Direct conversion of CO2 to solid carbon by Gallium-based liquid metals
265 comments·January 18, 2022
Cool to see this research continued. I've spoken to the authors of a very similar paper, also based in Australia: https://onlinelibrary.wiley.com/doi/epdf/10.1002/adma.202105...
One thing I learned is that these experiments are generally done with a pure CO2 stream. The reactions taking place often compete with other molecules (especially O2), making this approach best for capturing carbon from tailpipes and smokestacks. There's still a lot of work to be done to get this sort of thing to work reliably for direct air capture (which IMO will be needed to actually draw down carbon from the atmosphere).
Anyway, this research is being commercialized via a company called LMPlus (https://lmplus.com.au). While I appreciate that the technology is being brought to market, it probably means that future breakthroughs will occur in private labs, protected by patents and trade secret laws.
We have a an urgent need to conduct more of this kind of research out in the open and to make breakthrough carbon capture technology available to everyone, for free. If you're interested in helping out on this front, please consider joining me in the OpenAir Collective discord (https://openaircollective.cc/).
> One thing I learned is that these experiments are generally done with a pure CO2 stream.
According to this comment three months ago  "they used 6%-20% CO2 mixtures with balance nitrogen and some water to try and approximate different exhausts including flue gas".
I.e. no oxygen. It would probably not be suitable for direct air capture.
Focusing scrubbing smokestacks (where large amounts of new CO2 is concentrated) should be exactly where the focus should be. Direct open air capture really only makes sense once we've learnt to deal with the low hanging fruit.
> One thing I learned is that these experiments are generally done with a pure CO2 stream.
That's probably why these kinds of things will likely only be super effective if taking the immediate exhaust of carbon fuel burning.
1. Legally require all fossil fuel plants capture the carbon from their exhaust, which will have the knock on effect of increasing the economic cost of burning fuel, which means economic forces will naturally start to more strongly prefer carbon free sources, because they're cheaper.
2. Keep pushing for the migration to full BEVs and possibly have a similar requirement of carbon capture devices added to industrial vehicles that can't reasonably be EVs. I could see things like mining and construction vehicles go out with a tank full of fuel and return with a tank full of captured carbon to leave at a dump.
> it probably means that future breakthroughs will occur in private labs, protected by patents and trade secret laws.
This does well to incentivize the inventors; perhaps they could be purchased rather than nationalizing the successful innovation models.
I would almost suggest that gov't should be funding this, but it is primarily private corps doing the polluting. Maybe have gov't fund it, then force polluters to pay gov't back from actual carbon tax?
Ah.. independant invention without significant capital/money based support is very rare in the modern day world. So I'm sure it will support only inventors at an organizational level and at that level, there are lots of skewed incentives for the CXO level peeps to do more marketing that inventing and ensuring most value is captured for the organization than passed on.
Gallium is not cheap at scale. We can only produce trivial amounts, and it is cheap because we don't have many uses for what we produce. It does not form concentrated ores which means it cannot be mined in any conventional sense.
We extract it from aluminum (and zinc) refinery waste because it is relatively convenient in an industrial process sense, but the amount that can be produced this way is inconsequential. The gallium in all the known aluminum ore reserves is measured in kilotons. For this application, we would need something closer to gigatons. We could produce the necessary quantities if that was the sole objective, but the cost per gram would be closer to gold which is not great when you need gigatons.
The alloy is completely reusable as per TFA, I'm not sure why gigatons are needed...
The throughput of carbon sequestration per unit of time is a direct function of the quantity of gallium and the time it takes to execute a complete reaction cycle. Whether or not you recycle the gallium, the time it takes to complete the process cycle limits the number of iterations you can execute per year, which in turn limits the amount of carbon a ton of gallium can sequester per year.
If you need to remove tens of gigatons of carbon per year, you are going to need similar scales of gallium to have enough reactant operating concurrently.
How fo you turn gallium oxide back into gallium?
Hiw do you separate it from carbon or from indium?
Hmm.. i was wondering how cheap it actually was .. Can we re-use the Gallium used in this process?? Can we recover it?? Sorry I haven't yet had time to deep-dive on the article.
ca 720t in 2019
Its not fraudulent, its a good idea, and likely useful in niche large chemical plants. It is also useless against climate change.
The the paper uses a simple input of CO2 and nitrogen. Though O2 could be added as well, as the gallium will simply react with it too. After all, More energy is released by gallium reacting with O2, than by carbon reacting by O2, or it would not happen in the first place. In the atmosphere, CO2:O2 is say 400:1 meaning you need 400 times more gallium.
The next step would be to separate the gallium from the gallium oxide, which is energy intensive, though this can be climate neural using nuclear. To do so would require hundreds of times the global energy production from coal, oil, and gas. And if you have built these, why not just use them instead, global warming would already be effectively stopped.
It also cannot be used in car catalyzes, providing an anoxic reaction chamber which can collect and separate the outputs simply isn't feasible at small scale. The carbon would be in a fine powder, likely to spontaneously react with O2 if exposed to air, either slowly, or if given a spark, very rapidly indeed. The gallium also should not be released into the environment. Not only would large scale use of this tech make it very expensive, gallium is also mildly toxic. However, because it is rare in our environment, there are few if any studies on the impact of long term low dose gallium poisoning. As a result, its a comparable to lead, a few decades ago. Not saying its as deadly, just saying its a metal we are rarely if ever exposed to, known to be mildly poisonous in the short term. And this is before you add the problems of contaminants in fuel, and partial combustion. For similar reasons it also cannot be used in aviation.
At first I ignored the entire HN submission because I've been taught that carbon sequestration will never be a solution to atmospheric carbon.
On a whim I clicked on the comments though expecting the first comment to be, "This will never work, because..." So I was happy to see that the top comment was actually positive! Then I saw your response.
I get that pain -_-'
One of the things which I've seen but feel is rarely talked about is that the planet is sequestering a large amount of CO2 and this increases positively proportionally to CO2 concentration. Over longer periods of time, the CO2 levels drop, and sudden shocks such as volcanos and cataclysmic massive forest fires, are parried by the increase, quickly returning to previous levels. This proves that sequestration does work which is nice. There are many problems with this. At historic CO2 levels on human timelines the natural sequestration is effectively in balance with CO2 production. So its not enough to sequester the additional modern human production, and the prop increase is less than 1, so it wont be. Two, at least some of the systems which are sequestering are doing so as a side effect of severe to catastrophic ecological damage. For rare events this isn't a problem, but our current production is not one year in a century.
It does mean that if we stop using fossile, CO2 levels will revert on their own. Though it will take a long time.
This still is pretty positive- it seems like it can be used as carbon capture inside of existing plants that produce CO2 as a byproduct. It isn't going to solve climate change but it could give us another tool to use.
Getting C out of CO2 require more energy than burning C to get CO2, otherwise you'd get perpetual motion. So, if you want to reduce CO2 emissions, you're better off not burning the C to begin with rather than trying to split the CO2 after the C was burnt.
Even if capturing uses more energy than emitting, it can still be useful since we can Maxwell's Demon the whole process.
We can decide only to emit CO2 at the times when we don't have clean energy ready to use and decide to recapture CO2 when we do have clean energy to use.
Unfortunately we have collectively decided that solving hashes is a better use for clean energy
The reason we don't have clean energy ready to use is that the cost of renewable generation plus storage is currently higher than the cost of burning natural gas. If you add the cost of capture to the cost of natural gas, that's plausibly not going to be the case anymore. And then it's more cost effective to build storage than capture.
No. That doesn't follow at all, and I wish people would stop repeating this overclever "proof".
Burning fossil fuels for energy: hydrocarbon + O2 -> atmospheric CO2 + other stuff
Sequestration: atmospheric CO2 -> not-amospheric-CO2 e.g. solid C + other stuff
Notice that those are not exact reverses of each other, so there is no requirement that the energy released in one be the same quantity as the energy absorbed in the other.
Hydrocarbon fuels have very high Gibbs free energy. Solid carbon does not. We're only trying to get the CO2 out of the atmosphere, not return it to the form of the original fuel that was burned. Therefore, sequestration can potentially require less energy than was originally released from burning the hydrocarbons.
Combustion is an energetic oxidation. The primary chemical energy change is combining the oxygen with other stuff to produce stuff-oxide. If you want the stuff back out in its pure form you absolutely do have to put in more energy in to remove the oxygen than the combustion result produced. Now, depending on the chemical reaction, sometimes the "more energy in" is "waaay more energy in", which can certainly be improved with catalysts and the like, but that can only create improvements like 100x to 1.1x, which is great!, but always >1.
It's not a general rule for all capture processes, but on the case of solid carbon you do need to put back most of the energy yo get by burning the fuel.
Solid carbon is basically coal. It has more usable energy per mass than any hydrocarbon, and just a little bit less per carbon.
Sure, but dont underestimate the inefficiency of power production and sequestration.
Take take the gallium example, for atmospheric levels of CO2, is 400:1. Add the thermodynamic minimum to reverse GalliumOxide to Gallium and O2, and its 800:1, at an absolute minimum. This crushes the difference to methane.
More importantly its a pointless argument, we have practically endless nuclear fuel available, enough to supply the world with power entirely for millions of years. So it does not matter if its inefficient, its climate neutral anyways as long as we stop being idiots and use what we have better. That said there are way simpler more cost effective methods than using a rare earth metal.
You're on to something.
To make it concrete, let's look at methane, the hydrocarbon with most hydrogen.
One kg of methane contains 0.75 kg of C and 0.25 kg of H. When it burns it realeases 55.6 MJ (per wikipedia ). Carbon's energy density is 32.7 MJ/kg, so the 750 g release 24.6 MJ. More importantly, you could in principle split the CO2 for the cost of 24.6 MJ and be left with a net of 31 MJ. Q.E.D.
Well, except in real life nothing is done with 100% efficiency. You can't just use all the 55.6 MJ released by burning CH4, you first convert it to electricity, and the best you get is 63%, so you make 35 MJ of electricity. If you get 70% efficiency in splitting CO2, you get exactly the 24.6 MJ you need. But you don't get that efficiency. But let's just say you get even more than that, let's say you get a whooping 85%. That means you need 30 MJ for the splitting, and you are left with a net of 5 MJ of electricity. Which is another way of saying you increased the cost of electricity you generate by a factor of 6 (=30/5), this ignoring altogether the capital cost associated with the splitting of CO2.
But all is not lost.
If we get back to 1kg CH4 = 0.75 kg C + 0.25 kg H2, burning the CH4 we get 55.6 MJ, burning the C and H2 separately, we get 24.6 MJ + 35.5 MJ = 60.1 MJ, which is 4.5 MJ more. That's a different way of saying you need 4.5 MJ to split CH4 into C and H2. That reaction is called methane pyrolysis . Let's say you manage to deliver this with only 30% efficiency, i.e. for 15 MJ. You are left with 35.5-15=20.5 MJ net energy. If you convert this to electricity you get about 13 MJ, which is not that great. But hydrogen is valuable in itself. If we move towards the hydrogen economy, this method of generating hydrogen may be the winner.
Agreed, the only way this can be useful is to do both: stop burning carbon; use processes like this to recover the carbon from processes where there isn't a good alternative to producing CO2 output. Cement production, perhaps.
There are a number of marine processes which permanently bind carbon, and they do increase with increased CO2 concentration. Problem is that they are overwhelmed by current production. If we reduce CO2 production to zero, the earth will mostly bounce back on its own in a century or two.
Yes, recapturing CO2 at the powerplant is pointless because of conservation of energy. It's always better not to burn the carbon in the first place.
But recapturing CO2 from air isn't pointless - because we can use surplus renewable energy to do so (and then burn C later as fuel). Basically it would use air as infinite capacity (but low-efficiency) rechargeable battery. I've seen estimates of about 13% efficiency over the whole cycle (capturing + burning).
Is there any known or predicted capture technology that does this more efficiently than by growing plants or algae and then burying them underground and/or using them as biofuels?
> surplus renewable energy
is where you lost me. Hell coal is projected to get burned by the megaton for another half century or so because China and India have those resources and that demand. Mankind does not currently have surplus renewable energy.
Yeah, I was about to say... as great as this seems like it may be, surely turning CO2 into coal isn't a net gain versus not burning the coal.
But there are plenty of point emitters of CO2, so if we imagine a grid that's actually renewable sure. Capture though? It could always be paired with another capture method to concentrate it first, it's usually cheap to concentrate things a little and expensive to concentrate them a lot.
Even if we stop all emissions right now, the CO2 already in the air will stay there for a while. We need some way to recapture it if we want to return to pre-industrial levels.
And that might take centuries to do, so we'll probably end up doing some kind of geo-engineering to interrupt the buildup of heat to give us time to extract the carbon.
> We need some way to recapture it if we want to return to pre-industrial levels
To return we probably need to capture well below the "safe" boundary of 350 ppm of CO2, to engage all the feedback loops that will refreeze the arctic etc.
But also, why bother? Biggest problems from climate change are due to the fast change, which causes destruction of ecosystems (including human habitat). Even if we refreeze, we won't revert these losses.
That's a good reason not to apply this technology to smokestacks, since that would make a power plant into a power consumer.
However, in some cases it's quite difficult to stop burning C. Long-haul jets are one example. So for those, it makes sense to make carbon-neutral liquid fuels, even though there's an energy penalty. Pull CO2 from the atmosphere, use renewables or nuclear as an energy source to turn it into fuel. CO2 emissions from steel plants might be another good application.
Ideally we might be better off using clean energy sources to displace fossil plants, but that doesn't happen as fast as we'd like for all sorts of political and economic reasons. So we might as well get started now on other reductions, because it'll still help some, and we can get it scaled up by the time we've decarbonized the energy grid and want to decarbonize everything else.
We don't burn pure C to make CO2 though. We burn hydrocarbons. I am not sure what the effects of the longer carbon chain is. But I feel certain the other extra parts of hydrocarbons leave more energy on the table.
The output might be quite low. But I doubt it would be negative.
Coal is pure carbon more or less. You are right that the math is a bit different for gas and oil.
A lot of people think carbon capture and storage has the potential to save us from climate change, but that idea ignores the scale of the situation.
The world generates ~40 billion tons of CO2 a year, meanwhile the total mass of everything transported in the entire world is only around ~12b tons.
All of the carbon we put in the atmosphere was transported (oil, coal, etc) to the location where it was used so this rings false. Presumably the carbon will be pulled from the atmosphere on site and stored on site anyway.
Maybe something like building solar arrays in semi-arid regions and mixing up the resulting solid carbon into the local soil for some ecological engineering or filling up old mining pits.
The carbon gains 2 oxygens when combusted, tripling the weight of the original fuel
Well we do move that 40 billion tons to the atmosphere. If we can do that, then in principle we can do the reverse. We just need an economic framework to pay for it.
It's certainly going to be cheaper, in most cases, to avoid emissions in the first place. But there will also be cases, like long-haul jets, where it may be cheaper to remove the CO2 later. Plus the CO2 is already above a safe level, so we'll have to draw it down somehow anyway.
We only move a small portion of the 40 billion tons. Most of the mass of CO2 is oxygen, which was already present in the atmosphere before we combined it with carbon.
Related, for comparison: apparently planting 20B trees/year requires only $80M/year (!) and would pull down a trillion tons of carbon over 50 years (!): https://www.youtube.com/watch?v=QyQvfaW54NU&t=14m53s
But they would decay, unless something is done to sequester enough of the wood, and release much of it back into atmosphere, right?
Most trees are planted in order to be harvested as timber. Most of that timber turns back into CO2 in 100 years or so. Tree-planting is not a long-term solution to excess atmospheric carbon. Combined with carbon-trading, tree-planting is a rather obvious channel for greenwashing.
But only about 16% of that mass of CO2 was ever "transported" in the first place: the carbon atom. The O2 part appears for free because it comes out of the air the fuel is burned in. And that oxygen wouldn't be transported with carbon capture either, it would just be vented into the atmosphere again.
I mean think about it, we can't be emitting more waste than we are transporting, because that waste comes from fuel, and the fuel itself is part of what we're transporting.
And then there's the fossil fuel dependency. We still need more than will be produced, even ignoring CO2 levels.
> This process runs at 200C and 1 Bar (=0.98 ATM).
Let's not be misleadingly precise. It runs at 1 atmosphere.
The process oxidizes the gallium, which then needs to be reduced. That's a lot of energy input.
There’s also a massive sun, which has alot of energy output.
So many of the comments here talk about using abundant solar energy as a solution to the atmosphric CO2 problem. But unfortunately, solar energy isn't even close to providing the energy we need for home-heating, transport and industry.
We have to start from where we are: we can't just warp into a future where Dyson Spheres are realistic. We have less than thirty years. I suspect it's actually already too late. Using solar to reduce Gallium, so we can use the Gallium to reduce CO2, while we continue burning hydrocarbons because the solar is all being used to reduce Gallium (or solve hashes), doesn't look to me like the way forward.
So use that energy directly. Why burn carbon, release CO2, capture the CO2, and use the sun to turn it back into carbon?
The creation of solar cells to capture that sun takes time, and rare earth elements which have to be extracted from the environment. To use solar to capture carbon, you'd have to make enough solar panels to replace all the fossil fuel sources, then more. That's not happening any time soon, it can't.
Even if this works and this were marketable today, we still have to take into account the amount of time to scale up production capacities for the reactants and the devices which would to the extraction. On top of that the necessary subsidies to make it economically interesting to have it be deployed far and wide.
Don't get me wrong, if this works we HAVE to do this. I'm just not very optimistic that it could save us at this point. We'd still be fucked, just a little less so.
Can’t use just reduce the gallium oxide with renewable energy when it is available? So your process looks something like:
1. Emit carbon when there is no wind or sun and capture it with gallium.
2. Reduce the resulting gallium oxide when clean energy is available.
That makes this essentially a gallium/CO2 battery.
Also, it’s not true that cycles like this necessarily require more energy than is generated from the CO2 source. I can’t speak to this one in particular, but there are carbon capture techniques that use less energy than is generated by the combustion source. That’s because the carbon ends up in an energy state that’s lower than it’s energy state in the fuel source (e.g. solid carbon is higher energy than CO2 gas but lower energy than hydrocarbons) and because carbon isn’t the only molecule oxidizing in the combustion reaction, hydrogen is too.
> Can’t use just reduce the gallium oxide with renewable energy when it is available?
Renewable energy is not available. It's needed now, in much greater quantities than are available, to supplant hydrocarbons and coal.
It's not needed in peak production periods. Energy price then drops below zero.
Real trouble is that as long as this excess energy is useless there's no economic incentives to build more renewables.
I’m genuinely having trouble following what you’re saying. Can you simplify.
Too much co2 in the atmosphere is assumed to be a bad thing.
Someone says they can sequester co2 from the atmosphere at a certain energy cost.
Is your point that this energy cost is unrealizable?
This solution isn't about pulling it from the atmosphere.. its about pulling it from a clean direct source.
None of this matters.
This is literally burning $300,000 to save $100. No matter how well you optimise that $300K, it will never ever EVER be less than $100. It can't possibly be, that would break the laws of physics and you'd have essentially a perpetual motion (free energy) machine.
If you have 'x' units of carbon (coal), which you burn to make 'e(x)' units of energy and some CO2, turning the resulting CO2 back into 'x' units of carbon will always take more than 'e(x)' units of energy. There is no magical future science that can enable this. It's a fundamental law of thermodynamics!
If it were possible, then you could turn 'x' units of carbon back into 'x' units of carbon PLUS have some free energy left over by running the above in a loop. That's absurd. That's literally a perpetual motion machine. You could start with one ton of carbon and never need more to make infinite energy.
There is an endless series of "science" papers like this published every year, because it gets funding and attention from people that just don't understand that "there ain't no such thing as a free lunch". They're a total waste of time, and have the same logic as trying to power your ceiling lights by using solar cells as your wallpaper and then connecting the lights to the wallpaper. Isn't that brilliant!? The photons from the ceiling lights will produce electricity in the solar cells, which in turn will power your lights! Never pay electricity bills ever again!
NOTE: There are ways of capturing (not "un-burning"!) carbon dioxide and trapping it for less energy that was released by burning the carbon that went into it. That's fundamentally different, and potentially economically viable.
 Literally! Not figuratively! This process literally burns expensive gallium metal to produce cheap carbon and burnt gallium (gallium oxide).
> If you have an energy source that exceeds the world total carbon usage... then you don't need to burn carbon any more!
You would though? Carbon is dense and easy to store and transport in bulk, in your car/plane/etc. Whereas solar/nuclear/etc. aren't.
Hmmm... This is the same result basically. Weirdly, the OP paper doesn't cite this paper in Advanced Materials at all, although the OP paper was submitted 2 days before the Advanced Materials paper was published.
Anyway, good to see it works in two different labs.
EDIT: The Advanced Materials paper's process seems to produce carbon oxides, whereas the OP paper produces solid carbon. I suppose this is an important difference, the latter being more useful.
You can capture the CO2 and even separate the C and the O2 without using more energy than was released by the combustion for two reasons:
1. The hydrogen in the fuel remains oxidized.
2. Captured carbon (compressed CO2 gas, solid carbon) is still at a lower energy state than the source fuel (octane, etc.)
Thank you, that makes sense.
While I agree with you, upon quick search it seems Ga2O3 is an intermediate in the purification of gallium
So, process can be created wherein
basically, the Gallium oxide is reconverted to Gallium to be used in CO2 capture.
3CO2 + 4Ga => 3C + 2Ga2O3 => 4Ga + ... => 3CO2 + 4Ga ....
I believe that converting Ga2O3 back into gallium metal should not necessarily release any CO2, just the O2 that I suppose was originally in the atmosphere anyway.
You should be able to do it with heat, electrolysis hydrogen + heat to reduce it, or electrolysis itself. Theoretically you can do these all with electricity.
It makes way more sense than when I see carbonates brought up as a way to sequester things at least, since the thing being cycled has no carbon in it at least.
Though, if we have enough clean electricity generation capacity to convert that gallium oxide back to gallium without releasing more CO2 than we had sunk in the first place, then one wonders why we're rube goldberging it rather than eliminating the carbon-intensive processes altogether.
In other words, this doesn't sound to me like a solution to our clean energy problems, so much as a cool thing we might be able to do on a mass scale after our clean energy problems have already been solved.
Typically, the solid oxides would be heated in a reducing (H2) environment, forming water vapor and liquid metal (or suboxide).
You could get that reducing gas from hydrolysis, and solar, but it's not energy cheap (~50% efficiency).
This is a good question. Metal oxides tend to decompose at high temperatures, when exposed to reducing agents, under vacuum, or with electricity. I can’t immediately find good sources about which of these apply to gallium and would be most energetically favorable.
But regardless it is certainly possible to regenerate it using only electricity possibly with intermediate but renewable substances (water, sodium chloride, etc)
Probably O2 is a problem because it will oxidize gallium as well.
it has a very high melting point (1900 ˚C) and their experiment capped out at 200 °C so presumably it crystalizes and can be scooped out as well?
The high-value hydrocarbons combust to produce CO2 and energy. So at least as much energy is needed to turn that CO2 back into useful hydrocarbons. Where is that energy going to come from? I believe so far digging these hydrocarbons from the earth is still cheaper than using any energy source to turn CO2 back into useful hydrocarbons.
metallic gallium is also remarkably nontoxic. it’s the halides that’ll getcha.
Funny for a metal that seems to flow like Terminator and can melt an aluminum padlock, see LPL: https://youtu.be/jeghGhVdt9s
(It’s doesn’t melt it exactly, it destroys the structure because Gallium atoms are very small and insert themselves through).
It's aluminum. Ga atoms aren't small compared to Al. It's one step down in the periodic table below Al.
Seems to be basically a pure carbon substance similar to charcoal.
The carbon byproduct is essentially coke, and coke already has a lot of industrial uses.