The smallest element, hydrogen, can be the key to the implementation of a CO2-free society. Hydrogen can readily be prepared from renewable sun, wind, or water energy by electrolytic water splitting. The energy stored in the produced hydrogen can be released by reacting with oxygen, and water is the only exhaust gas. Thus, the hydrogen based energy system is clean and environmentally friendly. Hydrogen can store fluctuating renewable energy so that it can be used at another place and time than where and when it was harvested. However, hydrogen is a gas at ambient conditions and new methods for hydrogen storage are important in particular for mobile applications, such as a car. We have recently discovered a variety of new types of hydrides, which contain large amounts of hydrogen, which is a new approach for storage of huge amounts of renewable energy.
Hydrogen for Storage of Renewable Energy – A Sustainable Energy System
Clearly, the consequences of burning fossil fuels, i.e. coal, oil, and natural gas are increasing levels of carbon dioxide in the atmosphere along with local air pollution and smog. There is increasing evidence suggesting that this may lead to global climate changes. This is the primary reason for the increasing interest in development of a new energy system based on renewable energy. Associated Professor Torben R. Jensen explains:
"The fundamental problem is that sun and wind are fluctuating energy sources, which needs to be levelled out to cover our energy consumption. Efficient, large-scale energy storage is therefore needed, which can cover the entire country’s energy consumption for days, weeks, and maybe months; consider a night with no wind."
Hydrogen, H2, is suggested as a sustainable energy carrier. Hydrogen can be produced by water splitting (H2O), which produces hydrogen, H2, and oxygen, O2. The energy for this process is provided by wind, sun, or hydro power. Energy is again released when hydrogen at some other time or place is reacted with oxygen and converted back to water. Therefore, hydrogen is an energy carrier – not an energy source.
Figure 1: Two sustainable closed energy cycles are illustrated by green and blue arrows. Hydrogen may be produced from water and renewable energy, such as sun, wind, or hydropower. Carbon dioxide from air may react with hydrogen to form carbon-based fuels, i.e. sustainable gasoline (blue arrows), an alternative to the hydrogen energy cycle where hydrogen is used directly. The present fossil fuel based energy system is illustrated by the black arrow. Figure modified from M. B. Ley et al. Mater. Today, 2014, 17, 122–128.
A major advantage of using hydrogen is that it is carbon-free and non-poisonous. The reaction with oxygen provides energy and water and no harmful substances. Hydrogen is the lightest element of all, and H2 has the highest gravimetric energy content of all known substances. Therefore, hydrogen is the ideal fuel for mobile applications, such as cars, rockets, etc.
Hydrogen Storage in Metal Hydrides
However, the volumetric energy content is low because hydrogen is a gas, which takes up a significant amount of space. Current hydrogen technology uses pressurised hydrogen at 700 bar. A major advantage is fast refuelling of vehicles, which may take just 2-3 minutes. In order to increase the volumetric energy density solid state hydrogen storage is considered.
Table 1: Hydrogen and energy density along the temperatures of hydrogen release for selected complex metal hydrides. The values are compared to the current technology using gasoline and hydrogen pressurised to 700 bar.
Hydrogen may form new compounds with other lightweight elements, such as boron, nitrogen or aluminium as shown in the table. Synthesis and investigation of new hydrogen containing compounds is a major part of our current research activities. Associated professor Torben R. Jensen explains:
“We have recently discovered completely new types of metal hydrides. As an example: Consider a modification of the compound Mg(BH4)2, denoted d-Mg(BH4)2, with extreme volumetric energy density of 147 g H2/L. This is more than twice the density of pure liquid hydrogen, which has a density of 71 g H2/L (at T = -253 °C). This observation reveals significant perspectives in hydrogen storage in the solid state.”
He adds:
“If we assume that a normal car needs 5 kg hydrogen to obtain a driving range of 500 km, then this amount of hydrogen gas would have a volume of 60 m3 at room temperature and atmospheric pressure (1 bar), i.e. a balloon with a diameter of 5 m. The same amount of hydrogen would only take up a volume of 34 L and have a weight of 34 kg if the hydrogen was stored as the new compound Mg(BH4)2.”
New Types of Lithium-Ion Conductors Cased on Rare Earth Metals
Besides hydrogen storage, the new compounds discovered by Associated Professor Torben R. Jensen and his research group may have fascinating structures and properties. Some metal hydrides store significant amounts of hydrogen and simultaneously are new fast lithium-ion conductors, which may be used in batteries. Other hydrides have optical properties and change from transparent windows to reflecting mirrors, which may be used in “intelligent windows”.
“At Aarhus University we have prepared new types of fast ion conductors, such as the new lithium ion conductor LiM(BH4)3Cl (where M may be the rare earth metals La, Gd og Ce),” explains Torben R. Jensen.
Figure 2: The structure of a new lithium-ion conductor, LiCe(BH4)3Cl. Li, Ce and Cl are red, blue or yellow spheres, while BH4- is dark blue. Li+ ions can “jump” around in the fascinating structure due to empty cation positions.
The first nanoporous metal hydride, g-Mg(BH4)2, was discovered at Aarhus University. The compound has 30% ’empty space’ and can absorb different smaller molecules, also hydrogen, and form the compound g-Mg(BH4)2×2H2, which has an extremely high hydrogen content of 20 weight percent.
The research in new hydrides has also led to the discovery of series of perovskite-type compounds, which all have a structure similar to a known mineral. Compounds with perovskite-type structure may have a range of important properties, e.g. piezo electricity (where a mechanical stimulus can be converted to electric current, which is used in lighters), super conductivity (where electric current can run without resistance), and optical properties such as light emitting diodes with different colours.
Figure 3: Pictures of perovskite-type compounds with europium under white light and UV-light (365 nm). Variation of composition can tailor the colour of the light that is emitted in different blue colours. From P. Schouwink et al. Nature Comm., 2014, 5.
New Improved Properties by Tailoring Materials Composition
The multi-functionality of the new materials that we discover is very inspiring and a significant step towards rational materials design. Associate Professor Torben R. Jensen explains:
“We have developed a wide range of new synthesis methods, which may also be useful for preparation of other types of materials. New series of metal-borohydrides with different number of ammonia molecules as ligands, e.g. Y(BH4)3nNH3, n = 0, 1, 2, 4, 5, 6 or 7. The new hydrogen-containing perovskite materials have extreme flexibility on their composition. The research results provide new methods for controlled modification of materials composition and tailoring materials properties, which has attracted significant international attention.”
Figure 4: Illustration of different applications of metal hydrides. Figure modified from M. B. Ley et al. Mater. Today, 2014, 17, 122–128.
Associate Professor Torben R. Jensen about the Grant from of the Carlsberg Foundation
“The Carlsberg Foundation has supported me with research materials at iNANO and Chemistry Department, Aarhus University, with important equipment for X-ray diffraction, e.g. a silver X-ray source and a unique vacuum diffractometer. Furthermore, the Carlsberg Foundation has supported equipment of thermal analysis, which can measure mass loss as a function of temperature, purity, and composition of released gasses and heat exchange with surroundings. This equipment is extremely important for the investigation of the properties of new materials and it has been running day and night since the equipment was received. The new equipment has also expanded the international collaborations further. In addition, I was granted with a Carlsberg Foundation research grant, which ran for two years. This helped me to start up the very successful research conducted today.”
Torben R. Jensen says further:
“The support from the Carlsberg Foundation has provided me with new equipment, which is very important for teaching and further education of new scientists to the highest level. Mainly energy storage as hydrogen in a solid or as electricity in a battery has been the focus of our research, which is considered of key-importance for development of a sustainable society based on renewable energy.”
References
Structure and properties of complex hydride perovskite material, Pascal Schouwink, Morten B. Ley, Antoine Tissot, Hans Hagemann, Torben R. Jensen, Lubomir Smrčok and Radovan Černý, Nature Comm., 2014, 5, 5706. DOI: 10.1038/ncomms6706.
Trends in Syntheses, Structures, and Properties for Three Series of Ammine Rare-Earth Metal Borohydrides, M(BH4)3·nNH3 (M = Y, Gd, and Dy), Lars H. Jepsen, Morten B. Ley, Radovan Černý, Young-Su Lee, Young Whan Cho, Dorthe Ravnsbæk, Flemming Besenbacher, Jørgen Skibsted, Torben R. Jensen, Inorg. Chem. 2015, 54, 7402−7414.
LiCe(BH4)3Cl, a New Lithium-Ion Conductor and Hydrogen Storage Material with Isolated Tetranuclear Anionic Clusters, Morten B. Ley, Dorthe B. Ravnsbæk, Yaroslav Filinchuk, Young-Su Lee, Raphaël Janot, Young Whan Cho, Jørgen Skibsted and Torben R. Jensen, Chem. Mater. 2012, 24, 1654−1663.
References to Popular Science Papers
Complex hydrides for hydrogen storage – new perspectives, Morten B. Ley, Lars H. Jepsen, Young-Su Lee, Young Whan Cho, José Bellosta von Colbe, Martin Dornheim, Masoud Rokni, Jens Oluf Jensen, Michael Sloth, Yaroslav Filinchuk, Jens Erik Jørgensen, Flemming Besenbacher, Torben R. Jensen, Mater. Today, 2014, 17(3), 122-128 DOI: 10.1016/j.mattod.2014.02.013 (invited review).
Komplekse metalhydrider - Nye multifunktionelle materialer, Elisabeth Grube, Lars H. Jepsen, Torben R. Jensen, Dansk Kemi, 2015, 96, 6/7, 18-21. (In Danish).
Energi opbevaring – nøglen til en fossilfri fremtid, Lars H. Jepsen og Torben R. Jensen, Aktuel Naturvidenskab, 2014, 6, 20-24. (In Danish).