Michael Felderhoff

Chemical and Physical Solutions for Hydrogen Storage

Hydrogen is a promising energy carrier in future energy systems. However, storage of hydrogen is a substantial challenge, especially for applications in vehicles with fuel cells that use proton-exchange membranes (PEMs). Different methods for hydrogen storage are discussed, including high-pressure and cryogenic-liquid storage, adsorptive storage on high-surface-area adsorbents, chemical storage in metal hydrides and complex hydrides, and storage in boranes. For the latter chemical solutions, reversible options and hydrolytic release of hydrogen with off-board regeneration are both possible. Reforming of liquid hydrogen-containing compounds is also a possible means of hydrogen generation. The advantages and disadvantages of the different systems are compared.

High Temperature Metal Hydrides as Heat Storage Materials for Solar and Related Applications

For the continuous production of electricity with solar heat power plants the storage of heat at a temperature level around 400 °C is essential. High temperature metal hydrides offer high heat storage capacities around this temperature. Based on Mg-compounds, these hydrides are in principle low-cost materials with excellent cycling stability. Relevant properties of these hydrides and their possible applications as heat storage materials are described.

Investigation of hydrogen discharging and recharging processes of Ti-doped NaAlH4 by X-ray diffraction analysis (XRD) and solid-state NMR spectroscopy

Abstract The processes occurring in the course of two sequential hydrogen discharging and recharging cycles of Ti-doped sodium alanate were investigated in parallel using XRD analysis and solid-state NMR spectroscopy. Both methods demonstrate that in hydrogen storage cycles ( Eq. (1) ) the majority phases involved are NaAlH 4 , Na 3 AlH 6 , Al and NaH. Only traces of other, as yet unidentified phases are observed, one of which has been tentatively assigned to an Al–Ti alloy on the basis of XRD analysis. The unsatisfactory hydrogen storage capacities heretofore observed in cycle tests are shown to be due entirely to the reaction of Na 3 AlH 6 with Al and hydrogen to NaAlH 4 ( Eq. (1) , 2nd hydrogenation step) being incomplete. Using XRD and NMR methods it has been shown that a higher level of rehydrogenation can be achieved by adding an excess of Al powder.

Solid-state hydrogen storage for mobile applications: Quo Vadis?

In times of severe shortage of fossil fuels new strategies have to be developed to assure future mobility. Fuel cell driven automotives with hydrogen as an energy carrier is one alternative discussed for the substitution of gasoline in the long term. Both the generation as well as the storage of hydrogen are technical challenges which have to be solved before hydrogen technology can be a real alternative for mobile applications. This perspective paper highlights the state-of-the art in the field of hydrogen storage, especially in solids, including the technical limitations. New potential research fields are discussed which may contribute to future energy supply in niche applications.

Nanostructured Ti-catalyzed MgH2 for hydrogen storage

Nanocrystalline Ti-catalyzed MgH(2) can be prepared by a homogeneously catalyzed synthesis method. Comprehensive characterization of this sample and measurements of hydrogen storage properties are discussed and compared to a commercial MgH(2) sample. The catalyzed MgH(2) nanocrystalline sample consists of two MgH(2) phases-a tetrahedral β-MgH(2) phase and an orthorhombic high-pressure modification γ-MgH(2). Transmission electron microscopy was used for the observation of the morphology of the samples and to confirm the nanostructure. N(2) adsorption measurement shows a BET surface area of 108 m(2) g(-1) of the nanostructured material. This sample exhibits a hydrogen desorption temperature more than 130 °C lower compared to commercial MgH(2). After desorption, the catalyzed nanocrystalline sample absorbs hydrogen 40 times faster than commercial MgH(2) at 300 °C. Both the Ti catalyst and the nanocrystalline structure with correspondingly high surface area are thought to play important roles in the improvement of hydrogen storage properties. The desorption enthalpy and entropy values of the catalyzed MgH(2) nanocrystalline sample are 77.7 kJ mol(-1) H(2) and 138.3 J K(-1) mol(-1) H(2), respectively. Thermodynamic properties do not change with the nanostructure.

Molecular Suppression of the Pimerization of Viologens (=4,4′-Bipyridinium Derivatives) Attached to Nanocrystalline Titanium Dioxide Thin-Film Electrodes

Twelve viologens, i.e., 4,4′-bipyridinium derivatives 1 – 12, were examined for their use as electrochromic material when attached to nanocrystalline titanium dioxide thin-film electrodes. Eight of these (1 – 4, 7 – 8, 10, and 12) are new, and their synthesis is included. The modifier compounds consist of one to four bipyridinium subunits with linear or dendritic architecture, equipped with one to three TiO2-anchoring phosphonate groups. They are tailored for high electrochromic dynamics (large absorbance change upon reduction) and low extent of pimerization (=charge-transfer (CT) complexation of viologen cation radicals). A new graphical method is presented for the discrimination of simple dilution phenomena and more complex structural effects on the extent of pimerization in the surface-attached viologen layer.

Thermochemical Heat Storage for High Temperature Applications – A Review

Abstract Heat storage for high temperature applications can be performed by several heat storage techniques. Very promising heat storage methods are based on thermochemical gas solid reactions. Most known systems are metal oxide/steam (metal hydroxides), carbon dioxide (metal carbonates), and metal/hydrogen (metal hydrides) systems. These heat storage materials posses high gravimetric and volumetric heat storage densities and because of separation of the reaction products and their storage in different locations heat losses can be avoided. The reported volumetric heat storage densities are 615, 1340 and 1513 [kWh m−3] for calcium hydroxide Ca(OH)2, calcium carbonate CaCO3 and magnesium iron hydride Mg2FeH6 respectively. Additional demands for gas storage decrease the heat storage density, but metal hydride systems can use available hydrogen storage possibilities for example caverns, pipelines and chemical plants.

Hydrogen storage in complex metal hydrides

Complex metal hydrides such as sodium aluminohydride (NaAlH4) and sodium borohydride (NaBH4) are solid-state hydrogen-storage materials with high hydrogen capacities. They can be used in combination with fuel cells as a hydrogen source thus enabling longer operation times compared with classical metal hydrides. The most important point for a wide application of these mate- rials is the reversibility under moderate technical conditions. At present, only NaAlH4 has favourable thermodynamic properties and can be employed as a ther- mally reversible means of hydrogen storage. By contrast, NaBH4 is a typical non- -reversible complex metal hydride; it reacts with water to produce hydrogen.

Complex Rare-Earth Aluminum Hydrides: Mechanochemical Preparation, Crystal Structure and Potential for Hydrogen Storage

A novel type of complex rare-earth aluminum hydride was prepared by mechanochemical preparation. The crystal structure of the REAlH(6) (with RE = La, Ce, Pr, Nd) compounds was calculated by DFT methods and confirmed by preliminary structure refinements. The trigonal crystal structure consists of isolated [AlH(6)](3-) octahedra bridged via [12] coordinated RE cations. The investigation of the rare-earth aluminum hydrides during thermolysis shows a decrease of thermal stability with increasing atomic number of the RE element. Rare-earth hydrides (REH(x)) are formed as primary dehydrogenation products; the final products are RE-aluminum alloys. The calculated decomposition enthalpies of the rare-earth aluminum hydrides are at the lower end for reversible hydrogenation under moderate conditions. Even though these materials may require somewhat higher pressures and/or lower temperatures for rehydrogenation, they are interesting examples of low-temperature metal hydrides for which reversibility might be reached.

Development of Hydrogen Storage Tank Systems Based on Complex Metal Hydrides

This review describes recent research in the development of tank systems based on complex metal hydrides for thermolysis and hydrolysis. Commercial applications using complex metal hydrides are limited, especially for thermolysis-based systems where so far only demonstration projects have been performed. Hydrolysis-based systems find their way in space, naval, military and defense applications due to their compatibility with proton exchange membrane (PEM) fuel cells. Tank design, modeling, and development for thermolysis and hydrolysis systems as well as commercial applications of hydrolysis systems are described in more detail in this review. For thermolysis, mostly sodium aluminum hydride containing tanks were developed, and only a few examples with nitrides, ammonia borane and alane. For hydrolysis, sodium borohydride was the preferred material whereas ammonia borane found less popularity. Recycling of the sodium borohydride spent fuel remains an important part for their commercial viability.