Electroreduction of CO2 into CO Using Amine-Modified Diamond Electrode

Abstract

Electroreduction of CO2 is promising to convert CO2 into value-added compounds since electroreduction does not necessarily require a catalyst because an electrochemical energy itself can activate the reactivity of CO2. Most of the reported studies on CO2 electroreduction use a metal electrode material, and the selectivity of reduction products depends on the metal species. However, as the use of noble or toxic metals should be avoided from the viewpoint of sustainability, a metal-free carbon-based material are desirable. Along these lines, we have focused on boron-doped diamond (BDD) as a carbon-based electrode in CO2 electroreduction. On the other hand, molecular modification of electrode surface is an important technique in various fields. Functional molecules can be covalently immobilized onto carbon electrodes by an electrografting method. Along these lines, we decided to immobilize amine on BDD surface, which integrates CO2 capture and storage technologies and CO2 electroreduction. Here, we prepared amine-modified BDD (NH2-BDD) to elucidate an effect of amine modification on CO2 electroreduction.Prior to the electrolysis experiments, linear sweep voltammetry (LSV) was performed to investigate the electrochemical difference between bare- and NH2-BDD. As an energetically equivalent criterion for the CO2 electroreduction, E red was defined as the potential at which the current density reached -30 µA/cm2; E red (vs. Ag/AgCl) were determined to be -1.56 and -1.16 V for bare-BDD and NH2-BDD, respectively. A positive shift of E red in the NH2-BDD electrode is probably because CO2 molecules form the C-N bond with the amino group on the electrode surface, which results in enhancing the electrophilicity of the carbon atom. Next, we investigated how amine modification affects the product selectivity in CO2 electroreduction. Products were CO, HCOOH, and H2 regardless of the type of electrode and applied potentials. Difference between the Faraday efficiencies (FE) of HCOOH and CO production was dependent on applied potentials in NH2-BDD. Particularly, in the most prominent case, the selectivity of CO production was 8 times higher for NH2-BDD than the case of bare-BDD. Since CO production requires the adsorption of intermediate species, CO2 • -, on the electrode surface, the adsorption of CO2 and CO2 • - would be promoted on NH2-BDD through the formation of C-N bond. In order to obtain the direct evidence for CO2 capturing by NH2-BDD during the electroreduction, in situ ATR-IR measurements were performed. We focused on the C=O (carbonyl) stretching vibration of the carbamate anion, observed in the region of 1700-1500 cm-1. In NH2-BDD, a broad peak attributed to the C=O stretching vibration was observed at around 1640 cm-1, and the peak intensity decreased as the applied potential became negative. This result strongly supports that CO2 was captured by amine at the BDD surface to form the carbamate anion and reduced to CO.The above discussion can be explained by the behavior of LSV of NH2-BDD, in which two drops were observed. The first drop at around -1.20 V (vs. Ag/AgCl) is probably ascribed to the reduction of CO2 captured by amine, and the second drop at around -1.70 V (vs. Ag/AgCl) is ascribed to the reduction of free CO2. Therefore, CO production would be favored at potentials between -1.20 and -1.70 V (vs. Ag/AgCl) and HCOOH production would be favored at potentials more negative than -1.70 V (vs. Ag/AgCl). These threshold potentials are in good agreement with the potentials at which the product selectivity switched. It is noted that, in bare-BDD electrodes showing the different E red, the selectivity of CO production was almost unchanged, which suggests that the potential dependence of product selectivity in CO2 electroreduction cannot be explained only by differences in E red. Therefore, the product selectivity was driven by the interaction between the surface amine groups and CO2, i.e. the reaction via carbamate formation. Figure 1