Scientists split over artificial photosynthesis development methods

As oil and other fossil fuel reserves are expected to dry up in the future, scientists around the world are eyeing artificial photosynthesis as a new potential source of clean energy.

Artificial photosynthesis uses sunlight to generate clean energy resources, such as hydrogen and ethanol, from carbon dioxide and water. Society as a whole stands to benefit if it can be put into commercial use.

However, there are a host of challenges involved. In Japan, researchers are divided over two different development methods for achieving higher solar energy conversion efficiency, as well as production costs.

Two main approaches

Toru Setoyama, an executive officer of Mitsubishi Chemical, heads a government-backed artificial photosynthesis project. It is a 10-year initiative launched in fiscal 2012 by Japan’s Ministry of Economy, Trade and Industry.

As an expert on artificial photosynthesis and its use in society, Setoyama believes that using photocatalysts made of titanium oxide will be the key to the development of artificial photosynthesis.

Baking photocatalysts coated with titanium oxide, he said, will produce separation membranes — a core component that can break down water into hydrogen and oxygen. It is estimated to cost only hundreds of yen (a few dollars) to make such a membrane per square meter.

“To put artificial photosynthesis into practical use, we should use only a technology that can help cut production costs and create large photocatalysts easily. The method using expensive semiconductor devices is counterproductive,” he said.

In March, he worked with project partners, including the University of Tokyo and toilet-maker Toto, to make 10cm-size photocatalyst square sheets. They successfully achieved a 1.1% solar energy conversion efficiency rate, exceeding the rate of 0.2% to 0.3% by plants. They aim to raise that conversion efficiency to around 10%, a level high enough for conducting feasibility tests, by the project’s final year, fiscal 2021.

The other method — a technology that uses a set of several semiconductor devices to electrolyze water and synthesize useful substances, such as formic acid and ethanol — may seem less appealing. This method is more costly as semiconductors require expensive production equipment, such as vacuum devices. Toyota Central R&D Labs., a research arm of Toyota Motor group based in Aichi Prefecture, Panasonic and Toshiba are working on the method to develop artificial photosynthesis technologies.

Although these three companies have yet to make production cost estimates, Setoyama believes it is very difficult to lower production costs below 20,000 yen ($183) per square meter based on the production costs of silicon-based solar cells. This means the semiconductor method is roughly 100 times more expensive than the photocatalyst method.

Still, Toyota Central R&D Labs. has achieved a 4.6% solar energy conversion efficiency rate, a record high in the world, by way of the semiconductor method. Takeshi Morikawa, a senior fellow at the company, acknowledges such issues as production costs and the difficulty of scaling up artificial photosynthesis, but he still holds his own against criticism.

“We have increased the efficiency rate to 4.6% from 0.04% in 2011. We have gained much knowledge from our research efforts. We can’t accept those who deny the semiconductor method,” he said.

Different methods, different usages

Indeed, there is a difference in artificial photosynthesis usage between the two methods.

Setoyama and his fellow researchers envisage using artificial photosynthesis in a large chemical plant-like system to produce a great volume of hydrogen, ethylene and other substances — chemicals that can be used as an energy source or for raw materials in the chemical industry. These scientists put focus on cost reduction as well.

In contrast, Panasonic and Toshiba are looking to create a smaller system that can be easily installed at carbon dioxide-emitting factories and power plants.

Plants use sunlight to break down water into hydrogen and oxygen and absorb carbon dioxide in the air to synthesize sugar — an extremely precise and efficient reaction created by Mother Nature.

In 2010, Ei-ichi Negishi, a distinguished professor at Purdue University, won the Nobel Prize in chemistry for his research on cross-coupling reactions in which palladium is used as a catalyst to combine carbon atoms. Since he has promoted artificial photosynthesis, Japan’s education, science and technology ministry and the industry ministry have moved to make investments, spurring research in this field. But observers say that the actual use of artificial photosynthesis will not happen until around 2030, at the earliest.

While there are similar projects underway in the U.S., Europe, South Korea and elsewhere, Japan has its own unique approach; not just universities and public research institutes but also private companies are involved in the research activities. Setoyama, a scientist from the private sector, is now spearheading the national research project, which in turn has fueled debate over artificial photosynthesis development methods.

Haruo Inoue, a professor at Tokyo Metropolitan University, is the head of an artificial photosynthesis project led by the education ministry. He noted that it was necessary to have an environment where researchers in a broad range of fields can engage in discussions beyond their disciplines.

But he raised a question about those who are trying to narrow the focus of artificial photosynthesis research down to only one method at this early stage. “Research will continue toward putting artificial photosynthesis into practice,” Inoue said. “We need to carry it on just as we pass a baton from one person to another.”

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