Shining a light on new carbon-free fuels

Governments are said to be investing huge sums of money in a so-called 'ammonia economy.' That's where they, industry and some scientists see a future class of carbon-free liquid fuels — fuels that contribute less to climate change.

But they're not looking for the ammonia itself, they're after hydrogen. That's what some say is a fuel of the future. And if you get it from ammonia, among the only byproducts are water and nitrogen, the latter of which already makes up most of our atmosphere.

Dayne Swearer and his colleague, Linan Zhou, have been using light to break down ammonia into its constituent parts with less energy and more efficiency. Zhou is the postgraduate lead author on a new study they have published in Science. DW spoke to Swearer, whom we first met at the 2017 Lindau Nobel Laureate Meetings, so it was like catching up on his progress.

DW: Why do governments consider ammonia as an alternative to carbon-based liquid fuels? What's so special about ammonia?

Dayne Swearer: Right now the world is based on a liquid fuel economy. As everybody knows we go to the pump and fuel up with gasoline. But that is carbon-based, and so when it burns it produces carbon dioxide and some other byproducts, which aren't very good for the atmosphere, the ocean or our health, generally. So there's been a big push by governments around the world — including Australia and Japan — to look into using liquid ammonia.

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Ammonia is normally a gas, but if you increase its pressure slightly, and lower its temperature, it forms a liquid. So it could be used in the existing infrastructure throughout the world as a potential liquid fuel source. And when you break ammonia down, you get nitrogen, which makes up about 80 percent of the Earth's atmosphere, and hydrogen, which is what's used to actually drive a chemical reaction — the fuel. And that leaves nitrogen and water as the sole byproducts of ammonia-based liquid fuels.

Is ammonia sufficiently abundant?

Ammonia isn't actually particularly abundant, but we do have the ability to make it on large scales. There's a really important process, called the Haber-Bosch process, that was invented in Germany in the early 20th century. And while it is pretty energy intensive, there is research going on around the world, which we hope to push in a direction so that we can make fuels synthetically.

That dovetails quite nicely with the 2018 Nobel Prizes, which were announced at the start of October and mirrored a move into a more synthetic approach to producing fuels and other resources. So it seems your research is adding to a growing body of work, a consensus.

Yes, and particularly in regard to the physics Nobel Prize. The winners used a lot of pulsed optical laser sources. And the same way you would use a laser for eye surgery, we use a laser in our study, too.

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We discovered that when you shine a light onto a chemical bond, such as ammonia, you can break its hydrogen-nitrogen bonds, and you can release the nitrogen at much lower overall energy levels than if you used a traditional chemical reaction that is driven by heat, for instance by burning other fossil fuels. The light opens up new pathways for that reaction to take place.

So you're investing less to begin with to get out more at the other end. Is that a fair way of putting it?

That is one way to look at it. By using light you are reducing the overall amount of energy you need to break the bonds. In this case, it's about a 75 percent decrease in the energy you need for a single step.

However, that's not to say that if we tried this in a car or in an engine, it would automatically be 75 percent more efficient. So there are some large-scale engineering issues to consider. But it is a really interesting and enticing first step to understand that light can have this type of effect on chemical reactions and that it reduces energy barriers for particular reactions. And this is just one case study. We think it could also apply to other types of reactions, and that could be very interesting.

When you pick your research areas as a PhD candidate and beyond, do you have to be so conscious of the industrial application of the science you choose, or is there still room for really basic, fundamental science? Would you have picked this project if someone had told you it was "a nice little curiosity, but it won't make any money down the track?" Would you have still opted for this science?

I think I would have. I've always been driven in science with the desire to help people. And often those basic curiosities do turn into something that someone else down the line might be able to engineer or manipulate into something that is widely applicable. So I think there is room for basic science, and actually a lot more should be invested in it. You might not be able to see around the next corner, but an invention or discovery in something very fundamental can lead to large-scale changes down the line.

Dayne Swearer is a graduate student in Professor Naomi Halas' Laboratory for Nanophotonics at Rice University, USA. Swearer co-authored "Quantifying hot carrier and thermal contributions in plasmonic photocatalysis" with lead, Linan Zhou, and others. The paper was published in 'Science.'

The interview has been edited for length and clarity.