Solar-to-fuel system recycles CO2 to make ethanol and ethylene

September 18, 2017
Schematic of a solar-powered electrolysis cell which converts carbon dioxide into hydrocarbon and oxygenate products with an efficiency far higher than natural photosynthesis. Power-matching electronics allow the system to operate over a range of sun conditions. Credit: Clarissa Towle/Berkeley Lab

Scientists at the Department of Energy's Lawrence Berkeley National Laboratory (Berkeley Lab) have harnessed the power of photosynthesis to convert carbon dioxide into fuels and alcohols at efficiencies far greater than plants. The achievement marks a significant milestone in the effort to move toward sustainable sources of fuel.

Many systems have successfully reduced to chemical and fuel precursors, such as or a mix of carbon monoxide and hydrogen known as syngas. This new work, described in a study published in the journal Energy and Environmental Science, is the first to successfully demonstrate the approach of going from carbon dioxide directly to target products, namely ethanol and ethylene, at energy conversion efficiencies rivaling natural counterparts.

The researchers did this by optimizing each component of a photovoltaic-electrochemical system to reduce voltage loss, and creating new materials when existing ones did not suffice.

"This is an exciting development," said study principal investigator Joel Ager, a Berkeley Lab scientist with joint appointments in the Materials Sciences and the Chemical Sciences divisions. "As rising atmospheric CO2 levels change Earth's climate, the need to develop sustainable sources of power has become increasingly urgent. Our work here shows that we have a plausible path to making fuels directly from sunlight."

That sun-to-fuel path is among the key goals of the Joint Center for Artificial Photosynthesis (JCAP), a DOE Energy Innovation Hub established in 2010 to advance solar fuel research. The study was conducted at JCAP's Berkeley Lab campus.

The initial focus of JCAP research was tackling the efficient splitting of water in the photosynthesis process. Having largely achieved that task using several types of devices, JCAP scientists doing solar-driven carbon dioxide reduction began setting their sights on achieving efficiencies similar to those demonstrated for water splitting, considered by many to be the next big challenge in .

Another research group at Berkeley Lab is tackling this challenge by focusing on a specific component in a photovoltaic-electrochemical system. In a study published today, they describe a new catalyst that can achieve carbon dioxide to multicarbon conversion using record-low inputs of energy.

Not just for noon

For this JCAP study, researchers engineered a complete system to work at different times of day, not just at a light energy level of 1-sun illumination, which is equivalent to the peak of brightness at high noon on a sunny day. They varied the brightness of the light source to show that the system remained efficient even in low light conditions.

When the researchers coupled the electrodes to silicon photovoltaic cells, they achieved solar conversion efficiencies of 3 to 4 percent for 0.35 to 1-sun illumination. Changing the configuration to a high-performance, tandem solar cell connected in tandem yielded a conversion efficiency to hydrocarbons and oxygenates exceeding 5 percent at 1-sun illumination.

At left is a surface view of a bimetallic copper-silver nanocoral cathode taken from a scanning electron micrograph. To the right is an energy-dispersive X-ray image of the cathode with the copper (in pink/red) and silver (in green) highlighted. Credit: Gurudayal/Berkeley Lab

"We did a little dance in the lab when we reached 5 percent," said Ager, who also holds an appointment as an adjunct professor at UC Berkeley's Materials Science and Engineering Department.

Among the new components developed by the researchers are a copper-silver nanocoral cathode, which reduces the carbon dioxide to hydrocarbons and oxygenates, and an iridium oxide nanotube anode, which oxidizes the water and creates oxygen.

"The nice feature of the nanocoral is that, like plants, it can make the target products over a wide range of conditions, and it is very stable," said Ager.

The researchers characterized the materials at the National Center for Electron Microscopy at the Molecular Foundry, a DOE Office of Science User Facility at Berkeley Lab. The results helped them understand how the metals functioned in the bimetallic cathode. Specifically, they learned that silver aids in the reduction of carbon dioxide to carbon monoxide, while the copper picks up from there to reduce carbon monoxide further to hydrocarbons and alcohols.

Seeking better, low-energy breakups

Because dioxide is a stubbornly stable molecule, breaking it up typically involves a significant input of energy.

"Reducing CO2 to a hydrocarbon end product like ethanol or ethylene can take up to 5 volts, start to finish," said study lead author Gurudayal, postdoctoral fellow at Berkeley Lab. "Our system reduced that by half while maintaining the selectivity of products."

Notably, the electrodes operated well in water, a neutral pH environment.

"Research groups working on anodes mostly do so using alkaline conditions since anodes typically require a high pH environment, which is not ideal for the solubility of CO2," said Gurudayal. "It is very difficult to find an anode that works in neutral conditions."

The researchers customized the anode by growing the iridium oxide nanotubes on a zinc oxide surface to create a more uniform surface area to better support chemical reactions.

"By working through each step so carefully, these researchers demonstrated a level of performance and efficiency that people did not think was possible at this point," said Berkeley Lab chemist Frances Houle, JCAP deputy director for Science and Research Integration, who was not part of the study. "This is a big step forward in the design of devices for efficient CO2 reduction and testing of new materials, and it provides a clear framework for the future advancement of fully integrated solar-driven CO2-reduction devices."

Explore further: Copper catalyst yields high efficiency CO2-to-fuels conversion

More information: Gurudayal Gurudayal et al, Efficient solar-driven electrochemical CO2 reduction to hydrocarbons and oxygenates, Energy Environ. Sci. (2017). DOI: 10.1039/c7ee01764b

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MR166
1 / 5 (1) Sep 18, 2017
Solar powered my A$$. They are using electricity to convert water and Co2 into a hydrocarbon at 5% efficiency at best.
Sonhouse
not rated yet Sep 18, 2017
Yeah, take 100 watts of solar energy, get 5 watts of fuel out of it that when burned, is only good for what, 30%? What am I missing here?
omegatalon
not rated yet Sep 18, 2017
When they can make in excess of 50-100 Million gallons of Ethanol a day then we're talking as the Ethanol can be used in fuel cell to generate electricity in cars and even homes.
gkam
1 / 5 (3) Sep 18, 2017
Cars will scare the horses.

We don't need no computers.
Eikka
5 / 5 (2) Sep 19, 2017
Yeah, take 100 watts of solar energy, get 5 watts of fuel out of it that when burned, is only good for what, 30%? What am I missing here?


It doesn't mean taking 100 Watts of power from, say, the electric grid and turning it into 5 Watts of fuel, but taking 100 Watts of sunlight and turning it into 5 Watts of fuel. The actual catalytic step of turning electric power to fuel must be around 30-40% efficient which is fairly high.

It's comparable to other power-to-hydrocarbon methods which produce methane at 40-60% efficiencies, with the disadvantage that methane is a gas and not a convenient liquid like ethanol.

Liquid fuels are high value, so if you suppose you can get solar electricity at 6 cents a kWh in Saudi Arabia, and turn it to ethanol at 30% efficiency, you pay about $4.30 a gallon. That's not very far off from being economically sensible.

5 c/kWh and 60% efficiency makes it $1.80/gal
Eikka
5 / 5 (1) Sep 19, 2017
Low efficiency can be tolerable if the end product is worth more than the electricity you waste.

For example, the higher price of an electric vehicle negates the lower cost of using the electricity directly, so it can be more sensible to turn the power into fuel than to manufacture batteries.

After all, all the money you spend turns up as economic activity elsewhere, and that activity consumes energy as if you had burned it up yourself.
dnatwork
3 / 5 (2) Sep 19, 2017
Several things folks are missing:

Most solar electricity is sold for next to $0.00 because there's too much of it at noon. People can't use it, the grid can't handle it, etc. There have been instances when solar electricity actually had a negative price. This process could use the excess solar to make liquid fuels that sell for higher value, stabilizing prices and improving return on investment.

It's very low voltage. It would not subtracting all the the power from the grid, it could use the marginal energy and improve the overall efficiency of the system.

CO2 is a problem in itself. Climate change will be a catastrophe if left to the current course. This process could take CO2 out of the atmosphere, mitigating the fossil fuel burning that lines the pockets of moneyed interests.

And it would produce valuable chemicals, thereby paying for itself, so it should not be any skin off anybody's nose.
Eikka
not rated yet Sep 19, 2017

Most solar electricity is sold for next to $0.00 because there's too much of it at noon.


True, but money is still being paid for it because of subsidies, and even without subsidies the cost of the system has to be recovered somehow or else there's absolutely no point in building it.

From the individual producer's point of view, the power may be $0 but from everyone's point of view it's not because the panels cost labor, resources, and time - all of which translates to money. Therefore, everyone else is paying the one guy who's getting free electricity, and they have to ask - is it worth it?

From the society's point of view, the situation is regressive wealth redistribution - it's taking money from the many and giving it to the few who are already rich enough to own chemical processing plants. Then they also get to sell the product at a profit.

Fortunately, if there is this huge power sink, the price at noon won't be $0 because competitors will appear.

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