Theoretical Study of Oxygen Reduction Reaction Mechanism in Metal-Free Carbon Materials

Abstract
Metal-free carbon materials are attractive Pt-based catalyst alternatives. However, despite efforts, the reaction mechanism remains elusive. Thus, we investigated the role of defects (dopant nitrogen and carbon vacancy) on the catalytic oxygen reduction reaction in a metal-free carbon material focusing on the effect of structural flexibility. Crucially, defects lower the energy barrier for the sp2/sp3 transition of the carbon-centered O2-adsorption sites by releasing structural strain during the reaction. In particular, low-coordinated pyridinic-N displaces from the carbon plane to release the strain, whereas weak C–C bonds around the carbon vacancy change the bond lengths to release the strain. Defects indirectly promote the adsorption of oxygen by enhancing structural flexibility. Thus, the nonlocal structural environment is as critical as the direct interaction between adsorption sites and adsorbate in the chemical reaction. Molecular dynamics simulations reveal that pyridinic-N doping is a facile route to introduce stable catalytic active sites. Overall, our results provide a deeper understanding of chemical processes on defective carbon materials.

Carbon-based catalysts have garnered extensive attention over the past decades as an economical alternative to noble metal catalysts for renewable energy systems.

A recent study, affiliated with UNIST has unveiled the theoretical principle that carbon-based catalysts promote electrochemical reactions. The key to this is that the presence of carbon defects. along with the structural flexibility and chemical reactions are combined to enable the improvement of catalytic activity even without the need of precious metal catalysts, such as platinum (Pt).

The oxygen reduction reaction (ORR) is an important electrochemical reaction and has been widely applied in renewable energy applications, such as hydrogen fuel cells, water splitting, and metal–air batteries (MABs). Currently, commercial Pt-based catalysts (PBCs) are used extensively for this purpose due to their superior catalytic activity. However, the high price, facile CO poisoning, and low durability of Pt hamper the large-scale applications of PBCs, and thus noted the research team.

Carbon-based catalysts are being actively studied as a leading alternative material, and the efficiency and composition of catalysts are being steadily improved. However, the cause of carbon-based catalysts promoting electrochemical reactions is not clear, slowing the development of carbon-based catalysts.

Figure 1. Schematic image, showing the O2 adsorption in defective carbon.

In this study, the research team demonstrated how two kinds of defects, carbon vacancies and pyridinic-N dopants, in metal-free carbon materials enhance O2 adsorption, a critical reaction step for activating the ORR.

“Our calculations and structural analysis revealed that defects enable the neighboring edge carbon sites to adsorb O2 by enhancing the structural flexibility and, thus, lowering the energy barrier for the sp2/sp3 transition of the carbon atom,” noted the research team. “Thus, the nonlocal structural environment is as critical as the direct interaction between the adsorption site and adsorbate in the chemical reaction.”

Their findings further show that pyridinic-N has advantages over carbon vacancies in terms of structural stability. “Our study is based on numerous theoretical and experimental investigations of carbon materials and is consistent with catalytic experimental results,” added the research team. “Moreover, our results can be extended to general chemical reactions in defective carbon materials and will spur research in defect engineering.

Their findings have been published in the October 2022 issue of ACS Nano.

Journal Reference
Keunsu Choi and Seungchul Kim, “Theoretical Study of Oxygen Reduction Reaction Mechanism in Metal-Free Carbon Materials: Defects, Structural Flexibility, and Chemical Reaction,” ACS Nano, (2022).