Catalytic Reactions on the Open-Edge Sites of Nitrogen-Doped Carbon Nanotubes as Cathode Catalyst for Hydrogen Fuel Cells (original) (raw)
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Catalysis Communications, 2015
We utilized first-principles spin-polarized density functional theory (DFT) calculations to study the electrocatalytic reaction steps on FeN 4 /C site of carbon nanotubes. O 2 molecule can be adsorbed and partially reduced on FeN 4 /C site without any activation energy barrier. The partially reduced O 2 further reacts with H + and e − through a direct pathway (DPW) and form two water molecules without any activation energy barrier. Through an indirect pathway (IDPW), there is an activation energy barrier of~0.15 eV for the formation of the first H 2 O molecule. The formation of the second H 2 O molecule through IDPW does not have any activation energy barrier.
ACS Nano, 2012
Heat treating nitrogen-doped multiwalled carbon nanotubes containing up to six different types of nitrogen functionalities transforms particular nitrogen functionalities into other types which are more catalytically active toward oxygen reduction reactions (ORR). In the first stage, the unstable pyrrolic functionalities transform into pyridinic functionalities followed by an immediate transition into quaternary center and valley nitrogen functionalities. By measuring the electrocatalytic oxidation reduction current for the different samples, we achieve information on the catalytic activity connected to each type of nitrogen functionality. Through this, we conclude that quaternary nitrogen valley sites, N-Q(valley), are the most active sites for ORR in N-CNTs. The number of electrons transferred in the ORR is determined from ring disk electrode and rotating ring disk electrode measurements. Our measurements indicate that the ORR processes proceed by a direct four-electron pathway for the N-Q(valley) and the pyridinic sites while it proceeds by an indirect two-electron pathway via hydrogen peroxide at the N-Q(center) sites. Our study gives both insights on the mechanism of ORR on different nitrogen functionalities in nitrogen-doped carbon nanostructures and it proposes how to treat samples to maximize the catalytic efficiency of such samples.
Activity and active sites of nitrogen-doped carbon nanotubes for oxygen reduction reaction
Journal of Applied Electrochemistry, 2013
Nitrogen-doped carbon (CNx) nanotubes were synthesized by thermal decomposition of ferrocene/ethylenediamine mixture at 600-900°C. The effect of the temperature on the growth and structure of CNx nanotubes was studied by transmission electron microscopy, X-ray photoelectron spectroscopy, and Raman spectroscopy. With increasing growth temperature, the total nitrogen content of CNx nanotubes was decreased from 8.93 to 6.01 at.%. The N configurations were changed from pyrrolic-N to quaternary-N when increasing the temperature. Examination of the catalytic activities of the nanotubes for oxygen reduction reaction by rotating disk electrode measurements and single-cell tests shows that the onset potential for oxygen reduction in 0.5 M H 2 SO 4 of the most effective catalyst (CNx nanotubes synthesized at 900°C) was 0.83 V versus the normal hydrogen electrode. A current density of 0.07 A cm-2 at 0.6 V was obtained in an H 2 /O 2 proton-exchange membrane fuel cell at a cathode catalyst loading of 2 mg cm-2. Keywords Nitrogen-doped carbon nanotubes Á Thermal decomposition Á Oxygen reduction reaction Á Non-precious metal catalysts
Journal of Physical Chemistry C, 2009
One of the main challenges in the commercialization of low temperature fuel cells is the slow oxygen reduction reaction (ORR) kinetics and the high cost and scarcity of platinum (Pt)-based catalysts. As a result, alternative non-noble electrocatalysts to Pt materials for ORR is needed to realize the practical application of fuel cells. In this study, nitrogen-doped carbon nanotubes (NCNTs) were synthesized as a non-noble electrocatalyst for the ORR using ethylenediamine (EDA-NCNT) and pyridine (Py-NCNT) as different nitrogen precursors by a single-step chemical vapor deposition (CVD) process. The resulting EDA-NCNT has shown similar ORR performance compared to platinum on carbon support in terms of onset and half-wave potentials. Moreover, EDA-NCNT showed superior ORR performance in terms of limiting current density, number of electrons transferred, and H 2 O selectivity. The effects of nitrogen content on ORR performance of NCNT were investigated by comparing EDA-NCNT with Py-NCNT. The ORR performance of Py-NCNT was inferior compared to EDA-NCNT in terms of onset and half-wave potentials, limiting current density, number of electrons transferred, and H 2 O selectivity. Further material characterizations by X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy illustrated a higher nitrogen content and more defects in EDA-NCNT compared to that of Py-NCNT which indicates the important role of the nitrogen precursor on nitrogen content and structure of NCNT. By combining the results of ORR activity and material characterization, it is concluded that higher nitrogen content and more defects of NCNT lead to high ORR performance.
Catalysis Today, 2015
Nitrogen-doped carbon nanotubes (N-CNT) were synthesized at 700 • C via the chemical vapour deposition (CVD) method and were used as catalysts in the oxygen reduction reaction (ORR) in 0.1 M KOH. The activity toward the ORR and the stability of these N-CNTs in alkaline solution were studied as a function of the reaction temperature and of the chemical treatment applied to the catalyst. The kinetic analysis of these catalysts was also carried out and compared to the ORR performance of the commercial Pt/C Vulcan XC72 catalyst. N-CNT-700BW catalyst without any chemical treatment after the CVD synthesis, possesses a half-wave potential E 1/2 of approximately 0.82 V vs. RHE, 50 mV lower than the E 1/2 value of Pt/C catalyst and a specific current density J k at 0.9 V = 5.46 mA/mg at T = 25 • C. Removal of the major part of the iron growth catalyst by a chemical treatment resulted in a strongly decreased but still measurable activity. The activation energy of the N-CNT-based catalyst was calculated and is around 38 kJ mol −1 at an ORR overpotential of 300 mV. Increasing the temperature of the electrolyte up to 75 • C leads to a positive shift of the half-wave potential of the reaction as well as an increase of the H 2 O 2 escape. The long-term stability test has also been conducted and indicates a good stability of the activity of the N-CNT-based catalysts under operation in alkaline media.
Electrochimica Acta, 2016
Iron-and cobalt-containing nitrogen-doped carbon catalysts based on multi-walled carbon nanotubes are prepared using pyrolysis of dicyandiamide and CoCl 2 or FeCl 3 at 800 C. The electroreduction of oxygen is studied in 0.5 M H 2 SO 4 solution by the rotating disk electrode method and the surface morphology and composition of the catalysts are characterised by scanning and transmission electron microscopies and X-ray photoelectron spectroscopy. The as-prepared catalysts show mediocre electrocatalytic activity for oxygen reduction reaction (ORR), which increases as a result of acid treatment and second pyrolysis. The electrocatalytic activity of Fe-containing catalyst towards the ORR surpasses that of Co-based material and it supports a 4-electron reduction of O 2. Co-containing catalyst, in turn, shows higher stability. Both catalysts are highly methanol tolerant in acid media. The transition metal-containing N-doped carbon materials are promising cathode catalysts for low-temperature fuel cells.
RSC Advances, 2014
Although nitrogen-doped nanocarbon systems have recently received intense attention, the mechanism for the observed highly efficient oxygen reduction is still under debate. To address this issue, we investigated the adsorption and dissociation of an oxygen molecule on three pristine or nitrogen-doped nanocarbon systems: graphene, single-walled and double-walled carbon nanotubes using density functional theory calculations. The adsorption and dissociation energies were determined for both pristine and N-doped single-walled carbon nanotubes of different diameters with graphitic-like N substitutions in order to see the effect of diameter on oxygen dissociation. It was found that the energy barrier for oxygen dissociation, chemisorption energy and reaction energy are a function of carbon nanotube diameter, but independent of the number of walls. We also investigated the energy barrier of oxygen dissociation on single-walled carbon nanotubes with different types of nitrogen doping (i.e. pyridinic and graphitic). It was observed that higher nitrogen concentrations greatly reduce the energy barrier for graphitic nitrogen. Our results contribute towards a better understanding of the reaction mechanism for nitrogen-doped carbon nanomaterials involving oxygen molecule dissociation in the first step.
Journal of Power Sources, 2016
In this work, the electroreduction of oxygen on nitrogen-doped graphene and multi-walled carbon nanotube (MWCNT) composite catalysts is investigated. Acid-treated MWCNTs and graphite oxide were doped using biuret, carbohydrazide and semicarbazide hydrochloride as the nitrogen precursors. The reactants were mixed with carbon nanomaterials and pyrolysed in an inert atmosphere at 800 C. Scanning electron microscopy was used to characterise the surface morphology of catalysts and X-ray photoelectron spectroscopy (XPS) was used to determine the surface content of the catalysts. XPS revealed different contents of nitrogen gained by using different nitrogen precursors, which were tied to electrochemical activities observed in this work by using the rotating disk electrode (RDE) method. The catalysts revealed high oxygen reduction reaction (ORR) activity even at low loadings and excellent stability over 1000 potential cycles. This indicates their applicability as cathode materials in alkaline anion exchange membrane fuel cells.