Carbon sponge sucks CO2 out of the air

Used in countries with plentiful renewable energy sources, it could help combat climate change...
07 June 2024

Interview with 

Alexander Forse, University of Cambridge

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A Forse carbon sponge

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To chemistry now, and researchers at the University of Cambridge have developed a low-cost and energy-efficient way of making materials that can capture carbon dioxide directly from the air. The method is not dissimilar to charging a battery - but instead uses activated carbon sponge. To find out more, I went to meet Alexander Forse at the University of Cambridge…

Alexander - So here I've got some of our new sponge material, a black piece of fabric-like material that's a couple of centimetres square, here. This is what we've developed in our lab over the past three years. We effectively take these cheap carbon sponge materials and we charge them just like how you charge a battery. After we've done that, we find we're able to suck up carbon dioxide with the material directly from the air around us.

Chris - It's really thin and it looks like the material might stitch into the lining of a coat, for example. It's jet black. What's it made of? Is it just sponge?

Alexander - It's a layered carbon material. We've got sheets of carbon, like what you've got in a pencil which is graphite, except in this material there are spaces between the layers of the carbon, very tiny pores which give it its sponge-like properties.

Chris - It's bendy and flexible isn't it? But you are saying you can also charge it or you have charged it. So what was that process and how does that work?

Alexander - We take this carbon material and charge it up. The sponge here is one of the electrodes in a battery and when you charge it up, you store ions in the tiny pores of the sponge and these ions are going to become important later when we try to bind carbon dioxide.

Chris - When you say you charge it up, you literally put electricity through it? And what are the ions that go in then?

Alexander - Yeah, so in this case we charge in hydroxide ions. This is an alkaline species. Those hydroxides go into the tiny pores of the carbon.

Chris - So you'll get something which is a carbon matrix and the holes are filled with these hydroxide ions, the alkaline substances, and that's now stable? It'll just sit there until you want to do something with it?

Alexander - Yeah, exactly. Overall this material on the solid is positive, but then we've got these negative ions, the hydroxides, sitting in the pores.

Chris - And how does that make it do what you want it to do?

Alexander - What we want it to do here is to suck up carbon dioxide from the atmosphere. These hydroxides can directly bond with carbon dioxide very strongly and thereby remove it from the air.

Chris - What does that make in the sponge, then, when that chemical reaction happens?

Alexander - We looked at this with a technique similar to MRI that's done in hospitals. We do a type of spectroscopy where we see the kinds of bonds that are forming and we can see that we actually form a bicarbonate. So a little bit like baking soda you've got in your kitchen, it's a similar kind of chemical forming inside the sponge.

Chris - And how much carbon dioxide will it soak up and how quickly can it do that?

Alexander - This material can take up about one gram of carbon dioxide per 100 grams of the material. It's quite a small amount, but what we can do is wring out our sponge and get it ready for repeated use so we can potentially do a hundred cycles in a day of absorption and collection of carbon dioxide.

Chris - It's fully reversible? You can expel or push off the CO2 again and it's reset back to reuse?

Alexander - Yeah, exactly. So when we want to collect CO2 off the material and get the sponge ready for another step, we heat it up and that releases the carbon dioxide collection and gets our sponge ready to go again.

Chris - How can you heat it? Because is that not quite energy hungry, potentially? This is all about trying to pull down CO2 from the atmosphere and rescue the planet from climate change and part and parcel of the cause of that is us using too much energy.

Alexander - This process of sucking up CO2 from the atmosphere does use a lot of energy. Our new materials can be reactivated or regenerated at quite low temperatures of around 100 degrees C compared to some of the existing materials where you need to use much higher temperatures approaching 1000 degrees C. We're trying to cut down the amount of energy you'd need to use here. Also, our materials are conductive, so we can heat them up in a way similar to how your toaster works. If you flow current through a resistor, it heats up and we can do that in our material to heat the material very rapidly and regenerate it for another step.

Chris - That's neat. So rather than having to take the material away, do something to it to chemically reset it, it could all be done in situ. You could have a system where you just reverse the cycle, put a current through it, it gets hot, CO2 comes off, you grab that and you're back ready to go again.

Alexander - Exactly. All we need is to be able to plug in to the wall with electricity and then we can heat the material very quickly.

Chris - So where would you use it then? Where would you see this being deployed?

Alexander - Currently there's only a relatively small amount of CO2 being sucked out of the air. A lot of that activity is based in places where you've got very cheap renewable electricity. For example, there's a plant in Iceland that's capturing around 5000 tons of CO2 per year from the air. You really want to make use of cheap electricity, but also you need to be close to places where you can store that carbon dioxide. In Iceland they've got favourable rock formations where CO2 can be stored and permanently trapped in the rock.

Chris - It's all very well grabbing the CO2, but you've got to do something with it once you've caught it. What's the plan there?

Alexander - Either store it in the ground in a way that's safe, where it's permanently stored, or make something useful out of the CO2. Currently we produce so much carbon dioxide, around 40 billion tons per year. The big potential here is to store the carbon dioxide in the ground because if you start making stuff out of it, you end up making a lot more stuff than people would actually need.

Chris - Can you take it a step further and do this for other gases?

Alexander - I think so, yeah. To take one example, during the Covid pandemic, we needed oxygen supplies in hospitals and there were big shortages that were well documented. Things like separating oxygen out of air, oxygen from nitrogen, is really important. We've got a new way to make materials now with this charging process, so maybe we can extend it to some other areas like that.

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