Podcast: Super-efficient catalyst boosts hopes for hydrogen fuel

Host: Nick Howe

Welcome back to the Nature Podcast. This week, splitting water with light…

Host: Shamini Bundell

And a measure of matter in the Universe. I’m Shamini Bundell.

Host: Nick Howe

And I’m Nick Howe.

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Interviewer: Nick Howe

First up, for decades, hydrogen has been touted as a sustainable fuel for the future. When hydrogen burns, it releases a lot of energy but only emits pure water – no greenhouse gas or pollutants to worry about. The problem is that most methods of producing hydrogen fuels either rely on fossil fuels or are too energy- and cost-intensive to be feasible on a large scale. But what if you could produce it cleanly using nothing but water and sunlight?

Interviewee: Kazunari Domen

Hydrogen from pure water and solar energy – it can be a very clean and renewable hydrogen.

Interviewer: Nick Howe

This is Kazunari Domen. Water is made up of one oxygen atom and two hydrogen atoms, and Kazunari specialises in developing materials which can use light to split water, releasing the hydrogen. These materials are called photocatalysts, and they’ve been around for decades. But so far, they have been too inefficient to be useful at scale. This week in Nature, though, Kazunari has developed a photocatalyst with almost 100% quantum efficiency, meaning nearly every absorbed photon is used to make hydrogen.

Interviewee: Kazunari Domen

I myself was actually surprised. Two years ago, we achieved almost 70% quantum efficiency, and we thought that that is almost the upper limit.

Interviewer: Nick Howe

So, how do you achieve this near-perfect efficiency? The key was to prevent photons being wasted on reactions that went nowhere, and Kazunari used a few techniques to achieve this. First, the catalyst must have as few defects in its structure as possible. When light hits the catalyst, it liberates an electron and a quasi-particle. It is these particles which go on to catalyse the splitting of water. But if any defects are present in the catalyst, these particles can lose energy and be lost, meaning they can no longer catalyse the process. In other words, fewer defects equals a more efficient catalyst. To eliminate defects, Kazunari used several methods, including the incorporation of aluminium atoms into the photocatalyst to improve the structure. Another important part of Kazunari’s approach was to use cocatalysts. These are additional materials that are placed strategically on the photocatalyst, which direct the electrons and quasi-particles to the production of either hydrogen or oxygen from the water. These ideas aren’t new in this field but by combining them, along with a few other techniques, Kazunari was able to reach very high efficiency.

Interviewee: Simone Pokrant

I think it’s really great progress.

Interviewer: Nick Howe

This is Simone Pokrant, a materials scientist who has written a News and Views article on this latest research.

Interviewee: Simone Pokrant

This is the first time that several methods have been combined and implemented and with such a success.

Interviewer: Nick Howe

One important caveat of Kazunari’s new photocatalyst is that it only works with UV light. Now, UV only makes up a tiny amount of the light that falls on the Earth so, ideally, if you wanted to make lots of hydrogen for fuel, you’d want to be able to use the far more prevalent visible light. But Simone thinks that this work could be the blueprint for the development of catalysts that do use visible light.

Interviewee: Simone Pokrant

There are other visible-light-absorbing photocatalysts which have a similar structure, for example, so there is no reason why we shouldn’t be able to apply the same concepts to visible-light-absorbing catalysts.

Interviewer: Nick Howe

Development of photocatalysts like these would allow hydrogen fuel to be produced sustainably and, importantly, cost-effectively. And whilst we may not see them arrive tomorrow, with this latest work, we may be getting close.

Interviewee: Simone Pokrant

I would say that we have a good chance to get something reasonable in a few decades.

Interviewer: Nick Howe

Well, close-ish. For Kazunari, he’s hopeful that this research will energise the field – pun very much intended – and encourage more researchers to look into photocatalysts as a way to make hydrogen fuel a reality.

Interviewee: Kazunari Domen

To produce a large amount of solar hydrogen, we need to extend our solar energy conversion system to a very wide area. That’s why I hope that many more people will be able to work photocatalysts.

Interviewer: Nick Howe

That was Kazunari Domen from Shinshu University and Tokyo University, both in Japan. You also heard from Simone Pokrant from the University of Salzburg in Austria. We’ll put a link to Kazunari’s paper in the show notes, along with a link to a News and Views article written by Simone.

Host: Shamini Bundell

Later on, we’ll be hearing about how researchers have been on the trail of missing matter in the Universe. Before that, though, we’ve got the Research Highlights for you, read this week by Dan Fox.

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Dan Fox

Deep near the centre of the Earth could be a secret reservoir containing the majority of the planet’s water. More than 4.5 billion years ago, as the planet coalesced from hydrogen gas, dust and other material swirling around our newborn Sun, hydrogen – a component of water – might have moved into the planet’s developing core. Because we can’t see directly into the Earth’s core, scientists have tried to simulate what happened by analysing how hydrogen behaves under high pressures and temperatures similar to those found at the boundary between the Earth’s mantle and core. The team concluded that more than three quarters of the early Earth’s hydrogen went into the core, depending on how the core was formed, which could translate to over 20 times more water there than at the Earth’s surface. Drink in the rest of that research at Nature Geoscience.

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Dan Fox

Working memory has been called the brains sticky note – a place to store information short-term, like a phone number or directions to a shop. Now, researchers have found that activity in a specific region of a child’s brain predicts the strength of their working memory. The team of researchers analysed brain scan data and performance scores from memory and cognitive tests for around 11,000 children aged nine and ten. They found that children with a stronger working memory had higher activity in the region of the brain called the frontoparietal cortex. Children with strong working memory also tended to have better language skills and better ability to solve problems in new situations. The team hopes the findings could help to explain how memory and cognition change as children develop. Remember to read that research in full at the Journal of Neuroscience.

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Host: Shamini Bundell

Next up, reporter Adam Levy has been investigating a curious case.

Interviewer: Adam Levy

Today, I’d like to share a mystery of missing matter – missing baryons, to be precise. Whether you’ve heard of them or not, believe me, you have a very familiar relationship with baryons. The most famous are protons and neutrons, which make up the majority of visible matter in the Universe, including the majority of the Sun, the Earth and, yes, your very own body. But the question is, just how much baryonic matter – how much proton and neutron stuff – is out there in the Universe? Physicists know how much should exist in the Universe. They know this by studying properties of the Big Bang. You can get an answer by peering at the Big Bang’s afterglow, also known as the cosmic microwave background, as well as the relative amount of the lightest atoms created by the Big Bang. But when you try to directly measure baryons by actually working out how much you can see in the Universe, things don’t seem to add up.

Interviewee: Jean-Pierre Macquart

So, roughly half has been missing, which is a bit of an embarrassment.

Interviewer: Adam Levy

This is astrophysicist Jean-Pierre Macquart of Curtin University in Perth, Australia. So, we know how many baryons – in other words, protons and neutrons – should be in the Universe based on the Big Bang, but when we look, we can’t find a hefty chunk of it. Well, Jean-Pierre has been hunting for the hidden stuff using fast radio bursts as a probe. I gave him a call and asked for a quick refresher on what fast radio bursts actually are.

Interviewee: Jean-Pierre Macquart

Well, no one really knows exactly what they are, but all we care about for the purposes of this kind of physics is that they’re very bright lights and they’re very impulsive. So, they last about a millisecond and they’re so bright that we can detect them over the other side of the Universe. We can use them as like a cosmological Swiss Army knife. They’re so impulsive – their radiation – that it’s highly susceptible to effects that occur when they travel through this cosmic gas, even when it’s very diffuse, and that process is dispersion. It’s the same kind of process that occurs when you shine sunlight through a prism and it disperses the radiation out in towards different colours of the spectrum. So, as it travels through this intergalactic gas, the longer wavelengths, the redder wavelengths get delayed more than the bluer wavelengths. So, when you actually observe this with a radio telescope, you don’t find that the pulse all arrives at once. It actually dribbles in.

Interviewer: Adam Levy

So, you’re looking at these fast radio bursts to see how they disperse. I mean how difficult of a task is that to actually do, to firstly, track them down and to get that data in sufficient detail to get answer?

Interviewee: Jean-Pierre Macquart

It’s been relatively easy to detect the dispersion that’s associated with the bursts themselves. What’s been the stumbling block is getting the precise position of these things to actually point at an optical telescope and go and measure the red shift and hence the distance to that thing, which you need to do if you want to figure out the density of this baryonic matter in the Universe. So, that’s been the key problem.

Interviewer: Adam Levy

And how did you overcome that issue so you could actually not only see these fast radio bursts but understand how far away they are?

Interviewee: Jean-Pierre Macquart

Well, this involves technology on the Australian SKA Pathfinder, which enable us to see 30 square degrees of the sky all at once, so it means you can capture these fast radio bursts in sufficient numbers. And so, once you’ve detected one of these bursts, you have a buffer inside the telescope and you say, ‘Aha, we’ve detected the burst, now please dump all of the high-resolution data from that burst’, and that enables us to go back after the fact and to triangulate on the position on the precisely.

Interviewer: Adam Levy

Now that you have this data and you crunched the numbers, what answer do you actually get?

Interviewee: Jean-Pierre Macquart

Well, it’s both a relief and exciting that the number we get is actually very close to the number that you expect for the density of baryonic matter in the Universe.

Interviewer: Adam Levy

Were you expecting this in your heart of hearts? Were you expecting that you’d get the same answer predicted by the theory?

Interviewee: Jean-Pierre Macquart

I’ve learnt not to expect anything in science, but what was more surprising to me was that these fast radio bursts are much better cosmological probes than we had dared to think, and that was the real surprise.

Interviewer: Adam Levy

Is this now case closed for the missing protons and neutrons in the Universe or do you think this result will be at all controversial for the community?

Interviewee: Jean-Pierre Macquart

It won’t be controversial but it is but the beginning because we’ve said, ‘Okay, now we know that those baryons are there, we can account for them.’ What we haven’t done is say where they are, so are they distributed completely diffusely throughout the Universe or do they hang around large groups of galaxies? Now that we know the gas is there, where exactly is it and what’s it doing? And this is critical because you want to know how all of these galaxies that you see about you in the Universe are forming. There are violent processes in those galaxy star formations, black hole spewing jets out, and they’re throwing that baryonic matter back out into intergalactic space, whereas there’s also cool baryonic matter raining down on these galaxies to form next generational stars. So, it’s a whole ecosystem here, and we have many more puzzles to resolve from this point onwards, but now we’ve got a tool to do it.

Host: Shamini Bundell

That was Jean-Pierre Macquart of Curtin University in Australia. We’ll put a link to his paper in the show notes.

Host: Nick Howe

Finally, it’s time for a quick look at some other non-corona science stories. To do that, Shamini and I have been using the Nature Briefing – that’s Nature’s daily pick of science news and stories, and Shamini, what have you found this time that you’d like to share?

Host: Shamini Bundell

Okay, so, Nick, do you know anything about Planet Nine?

Host: Nick Howe

Planet Nine sounds like a sci-fi novel. I swear I saw a film about Planet Nine.

Host: Shamini Bundell

No, Planet Nine is real or it might be real. We don’t know. Lots of people are looking for it. It’s a hypothetical ninth planet – we’re not counting Pluto in this, sorry Pluto – out beyond Pluto and Neptune, a mass big enough to be defined as a planet that is orbiting our Sun that we think might be there but that no one can quite prove is there.

Host: Nick Howe

Okay, so why do scientists think there might be a planet out in the far reaches of the Solar System?

Host: Shamini Bundell

Well, so, even though we’ve never seen it, we do know of certain objects, Kuiper Belt objects, that do orbit the sun, but they orbit the Sun in a really weird way. They’ve got elliptical orbits but they’re all sort of lined up and they’re lined up along the same plane, so the idea is to explain their weird aligned orbits, there must be some source of gravity that’s affecting them that we haven’t spotted yet.

Host: Nick Howe

Oh, right, so there’s some massive object, maybe a planet, affecting these guys’ orbits, but how are scientists trying to identify or find this thing, whatever it is?

Host: Shamini Bundell

Well, I mean people have been looking for it with telescopes and so on for a while, but the reason this came up in the Nature Briefing is that there’s been an idea that maybe Planet Nine isn’t a planet. Maybe it’s not a huge object. Maybe it’s a very small object with a very strong gravitational pull. In other words, a very young black hole, smaller than the size of your fist, that could be out there.

Host: Nick Howe

Wow, so just the other week I was saying that oh, ‘We’ve found the closest black hole ever. It’s only 1,000 light years away.’ And now there might be one in our Solar System? How can we confirm it?

Host: Shamini Bundell

Well, this is hugely hypothetical at this point. It could be a planet that we haven’t seen yet. It could be a black hole. If it is a black hole, then sending a bunch of tiny probes out there, but that would still be a whole big project and the probes would still take maybe ten years to get there. So, I think we’re unlikely to know for some time, but certainly an intriguing idea – either a hidden planet or a hidden tiny black hole lurking out there on the edges of the Solar System.

Host: Nick Howe

The Universe just gets weirder and weirder. For my story this week, it’s very much back down to Earth, and I’ve been looking into a strange thing that bumble bees seem to be doing in order to force plants to bloom. They’re biting their leaves in order to make them bloom more quickly.

Host: Shamini Bundell

Wait, the bumble bees are attacking the flowers that they get the pollen from?

Host: Nick Howe

Basically, yeah. So, this was an observation some researchers made when doing a completely unrelated experiment, and they saw that some bumble bees were biting leaves of flowers and, like you, they were wondering what on Earth are these bees doing? So, what they did is they starved them of pollen and observed what they did, and when they were starved of pollen, they bit the leaves of these plants and want that did is it actually forced the plants to bloom much, much more quickly than they would have otherwise done.

Host: Shamini Bundell

So, rather than living in beautiful harmony, they’re basically stressing the plants out in order to trick them into making food quicker.

Host: Nick Howe

Yeah, so it’s sort of understood that during stress, plants will bloom more quickly to just try and get their genes out there as quickly as possible, and so the bumble bees are taking advantage of that. What’s interesting as well is the researchers actually tried to replicate it by stabbing the plants’ leaves with a scalpel, and the plant didn’t bloom as quickly as when the bees bit it, so there may actually be some sort of chemical in a bee’s saliva that also helps the plant bloom more quickly.

Host: Shamini Bundell

Oh, that’s super mysterious. I love it. I’m interested to know whether, long term, that’s actually harmful to the plants, you’d have thought to keep getting stressed out by these bees. But the bees are depending on the plants for their food.

Host: Nick Howe

Yeah, it also seems like a short-term strategy, but it’s not really clear. So, this was observed by accident, as I said, and so there’s a lot of open questions about it and it’s never been observed before, so I think a lot of ecologists in the article I was reading in Scientific American were saying, ‘This is really interesting. We need to do more research on it to see how widespread this is because this was just in one species of bumble bee – Bombus terrestris.

Host: Shamini Bundell

You did your PhD on something about bees, didn’t you?

Host: Nick Howe

I did, yes. I actually worked with this specific species of bees for a while, so I remember fondly feeding them pollen in the lab.

Host: Shamini Bundell

And that whole time, you didn’t realise how violent they really were.

Host: Nick Howe

Laughs. I certainly realised how violent they were. I did get stung a couple of times. But considering I worked with them for three and a half years and I only got stung three times, I think that’s pretty good odds.

Host: Shamini Bundell

That’s not too bad.

Host: Nick Howe

Yeah, it wasn’t too bad. Thanks for chatting to me about this, Shamini, and listeners, if those stories have peaked your interest and you’d like more of them, but instead in a daily email, then make sure you check out the Nature Briefing. We’ll put a link to that in the show notes along with links to the articles we discussed.

Host: Shamini Bundell

Well, that is all for this week. If you want to get in contact with us then you can find us on Twitter – we’re @NaturePodcast – or if you are more email-inclined then we’re all ears over at podcast@nature.com. I’m Shamini Bundell.

Host: Nick Howe

And I’m Nick Howe. Thanks for listening.