Chemical hydrogen storage

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Chemical hydrogen storage is being researched as a new medium for storing and transporting hydrogen . These substances only serve as a transport medium and are not consumed, but rather circulated. Examples of such substances are liquid hydrogen carrier materials ("Liquid Organic Hydrogen Carriers", LOHC ).

Scheme of energy storage via reversible, chemical hydrogen storage

The principle of chemical hydrogen storage is to use energy generated in an "energy-rich" location to convert the low-energy form of the carrier material in a chemical reaction with energy, e.g. B. electrolytically produced hydrogen to load. The medium stores its chemical energy by enriching it with hydrogen . This hydrogen-enriched material can be stored over long periods of time without loss, transported and distributed with a high energy density. At the place and at the time of the energy demand, the high-energy form is energetically discharged with the release of hydrogen and brought back to the place of energy generation. There it is ready for renewed energy absorption.

This allows power fluctuations to be absorbed when generating energy. Thus, the energy originally available as electrical power can be stored, transported and z. B. be converted back into electricity in connection with fuel cells . With suitable integration of the corresponding storage system, for example in residential buildings, the waste heat from the process can be used to heat the building.

Hydrogen storage in the narrower sense

Chemical conversions of hydrogen can be counted as hydrogen storage in the narrower sense, in which the hydrogen is recovered as such and is not irreversibly converted into another fuel. The most important types are metal hydrides and liquid organic hydrogen carriers .

N-ethyl carbazole

Hydrogenation and dehydrogenation of N -ethylcarbazole

At the moment , the core of current research on liquid organic hydrogen carriers is a substance, N -ethylcarbazole , which was proposed as a hydrogen storage medium by the US company Air Products in the mid-2000s , patented and investigated in the context of publicly funded projects. This is hydrogenated in an exothermic process under pressure and at an elevated temperature into the hydrogen “loaded” perhydro compound. This could be done on a wind turbine or a photovoltaic system . This material, which is now enriched with energy, can be stored over long periods of time without loss, transported with a high energy density and distributed using today's infrastructure ( pipeline , tanker , tank farm, gas station ). At the point of consumption, e.g. B. in a fuel cell vehicle with the supply of heat at between 100 and 200 ° C, the hydrogen is released again. New types of catalysts are used in both processes .

The perhydro compound has a calorific value of 1.9 kWh / kg. For the generation from N -ethylcarbazole, 2.8 kWh / kg must be added to the process. The difference falls (0.9 kWh / kg) as a waste heat and could be used in place to the efficiency to increase. The calorific value of the perhydro compound is around a fifth of that of gasoline . Since fuel cells work much more efficiently than combustion engines , Carbazole can be used to achieve the range of conventional cars with double the tank volume. When storing electrical energy with the help of N- ethyl carbazole, a significantly higher energy efficiency can be achieved than with comparable approaches such as the Sabatier process .

The perhydro form of N -ethylcarbazole is much safer than the highly flammable hydrogen.

Methylcyclohexane

As early as the 1980s, there were extensive experiments with toluene , which is converted to methylcyclohexane by hydrogenation . The basic idea of ​​this variant came from the USA in 1975 and was further developed in 1979 at the Paul Scherrer Institute in Switzerland together with the ETH Zurich . The entire circuit is as M ethylcyclohexan- T oluol- H 2 system (MTH).

However, due to the difficulty of dehydrating non-heterocyclic compounds, the focus of research has shifted to other carrier materials.

Ammonium boranes

In principle, ammonia boranes can also be hydrogenated and dehydrogenated. The simplest form of this is ammonium borane (NBH 4 or NBH 6 in the hydrogenated state). Hydrogenated ammonium borane can be hydrolyzed in an acidic medium, releasing hydrogen. However, regeneration from the ammonium - boric acid solution produced is technically not feasible. Accordingly, it is more of a disposable hydrogen storage device.
A catalytic dehydrogenation without decomposition of the ammonium borane is possible in principle. The regeneration is still difficult. As an alternative, heterocyclic amine boranes have therefore been investigated as potential hydrogen storage for some time . The focus of the relevant research is currently still on the synthesis of the compounds, which is very demanding.
The storage density of ammonia boranes would in principle be very high (12.1% by weight for NBH 6 ; 7.1% by weight for C 4 NBH 12 ). Since ammonium boranes cannot be handled without a solvent, the actual storage density is considerably lower.

Formic acid

In principle, formic acid can also serve as a carrier substance for hydrogen. Hydrogen can be released from it through catalytic decomposition. However, the formation of formic acid from hydrogen and carbon dioxide is thermodynamically very unfavorable and the synthesis is therefore associated with a high energy requirement.

Dibenzyltoluene

Dibenzyltoluene , also known as Marlotherm, has been investigated as a hydrogen storage device since 2016 . The storage capacity is 600 liters of hydrogen per liter of dibenzyltoluene.

Hydrogen storage in the broader sense

There are also approaches to chemical hydrogen storage in which the hydrogen is not recovered in the usage phase, but the "hydrogen storage" is converted (burned). In all of these approaches, the carrier is not recycled. Examples of possible conversion methods are:

Because no elemental hydrogen is recovered, the type of use of the stored energy may differ from hydrogen storage in the narrower sense.

Individual evidence

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  2. Erlangen researchers are testing future energy storage systems. In: nordbayern.de. August 11, 2012, accessed August 24, 2012 .
  3. Electricity can be stored - research with hydrogen is in full swing. In: vdi nachrichten.com. August 12, 2011, accessed January 11, 2014 .
  4. Bernd Otterbach: Carbazole miracle drug: The long way to the automotive industry series online, July 7, 2011.
  5. B. Müller, K. Müller, D. Teichmann, W. Arlt: Energy storage by means of methane and energy-carrying substances - a thermodynamic comparison. In: Chemical Engineer Technology. 83, no. 11, 2011, pp. 1–13, doi: 10.1002 / cite.201100113
  6. ^ FH Stephens, V. Pons, RT Baker: Ammonia – borane: the hydrogen source par excellence? In: Dalton Transactions. 25, 2007, pp. 2613-2626. doi: 10.1039 / B703053C
  7. PG Campbell, LN Zakharov, DJ Grant, DA Dixon, S.-Y. Liu: Hydrogen Storage by Boron − Nitrogen Heterocycles: A Simple Route for Spent Fuel Regeneration. In: Journal of the American Chemical Society. Vol. 132, 10, 2010, pp. 3289-3291. doi: 10.1021 / ja9106622
  8. ^ AJV Marwitz, ER Abbey, JT Jenkins, LN Zakharov, S.-Y. Liu: Diversity through Isosterism: The Case of Boron-Substituted 1,2-Dihydro-1,2-azaborines. In: Organic Letters. Vol. 9, 3, 2010, pp. 4905-4908. doi: 10.1021 / ol702383u
  9. ^ B. Loges, A. Boddien, F. Gärtner, H. Junge, M. Beller: Catalytic Generation of Hydrogen from Formic acid and its Derivatives: Useful Hydrogen Storage Materials. In: Topics in Catalysis . Vol. 53, No. 13-14, 2010, pp. 902-914 doi: 10.1007 / s11244-010-9522-8 .
  10. ^ I. Schmidt, K. Müller, W. Arlt: Evaluation of Formic-Acid-Based Hydrogen Storage Technologies. In: Energy & Fuels . Vol. 28, No. 10, 2014, pp. 6540-6544 doi: 10.1021 / ef501802r .
  11. LOHC - A deposit bottle for hydrogen , HZwei blog, May 11, 2016