CO2 Reduction on Boron-Doped Diamond Electrode in Aqueous Ammonia Solution

Abstract

Introduction The reduction of CO2 has attracted considerable attentions since its abundance in atmosphere and large amount of industrial CO2 gas emission. However, the process has so far been difficult because of its high thermodynamic stability of CO2. The high over potential for CO2 reduction promotes hydrogen evolution, which inhibits CO2 reduction process. In our previous work[ 1], electrochemical CO2 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 CO2 absorber with high CO2 loading capacity. Thus, a high concentration of CO2 can be achieved by using this solution. Herein, we performed the study of electrochemical CO2 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. N2 bubbling during 30 minutes followed by CO2 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 sp3 peak at 1332 cm-1. The sp2 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 4~7 μm. 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 CO2 was confirmed by measuring the CO2 concentration in the solution and compared to KOH and NaCl aqueous solution. The highest CO2 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, CH4, CO, and H2 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, CH4 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 CO2 to form bicarbonate ion at pH 7~8[ 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 CO2 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.