Hydrogen, which does not emit CO2 into the atmosphere when it is used, is of huge significance as an energy source. It can be used for a variety of different purposes and can also be used as an energy store. As a doctoral researcher at the Institute of Material Science at the University of Stuttgart, Efi Hadjixenophontos worked together with other doctoral students on the Ecostore project run by the Marie Skłodowska-Curie Actions (MSCA) research fellowship program to carry out research into how hydrogen can be used as an energy store and in solid-state batteries. The group published its findings in MDPI Open Access Journals in March 2020. The paper has been one of the University of Stuttgart’s most-cited publications this year.
- Original publication
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Hadjixenophontos, Efi; Dematteis, Erika Michela; Berti, Nicola; Wolczyk, Anna Roza; Huen, Priscilla; Brighi, Matteo; Le, Thi Thu; Santoru, Antonio; Payandeh, Seyed Hosein; Peru, Filippo; Dao, Anh Ha; Liu, Yinzhe; Heere, Michael. (2020) A Review of the MSCA ITN ECOSTORE-Novel Complex Metal Hydrides for Efficient and Compact Storage of Renewable Energy as Hydrogen and Electricity. In: MDPI Open Access Journals, https://doi.org/10.3390/inorganics8030017.
Space-saving and safe
Hydrogen has a lower volume if it is used in a solid state in metal hydride instead of as a gas. When used in energy stores or batteries, metal hydrides take up about a third less space than hydrogen in gaseous form. Hydrogen in a solid state is also easier to handle than in gaseous form, which is highly explosive.
Which are the most suitable materials?
“One of the most promising materials which is considered a pure metal hydride is magnesium hydride (MgH2)”, explains Dr. Efi Hadjixenophontos. In order to produce a thin layer of the material in nanoscale, metal is bombarded with ions in a so-called ion-beam sputter chamber so that metal ions are released which form a thin layer of magnesium hydride. This is the most suitable form of the material for investigating the fundamental behaviour of hydrogen motion. With the help of specially developed hydrogen reactors and microscopy techniques conducted at the Institute of Material Science, Hadjixenophontos was able to evaluate the diffusion over a wide range of temperatures. Furthermore, the researchers synthesized other hybrid materials such as amides, imides, boron hydrides and rare-earth metal hydrides.
New insights into the processes observed in batteries
Batteries which use magnesium hydride belong to the family of so-called conversion reaction batteries. In conventional Li-ion batteries, the lithium ions move to and fro between the negative and positive electrodes when charging and discharging without a chemical reaction taking place. “In the conversion batteries which we looked into though, a chemical reaction does happen. When the batteries are being charged, the magnesium hydride reacts with the lithium hydride. This reaction unfortunately isn’t always reversible though, which poses a challenge in terms of the lifespan of the battery”, is how the researcher describes the processes observed in the new types of battery.
Batteries can be produced without lithium
If metal hydrides such as magnesium hydride are used in batteries, it is also possible to use batteries which aren't based on lithium but instead for example with sodium, as is discussed in the published work. This is a big advantage, because lithium is extremely rare.
Larger capacity
Batteries with metal hydride have a larger capacity than conventional batteries. “This means a longer range if they are used in vehicles in the future”, says Dr Hadjixenophontos talking about the advantages they offer, “because the batteries take up less space and are lighter despite the higher capacity, this means that it will also become viable to use them for emergency power generators or other mobile devices.”
Creating a network
As well as carrying out research, also on the agenda for the Marie Skłodowska-Curie Actions fellowship program is creating a network. A total of twelve doctoral students and two postdocs from different universities, companies and scientific institutions from Germany and abroad were involved in the Ecostore project. Reciprocal research stays for participants were also integrated into the project. This meant that Efi Hadjixenophontos was able to spend two months as a guest researcher at both Tohoku University in Japan and at the French National Centre for Scientific Research (CNRS) in Paris. In addition, she visited the French subsidiary of the company SAFT Batteries. Dr. Efi Hadjixenophontos completed her doctorate at the Institute of Material Science at the University of Stuttgart under Prof. Guido Schmitz. “I’m very thankful for the fantastic research environment at the institute and for the commitment shown by Prof. Schmitz”, she points out. Since 2019 she has been working as a materials scientist at the Institute of Engineering Thermodynamics at the German Aerospace Center (DLR).
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