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Nanosized-Fe3PtN supported on nitrogen-doped carbon as electro-catalyst for oxygen reduction reaction
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Nanosized-Fe3PtN supported on nitrogen-doped carbon as electro-catalyst for oxygen reduction reaction Yu-Kai Huang a, Anirudha Jena a,b, Yu-Ting Chen a, Mu-Huai Fang a, Nai-Hsuan Yang a, Ho Chang b,**, Ru-Shi Liu a,b,* a
Department of Chemistry, National Taiwan University, Taipei 106, Taiwan Department of Mechanical Engineering and Graduate Institute of Manufacturing Technology, National Taipei University of Technology, Taipei 106, Taiwan
b
article info
abstract
Article history:
Novel nano-crystalline Fe3PtN supported on nitrogen-doped carbon materials are syn-
Received 23 November 2016
thesised via double pyrolysis, under Ar and NH3 with two sets of temperatures namely, 800
Received in revised form
and 900 C. An improved catalytic activity has been observed in terms of higher values of
16 February 2017
onset and half-wave potentials, with larger kinetic currents and low hydrogen peroxide
Accepted 24 March 2017
yields. The activity upon comparing with Pt/C with simultaneous experiments shows
Available online xxx
impressive results. Within the double annealed samples a comparative study has been done on the basis of active sites available for the oxygen reduction reactions. We have
Keywords:
revealed the origin of its activity by intensively investigating the composition and the
Iron-based catalyst
structure of the catalyst and their correlations with the electrochemical performance.
Oxygen reduction reaction
© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Nitrogen-doped carbon
Introduction Arising demands for energy in an environmental favourable manner is a current genuine concern. Hence developing clean, renewable and sustainable technologies for storage of energy is one of the most widely approached areas of research. Metaleair batteries in particular LieO2, among other energy storing devices, bear characteristics of eco-friendly and higher energy density (11.68 kWh kg1) which is capable of powering mobile vehicles [1]. The operation of the LieO2 battery principally banks on electrochemical reduction of oxygen in presence of a catalytic species. However, factors such as
electrolyte poisoning and production cost have limited the technological advancement [2]. Commercialization of these battery technologies mainly requires efficient electrocatalysts for oxygen reduction reaction (ORR). Varieties of materials have been examined as catalysts for the electrochemical reduction of oxygen including Pt-group and non-Pt group metals. However, the non-Pt group metals can only reach the performance of Pt-group up to 40e50% [3]. Beginning from the work by Jasinski, ‘N’ coordinated to transition metals like Fe, Co has been emerging as promising catalytic materials for ORR [4]. Jiang et al. have reported the role of FeeNx linkage and presence of FeeFe3C nanocrystals in the boost in the catalytic activity [5]. N-coordinated metal
* Corresponding author. Department of Chemistry, National Taiwan University, Taipei 106, Taiwan. Fax: þ886 2 23636359. ** Corresponding author. E-mail addresses: [email protected] (H. Chang), [email protected] (R.-S. Liu). http://dx.doi.org/10.1016/j.ijhydene.2017.03.175 0360-3199/© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Huang Y-K, et al., Nanosized-Fe3PtN supported on nitrogen-doped carbon as electro-catalyst for oxygen reduction reaction, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.03.175
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catalysts supported carbon (MeNeC), especially FeeNeC has shown to have outstanding performance in alkaline media [6e8]. However, their practical implementation still remains a serious challenge [9]. Carbonaceous composites Pt/C, catalysts derived from Ru, and Pd are also commonly studied electrocatalyst for ORR both in alkaline and acidic media. Other modifications to tune the properties of Pt-based materials to further improve electrochemical performance include controlling the active facets [10], creating multi-metallic nanocrystals and using better support materials [11]. Among these multi-metallics, nanocrystals are attractive and practical not only because they can get combined properties from distinct metals but also that some possible synergistic effects may appear. Due to such synergistic effects in case of multimetallic and hybrid catalytic systems are very helpful in reducing catalyst agglomeration and poisoning. This also can alter the electronic structure of the resultant catalyst to stabilise the intermediate species [12]. Lu et al. have reported enhancement of the catalytic performance of CoTe2/CNT hybrid by channelling the charge transfer via CNTs at the interface [13]. So far, many Pt-based multi-metallic alloy systems from the combination with other noble metals or 3d transition metals have been synthesised and studied in acidic media. Mixed oxides, carbonaceous oxide composite and alloy systems especially 3d transition metals in the alkaline medium are also examined in several reports [14,15]. Oezaslan et al. synthesised PtxCuy systems and PtxCoy systems, respectively, and analysed the electrochemical performances both in acidic media and alkaline media [16]. In this work, we have synthesised Fe3PtN nano-crystallites supported on nitrogen doped carbon framework, as the electro-catalyst for ORR in alkaline media in a two-step annealing process. The catalysts synthesised with different annealing temperatures of 800 C and 900 C are compared with their catalytic activities towards ORR. We have also drawn a mechanistic pathway for the active sites of the catalysis.
Experimental Synthesis of (Fe3PtN)eNeC The catalyst material Fe3PtN/C was synthesised following a modified method using sacrificial support involving two annealing steps [17]. All the AR grade precursors were purchased from various commercial sources e.g. FeCl3,6H2O, KSCN, H2PtCl and 1, 10-phenanthroline. Two-step annealing, first under Ar and second under NH3 was used to obtain the final catalyst powder. Such two-step pyrolysis helps in enhancing the activity of catalyst [18]. Briefly, 0.545 g of 1, 10phenanthroline anhydride was dissolved in 10 ml ethanol and subjected to ultra-sonication for few minutes. Then 7.0 g silica colloid (40 wt. %) was added to the resultant solution keeping the mass ratio of phenanthroline to silica be 1:5. The mixture was then stirred at room temperature for about 2 h to form a uniform slurry (Solution A). FeCl3,6H2O of 0.297 g and 0.321 g KSCN were then mixed in aqueous ethanol solution ultrasonically and the resultant solution was then added into
3.363 ml 0.1 M H2PtCl6 solution (solution B). Both the solution A and B are then mixed to get final dispersion which was dried at 60 C to get a solid product. The dried powder (1 g) was then annealed under Ar at a rate of 10 C/min to two temperatures 800 and 900 C for 2 h and are denoted hereafter as IPN8 and IPN9. The remnant silica was etched out using 15% HF and the product then washed repeatedly and dried. The second step annealing of both the samples was done under NH3 at 700 C for 2 h. Both the two-step annealed samples were compared with N8 sample which was synthesised without any Fe and Pt content with first annealing temperature at 800 C.
Characterization of catalyst materials X-ray diffraction (XRD) patterns were collected by a Bruker D2 PHASER XRD with Cu Ka radiation (l ¼ 1.54178 A). Raman spectra were recorded by Thermo-Scientific DXR Raman instrument with a laser wavelength of 532 nm. Transmission electron microscopic (TEM) images were recorded on JEOL1200EX II instrument. Scanning electron microscopy (SEM) images were collected with Hitachi S-4800 instrument. Core level X-ray photoelectron spectra (XPS) were acquired from ULVAC-PHI, Inc. using Al Ka source. ORR activity was studied using a rotating disk electrode (RDE) and a rotating ring-disk electrode (RRDE) in a standard three-electrode configuration. Saturated calomel electrode (SCE) used as a reference electrode and a Pt foil (1 cm2) as a counter electrode. A slurry ink was prepared by dispersing 5 mg catalyst in 20 mL Nafion (5 wt. %) solution, 500 mL isopropanol and 250 mL deionized water. The homogeneous dispersion was then drop-casted on the glassy carbon electrode surface with a loading of 0.2 mg cm2 and 0.25 mg cm2 for RRDE and RDE experiments respectively. To compare the results, Pt/C (20 wt.%) were also prepared in a similar manner with the loading of 0.1 mg cm1 and 0.125 mg cm1 for the RRDE and RDE experiments respectively. All RDE and RRDE experiments were conducted at room temperature in oxygensaturated 0.1 M KOH electrolyte. After initial 20 cycles in oxygen-saturated 0.1 M KOH in a potential window of 1.1 to 0 V vs RHE at a scan rate of 20 mV s1, RRDE tests were conducted at a scan rate of 10 mV s1 at a rotation speed of 1600 rpm. RDE tests were performed in a similar manner, with varying speed of rotation of 800 rpm, 1200 rpm, 1600 rpm, 2000 rpm and 2400 rpm. The durability tests were tested by cycling in oxygen-saturated 0.1 M KOH in the potential range from 1.1 V to 0 V versus RHE. The initial LSVs were compared with those after 3000 cycles. The electron transfer number and HO 2 yield derived from RRDE tests are calculated using equations (1) and (2), [19]. HO2 ð%Þ ¼ 200
n¼4
ID ID þ IR =N
IR =N ID þ IR =N
(1)
(2)
where ID is the disk current, IR is the ring current which came from the re-oxidation of HO2 species back to O2 at 1.3 V versus RHE and N is the collection efficiency of 0.37.
Please cite this article in press as: Huang Y-K, et al., Nanosized-Fe3PtN supported on nitrogen-doped carbon as electro-catalyst for oxygen reduction reaction, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.03.175
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The kinetic current density JK was acquired from the intercept of the KouteckyeLevich plots basing on the KeL equation (3) as below, [19]: 1 1 1 1 1 ¼ þ ¼ þ J JL JK JK Bu1=2
(3)
where J is the experimentally measured current density, JL is the limiting current density, JK is the kinetic current, B is a parameter and u is the rotating speed of the electrode.
Results and discussion XRD patterns of both IPN8 and IPN9 are shown in Fig. 1. Both the XRD profiles follow similar broad patterns indicating Fe3PtN particles composed of nanometre-sized grains. As calculated using Scherer's equation the crystallite sizes are in the range of ~9 nm for both the IPN8 and IPN9. The XRD profiles further refined using Pawley refinement using Total Pattern Analysis Solution (TOPAS) software. Profiles of both the samples are indexed to the cubic Fe3PtN phase space group of Pm3m (Fig. S1). The lattice parameter a for the Fe3PtN crystals in IPN8 and IPN9 are 3.87142(44) and 3.88039(69) A respectively. The absence of peaks related to carbon around two theta 20 is an indication of amorphous carbon in both the sample IPN8 and IPN9.
Fig. 2 e Raman spectra of powder samples IPN8 and IPN9.
Raman spectroscopy was used to investigate the nature of the carbon in both the samples. The spectra of IPN8 and IPN9 are similar and show two main peaks assigned as D and G of carbon (Fig. 2). The D band at 1360 cm1 is related to the carbon with structural disorders and defects (sp3); as for the G band at 1590 cm1, it represents the ordered graphitic carbon
Fig. 1 e XRD patterns of the catalyst powder samples (a) IPN8, (b) IPN9, and (c) atomic arrangements in Fe3PtN crystal. Please cite this article in press as: Huang Y-K, et al., Nanosized-Fe3PtN supported on nitrogen-doped carbon as electro-catalyst for oxygen reduction reaction, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.03.175
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sheet (sp2) [20]. Broadening of the D band and G band in “carbon peaks” indicates that carbon present in very fine particle size. The peak intensity ratio of D band and G band, namely ID/IG, provides an identification of overall carbon framework about the degree of disorder. The ID/IG ratio of IPN8 and IPN9 are 1.138 and 0.962, respectively. Therefore, the carbon framework of IPN9 is more ordered than the counterpart of IPN8. This suggests that higher first pyrolysis temperature induces a higher degree of graphitization and promote the formation of more ordered graphitic structure, as explained in the previous report [19]. Subsequent pyrolysis under NH3 creates disordered C-centres which facilitate catalytic activities. But, in this study, both the carbon frameworks of IPNs are still pretty ordered. In Fig. S1, the effect of the second pyrolysis under NH3 atmosphere is then clarified to mainly promote the formation of Fe3PtN nano-crystallites. However, the durability of IPN9 can be expected to be better than IPN8, because higher ordered graphitic structure which has a positive effect on durability. SEM and TEM were used to check the morphology of the samples. From low-resolution SEM images in Fig. 3a and c, both the samples show agglomerations collaging to sheet-like structures. Such agglomeration may be brought about by the strong interaction between carbon nano-crystallites. Notably, there is no significant effect of different first pyrolysis temperatures on the morphology. High-resolution SEM images show the presence of bright spots on each of the agglomerated sheets as shown in Fig. 3b and d indicating embedded nanocrystallites on the carbon matrix. Low-resolution TEM images in Fig. 4a and b, show a uniform distribution of nano-crystalline particles embedded in the amorphous matrix. The calculated particles size can be seen in the distribution plot in the inset which is in the range 7e13 nm. Comparing between the contrasts it can be assumed
that the nano-crystallites are holding the carbon sheets which are very thin and transparent to the electron beam as shown by the arrow in the TEM images. High-resolution TEM images in Fig. 4c and d further show resolved fringes of the nanocrystalline IPN8 and IPN9. They are indexed to the (111) plane corresponding d spacing of 2.22 A of Fe3PtN. From both SEM and TEM it can be confirmed that the nano-crystalline Fe3PtN have been successfully implanted on the carbon matrix using the solution technique. Core level XPS were recorded to analyse elemental compositions and the nature of N-bonding in both the catalyst samples (Fig. S2). Nitrogen content decreases from at% 4.8 to 2.7 when the first pyrolysis temperature increase from 800 C to 900 C, which can be expected because higher pyrolysis temperature is likely to induce decomposition of nitrogen species in carbon frameworks [21]. The N1s spectra of the catalysts were further deconvoluted to examine different chemical states of nitrogen species with different binding energies, including pyridinic-N (398.5 eV), pyrrolic-N (400.2 eV), graphitic-N (401.4 eV) and oxidized-type-N (402 eVe405 eV), which also suggests the formation of nitrogen doped carbon framework (Fig. 5). The peak at a binding energy of 398.5 eV is supposed to also include a contribution from nitrogen bound to the metal (metalenitrogen), because of the small difference between the binding energies of nitrogenemetal and pyridinic-N. The contents of different Nbonding configurations can be quantified via the integrated area of the peaks (Table S1). Notably, the relative contents of graphitic-N dominate over those of other N-species both in IPN8 and IPN9. The relative content of pyridinic-N diminishes with higher first pyrolysis temperature, which indicates that pyridinic-N are less thermal stable than graphitic-N [22]. The higher overall content of pyridinic-N and graphitic-N suggest that the ORR performance of IPN8 may be better than that of
Fig. 3 e SEM images of samples IPN8 and IPN9 (a, c) low resolution, and (b, d) high-resolution images. Please cite this article in press as: Huang Y-K, et al., Nanosized-Fe3PtN supported on nitrogen-doped carbon as electro-catalyst for oxygen reduction reaction, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.03.175
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Fig. 4 e TEM images of samples IPN8 and IPN9 (a, b) low resolution, particle size distribution (inset) and (c, d) high-resolution images.
Fig. 5 e Deconvoluted XPS core level spectra of N 1s of (a) IPN8, and (b) IPN9.
IPN9 because graphitic and pyridinic-N sites (metal-nitrogen may also be included) are known to play important roles in elevating the ORR catalytic performance owing to their low barriers for electron transfer and high selectivity toward the desirable four-electron pathway [23]. In addition, higher relative contents of pyrrolic-N and oxidized-type-N which are less desirable species are found at the surface of IPN9. This fact can further assure the superior catalytic activity of IPN8 to IPN9. Optimising the pyrolysis condition at 800 C may be
applied to increase the relative contents of pyridinic-N and graphitic-N to further improve the catalytic performance of the catalyst. The electro-catalytic properties of the IPNs towards ORR were evaluated using RRDE and RDE technique in oxygen saturated 0.1 M KOH at room temperature. The linear sweep voltammetry curves of the catalysts show that the IPNs follow a similar trend and hence their activities to that of Pt/C (Fig. 6a). Within the samples of IPNs i.e. IPN8 and N8, Fe3PtN
Please cite this article in press as: Huang Y-K, et al., Nanosized-Fe3PtN supported on nitrogen-doped carbon as electro-catalyst for oxygen reduction reaction, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.03.175
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Fig. 6 e (a) ORR polarisation curves of the catalysts in O2-saturated 0.1 M KOH acquired from RDE experiments, (b), (c) plots for electron transfer number and HO¡ 2 yields, respectively, acquired from RRDE experiments, and (d) kinetic current at 0.4 V acquired from KeL equation.
nano-crystalline were confirmed to have a great contribution to catalytic activity. And the N8 sample with single annealed particles shows the least activity. The onset potential, E0, of IPN8, IPN9 and Pt/C are calculated to be 0.947 V, 0.95 V and 0.945 V, respectively, versus RHE. The half-wave potential, E1/ 2, of IPN8, IPN9 and Pt/C are 0.864 V, 0.841 V and 0.831 V, respectively vs RHE. From the E0 and E1/2 values, it can be indicated that the IPN8 and IPN9 both have similar activity in comparison to that of Pt/C with slight lower in the limiting current. The electron transfer number, n, and hydrogen peroxide yield are calculated using disk current and ring current (Fig. 6b and c) in RRDE experiments. Both the IPN8 and IPN9 have the electron transfer number about 4 and peroxide yield about 2e4%, which are very close to those of Pt/C, indicating that the more desirable four electron transfer pathway are taken. The experiment results above have already shown that IPN8 have better catalytic activity than IPN9 and N8 do. It can hence be assumed that active sites number created by the two-step annealing vary with the change in first annealing temperature and that second annealing has a decisive effect on creating Fe3PtN. Lowering the temperature has an adverse effect on the performance of the catalyst. As reported earlier, in such catalyst material, 800 C serves as an optimal temperature [24e27]. In order to understand the mechanism of the electrochemical activity of our catalyst i.e. the most favourable binding sites of the catalyst, further RDE experiment with cyanide ions (CN) in the KOH solution is conducted. CN ion has a stronger tendency of coordinating with Fe and hence preventing O2 molecules to adsorb on the Fe sites for the ORR. RDE tests were carried out with IPN8 in oxygen-saturated 0.1 M KOH in the absence and presence of 10 mM KCN [7].
The ORR polarisation curves (Fig. 7) showed that there was a significant 79 mV negative shift of the half-wave potential of IPN8 when KCN was added to the 0.1 M KOH solution, along with about 12% diminishment of the diffusion-limited current. In addition, the half-wave potential of IPN8 with CN poisoning is close to and even little worse than that of N8 but with much larger current density. These results suggest that (i) Fe sites are definitely involved in ORR as active sites (ii) Pt in the nano-crystallites has a certain effect on promoting the ORR catalytic activity of IPN8 but Pt sites may not be main active sites. As reported earlier [28]: (i) In alkaline media, a special outer sphere electron transfer mechanism which comes from the specifically absorbed hydroxyl species in inner Helmholtz plane (IHP) can bring about a characteristic peak-shaped ring current in the potential range of 0.6 Ve0.9 V. (ii) Reports about Pt-3d transition metal alloy electro-catalysts in alkaline media are fewer than in acidic media, which may because 3d transition metals tend to remain as oxide or hydroxide forms, due to easily absorbed hydroxyl species, with low conductivity and stability at high pH value and this fact will also make the outer sphere electron transfer mechanism dominate. However, from the current experiments on the electro-catalytic activity of Fe3PtN (Figs. 6, 7a and S3), those phenomena weren't observed for IPN8. We deduce that the nitrogen in Fe3PtN play the key role to further modify the alloy system and therefore ease the problems above to give good catalytic performances. The Tafel plots of Pt/C, IPN8 and IPN9 are shown (Fig. 7b). The Tafel slopes of Pt/C, IPN8 and IPN9 are calculated to be 78 mV/dec, 67 mV/dec and 68 mV/dec respectively, suggesting that the as-fabricated electro-catalysts have different rate-
Please cite this article in press as: Huang Y-K, et al., Nanosized-Fe3PtN supported on nitrogen-doped carbon as electro-catalyst for oxygen reduction reaction, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.03.175
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Fig. 7 e (a) ORR polarisation curves of N8 and IPN8 with and without 10 mM CN¡ in O2-saturated 0.1 M KOH, (b) Tafel plots of Pt/C, IPN8 and IPN9.
determining step from that of Pt/C when they catalyse ORR, although all of them were already confirmed to take the fourelectron pathway. As reported earlier [29], Pt are likely to bind oxygen strongly, hence rate-determining step of the whole catalytic reaction would be the second proton transfer and subsequent desorption of an oxygen containing species. The apparent difference of Tafel slopes between IPNs and Pt/C shows the rate-determining step of the IPNs may be the adsorption of gaseous oxygen and subsequent first protonation of an oxygen atom. The smaller Tafel slopes also show that IPNs have better catalytic activity and may be attributed to the Fe3PtN nano-crystalline that speed up the original wellknown rate-determining step of first electron reduction of oxygen, which means Fe3PtN nano-crystalline may have superior back donation ability.
RDE technique was used to further analyse the catalytic ORR process. After doing LSV at various rotating speed, KouteckyeLevich (KeL) plots are obtained at a different potential from 0.3 V to 0.8 V versus RHE (Fig. 8). To more clearly evaluate the potential suitability of IPNs as ORR catalysts, the kinetic limiting current density (JK) was acquired from the intercept of the linearly fitted KeL plots at 0.4 V (Fig. 6d) and at other potentials (Fig. S4). The JK value of IPN8 exceeds the values of IPN9 and Pt/C in whole potential range, especially at low over potential. IPN8 has the JK values of 10.9 mA cm2, 24.6 mA cm2 and 30.9 mA cm2 at 0.8 V, 0.7 V and 0.6 V, respectively, which are significantly larger than the values of Pt/C, indicating that IPN8 can more effectively catalyse the ORR in terms of reaction kinetics. The better catalytic activity of IPN8 than IPN9 may be attributed to the nature and content
Fig. 8 e RDE experiments at different rotating speeds of (a) IPN8, (c) IPN9 and (e) Pt/C, and linearly fitted KeL plots of (b) IPN8, (d) IPN9 and (f) Pt/C. Please cite this article in press as: Huang Y-K, et al., Nanosized-Fe3PtN supported on nitrogen-doped carbon as electro-catalyst for oxygen reduction reaction, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.03.175
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Fig. 9 e Schematic representation of the ORR activity on the N-doped carbon surface in presence of catalyst.
of N species in the catalyst. The presence of pyridinic-Nspecies enhances the performance of IPN8 [30]. Also, presence of pyridinic-N in the carbon facilitates electron transfer from the carbon platform to the oxygen via catalyst surface, hence alleviates the synergic effect between the Fe3PtN particles and the nitrogen doped carbon framework or better joint performance of them which is represented schematically in Fig. 9. Evaluating the data of ORR polarisation curves and kinetic current, expectation can be made that electrochemical performance of IPN8 can be further improved via better structural modification to boost the accessibility of active sites, namely smoother mass transfer of O2 molecules. The durability of as-fabricated IPNs is tested by cycling in oxygen-saturated 0.1 M KOH in the potential range from 1.1 V to 0 V versus RHE. From the results of durability tests, there are potential shifts of 20 mV and 18 mV, respectively, after 3000 cycles (Fig. S5). And current losses about 3.6% and 5.3% at 0.4 V for IPN8 and IPN9, respectively, are also observed. The slightly better durability of IPN9 over IPN8 can be attributed to the higher graphitization degree and order of carbon in IPN9.
Conclusions Novel Fe3PtN nano-crystalline supported on nitrogen-doped carbon materials has been obtained double pyrolysis, under Ar and NH3 atmosphere, sequentially. The first pyrolysis under flowing Ar with different temperatures induced the formation of nitrogen doped carbon framework and affected the N-bonding configurations at the surface. The second pyrolysis under NH3 then further promoted the formation of Fe3PtN nano-crystalline. The electrochemical analysis suggested that the as-prepared catalysts have high catalytic activities, attributed to both nano-crystalline and nitrogen doped carbon, toward ORR in alkaline media with fouryields. More electron pathway taken and low HO 2
importantly, the special electric structure of Fe3PtN nanocrystalline makes Pt-3d transition metal alloy possible to have good catalytic performance and also strengthen the back donation ability.
Acknowledgements This work was supported by the Ministry of Science and Technology of Taiwan (Contract No. MOST 104-2113-M- 002012-MY3).
Appendix A. Supplementary data Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.ijhydene.2017.03.175.
references
[1] Bhatt MD, Geaney H, Nolan M, O'Dwyer C. Key scientific challenges in current rechargeable non-aqueous Li-O2 batteries: experiment and theory. Phys Chem Chem Phys 2014;16(24):12093e130. [2] Debe MK. Electrocatalyst approaches and challenges for automotive fuel cells. Nature 2012;486(7401):43e51. [3] Jaouen F, Proietti E, Lefevre M, Chenitz R, Dodelet J-P, Wu G, et al. Recent advances in non-precious metal catalysis for oxygen-reduction reaction in polymer electrolyte fuel cells. Energy Environ Sci 2011;4(1):114e30. [4] Jasinski R. A new fuel cell cathode catalyst. Nature 1964;201(4925):1212e3. [5] Jiang W-J, Gu L, Li L, Zhang Y, Zhang X, Zhang L-J, et al. Understanding the high activity of FeeNeC electrocatalysts in oxygen reduction: Fe/Fe3C nanoparticles boost the activity of FeeNx. J Am Chem Soc 2016;138(10):3570e8.
Please cite this article in press as: Huang Y-K, et al., Nanosized-Fe3PtN supported on nitrogen-doped carbon as electro-catalyst for oxygen reduction reaction, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.03.175
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 7 ) 1 e9
[6] Liu J, Sun X, Song P, Zhang Y, Xing W, Xu W. Highperformance oxygen reduction electrocatalysts based on cheap carbon black, nitrogen, and trace iron. Adv Mater 2013;25(47):6879e83. [7] Niu W, Li L, Liu X, Wang N, Liu J, Zhou W, et al. Mesoporous N-doped carbons prepared with thermally removable nanoparticle templates: an efficient electrocatalyst for oxygen reduction reaction. J Am Chem Soc 2015;137(16):5555e62. [8] Xiao M, Zhu J, Feng L, Liu C, Xing W. Meso/macroporous nitrogen-doped carbon architectures with iron carbide encapsulated in graphitic layers as an efficient and robust catalyst for the oxygen reduction reaction in both acidic and alkaline solutions. Adv Mater 2015;27(15):2521e7. [9] Liu J, Li E, Ruan M, Song P, Xu W. Recent progress on Fe/N/C electrocatalysts for the oxygen reduction reaction in fuel cells. Catalysts 2015;5(3):1167. [10] Hitotsuyanagi A, Nakamura M, Hoshi N. Structural effects on the activity for the oxygen reduction reaction on n(111)e(100) series of Pt: correlation with the oxide film formation. Electrochim Acta 2012;82:512e6. [11] Gupta G, Slanac DA, Kumar P, Wiggins-Camacho JD, Kim J, Ryoo R, et al. Highly stable Pt/ordered graphitic mesoporous carbon electrocatalysts for oxygen reduction. J Phys Chem C 2010;114(24):10796e805. [12] Singh AK, Xu Q. Synergistic catalysis over bimetallic alloy nanoparticles. ChemCatChem 2013;5(3):652e76. [13] Lu T-H, Chen C-J, Lu Y-R, Dong C-L, Liu R-S. Synergisticeffect-controlled CoTe2/carbon nanotube hybrid material for efficient water oxidation. J Phys Chem C 2016;120(49):28093e9. [14] Shi K-M, Cheng X, Jia Z-Y, Guo J-W, Wang C, Wang J. Oxygen reduction reaction of Fe-polyaniline/carbon nanotube and Pt/C catalysts in alkali media. Int J Hydrogen Energy 2016;41(38):16903e12. rrez A, Ferna ndez AM, Rodrı´guez Varela FJ. [15] Altamirano-Gutie Preparation and characterization of Pt-CeO2 and Pt-Pd electrocatalysts for the oxygen reduction reaction in the absence and presence of methanol in alkaline medium. Int J Hydrogen Energy 2013;38(28):12657e66. F, Strasser P. PtCu3, PtCu and Pt3Cu alloy [16] Oezaslan M, Hasche nanoparticle electrocatalysts for oxygen reduction reaction in alkaline and acidic media. J Electrochem Soc 2012;159(4):B444e54. [17] Serov A, Stariha S, Artyushkova K, Andersen N, Atanassov P. Synthesis of Non-PGM ORR catalysts: control of chemical, morphological and catalytic properties. Meet Abstr 2015;(37):1293. MA2015-02.
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[18] Wu G, More KL, Johnston CM, Zelenay P. High-performance electrocatalysts for oxygen reduction derived from polyaniline, iron, and cobalt. Science 2011;332(6028):443e7. [19] Sanetuntikul J, Chuaicham C, Choi Y-W, Shanmugam S. Investigation of hollow nitrogen-doped carbon spheres as non-precious Fe-N4 based oxygen reduction catalysts. J Mater Chem A 2015;3(30):15473e81. [20] Tuinstra F, Koenig JL. Raman spectrum of graphite. J Chem Phys 1970;53(3):1126e30. [21] Wan K, Long G-F, Liu M-Y, Du L, Liang Z-X, Tsiakaras P. Nitrogen-doped ordered mesoporous carbon: synthesis and active sites for electrocatalysis of oxygen reduction reaction. Appl Catal B 2015;165:566e71. [22] Lai L, Potts JR, Zhan D, Wang L, Poh CK, Tang C, et al. Exploration of the active center structure of nitrogen-doped graphene-based catalysts for oxygen reduction reaction. Energy Environ Sci 2012;5(7):7936e42. [23] Gong K, Du F, Xia Z, Durstock M, Dai L. Nitrogen-doped carbon nanotube arrays with high electrocatalytic activity for oxygen reduction. Science 2009;323(5915):760e4. [24] Huang H, Wang Q, Wei Q, Huang Y. Nitrogen-doped mesoporous carbon derived from copolymer and supporting cobalt oxide for oxygen reduction reaction in alkaline media. Int J Hydrogen Energy 2015;40(18):6072e84. [25] Serov A, Artyushkova K, Atanassov P. Fe-N-C oxygen reduction fuel cell catalyst derived from carbendazim: synthesis, structure, and reactivity. Adv Energy Mater 2014;4(10). 1301735-n/a. [26] He Q, Yang X, Ren X, Koel BE, Ramaswamy N, Mukerjee S, et al. A novel CuFe-based catalyst for the oxygen reduction reaction in alkaline media. J Power Sources 2011;196(18):7404e10. [27] Kong A, Kong Y, Zhu X, Han Z, Shan Y. Ordered mesoporous Fe (or Co)eNegraphitic carbons as excellent non-preciousmetal electrocatalysts for oxygen reduction. Carbon 2014;78:49e59. [28] Ramaswamy N, Mukerjee S. Fundamental mechanistic understanding of electrocatalysis of oxygen reduction on Pt and non-Pt surfaces: acid versus alkaline media. Adv Phys Chem 2012;2012:17. [29] Rossmeisl J, Karlberg GS, Jaramillo T, Norskov JK. Steady state oxygen reduction and cyclic voltammetry. Faraday Discuss 2009;140(0):337e46. zaro Elorri MJ, [30] Iannaci A, Mecheri B, D'Epifanio A, La Licoccia S. Ironenitrogen-functionalized carbon as efficient oxygen reduction reaction electrocatalyst in microbial fuel cells. Int J Hydrogen Energy 2016;41(43):19637e44.
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Nanosized-Fe3PtN supported on nitrogen-doped carbon as electro-catalyst for oxygen reduction reaction Yu-Kai Huang a, Anirudha Jena a,b, Yu-Ting Chen a, Mu-Huai Fang a, Nai-Hsuan Yang a, Ho Chang b,**, Ru-Shi Liu a,b,* a
Department of Chemistry, National Taiwan University, Taipei 106, Taiwan Department of Mechanical Engineering and Graduate Institute of Manufacturing Technology, National Taipei University of Technology, Taipei 106, Taiwan
b
article info
abstract
Article history:
Novel nano-crystalline Fe3PtN supported on nitrogen-doped carbon materials are syn-
Received 23 November 2016
thesised via double pyrolysis, under Ar and NH3 with two sets of temperatures namely, 800
Received in revised form
and 900 C. An improved catalytic activity has been observed in terms of higher values of
16 February 2017
onset and half-wave potentials, with larger kinetic currents and low hydrogen peroxide
Accepted 24 March 2017
yields. The activity upon comparing with Pt/C with simultaneous experiments shows
Available online xxx
impressive results. Within the double annealed samples a comparative study has been done on the basis of active sites available for the oxygen reduction reactions. We have
Keywords:
revealed the origin of its activity by intensively investigating the composition and the
Iron-based catalyst
structure of the catalyst and their correlations with the electrochemical performance.
Oxygen reduction reaction
© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Nitrogen-doped carbon
Introduction Arising demands for energy in an environmental favourable manner is a current genuine concern. Hence developing clean, renewable and sustainable technologies for storage of energy is one of the most widely approached areas of research. Metaleair batteries in particular LieO2, among other energy storing devices, bear characteristics of eco-friendly and higher energy density (11.68 kWh kg1) which is capable of powering mobile vehicles [1]. The operation of the LieO2 battery principally banks on electrochemical reduction of oxygen in presence of a catalytic species. However, factors such as
electrolyte poisoning and production cost have limited the technological advancement [2]. Commercialization of these battery technologies mainly requires efficient electrocatalysts for oxygen reduction reaction (ORR). Varieties of materials have been examined as catalysts for the electrochemical reduction of oxygen including Pt-group and non-Pt group metals. However, the non-Pt group metals can only reach the performance of Pt-group up to 40e50% [3]. Beginning from the work by Jasinski, ‘N’ coordinated to transition metals like Fe, Co has been emerging as promising catalytic materials for ORR [4]. Jiang et al. have reported the role of FeeNx linkage and presence of FeeFe3C nanocrystals in the boost in the catalytic activity [5]. N-coordinated metal
* Corresponding author. Department of Chemistry, National Taiwan University, Taipei 106, Taiwan. Fax: þ886 2 23636359. ** Corresponding author. E-mail addresses: [email protected] (H. Chang), [email protected] (R.-S. Liu). http://dx.doi.org/10.1016/j.ijhydene.2017.03.175 0360-3199/© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Huang Y-K, et al., Nanosized-Fe3PtN supported on nitrogen-doped carbon as electro-catalyst for oxygen reduction reaction, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.03.175
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catalysts supported carbon (MeNeC), especially FeeNeC has shown to have outstanding performance in alkaline media [6e8]. However, their practical implementation still remains a serious challenge [9]. Carbonaceous composites Pt/C, catalysts derived from Ru, and Pd are also commonly studied electrocatalyst for ORR both in alkaline and acidic media. Other modifications to tune the properties of Pt-based materials to further improve electrochemical performance include controlling the active facets [10], creating multi-metallic nanocrystals and using better support materials [11]. Among these multi-metallics, nanocrystals are attractive and practical not only because they can get combined properties from distinct metals but also that some possible synergistic effects may appear. Due to such synergistic effects in case of multimetallic and hybrid catalytic systems are very helpful in reducing catalyst agglomeration and poisoning. This also can alter the electronic structure of the resultant catalyst to stabilise the intermediate species [12]. Lu et al. have reported enhancement of the catalytic performance of CoTe2/CNT hybrid by channelling the charge transfer via CNTs at the interface [13]. So far, many Pt-based multi-metallic alloy systems from the combination with other noble metals or 3d transition metals have been synthesised and studied in acidic media. Mixed oxides, carbonaceous oxide composite and alloy systems especially 3d transition metals in the alkaline medium are also examined in several reports [14,15]. Oezaslan et al. synthesised PtxCuy systems and PtxCoy systems, respectively, and analysed the electrochemical performances both in acidic media and alkaline media [16]. In this work, we have synthesised Fe3PtN nano-crystallites supported on nitrogen doped carbon framework, as the electro-catalyst for ORR in alkaline media in a two-step annealing process. The catalysts synthesised with different annealing temperatures of 800 C and 900 C are compared with their catalytic activities towards ORR. We have also drawn a mechanistic pathway for the active sites of the catalysis.
Experimental Synthesis of (Fe3PtN)eNeC The catalyst material Fe3PtN/C was synthesised following a modified method using sacrificial support involving two annealing steps [17]. All the AR grade precursors were purchased from various commercial sources e.g. FeCl3,6H2O, KSCN, H2PtCl and 1, 10-phenanthroline. Two-step annealing, first under Ar and second under NH3 was used to obtain the final catalyst powder. Such two-step pyrolysis helps in enhancing the activity of catalyst [18]. Briefly, 0.545 g of 1, 10phenanthroline anhydride was dissolved in 10 ml ethanol and subjected to ultra-sonication for few minutes. Then 7.0 g silica colloid (40 wt. %) was added to the resultant solution keeping the mass ratio of phenanthroline to silica be 1:5. The mixture was then stirred at room temperature for about 2 h to form a uniform slurry (Solution A). FeCl3,6H2O of 0.297 g and 0.321 g KSCN were then mixed in aqueous ethanol solution ultrasonically and the resultant solution was then added into
3.363 ml 0.1 M H2PtCl6 solution (solution B). Both the solution A and B are then mixed to get final dispersion which was dried at 60 C to get a solid product. The dried powder (1 g) was then annealed under Ar at a rate of 10 C/min to two temperatures 800 and 900 C for 2 h and are denoted hereafter as IPN8 and IPN9. The remnant silica was etched out using 15% HF and the product then washed repeatedly and dried. The second step annealing of both the samples was done under NH3 at 700 C for 2 h. Both the two-step annealed samples were compared with N8 sample which was synthesised without any Fe and Pt content with first annealing temperature at 800 C.
Characterization of catalyst materials X-ray diffraction (XRD) patterns were collected by a Bruker D2 PHASER XRD with Cu Ka radiation (l ¼ 1.54178 A). Raman spectra were recorded by Thermo-Scientific DXR Raman instrument with a laser wavelength of 532 nm. Transmission electron microscopic (TEM) images were recorded on JEOL1200EX II instrument. Scanning electron microscopy (SEM) images were collected with Hitachi S-4800 instrument. Core level X-ray photoelectron spectra (XPS) were acquired from ULVAC-PHI, Inc. using Al Ka source. ORR activity was studied using a rotating disk electrode (RDE) and a rotating ring-disk electrode (RRDE) in a standard three-electrode configuration. Saturated calomel electrode (SCE) used as a reference electrode and a Pt foil (1 cm2) as a counter electrode. A slurry ink was prepared by dispersing 5 mg catalyst in 20 mL Nafion (5 wt. %) solution, 500 mL isopropanol and 250 mL deionized water. The homogeneous dispersion was then drop-casted on the glassy carbon electrode surface with a loading of 0.2 mg cm2 and 0.25 mg cm2 for RRDE and RDE experiments respectively. To compare the results, Pt/C (20 wt.%) were also prepared in a similar manner with the loading of 0.1 mg cm1 and 0.125 mg cm1 for the RRDE and RDE experiments respectively. All RDE and RRDE experiments were conducted at room temperature in oxygensaturated 0.1 M KOH electrolyte. After initial 20 cycles in oxygen-saturated 0.1 M KOH in a potential window of 1.1 to 0 V vs RHE at a scan rate of 20 mV s1, RRDE tests were conducted at a scan rate of 10 mV s1 at a rotation speed of 1600 rpm. RDE tests were performed in a similar manner, with varying speed of rotation of 800 rpm, 1200 rpm, 1600 rpm, 2000 rpm and 2400 rpm. The durability tests were tested by cycling in oxygen-saturated 0.1 M KOH in the potential range from 1.1 V to 0 V versus RHE. The initial LSVs were compared with those after 3000 cycles. The electron transfer number and HO 2 yield derived from RRDE tests are calculated using equations (1) and (2), [19]. HO2 ð%Þ ¼ 200
n¼4
ID ID þ IR =N
IR =N ID þ IR =N
(1)
(2)
where ID is the disk current, IR is the ring current which came from the re-oxidation of HO2 species back to O2 at 1.3 V versus RHE and N is the collection efficiency of 0.37.
Please cite this article in press as: Huang Y-K, et al., Nanosized-Fe3PtN supported on nitrogen-doped carbon as electro-catalyst for oxygen reduction reaction, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.03.175
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The kinetic current density JK was acquired from the intercept of the KouteckyeLevich plots basing on the KeL equation (3) as below, [19]: 1 1 1 1 1 ¼ þ ¼ þ J JL JK JK Bu1=2
(3)
where J is the experimentally measured current density, JL is the limiting current density, JK is the kinetic current, B is a parameter and u is the rotating speed of the electrode.
Results and discussion XRD patterns of both IPN8 and IPN9 are shown in Fig. 1. Both the XRD profiles follow similar broad patterns indicating Fe3PtN particles composed of nanometre-sized grains. As calculated using Scherer's equation the crystallite sizes are in the range of ~9 nm for both the IPN8 and IPN9. The XRD profiles further refined using Pawley refinement using Total Pattern Analysis Solution (TOPAS) software. Profiles of both the samples are indexed to the cubic Fe3PtN phase space group of Pm3m (Fig. S1). The lattice parameter a for the Fe3PtN crystals in IPN8 and IPN9 are 3.87142(44) and 3.88039(69) A respectively. The absence of peaks related to carbon around two theta 20 is an indication of amorphous carbon in both the sample IPN8 and IPN9.
Fig. 2 e Raman spectra of powder samples IPN8 and IPN9.
Raman spectroscopy was used to investigate the nature of the carbon in both the samples. The spectra of IPN8 and IPN9 are similar and show two main peaks assigned as D and G of carbon (Fig. 2). The D band at 1360 cm1 is related to the carbon with structural disorders and defects (sp3); as for the G band at 1590 cm1, it represents the ordered graphitic carbon
Fig. 1 e XRD patterns of the catalyst powder samples (a) IPN8, (b) IPN9, and (c) atomic arrangements in Fe3PtN crystal. Please cite this article in press as: Huang Y-K, et al., Nanosized-Fe3PtN supported on nitrogen-doped carbon as electro-catalyst for oxygen reduction reaction, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.03.175
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sheet (sp2) [20]. Broadening of the D band and G band in “carbon peaks” indicates that carbon present in very fine particle size. The peak intensity ratio of D band and G band, namely ID/IG, provides an identification of overall carbon framework about the degree of disorder. The ID/IG ratio of IPN8 and IPN9 are 1.138 and 0.962, respectively. Therefore, the carbon framework of IPN9 is more ordered than the counterpart of IPN8. This suggests that higher first pyrolysis temperature induces a higher degree of graphitization and promote the formation of more ordered graphitic structure, as explained in the previous report [19]. Subsequent pyrolysis under NH3 creates disordered C-centres which facilitate catalytic activities. But, in this study, both the carbon frameworks of IPNs are still pretty ordered. In Fig. S1, the effect of the second pyrolysis under NH3 atmosphere is then clarified to mainly promote the formation of Fe3PtN nano-crystallites. However, the durability of IPN9 can be expected to be better than IPN8, because higher ordered graphitic structure which has a positive effect on durability. SEM and TEM were used to check the morphology of the samples. From low-resolution SEM images in Fig. 3a and c, both the samples show agglomerations collaging to sheet-like structures. Such agglomeration may be brought about by the strong interaction between carbon nano-crystallites. Notably, there is no significant effect of different first pyrolysis temperatures on the morphology. High-resolution SEM images show the presence of bright spots on each of the agglomerated sheets as shown in Fig. 3b and d indicating embedded nanocrystallites on the carbon matrix. Low-resolution TEM images in Fig. 4a and b, show a uniform distribution of nano-crystalline particles embedded in the amorphous matrix. The calculated particles size can be seen in the distribution plot in the inset which is in the range 7e13 nm. Comparing between the contrasts it can be assumed
that the nano-crystallites are holding the carbon sheets which are very thin and transparent to the electron beam as shown by the arrow in the TEM images. High-resolution TEM images in Fig. 4c and d further show resolved fringes of the nanocrystalline IPN8 and IPN9. They are indexed to the (111) plane corresponding d spacing of 2.22 A of Fe3PtN. From both SEM and TEM it can be confirmed that the nano-crystalline Fe3PtN have been successfully implanted on the carbon matrix using the solution technique. Core level XPS were recorded to analyse elemental compositions and the nature of N-bonding in both the catalyst samples (Fig. S2). Nitrogen content decreases from at% 4.8 to 2.7 when the first pyrolysis temperature increase from 800 C to 900 C, which can be expected because higher pyrolysis temperature is likely to induce decomposition of nitrogen species in carbon frameworks [21]. The N1s spectra of the catalysts were further deconvoluted to examine different chemical states of nitrogen species with different binding energies, including pyridinic-N (398.5 eV), pyrrolic-N (400.2 eV), graphitic-N (401.4 eV) and oxidized-type-N (402 eVe405 eV), which also suggests the formation of nitrogen doped carbon framework (Fig. 5). The peak at a binding energy of 398.5 eV is supposed to also include a contribution from nitrogen bound to the metal (metalenitrogen), because of the small difference between the binding energies of nitrogenemetal and pyridinic-N. The contents of different Nbonding configurations can be quantified via the integrated area of the peaks (Table S1). Notably, the relative contents of graphitic-N dominate over those of other N-species both in IPN8 and IPN9. The relative content of pyridinic-N diminishes with higher first pyrolysis temperature, which indicates that pyridinic-N are less thermal stable than graphitic-N [22]. The higher overall content of pyridinic-N and graphitic-N suggest that the ORR performance of IPN8 may be better than that of
Fig. 3 e SEM images of samples IPN8 and IPN9 (a, c) low resolution, and (b, d) high-resolution images. Please cite this article in press as: Huang Y-K, et al., Nanosized-Fe3PtN supported on nitrogen-doped carbon as electro-catalyst for oxygen reduction reaction, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.03.175
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Fig. 4 e TEM images of samples IPN8 and IPN9 (a, b) low resolution, particle size distribution (inset) and (c, d) high-resolution images.
Fig. 5 e Deconvoluted XPS core level spectra of N 1s of (a) IPN8, and (b) IPN9.
IPN9 because graphitic and pyridinic-N sites (metal-nitrogen may also be included) are known to play important roles in elevating the ORR catalytic performance owing to their low barriers for electron transfer and high selectivity toward the desirable four-electron pathway [23]. In addition, higher relative contents of pyrrolic-N and oxidized-type-N which are less desirable species are found at the surface of IPN9. This fact can further assure the superior catalytic activity of IPN8 to IPN9. Optimising the pyrolysis condition at 800 C may be
applied to increase the relative contents of pyridinic-N and graphitic-N to further improve the catalytic performance of the catalyst. The electro-catalytic properties of the IPNs towards ORR were evaluated using RRDE and RDE technique in oxygen saturated 0.1 M KOH at room temperature. The linear sweep voltammetry curves of the catalysts show that the IPNs follow a similar trend and hence their activities to that of Pt/C (Fig. 6a). Within the samples of IPNs i.e. IPN8 and N8, Fe3PtN
Please cite this article in press as: Huang Y-K, et al., Nanosized-Fe3PtN supported on nitrogen-doped carbon as electro-catalyst for oxygen reduction reaction, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.03.175
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Fig. 6 e (a) ORR polarisation curves of the catalysts in O2-saturated 0.1 M KOH acquired from RDE experiments, (b), (c) plots for electron transfer number and HO¡ 2 yields, respectively, acquired from RRDE experiments, and (d) kinetic current at 0.4 V acquired from KeL equation.
nano-crystalline were confirmed to have a great contribution to catalytic activity. And the N8 sample with single annealed particles shows the least activity. The onset potential, E0, of IPN8, IPN9 and Pt/C are calculated to be 0.947 V, 0.95 V and 0.945 V, respectively, versus RHE. The half-wave potential, E1/ 2, of IPN8, IPN9 and Pt/C are 0.864 V, 0.841 V and 0.831 V, respectively vs RHE. From the E0 and E1/2 values, it can be indicated that the IPN8 and IPN9 both have similar activity in comparison to that of Pt/C with slight lower in the limiting current. The electron transfer number, n, and hydrogen peroxide yield are calculated using disk current and ring current (Fig. 6b and c) in RRDE experiments. Both the IPN8 and IPN9 have the electron transfer number about 4 and peroxide yield about 2e4%, which are very close to those of Pt/C, indicating that the more desirable four electron transfer pathway are taken. The experiment results above have already shown that IPN8 have better catalytic activity than IPN9 and N8 do. It can hence be assumed that active sites number created by the two-step annealing vary with the change in first annealing temperature and that second annealing has a decisive effect on creating Fe3PtN. Lowering the temperature has an adverse effect on the performance of the catalyst. As reported earlier, in such catalyst material, 800 C serves as an optimal temperature [24e27]. In order to understand the mechanism of the electrochemical activity of our catalyst i.e. the most favourable binding sites of the catalyst, further RDE experiment with cyanide ions (CN) in the KOH solution is conducted. CN ion has a stronger tendency of coordinating with Fe and hence preventing O2 molecules to adsorb on the Fe sites for the ORR. RDE tests were carried out with IPN8 in oxygen-saturated 0.1 M KOH in the absence and presence of 10 mM KCN [7].
The ORR polarisation curves (Fig. 7) showed that there was a significant 79 mV negative shift of the half-wave potential of IPN8 when KCN was added to the 0.1 M KOH solution, along with about 12% diminishment of the diffusion-limited current. In addition, the half-wave potential of IPN8 with CN poisoning is close to and even little worse than that of N8 but with much larger current density. These results suggest that (i) Fe sites are definitely involved in ORR as active sites (ii) Pt in the nano-crystallites has a certain effect on promoting the ORR catalytic activity of IPN8 but Pt sites may not be main active sites. As reported earlier [28]: (i) In alkaline media, a special outer sphere electron transfer mechanism which comes from the specifically absorbed hydroxyl species in inner Helmholtz plane (IHP) can bring about a characteristic peak-shaped ring current in the potential range of 0.6 Ve0.9 V. (ii) Reports about Pt-3d transition metal alloy electro-catalysts in alkaline media are fewer than in acidic media, which may because 3d transition metals tend to remain as oxide or hydroxide forms, due to easily absorbed hydroxyl species, with low conductivity and stability at high pH value and this fact will also make the outer sphere electron transfer mechanism dominate. However, from the current experiments on the electro-catalytic activity of Fe3PtN (Figs. 6, 7a and S3), those phenomena weren't observed for IPN8. We deduce that the nitrogen in Fe3PtN play the key role to further modify the alloy system and therefore ease the problems above to give good catalytic performances. The Tafel plots of Pt/C, IPN8 and IPN9 are shown (Fig. 7b). The Tafel slopes of Pt/C, IPN8 and IPN9 are calculated to be 78 mV/dec, 67 mV/dec and 68 mV/dec respectively, suggesting that the as-fabricated electro-catalysts have different rate-
Please cite this article in press as: Huang Y-K, et al., Nanosized-Fe3PtN supported on nitrogen-doped carbon as electro-catalyst for oxygen reduction reaction, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.03.175
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Fig. 7 e (a) ORR polarisation curves of N8 and IPN8 with and without 10 mM CN¡ in O2-saturated 0.1 M KOH, (b) Tafel plots of Pt/C, IPN8 and IPN9.
determining step from that of Pt/C when they catalyse ORR, although all of them were already confirmed to take the fourelectron pathway. As reported earlier [29], Pt are likely to bind oxygen strongly, hence rate-determining step of the whole catalytic reaction would be the second proton transfer and subsequent desorption of an oxygen containing species. The apparent difference of Tafel slopes between IPNs and Pt/C shows the rate-determining step of the IPNs may be the adsorption of gaseous oxygen and subsequent first protonation of an oxygen atom. The smaller Tafel slopes also show that IPNs have better catalytic activity and may be attributed to the Fe3PtN nano-crystalline that speed up the original wellknown rate-determining step of first electron reduction of oxygen, which means Fe3PtN nano-crystalline may have superior back donation ability.
RDE technique was used to further analyse the catalytic ORR process. After doing LSV at various rotating speed, KouteckyeLevich (KeL) plots are obtained at a different potential from 0.3 V to 0.8 V versus RHE (Fig. 8). To more clearly evaluate the potential suitability of IPNs as ORR catalysts, the kinetic limiting current density (JK) was acquired from the intercept of the linearly fitted KeL plots at 0.4 V (Fig. 6d) and at other potentials (Fig. S4). The JK value of IPN8 exceeds the values of IPN9 and Pt/C in whole potential range, especially at low over potential. IPN8 has the JK values of 10.9 mA cm2, 24.6 mA cm2 and 30.9 mA cm2 at 0.8 V, 0.7 V and 0.6 V, respectively, which are significantly larger than the values of Pt/C, indicating that IPN8 can more effectively catalyse the ORR in terms of reaction kinetics. The better catalytic activity of IPN8 than IPN9 may be attributed to the nature and content
Fig. 8 e RDE experiments at different rotating speeds of (a) IPN8, (c) IPN9 and (e) Pt/C, and linearly fitted KeL plots of (b) IPN8, (d) IPN9 and (f) Pt/C. Please cite this article in press as: Huang Y-K, et al., Nanosized-Fe3PtN supported on nitrogen-doped carbon as electro-catalyst for oxygen reduction reaction, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.03.175
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Fig. 9 e Schematic representation of the ORR activity on the N-doped carbon surface in presence of catalyst.
of N species in the catalyst. The presence of pyridinic-Nspecies enhances the performance of IPN8 [30]. Also, presence of pyridinic-N in the carbon facilitates electron transfer from the carbon platform to the oxygen via catalyst surface, hence alleviates the synergic effect between the Fe3PtN particles and the nitrogen doped carbon framework or better joint performance of them which is represented schematically in Fig. 9. Evaluating the data of ORR polarisation curves and kinetic current, expectation can be made that electrochemical performance of IPN8 can be further improved via better structural modification to boost the accessibility of active sites, namely smoother mass transfer of O2 molecules. The durability of as-fabricated IPNs is tested by cycling in oxygen-saturated 0.1 M KOH in the potential range from 1.1 V to 0 V versus RHE. From the results of durability tests, there are potential shifts of 20 mV and 18 mV, respectively, after 3000 cycles (Fig. S5). And current losses about 3.6% and 5.3% at 0.4 V for IPN8 and IPN9, respectively, are also observed. The slightly better durability of IPN9 over IPN8 can be attributed to the higher graphitization degree and order of carbon in IPN9.
Conclusions Novel Fe3PtN nano-crystalline supported on nitrogen-doped carbon materials has been obtained double pyrolysis, under Ar and NH3 atmosphere, sequentially. The first pyrolysis under flowing Ar with different temperatures induced the formation of nitrogen doped carbon framework and affected the N-bonding configurations at the surface. The second pyrolysis under NH3 then further promoted the formation of Fe3PtN nano-crystalline. The electrochemical analysis suggested that the as-prepared catalysts have high catalytic activities, attributed to both nano-crystalline and nitrogen doped carbon, toward ORR in alkaline media with fouryields. More electron pathway taken and low HO 2
importantly, the special electric structure of Fe3PtN nanocrystalline makes Pt-3d transition metal alloy possible to have good catalytic performance and also strengthen the back donation ability.
Acknowledgements This work was supported by the Ministry of Science and Technology of Taiwan (Contract No. MOST 104-2113-M- 002012-MY3).
Appendix A. Supplementary data Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.ijhydene.2017.03.175.
references
[1] Bhatt MD, Geaney H, Nolan M, O'Dwyer C. Key scientific challenges in current rechargeable non-aqueous Li-O2 batteries: experiment and theory. Phys Chem Chem Phys 2014;16(24):12093e130. [2] Debe MK. Electrocatalyst approaches and challenges for automotive fuel cells. Nature 2012;486(7401):43e51. [3] Jaouen F, Proietti E, Lefevre M, Chenitz R, Dodelet J-P, Wu G, et al. Recent advances in non-precious metal catalysis for oxygen-reduction reaction in polymer electrolyte fuel cells. Energy Environ Sci 2011;4(1):114e30. [4] Jasinski R. A new fuel cell cathode catalyst. Nature 1964;201(4925):1212e3. [5] Jiang W-J, Gu L, Li L, Zhang Y, Zhang X, Zhang L-J, et al. Understanding the high activity of FeeNeC electrocatalysts in oxygen reduction: Fe/Fe3C nanoparticles boost the activity of FeeNx. J Am Chem Soc 2016;138(10):3570e8.
Please cite this article in press as: Huang Y-K, et al., Nanosized-Fe3PtN supported on nitrogen-doped carbon as electro-catalyst for oxygen reduction reaction, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.03.175
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 7 ) 1 e9
[6] Liu J, Sun X, Song P, Zhang Y, Xing W, Xu W. Highperformance oxygen reduction electrocatalysts based on cheap carbon black, nitrogen, and trace iron. Adv Mater 2013;25(47):6879e83. [7] Niu W, Li L, Liu X, Wang N, Liu J, Zhou W, et al. Mesoporous N-doped carbons prepared with thermally removable nanoparticle templates: an efficient electrocatalyst for oxygen reduction reaction. J Am Chem Soc 2015;137(16):5555e62. [8] Xiao M, Zhu J, Feng L, Liu C, Xing W. Meso/macroporous nitrogen-doped carbon architectures with iron carbide encapsulated in graphitic layers as an efficient and robust catalyst for the oxygen reduction reaction in both acidic and alkaline solutions. Adv Mater 2015;27(15):2521e7. [9] Liu J, Li E, Ruan M, Song P, Xu W. Recent progress on Fe/N/C electrocatalysts for the oxygen reduction reaction in fuel cells. Catalysts 2015;5(3):1167. [10] Hitotsuyanagi A, Nakamura M, Hoshi N. Structural effects on the activity for the oxygen reduction reaction on n(111)e(100) series of Pt: correlation with the oxide film formation. Electrochim Acta 2012;82:512e6. [11] Gupta G, Slanac DA, Kumar P, Wiggins-Camacho JD, Kim J, Ryoo R, et al. Highly stable Pt/ordered graphitic mesoporous carbon electrocatalysts for oxygen reduction. J Phys Chem C 2010;114(24):10796e805. [12] Singh AK, Xu Q. Synergistic catalysis over bimetallic alloy nanoparticles. ChemCatChem 2013;5(3):652e76. [13] Lu T-H, Chen C-J, Lu Y-R, Dong C-L, Liu R-S. Synergisticeffect-controlled CoTe2/carbon nanotube hybrid material for efficient water oxidation. J Phys Chem C 2016;120(49):28093e9. [14] Shi K-M, Cheng X, Jia Z-Y, Guo J-W, Wang C, Wang J. Oxygen reduction reaction of Fe-polyaniline/carbon nanotube and Pt/C catalysts in alkali media. Int J Hydrogen Energy 2016;41(38):16903e12. rrez A, Ferna ndez AM, Rodrı´guez Varela FJ. [15] Altamirano-Gutie Preparation and characterization of Pt-CeO2 and Pt-Pd electrocatalysts for the oxygen reduction reaction in the absence and presence of methanol in alkaline medium. Int J Hydrogen Energy 2013;38(28):12657e66. F, Strasser P. PtCu3, PtCu and Pt3Cu alloy [16] Oezaslan M, Hasche nanoparticle electrocatalysts for oxygen reduction reaction in alkaline and acidic media. J Electrochem Soc 2012;159(4):B444e54. [17] Serov A, Stariha S, Artyushkova K, Andersen N, Atanassov P. Synthesis of Non-PGM ORR catalysts: control of chemical, morphological and catalytic properties. Meet Abstr 2015;(37):1293. MA2015-02.
9
[18] Wu G, More KL, Johnston CM, Zelenay P. High-performance electrocatalysts for oxygen reduction derived from polyaniline, iron, and cobalt. Science 2011;332(6028):443e7. [19] Sanetuntikul J, Chuaicham C, Choi Y-W, Shanmugam S. Investigation of hollow nitrogen-doped carbon spheres as non-precious Fe-N4 based oxygen reduction catalysts. J Mater Chem A 2015;3(30):15473e81. [20] Tuinstra F, Koenig JL. Raman spectrum of graphite. J Chem Phys 1970;53(3):1126e30. [21] Wan K, Long G-F, Liu M-Y, Du L, Liang Z-X, Tsiakaras P. Nitrogen-doped ordered mesoporous carbon: synthesis and active sites for electrocatalysis of oxygen reduction reaction. Appl Catal B 2015;165:566e71. [22] Lai L, Potts JR, Zhan D, Wang L, Poh CK, Tang C, et al. Exploration of the active center structure of nitrogen-doped graphene-based catalysts for oxygen reduction reaction. Energy Environ Sci 2012;5(7):7936e42. [23] Gong K, Du F, Xia Z, Durstock M, Dai L. Nitrogen-doped carbon nanotube arrays with high electrocatalytic activity for oxygen reduction. Science 2009;323(5915):760e4. [24] Huang H, Wang Q, Wei Q, Huang Y. Nitrogen-doped mesoporous carbon derived from copolymer and supporting cobalt oxide for oxygen reduction reaction in alkaline media. Int J Hydrogen Energy 2015;40(18):6072e84. [25] Serov A, Artyushkova K, Atanassov P. Fe-N-C oxygen reduction fuel cell catalyst derived from carbendazim: synthesis, structure, and reactivity. Adv Energy Mater 2014;4(10). 1301735-n/a. [26] He Q, Yang X, Ren X, Koel BE, Ramaswamy N, Mukerjee S, et al. A novel CuFe-based catalyst for the oxygen reduction reaction in alkaline media. J Power Sources 2011;196(18):7404e10. [27] Kong A, Kong Y, Zhu X, Han Z, Shan Y. Ordered mesoporous Fe (or Co)eNegraphitic carbons as excellent non-preciousmetal electrocatalysts for oxygen reduction. Carbon 2014;78:49e59. [28] Ramaswamy N, Mukerjee S. Fundamental mechanistic understanding of electrocatalysis of oxygen reduction on Pt and non-Pt surfaces: acid versus alkaline media. Adv Phys Chem 2012;2012:17. [29] Rossmeisl J, Karlberg GS, Jaramillo T, Norskov JK. Steady state oxygen reduction and cyclic voltammetry. Faraday Discuss 2009;140(0):337e46. zaro Elorri MJ, [30] Iannaci A, Mecheri B, D'Epifanio A, La Licoccia S. Ironenitrogen-functionalized carbon as efficient oxygen reduction reaction electrocatalyst in microbial fuel cells. Int J Hydrogen Energy 2016;41(43):19637e44.
Please cite this article in press as: Huang Y-K, et al., Nanosized-Fe3PtN supported on nitrogen-doped carbon as electro-catalyst for oxygen reduction reaction, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.03.175