Hydrogen is a potential renewable fuel because it can easily be generated from water using electrolysis. It also burns cleanly to produce water vapor. The hope is that it could also be distributed using the same global network of liquid fuel transport that moves petrol around the planet.
But there numerous problems with this dream of a hydrogen-based economy. One is that hydrogen is difficult to store efficiently. Hydrogen gas has a poor energy density by volume compared to petrol. In fact, there is at least 60 percent more hydrogen in a liter of gasoline then there is in a liter of pure liquid hydrogen. In other words, hydrogen will always require bigger tanks.
So finding ways to store more of it is a huge challenge. One option is to store it as a liquid but hydrogen boils temperatures above -250 °C and so requires bulky insulation to keep it in this state.
Another idea is to compress it. But this raises issues of safety should a hydrogen-fueled car be involved in a collision.
That is why much of the material science research in this area has focused on chemical storage: finding materials that adsorb hydrogen efficiently and then release it again when it is required.
Now Viney Dixit and buddies at the Hydrogen Energy Center of Banaras Hindu University in India say they have discovered that carbonized coconut flesh is particularly good at this task. Today, they show that it outperforms a number of other hydrogen storage materials, particularly in its ability to work over many charging cycles.
To help evaluate hydrogen storage materials, the US Department of Energy has set a number of targets that these materials must meet to be considered viable technologies for future transport systems. For example, the current criteria is that a hydrogen storage system must store at least 5.5 percent of hydrogen by mass (5.5 wt %).
This is the mass of the entire storage system and not just the mass of the storage material. So clearly the mass fraction of the storage material must be considerably higher.
Material scientists originally focused their efforts on metal hydrides, some of which can store hydrogen at higher fractions than the DoE criteria. However, these materials have a number of disadvantages. First, they need to be heated to release the hydrogen and this takes energy. Worse, the materials tend to physically break down as the number of charging cycles increase beyond 100 or so.
So in recent years, researchers have turned their attention to carbon. The bond between hydrogen and carbon is known to be quick and reversible. What’s more, it is relatively straightforward to create strong, porous carbon with a high surface area.
One way of doing this is to carbonize biological material, such as fruit or coconut shell. This means heating the material to few hundred degrees centigrade in a nitrogen atmosphere which ensures that the carbon retains its porous biological structure.
Instead of coconut shell, Dixit and co carbonized coconut flesh. They say this has the advantage of containing a wide variety of additional elements, such as potassium, sodium, calcium and magnesium, which are evenly distributed throughout the carbon matrix. And they say this turns out to be significant in their experiments.
These guys have measured the amount of hydrogen that carbonized coconut flesh can hold and say it compares well with more conventional materials. “The synthesized material adsorbs 2.30 wt % at room temperature and 8.00 wt % at liquid nitrogen temperature under 70 atm pressure,” say Dixit and co.
What’s more, the material releases hydrogen quickly and efficiently and does not appear to degrade over many charging cycles.
Whether that is good enough to meet the DoE’s 5.5 wt % criterion for an entire storage system has yet to be seen.
The team spent some time studying the microstructure of the carbonized coconut flesh to work out why it performs so well. And they have pinpointed two mechanisms.
The first is that the carbonized coconut flesh contains a significant amount of potassium chloride, which polarizes the carbon matrix in which it is embedded. “This will enhance the hydrogen adsorption capacity,” they say.
The second is that the carbon matrix also contains significant amounts of magnesium, which is known to enhance the dissociation of hydrogen molecules, making them easier to adsorb.
That is an interesting result that suggests some promising avenues for future research. The presence of molecules that catalyze the adsorption of hydrogen looks to be an important mechanism. It may even be possible to adjust these proportions by growing coconuts in different environments. Another possibility might be to artificially synthesize carbon that matches some of the characteristics of carbonized coconut flesh.
Either way, material scientists might profitably hang their hammocks between some coconut trees in future.
Ref: arxiv.org/abs/1409.7219 : Hydrogen Storage in Carbon Derived from Solid Endosperm of Coconut
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