Liquid organic hydrogen carriers (LOHC) are organic compounds that can absorb and release hydrogen through chemical reactions. LOHCs can therefore be used as storage media for hydrogen. In principle, every unsaturated compound (organic molecules with C-C double or triple bonds) can take up hydrogen during hydrogenation. The sequence of endothermal dehydrogenation followed by hydrogen purification is considered as the main drawback which limits the overall efficiency of the storage cycle. LOHC shipping without heat recycling has an energy efficiency of 60–70%, depending on the dehydrogenation rate, which is equivalent to liquid hydrogen shipping. With heat recycling, the energy efficiency increase to 80–90%. In 2020, Japan built up the world's first international hydrogen supply chain between Brunei and Kawasaki City utilizing toluene-based LOHC technology. Hyundai Motor invests in the development for stationary and on-board LOHC-systems.
Principle of LOHC-based hydrogen storage To absorb hydrogen, the dehydrogenated form of LOHC (an unsaturated, mostly aromatic compound) reacts with the hydrogen in a hydrogenation reaction. The hydrogenation is an exothermic reaction and is carried out at elevated pressures (approx. 30–50 bar) and temperatures of approx. 150–200°C in the presence of a catalyst. The corresponding saturated compound is thereby formed, which can be stored or transported under ambient conditions. If the hydrogen is needed again, the now hydrogenated, hydrogen-rich form of the LOHC is dehydrogenated, with the hydrogen being released again from the LOHC. This reaction is endothermic and takes place at elevated temperatures (250–320°C) again in the presence of a catalyst. Before the hydrogen can be used, it may have to be cleaned of LOHC steam. To increase efficiency, the heat contained in the hot material flow exiting the release unit should be transferred to the cold material flow consisting of hydrogen-rich LOHC entering the release unit in order to keep the energy requirement for preheating it before the reaction low. In particular, the heat released by the hydrogenation reaction when the hydrogen is absorbed can in principle be used for heating purposes or as process heat.
Requirements for LOHC materials LOHC materials need to have a high hydrogen storage capacity, ideally able to store over 5 wt% of hydrogen. Second, the materials should exhibit reversible hydrogenation and dehydrogenation properties, maintaining stability over multiple cycles without significant degradation. Additionally, the thermodynamic and kinetic properties of the materials must be optimized to ensure low energy consumption and rapid reactions during hydrogenation and dehydrogenation. Long-term stability and good catalyst compatibility are also crucial for the effectiveness of LOHC materials. LOHC materials must have low volatility and high boiling points to minimize loss during storage and transport. Finally, the materials need to be non-toxic, environmentally friendly, and economically viable for large-scale applications.
Determination of the degree of hydrogenation The determination of the degree of hydrogenation is a critical aspect in the analysis of unsaturated compounds, particularly in polymer chemistry. The 1 H NMR technique is a primary method used to confirm hydrogenation processes and to quantify the degree of hydrogenation over varying reaction times. This technique is particularly effective when hydrogenation follows the esterification of hydroxyl groups, as seen in the hydrogenation of hydroxyl terminated polybutadiene (HTPB) using a catalyst system of diisobutylaluminum hydride (DIBAL-H) and Co III acetylacetonate in cyclohexane as a solvent. The degree of hydrogenation itself refers to the extent to which unsaturated fats are converted to saturated fats through hydrogen addition, which is a fundamental chemical reaction in organic synthesis. Additionally, advanced methods such as emission spectroscopy can be employed to measure the degree of molecular dissociation in hydrogen plasmas, which is relevant for understanding hydrogenation processes. This approach allows for a more nuanced analysis of hydrogenation, particularly in non-thermal environments, enhancing the accuracy of hydrogenation degree assessments. In summary, the combination of 1 H NMR and emission spectroscopy provides a robust framework for determining the degree of hydrogenation, facilitating improvements in material properties and expanding the applications of hydrogenated polymers.
Direct LOHC fuel cell An alternative, innovative and highly promising approach to convert LOHC-bound hydrogen into electricity is proposed recently. The new unloading sequence consists of an almost thermoneutral catalysed transfer hydrogenation step converting ketone (acetone) to secondary alcohol (2-propanol) by contacting hydrogen-rich carrier (H18-DBT), and the secondary alcohol is then directly consumed in a PEMFC (direct isopropanol fuel cell; DIPAFC). It is a CO2 emission-free, external energy input-free, and safe sequence with no molecular hydrogen at any point during hydrogen releasing. The "direct LOHC fuel cell" based on the LOHC-DIPAFC coupling concept is a very attractive solution for the on-board generation of electric energy in mobile applications, and it's driving researchers to focus on the topic.
Examples of LOHC materials
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