• Higher Solar-To-Chemical Conversion Efficiency than Plants Using Practically Sized Cell for Solar-Driven CO2 Conversion


Higher Solar-To-Chemical Conversion Efficiency than Plants Using Practically Sized Cell for Solar-Driven CO2 Conversion

Naohiko Kato

Artificial photosynthesis uses solar energy to convert CO2 into useful resources.
We are working to improve the solar-to-chemical conversion efficiency of practical-sized cells to enable solar-driven CO2 conversion that can be implemented across a wide range of fields, including reducing the CO2 emissions from factories.

As part of its efforts to help resolve global environmental issues, TCRDL is aiming to realize a system that captures CO2 emissions from factories and other facilities and recycles them into practical resources. One example is solar-driven CO2 conversion, which produces useful organic compounds using solar energy, and stores them as chemical energy. After starting research in 2000, we have recently made major progress toward the adoption of this technology in the real world. We have achieved a solar-to-formate efficiency of 7.2% using a large 36 × 36 cm solar-driven CO2 conversion cell, the largest such cell that we have been able to fabricate. We are now continuing basic research to firmly establish the base technology of this system, with the objective of realizing a practical system in around the year 2030.

Illustration of artificial photosynthetic system
Illustration of artificial photosynthetic system

World’s First Demonstration of Artificial Photosynthesis Using only H2O, CO2, and Sunlight

Artificial photosynthesis is a technology that uses solar energy to produce organic compounds from H2O and CO2. In 2011, TCRDL was the first in the world to demonstrate the concept of artificial photosynthetic reactions driven solely by these three elements. This technology is characterized by the use of a semiconductor/metal-complex hybrid catalyst, which has a lower environmental impact because it allows the system to operate at ordinary temperatures and pressures. The solar-to-chemical efficiency* of the technology was 0.04% in 2011. Continuous research and development efforts pushed this efficiency to 4.6% in 2015, which is higher than the solar-to-chemical conversion efficiency of plants.

*This refers to the proportion of energy that can be stored in organic compounds, out of the total solar energy radiated by the sun.

2015 artificial photosynthetic cell (1 × 1 cm)
2015 artificial photosynthetic cell (1 × 1 cm)

Breakthrough for Improvement of Efficiency with a Practical-Sized Cell for Solar-Driven CO2 Conversion

Despite the successful demonstration of the concept of artificial photosynthesis using a 1 × 1 cm cell, we needed to realize a high solar-to-chemical conversion efficiency with a practical-sized cell to enable implementation of this technology. However, simply increasing the size of the cell would cause a significant drop in the solar-to-chemical conversion efficiency. Therefore, focusing on various factors such as the reaction rates of both CO2 reduction and the oxidation of water, as well as the transfer of electrons and hydrogen ions (H+), we studied novel cell structures with high reaction efficiencies. As a result, in 2020, we realized a solar-to-formate efficiency of 7.2% with a 36 × 36 cm cell. This represents a rate of CO2 absorption about 100 times greater than a cedar wood with the same area.

2020 artificial photosynthesis cell (36 × 36 cm)
2020 artificial photosynthesis cell (36 × 36 cm)

While developing a practical-sized cell for solar-driven CO2 conversion, three major technical issues were identified and a structure with a high reaction efficiency was designed.

Issues and countermeasures for realizing a practical-sized cell
Since the reaction rate is slower than the rate of generation of electrons, the number of available electrons cannot be fully utilized.

The area of the electrode catalysts was increased compared to the area of the solar cell using five stacked electrode catalysts to increase the amount of formate generation.

The electrons could not reach the ends of the reduction electrode due to high electrical resistance.

The substrate material of the reduction electrode was changed to a titanium, which creates less resistance to electron transfer.

High resistance to H+ transfer prevented H+ reaching the reduction electrode.

The electrodes were aligned to shorten the transfer distance of the H+.

Practical-sized cell structure
Sunlight shines on the solar cell, generating electrons (e-).
Water (H2O) is oxidized at the oxidation electrode, generating hydrogen ions (H+) and oxygen (O2).
Formate (HCOO-) is generated at the reduction electrode by reactions between the H+, e-, and carbon dioxide (CO2).

Tireless Exploration and Knowledge Combining to Realize Breakthrough for Implementation

“A whole range of other continuous improvements and innovations helped to create our highly efficient and practically sized cell for solar-driven CO2 conversion. In addition to myself, a whole team of researchers across many fields was involved. A combination of their knowledge, experience, and ideas led to this breakthrough of a practical artificial photosynthetic system. It’s also no exaggeration to say that the long history of research into catalysts by TCRDL also provided an essential basis for this technology. We intend to carry on working with various researchers to realize the practical adoption of this artificial photosynthetic system, which can help to resolve some of the global environmental issues we are facing.” (Interview with Naohiko Kato)

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