Green plants utilize sunlight to synthesize the necessary organic matter, such as glucose, out of carbon dioxide and water. This process is called photosynthesis. Mankind has taken steps to emulate natural photosynthesis to transform carbon dioxide (currently regarded as a greenhouse gas) into resources. This is called artificial photosynthesis.
Artificial photosynthesis will enable us to produce hydrogen, a clean energy resource, by combining solar battery technology and water electrolysis technology; and further allow us to generate various useful compounds (e.g., carbon containing fuels or raw materials for medicine) by reducing carbon dioxide. In this sense, artificial photosynthesis can be considered 21st century alchemy in the transformation of carbon dioxide.
Developing artificial photosynthesis technology is currently the subject of fierce competition among many countries, including the U.S., Europe, and Japan. In the development of artificial photosynthesis technology, Joint Center for Artificial Photosynthesis (JCAP) in the U.S., Max Planck Institute in Germany, and Ministry of Education, Culture, Sports, Science and Technology (MEXT) in Japan are currently at the cutting edge. Korea Center for Artificial Photosynthesis (KCAP) in Korea, too, has made gains in artificial photosynthesis research.
Solar energy would probably be the most sustainable energy on earth. Solar energy is infinite as long as the sun exists because energy from the sun continually comes to the earth. Without solar energy, other energy sources such as wind, water flow or tide, cannot be sustained. This immense energy source has been utilized by living organisms, including plants, which have evolved for billions of years and developed the mechanism to use the energy. That mechanism is called photosynthesis.
Photosynthesis is a process in which living organisms produce chemical energy out of solar energy, which cannot be directly used. Let’s think of changing a battery, for example. Batteries should be changed with a power supply. But the problem is that you need to change the voltage to a level usable at home since the voltage of electricity entering the house is too high to charge the battery. A similar mechanism can be applied to photosynthesis. At the first stage of photosynthesis, light energy is converted into and stored in an energy form that's usable for a chemical reaction within the cell; at the second stage of photosynthesis, the stored energy is used to produce glucose, a fuel for living organisms.
The first stage requires light, which we called a light-dependent reaction. The second stage can proceed without light, which we called a light-independent reaction. Simply put, the light-dependent reaction is similar to lowering the voltage to use electricity for the charger; the light-independent reaction is similar to the actual charging of the battery.
Scientists have paid close attention to light-dependent reactions. In the light-dependent reaction, light is used to generate ATP (Adenosine Tri-Phosphate), an energy source of life phenomenon; and the enzyme called NADP (Nicotinamide Adenine Dinucleotide Phosphate) is combined with hydrogen ions (H+). ATP and NADPH provide energy to a complicated chemical reaction called the Calvin Cycle in light-independent reactions to synthesize glucose out of carbon dioxide. In sum, the light-dependent reaction is a prerequisite for synthesizing chemical compounds.
The light-dependent reaction is one of the most complicated biochemical reactions, and consists of two photosystems that absorb light and produce electrons. Photosystem I, chlorophyll P700 absorbs light with a wavelength of 700nm (nanometer, where l nanometer=1/1 billionth of a meter), and produces high-energy electrons. These electrons transfer energy to various enzymes, oxidizing NADP into NADPH, before reverting to chlorophyll. This is why the Photosystem I is called “cyclic photophosphorylation.”
In the Photosystem II, chlorophyll P680 absorbs light with a wavelength of 680nm, and dissolves water into hydrogen ions, electrons, and oxygen. These electrons go through a series of enzymes to enter the Calvin Cycle in light-independent reactions. The electrons, after going into the Calvin Cycle, do not return to P680. This is why the Photosystem II is called “non-cyclic photophosphorylation.”
If the two photosystems could be artificially created and emulated, we would be able to utilize the generated electrons and NADPH to induce diverse chemical reactions. This was where research into artificial photosynthesis all started. NADP, an important enzyme in light-dependent reactions, is a major reductant in the energy transfer; and it is involved in diverse reactions from photosynthesis to steroid synthesis. The question is, “how can we emulate light-dependent reactions?”
A representative case for emulating light-dependent reactions is light reactive dye, which is made using light reactive quantum dots or nano-particles. Light reactive dyes are mostly made of cadmium sulfide (CdS), which receives light just like chlorophyll, and generates high-energy electrons. When the dye is surrounded by TiO2 semiconductors, the energy level of the semiconductors is slightly lower than the electrons, making it easier for the electrons to move to the semiconductor. There is a wire connecting the semiconductors that electrons move through, generating electricity. Once the electrons have consumed all the available energy, they return to the dye, and then move again, just like the electrons in the Photosystem I. As long as there is light, high-energy electrons are generated, and so is electricity. This is how the dye-sensitized solar cell is created. Produced in the form of dye, dye-sensitized solar cells can be widely adopted to diverse forms of surface.
Light-dependent reactions can be imitated by copying just one photosystem. However, light-dependent reactions are much more efficient when the two photosystems coexist. The entire light-dependent reaction is copied by a photo-electrochemical cell (PEC). The photo-electrochemical cell uses light to dissolve water, and generate electricity and hydrogen. The PEC can be made by putting the positive and negative poles (made respectively of titanium dioxide and platinum) into the electrolyte aqueous solution where electricity flows. When the positive pole receives ultra-violet rays, it dissolves water, generating electrons, and the electrons flow through the wire, causing a reduction reaction at the negative pole. When the water is dissolved, oxygen is emitted into the air, and the hydrogen ion remains in the water. This hydrogen ion causes the reduction reaction, transforming itself into hydrogen gas. Thus the positive pole acts as the Photosystem II; the negative pole acts as the Photosystem I.
Meanwhile, instead of using the positive and negative poles, photo-electrochemical cells can also be made using colloid and catalysts. This method uses the colloid gallium nitride-zinc oxide alloy (GaN:ZnO) that highly absorbs sunlight as a colloid. On the surface of a colloid, there are some catalysts that dissolve water and others that show the reduction reaction. The dissolved water generates electrons, which absorb light energy and transform hydrogen ions into hydrogen gas through the reduction reaction caused by catalysts.
The technology that emulates light-dependent reactions can be applied to diverse fields, from reductants for chemical reactions to hydrogen fuels and electricity. Among many, hydrogen, a product of artificial photosynthesis, is considered a next generation clean fuel. The production of hydrogen using artificial photosynthesis is an area of research led by the U.S. The U.S. has already developed technologies for hydrogen-fueled cars, and the US department of energy (DOE) is leading research on all areas of hydrogen fuel, from production to storage and management. Research into hydrogen production technology using artificial photosynthesis has been actively conducted and led by the Lawrence Berkeley Laboratory, while the JCAP established two research centers in Pasadena and Berkeley, California. The artificial photosynthesis research center of MIT recently showed results in creating artificial leaves that dissolve water by coating the leaves of plants with the catalyst made of nickel and cobalt.
European countries that have heavily invested in renewable energy are also playing an important role in artificial photosynthesis research. The Max Planck Institute for Chemical Energy Conversion in Germany has taken a big step in the field. Researchers at the institute conducted important base studies on the process of water oxidization and the process of energy transformation in artificial photosynthesis. In the U.K., the research team of professor Richard Cogdell from Glasgow University of Glasgow recently made artificial leaves that can change solar energy into liquid fuel.
In Japan, research in this field has been led by the MEXT. The main research has been led by professor Negishi Ei-ichi, Nobel laureate in chemistry in 2010. Professor Negishi Ei-ichi from Purdue University was invited from Hokkaido University for artificial photosynthesis research. The representative work of the professor, `Negishi Coupling’ is regarded as one of the core theories in artificial photosynthesis.
Businesses are also showing huge interest in artificial photosynthesis. A representative artificial photosynthesis corporation is Sun Catalytix in the U.S., which enjoys the direct support from of the Obama administration. The founder of Sun Catalytix Daniel George Nocera, professor from Harvard University is setting up plans to mass produce hydrogen fuel using sunlight to dissolve water into hydrogen and oxygen. If artificial leaves are widely commercialized, it is expected that every household could be self-sufficient in electricity production at an affordable price.
Mitsui Chemicals in Japan started research on the technology in 2008 that uses carbon dioxide to synthesize hydrogen through photolysis of water. They are currently focusing on commercialization to produce over ten billion tons of methanol each year. These active moves among businesses are a clear signal in light of favorable prospects for the future artificial photosynthesis market.
The light-independent reaction that synthesizes organic matter is also an important research subject. Scientists have studied how to use the products of light-dependent reactions to produce not glucose but liquid fuels, such as methanol or organic compounds that can be raw materials for new drugs. If they can combine this technology with artificial light-dependent reactions, they can create a factory able to produce chemical compounds using light.
A good case in point is dissolving carbon dioxide together with water in an aqueous solution of the photoelectrochemical cell. In this state, when the catalyst is coated on the surface of the pole where the reduction reaction occurs, carbonate (CO3-) in the water is revivified into organic matter on the surface of the pole. The representative reductants of carbon dioxide are methanol and olefin, which are considered next generation energy sources. Major research institutes, including Lawrence Berkeley National Laboratory and California Institute of Technology (Caltech), are pursuing projects to produce liquid fuel using artificial photosynthesis, and in Japan, a public-private partnership called the Association of Artificial Photosynthetic Chemical Process (ARPChem) is conducting research on establishing a system for the mass-production of olefin.
The Calvin Cycle can be applied as well. In the Calvin Cycle, there is a process where DPGA is reduced into PGAL by NADPH. PGAL plays an important role as a raw material for drugs because it can be transformed into organic acid, such as aspartic acid, oxalic acid or pyruvic acid. Thus a little change to the Calvin Cycle can produce the organic matter that we want.
Artificial photosynthesis encompasses both the artificial light-dependent reaction and artificial light-independent reaction. It is a complicated field as it relates to diverse research areas both in light-dependent reactions and light-independent reactions. As of now, research is conducted only into specific elements of artificial photosynthesis. It does not cover the whole system. It will take time to commercialize artificial photosynthesis. But once it is available for commercialization, and when it is applied with photovoltaic production, we will be able to use solar energy more efficiently. Artificial photosynthesis is a future engine for green technology, ranging from chemical factories running on light and chemical fuels produced in water to the infinite production of electricity. Infinite, of course, for as long as the sun exists.
* The founder of Sun Catalytix, a professor Daniel George Nocera, Harvard University plans to produce hydrogen fuel using sunlight to dissolve water into hydrogen and oxygen. If artificial leaves were to become widely commercialized, then every household would be self-sufficient in electricity production at an affordable price.
Korean researchers have made remarkable achievements in the field of artificial photosynthesis. Artificial photosynthesis research is currently led by the Korea Center for Artificial Photosynthesis (KCAP), while Korea Institute of Science and Technology (KIST), Korea Research Institute of Chemical Technology (KRICT), Seoul National University (SNU), Korea Advanced Institute of Science and Technology (KAIST), Kyungpook National University (KNU) are showing impressive performances.
“After Korea established KCAP, the US started to set up JCAP, and Japan also started to gather related researchers together. The Korean government, with a long term vision, has been investing in artificial photosynthesis for more than five years. I am confident in saying that our center is leading global research on artificial photosynthesis,” said professor KyungByung Yoon, a professor of Chemistry at Sogang University and a president of KCAP.
In Korea, researchers not only at KCAP but also in universities and institutions funded by the government have taken great steps in the field of artificial photosynthesis. So far businesses have taken no such steps toward commercialization, while research institutions and universities are conducting basic research and development.
KCAP was established in 2009 as part of the Project for Developing Basic & Original Technology for Climate Change Response at the Ministry of Science, ICT, and Future Planning, Korea. Led by professor KyungByung Yoon, about 100 researchers at Lawrence Berkeley National Laboratory in the U.S., KAIST and Pohang University of Science and Technology (POSTECH) are conducting artificial photosynthesis research. KCAP is scheduled to receive total government funding in 50 billion won over 10 years.
KCAP is located in Sogang University with the support of POSCO. Sogang University signed a MOU with POSCO for joint research into the commercialization of artificial photosynthesis in August, 2010. In January, 2013 and with the support of POSCO, a special research building for KCAP was constructed. The building, POSCO Francisco Hall, consists of eight ground floors and two basement floors with the area of 6,700m2 and 30 laboratories.
Yoon mentioned, “Artificial photosynthesis research utilizes carbon dioxide, sunlight, and water in order to make useful chemical compounds such as carbon monoxide, formic acid, formaldehyde, and methanol. But there is a long way to go before we commercialize artificial photosynthesis because of the high price of device installation and related materials, and the low production efficiency.”
KCAP aims to achieve the goal of artificial photosynthesis by combining previously commercialized technologies. When solar cell technology and water electrolysis technology are combined to produce hydrogen, we can mix hydrogen and carbon dioxide to make fuel. According to Yoon, the issue here is economic feasibility, and the key to improving economic feasibility lies in water electrolysis technologies. Previous hydrogen generation has mostly used fossil fuel, which emits carbon dioxide, and uses expensive platinum as a pole for water electrolysis. KCAP is currently working on ways to replace platinum with cheaper materials while improving the efficiency of hydrogen production.
He also mentioned, “In the long-term, KCAP aims to develop artificial leaves that absorb water, carbon dioxide, and solar energy to produce fuel and the molecular catalyst that can transform carbon dioxide, once injected into the pond, into fuel. Compared to the complicated artificial leaves, the molecular catalyst, such as artificial enzymes, can be an affordable and more efficient technology. But it will take about 30 years for the commercialization of artificial leaves alone.”
As for the major achievement of KCAP, Yoon elaborated KCAP developed the molecule separation membranes with zeolite which can distinguish 0.1nm (nanometer, where 1 nanometer = 1/1 billionth of a meter), and published a paper in Nature. However, the most important issue is to reduce the price of the pole for water electrolysis and improve efficiency. KCAP has developed an affordable yet efficient pole compared to platinum. It has also succeeded in increasing the efficiency of the production of hydrogen and oxygen (by more than 18%) using solar cells and water electrolysis electrodes.
* “Artificial photosynthesis research aims to use carbon dioxide, sunlight, and water to make useful chemical compounds such as carbon monoxide, formic acid, formaldehyde, and methanol. But there is a long way to go before we commercialize artificial photosynthesis because of expensive device installation and materials fee, and the low production efficiency.
By reducing carbon dioxide via the already-known thermochemical method that uses hydrogen (generated when water is dissolved by solar energy), one can admittedly produce fuel. However, it is more ideal to produce fuel that contains carbon by reducing carbon dioxide directly. In this respect, studies have been conducted that use the electric energy from solar energy to reduce carbon dioxide.
However, it has not been easy to produce liquid fuel such as methanol, so recently research has been conducted into changing carbon dioxide to carbon monoxide because liquid fuel can be easily produced using the latter. Carbon monoxide, mostly used as a chemical material, is an expensive chemical compound that costs 1.32 million won per ton. Currently, the highest level of efficiency attained in changing carbon dioxide into carbon monoxide is around 6%. KCAP is also showing about 6% efficiency in generating carbon monoxide out of solar cells, water, and carbon dioxide.
KIST has also developed a world-class artificial photosynthesis device that can produce high value-added chemical compounds such as carbon monoxide and formic acid. A research team led by Dr. Byoung Koun Min and Yun Jeong Hwang at the Clean Energy Research Center, KIST developed an integral artificial photosynthesis device, which shows higher efficiency in photosynthesis than natural leaves. They published a paper as a rear-cover paper in the Journal of Materials Chemistry A on March 21st. Plant leaves and algae usually show 1% and 3-4% of photosynthesis efficiency respectively, while the recently developed artificial photosynthesis device shows 4.23%.
The keys to completing artificial photosynthesis are the photo-electrode technology that produces electrons by absorbing sunlight, the catalyst technology that dissolves water, and the catalyst technology that changes carbon dioxide into useful compounds. The KIST research team has improved the stability of photo-electrodes by combining low-price thin film solar cell technology with catalyst technology. The team has made a device that can produce high value-added compounds by changing the catalyst for the reduction pole. For example, when you use gold or silver as a catalyst, you will mostly get carbon monoxide; when you use bismuth, you will mostly get formic acid. Formic acid, a natural material found in ants, is widely used in the textile industry, food industry, and pharmaceutical industry because it is easily decomposed during the biological process.
The research team at KNU developed a catalyst in the form of an electrode that can change carbon dioxide into formic acid using sunlight. The research team, led by professor Hyun-woong Park at School of Energy Engineering, KNU succeeded in changing dissolved carbon dioxide in water into formic acid using silicon semiconductors that absorb sunlight to generate electricity. The team published a front cover of Advanced Energy Materials in August, 2014. The team increased the sunlight absorption rate by making a cylinder type of positive semiconductor silicon, which has a diameter of just a few nanometers, and increased the production efficiency of formic acid by combining tin nanoparticles to the surface of the silicon cylinder.
The research team at SNU has developed an artificial photosynthesis system that produces hydrogen in water using only light energy. The team, led by professor Ki Tae Nam in the Department of Materials Science and Engineering, SNU, developed a system that produces hydrogen by combining the photosynthetic protein of plants with the particles of semiconductors. They published a paper at the front cover of Advanced Functional Materials on April 22nd.
The team made a hybrid system that combines bismuth vanadate (BiVO4), a material from semiconductors that dissolves water, with the protein from spinach; and succeeded in producing hydrogen in water by exposing the system to visible rays. professor Nam said, “The newly developed artificial photosynthesis system is the first case of true eco-friendly generation of hydrogen because it uses protein as a material with water and light. This system will be widely applied to diverse areas, including renewable energy development utilizing sunlight, light censor design and bioenergy development.”
The research team at KAIST, led by professor ChanBeum Park at the Department of Materials Science and Engineering, developed the original technology in artificial photosynthesis using nano-sized light sensitive materials in 2010. The team used light sensitive materials such as solar cell quantum dots (measuring a few nanometers in length) to emulate the light reaction of a natural state. They changed light energy into electric energy efficiently, and used that energy to regenerate a cofactor. In addition, the team replaced the complicated Calvin Cycle with the oxidation-reduction enzyme reaction for cofactor regeneration, creating a reactive system able to produce sophisticated chemical materials out of light energy.
The team published a paper in Advanced Materials, 2011 after developing an artificial photosynthesis system that uses a biocatalyst rather than photocatalyst. When researchers put photoelectrodes of dye-sensitive solar cells in water, and added biocatalyst powder together with cofactor (NADH) for natural photosynthesis, the oxidation-reduction enzyme reaction of photosynthesis resulted. It is expected that this technology will be applied to the production of high value-added compounds, including methanol, artificial amino acids and raw materials for new drugs. When sunlight reaches mineral dyes that act as chlorophyll, such as “cadmium sulfide (CdS) quantum dots,” dye-sensitive solar cells release electrons, and produce electricity.
The research team at KRICT is aiming to realize the vision of a Photovoltaic Factory that can produce the diverse chemical materials necessary for our lives out of sunlight and carbon dioxide. In December last year, the research team, led by Dr. JinWook Baek of Green Chemistry and Engineering Division at KRICT, succeeded in selectively producing methanol out of carbon dioxide using only sunlight and enzymes based on the principle of artificial photosynthesis. They published a paper to highlight the results in the online Journal of the American Chemical Society. The team’s results were selected as one of the ten major research achievements among government funded institutions in 2014.
In 2012, the team used sunlight and carbon dioxide to make formic acid and allyl alcohol, which is a raw material for new drugs. The process is almost the same to that of methanol. Carbon dioxide and an enzyme go into the photovoltaic reaction device as well as a graphene-based photocatalyst developed by the team. When sunlight reaches the device, light energy is converted into electric energy, causing a reaction in carbon dioxide and the enzyme, which produces methanol. When you want to produce other materials, you need to put in other types of enzyme. This device can produce substances that are in need, but only in small quantities. For commercialization, the photocatalyst needs to be more efficient while economic feasibility needs to be improved by introducing a bigger device.
* This is the concept and architecture of the artificial photosynthesis device developed by Dr. Byoung Koun Min and Yun Jeong Hwang of KIST. The device produces either carbon dioxide or formic acid, depending on the catalyst used for the reduction electrode.
* Dr. JinWook Baek (left) at KRICT is aiming to realize the vision of creating a Photovoltaic Factory that can produce diverse chemical materials out of sunlight and carbon dioxide. His research team also developed a graphene-based photocatalyst for the photovoltaic reaction device (right).
* It is expected that this technology will be applied to the production of high value-added compounds, including methanol, artificial amino acids and raw materials for new drugs. When sunlight reaches mineral dyes that can act as chlorophyll, such as cadmium sulfide (CdS) quantum dots, dye-sensitive solar cells release electrons, and produce electricity.