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Performance enhancement of high-temperature polymer electrolyte membrane fuel cells using Pt pulse electrodeposition
Journal of Power Sources 438 (2019) 227022
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Performance enhancement of high-temperature polymer electrolyte membrane fuel cells using Pt pulse electrodeposition Dong-Kwon Kim a, Hoyoung Kim a, Hyanjoo Park a, SeonHwa Oh a, Sang Hyun Ahn b, *, Hyoung-Juhn Kim c, Soo-Kil Kim a, ** a b c
School of Integrative Engineering, Chung-Ang University, 84 Heukseok-ro, Dongjak-gu, Seoul, 06974, Republic of Korea School of Chemical Engineering and Material Science, Chung-Ang University, 84 Heukseok-ro, Dongjak-gu, Seoul, 06974, Republic of Korea Fuel Cell Research Center, Korea Institute of Science and Technology, 5 Hwarang-ro 14-gil, Seongbuk-gu, Seoul, 02792, Republic of Korea
H I G H L I G H T S
G R A P H I C A L A B S T R A C T
� Pulse deposition modifies the wetting property of a commercial Pt-based electrode. � Maximum power density shows a vol cano relationship with deposition pulse number. � Hydrophilicity control on the anode significantly affects HT-PEMFC performance. � A specific maximum power of 437.2 mW mgPt1 is achieved under optimized conditions.
A R T I C L E I N F O
A B S T R A C T
Keywords: High-temperature PEM fuel cell Phosphoric acid distribution Pt pulse electrodeposition Hydrophilic surface
The development of high-performance polymer electrolyte membrane fuel cells that operate at elevated tem peratures is urgently required to overcome the technical problems associated with operation at low tempera tures. Here, we report an effective way to enhance electrode performance via simple Pt pulse electrodeposition at room temperature. This electrodeposition process enables both the formation of additional Pt electrocatalysts with extremely low loading (i.e., ~0.05 mg cm 2 for 100 pulse cycles) as a function of pulse number and control of the wetting properties of commercial Pt-based electrodes. Following optimization of the electrode conditions and configurations, the controlled hydrophilicity of the anode enhances the phosphoric acid distribution and the formation of a triple phase boundary in the catalyst layer, resulting in lowered ohmic and charge transfer re sistances, respectively. The mass activities of the membrane electrode assembly with the anode modified by Pt pulse electrodeposition is 437.2 mW mgPt1 (H2/O2), which is approximately 1.36 times higher than that of the pristine membrane electrode assembly. The controlled hydrophilicity allows moderate improvement of the performance, even without additional Pt. The results presented herein demonstrate the importance of surface property control for electrode preparation to achieve enhanced performance of high-performance polymer electrolyte membrane fuel cells.
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (S.H. Ahn), [email protected] (S.-K. Kim). https://doi.org/10.1016/j.jpowsour.2019.227022 Received 24 November 2018; Received in revised form 15 May 2019; Accepted 14 August 2019 Available online 17 August 2019 0378-7753/© 2019 Elsevier B.V. All rights reserved.
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Journal of Power Sources 438 (2019) 227022
1. Introduction
unfavorable for the kinetics of the oxygen reduction reaction (ORR) at the cathode [12–14] and CO poisoning resistance during the hydrogen oxidation reaction (HOR) at the anode [15–18]. In addition, water management to minimize cathodic flooding remains challenging [17–20]. When the operating temperature is increased above 120 � C, most issues can be overcome by using a polybenzimidazole (PBI) membrane doped with phosphoric acid (PA) [21,22]. Obvious increases of the ORR kinetics and CO tolerance occur on Pt electrocatalysts at elevated temperatures [23,24]. Further, the system design can be simplified to exclude water management [23] and the waste heat can be
Tremendous efforts have been focused on the development of poly mer electrolyte membrane fuel cells (PEMFCs) for portable [1,2], vehicle [3,4], and stationary [5,6] applications. As a result, recent PEMFC technologies based on the Nafion membrane have been realized that meet acceptable energy efficiencies and commercialization costs [7, 8]. However, to maintain a reasonable proton conductivity, the mem brane must be hydrated using a humidifier, typically limiting the operating temperature range to 60–80 � C [9–11]. These conditions are
Fig. 1. FESEM images of prepared (a–e) anodes and (f–j) cathodes: (a) Pt/C-a, (b) Pt50/Pt/C-a, (c) Pt100/Pt/C-a, (d) Pt300/Pt/C-a, (e) Pt500/Pt/C-a, (f) PtNi/C-c, (g) Pt50/PtNi/C-c, (h) Pt100/PtNi/C-c, (i) Pt300/PtNi/C-c, and (j) Pt500/PtNi/C-c. 2
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Journal of Power Sources 438 (2019) 227022
Fig. 2. XRD patterns of prepared (a) anodes and (b) cathodes. Dependence of contact angle on the number of deposition pulses for (c) anodes and (d) cathodes. Insets: contact angle images.
Pt loading mass was fixed at 1.0 mg cm 2, whereas the PTFE content was varied from 2% to 40%. Following PA uptake from a PA solution, the CL with 5% PTFE exhibited the highest performance. A similar experiment performed by Jeong et al. showed that the formation of a triple phase boundary (TPB) is accelerated at the optimized PTFE content of 20 wt% [39]. In addition, it has been reported that a crackless microporous layer (MPL) physically prevents PA leakage, as confirmed by X-ray tomogra phy [41]. On the contrary, a nonuniform distribution of PA in the CL causes insufficient TPB formation. Despite these many efforts, it is still difficult to understand the interrelation among membrane conductivity, TPB formation, and Pt poisoning during PA bleed-out. Therefore, larger amounts of Pt electrocatalysts have been loaded in the CL to obtain HT-PEMFCs with reasonable performance, thus inhibiting commercial ization [42,43], although the optimization of Pt loading in the ano de/cathode and PA content has been investigated [44]. The electrodeposition method facilitates simple, fast, and one-pot fabrication of GDEs at room temperature [45]. However, owing to the insufficient formation of a TPB, its use in the preparation of electrodes has been limited and only a few studies [46–48] have been reported. By contrast, recent reports on self-terminating electrodeposition (SED) of platinum group metals (PGMs) have opened a new research area on wet forms of atomic layer deposition with rapid and inexpensive deposition processes [49]. Under a highly negative deposition overpotential, where H passivation is enabled, SED facilitates precise control at extremely low
easily utilized [23,25]. Unfortunately, despite these advantages, increasing the operating temperature leads to new issues [26]. A major problem encountered in high-temperature PEMFCs (HT-PEMFCs) is bleeding out of PA from the PBI membrane through the gas diffusion electrode (GDE) [27,28]. The leached phosphate species induce Pt poisoning, which degrades the HT-PEMFC performance [29,30]. Spectroscopic analysis of half-cell re action has demonstrated that the phosphate species are present as H3PO4, H2PO4 , HPO24 , and PO34 in aqueous PA solution [31–33]. Among these species, the H2PO4 ion adsorbs strongly on the surface of Pt electrocatalysts, reducing their electrochemical active sites [34]. In response to this issue, Pt–M alloy electrocatalysts (M: Ni [35], Co [36], Cu [34], and Au [33]) have been developed to increase the tolerance toward phosphate species and obtain a higher Pt mass activity relative to �n et al. have reported the performance of an that of pure Pt. Milla HT-PEMFC single cell with a PtCo cathode [36]. Owing to the higher alloying degree of this electrocatalyst, the performance of HT-PEMFC was maintained for 150 h. Another problem associated with PA leakage is decreased proton conductivity owing to the lowered PA content in the membrane [37,38]. Hydrophilicity control on the catalyst layer (CL) neighboring the membrane is known as a promising way to prevent PA leakage [39,40]. Mack et al. have demonstrated the effect of the polytetrafluoroethylene (PTFE) content in the CL on HT-PEMFC performance [40]. In the CL, the 3
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Journal of Power Sources 438 (2019) 227022
Fig. 3. Dependence of HT-PEMFC performance on the number of deposition pulses for the anode. The cathode was fixed as pristine PtNi/C-c. (a) i–V curves and (b) maximum power densities under H2/O2 streams. (c) i–V curves and (d) maximum power densities under H2/air streams.
PGM loading masses (~ng cm 2) that exhibit high catalytic activity for fuel cell half reactions [50,51] and water electrolysis [51,52]. The SED process has been expanded to fabricate Pt GDEs with advantageous structures that provide good mass-transfer pathways for the reactant/ product in the high current density region during PEM water electrolysis operation [53]. However, as far as we know, this promising method has not been applied to the fabrication of electrodes for PEMFCs. The results presented herein demonstrate that the performance of HT-PEMFCs is enhanced by employing commercial electrodes modified by simple Pt SED. The controllable formation of extremely low amounts of Pt on the CL surface in both the anode and cathode was used to un derstand the changes in hydrophilicity and TPB formation in the pres ence of PA, as well as the performance of HT-PEMFCs under both H2/air and H2/O2 streams. The differing effects of Pt SED on the anode and cathode were also examined by analyzing the physical and electro chemical properties of the modified electrodes.
(Daejung, 7548–4100) [49]. A saturated calomel electrode (SCE) and a Pt wire were adopted as the reference and counter electrodes, respec tively. The deposition pulse consisted of the following sequence: 0.4 VSCE for 2 s, 0.9 VSCE for 10 s, and 0.4 VSCE for 3 s [53]. The deposition pulse was repeated 50, 100, 300, or 500 times to control the Pt loading amount. All deposition processes were performed at room temperature and under ambient pressure. 2.2. Characterization Before and after Pt pulse electrodeposition, the surface morphologies of the commercial electrodes were imaged using field emission scanning electrode microscopy (FESEM, SIGMA, Carl Zeiss) and their composi tions were analyzed using energy-dispersive spectroscopy (EDS, Thermo Scientific, NORAN System 7). The crystal structures were analyzed using X-ray diffraction (XRD, New D8-Advance, Bruker AXS) at a rate of 5� min 1 from 30� to 90� . The hydrophilicities were examined using a deionized water droplet (3 μL) and a contact angle analyzer (FM40Mk2 EasyDrop, KRÜSS GmbH). The Pt loading mass was measured using inductively coupled plasma mass spectroscopy (ICPMS, Nexion 300, PerkinElmer).
2. Experimental 2.1. Pt pulse electrodeposition Pt pulse electrodeposition was conducted in a conventional threeelectrode cell connected to a potentiostat (CS310, Wuhan CorrTest In struments Corp., Ltd.). Commercial BASF Celtec® electrodes (Pt/C for anode and Pt alloy/C for cathode) were used to prepare working elec trodes with exposed areas of 1.53 cm2 using a N2-purged deposition electrolyte containing 3 mM K2PtCl4 (Alfa Aesar, 43946) and 0.5 M NaCl
2.3. Fabrication of HT-PEMFC single cells As a pretreatment, a commercial PA-doped PBI membrane (BASF Celtec® membrane, thickness of 400 μm, acid concentration 55 wt%) was dried at 120 � C for 1 h in a dryer. Then, the membrane was 4
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Journal of Power Sources 438 (2019) 227022
Fig. 4. Dependence of HT-PEMFC performance on the number of deposition pulses for the cathode. The anode was fixed as pristine Pt/C-a. (a) i–V curves and (b) maximum power densities under H2/O2 streams. (c) i–V curves and (d) maximum power densities under H2/air streams.
sandwiched between the anode and the cathode to fabricate a mem brane electrode assembly (MEA) with an active area of 1.0 cm2. To avoid physical leakage of PA from the membrane, the MEA was fabricated without hot-pressing [44,54,55]. The serpentine-type flow channel on the bipolar plate had a channel width of 0.5 mm and a depth of 1.0 mm.
3. Results and discussion The performances of the HT-PEMFC single cells have been investi gated by using commercial electrodes modified by Pt SED as a function of the number of deposition pulses. The pulse profile and corresponding current vs. time plot are shown in Fig. S1. The Pt SED process facilitates the formation of additional Pt and the loading is controllable within extremely small amounts [49,53]. The modified electrodes have been named as Pt#/Pt/C-a for anodes and Pt#/PtNi/C-c for cathodes, where # is the number of Pt deposition pulses. Fig. 1 shows FESEM images of the electrodes before and after Pt SED. For the pristine anode (BASF Celtec® anode), the typical morphology of a Pt/C CL is observed (Fig. 1a) [39,56]. After 50 deposition pulses (Fig. 1b), the density of Pt nanoparticles obviously increases. The size of the Pt nanoparticles is approximately 12 nm, but some deviation exists because the Pt nucle ation process during Pt SED might differ on the surfaces of existing Pt nanoparticles and bare C spheres. Further increasing the pulse number results in growth of the Pt nanoparticles: 23 � 3.1 nm for Pt100/Pt/C-a (Figs. 1c), 28 � 4.1 nm for Pt300/Pt/C-a (Figs. 1d), and 42 � 8.3 nm for Pt500/Pt/C-a (Fig. 1e). The pristine PtNi/C cathode (BASF Celtec® cathode) shows a similar morphology to the pristine Pt/C anode (Fig. 1f). Compared with the Pt#/Pt/C-a electrodes, much larger Pt nanoparticles form on the Pt#/PtNi/C-c electrodes (Fig. 1g–j), indi cating that Pt galvanic displacement with Ni on the surface of PtNi/C occurs in addition to Pt SED [52]. After 500 deposition pulses, most of the CL surface is covered by agglomerated Pt spheres. Fig. 2a shows XRD patterns of the pristine Pt/C anode and Pt#/Pt/C-
2.4. Performance evaluation of HT-PEMFC single cells To eliminate any moisture inside the cell, N2 was flowed into the HTPEMFC single cell at a temperature of 150 � C for 20 min. After drying, a H2 stream was supplied to the anode at a constant volumetric flow rate of 0.1 L min 1, whereas an O2 or air stream was supplied to the cathode at a constant volumetric flow rate of 0.3 L min 1. After 30 min, to sta bilize the single cell, the MEA was activated at 0.6 V for 3 h. Then, the current density was gradually increased with an interval of 20 mA cm 2 for 1 min. All performance evaluations on single cells were performed using a fuel cell test station (CNL Energy). 2.5. Characterization of HT-PEMFC single cells Electrochemical impedance spectroscopy (EIS) was conducted using a potentiostat (Autolab PGSTAT302 N, Metrohm) connected to the sin gle cell. At a constant cell voltage of 0.75 V, the frequency was changed from 10 kHz to 0.01 Hz at an amplitude of 5 mV under a H2 stream at the anode and an O2 (or air) stream at the cathode. 5
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Journal of Power Sources 438 (2019) 227022
Fig. 5. Dependence of HT-PEMFC performance on the number of deposition pulses for the cathode. The anode was fixed as Pt100/Pt/C-a. (a) i–V curves and (b) maximum power densities under H2/O2 streams. (c) i–V curves and (d) maximum power densities under H2/air streams.
a electrodes. Three peaks are observed for all samples, demonstrating the presence of Pt(111) at 39.763� , Pt(200) at 46.243� , and Pt(220) at 67.454� (JCPDS 04–0802). Based on the Scherrer equation, the average crystalline size is calculated as 9.49 nm for the pristine anode. On increasing the pulse number, the size gradually increases to 10.08 nm (Pt50/Pt/C-a), 10.21 nm (Pt100/Pt/C-a), 10.77 nm (Pt300/Pt/C-a), and 12.46 nm (Pt500/Pt/C-a). For the pristine PtNi/C cathode, the peak observed at 41.113� is located between the Pt(111) and Ni(111) peaks, indicating the presence of a PtNi alloy phase. As the pulse number in creases, the intensity of the Pt(111) peak increases, whereas the in tensity of the original peak at 39.763� is maintained, indicating that deposited Pt and the PtNi alloy exist separately after the Pt SED process. The Scherrer equation provides average crystalline sizes of 10.21 nm (pristine PtNi/C cathode), 10.38 nm (Pt50/PtNi/C-c), 10.48 nm (Pt100/ PtNi/C-c), 13.21 nm (Pt300/PtNi/C-c), and 15.46 nm (Pt500/PtNi/C-c), indicating a faster increase in size than in the case of the anodes. In addition, the hydrophilicities of the pristine and modified electrodes have been analyzed using contact angle measurements, as shown in Fig. 2c and d. The contact angle between deionized water and the electrode surface gradually decreases as the deposition pulse number increases, from 137.6� to 63.8� for the anodes (Fig. 2c) and from 141.5� to 45.2� for the cathodes (Fig. 2d). This finding indicates that the hy drophilicity of the electrode gradually increases after the Pt SED process. Fig. 3 demonstrates the polarization curves of HT-PEMFCs operated at 150 � C with anodes prepared by varying the number of deposition pulses. As a reference, the pristine anode and cathode have been used to fabricate a pristine MEA. In order to avoid mass-transfer limitation of
reactant, excessive O2 was injected into HT-PEMFCs. Under the H2/O2 streams, the HT-PEMFC with the pristine MEA exhibits a maximum power density of 532.5 mW cm 2 (Fig. 3a), which is similar to the values previously reported in the literature [57,58], confirming the reliability of the operating conditions. The maximum power density shows para bolic behavior as a function of the pulse number, with the highest value of 748.3 mW cm 2 occurring at 100 deposition pulses, which is 1.4 times larger than that in the pristine case (Fig. 3b). Under the H2/air streams, a maximum power density of 319.6 mW cm 2 is obtained with the pristine MEA (Fig. 3c). Similar parabolic behavior depending on the number of pulses is observed, with the highest value of 473.3 mW cm 2 obtained using the Pt100/Pt/C-a electrode, which is 1.5 times larger than that in the pristine case (Fig. 3d). Fig. 4 shows the polarization curves of HT-PEMFCs with MEAs consisting of Pt#/PtNi/C-c and pristine Pt/C-a. In contrast to the anode modification case, less significant performance enhancement is observed, with no strong correlation to the number of deposition pulses under both H2/O2 (Fig. 4a and b) and H2/air (Fig. 4c and d). Further optimization of the MEA configuration has been attempted by controlling the number of deposition pulses for the cath ode while using Pt100/Pt/C-a as the best performing anode. Fig. 5a demonstrates the polarization curves of such HT-PEMFCs under H2/O2 streams. The highest maximum power density is obtained without Pt SED on the cathode. The maximum power density decreases to 483 mW cm 2 after 100 deposition pulses and then increases again, as shown in Fig. 5b. Under H2/air streams, similar behavior is observed, as shown in Fig. 5c and d. In summary, for single cells with various MEA configurations, the Pt 6
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Journal of Power Sources 438 (2019) 227022
Fig. 6. Performances of HT-PEMFCs with optimal anode and cathode config urations. Specific i–V curves and powers under (a) H2/O2 and (b) H2/ air streams.
Fig. 7. Nyquist plots of HT-PEMFCs with optimal anode and cathode config urations at 0.75 V under (a) H2/O2 and (b) H2/air streams.
SED process is effective for anode modification with the maximum power density showing parabolic behavior with the number of deposi tion pulses. The best HT-PEMFC performance is obtained with 100 deposition pulses. By contrast, cathode modification by Pt SED has no significant effect, regardless of the anode material and number of pulses. Fig. 6 demonstrates the specific polarization curves of HT-PEMFCs with selected MEA configuration according to the Pt mass activity (using the total Pt mass from both electrodes). The pristine MEA shows a specific power of 321.4 mW mgPt1 under H2/O2 streams (Figs. 6a) and 192.9 mW mgPt1 under H2/air streams (Fig. 6b). With Pt100/Pt/C-a and the pristine cathode, the specific power is obviously enhanced to 437.2 mW mgPt1 (O2) and 276.6 mW mgPt1 (air). Other MEA configurations also show enhancement of the specific power relative to that of the pristine MEA. Such increases in the performance per mass unit indicate that the Pt SED process modifies the structure and surface properties of the electrode to change the behavior of PA at the electrode/membrane interface or in side the electrode. Considering that the Pt nanoparticles obtained by SED are larger than those in the pristine electrode, a great part of the mass activity enhancement originates from the optimization of the structure of existing (pristine) electrode through the SED process. To determine the PA behavior in the various MEA configurations, EIS measurements have been conducted at a constant cell voltage of 0.75 V under H2/O2 streams, as shown in Fig. 7a. For the pristine MEA, the ohmic resistance is measured as 0.896 Ω cm2. With Pt100/Pt/C-a and the pristine cathode, the ohmic resistance significantly decreases to 0.417 Ω cm2, whereas Pt SED on the cathode is ineffective for changing the ohmic resistance. A similar change in ohmic resistance is observed under the H2/air streams (Fig. 7b). Evidently, compared with the
Fig. 8. Dependence of the maximum power density on the contact angle for pristine and treated anodes. The cathode was fixed as pristine PtNi/C-c. Insets: contact angle images.
pristine MEA, Pt#/Pt/C-a demonstrates much lower slopes in the ohmic loss region of the polarization curves (Fig. 3), whereas Pt#/PtNi/C-c show more similar slopes (Fig. 4). These different changes in the ohmic resistances are directly related to changes in the PA behavior and 7
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the TPB interface inside the CL. According to Liu et al. [44], the per formance of HT-PEMFCs is significantly affected by interactions be tween the electrode and the membrane, as examined by controlling the PA content and CL thickness. For the anode in this study, the ohmic resistance can be decreased by increasing the PA content in the MEA. Therefore, after 100 deposition pulses on the anode, the gradually increased hydrophilicity of the anode facilitates the uniform distribution of PA as well as the formation of a TPB in the CL. However, the Pt poisoning phenomenon caused by overwetted PA surely became domi nant at excessive surface hydrophilicities with further pulses (Fig. 2c), resulting in the observed volcano behavior of the maximum power density as a function of the pulse number (Fig. 3b and d). It also should be noted that, likewise the change in the membrane conductivity for LT-PEMFC (represented as membrane resistance) according to the cur rent density [59], the conductivity of PA-doped PBI for HT-PEMFC is also a function of the potential [55]. This is the reason for the discrep ancy in the ohmic resistances of MEAs according to the potential region; The ohmic resistance of 0.417 Ω cm2 at 0.75 V for MEA with Pt100/Pt/C-a and the pristine cathode (Fig. 7a) was decreased to 0.177 Ω cm2 at 0.63–0.54 V region (slope at 500–1000 mA/cm2 region in Fig. 3a). In contrast with the anode case, increasing the hydrophilicity of the cathode is less effective because the PA concentration is diluted by generated H2O [44]. In addition, the charge transfer resistances of the pristine MEA are 2.083 Ω cm2 under H2/O2 streams and 3.768 Ω cm2 under H2/air streams, similar to that of the MEA employing a modified cathode. However, anode modification results in obvious decreases in the charge transfer resistances (~1.523 Ω cm2 under H2/O2 streams and ~2.617 Ω cm2 under H2/air streams), conceivably indicating enhanced TPB formation at the anode. It should be noted that the interpretation of this phenomenon still remains controversial [42,60]. As indicated above, controlling the hydrophilicity of the electrode surface is a key factor in enhancing the MEA performance. Therefore, the origin of the hydrophilicity changes and the isolated effects without additional Pt deposits have been further investigated. Control of the wetting ability has been examined by immersing the pristine anode in the electrolyte without the Pt precursor (0.5 M NaCl, pH ¼ 7.0) and applying 100 pulses (Pt-free100/Pt/C-a) or the open circuit potential (OCP) for 1000 s (Pt-free OCP/Pt/C-a). From the polarization curves under H2/O2 streams (Fig. S2a) and H2/air streams (Fig. S2b), it is notable that substantial increases in performance are obtained in the case of Pt-free100/Pt/C-a, suggesting that a great part of the enhanced performance originates not from the additionally deposited Pt but from changes in the surface characteristics. For a more precise comparison, the maximum power densities have been summarized as a function of the anode contact angles (Fig. 8). In general, the maximum power density shows a strongly proportionality with the anode hydrophilicity. Compared with the pristine anode, there is no significant change of contact angle at the OCP; thus, a similar maximum power density is obtained. After the application of 100 pulses in the Pt-free electrolyte, the hydrophilicity increases (decreased contact angle of 127.5� ) and the maximum power density also increases to 658.0 mW cm 2 under H2/O2 streams. A further increase in the maximum power density occurs in the case of the most hydrophilic anode, Pt100/Pt/C-a. Elemental analysis of the different anodes cases reveals increased Pt, O, and Na components (Table S1), which could affect the wetting ability [61–63]. Considering the Pt mass loading, the specific maximum power density is 397.1 mW mgPt1 for Pt-free100/Pt/C-a, which is slightly lower than that of Pt100/Pt/C-a (437.2 mW mgPt1). This result also demonstrates that the HT-PEMFC performance can be enhanced simply by controlling the hydrophilicity of the anode without additional Pt loading, although loading additional Pt through pulse electrodeposition gives a better mass activity. Although the hydrophilicity control provide improvement of HT-PEMFC performance at initial stage, potential risk of PA bleeding from the MEA, PA flooding in the CL, and Na content at the CL surface should be considered for the long-term stability of MEA.
4. Conclusion In summary, we have demonstrated that the performance of HTPEMFCs can be enhanced by a facile Pt SED method. Pt SED had no significant effect when the cathode was modified. By contrast, for anode modification, the maximum power density exhibited volcano behavior as a function of the deposition pulse number. Based on various analyses, in addition to the effects of additional Pt loading, the increase in hy drophilicity during the Pt SED process facilitated an enhanced PA dis tribution and TPB formation in the CL, until 100 deposition pulses. Treatment of the pristine anode in a Pt-free electrolyte showed that hydrophilicity control without the formation of additional Pt was also effective for improving the performance of HT-PEMFCs to some extent. Notably, the mass activities of Pt100/Pt/C-a and Pt-free100/Pt/C-a were 437.2 mW mgPt1 and 397.1 mW mgPt1 under H2/O2 streams, approximately 1.36 and 1.24 times higher, respectively, than that of the pristine HT-PEMFC MEA. The strategy of performance improvement by PED will be extended to the cathode in near future, where small amounts of Pt-transition metal alloys instead of pure Pt can be formed on top of the pristine cathode through PED. A different aspect for the effects of Pt alloy PED, such as enhancement of ORR activity, is expected compared with the negligible effects of pure Pt PED during the cathode modification. Acknowledgements This work was supported by the National Research Foundation (NRF) of Korea Grant funded by the Korean Government MSIT (grant number 2016M1A2A2937146 & 2018M1A2A2062000), and by the Korea CCS R&D Center (KCRC) (grant number 2014M1A8A1049349). This research was supported by Korea Electric Power Corporation (Grant number: R18XA06-27). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.jpowsour.2019.227022. References [1] S. Shimpalee, M. Ohashi, J.W. Van Zee, C. Ziegler, C. Stoeckmann, C. Sadeler, C. Hebling, Experimental and numerical studies of portable PEMFC stack, Electrochim. Acta 54 (2009) 2899–2911. [2] H.-P. Chang, C.-L. Chou, Y.-S. Chen, T.-I. Hou, B.-J. Weng, The design and cost analysis of a portable PEMFC UPS system, Int. J. Hydrogen Energy 32 (2007) 316–322. [3] Y. Li, J. Yang, J. Song, Structure models and nano energy system design for proton exchange membrane fuel cells in electric energy vehicles, Renew. Sustain. Energy Rev. 67 (2017) 160–172. [4] C. Bernay, M. Marchand, M. Cassir, Prospects of different fuel cell technologies for vehicle applications, J. Power Sources 108 (2002) 139–152. [5] F.-C. Wang, P.-C. Kuo, H.-J. Chen, Control design and power management of a stationary PEMFC hybrid power system, Int. J. Hydrogen Energy 38 (2013) 5845–5856. [6] J. Hamelin, K. Agbossou, A. Laperri�ere, F. Laurencelle, T.K. Bose, Dynamic behavior of a PEM fuel cell stack for stationary applications, Int. J. Hydrogen Energy 26 (2001) 625–629. [7] G. Li, P.G. Pickup, Ionic conductivity of PEMFC electrodes effect of Nafion loading, J. Electrochem. Soc. 150 (2003) C745–C752. [8] J. Peron, A. Mani, X. Zhao, D. Edwards, M. Adachi, T. Soboleva, Z. Shi, Z. Xie, T. Navessin, S. Holdcroft, Properties of Nafion® NR-211 membranes for PEMFCs, J. Membr. Sci. 356 (2010) 44–51. [9] K.-H. Kim, K.-Y. Lee, S.-Y. Lee, E.A. Cho, T.-H. Lim, H.-J. Kim, S.-P. Yoon, S.H. Kim, T.W. Lim, J.H. Jang, The effects of relative humidity on the performances of PEMFC MEAs with various Nafion® ionomer contents, Int. J. Hydrogen Energy 35 (2010) 13104–13110. [10] C. Chen, T.F. Fuller, The effect of humidity on the degradation of Nafion® membrane, Polym. Degrad. Stab. 94 (2009) 1436–1447. [11] S.-K. Park, E.A. Cho, I.-H. Oh, Characteristics of membrane humidifiers for polymer electrolyte membrane fuel cells, Korean J. Chem. Eng. 22 (2005) 877–881. [12] J.K. Nørskov, J. Rossmeisl, A. Logadottir, L. Lindqvist, J.R. Kitchin, T. Bligaard, H. J� onsson, Origin of the overpotential for oxygen reduction at a fuel-cell cathode, J. Phys. Chem. B 108 (2004) 17886–17892.
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9
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Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
Performance enhancement of high-temperature polymer electrolyte membrane fuel cells using Pt pulse electrodeposition Dong-Kwon Kim a, Hoyoung Kim a, Hyanjoo Park a, SeonHwa Oh a, Sang Hyun Ahn b, *, Hyoung-Juhn Kim c, Soo-Kil Kim a, ** a b c
School of Integrative Engineering, Chung-Ang University, 84 Heukseok-ro, Dongjak-gu, Seoul, 06974, Republic of Korea School of Chemical Engineering and Material Science, Chung-Ang University, 84 Heukseok-ro, Dongjak-gu, Seoul, 06974, Republic of Korea Fuel Cell Research Center, Korea Institute of Science and Technology, 5 Hwarang-ro 14-gil, Seongbuk-gu, Seoul, 02792, Republic of Korea
H I G H L I G H T S
G R A P H I C A L A B S T R A C T
� Pulse deposition modifies the wetting property of a commercial Pt-based electrode. � Maximum power density shows a vol cano relationship with deposition pulse number. � Hydrophilicity control on the anode significantly affects HT-PEMFC performance. � A specific maximum power of 437.2 mW mgPt1 is achieved under optimized conditions.
A R T I C L E I N F O
A B S T R A C T
Keywords: High-temperature PEM fuel cell Phosphoric acid distribution Pt pulse electrodeposition Hydrophilic surface
The development of high-performance polymer electrolyte membrane fuel cells that operate at elevated tem peratures is urgently required to overcome the technical problems associated with operation at low tempera tures. Here, we report an effective way to enhance electrode performance via simple Pt pulse electrodeposition at room temperature. This electrodeposition process enables both the formation of additional Pt electrocatalysts with extremely low loading (i.e., ~0.05 mg cm 2 for 100 pulse cycles) as a function of pulse number and control of the wetting properties of commercial Pt-based electrodes. Following optimization of the electrode conditions and configurations, the controlled hydrophilicity of the anode enhances the phosphoric acid distribution and the formation of a triple phase boundary in the catalyst layer, resulting in lowered ohmic and charge transfer re sistances, respectively. The mass activities of the membrane electrode assembly with the anode modified by Pt pulse electrodeposition is 437.2 mW mgPt1 (H2/O2), which is approximately 1.36 times higher than that of the pristine membrane electrode assembly. The controlled hydrophilicity allows moderate improvement of the performance, even without additional Pt. The results presented herein demonstrate the importance of surface property control for electrode preparation to achieve enhanced performance of high-performance polymer electrolyte membrane fuel cells.
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (S.H. Ahn), [email protected] (S.-K. Kim). https://doi.org/10.1016/j.jpowsour.2019.227022 Received 24 November 2018; Received in revised form 15 May 2019; Accepted 14 August 2019 Available online 17 August 2019 0378-7753/© 2019 Elsevier B.V. All rights reserved.
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Journal of Power Sources 438 (2019) 227022
1. Introduction
unfavorable for the kinetics of the oxygen reduction reaction (ORR) at the cathode [12–14] and CO poisoning resistance during the hydrogen oxidation reaction (HOR) at the anode [15–18]. In addition, water management to minimize cathodic flooding remains challenging [17–20]. When the operating temperature is increased above 120 � C, most issues can be overcome by using a polybenzimidazole (PBI) membrane doped with phosphoric acid (PA) [21,22]. Obvious increases of the ORR kinetics and CO tolerance occur on Pt electrocatalysts at elevated temperatures [23,24]. Further, the system design can be simplified to exclude water management [23] and the waste heat can be
Tremendous efforts have been focused on the development of poly mer electrolyte membrane fuel cells (PEMFCs) for portable [1,2], vehicle [3,4], and stationary [5,6] applications. As a result, recent PEMFC technologies based on the Nafion membrane have been realized that meet acceptable energy efficiencies and commercialization costs [7, 8]. However, to maintain a reasonable proton conductivity, the mem brane must be hydrated using a humidifier, typically limiting the operating temperature range to 60–80 � C [9–11]. These conditions are
Fig. 1. FESEM images of prepared (a–e) anodes and (f–j) cathodes: (a) Pt/C-a, (b) Pt50/Pt/C-a, (c) Pt100/Pt/C-a, (d) Pt300/Pt/C-a, (e) Pt500/Pt/C-a, (f) PtNi/C-c, (g) Pt50/PtNi/C-c, (h) Pt100/PtNi/C-c, (i) Pt300/PtNi/C-c, and (j) Pt500/PtNi/C-c. 2
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Fig. 2. XRD patterns of prepared (a) anodes and (b) cathodes. Dependence of contact angle on the number of deposition pulses for (c) anodes and (d) cathodes. Insets: contact angle images.
Pt loading mass was fixed at 1.0 mg cm 2, whereas the PTFE content was varied from 2% to 40%. Following PA uptake from a PA solution, the CL with 5% PTFE exhibited the highest performance. A similar experiment performed by Jeong et al. showed that the formation of a triple phase boundary (TPB) is accelerated at the optimized PTFE content of 20 wt% [39]. In addition, it has been reported that a crackless microporous layer (MPL) physically prevents PA leakage, as confirmed by X-ray tomogra phy [41]. On the contrary, a nonuniform distribution of PA in the CL causes insufficient TPB formation. Despite these many efforts, it is still difficult to understand the interrelation among membrane conductivity, TPB formation, and Pt poisoning during PA bleed-out. Therefore, larger amounts of Pt electrocatalysts have been loaded in the CL to obtain HT-PEMFCs with reasonable performance, thus inhibiting commercial ization [42,43], although the optimization of Pt loading in the ano de/cathode and PA content has been investigated [44]. The electrodeposition method facilitates simple, fast, and one-pot fabrication of GDEs at room temperature [45]. However, owing to the insufficient formation of a TPB, its use in the preparation of electrodes has been limited and only a few studies [46–48] have been reported. By contrast, recent reports on self-terminating electrodeposition (SED) of platinum group metals (PGMs) have opened a new research area on wet forms of atomic layer deposition with rapid and inexpensive deposition processes [49]. Under a highly negative deposition overpotential, where H passivation is enabled, SED facilitates precise control at extremely low
easily utilized [23,25]. Unfortunately, despite these advantages, increasing the operating temperature leads to new issues [26]. A major problem encountered in high-temperature PEMFCs (HT-PEMFCs) is bleeding out of PA from the PBI membrane through the gas diffusion electrode (GDE) [27,28]. The leached phosphate species induce Pt poisoning, which degrades the HT-PEMFC performance [29,30]. Spectroscopic analysis of half-cell re action has demonstrated that the phosphate species are present as H3PO4, H2PO4 , HPO24 , and PO34 in aqueous PA solution [31–33]. Among these species, the H2PO4 ion adsorbs strongly on the surface of Pt electrocatalysts, reducing their electrochemical active sites [34]. In response to this issue, Pt–M alloy electrocatalysts (M: Ni [35], Co [36], Cu [34], and Au [33]) have been developed to increase the tolerance toward phosphate species and obtain a higher Pt mass activity relative to �n et al. have reported the performance of an that of pure Pt. Milla HT-PEMFC single cell with a PtCo cathode [36]. Owing to the higher alloying degree of this electrocatalyst, the performance of HT-PEMFC was maintained for 150 h. Another problem associated with PA leakage is decreased proton conductivity owing to the lowered PA content in the membrane [37,38]. Hydrophilicity control on the catalyst layer (CL) neighboring the membrane is known as a promising way to prevent PA leakage [39,40]. Mack et al. have demonstrated the effect of the polytetrafluoroethylene (PTFE) content in the CL on HT-PEMFC performance [40]. In the CL, the 3
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Journal of Power Sources 438 (2019) 227022
Fig. 3. Dependence of HT-PEMFC performance on the number of deposition pulses for the anode. The cathode was fixed as pristine PtNi/C-c. (a) i–V curves and (b) maximum power densities under H2/O2 streams. (c) i–V curves and (d) maximum power densities under H2/air streams.
PGM loading masses (~ng cm 2) that exhibit high catalytic activity for fuel cell half reactions [50,51] and water electrolysis [51,52]. The SED process has been expanded to fabricate Pt GDEs with advantageous structures that provide good mass-transfer pathways for the reactant/ product in the high current density region during PEM water electrolysis operation [53]. However, as far as we know, this promising method has not been applied to the fabrication of electrodes for PEMFCs. The results presented herein demonstrate that the performance of HT-PEMFCs is enhanced by employing commercial electrodes modified by simple Pt SED. The controllable formation of extremely low amounts of Pt on the CL surface in both the anode and cathode was used to un derstand the changes in hydrophilicity and TPB formation in the pres ence of PA, as well as the performance of HT-PEMFCs under both H2/air and H2/O2 streams. The differing effects of Pt SED on the anode and cathode were also examined by analyzing the physical and electro chemical properties of the modified electrodes.
(Daejung, 7548–4100) [49]. A saturated calomel electrode (SCE) and a Pt wire were adopted as the reference and counter electrodes, respec tively. The deposition pulse consisted of the following sequence: 0.4 VSCE for 2 s, 0.9 VSCE for 10 s, and 0.4 VSCE for 3 s [53]. The deposition pulse was repeated 50, 100, 300, or 500 times to control the Pt loading amount. All deposition processes were performed at room temperature and under ambient pressure. 2.2. Characterization Before and after Pt pulse electrodeposition, the surface morphologies of the commercial electrodes were imaged using field emission scanning electrode microscopy (FESEM, SIGMA, Carl Zeiss) and their composi tions were analyzed using energy-dispersive spectroscopy (EDS, Thermo Scientific, NORAN System 7). The crystal structures were analyzed using X-ray diffraction (XRD, New D8-Advance, Bruker AXS) at a rate of 5� min 1 from 30� to 90� . The hydrophilicities were examined using a deionized water droplet (3 μL) and a contact angle analyzer (FM40Mk2 EasyDrop, KRÜSS GmbH). The Pt loading mass was measured using inductively coupled plasma mass spectroscopy (ICPMS, Nexion 300, PerkinElmer).
2. Experimental 2.1. Pt pulse electrodeposition Pt pulse electrodeposition was conducted in a conventional threeelectrode cell connected to a potentiostat (CS310, Wuhan CorrTest In struments Corp., Ltd.). Commercial BASF Celtec® electrodes (Pt/C for anode and Pt alloy/C for cathode) were used to prepare working elec trodes with exposed areas of 1.53 cm2 using a N2-purged deposition electrolyte containing 3 mM K2PtCl4 (Alfa Aesar, 43946) and 0.5 M NaCl
2.3. Fabrication of HT-PEMFC single cells As a pretreatment, a commercial PA-doped PBI membrane (BASF Celtec® membrane, thickness of 400 μm, acid concentration 55 wt%) was dried at 120 � C for 1 h in a dryer. Then, the membrane was 4
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Journal of Power Sources 438 (2019) 227022
Fig. 4. Dependence of HT-PEMFC performance on the number of deposition pulses for the cathode. The anode was fixed as pristine Pt/C-a. (a) i–V curves and (b) maximum power densities under H2/O2 streams. (c) i–V curves and (d) maximum power densities under H2/air streams.
sandwiched between the anode and the cathode to fabricate a mem brane electrode assembly (MEA) with an active area of 1.0 cm2. To avoid physical leakage of PA from the membrane, the MEA was fabricated without hot-pressing [44,54,55]. The serpentine-type flow channel on the bipolar plate had a channel width of 0.5 mm and a depth of 1.0 mm.
3. Results and discussion The performances of the HT-PEMFC single cells have been investi gated by using commercial electrodes modified by Pt SED as a function of the number of deposition pulses. The pulse profile and corresponding current vs. time plot are shown in Fig. S1. The Pt SED process facilitates the formation of additional Pt and the loading is controllable within extremely small amounts [49,53]. The modified electrodes have been named as Pt#/Pt/C-a for anodes and Pt#/PtNi/C-c for cathodes, where # is the number of Pt deposition pulses. Fig. 1 shows FESEM images of the electrodes before and after Pt SED. For the pristine anode (BASF Celtec® anode), the typical morphology of a Pt/C CL is observed (Fig. 1a) [39,56]. After 50 deposition pulses (Fig. 1b), the density of Pt nanoparticles obviously increases. The size of the Pt nanoparticles is approximately 12 nm, but some deviation exists because the Pt nucle ation process during Pt SED might differ on the surfaces of existing Pt nanoparticles and bare C spheres. Further increasing the pulse number results in growth of the Pt nanoparticles: 23 � 3.1 nm for Pt100/Pt/C-a (Figs. 1c), 28 � 4.1 nm for Pt300/Pt/C-a (Figs. 1d), and 42 � 8.3 nm for Pt500/Pt/C-a (Fig. 1e). The pristine PtNi/C cathode (BASF Celtec® cathode) shows a similar morphology to the pristine Pt/C anode (Fig. 1f). Compared with the Pt#/Pt/C-a electrodes, much larger Pt nanoparticles form on the Pt#/PtNi/C-c electrodes (Fig. 1g–j), indi cating that Pt galvanic displacement with Ni on the surface of PtNi/C occurs in addition to Pt SED [52]. After 500 deposition pulses, most of the CL surface is covered by agglomerated Pt spheres. Fig. 2a shows XRD patterns of the pristine Pt/C anode and Pt#/Pt/C-
2.4. Performance evaluation of HT-PEMFC single cells To eliminate any moisture inside the cell, N2 was flowed into the HTPEMFC single cell at a temperature of 150 � C for 20 min. After drying, a H2 stream was supplied to the anode at a constant volumetric flow rate of 0.1 L min 1, whereas an O2 or air stream was supplied to the cathode at a constant volumetric flow rate of 0.3 L min 1. After 30 min, to sta bilize the single cell, the MEA was activated at 0.6 V for 3 h. Then, the current density was gradually increased with an interval of 20 mA cm 2 for 1 min. All performance evaluations on single cells were performed using a fuel cell test station (CNL Energy). 2.5. Characterization of HT-PEMFC single cells Electrochemical impedance spectroscopy (EIS) was conducted using a potentiostat (Autolab PGSTAT302 N, Metrohm) connected to the sin gle cell. At a constant cell voltage of 0.75 V, the frequency was changed from 10 kHz to 0.01 Hz at an amplitude of 5 mV under a H2 stream at the anode and an O2 (or air) stream at the cathode. 5
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Journal of Power Sources 438 (2019) 227022
Fig. 5. Dependence of HT-PEMFC performance on the number of deposition pulses for the cathode. The anode was fixed as Pt100/Pt/C-a. (a) i–V curves and (b) maximum power densities under H2/O2 streams. (c) i–V curves and (d) maximum power densities under H2/air streams.
a electrodes. Three peaks are observed for all samples, demonstrating the presence of Pt(111) at 39.763� , Pt(200) at 46.243� , and Pt(220) at 67.454� (JCPDS 04–0802). Based on the Scherrer equation, the average crystalline size is calculated as 9.49 nm for the pristine anode. On increasing the pulse number, the size gradually increases to 10.08 nm (Pt50/Pt/C-a), 10.21 nm (Pt100/Pt/C-a), 10.77 nm (Pt300/Pt/C-a), and 12.46 nm (Pt500/Pt/C-a). For the pristine PtNi/C cathode, the peak observed at 41.113� is located between the Pt(111) and Ni(111) peaks, indicating the presence of a PtNi alloy phase. As the pulse number in creases, the intensity of the Pt(111) peak increases, whereas the in tensity of the original peak at 39.763� is maintained, indicating that deposited Pt and the PtNi alloy exist separately after the Pt SED process. The Scherrer equation provides average crystalline sizes of 10.21 nm (pristine PtNi/C cathode), 10.38 nm (Pt50/PtNi/C-c), 10.48 nm (Pt100/ PtNi/C-c), 13.21 nm (Pt300/PtNi/C-c), and 15.46 nm (Pt500/PtNi/C-c), indicating a faster increase in size than in the case of the anodes. In addition, the hydrophilicities of the pristine and modified electrodes have been analyzed using contact angle measurements, as shown in Fig. 2c and d. The contact angle between deionized water and the electrode surface gradually decreases as the deposition pulse number increases, from 137.6� to 63.8� for the anodes (Fig. 2c) and from 141.5� to 45.2� for the cathodes (Fig. 2d). This finding indicates that the hy drophilicity of the electrode gradually increases after the Pt SED process. Fig. 3 demonstrates the polarization curves of HT-PEMFCs operated at 150 � C with anodes prepared by varying the number of deposition pulses. As a reference, the pristine anode and cathode have been used to fabricate a pristine MEA. In order to avoid mass-transfer limitation of
reactant, excessive O2 was injected into HT-PEMFCs. Under the H2/O2 streams, the HT-PEMFC with the pristine MEA exhibits a maximum power density of 532.5 mW cm 2 (Fig. 3a), which is similar to the values previously reported in the literature [57,58], confirming the reliability of the operating conditions. The maximum power density shows para bolic behavior as a function of the pulse number, with the highest value of 748.3 mW cm 2 occurring at 100 deposition pulses, which is 1.4 times larger than that in the pristine case (Fig. 3b). Under the H2/air streams, a maximum power density of 319.6 mW cm 2 is obtained with the pristine MEA (Fig. 3c). Similar parabolic behavior depending on the number of pulses is observed, with the highest value of 473.3 mW cm 2 obtained using the Pt100/Pt/C-a electrode, which is 1.5 times larger than that in the pristine case (Fig. 3d). Fig. 4 shows the polarization curves of HT-PEMFCs with MEAs consisting of Pt#/PtNi/C-c and pristine Pt/C-a. In contrast to the anode modification case, less significant performance enhancement is observed, with no strong correlation to the number of deposition pulses under both H2/O2 (Fig. 4a and b) and H2/air (Fig. 4c and d). Further optimization of the MEA configuration has been attempted by controlling the number of deposition pulses for the cath ode while using Pt100/Pt/C-a as the best performing anode. Fig. 5a demonstrates the polarization curves of such HT-PEMFCs under H2/O2 streams. The highest maximum power density is obtained without Pt SED on the cathode. The maximum power density decreases to 483 mW cm 2 after 100 deposition pulses and then increases again, as shown in Fig. 5b. Under H2/air streams, similar behavior is observed, as shown in Fig. 5c and d. In summary, for single cells with various MEA configurations, the Pt 6
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Fig. 6. Performances of HT-PEMFCs with optimal anode and cathode config urations. Specific i–V curves and powers under (a) H2/O2 and (b) H2/ air streams.
Fig. 7. Nyquist plots of HT-PEMFCs with optimal anode and cathode config urations at 0.75 V under (a) H2/O2 and (b) H2/air streams.
SED process is effective for anode modification with the maximum power density showing parabolic behavior with the number of deposi tion pulses. The best HT-PEMFC performance is obtained with 100 deposition pulses. By contrast, cathode modification by Pt SED has no significant effect, regardless of the anode material and number of pulses. Fig. 6 demonstrates the specific polarization curves of HT-PEMFCs with selected MEA configuration according to the Pt mass activity (using the total Pt mass from both electrodes). The pristine MEA shows a specific power of 321.4 mW mgPt1 under H2/O2 streams (Figs. 6a) and 192.9 mW mgPt1 under H2/air streams (Fig. 6b). With Pt100/Pt/C-a and the pristine cathode, the specific power is obviously enhanced to 437.2 mW mgPt1 (O2) and 276.6 mW mgPt1 (air). Other MEA configurations also show enhancement of the specific power relative to that of the pristine MEA. Such increases in the performance per mass unit indicate that the Pt SED process modifies the structure and surface properties of the electrode to change the behavior of PA at the electrode/membrane interface or in side the electrode. Considering that the Pt nanoparticles obtained by SED are larger than those in the pristine electrode, a great part of the mass activity enhancement originates from the optimization of the structure of existing (pristine) electrode through the SED process. To determine the PA behavior in the various MEA configurations, EIS measurements have been conducted at a constant cell voltage of 0.75 V under H2/O2 streams, as shown in Fig. 7a. For the pristine MEA, the ohmic resistance is measured as 0.896 Ω cm2. With Pt100/Pt/C-a and the pristine cathode, the ohmic resistance significantly decreases to 0.417 Ω cm2, whereas Pt SED on the cathode is ineffective for changing the ohmic resistance. A similar change in ohmic resistance is observed under the H2/air streams (Fig. 7b). Evidently, compared with the
Fig. 8. Dependence of the maximum power density on the contact angle for pristine and treated anodes. The cathode was fixed as pristine PtNi/C-c. Insets: contact angle images.
pristine MEA, Pt#/Pt/C-a demonstrates much lower slopes in the ohmic loss region of the polarization curves (Fig. 3), whereas Pt#/PtNi/C-c show more similar slopes (Fig. 4). These different changes in the ohmic resistances are directly related to changes in the PA behavior and 7
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the TPB interface inside the CL. According to Liu et al. [44], the per formance of HT-PEMFCs is significantly affected by interactions be tween the electrode and the membrane, as examined by controlling the PA content and CL thickness. For the anode in this study, the ohmic resistance can be decreased by increasing the PA content in the MEA. Therefore, after 100 deposition pulses on the anode, the gradually increased hydrophilicity of the anode facilitates the uniform distribution of PA as well as the formation of a TPB in the CL. However, the Pt poisoning phenomenon caused by overwetted PA surely became domi nant at excessive surface hydrophilicities with further pulses (Fig. 2c), resulting in the observed volcano behavior of the maximum power density as a function of the pulse number (Fig. 3b and d). It also should be noted that, likewise the change in the membrane conductivity for LT-PEMFC (represented as membrane resistance) according to the cur rent density [59], the conductivity of PA-doped PBI for HT-PEMFC is also a function of the potential [55]. This is the reason for the discrep ancy in the ohmic resistances of MEAs according to the potential region; The ohmic resistance of 0.417 Ω cm2 at 0.75 V for MEA with Pt100/Pt/C-a and the pristine cathode (Fig. 7a) was decreased to 0.177 Ω cm2 at 0.63–0.54 V region (slope at 500–1000 mA/cm2 region in Fig. 3a). In contrast with the anode case, increasing the hydrophilicity of the cathode is less effective because the PA concentration is diluted by generated H2O [44]. In addition, the charge transfer resistances of the pristine MEA are 2.083 Ω cm2 under H2/O2 streams and 3.768 Ω cm2 under H2/air streams, similar to that of the MEA employing a modified cathode. However, anode modification results in obvious decreases in the charge transfer resistances (~1.523 Ω cm2 under H2/O2 streams and ~2.617 Ω cm2 under H2/air streams), conceivably indicating enhanced TPB formation at the anode. It should be noted that the interpretation of this phenomenon still remains controversial [42,60]. As indicated above, controlling the hydrophilicity of the electrode surface is a key factor in enhancing the MEA performance. Therefore, the origin of the hydrophilicity changes and the isolated effects without additional Pt deposits have been further investigated. Control of the wetting ability has been examined by immersing the pristine anode in the electrolyte without the Pt precursor (0.5 M NaCl, pH ¼ 7.0) and applying 100 pulses (Pt-free100/Pt/C-a) or the open circuit potential (OCP) for 1000 s (Pt-free OCP/Pt/C-a). From the polarization curves under H2/O2 streams (Fig. S2a) and H2/air streams (Fig. S2b), it is notable that substantial increases in performance are obtained in the case of Pt-free100/Pt/C-a, suggesting that a great part of the enhanced performance originates not from the additionally deposited Pt but from changes in the surface characteristics. For a more precise comparison, the maximum power densities have been summarized as a function of the anode contact angles (Fig. 8). In general, the maximum power density shows a strongly proportionality with the anode hydrophilicity. Compared with the pristine anode, there is no significant change of contact angle at the OCP; thus, a similar maximum power density is obtained. After the application of 100 pulses in the Pt-free electrolyte, the hydrophilicity increases (decreased contact angle of 127.5� ) and the maximum power density also increases to 658.0 mW cm 2 under H2/O2 streams. A further increase in the maximum power density occurs in the case of the most hydrophilic anode, Pt100/Pt/C-a. Elemental analysis of the different anodes cases reveals increased Pt, O, and Na components (Table S1), which could affect the wetting ability [61–63]. Considering the Pt mass loading, the specific maximum power density is 397.1 mW mgPt1 for Pt-free100/Pt/C-a, which is slightly lower than that of Pt100/Pt/C-a (437.2 mW mgPt1). This result also demonstrates that the HT-PEMFC performance can be enhanced simply by controlling the hydrophilicity of the anode without additional Pt loading, although loading additional Pt through pulse electrodeposition gives a better mass activity. Although the hydrophilicity control provide improvement of HT-PEMFC performance at initial stage, potential risk of PA bleeding from the MEA, PA flooding in the CL, and Na content at the CL surface should be considered for the long-term stability of MEA.
4. Conclusion In summary, we have demonstrated that the performance of HTPEMFCs can be enhanced by a facile Pt SED method. Pt SED had no significant effect when the cathode was modified. By contrast, for anode modification, the maximum power density exhibited volcano behavior as a function of the deposition pulse number. Based on various analyses, in addition to the effects of additional Pt loading, the increase in hy drophilicity during the Pt SED process facilitated an enhanced PA dis tribution and TPB formation in the CL, until 100 deposition pulses. Treatment of the pristine anode in a Pt-free electrolyte showed that hydrophilicity control without the formation of additional Pt was also effective for improving the performance of HT-PEMFCs to some extent. Notably, the mass activities of Pt100/Pt/C-a and Pt-free100/Pt/C-a were 437.2 mW mgPt1 and 397.1 mW mgPt1 under H2/O2 streams, approximately 1.36 and 1.24 times higher, respectively, than that of the pristine HT-PEMFC MEA. The strategy of performance improvement by PED will be extended to the cathode in near future, where small amounts of Pt-transition metal alloys instead of pure Pt can be formed on top of the pristine cathode through PED. A different aspect for the effects of Pt alloy PED, such as enhancement of ORR activity, is expected compared with the negligible effects of pure Pt PED during the cathode modification. Acknowledgements This work was supported by the National Research Foundation (NRF) of Korea Grant funded by the Korean Government MSIT (grant number 2016M1A2A2937146 & 2018M1A2A2062000), and by the Korea CCS R&D Center (KCRC) (grant number 2014M1A8A1049349). This research was supported by Korea Electric Power Corporation (Grant number: R18XA06-27). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.jpowsour.2019.227022. References [1] S. Shimpalee, M. Ohashi, J.W. Van Zee, C. Ziegler, C. Stoeckmann, C. Sadeler, C. Hebling, Experimental and numerical studies of portable PEMFC stack, Electrochim. Acta 54 (2009) 2899–2911. [2] H.-P. Chang, C.-L. Chou, Y.-S. Chen, T.-I. Hou, B.-J. Weng, The design and cost analysis of a portable PEMFC UPS system, Int. J. Hydrogen Energy 32 (2007) 316–322. [3] Y. Li, J. Yang, J. Song, Structure models and nano energy system design for proton exchange membrane fuel cells in electric energy vehicles, Renew. Sustain. Energy Rev. 67 (2017) 160–172. [4] C. Bernay, M. Marchand, M. Cassir, Prospects of different fuel cell technologies for vehicle applications, J. Power Sources 108 (2002) 139–152. [5] F.-C. Wang, P.-C. Kuo, H.-J. Chen, Control design and power management of a stationary PEMFC hybrid power system, Int. J. Hydrogen Energy 38 (2013) 5845–5856. [6] J. Hamelin, K. Agbossou, A. Laperri�ere, F. Laurencelle, T.K. Bose, Dynamic behavior of a PEM fuel cell stack for stationary applications, Int. J. Hydrogen Energy 26 (2001) 625–629. [7] G. Li, P.G. Pickup, Ionic conductivity of PEMFC electrodes effect of Nafion loading, J. Electrochem. Soc. 150 (2003) C745–C752. [8] J. Peron, A. Mani, X. Zhao, D. Edwards, M. Adachi, T. Soboleva, Z. Shi, Z. Xie, T. Navessin, S. Holdcroft, Properties of Nafion® NR-211 membranes for PEMFCs, J. Membr. Sci. 356 (2010) 44–51. [9] K.-H. Kim, K.-Y. Lee, S.-Y. Lee, E.A. Cho, T.-H. Lim, H.-J. Kim, S.-P. Yoon, S.H. Kim, T.W. Lim, J.H. Jang, The effects of relative humidity on the performances of PEMFC MEAs with various Nafion® ionomer contents, Int. J. Hydrogen Energy 35 (2010) 13104–13110. [10] C. Chen, T.F. Fuller, The effect of humidity on the degradation of Nafion® membrane, Polym. Degrad. Stab. 94 (2009) 1436–1447. [11] S.-K. Park, E.A. Cho, I.-H. Oh, Characteristics of membrane humidifiers for polymer electrolyte membrane fuel cells, Korean J. Chem. Eng. 22 (2005) 877–881. [12] J.K. Nørskov, J. Rossmeisl, A. Logadottir, L. Lindqvist, J.R. Kitchin, T. Bligaard, H. J� onsson, Origin of the overpotential for oxygen reduction at a fuel-cell cathode, J. Phys. Chem. B 108 (2004) 17886–17892.
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