Simple hydrogen storage solution is powered by solar energy

June 19, 2015 by Lisa Zyga feature
simple hydrogen storage
The new reversible hydrogen storage method stores hydrogen atoms in cyclohexane and uses solar energy to release the hydrogen atoms, turning the cyclohexane molecule into benzene. The use of solar energy avoids the need for high temperatures to release the hydrogen. Credit: Li, et al. ©2015 American Chemical Society

(Phys.org)—By using solar energy to reversibly attach and detach hydrogen atoms on a 6-carbon ring called benzene, scientists have developed a simple and efficient method to store, transport, and release hydrogen potentially on a large scale. The hydrogen storage problem is currently one of the biggest challenges facing the development of hydrogen as a widespread energy carrier, and the researchers hope that the new strategy may lead to a safe and inexpensive solution to this problem.

The scientists, led by Professor Chao-Jun Li and Associate Professor Zetian Mi at McGill University in Montreal, have published a paper on the new system in a recent issue of the Journal of the American Chemical Society.

As the researchers explain, hydrogen has a very high mass energy density but a very low volumetric energy density. The high mass energy density, which is at least three times higher than that of other chemical fuels, is what makes hydrogen such an attractive energy carrier. However, its low volumetric under ambient conditions makes it difficult to store large amounts of hydrogen in small spaces. To overcome this problem, hydrogen is often stored at high pressures or low temperatures, but these storage methods present their own challenges.

The hydrogen storage system demonstrated in the new paper works under and stores the hydrogen in abundant, lightweight, and inexpensive molecules called hydrocarbons. The researchers demonstrated that six can be added to benzene (C6H6) in a "hydrogenation" process that forms cyclohexane (C6H12), which serves as the hydrogen carrier. In the reverse process, cyclohexane is "dehydrogenated" as the six carbons are removed and available for use in energy storage devices and other applications.

This method of storing hydrogen atoms in hydrocarbons is not new, but because the dehydrogenation process requires a large amount of energy to proceed, current versions always require high temperatures to release the hydrogen.

Since performing the reaction at high temperatures is not suitable for practical applications, here the researchers demonstrated that can be used to drive the dehydrogenation reaction at ambient temperatures. This process involves using platinum-based nanoparticles as photocatalysts. After absorbing incoming photons, the platinum nanoparticles temporarily donate their photoexcited electrons to the cyclohexane molecules, breaking the carbon-hydrogen bonds and releasing the hydrogen atoms without the need for elevated temperatures.

Tests showed that this photo-driven dehydrogenation process occurs rapidly (within a few seconds), converts 99% of the cyclohexane to benzene, and has a quantum efficiency (H2 produced per photon consumed) of 6.0%, which rivals the current top-performing solar water splitting devices without an external voltage. To start the hydrogenation process, the researchers simply removed the light source, causing the hydrogen atoms to reattach back onto the benzene. Using this method, 97% of the benzene could be converted back to cyclohexane, and the cycle could be repeated.

The researchers expect that this strategy is more suitable for stationary applications—for instance, for storing and transporting energy produced by wind turbines or other alternative sources—rather than vehicles because of the fact that it requires sunlight to release the hydrogen.

"The applications may include the storage and transport of hydrogen generated from other sources, such as water splitting and water electrolysis, using renewable energies (hydro, wind, nuclear, etc.)," Li told Phys.org.

Taking the next steps forward, McGill University has filed a provisional patent on this technology. In the future, the scientists plan to improve the storage system by reducing the amount of platinum required in the photocatalysts and developing other less expensive alternatives.

"Our future research is focused on developing cheaper and more earth-abundant metal catalysts, such as iron, and to further increase the quantum efficiency," Li said.

Explore further: Muons help understand mechanism behind hydrogen storage

More information: Lu Li, et al. "Simple and Efficient System for Combined Solar Energy Harvesting and Reversible Hydrogen Storage." Journal of the American Chemical Society. DOI: 10.1021/jacs.5b03505

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MR166
1 / 5 (2) Jun 19, 2015
The overall efficiency of the process of water to H2, H2 to cyclohexane and cyclohexane to H2 looks to be terrible. Batteries still look to be the only viable storage solution.

I wonder what the total energy efficiency of a Lithium Ion battery is from cradle to grave. IE, add all of the energy needed to create the battery, charge it during it's lifetime time and from that number subtract the amount on energy it releases to do actual work.
katesisco
1 / 5 (1) Jun 19, 2015
Platinum itself stores 200x times its volume in hydrogen. As noted in the popular science book, The Disappearing Spoon by Sam Keane.
greenonions
5 / 5 (2) Jun 19, 2015
Efficiency is not the only metric. A system like this may have some advantage - if it is able to store large quantities of energy - and give the option of storing that energy for long periods of time, and also transporting that energy. Example - far off shore wind/solar/wave/otec plants - that can turn excess power into cyclohexane - to be transported by pipe - to onshore power plants. Do that with batteries!
MR166
not rated yet Jun 19, 2015
Onions I agree with you that this has some potential but it only makes sense if the energy being stored is inexpensive on a cost basis and on an EROEI basis. Otherwise the entire process could waste more energy than it saves.
jeremy_h
5 / 5 (1) Jun 19, 2015
This is interesting.

There is also another approach to do that other than just using solar energy. The trick is to use a regenerative fuel cell that oxidize/reduce cyclohexane/benzene directly.
In this way, you can generate AND store hydrogen at the same time in a very convenient form (a liquid). If you need energy, run it in reverse.
A strong point is that to increase storage, you just need a bigger tank and it is storable for years, if you want, unlike batteries.
This technology is also more scalable and can run without solar energy (at night).
Another big advantage, is that the liquid chemical could also be used in FCEVs without any pressurized tanks.
However, benzene is a known carcinogen but this can be replaced by other more benign liquids listed in the joint paper.
http://zhuqilong....690e.pdf
jeremy_h
not rated yet Jun 19, 2015
The overall efficiency of the process of water to H2, H2 to cyclohexane and cyclohexane to H2 looks to be terrible.

The 6% figure is a photo-efficiency, which is different from the electrical efficiency you are referring. Here, photons are converted into photo-electrons which assists the reaction. Those electrons do the job that the heat and pressure would have done.
The electrical efficiency will be the one of the fuel cell (60%) or turbine (30-35%) or whatever system you use, not the photo-efficiency.
Eikka
3.7 / 5 (3) Jun 19, 2015
However, its low volumetric energy density under ambient conditions makes it difficult to store large amounts of hydrogen in small spaces.


"Difficult" is an understatement. Using liquid hydrogen at -196 C, even a tiny economy size car - something like a VW rabbit - using a state of the art hydrogen fuel cell, would only do about 10 miles to the gallon. The fuel tank with the necessary insulation would take up the entire trunk and parts of the back seat.

solar energy can be used to drive the dehydrogenation reaction at ambient temperatures


But this is useless, because you're only getting the hydrogen out when the sun is up, which is the exact opposite of what you want. You want to store solar energy and then release it when the sun is -not- shining.
TheGhostofOtto1923
4.2 / 5 (5) Jun 19, 2015
NoTennisNow
5 / 5 (1) Jun 19, 2015
Not to forget that benzene is a carcinogen.
marko
not rated yet Jun 19, 2015
Why have the dehydrogenation process.

Just burn the cyclohexane as it would carry the extra energy and energy density of hydrogen and benzene.
Eikka
5 / 5 (1) Jun 20, 2015
Why have the dehydrogenation process.

Just burn the cyclohexane as it would carry the extra energy and energy density of hydrogen and benzene.


Because then you have to syntethize more benzene, which comes at a cost.
Eikka
5 / 5 (1) Jun 20, 2015
Here's BMW's take on a hydrogen vehicle:

http://www.motive...er_1.jpg

I misremembered the temperature of liquid hydrogen. It's actually -254 C instead of -196 C which is the boiling point of liquid nitrogen.

There's 8 kg of hydrogen in that tank, which is equivalent to 8 gallons of gasoline.
Eikka
5 / 5 (2) Jun 20, 2015
quantum efficiency (H2 produced per photon consumed) of 6.0%


What the quantum efficiency means in practice:

We need ~17 photons for each released molecule, and there are 6.022e23 molecules in a mole, so we need approximately 1e25 photons to release a mole of hydrogen. The energy of a single photon at 555 nm (peak sunlight) is E=hf, which comes out at 3.57918e-19. Multiply the two together, and you're left with approximately 3.6 million Joules, or one kWh.

Since one mole of hydrogen weighs approximately 1 grams, you need approximately 1 kWh worth of sunlight to release a single gram of hydrogen out of the cyclohexane.

tear88
not rated yet Jun 20, 2015
Like NoTennisNow said, benzene is a carcinogen. And what are the chances that there WOULDN'T be leaks into the environment?

This is a terrible idea.
MR166
1 / 5 (1) Jun 20, 2015
That was great Eikka. There is nothing like putting a hoax in perspective.

Let me add this to your fine dissertation. 20 Mega Joules of of energy hits each square meter of the earth each day so each square meter of panel would release 5.5 grams of H2 per day.

The green hyperbole is unbounded it is the only thing greater than space and time which have limits.
TheGhostofOtto1923
3.4 / 5 (5) Jun 20, 2015
benzene is carcinogenic
Heres some news for ya

"...the occurrence of liver cancer in female mice and kidney cancer in male rats provides 'sufficient' evidence in animals that inhalation of wholly aerosolized gasoline is carcinogenic; and (3) gasoline vapors from vehicle refueling might be less carcinogenic than indicated by animal experiments using wholly aerosolized gasoline, if the less volatile components, which are apparently responsible for acute kidney toxicity, also contribute to the observed carcinogenic response."

-Gasoline is carcinogenic because it contains many carcinogenic materials, INCLUDING benzene. The above process eliminates all but benzene.

So you'd have to consider the amount it does produce, and the chances of exposure to it.
MR166
not rated yet Jun 20, 2015
"Gasoline is carcinogenic because it contains many carcinogenic materials, INCLUDING benzene. The above process eliminates all but benzene.

So you'd have to consider the amount it does produce, and the chances of exposure to it."

As Eikka so kindly pointed out, the process has a lot more wrong with it than a possible benzene leak. Huge energy inefficiencies are nothing to take lightly. I am sure that these platinum catalyzed panels will not be cheap.

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