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Palladium is a crucial element for initiating a hydrogen-centric energy industry. This lustrous metal acts as a natural barrier against all gases except hydrogen, which it easily allows to pass through. Due to its remarkable selectivity, palladium is regarded as one of the most efficient materials for filtering gas mixtures to yield pure hydrogen.
Currently, membranes composed of palladium are utilized on a commercial level to supply pure hydrogen for industries like semiconductor fabrication, food processing, and fertilizer creation, among others, where these membranes function at moderate temperatures. If palladium membranes exceed temperatures of around 800 kelvins, they may disintegrate.
Recently, engineers at MIT have crafted a novel palladium membrane that remains robust at significantly higher temperatures. Unlike traditional membranes, which are usually produced as a continuous layer, this innovative design employs palladium that is deposited as “plugs” within the pores of a foundational supporting material. At elevated temperatures, these snugly fitting plugs maintain stability and continue to extract hydrogen, unlike a surface film that might deteriorate.
The thermally stable structure opens avenues for membranes to be integrated into hydrogen-fuel-generating technologies such as compact steam methane reforming and ammonia cracking — technologies designed to function at much higher temperatures to generate hydrogen for zero-carbon-emitting fuel and electricity.
“With additional work on scaling and confirming performance under realistic industrial conditions, this design could pave the way for practical membranes aimed at high-temperature hydrogen production,” remarks Lohyun Kim PhD ’24, a previous graduate student from MIT’s Department of Mechanical Engineering.
Kim and his collaborators divulge details about the new membrane in a study published today in the journal Advanced Functional Materials. The co-authors include Randall Field, research director at the MIT Energy Initiative (MITEI); former MIT chemical engineering graduate student Chun Man Chow PhD ’23; Rohit Karnik, the Jameel Professor in MIT’s Department of Mechanical Engineering and director of the Abdul Latif Jameel Water and Food Systems Lab (J-WAFS); and Aaron Persad, a former MIT research scientist in mechanical engineering, now an assistant professor at the University of Maryland Eastern Shore.
Compact Future
The team’s innovative design emerged from a MITEI initiative related to fusion energy. Future fusion power facilities, including those being developed by MIT spinout Commonwealth Fusion Systems are designing, will entail circulating hydrogen isotopes of deuterium and tritium at exceedingly high temperatures to harness energy from their fusion. The reactions will inevitably generate other gases that must be separated, with the hydrogen isotopes recirculated back into the main reactor for further fusion.
Similar challenges exist in various other methods of hydrogen production, where gases require separation and recirculation into a reactor. Concepts for such recirculating systems would necessitate initially cooling down the gas before it could pass through hydrogen-separating membranes — a costly and energy-consuming process that would require additional machinery and equipment.
“One of the questions we contemplated was: Can we devise membranes that could be positioned as close to the reactor as possible, and operate at elevated temperatures, negating the need to extract and cool the gas first?” Karnik states. “This would enable more energy-efficient and consequently cheaper, compact fusion systems.”
The researchers sought methods to enhance the temperature resistance of palladium membranes. Palladium is currently the most effective metal for separating hydrogen from a wide array of gas mixtures. It naturally draws hydrogen molecules (H2) to its surface, where the metal’s electrons engage with and weaken the bonds of the molecule, causing H2 to temporarily dissociate into its individual atoms. These separate atoms then permeate through the metal and recombine on the opposite side as pure hydrogen.
Palladium is exceptionally skilled at permeating solely hydrogen from streams of diverse gases. Yet, standard membranes are usually restricted to operating at temperatures up to 800 kelvins before the film develops holes or clusters into droplets, permitting other gases to pass through.
Plugging In
Karnik, Kim, and their team adopted a distinct design methodology. They noticed that at elevated temperatures, palladium begins to shrink. From an engineering perspective, the material attempts to minimize surface energy. In doing so, palladium, along with most other materials and even water, will separate and form droplets that minimize surface energy. The lower the surface energy, the more stable the material can remain against further heating.
This prompted the team to ponder: If the pores of a supporting material could be “plugged” with palladium deposits — effectively creating a droplet with optimum surface energy — the confined environment might significantly enhance palladium’s heat resistance while preserving the membrane’s selectivity for hydrogen.
To validate this concept, they produced small chip-sized membrane samples using a porous silica supporting layer (each pore measuring about half a micron in width), onto which a very thin palladium layer was deposited. They employed techniques to essentially grow the palladium into the pores, subsequently polishing the surface to remove the palladium layer and leave only Palladium in the pores.
They placed the samples in a custom apparatus, flowing hydrogen-containing gas of various mixtures and temperatures to evaluate its separation performance. The membranes maintained stability and continued to filter hydrogen from other gases, even after enduring temperatures of up to 1,000 kelvins for over 100 hours — a notable enhancement over traditional film-based membranes.
“The usage of palladium film membranes is generally restricted to about 800 kelvins, at which stage they degrade,” Kim notes. “Our plug design therefore extends palladium’s effective heat resilience by at least 200 kelvins and upholds integrity far longer under extreme circumstances.”
These conditions are compatible with hydrogen-producing technologies such as steam methane reforming and ammonia cracking.
Steam methane reforming is an established technique that has necessitated intricate, energy-intensive systems to preprocess methane into a form from which pure hydrogen can be extracted. Such preprocessing stages could be replaced by a compact “membrane reactor,” allowing methane gas to flow directly through it, with the membrane inside filtering out pure hydrogen. Such reactors would drastically reduce the size, complexity, and expense of generating hydrogen from steam methane reforming, and Kim estimates that for a membrane to function reliably, it would need to withstand temperatures nearing 1,000 kelvins. The team’s new membrane appears to be suitably effective within such parameters.
Ammonia cracking provides another method for hydrogen production by “cracking” or breaking down ammonia. Given that ammonia is highly stable in liquid form, scientists envision using it as a hydrogen carrier, safely transporting it to a hydrogen fuel station, where the ammonia could be introduced into a membrane reactor that extracts hydrogen and directly feeds it into a fuel cell vehicle. Ammonia cracking remains primarily in the pilot and demonstration phases, and Kim asserts that any membrane in an ammonia cracking reactor would likely operate at temperatures around 800 kelvins — well within the capabilities of the group’s novel plug-based design.
Karnik emphasizes that their findings are merely a starting point. Implementing the membrane in operational reactors necessitates further development and testing to guarantee that it remains reliable over extended durations.
“We demonstrated that rather than fabricating a film, constructing discretized nanostructures can yield much more thermally stable membranes,” Karnik explains. “It provides a blueprint for designing membranes that can endure extreme temperatures, with the additional possibility of utilizing smaller quantities of costly palladium, thus making hydrogen production more efficient and economical. There is considerable potential there.”
This research was supported by Eni S.p.A. through the MIT Energy Initiative.
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