Electrochemical synthesis of formate from CO2 using a Sn/reduced graphene oxide catalyst

[Background]

Decreasing the emission and efficient utilization (fixation) of carbon dioxide (CO₂) are worldwide issues to prevent global warming. Promotion of the use of renewable energy is effective in reducing CO₂ emissions. However, since there are large time-dependent fluctuations and large regional differences in renewable energy production, it is necessary to establish a fixation technology to allow efficient energy transportation and storage. Thus, there is increasing interest in technologies for synthesizing useful chemicals from CO₂ using electricity derived from renewable energy. In particular, formic acid is attracting much attention as an energy (hydrogen) carrier because it is liquid and non-toxic at room temperature. Establishment of this technology will contribute to the efficient transportation and storage of renewable energy and to the fixation of CO₂, and allow energy storage with high environmental compatibility to be achieved.

In the electrochemical reduction of CO₂, it is known that formic acid can be obtained with a Faradic efficiency*1) of about 50 to 60% by using tin (Sn) as a cathode catalyst. However, in order to develop this technology for practical use, further improvement of Faradic efficiency and a reduction of overpotential*2) are necessary. There is much active interest in research to understand the design principles of catalysts to enable these goals to be achieved.

[Results]

The present research team led by Prof. Tsujiguchi and his colleagues of Kanazawa University in collaboration with scientists from University of Tsukuba and Osaka University prepared a tin/reduced graphene oxide*3) (Sn/rGO) catalyst in which Sn was supported on reduced graphite oxide by thermal reduction of tin chloride (SnCl₂) and graphene oxide (GO) obtained by oxidizing graphite powder using the improved Hummers method*4). In the catalyst thus prepared, Sn is uniformly dispersed in the rGO layer, and the composite is stacked to form a 3D morphology, i.e. rGO/Sn/rGO (Fig. 1). This catalyst is characterized as a carrier with functional group containing a much larger amount of oxygen than the tin/graphite catalyst used for a comparison. When we performed the electrochemical reduction of CO₂ using these catalysts with CO₂ dissolved in a solution of potassium hydrogen carbonate (KHCO₃), it was found that the Sn/rGO catalyst significantly decreased the overpotential and allowed a high current density to be obtained compared to the Sn catalyst. In addition, when the reduction of CO₂ was performed at a constant potential, almost no products other than formic acid, such as H₂ and CO, were detected and we succeeded in obtaining formic acid with a Faradic efficiency of 98% (1.8 times that with Sn catalyst alone) (Fig. 1).

The reason for the highly efficient formic acid production obtained using Sn/rGO catalyst is its high CO₂ adsorption capacity. Sn/rGO can adsorb four times as much CO₂ as Sn catalyst alone. Further, the rate of CO₂ adsorption is eight times that of Sn catalyst alone. Computational chemistry predicted that this high CO₂ adsorption capacity would be due to the oxidized functional groups of rGO and that the production of hydrogen and carbon monoxide would be suppressed since the CO₂ adsorbed by the oxidized functional group of rGO is quickly and efficiently supplied to the adjacent Sn surface (Fig. 2). In order to experimentally confirm this mechanism, our team attempted electrochemical imaging of the catalytic activity with a scanning electrochemical cell microscope*5). It was revealed that a significantly higher reduction current density was observed at the interface between Sn and rGO than on the Sn or rGO surfaces (Fig. 3) suggesting that a large amount of formic acid is synthesized on Sn adjacent to rGO, in support of the above prediction by computational chemistry. This is the first experimental demonstration using a scanning electrochemical cell microscope that formic acid synthesis is actively occurring at the interface between the catalyst and the support. Thus, more efficient formic acid synthesis would be possible by combining a support with a high CO₂ adsorption capacity with a catalyst for electrochemical reduction of CO₂*6). This provides an important framework that could be applied to all catalysts available so far.

[Future prospects]

The results of the present study provide new insights into the development of catalysts for formic acid synthesis by the reduction of CO₂, and dramatic progress is expected in the development of formic acid synthesis technology by the electrochemical reduction of CO₂. In addition, we have demonstrated an improvement of selectivity due to the excellent CO₂ adsorption capacity of the support as well as elucidating its reaction mechanism. This should have a great impact on electrochemical reduction technology related to CO₂, including the synthesis of methanol, methane and olefins. Therefore, it has the potential to be a useful basic technology in the synthesis of chemicals from CO₂. In the future, we expect that the development of electrochemical reduction cells using this catalyst will be encouraged, leading to the creation of energy storage devices with high environmental compatibility that can contribute to the fixation of CO₂ and promotion of the efficient use of renewable energy.

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[Glossary]

*1) Faradic efficiency

The ratio of the partial current that contributes to the synthesis of the object to the total current.

*2) Overpotential

The difference between the theoretically calculated equilibrium potential (voltage) and the actual potential (voltage) when the reaction takes place.

*3) Reduced graphene oxide

Graphene that is oxidized to have a functional group such as a hydroxyl, carboxyl, or epoxy group is called graphene oxide. The reduction product of graphene oxide is called reduced graphene oxide. While graphene oxide is an insulator, reduced graphene oxide is a conductor and is expected to be useful as an electrode material.

*4) Improved Hummers method

The Hummers method is a method in which graphite is used as a starting material, oxidized with potassium permanganate and sulfuric acid, peeled off as graphene oxide as a monoatomic layer, and then reduced to obtain a multilayer graphene sheet. The improved Hummers method is a modification of the Hummers method for the purpose of improvement of the yield of graphene oxide as a monoatomic layer or simplification.

*5) Scanning electrochemical cell microscope

A nanoscale electrochemical cell is formed on the sample surface using a nanopipette filled with an electrolyte, a meniscus-like electrochemical cell is formed between the nanopipette and the sample surface, and a local electrochemical measurement is performed. By scanning the nanopipette, an electrochemical image of the catalytically active site can be obtained.

*6) Electrochemical reduction of CO₂

Technology that synthesizes useful chemicals by electrolyzing CO₂. Various substances such as CO and methanol can be synthesized but the efficiency is currently poor.

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