Breakthrough in designing cheaper, more efficient catalysts for fuel cells

Feb 23, 2012
Molybdenite (top) is a popular catalyst, but reactions take place only at edge sites (circle) where a molybdenum-sulfur-sulfur triangle protrudes. UC Berkeley/LBNL chemists have created molecules (bottom) with only an active site triangle that efficiently convert water to hydrogen. Credit: Christopher Chang and Jeffrey Long, UC Berkeley

University of California, Berkeley, chemists are reimagining catalysts in ways that could have a profound impact on the chemical industry as well as on the growing market for hydrogen fuel cell vehicles.

Catalysts are materials typically metals that speed up and are widely used in the of chemicals and drugs. They also are employed in automobile to change chemicals into less-polluting emissions and in fuel cells to convert water into hydrogen.

The problem with catalysts, however, is that reactions occur only at edges of or defects in the material, while the bulk of the – often expensive platinum – is inactive and wasted.

In an article appearing this week in the journal Science, UC Berkeley show how to construct a composed only of edges and demonstrate that it can catalyze the production of hydrogen from water as readily as the edges and defects in regular catalysts.

"This is a conceptual advance in the way we think about generating hydrogen, a clean-burning fuel, from water, a sustainable source," said Christopher Chang, associate professor of chemistry and Howard Hughes Medical Institute Investigator at UC Berkeley. "Our new catalyst is just first generation, but the research gives us and the community a path forward to thinking about how to increase the density of functional active sites so that molecules and materials can be more effective catalysts."

At the moment, creating these catalysts in the lab is not cheaper than using traditional catalysts, but efforts by Chang and others to simplify the process and create materials with billions of active sites on a ridged wafer much like a Ruffles potato chip could allow cheaper, commercially viable fuel cell catalysts.

"The development of new earth-abundant catalysts for water splitting is an essential component of the global effort to move away from fossil fuels and towards solar energy," said coauthor Jeffrey Long, UC Berkeley professor of chemistry and faculty scientist at Lawrence Berkeley National Laboratory.

Chang and his UC Berkeley colleagues worked with a common catalyst, molybdenite, that is less expensive than platinum and of increasing interest as a catalyst. Composed of molybdenum and sulfur (MoS2), the material catalyzes reactions like the splitting of water into hydrogen and oxygen only at the edges, where triangles of molybdenum and two sulfur atoms stick out like pennants.

"These edge sites look like little MoSS triangles, and the triangular area does the business," Chang said.

Using complex organic synthesis techniques, Chang said he and his colleagues created a small carbon framework to hold the MoSS triangle so that "every molecule has a discrete edge site that is a catalytically active unit."

When lots of these single-molecule catalysts were dumped into acidic water and even seawater, they generated hydrogen for several days without letup.

In future research, Chang hopes to assemble billions of these molecules on a thin, ridged wafer, maximizing the number of catalytic sites for a given volume and boosting ultimate efficiency.

"There are many other types of materials out there for which people might want to generate edge-site fragments rather than use a bulk material with just a few edge or defect sites," Chang said. "With hydrogen being touted as a clean burning fuel that generates no CO2, creating cheaper and better catalysts has become a big and important field now. The main push is toward more earth-abundant materials than the rare metals like platinum."

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Sanescience
not rated yet Feb 24, 2012
Yes, the "growing market" for hydrogen fuel cell vehicles. No thanks. I'll keep my vehicle that I know how to work on or can take to my trusted mechanic. When biofuel based gas is available I'll be just as environmentally friendly as a fuel cell vehicle but with lower maintenance costs, lower fuel costs, and probably lower overall environment impact for total production and lifetime.
antialias_physorg
5 / 5 (2) Feb 24, 2012
The problem with catalysts, however, is that chemical reactions occur only at edges

This screams for molecules which best conform to a fractal structure.

I'll keep my vehicle that I know how to work on or can take to my trusted mechanic

The thing is: A fuel cell vehicle has far less parts that you'd need to work on. Electric motors - which the fuel cell would drive - are pretty much service free (and the things that would need replacing you can still do yourself)

The fuel cell itself is just stacks of catalyst covered plates and hydrogen in and out pipes. Nothing to service there. In the event that the stack ages you cold replace individual plates (which is an operation you could do yourself, easily)

Fuel cells aren't magic nor are they complex. In fact: the system is much less complex and service intensive than a combustion engine.
Eikka
not rated yet Feb 24, 2012

The fuel cell itself is just stacks of catalyst covered plates and hydrogen in and out pipes. Nothing to service there. In the event that the stack ages you cold replace individual plates (which is an operation you could do yourself, easily)


The fuel cell stack is really really expensive to replace though, and pulling it apart probably needs to be done in a cleanroom. I wonder how extensive filtering you need to not foul up the stack with dust and aerosols.

That's the problem with these new powerplants. They rely on a single very intricate and very expensive part, and when it fails, you might as well buy a new car.
antialias_physorg
5 / 5 (1) Feb 24, 2012
I wonder how extensive filtering you need to not foul up the stack with dust and aerosols.

Since you'd be pumping through hydrogen from a, presumably, clean source (pressurized hydrogen tank or metal hydride release system) and the resulting 'exhaust' is just pure H2O I'm not sure that aerosols or dust are a problem.

The videos of stack assembly I found dont show any need for a clean room (though I'm pretty sure the initial assembly would benefit from one)

They rely on a single very intricate and very expensive part, and when it fails, you might as well buy a new car.

You mean unlike the engine in a conventional car?

And there is nothig 'intricate' about a fuel cell. It has no moving parts. The only way it can break down is if you physically break it or if the catalyst wears down.

As for expensive: They are expensive because today we use platinum or palladium. Molybdenite is far cheaper.
roboferret
not rated yet Feb 24, 2012
Agreed, electric cars are the model of simplicity and efficiency. If we had a suitably dense energy storage medium we would have never had invented the combustion engine, which is sinfully complex and inefficient (max 30%). Electric motors can run for years without maintenance, and are up to 96% efficient.