Hydrogen is an extremely promising form of energy storage. It is not an inherently renewable source of energy like solar power, nor is it a naturally occurring fuel like methane or coal. However, the process by which it releases its energy is very efficient, and the only exhaust gas produced is pure water.
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Figure 1. Hydrogen could power fuel cells in vehicles and combined heat and power (CHP) systems. Top Image Credits: Sandia.gov. Bottom Image Credits: ORNL.gov
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Whilst hydrogen can be burnt like any other fuel (it is the fuel of choice for space rockets), it can also be used to produce electricity directly in a hydrogen fuel cell. Whilst fuel cells will run on other fuels (methanol, natural gas, etc.), they run less cleanly than on hydrogen, and require higher operating temperatures, more expensive materials, and more careful design to avoid fouling and achieve good long-term performance.
Issues with Hydrogen Infrastructure
The problems with adoption of hydrogen thus far have resulted from the production and storage of hydrogen gas. Existing processes for producing hydrogen require a large amount of energy which typically comes from fossil fuels, negating the environmental benefits of the otherwise carbon-free and pollution-free fuel.
Storage of hydrogen is also an issue, as it is highly flammable in its free gaseous form. Storing and transporting the fuel safely is therefore a priority. Much research effort has gone into developing a viable commercial solution to these problems, but so far to no avail.
Nanotechnology could provide more of the answers, however. The most promising recent developments in hydrogen production and storage have involved the use of novel nanomaterials to produce hydrogen from water more efficiently, and to enhance the performance of hydrogen storage technologies.
Production of Hydrogen
The majority of hydrogen is currently produced by steam reformation of natural gas. Despite being a well developed, relatively efficient process when operating on a large scale, steam reformation is highly energy intensive, and obviously requires a source of non-renewable natural gas.
Hydrogen can also be produced by electrolysis of water. This also requires large amounts of energy, typically a combination of heat and electrical energy.
As the energy for these processes is most likely to come from fossil fuel sources, hydrogen is not an especially "green" way to store energy. There has been a major push in the last few years to develop some methods of using renewable energy - primarily solar energy - to directly produce hydrogen, thereby making the fuel what it should be: an efficient alternative to batteries for storing clean energy on a large scale.
Photocatalytic Nanoparticles
Some kinds of nanoparticles (titanium dioxide, which is a common white pigment in its bulk form) have shown very strong photocatalytic activity - the ability to use the energy from sunlight to decompose molecules. So far, this has mostly been applied to self-cleaning surfaces, but a good deal of research has gone into investigating and optimizing the photocatalytic properties of various nanomaterials for splitting water into hydrogen and oxygen.
In 2009, researchers at Northeastern University and NIST discovered that titanium dioxide nanotubes have extremely good photocatalytic activity - much more so than what should be expected from the simple increase in available surface area over nanoparticles. The cause was found to be a very small amount of potassium ions on the nanotube surfaces, left accidentally by the fabrication process.
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Figure 2. Titanium Dioxide nanotubes can convert water into hydrogen and oxygen using the power of sunlight. Further research will try to optimize the technology so it can compete with established natural gas-based routes for manufacturing hydrogen. Image Credits: Menon, Northeastern University.
Storage of Hydrogen
Safe, practical storage of hydrogen has been a major roadblock to widespread use of the fuel. Storing hydrogen as a compressed gas or liquid requires extremely high pressures, which results in expensive tanks and risks of leaks or explosions.
Locking hydrogen into solid materials has long been seen as the way forward. Hydride-forming materials can absorb hydrogen and store it securely at much higher densities than are available by other means. However, conventional materials cannot store very large amounts of hydrogen, and require extremes of temperature to make the capture and release of hydrogen efficient enough for commercial applications.
Nanostructured Storage Materials
Nanostructured materials have unique and tunable properties which are much more suited to this application. Research teams around the world have hunted down nanomaterials which are capable of storing hydrogen at high densities. The key is to find a material which has controllable hydrogen affinity, and can absorb and release its full capacity of fuel in the shortest time possible.
In 2011, scientists at Lawrence Berkley National Laboratory developed a composite material composed of magnesium nanoparticles embedded in a flexible organic polymer matrix. The material is capable of selectively absorbing hydrogen gas, storing it safely at high densities as magnesium hydride, and releasing it rapidly when required.
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Figure 3. Magnesium nanoparticles in a polymer matrix could contain hydrogen safely and in high density, allowing it to be stored and transported efficiently. Image Credits: Lawrence Berkeley National Laboratory.
Conclusion
Nanotechnology seems to hold the key to viable solutions to the two biggest problems holding back the hydrogen economy: production of hydrogen using renewable energy sources, and safe storage and distribution of hydrogen around the world.
Photocatalytic production of hydrogen from water and storage of hydrogen using novel nanomaterials could form the basis of a viable distribution infrastructure, effectively allowing vehicles and local combined heat and power (CHP) systems to run on solar energy, stored as clean and efficient hydrogen fuel.
Sources and Further Reading