Charge induced enhancement of adsorption for hydrogen storage materials

The rising concerns about environmental pollution and global warming have facilitated research interest in hydrogen energy as an alternative energy source. To apply hydrogen for transportations, several issues have to be solved, within which hydrogen storage is the most critical problem. Lots of materials and devices have been developed; however, none is able to meet the DOE storage target. The primary issue for hydrogen physisorption is a weak interaction between hydrogen and the surface of solid materials, resulting negligible adsorption at room temperature. To solve this issue, there is a need to increase the interaction between the hydrogen molecules and adsorbent surface. In this study, intrinsic electric dipole is investigated to enhance the adsorption energy. The results from the computer simulation of single ionic compounds with hydrogen molecules to form hydrogen clusters showed that electrical charge of substances plays an important role in generation of attractive interaction with hydrogen molecules. In order to further examine the effects of static interaction on hydrogen adsorption, activated carbon with a large surface area was impregnated with various ionic salts including LiCl, NaCl, KCl, KBr, and NiCl and their performance for hydrogen storage was evaluated by using a volumetric method. Corresponding computer simulations have been carried out by using DFT (Density Functional Theory) method combined with point charge arrays. Both experimental and computational results prove that the adsorption capacity of hydrogen and its interaction with the solid materials increased with electrical dipole moment. Besides the intrinsic dipole, an externally applied electric field could be another means to enhance hydrogen adsorption. Hydrogen adsorption under an applied electric field was examined by using porous nickel foil as electrodes. Electrical signals showed that adsorption capacity increased with the increasing of gas pressure and external electric voltage. Direct measurement of the amount of hydrogen adsorption was also carried out with porous nickel oxides and magnesium oxides using the piezoelectric material PMN-PT as the charge supplier due to the pressure. The adsorption enhancement from the PMN-PT generated charges is obvious at hydrogen pressure between 0 and 60 bars, where the hydrogen uptake is increased at about 35% for nickel oxide and 25% for magnesium oxide. Computer simulation reveals that under the external electric field, the electron cloud of hydrogen molecules is pulled over to the adsorbent site and can overlap with the adsorbent electrons, which in turn enhances the adsorption energy Experiments were also carried out to examine the effects of hydrogen spillover with charge induced enhancement. The results show that the overall storage capacity in nickel oxide increased remarkably by a factor of 4

Final Report: Metal Perhydrides for Hydrogen Storage

Hydrogen is a promising energy source for the future economy due to its environmental friendliness. One of the important obstacles for the utilization of hydrogen as a fuel source for applications such as fuel cells is the storage of hydrogen. In the infrastructure of the expected hydrogen economy, hydrogen storage is one of the key enabling technologies. Although hydrogen possesses the highest gravimetric energy content (142 KJ/g) of all fuels, its volumetric energy density (8 MJ/L) is very low. It is desired to increase the volumetric energy density of hydrogen in a system to satisfy various applications. Research on hydrogen storage has been pursed for many years. Various storage technologies, including liquefaction, compression, metal hydride, chemical hydride, and adsorption, have been examined. Liquefaction and high pressure compression are not desired due to concerns related to complicated devices, high energy cost and safety. Metal hydrides and chemical hydrides have high gravimetric and volumetric energy densities but encounter issues because high temperature is required for the release of hydrogen, due to the strong bonding of hydrogen in the compounds. Reversibility of hydrogen loading and unloading is another concern. Adsorption of hydrogen on high surface area sorbents such as activated carbon and organic metal frameworks does not have the reversibility problem. But on the other hand, the weak force (primarily the van der Waals force) between hydrogen and the sorbent yields a very small amount of adsorption capacity at ambient temperature. Significant storage capacity can only be achieved at low temperatures such as 77K. The use of liquid nitrogen in a hydrogen storage system is not practical. Perhydrides are proposed as novel hydrogen storage materials that may overcome barriers slowing advances to a hydrogen fuel economy. In conventional hydrides, e.g. metal hydrides, the number of hydrogen atoms equals the total valence of the metal ions. One LiH molecule contains one hydrogen atom because the valence of a Li ion is +1. One MgH2 molecule contains two hydrogen atoms because the valence of a Mg ion is +2. In metal perhydrides, a molecule could contain more hydrogen atoms than expected based on the metal valance, i.e. LiH1+n and MgH2+n (n is equal to or greater than 1). When n is sufficiently high, there will be plenty of hydrogen storage capacity to meet future requirements. The existence of hydrogen clusters, Hn+ (n = 5, 7, 9, 11, 13, 15) and transition metal ion-hydrogen clusters, M+(H2)n (n = 1-6), such as Sc(H2)n+, Co(H2)n+, etc., have assisted the development of this concept. Clusters are not stable species. However, their existence stimulates our approach on using electric charges to enhance the hydrogen adsorption in a hydrogen storage system in this study. The experimental and modeling work to verify it are reported here. Experimental work included the generation of cold hydrogen plasma through a microwave approach, synthesis of sorbent materials, design and construction of lab devices, and the determination of hydrogen adsorption capacities on various sorbent materials under various electric field potentials and various temperatures. The results consistently show that electric potential enhances the adsorption of hydrogen on sorbents. NiO, MgO, activated carbon, MOF, and MOF and platinum coated activated carbon are some of the materials studied. Enhancements up to a few hundred percents have been found. In general, the enhancement increases with the electrical potential, the pressure applied, and the temperature lowered. Theoretical modeling of the hydrogen adsorption on the sorbents under the electric potential has been investigated with the density functional theory (DFT) approach. It was found that the interaction energy between hydrogen and sorbent is increased remarkably when an electric field is applied. This increase of binding energy offers a potential solution for DOE when looking for a compromise between chemisorption and physisorption for hydrogen storage. Bonding of chemisorption is too strong and requires high temperature for the release of hydrogen. Bonding for the physisorption is too weak for sufficient uptake of hydrogen. Electric field potentials can enhance the physisorption and can be adjusted to yield reversibility required in a system at room temperature

Quantenchemische Untersuchungen von Photokatalysatoren auf der Basis binärer und ternärer Übergangsmetallverbindungen

In dieser Arbeit wurden die absoluten Bandlagen von verschiedenen niedrigindizierten Oberflächen der Systeme TiO$_2$, TaON und Ta$_3$N$_5$ mittels DFT-Methoden berechnet. Dabei wurden folgende Funktionale verwendet: PBE, PBE0, PW1PW, HSE06 und HISS. Bis auf das HISS-Funktional wurde zusätzlich die Auswirkung einer Dispersionskorrektur berücksichtigt. Für die Berechnungen wurde das CRYSTAL-Programmpaket verwendet. Die Bandlagen relativ zum Vakuumreferenzniveau für die freien Oberflächen wurden experimentellen Ergebnissen gegenübergestellt. Zusätzlich wurde der Einfluss der Adsorption von intakten und dissoziierten Wassermolekülen auf ausgewählten Oberflächen untersucht. Durch den Vergleich mit experimentellen und theoretischen Arbeiten wurde eine geeignete Methode und Vorgehensweise für die Vorhersage der photokatalytischen Aktivität dieser Oberflächen gefunden. Zu den Vorarbeiten für diese Untersuchungen gehörte die Berechnung energetischer und struktureller Eigenschaften der betrachteten Oberflächen. Neben der erstmaligen systematischen Untersuchung der niedrigindizierten Oberflächen von TaON und Ta$_3$N$_5$ wurde auch eine hypothetische Inversion der Anionenpositionen bei TaON untersucht. Ebenfalls wurde für alle Oberflächen das Konvergenzverhalten von Oberflächenenergie, Relaxation und der Bandpositionen in Abhängigkeit von der Schichtanzahl untersucht. Abschließend wurde Magnesiumperoxid als binäres Hauptgruppenmetalloxid untersucht, da es bei der elektrochemischen Sauerstoffreduktion auf den Kathodenoberflächen von Mg-Luft-Batterien eine Rolle spielt. Zuvor wurden umfangreiche Methoden- und Basissatztests an der Referenzverbindung MgO durchgeführt, um die Verlässlichkeit der verwendeten Methoden zu prüfen. MgO und MgO$_2$ wurden insgesamt mit sieben verschiedenen Methoden untersucht. Erstmals wurden zwei niedrigindizierte Oberflächen von MgO$_2$, (001) und (011), strukturell und elektronisch charakterisiert. Für die (001)-Oberfläche wurden die Bindungsenergien neutraler Sauerstoffdefekte berechnet und mit denen der (001)-Oberfläche von MgO verglichen