January 11, 2024

Argonne researchers use AI to downselect candidates
 for liquid organic hydrogen carriers from 160 billion to 41

In a computational study leveraging artificial intelligence (AI), scientists at the US Department of Energy’s (DOE) Argonne National Laboratory assessed 160 billion molecules to screen the molecules for suitability as liquid carriers of hydrogen.

The team whittled down the candidates to a mere 41; the task passes into the hands of experimentalists now to test the promising ones. An open-access paper on the work is published in the RSC journal Digital Discovery.

We present a comprehensive, in silico-based discovery approach to identifying novel liquid organic hydrogen carrier (LOHC) candidates using cheminformatics methods and quantum chemical calculations. We screened over 160 billion molecules from ZINC15 and GDB-17 chemical databases for structural similarity to known LOHCs and employed a data-driven selection criterion connecting molecular features with dehydrogenation enthalpy. This scoring criterion effectively predicts dehydrogenation enthalpies from SMILES strings, streamlining the LOHC screening process.

After rigorous screening and down-selection, we compiled a database of 3000 dehydrogenation reactions for the most promising LOHC candidates, setting the stage for future selection based on kinetics and catalysis. This work demonstrates the significant impact of integrating quantum chemistry and cheminformatics in materials discovery, accelerating the selection process while reducing experimental efforts and time.

By proposing new molecules as prospective LOHC candidates, our study provides a valuable resource for researchers and engineers in the development of advanced LOHC systems and showcases a successful approach for high-throughput discovery, contributing to more efficient and sustainable energy storage solutions.

—Harb et al.

 

Hydrogen in its pure form exists as a gas under normal conditions. For use as a fuel, one of the challenges is shipping this gas safely to refueling stations and storing it. Hydrogen carrier compounds in liquid form offer several advantages. They have a much better safety profile because they are not as prone to leaking and explosion. They also have a much higher energy content per unit volume, making storage and transportation far easier.

The most visible form of a liquid hydrogen carrier compound is water—two atoms of hydrogen and one of oxygen. Another form is organic molecules, essentially an endless number of possible combinations of hydrogen and carbon atoms, in addition to other atoms such as nitrogen and oxygen.

Among the billions of possible liquid hydrogen carriers, common examples include chemicals like ammonia and methanol. However, the relatively few candidates tested in the laboratory to date have suffered from chemical instability and unwanted side reactions.

The team screened the candidate molecules based on four factors.

1. Structural similarity to known liquid hydrogen carriers.

2. Desirable physical properties, such as melting and boiling points—the liquid must stay liquid when the hydrogen has been added or extracted.

3. Storage capacity.

4. Dehydrogenation enthalpy.

Visual representation of the search space and approach for identifying suitable LOHCs. The large outer circle represents the entire chemical compound space, estimated to contain around 10¹¹ molecules. The smaller circle within this represents the subset of these molecules that are unsaturated. Within this subset, a four-circle Venn diagram represents additional selection criteria, labeled as Structure, Practicality, Capacity, and Enthalpy, and they correspond to structural similarity to known LOHCs, desirable physical properties and molecule synthesizability, a gravimetric capacity of 5.5% or higher, and a dehydrogenation enthalpy in the 40–70 kJ per mol H₂ range, respectively. The researchers hypothesize that the intersection of these four criteria indicates the most promising candidates for LOHCs. Harb et al.

The team’s calculations necessitated access to supercomputers available at few places in the world. One of them is Argonne, home to the Argonne Leadership Computing Facility, a DOE Office of Science user facility. The team also relied on Bebop, a computing cluster operated by the Laboratory Computing Resource Center at Argonne.

Even with these powerful resources available, if one allots one millisecond of compute time per molecule, that translates into five years of compute time for 160 billion molecules. For that reason, the team developed an AI-based screening approach that sped up the computations to three million molecules per second, or about 14 hours for the 160 billion.

Resources

• Hassan Harb, Sarah N. Elliott, Logan Ward, Ian T. Foster, Stephen J. Klippenstein, Larry A. Curtiss and Rajeev Surendran Assary (2023) “Uncovering novel liquid organic hydrogen carriers: a systematic exploration of chemical compound space using cheminformatics and quantum chemical methods” Digital Discovery doi: 10.1039/D3DD00123G