Electrochemical Reduction of Carbon Dioxide on Single-Crystal Copper Membrane
Authors: Yoshihara Naoki , Arita Mai , Noda Masaru 
Published: February 2020

The electrochemical reduction of carbon dioxide (CO2) on copper electrodes has been attracting great interest as an intermittent CO2 conversion system, which can synthesize hydrocarbons at room temperature and under ambient pressure. Many extensive efforts clarified the synthesis mechanism of hydrocarbons in the electrochemical reduction of CO2[1-2]. Recent progress of the selective synthesis utilizing the crystal structure on a copper electrode offers a promising route to produce the desired hydrocarbon product. Although this system converts CO2 to hydrocarbons effectively, it is difficult to construct the manufacturing process because a large-area single crystal electrode is expensive. In practice, the past efforts were almost utilized a fine single crystal electrode, such as bead or rod. In this research, we reports the electrochemical reduction of CO2 on a ”large-area” single crystal copper membrane epitaxially deposited on sapphire and magnesium oxide (MgO) as a support substrate.
In the experiments, copper membranes were deposited onto glass, c-plane sapphire, and MgO substrates in argon atmosphere by the radio frequency magnetron sputtering machine. A series of copper membranes were characterized by X-ray diffraction (XRD) analysis to confirm the copper crystal structure. Figure 1 shows the experimental results, where the commercial copper plate (Aldrich, 1.0 mm thick 99.999% purity) and the copper membrane deposited on the glass substrate have three peaks of the crystal structure, Cu(111), Cu(100), and Cu(110). On the other hand, copper membranes deposited on c-plane sapphire, MgO(100) and MgO(110) substrate are highly crystalline with Cu(111), Cu(100), and Cu(110), respectively. This suggests that the copper membranes epitaxially grew along a crystal structure on the single crystal support substrates.
The electrochemical reduction of CO2 on the single crystal copper membranes was performed in an electrochemical cell with a three-electrode assembly at room temperature and ambient pressure. Figure 2 shows the time dependence of the methane concentration in the effluent gas on Cu(111) membrane and the commercial copper plate as electrodes, respectively. Only a trace amount of ethylene was observed with the concentration of < 1 ppm. It is noted that the methane emission is strongly dependent on the crystalline of the copper electrode. Moreover, significantly higher CO2 conversion in sodium hydroxide (NaOH) electrolyte (pH=13.8) than that in phosphate buffer (pH=7.2) was observed on Cu(111) membrane. The electrochemical conversion of CO2 to methane reveals to be favored in strong alkali electrolyte. However, methane concentration in this electrolyte decreased with the electrolyte pH drop during electrochemical reduction (pH=13.8→7.4), which is accounted for by the neutralization between CO2 and NaOH.
We will also show the electrochemical reduction of CO2 on another copper crystal structure, and the electrochemical mechanism based on dissolved carbon oxide concentration in the electrolyte discussed.

Acknowledgement

This work is supported by the Sasakawa Scientific Research Grant from The Japan Science Society and the Shiraishi Scientific Research Grant.

Reference

[1] Y. Hori, I. Takahashi, O. Koga, N. Hoshi, “Electrochemical reduction of carbon dioxide at various series of copper single crystal electrodes”, Journal of Molecular Catalysis A: Chemical, 2003, 199, 39–47.
[2] K. J. P. Schouten, E. P. Gallent, and M. T. M. Koper, "Structure Sensitivity of the Electrochemical Reduction of Carbon Monoxide on Copper Single Crystals", ACS Catal., 2013, 3, (6), 1292–1295.

Figure 1. XRD profiles of copper electrode in this study. (a)commercial copper plate, (b)copper membrane on glass substrate, (c)copper membrane on c-sapphire substrate, (d)copper membrane on MgO(100) substrate, (e)copper membrane on MgO(110) substrate.

Figure 2. Methane concentration curves plotted as function of reaction time for the electrochemical reduction of CO2 on Cu(111) membrane and Cu plate at an electrode potential of -0.9 V vs. RHE.

Figure 1