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papaya
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In computation you supposed that your reaction goes to completion, this may be not true in reality (thus lower yield of energy). And why would anyone
mix glycerol with H2O2 if it's better to use separated cells in battery?
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deltaH
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| Quote: | | Doesn't adding an oxidizer to glycerol just cause it to burn? I know this happens if you add potassium permanganate to glycerol.
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My worry too when I first thought about, so I tried it and the answer is no. As bismuthate pointed out, the
amount is probably too little. That said, I was worried of partial oxidation, however, this appears slow in the absence of catalysts and dilution of
peroxide. Remember the glycerine dilutes it as well!
As an aside, if I remember correctly, there may have been a very slight warming instantly upon mixing, but I believe that might be a simple heat of
mixing because I was using pure glycerine (b.p. grade used on hair and bought from a pharmacy) and 50% H2O2 (200 volume) for swimming pools,
presumably stabilized, but I was a bad scientist and didn't keep notes so I can't be sure of the warming, might just be confusing it with something
else I may have mixed at the time! Lesson learned!
What is a worry is peroxide's self decomposition to oxygen and water, however, this is also slow and these days can be slowed to a crawl by the
addition of small amounts of stabilisers and proper pH.
Anyhow, so long as you don't leave it in a closed container but use a vented one instead and use it within a reasonable time period (months?), then
probably ok, but I don't know what the self partial oxidation (uncatalysed) would be over a long time, maybe not negligible.
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deltaH
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| Quote: | | In computation you supposed that your reaction goes to completion, this may be not true in reality (thus lower yield of energy). And why would anyone
mix glycerol with H2O2 if it's better to use separated cells in battery? |
Absolutely, if I cannot get
glycerine to be oxidised fully to CO2, then it must decrease the amount of usable work I get overall. But this is just an inefficiency. Even a partial
oxidation of glycerine is exothermic, it just wouldn't be good use of the fuel if I only partially oxidise it overall, but I will still get net work.
However, this is one of the reasons for using the hydrogen peroxide (strictly speaking on overall arguments it shouldn't be necessary).
Whether one oxidises the glycerine fully on the anode or not is a kinetic problem. The best you can do is employ a good electrocatalyst here to try
and minimise barriers so that you don't have a large overpotential for complete oxidation. BUT there is one other thing you can do and I think this
can really help, it's the peroxide. Normally on the anode you would be oxidising glycerine with water, this is energetically not favorable in
intermediate steps but ok overall, when you get to making CO2 which really drives the whole thing home.
But now you have [some] peroxide instead of water, so you now are oxidising glycerine with peroxide and some water at the anode, this should be far
easier to proceed on intermediate steps because of the low activation energy for peroxide as oxidant (compared to water).
What do you think?
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deltaH
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| Quote: | | And why would anyone mix glycerol with H2O2 if it's better to use separated cells in battery? |
Appologies,
but I want to specifically zero in on this point from another angle as well.
Consider steam reforming, say with methane and superheated steam, from wikipedia's article on it, I quote:
"At high temperatures (700 – 1100 °C) and in the presence of a metal-based catalyst (nickel), steam reacts with methane to yield carbon monoxide
and hydrogen. These two reactions are reversible in nature.
CH4 + H2O ⇌ CO + 3 H2"
They then go on to say that you can use the water gas shift reactions at lower temperature to make even more hydrogen and convert the CO to CO2.
Okay, the point is that the first steam reforming step is very endothermic because it makes so much hydrogen.
Industrially, you have two choices to solve this problem. You either burn fuel on the outside in a furnace with these gases flowing in pipes on the
inside OR as is often the case and simpler, you burn some of the methane internally with oxygen to make the heat for you!
How much? Just enough for this to tick over.
So you feed CH4, superheated H2O and some O2 and it works, you still get out CO and H2, but because of the oxygen, you change the CO to H2 ratio that
you get. Sometimes having a lower CO:H2 ratio is imperative for the next down stream step (whatever you were making this mixture for), in that case,
you have no choice but to run the reformer without co feeding oxygen and worry about the extreme heat transfer that is needed from the outside through
the pipe walls in your furnace.
Anyway, as I've often said before, my system is similar, except that we go the low temperature equivalent and therefore have to use more reactive
things for kinetic reasons. So instead of methane or hydrocarbons (too inert at low T), I propose glycerine as a compromise between hydrogen making
capacity and reactivity (sacrifice some hydrogen making capacity compared to pure hydrocarbons but gain reactivity). However, just like steam
reforming methane, the reaction:
C3H8O3 + 3H2O => CO2 + 7H2
is very endothermic, so you 'fix' this by the same trick as in steam reforming, you co feed an oxidant. However, you want your oxidant to also be nice
and kinetically active (by the way oxygen is not as kinetically active as you might think, it has the slowest kinetics in a fuel cell even with
platinum catalysts), so I propose using H2O2, which is in between H2O and O2 but has fantastic chemical kinetics on a catalyst.
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deltaH
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| Quote: | | You would get more energy, and the same waste products, if you burned glycerol with oxygen. An oxygen cylinder would be more practical to carry around
with you than an electrolysis cell. |
You would make more heat, but heat alone is useless, you need to
convert it into useful work. You can do so by using an engine and this is where you end up only converting only a fraction of your heat into useful
work because of the inherent thermodynamic inefficiency of an internal combustion engine or heat engine. Fuel cells convert chemical potential energy
into electrical work and as they operate at low temperatures with catalysts, can theoretically do so in a far more efficient manner than what a heat
engine can (albeit at a higher $ cost for now). Still not convinced, see this article on 'exergy'.
High efficiency electric motors are also a particularly efficient way to convert electrical work to mechanical work because they too do not need to
operate at high temperatures.
Need any more proof that running at low temperature with catalysts is the best way to go? Consider how nature does it with lifeforms 
There's a reason our bodies don't run on V8 engines, though it does make for entertaining snail racing movies!
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papaya
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Quote: Originally posted by deltaH  | | Quote: | | And why would anyone mix glycerol with H2O2 if it's better to use separated cells in battery? |
Appologies,
but I want to specifically zero in on this point from another angle as well.
Consider steam reforming, say with methane and superheated steam, from wikipedia's article on it, I quote:
"At high temperatures (700 – 1100 °C) and in the presence of a metal-based catalyst (nickel), steam reacts with methane to yield carbon monoxide
and hydrogen. These two reactions are reversible in nature.
CH4 + H2O ⇌ CO + 3 H2"
They then go on to say that you can use the water gas shift reactions at lower temperature to make even more hydrogen and convert the CO to CO2.
Okay, the point is that the first steam reforming step is very endothermic because it makes so much hydrogen.
Industrially, you have two choices to solve this problem. You either burn fuel on the outside in a furnace with these gases flowing in pipes on the
inside OR as is often the case and simpler, you burn some of the methane internally with oxygen to make the heat for you!
How much? Just enough for this to tick over.
So you feed CH4, superheated H2O and some O2 and it works, you still get out CO and H2, but because of the oxygen, you change the CO to H2 ratio that
you get. Sometimes having a lower CO:H2 ratio is imperative for the next down stream step (whatever you were making this mixture for), in that case,
you have no choice but to run the reformer without co feeding oxygen and worry about the extreme heat transfer that is needed from the outside through
the pipe walls in your furnace.
Anyway, as I've often said before, my system is similar, except that we go the low temperature equivalent and therefore have to use more reactive
things for kinetic reasons. So instead of methane or hydrocarbons (too inert at low T), I propose glycerine as a compromise between hydrogen making
capacity and reactivity (sacrifice some hydrogen making capacity compared to pure hydrocarbons but gain reactivity). However, just like steam
reforming methane, the reaction:
C3H8O3 + 3H2O => CO2 + 7H2
is very endothermic, so you 'fix' this by the same trick as in steam reforming, you co feed an oxidant. However, you want your oxidant to also be nice
and kinetically active (by the way oxygen is not as kinetically active as you might think, it has the slowest kinetics in a fuel cell even with
platinum catalysts), so I propose using H2O2, which is in between H2O and O2 but has fantastic chemical kinetics on a catalyst.
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I don't understand your arguments here, I supposed you not to exclude peroxide or change your overall equation, but separate glycerol & H2O2 to
anodic/cathodic cells - for the SAME amounts of reagents you get HIGHER concentrations => higher potential difference, isn't it?
But of course, the thing overall is too complex - peroxide needs to be produced in another plant (by electrolysis for example), H2O2/glycerol mixture
looks like a rocket fuel (who knows if oxidation is auto-catalytic then it may go off some day, etc) and then you want to build a vehicle! If you were
to build an electricity generation plant then maybe this would be accepted more calmly.
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deltaH
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Thanks Papaya, I love your questions, they really challenge me!
| Quote: | | I don't understand your arguments here, I supposed you not to exclude peroxide or change your overall equation, but separate glycerol & H2O2 to
anodic/cathodic cells - for the SAME amounts of reagents you get HIGHER concentrations => higher potential difference, isn't it?
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I see your point and frankly I'm not sure about this point at all, I think because we are using the peroxide
in substoichiometric amounts to 'fix' the system, then it's placed on the anode side together with glycerine, in fact what would happen if you used a
fairly concentrated sulfuric acid electrolyte as what I'm currently thinking of using (because the peroxide would quickly form persulfuric acid which
as anions would concentrate at anode, and anions is exactly what nearly all would be at around 70% sulfuric acid strength!)
But maybe this is a big problem, I don't know, if at first I absolutely cannot get it to work one way, I will try the other.
As for your safety issues, ANY energetic fuel is dangerous... how many times do people die because gasoline catches fire in car accidents? To be
honest though, I don't think such a sub stoichiometric mixture of glycerine would be as flammable as gasoline. In fact when I made it, I couldn't like
it easily with a flame!!!
I say SAFER than gasoline and non toxic and non-volatile and non-smelly!!! In fact, probably have to purposefully make it smelly, bitter and coloured
so that people don't accidentally drink it. Off course strong peroxide can still damage tissue on contact... certainly will make skin temporarily
white but that's harmless. Never tried getting 30% H2O2 solutions on my skin though, maybe somebody can advise if this does major damage or just makes
skin white temporarily.
I know that peroxidases in our bodies neutralise peroxide very quickly, but does high concentrations overwhelm it?
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deltaH
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Did some more simple thermo calculations in EXCEL using that data from before.
I was interested to find out how much peroxide to water was necessary to make the hydrogen generation dGrx = 0.
Turns out you need very little:
C3H8O3(l) + 0.2H2O2(l) + 2.6H2O(l) <=> 3CO2(g) + 6.8H2(g)
Has a dGrx standard = 0!
Ok in reality maybe you want a little more to help it alone, but this is the theoretical value, don't know if it is optimal though.
[Edited on 4-10-2013 by deltaH]
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papaya
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Discard my last posts, I thought about H2O2-glycerol "battery" when suggesting separated cells, not the electrolysis...
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deltaH
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| Quote: | | Discard my last posts, I thought about H2O2-glycerol "battery" when suggesting separated cells, not the electrolysis... |
Might make a nice battery though, alkaline medium so no gases generated, microvent for peroxide... would need come catalyst on the
anode though for the glycerine. What metal to use for cathode not to catalyse H2O2 decomposition, zirconium? Pricey... but oh so cool!
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blogfast25
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Yeah, I worked out yesterday that the ΔGReaction for that reaction but using O2:
C3H8O3 + 3/2 O2 === > 3 CO2 + 4 H2
… is very close to 0. It's not a surprise: you lose a lot of bonding energy and gain relatively little of it in return. So those reactions are more
or less 'energy neutral', at least on paper.
So your idea is to carry out this reaction (but with peroxide) by electrolysis, then use the H2 in a separate fuel cell to power the electrolysis of
glycerol (plus some oxidant) AND power propulsion of the vehicle TOO? Is that the idea?
[Edited on 4-10-2013 by blogfast25]
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papaya
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| Quote: Originally posted by papaya | | Discard my last posts, I thought about H2O2-glycerol "battery" when suggesting separated cells, not the electrolysis... |
Hmm, but I thought a bit harder - I was right? Because if you plug that "battery" to external voltage REVERSED polarity then it certainly should
help(hmm, you might generate hydrogen just shorting that battery?).
Also you have to separate H2O2 and glycerol, because if the oxidation and reduction takes place in the same compartment (on the same electrode) then
you don't have opportunity to get that electrons flow to external circuit.
Now I feel totally lost, I must have had many errors here...
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deltaH
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@blogfast
Thanks for the confirmation on the calculation, always good to have a check or related check.
Yes that is correct, did you read my long thermodynamic calculations post I wrote in reply to you earlier (long post on page one)? It may have gotten
lost in the wash of posts since.
[Edited on 4-10-2013 by deltaH]
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deltaH
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| Quote: | Hmm, but I thought a bit harder - I was right? Because if you plug that "battery" to external voltage REVERSED polarity then it certainly should
help(hmm, you might generate hydrogen just shorting that battery?).
Also you have to separate H2O2 and glycerol, because if the oxidation and reduction takes place in the same compartment (on the same electrode) then
you don't have opportunity to get that electrons flow to external circuit.
Now I feel totally lost, I must have had many errors here... |
Yeah this is a tough topic to grapple with, I
struggling too if that makes you feel better.
Anyway, I am not exactly sure what you are trying to say here, but maybe this helps... at the moment with just a little peroxide, we have dG = 0, so
it's neither battery nor electrolysis, but we can force it to be electrolysis. Add more peroxide, then dG drops dramatically the more you add and it
becomes more and more a battery system but less and less hydrogen to produce, to the extreme where you add stoichiometric amounts of peroxide
(separately off course) to maximise battery power and then your equation becomes:
C3H8O3 + 7H2O2 => 3CO2 + 11H2O + Huge energy
Hold on... I think I had a brainwave from that... maybe total nonsense, but isn't it that because we are targeting dG = 0 operation mode, it doesn't
matter where the peroxide goes, annalyte or catholyte side. This is only a issue when targeting some distance away from dG, so if you want to minimise
dG (i.e. make a battery) then you MUST put peroxide on cathode side and glycerine on anode side?
I declare that this topic is now officially causing me a critical mass in my brain lol
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blogfast25
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deltaH:
Yes, I read your calculations w/o revising them in detail. At first glance they're sound.
Whatever oxidant you use, the real challenge will be to stop the oxidation from going all the way:
C3H8O3 + [oxidant] === > 3 CO2 + 4 H2O
... because the ΔG of that reaction really is strongly negative.
Adding whatever oxidant in drips and draps won't help: with an excess glycerol present the oxidation will always go to the end, i.e. water instead of
hydrogen.
I no longer think you were peddling pseudo-science, only that extracting hydrogen from glycerol by whatever mechanism will be extremely challenging
(and may cost more energy than paper calculations suggest).
You may want to consider 'something enzymatic'...
You chose glycerol because that way the idea is carbon-neutral, right? Glycerol is NOT very hydrogen-dense though...
[Edited on 4-10-2013 by blogfast25]
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deltaH
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Thanks blogfast, I appreciate it greatly.
You raise an interesting and problematic point about the complete oxidation of a small amount of glycerol that could mean bypassing the
electrochemical mechanism altogether and thus just create heat and drop hydrogen generation capacity. I will need to give this more thought and
consider exactly what could occur on the surface of the anode catalyst, as really this is where all the action will presumably be occurring. I'll have
to get back to you on that!
I recall reading an article a while back about a proposed enzymatic route for glycerine oxidation electrochemically. Off the top of my head, that was
for a partial oxidation. I think my worry there was that enzymes are too specific, we really want to take it all the way here and that's a very large
number of reaction steps before you end up with CO2. Also, I don't like the very narrow operating range where you can use enzymes, I think it can be a
problem with such brute force devices as I am considering, after all, I'm going to be driving these electrolysers crazy hard if I can ever get this to
work, I just don't think enzymes will survive such operation. If I remember correctly, they were operating at very low current densities with those
enzymes on the anodes.
As for your final point about why glycerol, yes you are correct, as organic fuels go, it's far from ideal. The point here is that for reactivity
reasons you need at least every carbon functionalised to the very least to be an alcohol. So on the table are things like methanol, ethylene glycol,
glycerine... etc. So some are out: methanol... too toxic and volatile for my liking, plus synthetic (mostly), ethylene glycol (toxic, synthetic), that
leaves glycerine and sugars, bio glycerine prices have plummeted because of the surplus from biodiesel industry, so that's great, plus it's completely
non-toxic and a liquid. If you're wondering why I didn't go for ethanol, that's because of the poor kinetic again of not having every carbon at least
once functionalised, though I have read a while back that a research group has prepared a rhodium electrocatalyst that works well for direct ethanol
fuel cells... well being VERY relative here or else we would all already be running our car on a direct ethanol fuel cell lol
In fact, I will put the open question to the SM community, can you suggest a better fuel than glycerine for generating hydrogen by electrolysis. Think
of many factors, not just capacity. Think of capacity, sustainability, toxicity, cost, availability, chemical reactivity etc. Maybe I'm missing
something better?
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blogfast25
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deltaH:
Recalculating ΔG for:
C3H8O3 + 3/2 O2 === > 3 CO2 + 4H2
... using data from that *.pdf you linked too, I now got quite a negative value at 298 K (about - 400 kJ/mole) but in essence that doesn't change
much.
[Edited on 4-10-2013 by blogfast25]
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blogfast25
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I don't know of a better fuel than glycerol but something with the empirical formula CnO2nHm would have enough O to
split it into n CO2 and m/2 H2. No oxidant needed. Totally hypothetical at this point of course...
[Edited on 4-10-2013 by blogfast25]
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bfesser
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Threads Merged
4-10-2013 at 09:48 |
bfesser
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Threads Merged
After getting a better perspective on this from another moderator, I've decided to re-open and merge the original topic. I get tired of saying this,
but; <em>please keep the conversation civil</em>.
<strong>deltaH</strong>, I suggest that you learn to deal with criticism a little better. Stop imagining and/or complaining that you or
your idea is being unfairly dismissed, insulted, or attacked. Someone can call your idea idiotic without calling you an idiot. I have idiotic ideas
all the damn time, but that doesn't make me an idiot—other things do! 
<strong><a href="viewthread.php?tid=19143">The ScienceMadness Guidelines</a></strong>
[Edited on 4.10.13 by bfesser]
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deltaH
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Yay, threads merged again... now I can refer to earlier posts and it doesn't all become a garbled mess. Thanks bfesser!
Anyhow, now back to more serious matters...
@blogfast25
| Quote: | | I don't know of a better fuel than glycerol but something with the empirical formula CnO2nHm would have enough O to split it into n CO2 and m/2 H2. No
oxidant needed. Totally hypothetical at this point of course... |
I love your thinking here, approach from
the outside and then zeroing in. Starting with an empirical formula and the see what fits the bill is smart.
The classic case here is formic acid, H2CO2 BTW and I've touched on this earlier on in this thread.
You're right, kinetically, it's relatively 'easy' to get formic acid to decompose to H2 and CO2 because the carbon is so close to being fully
oxidised. There's a VERY large body of literature on many different catalysts to do so... including my friend ruthenium!
I quote from wiki's article on formic acid thus:
"In the presence of platinum, it decomposes with a release of hydrogen and carbon dioxide. Soluble ruthenium catalysts are also effective.[22][23]
Carbon monoxide free hydrogen has been generated in a very wide pressure range (1–600 bar).[22] Formic acid has even been considered as a material
for hydrogen storage.[24] The co-product of this decomposition, carbon dioxide, can be rehydrogenated back to formic acid in a second step. Formic
acid contains 53 g L−1 hydrogen at room temperature and atmospheric pressure, which is three and a half times as much as compressed hydrogen gas
can attain at 350 bar pressure (14.7 g L−1). Pure formic acid is a liquid with a flash point of +69 °C, much higher than that of gasoline
(–40 °C) or ethanol (+13 °C)."
Refs:
22. a b C. Fellay, P. J. Dyson, G. Laurenczy (2008). "A Viable Hydrogen-Storage System Based On Selective Formic Acid Decomposition with a Ruthenium
Catalyst". Angew. Chem. Int. Ed. 47 (21): 3966–3970. doi:10.1002/anie.200800320. PMID 18393267.
23. G. Laurenczy, C. Fellay, P. J. Dyson, Hydrogen production from formic acid. PCT Int. Appl. (2008), 36pp. CODEN: PIXXD2 WO 2008047312 A1 20080424
AN 2008:502691
24. Joó, Ferenc (2008). "Breakthroughs in Hydrogen Storage-Formic Acid as a Sustainable Storage Material for Hydrogen". ChemSusChem 1 (10): 805–8.
doi:10.1002/cssc.200800133. PMID 18781551.
This is just a snippet, a quick literature search on this topic turns out masses and masses of papers on the topic...
Now as I mentioned right back in the beginning of this thread, I think formic acid is no good...
It's corrosive, it's smelly as hell and you get a meager 4.4% hydrogen out of it by mass.
Guys, coming back to the big black box approach, you just have to look up the heat of combustion and get a quick idea whether you have a good fuel!
This holds even for this kind of system!
Formic acid's heat of combustion is a meagre 5.5 MJ/kg
That's pathetic as fuels go, compare to
gasoline's heat of combustion is a wopping 47.3MJ/kg
So I would conclude that CnO2nHm types are over oxidised and can't perform well on energy density arguments.
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blogfast25
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deltaH:
"It's corrosive, it's smelly as hell and you get a meager 4.4% hydrogen out of it by mass."
With hydrogen you're always going to get that: for glycerol it's only 8.7 %, not brilliant either. Even for water it's only 11 %. Try deuterium (just
kiddin')
How about di or tri carboxylic acids: oxalic acid, citric acid? Just 'off the cuff' here... These have low hydrogen contents too of course but it
might be easier to snip it off, vis-à-vis glycerol?
[Edited on 4-10-2013 by blogfast25]
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deltaH
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@blogfast25
I'll stick to my 8.7% thank you lol
By the way, before I considered polyols, I had the 'brainwave' of realising this was actually all about energetic materials, so I started thinking
about along nitrogen containing lines...
One option was nitromethane in the don't-need-to-add-water-class-of-things:
2CH3NO2 => 2CO2 + 3H2 + N2!!!
But it's not a renewable, it's dangerous and and and... but you see how energetic materials can come into play
Hydrogen content of nitromethane is 5% and heat of combustion is 11.6MJ, so as liquid things that come prepackaged with their own required oxygen go,
I don't think you can beat nitromethane...
Now calling all the energetic materials experts to run on over to this thread LOL
[Edited on 4-10-2013 by deltaH]
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papaya
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Urea ? (piss into car )
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deltaH
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| Quote: | | Urea ? (piss into car ) |
lol, yeah I
actually looked into urea... urea for all intents and purposes is a clever non nasty form of ammonia, well that's what it behaves like in these kind
of systems so since people thought ammonia was smart, certainly urea is smarter...
The corresponding reaction is (NH2)2CO + H2O => CO2 + 3H2 + N2
Pure urea is 6.7% H2 in theory, BUT it's not a liquid, though you can dissolve a lot in water. But it fails when it comes to energy content!!!
Incidentally, the urea as a source of hydrogen idea is commercially used in Diesel exhaust fluid exactly because it's pretty decent.
Germans...
Basically diesel engines run with a slight oxygen excess to minimise soot, but that means that there's not reducing agents in the fumes for the
catalytic convert to reduce the NOX's with. So they use a solution of urea in water sprayed on the catalyst in the catalytic converter full of PGMs,
off course the PGMS quickly decompose the urea which is 'desguided solid ammonia' into hydrogen and voila... pretty clever!
See this wiki: http://en.wikipedia.org/wiki/Diesel_exhaust_fluid
Anyhow, as for this threads topic, I think we can do better than urea...
BTW while we at it on solid nitrogens, I had also considered trimethylamine N-oxide as a safe 'disguised methanol' that would be easy and cheap to
make, but also discarded by me.
[Edited on 4-10-2013 by deltaH]
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papaya
National Hazard
Posts: 615
Registered: 4-4-2013
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urea + choline chloride = deep eutectic solvent (liquid at RT)
google finds many things http://www.erc.arizona.edu/seminar/Current-2011/DineshThanu_...
What is your final purpose? Do you intend to build such a cell, or convince some company to develop your ideas there or what? You seem to be more
serious with it than just an amateur project (I find it impossible to build these hi-techs for someone alone).
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