Storing Hydrogen in Oil-like Liquid Could Allow Easy Transport in Trucks and Ships

Cheap catalyst could improve global access to costly green fuel

As a fuel, hydrogen has one major attraction. When it burns or powers a fuel cell, it creates only water—and no climate-warming carbon dioxide. After that, the caveats start. To ship it or store it, the gas must be crushed under intense pressures or liquefied at ultracold temperatures, which raises costs. Now, researchers report the discovery of a cheap catalyst that adds hydrogen atoms to oil-like molecules that are liquid at ambient temperature and pressure. That means hydrogen could be stored and shipped in existing tanks, trucks, and pipelines, much like gasoline.

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"This is a significant step forward," says Peter Wasserscheid, a chemist at the Friedrich-Alexander University of Erlangen-Nuremberg. The new catalyst, reported on 10 July in Nature Energy, is also able to handle impure sources of hydrogen—waste gases from steelmaking and other fossil fuel–powered heavy industries. That could help kick-start a broader hydrogen economy by opening "the way for significantly cheaper hydrogen production," Wasserscheid adds, before the industry transitions to cleaner hydrogen sources.

For decades, industry has touted hydrogen’s promise as a green fuel. The gas can be produced cleanly by using renewable electricity to split water into hydrogen and oxygen. But the process is still too costly. Today, nearly all the estimated 100 million tons of hydrogen made each year comes from fossil fuels. The high cost of storing or shipping hydrogen means most is used near where it is produced, as an additive in refineries and an ingredient in ammonia fertilizer.

So-called liquid organic hydrogen carriers (LOHCs) offer a solution to the storage and transport problem. But inserting and extracting hydrogen into LOHCs requires catalysts that are often “poisoned” and rendered useless by carbon monoxide and other impurities found in hydrogen gas streams. And releasing hydrogen from LOHCs typically requires temperatures of 250°C or more, heat that adds to the cost and can also cause the hydrogen to bind to other gases and create unwanted byproducts.

To get around the poisoning problem, researchers typically turn to resistant precious metal catalysts such as platinum and palladium. Though effective, the cost and limited supply of these metals will likely make the technology harder to scale, says Ding Ma, a chemist at Peking University.

Hoping to find a more abundant alternative, researchers led by Ma and Yifeng Zhu, a chemist at Fudan University, decided to test combinations of cheaper metals. They also screened for catalysts that would work below temperatures of 170°C, minimizing the unwanted byproducts. Out of dozens of possibilities, they identified one catalyst, a combination of copper and aluminum oxide, that was highly efficient at attaching hydrogen to an LOHC called gamma-butyrolactone (GBL), a cheap organic solvent made from petroleum or agricultural waste. The same catalyst proved even better at the reverse process, causing the LOHC to give the hydrogen back up.

By bombarding the catalyst with x-rays and studying the energy of the electrons it emitted, the researchers revealed its structure and clues to how it resists poisoning by carbon monoxide. The clusters of copper atoms were mostly shrouded by aluminum oxide, altering the electronic structure of the copper clusters in a way that prevented carbon monoxide from binding. As a result, the catalyst could operate for 370 hours with virtually no degradation, a number that has been increased to 8000 hours in subsequent work, Zhu says.

The researchers also tested their catalyst with different hydrogen-rich waste gases typical of industrial processes. Today, those waste gases are typically either burned off or vented into the atmosphere, because filtering out the hydrogen would cost more than the gas is worth. The team found that the catalyst could selectively bind hydrogen to GBL, even when the amount of carbon monoxide reached up to 50% of the waste stream. The group’s analysis suggests that if commercialized, such a setup could produce hydrogen from industrial waste gases for about $0.50 per kilogram, far below the existing price of hydrogen.

Given the initial success, "there is potential to scale this up," says Rajeev Assary, an LOHC expert at Argonne National Laboratory. But he cautions the catalyst still needs to be proved stable and effective for years.

Zhu says his team has already developed a new version of the catalyst that performs just as well and can be made in kilogram quantities, which they plan to test in scaled-up reactors with industrial partners. Their success could lead to an ironic situation: plentiful clean hydrogen burned instead of fossil fuels, piggybacking on the infrastructure that Big Oil built.