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Science

How artificial photosynthesis could cut emissions

Panasonic started its artificial photosynthesis research in 2009. In 2012, it successfully achieved a 0.2% conversion efficiency, the highest in the world at the time.

TOKYO -- In recent years, two Japanese electronics makers have made big advances in artificial photosynthesis research.

     With the help of government funding, companies and universities are hoping to develop the artificial photosynthesis technology needed to convert carbon dioxide into fuel and chemical feedstock sometime in the 2020s.

     In November last year, Toshiba achieved the artificial photosynthesis conversion at the highest efficiency to date. Its 1.5% conversion rate far surpassed the 0.2% recorded by Panasonic in 2012. The news surprised many industry experts.

     Artificial photosynthesis is generally divided into oxidation reaction and reduction reaction. Oxidation reaction occurs when water is decomposed into oxygen, electrons and hydrogen ions (or protons) by sunlight. Reduction reaction occurs when these electrons and protons are used to generate organic substances, such as carbon monoxide and formic acid, from CO2.

     Conventional oxidation catalyst technology can use only 3% of ultraviolet light in photovoltaic energy. Toshiba uses a multijunction semiconductor -- comprised of an oxidation electrode grafted with three types of amorphous silicon -- that can use a wider range of wavelengths in photovoltaic energy, including visible and infrared light.

     However, this technology still cannot generate the level of power voltage needed for oxidation reaction. So, the company has adopted a gold nanocatalyst based on the nanoscale structural control technology as a catalyst of a reduction electrode, thereby lowering overvoltage. By lowering the reaction's electric potential with a gold nanocatalyst, Toshiba has successfully developed the technology to reduce CO2 using semiconductors, which in turn helps improve the conversion efficiency.

Toshiba uses a gold catalyst for energy conversion.

     Since Toshiba manufactures thermal and other power plants emitting huge amounts of CO2, it must cut CO2 emissions. As such, it plans to raise the conversion efficiency to 10% in the 2020 so that it can use artificial photosynthesis to help cut highly concentrated CO2 emissions near power plants.

$124 million investment

Historically, Japan has been a world leader in catalyst research, a technology essential for artificial photosynthesis. In 1972, Akira Fujishima, then a graduate student at the University of Tokyo, and another researcher published an article about the so-called Honda-Fujishima effect in the U.K. science magazine Nature. In the report, they discovered a way to decompose water into hydrogen and oxygen by irradiating titanium oxide electrode to sunlight. Fujishima is now the president of Tokyo University of Science.

     The government embarked on artificial photosynthesis research at the suggestion of Purdue University Distinguished Professor and 2010 Nobel Prize laureate Ei-ichi Negishi.

     The Ministry of Economy, Trade and Industry is spending about 15 billion yen ($124 million) on artificial photosynthesis over 10 years from fiscal 2012. Businesses and universities are working together to find ways to put artificial photosynthesis to commercial use in 10 years' time.

     In 2009, Panasonic launched a team led by Satoshi Yotsuhashi, chief researcher at its eco-materials research department. Yotsuhashi had been studying fuel cell catalysts.

     Conventional photocatalyst technology generally uses a powdered catalyst of oxide system, such as oxidized titanium. Instead, Yotsuhashi and his fellow researchers see the potential for gallium nitride as a catalyst for oxidation reaction.

     As for reduction reaction, they use different kinds of metal catalysts depending on which substances are to be produced, such as formic acid and methane. "We use research by university professors to gain further insight into reduction reaction," he said.

     Still, there is a problem. Gallium nitride reacts only to ultraviolet light in photovoltaic energy. Yet, Panasonic has created the technology to create multilayered gallium nitride through the development of high-efficiency light-emitting diode. With advice from Panasonic engineers, Yotsuhashi has created an extra layer with a different type of multilayered gallium nitride semiconductor, which can absorb a broader range of photovoltaic energy wavelengths.

     The conversion efficiency is merely 0.13% when aluminum-contained gallium nitride layers are used. But the research team was able to increase it to 0.91% by using indium-contained gallium nitride layers. In 2014, the team successfully raised it to 0.97% by using an improved electrolyte.

     Panasonic hopes to use this artificial photosynthesis technology to produce fuels in areas with large CO2 emissions, such as waste incineration plants.

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