Photovoltaic can "carbonize" carbon dioxide?

StuartLicht designed the final recycling machine. The solar reactor he and his colleague built at the University of Washington laboratory can use sunlight to convert carbon dioxide in the air, a by-product of the oxidation of fossil energy, into fuel again. There are several steps in the process: water is used during this reaction, water can be decomposed into hydrogen and carbon monoxide; then the decomposition products can be mixed with liquid hydrocarbon fuels. It can be said that Licht's device is the most effective conversion device in the world so far.

In fact, Licht's approach is just one example of the use of solar energy for carbon dioxide conversion by various laboratories around the world. These technologies represent a dream: One day, they can bypass fossil fuels and generate fuel for transportation from sunlight, air, and water, and in the process, get rid of human emissions to the air because of their dependence on fossil fuels. carbon dioxide.

These technologies have not yet threatened the oil industry. In Licht's design, part of the reactor temperature is as high as 1000°C. This high temperature requires special materials to hold the relevant components. Other researchers are also exploring alternatives to develop catalysts that can use sunlight or other catalysts powered by renewable energy to perform the same chemical reaction, or that can perform chemical reactions at room temperature.

One of the obstacles is economics. At present, oil prices are still low, so it is difficult to motivate other high-end, costly options. However, unstoppable climate change and its related effects have attracted researchers all over the world to explore solar fuels. “This is a very hot area,” said Omar Yaghi, a chemist at the University of California, Berkeley. As confirmed by Licht's reactors, relevant research continues to advance. "We haven't gotten there yet, but we're heading in the right direction," says Andrew Bocarsly, a chemist at Princeton University who is studying cryogenic catalysts.

Enthusiastic researchers have even seen a glimmer of light, making this technology more economical and practical: the stable development of renewable energy such as wind power and solar energy. Now, wind turbine blades and solar cells have been able to provide more than the amount of electricity used in some areas. If this excess energy can be stored as a chemical fuel, experts say, perhaps the equipment supplier can save energy anytime, anywhere, resulting in additional revenue.

Technical and economic challenges

Despite concerns about climate change, the demand for liquid fuels cannot be finalized. The high energy density and easy-to-transport properties of petroleum and other liquid hydrocarbons make it a major dependency of the global transportation infrastructure. Researchers are continuing to explore the use of low-carbon gases, such as the use of methane and hydrogen as transportation fuels, which has led to a dramatic increase in electric vehicles. But for long-haul trucks and other heavy-duty vehicles and the aviation industry, there is no better choice than liquid fuels. Those who support solar energy say they should find a way to make carbon dioxide brewing liquids using available compounds such as water and carbon dioxide.

This goal can be attributed to the reverse oxidation reaction, that is, to obtain energy from the sun or other renewable energy sources, and then turn it into a compound. "This is a very challenging issue and it is a difficult battle," said chemist John Keith of the University of Pittsburgh in Pennsylvania. It can be said that this is just like plants need to make the sugar needed for growth, but plants only convert about 1% of their energy into chemical energy. In order to drive industrial development, researchers have to do more than this. Keith likens this challenge to the human moon project.

The problem is that carbon dioxide is a very stable molecule that can hardly produce chemical reactions. Chemists can force them to react by electricity or heat. Among them, the first step is usually to peel off an oxygen atom from the carbon dioxide molecule to form carbon monoxide. Carbon monoxide can then be mixed with hydrogen to form a mixed gas containing carbon monoxide and hydrogen, which can be converted to methanol, a liquid alcohol that can be used directly or converted into valuable chemicals and fuels. Large chemical plants can carry out this process, but they are not making mixed gas from the air, but use a lot of cheap natural gas to synthesize the gas. Therefore, the chemist's challenge is to synthesize a mixture of gases from renewable energy that is cheaper than current energy prices.

From laboratory to application

Licht called his device that produces a mixture of carbon monoxide and hydrogen from solar energy "solar gas," and stated that his goal is to use the heat and electricity challenges from the sun. In an article published in the Journal of Advanced Science, he detailed the designed device. The device utilizes a sophisticated solar cell called Condensed Photovoltaic (PV) solar power generation technology. The battery can concentrate a large amount of solar energy on a semiconductor board surface, and then convert 38% of the input energy into high-voltage power.

This energy is then diverted to the electrodes of two electrochemical cells: one group of energy is used to decompose water molecules, and the other group is used to decompose carbon dioxide. At the same time, other remaining solar energy collected in the battery is used as a source of thermal energy to preheat the two batteries to thousands of degrees Celsius. This step can reduce the power required to decompose water and carbon dioxide molecules, which can be reduced by approximately 25%. Licht said that eventually about 50% of solar energy can be converted into chemical substances.

At present, it is unclear whether the cost of synthesizing mixed gas through this process is as low as that of natural gas mixed gas. But Licht emphasized that the separate cost analysis of his solar water splitting facility in 2010 was that if it cost $2.61, it would break out 1 kilogram of hydrogen—this energy is equivalent to 4 liters of gasoline.

Taking into account the costs involved, Bocarsly and other scientists tried to continue to decompose carbon dioxide at lower temperatures. One of these methods has been implemented commercially. In Iceland, a company called International Carbon Recycling opened a factory in 2012 to use renewable energy to synthesize mixed gases. The company uses Iceland’s abundant geothermal energy to generate electricity, which is then used to drive electrolysis machines that decompose carbon dioxide and water. The resulting synthesis gas is then converted to methanol.

Everything is possible

Of course, many regions in the world do not have as much geothermal energy as Iceland to drive this process. For this reason, researchers are looking for new catalysts that can use less energy to decompose carbon dioxide. These catalysts are generally located on the negative electrode (ie, the aqueous side of the two electrodes of the electrochemical cell). At the opposite electrode, water molecules are broken down into electrons, protons, and oxygen, and the oxygen fuses into the air after it turns into a bubble. The electrons and protons are transmitted to the negative electrons, where the carbon dioxide molecules are decomposed into carbon monoxide and oxygen atoms, and the oxygen atoms and electrons and protons combine to form more water.

At present, the best standard for this catalyst is "gold". In the 1980s, Japanese scientists discovered that electrodes made of gold have the highest efficiency of decomposing carbon dioxide into carbon monoxide in cryogenic devices. In 2012, Stanford University chemist Matthew Kanan and colleagues discovered a better material: the thin gold layer was converted into nano-sized crystals, which were then used to make electrodes. The research results published in the "Journal of the American Chemical Society" show that this material can reduce the required electricity by more than 50% and increase the catalyst activity by 10 times.

However, the price per kilogram of gold is 36,000 US dollars, and the large-scale use of this metal is too expensive. Last year, the University of Delaware chemist Feng Jiao stated in the research results of Nature-Communication that the catalyst made of silver nanoparticles is equally effective. This year, they published a study published in the Journal of the American Chemical Society's "Catalysis", introducing a cheaper, more efficient catalyst for the decomposition of carbon monoxide: dendrites made from small zinc nails.

Researchers around the world are still exploring other “bonanzas”: ways to use solar energy to directly drive carbon dioxide and water to low-temperature electrolysis. Many studies have focused on light-absorbing semiconductors, such as the use of titanium-based carbon dioxide nanotubes to decompose carbon monoxide, methane, and other hydrocarbons. To date, similar devices are still not efficient enough; many times they can only convert less than 1% of input solar energy into compounds. Bocarsly and some people have used the sun's ultraviolet rays to make better results. But at the American Chemical Society meeting in Boston in August this year, the University of Delaware chemist Joel Rosenthal reported that he and his colleagues have developed a ruthenium-based photocatalyst that can convert 6.1% of the collected solar energy into compounds.

Although these cutting-edge technologies are advancing, Kanan warns that there is still a long way to go for solar fuels and liquid fossil fuels. In particular, the price of oil per barrel has dropped below US$50. This hinders the global government from forming a concerted effort to impose caps on carbon dioxide emissions or impose carbon taxes, so solar energy may never beat fossil energy if solely on price. "This is a difficult task to accomplish." Kanan said.

Nonetheless, Kanan said that one day, if the application of renewable energy is broad enough, and the technology for making renewable fuels is also improved, people may be able to consume large amounts of energy without guilt, because people know that they are only burning solar energy.

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