Never Mind the Hype: This New Solar Fuel Paint is Interesting Anyway

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This article is a part of KCET and Link TV's “Summer of the Environment,” which offers a robust library of content on multiple platforms from June-August intended to ignite compassion and action for helping to save and heal our planet.

Painting a landscape the easy way | Photo; CreativaImages/iStockPhoto
Going green with solar fuel paint won't be quite this easy.  | Photo: CreativaImages/iStockPhoto, by cclarke

A team of Australian researchers has discovered a material that absorbs water vapor from the air, and then uses the energy of sunlight to split those water molecules into hydrogen and oxygen.

The discovery by chemist Torben Daeneke and his colleagues at Australia's Royal Melbourne Institute of Technology, was described in a paper published June 14. The new material might offer a way around one of the biggest obstacles to widespread adoption of hydrogen as a clean, climate-safe fuel: the large amount of energy it takes to create hydrogen gas.

It's an intriguing discovery. And predictably, it's being hyped to the public in unrealistic terms, as a "solar paint" that could allow the walls of your home to generate fuel. That's almost certainly not going to happen, but the actual possibilities offered by the new technology are only a little bit less gee-whiz-worthy. 

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As described in the study, published in the American Chemical Society's journal ACSNano, the material consists of a polymer called sulfur-rich molybdenum sulfides bonded to a layer of titanium oxide. The molybdenum compound absorbs water vapor from the air and holds it indefinitely. The titanium oxide then captures solar energy and turns it to electrical energy in a manner similar to the way standard photovoltaic cells work.

How the new paint works: a schematic | Image: Daeneke et al
How the new paint works: a schematic | Image: Daeneke et al

The electrical energy then splits the absorbed water molecules into hydrogen and oxygen, which can conceivably be collected and used either as fuel, or as raw materials to make other chemicals. Once a water molecule is split the gases escape the hold of the molybdenum sulfides, which frees up room for another water molecule.

It's a fascinating bit of technology, and if it can work on an industrial scale it could actually revolutionize the way we fuel our machines. At present, most hydrogen gas used in industry is created in a highly energy-intensive process called steam reforming, in which steam at high temperature and pressure reacts with hydrocarbon fuels — usually methane — to split hydrogen atoms from the water and methane molecules. Oxygen from the water combines with the carbon in the methane to create carbon monoxide, which can then be combined with more steam to produce carbon dioxide and hydrogen. 

Steam reforming is an expensive process, and it consumes a whole lot of energy, usually derived from burning fossil fuels, which isn't good for the planet's climate. The fact that steam reforming produces CO2 as a waste product isn't great, either. About 95 percent of the hydrogen we use on the planet is produced by this messy process, with the remaining five percent coming from oil refineries.

That's most of the reason why climate activists haven't gotten behind hydrogen-powered cars and similar technologies: the way we make it today, hydrogen has a big greenhouse gas footprint. But if Daeneke's work offers a way to produce hydrogen that doesn't rely on huge fossil fuel inputs, that could change things in a big way. Solar fuel panels just sitting there quietly soaking up both sun and water vapor and producing hydrogen fuel sounds like a really good alternative to steam reforming. 

So in a way, the Royal Melbourne Institute of Technology can almost be forgiven for this little bit of hype:

Solar Fuel Paint | Video: RMIT University

Despite the claims made by RMIT's public information office — whose mention of "water atoms" in the video above does not particularly inspire confidence — you're not going to be able to paint your house and thereby fuel your hydrogen-powered car. Much of the reason has to do with infrastructure. If you merely painted your walls with Daeneke's team's solar fuel paint and sat back, the matierial might well make hydrogen just fine — but it'd diffuse out into the atmosphere and do no one any good. You'd need a way of keeping the hydrogen contained, which would require sealing the walls in a substance far more airtight than most buildings ever are.

Researchers Kourosh Kalantar-zadeh and Torben Daeneke with their solar fuel paint | Photo: RMIT
Researchers Kourosh Kalantar-zadeh and Torben Daeneke with their solar fuel paint | Photo: RMIT

You'd then need a mechanism to separate the hydrogen from the oxygen, because the hydrogen and oxygen from water, when combined as gases, form a highly volatile substance called oxyhydrogen, which you do not want in significant quantities anywhere near your house. Hydrogen has a long-standing reputation for dangerous flammability, in part due to the effect the Hindenberg disaster had on popular culture. But if the Hindenberg had been full of oxyhydrogen, radio announcer Herbert Morrison might well have been incinerated before he could say "oh, the humanity."

That's a lot of infrastructure to add to an existing home, and the fuel payoff wouldn't be all that great. The atmosphere is about a quarter of a percent water vapor. Obviously, the amount of water vapor in your local air is going to vary depending on your location — rainforest or desert? — and time of year. But at a quarter of a percent, you'd need to suck up all the water vapor in around 3,000 cubic meters of air to make enough hydrogen to equal the energy provided by a gallon of gasoline. Not metric-savvy? Imagine a cube 47 feet and four inches tall, filled with air. And that's assuming every bit of water vapor s converted to hydrogen and oxygen, and that none of the hydrogen leaks out into the atmosphere. Hydrogen, the universe's smallest molecule, is notoriously hard to contain.

As is often the case when researchers aren't running a university's publicity machine, Daeneke himself is far more restrained than his school's PR team when describing the potential uses of his team's solar paint. Painting your existing house with his solar paint and hooking your fuel tank up to the downspout ain't gonna happen. Far more plausible: imagine racks of solar fuel panels catching excess humidity in places like sewage treatment plants, or providing shelter for people in perennially foggy public spaces, and producing a slow, steady supply of a clean-burning fuel with no climate impact. That's a fantastic enough prospect all by itself. 

So, a note to university public affairs offices: sometimes the truth of a new discovery is interesting enough that you don't need to hype it.

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