Electroreduction of CO2 into CO Using Amine-Modified Diamond Electrode
At a Glance
Section titled āAt a Glanceā| Metadata | Details |
|---|---|
| Publication Date | 2023-12-22 |
| Journal | ECS Meeting Abstracts |
| Authors | Takashi Yamamoto, Tatsuhiko Mikami, Mai Tomisaki, Yasuaki Einaga |
| Institutions | Keio University, Kyushu University |
Abstract
Section titled āAbstractāElectroreduction of CO 2 is promising to convert CO 2 into value-added compounds since electroreduction does not necessarily require a catalyst because an electrochemical energy itself can activate the reactivity of CO 2 . Most of the reported studies on CO 2 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 CO 2 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 CO 2 capture and storage technologies and CO 2 electroreduction. Here, we prepared amine-modified BDD (NH 2 -BDD) to elucidate an effect of amine modification on CO 2 electroreduction. Prior to the electrolysis experiments, linear sweep voltammetry (LSV) was performed to investigate the electrochemical difference between bare- and NH 2 -BDD. As an energetically equivalent criterion for the CO 2 electroreduction, E red was defined as the potential at which the current density reached -30 ĀµA/cm 2 ; E red ( vs. Ag/AgCl) were determined to be -1.56 and -1.16 V for bare-BDD and NH 2 -BDD, respectively. A positive shift of E red in the NH 2 -BDD electrode is probably because CO 2 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 CO 2 electroreduction. Products were CO, HCOOH, and H 2 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 NH 2 -BDD. Particularly, in the most prominent case, the selectivity of CO production was 8 times higher for NH 2 -BDD than the case of bare-BDD. Since CO production requires the adsorption of intermediate species, CO 2 ā¢ - , on the electrode surface, the adsorption of CO 2 and CO 2 ā¢ - would be promoted on NH 2 -BDD through the formation of C-N bond. In order to obtain the direct evidence for CO 2 capturing by NH 2 -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 NH 2 -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 CO 2 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 NH 2 -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 CO 2 captured by amine, and the second drop at around -1.70 V ( vs. Ag/AgCl) is ascribed to the reduction of free CO 2 . 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 CO 2 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 CO 2 , i.e. the reaction via carbamate formation. Figure 1