appropriately engineered nanoporous carbons.The dense films occur at a temperature more than twice the liquid-gas critical temperature of hydrogen, 33 K, above which no bulk liquid exists at any pressure. The high-density film above 33 K does not contradict the non-existence of bulk liquid: the film is not a bulk, 3D phase, but a monomolecular 2D phase. Monte Carlo simulations confirm the observed high density and small film thickness. The film density and volume remain constant up to gas densities ~80% of the film density. A discussion in terms of competing forces acting on adsorbed molecules will be given.

Structural transformations of nano-systems show properties different from their 3D bulk analogs. The mechanism of transformation is different due to the nano-object size or nanometric confinement. This field has not yet been fully explored and requires more fundamental studies which may lead to future application, e.g., in nanoporous systems characterization. Properties of methane adsorbed in confined geometries are interesting from both fundamental and practical points of view. At ambient temperatures supercritical adsorption is studied for possible methane storage applications. At the same time, the analysis of methane’s low temperature adsorption properties is essential for understanding the mechanism of adsorption as a function of temperature, pore size, pore topology and heterogeneity. The temperature dependence of structures of adsorbed phases allows one to understand evolution of confined phases in nano-space. In this work we compare the mechanism of adsorption and structural transformations of methane confined in (i) homogeneous carbon slit pores of widths between 1nm and 4nm and (ii) heterogeneous MOF pores. We discuss the mechanism of layering transition, melting and capillary condensation in subcritical conditions, for temperatures between 80 K and 180 K. The mechanism of layer formation is strongly temperature dependent. At the same time, a commensurability between the methane and the pore sizes is important factor. In slit pores it evolves from a sharp layers formation at 80 K to continuous adsorption at higher temperatures. The capillary condensation is observed clearly in slit pores bigger than 2 nm. The melting transition of methane in slit pores shows complicated mechanism where the interior of the pore melts at lower temperature than the layers adsorbed closer the pore wall. We compare this behavior with the mechanism of adsorption in MOF porous structures where the adsorbing walls are strongly heterogeneous both structurally and energetically. The comparison between the slit pores and the MOFs will allow us to discuss an influence of the heterogeneous wall topology and intermolecular interactions on the thermodynamic properties of nano-confined structures.

La compréhension du mécanisme de la formation du verre est l'un des importants problèmes ouverts en recherche sur la matière condensée. De nombreuses questions restent sans réponse, en raison d'une énorme augmentation des temps de relaxation pendant le processus de refroidissement qui ne permet pas l'exploration des propriétés d'équilibre des liquides surfondus à très basses températures. Les simulations numériques des liquides surfondus sont actuellement en mesure d'atteindre l'équilibre à des températures comparables à la température du crossover de la théorie de couplages de modes, qui est bien supérieure à la température de transition vitreuse expérimentale. En conséquence, les simulations plus lentes que les expériences pour équilibrer un liquide surfondu par un facteur d'environ huit ordres de grandeur. Les progrès réalisés pour combler cet écart ont été lents et résultent essentiellement d'améliorations de l'architecture des ordinateurs. Dans cette thèse, nous résolvons en partie le problème de la thermalisation à basse température de liquides surfondus dans des simulations numériques. Nous combinons l'utilisation d'un algorithme Monte Carlo, connu sous le nom d'algorithme de swap, avec la conception de nouveaux modèles de formateurs de verre. Nous examinons systématiquement des nombreux systèmes, à la fois des mélanges discrets de particules, ainsi que des systèmes a polydispersité continue. Nous discutons le rôle que la polydispersité et la forme du potentiel entre particules jouent pour éviter la cristallisation et parvenir efficacement à des régimes de température inexplorés. De plus, nous étudions les processus dynamiques à l’œuvre pendant une simulation de swap Monte Carlo. Nous démontrons que, dans certains cas, notre technique permet de produire des configurations équilibrées à des températures inaccessibles même dans des expériences. Dans ce régime de température complètement nouveau, nous examinons plusieurs questions ouvertes concernant la physique de la transition vitreuse. Nous montrons qu'un fluide de sphères dures peut être équilibré jusqu'à la densité critique du jamming, et même au-delà. Nous mesurons l'entropie configurationelle dans un liquide refroidi à très basse température. Nous mettons en évidence une forte dépendance dimensionnelle, qui suggère l'existence d'une transition vitreuse idéale à une température finie en trois dimensions et à son absence en deux dimensions. Nous détectons l'augmentation de l'ordre amorphe quantifié par une longueur statique point-to-set pendant la formation du verre. Nous mesurons les exposants critiques introduits dans la théorie de champ moyen des verres beaucoup plus proche de la température critique prédite dans la théorie. Enfin, nous révélons l'absence de transition géométrique caractérisant le paysage d’énergie potentiel au travers de la température du crossover de la théorie de couplages de modes.Les modèles et les algorithmes développés dans cette thèse déplacent les études des liquides surfoundus vers un territoire entièrement nouveau, en réduisant l'écart entre la théorie et les expériences, ce qui nous amène plus proche de la solution du problème de la transition vitreuse.

The world is running out of fossil fuels and the products of their burning in air (mostly CO2) have already impacted global climate. Therefore, we still need a lot effort to search how to convert the global energy economy towards cleaner and renewable fuels (like hydrogen), in particular, for mobile applications. It is clear now, that to efficiently store hydrogen at ambient temperature and not too high pressures, we need to develop hydrogen sorbent and optimize simultaneously both its specific surface and adsorption energy. In this project we propose to explore the potential effectiveness of arc-discharge procedure to synthetize nanoporous, carbon based sorbents with characteristics required in vehicular applications. The technique was successfully used in the past to synthetize fullerenes and nanotubes. Therefore we have assumed that we can adjust the synthesis parameters to prepare other graphene-based structures, with a variety graphene fragment sizes, forms, and interconnections between them. In addition, arc-discharge technique provides a relatively easy way to incorporate heteroatoms into pure carbon structures.The first characterizations of boron-substituted carbons obtained by this method are promising: the energy of hydrogen adsorption is the highest ever observed experimentally in carbon-based sorbents. However, the specific surface is relatively low, ~ 200 m2/g, suggesting that the samples need to be activated to increase their accessible for adsorption surface and, in consequence, their capacity to store hydrogen.