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October 2012

How Does Hydrogen Hide in Metals?

Look to the areas of intermixing

Aleksandr Chernatynskiy

High-resolution transmission electron microscopy image of the magnesium/palladium interface, showing the intermixing at the interface. At the top of the picture one can see atomic planes separated by the 0.226 nanometers, which is characteristic of the palladium metal. At the bottom, atomic planes are separated by 0.241, identifying pure magnesium metal. In the middle, there is no ordered structure, indicating a mixture of the magnesium and palladium with thickness of about 10 nanometers.

Discovering the mechanism of hydrogen uptake by thin films of magnesium and palladium expands the possibilities for an efficient hydrogen storage system, which could turn intermittent energy from solar and wind farms into chemical bonds. This breakthrough was made by researchers at the Center on Nanostructuring for Efficient Energy Conversion (CNEEC).

Humankind is projected to utilize ever-increasing amounts of energy in the future. Current sources, in particular hydrocarbons (oil and gas), are limited in supply and their usage is unsustainable. In addition, in the process of obtaining energy from them, our technologies emit carbon dioxide into the atmosphere, contributing to climate change. There are plenty of alternative sources of energy available; however, for many of them, it is highly desirable to have a capability to store the energy produced. This is where hydrogen enters the stage: storing energy in the form of hydrogen is an attractive proposition from many perspectives. First, hydrogen is plentiful on Earth, as it is a constituent part of water. Second, an efficient, safe and environmentally friendly way to produce electricity from hydrogen exists in the form of hydrogen fuel cells. Such cells have already found commercial applications in automobiles and in autonomous power generators for buildings. The major obstacle for hydrogen is the absence of an efficient storage medium; this is where  CNEEC's discovery made a contribution.

Compounds of metals and hydrogen, called metal hydrides, have long attracted the attention of researchers as a possible medium for hydrogen storage. In particular, magnesium hydride is an attractive option, due to the relatively large amount of hydrogen that might be stored: 7.6 percent by weight. To put this in perspective, magnesium hydride contains more hydrogen atoms than an equal volume of solid hydrogen! The problematic part is that in magnesium hydride, hydrogen atoms are tightly bonded to the magnesium atoms, and while it is easy to put hydrogen in there, it is far more difficult to extract it. In other words, equilibrium hydrogen pressure is very low.

The situation changes if the storage system consists of the magnesium thin film covered with the film of another metal, palladium: The equilibrium hydrogen pressure was found to increase 100-fold!  Some scientists thought that the palladium film acts as a "clamp," strains the underlying magnesium film, and pushes hydrogen atoms out, increasing equilibrium pressure. However, the CNEEC scientists paint a completely different picture.

Via a combination of theoretical calculations and experimental techniques, Chia-Jung Chung and her coworkers at CNEEC demonstrated that the hydrogen pressure increase is due to the intermixing of magnesium with palladium, forming a thin alloy layer. Formation of the alloy layer is convincingly demonstrated by high-resolution transmission electron microscopy, a technique that shows individual atoms. Scientists can clearly identify pure palladium on top, pure magnesium at the bottom, and the intermixing region in the middle. Upon heating and cooling the film, a process called annealing, the intermixing region increases, as does equilibrium hydrogen pressure. Furthermore, theoretical calculations based on this picture estimate increases in the equilibrium hydrogen pressure in good agreement with the experimental results.

The research team suggestes that the strong interaction of  palladium with magnesium reduces the strength of hydrogen-magnesium interaction and makes hydrogen release easier. By identifying the correct mechanism for the observed increase in hydrogen pressure, this work helps direct researchers toward an effective means for controlling the strength of hydrogen interaction with the storage medium for practical applications.

Chia-Jung Chung said, "This work really answered a lot of people's questions.  We did a lot of characterizations, came up with a simple thermodynamic model, and then finally explained the real mechanism of the huge increase in equilibrium hydrogen pressure."  

Acknowledgments

This work was supported as part of the Center on Nanostructuring for Efficient Energy Conversion at Stanford University, an Energy Frontier Research Center funded by the Department of Energy, Office of Science, Office of Basic Energy Sciences. James R. Groves acknowledges funding from Los Alamos National Laboratory, and Edwin N. Brower acknowledges support from the Vice Provost for Undergraduate Education at Stanford University in the form of a Research Experience for Undergraduates fellowship. The Stanford Nanocharacterization Laboratory and Stanford Nano Center was used.

More Information

Chung CJ, SC Lee, JR Groves, EN Brower, R Sinclair, and BM Clemens. 2012. “Interfacial Alloy Hydride Destabilization in Mg/Pd Thin Films.” Physical Review Letters 108:106102. DOI: 10.1103/PhysRevLett.108.106102

About the author(s):

  • Aleksandr Chernatynskiy is a member of the Center for Materials Science of Nuclear Fuel and a postdoctoral research associate at the University of Florida. He received his B.S. in Theoretical Physics from Perm State University, Russia, in 1999 and his Ph.D. in Physical Chemistry from the University of Louisville in 2005. His research focuses on simulations of the materials properties based on atomistic techniques, including first-principles methods. Particular applications of interest are phonons thermal transport in technologically important systems such as nuclear fuels and thermal barrier coatings.

When It Comes to Holding Hydrogen, Mixing Matters

Thin films with comingled palladium and magnesium easily release possible fuel

High-resolution transmission electron microscopy image of the area where the magnesium and palladium intermix.

A fossil-fuel-free way to produce electricity is with fuel cells powered by hydrogen. The challenge to using these cells more widely is creating a material that can store and release hydrogen when needed.  Using theoretical and experimental techniques, scientists showed the inner workings of a promising magnesium and palladium film that can store and release hydrogen. The scientists showed that the release rate is determined by a thin layer that forms where the bottom magnesium layer and top palladium layer mix together. Identifying how the material functions directs researchers toward more promising means of bonding and releasing hydrogen for fuel cells and other uses. Scientists at the Center on Nanostructuring for Efficient Energy Conversion, led by Stanford University, conducted the research. 

More Information

Chung CJ, SC Lee, JR Groves, EN Brower, R Sinclair, and BM Clemens. 2012. “Interfacial Alloy Hydride Destabilization in Mg/Pd Thin Films.” Physical Review Letters 108:106102. DOI: 10.1103/PhysRevLett.108.106102

Disclaimer: The opinions in this newsletter are those of the individual authors and do not represent the views or position of the Department of Energy.