The Role of Nitrogen and Boron Dopants Incorporated in Carbon Nano Onions Toward Electrocatalytic Oxygen Reduction Reaction
Authors: Kodithuwakku Udari Shyamika , Wanninayake Namal , Thomas Melonie , Guiton Beth , Kim Doo Young 
Published: February 2020

Oxygen reduction reaction (ORR) is an essential reaction in fuel cells and metal-air batteries. One main challenge in these energy conversion and storage devices is the sluggish kinetics of ORR at the cathode. It is required that ORR electrocatalysts should operate under low overpotential giving high current density for an extended running time. To date, platinum (Pt)-based catalysts are the most efficient electrocatalyst for ORR with high current density. However, Pt catalysts still suffer from high over-potential, instability and surface poisoning during their electrochemical operations. Furthermore, Pt is expensive and has a limited abundance on earth. Thus, the development of an alternative catalyst is imperative. A novel material should be metal-free, cheap, efficient and stable over a long period of time. Now, intensive endeavors have been taken to replace Pt with heteroatom co-doped carbon catalysts. For instance, nitrogen (N) and boron (B) heteroatoms are incorporated in carbon-based host materials such as graphene sheets, graphene nanoribbon, and carbon nano-onions (CNOs). However, the fundamental understanding of catalyst active sites associated with doped heteroatoms is limited, and further studies are necessary to develop catalysts to replace Pt-based catalysts.

CNOs are an emerging type of carbon allotropes. CNOs are comprised of concentric sp2-carbon bonded, graphitic shells surrounding a hollow core. Especially, nanodiamond-derived CNOs are <10 nm in diameter. These particular CNOs have a high specific surface area (~ 300 m2/g) and a good electrical conductivity. Furthermore, the curved graphitic shells of CNOs elevate the electron density of outer surfaces and generate a strain, promoting electrocatalytic activity. In the present study, CNOs was selected as a substrate for the incorporation of nitrogen (N) and boron (B) atoms. Briefly, N-doped CNOs (N-CNOs) were first prepared at 700 ºC. Then, B was introduced into N-CNOs. The incorporation of B into N-CNOs was done at varied temperature ranging from 600 ºC to 1000 ºC. The effect of temperature on the chemical states of B and N co-dopants was thoroughly studied. Furthermore, the curvature effect of CNOs on the chemical reactivity toward doping and catalytic activity toward ORR was investigated.

The morphology, microstructure, and chemical states of undoped and N, B-doped CNOs (NB-CNOs) were probed by high resolution transmission electron microscopy (HR-TEM), Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS). The electrocatalytic performance of NB-CNOs toward ORR were assessed by a series of electrochemical characterizations including cyclic voltammetry, linear sweep voltammetry, rotating disk electrode (RDE), and rotating ring disk electrode (RRDE) measurements. The electrochemical characterizations were performed in aqueous electrolyte, 0.1 M KOH. RDE measurements were performed at 5 mV/s scan rate and with the rotation speeds between 400 and 3600 rpm. The electron transfer number for the ORR pathway was determined from the Koutecky-Levich analyses of RDE results. Overall, the NB-CNOs prepared at 700 ºC showed the best performance among the prepared NB-CNOs such as highest current density (~ 6 mA/cm2), low onset potential ( -0.05 V vs. Ag/Ag Cl), and the dominant 4-electron transfer. The performance of NB-CNOs prepared at 700 ºC was comparable to that of commercial Pt/C catalyst. Furthermore, N, B-CNOs showed a remarkable long-term stability. This performance was related to active sites formed on CNOs associated with N and B dopants. It was noted that the formation of separated N and B atoms is favored at low temperature and constitutes active sites for ORR. However, the number of these active sites was reduced at elevated temperatures due to catalyst sintering and the aggregation of N and B. As a result, the catalyst performance was degraded with the increase of the annealing temperature.

This presentation will address unique synthesis, chemical structure, and electrochemical performances of NB-CNOs. Fundamental knowledge gained on the role of curvature in the heteroatom doping and structure-catalytic activity relations will be highlighted. This knowledge will pave the way for efficient and robust electrocatalysts towards ORR.