Electrochemical Reduction from CO2 to CH4 By Electrodeposited Cu/L-Histidine Hybrid Materials
Authors: Tsuda Yuki , Takeda Sahori , Takeichi Nobuhiko 
Published: February 2024

The electrochemical reduction technology of carbon dioxide (CO2) is attracting attention due to it can convert CO2 into a useful resource with zero emissions by using surplus electricity and renewable energy. For achieving efficient electrolytic reduction of CO2, the development of a high active 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 the present study, we have demonstrated electrochemical reduction of CO2 by electrodeposited Cu loading L-Histidine (L-His) which is expecting stabilization of CO2 reductive intermediates hybrids.
The electrodeposited Cu/L-His hybrids was obtained by a constant current electrolysis at -1.0~-4.5 mA cm-2 for 1 C cm-2 from the aqueous solutions containing 10 mmol dm-3, CuSO4･5H2O, 0.1 mol dm-3 Na2SO4, and 0.5~20 mmol dm-3 L-His 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 controlled between 1.10 and 4.07 by adding H2SO4. The electrochemical reduction of CO2 has been carried out 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 GC-MS and GC-TCD.
The changing the color from pale blue to deep blue was observed by adding L-His into electrolytic bath containing Na2SO4 and CuSO4･5H2O. As increasing concentration of L-His, absorption peak measured by UV-Vis was shifting from around 800 nm originated [Cu(H2O)4]2+ to around 630 nm. The shift was not observed above Cu2+ : L-His = 1 : 2 for molar concentration ratio, therefore, it was considered that Cu2+ and L-His form a complex of [Cu(L-His)2]2+ in aqueous solution. Thus, electrodeposition of Cu/L-His has been tried from Cu2+ : L-His = 1 : 2 aqueous solutions but electrodeposited Cu/L-His cannot achieve. As decreasing pH of electrolytic bath with fixing Cu2+ : L-His = 1 : 2, the absorption peak originated [Cu(L-His)2]2+ was shifting to [Cu(H2O)4]2+
absorption peak. Below pH = 1.7, the absorption peak had been shifted to [Cu(H2O)4]2+ completely and the precipitate was obtained in this condition. The hybridization Cu and L-His was confirmed by Raman spectra. Not only Cu, L-His peaks but also new peak originated L-His adsorbs to Cu were observed. CP has fiber like structure but electrodeposited Cu and Cu/L-His were precipitated such coating CP fiber. The particle size of Cu/L-His hybrids was more homogeneous, and the particles were more densely packed together compared to the pure Cu. However, changing diffraction patterns and peak broadening due to loading organic components in XRD were not observed.
The electrochemical reductive reaction of CO2 on ED Cu has been conducted and the faradic efficiency (FE) of CO2 reductive gas product were calculated (Figure). Electrodeposited Cu (ED Cu) showed dramatically improvement of FE from CO2 to CH4 compared to copper foil (Cu foil). Furthermore, the L-His loading electrodeposited Cu (ED Cu/L-His) was more improved than ED Cu. Worthy of a special mention, the amount of hydrogen generation was reduced dramatically by introducing L-His. It can be considered that a reaction field was formed in which the CO2 reduction reaction preferentially takes place over hydrogen evolution. As increasing the concentration of L-His between 0.5~20 mmol dm-3, the FE from CO2 to CH4 goes high until 1.0 mmol dm-3 and then goes down, therefore, it can be observed that high L-His loading inhibits CO2 electrochemical reduction.
[1] S. Jia et al., Angew. Chem. Int. Ed. 2021, 60, 10977–10982.

Figure 1