An electric route to safe, practical hydrogen storage

Fuel cells powered by hydrogen have the potential to reduce carbon emissions from trucks, airplanes, and ships. They’re already helping passenger cars and buses shrink their carbon footprints. But hydrogen is stored today as compressed gas in high-pressure vessels or as a liquid in cryogenic tanks. Searching for a less complex and more practical storage method, researchers have now found a way to pack hydrogen into a solid material at low temperatures (Science 2025, DOI: 10.1126/science.adw1996).

The material the team used is magnesium hydride (MgH₂), which, like other metal hydrides, forms when hydrogen chemically bonds to the metal. MgH₂ has a high storage capacity—a mass fraction of 7.6% hydrogen—“and is considered an ideal hydrogen-storage material,” says Naoki Matsui, a solid-state battery researcher at the Institute of Science Tokyo and one of the authors of the study.

By comparison, lanthanum pentanickel, the compound used for storing hydrogen in the nickel–metal hydride batteries that drive hybrid vehicles, can store a mass fraction of only 1.4%. For years, researchers have been looking for solid materials to store hydrogen at low temperatures and pressures. Magnesium hydride can take up a lot of hydrogen, but it needs to be heated above 300 °C to release the stored fuel, which has restricted its applications.

So Matsui and colleagues developed a new solid electrolyte that helps pump hydrogen into and out of MgH₂ at low temperatures. Made of barium, calcium, and sodium (Ba_0.5Ca_0.35Na_0.15H_1.85), the electrolyte’s ability to help with hydrogen storage comes from its exceptional capacity for conducting hydride ions.

The researchers made a battery-like device by sandwiching the solid electrolyte between MgH₂, which serves as an anode, and a cathode made of lanthanum hydride. The cathode is connected to a hydrogen gas reservoir. When the device is being charged, hydrogen gas is reduced to hydride ions, which the electrolyte shuttles into the MgH₂ for storage. The process is reversed during discharge.

These electrochemical reactions occur at 90 °C, and the cell is able to store and retrieve hydrogen 10 times before its capacity drops. The MgH₂ electrode’s energy storage capacity is 2,030 mA·h/g, which corresponds to its theoretical capacity. But the cell is able to deliver only a fraction of that energy. That’s partly because of the thick electrode and the relatively low mass fraction of 20% metal hydride that the team loaded into the electrode.

Matsui says that the team plans to improve the solid electrolyte and electrodes to “develop hydrogen storage devices that operate at even lower temperatures with higher capacity.”

Practical storage systems need to be stable for more than 1,000 cycles, says Ryan O’Hayre, a materials scientist and engineer at the Colorado School of Mines who was not involved in the study. The chemical and mechanical stability of the electrodes, electrolytes, and their interfaces over the course of thousands of charging cycles remains unproven, he says.

“It is too early to tell if this will ultimately be viable,” O’Hayre says. But this is a brand-new approach that “elegantly circumvents the principal obstacle for magnesium hydride” to be used for hydrogen storage, so it warrants further research, he says. “It certainly opens up interesting new directions for the technology.”