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Electrochimica Acta 139 (2014) 111–116

Contents lists available at ScienceDirect

Electrochimica Acta

j ourna l ho me page: www.elsev ier .com/ locate /e lec tac ta

lectrospun Iron–Polyaniline–Polyacrylonitrile Derived Nanofibers ason–Precious Oxygen Reduction Reaction Catalysts for PEM Fuel Cells

ouyan Zamani, Drew Higgins, Fathy Hassan, Gaopeng Jiang, Jason Wu, Salah Abureden,hongwei Chen ∗

epartment of Chemical Engineering, University of Waterloo, 200 University Ave. W., Waterloo, Ontario, Canada, N2L 3G1

r t i c l e i n f o

rticle history:eceived 16 May 2014eceived in revised form 4 July 2014ccepted 4 July 2014vailable online 12 July 2014

eywords:xygen Reduction Reaction

a b s t r a c t

Replacing high cost platinum (Pt) catalysts with alternative non-precious materials for the oxygen reduc-tion reaction (ORR) is highly desirable to reduce the cost of polymer electrolyte membrane fuel cell(PEMFC) systems. Transition metal-nitrogen-carbon complexes (M-N-C) prepared by high tempera-ture heat treatment techniques are the most promising class of non-precious materials with respectto ORR activity and stability. In this work, one-dimensional nanofibers are prepared by electrospinningan iron–polyaniline/polyacrylonitrile (Fe–PANI-PAN) metal-polymer blend, followed by subsequent heattreatment. This represents the first time a blend of polymers (PANI-PAN) has been used to prepare M-

on-precious catalystEM fuel cellron-polyaniline-polyacrylonitrile blendlectrospun nanofiber

N-C nanofiber catalysts via electrospinning. The addition of 10 wt. % PANI to the electrospinning mixtureprovides 100 and 70 mV improvements to the ORR onset potential and half-wave potential, respectively,rendering the most active non-precious ORR nanofiber catalysts prepared by electrospinning to date. Thehigh activity is attributed to the porous structure of the nanofibers, combined with increased nitrogencontents provided by the PANI incorporation.

© 2014 Elsevier Ltd. All rights reserved.

. Introduction

Fuel cells that operate by efficiently converting chemical energynto electrical energy are considered highly promising clean energyevices, offering potential replacement to conventional fossil fuelased technologies such as in the automotive industry or for backupower [1,2]. It is highly desirable to replace expensive platinumatalysts with non–precious catalysts, particularly at the cathodehich requires excessive platinum loadings to facilitate the slug-

ish oxygen reduction reaction (ORR) [3,4]. To this end, transitionetal-nitrogen-carbon (M-N-C) nanomaterial complexes synthe-

ized by high temperature pyrolysis techniques are considered theost promising in terms of ORR activity and stability [1,2,5–9].

hrough extensive investigations, it is clear that in this class ofatalyst the active site structure identities and concentrations areoverned by precursor and synthesis technique selection [1,2,6];owever the exact role and coordination chemistries of the individ-

al atoms remain a subject of debate. For different non–preciousatalyst technologies, it is therefore important to optimize syn-hetic conditions in order to increase ORR active site densities and

∗ Corresponding author. Tel.: +1-(519)888-4567x38664.E-mail address: zhwchen@uwaterloo.ca (Z. Chen).

ttp://dx.doi.org/10.1016/j.electacta.2014.07.007013-4686/© 2014 Elsevier Ltd. All rights reserved.

turnover frequencies, while simultaneously providing high surfaceareas and porosity for improved catalyst utilization [10,11].

Recently, one–dimensional nanostructured carbon materialshave been applied as electrocatalyst materials due to their excel-lent conductivity, tunable surface properties and the ability to beintegrated into interconnected, highly porous electrode structures[12–14]. Nanofibers have been fabricated via several techniques,such as template synthesis, self–assembly, phase separation, andelectrospinning [15]. Among these techniques, electrospinningis a highly attractive technique to reliably, and cost-effectivelyfabricate continuous polymer nanofibers into porous web-likestructures [15]. Furthermore, its simplicity renders electrospinninghighly applicable to the development of non-precious M-N-Cnanofibers for fuel cell applications [11,16–19]. Previous stud-ies have however been confined to metal-polyacrylonitrile (PAN)based nanofibers, that has been demonstrated capable in the pres-ence of iron to provide ORR activity in alkaline [11,19,20] and acidicelectrolytes [18,20–23]. It is therefore highly desirable to investi-gate new synthetic strategies to prepare materials with improvedORR activity.

Herein, we investigate the incorporation of polyaniline (PANI)precursors into Fe-PAN polymer blends (Fe-PANI-PAN) for elec-trospinning, followed by pyrolysis. This comprises the first timea blend of polymers with complementary properties have been

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sed for electrospinning to prepare M-N-C nanofiber catalysts,ith improved ORR activity demonstrated in acidic electrolytes.

ANI was selected as it has previously been demonstrated as andeal nitrogen precursor to produce some of the most active M--C catalysts to date, owing to its aromatic ring structure with

high content and uniform distribution of nitrogen species thatan readily form nitrogen-doped graphitic carbon structures dur-ng heat treatments [6,8,24–26]. PAN was also helpful as a low-costolymer carrier to overcome the poor solubility of PANI in solu-ion and as a secondary source of nitrogen. We demonstratehat the addition of 10 wt. % PANI to the Fe-PAN electrospinningolution significantly enhances the ORR activity of the resul-ant M-N-C nanofibers in terms of onset potential and half-waveotential, while demonstrating high selectivity towards the four-lectron reduction of oxygen. This unique synthetic approachherefore provides practical progress towards the development ofne-dimensional non-precious metal based catalysts for fuel cellpplications.

. Experimental

.1. Fe–PANI polymerization.

In a 500 ml beaker, 5 mL of aniline and 50 mg of iron chlorideFeCl3) were sequentially added to 300 mL of 1.0 M hydrochloriccid (HCl) under gentle stirring while keeping it below 10 ◦C usingce bath. In another beaker, 11.4 g of ammonium persulfate wasissolved in 200 mL of 1.0 M HCl. This oxidant solution was thenlowly added to the aniline mixture, and stirring was maintainedor 24 h to allow complete polymerization. The liquid was removedsing a rotary evaporator and the Fe–PANI complex was collected.

.2. Synthesis of Fe-PANI-PAN electrospun nanofiber catalysts.

In a 25 ml vial, 1.0 g PAN (Mw 150,000), 100 mg of Fe–PANI and15 mg of FeCl3 were dispersed in 23 mL of DMF by sonication forwo days. Electrospinning was carried out at a needle–collector dis-ance of 15 cm, solution flow rate of 200 �L.h−1 and applied voltagef 23 kV. The electrospun polymer nanofibers then subjected toalcination at 250 ◦C for two hours in air using a heating rate of◦Cmin−1. The materials were then annealed in argon at 850 ◦C for

wo hours to obtain the final Fe-PANI-PAN catalyst. Reference Fe-AN and PANI-PAN samples were also prepared by similar methodsithout the incorporation of PANI and Fe precursors, respectively.

.3. Catalyst characterization

Catalyst morphology was investigated using scanning electronicroscopy (SEM) and transmission electron microscopy (TEM),hich provided elucidation of the effect of electrospinning param-

ters and heat treatments on the nanofiber structure. Catalysturface areas were determined using Brunauer–Emmett–TellerBET) analysis based on the isothermal adsorption of nitrogen afterhat samples were degassed for 12 hours at 250 ◦C. X-ray photoelec-ron spectroscopy (XPS) with a K-alpha X-ray source was conductedo determine the relative elemental surface contents in Fe-PAN ande-PANI-PAN catalysts and to elucidate the identity of nitrogenurface species. Raman spectroscopy was applied to evaluate thetructure of the nanostructured carbon materials.

.4. Electrochemical characterizations

ORR activity was evaluated through rotating disk electrodeRDE) and rotating ring disk electrode (RRDE) testing. In order torepare the catalyst ink, 2.0 mg of the catalyst was dispersed in00 �l of 0.03 wt% Nafion/ethanol by sonication for two hours. A

Acta 139 (2014) 111–116

glassy carbon disk (0.19635 cm2) was then coated with 30 �l ofthe ink to achieve a catalyst loading of 0.6 mgcm−2. Linear sweeptesting was performed in oxygen saturated 0.1 M perchloric acid(HClO4) electrolyte at rotational speeds of 100, 400, 900, 1600 rpm,and a scan rate of 10 mVs−1. Background currents measured inthe same scan rate and under nitrogen saturated electrolyte wereremoved to account for only Faradaic ORR processes. Cyclic voltam-metry (CV) was done under oxygen and nitrogen saturated 0.1 MHClO4 and scan rate of 50 mVs−1. A reversible hydrogen electrode(RHE) and a Pt wire were used as reference and counter electrodes,respectively for all the electrochemical measurements. For RRDEtesting, the ring potential was set to 1.2 V to effectively oxidize anyperoxide species reaching its surface.

3. Results and discussion

The first step of the present study was to optimize the for-mation of electrospun nanofibers by modifying the Fe-PANI-PANconcentrations within the DMF precursor solution. As shown inSEM images Fig. 1 a and Fig. 1b, at relatively low Fe-PANI-PAN con-centrations of 1 and 2 wt. %, respectively, only blood–cell shapeddroplets were observed, indicating that the surface tension at thissolution concentration was likely too high [27]. By increasing theFe-PANI-PAN concentration to 5 wt. %, nanofiber formation wasclearly observed (Fig. 1c). Diameters varying from ca. 30 to 150 nm(average of 100 nm) were observed along with some bead forma-tion along the lengths of the nanofibers. This highlights the impactPANI incorporation has on the electrospinning process in compar-ison to the smoother and relatively uniform electrospun Fe-PANnanofibers shown in Fig. 1e. It should be noted that higher Fe-PANI-PAN concentrations were not appropriate due to the poor solubilityof PANI that limited its good dispersion in the DMF solution.

The Fe-PANI-PAN nanofibers (Fig. 1c) and Fe-PAN (Fig. 1e) pre-pared from the 5 wt. % solution were then subjected to calcinationin air at 250 ◦C and subsequent carbonization in argon at 850 ◦C(Fig. 1d and Fig. 1f, respectively). Based on TEM imaging (Fig. 2), thisheat treatment was found to result in the formation of Fe nanoparti-cles, along with the evolution of porosity throughout the nanofiberstructure as shown from the changes occurring to the nanofibermaterial structure before (Fig. 2a) and after (Fig. 2b) pyrolysis. Thenanofiber porosity is likely a result of the evaporation of any resid-ual solvent or volatile species embedded in the nanofiber networkstructure after electrospinning.

BET analysis (Fig. 3) indicated a higher specific surface areaof 355 ◦C was obtained for Fe-PANI-PAN compared to 330 ◦C forFe-PAN. These values are significantly higher than those reportedfor non-porous carbon blacks and non-porous carbon nanofibers[11,12] and are similar to those of porous carbon nanofibersreported previously [11]. A highly porous morphology of pyrolyzedFe-PANI-PAN was also shown by TEM imaging (Fig. 2b), supportingthe BET surface areas observations.

The effect of adding PANI was investigated by conducting XPSanalysis of Fe-PAN-PANI and Fe-PAN nanofibers. The overall XPSspectra in Fig. 4a show the existence of carbon, oxygen, nitrogenand iron in the catalysts. The C 1s spectra provided in Fig. 4b showsasymmetric graphitic carbon peaks at 284.5 eV for both pyrolyzedFe-PANI-PAN and Fe-PAN, indicating that the graphitization of bothnanofibers was successful despite the different atomic structuresof the precursor solutions. The Raman spectra shown in Fig. 4calso demonstrate the graphitic carbon resonances (G band) at ca.1580 cm−1 and defect band (D band) at ca. 1340 cm−1 for both sam-

ples. These results indicate that, after pyrolosis, the Fe-PAN andFe-PANI-PAN materials had a graphitic structure with a certainamount of disorder. An estimation of the total order related to thecarbon structures can be given by the ratio of these peaks (i.e. ID:IG)

P. Zamani et al. / Electrochimica Acta 139 (2014) 111–116 113

F t. % (b)i d fromb

[FoAFnoppagc

ig. 1. SEM images of Fe-PANI-PAN electrospun nanomaterials derived from (a) 1 wn the DMF precursor solution and (d) Fe-PANI-PAN electrospun nanofibers deriveefore and (f) after heat treatments.

28]. Based on the ID:IG peak ratios of 0.69 for Fe-PAN and 1.09 fore-PANI-PAN, it is clear that Fe-PANI-PAN had an increased amountf disorder, most likely due to a higher content of nitrogen dopants.ccordingly, the high resolution N 1s spectra provided in Fig. 4d andig. 4e for Fe-PAN and Fe-PANI-PAN, respectively, revealed that theitrogen content increased from 3.63 to 5.35 at. % with the additionf PANI (Fig. 4f). Based on peak deconvolution, it was found thatyridinic (∼398.5 eV) and graphitic/pyrrolic N (∼401.0 eV) were

resent in higher concentrations in Fe-PANI-PAN (pyridinic∼2.50nd graphitic∼2.45 at. %) compared to Fe-PAN (pyridinic∼1.08 andraphitic∼1.93 at. %). The increased content of these structuresould likely arise during pyrolysis due to the unique aromatic

Fig. 2. TEM images of Fe-PANI-PAN electrospun nano

2 wt. % and (c) Fe-PANI-PAN nanofiber derived from 5 wt. % polymer concentration 5 wt.% after heat treatments. SEM images of Fe-PAN electrospun nanofibers (e)

structure of PANI which is linked via nitrogen sites in a very largechain (a rich source of nitrogen atoms) [6,24]. Increased nitrogencontents of non-precious catalysts have previously been linked toORR activity both in alkaline and acidic environment [7,10,29,30],potentially underlying the beneficial impact of PANI incorporationtowards the improved electrochemical performance highlightedlater on.

Fig. 5a shows CV plots of Fe-PANI-PAN in O2 and N2 saturated

0.1 M HClO4. From this graph, ORR activity is clearly observed dueto the emergence of a distinct reduction peak that is not observed inthe absence of oxygen [31]. Polarization curves of Fe-PANI-PAN areshown in Fig. 5b along with the related Koutecky–Levich (K–L) plot

fibers (a) before and (b) after heat treatments.

114 P. Zamani et al. / Electrochimica

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Fig. 3. BET analysis of Fe-PANI-PAN and Fe-PAN derived catalysts.

Fig. 5c). The number of electrons transferred in RDE is calculatedsing the K–L equation (Equation 1) [32].

1i

= 1ik

+ 1m

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(1)

= 0.201nFCo2 D2/3o2

v−1/6 (2)

Here, ik is the kinetic current density, i is the experimental

urrent density, m is the Levich slope (Equation 2) [32] and ωs the rotation speed. In Equation 2, n is the number of electronransferred, F is the Faraday constant, C is the dissolved oxygenoncentration, D is the oxygen diffusion coefficient, and � is the

ig. 4. (a) XPS elemental survey and (b) C 1s spectra, and (c) Raman spectra of Fe-PANI-e-PANI-PAN, and (f) nitrogen content and species differentiation, of Fe-PANI-PAN and Fe

Acta 139 (2014) 111–116

electrolyte solution kinematic viscosity [32]. The number of elec-trons transferred was calculated to be ca. 3.85 ± 0.05 at all electrodepotentials investigated, demonstrating high selectivity towards anoverall 4 electron reduction mechanism occurring on the Fe-PANI-PAN catalysts. Ring currents from RRDE measurements were alsoused to determine the number of electrons transferred using Equa-tion 3.[9] At all potentials below ca. 0.65 V vs RHE, the number ofelectrons transferred was calculated to be ca. 3.89 ± 0.02 (Fig. 5d),which is in good agreement with the results from K-L analysis.Accordingly, the peroxide yield (Equation 4) [25] for Fe-PANI-PANcatalyst was also less than ca. 5.0 ± 1.0%, indicating good selectivitytoward the four electron ORR to produce H2O.

n = 4 × ID(IR/N) + ID

(3)

H2O2(%) = 200 × IR/N

(IR/N) + ID(4)

Where N is the ring collection efficiency constant, and IR and ID arerespectively ring and disk electrode current.

In Fig. 6a, ORR polarization curves collected at 1600 rpm forcatalysts derived from PAN-PANI, Fe-PAN, and Fe-PANI-PAN aredemonstrated, with comparison of the collected electrokinetic datasummarized in Fig. 6b. From these plots, it is seen that the additionof 10 wt. % PANI to Fe-PAN significantly improves the onset poten-tial from 0.80 to 0.90, and half–wave potential from 0.63 ± 0.01to 0.70 ± 0.01 V vs RHE. Based on a measured potential differenceof 0.26 V between RHE and Ag/AgCl reference electrodes, these

results show greater than ca. 40 mV improvements in the onset andhalf–wave potentials in acidic media compared to the best resultspreviously reported for M-N-C non-precious catalysts prepared byelectrospinning and subsequent high temperature heat treatments

PAN and Fe-PAN derived catalysts. (d) N 1s spectra of Fe-PAN, (e) N 1s spectra of-PAN.

P. Zamani et al. / Electrochimica Acta 139 (2014) 111–116 115

Fig. 5. (a) CV curves of Fe-PANI-PAN in N2 and O2 saturated 0.1 M HClO4, (b) Polarization curves performed in O2 saturated 0.1 M HClO4 of Fe-PANI-PAN at different rotationalspeed, and (c) the associated Koutecky–Levich plot. (d) The peroxide yield and n, number of electron transferred for Fe-PANI-PAN in O2 saturated 0.1 M HClO4.

F of Feo s of thc

[tnbMbn[m

ig. 6. (a) Linear sweep voltammetry curves performed in O2 saturated 0.1 M HClO4

f onset potential, half–wave potential and limiting current density. The error rangeurrent densities, respectively.

20–22]. Coupled with the high selectivity towards the 4 elec-ron reduction mechanism, Fe-PANI-PAN comprises the most activeon-precious metal catalysts prepared by electrospinning that haveeen reported to date. Although the exact origin of ORR activity in-N-C catalysts is still unclear, the enhancements could potentially

e linked to the relatively higher pyridinic and graphitic/pyrrolicitrogen contents (Fig. 4f) of Fe-PANI-PAN in comparison to Fe-PAN25,26,29,31], along with the slightly higher surface area deter-

ined by BET analysis (Fig. 3).

-PAN, Fe-PANI-PAN and PANI-PAN at 1600 rpm, and (b) Corresponding comparisone values are ±0.01 V and ±0.1 mA cm−2 for all the half-wave potentials and reported

4. Conclusions

Fe–PANI-PAN nanofibers prepared via electrospinning and sub-sequent heat treatments were prepared as non-precious ORRcatalysts. From SEM and TEM imaging, highly porous nanfibers

with an average diameter of 100 nm and high specific surfacearea of 355 m2 g−1 were successfully prepared. ElectrochemicalRDE and RRDE characterization revealed that the addition of10 wt% PANI to PAN significantly improved the ORR activity of

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e-PANI-PAN in comparison to Fe-PAN prepared in the absencef PANI. Based on XPS analysis, these enhancements can likely bettributed to the increased surface concentration of pyridinic andraphitic nitrogen species, likely arising due to the nitrogen right,romatic structure of PANI precursor. The preparation of Fe-PANI-AN therefore provides a unique synthetic approach to preparehe most active one-dimensional non-precious catalysts by elec-rospinning reported to date for the ORR.

cknowledgements

This work was partially supported by the University of Water-oo and the Waterloo Institute for Nanotechnology. This research

as conducted as part of the Catalysis Research for Polymer Elec-rolyte Fuel Cells (CaRPE FC) Network administered from Simonraser University and supported by Automotive Partnership CanadaAPC) Grant No. APCPJ 417858 - 11 through the Natural Sciences andngineering Research Council of Canada (NSERC).

eferences

[1] Z. Chen, D. Higgins, A. Yu, L. Zhang, J. Zhang, A review on non-precious metalelectrocatalysts for PEM fuel cells, Energy Environ. Sci. 4 (2011) 3167–3192.

[2] D.C. Higgins, Z. Chen, Recent progress in non-precious metal catalysts for PEMfuel cell applications, Can. J. Chem. Eng. 91 (2013) 1881–1895.

[3] A. Serov, M.H. Robson, K. Artyushkova, P. Atanassov, Templated non-PGM cath-ode catalysts derived from iron and poly(ethyleneimine) precursors, Appl.Catal. B Environ. 127 (2012) 300–306.

[4] A. Serov, M.H. Robson, B. Halevi, K. Artyushkova, P. Atanassov, Highly active anddurable templated non-PGM cathode catalysts derived from iron and aminoan-tipyrine, Electrochem. Commun. 22 (2012) 53–56.

[5] M. Lefevre, E. Proietti, F. Jaouen, J.P. Dodelet, Iron-based catalysts with improvedoxygen reduction activity in polymer electrolyte fuel cells, Science 324 (2009)71–74.

[6] F. Jaouen, E. Proietti, M. Lefèvre, R. Chenitz, J.-P. Dodelet, G. Wu, H.T. Chung,C.M. Johnstonb, P. Zelenay, Recent advances in non-precious metal catalysisfor oxygen-reduction reaction in polymer electrolyte fuel cells, Energy Environ.Sci. 4 (2011) 114–130.

[7] H.T. Chung, C.M. Johnston, K. Artyushkova, M. Ferrandon, D.J. Myers, P. Zele-nay, Cyanamide-derived non-precious metal catalyst for oxygen reduction,Electrochem. Commun. 12 (2010) 1792–1795.

[8] G. Wu, K.L. More, C.M. Johnston, P. Zelenay, High-performance electrocatalystsfor oxygen reduction derived from polyaniline, iron, and cobalt, Science 332(2011) 443–447.

[9] J. Wu, W. Li, D. Higgins, Z. Chen, Heat-Treated Nonprecious Catalyst Using Fe andNitrogen-Rich 2,3,7,8-Tetra(pyridin-2-yl)pyrazino[2,3-g]quinoxaline Coordi-nated Complex for Oxygen Reduction Reaction in PEM Fuel Cells, J Phys ChemC 115 (2011) 18856–18862.

10] Z. Chen, D. Higgins, H.S. Tao, R.S. Hsu, Z.W. Chen, Highly Active Nitrogen-DopedCarbon Nanotubes for Oxygen Reduction Reaction in Fuel Cell Applications, JPhys Chem C 113 (2009) 21008–21013.

11] J. Wu, H.W. Park, A. Yu, D. Higgins, Z. Chen, Facile Synthesis and Evaluation

of Nanofibrous Iron–Carbon Based Non-Precious Oxygen Reduction ReactionCatalysts for Li–O2Battery Applications, J Phys Chem C 116 (2012) 9427–9432.

12] A. Kongkanand, S. Kuwabata, G. Girishkumar, P. Kamat, Single-wall carbonnanotubes supported platinum nanoparticles with improved electrocatalyticactivity for oxygen reduction reaction, Langmuir 22 (2006) 2392–2396.

[

Acta 139 (2014) 111–116

13] K.P. Gong, F. Du, Z.H. Xia, M. Durstock, L.M. Dai, Nitrogen-Doped Carbon Nano-tube Arrays with High Electrocatalytic Activity for Oxygen Reduction, Science323 (2009) 760–764.

14] W.Z. Li, C.H. Liang, W.J. Zhou, J.S. Qiu, Z.H. Zhou, G.Q. Sun, Q. Xin, Prepara-tion and characterization of multiwalled carbon nanotube-supported platinumfor cathode catalysts of direct methanol fuel cells, J Phys Chem B 107 (2003)6292–6299.

15] Z.-M. Huang, Y.Z. Zhang, M. Kotaki, S. Ramakrishna, A review on polymernanofibers by electrospinning and their applications in nanocomposites, Com-pos. Sci. Technol. 63 (2003) 2223–2253.

16] S. Uhm, B. Jeong, J. Lee, A facile route for preparation of non-noble CNF cathodecatalysts in alkaline ethanol fuel cells, Electrochim. Acta 56 (2011) 9186–9190.

17] D. Shin, B. Jeong, B.S. Mun, H. Jeon, H.-J. Shin, J. Baik, J. Lee, On the Origin ofElectrocatalytic Oxygen Reduction Reaction on Electrospun Nitrogen–CarbonSpecies, J Phys Chem C 117 (2013) 11619–11624.

18] N. Nakagawa, M.A. Abdelkareem, D. Takino, T. Ishikawa, T. Tsujiguchi, <PANbased carbon nanofibers as an active ORR cat for DMFC - Nakagawa - ECST -2011.pdf>, ECS Trans. 41 (2011) 2219–2229.

19] B. Jeong, S. Uhm, J. Lee, <Iron-cobalt modified electrospun CNF as ORR cat inalkakline - Jeong - ECST - 201.pdf>, ECS Trans. 33 (2010) 1757–1767.

20] Y. Qiu, J. Yu, W. Wu, J. Yin, X. Bai, Fe–N/C nanofiber electrocatalysts withimproved activity and stability for oxygen reduction in alkaline and acid solu-tions, J. Solid State Electrochem. 17 (2012) 565–573.

21] J. Yin, Y. Qiu, J. Yu, Nitrogen-doped Carbon Nanofibers as Highly Active Metal-free Electrocatalysts for Oxygen Reduction Reactions in Acidic Media, Chem.Lett. 42 (2013) 413–415.

22] Y. Qiu, J. Yin, H. Hou, J. Yu, X. Zuo, Preparation of nitrogen-doped carbon sub-microtubes by coaxial electrospinning and their electrocatalytic activity foroxygen reduction reaction in acid media, Electrochim. Acta 96 (2013) 225–229.

23] J. Yin, Y. Qiu, J. Yu, Onion-like graphitic nanoshell structured Fe–N/C nanofibersderived from electrospinning for oxygen reduction reaction in acid media, Elec-trochem. Commun. 30 (2013) 1–4.

24] V.G. Khomenko, V.Z. Barsukov, A.S. Katashinskii, The catalytic activity ofconducting polymers toward oxygen reduction, Electrochim. Acta 50 (2005)1675–1683.

25] G. Wu, Z.W. Chen, K. Artyushkova, F.H. Garzon, P. Zelenay, Polyaniline-derivedNon-Precious Catalyst for the Polymer Electrolyte Fuel Cell Cathode, ECS Trans.16 (2008) 159–170.

26] G. Wu, K. Artyushkova, M. Ferrandon, J. Kropf, D. Myers, P. Zelenay, PerformanceDurability of Polyaniline-derived Non-Precious Cathode Catalysts, ECS Trans.25 (2009) 1299–1311.

27] T. Subbiah, G.S. Bhat, R.W. Tock, S. Parameswaran, S.S. Ramkumar, Electro-spinning of nanofibers, J. Appl. Polym. Sci. 96 (2005) 557–569.

28] G. Wu, C.M. Johnston, N.H. Mack, K. Artyushkova, M. Ferrandon, M. Nel-son, J.S. Lezama-Pacheco, S.D. Conradson, K.L. More, D.J. Myers, P. Zelenay,Synthesis–structure–performance correlation for polyaniline–Me–C non-precious metal cathode catalysts for oxygen reduction in fuel cells, J. Mater.Chem. 21 (2011) 11392–11405.

29] L. Lai, J.R. Potts, D. Zhan, L. Wang, C.K. Poh, C. Tang, H. Gong, Z. Shen, J. Lin, R.S.Ruoff, Exploration of the active center structure of nitrogen-doped graphene-based catalysts for oxygen reduction reaction, Energy Environ. Sci. 5 (2012)7936–7942.

30] P. Matter, L. Zhang, U. Ozkan, The role of nanostructure in nitrogen-containingcarbon catalysts for the oxygen reduction reaction, J. Catal. 239 (2006) 83–96.

31] Q. Liu, Z. Pu, C. Tang, A.M. Asiri, A.H. Qusti, A.O. Al-Youbi, X. Sun, N-dopedcarbon nanotubes from functional tubular polypyrrole: A highly efficient elec-

trocatalyst for oxygen reduction reaction, Electrochem. Commun. 36 (2013)57–61.

32] D.C. Higgins, Z. Chen, Z.W. Chen, Nitrogen doped carbon nanotubes synthesizedfrom aliphatic diamines for oxygen reduction reaction, Electrochim. Acta 56(2011) 1570–1575.