Carbon matrix effects on the micro-structure and performance of Pt nanowire cathode prepared by decal transfer method

Journal of Energy Chemistry 24(2015)213–218

Carbon matrix effects on the micro-structure and performance of Pt nanowire cathode prepared by decal transfer method Zhaoxu Wei,

An He,

Kaihua Su,

Sheng Sui∗

Institute of Fuel Cell, Shanghai Jiao Tong University, Shanghai 200240, China [ Manuscript received September 29, 2014; revised November 15, 2014 ]

Abstract High performance cathode for polymer electrolyte membrane fuel cell was prepared by depositing Pt nanowires in a carbon matrix coated on a substrate, and using decal transfer method to fabricate the membrane electrode assembly. The effects of carbon and ionomer contents on the electrode micro-structure and fuel cell performance are investigated by physical characterization and single cell testing. The Pt nanowires are gradient distributed across the cathode thickness, and more Pt exists near the membrane. Both the carbon and ionomer contents can affect the Pt nanowires distribution and aggregation. In addition, the carbon loading dominates the transport distance of gas and proton, and the ionomer content affects the triple phase boundaries and porosity in the cathode. The optimal structure of Pt nanowire cathode is obtained at 0.10 mg·cm−2 carbon loading and 10 wt% ionomer. Key words Pt nanowire; carbon matrix; ionomer; decal transfer method; polymer electrolyte membrane fuel cell

1. Introduction The polymer electrolyte membrane fuel cell (PEMFC) is a promising power generation device because of its high efficiency and zero-emission. However, its high catalyst cost still hampers its commercialization [1]. Many research efforts have been taken to solve this issue, such as developing high active Pt-based catalysts [2−4], and optimizing the electrode structure [5,6]. Recently, Pt nanowire (Pt-NW) catalyst has attracted much attention in PEMFC application because of its inherent characters, including high resistance to Ostwald ripening and preferential exposure of highly active crystal facets [7,8]. Sun et al. [9] prepared Pt-NW/C catalyst at room temperature using the formic acid (HCOOH) to reduce the hexachloroplatinic acid (H2 PtCl6 ), and the Pt-NW/C showed better catalytic activity for oxygen reduction reaction as well as higher stability than normal Pt nanoparticle/C catalyst. With the same method, the star-like [10] and the flower-like [11] Pt-NWs were fabricated. Du et al. [12] has in-situ grown Pt-NWs directly onto gas diffusion layer (GDL) to make gas diffusion electrode (GDE) and also got better performance than the commercial GDE. Weissmann et al. [13] used the similar method to grow Pt-NWs onto different polymer elec∗

trolyte membranes. Besides the catalyst, the electrode structure also has significant effect on performance. The thickness, porosity and ionomer content are the main structural parameters. The thickness determines the gas and proton transport distance. The ionomer can extend the triple phase boundary (TPB), bind the catalyst layer and retain moisture in electrode [14,15]. But excessive ionomer would decrease the porosity, resulting in increased mass transfer resistance [16]. Suzuki et al. [17] studied catalyst layers (CLs) with various ionomer to carbon weight ratios, and found that the thickness and porosity of the CLs were chiefly determined by carbon and ionomer content, respectively. Soboleva et al. [18] evaluated the volumes of primary pores and secondary pores in CLs with various ionomer contents by N2 physisorption measurements, and found that the primary pores volume decreased with increasing ionomer content. In our previous work [19−21], we developed a novel method to make membrane electrode assembly (MEA) by insitu growing Pt nanowires in a carbon matrix coated on a polymer electrolyte membrane. The Pt-NWs have a gradient distribution, in which more Pt exists near the GDL. It has been demonstrated that the performance of the homemade MEA is better than the commercial GDE. However, Antoine et al. [22]

Corresponding author. Tel/Fax: +86-21-34206249; E-mail: [email protected]

Copyright©2015, Science Press and Dalian Institute of Chemical Physics. All rights reserved. doi: 10.1016/S2095-4956(15)60303-5

214

Zhaoxu Wei et al./ Journal of Energy Chemistry Vol. 24 No. 2 2015

reported that more Pt locating near the membrane rather than the GDL can reduce the proton migration resistance and improve the cell performance. Therefore, it is very attractive to change the Pt-NWs gradient which is reverse to our previous works to further improve the MEA performance. In this study, we in-situ grew Pt-NWs in a carbon matrix coated on a decal substrate, and used the decal transfer method to prepare the MEA. With this method, more Pt-NWs exist near the membrane. The effects of carbon loading and ionomer content on the MEA performance are investigated, and the influence mechanisms are detailed discussed. 2. Experimental 2.1. Preparation of catalyst layer and MEA The cathode catalyst layer was prepared by in situ growing Pt nanowires in a carbon matrix coated on a decal substrate. First, carbon matrix ink was prepared by mixing carbon powder (Vulcan XC-72R, Shanghai Cabot Chemical Co., Ltd), Nafionr iomomer solution (DE1020, 10 wt%, Dupont) and isopropyl alcohol (Sinopharm Chemical Reagent Co., Ltd), and then the ink was sonicated for 5 min. The carbon matrix ink was sprayed onto a polytetrafluoroethylene (PTFE) substrate (ultra-premium grade, CS Hyde Company) through a spraying gun under an infrared light, then drying the coated PTFE substrate under the infrared light for 30 min. After that, the coated PTFE substrate was rinsed by mixed solution of alcohol and deionized water for 3 times, and immersed in an aqueous solution of hexachloroplatinic acid (H2 PtCl6 ) (99.95%, Sinopharm Chemical Reagent Co., Ltd) and formic acid (HCOOH) (88%, Sinopharm Chemical Reagent Co., Ltd) in a glass Petri dish. The sample was kept at room temperature for 72 h. Generally, for a 10 cm2 PTFE substrate, to deposit 0.30 mg·cm−2 Pt-NWs would require adding 8.77 mg H2 PtCl6 ·6H2 O, 200 µL formic acid and 20 mL deionized water. Finally, the sample was rinsed by deionized water for six times, and dried under the infrared light for 2 h. In order to extend the triple phase boundary in electrode, a diluted ionomer was sprayed onto the catalyst layer surface at an amount of 0.10 mg·cm−2. The anode catalyst layer was prepared with commercial Pt/C catalyst. The catalyst ink was prepared by mixing 40 wt% Pt/C (HiSPECTM 4000, Johnson Matthey), Nafionr ionomer solution and isopropyl alcohol, and then the ink was sprayed onto a PTFE substrate. The Pt and ionomer loading in the anode are fixed at 0.30 mg·cm−2 and 0.10 mg·cm−2 , respectively. A pair of catalyst layers on the PTFE substrates were hot pressed onto a Nafionr membrane (NR212, Dupond) at 145 ◦ C for 3 min under 0.4 MPa. At last, the PTFE substrates were peeled off and a MEA was gotten. In this study, the deposited Pt-NW loading was fixed at 0.30 mg·cm−2 . The carbon loading in the matrix was varied from 0.05 to 0.30 mg·cm−2 with the ionomer content fixed at 10 wt% based on the carbon loading. In ionomer content investigation, the

ionomer content was varied to be 5, 10, 15 and 20 wt% with the carbon loading fixed at 0.10 mg·cm−2. 2.2. Morphological characterization The morphologies of the catalyst layer surfaces were observed by a scanning electron microscopy (SEM) (S-4800, Hitachi) operating at an accelerating voltage of 10 kV. The cathode cross-sectional samples were cut from the MEAs, embedded into epoxy resins, and then sliced by the cryosection system to obtain smooth cross-sections. The cross-sectional morphologies were observed with a biology transmission electron microscope (TEM) (Tecnai G2 Spirit Biotwin, FEI) operating at an accelerating voltage of 120 kV. Pt distribution across the catalyst layer thickness was analyzed by energydispersive X-ray spectroscopy. Pt loadings of the catalyst layers were confirmed by inductively coupled plasma-atomic emission spectrometer (ICP-AES) (7500a, Agilent). 2.3. Fuel cell tests All cells were tested on the 850e Multi-Range Fuel Cell Test System (Scribner Associates Inc.). To activate the cells, the following procedure has been repeated for two times: 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 voltage (OCV) @20 min, subsequently the following procedure has been repeated for eight times: 0.20 V@10 min, OCV@30 s. Pure hydrogen and air were humidified at 70 ◦ C, with a fixed flow of 200 mL·min−1 and 400 mL·min−1, and the cell temperature was 75 ◦ C. The polarization curves were tested at the cell temperature of 70 ◦ C. Pure hydrogen (99.999%) and bottle air (99.999%) were humidified at 65 ◦ C, and were used as the anode fuel and cathode oxidant, respectively. The initial flow rates of H2 and air were 150 mL·min−1 and 300 mL·min−1, and the stoichiometric ratios of hydrogen and air were 1.5 and 2.0, respectively. The backpressures of both sides were 1.0 bar. The measurement was conducted by voltage sweeping from OCV to 0.30 V at a rate of 2 mV·s−1 . The electrochemical impedance spectroscopy (EIS) was carried out by 885 Fuel Cell Potentiostat from Scribner Associates Inc. The test condition is the same as the polarization test. The measurement was conducted in a frequency range of 10 kHz to 0.1 Hz with the AC amplitude of 10% DC current at 0.8 V and 0.4 V. The cyclic voltammogram was recorded by SI1287 (Solartron Analytical Inc.) and the sweeping voltage was between 0.05 and 1.00 V versus reversible hydrogen electrode (RHE) with a rate of 25 mV·s −1 at 35 ◦ C. N2 and H2 flowed to the cathode and anode at a rate of 75 mL·min−1 and 300 mL·min−1, respectively. The electrochemical active surface area (ECSA) of Pt was calculated from the charge density for hydrogen absorption under nitrogen atmosphere, assuming that 210 µC·cm−2 was needed to form a monolayer of absorbed H on polycrystalline Pt surface.

Journal of Energy Chemistry Vol. 24 No. 2 2015

3. Results and discussion 3.1. The ef fect of carbon loading in the matrix In order to figure out how the carbon loading affects the physical characteristics of the cathodes, the cathode crosssectional samples were observed by TEM. The holistic cathode morphology and Pt nanowire dispersion near the membrane are exhibited in Figure 1 and Figure 2, respectively.

215

the thickness increases, the path for mass transport and proton conduction is longer, which would increase the mass and charge resistance consequently. As shown in Figure 2(a), at the carbon loading of 0.05 mg·cm−2, where there is only 1.8 µm in thickness for the Pt nanowires to grow, the Pt nanowires tend to aggregate severely, especially near the membrane, leading to blocked micro-pores in catalyst layer. As the carbon layer becomes thicker, the aggregation tendency reduces due to the increased sites for Pt nanowires growing. Therefore, the optimal thickness should balance the transport distance and the catalyst aggregation. The optimal thickness is demonstrated by the electrochemical testing in the following.

Figure 1. TEM images of the holistic cathode morphologies: (a) 0.05 mg·cm−2 , (b) 0.10 mg·cm−2 and (c) 0.30 mg·cm−2 carbon loading

Figure 1 has illustrated the thicknesses and Pt nanowire distributions in the cathodes with different carbon loadings. The Pt nanowires exhibit a gradient distribution across the catalyst layer thickness, i.e. more Pt exists near the membrane, as shown in the inset of Figure1 (a) (the EDS analysis of Pt). According to our previous study [19], the Pt nanowire is 3−4 nm in diameter and 10−20 nm in length. The thicknesses of the catalyst layers with 0.05, 0.10 and 0.30 mg·cm−2 carbon loading are about 1.8, 3.0 and 8.0 µm, respectively, suggesting that the thickness is proportional to the carbon loading. As

Figure 2. TEM images of Pt nanowire dispersion near the membranes: (a) 0.05 mg·cm−2 , (b) 0.10 mg·cm−2 and (c) 0.30 mg·cm−2 carbon loading

Figure 3 shows the polarization curves for cathodes with 0.05, 0.10, 0.20 and 0.30 mg·cm−2 carbon loading. When the carbon loading is less than 0.10 mg·cm−2 , the cell performance increases gradually with the carbon loading; reversely, the MEA performance decreases considerably. The maximum power density, 0.93 W·cm−2 , was achieved at 0.10 mg·cm−2. This result is further explained by EIS and CV measurements.

216

Zhaoxu Wei et al./ Journal of Energy Chemistry Vol. 24 No. 2 2015

Figure 3. Polarization curves of the cathodes with various carbon loadings

the catalyst layer surface (contacting with the membrane after the hot-pressing transfer) in the final step of preparing the PtNW cathode. The sprayed ionomer has definite permeation depth in the electrode, and only in the impregnated region, the Pt-NWs can be used effectively. Therefore thin catalyst layer is benefit for the Pt utilization and reducing charge transfer resistance. However, from the perspective of Pt aggregation, too thin carbon matrix will lead to decreased Pt utilization and increased charge transfer resistance. It is founded that the intercept impedances on the Z’real axis, which represent the cell ohmic resistances, are different from each other. In PEMFC, the proton resistance of the polymer electrolyte membrane contributes the most to the total ohmic resistance. In this work, the same type of Nafionr membrane was used, so the ohmic resistance difference may come from the decal transfer process and cell assembling. The same thing happens again in the following section (Figure 8). At 0.4 V there are two semi-circles in the Nyquist plot, where the left arc (high frequency) is affected by the charge transfer impedance and double layer capacitance, and the right arc (low frequency) is dominated by the mass transfer impedance. The cathode with 0.10 mg·cm−2 also shows the smallest mass transfer impedance. Higher carbon loading will increase the catalyst thickness, and the mass transfer is worsened by the long distance. When the carbon matrix is too thin, more Pt nanowires will aggregate near the membrane, as shown in Figure 2. The severely aggregated Pt and the spayed ionomer would form a dense region near the membrane, which may deteriorate the mass transfer property. The ECSA values were calculated from the cyclic voltammograms and used to study the effect of carbon loading on the catalyst utilization. As shown in Figure 5, the maximum ECSA value is 33 m2 ·g−1 achieved at 0.10 mg·cm−2, indicating that at this carbon loading the cathode has the largest catalyst utilization. These results are in well agreement with the polarization curves shown in Figure 3, in which the cathode with 0.10 mg·cm−2 shows the best performance.

Figure 4. EIS of the cathodes with various carbon loadings at (a) 0.8 V and (b) 0.4 V

Electrochemical Impedance Spectroscopy at the voltage of 0.8 V and 0.4 V for the cathodes with carbon loading of 0.05, 0.10 and 0.30 mg·cm−2 is shown in Figure 4. In the EIS pictures, at the voltage of 0.8 V, due to the small current density, the arc diameter represents the charge transfer resistance. As shown in Figure 4(a), the cathode with 0.10 mg·cm−2 shows the smallest charge transfer resistance. It is noted that in present work, a diluted ionomer solution was sprayed onto

Figure 5. ECSA results of the cathodes with various carbon loadings

Journal of Energy Chemistry Vol. 24 No. 2 2015

3.2. The ef fect of ionomer content in the matrix To understand the influence of ionomer content on the micro-structure of Pt-NW cathode, the SEM analysis was conducted to the upside surface of the catalyst layer coated on the PTFE substrate, i.e. the side contacting with the membrane after hot-pressing transfer, as shown in Figure 6. When the ionomer content is low and the micro-pores are less blocked, the H2 PtCl6 is able to penetrate deeply into the carbon matrix. Therefore, the cathode with 5 wt% ionomer has less Pt near the catalyst layer surface than that of the cathode with 10 wt% and 20 wt% ionomer, as shown in Figure 6. And because Pt nanowires tend to grow on the rough carbon surface without covered by ionomer, the cathode with 5 wt% ionomer has more Pt-NW growth sites and less catalyst aggregation compared with the others. However, with insufficient ionomer content, the catalyst layer would suffer from less triple phase boundary and lower ability to retain moisture.

217

penetrating deeply into the carbon matrix, leading to Pt nanowires aggregation near the catalyst layer surface, which may block the gas transport. And some of the electronic and gas paths would also be interdicted by the excessive ionomer in the catalyst layer [16]. The optimal ionomer content should consider all the factors discussed above, and the electrochemical characterizations are used to find the optimal content as illustrated in the following. Figure 7 depicts polarization curves of cathodes with ionomer contents of 5, 10, 15 and 20 wt% in the matrices. When the ionomer content is blow 10 wt%, the performance improves with increasing ionomer content; reversely, the performance degrades considerably. The cathode with 10 wt% ionomer has the maximum power density of 0.93 W·cm−2 .

Figure 7. Polarization curves of the cathodes with various ionomer contents

Figure 6. SEM images of the catalyst layer surfaces with various ionomer contents: (a) 5 wt%, (b) 10 wt% and (c) 20 wt%

When the ionomer content is excessive, such as 20 wt%, a bulk of ionomer in the carbon matrix hampers the H2 PtCl6

EIS results at 0.8 V and 0.4 V of cathodes with ionomer contents of 5, 10, 20 wt% have been presented in Figure 8. As mentioned above, at 0.8 V the arc diameter represents the charge transfer resistance, and the sample with 10 wt% ionomer has the smallest one. When the ionomer content is less than 10 wt%, the activation polarization is deteriorated by the poor proton conduction, resulting in increased charge transfer resistance. When the ionomer content is beyond 10 wt%, the charge transfer resistance is also larger, because the excessive ionomer causes serious Pt aggregation (as shown in Figure 6c), which decreases the catalyst electrochemical specific surface as discussed in the following ECSA analysis. At the voltage of 0.4 V, as shown in Figure 8(b), the order of the polarization impedance is 10 wt%<5 wt%<20 wt% ionomer. The largest impedance can be observed at 20 wt% ionomer which is about twice over the smallest one at 10 wt%, as excessive ionomer would deteriorate the mass transfer property [23]. Less ionomer amount leads to small TPB, which can enlarge the polarization impedance as well. To further investigate the ionomer effect, the ECSA is measured and the result is exhibited in Figure 9. The largest ECSA value is obtained at 10 wt% ionomer content, reaching 33 m2 ·g−1 . The cathode with 20 wt% ionomer content shows the smallest ECSA value of 18 m2 ·g−1 , which confirms the

218

Zhaoxu Wei et al./ Journal of Energy Chemistry Vol. 24 No. 2 2015

severe catalyst aggregation at excessive ionomer content. The results have a good agreement with the EIS analysis above.

MEA. The carbon matrix effects on the micro-structure and performance of the Pt-NW cathode are investigated. TEM analysis proves that more Pt-NWs exist near the membrane instead of the GDL. Low carbon loading is benefit for the Pt utilization and reducing mass transfer resistance. However, from the perspective of Pt aggregation, too low carbon loading will lead to decreased Pt utilization and increased charge transfer resistance. As for ionomer in the matrix, insufficient ionomer leads to small TPB and less Pt-NWs concentrating near the membrane, which enlarge the polarization impedance; excessive ionomer can block the gas diffusion and water draining, and also causes Pt nanowire aggregation which further deteriorates the mass transfer property. The cathode with 0.10 mg·cm−2 carbon loading and 10 wt% ionomer obtains the optimal performance. References

Figure 8. EIS of the cathodes with various ionomer contents at (a) 0.8 V and (b) 0.4 V

Figure 9. ECSA results of the cathodes with various ionomer contents

4. Conclusions In this work, we prepared a Pt gradient electrode by insitu grown Pt nanowires in a carbon matrix coated on a substrate, and used the decal transfer method to fabricate the

[1] Yu X W, Ye S Y. J Power Source, 2007, 172: 133 [2] Woo S, Kim I, Lee J K, Bong S, Lee J, Kim H. Electrochim Acta, 2011, 56: 3036 [3] Stamenkovic V R, Mun B S, Arenz M, Mayrhofer K J J, Lucas C A, Wang G F, Ross P N, Markovic N M. Nat Mater, 2007, 6: 241 [4] Lim B, Jiang M J, Camargo P H C, Cho E C, Tao J, Lu X M, Zhu Y M, Xia Y N. Science, 2009, 324: 1302 [5] Qiu Y L, Zhang H M, Zhong H X, Zhang F X. Int J Hydrogen Energy, 2013, 38: 5836 [6] Su H N, Liao S J, Wu Y N. J Power Sources, 2010, 195: 3477 [7] Yan Z Y, Li B, Yang D J, Ma J X. Chin J Catal (Cuihua Xuebao), 2013, 34: 1471 [8] Liang H W, Cao X A, Zhou F, Cui C H, Zhang W J, Yu S H. Adv Mater, 2011, 23: 1467 [9] Sun S H, Jaouen F, Dodelet J P. Adv Mater, 2008, 20: 3900 [10] Wang L, Guo S J, Zhai J F, Dong S J. J Phys Chem C, 2008, 112: 13372 [11] Sun S H, Yang D Q, Villers D, Zhang G X, Sacher E, Dodelet J P. Adv Mater, 2008, 20: 571 [12] Du S F. J Power Sources, 2010, 195: 289 [13] Weissmann M, Coutanceau C, Brault P, Leger J M. Electrochem Commun, 2007, 9: 1097 [14] Passalacqua E, Lufrano F, Squadrito G, Patti A, Giorgi L. Electrochim Acta, 2001, 46: 799 [15] Song J M, Cha S Y, Lee W M. J Power Sources, 2001, 94: 78 [16] Paganin V A, Ticianelli E A, Gonzalez E R. J Appl Electrochem, 1996, 26: 297 [17] Suzuki T, Tsushima S, Hirai S. Int J Hydrogen Energy, 2011, 36: 12361 [18] Soboleva T, Malek K, Xie Z, Navessin T, Holdcroft S. ACS Appl Mater Interfaces, 2011, 3: 1827 [19] Yao X Y, Su K H, Sui S, Mao L W, He A, Zhang J L, Du S F. Int J Hydrogen Energy, 2013, 38: 12374 [20] Su K H, Sui S, Yao X Y, Wei Z X, Zhang J L, Du S F. Int J Hydrogen Energy, 2014, 39: 3397 [21] Su K H, Sui S, Yao X Y, Wei Z X, Zhang J L, Du S F. Int J Hydrogen Energy, 2014, 39: 3219 [22] Antoine O, Bultel Y, Ozil P, Durand R. Electrochim Acta, 2000, 45: 4493 [23] Ahn C Y, Cheon J Y, Joo S H, Kim J. J Power Sources, 2013, 222: 477