Cooperation catalyzes the evolution of cleaner cars

style="float: right; margin-bottom: 10px; font-weight: 600;"Tue 10th Nov, 2015

Cars generate lots of pollution, as the Volkswagen Diesel scandal lately reminded us.

From the same two countries where the incident emerged---the US and Germany---come some recent advances that bring us closer to a long-awaited clean alternative: hydrogen cars.

Capitalizing on the combined efforts of many scientists from several different fields, in fact, huge strides are being made in the development of nano-structured materials, particularly useful as catalysts.

Catalysts are a general class of chemical agents, whose role is to facilitate reactions.

Typically, they nudge the reaction over some energy hump, making it happen faster, or even at all.

For example, when we hear the word "catalyst" we often think of a device in the exhaust system of our cars (the catalytic converter), This device contains some materials that facilitate the reactions turning some products of the combustion into less harmful ones, thus cleaning up the fumes.

Hydrogen cars don't emit polluting fumes, but they, too, need catalysts.

That is because they must accelerate the chemical reactions in their "fuel cells", where hydrogen and oxygen combine to become water, heat, and the electricity that powers the car.

Usually, this catalyst is a platinum surface, which is both efficient and very durable, but also extremely expensive.

This attracted some skepticism towards fuel cells, although, as Professor Aliaksandr Bandarenka a chemical physicist of the Technical University of Munich (TUM) points out, "In the catalytic converter of normal combustion engines we use a lot of precious metals, and the amount is comparable to what we need for fuel cells, even with the current technology." 

Prof Bandarenka has led a large, Europe-wide collaboration that included theoreticians, surface scientists, electrochemists, and catalysis specialists.

Their work focused on the so-called active sites of the surface: a few, particularly industrious patches, which do nearly all the work of the catalyst.

Predicting the position and activity of these spots would improve and simplify the design of platinum catalysts, reducing the amount of stress on the materials in fuel cells.

The linchpin of a site's activity is the electronic structure, that is, where electrons are and how much they can move around.

Prof Bandarenka and his team found a way to predict and manipulate this structure, by exploiting a few simple geometric principles: they counted the number of direct neighbors each atom on the surface had, and how many next to those.

From this count, they could tell how placing a new atom on the surface would shift electrons around and change the surface properties.

Thus, they could control where to put the active sites.

"We know the sites where the reaction is localized," says Prof Bandarenka, "We can tell the people that build nano-structures, 'if you change an atom here, it will increase the activity'."

And so they did: the team constructed three platinum catalytic surfaces, each with a different production technique, but all designed based on that simple counting idea.

All three catalysts proved to be competitive with state-of-the art platinum surfaces, and more durable than the most active alloys.

The concept is not limited to platinum, though: the process could be used to design catalysts based on different materials as well.

This expansion potential is fundamental because most research in catalysts is devoted to finding cheap, sustainable alternatives to the materials currently used.

And this is where the second partner comes in. Dr. Stan Chou and his colleagues at the SANDIA laboratories in New Mexico, for example, worked on a cheap catalyst that would make hydrogen fuel cells technology actually green.

This might sound surprising, given that hydrogen cars famously produce zero carbon emissions, yet the problem is where the hydrogen comes from.

Currently, in fact, most of it is obtained from natural gas (through a process called vapor reformation) that makes hydrogen production depend on fossil fuels and emits substantial amounts of greenhouse gases.

Extracting the hydrogen from water---the ideal alternative---is wasteful, because the electricity it requires may be used more efficiently to recharge batteries of electric vehicles.

But what if we could do it without electricity?

After all, plants separate hydrogen and oxygen all the time as part of photosynthesis, and use catalysts (in this case, enzymes) instead of electricity.

"It would be fantastic if we were able to do artificial photosynthesis cheaply," says Prof Bandarenka, "this is like the Holy Grail."

Unfortunately, these enzymes are too slow and too fragile for industrial use, not to mention very hard to extract from plants.

The researchers at SANDIA worked on molybdenum disulfide, which they nicknamed Molly that catalyzes the reaction separating water into hydrogen and oxygen.

It is an abundant, extremely cheap material, and its activity is triggered by sunlight. The ideal candidate, right? Not quite!

This material is somewhat uncooperative: the bulk of it is useless since, as Dr. Chou explains, "The material... is traditionally only active on the edges."

Therefore to put Molly's full potential to use, he says, "[We] moved the ... atoms there just a tiny bit with chemicals, ... that made a new atomic structure that is active for hydrogen catalysis."

In other words, they "tricked" the material into behaving as if it had more edges than it actually did.

Through this procedure, he adds, "the previously inactive, wasted areas are now active," making Molly a competitive, robust, and affordable catalyst to obtain hydrogen from water without additional power.

Despite the success, the researchers point out that the process is still unsuitable for full-fledged industrial application, as it had never been carried out in its entirety before.

Dr. Chou and his colleagues were the first to execute it from start to finish, which allowed them to find precisely what was happening at each stage, instead of some partial and confusing results.

Similar to the TUM researchers, the key to their success was being able to tap into many different talents.

As Prof Bandarenka puts it, "The current interest to solve our energy problems has attracted many bright brains, both experimentalists and theoreticians, and the exchange of ideas results in great output."

His results, along with those of Dr. Chou's decisively back this claim.

The work of this driven, diverse community of scientists takes us closer every day to a future of near-zero-emissions mobility---without anyone needing to cheat on their tests for it!


Image credit:RayBay/Unsplash 




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