Ionic liquid improves speed and efficiency of hydrogen-producing catalyst

[OKIsItJustMe]

(Please note, US Government lab release—copyright concerns are nil.)

http://www.pnnl.gov/news/release.aspx?id=928

[font face=Serif][font size=5]Ionic liquid improves speed and efficiency of hydrogen-producing catalyst[/font]

June 15, 2012

[font size=4]Ongoing saga of building a better fuel cell catalyst goes holistic[/font]

[font size=3]RICHLAND, Wash. – The design of a nature-inspired material that can make energy-storing hydrogen gas has gone holistic. Usually, tweaking the design of this particular catalyst — a work in progress for cheaper, better fuel cells — results in either faster or more energy efficient production but not both. Now, researchers have found a condition that creates hydrogen faster without a loss in efficiency.

And, holistically, it requires the entire system — the hydrogen-producing catalyst and the liquid environment in which it works — to overcome the speed-efficiency tradeoff. The results, published online June 8 in the Proceedings of the National Academy of Sciences, provide insights into making better materials for energy production.

"Our work shows that the liquid medium can improve the catalyst's performance," said chemist John Roberts of the Center for Molecular Electrocatalysis at the Department of Energy's Pacific Northwest National Laboratory. "It's an important step in the transformation of laboratory results into useable technology."

The results also provide molecular details into how the catalytic material converts electrical energy into the chemical bonds between hydrogen atoms. This information will help the researchers build better catalysts, ones that are both fast and efficient, and made with the common metal nickel instead of expensive platinum.

…

In previous versions, the catalyst was either efficient but slow, making about a thousand hydrogen molecules per second; or inefficient yet fast — clocking in at 100,000 molecules per second. (Efficiency is based on how much electricity the catalyst requires.) The previous work didn't get around this pesky relation between speed and efficiency in the catalysts — it seemed they could have one but not the other.

Hoping to uncouple the two, Roberts and colleagues put the slow catalyst in a medium called an acidic ionic liquid. Ionic liquids are liquid salts and contain molecules or atoms with negative or positive charges mixed together. They are sometimes used in batteries to allow for electrical current between the positive and negative electrodes.

The researchers mixed the catalyst, the ionic liquid, and a drop of water. The catalyst, with the help of the ionic liquid and an electrical current, produced hydrogen molecules, stuffing some of the electrons coming in from the current into the hydrogen's chemical bonds, as expected.

As they continued to add more water, they expected the catalyst to speed up briefly then slow down, as the slow catalyst in their previous solvent did. But that's not what they saw.

"The catalyst lights up like a rocket when you start adding water," said Roberts.

The rate continued to increase as they added more and more water. With the largest amount of water they tested, the catalyst produced up to 53,000 hydrogen molecules per second, almost as fast as their fast and inefficient version.

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http://dx.doi.org/10.1073/pnas.1120208109

[longship]

1. Is it as efficient as electrical separation

Every grade school science student knows this process. DC voltage is applied to plain water with the electrodes isolated under test tubes. One pole gathers oxygen, the other gathers hydrogen. It is an efficient process because the electric energy breaks the bonds in the water and deposits the two gases in their respective containers.

I would think that a strictly chemical process would be less efficient, maybe much less efficient. However, electrical separation requires a source of electricity and a chemical process might not.

This seems interesting if, and only if, the energy used to make the catalyst and make the chemical process work is less than the energy needed for very simple electrical separation of water into H2 and O2.

[OKIsItJustMe]

2. Makes you wonder why researchers are working so hard then… (doesn’t it?)

Energy is typically lost in electrolysis, in the form of heat.

Catalysts can make electrolysis more efficient, by generating less heat.

[longship]

3. Well, there's Thermodynamics 101

I am skeptical that the energy required for the production of the catalyst makes up for energy gained by more efficient electrical separation of water into H2 and O2.

This is something every undergraduate physics student should understand in principle.

The law that entropy always increases holds, I think, the supreme position among the laws of Nature. If someone points out to you that your pet theory of the universe is in disagreement with Maxwell's equations — then so much the worse for Maxwell's equations. If it is found to be contradicted by observation — well, these experimentalists do bungle things sometimes. But if your theory is found to be against the second law of thermodynamics I can give you no hope; there is nothing for it but to collapse in deepest humiliation.

Sir Arthur Eddington (1882-1944), an astrophysicist and philosopher of science of some note.

[eppur_se_muova]

4. It's more complicated than that -- this IS electrical separation, i.e. electrolysis.

Electrolysis suffers from the problem of "hydrogen overvoltage", which cuts into the energy efficiency of the process. This is the big fly in the ointment they don't teach you about in grade school.

The "strictly chemical process" these authors are describing is actually a catalytic redox cycle, driven by an electrical potential, but mediated through a different series of chemical reactions than those that normally take place at an electrode. Electrode interfacial physics/chemistry is extremely detailed technogeek stuff, and NMAOE. Serious tweaking is involved here.

[eppur_se_muova]

5. The link at the end of the OP will take you to the full paper (free!)

or click --> http://www.pnas.org/content/early/2012/06/07/1120208109.full.pdf