The Effect of O-Functionalities for the Electrochemical Reduction of Oxygen on MWCNTs in Acid Media

Abstract

The effect for oxygen reduction characteristics of O-functionalities formed on the edge planelike defects at the open ends of multiwalled carbon nanotubes ͑MWCNTs͒ and at the hole defects on the tube walls was investigated by using the rotating disk electrode technique. The onset potential of the electrochemical reduction of oxygen for MWCNTs shifts to the positive direction of 0.19 V, while the O-functionalities were removed from the defects sites. These results suggest that the control of functional groups is important for the control of the oxygen reduction reaction catalyst activity.In recent years, carbon nanotubes ͑CNTs͒ have attracted a great deal of attention as a new type of electrocatalyst material owing to their unique structural, electronic, chemical, and mechanical properties. 1 So, numerous studies have been made to elucidate the electrochemistry of CNTs, including the oxygen reduction reaction ͑ORR͒ characteristics in both acid 2-5 and alkaline, 6-11 because of the possibility of using them as support materials for various catalysts and catalyst materials themselves.Britto et al. 2 confirmed by cyclic voltammograms ͑CVs͒ that the multiwalled carbon nanotube ͑MWCNT͒ past electrode demonstrate a well-defined peak at Ϫ0.31 V vs saturated calomel electrode ͑SCE͒ for ORR in H 2 SO 4 ͑pH = 2͒ compared with graphite powder and the potential of using nanotubes in electrodic applications such as fuel cells. Also, the results of the density functional theory calculations indicated that the pentagon and heptagon sites could act as favorable sites for the electron transfer for ORR. However, the effect of the O-functionalities formed on the edge planelike defects at the open ends of the MWCNTs and at the hole defects on the tube walls for the oxygen reduction characteristics has not been unambiguously identified yet in acid media because the defect sites on the surface of the MWCNTs introduced by the oxidation process are decorated with the O-functionalities.The aim of this work is to investigate the effect of the O-functionalities on the edge planelike defects and hole defects on MWCNTs for the oxygen reduction characteristics by a comparison of the ORR characteristics for MWCNTs before and after the Ar treatment. ExperimentalMWCNTs with a hollow structure ͑Nano Laboratory, purity Ͼ 95%, diameter: 30 Ϯ 15 nm, length: 1-5 m͒, which have iron and sulfur as residuals, 12 were treated by the following oxidation procedure: MWCNTs were heated at 500°C in Ar to remove amorphous carbon, and were refluxed in a mixture of 2 M sulfuric acid and concentrated nitric acid ͑1:1, v/v͒ at 120°C for 4 h to form the O-functionalities on MWCNTs and to remove the impurity in MWCNTs. Then, the O-functionalities of the acid-treated MWCNTs were removed by heating them in Ar at 900°C for 1 h. For brevity, acid-treated MWCNTs as MWCNT-A and acid-treated MWCNTs, followed by Ar treatment MWCNTs, as MWCNT-B were indicated hereafter. It was examined by inductively coupled plasma-mass spectrometry ͑ICP-MS͒ that the iron content for the untr...

MAPbI3 Self‐Recrystallization Induced Performance Improvement for Oxygen‐Containing Functional Groups Decorated Carbon Nanotube‐Based Perovskite Solar Cells

“…We adopted temperature-programmed desorption analysis under ultrahigh vacuum to confirm the surface functional groups on the CNTs through analysis of the evolved CO and CO 2 ( Figure S1, Supporting Information). It is known that carboxyl, carboxylic anhydride, and lactone groups desorb CO 2 , whereas ketone, carboxylic anhydride, phenol, and carbonyl (quinone) desorb CO. [28][29][30][31] The peak at around 280 C in the CO 2 profile can be assigned to the carboxyl functional group, [31,32] so we conclude that the acid treatment conducted in this article introduced -COOH as one of the dominant functional groups. The peaks at around 680 and 880 C in the CO profile are assigned to the phenol and carbonyl functional groups, [31,32] which indicates that -C-OH and -C(═O)are also dominating functional groups.…”

Heterogeneous nanocarbon materials for oxygen reduction reaction

“…Siteselective oxidation of CNTs at the edge sites (open end of the nanotube) and at the hole defects was conducted to evaluate the ORR activity. 115 The excess O-functionalities on the CNT surface inhibited the electron transfer and occupied the adsorption sites for molecular O 2 , which eventually caused additional resistance and adsorption overpotential to the overall ORR. 115 Aer removing the O-functionalities from the non-defective sites but preserving those on defects the overall overpotential was decreased by 0.19 V. 115 The catalytic effect of the O-groups was conrmed in a blend consisting of graphene oxide (GO) and CNTs.…”

“…115 The excess O-functionalities on the CNT surface inhibited the electron transfer and occupied the adsorption sites for molecular O 2 , which eventually caused additional resistance and adsorption overpotential to the overall ORR. 115 Aer removing the O-functionalities from the non-defective sites but preserving those on defects the overall overpotential was decreased by 0.19 V. 115 The catalytic effect of the O-groups was conrmed in a blend consisting of graphene oxide (GO) and CNTs. 116 An improved ORR performance was observed on small graphene sheets which had higher oxygen content and more exposed edges (Fig.…”

Non-nitrogen doped and non-metal oxygen reduction electrocatalysts based on carbon nanotubes: mechanism and origin of ORR activity

“…12,14 Previously, we have found that aer the removal of functional groups in functionalized hollow multi-walled carbon nanotubes (MWNTs), the onset potential shied to the positive direction up to 0.55 V vs. RHE. 17 This result suggested that functional groups themselves are not responsible for such enhancement of the ORR catalyst performance. However, the nature of the active sites has not been well investigated.…”