D4RR3N
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Storing hydrogen gas in a carbide ?
Many of those proposing cars run on hydrogen gas have promoted the idea of storing the gas in pellets or powders such as metal hydride which can be
later released for usage.
I was thinking a carbide might work better, whats your thoughts on this, are there any carbides that have an affinity for hydrogen?
Silicon carbide
http://pubs.acs.org/doi/abs/10.1021/jp106509g
Boron carbide
http://www.hydrogen.energy.gov/pdfs/review12/bes008_pfeifer_...
Doped carbon nano powder
http://www.hindawi.com/journals/jnm/2013/742075/
Carboranes
http://onlinelibrary.wiley.com/doi/10.1002/er.1886/abstract
http://www.google.es/patents/US7521564
[Edited on 8-10-2013 by D4RR3N]
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D4RR3N
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"The invention relates generally to high density propellant fuels, and more specifically, it relates to a new group of propellant fuels and fuel
additives that use heterocyclic borane and carborane salt ingrediants.
Neutral borane and carborane compounds are known to be high energy compounds and have been considered for use in both formulated rocket propellant and
explosives applications."
"Additional invention novelty would come from their unprecedented use as potential propellant fuels, fuel additives, or in fuel cell, gas generator
and hydrogen storage applications."
Could Ortho-carborane B10C2H12 be used to store hydrogen?
When it releases hydrogen would it return to Carbon boride?
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deltaH
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Hi D4RR3N
Well done for thinking about hydrogen storage... it's a topic close to my heart. I too started out thinking about many substances. There's so much to
say about this topic! Some basic points in general (not necessarily geared to you):
1. There's two trains of though when it comes to chemically storing hydrogen, the first school says absorb it into transistion metal powders to form
[usually] non-stoichiometric metal hydrides. These systems are often very easily reversible, but they have the down side of only being able to store a
small amount of H2 and they are also particularly heavy too boot.
The practical application for these in my opinion in the context of fuel cells is not as primary hydrogen storage, but as a buffer tank between your
primary and your fuel cells.
2. The second school makes use of the much stronger inorganic to hydrogen covalent bond to store much more hydrogen. Here you really go for maximising
hydrogen storage density. That is why one focuses on boron, carbon, nitrogen and oxygen. Boron compounds are of great interest because of their very
high reactivity and energy content, so yes then can be used, but they are not very practical in my opinion compared to other compounds which are
cheaper and safer to work with and have similar capacities.
The classic borane based source is sodium borohydride:
NaBH4 + 4H2O => NaB(OH)4 + 4H2
Water vapour could simply be supplied from your fuel cell exhaust downstream and there's nothing particularly weird you need to do for this reaction
to proceed.
Also NaBH4 has a hydrogen generation capacity of 21%, which is awesome really, BUT such a system is still not considered very practical because of the
high cost of manufacturing it, plus it's a solid which is not as convenient for 'tanking up'.
OK, so back to your carboranes, I am no expert here, but I will take a stab at it, I think the reaction you are expecting is:
B10C2H12 + 34H2O => 10B(OH)3 + 2CO2 + 25H2
Therefore H2 production ability is also about 21%
I'd go with the much simpler sodium borohydride, though that probably won't fly on costs and for other reasons.
Finally, you mentioned a whole bunch of other materials and potentially could do many more... so I will give you a general approach for evaluating the
feasibility of any such candidate.
Let's use silicon carbide as an example:
STEP ONE
Evaluate the thermodynamic feasibility of your reaction, so say you want to react it with water, consider:
SiC + 4H2O => SiO2 + CO2 + 4H2
for example.
Calculate dG standard of reaction. If this is highly positive, then your reaction is thermodynamically unfavoured in that direction and thus not a
good candidate.
STEP TWO
Consider kinetic issues. How easily does your reaction proceed? For example, borane derivatives are generally reactive enough to react straight with
water, but the majority of simple organic compounds are not (C-H bonds are damn strong and so the activation energies in breaking them are huge
without catalysts)
If the kinetics don't sounds so fantastic, then you need to consider catalytic routes. Consider what's available and known for your system.
STEP THREE
Economics and other factors. This one is really a huge step, but basically it boils down to how practical is your proposed fuel: what is its hydrogen
production capacity on a mass basis? is it readily available? Is it cheap? Is it sustainable? Is it safe? etc.
This is the process I went through with my glycerine as hydrogen source topic before deciding on it as a good candidate 
[Edited on 9-10-2013 by deltaH]
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D4RR3N
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Thanks deltaH
Boron doped activated carbon is supposedly good at absorbing H2
As for the price of Boron, if you could start with something like boric acid that's dirt cheep.
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bismuthate
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You could even start with borax.
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PeeWee2000
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I know the topic is hydrogen storage but why not just use calcium carbide and react it with water, that gives you plenty of acetylene to run your
engine off of and is fairly easy to regenerate assuming you have the proper equipment (boy the many uses of an arc furnace!) as well as having more
potential thermal energy (not sure about energy density) thanks to the carbon carbon triple bond. This seems much more simple and practical for a do
it yourselfer than hydrogen production from fancy pants doped nanotubes. The only downfall I see with acetylene is that you have to find more carbon
to use to replenish your calcium carbide, not that thats a difficult task however.
Using acetylene would also avoid any risk of hydrogen embrittlement although that would not be likely to happen as most engine blocks are cast iron.
Very interesting topic though, I've been wanting to run one of my cars off an alternative fuel for a while waste oil is probably what I'll settle with
though as I have a nearly unlimited supply at my hands already
“Everything is relative in this world, where change alone endures.”
― Leon Trotsky
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bfesser
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Quote: Originally posted by PeeWee2000  | | The only downfall I see with acetylene is that you have to find more carbon to use to replenish your calcium carbide, not that thats a difficult task
however. |
Conveniently, we're currently faced with an uncomfortable surplus of atmospheric
CO<sub>2</sub>. One of the major problems that I see with your suggestion is that acetylene does not easily burn cleanly—at
least as far as I'm aware. I've long been a fan of the <a href="viewthread.php?tid=8904#pid102527">Al-Ga system</a> for generating
hydrogen gas from water.
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