National / Science & Health | ADVANCES IN PROGRESS

Lab photosynthesis begins to bloom

by Kazuaki Nagata

Staff Writer

The world’s fight against global warming would be dramatically easier if a cheap technology was available to turn carbon dioxide into something useful.

In fact, researchers and firms are trying to do just that with artificial photosynthesis, a process that uses sunlight, water and carbon dioxide to produce oxygen and substances that can be turned into energy, just as plants and algae do via chlorophyll.

Expectations in this field are rising because the technology might help meet globally rising demand for clean energy and, more importantly, reduce carbon dioxide emissions.

Electronics giants Toshiba Corp. and Panasonic Corp. have been making encouraging strides in this area. While acknowledging the scientific and financial hurdles, they say artificial photosynthesis has a potential and could be ready for practical use or for field testing within a decade or so.

In December, Tokyo-based Toshiba announced it had achieved an energy conversion efficiency of 1.5 percent — the highest in the world.

“Artificial photosynthesis is the same as photosynthesis in terms of using solar energy to produce oxygen with water. But we are trying to produce something that is useful and in high demand,” said Akihiko Ono, a researcher at the organic materials laboratory of Toshiba’s Corporate Research and Development Center.

Artificial photosynthesis systems use two components — semiconductors and metal catalysts — to generate substances that can be converted into energy.

The process is carried out using two connected tanks, one containing water and a semiconductor, the other containing carbon dioxide-infused water and a metal catalyst. The semiconductor and the catalyst are also linked by a wire.

When light hits the semiconductor, it induces oxidation, splitting the water and generating electrons that flow to the metal catalyst via the wire.

When the electrons hit the catalyst, a chemical reaction takes place involving the surrounding carbon dioxide, hydrogen ions and electrons, producing carbon monoxide, formic acid, methane and ethanol. These substances can all be used as fuel.

The energy conversion efficiency of natural plants varies, but many are lower than 1 percent. For instance, corn is said to have a high conversion rate, but it’s only 0.8 percent.

A conversion rate of 0.8 percent means that, if the solar energy input is 100, artificial photosynthesis can create chemical substances that produce 0.8 percent of that energy.

Toshiba’s aim is to produce carbon monoxide, a versatile substance that can be processed into methanol, and then gasoline, PET bottles and medical products.

To do this, Toshiba uses a germanium semiconductor layered with amorphous silicon, which is used in solar panels, to absorb sunlight.

But Ono said Toshiba’s main strength can be found in its catalyst, which is made of gold.

Toshiba found a way to modify gold at the atomic level using nanotechnology. When elements are modified on such a small scale, their innate properties can change in a way that turns them highly active catalysts.

Normally, a high voltage is needed to get carbon dioxide to change into other compounds, but a highly active gold nanocatalyst allows the gas to complete the change at a lower voltage. That’s how Toshiba achieved its record high 1.5 percent energy efficiency rate, Ono said.

Can it go higher?

“There is still plenty of room to improve the efficiency rate,” he said, explaining that his goal is 10 percent.

If artificial photosynthesis devices with that kind of efficiency were set up on a parcel of land measuring 10,000 sq. meters, it would be capable of producing 3,700 liters of methanol, the company estimates.

As a result, such devices would likely be set up close to Toshiba factories and thermal power plants that emit large amounts of carbon dioxide, generating synergy between its energy business and its so-called carbon dioxide capture and storage technology.

The key to achieving this is developing inexpensive metal catalysts that can lead to high energy conversion rates.

“There are various ways to increase the efficiency but we need to balance the cost and longevity of the system,” Ono said. “It would be pointless to have something that can work for only half a year and achieve 10 percent at a really high cost.”

Toshiba is hoping to put artificial photosynthesis into practical use sometime in the 2020s.

Panasonic, another electronics giant, is pursuing a similar goal.

“We are an electronics maker and provide electronics products to our customers, which means (our products) use power generated from somewhere and emit carbon dioxide,” said Masanori Iida, manager of the technology public relations section.

“So, the project started with the idea of somehow turning carbon dioxide into something useful to fulfill our social responsibility as a manufacturer,” he said.

Osaka-based Panasonic has come up with an artificial photosynthesis system that achieves an energy conversion efficiency of 0.3 percent.

Whereas Toshiba aims to produce carbon monoxide, Panasonic is focused on making methane and ethanol, which can be used directly as energy.

For its semiconductor, Panasonic uses gallium nitride mixed with indium and layered with silicon. The catalyst is copper.

Panasonic’s energy conversion rate is 0.2 percent for methane and 0.1 percent for ethanol, Iida said, adding that it is the only firm to achieve such high efficiency levels with the two substances.

Since gallium nitride is used in LED lights, which Panasonic makes, it is using this know-how to its advantage.

While Panasonic hopes to start field experiments around 2020, Iida said that the technology is still under development and that its prospects for contributing to the reduction of carbon dioxide emissions remain unclear.

“When thinking about environmental measures, it is unlikely that one measure will solve everything. Rather, various solutions need to work together to create the best ecosystem,” he said.

Ono of Toshiba agrees.

“I think this will become one choice among all the renewable energy options. And most renewables are for generating power, which is hard to store and distribute, so (artificial photosynthesis) will be a bit different from those,” he said.

This section, appearing on the second Monday of each month, features new technologies that are still under research and development but expected to hit the market in coming years.

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