Caught in a trap
New understanding of hydrogen interactions with metal nanoparticles may pave the way for future hydrogen storage
Figure 1: A schematic depiction of hydrogen storage of palladium (Pd) and platiunum (Pt) nanoparticles (green, hydrogen; red, Pd; blue Pt).
Reproduced with permission from Ref. 1 © 2008 by the American Chemical Society
The environmental impact of the use of hydrocarbon as fuels has led to a global search for cleaner energy sources. Hydrogen offers a greener alternative for transportation fuels, being easily generated from renewable energy sources and creating only environmentally benign water as a byproduct of electricity generation. However, a critical issue is the requirement of a safe and reliable hydrogen storage medium.
Current options for hydrogen storage include intensely pressurizing hydrogen or storing it as a liquid at cryogenic temperatures. These options are not practical for everyday use, so new approaches are being investigated. For example, hydrogen can be stored chemically in the form of hydrides. Another approach is to weakly adsorb hydrogen onto materials of large surface area. Nanoscale particles are a good example, they are advantageous over bulk materials as they have a larger solid/gas interface area and shorter hydrogen diffusion paths, yielding potentially faster kinetics for gas absorption and desorption.
In two studies recently published in the Journal of the American Chemical Society, Masaki Takata from the RIKEN SPring-8 Center, Harima, and his colleagues including Hiroshi Kitagawa from Kyushu University have revealed hydrogen trapping in nanoparticles made of metals that can form hydrides. Their findings were made by exploring the chemical and structural changes on exposure of the particles to high hydrogen pressures.
Figure 2: Transmission electron microscope image of palladium nanoparticles.
Reproduced with permission from Ref. 2 © 2008 by the American Chemical Society
The hydrogen storage capability of bulk palladium has been shown previously to be higher than that of nanoparticulate palladium, but that nanoparticles of platinum stored more hydrogen than in the bulk form.
Takata, Kitagawa and colleagues felt that the strikingly different behavior of these materials could be exploited in core-shell type materials, which can be regarded as an alloy phase separated into a core of one metal surrounded by a shell of another. And so, for the first time, an investigation into the hydrogen storage properties of core-shell nanoparticles was undertaken1. They chose palladium as the core and platinum as the shell materials of the nanoparticles.
The team used a stepwise growth method to prepare structures with crystalline palladium cores of 6 nm diameter, using poly (N-vinyl-2-pyrrolidone), PVP, to prevent aggregation of the nanomaterials and control their shape and shape distribution. PVP does not show any hydrogen storage behavior of its own.
Crystalline platinum shells of thickness around 2 nm were then deposited around the palladium cores using hydrogen as a reducing agent, and the resulting core-shell structures were characterized using a variety of techniques. High-resolution transmission electron microscopy and energy dispersive spectroscopy revealed the core-shell structure, while x-ray diffraction revealed that both the core and the shell were crystalline. Pressure-composition isotherms showed that the core-shell nanoparticles absorbed the same amount of hydrogen as homogenous palladium nanoparticles.
Takata, Kitagawa and colleagues then performed solid state nuclear magnetic resonance (NMR) measurements with deuterium, a hydrogen isotope, to identify the absorption site of hydrogen. Surprisingly, they have found that while deuterium was dispersed in both palladium and platinum lattices, it was concentrated in the boundary region between core and shell (Fig. 1).
The atomic arrangements or chemical potential at the interfacial region are more favorable for the generation of hydrides than within the palladium core or the platinum shell of the nanoparticle, according to the researchers.
The ‘big and small’ of hydrogen storage with palladium
In their second study2, Takata, Kitagawa and colleagues investigated the differences in storage behavior between the bulk palladium and palladium nanoparticles of 6 nm diameter coated with PVP (Fig. 2). Bulk palladium has been intensively studied because of its potential to store hydrogen as a hydride.
Composition isotherms of hydrogen pressure showed that the palladium nanoparticles require higher hydrogen pressures to reach the same hydrogen absorption levels as bulk palladium. On release of the high hydrogen pressure, the hydrogen pressure-composition isotherm for the bulk system was shown to be completely reversible; however, this was not the case for the nanoparticles. The atomic hydrogen somehow got trapped on, or inside, the palladium nanoparticles causing the hysteresis, the authors hypothesized.
To test this theory, the authors carried out several additional experiments. They used x-ray diffraction and found that the lattice constant of the nanoparticles increases with exposure to 101.3 kPa of hydrogen pressure. However, on evacuation of the hydrogen, the lattice does not return to its original value, it remains slightly larger. Then, again using solid state NMR measurements with deuterium, the researchers found that some deuterium atoms remain within the palladium lattice after evacuation of ‘free’ deuterium from the system. The results suggest that hydrogen atoms are not deposited on the surface of the nanoparticles, or clustered, but are distributed throughout the nanoparticles.
Takata, Kitagawa and colleagues propose that some hydrogen atoms are stabilized and trapped firmly within the lattice strongly bound as hydrides, which expands the crystal lattice and hence the lattice constant of palladium. This, they say, explains why hydrogen absorption in these materials is not completely reversible on the removal of high pressure hydrogen from palladium materials.
A step in the right direction
The metal nanoparticles investigated in the study have potential for combining the hydrogen storage capabilities of hydrides with the kinetic benefits of large-surface-area nanomaterials. As such, the research team’s detailed observations of strong hydrogen trapping in palladium nanoparticles, and of higher hydrogen accumulation at the interfaces between the core and shell of core-shell nanoparticles, could aid in the development of practical hydrogen storage materials.
Kobayashi, H., Yamauchi, M., Kitagawa, H., Kubota, Y., Kato, K. & Takata, M. Hydrogen absorption in the core/shell interface of Pd/Pt nanoparticles. Journal of the American Chemical Society 130, 1818–1819 (2008). | |
Kobayashi, H., Yamauchi, M., Kitagawa, H., Kubota, Y., Kato, K. & Takata, M. On the nature of strong hydrogen atom trapping inside Pd nanoparticles. Journal of the American Chemical Society 130, 1828–1829 (2008). | |
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