CO2 Reduction on Boron-Doped Diamond Electrode in Aqueous Ammonia Solution
At a Glance
Section titled āAt a Glanceā| Metadata | Details |
|---|---|
| Publication Date | 2016-09-01 |
| Journal | ECS Meeting Abstracts |
| Authors | Prastika Krisma Jiwanti, Keisuke Natsui, Kazuya Nakata, Yasuaki Einaga |
| Institutions | Keio University, Tokyo University of Science |
Abstract
Section titled āAbstractāIntroduction The reduction of CO 2 has attracted considerable attentions since its abundance in atmosphere and large amount of industrial CO 2 gas emission. However, the process has so far been difficult because of its high thermodynamic stability of CO 2 . The high over potential for CO 2 reduction promotes hydrogen evolution, which inhibits CO 2 reduction process. In our previous work [ 1] , electrochemical CO 2 reduction was carried out in NaCl and methanol solution and achieved high faradaic efficiency of product using the Boron-doped Diamond (BDD) electrode, which has chemical inertness and wide potential window. Meanwhile, ammonia solution has been known as a strong CO 2 absorber with high CO 2 loading capacity. Thus, a high concentration of CO 2 can be achieved by using this solution. Herein, we performed the study of electrochemical CO 2 reduction in aqueous ammonia solution using BDD electrode. Methods The BDD electrode was prepared by depositing BDD film onto a silicon wafer substrate with Microwave Plasma Assisted Chemical Vapor Deposition (MPA-CVD) during 6 hours. The electrochemical measurements were performed in two compartment cells divided by nafion membrane using Pt as counter electrode, Ag/AgCl as reference electrode, and BDD as working electrode. N 2 bubbling during 30 minutes followed by CO 2 bubbling during 2 hours were carried out everytime before 2 hours electrochemical reduction at potential ranging from -1.2 V to -1.5 V vs. Ag/AgCl. The liquid product was analyzed using GC-MS, and gas products were analyzed using GC with FID/TCD detector. Results and Discussion BDD film was successfully deposited on silicon wafer. The raman spectrum showed sp 3 peak at 1332 cm -1 . The sp 2 peak at 1500 cm -1 was not observed. Characterization of the BDD surface morphology was also carried out using Scanning Electron Microscopy (SEM) and the grain size of BDD was around 47 Ī¼m. In addition, SEM image showed no change of the BDD surface after more than 30 hours electrochemical reduction. Thus, the high durability of BDD electrode was proved. The great advantage of ammonia solution to absorb CO 2 was confirmed by measuring the CO 2 concentration in the solution and compared to KOH and NaCl aqueous solution. The highest CO 2 concentration was achieved by ammonia solution. The absorbance capacity was also increased with increasing the concentration of ammonia. The products achieved from this reduction process were methanol, CH 4 , CO , and H 2 gas. The maximum amount of methanol production was 0.25 ppm (24% faradaic efficiency) at the potential -1.3 V vs. Ag/AgCl. This faradaic efficiency is based on the 6 electrons involved in the reaction. On the other hand, CH 4 and CO produced at low efficiency. Faradaic efficiencies of products at the various potentials are described in Fig. 1. Aqueous ammonia solution, which is used as a supporting electrolyte, reacts rapidly with CO 2 to form bicarbonate ion at pH 78 [ 2] . In our experiment, the pH of the solution was achieved near the value. Therefore, it is assumed that the reduced species are bicarbonate ions. The electrochemical reduction of aqueous ammonium bicarbonate solution was carried out at the potential -1.3 V vs. Ag/AgCl for 2 hours to reveal the mechanism. As the result, the pH of the aqueous ammonium bicarbonate solution was around 7.9 and methanol was achieved as a main product. On the other hand, study about the importance of ammonia was also performed by reducing CO 2 gas in KOH and NaCl aqueous solution in the same condition. We assumed that there is important effect of ammonia since high amount of methanol was only achieved by the presence of ammonia in this condition. In addition, reduction on other electrode was performed on glassy carbon electrode as a carbon based electrode. No methanol was analyzed and high amount of hydrogen gas was produced. References [1] K. Nakata, T. Ozaki, C. Terashima, A. Fujishima, Y. Einaga, Angew. Chem. Int. Ed . 53 , 871-874 (2014). [2] F. Mani, M. Peruzzini, P. Stoppioni, Green. Chem. 8 , 995-1000 (2006). Figure 1