Nanostructured Materials for a Cleaner Future

By Daria Setman, University of Vienna

Image Caption: A highly deformed metal, with small grains and sharp grain boundaries. The fuzzy regions are represented by a high density of dislocations.

Modern society can be characterized by the desire for individual mobility and for independence from public transportation. However, that freedom is now dependent on fossil fuels, with the added cost of environmental pollution.

Carbon dioxide, which is a by-product of fossil fuel combustion, now attracts attention as a greenhouse gas, but it is not the only pollutant: even more serious for individual health are soot and other chemicals, which also contribute to smog. A safe and easily available alternative to fossil fuels is therefore essential, and the best so far is the fuel-cell-based car.

Every major car maker has already built prototypes of such a car, which are being field-tested. The reasons are obvious: fuel cells have already proved their value in space flights by producing electricity very efficiently, which can then be converted into all sorts of energy with little effort. Another major advantage of fuel cells is the energy source they use: it is hydrogen, available in abundance over 75% of the earth’s surface, the part covered with water. The beauty of the fuel cell is that it does exactly the opposite: it recombines hydrogen and oxygen catalytically to form water and electricity—and nothing else, which means zero pollution. Thus fuel cells are the best alternative source of energy for future generations.

However, it is not easy to store hydrogen. It is the smallest molecule, and ‘escapes’ easily through even the most minute gaps: for hydrogen, the walls and gaskets of the usual storage tanks are like sieves, and storing hydrogen in ordinary gas bottles is useless; storing it as a compressed gas is risky because it can explode and burns readily to boot. It may seem counterintuitive, but there is more storage space for hydrogen within the pores of a block of solid metal than in storage containers meant for storing hydrogen as a liquid or as a gas. This is possible because metals represent by far the most densely packed matter. Therefore storing hydrogen inside metal blocks is an excellent solution to problem of storing hydrogen.

However, the process of loading and then unloading the metal blocks presents a challenge. As molecules of the metal are very closely packed, hydrogen molecules take some time to distribute themselves within the block of metal. Usually the upper layer is penetrated fast, but the process soon slows down; depending on the metal used, the filling procedure can last up to several hours — and nobody wants to wait that long merely to top up a fuel tank. One solution to the problem is to make small tunnels within the storage block to make hydrogen move freely. It is well known in materials science that the so-called grain boundaries are such tunnels. A grain boundary is the less densely packed interface between the small regular crystallites, or ‘grains’, that make up the solid. Therefore, the more the grain boundaries within a block of metal, the faster the loading and unloading. More grain boundaries also mean smaller grains, which again means faster refuelling.

This is where nanostructured metals enter the stage: nanomaterials produced by means of the severe plastic deformation (SPD) have a huge number of grain boundaries and very small grains (well below 100 nm). Additionally, nanostructured metals produce a large number of lattice defects such as dislocations and, especially, vacancies. These defects widen the lattices, creating empty spaces for hydrogen and allowing even higher volumes of hydrogen to be stored.

SPD is a technique where materials are subjected to very high levels of strain but are prevented from fracturing by confining them using hydrostatic pressure. Several methods of SPD have been developed, most notably ECAP (equal channel angular pressing) and HPT (high pressure torsion); even commercial products — tooth implants, for example — made by using SPD are available.

The group on physics of nanostructured materials (http://physnano.univie.ac.at) at the University of Vienna is engaged in answering some questions related to materials produced using SPD: How large are the grains? What sort of defects, and how many of them, are present in the sample? Of course the macroscopic physical properties and how they are affected by micro-structural parameters are also matters of great interest. The next step in exploiting SPD is to optimize the process for specific applications. In the case of fuel cell research, for example, different materials are subjected to different types of SPD processing, and the microstructure of the resulting product is analysed and its mechanical properties are determined. Each time, the amount of hydrogen that could be stored and the loading/unloading characteristics are investigated. After many experiments, the group has managed to find the right combination of materials and the process that offers one of the best loading/unloading kinetics so far. A greener future may be just around the corner.

Daria Setman
University of Vienna
www.atomiumculture.eu

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