Learning from Nature to Build the Future--the Challenge of Artificial Photosynthesis (White Paper on Science and Technology 2020)

The White Paper on Science and Technology, compiled by the Ministry of Education, Culture, Sports, Science and Technology (MEXT) in June, is a special white paper that forecasts the future. One of the key factors for the energy technology used by society in the year 2040, as depicted in the white paper, is artificial photosynthesis. Japan currently leads the world researching this ambitious initiative to create helpful materials for humanity by learning from the mechanism of photosynthesis used by plants. One expert who has long played a role in leading the cutting edge of this research stresses that "it will assist humanity to escape from the single-minded reaping of global resources for continued survival."

"It is the ultimate answer to escaping from the devouring of the planet."

The special in the white paper focuses on S&T Foresight (see column below), a survey on future science and technology released by the National Institute of Science and Technology Policy (NISTEP). The survey includes the prediction that artificial photosynthesis "will enable recycling (synthesizing fuel or chemicals) through carbon dioxide reduction at an energy efficiency of 20% or greater." The technology is predicted to reach "technical feasibility," or the point at which the technology will become feasible at the research level, by 2036, with "social feasibility," or commercial applications, possible by 2039. The technology is positioned as one which will contribute to "affordable and clean energy" for all, one of the 17 Sustainable Development Goals established by the United Nations.

Haruo Inoue, senior leading professor at Tokyo Metropolitan University and the number one expert on artificial photosynthesis, speaks about the importance of the technology. "Humanity has so far survived by 'consuming the planet' by gathering and consuming fossil fuels such as coal, oil, and natural gas. This has caused the constant release of greenhouse gasses such as carbon dioxide, and we are now facing concerns over climate change. Given this situation, the ultimate answer is to obtain helpful materials for artificial synthesis, a process that not only does not produce carbon dioxide, in fact it makes use of it." Inoue served as the Research Supervisor on the "Chemical Conversion of Light Energy" research program, PRESTO(Precursory Research for Embryonic Science and Technology) program of the Japan Science and Technology Agency (JST), from 2009 through 2016, and has led numerous other major research projects on artificial photosynthesis.

What exactly is artificial photosynthesis? Most likely you recall learning about photosynthesis at school. Photosynthesis is the process by which plants absorb carbon dioxide from the air and water via their roots and then create carbohydrates, such as starches, and emit oxygen using energy from the sun via chloroplasts present in the cells of their leaves. The carbohydrates are used as nutrients and to form the structure of the plants. Accordingly, artificial photosynthesis refers to technologies which learn from photosynthesis to extract useful materials.

"However, the actual research being carried out does not always faithfully imitate plants. It takes a wider approach," explains Inoue. He goes on to explain the three criteria for artificial photosynthesis. (1) Use visible light, which comprises the majority of sunlight, (2) use water, and (3) convert the light energy to chemical energy to produce useful materials. In this process, "catalysts" are used which do not change themselves but promote reactions in other materials. The materials produced by plants through photosynthesis are carbohydrates. However, in the case of artificial photosynthesis, it is more realistic to generate other materials such as hydrogen, hydrogen peroxide (In Japanese, Oxydol), or carbon monoxide.

Catalysts that work by absorbing light are sometimes called "photocatalysts."
The search for a catalyst to achieve practical applications

Researchers have been taking on numerous challenges in the search to develop a catalyst that will leverage light efficiently to produce useful materials. One leading example of a catalyst is the semiconductor. Just last May, a group of research organizations in Japan, including the New Energy and Industrial Technology Development Organization (NEDO), announced that they had developed a semiconductor capable of using almost 100% of the energy in light, though in this case it was ultraviolet rather than visible light, to break down water. Prior to this it had been rare to ever achieve an efficiency over 50%. The world was astounded by this revolutionary advancement that opened a path towards the development of advanced catalysts.

To take a closer look at photosynthesis in plants, the process could be described as the motion of electrons, subatomic particles with a negative electric charge. The application of light energy causes water molecules to release electrons and break down into hydrogen and oxygen. Through a complex process, the work of the released electrons results in the creation of carbohydrates based on that hydrogen and carbon dioxide. In comparison, though artificial photosynthesis does not produce carbohydrates, it learns from the system by which electrons are moved using light energy.

In addition to semiconductors, there is active research on using metallic complexes as catalysts that combine numerous molecules and ions with metal ions. The aim of Inoue's research group is to develop a metallic complex that adroitly absorbs light in order to extract electrons, the leading actor in photosynthesis, from water. Between 2017 and 2019, they were able to do just that, garnering great interest, using aluminum as one of the most common metals. The technology is likely to be very useful in generating hydrogen peroxide, which is widely used for such applications as sterilization, disinfection, and purification.

Progress has also been made in research to modify organisms that carry out photosynthesis to make them easier to utilize. These are just a small part of the efforts that are being carried out, and it is still too early to tell which of the technologies will turn out to be useful.

Artificial photosynthetic panels opening the doorway to a hydrogen-based society

Artificial photosynthetic panels are one effective form in which artificial photosynthesis can be applied. The panel surface is coated with a catalyst which breaks down water when sunlight is applied. The resulting hydrogen can then be used as a fuel or for other purposes. The panels will likely be very similar to solar panels in appearance. This approach would avoid the use of fossil fuels when producing hydrogen, and would avoid the production of waste products because the hydrogen simply reacts with oxygen to become water again when used as fuel.

A factory could be built with arrays of these panels to produce hydrogen. That hydrogen could be used to produce a reaction in carbon dioxide to produce useful materials such as methanol or olefin which can be used to create plastics and pharmaceuticals. This would be a revolutionary shift where the carbon dioxide, for which reduction of emissions is becoming urgent, would become a raw material put to use in manufacturing. Inoue explains, "There are calls to achieve a carbon neutral state where the same amount of carbon dioxide is emitted and absorbed, but if we began using the carbon dioxide in the air, we could achieve not only a carbon neutral state but a carbon negative state."

In recent years, fuel cells which generate electricity and heat from hydrogen are starting to be used in the home. One effective way to generate this hydrogen would be using artificial photosynthetic panels installed on roofs, rather than through the use of fossil fuels such as liquid petroleum gas. There are also expectations for the use of fuel cell vehicles (FCVs). These are just a few of the possible applications, and the technology is likely to help open a path towards the hydrogen society concept that Japan is taking the lead in promoting.

Prime Minister Yoshihide Suga gave a policy statement to the Diet in October, declaring the target to achieve effectively zero emissions of greenhouse gasses in Japan by 2050. Inoue stresses that "artificial photosynthesis can greatly contribute" to the goal.

Narrowing down the technology a decade away, now is the time for diversity

Researchers describe the performance of photosynthesis in terms of "energy conversion efficiency," which is an indicator of just how much of the total energy contained in sunlight can be converted and stored. Most plants only manage less than 1%, but experiments in artificial photosynthesis that ignore costs have already achieved a level beyond 1%. According to Inoue, at the current rate, a level of "10% should be achieved by 2030" experimentally.

Researchers aim to achieve the practical application of artificial photosynthesis for society by 2050. To that end, they say it will be necessary to narrow down the most hopeful technologies around 2030, and begin building the necessary infrastructure by 2040. Inoue adds, "We will not be able to narrow down the technologies for some time, so the diversity of research is important. To compare it to climbing a mountain, the goal of which point to reach by when is important, but if you set the route too early, you might get stuck when that route becomes unpassable."

With regard to the target in the white paper to "achieve 20% efficiency between 2036 and 2039," Inoue responds seriously that, "It is a very high target. The government, researchers, and public will have to be determined and prepare carefully."

He explains that the solar panels now commonly used in the more familiar form of renewable energy known as solar power have an efficiency of about 10%. However, he adds, batteries are required to store the electricity generated during daylight hours, and the weight thereof create issues for practical use. In comparison, hydrogen is easy to store. Also, the generation of hydrogen using solar power requires a two-step process of first generating electricity and then using that electricity for electrolysis. The single step process of artificial photosynthesis will likely be more efficient, including the cost of building the system, according to Inoue. Another potential option is the combination of solar power with artificial photosynthesis.

Japan contributes to the world by leading research

The Honda-Fujishima effect, a process where the application of light to titanium oxide causes the breakdown of water, was discovered in Japan in 1967, helping Japan pioneer the study of artificial photosynthesis. It was also Japan that first identified the crystal structure of proteins required by plants for photosynthesis. Inoue explains that Japan is in a position to lead the world in the field, and has many researchers involved. "However, Sweden and the US are working hard, and China is also catching up. Japan must continue to work very hard to contribute to the world. We always welcome talented young people."

Constantly at the forefront, Inoue explains the appeals of the technology, explaining that, "The most interesting thing about this research is the ability to contribute to the SDGs and to the continued survival of humanity. Sunlight falls everywhere on the planet, though with some degree of variation. Throughout history, humanity has continued to fight over resources, but if there is enough already where you are, there will no longer be any reason to fight, and we will achieve true security."

Column: S&T Foresight - forecasting the future by asking the public and experts

The S&T Foresight survey that covered practical applications for artificial photosynthesis has been carried out by the NISTEP roughly every five years for the purpose of assisting in preparing the government's basic science and technology plan. The survey included in the recent White Paper on Science and Technology was the 11th survey released last November. The survey compiles discussions from numerous citizens and the understanding of researchers to give a forecast of the future of society in a manner that can invoke interest in anyone.

The survey was first carried out in 1971. Shinichi Akaike, a senior fellow at NISTEP and overall director of the most recent survey, explains that, "Originally, the survey was more intended to 'guess' the future of science and technology, but there was a major shift around the year 2000 to compiling the understanding of the public and experts in light of the needs of society." The emphasis of the survey now is on the desired shape of society.

Work for the current survey was begun immediately after the previous one finished in 2015. Workshops were held on themes of the future of the world, Japanese society, and regions participated in by the general public and by experts from overseas. Fifty futures for Japanese society were identified, including a society where everyone is a creator, and a borderless society.

Meanwhile, a total of 702 challenges facing future research and development were identified through discussions by 74 experts. Then a survey was distributed to researchers in each field to inquire about the importance and feasibility of each. The survey was run using the Delphi method, in which each person was asked to respond twice, to obtain reliable answers. Respondents numbered 6,697 people in the first round and 5,352 in the second.

After these responses were compiled, the basic scenario that emerged was that of a flexible society achieved through the reviving and reconsideration of human nature. TheMEXT created this original large-scale illustration when including the survey results in the white paper. With an entertaining touch, it depicts how people in 2040 will enjoy 38 dream technologies and services such as telemedicine, instant disaster prediction, and advanced agricultural robots on the streets and throughout their lives.

The survey results are said to be used not only by the government, but by companies tracking technological trends and young people choosing their career path. Akaike explains, "One of the major purposes of the survey is to deepen the understanding and interest of the citizens in science and technology."

Haruo Inoue

Senior Leading Professor, Department of Applied Chemistry for Environment, Graduate School of Urban Environmental Sciences, Tokyo Metropolitan University.
Prof. Inoue completed the engineering graduate course at University of Tokyo in 1972. He became a full professor at the Tokyo Metropolitan University in 1991 after serving there as an assistant professor, and again in 2005 when the university changed its Japanese name. He has also served as a visiting professor at Tohoku University and at the University of Florida, as well serving as the director of the Artificial Photosynthesis Research Center at Tokyo Metropolitan University. He became a Specially Appointed Professor in 2012, a Special Leading Professor in 2018, and took his current position in 2020.
His research themes include building artificial photosynthesis systems and chemical reaction energy coupling.

Shinichi Akaike

Senior Fellow at the NISTEP, MEXT.
Completed the doctorate program, School of Environment and Society, Tokyo Institute of Technology in 2008. He joined the Science and Technology Agency in 1992. Before taking his current position in 2018, he served at the Ministry of Education, Culture, Sports, Science and Technology(MEXT), the Embassy of Japan in Sweden, as acting assistant director of the Center for Research and Development Strategy, Japan Science and Technology Agency, as the professor of Hitotsubashi University, and as the director of the NISTEP S&T Foresight Center.
His research themes include forecasting science and technology and science and technology innovation policy