Influence of nitrogen doping on carbon nanotubes towards the structure, composition and oxygen reduction reaction
Highlights
► NCNT synthesised from different nitrogen precursors. ► Investigate ORR mechanisms on NCNT via cyclic voltammetry. ► ORR active sample shows highest at% of pyridinic-N and N-oxides in NCNT. ► ORR on NCNT proceed via indirect four-electron transfer.
Introduction
The development of proton exchange membrane fuel cells (PEMFCs) is facing crucial issues attributed to the utilisation of expensive materials in its components, which are hindering their adoption in energy-related technology. To date, platinum and its alloys have been the most promising catalysts for both the oxygen reduction reaction (ORR) at the fuel cell cathode and the hydrogen oxidation reaction at the fuel cell anode. Nevertheless, the degradation of platinum catalysts remains a barrier to the development of the PEMFC, especially at the cathode, because of its susceptibility to time-dependant drift and carbon monoxide deactivation [1], [2], [3]. Numerous investigations have been conducted in recent decades to find a complete replacement for platinum catalysts at the cathode. One challenge in this area of study is the development of a lower-cost catalyst with catalytic activity comparable to that of platinum and long-term durability in the harsh environment at the PEMFC cathode.
The peculiar electronic properties of nitrogen-doped carbon compounds have been shown to impart oxygen reduction ability in both alkaline and acidic environments despite the ambiguity concerning the underlying reaction mechanisms [4], [5], [6]. Nonetheless, the surface defects are generally accepted as creating edge-plane sites due to the nitrogen doping into the carbon matrix; these defects are believed to be responsible for the ORR activity [1], [7], [8], [9]. In this regard, nitrogen-doped carbon nanostructures such as carbon nanotubes, carbon nanofibers and carbon nanospheres possess the potential to increase the ORR activity because of their ability to maximise the edge-plane exposure per unit volume. Matter et al. [7] synthesised nitrogen-doped carbon nanofibers and demonstrated that the pyridinic nitrogen in the nanostructures is responsible for the edge-plane exposure; the common consensus is that the pyridinic nitrogen serves as the active sites for the ORR. In fact, the ORR of N-doped carbon nanostructures in alkaline media [7], [10], [11], [12] have been extensively studied because of its promising activity in alkaline fuel cells and batteries, although several drawbacks remained unresolved. Less effort has been devoted to understanding the ORR activity of such a pure organic nanocatalyst in acidic media because of the general presumption of its low intrinsic activity. Fewer studies [13], [14], [15], [16], [17], [18], [19] have been focused on the fabrication of metal/nitrogen-doped carbon compound composite with the goal of increasing the activity; such composites have shown promising activity in PEMFC applications. In this sense, a fundamental study on the oxygen reduction reaction in acidic media using nitrogen-doped carbon compounds is necessary before the method can be fine-tuned to improve the catalytic activity of the sample a PEMFC, and determine whether they offer improved performance relative to its metal alloy composites.
The use of different nitrogen precursors in the synthesis of NCNT has led to different structural and electronic properties of NCNTs [20], and these properties have shown an explicit effect on ORR activity. Gong et al. [21] and Higgins et al. [22] employed ethylenediamine as nitrogen precursors for NCNT growth, and the resulting NCNT was shown to be active towards the ORR. An increase in the nitrogen content has also been hypothesized to enhance the catalytic activity of the sample. A systematic study by Higgins et al. [22], who synthesised NCNT from different aliphatic amines, revealed that an increase in the nitrogen content of the samples resulted in higher ORR activity. These results demonstrate a positive correlation between the ORR activity and the overall nitrogen content in the samples, irrespective of the media used for the analysis. However, the ORR activity and whether pyridinic or quaternary nitrogen serves as the active sites are still matters of debate [23], [24], [25], [26]. Recently, Ikeda et al. [15]and Niwa et al. [27] have proposed that the carbon atom adjacent to the quaternary nitrogen atom is responsible for ORR because of its better stability in acidic media. In controversy, Liu et al. [8] have suggested that both pyridinic and quaternary nitrogen atoms serve as the active sites.
An investigation of the intrinsic activity of NCNTs requires a set of experimental parameters to be designed investigate the effect of the surface structure, the nitrogen content, and the nitrogen functional groups on the ORR activity. Rotating-ring disc electrode (RRDE) analysis was employed to study the electron transfer mechanisms in the catalysts and the samples were found to be under mixed kinetic control with no limiting current plateau.
In this study, we employed a systematic approach to examine the previously discussed relationships with the aid of cyclic voltammetry and rotating-ring disc electrode analysis to obtain new insight into the catalytic mechanisms. We investigated the nature of the nitrogen functional groups in the NCNT samples prepared from three different chemical precursors. Our results surprisingly show that the samples that exhibit the greatest activity have an observable pyridinic-nitrogen-oxides peak in the XPS spectrum, which has not, to our knowledge, been previously observed. Moreover, we revealed that different chemical precursors result in different nitrogen functional groups in the NCNT samples and that these functional groups explicitly affect the ORR activity of the catalysts in acidic media.
Section snippets
Preparation of the NCNT catalysts
Vertically aligned NCNTs were grown using the chemical vapour deposition (CVD) method in a metre-length horizontal quartz tube inserted in a temperature-programmable flow furnace (Carbolite, USA). A smaller, cut quartz tube was placed at the centre as a substrate for NCNT growth. Iron (II) phthalocyanine, which served as a catalyst for NCNT growth, was placed at the entrance of the furnace. Three different NCNT samples were synthesised using aniline, diethylamine (DEA) and ethylenediamine (EDA)
SEM and TEM
Physical characterisations of the NCNTs were performed to study the effect of the nitrogen-containing precursor on the structural morphology, nitrogen bonding nature and doping content of the NCNT products. The surface morphologies of the synthesised NCNTs were investigated using scanning electron microscopy (SEM). As shown in Fig. 1, vertically aligned nanotubes forests were clearly observed and the synthesis procedure successfully produced carbon nanotubes. In this synthesis process, iron
Conclusion
In this study, a class of vertically aligned NCNTs with nitrogen content between 4.33 and 6.58 at% were synthesised via the CVD method using aniline, DEA and EDA as chemical precursors. The EDA-NCNT was the most electrochemically active sample towards the ORR in acidic media among the investigated catalysts. The highly active sample possessed the unique properties of a highly corrugated nanotube structures with sufficiently high surface defects, along with a high percentage of pyridinic-N and
Acknowledgements
The authors gratefully acknowledge the financial support for this work from the Malaysian Ministry of Higher Education through a Fundamental Research Grant Scheme (UKM-RF-03-FRGS0138-2010), the Universiti Kebangsaan Malaysia through a Universiti Research Grant (UKM-AP-TK-05-2009) and the Malaysian Ministry of Science, Technology and Innovation through a National Science Fellowship awarded to Ms. Wong Wai Yin.
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2019, International Journal of Hydrogen Energy