Improved durability of Pt catalyst supported on N-doped mesoporous graphitized carbon for oxygen reduction reaction in polymer electrolyte membrane fuel cells

Accepted Manuscript Improved durability of Pt catalyst supported on N-doped mesoporous graphitized carbon for oxygen reduction reaction in polymer electrolyte membrane fuel cells Won Suk Jung, Branko N. Popov PII:

S0008-6223(17)30711-X

DOI:

10.1016/j.carbon.2017.07.028

Reference:

CARBON 12194

To appear in:

Carbon

Received Date: 25 March 2017 Revised Date:

21 June 2017

Accepted Date: 10 July 2017

Please cite this article as: W.S. Jung, B.N. Popov, Improved durability of Pt catalyst supported on Ndoped mesoporous graphitized carbon for oxygen reduction reaction in polymer electrolyte membrane fuel cells, Carbon (2017), doi: 10.1016/j.carbon.2017.07.028. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Graphical Abstract

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Won Suk Jung*, Branko N. Popov*

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Improved Durability of Pt Catalyst Supported on N-doped Mesoporous Graphitized Carbon for Oxygen Reduction Reaction in Polymer Electrolyte Membrane Fuel Cells

Center for Electrochemical Engineering, Department of Chemical Engineering, University of

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Carbon

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South Carolina, Columbia, SC, USA 29208

March 2017

__________________________________ ___ *Corresponding author. E-mail: [email protected] (W.S. Jung), [email protected] (B.N. Popov) 1

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Abstract N-doped mesoporous graphitized carbon (NMGC) was prepared by the pyrolysis of chelating

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complex adsorbed on carbon. The NMGC shows a higher degree of graphitization than carbon black as evidenced by the change of specific surface area, interlayer spacing, and ID/IG value. The stable quaternary nitrogen type is dominantly observed in the NMGC incorporated with Fe

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(NMGC-Fe). Pt particles are well-distributed on the NMGC-Fe in the range of 2-3 nm. The Pt/NMGC-Fe catalyst shows remarkably improved durability after 30,000 potential cycles

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between 0.6 and 1.0 V. The maximum power density and the potential loss of Pt/NMGC-Fe catalyst decrease by 26% and 83 mV at 600 mA cm−2. The commercial Pt/C catalyst shows the very poor durability of 52% loss of maximum power density and zero potential at 600 mA cm−2. The electrochemical surface area of Pt/NMGC-Fe catalyst is more stable than the commercial Pt/C catalyst. The interaction between Pt and NMGC-Fe contributes strongly to the observed

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high durability under practical operating conditions.

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1. Introduction Polymer electrolyte membrane fuel cells (PEMFCs) are attracting attention as new

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power sources for automotive and stationary applications, due to intrinsic advantages such as low emission, high energy density, and high efficiency. The Pt exhibits the highest activity and selectivity in the pure metals for oxygen reduction reaction (ORR) [1, 2]. However, there are still

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activity and durability issues to be solved for the commercialization. In general, the Pt is rarely present in the world and an expensive precious metal, which causes the necessity for the research

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of increasing the activity of Pt.

Many efforts such as Pt-alloy [3, 4], Pt skin [5, 6], dealloyed Pt core-shell [7, 8] catalysts have been made aiming to decrease the amount of Pt and increase the catalytic activity for ORR. Non-noble metal catalysts are an alternative for the Pt catalyst but their activity and durability should be increased for the commercialization. Recently, researchers have been attracted by N-

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doped carbon as a support for Pt catalyst since it has been known that N-doping increases conductivity [9, 10], mesoporosity [11] and catalytic activity [12-15]. Cheng et al. reported the

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N-self-doped 3-dimensional graphene-like networks (N-3D GNs) can be prepared by improved ion-exchange/activation method [13]. Pt/N-3D GNs showed 2.6 times higher catalytic activity

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than the commercial Pt/C catalyst. Zhang et al. prepared N-doped carbon black (NCB) by treating Vulcan XC-72R with NH3 gas at 600 °C to dope ca. 1 at% surface nitrogen content [14]. Pt/NCB catalyst exhibits simultaneously smaller particle sizes and higher electrochemical surface area and electrocatalytic activity for the ORR than the Pt/CB catalyst prepared with the same method. They explained that the oxygenated adsorbates formed on Pt are transported to surface of the NCB support, which may be responsible for the increased area specific activity of the Pt/NCB catalyst. Nitrogen-doped onion-like carbon (N-OLC) prepared by arc discharge in a 3

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liquid phase in the presence of different ammonia concentrations was used as a support of Pt [16]. Pt/N-OLC catalyst with 1.7 at% nitrogen exhibited the higher electrochemically active surface

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area, specific current density and half-wave potential than the commercial Pt/C catalyst. The enhanced oxygen reduction could be ascribed to the defective outermost layers and the electronic modification. X-ray photoemission spectroscopy (XPS) has also been used to explain the effects

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of N-doped carbon [17]. In contrast to undoped carbon, the N-doped carbon-supported Pt or Pd showed a clear binding energy shift after the deposition of metals, indicating a better electronic

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interaction between the metal and the N-doped mesoporous carbon support due to the destabilization induced by nitrogen atoms on the delocalized double bond present in the undoped structure [18]. In particular, the interaction between Pt nanoparticles and N-doped mesoporous carbon resulted in high ORR activity and high stability in acidic solutions as compared to commercial Pt/C catalyst. Recently, a research for the direct synthesis of N-doped carbon

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aerogels (NCAs) was reported [19]. Compared with a Pt on carbon aerogel synthesized by a conventional reduction method, the Pt/NCA showed enhanced electrochemical performance with

reduction.

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a high electrochemically active surface area and electrocatalytic activity towards oxygen

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However, it is not clear that the role of transition metal in the synthesis of N-doped carbon supports on improving the performance and durability of Pt catalyst under the practical conditions. In this study, the N-doped mesoporous graphitized carbon (NMGC) used as supports for the Pt deposition was prepared by the pyrolysis of M-chelate complex (M=Ni or Fe) adsorbed on carbon black (CB). Metal salt was initially chelated with ethylenediamine as a nitrogen source, followed by mixing with CB. The M-chelate complex thus prepared went through high temperature pyrolysis to graphitize the carbon sources in an inert atmosphere. After removal of 4

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unstable metal particles on the carbon surface, the resultant was used as a support for the Pt. Performance and durability of Pt/NMGC catalysts were measured in a single cell supplying

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H2/air with 40% RH at 80 °C at a catalyst loading of 0.1 mgPt cm−2. Pt/NMGC catalysts showed significantly improved durability suppressing the particle size growth and the aggregation as compared to the commercial Pt/C catalyst. For comparison, the commercial state-of-the-art Pt/C

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catalyst was examined in a single cell under the same experimental condition.

2.1 Preparation of support and catalyst

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2. Experimental

The NMGC was prepared with the chelation method. First, the CB (Ketjen Black EC 300J, Akzo Nobel) was refluxed with concentrated nitric acid at 85 °C. 0.15 M ethylenediamine (Aldrich) and 0.017 M Fe(NO3)3 or Ni(NO3)2 solution in 250 ml isopropyl alcohol (IPA, BDH)

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as a nitrogen source and catalyst for the graphitization, respectively, were mixed with oxygenfunctionalized CB homogeneously. Then the mixture was refluxed at 85 °C for 3 hr to anchor the

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metal-chelated complexes on the carbon surface. The rotary evaporator (Rotavapor® R-210, Buchi) was used to remove the volatile solvent. After fully dried, the resultant was transferred to

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the tubular furnace (OTF-1200X-SNT-110, MTI corp.) to form the graphitic carbon structure containing nitrogen at 1100 °C for 1 hr under N2 atmosphere. The pyrolyzed sample was leached with 0.5 M H2SO4 at 80 °C for 4 hr to remove unstable metal species. After filtering and washing with de-ionized (DI) water, the sample was used as a support of Pt catalyst. NMGCs prepared with Fe or Ni as transition metal source are denoted as NMGC-Fe or NMGC-Ni, respectively. Pt deposition was carried out using the polyol method. Noncovalently functionalized support by the pyrenecarboxylic acid was mixed with the desired amount of PtCl4 (Alfa Aesar). 5

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After adjusting to pH=11 with 0.5 M NaOH solution, the resulting solution was refluxed at 160 o

C for 3 hr and allowed to cool to room temperature. Then, the solution was filtered, washed with

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DI water, and dried at 160 °C for 1 h.

2.2 Physical characterization

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The nitrogen adsorption/desorption isotherms were obtained at -196

Quantachrome NOVA 2000 BET analyzer. Specific surface area was determined by a multipoint

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Brunauer-Emmett-Teller (BET) analysis. Pore size distribution (PSD) curves were calculated by the Barrett–Joyner–Halenda (BJH) method using the adsorption/desorption branch. X-ray diffraction (XRD) analysis was performed using a Rigaku D/Max 2500 V/ PC with a Cu Kα radiation. A tube voltage of 30 kV and a current of 15 mA were used during the scanning. To estimate the particle size of samples, we employed the following Scherrer equation: kλ 10 B cos θ

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where D is the particle size in nm, k is a coefficient taken here as 0.9, λ is the wavelength of X-

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ray (0.15404 nm), B is the line broadening at half the maximum intensity in radians. And θ is the angle at the position of the maximum peak known as Bragg angle. Raman spectroscopy was used

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to evaluate the degree of graphitization of the carbon supports using HORIBA "LABRAM 1B” (He-Ne 20mW laser, wavelength 632.817 nm). XPS was carried out with a Kratos AXIS 165 high-performance electron spectrometer on samples to determine the elemental surface composition. High resolution transmission electron microscope (HR-TEM) was taken by Hitachi 9500 HRTEM operated at 300 kV accelerating voltage.

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2.3 Electrochemical characterization A glassy carbon disk electrode (0.247 cm2) was acted as a working electrode. The

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Ag/AgCl electrode and platinum mesh were used as a reference and counter electrodes, respectively. All electrode potentials reported here were converted into the RHE. In a typical rotating disk electrode (RDE) experiment, RDE tests were performed in 0.1 M HClO4 solution as

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an electrolyte at room temperature using a Pine bi-potentiostat (Model AFCBP1). Pt catalyst was mixed with IPA and DI water ultrasonically. The catalyst ink was deposited onto the glassy

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carbon electrode, leading to a catalyst loading of 20 µgPt cm−2. For all RDE tests, 5 µl of 0.25 wt% ionomer (Alfa Aesar) was additionally deposited on the catalyst layer to ensure good adhesion of the catalyst onto the glassy carbon electrode. The linear sweep voltammetry (LSV) was measured at a scan rate of 5 mV s−1 by sweeping the potential between 0.2 V and 1.05 V under O2 purging. The LSV curves presented in this work were properly corrected using the

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background capacitance current that was measured in the N2 atmosphere at a scan rate of 5 mV s−1. The cyclic voltammetry (CV) was swept at a scan rate of 50 mV s−1 from 0.005 to 1.0 V in

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deaerated electrolyte under N2 atmosphere.

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2.4 MEA fabrication and single cell operation The Pt/NMGC catalyst was employed as the cathode catalyst, while commercial Pt/C

catalyst (TEC10E50E, Tanaka Kikinzoku Kogyo K.K.) was used as a catalyst for the anode. Catalyst inks were prepared by ultrasonically mixing the appropriate amount of catalysts, IPA, Nafion® ionomer (5% solution, Alfa Aesar), and DI water. The ionomer content was 30% and 20% in the anode and cathode inks, respectively. The catalyst inks were sprayed directly on the Nafion® 212 membrane covering an active area of 25 cm2. The Pt loading on the anode and 7

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cathode electrodes is kept at 0.1 and 0.1 mg cm−2, respectively. The catalyst coated membrane was then hot pressed at 140 °C using a pressure of 20 kg cm−2 for 6 min. in between the gas

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diffusion layers (Sigracet GDL 10BC, SGL) and Teflon gaskets to prepare the MEA for the performance evaluation studies in fuel cells.

The fuel cell polarization was conducted using a fully automated fuel cell test station

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(Scribner Associates Inc., model 850e) at 80 °C. H2 and air were supplied to the anode and cathode at the constant stoichiometry of 1.5 and 1.8 applying the backpressure of 150 kPaabs. The

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measurement was carried out at 40 % relative humidity (RH). The electrochemical surface area (ECSA) was estimated using CV experiments carried out between 0.05 and 0.6 V (vs. RHE) at 80 °C under fully humidified H2 and N2 supply to the anode and the cathode, respectively. The ECSA measurements were performed periodically up to 30,000 potential cycles. The accelerated stress test were conducted by supplying 200 sccm H2 and 75 sccm N2 to the anode and cathode,

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respectively and sweeping the potential between 0.6 and 1.0 V (vs. RHE) at 50 mV s−1 in a triangle profile for up to 30,000 potential cycles, since it has been estimated that the cathode

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catalysts undergo 30,000 potential cycles under operating conditions. For comparison purposes, MEAs with commercial Pt/C catalyst as cathode catalysts was also prepared and evaluated under

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the same experimental conditions.

3. Results and discussion

3.1 Characterization of supports Fig. 1 shows the nitrogen adsorption-desorption isotherms and BJH PSD curves of CB, NMGC-Fe, and NMGC-Ni. Fig. 1a shows that the CB, NMGC-Fe and NMGC-Ni exhibit characteristic Type IV indicating the mesoporous nature [20]. The isotherms show hysteresis 8

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loop with sharp adsorption and desorption branches over a relative pressure range of 0.4-0.8. The nitrogen uptake is observed when (P/P0) ratio is 0.94-1.0, which indicates the presence of

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mesopores. The specific surface area of NMGC-Fe and NMGC-Ni were 230 and 308 m2 g−1, while the CB has 826 m2 g−1. After the pyrolysis, the peak pore diameter which provides the maximum differential pore volume in BJH PSD did not change, as shown in Fig. 1b inset. At the

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pore diameter of 3 nm or less, the pore volume of CB is increased, while those of NMGC-Fe and NMGC-Ni are decreased. Thus, the results indicated a tendency for formation of the highly

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crystallized surface as a result of the removal of amorphous carbon by the catalyzed graphitization of carbon sources.

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Relative Pressure [P/P0 ] Fig. 1. (a) N2 adsorption/desorption isotherms and (b) BJH pore size distribution curves obtained from the adsorption branch of NMGC-Fe, NMGC-Ni, and CB. The inset in (b) compares the pore size distribution in the range 0-10 nm.

The Raman spectra for the NMGC-Fe, NMGC-Ni, and CB in Fig. 2 exhibit the D band

and G band at ca. 1350 and1580 cm−1, respectively. The D band originates from structural defects and disorder-induced features on carbon, while the G band corresponds to the stretching vibration mode of graphite crystals [21, 22]. The relative ratio of D band to the G band (ID/IG) for

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the NMGC-Fe, NMGC-Ni, and CB is estimated to be 1.47, 1.71 and 2.75, respectively, indicating that the NMGC-Fe is more graphitized than the NMGC-Ni and CB. G

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Fig. 2. Raman spectra of NMGC-Fe, NMGC-Ni, and CB.

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For further analysis on the degree of graphitization, XRD patterns of NMGC-Fe,

NMGC-Ni, and CB are shown in Fig. 3a. The diffraction peaks of CCC are sharper with increased intensity and shift to more positive angles. Consequently, the interlayer spacing (d(002)) calculated by Bragg’s law based on (002) planes of carbon showed 0.3385, 0.3417 and 0.3613 nm for NMGC-Fe, NMGC-Ni, and CB, respectively. The previous results indicated that the Fe is more efficient catalyst than the Ni for the graphitization of carbon surface since the theoretical

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d(002) of graphite crystallite is 0.3354 nm [23-25]. These results are supported by HR-TEM images. HR-TEM images of NMGC-Fe, NMGC-Ni and CB are shown in Fig. 3b-d, respectively.

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NMGC-Fe clearly shows the well-defined graphitic layers when compared to NMGC-Ni and CB. Thus, the degree of graphitization is also determined by the nature of the catalyst used in the synthesis. It is well-known that the Fe catalyst shows the highest degree of graphitization in the

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preparation of carbon nanotubes in a wide range of temperature [26]. Raman spectroscopy, XRD and HR-TEM images are in good agreement with their results indicating that the Fe is a more

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efficient catalyst for the graphitization than the Ni.

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(b)

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Intensity [a.u.]

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Fig. 3. (a) XRD patterns of NMGC-Fe, NMGC-Ni, and CB. HR-TEM images of (b) NMGC-Fe, (c) NMGC-Ni and (d) CB. Scale bar represents 10 nm.

To investigate the near-surface chemical analysis on the NMGC-Fe and NMGC-Ni, XPS

analysis is employed. The elemental analysis on the carbon surface exhibited that ca. 1% nitrogen was doped and no metal was found on the surface for both NMGC-Fe and NMGC-Ni as shown in Table 1. Fig. 4a and b show the N1s XPS spectra of NMGC-Fe and NMGC-Ni, 12

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respectively. The N1s XPS spectra are deconvoluted into major 4 types ascribed as NI, NII, NIII and NIV [27]. NI represents the pyridinic-N, that is, the nitrogen bonded to two carbon atoms in

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six-membered rings at the edge of graphene layer [28]. NII is the pyrrolic-N in a five-membered ring and/or pyridonic-N that is pyridinic-N in association with phenolic or carbonyl group on the neighbor carbon atom of the ring [28]. NIII is attributed to the quaternary-N, that is, a nitrogen

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atom replacing a carbon atom within a graphite plane and being bonded to three carbon atoms [29-31]. NIV is oxidized pyridinic nitrogen, which is bonded to two carbon atoms and one

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oxygen atom [32-34]. Peaks at ca. 398, 400, 401 and 403 eV are ascribed to the presence of NI, NII, NIII, and NIV, respectively. Based on the N1s XPS spectra, the relative ratios of NI, NII, NIII, and NIV in the NMGC-Fe are 16.2, 19.4, 62.2, and 2.2 %, respectively. For the NMGC-Ni, NI, NII, NIII, and NIV account for 41.5, 20.3, 27.5, and 10.7 %, respectively. It clearly shows that the nitrogen types on carbon surfaces can be determined by different transition metals. NIII species

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can be dominantly formed by the Fe, while the Ni produced NI-rich carbon surface. Also, NI and NIV are formed less by the Fe than the Ni. This tendency is very similar to the temperature effect indicating that the amount of NI type decreases while that of NIII increased, as the temperature

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increased [35, 36]. According to the literature [37, 38], the NIII type increases and the NI type

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decreases as the degree of graphitization increases. The results observed in this study agree with graphitization degree of the N-doped carbon supports reported in the literature.

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(b)

III

N N

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IV

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Binding Energy [eV] Binding Energy [eV] Fig. 4. Deconvoluted N1s XPS spectra of (a) NMGC-Fe and (b) NMGC-Ni.

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Table 1. Summary of near-surface atomic compositions of supports.

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3.2 Characterization of Pt catalysts

The XRD patterns of the Pt/NMGC-Fe, Pt/NMGC-Ni, and commercial Pt/C catalysts are

shown in Fig. 5. All the Pt samples exhibit the characteristics of the Pt face-centered cubic (fcc) structure. The characteristic diffraction peaks at 39.8, 46.25, and 67.7° correspond to the (111), (200), and (220) planes of Pt nanoparticles, respectively, while (002) plane of carbon is located at ca. 26°. Based on Pt (220) plane, the particle sizes calculated using a Scherrer equation are 2.3, 2.6 and 2.0 nm for the Pt/NMGC-Fe, Pt/NMGC-Ni and commercial Pt/C catalysts, respectively. 14

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HR-TEM images of Pt/NMGC-Fe, Pt/NMGC-Ni, and commercial Pt/C catalysts are shown in Fig. 5b-d, respectively. As can be seen, the Pt particles are well-distributed on Pt/NMGC-Fe and

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Pt/NMGC-Ni catalysts. The mean particle size and the particle size distribution of Pt were measured using the values obtained from over 100 nanoparticles. The mean particle size is approximately 2.4 and 2.1 and 2.4 nm for the Pt/NMGC-Fe, Pt/NMGC-Ni and commercial Pt/C

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catalysts, respectively. The mean particle size and distribution of Pt particles are a similar degree with those of commercial Pt/C catalyst. Pt nanoparticles in all catalysts are dominantly deposited

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on both supports in the range of 2-3 nm around with the standard deviation (SD) of 0.4 nm approximately.

XPS technique has been used to study the chemical states of Pt and their relative intensities as determined by the relative peak area of the doublets. Figure 6a and 6b show the Pt 4f spectra for the Pt/NMGC-Fe and Pt/NMGC-Ni catalysts, respectively. The Pt 4f spectra can be

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deconvoluted to three pairs of doublets, which can be attributed to the Pt0, PtII, and PtIV states. The first doublet (Pt0) corresponds to the metallic state, while the second doublet (PtII) is assigned to the Pt2+ chemical state such as PtO or Pt(OH)2. The third doublet (PtIV) observed at

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the highest binding energies is caused by Pt4+ such as PtO2. Three deconvoluted doublets are

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shown at binding energies of ca. 71.1/74.4, 72.1/75.3, and 74.2/77.3 eV corresponding to the Pt0, PtII, and PtIV states, respectively. The percentage of Pt0 in the Pt/NMGC-Fe catalyst is 60%, which is higher than that observed in Pt/NMGC-Ni catalyst (51%), which is a good agreement with literature data [37, 39, 40]. These studies indicated that the percentage of the metallic state increases with the graphitization degree of carbon supports. The Pt and Pd on highly graphitized carbon support exhibited a metallic state of ca. 60-70%. Since the graphitization degree of NMGC-Fe is higher than that of NMGC-Ni, the Pt/NMGC-Fe catalyst exhibits higher Pt0 15

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concentration.

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Fig. 5. (a) XRD patterns of Pt/NMGC-Fe, Pt/NMGC-Ni and commercial Pt/C. HR-TEM images of fresh (b) Pt/NMGC-Fe, (c) Pt/NMGC-Ni and (d) commercial Pt/C. Scale bar represents 20 nm.

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Pt4f7/2

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Fig. 6. Deconvoluted XPS spectra of Pt 4f in (a) Pt/NMGC-Fe and (b) Pt/NMGC-Ni catalysts.

The electrochemical experiments were carried out in a three-electrode electrochemical

cell. The CV was performed at a scan rate of 50 mV s−1 by sweeping the potential between 0.005 V and 1.0 V in N2-saturated electrolyte as shown in Fig. 7a. The ECSA was calculated from the integrated charge in the hydrogen desorption peak using the following equation:

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ECSA =

ܳୌ 0.21 × ‫ܮ‬௉௧

where, QH (mC cm−2) is the coulombic charge for hydrogen desorption, LPt (mgPt cm−2)

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represents the Pt loading on the glassy carbon electrode and 0.21 mC cm−2 is the charge required to oxidize a monolayer of H2 on the Pt site [41]. The initial ECSA values of Pt/NMGC-Fe, Pt/NMGC-Ni, and commercial Pt/C catalysts were 61.7, 60.5 and 78.9 m2 g−1, respectively.

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LSVs in Fig. 7b are measured at a scan rate of 5 mV s−1 in O2-saturated electrolyte by sweeping the potential between 0.2 and 1.1 V in the anodic direction. The LSV curves presented in this

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work were properly corrected using the background capacitance current that was measured in the N2 atmosphere at a scan rate of 5 mV s−1. Fig. 7b shows that the Pt/NMGC-Fe catalyst exhibits better performance for the ORR than the Pt/NMGC-Ni and commercial Pt/C catalysts. The Pt/NMGC-Fe catalyst shows higher onset potential and diffusion-limited current density than the

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commercial Pt/C catalyst. Mass activity and specific activity of catalysts are shown in Fig. 7c. Interestingly, the Pt/NMGC-Ni catalyst doesn’t show a significant improvement in the ORR such as onset potential, diffusion-limited current density. It may result from the less activity of

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NMGC-Ni since it was reported that the more active the N-doped carbon is, the higher the electrocatalytic factors are observed in the Pt on N-doped carbon [42-44]. On the other hand, the

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mass and specific activities of Pt/NMGC-Fe catalyst are increased as compared to the commercial Pt/C catalyst. We speculate that the enhancement of ORR is caused by the interaction between Pt and support, by improvement of electronic properties, as well as the synergistic effect [45-48]. The Pt/NMGC-Fe catalyst, which shows better electrocatalytic and physical properties, was further analyzed by MEA tests.

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Current Density [mA cm−2 ]

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Specific Activity [µA cmPt

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Potential [Vvs. RHE]

Fig. 7. (a) CV diagrams of Pt/NMGC-Fe, Pt/NMGC-Ni and commercial Pt/C in N2-saturated 0.1 M HClO4 at room temperature and a scan rate of 50 mV s−1. (b) LSV curves of Pt/NMGC-Fe, 19

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Pt/NMGC-Ni and commercial Pt/C in O2-saturated 0.1 M HClO4 at room temperature and a scan rate of 5 mV s−1 with 1600 rpm. (c) Mass activity and specific activity of Pt/NMGC-Fe, Pt/NMGC-Ni, and commercial Pt/C.

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3.3 MEA tests and durability

The PEMFC polarization was conducted at 80 oC at the low relative humidity. Fig. 8a and (b) show performances of the Pt/NMGC-Fe and commercial Pt/C catalysts before and after

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30,000 potential cycles, respectively, according to the accelerated stress test protocol suggested by U.S DRIVE Fuel Cell Tech Team. As shown in Fig. 8, the initial performance of Pt/NMGC-

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Fe catalyst shows similar with that of the commercial Pt/C catalyst. The maximum power density of Pt/NMGC-Fe catalyst exhibits ca. 510 mW cm−2. After 30,000 potential cycles, the maximum power density of Pt/NMGC-Fe catalyst decrease by 26% and 83 mV at 600 mA cm−2. However, the commercial Pt/C catalyst shows the devastated performance after 30,000 potential cycles. No

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activity is observed at 600 mA cm−2 and the maximum power density decreased by 52%, which indicates that the commercial Pt/C catalyst layer is totally destroyed when subjected to potential cycling.

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To further analyze the durability of catalysts, the normalized ECSAs calculated for the Pt/NMGC-Fe and commercial Pt/C catalysts as a function of cycle number are shown in Fig. 8c.

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Initial ECSA values of 42 and 63 m2 gPt−1 are measured for the Pt/NMGC-Fe and commercial Pt/C catalysts, respectively. After 30,000 potential cycles, 31% and 12% of initial ECSA are remained for the Pt/NMGC-Fe and commercial Pt/C catalysts, respectively. These results indicate that the Pt/NMGC-Fe catalyst is remarkably durable as compared to the commercial Pt/C catalyst. It indicates that the employment of NMGC-Fe as a support for Pt catalyst can be a promising approach to increase the catalyst durability.

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600

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Normalized ECSA [%]

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Pt/NMGC-Fe commercial Pt/C

80 60 40 20 0

0

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No. Cycles Fig. 8. PEMFC polarization and power density curves of a) Pt/NMGC-Fe and b) commercial Pt/C before and after AST. AST was performed under 30,000 potential cycles between 0.6 and 21

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1.0 V supplying fully humidified H2/N2 to anode and cathode at 80 °C, respectively. c) Normalized ECSA plot of Pt/NMGC-Fe and commercial Pt/C as a function of cycle number.

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The HR-TEM images of Pt/NMGC-Fe and commercial Pt/C catalysts after 30,000 potential cycles are shown in Fig. 9a and 9b, respectively. The mean particle sizes of Pt/NMGCFe and commercial Pt/C catalysts after 30,000 potential cycles are 6.1 and 7.3 nm, corresponding

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to 255 and 305% increase, respectively. According to the images of commercial Pt/C catalyst after 30,000 potential cycles, the aggregation of Pt particles is clearly observed, while the

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alleviated aggregation of Pt particles is found in the Pt/NMGC-Fe catalyst after 30,000 potential cycles. Additionally, Pt/NMGC-Fe catalyst after 30,000 potential cycles shows the relatively narrow range of Pt particle size in comparison to the commercial Pt/C catalyst. The SD of Pt particle size for the Pt/NMGC-Fe catalyst is 1.5 nm, while that for the commercial Pt/C catalyst shows 2.2 nm. It has been reported in the literature that the durability of the catalyst increases

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with the degree of graphitization due to a strong interaction of the Pt particles and delocalized sites (sp2-hybridized carbon) on graphitic carbon [49]. The delocalized sites are formed by an

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overlap of the delocalized electrons in the p orbital of the π sites on graphitic carbon and the d orbital of the Pt [25, 50]. In addition, the N-doped carbon support can provide enhanced

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structural durability of a Pt particle and suppress Pt particle coarsening during AST [51]. Therefore, the observed high durability of Pt/NMGC-Fe catalyst may relate to the interaction of Pt and NMGC-Fe due to the enhanced π sites on carbon induced by a high degree of graphitization and the structurally improved stability of Pt particle. These effects can impede the Pt dissolution/re-deposition, Ostwald ripening, migration, and coalescence during AST [52-56].

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(b) 90

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Particle size [nm] Fig. 9. HR-TEM images and Pt particle size distribution histogram for (a, b) Pt/NMGC-Fe and (c, d) commercial Pt/C catalysts after AST. Scale bar represents 10 nm.

4. Conclusion

In this study, a mesoporous graphitic carbon doped with nitrogen was prepared by the pyrolysis of M-chelate complex adsorbed on CB. The change of specific surface area, interlayer spacing, and ID/IG value indicated that the NMGC showed a higher degree of graphitization than 23

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the CB. The stable quaternary nitrogen type was dominantly observed in the NMGC-Fe by XPS. HR-TEM studies indicated uniform Pt particle sizes for Pt/NMGC-Fe and Pt/NMGC-Ni catalysts

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with a mean particle size of ca. 2.3 nm. The Pt/NMGC-Fe catalyst showed the nearly same initial performance as the commercial Pt/C catalyst in MEA tests. However, Pt/NMGC-Fe catalyst showed remarkable durability after 30,000 potential cycles between 0.6 and 1.0 V. While the

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commercial Pt/C catalyst showed 52% loss of maximum power density and was unable to be measured at 600 mA cm−2, Pt/NMGC-Fe catalyst decreased by 26% loss of the maximum power

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density and 83 mV loss at 600 mA cm−2. ECSA of Pt/NMGC-Fe catalyst also was more stable than the commercial Pt/C catalyst. It can be concluded that the NMGC-Fe is highly promising as supporting materials for PEM fuel cells.

Acknowledgements

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acknowledged.

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The financial support of U.S. Department of Energy (contract no. DE-EE0000460) is gratefully

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