Achieving High-Power PEM Fuel Cell Performance with an Ultralow-Pt

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Achieving High-Power PEM Fuel Cell Performance with an Ultralow Pt Content Core-Shell Catalyst Anusorn Kongkanand, Nalini P Subramanian, Yingchao Yu, Zhongyi Liu, Hiroshi Igarashi, and David A. Muller ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.5b02819 • Publication Date (Web): 26 Jan 2016 Downloaded from http://pubs.acs.org on February 4, 2016

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Achieving High-Power PEM Fuel Cell Performance with an Ultralow Pt Content Core-Shell Catalyst Anusorn Kongkanand*,a, Nalini P. Subramaniana, Yingchao Yub, Zhongyi Liua, Hiroshi Igarashic, David A. Mullerb a

Electrochemical Energy Research Lab, General Motors Global Research and Development, Honeoye Falls, NY 14472 b School of Applied and Engineering Physics, Cornell University, Ithaca, New York 14853, United States c Catalysts Development Center, N.E. Chemcat Corporation, Ibaraki 306-0608, Japan

* Email: [email protected]; Phone: +1 585 953 5538; Fax: +1 248 857 4054

ACS Paragon Plus Environment

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ABSTRACT The high-power performance of proton exchange membrane fuel cells (PEMFC) decreases as the Pt loading or Pt surface area decreases due to oxygen transport constraints. This has limited the Pt reduction of a fuel cell below the current ~0.2 mgPt/cm2MEA. In this paper, the performance of a Pt monolayer core-shell catalyst (PtML/Pd/C) was studied with a particular focus on high-current-density operation. Although conventional Pt/C electrodes with low Pt loading showed a substantial voltage fall-off at high current densities, PtML/Pd/C showed superior performance due to its greater Pt surface area. We show that Pt loading can be reduced to a level as low as 0.025 mgPt/cm2 without noticeable transport-related losses. This suggests considerable potential for further fuel cell cost reduction. The performance and microscopic properties of the catalyst were also studied after accelerated stability tests. Degradation mechanism and pathways for future development are also discussed. Keywords: Oxygen reduction electrocatalyst, proton exchange membrane fuel cell, local oxygen transport, catalyst degradation, core-shell catalyst, platinum monolayer catalyst, Pt dissolution

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INTRODUCTION The development focus for PEMFC for automotive application is cost reduction, with a particular emphasis on reducing the cathode platinum loading. Prior research has been successful in demonstrating catalysts with high oxygen reduction reaction (ORR) activities that exceed the US Department of Energy (US DOE) ORR activity target, which is measured at low current density (LCD). Improvements in the ORR activity were achieved by various approaches including Pt alloying, Pt core-shell structures, and extended-surface catalysts.1,2 Using these higher activity catalysts, a state-of-the-art fuel cell can now operate at a level of 0.35 g/kW (~0.2 mg-Pt/cm2MEA cathode, where MEA stands for the membrane-electrode assembly). However, if one were to reduce the Pt loading further, as several groups

1,3–6

have found, the voltage loss at high current

density (HCD) would become significantly larger than one would expected from known oxygen reduction kinetics or oxygen transports. This impedes further reduction of fuel cell stack size and cost. To explain the large voltage losses observed at higher current density with lower loadings, researchers have introduced a model resistance that is local to Pt. There are differences in the physical interpretation of how the loss terms are constructed among the proposed models.3–7 Unfortunately, there is little experimental data that supports or distinguishes any of the scenarios. Irrespective of the physical interpretation of the causes of voltage loss at HCD, the two most important observations are that the voltage loss: (a) increases with Pt area-specific current density (A/cm2-Pt), and (b) increases with decreasing O2 partial pressure.1 In addition, detailed analysis has shown that the local electrode oxygen transport resistance is inversely proportional to the available Pt surface area.3 This strongly suggests that the voltage loss is caused by an oxygen transport resistance that is very close to the Pt surface, hence the term ‘local-Pt resistance’. The available Pt surface area, or roughness factor (r.f.), is the product of Pt-mass specific surface area (ECSA) and Pt loading. Performance loss due to the local-Pt resistance is large in a low-Pt-loaded electrode because of its low r.f.. One logical way to reduce voltage loss is to increase the ECSA of the catalyst. To do this, one can reduce the size of the Pt particles. However, as the particle size is reduced, the dissolution rate increases8 and combined with a low are-specific activity9, practical use is limited to particles larger than about 3 nm in diameter. An attractive proposition by Adzic and coworkers is to deposit a Pt monolayer on a suitable metal nanoparticle core.10 This approach not only increases catalyst activity by modifying the electronic properties of the Pt monolayer when coupled with the underlying core, but it also maximizes Pt dispersion by locating all of the Pt atoms on the surface of the particles. Depending on the structural arrangement of the deposited Pt 3


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monolayer, a Pt ECSA of 205-240 m2/gPt or about a factor of four over a 4 nm Pt/C catalysts could be obtained. This catalyst strategy thus presents an excellent platform to demonstrate HCD fuel cell performance for a very low Pt loaded electrode. Although tremendous progress has been made in improving and stabilizing the mass activity of Pt-monolayer catalysts,11 little attention has been given to the performance of these catalysts at HCD in an operating fuel cell. Meeting both the efficiency and power targets for the fuel cell vehicles, performance at both the low and high current density, respectively, are critical. This has largely been limited by challenges in preparing the catalyst at a sufficiently large scale to enable high quality and reproducible MEA testing. N.E. Chemcat has developed such scale-up capability and made its first-generation PtML/Pd/C catalyst available in 2011. In this study, here we show that Pt loading in the cathode can be reduced to a level as low as 0.025 mg-Pt/cm2MEA without noticeable transport-related loss. Such a low cathode loading translates to about 2 g of Pt per vehicle (80 kW light duty vehicle), which is equal or lower than the Pt amount used in an exhaust catalyst of a recent internal combustion engine vehicle. The stability of the Pt-monolayer catalyst and potential further development is also discussed.

EXPERIMENTAL Materials The cathode catalyst was PtML/Pd/C core-shell catalyst (Gen1, 15.2 wt.% Pt, 25.4 wt.% Pd) supported on NE-H carbon black received from N.E.ChemCat, Japan or 50 wt.% Pt catalyst supported on Vulcan® XC-72 carbon black (called here conventional Pt/C) received from Tanaka Kikinzoku Kogyo K. K. (TKK), Japan in 2011. Perfluorosulfonic acid with an equivalent weight of 900 g/equiv was utilized with an ionomer to carbon weight ratio of 0.95 and 0.6 in the cathode and anode, respectively. The gas diffusion medium used was carbon fiber paper backings (~200 µm) coated with a 30 µm microporous layer. Nafion® NRE211 (25 µm thick) was used as the membrane. 5 cm2 catalyst-coated membranes (CCMs) were prepared using the decal transfer method. General procedure for electrode and Membrane Electrode Assembly (MEA) fabrication can be found in previously published literature.12 Methods High gas stoichiometry and flow fields with low pressure drop were used to ensure negligible variation across the active area.13 The details of this break-in procedure are given in the appendix. The conditions used for the fuel cell testing were 80°C, 100% relative humidity, 150 4


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kPaabs. Either H2/O2 or H2/air was used as reactant gases in anode/cathode. The ORR mass activity was calculated at 0.9V high frequency resistance-corrected potential from the H2/O2 polarization curve.12 Cathode Pt ECSA was measured by integrating hydrogen adsorption peaks (assuming 210 µC/cm2Pt) from a cyclic voltammogram taken at room temperature with liquid water filled cathode channels.14 The membrane and contact resistances, the electrode proton transfer resistances, and the diffusion medium transport resistance was measured as described elsewhere.13,15 Electrode thickness was measured from scanning electron microscopy of MEA cross-sections. Voltage cycling experiments were performed in 200/50 SCCM of H2/N2 at 80°C, 100% RH and ambient pressure for 20,000 cycles at a scan rate of 50 mV/s. The upper voltage limit was 0.925 V and the lower voltage was 0.02 or 0.6 V. BOL (beginning of life) and EOT (end of test) will be denoted for the stages before and after these voltage-cycling tests. Microscopy The EELS data was acquired on a 5th-order aberration-corrected scanning transmission electron microscope (Nion UltraSTEM) operated at 100 kV, with a convergence angle αmax = ~30 mrad. The upper limit of beam dose was calculated to be 5.8×106 e-/nm2 for a complete EELS mapping experiment and 2.0×103 e-/(nm2s) for each second of electron beam exposure. Due to the dual-EELS detector, both Pt M4.5 edges and Pd M4,5 edges were acquired and analyzed.

RESULTS AND DISCUSSION Fuel Cell Performance of PtML/Pd/C Catalyst H2/air polarization curves of different catalysts at multiple cathode loadings are compared in Figure 1. As discussed earlier for Pt/C catalyst (gray dashed lines), although the voltage losses at LCD are consistent with lower ORR activity due to low Pt loading (21 mV per every half Pt loading), voltage losses at HCD are significantly larger. PtCo/C catalyst (blue solid line), thanks to its high ORR activity, shows higher voltage at LCD than Pt/C catalyst at the same Pt loading. However, because of its limited Pt ECSA (about 45 m2/gPt), the voltage loss rises at HCD, restricting its overall performance. In contrast, having more than twice the Pt ECSA (110 m2/gPt), Pt-ML/C shows negligible voltage losses at HCD. It shows comparable performance to Pt/C with a factor of 3-4 higher Pt loading. These results demonstrate great potential in further reduction of Pt use in a fuel cell.

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Current Density (A/cm2) Figure 1. H2/air polarization curves of different catalysts at multiple cathode loadings. Gray dashed lines, blue solid line, and red solid lines represent Pt/C, PtCo/C, and Pt-ML/C catalysts, respectively. Numbers in the legends are cathode Pt loadings in mg/cm2MEA. Error bars represent one standard deviation of at least 3 measurements.

In order to compare the HCD performance of the different catalysts, voltages at 1.75 A/cm2MEA

were summarized in Figure 2 as a function of available Pt area (roughness factor, r.f.)

which is the product of Pt ECSA and Pt loading. Under pure O2 (filled symbols), the voltage losses were small, predominantly driven by decreasing ORR activity with lowering Pt loading. At a given r.f., Pt-ML and PtCo catalysts give higher voltage due to their higher ORR activities. Under air (open symbols), oxygen transport to the Pt surface becomes a limiting factor especially at low r.f., resulting in a large voltage loss. It is clear that voltage at HCD is strongly dependent on the r.f., regardless of the catalyst type.

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Voltage at 1.75 A/cm2MEA (V)

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Pt/C PtCo/C

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r.f. (cm2-Pt/cm2MEA) Figure 2. Voltage at 1.75 A/cm2 as a function of available Pt area (r.f.) for different catalysts. Error bars represent one standard deviation of at least 3 measurements.

To quantitatively evaluate the local-Pt transport resistance of these catalysts, we utilized the oxygen limiting current measurement.13 In this measurement, oxygen concentration and total pressure were systematically varied in order to deconvolute non-Fickian oxygen resistance (RO2,NF) from the Fickian one (RO2,F). RO2,F primarily represents gas phase transport through larger pores while RO2,NF represents transport through small pores as well as non-gas phase transport. It is this RO2,NF that strongly correlates with HCD performance. Figure 3 summarizes the measured RO2,NF for different catalysts as a function of r.f. Regardless of the type of the catalysts, RO2,NF increases with decreasing r.f. as more oxygen must be delivered to a smaller area of Pt. As discussed in previous studies,3,6 one could determine an intrinsic Pt-area-specific oxygen transport resistance RO2,Pt, from the slope of a plot of RO2,NF and 1/r.f. (see inset). All catalyst types we studied in this work, including the Pt-ML/C, gave a single RO2,Pt of 11.2 sec/cm, comparable to previous works.3,6 These analyses suggest that the high HCD performance we observed on Pt-ML catalyst even at a very low Pt loading is predominantly attributed to its high Pt ECSA.

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O2 RO2,NF (sec/cm)

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1.5

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Roughness Factor (cm2Pt/cm2MEA) Figure 3. Non-Fickian O2 transport resistance (RO2,NF) as a function of r.f. for different catalysts. Inset is a plot of RO2,NF vs 1/R.F. Pt/C (×), PtCo/C (○), Pt-ML/Pd/C (+). PtCo on two types of carbon support are provided (red and blue circles).

Activity and Stability of PtML/Pd/C Catalyst ORR activities measured in a rotating-disk electrode (RDE) and in an MEA more often do not agree due to not only testing condition and platform but also the stability of the catalyst.16,17 MEA is considered to be a more severe condition while reflecting a realistic fuel cell operation. Unfortunately, only few MEA ORR activity results has been reported.11 For a lab-scale PtML/Pd/C catalyst, Pt-mass activity of 0.3 A/mgPt and a Pt ECSA of 120 m2/gPt were reported, a factor of 1.63.2 lower than what were measured in RDE (0.96 A/mgPt and 192 m2/gPt)18. Our ECSA measurements, on a large-scale catalyst, in MEA and RDE were 110 and 171 2

m /gPt, similar to previous studies. The lower ECSA in MEA indicates reorganization or degradation of the Pt monolayer surface. We will discuss more in detail later. Our ORR activity in MEA was actually 50% higher than previously report from a lab-scale materials (0.45 vs 0.3 A/mgPt) and comparable to our RDE measurement (0.55 A/mgPt). On a Pt-mass basis, we found that the ORR activity of the Pt-ML catalyst was about a factor of four higher than that of Pt/C catalyst (the first two groups in Figure 4). This result agree well with the improved performance observed in Figures 8


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1 and 2. However, when taking into account of the cost of Pd, the cost-normalized mass activity (light blue bars) of the Pt-ML catalyst become less appealing. This highlights the need to further

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0

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PtML, cycled 0V

Pt Surface Area [m2-Pt/g-Pt] Mass Activity [A/mg-Pt] Cost-normalized Mass Activity [A/mg-Pt-equiv] Specific Activity [mA/cm2-Pt] Figure 4. Pt-surface area and ORR measurements12 for PtML/Pd/C and Pt/C catalysts. Two lowervoltage limits were used for the voltage cycling stability tests. 20,000 triangle voltage cycles, 0.05 V/sec, upper-voltage limit of 0.925 V, H2/N2, 100% RH.

Pt ECSA was measured by hydrogen adsorption charge in a voltammogram. ECSA of PtML/Pd/C was about twice as high as that of Pt/C (110 vs 50 m2-Pt/g-Pt) and was in good agreement with previous publication11. However, the Pt surface area is still lower than what expected for an ideal Pt monolayer coverage (ca. 200 m2/gPt). The lower value is commonly observed for PtML/Pd/C in an MEA. The lower surface area suggests a non-uniform Pt layer in a form of either Pt-oligolayer or Pt-cluster formation. Note that hydrogen also adsorbs on Pd surface in the same potential region as Pt,19 and hence it is practically impossible to distinguish between the two surfaces using an electrochemical method. Despite this complication, all surface area measured was assigned to that of Pt in this study and it may result in overestimation of Pt area. Development of new

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characterization technique is required to enable distinction and further insight. Interestingly, Van Der Vliet et al.20 have shown that electronic effect by foreign subsurface metal could largely suppress the adsorption of hydrogen on a Pt-skin surface. This offers another explanation for the observed lower-than-expected surface area determined by the hydrogen adsorption in the Ptmonolayer catalysts. After the voltage-cycling stability tests, the ECSA of the monolayer catalyst decreased from 110 to about 55 m2/gPt. As a result, its fuel cell performance decreased as shown in Figure 5. In fact its performance at EOT is now comparable to that of Pt/C at BOL at the same Pt loading (see Figure 1), because they now have comparable r.f.. The voltages at 1.75 A/cm2 of these catalysts are compared in Figure 2 (squares for EOT PtML).

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Current Density (A/cm2) Figure 5. H2/air polarization curves of PtML/Pd/C catalysts before (solid lines) and after 20,000 voltage cycles (dashed lines). Two lower-voltage limits were tested during the voltage cycling tests. Numbers in the legends are cathode Pt loadings in mg/cm2MEA and lower-voltage limits. Error bars represent one standard deviation of at least 3 measurements.

Pd can undergo hydride formation at low potential that creates lattice expansion and fracture of the particle, so-called H2 embrittlement.21,22 This could potentially disrupt the structure of monolayer catalyst. Attempt to investigate the stability of the catalyst against this kind of stress was done by applying different lower-voltage limits (0.6 and 0 V) during the voltage-cycling test while 10


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maintaining the upper-voltage limit of 0.925 V. The results of voltage-cycling tests from our study are summarized in Figure 4 and 5. Despite different LVLs and voltage-cycling time, similar losses in ORR activity, ECSA, and voltage were observed. The lack of sensitivity of catalyst stability to the LVL for the PtML/Pd catalyst compared to a non-protected Pd nanoparticle catalyst under similar test22 suggests that the Pt layer may improve the stability against hydride formation under the Vcycling tests studied here. The stability of the catalyst in real conditions (i.e., longer cycles/durability tests) is yet to be determined. Electron probe microanalysis (EPMA) was performed on cross-sectioned MEAs at different stages of life. Figure 6 shows Pd mappings for an as-made MEA, MEA after 1 day of fuel cell testing, and MEA after 20,000 voltage cycles between 0.6 and 0.925 V. It is clear that while there was no Pd migrated out from the cathode at the beginning, after only 1 day of MEA testing a substantial amount of in the cathode Pd dissolved and redeposited in the membrane and anode. TEM shows single nanocrystals of pure Pd in a range of 8-50 nm forming Pd bands in the membrane. Interestingly, there were two distinct Pd bands in the membrane, probably due to different sets of H2/air front that the MEA experienced throughout the test. Since it was shown that Pt band in the membrane could increase23 or decrease24–27 the degradation of membrane ionomer, it is plausible that Pd band may behave similarly. More study is needed to confirm this. The amount of Pd leaving the cathode was determined to be about 50% at BOL and 70% at EOT. The substantial amount of Pd loss from the cathode may be responsible for the lower ECSA when compared with that measured in RDE (110 vs 171 m2/gPt).

Some Pd was also found to

redeposit on the anode. But due to the high activity of Pd toward H2 oxidation reaction, degradation of anode kinetic is not expected.

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Figure 6. Electron probe microanalysis mapping of cross-sectional MEAs showing Pd distribution on a fresh (a), BOL (b), and EOT (c) samples. Yellow arrows indicate the location of redeposited Pd particle bands in the membranes. Red arrow indicate Pd redeposition in the anode.

Despite the large amount of Pd loss and the subsequent ECSA loss, it is interesting to point out that the ORR mass activity did not deteriorate as much, thanks to increased area-specific activity (green bars in Figure 4). This was discussed earlier by Sasaki et al.11 that as Pd dissolution decreases the particle size, it causes the Pt shell to contract its lattice slightly and in turn improve specific activity. This helps to compensate for the ECSA loss.

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Atomic-level chemical mapping enabled by STEM-EELS was performed to gain insight in the microstructural changes of the monolayer catalyst. Figure 7a shows the particle size distribution of BOL and EOT samples. The average particle diameter decreased from 4.2 nm to 3.4 nm after 20,000 voltage cycles. This is in contrast to the case of a conventional solid nanoparticle where particle size usually grows as a result of electrochemical dissolution and redeposition (electrochemical Ostwald ripening). For PtML/Pd/C, Pd dissolution results in particle size reduction. As shown in Figures 7b and 8c, the EELS maps suggest that the Pt shell becomes thicker and more uniform after voltage cycling. Sporadically, some small amount pure Pt particles (< 2 nm) were also observed. These are likely a result of complete Pd depletion from the particle leaving alone the Pt shell to coalescence.

Figure 7. (a) Particle size distribution of BOL and EOT samples as determined by HAADF-STEM. (b) and (c) show the large area of EELS maps for BOL and EOT catalysts, respectively. Arrows in (c) mark particles without Pd.

At higher resolution with aid of STEM-EELS, one can determine the location of each element at an atomic level in relation to the others. Figure 8 shows a typical analysis of how Pt shell thickness can be determined. Figure 8e shows a line profile of the area indicated in Figure 8d. It was found that the Pt shell thickness is not truly uniform. Areas with thick and thin Pt and, infrequently, without Pt could be observed.

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Figure 8. (a) HAADF-STEM of an as-received PtML/Pd/C nanoparticle along a zone axis where the atoms were not directly visible, along with EELS analysis of (b) Pd M4,5 edges; (c) Pt M4,5 edges and (d) a composite showing both Pd and Pt elemental distribution. The line profile in (e) was drawn to measure the Pt shell thickness across the direction which was shown in (d).

We determined that about half of the nanoparticles of the as-received catalyst powder had incomplete Pt shell coverage. After 1 day of MEA testing (BOL), almost all of the nanoparticles showed core-shell structure. This result along with the above-discussed EPMA result suggest that incomplete Pt shell coverage leads to the rapid Pd dissolution within 1 day of fuel cell operation. After voltage cycling (EOT), despite slightly smaller particle size, the Pt shell thickness was mostly unchanged from the BOL. There was some increase in small pure Pt nanoparticles (< 2 nm) which is likely a result of complete Pd depletion. These results suggest that the stability of the catalyst can be improved substantially if the quality of the Pt shell is improved. It is important to note that the catalyst used in this study was the first-generation scaled up catalyst and should not be considered as a limitation of the approach but as a potential for further improvement. Progresses in improving the Pt shell quality as well as the stability and cost of the core materials are underway and will be reported elsewhere.

CONCLUSIONS Fuel cell performances of PtML/Pd/C and conventional Pt/C were compared at different levels of Pt loading. Pt/C electrodes with low Pt loading showed substantial voltage fall-off 14


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particularly at high current density. In contrast, at the same Pt loading PtML/Pd/C showed enhanced fuel cell performance. Future catalyst and electrode development efforts should be made to meet both efficiency and power requirements at both low and high current densities. Improving and stabilizing the catalyst surface area is as important as the ORR activity. The stability of the Pd core is also an obvious concern and the cost of Pd cannot be ignored. Search for a lower cost and durable core should be one of the focal research areas.

ACKNOWLEDGMENTS The authors thank Roland Koestner for facilitating electrode optimization; Puneet Sinha, Thomas Greszler, and Wenbin Gu for providing 1-D fuel cell-model and helpful discussion; Frederick Wagner, Mark Mathias, and Frank Coms for insightful comments.

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REFERENCES (1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12) (13) (14) (15) (16) (17) (18) (19) (20) (21) (22) (23) (24) (25) (26) (27)

Wagner, F. T.; Lakshmanan, B.; Mathias, M. F. J. Phys. Chem. Lett. 2010, 1 (14), 2204– 2219. Debe, M. K. Nature 2012, 486 (7401), 43–51. Greszler, T. a.; Caulk, D.; Sinha, P. J. Electrochem. Soc. 2012, 159 (12), F831–F840. Ohma, A.; Mashio, T.; Sato, K.; Iden, H.; Ono, Y.; Sakai, K.; Akizuki, K.; Takaichi, S.; Shinohara, K. Electrochim. Acta 2011, 56 (28), 10832–10841. Nonoyama, N.; Okazaki, S.; Weber, A. Z.; Ikogi, Y.; Yoshida, T. J. Electrochem. Soc. 2011, 158 (4), B416. Owejan, J. P.; Owejan, J. E.; Gu, W. J. Electrochem. Soc. 2013, 160 (8), F824–F833. Weber, A. Z.; Kusoglu, A. J. Mater. Chem. A 2014, 2 (c), 17207–17211. Darling, R. M.; Meyers, J. P. J. Electrochem. Soc. 2003, 150 (11), A1523-A1527. Peuckert, M.; Yoneda, T.; Betta, D.; Boudar, M. J. Electrochem. Soc. 1986, 133 (5), 944947. Zhang, J.; Mo, Y.; Vukmirovic, M. B.; Klie, R.; Sasaki, K.; Adzic, R. R. J. Phys. Chem. B 2004, 108 (30), 10955–10964. Sasaki, K.; Naohara, H.; Cai, Y.; Choi, Y. M.; Liu, P.; Vukmirovic, M. B.; Wang, J. X.; Adzic, R. R. Angew. Chemie Int. Ed. 2010, 49 (46), 8602–8607. Gasteiger, H. A.; Kocha, S. S.; Sompalli, B.; Wagner, F. T. Appl. Catal. B Environ. 2005, 56 (1-2), 9–35. Baker, D. R.; Caulk, D. A.; Neyerlin, K. C.; Murphy, M. W. J. Electrochem. Soc. 2009, 156 (9), B991-B1003. Subramanian, N. P.; Greszler, T. a.; Zhang, J.; Gu, W.; Makharia, R. J. Electrochem. Soc. 2012, 159 (5), B531-B540. Liu, Y.; Ji, C.; Gu, W.; Baker, D. R.; Jorne, J.; Gasteiger, H. A. J. Electrochem. Soc. 2010, 157 (8), B1154-B1162. Debe, M. K. Advanced Cathode Catalysts and Supports for PEM Fuel Cells; DOE Annual Merit Review, 2009. Huang, Y.; Zhang, J.; Liu, Y.; Subramanian, N.; Wagner, F. T.; Jorne, J.; Li, J. C. M. ECS Trans. 2011, 41, 1009–1020. Wang, J. X.; Inada, H.; Wu, L.; Zhu, Y.; Choi, Y.; Liu, P.; Zhou, W. P.; Adzic, R. R. J. Am. Chem. Soc. 2009, 131 (18), 17299–17302. Gabrielli, C.; Grand, P. P.; Lasia, A.; Perrot, H. J. Electrochem. Soc. 2004, 151 (11), A1937A1942. Van Der Vliet, D. F.; Wang, C.; Li, D.; Paulikas, A. P.; Greeley, J.; Rankin, R. B.; Strmcnik, D.; Tripkovic, D.; Markovic, N. M.; Stamenkovic, V. R. Angew. Chemie - Int. Ed. 2012, 51 (111), 3139–3142. Yun, S.; Ted Oyama, S. J. Memb. Sci. 2011, 375 (1-2), 28–45. Montes de Oca, M. G.; Kumarakuru, H.; Cherns, D.; Fermín, D. J. J. Phys. Chem. C 2011, 115 (21), 10489–10496. Ohma, A.; Yamamoto, S.; Shinohara, K. ECS Trans., 2007, 11, 1181–1192. Endoh, E.; Hommura, S.; Terazono, S.; Widjaja, H.; Anzai, J. ECS Trans. 2007, 11 (1), 1083–1091. Macauley, N.; Ghassemzadeh, L.; Lim, C.; Watson, M.; Kolodziej, J.; Lauritzen, M.; Holdcroft, S.; Kjeang, E. ECS Electrochem. Lett. 2013, 2 (4), F33–F35. Macauley, N.; Alavijeh, a. S.; Watson, M.; Kolodziej, J.; Lauritzen, M.; Knights, S.; Wang, G.; Kjeang, E. J. Electrochem. Soc. 2014, 162 (1), F98–F107. Macauley, N.; Wong, K. H.; Watson, M.; Kjeang, E. J. Power Sources 2015, 299, 139–148.

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Achieving High-Power PEM Fuel Cell Performance with an Ultralow-Pt

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