At the Florida Solar Energy Center (FSEC), a research program is underway for developing a high-density hydrogen storage system based on amine-borane (AB) complexes. Due to their high hydrogen capacity, these hydrides have been employed, in the past, as disposable hydrogen sources for fuel cell applications. However, to meet the requirements for hydrogen storage onboard vehicles, it is essential that cost effective and energy efficient methods for the regeneration (i.e. hydrogenation) of the spent (dehydrogenated) AB complexes can be found that utilize only hydrogen and/or electricity (i.e. the only plausible hydrogen economy energy carriers).

We are studying two ammoniaborane (NH3BH3)-based systems with high hydrogen storage capacity. The first system employs a borazine-cyclotriborazane cycle. Borazine is a product of NH3BH3 thermolysis. Cyclotriborazane is the inorganic analog of cyclohexane. The second system employs polymeric AB complexes such as poly-(aminoborane) and polyborazylene. Poly-(aminoborane), an inorganic analog of polyethylene, is also a product of amoniaborane thermolysis whilepolyborazylene is the product of borazine thermolysis.

For the two systems above, we are developing regeneration (i.e. reduction of borazine, poly-(aminoborane) and polyborazylene) schemes based on: 1) catalytic hydrogenation and 2) indirect (multi-step) synthesis techniques.

The attribute of solid state hydrogen storage materials that is most commonly the focus of evaluations is reversible hydrogen weight percent. Other material characteristics, including density, charging pressure, enthalpy and conductivity can influence the weight of storage system components and hence the overall hydrogen weight percent that is ultimately of interest. However, accounting for these effects involves some level of storage system representation that typically is not undertaken when making material assessments and comparisons.

The current paper will present a simple model that represents system elements and trade-offs on a high level so that overall system performance can be estimated without the burden of detailed design studies. The model should be useful to evaluate novel materials in a more complete manner for a better assessment of their potential when implemented in a storage system. While the model has been derived based on the design of a particular NaAlH4 system, the key attributes are sufficiently general to be applicable to a range of system designs. Using this approach, the properties of materials can be related more precisely to goals for overall system performance with modest additional effort.

The effective hydrogen capacity of TixCr2-yMny [≫X λ1.1 (1.08≤0x≤1.16), y λ1.0 (0.96≤y≤1.08)rsqb; exhibited the maximum value of 1.8 wt% in the pressure range of 33 MPa and 0.1 MPa at 296K (dissociation pressure: 5-11 MPa), and the alloy provided over 10% more capacity than conventional Ti-Cr-Mn (Ti1.2CrMn: 1.6 wt%, Ti1.2Cr1.9Mn0.1: 1.3 wt%). At the low temperature of 233 K, the alloy absorbed 2.0 wt% of hydrogen and the hydrogen desorption capacity at 0.1 MPa was 1.6 wt%. The dissociation pressure decreased with the Ti and the Mn contents and was explained by the function of the bulk modulus and the cell volume. According to the van't Hoff plots, the standard enthalpy differences (heat of formation) of the Ti1.16Cr0.92Mn1.08 and Ti1.08Cr1.04Mn0.96 hydrides were -21 and -22 kJ/molH2, respectively. These absolute values were about 10 kJ/molH2 smaller than those of LaNi5 and Ti-Cr-V. The alloy had sufficient hydriding and dehydriding kinetics. In the pressure range of 33 MPa and 0.1 MPa at 296 K, the alloy absorbed and desorbed 1.8 wt% of hydrogen in 60 sec and 300 sec, respectively. The hydrogen capacity changed gradually over many cycles and that after 1000 cycles was 94 % of the initial capacity. Thus Ti1.1CrMn can be utilized for a high- pressure MH tank which contains a hydrogen absorbing alloy with high dissociation pressure and compressed hydrogen.

Presence of ∼2-4 % Ti is critical for reversible hydrogenation/rehydrogenation in NaAlH4. We have investigated the probable catalytic role of Ti in this complex multi-step process. The present part of our study concentrates on the rehydrogenation reaction, i.e., the reverse reaction that forms NaAlH4 from its constituent binary hydrides. First principles calculations using density functional theory (DFT) show that a particular arrangement of Ti atoms on the surface of Al metal promotes the chemisorption of molecular hydrogen. We also present comparisons with existing experimental data (EXAFS and TEM) to support the existence of such an arrangement on the surface.

Influence of hydrogen intercalation on indium monoselenide is investigated measuring electrical properties of based on it such photosensitive structures as metal-insulator-semiconductor (MIS) and Mott diode (MD). It is established that intercalation of the basic semiconductor with hydrogen leads to the improvement of diodic properties of the MIS structures whereas those for the MD structure becomes worse. For the both structures charge transfer mechanisms are analyzed on the basis of their band diagrams.

The inclusion of multiwalled carbon nanotubes in a conjugated polymer matrix results in extensive alterations in the polymer morphology. When the physical state of a substance is changed, heat is either absorbed or liberated but the temperature remains constant. The flexibility of chain molecules arises from rotation round the saturated chain bond moreover the potential energy barriers hindering this rotation. It is not surprising therefore that the flexibility of polymer chains is an important factor in determining their melting points and stability. If the substitution of carbon nanotube is random the primary effect is a decrease in the degree of crystallinity. These microstructures are governed by the balance of interactions between hydrodynamic forces (both viscous and elastic) and the forces working to retain the integrity of the disperse particles, such as interfacial tension or, in the case of solid filler and their mechanical strength. Carbon materials have long been shown to absorb as much as 60 wt% hydrogen due to their large surface areas as well as their high surface to volume ratios. In the present approach conducting polyaniline was doped with metal oxide, such as SnO2, as well as carbon nanotubes. The resulting carbon nanocomposite was made in both gel- and solid-state to study the effect of physical state on hydrogen adsorption and desorption by weight percentage. The morphology formation process and its impact on the rheological properties of complex polymer systems, i.e. polymer blends and composites are being studied. The materials are also compared with regard to their thermal stability using DSC, and further characterized using various techniques such as FTIR. Further experiments are in progress to better understand the nature of the hydrogen storage mechanism.

Thermal treatment of aqueous streams loaded with organics can be efficiently performed at pressures and temperatures above the critical data for water (Pc = 22.1 MPa, Tc = 374 °C). Two applications are under investigation using supercritical water (SCW) as solvent and reactant: supercritical water oxidation (SCWO) and supercritical water gasification (SCWG).

SCWO is typically operated at 25-35 MPa and 600-900 °C, because water, oxygen (or air), CO2 and most of the organics form a single fluid phase with rapid oxidation kinetics. Thus, SCWO can be processed with high space-time yield and in some cases self-sustaining.

Moreover, expensive off-gas treatment is prevented because NOx formation is suppressed. Other heteroatoms form acids like HCl, H2SO4 and H3PO4 or their corresponding salts. However, acids may lead to corrosion, formation or presence of salts to plugs.

To avoid these problems a transpiring wall reactor (TWR) has been developed and installed. Results of SCWO of different industrial effluents are very promising. Destruction of the organic waste compounds was close to 100 %, even for effluents containing solids and salts up to 5%wt., each.

In accompanied studies material tests have been performed. Long time runs clearly indicate that alloy 625 is most suited to withstand the aggressive environment at temperatures higher than about 500 °C. A corrosion mechanism has been proposed.

The SCWG process of biomass is performed under SCW conditions. The aim of this work is to study the conversion of biomass (e.g.) to fuel gas with high energetic value. R&D is focused on process optimization particularly with respect to energy efficiency as well as applicability to extended feedstocks even with high amounts of solids. At SCW conditions organic matter reacts with water to form a hydrogen containing gas. The feed carbon is converted preferentially to CO2, which can be separated by e.g. stripping, and to methane. While the organic carbon is oxidized to CO2, water is reduced to from hydrogen, e.g. for glucose:

C6H12O6 + 6 H2O → 6 CO2 + 12 H2

In basic studies of the biomass gasification, the main reaction pathways were identified. In addition the influence of ingredients of biomass and additives / catalysts was investigated. Here the influence of salts is of special interest, because biomass with an high water content, which is most suitable for the SCWG process, usually contain high salt contents. The changes in selectivity of different reaction pathways observed, opens the possibility to manipulate the chemistry of biomass gasification in supercritical water. Therefore high gas yields with different feedstock of various compositions can be achieved.

SCWG process is performed using efficient heat exchangers at SCW conditions.

Compression work is low since non compressible water slurry is pressurized. The reaction of the organic substances with water proceeds fast and completely, thus with high space-time yield. At about 600 °C and 25 MPa high gasification yield can be achieved. Advantageously, formation of CO, tar and char is low enhancing the efficiency of the SCWG process.

Consequently, based on lab scale plants, a 100 kg/h SCWG plant called Verena has been installed and is operated since few years. A high thermal efficiency of about 80% for diluted educt streams (only 5 wt% OM) has been measured. Experimental results with the Verena plant confirmed the production of a hydrogen rich gas and the high thermal efficiency of the process.

Two ternary alloys based on III-nitride semiconductor alloys are explored as potential components of photoelectrochemical cells (PECs) for the direct generation of hydrogen using solar energy. For In1-xGaxN, it will be shown using prior measurements of band offsets that spontaneous water splitting can occur for x up to 0.2 and potentially higher. Flat band potential and photocurrent measurements from an n-type epilayer with x = 0.37 will be presented. This initial data appears to indicate that the flat band potential lies just below the H+/H2 from pH 0 – 14. In the case of GaAsxN1-x we will demonstrate that the replacement of a few percent of As in N sublattice drives the bandgap down from the GaN value (3.4 eV) into a range that is attractive for PEC cells [1]. This band gap reduction is explained by the valence band anticrossing that pushes the valence band maximum up initially by 0.5 eV. From the point of view of a PEC cell, this reduces the gap (desirable for efficiency) without compromising the desired H+/H2 overpotential.

The development of light weight hydrogen storage systems with high volumetric and gravimetric hydrogen densities is indeed essential for the on-board fuel cell vehicular applications [1]. Among the different hydrogen storage systems designed and developed so far, Ti- doped sodium aluminum hydrides exhibit potential promise of reversible hydrogen storage capacity (4-5 wt.%) at moderate temperatures [2,3]. However, the poor cyclic stability of these hydrides due to the partial reversibility of the two step reactions necessitates the development of exotic materials or tailoring the known hydride systems. On the other hand, transition metal complex hydrides, TMHx (T = Mg; M = Fe, Co, Ni) have also been identified as potential candidates for hydrogen storage [4-6] and/or analogous to alanates [7]. These hydrides especially Mg2FeH6, have shown excellent cyclic capacities (more than 500 cycles) even without a catalyst [8]. Besides, Mg2FeH6 possesses the highest volumetric and gravimetric hydrogen densities of 150 kg/m3 and 5.6 wt.% respectively [9]. However, at low temperatures, the rate of release of hydrogen and the effective reversible hydrogen capacity seems poor. Recent reports declared that the enhancement in the cycling kinetics and reduction in the operating temperature is very much possible by using a distorted nano-scale Mg structure [10, 11], doping the host lattice with Ti- species and/or lattice substitution [12]. Keeping these facts in view, the present investigation aims to improve the sorption kinetics and thermodynamics of Mg2FeH6, by 1) preparing nano-scale Mg-Fe-H system using mechano-chemical synthesis process, 2) surface localized catalyst (Ti- species) doping and 3) cationic substitution of Na+/Li+ for Mg2+ by incorporating NaH/LiH. The synergistic behavior of the tailored nano-scale transition metal complex for hydrogen storage is outlined.

A new type of hydrogen-absorbing alloy tank has been developed. The high-pressure metal hydride (MH) tank has been designed based on a 35 MPa cylinder vessel. The heat exchanger module is integrated into the tank. Its advantage over high-pressure cylinder vessels is its large hydrogen storage capacity, for example 7.3 kg with a tank volume of 180 L. Cruising range is about 2.5 times longer than that of a 35 MPa cylinder vessel system with the same volume.

The hydrogen-charging rate of this system is equal to the 35 MPa cylinders without any external cooling facility. Furthermore, release of hydrogen at 243 K is enabled due to the use of a hydrogen-absorbing alloy with a high disassociation pressure, Ti-Cr-Mn alloy with AB2 laves phase. It is thought that the high-pressure MH system is one realistic option for fuel cell vehicles to achieve a cruising range of over 700 km.

Ti doped NaAlH4 hydride is proposed as a reversible hydrogen storage material. In this work, the microstructure of NaAlH4 with 2% TiCl3 additive was studied after 5 hydrogen cycles using a combination of transmission electron microscopy (TEM) techniques including energy dispersive spectroscopy (EDS) X-ray analysis. Selected area diffraction and high-resolution (HR) imaging confirmed the presence of the NaH phase in the material. Electron diffraction was dominated by Al. HRTEM showed the presence of edge dislocations, which might influence the hydrogen diffusivity process in these materials.

Mg-based thin films were successfully grown by Pulsed Laser Deposition. Dramatic optical changes were observed depending on the conditions of deposition. Films grown in vacuum were shiny metallic whereas the ones grown in a Ar/H2 gas mixture were highly transparent. Ex situ hydrogenation, by annealing the films at 200 °C in 15 bars of hydrogen, led to similar metallictransparent transformation. Mg-Cx (x ≤ 20 %) films show a faster hydrogenation associated with a significant decrease in oxygen content by carbon addition.

Sodium alanate has been studied as a promising candidate material for reversible hydrogen storage due to its intermediate temperature range and relatively high storage capacity. Its rates of desorption and absorption of hydrogen have been shown to be enhanced by the addition of Ti in various compounds. To date, the sorption kinetics, especially absorption kinetics, is not well understood. In this study, a practical sorption kinetics model for TiCl3 catalyzed NaAlH4 has been developed to assist in the engineering design and evaluation of a prototype hydrogen storage system.