Sputter-Deposited Pt PEM Fuel Cell Electrodes: Particles ... - CiteSeerX

Journal of The Electrochemical Society, 156 共5兲 B614-B619 共2009兲

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0013-4651/2009/156共5兲/B614/6/$23.00 © The Electrochemical Society

Sputter-Deposited Pt PEM Fuel Cell Electrodes: Particles vs Layers M. D. Gasda,a R. Teki,b T.-M. Lu,c,* N. Koratkar,d G. A. Eisman,a,* and D. Galla,z a Department of Materials Science and Engineering, bDepartment of Chemical and Biological Engineering, cDepartment of Physics, Applied Physics and Astronomy, and dDepartment of Mechanical, Aerospace and Nuclear Engineering, Rensselaer Polytechnic Institute, Troy, New York 12180, USA

Platinum catalyst layers with Pt loadings w = 0.05–0.40 mg/cm2 were deposited by magnetron sputtering from a variable deposition angle ␣ onto gas diffusion layer 共GDL兲 substrates and tested as cathode electrodes in proton exchange membrane 共PEM兲 fuel cells using Nafion 1135 membranes and Teflon-bonded Pt-black electrode 共TBPBE兲 anodes. Layers deposited at normal incidence 共␣ = 0°兲 are continuous and approximately replicate the rough surface morphology of the underlying GDL. In contrast, glancing angle deposition 共GLAD兲 with ␣ = 87° and continuous substrate rotation yields highly porous layers consisting of vertically oriented Pt particles, 100–500 nm high and 100–300 nm wide, that are separated by 20–100 nm. The particle electrodes exhibit a higher 共lower兲 mass-specific performance than the continuous-layer electrodes for a high 共low兲 current density i. This is attributed to a higher porosity but lower overall electrochemically active surface area for the particles compared to the continuous layer. Increasing w in particle cells from 0.05 to 0.10 to 0.18 mg/cm2 yields increasing potentials, but w = 0.40 mg/cm2 causes a voltage drop at i ⬎ 0.4 A/cm2, associated with the reduced pore density at large w. Comparison cells with a TBPBE cathode exhibit comparatively low Tafel slopes but a lower Pt mass specific performance than the sputtered catalysts. Quantitative analyses of kinetic and mass-transport losses in the polarization curves suggest a competing microstructural effect, favoring mass-transport performance and an efficient oxygen reduction reaction for particle and continuous layer electrodes, respectively. The overall results suggest that in addition to the well-known promise of sputter-deposited Pt catalysts as an approach to increase Pt utilization at low loading, GLAD provides the unique ability to control Pt porosity and to achieve efficient reactant flow for high-currentdensity operation. © 2009 The Electrochemical Society. 关DOI: 10.1149/1.3097188兴 All rights reserved. Manuscript submitted December 12, 2008; revised manuscript received February 2, 2009. Published March 24, 2009.

Polymer electrolyte membrane or proton exchange membrane 共PEM兲 fuel cells are energy conversion devices that show promise for future use in portable, stationary, and transportation applications.1 A key hurdle for mass-market adoption of fuel cell technology is overall system cost, driven in large part by materials and materials processing costs in the stack. Therefore, a considerable effort in fuel cell research is directed toward minimizing Pt loadings for a given target efficiency.2 A Pt catalyst loading w of 4.0 mg/cm2 is typical for Teflon-bonded Pt-black electrodes 共TBPBE兲. TBPBEs consist of a hydrophobic, porous structure of micrometer-sized Pt agglomerates and were used in early space applications due to their high efficiency and long lifetime.3,4 Further optimization and the addition of ionomer to the electrode surface resulted in the reduction of TBPBE loading to below w = 1 mg/cm2 without significant performance loss.5 Current fuel cell technology mostly employs ionomer-bonded thin-film electrodes with 2–5 nm wide Pt nanoparticles supported on carbon 共designated Pt/C兲, with typical w = 0.4 mg/cm2. However, the carbon support also introduces some challenges, including reduced operating lifetimes due to carbon corrosion,6 peroxide attack of the membrane,7 and sintering and growth of the Pt particles over time.8 Further reduction of w below 0.4 mg/cm2 typically results in voltage losses consistent with the fundamental kinetic losses associated with the oxygen reduction reaction 共ORR兲.9 Sputter deposition of Pt has been explored by various researchers as a potential path for manufacturing ultralow Pt loading electrodes in a controlled process. Pt has been sputtered onto the gas diffusion layer 共GDL兲,10-17 the membrane,17-22 or deposited onto poly共tetrafluoroethylene兲共PTFE兲 sheets or similar support substrates and subsequently transferred to the membrane.23,24 Most researchers found that the sputtered Pt morphology strongly affects fuel cell performance and have reported increases in catalyst utilization through roughening the membrane,19 patterning the catalyst,20,21 increasing the Pt porosity,13,14 depositing on polymer nanowhiskers,24 or selecting appropriately rough GDL substrates.10,11 Depositing ad-

* Electrochemical Society Active Member. z

E-mail: [email protected]

ditional conductive layers such as Cr,12 carbon,22,25 Au,23 or Nafioncarbon ink16-18 also affects performance by enhancing the gas transport or electron conduction within the electrode. Work is already in progress to characterize the angular distribution of the sputtered Pt flux for the possible application of such methods to commercial manufacturing.26 Rabat and Brault22 deposited carbon layers at 45° with respect to the substrate, coated the resulting columnar morphology with Pt, and found a higher open-circuit voltage of the assembled fuel cell as compared to a commercial electrode. Sputtering Pt onto carbon nanofibers grown directly on the GDL results in 3.6⫻ higher Pt utilization as compared to a commercial cathode.25 In this paper, we present an alternative approach to engineer the nanoscale structure of fuel cell electrodes by varying the deposition angle during sputtering of Pt onto the GDL. Normal-incidence deposition results in a conformal, nearly continuous layer that covers the porous surface of the GDL. In contrast, deposition from a highly oblique angle of ␣ = 87° causes Pt to nucleate preferentially on raised areas of the GDL surface and form a porous coating consisting of vertically oriented Pt particles. This approach is based on the glancing-angle deposition 共GLAD兲 technique27 which exploits atomic shadowing effects during line-of-sight physical vapor deposition to engineer nanorods into various shapes such as nanopillars,28-30 zigzags,31,32 nanospirals,33 nanotubes,34 twocomponent rods,35-37 and branched nanocolumns,38-40 with such potential applications as photonic crystals,33 sensors,32 catalyst supports,41 magnetic storage media,42 radiation-resistant coatings,43 and field emitters.44 We evaluate the deposited Pt layer and particle electrodes as cathodes in PEM fuel cells in order to understand the effect of GLAD on the Pt utilization. At high power, particle cells have higher Pt utilization than cells with “normally deposited” continuous layers of Pt. The superior mass-transport performance of the Pt-particle layers is due to the porous, columnar morphology. Highporosity Pt layers formed by GLAD offer a potential path to highperformance sputtered fuel cell electrodes. Experimental All electrodes were deposited onto the microporous layer 共MPL兲 of as-received, commercially available GDLs 共SIGRACET 35DC supplied by SGL Technologies, GmbH兲. The MPL is a Teflon-

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Figure 1. 共Color online兲 Schematics of Pt layer deposition onto MPL substrates with corresponding cross-sectional SEM micrographs from fuel cells after testing, for the case of 共a兲 normal deposition 共␣ = 0°兲 and 共b兲 GLAD 共␣ = 87°兲, yielding a continuous Pt layer and separated Pt particles, respectively. The dashed lines on the left in the micrographs indicate the position of the Pt 共bright contrast兲 which is deposited on the MPL 共bottom兲 and is partially embedded in the membrane electrolyte 共top兲.

bonded layer of carbon particles that provides mechanical support for the electrode. The MPL substrates were introduced into a magnetron sputter deposition system with a base pressure of ⬍8 ⫻ 10−7 Torr, placed 15 cm from a 7.6 cm diam Pt target 共Plasmaterials, 99.99% pure兲, tilted by ␣ = 0 and 87° for continuous layer and particle depositions, respectively, and rotated about the surface normal at 0.5 Hz 共30 rpm兲 in order to achieve particle orientation normal to the surface. It is important to clearly define our geometry by noting that ␣ is the angle between the substrate normal and a line drawn through the centers of the target and the substrate as in Ref. 27. Ar 共99.999% pure兲 was introduced into the system to reach a constant pressure of 2.3 mTorr. No external heating was applied to the substrate during the deposition process. A constant dc power of 200 W yielded deposition rates of 2 and 0.3 nm min−1 for the Pt layer and particles, respectively. This corresponds to Pt loading rates of 0.05 and 0.007 mg cm−2 min−1, as determined using differential weighting of 0.2 and 1.5 mg samples in an electronic semimicrobalance 共Sartorius 2024 MP6, standard deviation ⫾0.02 mg兲. The sputter-deposited electrodes were integrated as cathodes into fuel cells with an active area of 10 cm2. Nafion 1135 polymer electrolyte membranes 共Ion Power兲, 1100 equivalent weight and 89 ␮m thick, were activated by boiling in 0.5 M H2SO4 for 30 min, followed by three separate 30 min boiling steps in distilled water, and stored in distilled water until being assembled into a membraneelectrode assembly 共MEA兲. Anode 共counter兲 electrodes were TBPBE with 1 mg/cm2 Pt and were prepared following the procedure described in Ref. 5. To facilitate electrode/electrolyte interfacial contact, a 15 wt % dispersion of Nafion in water, isopropanol, and methanol 共Ion Power Liquion LQ-1115兲 was applied to the surface of the electrodes before bonding to the membrane. After this coating dried, the electrodes contained 5–10 wt % Nafion. Finally, electrodes were bonded to the membrane at a pressure and temperature of 350 psi 共2.4 MPa兲 and 126°C for 5 min. Fuel cells were operated at 70°C with atmospheric pressure H2 and air reactants that were humidified to a dew point of 70°C 共100% relative humidity兲. Prior to performance testing, all cells were “incubated” by applying a constant voltage of 0.4 V and allowing the output current to stabilize for 24 h. H2 and air flow rates were set to stoichiometric ratios of 1.2 and 2.0, respectively, for 1.0 A/cm2, corresponding to 84 and 333 sccm. The raw polarization curves were corrected for shorting, crossover, and internal ohmic resistance 共iR兲 in order to isolate the performance of the cathode electrode from confounding effects, as discussed in the Results section. Typical values for the current correction due to shorting and crossover were 5–6 mA/cm2, as determined by applying H2 and N2 to the anode and cathode, respectively, and measuring the steady-state H2 crossover current at high cathode potential.9 The iR correction for

the voltage, ranging from 0.15 to 0.30 ⍀ cm2, was obtained from the slope of the linear polarization curve of the cell in hydrogen pumping mode, as described in Ref. 5. Electrochemical surface area 共ECSA兲 measurements were performed via cyclic voltammetry 共CV兲 with a Parstat 2273 共Princeton Applied Research兲 under humidified N2 at 70°C and a scan rate of 20 mV/s. ECSA was calculated from the raw CV curve by integration under the H-underpotential deposition region after subtracting the doublelayer charging current as described in Ref. 45. After fuel cell testing, MEAs were prepared for scanning electron microscopy 共SEM兲 analysis by freeze-fracturing after immersion for 15 min in liquid N2. SEM micrographs were collected on a Carl Zeiss Supra SEM in secondary electron mode at 2.5 kV. Results and Discussion The schematics in Fig. 1 illustrate the geometry for 共a兲 normal and 共b兲 glancing-angle deposition of the Pt catalyst onto the MPL. In the case of normal deposition, where the angle between the substrate surface normal and the deposition flux ␣ = 0°, the Pt forms a continuous layer that approximately replicates the rough surface morphology of the underlying MPL, as shown on the left side in Fig. 1a. The corresponding cross-sectional scanning electron micrograph on the right is from an electrode with w = 0.25 mg/cm2. It shows at the bottom the 30–50 nm wide carbon nanoparticles from the uppermost part of the MPL and at the top the membrane electrolyte, which exhibits little contrast. The catalyst layer at the interface between MPL and membrane appears bright, because the high-Z Pt exhibits a higher secondary electron yield during SEM imaging. Its measured thickness is 50 nm, which is slightly less than the nominal thickness of 70 nm. We attribute this difference to the roughness of the layer which conforms to the rough MPL surface, resulting in an effective Pt layer area that is larger than the nominal 共flat兲 area. This observation shows that the morphology of the catalyst is strongly affected by the MPL surface, which may explain previous reports indicating a strong impact of substrate morphology on fuel cell performance.10,11 Figure 1b illustrates the case for an oblique deposition angle ␣ = 87°. The incoming Pt atoms preferentially deposit on substrate features that protrude out of the surface, as the flux to the lowerlying GDL is blocked as a result of a line-of-sight deposition process. During continued deposition, this atomic shadowing effect leads to the formation of well-separated particles which grow vertically due to a circular symmetry achieved by continuous substrate rotation about the surface normal. Statistical fluctuations in the substrate roughness and the deposition process cause differences in the growth rates of individual particles and lead to a competitive growth process. The taller particles capture a larger fraction of the incident


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Figure 2. 共Color online兲 Potential E 共iR-corrected兲 vs mass activity im polarization curves for PEM fuel cells with cathodes comprising Pt particles, a continuous Pt layer, and a TBPBE.

Figure 3. 共Color online兲 Potential E 共iR-corrected兲 vs current density i polarization curves for cells with cathodes comprising Pt particles with loadings w = 0.05, 0.10, 0.18, and 0.40 mg/cm2 Pt.

particle flux while their smaller neighbors are shadowed and terminate their growth prematurely, as previously reported for GLAD nanorods.46,47 Consequently, the particles exhibit a relatively large distribution in both height and width, as also observed in the corresponding SEM micrograph on the right of Fig. 1b from a cell with 0.40 mg/cm2 Pt loading. This cross-sectional micrograph shows Pt particles, 100–500 nm high and 100–300 nm wide, that have nucleated on the MPL surface and are embedded in the membrane. The gaps between the particles exhibit widths ranging from 20 to over 100 nm. These pores become partially filled with electrolyte during the drying of the Nafion dispersion and the membrane bonding process, as described above. Figure 2 shows polarization curves for three fuel cells that were assembled using a Pt layer, a Pt particle, and a conventional TBPBE cathode electrode with loadings of 0.25, 0.18, and 0.88 mg/cm2, respectively. The plot shows the iR-corrected potential E vs the mass activity im, which is the current density normalized to the amount of Pt. Therefore, the graph shows a direct comparison of the catalyst utilization efficiency. The potential for the continuous Pt layer decreases with increasing current density for im ⬍ 0.6 A/mg. Such a decrease is typical for PEM fuel cells48 and is consistent with activation polarization. However, E exhibits a distinct drop from 0.67 to 0.37 V as im increases from 0.6 to 1.0 A/mg. This drop is caused by a significant concentration overpotential that dominates the performance of the continuous layer at high current density, indicating poor electrode porosity, as discussed below. The fuel cell with the Pt-particle electrode shows a slightly steeper decrease in E at low current in comparison to the Pt-layer electrode, with E = 0.65 and 0.69 V at im = 0.45 A/mg for Pt particles and the Pt layer, respectively. However, the particle cell shows negligible concentration overpotential, leading to a mass activity of 2.7 A/mg at E = 0.5 V, which is more than a factor of 3 larger than 0.86 A/mg, the value for the continuous layer. Figure 2 also contains a plot of the mass activity from a cell with a TBPBE cathode, which serves in this study as comparison data. This cell with w = 0.88 mg/cm2 exhibits the highest mass activity out of a set of TBPBE cathodes with different values of w. Higher w values result in lower mass activity, because a smaller fraction of Pt/PTFE agglomerates is in ionic contact with the bulk electrolyte, while lower 共w ⬍ 0.88 mg/cm2兲 TBPBE loadings also result in a reduced mass activity, associated with an increasing fraction of the Pt/PTFE agglomerates that seep into the MPL during the application and drying process. This effect results in the smallest possible TBPBE loading within our processing ap-

proach of w ⬇ 0.5 mg/cm2, which is considerably higher than what is achieved by both types of sputtered Pt. The absolute 共nonnormalized current兲 performance of the TBPBE cell is higher than for any sputter-deposited electrode cell of this study. However, in a mass-specific comparison as shown in Fig. 2, the layer electrode V V = 0.068兲 outperforms the TBPBE 共i0.8 = 0.033兲 at a high 共i0.8 m m V = 2.7兲 outperpotential of 0.8 V, while the particle electrode 共i0.5 m 0.5 V forms the TBPBE 共im = 0.77兲 at a low potential E = 0.5 V. We attribute the high performance at low im of the layer electrode to a high Pt fraction that is in direct contact with the electrolyte, facilitated by the relative thinness of this electrode. In contrast, the relatively high voltage of the particle electrode at high current is associated with the excellent electrode porosity, which facilitates oxygen transport from the bulk GDL pores to catalyst sites throughout the electrode. Figure 3 shows plots of the cell potential E vs the current density i for Pt-particle fuel cells with increasing catalyst loadings w = 0.05, 0.10, 0.18, and 0.40 mg/cm2, obtained by increasing the GLAD time from 7.5 to 60 min. All cells show a characteristic decrease in E with increasing i. However, the slope of the E共i兲 curves is less steep for higher Pt loadings, corresponding to a higher current density at a given potential. This indicates that increasing the particle size leads to larger catalytically active surface areas. Adding a given amount of Pt has the greatest effect at low loadings. For example, i at 0.60 V doubles from 0.04 to 0.08 A/cm2 for an increase in w from 0.05 to 0.10 mg/cm2, but only increases by 23%, from 0.17 to 0.21 A/cm2, for an increase in w from 0.18 to 0.40 mg/cm2. The trend of increasing i with increasing w is eventually reversed for the two highest Pt loadings at high current. The voltage of the 0.40 mg/cm2 cell drops below the 0.18 mg/cm2 cell for i ⬎ 0.38 A/cm2. This indicates a mass-transport limitation for w = 0.40 mg/cm2, which we attribute to limited oxygen transport through the pores of the catalyst layer. Increasing w leads to longer and wider particles and, in turn, reduced pore density and longer diffusion paths, resulting in inefficient oxygen transport from the GDL to the active Pt-membrane interface. This causes a drop in E for i ⲏ 0.35 A/cm2 and w = 0.40 mg/cm2. Nevertheless, this drop is much less pronounced than for the electrode with the continuous Pt layer shown in Fig. 2. Figure 4 shows comparative plots of the mass activity vs Pt loading for both particle and layer cells in the low and high current density regimes defined by E = 0.80 and 0.50 V, respectively. The



Figure 4. 共Color online兲 Mass specific activity vs loading w for cells with cathodes comprising a continuous Pt layer or Pt particles at 共a兲 0.80 and 共b兲 0.50 V.

V high-potential mass activity i0.8 for the particle cells is largest for m the smallest loading, 0.11 A/mg for w = 0.05 mg/cm2, and decreases monotonically with increasing w to 0.024 A/mg for w = 0.40 mg/cm2, as shown in Fig. 4a. This decrease is associated with a decrease in the surface-area-to-volume ratio as the particles grow both longer and wider. Larger particles have a larger total surface area, resulting in higher absolute performance, but a smaller fraction of their Pt is at the surface, yielding a lower mass-specific performance. The plot in Fig. 4a also shows, for comparison, the current density for cells with continuous Pt layers. The absolute open-circuit voltage of the continuous layer cell with the lowest Pt V loading 共0.05 mg/cm2兲 was less than 0.80 V, and i0.8 was estim mated by extrapolation from the crossover- and short-corrected current values. This cell has approximately the same mass activity, 0.12 A/mg for w = 0.05 mg/cm2, as the particle cell with the same loading. The nearly identical mass activities of the particle and layer cells with the same w suggests that in the limit of low loading, the active surface area of the particles approaches that of the continuous V layer. The layer with w = 0.25 mg/cm2 exhibits i0.8 m 0.8 V = 0.068 A/mg. This is 30% higher than im for the particles at comparable loadings. That is, the continuous layer has a higher Pt utilization at 0.80 V than the particles. We attribute this, as noted above, to a higher fraction of Pt that is in direct contact with the membrane for the layer than for the particles. The TBPBE compariV = 0.034 A/mg. This value is higher than what son cell exhibits i0.8 m would be expected if the mass activity vs w curve for particle cells were extrapolated to a high w = 0.88 mg/cm2. However, all particle

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V cells with w 艋 0.18 mg/cm2 exhibit higher i0.8 -values than that of m the TBPBE cell, which are loadings that are too low to achieve with the TBPBE architecture, as discussed above. The plot of the low-potential 共high current兲 mass activity, shown in Fig. 4b for the same cells as in 共a兲, reveals different trends. The V particle electrodes exhibit a maximum in i0.5 of 2.7 A/mg at an m 2 intermediate loading of w = 0.18 mg/cm . The mass activity decreases slightly with decreasing w, indicating that flooding by product water of the smaller pores 共at smaller w兲 is more likely. As w is V decreases to 1.1 A/mg. This is atincreased to 0.40 mg/cm2, i0.5 m tributed to the decreased pore density and increased pore length with increasing w, because longer pores result in decreased oxygen flux, and lower pore density combined with wider particles increases the oxygen diffusion path through the electrolyte to access active sites on the particle tips that are embedded in the membrane. The lowpotential mass activity of the continuous layers, also shown in Fig. 4b, is by more than a factor of 2 lower than that for the particle cells with comparable w. This dramatic difference is attributed to masstransport-limiting effects of the continuous layers. The low porosity of the layer inhibits efficient reactant 共oxygen兲 distribution or product 共water兲 rejection. The TBPBE comparison cell has a mass activV for all particle ity of 0.76 A/mg at 0.5 V, which is lower than i0.5 m electrodes but within the range of the continuous layer electrodes. The polarization curves for all fuel cells shown in Fig. 2-4 were analyzed using a semiempirical model which provides insight into kinetic and mass-transport losses as described in greater detail in Ref. 49. The fuel cell voltage E at the current density i is described by

关1兴

E = E0 − iR − b log i − m exp共ni兲 where E0 is given by

关2兴

E0 = ER + b log i0

ER is the reversible open-circuit voltage which is 1.17 V at 70°C,50 R is the ohmic resistance which was measured independently and is controlled by proton transport in the membrane as well as electron transport within the cell components, i0 is the exchange current density, i.e., the equilibrium rate of reaction at the electrode/electrolyte interface, and b is the characteristic slope associated with the Tafel relationship of current and overpotential.51 The last term in Eq. 1 is an empirical quantity that describes the high current density overpotential which is associated with mass transport. A nonlinear curve fit routine was used to determine the values of the unknown parameters E0, b, m, and n in Eq. 1 for each polarization curve, as summarized in Table I and plotted with the measured data on a logarithmic scale in Fig. 5. The Tafel slope b is lowest for the cell with a continuous w = 0.25 mg/cm2 Pt layer and for the TBPBE comparison cell, with b = 97 and 101 mV/dec, respectively. The particle electrodes have considerably higher slopes, ranging from 142 to 171 mV/dec, and the low loading w = 0.05 mg/cm2 continuous layer Pt cell has 201 mV/dec. We attribute the higher Tafel slope for the particles to losses within the electrode,52 because the slope is expected to double when the ORR rate is determined by reaction kinetics plus either 共i兲

Table I. Kinetic and mass-transport parameters given by Eq. 1. R is the slope extracted from the hydrogen pumping polarization curve as discussed in the text, and E0, b, m, and n are obtained from a nonlinear curve fit.

Electrode Layer Particles

TBPBE

Loading 共mg/cm2兲

R 共⍀ cm2兲

E0 共mV兲

b 共mV/dec兲

m 共mV兲

0.05 0.25 0.05 0.10 0.18 0.40 0.88

0.294 0.156 0.241 0.204 0.200 0.152 0.310

810 927 869 922 943 934 946

201 97 171 170 153 142 101

1.08 ⫻ 10−1 4.54 6.50 ⫻ 10−2 4.71 ⫻ 10−2 4.65 ⫻ 10−1 1.74 ⫻ 10−1 1.09 ⫻ 10−6

n 共cm2 /mA兲 3.20 1.68 5.28 3.07 8.63 1.31 2.72

⫻ ⫻ ⫻ ⫻ ⫻ ⫻ ⫻

10−2 10−2 10−2 10−2 10−3 10−2 10−2


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Journal of The Electrochemical Society, 156 共5兲 B614-B619 共2009兲 Conclusion Cathode electrodes consisting of Pt layers and particles were deposited onto GDL substrates by sputtering from deposition angles of 0 and 87°, respectively, and integrated into PEM fuel cells. A comparison of polarization curves from cells with Pt layers and particles shows that the mass-specific activity is higher for Pt layers at low current densities but higher for Pt particles at high current densities. These trends are attributed to a higher catalytically active surface area for the Pt layer but a more efficient mass transport 共oxygen or water兲 for the particles. These results illustrate the performance tradeoff associated with the Pt surface area and porosity and demonstrate that the GLAD technique is a promising method to control the catalyst nanostructure in that regard. Acknowledgments

Figure 5. 共Color online兲 Tafel plots for cells with cathodes containing Pt particles, continuous Pt layers, and a TBPBE. The solid lines through the measured data points are from a fit using Eq. 1.

This work was performed within the Center for Fuel Cell and Hydrogen Research at Rensselaer Polytechnic Institute and was supported by the National Science Foundation through the Integrated Graduate Education and Research Traineeship on fuel cells with award no. DGE 0504361, as well as through award no. ECCS 0506738, ECCS 0403789, and CMMI 0727413. Rensselaer Polytechnic Institute assisted in meeting the publication costs of this article.

References oxygen transport within the electrode or 共ii兲 proton migration within the electrode.53 We expect the former, because the particles have been coated with a solution of ionomer and are therefore fully immersed in the membrane electrolyte 共Fig. 1兲, making the oxygen diffusion within the electrode the probable rate-determining step. We attribute the relatively high Tafel slope for the continuous layer cell with w = 0.05 mg/cm2 also to limited oxygen transport, because the fraction of Pt that is immersed in ionomer increases with decreasing w. The parameters m and n quantify the mass transport limitation at high i, revealing the enhanced reactant diffusion for particle vs continuous layer electrodes. The current density at which the mass transport overpotential m exp共ni兲 = 50 mV is ⬃4⫻ higher for particle cells 共0.54 A/cm2 at w = 0.18 mg/cm2兲 than for continuous layer cells 共0.14 A/cm2 at w = 0.25 mg/cm2兲. The ohmic resistance R increases with decreasing w, because lower Pt loadings yield smaller contact areas with the membrane electrolyte, as reported in Ref. 54. The effect of Pt loading on resistance highlights the importance of correcting for iR in order to isolate the cathode electrode; it is otherwise possible to misinterpret high ohmic resistance as poor kinetic or mass transport performance, particularly at high current density. ECSA measurements done by CV indicate that layer cells have more Pt in contact with the electrolyte than particle cells, in perfect agreement with the above discussion. For example, continuous layer electrodes with w = 0.05 mg/cm2 have ECSA values between 18.5 and 19.8 mC/cm2, whereas particles at the same low loading exhibit 15.9–18.0 mC/cm2. Increasing w results in higher ECSA, reaching 19.4–20.5 mC/cm2 for layers with w = 0.25 mg/cm2 and 19.0–19.9 mC/cm2 for particle cells with w = 0.40 mg/cm2. That is, particle electrodes require 60% more platinum than layer electrodes to achieve a comparable ECSA. The TBPBE comparison electrode 共w = 0.88 mg/cm2兲 exhibits 38.3–38.7 mC/cm2, which is almost a factor of 2 higher than the value for any sputtered electrode in this study, however, with 3.5⫻ more Pt. The higher ECSA of the continuous layers confirms that they have more Pt in contact with the membrane, which explains the superior efficiency of layer cells at high potential. The ESCA results also indicate that the high current at low potential of the particle electrodes cannot be explained by the presence of additional active catalyst area on the particles and therefore must be due to the enhanced reactant transport within these porous electrodes at high oxygen demand.

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