Sun, shade and clean energy

A polymer of molybdenum sulfide uses solar power to draw hydrogen from water

P. Rajendran

Kourosh Kalantar-zadeh

Kourosh Kalantar-zadeh (Courtesy: RMIT)

Kourosh Kalantar-zadeh’s research was all about gas.

In fact, his team at the Royal Melbourne Institute of Technology in Australia had gained media attention for making an ingestible capsule that measured gas production in the gastrointestinal tract. They found that, on average, low-fiber diets produce more hydrogen in the small intestine, while high-fiber diets was partial to methane generation in the large intestine.

But when trying out a new chemical to gauge gas levels the team got sidetracked.

“We wanted to make a hydrogen gas sensors from polymeric molybdenum sulfide,” Kalantar-zadeh told TC. “After a few tests we realized that this material is not useful as a hydrogen-sensing element since its response to the environmental moisture was much more that its response to any target gas.”

The team had spent a lot of time putting together the molybdenum sulfide (MoSx) and was loath to let the effort go to waste. According to Kalantar-zadeh, “After an hour of discussion with Torben Daeneke, a member of my team, we decided to look at the concept completely upside down.”

Flipping the idea on its head meant that instead of measuring gas, the water-absorbing MoSx – which fortuitously is also a catalyst in sunlight – could possibly be used to generate hydrogen by splitting the water into its component hydrogen and oxygen.

That was just the first of a series of serendipitous stumbles that led Kalantar-zadeh, Daeneke, PhD student Nripen Dahr, and others from biosensors and gas monitoring to a more efficient way to produce clean energy.

The problem

To ensure a simple gas-phase water-splitting, there were four criteria the team looked at: it had to be porous to ensure easy flow of the vapor, be a good adsorber (that is, water can easily bind to its surface), be able to conduct electricity to some degree, and be a good catalyst to split water.

Now there are good adsorbers (and absorbers) like silica gel or some polymers, but then they are insulators, too. Organic semiconductors and porous metal-organic composites might have been better options had they not been vulnerable to water damage. Besides, organic compounds are not very effective catalysts. Semiconducting clays can cover most of these needs, but are less porous, which means gases take longer to make their way through them.

Long-chain compounds of transition metals, such as molybdenum, seemed likely candidates if their water-adsorbing capability could be confirmed. They were good catalysts, thanks to their partially empty d-shells (the truly curious can check out this site for more information on shells and suchlike).

It so happened that Phong Tran and his colleagues at Nanyang Technological University had recently found that something they had assumed was molybdenum trisulfide (MoS3) behaved a bit more like it was MoS4. That seemed odd enough but they finally concluded that it was actually a string of repeating Mo3S13 units – that is, a polymer – a name that was conveniently reduced for research purposes to MoSx.

MoSx conducts electricity well, is a catalyst and was good at adsorbing water. The last property, if you remember, which was what made it such a bad gas sensor in the first place and sent the RMIT team down the chemical rabbit hole in the first place.

Sunlight, the best electrolytic agent

Kalantar-zadeh et al discovered that it was also very porous, weakly bound water that was as easily released. While the presence of water change the properties of MoSx, it absorbed light that only gave it 1.2 eV of energy, when 1.23 eV and higher are needed to split water up into its components. So the team made a paint of MoSx and titanium dioxide (the same thing found in sunscreens) in a 9:1 ratio, painted the mixture on a glass plate and exposed it to the sun.

As the paper put it, “Sulfur-rich MoS 3⅔ (or MoSx) captures water from moist air and then acts as a
photocatalyst in conjunction with TiO2.”

The team got good results, comparable to that of other researchers who did not use fuels to drive water-splitting, to get in the region of 4 millimoles of hydrogen per gram of their mix every hour.

Then their paint started doing the electrochemical version of choking and sputtering, and production reduced drastically. They wondered if the light was just splitting more water than could be recaptured since the problem persisted even in high humidity. And there was heat generated by the sunlight in the material, which tended to absorb a lot of it. So they kept the plate away from the light awhile to see if it their MoSx-TiO2 paint would be rejuvenated.

In sunlight, a MoSx-TiO2 layer breaks down water faster than it can be absorbed. A dark resting phase helps it replenish its supply of moisture.

In sunlight, a MoSx-TiO2 layer breaks down water faster than it can be absorbed. A resting phase in the dark allows it replenish its supply of moisture.

The paper put it rather candidly: “To our pleasant surprise, the film regained its catalytic activity upon storage in the dark, which allowed us to design a cyclic illumination pattern, which enables continued hydrogen production.” Indeed, that pushed up the average hydrogen production to 11.09 millimoles per gram paint per hour.

There is much work yet to be done, though.

“We are not sure about very long-term performance as we didn’t do a long-term stability test,” Kalantar-zadeh admits. “We only tested the system for two to three days.”

He hopes this will be a disruptive technology with the “possibility of changing the course of solar energy conversion and storage towards a hydrogen economy.” While MoSx is what his team relied on, he believes other “sulfur-containing metal compounds such as one-dimensional zinc sulfide [zinc being another transition metal] can be good options. Mixes of organometallic materials with semiconducting compounds are also great.”

Kalantar-zadeh is pretty frank that he does not see his own team taking the research further.

“This was more like a serendipitous outcome of another project,” he says. “I don’t think I can get any grant to continue this in my group as I am not recognized in the energy field (you have to work in a specific area for many years to get any grants).”

So what drives him to do this work? Pure curiosity? Or the need for a solution?

Neither really,” he says. “My interest is to explore and develop things that don’t exist.”

The research appeared in this month’s issue of ACS Nano.

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