Jul 29 2016

An Artificial Leaf

solar sell1We are currently in a transition period from an economy based largely on fossil fuels to one based largely on renewable or carbon-neutral fuels. Even if we put aside the question of global warming, there are many good reasons to make this transition. Fossil fuel pollution results in billions of dollars of health care costs and lost productivity each year. For any nation, the ability to create more of their own fuel and be less dependent on oil imports can have economic benefits.

Of course, if you accept the scientific consensus on anthropogenic global warming, we may just avoid some unwanted consequences of dumping billions of tons of previously sequestered carbon back into the atmosphere (40 billion tons in 2015 alone).

One interesting technology is often called an artificial leaf, because it uses light energy not to generate electricity directly (photovoltaics) but to synthesize fuel or fuel precursors from carbon dioxide in the atmosphere and other inputs (photosynthesis).

Photosynthesis is what plants do, hence the term “artificial leaf.” Plants use this process to make food in the form of carbohydrates from carbon dioxide and water, releasing oxygen in the process.

We can just use plants, which already do photosynthesis quite efficiently. This is the biofuel approach – grow a crop that contains a lot of a carbohydrate that can be conveniently converted into alcohol or some other fuel. Biofuels were of questionable benefit for a long time, but in the last few years have probably crossed over the line to being energy and carbon efficient (depending on all the variables of which crop and how it is grown and processed).

However, crop-based biofuels are problematic because they use land that could be used to grow crops for food. Our ability to make this trade-off in the future is questionable. We probably will not have the luxury of dedicating enough farmland to grow biofuels necessary to significantly reduce fossil fuel use. Some researchers are trying to develop algae as a biofuel source, but it remains to be seen if this will pan out.

An artificial leaf could therefore have several advantages. We can put them anywhere there is a lot of sunlight, such as the desert, without displacing any cropland. We could theoretically develop a process that is more efficient than current plants, in terms of the percent of sunlight that is converted into fuel. Finally, we may be able to create a system that produces more useful fuels or precursors than ethanol or other biofuels.

The Latest “Breakthrough”

I frequently see press releases and articles proclaiming the latest “breakthrough” in artificial leaf technology (just as with solar, battery, and many other technologies). They often claim to be a game-changer. In reality most of them are either incremental advances, or they are prototypes that are nothing more than a proof of concept.

These are still interesting science news items, and I follow them closely, but often the glowing press releases gloss over some important reality-checks. Often I read that the technology just needs to be “scaled up” in order to change the world. That is a non-trivial hurdle, however. That is often the most important question – can this technology be made efficient and cost effective at a useful scale?

It is one thing to make a process work in a carefully controlled laboratory environment, and it is quite another to make it work on an industrial scale. That is why, if you read the science and technology news, you will read about many game-changing breakthroughs that you then never hear about again. (To be fair, sometimes it’s because it just takes years to develop and you forget about it in the meantime.)

So here is the latest breakthrough, which prompted this article today:

Researchers at the University of Illinois at Chicago have engineered a potentially game-changing solar cell that cheaply and efficiently converts atmospheric carbon dioxide directly into usable hydrocarbon fuel, using only sunlight for energy.

The results were published in the journal Science, so that lends the claims some credibility. This artificial leaf system uses sunlight to create syngas, which is a mixture of hydrogen gas and carbon monoxide. This can be burned directly as a fuel (since both gases can be burned with oxygen to create H2O and CO2), or they can be used as precursors to create hydrocarbons – essentially forms of gasoline.

The specific advance was in finding a catalyst that is more efficient and cost effective than existing catalysts (as is often the case – catalysts are what make reactions go).

Salehi-Khojin and his coworkers focused on a family of nano-structured compounds called transition metal dichalcogenides — or TMDCs — as catalysts, pairing them with an unconventional ionic liquid as the electrolyte inside a two-compartment, three-electrode electrochemical cell.

The best of several catalysts they studied turned out to be nanoflake tungsten diselenide.

“The new catalyst is more active; more able to break carbon dioxide’s chemical bonds,” said UIC postdoctoral researcher Mohammad Asadi, first author on the Science paper.

In fact, he said, the new catalyst is 1,000 times faster than noble-metal catalysts — and about 20 times cheaper.

There is another aspect of this specific process that some may feel make the claims of an “artificial leaf” a bit misleading:

The UIC artificial leaf consists of two silicon triple-junction photovoltaic cells of 18 square centimeters to harvest light; the tungsten diselenide and ionic liquid co-catalyst system on the cathode side; and cobalt oxide in potassium phosphate electrolyte on the anode side.

So it’s not really an artificial leaf, which is defined as using sunlight to directly drive a chemical synthetic reaction. Rather, this is just a process that uses electricity to make hydrogen and CO from atmospheric CO2 and H2O. That electricity can be from any source, and they just chose to use photovoltaics. So really, the conversion of light to energy is still just using photovoltaic technology.

That doesn’t mean this process is not useful. It’s just not an artificial leaf. This is an energy storing system – it takes electricity and stores it in the form of burnable compounds (hydrogen and CO).

This may seem like a picky distinction, but I think it’s important. When communicating to the public about energy we should be clear and consistent about exactly what we are discussing. Hydrogen is not a fuel source, it is a storage medium, for example.

We should be crystal clear about what exactly is the source of energy, the process by which that energy is captured or converted, and the storage medium of that energy. I really dislike press releases that deliberately confuse these elements in order to make a technology seem more impressive or useful.

The specific technology that was developed and presented in this paper is not an artificial leaf. It does not capture sunlight nor use sunlight to drive any chemical reaction. It does not do photosynthesis, and yet the press release specifically presents it as if it is photosynthesis.

This is a system that uses energy to create fuel – it is an energy storage system. The photovoltaics are the energy source, and there was no particular need to use photovoltaics. This technology can work with any source of electricity. We may want to use photovoltaics because it is a renewable energy source, but that is incidental to the technology. This should have been presented as an energy storage system that can be coupled with renewable energies to make them more viable. (This can just as easily be coupled with a wind farm.)

In any case, I do think that this can be an important technology. The biggest limiting factor with relying upon wind and light as energy sources is that they are intermittent. Storing all that energy for use when needed is critical, and this can be one way to do that.

This storage method has the advantage that it can be used to create synthetic gasoline, and therefore used in our existing auto fleet without any change in infrastructure. Because it uses atmospheric carbon as an input, this system would also be carbon neutral.

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