Aqueous Electrocatalytic CO2 Reduction Using Metal Complexes and Carbon Material Support
Authors: Sato Shunsuke , Arai Takeo , Sekizawa Keita , Suzuki Tomiko M. , Morikawa Takeshi 
Published: December 2020

Electrocatalytic carbon dioxide (CO2) reduction using water is the key of artificial photosynthesis systems designed to produce fuels. However, a lot of catalysts for CO2 reduction have been studied using organic solvent, because hydrogen production is competitive reaction in the aqueous solution.
We have improved the CO2 reduction selectivity of inorganic semiconductor (SC) material by combining SC with metal complex catalyst (MC) (SC/[MC] hybrid photocatalyst).1,2 In addition, by conjugating the SC/[MC] hybrid photocatalyst for CO2 reduction with a SC capable of H2O oxidation, we have successfully achieved an artificial photosynthesis system, which reduces CO2 to HCOOH with no external electrical bias using H2O as electron and proton source.3,4 However, the device requires noble metal catalysts, such as a ruthenium complex polymer as a CO2 reduction catalyst, and iridium oxide as a water oxidation catalyst. In view of future practical applications, such a system requires efficient catalysts that consist of inexpensive and abundant elements. The system can be applied to many other SCs and MCs. Thus, development of MC is an important factor for improvement in reaction rates and product selectivity of the artificial photosynthesis.
Electrocatalytic CO2 reduction can be conducted using molecular catalysts (metal-complexes) under certain electrical biases. These electrocatalysts require a large electrical potential to achieve catalytic CO2 reduction, because the first step in the CO2 conversion is formation of a CO2
- radical anion intermediate during single-electron reduction. Therefore, hydrogen is likely to be generated preferentially by water splitting in aqueous solution. Current electrocatalysts facilitate proton-coupled multi-electron reactions (for example, CO2 + 2H+ + 2e- ➝ CO + H2O, -0.11 V vs. RHE), which require lower potentials than those for the single-electron reaction. However, many metal complex catalysts for CO2 reduction are limited by low product selectivity in the presence of water, due to preferential hydrogen generation which occurs at 0.0 V (vs. RHE).
Mn complex catalysts for CO2 reduction have been researched by a lot of researchers, because Mn is one of earth abundant elements. However, because of the high overpotential for CO2 reduction, the limited operation conditions in organic solvents with H2O additives only, and the instability under irradiation, new catalysts need to be developed. Therefore, we tried to develop a new metal complex catalyst for CO2 reduction even in an aqueous solution. As a result, we have successfully developed a new Mn complex electrocatalyst with carbon material support for CO2 reduction in water.5 The developed Mn complex/carbon electrode catalyzed selective CO2 reduction even in aqueous solution, at very low overpotential under room light conditions. On the other hand, without carbon material support, this Mn complex electrode cannot act as CO2 reduction catalyst. Therefore, we have developed a noble method to improve the CO2 reduction activity of Mn complex catalysts by carbon materials, both high CO2 selectivity and low reaction overpotential in aqueous solutions. We also confirmed that the carbon support effect can be applied to other metal complex catalysts.
We will discuss research progress of the new Mn complex electrocatalyst and development of new SC/[MC] hybrid photocatalyst for the artificial photosynthesis system.

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