📄 chemistry

Reversible hydrogen storage with aromatics: boosting LOHC effectiveness through oxygen functionality

This study demonstrates that oxygen functionalization in lignin-derived indane-based liquid organic hydrogen carriers significantly lowers reaction enthalpies and equilibrium temperatures, enabling efficient reversible hydrogen storage and release below 500 K.

Original authors: Sergey P. Verevkin, Arina V. Elbakari, Artemiy A. Samarov, Sergey V. Vostrikov, Anton V. Eremin, Riko Siewert

Published 2026-08-04

📖 5 min read🧠 Deep dive

CC BY 4.0

Original authors: Sergey P. Verevkin, Arina V. Elbakari, Artemiy A. Samarov, Sergey V. Vostrikov, Anton V. Eremin, Riko Siewert

Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine the world is trying to switch from dirty fossil fuels to clean, green energy. One of the biggest hurdles in this transition is hydrogen. Hydrogen is a fantastic fuel, but it's tricky to handle: it's incredibly light, meaning you need huge tanks to store enough of it, and it's so flammable that storing it under high pressure is risky. Scientists have been looking for a "magic sponge" that can soak up hydrogen like a liquid, making it safe and easy to transport using existing fuel trucks and pipelines. This is where Liquid Organic Hydrogen Carriers (LOHCs) come in. Think of them as chemical sponges: you fill them with hydrogen at a charging station (a process called hydrogenation), drive them around, and then squeeze the hydrogen out at your destination (dehydrogenation) to power a car or a factory.

However, there's a catch. The current best "sponges" are made of simple aromatic rings (like benzene), and squeezing the hydrogen out of them is like trying to pull a heavy magnet off a fridge—it takes a lot of heat energy (high temperatures) to do it. This makes the process expensive and energy-hungry. The big question scientists are asking is: Can we tweak the chemical structure of these sponges to make them let go of hydrogen more easily? Specifically, can we add oxygen atoms to the mix to lower the energy cost? This is the puzzle a team of researchers from Germany and Russia set out to solve.

The Oxygen Upgrade: Making Hydrogen Sponges Let Go

In this study, the researchers decided to play with the molecular architecture of their hydrogen sponges. Instead of using plain, boring rings, they looked at a family of molecules based on indane (a structure that looks like a hexagon fused to a pentagon). They then started adding oxygen-based decorations, like carbonyl groups (think of them as double-bonded oxygen "ears") or methoxy groups (oxygen "hats" with a methyl group attached). These oxygenated structures are interesting not just because they might work better, but because they can be made from lignin, a major part of wood and plant waste. This means we could potentially make our hydrogen carriers from renewable biomass, closing the carbon loop.

The team didn't just guess; they built a massive thermodynamic map using a mix of real-world experiments and high-level computer simulations (called ab initio calculations). They calculated exactly how much energy it would take to fill these new sponges with hydrogen and, more importantly, how much heat it would take to empty them.

The Big Discovery: Oxygen Lowers the Heat
The results were promising. The researchers found that adding oxygen groups to the indane structure acts like a lubricant for the hydrogen release.

The Old Way: Traditional hydrogen carriers (like plain indane) need to be heated to about 511 K (roughly 238°C) to release their hydrogen efficiently. That's hot!
The New Way: By adding oxygen, the team found they could drop that required temperature significantly. Some of the best oxygenated candidates, like certain methoxy-indanones, could release hydrogen at temperatures as low as 467 K (about 194°C). That's a drop of 30 to 60 K compared to the unmodified versions.

In the world of chemistry, a drop of 30–60 degrees is a huge deal. It means the process becomes much more energy-efficient and puts less stress on the expensive catalysts (the tiny particles that speed up the reaction) that usually break down at high heat.

The "Score" of the Reaction
To judge how good a candidate is, the scientists used a "scoring criterion." They looked at the reaction enthalpy (the energy change per mole of hydrogen).

A value around -68.5 kJ/mol (like in benzene) is considered too high; it's too hard to get the hydrogen out.
The ideal target is closer to -40 kJ/mol.
The oxygenated indane derivatives the team studied landed in a sweet spot, with values ranging from -57 to -64 kJ/mol. While not quite hitting the perfect -40, they were significantly better than the unmodified versions.

What Didn't Work (and What Did)
The study was careful to rule out some ideas.

Hydroxyl groups (alcohol-like): Simply adding a hydroxyl group (-OH) didn't help much. In fact, for 1-indanol and 2-indanol, the energy required to release hydrogen was actually slightly higher than for plain indane. So, just adding an alcohol group isn't the magic bullet.
Methyl groups: Adding simple methyl groups (-CH₃) didn't really change the energy game one way or the other. It was neutral.
The Winners: The real stars were the carbonyl (C=O) and methoxy (-OCH₃) groups. Specifically, placing a methoxy group at certain positions on the ring lowered the energy barrier the most. However, the team noted that if you put two methoxy groups right next to each other (in the 5,6-position), the benefit disappeared. The groups got in each other's way (steric repulsion), canceling out the good effects.

The Final Verdict: Feasible at Lower Temperatures
The researchers calculated the Gibbs free energy at 500 K (a practical operating temperature). For the best oxygenated candidates, this value was close to zero or even negative in the direction of releasing hydrogen. In plain English, this means that at temperatures well below 500 K, the chemical reaction naturally wants to happen. The hydrogen wants to leave the sponge without needing a massive furnace.

The study concludes that these oxygen-functionalized indane derivatives are thermodynamically feasible for real-world use. They offer a path to hydrogen storage that is not only more efficient but also potentially sustainable, as they can be derived from renewable plant materials. While the paper doesn't claim these are the final solution, it strongly suggests that tweaking aromatic molecules with oxygen is a winning strategy to make hydrogen storage cheaper, safer, and easier to scale up.

Technical Summary: Reversible Hydrogen Storage with Aromatics: Boosting LOHC Effectiveness through Oxygen Functionality

Problem Statement
Liquid Organic Hydrogen Carrier (LOHC) systems offer a promising solution for reversible hydrogen storage by leveraging existing liquid fuel infrastructure. However, traditional LOHCs based on simple aromatic compounds (e.g., benzene, toluene, naphthalene) face significant thermodynamic limitations. Their hydrogenation/dehydrogenation cycles exhibit high reaction enthalpies (typically around -65 to -68 kJ·mol⁻¹ per mole of H₂), necessitating high equilibrium temperatures (>573 K) for hydrogen release. These high temperatures lead to substantial energy consumption, catalyst deactivation, and undesirable side reactions that compromise system stability. While oxygen-containing functional groups are known to modulate electron density and potentially lower reaction barriers, a systematic thermodynamic evaluation of oxygen-functionalized aromatic systems derived from renewable feedstocks (specifically lignin valorization products like indane and indanone derivatives) has been lacking. Furthermore, experimental thermodynamic data for the hydrogen-rich (HR) counterparts of these systems are largely absent in the literature.

Methodology
This study employs a rigorous three-step methodology combining high-level quantum chemical calculations with experimental and empirical thermochemical data to evaluate 38 LOHC pairs based on indane and benzofuran structures.

Quantum-Chemical Calculations (Step I): The authors utilized the Gaussian 16 software package and the CREST tool for conformational sampling to determine gas-phase enthalpies of formation ( $\Delta_f H_m^\circ(g)$ ) and absolute entropies ( $S_m^\circ(g)$ ). The G3MP2 composite method was applied, with results calibrated against experimental data for structurally similar compounds using atomization reactions to ensure accuracy.
Vaporization Thermodynamics (Step II): To convert gas-phase data to the liquid phase (where reactions practically occur), the study compiled experimental vapor pressures and boiling points. Enthalpies and entropies of vaporization ( $\Delta_{l}^{g}H_m^\circ$ and $\Delta_{l}^{g}S_m^\circ$ ) were derived using a three-parameter vapor pressure equation. For compounds lacking sufficient experimental data, a "centerpiece approach" (CP) was employed. This empirical method uses a well-characterized reference molecule (e.g., indane) and adds structural fragment contributions (e.g., carbonyl or methoxy groups) to predict vaporization properties with high reliability.
Thermodynamic Analysis (Step III): Using the derived liquid-phase enthalpies of formation and entropies, the study calculated reaction enthalpies ( $\Delta_r H_m^\circ$ ), reaction entropies ( $\Delta_r S_m^\circ$ ), equilibrium temperatures ( $T_{eq}$ ), and Gibbs free energies ( $\Delta_r G_m^\circ$ ) for three reaction series: partial hydrogenation of the aromatic ring (R), hydrogenation of the resulting aliphatic ketone to an alcohol (R'), and complete hydrogenation to aliphatic alcohols (R'').

Key Contributions

Comprehensive Thermodynamic Dataset: The paper provides the first consistent set of standard molar enthalpies of formation and absolute entropies for both hydrogen-lean (HL) and hydrogen-rich (HR) indane and benzofuran derivatives, including many HR species for which no experimental data previously existed.
Validation of the Centerpiece Approach: The study demonstrates the efficacy of the "centerpiece approach" in predicting vaporization enthalpies for complex organic molecules, successfully reconciling discrepancies between various experimental methods and theoretical predictions.
Structure-Property Correlations: The work systematically correlates specific oxygen functionalities (carbonyl, methoxy, hydroxyl) with thermodynamic performance metrics, offering a clear guide for molecular design in LOHC development.

Results
The investigation reveals that oxygen functionalization significantly alters the thermodynamic landscape of indane-based LOHCs:

Reaction Enthalpies: The introduction of carbonyl and methoxy groups generally lowers the reaction enthalpy per mole of H₂. For instance, 1-indanone exhibits a reaction enthalpy of -60.4 kJ·mol⁻¹ H₂, compared to -62.6 kJ·mol⁻¹ H₂ for unsubstituted indane. Methoxy-substituted derivatives (e.g., 5-methoxy-1-indanone) show even lower values, reaching approximately -57.1 kJ·mol⁻¹ H₂. Conversely, certain configurations, such as the 6H-4-benzofuranone system, showed increased enthalpies (-80.4 kJ·mol⁻¹ H₂ for the ketone-to-alcohol step), indicating that structural context is critical.
Equilibrium Temperatures: The reduction in reaction enthalpy translates directly to lower equilibrium temperatures ( $T_{eq}$ ). Oxygen-functionalized derivatives exhibited $T_{eq}$ values 30–60 K lower than the unsubstituted indane reference (511 K). Several candidates, such as 5-methoxy-1-indanone, achieved $T_{eq}$ values as low as 467 K.
Gibbs Free Energy: Calculations of $\Delta_r G_m^\circ$ at 500 K indicate that many oxygen-functionalized systems are thermodynamically feasible for hydrogen release at temperatures well below 500 K. Several compounds (e.g., 1-indanone, methoxy-substituted indanones) displayed large negative Gibbs free energies for dehydrogenation, suggesting spontaneous hydrogen release under moderate conditions.
Entropy Trends: Reaction entropies for aromatic-to-aliphatic transitions (Series R and R'') were found to be largely independent of molecular structure, averaging around -126.8 J·K⁻¹·mol⁻¹ H₂. However, the hydrogenation of aliphatic ketones to alcohols (Series R') showed higher entropy values due to increased molecular flexibility.

Significance
The authors conclude that the deliberate introduction of oxygen-containing functional groups into indane-based structures is a viable strategy for enhancing LOHC effectiveness. The study demonstrates that these modifications can lower the energy barriers for hydrogen release, making the process thermodynamically feasible at temperatures significantly below the 573 K threshold typical of traditional aromatic LOHCs. By linking these findings to lignin valorization, the paper posits that such oxygenated indane derivatives represent a promising class of sustainable, renewable LOHC candidates. The work provides a thermodynamic foundation for the rational design of next-generation hydrogen storage systems that are more energy-efficient and compatible with practical operating conditions.

The Oxygen Upgrade: Making Hydrogen Sponges Let Go

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