Artificial leaf turns CO2 into fuel

One of the challenges we face when it comes to climate change is not just how to reduce carbon emissions, but how to scavenge back some of the carbon we’ve already put into the atmosphere... ideally without compromising our standards of living. And this week scientists at Cambridge University unveiled a technology that might help to achieve these aims: it’s an ‘artificial leaf’ that, like real leaves, uses light to turn carbon dioxide into a useful fuel - in this case a mixture of carbon monoxide and hydrogen called “syngas”. Katie Haylor went to see it in action with its inventor, Erwin Reisner...

Erwin - We have demonstrated that we can convert the greenhouse gas carbon dioxide together with water into an energy carrier which is known as syngas. Syngas is a gas mixture made out of carbon monoxide and hydrogen, currently used for methanol synthesis for example, you can also extract the hydrogen from syngas to make fertilizers. We are excited about syngas because syngas can be converted to liquid carbon-based fuels, essentially gasoline.

Katie - Let's take a closer look then. So on the desk in front of us is a box. Is that plastic? It's transparent in nature and then there's lots of nuts and bolts and things going on. Separately you've got a little square with some wires coming out of it. Can you explain what is happening step by step?

Erwin - Absolutely. So inside the box, we have this artificial leaf which means this is electrodes, and the electrodes now take up the light, and this energy from the light is being used to drive our catalysts.

Katie - Is the light being absorbed? What's going on? What materials are we talking about?

Erwin - Yes it's being absorbed by semiconductors, a semiconductor can absorb light, in our case the semiconductor - rather than producing electricity - we use the energy to drive a catalyst to make syngas.

Katie - It's been described as an artificial leaf. And on the cycle over here I couldn't help but pick up this beautiful, green, autumnal leaf in front of me. How does this relate to what you're doing?

Erwin - We try to adapt the concepts from photosynthesis and in our case we also have a leaf configuration, that's the quite thin platelet, if you like, known as an electrode. And here we also take the carbon dioxide, but we do not produce sugars. We want to produce gaseous or liquid products.

Katie - So you've got a box which has carbon dioxide pumped into it through some water or an aquaeous solution. Then you've got your “leaf”, which has got some semiconductors in it which absorb the light and does some clever chemistry with the light on some catalysts in the leaf. And that is how we get the syngas?

Erwin - That's correct. And it's really that the choice of all these materials and the combination that make this a functional system.

Katie - What about a cloudy day? What about winter in the UK?

Erwin - So one of the strengths of the system is that it does not rely on very strong or high light intensities. It can also operate under cloudy days or even in the winter. So quite low light intensity situations.

Katie - Now why hasn't this been done before? What are the current technological or scientific barriers?

Erwin - The artificial leaf contains a very large variety of different elements, very unproblematic elements, but it also contains a range of problematic or poisonous or rare elements. This would for example be lead or silver.

Katie - OK. So would the idea be to swap those out?

Erwin - Yes that's what we will do in the next stage. So we know precisely which layers to replace into substitutes and we will look for replacements.

Katie - And once we've got the syngas, how do you go from that to fuel? How green is that process?

Erwin - So this process is established. It's known as the Fischer–Tropsch process and with certain high temperature and catalysts you can convert the syngas into the liquid fuel. This is something we can in principle also do sustainably.

Katie - Is it scalable?

Erwin - We hope so. So there are many limitations at the moment. The stability is not high enough and also the efficiency is not high enough. But we hope with further development we can improve on these.

Katie - How would you see this kind of technology being used?

Erwin - We see there's still a lot of challenges ahead. We cannot promise that this will be implemented in a couple of years. We recognise for real commercial application there are probably decades of development needed. But I think there are many possibilities. One way will be to use it on a very large scale, centralised energy production. But there's also a possibility for off-grid or localised uses. So I don't see a reason why this can not be a smaller scale device that generates energy where there's no grid.