Research

Our work has demonstrated that metal complexes can serve as catalysts to promote the conversion of CO₂ into organic compounds via electrochemical reduction. The catalytic activity of these materials and their immobilization on solid surfaces can both be tuned based on molecular design. Catalytic activity under visible light has been achieved using a hybrid photocatalyst or a photoelectrode that combines the functions of a photoactive semiconductor and a metal complex catalyst. In this hybrid photosystem, the semiconductor absorbs visible light, following which the resulting photoexcited electrons are rapidly transferred to the catalyst at rates on the picosecond time scale. CO₂ is then selectively transformed to generate organic compounds via reduction in solution.

Our prior work developed a hybrid monolithic photoelectrode comprising a semiconductor and a metal complex catalyst and having a tablet construction, referred to as an “artificial leaf.” When immersed in an aqueous solution containing CO₂, this device was able to generate HCOOH from CO₂ and H₂O in response to irradiation with sunlight. During this process, one side of the electrode generated formic acid while bubbles of oxygen were produced from the opposite side. This artificial leaf functioned with a solar-to-chemical energy conversion efficiency of 4.6%, which exceeds the photosynthesis efficiency of plants. This highly efficient artificial leaf cannot be constructed simply by conventional combinations of various materials. Our newly-developed catalyst technology, which proceeds CO₂ reduction reaction at a low electrical potential close to the theoretical limit, contributes to the artificial leaf.

Techniques in which the CO₂ reduction reaction is promoted by photocatalyst particles suspended in aqueous solution are expected to allow the realization of practical, low cost and scalable artificial photosynthesis systems. In these systems, a complex catalyst/semiconductor hybrid photocatalyst reduces CO₂, a semiconductor photocatalyst oxidizes water, and another metal complex mediates electron transfer between these two types of particles. All these materials are suspended in an aqueous solution. In previous joint research, the first-ever visible-light-driven particle suspension system for CO₂ reduction was realized. In this system, the competitive hydrogen generation reaction was suppressed even in water, with the simultaneous generation of dioxygen. This system also recently exhibited a reaction rate that was increased by an order of magnitude.

In order to fully determine the reductions in CO₂ emissions associated with using artificial photosynthesis, a life cycle assessment of all aspects of the system is required, including manufacturing and operation. The use of Earth-abundant elements is also important. For this reason, we are presently researching semiconductors and catalysts that are primarily composed of elements such as iron (Fe) and manganese (Mn). This work has thus far developed a CO₂ electrolysis system using Fe and Mn-based electrodes together with a silicon (Si) solar cell that promotes the CO₂ reduction reaction with a solar conversion efficiency of over 10%.

CO₂ reduction is a chemical reaction that consists of multiple steps, including light absorption, electron excitation, electron transfer and protonation. These processes determine the overall efficiency of the reaction and the selectivity for the various possible products. Unfortunately, the details of these reactions have not yet been fully clarified. We have therefore examined the associated mechanisms and the electronic structures of the photocatalysts using ultra-fast transient absorption spectroscopy, electronic structure analysis at a synchrotron radiation facility, and theoretical computations, while collaborating with experts both inside and outside the company. Combined theoretical and experimental investigations of a N-doped Ta₂O₅/Ru-complex hybrid capable of functioning as a CO₂ reduction photocatalyst found that N doping raised the conduction band minimum of Ta₂O₅ and promoted the transfer of excited electrons to the Ru-complex mounted on the surface of the material.

A cooperating research team has begun efforts to accelerate the future implementation of artificial photosynthesis technologies in society. CO₂ reduction with a high efficiency of 7.2% has been demonstrated in a system with an electrode area approximately 1,000 times larger than the that of monolithic element. ＜*Link to Kato's page＞

News Release Solar-Driven CO₂ Recycling Technology! Dramatic Breakthrough in Artificial Photosynthesis

At present, we are also conducting research on technologies intended to realize high throughput CO₂ reduction for the synthesis of formic acid and carbon monoxide, and to demonstrate generation of molecules with chemical energy greater than them.