Nitrogen-doped porous carbon nanofiber based oxygen reduction reaction electrocatalysts with high activity and durability

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Nitrogen-doped porous carbon nanofiber based oxygen reduction reaction electrocatalysts with high activity and durability Yang Wang 1, Junhong Jin, Shenglin Yang, Guang Li*, Jianming Jiang State Key Laboratory for Modification of Chemical Fibers and Polymer Materials, College of Materials Science and Engineering, Donghua University, Shanghai, PR China

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abstract

Article history:

Sluggish oxygen reduction reaction (ORR) is one of the critical challenges in polymer

Received 29 February 2016

electrolyte membrane fuel cell (PEMFC) technologies. Carbon materials doped with various

Received in revised form

heteroatoms have been exactly proved as promising alternative catalysts and even the

26 April 2016

catalyst supports for the ORR in fuel cells. In this work, we have developed nitrogen-doped

Accepted 27 April 2016

porous carbon nanofibers (N-PCNF) as novel ORR catalysts. The obtained N-PCNF shows

Available online xxx

excellent electrocatalytic performance and durability for ORR both in basic and acid solutions. Furthermore, the N-PCNF acted as support to deposited platinum (Pt) nano-

Keywords:

particles, the Pt nanoparticles with uniform size were well dispersed not only on the

N-doped porous carbon nanofiber

surface but also the cross-section of N-PCNF. The electrocatalytic activity and stability of

(N-PCNF)

the resultant Pt/N-PCNF along with the commercial one (JM20) were investigated. As a

Electrospinning

result, the Pt/N-PCNF exhibited enhanced ORR activity when compared with Pt supported

Pt supported on nitrogen-doped

on PCNF (Pt/PCNF) as well as the state-of-the-art JM20. In addition, enhanced stability of Pt/

porous carbon nanofiber

N-PCNF, coupled with ORR activity and electrochemical surface area (ECSA) retention after

(Pt/N-PCNF)

accelerated durability test (ADT) in acid media was observed. These results indicated that

Oxygen reduction reaction (ORR)

the N-PCNF can not only work as the non-precious electrocatalysts toward ORR, but also a

Electrocatalysts

promising candidate as catalyst supports. © 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

Introduction Polymer electrolyte membrane fuel cells (PEMFCs) are regarded as the reliable sustainable energy conversion devices due to their outstanding advantages including excellent energy densities, high energy conversion efficiencies and zero byproduct emissions [1e4]. One of the most critical challenges for widespread commercialization of PEMFC especially in the automotive infrastructure is the development of high active, excellent durability, and low price electrocatalyst for the

sluggish oxygen reduction reaction (ORR) [5e8]. Until now, Ptbased electrocatalysts along with Pt-based alloys have been generally considered as the most suitable ORR electrocatalysts [9e11]. However, they still suffer from various disadvantages such as low earth-abundance, high cost, and poor durability during long time operation of fuel cells [1]. Thus, designing alternative ORR electrocatalysts with high catalytic performance to supersede Pt or reduce the Pt dosage is of great interests [9]. Unflagging efforts have been focused on several aspects, including Pt nano-structure controlling [12e15],

* Corresponding author. Tel.: þ86 21 67792830; fax: þ86 21 67792855. E-mail addresses: [email protected] (Y. Wang), [email protected] (G. Li). 1 Tel.: þ86 21 67792798. http://dx.doi.org/10.1016/j.ijhydene.2016.04.235 0360-3199/© 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Wang Y, et al., Nitrogen-doped porous carbon nanofiber based oxygen reduction reaction electrocatalysts with high activity and durability, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/ j.ijhydene.2016.04.235

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supports fabrication [4,16e18], and even non-precious metal electrocatalysts design [19e25]. Hitherto, carbonaceous materials doped with various heteroatoms have been confirmed as an efficient way to prepare non-precious electrocatalysts for ORR [10,26e29]. Numerous researchers reported that the non-precious electrocatalysts can be obtained by doping carbon-based materials with heteroatoms (including N, S, B, Fe or Co) [20,24]. Among them, nitrogen-doped carbon materials including N-doped carbon nanotubes (NCNT) [25,30], N-doped carbon nanographene (NG) [23,24,31e33], and N-doped carbon nanospheres [34] have been widely prepared, investigated and always exhibited excellent catalytic performance for ORR. Most of the studies are focused on the nitrogen-doped carbon nanotubes and graphenes to date, however, few investigations are conducted on the carbon nanofibers [35e39]. Carbon nanofibers are regarded as a promising conducting material because they act as efficient pathways for the electrons transporting through freely [40]. Liu et al. [35] found that urea-treated carbon nanofibers can be used as efficient catalytic materials for ORR. The N-doped carbon nanofibers exhibited much better catalytic activity and durability than the traditional carbon nanofibers. Qiu and coworkers [41] investigated three different surface modification methods such as acidification with a H2SO4/HNO3 mixture, heat treatment in NH3, and the combined treatment of the above two process to prepare the N-doped carbon nanofibers. As a result, the combined technique achieved much better catalytic performance both in acid solutions and alkaline solutions. These studies do confirmed the carbon nanofibers as efficient catalysts toward catalyzing ORR. Taking into account that the catalytic activity is largely depends on the catalytic sites density on the supports surface. Introducing porosity and defects on the surface of fibers for increasing the special surface area, thus improve the catalytic sites density seems to be effective [37]. Electrospinning is an easy, reliably, and cost-effectively technique to prepare fibers with thin diameter into nanometer size [37]. The porous carbon nanofibers can be fabricated by electrospinning suitable polymer blends solution containing carbon precursor polymer (CPP) and thermally decomposable polymer (TDP) followed by carbonization at relatively high temperatures. Dae-Soo Yang et al. [40] used the solution mixed poly(acrylonitrile) (PAN) with poly(ethylene oxide) (PEO) for electrospinning nanofibers and followed by carbonization and thus the N-doped porous carbon nanofibers have been successfully obtained. In their process the porosity can be easily tuned via varying the radio of the two materials. They found the favorable sample for improved ORR performance can be prepared when the solution ratio is 1:1 and carbonization at 1000  C. Their study indicated that the trade-offs between electrical conductivity and nitrogen content can be important for the active sites density which do befit the ORR activity. Our group has already successfully prepared the porous carbon nanofibers (PCNF) derived from electrospinning poly(acrylonitrile) (PAN)/poly (methyl methacrylate) (PMMA) followed by carbonization with suitable diameter and controlled porosity, and even used as the electromagnetic wave absorption materials [42], cosupport for commercial electrocatalyst [43], and the electrocatlyst support for PEMFC [44].

In the present work, we developed nitrogen-doped porous carbon nanofibers (N-PCNF) by thermal treatment PCNF with dicyandiamide (DCDA) as the nitrogen resource and investigated the electrocatalytic activity and durability of N-PCNF for ORR both in basic and acid media; meanwhile N-PCNF was used as the support to deposited Pt nanoparticles. The nature of Pt/N-PCNF, along with Pt/PCNF and JM20 was characterized. Their electrocatalystic activity and durability toward ORR were also measured and discussed.

Experimental Preparation of the nitrogen-doped carbon nanofibers (NPCNF) The PCNFs and pristine CNFs were prepared as we reported before [43]. Briefly, the spinning solution was consist of 70wt polyacrylonitrile (PAN) and 30wt polymethylmethacrylate (PMMA). The diameter of capillary tip was about 0.40 mm. Electrospinning process was carried out under a conduct difference of 18 kV provided by a variable high voltage power supply and an extrusion rate of 15 ml/min controlled by a syringe pump. The nanofibers were collected on the grounded aluminum foil placed on the surface of an adjustable lab jack as the target. The distance between the target and the tip of the syringe needle was 18 cm, ambient temperature and relative humidity were kept at 25  C and 30% respectively. The obtained mat was dried under vacuum overnight to remove the residual solvents. The pre-oxidation process was carried out at 280  C for 1 h and followed by carbonization. The N-PCNFs were prepared by the by thermal treatment PCNFs with dicyandiamide (DCDA) [24]. Typically, 50 mg PCNFs, 200 mg DCDA and 100 mg FeCl3$6H2O were added into 80 mL ethanol and followed by ultrasonic for 1 h. The mixture was continuously stirred for another 24 h at 60  C and dried. The dried sample was then calcined at 900  C for 1 h under N2 atmosphere. After cooling down, the sample was poured into 10% HCl solution and stirred for 24 h to remove the inactive metal species and subsequently the product was heat treated again as above.

Synthesis of N-PCNF supported platinum nanoparticles (Pt/ N-PCNF) Pt was deposited onto N-PCNF and PCNF using a traditional ethylene glycol (EG) technique [44]. Briefly, 80 mg of carbon materials were first added into a flask with 50 mL of EG under ultrasonic treatment for 1 h to form a homogeneous slurry, a certain amount of H2PtCl6$6H2O aqueous solution (7.53 mg mL1 of Pt) was then slowly added to the slurry to ensure 20wt% Pt content, followed by ultrasonic and stirred for 1 h. And then the slurry was heated to 130  C and maintained for 3 h in N2 atmosphere under reflux conditions. After cooling down, the slurry were separated in the centrifuge and washed several times. Finally, the sample was dried at 80  C in vacuum oven for 12 h.

Please cite this article in press as: Wang Y, et al., Nitrogen-doped porous carbon nanofiber based oxygen reduction reaction electrocatalysts with high activity and durability, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/ j.ijhydene.2016.04.235

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Physical characterization The morphology and structure of the samples were examined with JEOL-2100F transmission electron microscopy (TEM). Xray photoelectron spectrometry (XPS) (PHI-5400) was used to determine the electronic structure of surfaces for the supports as well as the valence state of Pt.

Electrochemical measurements Three-electrode cell coupled with an electrochemical workstation (CHI760D) was used for electrochemical experiments at room temperature. The working electrodes were prepared by dropping sample powders on the glassy carbon working ring disk electrode (RDE: ~0.247 cm2). A Pt wire was used as the counter electrode and saturated calomel electrode (SCE) was acted as the reference electrode. All potentials in this work are converted to the reversible hydrogen electrode (RHE), based on the conversion equation, ERHE ¼ ESCE þ 0.241 V þ 0.059  pH [45]. The peroxide percentage (H2O2%) was obtained by employing the RRDE measurement and calculating based on the disk current (Id) as well as the ring current (Ir) via the following equation: H2 O2 % ¼ 100  Id2IþIr =N ; The electron r =N transfer number (n) was based on the following equation: 4Id ; where N ¼ 0.37 is the current collection efficiency 5n ¼ Id þI r =N of Pt ring. The un-supported catalyst ink was prepared by ultrasonically blending 4 mg of catalyst in 1 mL ethanol with 8 mL of 5% Nafion® for 1 h. Before measurement, 30 mL of ink was dropped to the disk with the catalyst loading of about 0.485 mg cm2. Commercial electrocatalyst JM20 was used as the reference and the total loading was same as above. The CV experiments were carried out in the oxygen-saturated 0.1 M KOH (or 0.5 M H2SO4) solution for ORR with a scan rate of 50 mV s1 in the potential range 1.0 V to 0.2 V (or 0.24 V to 0.96 V). LSV measurement with RRDE were performed in the oxygensaturated 0.1 M KOH (or 0.5 M H2SO4) solution at a rotation speed of 1600 rpm with the scan rate of 5 mV s1 in the

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potential range 0.2 Ve1.0 V (or 0.8 V to 0.3 V). Accelerated degradation testing (ADT) was carried out by cycling the electrode potential 1000 times under oxygen saturated at a scan rate of 50 mV s1. To investigate the Pt supported catalysts, testing was carried out only in 0.5 M H2SO4 electrolyte at room temperature. For working electrode preparation, 4 mg of the electrocatalyst was dispersed in 2 mL of methanol/Nafion® solvent and ultrasonicated for 1 h to prepare a homogeneous catalyst ink. Then, 10 mL of ink was deposited onto the working ring disk electrode using a pipette and dried under ambient conditions. The loading of catalyst on the disk was ~16.16 mgPt cm2. CV and LSV measurements were the same as above.

Results and discussion As shown in Scheme 1, the synthesis of N-PCNF involves the electrospinning of PAN/PMMA nanofibers, followed by preoxidation and carbonization to yield the porous carbon nanofibers (PCNF), and then blend with the FeCl3 and DCDA and hydrothermally treated. The precipitate is then calcined under an inert atmosphere in order to introduce nitrogen atoms into the PCNF. Finally, the N-PCNF is totally prepared by etching the unstable and inactive species from the catalysts, followed by heat-treatment again. The morphologies of the prepared N-PCNF were examined by TEM as well as the PCNF and the traditional CNF. As shown in Fig. 1aef, the three samples were both keeping the nanofiber shape and the diameter of them was averagely 200 nm. Obviously, the surface of the traditional CNF (Fig. 1a and d) was particularly smooth without any roughness and pores. However, the PCNF (Fig. 1b and e) and N-PCNF (Fig. 1c and f) possess many mesopores and the surface was rough enough, due to the disappearance of TDP after the calcination. It can also be seen that the fiber length of the N-PCNF was much shorter than that of PCNF, which means that the N-PCNF was more defected and rough, that might expose more active sites

Scheme 1 e Illustration of the synthetic procedure for the NePCNF. Please cite this article in press as: Wang Y, et al., Nitrogen-doped porous carbon nanofiber based oxygen reduction reaction electrocatalysts with high activity and durability, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/ j.ijhydene.2016.04.235

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Fig. 1 e Typical TEM images of traditional CNF (a, d), PCNF (b, e) and N-PCNF (c, f).

[40] and benefit the deposition and distribution of Pt nanoparticles during the process of Pt catalyst preparation. The chemical structure of the resulting samples was elucidated by XPS measurement. C1s, N1s, and O1s signals can be clearly observed for all samples in Fig. 2a XPS spectra.

Some researchers reported that graphitic N can be benefit for increasing the limiting current density, and the onset potential and wettability can be affected by pyridinic N, and the rest N species including pyrrolic N or oxidized N exhibit little effect on carbon materials [22]. So two primary peaks for N1s spectra

Fig. 2 e XPS survey spectra (a) of CNF, PCNF, and N-PCNF, respectively; and high resolution N1s XPS spectra of N-PCNF (b), PCNF (c), and CNF (d). Please cite this article in press as: Wang Y, et al., Nitrogen-doped porous carbon nanofiber based oxygen reduction reaction electrocatalysts with high activity and durability, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/ j.ijhydene.2016.04.235

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Table 1 e Atomic content of all the catalysts calculated from the XPS survey spectra. Catalyst

N-PCNF PCNF CNF

C Content (at%)

O Content (at%)

N content (at%)

Graphitic N %

Pyridinic N %

Ratio (Graphitic/Pyridinic N)

91.32 91.92 92.85

2.91 4.79 4.47

5.77 3.29 2.67

66.70 78.05 81.88

33.33 21.95 18.12

2.00 3.56 4.52

can be generally deconvoluted such as pyridinic N (ca. 398.6 eV) and graphitic N (ca. 401.3 eV). The percentage was summarized in Table 1. It can be clearly seen that the radio of graphitic to pyridinic N of N-PCNF is about 2, much lower than others', suggesting N-PCNF may benefit the ORR. To inspect the activity of these carbonaceous catalysts for ORR, CV were first used in N2- and O2-saturated 0.1 M KOH aqueous solution at a scan rate of 50 mV s1. Typically rectangular voltammetric curves (Fig. 3a) can be observed under N2 atmosphere, which is ascribed as the carbon materials characteristic double-layer capacitive current. The area of traditional CNF was much smaller than the PCNF and the NPCNF, indicating that the special surface area of the CNF was smaller than the other two. In contrast, when performed in the O2-saturated electrolyte shown in Fig. 3b, profound cathodic current peaks emerged rapidly. The cathodic current peak of N-PCNF shifted positively in comparison with the two analogs, indicating that the N-PCNF has relatively high catalytic activity toward the ORR. Rotating disk electrode (RDE) measurements were employed to further evaluate the electrocatalytic activity of

these non-precious electrocatalysts toward ORR. Fig. 4a shows the polarization curves for different samples. For comparison, a linear sweep voltammogram (LSV) of commercial 20wt% Pt/ C (JM20) was also obtained. Obviously, the traditional CNF exhibited the worst ORR activity both in terms of the onset potential and the half-wave potential (Fig. 4a). While PCNF shifts half-wave potential positively more than 130 mV compared to CNF, indicating that the existence of various pores can markedly improve the ORR activity due to the increased active sites [40]. Notably, the addition of the nitrogen for N-PCNF can be helpful, under which a half-wave potential of 0.77 V is achieved (about 41 mV higher than the PCNF). The ORR activity of N-PCNF was much close to state-ofthe-art one (JM20) with only about 89 mV lower in half-wave potential than JM20. The improved performance may be ascribed to the increased surface area and the existence of the nitrogen as well as the high density of active sites, indicating that the N-PCNF is a promising candidate for catalyzing ORR. ORR catalytic pathway of the catalysts and the peroxide yield on the electrode were also investigated. As shown in Fig. 4b, the peroxide yield on the N-PCNF was found to be less

Fig. 3 e CV of electrocatalysts (a) in N2-saturated 0.1 M KOH; (b) in O2-saturated 0.1 M KOH; (c) in N2-saturated 0.5 M H2SO4; and (d) in O2-saturated 0.5 M H2SO4 at the rate of 50 mV s¡1 Please cite this article in press as: Wang Y, et al., Nitrogen-doped porous carbon nanofiber based oxygen reduction reaction electrocatalysts with high activity and durability, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/ j.ijhydene.2016.04.235

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Fig. 4 e (a) LSV of various electrocatalysts on RDE (1600 rpm) in O2-saturated 0.1 M KOH at a scan rate of 5 mV s¡1; (b) percentage of H2O2 produced; (c) electron transfer number; (d) LSV of various electrocatalysts on RDE (1600 rpm) in O2saturated 0.5 M H2SO4 at a scan rate of 5 mV s¡1; (e) percentage of H2O2 produced and (f) electron transfer number.

than 15% over the potential range of 0.2 Ve0.8 V whereas PCNF and CNF kept less than 30% and 50%, respectively. Fig. 4c shows that the corresponding electron transfer number of NPCNF was higher than 3.75, higher than PCNF (3.5) and CNF (3.15), suggesting the complete reduction of oxygen to water. These non-precious catalysts were also performed in the acidic medium. As shown in Fig. 3c and d, the electrochemical activities trend seems the same both in acid and alkaline media. The N-PCNF exhibited higher ORR onset potential and half-wave potential than the PCNF and CNF. In acidic media, the N-PCNF catalyst showed the highest ORR catalytic activity, with half-wave potential about 0.59 V and well-defined mass transport-limited current density in the RDE curve as shown in Fig. 4d. Moreover, N-PCNF also exhibited a 4-electron transfer pathway ORR process along with a low peroxide yield below 18% as shown in Fig. 4e and f.

For the long-time operating in fuel cells, the durability is extremely significant. In this regard, the durability of the nonprecious catalysts were assessed by accelerated durability test (ADT) cycling in O2-saturated electrolyte in potential range of 0.6e1.0 V (vs. RHE) at a scan rate of 50 mV s1. After 3000 continuous cycles, the half-wave potential performed only a small loss of about 21 mV for N-PCNF (Fig. 5a) whereas 53 mV for JM20 (Fig. 5b) in alkaline medium, while, about 34 mV loss in half-wave potential for N-PCNF (Fig. 5c) and 90 mV loss for JM20 (Fig. 5d) in acidic media. The stability of the N-PCNF is close to that of the reported before elsewhere [24,45]. We have demonstrated that N-PCNF can be used as the non-precious electrocatalysts either in alkaline medium or acidic medium as above. If the obtained N-PCNF was used as a support to prepare platinum catalyst, can we expect much enhanced electrocatalytic performance?

Please cite this article in press as: Wang Y, et al., Nitrogen-doped porous carbon nanofiber based oxygen reduction reaction electrocatalysts with high activity and durability, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/ j.ijhydene.2016.04.235

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Fig. 5 e LSV of N-PCNF and JM20 on RDE (1600 rpm) before and after 3000 cycles in O2-saturated electrolyte at a scan rate of 5 mV s¡1; (a) N-PCNF in 0.1 M KOH; (b) JM20 in 0.1 M KOH; (c) N-PCNF in 0.5 M H2SO4; (d) JM20 in 0.5 M H2SO4.

The TEM images (Fig. 6) could further reveal this difference and present the size of Pt nanoparticles more distinctly. As can be clearly seen, the nanoscaled Pt particles are uniformly deposited on both the surface and cross section of N-PCNF (Fig. 6a) and PCNF (Fig. 6b). Pt species chemical state can also be identified by X-ray photoelectron spectroscopy (XPS). Fig. 7 shows the Pt 4f regions of the XPS spectrum of different samples and the relative values given in percentage of total intensity are listed in Table 2. According to the existing reports, the Pt 4f signal consists of three pairs of doublets, which are namely 4f7/2 and 4f5/2 peaks [46]. The most intense doublet (~71.4 eV and ~74.8 eV) is due to metallic Pt. The second set of doublets (~72.4 eV and ~75.8 eV) could be belong to the Pt (Ⅱ) as in PtO and Pt(OH)2. The third doublet (~75.2 eV and ~78.6 eV) was regarded as the small amount of Pt (Ⅳ) species. From the values listed in Table 2, the percentages of Pt (0) is found to be the predominant specie for each catalyst, about 73%, 60% and 53% for Pt/N-PCNF, Pt/PCNF and JM20, respectively. It has been stated that Pt (0) is more active than other species [47]. The cyclic voltammograms (CVs) of Pt/N-PCNF, along with Pt/PCNF and JM20 as a comparison are given in Fig. 6d. As observed, Pt/N-PCNF displays larger hydrogen absorption area than that of others. The electrochemical active surface (ECSA) is a significant parameter used to represent the intrinsic activity of the Pt catalysts ad can be calculated based on the equation [44]: ECSA ¼ Q/(0.21  M), where Q (mc) is the electrical charge caused by hydrogen monolayer adsorption on Pt

with an assuming value of 210 mC/cm2, M is the Pt loading. The resultant ECSAs is about 62 m2 g1, 54 m2 g1 and 49 m2 g1 for Pt/N-PCNF, Pt/PCNF and JM20, respectively. According to ECSA values, the Pt/N-PCNF exhibited the best electrocatalytic activity. Beside the uniform Pt nanoparticles dispersion and the higher metallic Pt (0) content on the surface of N-PCNF, the defected and rough characteristics even the addition of nitrogen presented in N-PCNF may provide extra and more active sites which could enhance electrochemistry activity. LSV curves obtained for Pt/N-PCNF, Pt/PCNF, and stateof-the-art JM20 by half-cell testing in acidic electrolyte are displayed in Fig. 6e. As shown, all the samples exhibited significant ORR activity coupled with the characteristic regions for platinum-containing materials including kinetic, mixed, and diffusion limited regions. The Pt/N-PCNF exhibited superior ORR activity in comparison with the Pt/ PCNF and JM20 in terms of the onset potential as well as the half-wave potential. The half-wave potential and the onset potential of Pt/PCNF were found to positively shift about 24 mV and 23 mV comparison with JM20, indicating that the special surface characteristics included the various pores, defects and rough surface were advantageous for improving ORR activity as we reported before [44]. When contrast Pt/NPCNF with Pt/PCNF, the former one even showed better ORR activity in onset potential, about 32 mV higher than the latter. As given in Table 3, the mass activity (MA) of Pt/NPCNF showed almost 2.3 times higher than JM20 and 1.3

Please cite this article in press as: Wang Y, et al., Nitrogen-doped porous carbon nanofiber based oxygen reduction reaction electrocatalysts with high activity and durability, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/ j.ijhydene.2016.04.235

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Fig. 6 e Typical TEM images of Pt/N-PCNF (a), Pt/PCNF (b) and JM20 (c); and CV curves (d) of JM20, Pt/PCNF and Pt/N-PCNF in 0.5 M H2SO4 at ambient solution temperature with a potential scanning rate of 50 mV s¡1; LSV curves (e) of JM20, Pt/PCNF and Pt/N-PCNF in 0.5 M H2SO4 at ambient solution temperature with a potential scanning rate of 5 mV s¡1 and 1600 rpm electrode rotation speed; (f) mass activities and specific activities calculated at 0.85 V (vs.RHE).

higher than Pt/PCNF. Thus, the ORR activity enhancement of Pt/N-PCNF catalyst may be ascribed to the well-dispersed uniform-sized platinum nanoparticles, doped with nitrogen that are more electronegative than carbon [25]. In this case, the carbon atoms may themselves be the adsorption sites for oxygen. The synergistic effect between the platinum nanoparticles and support materials may also be helpful, where the N-PCNF support itself possess inherent catalytic activity as we discussed above, which can serve to ORR [24].

ECSA values were calculated from hydrogen adsorption change every 100 cycles ranging from 100 cycles to 1000 cycles, as shown in Fig. 8a. It can be clearly seen that the ECSA retention of Pt/N-PCNF is better than that of both Pt/PCNF and JM20, retaining about 58%, 38%, and 16% of their initial ECSA cycles, respectively. The improved stability of Pt/N-PCNF along with the other two is in consistence with above and primarily attributed to the enhanced Pt-p orbital bonding strength and the presence of functional anchoring groups [45]. Pt/N-PCNF also exhibited well ORR activity retention after

Please cite this article in press as: Wang Y, et al., Nitrogen-doped porous carbon nanofiber based oxygen reduction reaction electrocatalysts with high activity and durability, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/ j.ijhydene.2016.04.235

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Fig. 7 e XPS survey spectra (a) of Pt/N-PCNF, Pt/PCNF, and JM20, respectively; and high resolution Pt4f XPS spectra of Pt/NPCNF (b), Pt/PCNF (c), and JM20 (d).

Table 2 e Result of the fits of Pt 4f spectra, values given in percentage of total intensity. Catalyst

Pt (0) 71.4 eV

Pt (Ⅱ) 72.4 eV

Pt (Ⅳ) 75.2 eV

Pt/N-PCNF Pt/PCNF JM20

72.55% 59.92% 52.90%

22.12% 25.42% 35.26%

5.33% 5.10% 11.84%

ADT. After 1000 potential cycles, Pt/N-PCNF showed only 27 mV loss in the half-wave potential as shown in Fig. 8b. Meanwhile, the Pt/PCNF exhibited about 80 mV decrease in half-wave potential, whereas, commercial JM20 demonstrated a significant half-wave potential decrease about 101 mV. Once again, these results were consistent with ECSA loss measurements, the Pt/N-PCNF showed the best stability among the investigated materials, while Pt/PCNF and commercial Pt/ C exhibited comparatively poor stability.

Conclusions In conclusion, we have carefully and successfully synthesized a novel ORR catalyst based on nitrogen doped carbon nanofibers (N-PCNF) by electrospinning method and carbonization. The N-PCNF exhibited excellent electrocatalytic activity as well as stability toward ORR both in basic and acidic medium. The presence of defects, pores, roughness and nitrogen doping are responsible for enhancing the electrocatalytic performance of the N-PCNF. Furthermore, we have confirmed N-PCNF is a promising candidate as catalysts supports for the enhanced performance of catalyzing ORR. Much higher and more stability electrocatalytic activity of Pt/N-PCNF were observed compared to Pt/PCNF and JM20. The outstanding electrochemical performances indicated that the N-PCNF can not only work as the non-precious electrocatalysts toward ORR, but also a promising candidate as catalyst supports for the enhancement of PEMFC performance.

Table 3 e Totally results of CV and LSV measurement. Catalysts

ECSA (m2 g1)

Onset potential (V)

Half-wave potential (V)

MA (mA mg1 Pt )

SA (mA cm2 Pt )

Pt/N-PCNF Pt/PCNF JM20

62 54 49

0.949 0.917 0.894

0.773 0.773 0.749

51.38 39.15 22.12

0.083 0.073 0.045

Please cite this article in press as: Wang Y, et al., Nitrogen-doped porous carbon nanofiber based oxygen reduction reaction electrocatalysts with high activity and durability, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/ j.ijhydene.2016.04.235

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Fig. 8 e (a) normalized ECSA curves of the three catalysts; and LSV curves of on RDE (1600 rpm) before and after 1000 cycles in N2-saturated 0.5 M H2SO4at a scan rate of 5 mV s¡1; (b) Pt/N-PCNF; (c) Pt/PCNF; and (d) JM20.

Acknowledgments This work has been financially supported by the Fundamental Research Funds for the Central Universities (grant no. 16D310608).

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Please cite this article in press as: Wang Y, et al., Nitrogen-doped porous carbon nanofiber based oxygen reduction reaction electrocatalysts with high activity and durability, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/ j.ijhydene.2016.04.235