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Electrocatalytic activity and durability study of carbon supported Pt nanodendrites in polymer electrolyte membrane fuel cells
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 x x x ( 2 0 1 3 ) 1 e7
Available online at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/he
Electrocatalytic activity and durability study of carbon supported Pt nanodendrites in polymer electrolyte membrane fuel cells Woong Hee Lee, Hansung Kim* Department of Chemical and Biomolecular Engineering, Yonsei University, 134 Shinchon-Dong, Seodaemun-gu, 120-749 Seoul, Republic of Korea
article info
abstract
Article history:
In this paper, Pt nanodendrites are synthesized, and their use as an oxygen reduction
Received 18 December 2012
catalyst in polymer electrolyte membrane fuel cells is examined. When the Pt nano-
Received in revised form
particles are shape-controlled in a dendritic form, the Pt nanoparticles exhibit a high mass
22 February 2013
activity that is nearly twice as high as the commercial Pt/C catalyst for the oxygen
Accepted 2 April 2013
reduction reaction. This high activity is only achieved when the Pt nanodendrites are
Available online xxx
supported on carbon. The unsupported Pt nanodendrites exhibit very poor catalytic activity due to the limited accessibility of the active sites in the catalyst layer of the fuel cells. Based
Keywords:
on the durability study of Pt nanodendrites, however, the dendritic structure is not stable
Fuel cell
during repeated potential cycling test and its structure collapse is the primary reason for
Oxygen reduction reaction
the performance loss in the fuel cells.
Dendrite
Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights
Platinum
reserved.
Durability
1.
Introduction
Hydrogen-operated polymer electrolyte membrane (PEM) fuel cells have received a great deal of attention as a green alternative to conventional combustion engines due to their high efficiency and zero emission of pollutants [1e3]. Despite significant advances, a number of problems, such as the use of platinum, have yet to be overcome. The sluggish electrocatalytic kinetics of the oxygen reduction reaction (ORR) requires Pt as a catalyst, which is an expensive material. The high cost of Pt is a serious limitation to the commercialization of PEM fuel cells. Therefore, it is of great interest to synthesize advanced electrocatalysts for ORR. In an attempt to achieve this goal, various Pt-based alloy catalysts with less expensive 3d-transition metals, such as PtCo [4e6], PtNi [7e9], PtCu
[10,11] and PtFe [12,13], have been extensively developed to improve the activity toward ORR and to reduce the consumption of Pt. However, serious durability issues have been raised in the acidic environment of PEM fuel cells. Another important route for improving the activity of catalysts is to control the shape of the Pt nanoparticles. In general, the activity of metallic catalysts depends strongly on their morphology, which determines the distinct facets and atomic surface arrangements. Significant research has been devoted to synthesizing shape-controlled Pt nanoparticles and evaluating their electrocatalytic activity [14e20]. The tetrahexahedral Pt nanoparticles with 24 high-index facets were synthesized and found to exhibit high activity with respect to the electro-oxidation of formic acid and ethanol [21]. The enhanced catalytic activity was reported for one-dimensional
* Corresponding author. Tel.: þ82 2 2123 5753; fax: þ82 2 312 6401. E-mail address: [email protected] (H. Kim). 0360-3199/$ e see front matter Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijhydene.2013.04.002
Please cite this article in press as: Lee WH, Kim H, Electrocatalytic activity and durability study of carbon supported Pt nanodendrites in polymer electrolyte membrane fuel cells, International Journal of Hydrogen Energy (2013), http://dx.doi.org/ 10.1016/j.ijhydene.2013.04.002
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structures, such as Pt nanotubes [22,23] and Pt nanowires [24,25]. Because of the high surface to volume ratio, a hollow structure such as a nanocage was synthesized via galvanic replacement, and its improved activity toward electrochemical methanol oxidation was reported [26,27]. Recently, Pt nanodendrites with rich edges and corner atoms were prepared and found to exhibit superior electrocatalytic activity due to the electronic configurations effect [28e34]. Although there have been several reports regarding the improved electrocatalytic activity of shape-controlled Ptbased catalysts, it is unclear if such high activity could also be observed under actual fuel cell operating conditions. It is important to note that most of these studies evaluated the catalytic properties in an aqueous electrolyte using a conventional three-electrode system. Due to the difference in configuration between the three-electrode system and the fuel cell operating environment, it is necessary to demonstrate that the results drawn from the three-electrode system are applicable to actual fuel cells. In addition, the stability of a shape-controlled catalyst with respect to time has not been thoroughly examined. Therefore, an investigation of the catalytic behavior and durability of the shape-controlled Pt catalysts in PEM fuel cell environments is necessary. Here, the ORR activity of Pt nanodendrites in a fuel cell station under H2/O2 conditions is evaluated. The durability of the Pt nanodendrites is examined in the fuel cell station using an accelerated durability test (ADT) up to 20,000 cycles, and the performance loss after ADT is examined by measuring the changes in the morphology and the active surface area of the Pt.
2.
Experimental
2.1.
Preparation of Pt nanodendrites
Pt nanodendrites were synthesized using a previously published protocol [28]. Potassium tetrachloroplatinate (K2PtCl4, Kojima) and tetradecyltrimethylammonium bromide (TTAB, Aldrich) were dissolved in de-ionized water. The mixture was heated to 70 C with vigorous stirring until the mixture became clear. Then, ascorbic acid was added to the mixture under stirring. After 6 additional hours, the resulting solution was cooled down to room temperature. The products were separated by centrifugation at 14,000 rpm for 30 min and dispersed in an ethanol and water mixture (volume ratio 1:1). To prepare the carbon supported Pt nanodendrites (PtD/C), the appropriate amount of carbon supports (i.e., a carbon nanocage) was added to the solution under vigorous stirring to form 60 wt% carbon supported Pt nanodendrites. Subsequently, the solutions were stirred at room temperature for 12 h. The PtD/C was then filtered and dried in convection oven at 80 C for 30 min.
2.2.
Membrane electrode assembly fabrication
The membrane electrode assemblies (MEA) were fabricated using the catalyst-coated membrane (CCM) method. To prepare the MEA, the prepared catalysts were employed as the cathode, and the commercial Pt/C catalyst, which was
obtained from the Tanaka Co. (TKK, 46 wt% Pt), was employed as the anode. The catalyst powder was ultrasonically mixed with a Nafion solution (5 wt%) in isopropyl alcohol and sprayed directly onto a Nafion 212 membrane. Next, the sprayed membrane was hot pressed at 140 C for 150 s. Commercial SGL 10BC was employed as the gas diffusion layer. In all of the MEAs, the electrode area was 5 cm2 with a Pt loading of 0.1 mg cm2.
2.3.
Electrochemical and physical characterization
Polarization curves were measured at 80 C and 1 atm. The anode side was fed with pure hydrogen, and the cathode side was fed with pure oxygen. The flow rate of hydrogen and oxygen was 150 ccm. All of the gasses were humidified to 100% relative humidity. After completing the polarization curves, the mass activity was measured at 80 C and 1.5 atm 50 ccm of hydrogen and 150 ccm of oxygen. IR correction was done by correcting the cell voltage with measured ohmic resistance of the fuel cell by an impedance analyzer. An impedance analysis was performed at 0.8 V over a frequency range of 0.1 and 1 kHz. Cyclic voltammetry (CV) was performed from 0.05 to 1.2 V to measure the electrochemical surface area (ECSA) of the catalyst. During the CV, the cathode was deoxygenated via a nitrogen purge. The ECSA was also measured using a conventional three-electrode system in 0.5 M H2SO4 solution. A glassy carbon electrode with a thin film of the prepared sample was used as the working electrode. A platinum wire and standard Hg/HgSO4 electrode were used as the counter and reference electrodes, respectively. The catalyst ink was prepared by blending the catalyst powder into a solvent of isopropyl alcohol (IPA) and water containing 5 wt% Nafion ionomer. The catalyst ink was deposited onto the glassy carbon disk (24.3 mg cm2). The scan rate was 5 mV s1. The accelerated durability test (ADT) was conducted by potential cycling between 0.6 and 1.0 V for 20,000 cycles at a scan rate of 50 mV s1. The cathode was exposed to humidified N2 at a constant flow rate of 75 ccm, and humidified H2 at a flow rate of 200 ccm was supplied to the anode. After ADT, the polarization curves and CV were measured again. High-resolution transmission electron microscopy (HR-TEM) was performed to analyze the size and morphology of the Pt nanodendrites.
3.
Results and discussion
Fig. 1 shows HR-TEM images of the as-prepared Pt nanodendrites and the TKK commercial Pt/C catalysts. The image (Fig. 1(a)) indicates that the synthesized Pt nanodendrites have a well-oriented 3D dendritic structure with branches in various directions. The size of each Pt nanodendrites is in the range of 20e30 nm. Most of the shape-controlled electrocatalysts in the literature were tested in the unsupported form. However, in this study, the Pt nanodendrites are supported by carbon for use in fuel cells. Fig. 1(b) and (c) displays the Pt nanodendrites supported by carbon (PtD/C) and their higher magnification images. Based on the high Pt content (60 wt%), the number of Pt nanodendrite particles does not appear large due to the relatively large particle size of the Pt nanodendrites. For comparison, the TKK commercial Pt/C
Please cite this article in press as: Lee WH, Kim H, Electrocatalytic activity and durability study of carbon supported Pt nanodendrites in polymer electrolyte membrane fuel cells, International Journal of Hydrogen Energy (2013), http://dx.doi.org/ 10.1016/j.ijhydene.2013.04.002
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 x x x ( 2 0 1 3 ) 1 e7
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Fig. 1 e HR-TEM images of the (a) unsupported Pt nanodendrites, (b) carbon supported Pt nanodendrites, (c) carbon supported Pt nanodendrites with higher magnification and (d) TKK commercial Pt/C.
catalyst is shown in Fig. 1(d), where the particle size of the Pt nanoparticles is approximately 3 nm, which is much smaller than that of the Pt nanodendrites. Although the Pt content is 46 wt%, which is lower than that of PtD/C, the density of the particles in the images appears much higher compared to PtD/C. The activity of the catalyst for the ORR was evaluated by measuring the polarization curves of the MEAs under hydrogen and oxygen operating conditions. From Fig. 2, the unsupported Pt nanodendrites exhibit very poor performance compared to the carbon supported Pt nanodendrites over the whole potential range. The performance of the MEA prepared using the carbon supported Pt nanodendrites significantly increases to 1.48 A cm2 at 0.6 V, which is six times higher than that of the MEA with the unsupported Pt nanodendrites (0.24 A cm2). This result could be attributed to the change in the resistance in the MEA. According to the impedance analysis shown in Fig. 3, the unsupported Pt nanodendrites display a huge charge transfer resistance due to the limited accessible active area of the Pt nanodendrites. The Pt nanodendrites have needle-like branches, and the number of Pt nanodendrite particles is low due to the relatively larger particle
Fig. 2 e Polarization curves of MEAs using different electrocatalysts as the cathode, including the unsupported Pt nanodendrites and carbon supported Pt nanodendrites. The measurements were performed with H2/O2 at 80 C and 1 atm.
Please cite this article in press as: Lee WH, Kim H, Electrocatalytic activity and durability study of carbon supported Pt nanodendrites in polymer electrolyte membrane fuel cells, International Journal of Hydrogen Energy (2013), http://dx.doi.org/ 10.1016/j.ijhydene.2013.04.002
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Fig. 3 e Impedance analysis of MEA for the unsupported Pt nanodendrites and carbon supported Pt nanodendrites measured at 0.8 V in a frequency range of 0.1e1 kHz.
size. Therefore, the physical and electrical contacts between the Pt nanodendrite particles are poor, which results in the isolation of the active site on the Pt nanodendrites. When the Pt nanodendrite particles are supported by carbon, the charge transfer resistance significantly decreases from 0.34 to 0.07 U because the carbon support contributes to dispersion of PtD nanodendrite particles and provides the electrical connection between particles. As a result, the accessible active sites on the Pt nanodendrites are increased by the enhanced formation of triple-phase boundary. This explanation is supported by the observed difference in the electrochemical active surface area (ECSA). To investigate the ECSA of the catalysts, cyclic voltammogram (CV) experiments were performed, and the results are shown in Fig. 4. The CV curves of these catalysts were recorded in a fuel cell station under hydrogen and nitrogen purging for the anode and cathode, respectively, at a sweep rate of 50 mV s1. The ECSA was calculated from the charge collected in the hydrogen desorption region after doublelayer correction between 0.05 and 0.4 V. The electrochemical surface area was then converted to the specific active surface area using a conversion factor of 210 mC cm2. According to Fig. 4(a), the PtD/C exhibits the ECSA of 15.9 m2 g1. For the unsupported Pt nanodendrites, the ECSA is measured to be 3.5 m2 g1, which is much smaller than that measured for PtD/C indicating that the number of accessible active sites on the Pt nanodendrites is reduced in the absence of carbon supports. It should be noted that this big difference has not yet been observed from the conventional threeelectrode system. As shown in Fig. 4(b), the ECSA measured in the three-electrode system was 34.5 m2 g1 and 25.4 m2 g1 for PtD/C and PtD, respectively. When the ECSA is measured in the three-electrode system, electrolyte is a liquid phase and a tiny amount of Pt nanodendrite catalyst is employed to prepare the thin film on the rotating disk electrode. As a result, the catalyst utilization is improved from the threeelectrode system. Therefore, it could be concluded that the Pt nanodendrites should be supported by carbon for use in fuel cells.
Fig. 4 e Cyclic voltammetric profiles of the unsupported Pt nanodendrites and carbon supported Pt nanodendrites measured in (a) fuel cells with H2 and N2 purging of the anode and cathode, respectively at a scan rate of 50 mV sL1 and (b) a three-electrode system under N2 purging at a scan rate of 5 mV sL1.
Fig. 5 e Tafel plots measuring the mass activity of MEA with the carbon supported Pt nanodendrites and TKK commercial Pt/C catalysts. The measurements were performed with H2/O2 at 80 C and 1.5 atm.
Please cite this article in press as: Lee WH, Kim H, Electrocatalytic activity and durability study of carbon supported Pt nanodendrites in polymer electrolyte membrane fuel cells, International Journal of Hydrogen Energy (2013), http://dx.doi.org/ 10.1016/j.ijhydene.2013.04.002
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To contrast the kinetic activity of the PtD/C against that of TKK commercial Pt/C catalysts, the mass activity was measured by normalizing the IR corrected kinetic current of the MEA with respect to the Pt loading and presented in Fig. 5. This result clearly demonstrates that PtD/C has superior ORR activity. The absolute values for the PtD/C at 0.9 V is 0.54 A mg1, which is nearly twice that of the TKK commercial Pt/C catalyst (0.23 A mg1). It is surprising to note that the PtD/ C has higher catalytic activity despite its drastically increased PtD particle size and a 41% lower ECSA (the ECSA of TKK commercial Pt/C: 38.1 m2 g1). Fig. 6 illustrates the polarization curves of the MEAs with PtD/C and TKK commercial Pt/C. The Pt loading of PtD/C was 0.1 mg cm2 while that of TKK commercial Pt/C was 0.2 mg cm2. As shown in Fig. 6, the PtD/ C shows comparable performance even with only 1/2 of Pt loading of the TKK commercial Pt/C catalyst. Based on this observation, it is proved that the shape-controlled Pt nanodendrites possess high catalytic activity toward oxygen reduction reaction. In addition to the electrocatalytic activity, the durability of the shape-controlled electrocatalysts is critical. Nevertheless, investigations into the durability of shapecontrolled electrocatalysts have been rarely studied. To evaluate the durability of PtD/C for use in fuel cells, the accelerated durability test (ADT) was performed by cycling the potential between 0.6 and 1.0 V at 80 C. The ECSA and performance of fuel cells were then compared before and after the ADT. The ECSA results are shown in Fig. 7. After 20,000 cycles, the PtD/C lost 33.3% of the initial ECSA (from 15.9 to 10.6 m2 g1). It is necessary to confirm the durability of Pt nanodendrites by measuring the MEA performance loss after the ADT in the fuel cell system. Fig. 8 shows the polarization curves for the PEM fuel cells recorded before and after the ADT. Interestingly, the performance drop of the MEA containing PtD/C was higher considering the relatively small loss in ECSA. At a cell potential of 0.6 V, the PtD/C catalyst exhibited performance decay of approximately 59.5% at 0.6 V (from 1.48 to 0.6 A cm2). This result suggests
Fig. 6 e Polarization curves of MEAs using different electrocatalysts as the cathode, including the carbon supported Pt nanodendrites and TKK commercial Pt/C. The measurements were performed with H2/O2 at 80 C and 1 atm.
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Fig. 7 e Cyclic voltammograms of MEAs for the carbon supported Pt nanodendrites before and after the accelerated durability test employed up to 20,000 cycles at a scan rate of 50 mV sL1.
that the performance drop in the MEA was affected not only by the active surface area loss but also by other factors such as the morphology change. To explore the morphology change of the catalysts, HRTEM images were taken before and after the ADT and are shown in Fig. 9. After the potential cycling, the dendritic shape of the PtD/C disappeared, and the surface of the Pt nanodendrites became smoother, indicating that the Pt atoms located at the corners and edges were rearranged. However, a significant particle size change due to the sintering effect is not observed. Based on the high ORR activity of PtD/C originating from its dendritic structure, the destruction of the Pt nanodendrites’ morphology may be responsible for the higher performance drop despite the smaller drop in the ECSA. Therefore, it is necessary to develop shape-controlled electrocatalysts with high activity as well as structural stability under fuel cell operating conditions.
Fig. 8 e Comparison of the MEA performance of the carbon supported Pt nanodendrites before and after the accelerated durability test.
Please cite this article in press as: Lee WH, Kim H, Electrocatalytic activity and durability study of carbon supported Pt nanodendrites in polymer electrolyte membrane fuel cells, International Journal of Hydrogen Energy (2013), http://dx.doi.org/ 10.1016/j.ijhydene.2013.04.002
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Fig. 9 e HR-TEM images of the carbon supported Pt nanodendrites (a) before and (b) after the accelerated durability test.
4.
Conclusion [6]
In this study, the Pt nanodendrites were prepared and their oxygen reduction activity in a fuel cell system was explored. Unlike a three-electrode system, the Pt nanodendrites should be supported by carbon to prevent the isolation of the electrocatalytic active sites on the Pt nanodendrites in the catalyst layer of the fuel cells. The carbon supported Pt nanodendrites exhibited a mass activity of 0.54 A mg1, which is nearly two times higher than that of the TKK commercial Pt/C catalysts. Based on the durability test using a potential sweep up to 20,000 cycles, however, the PtD/C catalyst exhibited the performance loss of 59.5% at 0.6 V. The HR-TEM images revealed that the dendritic structure of Pt nanodendrites collapsed, which is responsible for the performance loss.
[7]
[8]
[9]
[10]
[11]
Acknowledgments This work was supported by the Priority Research Centers Program through the National Research Foundation of Korea (2009-0093823) and (NRF-2009-C1AAA001-0092926) funded by the Ministry of Education, Science and Technology.
references
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Please cite this article in press as: Lee WH, Kim H, Electrocatalytic activity and durability study of carbon supported Pt nanodendrites in polymer electrolyte membrane fuel cells, International Journal of Hydrogen Energy (2013), http://dx.doi.org/ 10.1016/j.ijhydene.2013.04.002
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[19] Lee C-L, Wu C-C, Chiou H-P, Syu C-M, Huang C-H, Yang C-C. Mesoporous platinum nanosponges as electrocatalysts for the oxygen reduction reaction in an acidic electrolyte. Int J Hydrogen Energy 2011;36:6433e40. [20] Zhang J, Yang H, Fang J, Zou S. Synthesis and oxygen reduction activity of shape-controlled Pt3Ni nanopolyhedra. Nano Lett 2010;10:638e44. [21] Tian N, Zhou ZY, Sun SG, Ding Y, Wang ZL. Synthesis of tetrahexahedral platinum nanocrystals with high-index facets and high electro-oxidation activity. Science 2007;316:732e5. [22] Chen Z, Waje M, Li W, Yan Y. Supportless Pt and PtPd nanotubes as electrocatalysts for oxygen-reduction reactions. Angew Chem Int Ed 2007;46:4060e3. [23] Alia SM, Zhang G, Kisailus D, Li D, Gu S, Jensen K, et al. Porous platinum nanotubes for oxygen reduction and methanol oxidation reactions. Adv Funct Mater 2010;20:3742e6. [24] Liang H-W, Cao X, Zhou F, Cui C-H, Zhang W-J, Yu S-H. A free-standing Pt-nanowire membrane as a highly stable electrocatalyst for the oxygen reduction reaction. Adv Mater 2011;23:1467e71. [25] Sun S, Jaouen F, Dodelet J-P. Controlled growth of Pt nanowires on carbon nanospheres and their enhanced performance as electrocatalysts in PEM fuel cells. Adv Mater 2008;20:3900e4. [26] Au L, Lu X, Xia Y. A comparative study of galvanic replacement reactions involving Ag nanocubes and AuCl 2 or AuCl 4 . Adv Mater 2008;20:2517e22.
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Please cite this article in press as: Lee WH, Kim H, Electrocatalytic activity and durability study of carbon supported Pt nanodendrites in polymer electrolyte membrane fuel cells, International Journal of Hydrogen Energy (2013), http://dx.doi.org/ 10.1016/j.ijhydene.2013.04.002
Available online at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/he
Electrocatalytic activity and durability study of carbon supported Pt nanodendrites in polymer electrolyte membrane fuel cells Woong Hee Lee, Hansung Kim* Department of Chemical and Biomolecular Engineering, Yonsei University, 134 Shinchon-Dong, Seodaemun-gu, 120-749 Seoul, Republic of Korea
article info
abstract
Article history:
In this paper, Pt nanodendrites are synthesized, and their use as an oxygen reduction
Received 18 December 2012
catalyst in polymer electrolyte membrane fuel cells is examined. When the Pt nano-
Received in revised form
particles are shape-controlled in a dendritic form, the Pt nanoparticles exhibit a high mass
22 February 2013
activity that is nearly twice as high as the commercial Pt/C catalyst for the oxygen
Accepted 2 April 2013
reduction reaction. This high activity is only achieved when the Pt nanodendrites are
Available online xxx
supported on carbon. The unsupported Pt nanodendrites exhibit very poor catalytic activity due to the limited accessibility of the active sites in the catalyst layer of the fuel cells. Based
Keywords:
on the durability study of Pt nanodendrites, however, the dendritic structure is not stable
Fuel cell
during repeated potential cycling test and its structure collapse is the primary reason for
Oxygen reduction reaction
the performance loss in the fuel cells.
Dendrite
Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights
Platinum
reserved.
Durability
1.
Introduction
Hydrogen-operated polymer electrolyte membrane (PEM) fuel cells have received a great deal of attention as a green alternative to conventional combustion engines due to their high efficiency and zero emission of pollutants [1e3]. Despite significant advances, a number of problems, such as the use of platinum, have yet to be overcome. The sluggish electrocatalytic kinetics of the oxygen reduction reaction (ORR) requires Pt as a catalyst, which is an expensive material. The high cost of Pt is a serious limitation to the commercialization of PEM fuel cells. Therefore, it is of great interest to synthesize advanced electrocatalysts for ORR. In an attempt to achieve this goal, various Pt-based alloy catalysts with less expensive 3d-transition metals, such as PtCo [4e6], PtNi [7e9], PtCu
[10,11] and PtFe [12,13], have been extensively developed to improve the activity toward ORR and to reduce the consumption of Pt. However, serious durability issues have been raised in the acidic environment of PEM fuel cells. Another important route for improving the activity of catalysts is to control the shape of the Pt nanoparticles. In general, the activity of metallic catalysts depends strongly on their morphology, which determines the distinct facets and atomic surface arrangements. Significant research has been devoted to synthesizing shape-controlled Pt nanoparticles and evaluating their electrocatalytic activity [14e20]. The tetrahexahedral Pt nanoparticles with 24 high-index facets were synthesized and found to exhibit high activity with respect to the electro-oxidation of formic acid and ethanol [21]. The enhanced catalytic activity was reported for one-dimensional
* Corresponding author. Tel.: þ82 2 2123 5753; fax: þ82 2 312 6401. E-mail address: [email protected] (H. Kim). 0360-3199/$ e see front matter Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijhydene.2013.04.002
Please cite this article in press as: Lee WH, Kim H, Electrocatalytic activity and durability study of carbon supported Pt nanodendrites in polymer electrolyte membrane fuel cells, International Journal of Hydrogen Energy (2013), http://dx.doi.org/ 10.1016/j.ijhydene.2013.04.002
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structures, such as Pt nanotubes [22,23] and Pt nanowires [24,25]. Because of the high surface to volume ratio, a hollow structure such as a nanocage was synthesized via galvanic replacement, and its improved activity toward electrochemical methanol oxidation was reported [26,27]. Recently, Pt nanodendrites with rich edges and corner atoms were prepared and found to exhibit superior electrocatalytic activity due to the electronic configurations effect [28e34]. Although there have been several reports regarding the improved electrocatalytic activity of shape-controlled Ptbased catalysts, it is unclear if such high activity could also be observed under actual fuel cell operating conditions. It is important to note that most of these studies evaluated the catalytic properties in an aqueous electrolyte using a conventional three-electrode system. Due to the difference in configuration between the three-electrode system and the fuel cell operating environment, it is necessary to demonstrate that the results drawn from the three-electrode system are applicable to actual fuel cells. In addition, the stability of a shape-controlled catalyst with respect to time has not been thoroughly examined. Therefore, an investigation of the catalytic behavior and durability of the shape-controlled Pt catalysts in PEM fuel cell environments is necessary. Here, the ORR activity of Pt nanodendrites in a fuel cell station under H2/O2 conditions is evaluated. The durability of the Pt nanodendrites is examined in the fuel cell station using an accelerated durability test (ADT) up to 20,000 cycles, and the performance loss after ADT is examined by measuring the changes in the morphology and the active surface area of the Pt.
2.
Experimental
2.1.
Preparation of Pt nanodendrites
Pt nanodendrites were synthesized using a previously published protocol [28]. Potassium tetrachloroplatinate (K2PtCl4, Kojima) and tetradecyltrimethylammonium bromide (TTAB, Aldrich) were dissolved in de-ionized water. The mixture was heated to 70 C with vigorous stirring until the mixture became clear. Then, ascorbic acid was added to the mixture under stirring. After 6 additional hours, the resulting solution was cooled down to room temperature. The products were separated by centrifugation at 14,000 rpm for 30 min and dispersed in an ethanol and water mixture (volume ratio 1:1). To prepare the carbon supported Pt nanodendrites (PtD/C), the appropriate amount of carbon supports (i.e., a carbon nanocage) was added to the solution under vigorous stirring to form 60 wt% carbon supported Pt nanodendrites. Subsequently, the solutions were stirred at room temperature for 12 h. The PtD/C was then filtered and dried in convection oven at 80 C for 30 min.
2.2.
Membrane electrode assembly fabrication
The membrane electrode assemblies (MEA) were fabricated using the catalyst-coated membrane (CCM) method. To prepare the MEA, the prepared catalysts were employed as the cathode, and the commercial Pt/C catalyst, which was
obtained from the Tanaka Co. (TKK, 46 wt% Pt), was employed as the anode. The catalyst powder was ultrasonically mixed with a Nafion solution (5 wt%) in isopropyl alcohol and sprayed directly onto a Nafion 212 membrane. Next, the sprayed membrane was hot pressed at 140 C for 150 s. Commercial SGL 10BC was employed as the gas diffusion layer. In all of the MEAs, the electrode area was 5 cm2 with a Pt loading of 0.1 mg cm2.
2.3.
Electrochemical and physical characterization
Polarization curves were measured at 80 C and 1 atm. The anode side was fed with pure hydrogen, and the cathode side was fed with pure oxygen. The flow rate of hydrogen and oxygen was 150 ccm. All of the gasses were humidified to 100% relative humidity. After completing the polarization curves, the mass activity was measured at 80 C and 1.5 atm 50 ccm of hydrogen and 150 ccm of oxygen. IR correction was done by correcting the cell voltage with measured ohmic resistance of the fuel cell by an impedance analyzer. An impedance analysis was performed at 0.8 V over a frequency range of 0.1 and 1 kHz. Cyclic voltammetry (CV) was performed from 0.05 to 1.2 V to measure the electrochemical surface area (ECSA) of the catalyst. During the CV, the cathode was deoxygenated via a nitrogen purge. The ECSA was also measured using a conventional three-electrode system in 0.5 M H2SO4 solution. A glassy carbon electrode with a thin film of the prepared sample was used as the working electrode. A platinum wire and standard Hg/HgSO4 electrode were used as the counter and reference electrodes, respectively. The catalyst ink was prepared by blending the catalyst powder into a solvent of isopropyl alcohol (IPA) and water containing 5 wt% Nafion ionomer. The catalyst ink was deposited onto the glassy carbon disk (24.3 mg cm2). The scan rate was 5 mV s1. The accelerated durability test (ADT) was conducted by potential cycling between 0.6 and 1.0 V for 20,000 cycles at a scan rate of 50 mV s1. The cathode was exposed to humidified N2 at a constant flow rate of 75 ccm, and humidified H2 at a flow rate of 200 ccm was supplied to the anode. After ADT, the polarization curves and CV were measured again. High-resolution transmission electron microscopy (HR-TEM) was performed to analyze the size and morphology of the Pt nanodendrites.
3.
Results and discussion
Fig. 1 shows HR-TEM images of the as-prepared Pt nanodendrites and the TKK commercial Pt/C catalysts. The image (Fig. 1(a)) indicates that the synthesized Pt nanodendrites have a well-oriented 3D dendritic structure with branches in various directions. The size of each Pt nanodendrites is in the range of 20e30 nm. Most of the shape-controlled electrocatalysts in the literature were tested in the unsupported form. However, in this study, the Pt nanodendrites are supported by carbon for use in fuel cells. Fig. 1(b) and (c) displays the Pt nanodendrites supported by carbon (PtD/C) and their higher magnification images. Based on the high Pt content (60 wt%), the number of Pt nanodendrite particles does not appear large due to the relatively large particle size of the Pt nanodendrites. For comparison, the TKK commercial Pt/C
Please cite this article in press as: Lee WH, Kim H, Electrocatalytic activity and durability study of carbon supported Pt nanodendrites in polymer electrolyte membrane fuel cells, International Journal of Hydrogen Energy (2013), http://dx.doi.org/ 10.1016/j.ijhydene.2013.04.002
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Fig. 1 e HR-TEM images of the (a) unsupported Pt nanodendrites, (b) carbon supported Pt nanodendrites, (c) carbon supported Pt nanodendrites with higher magnification and (d) TKK commercial Pt/C.
catalyst is shown in Fig. 1(d), where the particle size of the Pt nanoparticles is approximately 3 nm, which is much smaller than that of the Pt nanodendrites. Although the Pt content is 46 wt%, which is lower than that of PtD/C, the density of the particles in the images appears much higher compared to PtD/C. The activity of the catalyst for the ORR was evaluated by measuring the polarization curves of the MEAs under hydrogen and oxygen operating conditions. From Fig. 2, the unsupported Pt nanodendrites exhibit very poor performance compared to the carbon supported Pt nanodendrites over the whole potential range. The performance of the MEA prepared using the carbon supported Pt nanodendrites significantly increases to 1.48 A cm2 at 0.6 V, which is six times higher than that of the MEA with the unsupported Pt nanodendrites (0.24 A cm2). This result could be attributed to the change in the resistance in the MEA. According to the impedance analysis shown in Fig. 3, the unsupported Pt nanodendrites display a huge charge transfer resistance due to the limited accessible active area of the Pt nanodendrites. The Pt nanodendrites have needle-like branches, and the number of Pt nanodendrite particles is low due to the relatively larger particle
Fig. 2 e Polarization curves of MEAs using different electrocatalysts as the cathode, including the unsupported Pt nanodendrites and carbon supported Pt nanodendrites. The measurements were performed with H2/O2 at 80 C and 1 atm.
Please cite this article in press as: Lee WH, Kim H, Electrocatalytic activity and durability study of carbon supported Pt nanodendrites in polymer electrolyte membrane fuel cells, International Journal of Hydrogen Energy (2013), http://dx.doi.org/ 10.1016/j.ijhydene.2013.04.002
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Fig. 3 e Impedance analysis of MEA for the unsupported Pt nanodendrites and carbon supported Pt nanodendrites measured at 0.8 V in a frequency range of 0.1e1 kHz.
size. Therefore, the physical and electrical contacts between the Pt nanodendrite particles are poor, which results in the isolation of the active site on the Pt nanodendrites. When the Pt nanodendrite particles are supported by carbon, the charge transfer resistance significantly decreases from 0.34 to 0.07 U because the carbon support contributes to dispersion of PtD nanodendrite particles and provides the electrical connection between particles. As a result, the accessible active sites on the Pt nanodendrites are increased by the enhanced formation of triple-phase boundary. This explanation is supported by the observed difference in the electrochemical active surface area (ECSA). To investigate the ECSA of the catalysts, cyclic voltammogram (CV) experiments were performed, and the results are shown in Fig. 4. The CV curves of these catalysts were recorded in a fuel cell station under hydrogen and nitrogen purging for the anode and cathode, respectively, at a sweep rate of 50 mV s1. The ECSA was calculated from the charge collected in the hydrogen desorption region after doublelayer correction between 0.05 and 0.4 V. The electrochemical surface area was then converted to the specific active surface area using a conversion factor of 210 mC cm2. According to Fig. 4(a), the PtD/C exhibits the ECSA of 15.9 m2 g1. For the unsupported Pt nanodendrites, the ECSA is measured to be 3.5 m2 g1, which is much smaller than that measured for PtD/C indicating that the number of accessible active sites on the Pt nanodendrites is reduced in the absence of carbon supports. It should be noted that this big difference has not yet been observed from the conventional threeelectrode system. As shown in Fig. 4(b), the ECSA measured in the three-electrode system was 34.5 m2 g1 and 25.4 m2 g1 for PtD/C and PtD, respectively. When the ECSA is measured in the three-electrode system, electrolyte is a liquid phase and a tiny amount of Pt nanodendrite catalyst is employed to prepare the thin film on the rotating disk electrode. As a result, the catalyst utilization is improved from the threeelectrode system. Therefore, it could be concluded that the Pt nanodendrites should be supported by carbon for use in fuel cells.
Fig. 4 e Cyclic voltammetric profiles of the unsupported Pt nanodendrites and carbon supported Pt nanodendrites measured in (a) fuel cells with H2 and N2 purging of the anode and cathode, respectively at a scan rate of 50 mV sL1 and (b) a three-electrode system under N2 purging at a scan rate of 5 mV sL1.
Fig. 5 e Tafel plots measuring the mass activity of MEA with the carbon supported Pt nanodendrites and TKK commercial Pt/C catalysts. The measurements were performed with H2/O2 at 80 C and 1.5 atm.
Please cite this article in press as: Lee WH, Kim H, Electrocatalytic activity and durability study of carbon supported Pt nanodendrites in polymer electrolyte membrane fuel cells, International Journal of Hydrogen Energy (2013), http://dx.doi.org/ 10.1016/j.ijhydene.2013.04.002
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To contrast the kinetic activity of the PtD/C against that of TKK commercial Pt/C catalysts, the mass activity was measured by normalizing the IR corrected kinetic current of the MEA with respect to the Pt loading and presented in Fig. 5. This result clearly demonstrates that PtD/C has superior ORR activity. The absolute values for the PtD/C at 0.9 V is 0.54 A mg1, which is nearly twice that of the TKK commercial Pt/C catalyst (0.23 A mg1). It is surprising to note that the PtD/ C has higher catalytic activity despite its drastically increased PtD particle size and a 41% lower ECSA (the ECSA of TKK commercial Pt/C: 38.1 m2 g1). Fig. 6 illustrates the polarization curves of the MEAs with PtD/C and TKK commercial Pt/C. The Pt loading of PtD/C was 0.1 mg cm2 while that of TKK commercial Pt/C was 0.2 mg cm2. As shown in Fig. 6, the PtD/ C shows comparable performance even with only 1/2 of Pt loading of the TKK commercial Pt/C catalyst. Based on this observation, it is proved that the shape-controlled Pt nanodendrites possess high catalytic activity toward oxygen reduction reaction. In addition to the electrocatalytic activity, the durability of the shape-controlled electrocatalysts is critical. Nevertheless, investigations into the durability of shapecontrolled electrocatalysts have been rarely studied. To evaluate the durability of PtD/C for use in fuel cells, the accelerated durability test (ADT) was performed by cycling the potential between 0.6 and 1.0 V at 80 C. The ECSA and performance of fuel cells were then compared before and after the ADT. The ECSA results are shown in Fig. 7. After 20,000 cycles, the PtD/C lost 33.3% of the initial ECSA (from 15.9 to 10.6 m2 g1). It is necessary to confirm the durability of Pt nanodendrites by measuring the MEA performance loss after the ADT in the fuel cell system. Fig. 8 shows the polarization curves for the PEM fuel cells recorded before and after the ADT. Interestingly, the performance drop of the MEA containing PtD/C was higher considering the relatively small loss in ECSA. At a cell potential of 0.6 V, the PtD/C catalyst exhibited performance decay of approximately 59.5% at 0.6 V (from 1.48 to 0.6 A cm2). This result suggests
Fig. 6 e Polarization curves of MEAs using different electrocatalysts as the cathode, including the carbon supported Pt nanodendrites and TKK commercial Pt/C. The measurements were performed with H2/O2 at 80 C and 1 atm.
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Fig. 7 e Cyclic voltammograms of MEAs for the carbon supported Pt nanodendrites before and after the accelerated durability test employed up to 20,000 cycles at a scan rate of 50 mV sL1.
that the performance drop in the MEA was affected not only by the active surface area loss but also by other factors such as the morphology change. To explore the morphology change of the catalysts, HRTEM images were taken before and after the ADT and are shown in Fig. 9. After the potential cycling, the dendritic shape of the PtD/C disappeared, and the surface of the Pt nanodendrites became smoother, indicating that the Pt atoms located at the corners and edges were rearranged. However, a significant particle size change due to the sintering effect is not observed. Based on the high ORR activity of PtD/C originating from its dendritic structure, the destruction of the Pt nanodendrites’ morphology may be responsible for the higher performance drop despite the smaller drop in the ECSA. Therefore, it is necessary to develop shape-controlled electrocatalysts with high activity as well as structural stability under fuel cell operating conditions.
Fig. 8 e Comparison of the MEA performance of the carbon supported Pt nanodendrites before and after the accelerated durability test.
Please cite this article in press as: Lee WH, Kim H, Electrocatalytic activity and durability study of carbon supported Pt nanodendrites in polymer electrolyte membrane fuel cells, International Journal of Hydrogen Energy (2013), http://dx.doi.org/ 10.1016/j.ijhydene.2013.04.002
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Fig. 9 e HR-TEM images of the carbon supported Pt nanodendrites (a) before and (b) after the accelerated durability test.
4.
Conclusion [6]
In this study, the Pt nanodendrites were prepared and their oxygen reduction activity in a fuel cell system was explored. Unlike a three-electrode system, the Pt nanodendrites should be supported by carbon to prevent the isolation of the electrocatalytic active sites on the Pt nanodendrites in the catalyst layer of the fuel cells. The carbon supported Pt nanodendrites exhibited a mass activity of 0.54 A mg1, which is nearly two times higher than that of the TKK commercial Pt/C catalysts. Based on the durability test using a potential sweep up to 20,000 cycles, however, the PtD/C catalyst exhibited the performance loss of 59.5% at 0.6 V. The HR-TEM images revealed that the dendritic structure of Pt nanodendrites collapsed, which is responsible for the performance loss.
[7]
[8]
[9]
[10]
[11]
Acknowledgments This work was supported by the Priority Research Centers Program through the National Research Foundation of Korea (2009-0093823) and (NRF-2009-C1AAA001-0092926) funded by the Ministry of Education, Science and Technology.
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Please cite this article in press as: Lee WH, Kim H, Electrocatalytic activity and durability study of carbon supported Pt nanodendrites in polymer electrolyte membrane fuel cells, International Journal of Hydrogen Energy (2013), http://dx.doi.org/ 10.1016/j.ijhydene.2013.04.002