Exploring new liquid organic hydrogen carrier materials for a safer, more transportable energy source

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Optimization of LOHC molecules through molecular engineering approach. Modification of the position of methyl group in nitrogen-containing bicyclic LOHC (left) and hydrocarbon-based tricyclic LOHC (right). Credit: Korea Research Institute of Chemical Technology (KRICT)

To reduce CO2 emissions, the energy transition from a carbon-based energy system to a more sustainable system based on hydrogen energy is urgently needed. However, the nature of hydrogen (such as low volumetric density, flammability, and embrittlement) makes its use as a widespread energy source extremely challenging. Therefore, the key to establishing a hydrogen-based society is the safe and efficient use of hydrogen.

One way to do this is by using liquid organic hydrogen carrier (LOHC) technology, which can safely store and transport hydrogen in large quantities through chemical bonding.

LOHC technology offers a solution by allowing hydrogen to be stored in liquid organic compounds that remain stable at ambient temperature and pressure, much like gasoline or diesel fuel. This technology also streamlines hydrogen transportation by utilizing existing fossil fuel infrastructure, thereby reducing the costs associated with hydrogen distribution compared other hydrogen storage methods.

Significant efforts have been directed towards developing catalysts and new reactor designs to enhance the dehydrogenation and hydrogenation efficiency of LOHC-based systems. However, the most effective approach lies in addressing the inherent limitations of the LOHC material itself.

The key to LOHC technology relies on developing the proper organic compounds for hydrogen storage. The characteristics of LOHC materials are crucial in determining key factors such as hydrogen storage capacity, reaction kinetics, energy consumption during the dehydrogenation/hydrogenation process, and reversibility.

Comparison of hydrogen storage/release reaction performance between the structure-optimized LOHCs and the existing LOHCs. Credit: Korea Research Institute of Chemical Technology (KRICT)

In previous studies, the focus on meeting hydrogen storage capacity (>6 wt%) and physicochemical properties (a wide liquid range from subzero to 300°C) for aromatic LOHC carriers resulted in a lack of material diversity, limiting the potential for performance improvement.

A research team, led by Dr. Jihoon Park at the Korea Research Institute of Chemical Technology (KRICT), has developed advanced LOHC materials and has been actively exploring new LOHC compounds to increase the diversity of LOHC materials for improved performance.

The findings are published in the Chemical Engineering Journal.

The team focused on optimizing LOHC materials through a molecular engineering approach, redesigning their molecular structure to overcome its limitations. In 2018, the research team developed a new LOHC material (MBP, 2-(n-methylbenzyl)pyridine) that enhanced dehydrogenation performance by adding a N atom into the benzene ring of benzyltoluene.

However, through a combination of experimental and theoretical studies, the research team made a groundbreaking discovery: methyl groups (-CH3), previously thought to have little impact, played a crucial role in improving the performance of LOHC material. Unlike previous LOHC materials (MBP) that existed as isomer mixtures, the research team suggested a new synthetic method for a pure LOHC material (2-benzyl-6-methylpyridine, BMP) with precise control over the position of the methyl group.

Schematic dehydrogenation mechanism in which the bridge carbon and nitrogen atom facilitate hydrogen removal and migration (left). Reaction energy barriers of MBP dehydrogenation reaction catalyzed by Pd and Pt catalysts (right). Credit: Korea Research Institute of Chemical Technology (KRICT)

The new LOHC materials (BMP) increased hydrogen storage and release rates by 206% and 49.4%, respectively, compared to those of MBP.

Additionally, the research team developed a new LOHC candidate, benzyl-methylbenzyl-benzene (BMB), by rearranging the methyl group of dibenzyltoluene, one of the most promising commercial LOHC materials, to overcome the limitations of slow reaction kinetics due to its chemical structure.

BMB exhibits a hydrogenation rate 150% faster than DBT at 150°C and releases 170% more hydrogen compared to DBT at 270°C. Furthermore, the research team uncovered the dehydrogenation mechanism by which N-heterocyclic LOHC materials interact with various active metals in catalysts to facilitate hydrogen extraction.

Dr. Jihoon Park said, "Our research focuses on optimizing LOHC structures, enabling precise control over the placement of methyl groups as functional groups within LOHC material, unlocking new potential for LOHC systems. Also, these findings are expected to influence the design of next-generation hydrogen storage materials, paving the way for a safer and more efficient hydrogen energy-based society."

More information: Kwanyong Jeong et al, Benzyl-methylbenzyl-benzene: Improving hydrogen storage and release performance of dibenzyltoluene based liquid organic hydrogen carrier, Chemical Engineering Journal (2024). DOI: 10.1016/j.cej.2024.150927

Journal information: Chemical Engineering Journal

Provided by National Research Council of Science and Technology