Controlling Pt loading and carbon matrix thickness for a high performance Pt-nanowire catalyst layer in PEMFCs

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Controlling Pt loading and carbon matrix thickness for a high performance Pt-nanowire catalyst layer in PEMFCs Kaihua Su a, Sheng Sui a,*, Xianyong Yao a, Zhaoxu Wei a, Junliang Zhang a, Shangfeng Du b a b

Institute of Fuel Cell, Shanghai Jiao Tong University, 200240 Shanghai, China School of Chemical Engineering, University of Birmingham, Edgbaston, Birmingham B15 2TT, United Kingdom

article info

abstract

Article history:

Pt-nanowire (Pt-NW) catalyst layers were prepared by in-situ growing Pt nanowires onto

Received 3 October 2013

carbon matrix coated on electrolyte membrane surface and used as PEMFC cathodes. The

Received in revised form

performances of the catalyst layer with various catalyst loadings and carbon matrix

1 December 2013

thicknesses were evaluated. Scanning electron microscopy (SEM) was employed to observe

Accepted 5 December 2013

the morphology and thickness of the Pt-NW catalyst layers, energy-dispersive X-ray

Available online 4 January 2014

spectroscopy (EDS) was used to investigate the Pt distribution along the layers. The polarization curve, electrochemical impedance spectroscopy (EIS), and cyclic voltammetry

Keywords:

(CV) were performed in fuel cells to check the practical electrochemical activity of the Pt-

Pt nanowire

NW catalyst layers. The results showed that the electrochemical surface area (ECSA) and

Catalyst layer

the cell performance exhibited a volcano-type curve with different Pt loadings, while the

Catalyst loading

carbon matrix thickness had an influence on the Pt-NW gradient distribution, the mass

Carbon matrix

diffusion, and the charge transfer in the electrodes.

Proton exchange membrane fuel cell

Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.

1.

Introduction

Among various nanomaterials, one-dimensional (1D) nanostructures, such as wire, rod, tube, and belt, have attracted much research efforts owing to their intriguing properties in electrical and thermal conductivity and/or mechanical properties for mesoscopic physics and fabrication of nanoscale devices [1]. Some noble metal nanostructures with 1D morphologies are emerging as new energy materials due to their unique structural and electrochemical properties, which are highly relevant to energy storage and conversion devices in industrial catalysis and fuel-cell technology [2e4].

Among these 1D noble metal nanostructures, Pt nanowires, with their unique structure, surface properties and excellent catalytic activities, are recognized as promising candidates for electrocatalysts using in proton exchange membrane fuel cells (PEMFCs) [5,6]. At present, highly dispersed Pt nanoparticles (Pt-NPs, 2e5 nm) supported on active carbon are widely used as electrocatalysts in PEMFCs. However, the zero-dimensional (0D) morphology of nanoparticles is of high surface energy, high numbers of defect sites and lattice boundaries, and low coordination atoms at the surface, which induce severe Ostwald ripening and/or agglomeration of the nanoparticles and thus a low catalyst

* Corresponding author. Tel./fax: þ86 21 34206249. E-mail address: [email protected] (S. Sui). 0360-3199/$ e see front matter Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijhydene.2013.12.062

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activity towards oxygen reduction reaction (ORR) [7e10]. 1D structural Pt-NWs can potentially solve some of these problems and exhibit enhanced catalytic activity and stability. This is attributed to preferential exposure of highly active crystal facets and high surface aspect ratio, which bring an improved catalytic activity, high catalyst utilization, and low charge and mass transport resistance [11,12]. The smooth crystalline plane reduces the number of undesirable lowcoordination defect sites and promotes ORR activity [13]. In addition, anisotropic Pt-NWs mitigate agglomeration and improve stability during the fuel cell operation [14]. Up to now, many methods had been proposed for the synthesis of Pt-NWs, such as template method [15e17], electrospinning method [18,19], electrochemical deposited method [20], and wet-chemical method [5,12,14,21e23]. Polycrystalline and/or single crystal Pt-NWs were prepared with above methods and showed an excellent catalytic activity in PEM fuel cells. In our previous work, a novel high performance catalyst layer structure was prepared by in-situ growing PtNWs on the carbon matrix made of carbon powder and Nafion ionomer [24]. The particle sizes of the Pt-NWs were 10e30 nm length and about 4 nm diameter, the size of the carbon sphere was about 30 nm diameter. There is distinct difference between above catalyst layer and conventional Pt/C catalyst layer. The former had a continuous gradient distribution along the thickness direction of the catalyst layer, while the latter had a uniform Pt distribution. The thickness of PEMFC catalyst layers has influence on the mass and charge transfer in operation [25]. The conventional catalyst layer is usually made from spherical Pt catalyst nanoparticles supported on carbon black with thickness depending on how much Pt used is in a range of 10e20 mm [26]. The catalyst layer thickness increases with the Pt loading. A thick catalyst layer prolongs the reactant gas transport path, raises proton conduction resistance and is apt to flood. If such mass-transport-limiting in the catalyst layer becomes dominant, immediate cell performance degradation arises [27]. However, for the Pt-NW catalyst layer, the thickness mainly depends on the carbon powder loading in the carbon matrix, which is independent of the Pt loading. This is a significant difference between the Pt-NW catalyst layer and the conventional catalyst layer. To understand the structure of the Pt-NW catalyst layer and bring this novel technology into commercial application, it is necessary to carry out a systematic investigation of the effects of Pt loading and catalyst layer thickness on the performance. In this study, the Pt-NW catalyst layers with various Pt and carbon powder loadings were prepared and tested as cathodes in a single PEM fuel cell. The performance curves for various design parameters were analyzed. The influences and mechanisms of Pt loading and carbon matrix thickness on the electrode structure and performance were analyzed.

2.

Experimental section

2.1.

Materials

All chemicals were used as received without any further purification. Hexachloroplatinic acid (H2PtCl6$6H2O, 99.95%),

formic acid (HCOOH, 88%), and isopropyl alcohol were purchased from Sinopharm Chemical Reagent Co., Ltd. 50 wt% Pt/C electrocatalyst (HiSPEC 9100, Johnson Matthey) was used as catalyst in the anode. Carbon powder (Vulcan XC72R, Shanghai Cabot Chemical Co., Ltd.) and Nafion ionomer solution (DE1020, 10 wt%, DuPont) were taken for making the carbon matrix. Nafion NR212 membrane (DuPont) and Gas diffusion layers (GDLs) with water management layers (AvCarb GDS3250, Ballard Power Systems, Inc.) were used for MEA fabrication. All aqueous solutions were prepared with ultrapure water (18.2 MU cm) from Instrumental Analysis Center of Shanghai Jiao Tong University (IAC-SJTU).

2.2.

Catalyst layer and MEA fabrication

The carbon matrix layer was fabricated by a spraying procedure that is similar to the catalyst-coated membrane (CCM) processes. To fabricate the carbon matrix for the cathode PtNW catalyst layer, the carboneionomer mixture was prepared by mixing carbon powder, isopropyl alcohol and Nafion ionomer solution, and sonicating for 5 min to form homogeneous ink. Then under an infrared light, the ink was coated on a Nafion membrane by spraying method with a spray gun (Iwata HP-CH, Japan). The anode was prepared by mixing 50 wt% Pt/C, Nafion ionomer solution and isopropyl alcohol into ink, and spraying on the other side of the above membrane. In all the cases, the Pt loading and Nafion ionomer content at the anode were fixed at 0.50 mg cm2 and 20 wt%, respectively. Then the membrane coated with the carbon matrix and the anode was immersed in the aqueous solution of H2PtCl6 and HCOOH in a 10 cm glass Petri dish at room temperature under ambient atmosphere. For growing Pt in the carbon matrix, the membrane with the carbon matrix side up was attached to the dish bottom. Typically, 16.0 mg H2PtCl6$6H2O and 4.0 ml HCOOH in 40 ml water were used for growing 0.60 mg cm2 Pt-NWs in the carbon matrix with an active area of 10 cm2. After 72 h, the Pt reduction reaction was completed. The membrane was taken out, washed six times by ultrapure water, and then dried at 50  C for 30 min. Finally, diluted Nafion ionomer solution was sprayed onto the surface of the grown Pt-NWs to maximize the triple phase boundary (TPB). The dry basis loading of ionomer was 0.025 mg cm2. Either the Pt loading or carbon matrix thickness was a single variable in the present investigation. The Pt loadings were varied at 0.20e0.60 mg cm2 when the carbon powder loading and the Nafion ionomer content were fixed at 0.10 mg cm2 and 20 wt% in the carbon matrix, respectively. The carbon powder loadings were investigated from 0.03 to 0.15 mg cm2 based on the fixed Pt loading of 0.30 mg cm2 and Nafion ionomer content of 20 wt% in the catalyst layer, respectively. Two pieces of GDLs were sandwiched with the membrane coated with the catalyst layers and hot-pressed at 130  C for 2 min under 0.2 MPa to make a MEA. PTFE coated fabric (ultra premium grade, thickness 0.127 mm, CS Hyde Company) was used as sealant material in the PEM fuel cell hardware to prevent gas leakage.

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2.3.

Physical characterization

The morphology of the as-prepared catalyst layers and the cross-sections of the MEAs were characterized by a field emission scanning electron microscope (Hitachi S-4800, Japan), operating at an accelerating voltage of 10 KV. Platinum distribution across the thickness of the cathode catalyst layer was analyzed by energy-dispersive X-ray spectroscopy. The cross-sections for SEM analysis were prepared by freezing the MEAs in liquid nitrogen and fracturing them with tweezers. The Pt-NW loading of cathode catalyst layers was determined by inductively coupled plasma-atomic emission spectrometer (ICP-AES) (7500a, Agilent). The samples without the anode for ICP-AES detection were prepared by the above spraying and reduction process.

2.4.

Fuel cell tests

MEA tests were carried out in a PEMFC single cell with an active area of 10 cm2 at 70  C. H2 and air were used as anode fuel and the cathode oxidant with a stoichiometric ratio 1.5 (H2)/2.0 (Air) under atmospheric pressure, respectively. They were humidified at 65  C before entering the cell. All the electrochemical measurements were controlled and recorded using 850e Multi-Range Fuel Cell Test System plus 885 Fuel Cell Potentiostat from Scribner Associates Inc. The data analysis and curve fitting were processed using the software Z-view. Each MEA activation was done by repeating the following programs (a) and (b) for two times and eight times, respectively: (a) 0.60 V @ 20 min/0.70 V @ 20 min/0.80 V @ 20 min/ 0.85 V @ 20 min/0.90 V @ 20 min/open circuit potential (OCP) @ 20 min; and (b) 0.20 V @ 10 min/OCP @ 30 s. Then polarization measurements were performed with a sweep rate of 2 mV s1 from OCP to 0.30 V. Electrochemical impedance spectroscopy (EIS) was carried out at the applied potential of 0.80 V and 0.40 V in a frequency range of 10 kHze0.1 Hz with the AC amplitude of 10% of DC current. Before each EIS measurement, the single cell was stabilized at the applied potential for 20 min. During the impedance test, the operating conditions, including the cell

Fig. 1 e Polarization curves of MEAs with various Pt-NW loadings (unit: mg cmL2) in the cathode.

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temperature and feed flow rates, were all kept the same as in the polarization measurements. After the cathodic gas was changed to nitrogen and the cell temperature cooled down to 35  C, in-situ cyclic voltammetry (CV) was carried out to derive the electrochemical surface area (ECSA). In the measurements, N2 and H2 were fed into the working electrode (cathode) and the reference/counter electrode (anode), respectively, and a potential scanning was performed in the range of 0.05e1.00 V vs. reversible hydrogen electrode (RHE) with a sweep rate of 25 mV s1.

3.

Results and discussion

A series of the MEAs with various Pt loadings of 0.20, 0.30, 0.40, 0.50, and 0.60 mg cm2 in the cathode catalyst layers were tested in H2/air feed single cell. Fig. 1 shows the polarization curves of the MEAs. The cell performance was affected significantly by the Pt loading, and the maximum power density of 0.47 W cm2 was achieved at 0.30 mg cm2. The improvement of the cell performance below 0.30 mg cm2 could be attributed to the increasing catalytic kinetics with Pt loading [28]. By contrast, further increase of the Pt loading led to a decrease of cell performance. The cause was further investigated with the help of EIS and CV measurements. Fig. 2(a) and (b) illustrates the electrochemical impedance spectroscopy measured at 0.80 V and 0.40 V for the cathode catalyst layers with three Pt loadings (0.20, 0.30, and 0.60 mg cm2). As shown in Fig. 1, at 0.80 V, the current densities of the samples were very small. In this case, the cathode impedance was mainly dominated by the charge transfer

Fig. 2 e Electrochemical impedance spectra of MEAs measured at 0.80 V and 0.40 V with various Pt-NW loadings (unit: mg cmL2) in the cathode.

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resistance represented by the arc diameter in the Nyquist plot. Fig. 2(a) shows that the smallest semi-circles were obtained at the loading of 0.30 mg cm2, which corresponded to the best performance as shown in Fig. 1. Comparing that at 0.30 mg cm2, the impedance at Pt loading of 0.20 mg cm2 was larger due to its poor catalytic kinetics resulting from insufficient Pt loading. Mass transport effect appeared at a low voltage of 0.40 V and could be evaluated by the EIS. In Fig. 2(b), two semi-circles are present for each sample. The high frequency arc (left) was attributed to the combination of the charge transfer impedance and the double layer capacitance within the catalyst layer, and the low frequency arc (right) indicated to the mass transport impedance. In Fig. 2(b), it can be seen that the mass transport resistance at Pt-NW loading of 0.60 mg cm2 increased about ten-fold compared with that at 0.30 mg cm2. This resistance increase led to a rapid drop in cell performance, especially at a very large current density as shown in Fig. 1. Fig. 3 shows the cyclic voltammograms (CV) of the cathode catalyst layer with various Pt loadings. The ECSA data were calculated from the hydrogen evolution charge densities of the cyclic voltammograms, assuming a value of 210 mC cm2 for the adsorption of a hydrogen monolayer on the surface of polycrystalline Pt [29]. The normalized ECSA results are summarized in the inset of Fig. 3. In conventional catalyst layers with Pt/C catalyst, a higher catalyst loading usually results in a larger electrochemical active area. However, in the Pt-NW catalyst layers, a volcano-type relationship was obtained between the Pt loading and the ECSA. The maximum value of 28.44 m2 g1 was reached at a Pt-NW loading of 0.30 mg cm2 and then dropped to 23.61 m2 g1 at 0.60 mg cm2. This indicated a different influence mechanism of the catalyst loading to the Pt-NW catalyst layer as compared with a conventional electrode. In order to examine the electrode structure changing with Pt loading, SEM analysis was performed to the Pt-NW catalyst layers at Pt loadings of 0.20, 0.30, and 0.60 mg cm2 and the images are shown in Fig. 4. It could be seen that, at a low Pt

Fig. 3 e Cyclic voltammograms of MEAs with various PtNW loadings (unit: mg cmL2) in the cathode measured at 35  C with a scan rate of 25 mV sL1.

loading of 0.20 mg cm2, the Pt-NWs did not cover the whole surface of carbon matrix and part of the carbon particles’ surface was still bare. The whole structure was still quite porous. With the increase of the Pt loading, the carbon particles were gradually covered by thick Pt-NW clusters and the pores in the catalyst layer were gradually blocked. Fig. 4(c) further shows the morphology of the Pt-NW catalyst layer at a high loading at 0.60 mg cm2. Most Pt-NWs assembled into 3D sphere-shaped and cube-like superstructure with the carbon core. The dense Pt-NW clusters not only blocked reactant gas transport in the catalyst layer, but also hampered oxygen

Fig. 4 e SEM images of cathode catalyst layer with various Pt loadings at (a) 0.20, (b) 0.30, and (c) 0.60 mg cmL2.

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accessing to the Pt surface. These Pt-NWs aggregated to a large size, reduced the specific surface area, and thus led to a low catalytic activity and small ECSA as shown in Figs. 2(a) and 3. The partially blocked pores in the carbon matrix make it difficult for reactant gas transporting to the TPB and the generated water draining, thus a large mass transport resistance was as expected which thus cut down the catalyst layer performance, as shown in Fig. 2(b). In Fig. 4(b), at the optimal Pt-NW loading (0.30 mg cm2), the carbon matrix surface was partially covered by a thin layer of Pt-NW clusters comparing with that at 0.60 mg cm2. There were still enough exposure pores ‘existed’ for the diffusion of reactant gas. As a consequence, a successful compromise was reached between the high kinetics of ORR and the low mass transport resistance at the loading of 0.30 mg cm2. Fig. 5 shows the polarization curves of cathode catalyst layers with different carbon powder loadings in the carbon matrix. The PEMFC reached its best performance, with a peak power density of 0.47 W cm2, at a carbon powder loading of 0.10 mg cm2. In the examined range, the cell performance declined significantly as the carbon powder loading decreased from 0.10 to 0.03 mg cm2. Impedance spectra were also recorded at 0.80 V and 0.40 V as shown in Fig. 6(a) and (b). At a cell voltage of 0.80 V, it could be observed that the charge transfer resistance of the catalyst layer at a carbon powder loading of 0.10 mg cm2 was the lowest, which suggested that the catalyst layer gave more efficient electrochemical performance at this thickness. The catalyst layer at carbon powder loading of 0.03 mg cm2 had the largest charge resistance. Even when the cell operated at a low potential of 0.40 V, the cathode catalyst layer at carbon powder loading of 0.10 mg cm2 also exhibited the lowest impedance which meant that the best performance was achieved under mass transport control. The ECSA results at various carbon powder loadings were measured by cyclic voltammetry and summarized in Fig. 7. The ECSA value at carbon powder loading of 0.10 mg cm2 was about 28.23 m2 g1, which was the maximum. These results revealed that the catalyst layer with carbon powder loading at 0.10 mg cm2 had a large ECSA, high Pt utilization, as well as

Fig. 5 e Polarization curves of MEAs with different carbon powder loadings in the cathode carbon matrix (unit: mg cmL2).

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Fig. 6 e Electrochemical impedance spectra measured at 0.80 V and 0.40 V of MEAs with different carbon powder loadings in the cathode carbon matrix (unit: mg cmL2).

high ORR kinetics, which were in good agreement with the polarization performance shown in Fig. 5. To understand the effect of the carbon matrix thickness on the cell structure, cross-sections of three MEAs with different carbon powder loadings were checked using SEM. As shown in Fig. 8, the thickness of the catalyst layers was almost proportional to the carbon powder loading, where it was about 1.5 mm at 0.03 mg cm2 and 6.0 mm at

Fig. 7 e Cyclic voltammograms of MEAs with different carbon powder loadings in the cathode carbon matrix (unit: mg cmL2) recorded at 35  C with a scan rate of 25 mV sL1.

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0.15 mg cm2. The Pt distribution along the catalyst layer thickness direction was marked in red. Pt-NWs were uniformly distributed across the thin catalyst layer as shown in Fig. 8(a). However, with the increase of the thickness, PtNWs got a gradient distribution, as shown in Fig. 8(b) and (c). It could also be seen in Fig. 8(c) that the Pt-NWs mainly grow on the upper part of the catalyst layer within a depth

of 0e4.5 mm. In the range of 1.5 mm close to the membrane, there were nearly no Pt-NWs to be found. Growth of Pt-NWs on the carbon matrix has some similarity to that on carbon spheres [12]. In the carbon matrix, the ionomer did not penetrate into carbon agglomerates, but partially covered their surfaces [30]. The Pt nuclei formed through the reduction reaction in the solution might preferentially grow on the bare carbon nanospheres due to the rough surface. The gradient distribution of Pt-NWs was caused by the diffusion and migration of Pt nuclei, and their growth along the pore channels in the carbon matrix. Compared to that in the carbon matrix, Pt nuclei had a much smaller diffusion resistance in the solution. Therefore, the Pt nuclei mainly concentrated at the outer part of the carbon matrix at about 2.0 mm in depth and grew into nanowires. At a low carbon powder loading as shown in Fig. 8(a), the thin layer (1.5 mm) had a short diffusion path and Pt-NWs grew uniformly across the thickness direction. But, the low loading of the carbon powder provided less growth sites for Pt-NWs. Therefore, Pt-NWs were apt to aggregate into big sizes and thus blocked the micro-pores in the carbon matrix, which resulted in a larger mass transport resistance and smaller ECSA as seen in Figs. 6(b) and 7. When the catalyst layer became thicker, the growth sites for the Pt-NWs increased, which reduced the Pt-NW aggregation and pore blocking, leading to an increase of the ECSA and a decrease of the mass transport resistance. On the other hand, a thicker catalyst layer could increase the path lengths of the proton and oxygen transport [25]. Furthermore, the diluted ionomer solution was sprayed onto the surface of the catalyst layer and penetrated into the inside places to enlarge the TPB by contacting with PtNWs. This penetration, however, was very difficult and thus most of the ionomer were located near the catalyst layer surface [31]. The Pt-NWs located in the deep catalyst layer might therefore hardly access ionomer, which did not contribute to the TPB and ECSA resulted in a poor catalytic performance as shown in Figs. 5 and 7. Therefore, a tipping point of the catalyst layer thickness was as expected to get the optimal charge and mass transport. In this work, the optimal thickness was about 3.0 mm at 0.10 mg cm2 carbon powder loading when an excellent performance was achieved (Fig. 5).

4.

Fig. 8 e Cross-sectional SEM images and the profile of elemental Pt of cathode catalyst layer with different carbon powder loadings at (a) 0.03, (b) 0.10, and (c) 0.30 mg cmL2. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Conclusions

The Pt-NW catalyst layers with various Pt loadings and carbon matrix thicknesses were tested as cathode in a 10 cm2 hydrogen/air PEMFC. The Pt-NWs grown in the carbon matrix assembled into 3D sphere-shaped and cube-like superstructure on carbon spheres. A higher Pt loading caused a severe aggregation of Pt-NWs resulted in a decrease of the ECSA and an increased mass transport limitation. For a thinner carbon matrix, Pt-NWs had a uniform distribution across the catalyst layer cross-section, but aggregated into large size superstructures which blocked the pores. However, within a thick catalyst layer, Pt-NWs gave a gradient distribution in the catalyst layer and low catalyst utilization could be observed. A Pt loading of 0.30 mg cm2 and carbon powder loading of 0.10 mg cm2 were obtained with optimal performance for PEMFCs.

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Acknowledgements [15]

This work was funded by award from the Royal Academy of Engineering (UK) and Science and Technology Committee of Shanghai Municipality (Grant No. 12dz1200900). We also thank the IAC-SJTU for the help on ICP characterization. Prof. Kevin Kendall FRS at the University of Birmingham was acknowledged for his help on the manuscript English.

[16]

[17]

references [18] [1] Xia Y, Yang P, Sun Y, Wu Y, Mayers B, Gates B, et al. Onedimensional nanostructures: synthesis, characterization, and applications. Adv Mat 2003;15:353e89. [2] Arico` AS, Bruce P, Scrosati B, Tarascon J-M, Van Schalkwijk W. Nanostructured materials for advanced energy conversion and storage devices. Nat Mat 2005;4:366e77. [3] Zhou H, Zhou W-p, Adzic RR, Wong SS. Enhanced electrocatalytic performance of one-dimensional metal nanowires and arrays generated via an ambient, surfactantless synthesis. J Phys Chem C 2009;113:5460e6. [4] Yin Z, Zheng Q. Controlled synthesis and energy applications of one-dimensional conducting polymer nanostructures: an overview. Adv Energy Mat 2012;2:179e218. [5] Du S, Pollet BG. Catalyst loading for Pt-nanowire thin film electrodes in PEFCs. Int J Hydrogen Energy 2012;37:17892e8. [6] Du S. Pt-based nanowires as electrocatalysts in proton exchange fuel cells. Int J Low-Carbon Tech 2012;7:44e54. [7] Ferreira P, Shao-Horn Y, Morgan D, Makharia R, Kocha S, Gasteiger H. Instability of Pt/C electrocatalysts in proton exchange membrane fuel cells a mechanistic investigation. J Electrochem Soc 2005;152:A2256e71. [8] Shao Y, Yin G, Gao Y. Understanding and approaches for the durability issues of Pt-based catalysts for PEM fuel cell. J Power Sources 2007;171:558e66. [9] Peng Z, Yang H. Synthesis and oxygen reduction electrocatalytic property of Pt-on-Pd bimetallic heteronanostructures. J Am Chem Soc 2009;131:7542e3. [10] Liu Z, Zhang J, Yu P, Zhang J, Makharia R, More K, et al. Transmission electron microscopy observation of corrosion behaviors of platinized carbon blacks under thermal and electrochemical conditions. J Electrochem Soc 2010;157:B906e13. [11] Zhang J, Vukmirovic MB, Xu Y, Mavrikakis M, Adzic RR. Controlling the catalytic activity of platinum-monolayer electrocatalysts for oxygen reduction with different Substrates. Angew Chem Int Ed 2005;44:2132e5. [12] Sun S, Jaouen F, Dodelet JP. Controlled growth of Pt nanowires on carbon nanospheres and their enhanced performance as electrocatalysts in PEM fuel cells. Adv Mat 2008;20:3900e4. [13] Koenigsmann C, Santulli AC, Gong K, Vukmirovic MB, Zhou W-p, Sutter E, et al. Enhanced electrocatalytic performance of processed, ultrathin, supported PdePt coreeshell nanowire catalysts for the oxygen reduction reaction. J Am Chem Soc 2011;133:9783e95. [14] Sun S, Zhang G, Geng D, Chen Y, Li R, Cai M, et al. A highly durable platinum nanocatalyst for proton exchange

[19]

[20]

[21]

[22]

[23]

[24]

[25]

[26]

[27]

[28]

[29] [30]

[31]

3403

membrane fuel cells: multiarmed starlike nanowire single crystal. Angew Chem 2011;123:442e6. Choi SM, Kim JH, Jung JY, Yoon EY, Kim WB. Pt nanowires prepared via a polymer template method: its promise toward high Pt-loaded electrocatalysts for methanol oxidation. Electrochim Acta 2008;53:5804e11. Park I-S, Choi J-H, Sung Y-E. Synthesis of 3 nm Pt nanowire using MCM-41 and electrocatalytic activity in methanol electro-oxidation. Electrochem Solid-State Lett 2008;11:B71e5. Zhang X, Lu W, Da J, Wang H, Zhao D, Webley PA. Porous platinum nanowire arrays for direct ethanol fuel cell applications. Chem Commun 2009:195e7. Kim HJ, Kim YS, Seo MH, Choi SM, Kim WB. Pt and PtRh nanowire electrocatalysts for cyclohexane-fueled polymer electrolyte membrane fuel cell. Electrochem Commun 2009;11:446e9. Kim JM, Joh H-I, Jo SM, Ahn DJ, Ha HY, Hong S-A, et al. Preparation and characterization of Pt nanowire by electrospinning method for methanol oxidation. Electrochim Acta 2010;55:4827e35. Liang Z, Zhao T. New DMFC anode structure consisting of platinum nanowires deposited into a Nafion membrane. J Phys Chem C 2007;111:8128e34. Sun S, Zhang G, Zhong Y, Liu H, Li R, Zhou X, et al. Ultrathin single crystal Pt nanowires grown on N-doped carbon nanotubes. Chem Commun 2009:7048e50. Du S. A facile route for polymer electrolyte membrane fuel cell electrodes with in situ grown Pt nanowires. J Power Sources 2010;195:289e92. Wu R, Li B, Zhou K, Chan SH, Tey JN, Wei J, et al. Assembly of Pt nanowires into cubelike superstructures supported on aligned carbon nanotubes as highly stable electrocatalysts. Chem e Eur J 2013;19:9155e9. Yao X, Su K, Sui S, Mao L, He A, Zhang J, et al. A novel catalyst layer with carbon matrix for Pt nanowire growth in proton exchange membrane fuel cells (PEMFCs). Int J Hydrogen Energy 2013;38:12374e8. Su H-N, Liao S-J, Wu Y-N. Significant improvement in cathode performance for proton exchange membrane fuel cell by a novel double catalyst layer design. J Power Sources 2010;195:3477e80. Srinivasarao M, Bhattacharyya D, Rengaswamy R, Narasimhan S. Performance analysis of a PEM fuel cell cathode with multiple catalyst layers. Int J Hydrogen Energy 2010;35:6356e65. Passos RR, Paganin VA, Ticianelli EA. Studies of the performance of PEM fuel cell cathodes with the catalyst layer directly applied on Nafion membranes. Electrochim Acta 2006;51:5239e45. Leimin X, Shijun L, Lijun Y, Zhenxing L. Investigation of a novel catalyst coated membrane method to prepare lowplatinum-loading membrane electrode assemblies for PEMFCs. Fuel Cells 2009;9:101e5. Li G, Pickup PG. Ionic conductivity of PEMFC electrodes effect of Nafion loading. J Electrochem Soc 2003;150:C745e52. Holdcroft S. Fuel cell catalyst layers: a polymer science perspective. Chem Mat 2013. http://dx.doi.org/10.1021/ cm401445h. Poltarzewski Z, Staiti P, Alderucci V, Wieczorek W, Giordano N. Nafion distribution in gas diffusion electrodes for solid-polymer-electrolyte-fuel-cell applications. J Electrochem Soc 1992;139:761e5.