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Metal-support interactions of platinum nanoparticles decorated N-doped carbon nanofibers for the oxygen reduction reaction Julia Melke, Benedikt Peter, Anja Habereder, Juergen Ziegler, Claudia Fasel, Alexei Nefedov, Hikmet Sezen, Christof Wöll, Helmut Ehrenberg, and Christina Roth ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b06225 • Publication Date (Web): 17 Dec 2015 Downloaded from http://pubs.acs.org on December 19, 2015

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ACS Applied Materials & Interfaces

Metal-Support Interactions of Platinum Nanoparticles Decorated N-Doped Carbon Nanofibers for the Oxygen Reduction Reaction Julia Melke1,2*, Benedikt Peter3, Anja Habereder3, Juergen Ziegler3, Claudia Fasel3, Alexei Nefedov4, Hikmet Sezen4,5, Christof Wöll4, Helmut Ehrenberg1, Christina Roth2 1

Institut für Angewandte Materialien – Energiespeichersysteme (IAM-ESS), Karlsruher Institut für

Technologie (KIT), Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany 2

Freie Universität Berlin, Physikalische und Theoretische Chemie, Takustr. 3, 14195 Berlin, Germany 3

Technische Universität Darmstadt, Institut für Material- und Geowissenschaften, Alarich-Weiss-Strasse 2, 64287 Darmstadt, Germany

4

Institut für Funktionelle Grenzflächen (IFG), Karlsruher Institut für Technologie (KIT), Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany

5

Elettra-Sincrotrone Trieste S.C.p.A., Strada Statale 14, 34012 Basovizza, Trieste, Italy

*Corresponding author. E-mail: [email protected] (Dr. Julia Melke)

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KEYWORDS NEXAFS, XPS, fuel cell, polyaniline, electrocatalysis, PANI ABSTRACT N-doped carbon materials are discussed as catalyst supports for the electrochemical oxygen reduction reaction (ORR) in fuel cells. This paper deals with the preparation of Pt nanoparticles (NPs) supported on N-doped carbon nanofibers (N-CNF) from a polyaniline nanofiber (PANI NF) precursor, and investigates the ORR activity of the produced materials. Initially, Pt NPs are deposited on PANI NFs. The PANI NF precursors are characterized by near edge X-ray absorption fine structure (NEXAFS) and transmission electron microscopy (TEM) measurements. It is shown, that in the PANI NF precursor materials electrons from the Pt are being transferred towards the π-conjugated systems of the aromatic ring. This strong interaction of Pt atoms with PANI explains the high dispersion of Pt NPs on the PANI NF. Subsequently, the PANI NFs precursors are carbonized at different heat-treatment conditions resulting in structurally different N-CNFs which are characterized by NEXAFS, X-ray photoelectron spectroscopy (XPS) and TEM measurements. It is shown that an interaction between N-groups and Pt NPs exist in all investigated N-CNFs. However, the N-CNFs differ in the composition of the N-species and the PtNPs. A small mean Pt NPs sizes with a narrow size distribution is attributed to the presence of pyrdinic N-groups in the N-CNFs. Whereas, for the N-CNFs with mainly graphitic and pyrrolic Ngroups an increase in the average Pt NP size with a broad size distribution is found. The ORR activity in alkaline media investigated by Koutecky-Levich analysis of rotating disk electrode measurements showed a largely enhanced ORR activity in comparison to a conventional Pt/C catalyst.


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INTRODUCTION Polymer electrolyte membrane fuel cells are efficient and environmentally-friendly energy converters. However, cost and life time of these fuel cells are still insufficient for a broad commercialization1. A reduction of the cost is achieved by using carbon supported platinum (Pt) metal catalysts reducing the amount of noble metal required. However, large overpotentials leading to performance losses are observed for the oxygen reduction reaction (ORR) at the cathode side.2,3 Furthermore, the life time of these catalysts is still low and different degradation mechanisms, such as Oswald ripening and Pt dissolution, have been found depending on the operation conditions.4,5,6,7 In order to enhance catalytic activity and stability, N-doped carbons are used as catalyst support.8,9,10 The N-doping of carbons can be achieved by either post treatment of carbons, e.g. with urea or NH3, or direct synthesis by carbonization of N-containing polymers.11 Direct synthesis leads to a homogeneous distribution of N-groups throughout the whole material.11 Thus, the carbonization of nitrogen containing organic polymers seems to be a very promising approach. In particular, with polyaniline (PANI) the morphology of the resulting PANI material can be controlled by the polymerization conditions.12,13 Different morphologies, such as particulate, short and long fibres, can be obtained and their shape is being maintained during the carbonization step.14 Different carbon support shapes in a porous fuel cell electrode layer then influence the fuel cell performance significantly, as has been demonstrated by Peter et al.15 Furthermore, the carbonization is a multistep process and the resulting carbon network contains several different N-groups, such as pyridine or amino groups.14,16,17 The effect of Ndoping on the Pt nanoparticle (NP) dispersion and the catalytic activity are still under debate. It 3 ACS Paragon Plus Environment


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was shown, that Pt NPs supported on N-doped carbon materials are more stable and bind more strongly to the support in regions with a high degree of N-doping.18 Even the direct deposition of Pt on PANI and a subsequent carbonization at 750°C leads to N-doped carbon nanofiber (NCNF) electrodes with highly dispersed Pt NPs.19 Surprisingly, the heat-treatment does not lead to agglomeration of the Pt NPs. A first principle study of highly ordered pyrolytic graphite (HOPG) doped with nitrogen showed that Pt favorably nucleates on pyrrole and pyridinic defect sites.20 N atoms induce partial charges on neighboring C atoms acting as defect sites for Pt nucleation.20 An increase in the kinetic rate for ORR was demonstrated for mesoporous Ndoped carbons containing mainly quaternary (N-C-OH and N-C=O) N-groups due to a chemical interaction between the metal NPs and the N-defects.9 Furthermore, it was shown that the insertion of PANI in the electrode by impregnation or a layer-by-layer spraying process during the electrode fabrication increases the activity and stability of the electrode.21,22 Besides the change in the electronic structure of the metal NP, the specific capacitance, density of states and electronic conductivity all depend on the N-doping.23 Hence, for the ORR the types of Ngroups formed on the carbon support and the corresponding interaction with the metal NP are important in order to produce highly efficient and long-term stable catalysts. In this paper we discuss as a starting point the influence of certain types of N-groups on the electronic structure and distribution of Pt NPs on PANI materials, and the effect of such an interaction on the carbonization behavior. Therefore, two types of PANI NFs with different fiber length, named as PANIlong and PANIshort, are synthesized and impregnated with Pt NPs. The PANI samples with and without Pt NPs are investigated by transmission electron microscopy (TEM) and C and N K-edge near edge x-ray absorption fine structure (NEXAFS) spectroscopy. 4 ACS Paragon Plus Environment

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Subsequently, N-CNFs decorated with and without Pt NP are produced by carbonization of different PANI precursor materials at various heat-treatment conditions. The carbonization is followed by thermogravimetric analysis (TGA) and the resulting N-CNFs are characterized by C and N K-edge NEXAFS spectroscopy and TEM. Furthermore, the Pt binding energies are determined by X-ray photoelectron spectroscopy (XPS) for the N-CNFs produced from the PANIlong precursor. The differences in the carbonization conditions, such as heating rate and residence time, are correlated to the number and kind of N-groups, Pt binding energies and the Pt NP sizes. The ORR activities of the different CNFs are determined by rotating disc electrode (RDE) and CO stripping experiments in alkaline media. A conventional Pt/C catalyst was investigated for comparison. Finally, differences in the structure are correlated with the activity for the ORR.

2 EXPERIMENTAL 2.1 Sample preparation PANI precursor materials were synthesized by an oxidative polymerization reaction.14 Aniline (450 µl) and ammonium peroxodisulfate (APS, 1.428 g) were dissolved in 50 ml 1 M H2SO4 or 0.4 M acetic acid to produce PANIshort or PANIlong, respectively. The APS solution was quickly poured into the aniline solution under vigorous stirring at room temperature and then left overnight. The green precipitate was filtered and washed with distilled water until a clear filtrate with neutral pH was obtained. Thus, we ensure that no residual sulfuric acid is contained



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in the polymer. To maintain a good dispersion, the filter cake is stored in wet conditions until further use. The deposition of Pt on the PANI was prepared by a modified synthesis route proposed by Guo et al.24 In order to obtain approx. 450 mg Pt/PANI (11 wt.% Pt), 400 mg PANI was dispersed in 20 ml ultrapure water (MilliQ 18.2 MΩ @25°C) followed by the addition of 125 mg H2PtCl6 and two hours stirring, to allow a good contact between the support and the platinum precursor. Afterwards, 2 ml HCOOH were added. In contrast to the more routinely applied ethylene glycol reduction, HCOOH and a mild synthesis protocol were chosen due to the sensitive PANI materials. The mixture was stored at room temperature for 24 h to reduce the Pt precursor completely. Then, the solution was filtered, washed several times with MilliQ water and dried under vacuum. The Pt/PANIlong (PANI already decorated with Pt NPs) and PANIlong NF (without Pt NPs) are transformed into N-CNFs by a carbonization procedure in a tube furnace. The carbonization was done with a heating rate of 1 K/min under nitrogen flow and the final temperatures, 750°C and 1000°C, were held for 90 min. The PANIshort and the Pt/PANIshort NF were carbonized with a heating rate of 5 K/min up to 1000°C. The commercially available Pt/C (20 wt% Pt) catalyst, Hispec 3000, was obtained from Alfa Aesar GmbH & Co KG, Karlsruhe, Germany.


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2.2 Methods TEM was performed with a FEI CM20 operating at 200 kV acceleration voltage with a LaB6 filament. For sample preparation a small amount of the powder was dispersed in ethanol using ultrasonic dispersion and one drop was transferred to a standard holey carbon film-covered copper grid. NEXAFS spectra at the C K-edge and N K-edge were measured in partial electron yield (PEY) mode at the HESGM Beamline at BESSY II (Helmholtz Zentrum Berlin, Germany).25 The carbon sample powders were pressed into indium foil and mounted on a corresponding sample holder. NEXAFS data were recorded at an incidence angle of 55° using linearly polarized light. The spectra were measured with an energy resolution of 0.3 eV from 275 to 330 eV and 390 to 440 eV, respectively. The raw data were divided by a spectrum of a freshly sputtered Au wafer to account for the dependence of the direct beam intensity on the photon energy and carbon contamination of the beamline optics and normalized to an edge jump of 1.26 X-ray photoelectron spectroscopy (XPS) was performed using monochromated Al Kα radiation. The detector was a Phoibos 150 MCD hemispherical electron analyzer (SPECS Surface Nano Analysis GmbH). The spectrometer was calibrated using the Fermi edge and core line positions of the noble metals Cu, Ag and Au. In the following the energetic positions of the emission lines are displayed “as measured”. After a survey scan, the O 1s, C 1s, N 1s and Pt 4f are measured in detail. The spectra are analyzed with the software XPSPEAK 4.1. Therefore the background is corrected by the Shirley method and the intensity maxima are fitted by Voigt profiles.



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Thermogravimetric analysis was conducted in a thermobalance STA 449 C Jupiter (NETZSCH) under a constant nitrogen stream and heating rate of 2 K/min. Linear sweep voltammetry (LSV) and CO stripping were done with a Gamry potentiostat in a glass cell in 0.1 M KOH versus a saturated calomel electrode (SCE). A Pt wire served as counter electrode. An Autolab glassy carbon (GC) disk electrode (d = 3 mm) with the corresponding catalysts was used as working electrode. Inks with the dispersed catalyst powders were produced and 5 µl (for the Pt/C, 4 µl) were drop casted on the GC electrode. The inks were produced from 31.3 mg Pt/PANIlong1000, 17.6 mg Pt/PANIlong750 and 21.3 mg Pt/C powders dispersed in 1700 µl H2O, 200 µl isopropanol, 100 µl Nafion, respectively. CO adsorption was done at -0.7 V vs. SCE in a CO saturated solution. Subsequently, CO was stripped from the surface by cycling the potential between -0.95 and 0.1 V vs. SCE after purging with N2 for 1h. For the successive linear sweep voltammetry measurement (LSV), the solution was saturated with O2 and potential cycling was performed between -0.7 and 0 V vs. SCE at several rotation speeds (400, 600, 900, 1200, 1600 and 1800 rpm).

3. RESULTS 3.1 Characterization of PANI and Pt/PANI NFs The synthesized PANI NFs exhibit a green color indicating that the PANI chain is protonated.13 TEM images show a uniform and homogeneous Pt NP size dispersion (Figure 1). The average sizes of the Pt NPs, determined by TEM, are 2.7 ± 0.8 nm and 2.5 ± 0.7 nm for Pt/PANIlong and Pt/PANIshort, respectively. 8 ACS Paragon Plus Environment

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Figure 1. TEM images and histogram of the Pt NP size distribution of Pt/PANIlong (A) and PtPANIshort (B). Although both materials have a different morphology, very similar C K-edge and N K-edge NEXAFS spectra (Figure 2) are obtained. At the C K-edge, besides the π* transition at 285.5 eV for C=C, additionally two π*-transitions at 283.9 eV and 287 eV are observed, for carbon atoms bonded to an imine and amine group, respectively.27 However, the π* transitions related to the imine and amine are more pronounced for PANIshort. The N K-edge of PANIlong and PANIshort exhibits two π* transitions at 397.5 and 399.2 eV also related to the imine and amine groups, respectively. For the short PANI, in agreement with the C K-edge NEXAFS the π* peaks are more 9 ACS Paragon Plus Environment


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intense. The NEXAFS data show in principle, that the molecular structures of PANIshort and PANIlong are very similar. This observation is in agreement with the current literature reporting that the molecular structure is independent of the PANI morphology.28 However, the more intense feature related to the N-groups in the PANIshort could be attributed to a larger amount of N-groups due to cross linking N-groups between the PANI chains.12 Moreover, the Pt deposition on PANIshort and PANIlong induces almost the same effect on the PANI structure as shown in Figure 2. The peak intensity of the π* transition at 285.5 eV decreases, which indicates, that charge is transferred from the Pt metal towards the πconjugated systems of the aromatic ring. Furthermore, the peak at 287 eV (π*, C-NH) vanishes or decreases in intensity and shifts to lower energies for both Pt/PANIlong and Pt/PANIshort, respectively. From the N K-edge it can be clearly seen, that the intensity ratio between the π* peak at 397.5 and 399.2 eV and thus the ratio between imine and amine group changes. Hence, it can be concluded that amine groups act as nucleation centres for Pt NPs leading to an increase of the electron density of the π-conjugated systems of the aromatic ring as observed by the C K-edge NEXAFS spectra. This strong interaction explains the high dispersions of Pt NPs on PANI as they were found in the TEM measurement.


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Figure 2. C K-edge (A) and N K-edge (B) NEXAFS spectra of Pt/PANIlong, PANIlong, Pt/PANIshort and PANIshort.

3.2 Carbonization of PANI and Pt/PANI TGA data of PANIshort and PANIlong are depicted in Figure 3. The loss of organic material is about 45% for both samples, which is in agreement with other reports.14 The derivative of the thermo gravimetric curve (DTG) shows four pronounced weight losses (indicated by peak minima) at around 225°C, 290°C, 470°C and 700°C for PANIlong (Figure 3A, B). The weight loss between 200°C and 350°C originates from a deprotonation of PANI and the removal of the corresponding HSO4- and SO42- anions.17 Between 350 and 750°C, Yin et al.16 observed the release of SO3 and ammonia and attributed it to the destruction of PANI chains which in the case of Trochvá et al.14 were accompanied by the formation of phenazine-like segments. As shown in Figure 3A, the carbonization process for PANIshort and PANIlong is different. For PANIshort the minimum at 290°C in the DTG curve is not observed. That allows us to make a conclusion about an absence of enhanced weight loss at this temperature. Hence, from 11 ACS Paragon Plus Environment


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PANIshort compared with PANIlong less sulfate and hydrogen sulfate are removed between 200°C and 350°C indicating that both PANI are differently protonated. Although for PANI, synthesized in 1 M H2SO4 (here PANIshort) and PANI synthesized in 0.4 M acetic acid (here PANIlong) very identical anions have been found by FTIR measurement.12 Moreover, the weight loss of PANIshort with the minimum in the DTG curve at 470°C is more pronounced compared to the weight loss of PANIlong indicating that the anions, which were not able to leave the PANIshort at lower temperatures, might be removed now together with parts of segregated PANI chains.

Figure 3. Thermo gravimetric (TG) curve and derivative of the TG (DTG) curve comparing PANIlong with PANIshort (A) and PANIlong with Pt/PANIlong (B). The mass of Pt/PANIlong is corrected for the Pt content in the sample, which is about 14.3% and the absolute amount of Pt is supposed to remain constant during the carbonization. 12 ACS Paragon Plus Environment

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A similar DTG curve as for PANIshort is observed for Pt/PANIlong (Figure 3B). Only one broad minimum is measured instead of the two separate minima at 225°C and 290°C. The enhanced mass loss with the minimum in the DTG curve at around 470°C is not observed. Furthermore, the polymer mass loss of Pt/PANIlong is lower with 37.8% in comparison to the weight loss (44.6%) of PANIlong. This shows that for Pt NPs in the sample a larger amount of the PANI chains are cross linked to build-up the carbon network. Hence, the N-groups in the resulting NCNFs are likely to vary with the carbonization and the Pt content of the PANI samples. Differently carbonized PANI samples, N-CNFs, are analysed by NEXAFS spectroscopy in order to investigate the structural changes for various carbonization conditions. In Figure 4, C K-edge and N K-edge NEXAFS data are shown for PANIlong and Pt/PANIlong carbonized at 750°C and 1000°C and PANIshort and Pt/PANIshort carbonized at 1000°C. For comparison, a C K-edge NEXAFS spectrum of Pt/C is also measured. C K-edge NEXAFS spectra of the N-CNF show a broad peak at around 285 eV corresponding to π* transitions for the different carbon atoms in a defect-rich graphitic like structure similar to the spectra measured for Pt/C. The peak at 285 eV is narrower in PANIlong1000 and PANIshort1000 compared with PANIlong750 indicating that with increasing temperature the structural ordering increases. N K-edge spectra indicate π* transitions at 398.5, 400 and 401.4 eV, which can be related to pyridinic, amino and pyrrolic/graphitic type N-groups.29 The amount of pyridine and amino type N-groups decreases upon carbonization at 1000°C as found for PANIlong1000. For the PANIshort1000 N-CNFs, significant amounts of pyridinic and amino N-groups are still present which is related to the low residence time at 1000°C during carbonization. An effect of Pt on the N-groups is found for the Pt/PANIlong750 and the Pt/PANIshort1000. Hence, lower final temperatures and shorter 13 ACS Paragon Plus Environment


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residence times will lead to larger amounts of pyridinic and amino N-groups whereas pyrrolic/graphitic type N-groups start to dominate at higher carbonization temperatures. The N K-edge spectra show an increased intensity of the π* transition at 400 eV for Pt/PANIlong750 and Pt/PANIshort1000 compared to the corresponding PANIlong750 and PANIshort1000. This increased peak intensity for the Pt containing samples indicates an electronic interaction between Pt atoms and N-groups.

Figure 4. C K-edge (A) and N K-edge (B) NEXAFS spectra of the differently carbonized N-CNFs, Pt/N-CNFs.

3.3 Structural characterization and electrochemical activity of Pt/N-CNFs The interaction between Pt NPs with N-groups is investigated quantitatively by XPS analysis. Therefore, XPS spectra are recorded for PANIlong750, Pt/PANIlong750, Pt/PANIlong1000 and a conventional Pt/C (20 wt% Pt) catalyst. The N1s spectra and Pt4f spectra are shown in Figure 5 and Figure 6, respectively. At first, the N1s spectrum of PANIlong750 is deconvoluted by 14 ACS Paragon Plus Environment

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assuming the presence of five different N-species in the sample, namely pyridine, 298.2 eV, amine/amide 399.6 eV, pyrrolic 400.7 eV, graphitic/quaternary, 401.8 eV and oxidic N-species, 404.0 eV.29,30,31,32 In a second step, N1s spectra of Pt/PANIlong750 and Pt/PANIlong1000 are fitted using the same components as in the fitting procedure for PANI750 and an additional sixth contribution at 399 eV. This additional component is required for the fit and accounts for the Pt-N interaction. The binding energy (BE) of N-Pt is found to be similar as observed for metal nitrogen interactions in non-noble metal catalysts.33 All BEs, full width half maxima (FWHMs) and the relative amounts of the different components are given in Table 1. In agreement with the NEXAFS data (Fig. 4), the Pt/PANIlong1000 contains a lower amount of pyridinic and a larger amount of graphitic N-type groups. However, a very similar relative amount of the component reflecting the Pt-N interaction with 8 and 10% is found for Pt/PANIlong750 and Pt/PANIlong1000, respectively.



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Figure 5. N 1s XPS spectra and the corresponding deconvoluted single components of PANIlong750, Pt/PANIlong750 and Pt/PANIlong1000.

Figure 6. Pt 4f XPS spectra of Pt/PANIlong750, Pt/PANIlong1000 and Pt/C and their deconvolution into Pt0, Pt2+ and Pt4+ contributions.


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Table 1: Binding energy [eV], FWHM and peak areas A [%] of the single N 1s components of Pt/PANIlong750, Pt/PANIlong1000 and Pt/C. Peak

PANIlong750

Pt/PANIlong750

Pt/PANIlong1000

BE [eV]

FWHM [eV]

[%]

[%]

[%]

398.21

1.1

28

22

13

399.02

1.1

x

10

14

399.63

1.1

10

9

8

400.74

1.1

41

37

44

401.85

1.7

11

11

20

404.06

3.6

10

11

n.a.

1

2

N-species: pyridinic, Pt-N,

3

4

5

6

amino, pyrrolic, graphitic/quaternary higher oxidized

The Pt4f spectra of Pt/PANIlong750, Pt/PANIlong1000 and Pt/C are fitted with three transitions corresponding to the different oxidation states of Pt (Pt0, Pt2+ and Pt4+).34,35 The fit results are shown in Table 2. The oxidation state of Pt for both PANI based samples is largely increased in comparison with the reference Pt/C catalyst. As discussed recently, the presence of higher oxidation states of Pt can be attributed to Pt oxide phases, which vary with the NP size as the relative amount of surface atoms increases with decreasing NP size.36 However, the NP size of the Pt/C catalyst is about 2 nm.19 A very similar or larger NPs size is found by TEM measurements for Pt/PANIlong750 and the Pt/PANIlong1000, respectively (Figure 7). Thus, it is more likely that the observed increased oxidation state for Pt/PANIlong750 and 17 ACS Paragon Plus Environment


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Pt/PANIlong1000 is due to a shift of electrons towards the N-groups and the carbon network. The interrelation between the N-groups and the carbon network works most probably in a similar way as observed for the Pt/PANI, where also an increase in the electron density of the πconjugated systems is observed by C-K edge NEXAFS spectroscopy. However, in the case of the carbonized sample the amount of N-dopants is much lower and thus the increase in the electron density of the π-conjugated system is very small and therefore not visible in the C-K edge NEXAFS spectra. Beyond the oxidation state of the Pt, also the BE of the Pt is affected by an electronic interaction.21,37 Hence a decrease in the electron density of the Pt would result in an increase of the Pt4f BE. However, also the Pt4f BE for Pt/C might be shifted due to an interaction with oxygen functional groups existing on carbon blacks,38,39 thus it is difficult to draw any conclusion by comparing the Pt/C and Pt/N-CNFs Pt4f BEs.

Table 2 Binding energy [eV], FWHM and peak areas A [%] of the single Pt4f components of Pt/PANIlong750, Pt/PANIlong1000 and Pt/C. Pt/PANIlong750

Pt/PANIlong1000

Pt0

Pt2+

Pt4+

Pt0

Pt2+

BE [eV]

71.2

72.1

72.9

71.0

71.9

FWHM [eV]

0.8

1.1

2.3

0.8

A [%]

35

31

34

23

Pt4+

Pt/C Pt0

Pt2+

Pt4+

72.2

71.4

72.4

73.9

1.4

3.3

1.1

1.1

2.2

36

41

59

19

22

Besides electronic interaction also the NPs size is known to influence the ORR.40,41 Therefore, TEM measurements of Pt/PANIlong750 and Pt/PANIlong1000 (Figure 7A,B) are performed. In 18 ACS Paragon Plus Environment

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both N-CNF samples, carbon nanotubes are found with diameters ranging between 100-200 nm similar to the PANI NFs showing that the morphology of the samples is maintained. However, both N-CNFs exhibit huge differences in the Pt NP size distribution. The Pt/PANIlong750 has a mean particle size of 1.7 nm, and no increase in the particle size is observed compared to Pt/PANIlong. These findings are in agreement with a previously published study where the NP size distribution of a similarly produced sample is preserved after electrochemical aging.19 For the Pt/PANIlong1000 a much larger average particle size of 10 nm with a broad particle size distribution is observed. The dramatic growth in the Pt-NPs size for Pt/PANIlong1000 and the preserved particle size for Pt/PANIlang750 indicate that in both catalysts different N-type groups interact with the Pt-NPs. E.g. it is discussed that pyridinic edge sites prevent Pt agglomeration,8,20 which fits well with our observation that in the Pt/PANIlong1000 pyridine type N-groups exist to a smaller extent compared to the Pt/PANIlong750. Moreover, by TEM investigations it is also found for both N-CNFs, that most of the Pt NPs are either placed inside the carbon or are surrounded by a thin layer of carbon material. This is shown for Pt/PANIlong1000 in Figure 7B (left, top) and most probably affects the accessible Pt NP surface area.



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Figure 7. TEM images and histograms of the Pt NP size distribution of Pt/PANIlong750 (A) and Pt/PANIlong1000 (B).

The specific electrochemical activity of Pt/PANIlong750, Pt/PANIlong1000 and Pt/C for the ORR was determined in alkaline solution and is based on the assumption that the following reactions take place:42 O2 O2 (ads) O2 (ads) +H2O+2e− HOO− (ads) +OH−

HOO- (ads) + 2H2O + 2e- 3OH-,

with Eeq = 0.867 V.


(1)

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Therefore, RDE measurements for different rotation speeds ω (Figure 8) are performed and evaluated by Koutecky-Levich analysis.43 The Koutecky-Levich equation,

, describes

the current density i with the diffusion limited current density id and the kinetic current density ik. The kinetic current density ik is determined for various potentials by the interception of the linear fit of the reciprocal limiting current density 1/id plotted vs. ω-1/2 (in Figure 8A, B and C). Finally, the exchange current densities are evaluated from Figure 8D and decrease in the following order for Pt/PANIlong1000 > Pt/C > Pt/PANIlong750 with the respective observed values, 1.73 10 A cm-2 , 2.8 10 A cm-2 , 1.82 10 A cm-2.



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Figure 8. Linear sweep voltammetry obtained in O2 saturated KOH (0.1 M) at several rotational speeds ω for Pt/PANIlong750 (A), Pt/PANIlong1000 (B) and Pt/C (C). (D) Tafel plots of the kinetic limited current density ik determined by Koutecky-Levich analysis of the LSV measurements. (ik is obtained by normalizing Ik by the respective ECSAs, 36.1 cm2 mg-1 x 0.043 mg, 6.3 cm2 mg-1 x 0.063 mg and 16.0 cm2 mg-1 x 0.044 mg for Pt/C, Pt/PANIlong1000 and PtPANIlong750, determined by CO stripping experiments.)

The electrochemical analysis shows the following: 1. The obtained ECSAs are much smaller for the Pt/PANIlong750 and Pt/PANIlong1000 sample compared to Pt/C, although the carbonized PANI based catalysts have a similar Pt content with 24 wt% and 17°wt%, respectively. 2. The observed exchange current densities are found to be the highest for the Pt/PANIlong1000, while the value obtained for Pt/PANIlong750 was found to be very similar to the conventional Pt/C catalyst. In the case of Pt/PANIlong750 the low graphitization degree of the sample could be detrimental to its activity for the ORR.44 Pt/PANIlong1000 has a very similar graphitization level as the Pt/C catalysts as found by NEXAFS spectroscopy and the increase in ORR activity could be attributed to the N-doping of the catalyst support. However, also the larger Pt NP size is known to increase the ORR activity,40,41 but only a weak effect was found in KOH solution.40 Thus, it is more likely that N-doping is affecting the ORR here. Nevertheless, the effect of N-doping on the ORR is still under debate. Simply the presence of graphitic type N-groups affects the physical properties of 22 ACS Paragon Plus Environment

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the carbon support as electron density is added to the carbon network.45 Moreover, also the carbon support in the Pt/C catalysts has a lot of oxygen functional groups which might also influence the ORR activity as it is known that a spill-over of H and O-groups from carbon towards the Pt NP takes place,46 and a Pt-O bond also alters the ORR kinetics.47 The graphitic Ntype groups could just prevent the formation of O-type functional groups on carbon. Finally, the change in the electronic structure of the Pt as found by XPS could facilitate the reaction kinetics of the ORR.

CONCLUSIONS Pt NPs supported on N-CNFs are prepared through carbonization of PANI NF precursor materials with deposited Pt. The combination of NEXAFS, XPS, TEM and electrochemical measurements demonstrates on the one hand that highly active Pt supported N-doped CNF catalysts can be produced by this synthesis approach and on the other hand how Pt NP size and the kind of Ngroups are influenced by the synthesis. By NEXAFS spectroscopy it was shown that Pt interacts with the N-groups in the PANI shifting electrons from Pt NPs into the π-conjugated system thereby increasing its electron density. Hence, the N-groups work as nucleation centres for Pt NPs which explains the high Pt dispersions found by TEM measurements. Moreover, also the carbonization of the PANI NFs is altered by this strong electronic interaction influencing the structure of the produced N-CNFs. It is observed that in the presence of Pt NPs a larger amount of PANI chains are used to form the carbon network. By NEXAFS spectroscopy and XPS analyses of N-CNFs a change in the electronic 23 ACS Paragon Plus Environment


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structure of the Pt due to the interaction with N-groups is observed. The very different Pt dispersion obtained for both N-CNFs indicates that in both materials different N-type groups might interact with the Pt leading to a narrow particle size distribution by an interaction with pyridinic N-groups in Pt/PANIlong750. In agreement, a broad particle size distribution with a large mean particle size is found for N-CNFs containing mainly graphitic type N-groups (Pt/PANIlong1000). The ORR activity of this N-CNFs sample (Pt/PANIlong1000) reveals a largely enhanced activity compared with a conventional Pt/C catalyst and the effect of N-doping on the ORR kinetics is discussed.

ASSOCIATED CONTENT Supporting Information Available: XPS survey scan and the corresponding determined elemental compositions for PANIlong750, Pt/PANIlong750, Pt/PANIlong1000. This material is available free of charge via the Internet at http://pubs.acs.org.

ACKNOWLEDGEMENTS We thank Helmholtz Zentrum Berlin for the allocation of synchrotron radiation beamtime. A.N. and C.W. acknowledge funding from the "Science and Technology of Nanosystems" program (Project No. 431103-Molecular Building Blocks/Supramolecular Networks). B.P. acknowledges the financial support of the German Science Foundation (under contract number Ro2454/10-1). Furthermore, Lars Riekehr is thanked for the TEM measurements of the carbonized samples.


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