Electrochemical Reduction of Carbon Dioxide on Single-Crystal Copper Membrane
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.
This work is supported by the Sasakawa Scientific Research Grant from The Japan Science Society and the Shiraishi Scientific Research Grant.
 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.
 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.