Electrodeposition of Cu with Amino Acids Toward Electrocatalytic Enhancement of CO₂ Reduction Reactions
Authors: Tsuda Yuki , Yoshii Kazuki , Gunji Takao , Takeda Sahori , Takeichi Nobuhiko 
Published: December 2024

The electrochemical reduction of carbon dioxide (CO2) is attracting technology because it can convert CO2 into a useful resource with zero emissions by using surplus electricity and renewable energy. For achieving efficient electrolysis of CO2, the development of a high performance electrocatalysts is required. Copper (Cu) is the only metal that is capable of electrochemically converting CO2 to hydrocarbons, but the high overpotential of hydrocarbon formation and the low selectivity of the reduction products have been problems. One of the solutions to these problems is the hybridization of metal and organic materials, in which the metal serves as the CO2 reduction reaction sites and the organic material stabilizes the intermediates [1]. In this study, we have electrodeposited Cu with five different types of amino acids which is expecting stabilization of CO2 reductive intermediates and evaluated its catalytic activity of electrochemical reduction of CO2.

The electrodeposited Cu with amino acids was obtained by a constant current electrolysis at -2.0 mA cm-2 for 1.0 C cm-2 from the aqueous solutions containing 10 mmol dm-3 CuSO4･5H2O, 0.1 mol dm-3 Na2SO4, and 1.0 mmol dm-3 amino acids in a three-electrode cell with carbon paper (CP) as the working electrode, platinum wire as the counter electrode, and Ag/AgCl as the reference electrode. The pH of electrolytic bath was fixed at 1.10 by adding H2SO4. The obtained electrodeposited Cu samples were characterized by Raman measurement, high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), energy dispersive X-ray spectroscopy (EDS) measurement, scanning electron microscopy (SEM) and X-ray diffraction (XRD). The electrochemical reduction of CO2 has been conducted by a constant potential electrolysis at -1.27 V vs. RHE of 0.5 mol dm-3 KHCO3 aqueous solutions saturated with CO2. The amount of reductive gas products was quantified by gas chromatograph- mass spectrometer (GC-MS) and gas chromatograph thermal conductivity detector (GC-TCD).

The chronopotentiograms obtained during the electrodeposition of Cu, both in the presence and absence of 1.0 mmol dm−3 of the various amino acids showed different potential required to reach a current of −4.0 mA depending on the amino acids. However, considering the result of the open-circuit-potential measurement, the amino acids do not form complexes with Cu2+, suggesting that the electrolysis species remain consistent regardless of the amino acid present in the Cu electrodeposition bath. Therefore, the differences observed in the chronopotentiograms during Cu electrodeposition are attributed to the adsorption of amino acids on the electrodeposited Cu. The adsorption of amino acids on electrodeposited Cu was confirmed by Raman spectra and EDS spectra obtained from cross-sectional HAADF-STEM images. The peak of Raman spectra originating from amino acids was observed from electrodeposited Cu when Cu electrodeposition was conducted with amino acids, confirming the higher ratio of N and Cu on the surface compared to within the particle in the EDS spectra. Surface morphologies were observed by using SEM. CP has fiber-like structure but electrodeposited Cu without and with amino acids were deposited such coating CP fiber. The particle size of electrodeposited Cu with amino acids was more homogeneous, and the particles were more densely packed together than pure electrodeposited Cu. However, changing diffraction patterns and peak broadening due to loading amino acids in XRD were not observed.

The electrochemical reduction of CO2 on electrodeposited Cu samples was performed and the faradic efficiency (FE) of the CO2 reduction gas products were calculated. All electrodeposited Cu with and without amino acids showed a higher FE for the electrochemical reduction of CO2 to CH4 compared to Cu foil (24.2%) and different values depending on amino acids. In particular, electrodeposited with L-histidine containing imidazole groups showed a high FE of 67.6%, effectively suppressing the hydrogen evolution reaction. This finding highlights the important role of functional groups in amino acids, especially imidazole, in facilitating the electrochemical conversion of CO2 to CH4. This study shows that certain functional groups in amino acids have a crucial influence on the catalytic efficiency of electrodeposited Cu in CO2 reduction reaction applications.

[1] S. Jia et al., Angew. Chem. Int. Ed. 2021, 60, 10977–10982.