Materials Today Energy
Understanding the effect of functional groups on carbon nanotube towards oxygen reduction reaction
Introduction
Due to the increasing concern of global warming and shortage of fossil fuels, the huge research effort was devoted to the development of sustainable clean energy or more efficient utilization of existing energy. Then a series of energy storage and conversion systems were successfully fabricated aiming at lightning the heavy dependence on non-renewable resources, such as fuel cells, lithium-ion batteries and photovoltaic cells. However, these devices are only meaningful if the output energy is far more beyond what has been input to make reactions happen, in which sense developing catalysts with rather high activity and stability for certain reaction processes and studying their intrinsic mechanism are of significant importance. As one of the most promising solution, fuel cell has attracted a lot of research interest in the past few decades [[1], [2], [3]]. Thus, intensive research efforts were devoted to the development of cathode catalyst for oxygen reduction reaction (ORR), among which metal-free, non-nitrogen doped multi-wall carbon nanotubes (MWCNTs) play an important role regarding its relatively low cost and competitive activity compared with conventional noble metal Pt catalyst. To note, pure MWCNTs exhibit no electrocatalytic ORR activity unless introducing defects into it. It is suggested by molecular dynamics simulations [2] that topological defects in the form of pentagon-heptagon defect pairs are favorable for the enhanced ORR activity [4], which can be realized by the oxidation of carbon nanotubes followed by high-temperature annealing at 900 °C in Ar. In general, functionalization process involves strong oxidizing acid like HNO3 and H2SO4. However, these oxidants are so powerful that various types of oxygen-containing functional groups (OFGs) will be obtained, including carboxyl, anhydride, carbonyl and quinone. Furthermore, the formation of topological defects is ascribed to the removal of high-temperature CO desorbing OFGs [5]. In this regard, it is expected that elevated ORR performance can be achieved by creating functional groups with a higher proportion of carbonyl. However, this is quite hard to prove directly. Therefore, in this study, a two-step oxidation method is proposed to regulate the surface OFGs of MWCNTs aiming at adjusting the ratio of the carbonyl and phenol, which is then determined quantitively by multiple characterization techniques. By carefully choosing the combination and sequence of oxidants, a mild oxidizing atmosphere is established for the convenience of carbonyl construction. Then a rapid annealing process which could gradually remove the OFGs on the surface of MWCNTs has also been applied for further confirmation the OFGs influences on the ORR activity. Finally, to identify if the topological defect is the real key to the ORR activity, some hole defects were artificially made on MWCNTs to verify their contribution.
Section snippets
MWCNT preparation
In this study, different types of MWCNTs has been prepared to study the ORR activity with the removal of OFGs. The raw hollow multi-walled carbon nanotubes (MWNTs) used was provided by Showa Denko KK, Japan (VGFX-X 250001, diameter: 15 nm, approximate length: 3 mm; containing Fe impurities <1 wt%). Before the experiment, raw MWCNTs has been pre-treated by using heat and acid treatment in order to remove the amorphous carbon and metal impurities. Then, the pre-treated MWCNTs was oxidized by H2O2
Oxidation results and OFGs change
The temperature programmed desorption (TPD) has been applied to study the oxidation results, which will bring clear evidence for the OFGs change. To emphasize the change of OFGs after different treatment, in Fig. 1a and b, the intensity change (intensity subtraction with the sample after and before treatment) was used instead of the original intensity data. From the TPD analysis in Fig. 1a, after the H2O2 oxidation, the amounts of functional groups increase significantly. The CO2 desorption
Conclusion
This study investigated the effect of different OFGs on the ORR activity. The results break the “900 °C magic (900 °C annealing will always provide the highest ORR activity)” and suggest that removing the low-mid-temperature-decomposed OFGs such as carboxyl or phenol could also benefit the ORR activity. More importantly, this work proved that only the topological defects from removing OFGs but not the hole defects on the MWCNTs will help increase the ORR activity. Our specific adjustment
Credit author statement
Jiuting Chen: Supervision, Project administration, Investigation, Writing- Original draft preparation.
Cong Li.: Conceptualization, Methodology, Investigation, Writing- Reviewing and Editing.
Yuran Chen: Data curation, Investigation, Software.
Declaration of Interests
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This work partially was supported by COI STREAM from the Ministry of Education, Culture, Sports, Science, and Technology (MEXT) and Shin-Etsu Chemical Co., Ltd. Japan. Their contribution is greatly appreciated.
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These authors contributed equally to this paper.