Influence of the reaction temperature on the oxygen reduction reaction on nitrogen-doped carbon nanotube catalysts
Graphical abstract
Introduction
The oxygen reduction reaction (ORR) is of critical importance to support industrial development of fuel cells and metal-air batteries for the future energy mix infrastructure [1], [2], [3]. The slow kinetics of the ORR compared to that of the hydrogen oxidation at the anode severely limits the performance of low temperature fuel cells. To date, platinum-based materials are used as state-of-the art ORR catalysts due to their higher, compared to other materials, catalytic activity. However, they suffer from high cost, low tolerance to poisons such as methanol and CO, and low stability mostly due to the Pt particle agglomeration, Pt dissolution and to the corrosion of the carbon support under fuel cell operation conditions [4], [5]. A large number of investigations have been devoted to the decrease of the amount and to the replacement of platinum by other catalysts, including Pt-based alloys [6], [7], [8], transition metal–nitrogen–carbon M–N–C catalysts [9], [10], [11], [12], [13], [14], perovskite-type oxide catalysts [15], [16], [17] and carbon-based materials doped with various elements [18].
Application of M–N–C-type materials as the ORR catalysts dates back to the 1960–1970s when it was discovered that metal (e.g. Fe) porphyrins and Phthalocyanines can be pyrolyzed to produce highly active catalysts [19], [20]. The last decades saw a remarkable rise in the interest toward the M–N–C ORR materials, a diversification of their methods of preparation by using various transition metal and nitrogen precursors [10], [12], [21] and a significant progress in the understanding of the nature of the active sites and the mechanism of electrocatalysis [13], [14], [22].
Nitrogen-doped carbon composites such as nitrogen-doped carbon nanotubes/nanofibers and/or mesoporous carbon have received an increasing academic and industrial interest, as catalysts or as catalyst support with enhanced surface properties, in several catalytic applications during the last decades [23], [24], [25], [26], [27], [28]. These composites, either in a bulk or supported forms, are mostly synthesized via chemical vapor deposition (CVD) process using different sources of carbon and nitrogen precursors and iron as the growth catalyst [29], [30], [31], [32], [33], [34], [35] or synthesized by pyrolysis of a precursor containing nitrogen, carbon in the presence of a transition metal catalyst [36], [37]. Beside their use as catalyst support in several reactions, nitrogen-doped carbon nanotubes (N-CNTs) have also been considered as catalysts for replacing platinum in the cathode oxygen reduction reaction (ORR) for fuel cells application [38], [39], [40], [41], [42], [43], [44]. The issue on whether metal-free N-containing materials might act as catalysts in the ORR attracted much attention. It is widely accepted that while in alkaline media metal-free carbon materials possess the ORR activity, in acid the activity of metal-free materials is small. In recent studies, Singh et al. [45] and Masa et al. [46] compared N-containing carbon nanostructures and N-coordinated iron carbon catalysts and observed a significant positive effect of transition metal ions on the ORR activity. On the other hand, Singh et al. [45] reported that an initially a much higher activity of the iron-containing catalyst degraded more rapidly than that of iron-free catalyst in the fuel cell environment.
In general, the ORR is operated at temperature close to the maximum affordable temperature of the fuel cell membrane, ca. 80 °C for Nafion®. For the state-of-the-art anion exchange membranes the operating temperature is usually lower due to their so far insufficient thermal stability. It is thus of interest to compare the electrocatalytic activities of Pt and non-PGM (platinum group metal) catalysts as a function of the operation temperature. Furthermore, it is expected that an increase of the reaction temperature increases the materials corrosion which could directly impact the ORR performance during the operation. It is thus of interest to be able to correlate the stability of the N-doped carbon catalysts, after and before leaching of the iron catalyst with that of the Pt/C, not only at room temperature, but also at higher operating temperature. Deactivation of the Pt/C catalyst as a result of cycling was reported in a number of publications [5], [43], [44], [47], [48]. Herein, we report on the ORR activity and stability of the N-CNTs catalysts as a function of the reaction temperature (25–75 °C) and their comparison to a benchmark Pt-20 wt%/C catalyst. It is expected that such investigation will be of interest for the future development of non-PGM catalysts for fuel cell applications where the long-term stability is a prerequisite.
Section snippets
N-CNTs synthesis
The N-CNT sample was synthesized using the chemical vapor deposition (CVD) method with iron supported on alumina as a growth catalyst [23], [24], [49]. The Fe/Al2O3 catalyst was synthesized by impregnating the gamma alumina support with an aqueous solution containing Fe(NO3)3 and contained 20 wt% of Fe. The solid was dried at room temperature overnight and oven-dried at 110 °C for 24 h. It was then calcined in air at 350 °C for 2 h in order to transform the nitrate precursor into Fe2O3 oxide. The Fe2
Characterization of N-CNTs
Representative SEM and TEM micrographs of N-CNTs synthesized at 700 °C are presented in Fig. 1. SEM micrographs indicate the relatively high homogeneity of the diameter of the as-synthesized N-CNTs which is centered around 80 nm and length up to several tens of micrometers (Fig. 1A and B). According to the SEM analysis the N-CNTs are highly entangled forming bundles. In agreement with the recent TEM tomography analysis [52], TEM micrograph of the as-synthesized sample (Fig. 1C) shows the
Conclusions
Nitrogen-doped carbon nanotube (N-CNT) catalysts have been synthesized via the chemical vapor deposition (CVD) method at 700 °C using an Fe growth catalyst and their catalytic activity toward the oxygen reduction reaction (ORR) has been studied in alkaline electrolyte. The removal of the iron growth catalyst by a chemical treatment resulted in a significant decrease of the ORR currents as well as an increase up to 30% of the H2O2 amount escaping the catalytic layer during the ORR confirming that
Acknowledgements
The present work is financially supported by a European project (FREECATS) under a contract number NMP3-SL-2012-280658. LTP would like to thank the EU for the grant during his postdoctoral stay at the ICPEES. DVC would like to thank the Vietnamese government for the grant during his stay at the ICPEES. Dr. W. Baaziz and T. Romero (ICPEES) are gratefully acknowledged for performing TEM and SEM analysis of the sample. The SEM and TEM experiments were carried out at the facilities of IPCMS (UMR
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