The auto industry is eagerly waiting for a breakthrough in hydrogen storage for improved hydrogen fuel cell vehicle performance and scientists have taken a huge step in that direction.
Published in Advanced Materials Interfaces is a study that describes how nanoconfinement can be used to create materials that can efficiently store hydrogen in fuel cells and release it as and when necessary at optimal speeds. Researchers say that their findings have paved way for creation of high-capacity hydrogen storage materials that will enable quick refueling as well as improve performance of hydrogen powered cars.
Electrochemical reaction between hydrogen and oxygen inside a fuel cell drive hydrogen fuel cell vehicles. While oxygen is provided by air, the hydrogen must be stored separately on the vehicle. Current fuel cell electric vehicles store hydrogen as a high-pressure gas. While scientists have been looking for materials like sponge to be used for storage of hydrogen, there aren’t any materials that can do it efficiently.
A solid material can act like a sponge for the absorption and release of hydrogen, in chemical terms hydrogenation and dehydrogenation. Thus using such a hydrogen storage material could increase how much hydrogen can be stored. The material must be able to store enough hydrogen for the vehicle to go at least 300 miles before refueling.
However, there is a problem with using solid material for hydrogen storage and release. Most of these materials are not capable of soaking up enough hydrogen for cars and they do not release and absorb hydrogen fast enough, especially compared to the 5 minutes needed for fueling.
That’s where the new study comes in as scientists have synthesised, characterized and modelled improved lithium nitride, which is a promising hydrogen storage sponge. Scientists have also developed a fundamental understanding of why nanosizing improves the hydrogen storage properties of this material.
Using the idea of nanoconfinement, researchers enhanced hydrogen storage reactions in nitrogen-containing compounds. The team found that liquid ammonia could be used as a gentle and efficient solvent for introducing metals and nitrogen into the pockets of carbon nanoparticles, producing nanoconfined lithium nitride particles. This new material showed some unusual and unexpected properties.
First, the amount of lithium nitride in the carbon nanoparticle host was quite high for a nanoconfined system, about 40 percent. Second, the nanoconfined lithium nitride absorbed and released hydrogen more rapidly than the bulk material. Furthermore, once the lithium nitride had been hydrogenated, it also released hydrogen in only one step and much faster than the bulk system that took two steps.
The team discovered that the reason for the unusual behavior was the energy associated with two material interfaces. Since the lithium nitride nanoparticles are only 3 nanometers wide, even the smallest energetically unfavorable process is avoided in the hydrogen storage properties. For lithium nitride nanoparticles undergoing hydrogenation reactions, the avoidance of unfavorable intermediates — extra steps in the chemical process — increases efficiency.
Taking the path of least resistance, the material undergoes a single-step path to full hydrogenation. Similarly, once hydrogenated, the nanoparticles release hydrogen by the lowest energy pathway available, which in this case is direct hydrogen release back to lithium nitride.
“In this way, the nanointerfaces drive the hydrogen storage properties when the materials are made very small, for example with nanoconfinement,” said computational scientist Brandon Wood of LLNL. “The purposeful control of nanointerfaces offers a new way to optimize hydrogen storage reaction chemistry.”