(720g) Thermodynamic Analysis of Hydrogen Carrier Materials
Hydrogen storage by reversible chemical reactions is an attractive option to increase energy density and realize storage at ambient conditions. A group of very interesting storage materials are the so called Liquid Organic Hydrogen Carriers (LOHC). Examples currently under research are toluene, N‑ethylcarbazole or dibenzyl toluene.
One of the biggest challenges in this approach is the release of hydrogen due to unfavorable thermodynamics of the dehydrogenation reaction. Selection of carrier materials that exhibit small enthalpic difference between hydrogen rich and lean form supports hydrogen release. On the other hand, if the energy level of the hydrogen rich form is not sufficiently below that of the hydrogen lean form, storage becomes unstable and elevated pressures or low temperatures are required during storage time.
In addition to their use as hydrogen carrier for energy storage and transport purposes, LOHCs can be used as transport vectors for hydrogen in the chemical industry. In such a case the carrier material should be selected in a way that the enthalpy of hydrogenation of the transfer agent is less than that of the accepting substance. However, to achieve a good energy efficiencyaich the difference should not be too high. Hence, different carrier materials can be optimal for different processes.
In this contribution the thermodynamic aspects of the selection of LOHC materials are highlighted regarding energy systems as well as chemical processes.