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High performance polymer electrolyte membrane fuel cells (PEMFCs) with gradient Pt nanowire cathodes prepared by decal transfer method
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 4 0 ( 2 0 1 5 ) 3 0 6 8 e3 0 7 4
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
High performance polymer electrolyte membrane fuel cells (PEMFCs) with gradient Pt nanowire cathodes prepared by decal transfer method Zhaoxu Wei a, Kaihua Su a, Sheng Sui a,*, An He 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:
A gradient Pt nanowire (Pt-NW) cathode with a promoted mass transfer and Pt utilization
Received 12 August 2014
was developed by decal transfer method. The relationships of Pt loading and ionomer
Received in revised form
content with electrode performance were investigated by electrode structure character-
11 December 2014
ization and testing in polymer electrolyte membrane fuel cell (PEMFC). The results show
Accepted 2 January 2015
that an increasing Pt loading can improve the catalytic kinetic performance of Pt-NW
Available online 24 January 2015
electrode, but too higher a Pt loading leads to serious aggregation and thus low catalyst utilization. A similar trend was found to the ionomer content sprayed onto the Pt-NW
Keywords:
cathode. The ionomer extends the triple-phase boundary (TPB), but excessive amount
Pt nanowire
would cover part of the catalyst active sites and hinder the mass transfer. The optimal
Electrode
performance of the Pt-NW cathode is achieved at 0.30 mgPt cm2 and 33 wt% ionomer,
Triple-phase boundary (TPB)
where a maximum power density of 0.93 W cm2 is obtained, which is better than the
Decal transfer method
state-of-the-art commercial gas diffusion electrode (GDE) with 0.40 mgPt cm2.
Polymer electrolyte membrane fuel
Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
cell (PEMFC)
Introduction The polymer electrolyte membrane fuel cell (PEMFC) is a promising power generator for its high efficiency and zeroemission, but its cost and durability are the main barriers in the commercialization [1]. To satisfy the practical PEMFC application, much attention is paid on the cathode in membrane electrode assembly (MEA) because of its sluggish oxygen reduction reaction (ORR) kinetics and mass transfer problem. Recently, Pt nanowire (Pt-NW) catalyst has attracted much attention in PEMFCs. Compared with the conventional Pt nanoparticle catalyst, Pt-NW can provide several advantages
[2,3]: 1) it has more highly active crystal facets, 2) its highly ordered structure can facilitate the multiphase mass transfer in the electrode reaction, and 3) its one-dimensional structure can reduce Ostwald ripening. There are mainly three methods to prepare Pt-NWs: hard template [4,5], soft template [6,7] and wet-chemical method [8e11]. Sun et al. [9]synthesized Pt nanowires/C catalyst with carbon black as catalyst support in an aqueous solution of hexachloroplatinic acid (H2PtCl6) and formic acid (HCOOH), and better catalyst activity was demonstrated as compared with the Pt nanoparticles/C catalyst. Du et al. [12] fabricated Pt-NW gas diffusion electrode (GDE) by simply immersing carbon paper in a mixed aqueous solution of H2PtCl6, HCOOH and polyvinyl pyrrolidone (PVP),
* Corresponding author. Tel./fax: þ86 21 34206249. E-mail address: [email protected] (S. Sui). http://dx.doi.org/10.1016/j.ijhydene.2015.01.009 0360-3199/Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
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 4 0 ( 2 0 1 5 ) 3 0 6 8 e3 0 7 4
and also got improved performance in contrast with the commercial GDE. In our previous work, we developed a novel method for preparing Pt-NW electrode by in-situ growing PtNWs into a carbon matrix coated on the polymer electrolyte membrane [13e15]. In the case of electrode structure designing, optimization of the Pt distribution is an approach to improve the mass transfer and Pt utilization in electrodes. Roshandel [16] evaluated the effect of catalyst gradient distribution in the catalyst layer (CL) by numerical and analytical approaches, and found that more catalyst should be applied to the CL/membrane interface, where the reaction rate is higher than that at gas diffusion layer (GDL)/CL interface. Su et al. [17] designed a composite catalyst layer which had a higher Pt loading near the membrane and a lower Pt loading near the GDL, and better performance was achieved. Besides the Pt catalyst effect, the ionomer content and distribution in CL are also very important. Raistri [18] firstly introduced the ionomer into the PEMFC electrode to extend the triple-phase boundary (TPB), and a great breakthrough has been made in PEMFC. However, it has also been reported that excessive ionomor would cover the catalyst active sites, reduce the gas permeability and hinder the water draining [19,20]. In the fabrication of membrane electrode assemblies, the decal transfer method was introduced to overcome the drawbacks of the other two methods: e.g. the structural distortion during the hot pressing and catalyst loss in porous GDL by GDL-based method [21,22], and the serious membrane swelling in the catalyst coated membrane (CCM) approach [23,24]. In this work, we demonstrated the in-situ growth of PtNWs in the carbon matrix coated on the polytetrafluoroethylene (PTFE) substrate, and prepared the Pt-NW electrode by decal transfer method, as a comparison with our previous study on the Pt-NW electrodes using CCM method [15]. The PtNW loading and Nafion® ionomor content sprayed onto the electrode were detailed, and the influence mechanisms on the electrode performance were discussed as well.
Experimental In-situ growing Pt-NW cathode and MEA preparation by decal transfer method The cathode catalyst layer was prepared by in-situ growing Pt nanowires in the carbon matrix coated on a PTFE substrate. The preparation process was as follows: Firstly, carbon powder (XC-72R, Cabot), Nafion® ionomer solution (DE1020, 10 wt %, DuPont) and iso-propanol (Sinopharm Chem. Reagent) (carbon: Nafion® ionomer ¼ 9:1 wt%) were sonicated for 5 min to form an ink, then the ink was sprayed onto the decal substrate (PTFE coated fabric, ultra-premium grade, CS Hyde Company) with a spray gun (Iwata HP-CH, Japan). The carbon loadings of all samples were fixed at 0.10 mg cm2. Secondly, the substrate was pasted on the glass Petri dish, and then a mixed solution of H2PtCl6 (Sinopharm Chem. Reagent) and HCOOH (Sinopharm Chem. Reagent) was added evenly to grow Pt nanowires in the matrix [9,13e15]. The coated substrate was kept in the solution at room temperature for 72 h. After that, the substrate was rinsed for three times and
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immersed in the deionized water for 24 h to remove the remained ions, and then dried at 50 C for 30 min. Finally, a diluted Nafion® ionomer solution was sprayed onto the surface. Either the Pt loading or the Nafion® ionomer content sprayed onto the surface of the Pt-NWs was changed independently. The Pt loading was varied from 0.10 to 0.50 mg cm2, while the Nafion® ionomer content was fixed at 33wt% (based on the Pt loading). The Nafion® ionomer content was varied to be 17, 33, 67 and 100 wt% with a fixed Pt loading of 0.30 mg cm2. The anode catalyst layer was prepared with commercial Pt/ C catalyst. The well mixed ink of 40wt% Pt/C catalyst (HiSPEC™4000, Johnson & Matthey), Nafion® ionomer solution and isopropanol was sprayed onto the decal substrate. The Pt loadings and Nafion® ionomer contents in all anode samples were fixed at 0.30 mg cm2 and 25 wt%, respectively. The homemade MEA was fabricated by decal transfer method. A pair of anode decal substrate and cathode one was placed on both sides of a Nafion® membrane (NR212, DuPont) and hot-pressed at 145 C for 3 min under 0.4 MPa. Next, the decal substrates were peeled from the electrodes and left the MEA. For comparison, a commercial GDE from Johnson Matthey, with Pt nanoparticles at a loading of 0.40 mg cm2 screen printed on GDL 34BC, was used to fabricate a MEA under the same conditions as above.
Single cell test The MEA with an active area of 3.3 3.3 cm2 was assembled with GDLs (AvCarb GDS3250, Ballard Power SystemsInc.), PTFE gaskets, and graphite blocks. The cell performance test was operated on the 850e Multi-Range Fuel Cell Test System (Scribner Associates Inc.). The MEA was initially activated as reported in our previous work [13]. The polarization curves were recorded by voltage sweeping from open circuit voltage (OCV) to 0.30 V at a rate of 2 mV s1. The temperatures of the cell and humidifying cans were kept at 70 C and 65 C, respectively. The stoichiometric ratios of hydrogen (99.999% purity) and air (99.999% purity) were 1.5 and 2.0, respectively. The backpressures are 1.0 bar at both sides. Electrochemical impedance spectra(EIS) measurement (885 Fuel Cell Potentiostat, Scribner Associates Inc.) were conducted at the cell voltages of 0.80, 0.60 and 0.40 V in a frequency range of 10 kHze0.1 Hz with an AC amplitude of 10% DC current. Cyclic voltammograms (CV) (SI1287, Solartron Analytical Inc.) were recorded with a scan rate of 25 mV s1 at a cell temperature of 35 C. The flow rates of hydrogen and nitrogen were 300 and 75 sccm to the anode and cathode, respectively.
Physical characterization A 5 mm 0.5 mm strip cut from the homemade MEA was embedded in the epoxy resin. After solidification, the epoxy resin was sliced by a cryosection system to prepare a crosssectional sample for the cathode catalyst layer. The crosssectional morphologies were observed using a biology transmission electron microscope (TEM) (Tecnai G2 Spirit Biotwin, FEI) operating at an accelerating voltage of 120 kV. The surface
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morphologies of the cathode catalyst layers were characterized using a field emission scanning electron microscope (SEM) (S-4800, Hitachi) operating at an accelerating voltage of 10 KV. Pt distribution across the catalyst layer thickness was analysed by energy-dispersive X-ray spectroscopy in SEM (SEM-EDS). The Pt loading of the cathode catalyst layer was measured by inductively coupled plasma-atomic emission spectrometer (ICP-AES) (7500a, Agilent).
Results and discussion The effect of Pt loading on the performance of Pt-NW cathode The polarization curves of the Pt-NW cathodes with various Pt loadings from 0.10 to 0.50 mg cm2 are shown in Fig. 1. The maximum power density of 0.93 W cm2 is achieved at 0.30 mgPt cm2. At a lower Pt loading range, the improvement of the cell performance is mainly caused by the better ORR kinetics in the presence of higher Pt loading, however, this effect will be limited when the Pt loading is sufficiently high [25]. The performance will even deteriorate with further increasing the Pt loading. The peak power density at 0.50 mgPt cm2 is only 0.77 W cm2, which is 17.2% lower than the maximum value of 0.93 W cm2 at 0.30 mgPt cm2. Fig. 2 illustrates the EIS results measured at 0.80 and 0.60 V for various Pt loadings of 0.10, 0.20, 0.30 and 0.50 mg cm2. In a Nyquist plot, the high-frequency intercept on the real axis represents the total ohmic resistance of the cell, and the arc diameter is the sum of the charge transfer and mass transfer resistance [26]. At 0.80 V, due to the low current density, the arc diameter is mainly dominated by the kinetic (charge transfer) resistance. As shown in Fig. 2(a), the kinetic resistance is decreased with the increasing Pt loading, and this becomes very small when the Pt loading is above 0.30 mg cm2. At 0.60 V, the effect of mass transfer resistances in our homemade MEAs already appears because of the high current density, thus the dual or compressed arcs are formed in the Nyquist plot as shown in Fig. 2(b). The sample with
Fig. 1 e Polarization curves of different Pt loadings (mg cm¡2) in the cathodes.
Fig. 2 e EIS of the MEAs with various Pt loadings (mg cm¡2) in the cathodes at (a) 0.80 and (b) 0.60 V.
0.30 mgPt cm2 shows the smallest arc diameter over the others, indicating the minimum total resistance. This is in good agreement with the polarization curves as shown in Fig. 1. The arc diameters of 0.20 and 0.50 mgPt cm2 samples are almost the same at 0.60 V, but the 0.20 mgPt cm2 one exhibits a larger kinetic resistance and a smaller mass transfer resistance than that of 0.50 mgPt cm2. To study the effect of Pt-NW loading on the catalyst active sites in electrode, CV was recorded and the results are shown in Fig. 3. The charge density of hydrogen absorption was used to calculate the electrochemically active surface area (ECSA) of Pt, assuming that 210 mC cm2 is needed to form a monolayer of absorbed H on polycrystalline Pt [27]. The inset in Fig. 3 illustrates the ECSA results. ECSA reduces gradually with the increasing Pt loading. The maximum ECSA value is 45.06 m2 g1 obtained at 0.10 mgPt cm2, and dramatically drops to 23.85 m2 g1 at 0.50 mgPt cm2. These samples were further examined by SEM analysis to understand the structure changing with the Pt loading. Fig. 4 shows the SEM images of the catalyst layer surfaces near the membranes, and the Pt loading is 0.10, 0.20 and 0.50 mg cm2, respectively. With the increasing Pt loading, the aggregation of Pt-NWs (bright area) becomes serious and the pores (dark
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 4 0 ( 2 0 1 5 ) 3 0 6 8 e3 0 7 4
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Fig. 3 e CV of the cathodes with different Pt loadings (mg cm¡2).
area) are reduced. Therefore, if the Pt loading is too high, not only the ECSA is reduced, but also the gas and water transport are hindered.
The effect of Nafion® ionomer sprayed onto the Pt-NW cathode To extend the TPB, Nafion® ionomer was sprayed onto the PtNW cathode before transfer to the membrane. The ionomer content was varied to be 17, 33, 67 and 100 wt% of the catalyst mass, which is fixed at 0.30 mgPt cm2. The polarization curves of the cathodes with different ionomer contents are shown in Fig. 5. The optimal performance is achieved at 33wt % ionomer with a peak power density of 0.93 W cm2. In Fig. 6, the current densities of various ionomer contents are compared at different voltages (0.80, 0.60 and 0.40 V). At 0.80 V, the current densities of the MEAs have no obvious difference, suggesting that at the present range of ionomer content, it has no significant influence on the ORR kinetics. However, at a lower voltage, e.g. 0.60 or 0.40 V, the current densities rise with the increasing ionomer content at first and then descend. The rise can be attributed to the extending of the TPB. But excessive ionomer content would cover part of the catalyst active sites and increase mass transfer resistance [28], resulting in the decline of the current density. It is noted that the optimal ionomer content of 33wt% is much higher than that of our previous result, which is only 8wt% using the direct CCM method [15]. This can be understood by the different locations of the sprayed ionomer in the cathodes: the sprayed ionomer is closed to the GDL in our previous CCM method, thus the MEA is much more susceptible to mass transfer resistance; but in our decal transfer method here, the sprayed ionomer is closed to the membrane, hence more ionomer can be used to extend the TPB without heavily affecting the mass transfer. Fig. 7 shows the CV curves and ECSA values of the cathodes with different ionomer contents. The cathode with 33 wt% ionomer exhibits the largest ECSA value of 32.95 m2 g1, and with further increasing the ionomer content, the ECSA value is significantly reduced. This is probably because the excessive ionomer has covered some of the catalyst active sites.
Fig. 4 e Surface SEM images of the cathode catalyst layers with different Pt loadings of (a) 0.10, (b) 0.20, and (c) 0.50 mg cm¡2.
Comparison of Pt-NW electrode with the commercial GDE Fig. 8 shows the polarization curves of the MEAs with the optimal Pt-NW cathode and the commercial GDE. The Pt loadings of the Pt-NW cathode and the commercial GDE are 0.30 and 0.40 mg cm2, respectively. The maximum power density of the MEA with the Pt-NW cathode is 0.93 W cm2,
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Fig. 5 e Polarization curves of different ionomer contents (wt% based on the Pt loading of 0.30 mg cm¡2) spayed onto the Pt-NWs.
Fig. 6 e Current densities comparison of different ionomer contents (wt%) at 0.80, 0.60 and 0.40 V.
Fig. 7 e CV of the cathodes with different ionomer contents (wt%) spayed onto the Pt-NWs.
Fig. 8 e Performance comparison of the MEAs with the optimal Pt-NW cathode and the commercial GDE.
which is 6.90% higher than that of the commercial GDE. Their power performances are nearly the same at the voltage above 0.60 V. But below 0.60 V, the Pt-NW electrode has an obviously better performance. To further investigate the distinctions between the two MEAs at the high current density, EIS were recorded at 0.40 V as shown in Fig. 9. At 0.40 V, the EIS is mainly dominated by the mass transfer resistance. It can be seen that, although the ohmic resistance of the Pt-NW electrode is a little bigger than that of the commercial GDE, the mass transfer resistance is much smaller, and thus less concentration polarization loss and better performance are achieved. To understand more about the lower mass transfer resistance, the SEM-EDS results and cross-sectional TEM images are shown in Figs. 10 and 11, respectively. According to Fig. 10, the Pt-NWs are gradient distributed across the cathode thickness, and less Pt exists near the GDL. The cathode structure and Pt nanowire morphology are displayed in Fig. 11(a) and (b), respectively. It is found that the Pt-NWs assemble into large superstructures with a size of 50e100 nm. As reported in our previous work [15], the Pt-NWs are about 10e30 nm in length and 2e4 nm in diameter with single-crystal structures growing along the (111) facets. The
Fig. 9 e EIS at 0.40 V for the MEAs with the optimal Pt-NW cathode and the commercial GDE.
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improvement of the mass transfer and Pt utilization can be attributed to several factors: 1) the gradient distribution improves Pt utilization; 2) dense Pt-NWs near membrane provide abundant reaction sites; 3) affluent pores near GDL facilitate the reactant gas diffusion and water draining; 4) outstretched Pt-NWs on the carbon supports are easier for oxygen accessing [11].
Conclusions
Fig. 10 e SEM-EDS analysis of the Pt-NW cathode.
In this work, Pt-NW cathodes were prepared by in-situ growing Pt-NWs in the carbon matrices coated on the PTFE substrates, and the MEAs were fabricated by decal transfer method. The gradient distribution of Pt-NWs facilitates the mass transfer in the electrodes and thus improves the Pt utilization. The optimal Pt loading for Pt-NW electrode is 0.30 mg cm2, and a higher Pt loading results in serious aggregation and blocking pores. The ionomer sprayed onto the Pt-NW electrode locates near the membrane and facilitates the proton conduction. The optimal ionomer content is 33 wt %, and excessive amount would cover part of the catalyst active sites and hinder the mass transfer.
Acknowledgements 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 and TEM characterization.
references
Fig. 11 e Cross-sectional TEM images at (a) the lowmagnification and (b) high-magnification of the Pt-NW cathode.
[1] Steele BC, Heinzel A. Materials for fuel-cell technologies. Nature 2001;414(6861):345e52. [2] Yan Z, Li B, Yang D, Ma J. Pt nanowire electrocatalysts for proton exchange membrane fuel cells. Chin J Catal 2013;34(8):1471e81. [3] Liang HW, Cao X, Zhou F, Cui C-H, Zhang WJ. A free-standing Pt-nanowire membrane as a highly stable electrocatalyst for the oxygen reduction reaction. Adv Mater 2011;23(12):1467e71. [4] Zhang M, Li J, Pan M. Catalytic performance of Pt nanowire arrays for oxygen reduction. Acta Physico Chimica Sin 2011;27(7):1685e8. [5] Zhao Y, Xu L, Guo J, Li H. Template preparation of Pt nanowire array electrode on Ti/Si substrate for methanol electro-oxidation. Appl Surf Sci 2007;253(6):3242e6. [6] Song Y, Garcia RM, Dorin RM, Wang H, Qiu Y. Synthesis of platinum nanowire networks using a soft template. Nano Lett 2007;7(12):3650e5. [7] Li B, Higgins DC, Zhu S, Li H, Wang H. Highly active PteRu nanowire network catalysts for the methanol oxidation reaction. Catal Commun 2012;18:51e4. [8] Sun S, Yang D, Zhang G, Sacher E. Synthesis and characterization of platinum nanowireecarbon nanotube heterostructures. Chem Mater 2007;19(26):6376e8. [9] Sun S, Jaouen F, Dodelet JP. Controlled growth of Pt nanowires on carbon nanospheres and their enhanced
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performance as electrocatalysts in PEM fuel cells. Adv Mater 2008;20(20):3900e4. Sun S, Zhang G, Geng D, Chen Y, Banis MN. Direct growth of single-crystal Pt nanowires on Sn@CNT nanocable: 3D electrodes for highly active electrocatalysts. Chem A Eur J 2010;16(3):829e35. Sun S, Zhang G, Geng D, Chen Y. A highly durable platinum nanocatalyst for proton exchange membrane fuel cells: multiarmed starlike nanowire single crystal. Angew Chem 2011;123(2):442e6. Du S. A facile route for polymer electrolyte membrane fuel cell electrodes with in situ grown Pt nanowires. J Power Sources 2010;195(1):289e92. Yao X, Su K, Sui S, Mao L, He A, Zhang J. A novel catalyst layer with carbon matrix for Pt nanowire growth in proton exchange membrane fuel cells (PEMFCs). Int J Hydrogen Energy 2013;38(28):12374e8. Su K, Sui S, Yao X, Wei Z, Zhang J. Controlling Pt loading and carbon matrix thickness for a high performance Pt-nanowire catalyst layer in PEMFCs. Int J Hydrogen Energy 2014;39(7):3397e403. Su K, Yao X, Sui S, Wei Z, Zhang J, Du S. Ionomer content effects on the electrocatalyst layer with in-situ grown Pt nanowires in PEMFCs. Int J Hydrogen Energy 2014;39(7):3219e25. Roshandel Ramin, Ahmadi Farzad. Effects of catalyst loading gradient in catalyst layers on performance of polymer electrolyte membrane fuel cells. Renew Energy 2013;50:921e31. Su H, Liao S, Wu Y. Significant improvement in cathode performance for proton exchange membrane fuel cell by a novel double catalyst layer design. J Power Sources 2010;195(11):3477e80.
[18] Raistrick ID. Electrode assembly for use in a solid polymer electrolyte fuel cell. U.S. Patent No. 4876115, 24 Oct, 1989. [19] Passalacqua E, Lufrano F, Squadrito G, Patti A. Nafion content in the catalyst layer of polymer electrolyte fuel cells: effects on structure and performance. Electrochim Acta 2001;46(6):799e805. [20] Sasikumar G, Ihm JW, Ryu H. Dependence of optimum Nafion content in catalyst layer on platinum loading. J Power Sources 2004;132(1):11e7. [21] Frost JC, Gascoyne JM, Hards GA, Prater KB. Manufacture of electrodes. U. S Patent No. 5871860. 16 Feb, 1999. [22] Frey T, Linardi M. Effects of membrane electrode assembly preparation on the polymer electrolyte membrane fuel cell performance. Electrochim Acta 2004;50(1):99e105. [23] Mussell RD, Timothy J. Active layer for membrane electrode assembly. U. S Patent No. 5882810. 16 Mar, 1999. [24] Tang H, Wang S, Pan M. Performance of direct methanol fuel cells prepared by hot-pressed MEA and catalyst-coated membrane (CCM). Electrochim Acta 2007;52(11):3714e8. [25] Leimin X, Shijun L, Lijun Y. Investigation of a novel catalyst coated membrane method to prepare low-platinum-loading membrane electrode assemblies for PEMFCs. Fuel Cells 2009;9(2):101e5. [26] Yuan X, Wang H, Colin SJ. AC impedance technique in PEM fuel cell diagnosisda review. Int J Hydrogen Energy 2007;32(17):4365e80. [27] Ticianelli EA, Derouin CR, Srinivasan S. Localization of platinum in low catalyst loading electrodes to attain high power densities in SPE fuel cells. J Electroanal Chem Interfacial Electrochem 1988;251(2):275e95. [28] Holdcroft S. Fuel cell catalyst layers: a polymer science perspective. Chem Mater 2013;26(1):381e93.
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
High performance polymer electrolyte membrane fuel cells (PEMFCs) with gradient Pt nanowire cathodes prepared by decal transfer method Zhaoxu Wei a, Kaihua Su a, Sheng Sui a,*, An He 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:
A gradient Pt nanowire (Pt-NW) cathode with a promoted mass transfer and Pt utilization
Received 12 August 2014
was developed by decal transfer method. The relationships of Pt loading and ionomer
Received in revised form
content with electrode performance were investigated by electrode structure character-
11 December 2014
ization and testing in polymer electrolyte membrane fuel cell (PEMFC). The results show
Accepted 2 January 2015
that an increasing Pt loading can improve the catalytic kinetic performance of Pt-NW
Available online 24 January 2015
electrode, but too higher a Pt loading leads to serious aggregation and thus low catalyst utilization. A similar trend was found to the ionomer content sprayed onto the Pt-NW
Keywords:
cathode. The ionomer extends the triple-phase boundary (TPB), but excessive amount
Pt nanowire
would cover part of the catalyst active sites and hinder the mass transfer. The optimal
Electrode
performance of the Pt-NW cathode is achieved at 0.30 mgPt cm2 and 33 wt% ionomer,
Triple-phase boundary (TPB)
where a maximum power density of 0.93 W cm2 is obtained, which is better than the
Decal transfer method
state-of-the-art commercial gas diffusion electrode (GDE) with 0.40 mgPt cm2.
Polymer electrolyte membrane fuel
Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
cell (PEMFC)
Introduction The polymer electrolyte membrane fuel cell (PEMFC) is a promising power generator for its high efficiency and zeroemission, but its cost and durability are the main barriers in the commercialization [1]. To satisfy the practical PEMFC application, much attention is paid on the cathode in membrane electrode assembly (MEA) because of its sluggish oxygen reduction reaction (ORR) kinetics and mass transfer problem. Recently, Pt nanowire (Pt-NW) catalyst has attracted much attention in PEMFCs. Compared with the conventional Pt nanoparticle catalyst, Pt-NW can provide several advantages
[2,3]: 1) it has more highly active crystal facets, 2) its highly ordered structure can facilitate the multiphase mass transfer in the electrode reaction, and 3) its one-dimensional structure can reduce Ostwald ripening. There are mainly three methods to prepare Pt-NWs: hard template [4,5], soft template [6,7] and wet-chemical method [8e11]. Sun et al. [9]synthesized Pt nanowires/C catalyst with carbon black as catalyst support in an aqueous solution of hexachloroplatinic acid (H2PtCl6) and formic acid (HCOOH), and better catalyst activity was demonstrated as compared with the Pt nanoparticles/C catalyst. Du et al. [12] fabricated Pt-NW gas diffusion electrode (GDE) by simply immersing carbon paper in a mixed aqueous solution of H2PtCl6, HCOOH and polyvinyl pyrrolidone (PVP),
* Corresponding author. Tel./fax: þ86 21 34206249. E-mail address: [email protected] (S. Sui). http://dx.doi.org/10.1016/j.ijhydene.2015.01.009 0360-3199/Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
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 4 0 ( 2 0 1 5 ) 3 0 6 8 e3 0 7 4
and also got improved performance in contrast with the commercial GDE. In our previous work, we developed a novel method for preparing Pt-NW electrode by in-situ growing PtNWs into a carbon matrix coated on the polymer electrolyte membrane [13e15]. In the case of electrode structure designing, optimization of the Pt distribution is an approach to improve the mass transfer and Pt utilization in electrodes. Roshandel [16] evaluated the effect of catalyst gradient distribution in the catalyst layer (CL) by numerical and analytical approaches, and found that more catalyst should be applied to the CL/membrane interface, where the reaction rate is higher than that at gas diffusion layer (GDL)/CL interface. Su et al. [17] designed a composite catalyst layer which had a higher Pt loading near the membrane and a lower Pt loading near the GDL, and better performance was achieved. Besides the Pt catalyst effect, the ionomer content and distribution in CL are also very important. Raistri [18] firstly introduced the ionomer into the PEMFC electrode to extend the triple-phase boundary (TPB), and a great breakthrough has been made in PEMFC. However, it has also been reported that excessive ionomor would cover the catalyst active sites, reduce the gas permeability and hinder the water draining [19,20]. In the fabrication of membrane electrode assemblies, the decal transfer method was introduced to overcome the drawbacks of the other two methods: e.g. the structural distortion during the hot pressing and catalyst loss in porous GDL by GDL-based method [21,22], and the serious membrane swelling in the catalyst coated membrane (CCM) approach [23,24]. In this work, we demonstrated the in-situ growth of PtNWs in the carbon matrix coated on the polytetrafluoroethylene (PTFE) substrate, and prepared the Pt-NW electrode by decal transfer method, as a comparison with our previous study on the Pt-NW electrodes using CCM method [15]. The PtNW loading and Nafion® ionomor content sprayed onto the electrode were detailed, and the influence mechanisms on the electrode performance were discussed as well.
Experimental In-situ growing Pt-NW cathode and MEA preparation by decal transfer method The cathode catalyst layer was prepared by in-situ growing Pt nanowires in the carbon matrix coated on a PTFE substrate. The preparation process was as follows: Firstly, carbon powder (XC-72R, Cabot), Nafion® ionomer solution (DE1020, 10 wt %, DuPont) and iso-propanol (Sinopharm Chem. Reagent) (carbon: Nafion® ionomer ¼ 9:1 wt%) were sonicated for 5 min to form an ink, then the ink was sprayed onto the decal substrate (PTFE coated fabric, ultra-premium grade, CS Hyde Company) with a spray gun (Iwata HP-CH, Japan). The carbon loadings of all samples were fixed at 0.10 mg cm2. Secondly, the substrate was pasted on the glass Petri dish, and then a mixed solution of H2PtCl6 (Sinopharm Chem. Reagent) and HCOOH (Sinopharm Chem. Reagent) was added evenly to grow Pt nanowires in the matrix [9,13e15]. The coated substrate was kept in the solution at room temperature for 72 h. After that, the substrate was rinsed for three times and
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immersed in the deionized water for 24 h to remove the remained ions, and then dried at 50 C for 30 min. Finally, a diluted Nafion® ionomer solution was sprayed onto the surface. Either the Pt loading or the Nafion® ionomer content sprayed onto the surface of the Pt-NWs was changed independently. The Pt loading was varied from 0.10 to 0.50 mg cm2, while the Nafion® ionomer content was fixed at 33wt% (based on the Pt loading). The Nafion® ionomer content was varied to be 17, 33, 67 and 100 wt% with a fixed Pt loading of 0.30 mg cm2. The anode catalyst layer was prepared with commercial Pt/ C catalyst. The well mixed ink of 40wt% Pt/C catalyst (HiSPEC™4000, Johnson & Matthey), Nafion® ionomer solution and isopropanol was sprayed onto the decal substrate. The Pt loadings and Nafion® ionomer contents in all anode samples were fixed at 0.30 mg cm2 and 25 wt%, respectively. The homemade MEA was fabricated by decal transfer method. A pair of anode decal substrate and cathode one was placed on both sides of a Nafion® membrane (NR212, DuPont) and hot-pressed at 145 C for 3 min under 0.4 MPa. Next, the decal substrates were peeled from the electrodes and left the MEA. For comparison, a commercial GDE from Johnson Matthey, with Pt nanoparticles at a loading of 0.40 mg cm2 screen printed on GDL 34BC, was used to fabricate a MEA under the same conditions as above.
Single cell test The MEA with an active area of 3.3 3.3 cm2 was assembled with GDLs (AvCarb GDS3250, Ballard Power SystemsInc.), PTFE gaskets, and graphite blocks. The cell performance test was operated on the 850e Multi-Range Fuel Cell Test System (Scribner Associates Inc.). The MEA was initially activated as reported in our previous work [13]. The polarization curves were recorded by voltage sweeping from open circuit voltage (OCV) to 0.30 V at a rate of 2 mV s1. The temperatures of the cell and humidifying cans were kept at 70 C and 65 C, respectively. The stoichiometric ratios of hydrogen (99.999% purity) and air (99.999% purity) were 1.5 and 2.0, respectively. The backpressures are 1.0 bar at both sides. Electrochemical impedance spectra(EIS) measurement (885 Fuel Cell Potentiostat, Scribner Associates Inc.) were conducted at the cell voltages of 0.80, 0.60 and 0.40 V in a frequency range of 10 kHze0.1 Hz with an AC amplitude of 10% DC current. Cyclic voltammograms (CV) (SI1287, Solartron Analytical Inc.) were recorded with a scan rate of 25 mV s1 at a cell temperature of 35 C. The flow rates of hydrogen and nitrogen were 300 and 75 sccm to the anode and cathode, respectively.
Physical characterization A 5 mm 0.5 mm strip cut from the homemade MEA was embedded in the epoxy resin. After solidification, the epoxy resin was sliced by a cryosection system to prepare a crosssectional sample for the cathode catalyst layer. The crosssectional morphologies were observed using a biology transmission electron microscope (TEM) (Tecnai G2 Spirit Biotwin, FEI) operating at an accelerating voltage of 120 kV. The surface
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morphologies of the cathode catalyst layers were characterized using a field emission scanning electron microscope (SEM) (S-4800, Hitachi) operating at an accelerating voltage of 10 KV. Pt distribution across the catalyst layer thickness was analysed by energy-dispersive X-ray spectroscopy in SEM (SEM-EDS). The Pt loading of the cathode catalyst layer was measured by inductively coupled plasma-atomic emission spectrometer (ICP-AES) (7500a, Agilent).
Results and discussion The effect of Pt loading on the performance of Pt-NW cathode The polarization curves of the Pt-NW cathodes with various Pt loadings from 0.10 to 0.50 mg cm2 are shown in Fig. 1. The maximum power density of 0.93 W cm2 is achieved at 0.30 mgPt cm2. At a lower Pt loading range, the improvement of the cell performance is mainly caused by the better ORR kinetics in the presence of higher Pt loading, however, this effect will be limited when the Pt loading is sufficiently high [25]. The performance will even deteriorate with further increasing the Pt loading. The peak power density at 0.50 mgPt cm2 is only 0.77 W cm2, which is 17.2% lower than the maximum value of 0.93 W cm2 at 0.30 mgPt cm2. Fig. 2 illustrates the EIS results measured at 0.80 and 0.60 V for various Pt loadings of 0.10, 0.20, 0.30 and 0.50 mg cm2. In a Nyquist plot, the high-frequency intercept on the real axis represents the total ohmic resistance of the cell, and the arc diameter is the sum of the charge transfer and mass transfer resistance [26]. At 0.80 V, due to the low current density, the arc diameter is mainly dominated by the kinetic (charge transfer) resistance. As shown in Fig. 2(a), the kinetic resistance is decreased with the increasing Pt loading, and this becomes very small when the Pt loading is above 0.30 mg cm2. At 0.60 V, the effect of mass transfer resistances in our homemade MEAs already appears because of the high current density, thus the dual or compressed arcs are formed in the Nyquist plot as shown in Fig. 2(b). The sample with
Fig. 1 e Polarization curves of different Pt loadings (mg cm¡2) in the cathodes.
Fig. 2 e EIS of the MEAs with various Pt loadings (mg cm¡2) in the cathodes at (a) 0.80 and (b) 0.60 V.
0.30 mgPt cm2 shows the smallest arc diameter over the others, indicating the minimum total resistance. This is in good agreement with the polarization curves as shown in Fig. 1. The arc diameters of 0.20 and 0.50 mgPt cm2 samples are almost the same at 0.60 V, but the 0.20 mgPt cm2 one exhibits a larger kinetic resistance and a smaller mass transfer resistance than that of 0.50 mgPt cm2. To study the effect of Pt-NW loading on the catalyst active sites in electrode, CV was recorded and the results are shown in Fig. 3. The charge density of hydrogen absorption was used to calculate the electrochemically active surface area (ECSA) of Pt, assuming that 210 mC cm2 is needed to form a monolayer of absorbed H on polycrystalline Pt [27]. The inset in Fig. 3 illustrates the ECSA results. ECSA reduces gradually with the increasing Pt loading. The maximum ECSA value is 45.06 m2 g1 obtained at 0.10 mgPt cm2, and dramatically drops to 23.85 m2 g1 at 0.50 mgPt cm2. These samples were further examined by SEM analysis to understand the structure changing with the Pt loading. Fig. 4 shows the SEM images of the catalyst layer surfaces near the membranes, and the Pt loading is 0.10, 0.20 and 0.50 mg cm2, respectively. With the increasing Pt loading, the aggregation of Pt-NWs (bright area) becomes serious and the pores (dark
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Fig. 3 e CV of the cathodes with different Pt loadings (mg cm¡2).
area) are reduced. Therefore, if the Pt loading is too high, not only the ECSA is reduced, but also the gas and water transport are hindered.
The effect of Nafion® ionomer sprayed onto the Pt-NW cathode To extend the TPB, Nafion® ionomer was sprayed onto the PtNW cathode before transfer to the membrane. The ionomer content was varied to be 17, 33, 67 and 100 wt% of the catalyst mass, which is fixed at 0.30 mgPt cm2. The polarization curves of the cathodes with different ionomer contents are shown in Fig. 5. The optimal performance is achieved at 33wt % ionomer with a peak power density of 0.93 W cm2. In Fig. 6, the current densities of various ionomer contents are compared at different voltages (0.80, 0.60 and 0.40 V). At 0.80 V, the current densities of the MEAs have no obvious difference, suggesting that at the present range of ionomer content, it has no significant influence on the ORR kinetics. However, at a lower voltage, e.g. 0.60 or 0.40 V, the current densities rise with the increasing ionomer content at first and then descend. The rise can be attributed to the extending of the TPB. But excessive ionomer content would cover part of the catalyst active sites and increase mass transfer resistance [28], resulting in the decline of the current density. It is noted that the optimal ionomer content of 33wt% is much higher than that of our previous result, which is only 8wt% using the direct CCM method [15]. This can be understood by the different locations of the sprayed ionomer in the cathodes: the sprayed ionomer is closed to the GDL in our previous CCM method, thus the MEA is much more susceptible to mass transfer resistance; but in our decal transfer method here, the sprayed ionomer is closed to the membrane, hence more ionomer can be used to extend the TPB without heavily affecting the mass transfer. Fig. 7 shows the CV curves and ECSA values of the cathodes with different ionomer contents. The cathode with 33 wt% ionomer exhibits the largest ECSA value of 32.95 m2 g1, and with further increasing the ionomer content, the ECSA value is significantly reduced. This is probably because the excessive ionomer has covered some of the catalyst active sites.
Fig. 4 e Surface SEM images of the cathode catalyst layers with different Pt loadings of (a) 0.10, (b) 0.20, and (c) 0.50 mg cm¡2.
Comparison of Pt-NW electrode with the commercial GDE Fig. 8 shows the polarization curves of the MEAs with the optimal Pt-NW cathode and the commercial GDE. The Pt loadings of the Pt-NW cathode and the commercial GDE are 0.30 and 0.40 mg cm2, respectively. The maximum power density of the MEA with the Pt-NW cathode is 0.93 W cm2,
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Fig. 5 e Polarization curves of different ionomer contents (wt% based on the Pt loading of 0.30 mg cm¡2) spayed onto the Pt-NWs.
Fig. 6 e Current densities comparison of different ionomer contents (wt%) at 0.80, 0.60 and 0.40 V.
Fig. 7 e CV of the cathodes with different ionomer contents (wt%) spayed onto the Pt-NWs.
Fig. 8 e Performance comparison of the MEAs with the optimal Pt-NW cathode and the commercial GDE.
which is 6.90% higher than that of the commercial GDE. Their power performances are nearly the same at the voltage above 0.60 V. But below 0.60 V, the Pt-NW electrode has an obviously better performance. To further investigate the distinctions between the two MEAs at the high current density, EIS were recorded at 0.40 V as shown in Fig. 9. At 0.40 V, the EIS is mainly dominated by the mass transfer resistance. It can be seen that, although the ohmic resistance of the Pt-NW electrode is a little bigger than that of the commercial GDE, the mass transfer resistance is much smaller, and thus less concentration polarization loss and better performance are achieved. To understand more about the lower mass transfer resistance, the SEM-EDS results and cross-sectional TEM images are shown in Figs. 10 and 11, respectively. According to Fig. 10, the Pt-NWs are gradient distributed across the cathode thickness, and less Pt exists near the GDL. The cathode structure and Pt nanowire morphology are displayed in Fig. 11(a) and (b), respectively. It is found that the Pt-NWs assemble into large superstructures with a size of 50e100 nm. As reported in our previous work [15], the Pt-NWs are about 10e30 nm in length and 2e4 nm in diameter with single-crystal structures growing along the (111) facets. The
Fig. 9 e EIS at 0.40 V for the MEAs with the optimal Pt-NW cathode and the commercial GDE.
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improvement of the mass transfer and Pt utilization can be attributed to several factors: 1) the gradient distribution improves Pt utilization; 2) dense Pt-NWs near membrane provide abundant reaction sites; 3) affluent pores near GDL facilitate the reactant gas diffusion and water draining; 4) outstretched Pt-NWs on the carbon supports are easier for oxygen accessing [11].
Conclusions
Fig. 10 e SEM-EDS analysis of the Pt-NW cathode.
In this work, Pt-NW cathodes were prepared by in-situ growing Pt-NWs in the carbon matrices coated on the PTFE substrates, and the MEAs were fabricated by decal transfer method. The gradient distribution of Pt-NWs facilitates the mass transfer in the electrodes and thus improves the Pt utilization. The optimal Pt loading for Pt-NW electrode is 0.30 mg cm2, and a higher Pt loading results in serious aggregation and blocking pores. The ionomer sprayed onto the Pt-NW electrode locates near the membrane and facilitates the proton conduction. The optimal ionomer content is 33 wt %, and excessive amount would cover part of the catalyst active sites and hinder the mass transfer.
Acknowledgements 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 and TEM characterization.
references
Fig. 11 e Cross-sectional TEM images at (a) the lowmagnification and (b) high-magnification of the Pt-NW cathode.
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