Date of Degree

2-2018

Document Type

Dissertation

Degree Name

Ph.D.

Program

Chemistry

Advisor

Teresa J. Bandosz

Committee Members

Maria C. Tamargo

Malgorzata Ciszkowska

Elizabeth J. Biddinger

Subject Categories

Analytical Chemistry | Catalysis and Reaction Engineering | Environmental Chemistry | Materials Chemistry

Keywords

carbon dioxide, electrochemical reduction, nanoporous carbon, sulfur or/and nitrogen doping, carbon monoxide, methane

Abstract

For the first-time sulfur-doped, nitrogen-doped and sulfur, nitrogen-codoped nanoporous carbons were systematically studied as catalysts for CO2 electrochemical reduction reaction (CO2ERR). The Faradaic efficiencies (FE) of CO and CH4 formation were calculated to evaluate the performance of these carbons. The best catalysts showed the FE of CO and CH4 formation of 29% and 0.76%, respectively. It was found that the overall performance in CO2ERR dramatically increased upon the reduction pretreatment of the carbons in N2-saturated electrolyte before the CO2 reduction process. The pretreated carbon showed the maximum FE of CO and CH4 formation of 39% and 1.2%, respectively. The most stable carbon showed the unchanged FE of CO formation for 48h, which was followed by a gradual decrease in the FE up to 96h.

The performance of CO2ERR was affected by such carbon surface features as the type and number of functional groups, electric conductivity, porosity and surface acidity/basicity. Positively charged carbons induced by pyridinic-N, quaternary-N and thiophenic-S were identified as catalytically active groups. The reduction process intermediates CO2/COOH* were stabilized on these positively charged sites. The pyridinic-N induced positively charged carbon showed the highest activity for CO formation. It was found that the carbon surface suffered from some extent of oxidation after CO2ERR. The pyridinic-N was oxidized to pyridonic-N. However, when pyrazinic-N was present on the carbon surface, the pyridinic-N remained stable and pyrazinic-N was found to be decomposed after CO2ERR. This indicates that pyrazinic-N was a sacrificial species protecting the active pyridinic-N sites from oxidation during CO2ERR, which markedly increased the stability of the catalysts. A high electric conductivity of catalysts is a necessary factor for the desirable performance in CO2ERR. Porosity is a predominant factor for CH4 formation but not for CO formation. However, it increases the adsorption strength of CO2 and intermediates and thus enhances the electron transfer. The pores, especially the ultramicropores, which provide high adsorption potential, bind CO strongly. The adsorbed CO (intermediate CO*) likely accepts both electrons and protons simultaneously and thus CH4 is formed. Acidic surface also helps with the protonation of CO*, which is beneficial for CH4 formation. The basic surface of carbon favors CO formation since its basic feature suppresses the competing hydrogen evolution reaction. The mechanism of CH4 formation might not be an exclusively electrochemical process. With the existence of formed H2 and CO that are adsorbed in the pores, the system might act as a set of Fisher-Tropsch nanoreactors where CH4 is formed. Sulfur-containing nanoporous carbons also showed photoactivity when exposed to visible light during CO2ERR. It was found that the band-gap decreased with the increase in the amount of thiophenic-S on the surface and an increase in the contribution of sp2 configuration. These features were indicated to be responsible for the generation of photocurrent under visible light. The photoinduced electrons were accepted by CO2 and the partial current density for CO formation increased.

The study sheds new light on developing nanoporous carbon as metal-free catalysts for CO2ERR. The results obtained suggest that these materials have a potential be a low-cost alternative to the metal electrocatalysts. The investigated process is environmental friendly and economical. These findings also extend our understanding of the mechanism of CO2ERR. Further studies on improving the efficiency of CO2ERR in nanoporous carbons are needed. This can be achieved by exploring the diversity of their surface modifications.

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