How Does Hydrogen Hide in Metals?
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."
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
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.