Activity Report

February 22, 2013 The 21st Century Dream: Artificial Photosynthesis

When it comes to the issue of how to manage global scale climate change, advanced countries and developing countries make very different assertions, and within the former group, opinions clash between EU nations that are for proactive climate measures and the United States that is less so, which further stalls progress. However, it is clear that CO2 emissions are drastically on the rise due to industrial activity, and that it is having great impact on the earth’s environment. It is difficult to quantitatively estimate the actual increase in the earth’s temperature and extent of climate change that is caused by CO2 emissions, but the problem should be treated not in a manner of “innocent until proven guilty,” but rather with a “fail safe” approach, from a scientific point of view. As can be said for the Fukushima nuclear accident, it is reasonable to be on the safe side and have preventative measures in place when dealing with issues that can potentially lead to serious disaster.

One of the major themes in chemistry in the 21st century is “Green Sustainable Chemistry.” The aim is to realize a sustainable society by using renewable resources (although biomass seems to gather the most attention) to synthesize chemicals in an elegant manner (i.e. to decrease CO2 emissions).

As an ideal form of “Green Sustainable Chemistry,” The KAITEKI Institute (TKI) has been working on developing an industrial process of artificial photosynthesis. Existing approaches for artificial photosynthesis can largely be categorized into 3 different ways. The most classic is to reconstruct the active center of photosynthesis in the manner of organometallic chemistry, as represented by metal porphyrins. Next is the method of increasing the productivity of plants by metabolic engineering. The third, newest way is to apply semiconductor photocatalysts to split water molecules, which can be expressed as the chemical reaction H2O ⇒ H2 + 1/2O2. TKI is focusing on this third approach, with the objective of developing “water-splitting catalyst under visible light,” and at the time of the institute’s founding in 2009, started a joint research project with 7 universities in Japan and abroad, 8 research institutions, and Mitsubishi Chemicals Research Center (MCRC). The project evolved, and in November 2012, became a Japanese governmental project called the “Artificial Photo Synthetic Chemical Process” (ARPChem), under the direct rule of the Ministry of Economy, Trade and Industry (METI) with a planned duration of ten years.

Water splitting by optical semiconductor electrodes was first discovered and announced around 1970 as the Honda-Fujishima effect, and since then had been studied with great enthusiasm by scientists in Japan who viewed it as the jewel in the crown of Japanese science. The process yielded practical application as an environmental catalyst, with the titania catalyst used in the decomposition of harmful chemicals. In regards to water splitting, the process had achieved a level of efficiency of over 50% under ultraviolet light (very strong ultraviolet light with a wavelength of 260 nm) by the end of the 20th century. It can be said that this period in Japan established the scientific fundamental that has set the stage for the 21st century to see the dawn of water splitting technology under sunlight (visible light with wavelength of over 400nm).

In 2003, Kazunori Domen, Professor of Tokyo Institute of Technology Chemical Resources Laboratory (currently Professor of The University of Tokyo, Chemical System Engineering) reported the findings that by using GaN-ZnO, a particle catalyst based on photo semiconductors, under visible light of over 400nm, water is completely broken down at a quantum yield of roughly 3%, generating hydrogen and oxygen at a ratio of 2:1. The use of oxynitride to absorb visible light, and the achievement of double-digit increase in activity compared to catalysts used in earlier studies was revolutionary, and the announcement had a profound impact on the direction of subsequent research worldwide. (The catalyst is now known as the ‘Domen Catalyst’). The project led by TKI formed a research collective organized around Professor Domen’s network around the world, and continued to contribute to the cutting edge research.

The ARPChem government project that started at the end of last year ultimately aims for 10% energy conversion efficiency as shown in Diagram 1 below. This number is the estimated target at which the process is thought to be economically feasible to produce ethylene, propylene and other chemical materials out of CO2 and hydrogen generated from the water-splitting process. Currently, optical semiconductors that absorb light with 400~450nm wavelengths has an energy conversion efficiency of around 0.3%, so in order to reach the project’s target, activity will have to be improved by over 30 times, and optical semiconductors will have to become able to absorb light of over 600nm. This is quite a far-reaching target, but progress is being made, and we are beginning to gain more understanding of what factors are affecting performance, and the kind of catalyst that will enable the process. Much of the remaining questions regard procedures and methodology on what kind of catalysts are actually possible to create, which is an area for scientists to really show their skills. Although the duration of ARPChem is set for 10 years, we hope that the target will be reached with much less time.

Diagram 1 Light absorption wavelength and energy conversion efficiency

The issue of how to incorporate artificial photosynthesis into industry is as important as the development of the technology. In recent years, the “shale gas revolution” has rejuvenated the field of unconventional natural gas and chemical feedstock in the United States, which has resulted in a halt in the advancement of renewable energy such as solar power generation. In contrast, utilization of biomass resources is slowly but surely progressing in the EU region. There will most likely be many more changes in circumstances surrounding energy in the future, but “artificial photosynthesis” will continue to be the method of dreams for obtaining energy security, and the ultimate solution.

There is still a long way to go for the large-scale utilization of the technology to become a reality, and even if the technology were available, it would not come into practical use unless the infrastructure was ready in society. How then, are we supposed to deal with the situation right now? We think that a key is to focus on compatibility. Taking chemical products as an example, what is needed is the development of technology that is compatible with resource diversification, with the current outlook including oil, natural gas and other fossil fuels, biomass resources (with increased productivity), solar hydrogen obtained from artificial photosynthesis, and CO2 in the atmosphere, as shown in Diagram 2. The idea is to establish a flexible infrastructure by which resources can be converted to have less environmental impact. Bringing about this diversification and compatibility requires the large-scale verification of a revolutionary technology. As the expression “the lost 20 years” describes, there is a sense of stagnation and limitation in Japan today. Artificial photosynthesis holds the potential for this country to venture out again into the global arena and show its striking presence to the world with a revolutionary technology. The aim of the national project ARPChem that started off as a research project at TKI is not just to develop a useful technology, but also to provide the chance for Japan to contemplate a new way forward.

Diagram 2 Chemical production process using various fossil fuels/ renewable resources