Ionomer content effects on the electrocatalyst layer with in-situ grown Pt nanowires in PEMFCs

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 9 ( 2 0 1 4 ) 3 2 1 9 e3 2 2 5

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Ionomer content effects on the electrocatalyst layer with in-situ grown Pt nanowires in PEMFCs Kaihua Su a, Xianyong Yao a, Sheng Sui 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:

The effects of ionomer contents were investigated in composite electrodes with in-situ

Received 12 November 2013

grown single crystal Pt nanowires (Pt-NWs) for PEMFCs, including the amount in the carbon

Received in revised form

matrix and impregnated on the surface of the electrocatalyst layer. The electrocatalyst

10 December 2013

layer was prepared by growing Pt-NWs directly on the carbon matrix with a simple one-

Accepted 17 December 2013

step wet chemical approach at room temperature. Transmission electron microscopy

Available online 17 January 2014

(TEM), scanning electron microscopy (SEM), polarization curve tests, electrochemical impedance spectroscopy (EIS), and cyclic voltammetry (CV) were employed to evaluate the

Keywords:

ionomer effects. The experimental results showed that the ionomer in the carbon matrix

Ionomer

had an influence on the ionic conductivity and aggregation and distribution of the Pt-NWs,

Pt nanowire

and the ionomer impregnated on the surface of the electrocatalyst layer affected the mass

Distribution

transport and ionic conductivity. The performance of the MEA was improved by optimizing

Electrocatalyst layer

the ionomer contents.

Polymer electrolyte membrane fuel

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

cell

1.

Introduction

As an alternative clean power generator, the polymer electrolyte membrane fuel cell (PEMFC) is highly attractive for its wide applications ranging from automotive power, stationary power to microelectronics. However, several drawbacks are hampering its commercial realization, including the high cost, poor durability, and reliability [1e3]. In order to achieve low cost and high performance for profitable applications, the optimization is important for membrane-electrode assemblies (MEAs) in PEMFCs, as a core component in which the chemical energy directly converted to electrical energy through fuel oxidation and oxygen

reduction. In past years, many efforts have been made on fabricating high performing, robust, and durable MEAs [4]. As the key part of the MEA, the electrocatalyst layer in which precious metal (normally Pt supported on carbon) utilized as an electrocatalyst, has a great influence on the total cost of PEMFCs. Among various issues, preparing durable electrocatalyst, improving electrocatalyst utilization, and optimizing electrode microstructure has been given top priority in preparing the electrocatalyst layers [5]. Up to now, highly dispersed Pt nanoparticles (2e5 nm) supported on high surface area carbon are still widely used in the electrocatalyst layer, and a lot of attention has been given to improving their morphology and structure [6]. However, the high surface energy of nanosized Pt particles usually induced

* 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.2013.12.117

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the loss of the electrochemical surface area (ECSA) because of Oswald ripening and/or aggregation during fuel cell operation [7e9]. Pt nanowires, known by their anisotropy, unique onedimensional (1-D) nanostructure, and/or surface properties, have exhibited better catalytic activity and high stability compared to the nanosized Pt particles [10]. In recent years, many methods have been developed for the preparation of Ptbased nanowires, including electro-deposition [11,12], template approaches [13e16], electro-spinning [17,18], phase transfer method [19] and wet chemical methods [20e24]. In our previous work, the electrocatalyst layers with in-situ grown single crystal Pt-NWs on carbon matrix were prepared by a simple one-step wet chemical procedure [25]. This procedure was designed to directly reduce the Pt precursor with a weak reducing agent in aqueous solution at room temperature, without using any templates or surfactant. The results revealed that the preferential exposure of certain crystal facets and improved oxygen diffusion of the Pt-NW electrocatalyst layer contributed to the higher performance in comparison with conventional electrocatalyst layers. Besides electrocatalysts, hydrate poly-perfluorosulphonic acid polymers such as ionomer, serving as electrolyte, is also one of the fundamental materials for the electrocatalyst layer [26,27]. Ionomer in the electrocatalyst layer not only serves as a physical binder for the electrocatalyst/support particles, but also helps to form triple phase boundary (TPB), retain moisture and prevent the membrane from dehydration. A large TPB based on a good contact between electrocatalysts, electrolyte, and reactant gases is required for electrochemical reactions to occur. The ionomer is exposed to incoming gases, electrocatalyst, and product (water), and conduct protons from the electrolyte membrane. It also facilitates access of reactant gases to the TPB and the transport of water to and from the TPB [28]. Optimum ionomer content and distribution are very important for high MEA performance [29e34]. Compared to the conventional electrocatalyst layer with spherical nanoparticles, the electrocatalyst layer with in-situ growth Pt-NWs also faces the same issues [33]. Therefore, there is a need to carry out this investigation on the electrocatalyst layer with Pt-NWs, providing practical information for commercial development. Based on our preliminary results, the effects of ionomer contents in the cathode with a Pt-NW electrocatalyst layer were investigated in this work. The experiments were performed by fuel cell measurements in a 10 cm2 Hydrogen/Air PEMFC apparatus, including the polarization curve, EIS and CV. From our data, the electrochemical activity and the mass transport properties were analyzed and correlated with the single cell performance. Mechanisms through which ionomer influences Pt-NW growth and fuel cell performance are proposed.

2.

Experimental section

2.1.

Materials

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

formic acid (HCOOH, 88%), and isopropanol were used as received from Sinopharm Chemical Reagent Co., Ltd. 50 wt% Pt/C electrocatalyst (HiSPEC 9100) was purchased from Johnson Matthey. Carbon powder (Vulcan XC-72R) used for preparing the carbon matrix was provided from Shanghai Cabot Chemical Co., Ltd. Nafion NR212 membrane and Nafion ionomer solution (DE1020, 10 wt%) used for fabricating MEA was obtained from DuPont. Gas diffusion layer (GDL) with water proof treatment (AvCarb GDS3250) was purchased from Ballard Power Systems, Inc. All aqueous solutions were prepared with deionized water with resistivity not less than 18.2 MU cm from Instrumental Analysis Center of Shanghai Jiao Tong University (IACSJTU).

2.2.

Preparation of electrocatalyst layer and MEA

The carbon powder mixed with ionomer was coated on the membrane and used as the matrix to grow Pt-NWs. The matrix was prepared in a manner similar to the catalyst-coated membrane (CCM) spraying method. The procedure was described in our previous work [25]. Typically, carbon inks were prepared by ultrasonicating the required quantity of carbon powder, isopropanol, and ionomer solution for 5 min. For carbon matrix fabrication, an NR-212 membrane was directly used after the cover sheets on both its sides were removed. The carbon ink was sprayed onto one side of the membrane with a spray gun (Iwata HP-CH, Japan) under the heating of an infrared light. After the carbon matrix was made, the anode electrocatalyst layer was prepared by spraying a mixture of commercial Pt/C catalysts, isopropanol, and ionomer on another side of the membrane. In order to eliminate any possible influences of anode, the Pt loading and Nafion content at the anode was fixed at 0.50 mg cm2 and 20 wt%, respectively. Then membrane coated with matrix and anode electrocatalyst layer was immersed in an aqueous solution of H2PtCl6 and HCOOH in a 10 cm glass Petri dish at room temperature. For the Pt nuclei self-attachment to the surface of the carbon matrix, the membrane was fixed directly on the bottom of the dish and the matrix was kept on the up side. Normally, to grow 0.30 mg cm2 Pt-NWs on the carbon matrix, 8.0 mg H2PtCl6$6H2O, 4.0 ml HCOOH were added to 40 ml D.I. water. The sample was stored at room temperature for 72 h. Then the samples were rinsed with D.I. water six times, followed by drying at 50  C for 30 min. At last, a dilute solution of the ionomer was sprayed on the surface of the Pt-NWs to enlarge the TPB. In order to investigate the effects of the ionomer contents in the carbon matrix, the ionomer contents were controlled at 5, 10, 20, 30, and 35 wt% of the carbon loading in the matrix. The carbon loading and Pt-NW loading were fixed at 0.10 mg cm2 and 0.30 mg cm2, respectively. And the ionomer content coated on the surface of the Pt-NWs was fixed at 0.025 mg cm2. Then the ionomer contents coated on the surface of the Pt-NW layers were studied in the range from 1 to 25 wt% based on the Pt-NW loading of 0.30 mg cm2, while the carbon loading and the ionomer content in the carbon matrix was maintained constant at 0.10 mg cm2 and 20 wt% respectively.

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The catalyst-coated-membrane was “sandwiched” between two GDLs by hot pressing at 130  C under 0.2 MPa for 2 min to fabricate a MEA for single cell testing. Teflon-coated fiber gaskets were used in the PEM fuel cell hardware to prevent gas leakage.

2.3.

MEA characterization

TEM images were acquired on a transmission electron microscope operating at an accelerating voltage of 200 kV (JEOL 2100F, Japan). SEM analysis was performed using a field emission scanning electron microscope operating at 10 kV (Hitachi S-4800, Japan). Inductively coupled plasma-atomic emission spectrometer (ICP-AES) (7500a, Agilent) was used to determine Pt loading in the electrocatalyst layers.

2.4.

MEA testing

The MEAs with in-situ grown Pt-NW electrocatalyst layers as the cathode were tested in a 10 cm2 PEM fuel cell at 70  C under atmospheric pressure. Pure H2 and air were humidified at 65  C before entering the cell. Gas flows of H2 and air were 150 mL min1 and 300 mL min1 with a stoichiometry of 1.5/ 2.0, respectively. Tests were controlled and recorded by an 850e Multi-Range Fuel Cell Test System and 885 Fuel Cell Potentiostat from Scribner Associates Inc. The MEA activation was done by the following cycles repeating two times and eight times, respectively: 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 0.20 V 10 min/ OCP 30 s. After the activation, the polarization curves were recorded by potentiostatically decreasing the cell voltage with a sweep rate of 2 mV s1 from OCP to 0.30 V. The impedance spectroscopy measurements were performed in the frequency range of 10 kHze0.1 Hz with an AC amplitude of 10% of DC current at 0.80 V and 0.40 V respectively. The in-situ cyclic voltammetry experiments were conducted between 0.05 and 1.00 V vs. reversible hydrogen electrode (RHE) with a sweep rate of 25 mV s1 at 35  C by flushing the fuel cell cathode and anode with N2 and H2 at flow rates of 75 mL min1 and 300 mL min1, respectively.

3.

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to improve the proton transport by spraying the ionomer on the surface of the Pt-NWs. As shown in Fig. 1, 10 wt% ionomer content in the carbon matrix gave the best performance. While the content was below 10 wt%, the cell performance was improved with increasing ionomer content in the carbon matrix. The peak power density rose from 0.35 W cm2 at 5 wt% ionomer content to 0.45 W cm2 at 10 wt%. This could be explained by an increasing of the TPB in the electrocatalyst layer. When more ionomer was added into the carbon matrix, the performance dropped rapidly. Peak power density even decreased to 0.13 W cm2 at 35 wt%. To fully understand the effects of ionomer content in the carbon matrix on the cell performance, detailed morphological and electrochemical investigations were performed. The TEM image in Fig. 2 shows the representative Pt-NW crystal structure and size. It could be seen that as-synthesized Pt was flower-like clusters, being composed of single Pt-NW arms with lengths in the range of 10e30 nm, were grown on the surface of the carbon spheres in the carbon matrix. Highresolution (HR) TEM image, as shown in the inset of Fig. 2, gives the crystallographic alignment of the Pt-NWs. It could be observed that the Pt-NWs were about 2e4 nm in diameter. The crystallographic alignment of one branched nanowire revealed that the entire nanowires were one single-crystal atomic structure and grow along the {111} axis with a lattice spacing between the {111} planes of 2.26 A, which was in agreement with our preliminary works and the results reported by Sun and his co-workers [21,25]. Fig. 3 shows the top-view SEM images of the carbon matrix before and after Pt-NW growing. In Fig. 3(a)e(c), it could be observed that the plain carbon matrix with a low ionomer content displayed abundant pores and more bare carbon sphere surface without been covered by ionomer. These pore served as macro-porous channels for gas flow and the bare surface as Pt-NW growing sites. However, with increasing the ionomer content in the carbon matrix, more and more pores and carbon surface gradually got covered by ionomer. Fig. 3(d)e(f) shows the morphology and distribution of the PtNWs at the same Pt loading on the carbon matrix with different ionomer contents. It could be seen that, at the low ionomer content of 10 wt% in the carbon matrix in Fig. 3(d),

Results and discussion

In cathode electrocatalyst layers of PEMFCs, the oxygen reduction reaction (ORR) requires electron, proton, and reactant gas transport to TPB at the same time. The ionomer provides a proton transport route for expanding the TPB. The insufficient amount of the ionomer results in a poor ‘connection’ between the electrocatalysts and electrolyte membrane. Protons transferred through the membrane cannot migrate to every part of the cathode electrocatalyst layer. On the other hand, too much ionomer causes a coverage of the catalyst active sites and blocks the electronic conduction paths and gas transport channels [28]. Optimizing the ionomer content is important for promoting catalyst utilization and improving the ORR activity. As Pt-NWs in-situ grown on the carbon matrix had a higher aspect ratio and stretched out from the carbon nanosphere surface, it was also necessary

Fig. 1 e Polarization curves for MEAs with various ionomer contents in the carbon matrix.

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Fig. 2 e TEM images of the Pt-NWs in carbon matrix with the inset of HR-TEM.

the surface of the carbon matrix was densely covered by PtNW clusters. However, coverage became very different at a high ionomer content. As shown in Fig. 3(f), the Pt-NWs aggregated to form large size 3-D sphere-like or cube-shaped structures on the surface of the carbon matrix. Based on the above SEM results, it could be concluded that the ionomer content in the carbon matrix affected both the Pt-NW aggregation and their distribution. Growth of the Pt-NWs on the carbon matrix followed a similar process to that for Pt-NWs on carbon spheres [21]. The difference was that the ionomer existing here influenced the amount of the growth sites for Pt-NWs. The carbon matrix had

limited sites for Pt-NW growing and consequently the ionomer introduced partially covered the carbon support. Compared to smooth hydrophilic/hydrophobic interface of the electrolyte or the surface covered by ionomer, the Pt nuclei formed through the reduction of H2PtCl6 preferentially grew on the carbon nanospheres due to their high specific surface area and the rough surface. Therefore, these uncovered sites on the carbon surface were preferred by Pt-NW growth. With the increasing of the ionomer content, uncovered growth sites for Pt-NWs were decreased on the carbon nanospheres. At the same Pt loading, the fewer supplied growth sites induced more severe aggregation of Pt-NWs, as shown in Fig. 3(d)e(f).

Fig. 3 e SEM images of carbon matrix before and after Pt-NW grown at the same Pt loading of 0.3 mg cmL2 with various ionomer contents at 10 wt% ((a) and (d)); 20 wt% ((b) and (e)); 35 wt% ((c) and (f)).

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This was also demonstrated by cyclic voltammetry measurement results shown in Fig. 4, which were carried out to investigate the ECSA in the cathodes of the MEAs. The hydrogen reduction charge density was used to calculate the active surface area of Pt-NWs by assuming that 210 mC cm2 produced a monolayer of adsorbed H on the smooth surface of polycrystalline Pt [35]. The inset in Fig. 4 represented that ECSA increased with ionomer content and the largest ECSA was achieved at a content of 10 wt%, owing to the increase of TPB, and then rapidly decreased with a further increased ionomer content to 35 wt%. Therefore, the Pt-NW aggregate induced by the high ionomer content significantly reduced the ECSA of Pt-NWs and further decreased the cell performance. These results were consistent with the cell performance trend in Fig. 1. To gain more electrochemical information on the electrocatalyst layer behavior, electrochemical impedance spectroscopy traces were recorded. Fig. 5 shows the impedance data in the Nyquist plots obtained at 0.80 V and 0.40 V. The high frequency intercept on the real axis represented the sum total of all ohmic resistance of the single cell, including the ohmic resistance of each component (membrane, catalyst layer, GDL, and bipolar plates) and the interfacial contact ohmic resistance among them. In Fig. 5(a), at the high potential of 0.80 V, the Nyquist plots presented complete depressed semicircles, which were the kinetic loop and usually used as an index of the catalytic activity, including the catalyst utilization, catalyst loading, and the catalyst surface area [36]. The small loop diameter implied the high catalytic activity and low charge transfer impedance. At 10 wt% ionomer content in the carbon matrix, the MEA exhibited the lowest charge transfer impedance. The MEA with the 5 wt% ionomer content had a larger charge transfer resistance, indicating a poor TPB owing to the low ionomer content in the carbon matrix. The largest charge transfer impedance could be observed at the 35 wt% ionomer content, which showed a 4-fold change compared with the 10 wt%. This change was mainly due to the loss of

Fig. 4 e Cyclic voltammograms for MEAs with various ionomer contents in the carbon matrix measured at 35  C with a scan rate of 25 mV sL1.

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Fig. 5 e Electrochemical impedance spectra measured at 0.80 V and 0.40 V for MEAs with various ionomer contents in the carbon matrix.

electrochemical specific surface caused by the Pt-NW aggregation. At the low voltage of 0.40 V, two pronounced arcs could be observed, the high frequency arc of the resistance spectrum represented the charge transfer resistance, whereas the medium and low frequency arcs represented the mass transport resistance. The mass transfer process had become an important factor influencing the cell performance and its influence was more significant with the increase of the current density. In Fig. 5(b), it could be observed that the mass transport impedance of the MEA with 35 wt% ionomer content was around 8 times larger

Fig. 6 e Polarization curves for MEAs with different ionomer loadings (wt% based on the Pt-NW loading of 0.30 mg cmL2) impregnated on the surface of the Pt-NWs.

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Fig. 7 e Cyclic voltammograms for MEAs with different ionomer loadings (wt% based on the Pt-NW loading of 0.30 mg cmL2) impregnated on the surface of the Pt-NWs measured at 35  C with a scan rate of 25 mV sL1.

than that of the 10 wt%. At the optimized 10 wt% ionomer content in the carbon matrix, the high kinetic of ORR and the low mass transport resistance were balanced thus leading to the highest cell performance. Based on our previous study, the Pt-NWs were mainly grown on the surface of the carbon matrix [25]. It was necessary to spray the ionomer on the surface of the PtNWs to enlarge the TPB. This work was performed and the

results are shown in Figs. 6e8. The MEA with an ionomer loading of 8 wt% impregnated on the surface of the Pt-NWs exhibited a maximum power density (0.37 W cm2), the largest ECSA, and the lowest charge transfer and mass transport impedance. Poor MEA performance at 1 wt% ionomer loading was attributed to a lack of protons on the electrocatalytic sites and, as a consequence, a small TPB and catalytic activity, which were confirmed by the CV and EIS experiments as shown in Figs. 7 and 8(a). When ionomer was sprayed on the surface of the Pt-NWs, it penetrated into the electrocatalyst layer. However, this penetration was rather difficult and most of the ionomer was located near the electrocatalyst layer surface. Further supply of ionomer filled the pores and formed a film on the surface of the electrocatalyst layer [37]. Too much thick ionomer film on the surface of the Pt-NWs covered the catalyst sites, limited mass transport by retarding the gas access to actives sites, and increased mass transport polarization in the electrocatalyst layer, which induced a small ECSA and large mass transport resistance as shown in Figs. 7 and 8(b) at 25% ionomer content. Fig. 8(a) shows the impedance spectra measured at 0.80 V for different ionomer loadings on the surface of the Pt-NWs. It could be observed that the starting point of the loop shifted to the right side with increasing ionomer loading, indicating an increasing ohmic resistance of the electrocatalyst layer. This phenomenon could be explained by the increased contact resistance caused by the thicker ionomer layer on the surface of the electrocatalyst layer [35].

4.

Conclusions

The effects of ionomer contents in the carbon matrix and impregnated on the surface of the Pt-NW electrocatalyst layer were investigated for the composite Pt-NW electrodes. The electrodes were tested as cathode by the polarization, CV, and EIS experiments in a 10 cm2 PEMFC single cell hardware with H2/air feeds. It was found that the electrolyte ionomer content was an important parameter for optimizing the cell performance. The optimal ionomer content in the carbon matrix and impregnated on the surface of the Pt-NWs was 10 wt% of the carbon matrix and 8 wt% of the Pt loading, respectively. The ionomer content in the carbon matrix had a great influence on the aggregation and distribution of Pt-NWs grown on it, and therefore influenced the cell performance. Impregnated ionomer on the surface of the Pt-NWs improved the ionic conductivity but excess ionomer limited the mass transport characteristics.

Acknowledgments

Fig. 8 e Electrochemical impedance spectra measured at 0.80 V and 0.40 V for MEAs with different ionomer loadings (wt% based on the Pt-NW loading of 0.30 mg cmL2) impregnated on the surface of the Pt-NWs.

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

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