Technique turns carbon dioxide back into coal

03 March 2019

Interview with

Torben Daeneke, RMIT University

COAL

Pieces of coal

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An Australian research team have stumbled upon something that might be the answer in the long term to climate change caused by rising carbon dioxide levels; it sounds like it should be impossible, but it’s not... Speaking with Chris Smith, Torben Daeneke...

Torben - What we have discovered is a new chemical reaction that allows us to convert CO2 back into something that resembles coal. Why is that important? We can actually convert it into a solid safe product. So in the past people have tried to take the CO2 and pressure it up or compress it into a liquid and just pump it underground. But that relies on the assumption that it will stay down there forever. And that just might not be the case...

Chris - So tell us about the technique then. How are you doing what seems - at face value - impossible - to reverse the burning process and get CO2 to become coal again?

Torben - We are using a concept that is called electrocatalysis. It sounds quite complex, but what it actually is is a version of that high school experiment where you take two wires and you put them in a glass of water and you apply electricity and you create bubbles - oxygen and hydrogen. So what we're doing here is quite similar. So we have our liquid metal catalyst. We apply electricity and we use the energy from the electricity to convert CO2 back into coal.

Chris - Now tell us about this liquid metal catalyst; that's obviously the special feature here. So what's in it?

Torben - For all purposes, the liquid metal catalyst looks a lot like mercury but it's not mercury. It's based on gallium. Gallium is another metallic element with a very low melting point and it's completely non-toxic and we use this gallium-based alloy, which is liquid at room temperature, and we add another element to it which is cerium. So cerium is a highly reactive metal that is also used in the little spark stones in fire lighters and these sort of things.

Chris - And what's the chemistry behind this?

Torben - This entire setup is best described as sort of a beaker with a wire in it and then our liquid metal, which also connecting electrically to. And then we've got a fluid in there that's saturated with carbon dioxide. And when we apply electricity to it, we then separate the carbon dioxide molecules into oxygen and into carbon. The cerium inside of our liquid metal really helps to facilitate this process.

Chris - Do you know how the cerium's doing that?

Torben - That's quite a tricky question. We don't fully understand that yet. But our best guess at the moment is that the cerium actually forms an oxide during the reaction and then it gets electro-chemically reduced back to the cerium, so we cycle the cerium atoms between oxide and metallic cerium. 

Chris - And in the process you generate this carbon material, whatever it is. Is it a mixture of chemicals, or is it just pure carbon?

Torben - It is what we would call as a chemist a highly functional ice carbon sheet, which still has some oxygen and hydrogen molecules embedded in them. So it's not pure carbon, and chemically it probably resembles more brown coal and, you know, black graphite coal. 

Chris - But, critically, it's not gaseous carbon dioxide anymore is it. So you've got something which is in potentially a stable, depositable form that could be packed away somewhere safe?

Torben - Exactly. So other electrochemical processes they might create liquids, and a lot of these carbon-based liquids are actually quite toxic themselves, so you can't really store them; and the CO2 if you pump it underneath the ground you really just rely that there's no earthquakes, there's no cracks in the ground that will just simply release it back out of the atmosphere.

Chris - And how does the energy equation stack up? If we look at energy you've had to supply to make this happen, does it look favourable?

Torben - Unfortunately the laws of thermodynamics dictate that whatever energy you got out of burning the coal you will have to put that back into the CO2 to turn it back into coal. From what we have seen in our experiments we don't have to supply a huge amount more to it than what you have gotten out when we burned it. And this means that, ultimately, when we are reaching that point where we have to reduce the CO2 from our atmosphere and actively take it out and turn it back into coal, at that point in time we need to have large scale cheap renewable energies to actually drive this process.

Chris - And the process, obviously this is at the very earliest stages, but could you see this sort of process scaling. Would this work at scale in order to make a dent in what we've done to the planet?

Torben - Yes we do believe that this is possible. So at the moment we're working with a small beaker and a small droplet of liquid metal producing small amounts of solid carbon. But we do not see any fundamental issue why this cannot be scaled up.

Chris - Sounds slightly like I'm trivialising, but is there enough gallium and cerium on the planet to actually do this meaningfully?

Torben - We actually had a look into this, because a lot of people think that gallium is quite a rare material, and cerium as well is actually called rare earth metal. But when we looked at the actual abundance of these materials in the world, they're about as common as other bulk materials like lead or tin. So we do not think that there is a fundamental problem in terms of the availability of these metals in the Earth's crust...

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