Study on the Oxygen Reduction Reaction Activity Induced By Quick Oxidation of Carbon Nanotube

Abstract

1.IntroductionIn recent years, carbon nanotube (CNT)-based ORR electrocatalysts have attracted a lot of attention due to their appropriate physical and chemical properties. However, the pristine CNT is inert in ORR due to its electroneutrality of sp2 network. Creating topological defects in carbon materials is an efficient way to boost its electrochemical performance. Many techniques have been developed to break sp2 network on CNTs. Unzipping pristine CNTs by oxidants, such as KMnO4 [1], can form active defect sites. However, harsh oxidation process followed by a high temperature reduction of the carbon by removing the oxygen functional groups was generally required for preparing such kind of defective carbon materials. The active sites for this kind of defective carbon is considered to be the intrinsic defects of CNTs (edges or topological defects) It will be interesting if we can introduce intrinsic defects without intensive oxidation of CNTs with a well-controlled process, Aiming on direct forming the intrinsic defects to better understand the catalytic mechanism for the defective carbon nanotubes, In this work, The oxidation process to form defects in CNTs were precisely controlled by the rapid thermal annealing (RTA) strategy with CoOx as the oxidation catalyst to chemically drill defects on CNTs under relatively mild condition.2.Experimental MethodsCo(NO3)2 was deposited on the CNTs by ultrasound sonication and drying. The obtained powder was annealed at 300 °C under Ar atmosphere for 1h. To carefully control the oxidation condition, rapid thermal annealing (RTA) was used for oxidation the CNTs in several seconds by using IR-lamp heater. The ORR activity was evaluated by the cyclic voltammogram (CV) and the linear sweep voltammetry (LSV). The defective nature of DMWNT was studied by Raman.3.Results and DiscussionCobalt oxides can oxidize carbon at mild temperatures (250 °C) according to the TGA results [2]. First, 270°C and 30min was first selected as the oxidation condition. The Id/Ig ratio increased from 1.56 to 1.63, while no increase in the onset potential (0.5V) was observed after oxidation. This means that the CNTs became more defective but there is no impact on its electrochemical performance. The highly active site was not formed in the prolonged oxidation.To control oxidation extent of CNTs, RTA was employed to control the annealing process where temperature was raised up in 8~9 s. As a result, quick oxidized DMWNT showed improved onset potential of 0.68 V. The carbon defects in quick oxidized DMWNT with proper structure were expected to be beneficial for ORR activity by decreasing the overpotential.An obvious redox couple (C=O + 2H+ + 2e- ⇋C-OH) at ~0.6 V was showed in CV profiles of the DMWNT with prolonged oxidation but decreased significantly in quick oxidized DMWNT, confirming the presence of oxygen species on the surface. The small redox peak also indicated the slight oxidization degree in quick oxidation. The high ORR performance is benefit from these small defects and the resulted fewer OFGs around the carbon edges.Reference[1] L. Xue , Y. Li, X. Liu, Q. Liu, J. Shang, H. Duan, L. Dai, J. Shui, Nat. Commun., 9, 3819, (2018).[2] K.Waki, R. A. Wong, H. S. Oktaviano, T. Fujio, T. Nagai, K. Kimoto, K. Yamata, Energy Environ. Sci., 7, 1950 (2014). Figure 1