Issue No. 2, August

Abstract

This paper reviews two-step water splitting cycles by redox iron-based metal oxides to produce hydrogen utilizing concentrated solar high-temperature heat as energy source. The concentrated solar radiation is focused upon a windowed solar reactor where maximum temperatures can exceed 1500˚C in the greatly insolated “sun-belt” regions such as the South-western United States, southern Europe, north Australia, etc. The concentrated solar high-temperature heat has the potential to produce hydrogen from water thermochemically. A two-step thermochemical water splitting by iron-oxide redox pair is one of the most promising cycle. However, the cycle requires heat above 1500 K minimum. This provides a challenge to “windowed” solar reactor concepts, such as a monolithic device reactor, a foam-device reactor, a Counter-Rotating-Ring device reactor, and an internally-circulating fluidized bed reactor. In the reactors, the kinetics of the reactions must be an important key factor. Thus, further development of reactive redox materials and devices will have a very large impact on the improvement of these reactors. This paper reviews the working redox materials, the reactive devices and the reactors developed for the two-step thermochemical water splitting cycle by a redox iron-based oxide.

Abstract

In the presence of supercritical water, organic materials are converted into energetic gas, with high hydrogen content. In the present application, thermochemical conversion of humid wastes from wine or sugar production is reached at temperatures and pressures up to 550°C and 250 bars. Material is a key point for the development of SCWG process operating in such conditions. In supercritical water the durability of metals and alloys is limited by corrosion, but ceramics and ceramic composites are known to be stable under extreme conditions. For this reason, we have proceeded to an examination of a selection of ceramic materials that could be used as an anticorrosive wall of the gasification reactor. We show that, ultra-high temperature ceramics Si₃N₄, BN, Al₂O₃ and SiC are strongly corroded by humid waste, but carbon-based materials are resistant in both subcritical and supercritical conditions in the presence of pure or waste water. As a conclusion, graphite may be a very good material for SCWG reactor.

Abstract

Metal supported cells as developed at DLR for use as solid oxide fuel cells by applying plasma deposition technologies were investigated in operation of high temperature steam electrolysis. The cells consisted of a porous ferritic steel support, a diffusion barrier layer, a Ni/YSZ fuel electrode, a YSZ electrolyte and a LSCF oxygen electrode. During fuel cell and electrolysis operation the cells were electrochemically characterised by means of i-V characteristics and electrochemical impedance spectroscopy measurements including a long-term test over 2000 hours. The results of electrochemical performance and long-term durability tests of both single cells and single repeating units (cell including metallic interconnect) are reported. During electrolysis operation at an operating temperature of 850 °C a cell voltage of 1.28 V was achieved at a current density of -1.0 A cm^-2; at 800 °C the cell voltage was 1.40 V at the same operating conditions. The impedance spectra revealed a significantly enhanced polarisation resistance during electrolysis operation compared to fuel cell operation which was mainly attributed to the hydrogen electrode. During a long-term test run of a single cell over 2000 hours a degradation rate of 3.2% per 1000 hours was observed for operation with steam content of 43% at 800 °C and a current density of - 0.3 Acm^-2. Testing of a single repeating unit proved that a good contacting of cell and metallic interconnect is of major importance to achieve good performance. A test run over nearly 1000 hours showed a remarkably low degradation rate.

Abstract

Over the past several decades, many coating techniques have been developed for titania thin and thick film fabrication. Of these, electrophoretic deposition (EPD), anodic oxidation, gel oxidation, spray pyrolysis, ultrasonic spray pyrolysis, sol-gel spin coating, aerosol spraying, and sol-gel dipcoating have become of interest owing to the low cost of operation and the non-necessity of the use of a vacuum system. The present work briefly reviews some of the activities of the authors in the production of titania films.

Abstract

By Raman spectroscopy we provide direct evidence for successive exchange of D atoms in the tetrahedral BH₄ units of the molecular solid lithium borohydride. We prove the coexistence of all isotopomers by deconvolution of the D-stretching vibrations band of Raman spectra at 83 K and 5 K in partially D-exchanged LiBH₄ and comparison with first-principles Raman intensity calculations. This implies net transport of atomic hydrogen in LiBH₄ below the melting temperature. Tracer diffusion experiments of LiBH₄/LiBD₄ diffusion couples show a fast macroscopic diffusion of BH₄ ions of D ~ 7 x 10^-14 m²/s at 473 K. The direct exchange rate of hydrogen between BH₄ units is 10 orders of magnitude slower, i.e. the relatively fast effective hydrogen diffusion has its origin in the fast diffusion of BH₄ units. An effective tracer diffusion coefficient of deuterium in LiBH₄ of D ~ 7 x 10^-14 m²/s at 473 K is derived. We discuss the interdependencies of the diffusion of cations (Li⁺) and anions (BH₄ ^-).

Abstract

Lithium imide (Li₂NH) and amide (LiNH₂) belong to the Li-H-N system, which has been recently considered for on-board hydrogen storage applications. However the imide lowtemperature crystal structure is still highly controversial, with at least six options compatible with the diffraction experimental findings. A complementary study on low-temperature Li₂NH and LiNH₂ has been recently accomplished by the authors using neutron spectroscopy (with energy transfer in the 3-500 meV range). The rationale of these measurements was that crystal structures (especially their proton arrangements) affect in a strong way the neutron scattering spectra, so that a combined use of computer ab-initio simulations and inelastic neutron scattering could be a stringent validation method for the various models. Data analysis has pointed out broad and almost featureless proton-projected phonon densities of states for lithium imide, with large differences in the data sets derived from forward scattering and backscattering detector banks. On the contrary, a sharp phonon spectrum and much less discrepancy was found applying the same analytic procedure to lithium amide. This Li₂NH peculiarity has been interpreted as an effect of the fast proton jump diffusion among the available lattice sites, which smears out the phonon vibrational excitations in a momentum transfer-dependent way.

Abstract

Hydrogen storage in the solid state has shown increasing research and development, and recently an approach in mixing two hydride systems together by ball milling (reactive hydride composites) has been investigated in more detail, e.g. NaBH₄ plus MgH₂. Thermodynamic destabilization may occur by new compounds formation during dehydrogenation, e.g. MgB₂. A study of the the role of O₂/H₂O contamination for the reaction 2NaBH₄ + MgH2 ↔ 2NaH + MgB₂ + 4H₂ was conducted using in-situ X-ray powder diffraction. Desorption reaction is observed to begin by a competition of MgH₂ and NaBH₄ decomposition due to higher reactivity promoted by ball milling processing summed to O₂/H₂O contamination. Oxidation of NaBH₄ into NaBO₂ is observed  to happen in higher degree than MgH₂/Mg into MgO for the Na-Mg-B-H system.

Abstract

To address the issues of poor thermal conductivity and fragmentation of metal hydride particles undergoing hydriding/dehydriding reactions, a metal hydride-based composite material was developed. The active metal phase was embedded in a silica matrix and agraphite filler was incorporated by ball milling. A set of compact pellet samples at different composition were prepared and tested. Experimental data obtained from the thermal conductivity measurements shown that using powder graphite produced a quite linear increase in the thermal conductivity of the metal hydride –silica composite. Ongoing studies include composition optimization as well as longterm testing upon cycling of such metal hydride composites to evaluate their potentiality in technological hydrogen storage applications.

Abstract

One of the main challenges in the perspective of a hydrogen economy is the development of a storage system both safe and with high weight capacity. Among the most promising systems are the storage in metals and chemical hydrides and the high pressure storage in tanks made of composite materials. Both these technologies allow volumetric densities equal or higher than that of liquid hydrogen.
The present work deals with the results obtained in a Italian national project, whose objectives have been the development of innovative technologies in specific applications: large scale energy storage, stationary applications in distributed generation, and automotive (with a particular attention to the fluvial and the sea transportation in protected areas).
The theoretical, modellistic and experimental activities have been oriented to the development of innovative high capacity metal hydrides, the study of a regeneration method for chemical hydrides, the integration of intermediate pressure electrolyzers with advanced compressors and, finally, the development of thermomechanical models for executive design of storage systems. A number of prototypes has been realised and installed in a test facility in the Fusina (Venezia) power plant. The activity has been completed with an executive feasibility evaluation, in the perspective of industrial applications.

Abstract

Microstructure and catalytic properties of new nanometric-scale Ni powders produced by a carbonyl nickel chemical vapour deposition process, have been investigated in respect to kinetic destabilization of LiAlH₄ for hydrogen release during continuous heating conducted in a directpower-compensation differential scanning calorimeter. The endothermic only heat flow was observed for the nanonickiel-catalyzed LiAlH₄, which was prepared by short-time milling of 5 wt% nano-Ni with Li alanate. The endothermic heat that occurs around 160°C corresponds well to release 4.4 wt% H² into a 1 bar reservoir, as determined in the Hiden Model IGA gravimetric gas analyzer.
None of the three thermal events can be attributed to melting, and even 15 min milling resulted in receding of the melting peak at 170° C in the as-received LiAlH⁴. Therefore, nanonickel-catalyzed LiAlH⁴ desorbs hydrogen from solid crystal, without melting and with no exothermic desorption step reported in many previous studies. These results are discussed in terms of a catalytic promotion of thermodynamic destabilization that can occur in complex hydrides.

Abstract

Ab initio density functional calculations were performed on a toroidal carbon C₁₂₀ nanostructure with a single beryllium atom bonded to its outer surface. These calculations are based on DFT with the generalized gradient approximation PW91 (Perdew and Wang) as implemented in the Materials Studio v.4.3 code. The Dmol³ module was used to calculate, among others, total energy, charge density, HOMO-LUMO and Mulliken population analysis. On the basis of these results, the beryllium-coated toroidal carbon C120 nanostructure appears to be a good candidate for H₂ storage with moderate adsorption energy.

Abstract

The search for efficient hydrogen-storage materials has led to an increasing interest in hydrogen clathrate hydrates, since it has been demonstrated that an appreciable amount of molecular hydrogen can be stored in the water cages and released at melting. Different synthetic routes have been followed to maximize the quantity of trapped hydrogen and to speed up the kinetics of the clathrate formation.
Here, we describe two different synthetic routes for the production of hydrogen clathrate hydrates. Then we present the results of inelastic neutron scattering and Raman light scattering experiments on simple (i.e. containing only hydrogen) and binary (i.e. with a second guest molecule) clathrates. For each class of compounds, we have obtained spectroscopic information on the motion of hydrogen inside the cages, on the occupancy of the cages by hydrogens, and on lattice dynamics. Finally, we have investigated the clathrate crystal stability and the hydrogen release as a function of temperature by means of neutron diffraction.