F676 Journal of The Electrochemical Society, 159 (11) F676-F682 (2012)0013-4651/2012/159(11)/F676/7/$28.00 © The Electrochemical Society

Development of Dispersed-Catalyst/NSTF Hybrid ElectrodeAnusorn Kongkanand,a,∗,z Jeanette E. Owejan,a Scott Moose,b Matthew Dioguardi,cMahesh Biradar,b and Rohit Makhariaa

aElectrochemical Energy Research Lab, General Motors Global Research and Development, Honeoye Falls,New York 14472, USAbGeneral Motors Global Powertrain Engineering, Honeoye Falls, New York 14472, USAcTrison Business Solutions, Inc., Leroy, New York 14482, USA

Ultrathin electrodes, such as the 3M Nanostructured Thin Film (NSTF) electrode, provide a plausible pathway to reduce platinum costin low temperature fuel cells. However, several operational shortcomings, involving relatively poor electrode proton conduction andtendencies to collect water in the cathode, were observed in our fuel cell tests. This can be greatly mitigated when a few-micron thickdispersed-catalyst layer is placed adjacent to the NSTF layer, forming a dispersed-catalyst/NSTF hybrid electrode. This dispersed-catalyst layer is also called the “interlayer” because it is located between the NSTF layer and the microporous layer of the cathodediffusion medium. In this study, development of the hybrid electrode was pursued. Emphasis on developing lab-scale fabricationmethods that can easily translate to roll-to-roll manufacturing process was a key element of the hybrid electrode development. Thefuel cell performance of the electrode showed high sensitivity to fabrication methods. When the dispersed-catalyst layer was coateddirectly on the NSTF electrode, voltage at high current density dropped significantly. The voltage loss was surmised to be causedby ionomer seepage into the NSTF layer during the coating process. This voltage loss could be eliminated by placing the dispersed-catalyst layer on the gas-diffusion layer and then located adjacent to the NSTF cathode. Interaction between the dispersed-catalystand NSTF layers and how it affects the fuel cell performance is discussed.© 2012 The Electrochemical Society. [DOI: 10.1149/2.023211jes] All rights reserved.

Manuscript submitted June 28, 2012; revised manuscript received August 2, 2012. Published August 31, 2012.

In a proton exchange membrane fuel cell (PEMFC), Pt or Pt alloynanoparticles supported on high surface area carbon particles are gen-erally used in the cathode. The highly dispersed nature of the catalystallows for good utilization of the precious metal. However, the elec-trochemical stability of the carbon support as well as the dissolution ofPt nanoparticles raise concerns about the durability of the catalyst.1–4

3M’s Nanostructured Thin Film (NSTF) is an extended surface cat-alyst which consists of a thin film of Pt alloy coated on individ-ual lath-shaped single crystalline whiskers of an organic compound,perylene red (PR149).5–7 The ∼0.5–1 μm long support whiskers areelectrically non-conductive, thereby greatly reducing their suscepti-bility to electro-oxidative corrosion. The continuous layer of Pt servesas the electrical conductor for the electrode. Due to the low curvature(smaller number of low-coordination-number Pt atoms) and bulk-likecharacteristics of Pt in this form, NSTF electrodes exhibit higherchemical and electrochemical stability and 5-10 times higher Pt-area-specific ORR activity than dispersed carbon supported catalysts.7–10

In addition, owing to its unique structure of an extended surface of Ptcovering a support, one expects that the Pt or Pt-alloy layer thickness(and therefore the Pt loading) could be reduced while maintaining theelectrochemical properties of the catalyst.11 Therefore, NSTF catalysttechnology provides a plausible pathway for achieving higher ORRactivity using small amounts of Pt.

Despite the apparent potential advantages of NSTF electrodes, sev-eral operational shortcomings were observed in the fuel cell.12–15 TheNSTF membrane-electrode assembly (MEA) generally show highersusceptibility to cell reversal in transient operations such as currentramp-up or cold start-up tests16 compared to conventional dispersed-catalyst electrodes with similar Pt loading. Under steady-state op-eration although NSTF shows comparable fuel cell performance toconventional dispersed Pt/C electrodes at temperature higher than70◦C, it can show poor performance at lower temperatures as com-pared with conventional dispersed-catalyst cathode due to electrodeflooding.12 These operational shortcomings can result in a more com-plicated and costly fuel cell system. The lack of operational robustnesscan be attributed to the small electrode thickness and pore volume ofNSTF electrodes.16–18 The absence of ionomer in the metal-rich elec-trode makes the electrode very hydrophilic and susceptible to liquidwater accumulation. It also causes relatively poor electrode protonconduction and inefficient Pt utilization. 3M has shown that modifi-

∗Electrochemical Society Active Member.zE-mail: anusorn.kongkanand@gm.com

cation of the anode gas diffusion layer, utilization of thin membrane,and application of reduced anode reactant pressures can improve theoperational robustness by transporting product water to the anodeside.13,19,20 However, since most enhancement occurs at sub-ambientpressure, this poses a system trade-off which reduces fuel cell effi-ciency due to anode purges required at shorter frequencies and likelyincreases cost and complexity.

Recently we reported that a placement of an additional layer ofdispersed Pt/C catalyst between the NSTF layer and microporouslayer (Figure 1) could significantly enhance water removal capabilityand water storage capacity of the electrode.18 The layer is referredto as the interlayer (IL). Although the benefits of the IL were de-scribed, the great sensitivity of the preparation methods on the fuelcell performance was not discussed. This study describes challengesin processing and ultimate manufacturable method of producing ahigh performing interlayer/NSTF electrode. First, the initial proof-of-concept results of the interlayer made by hand-spray technique arepresented. Then, the development of a manufacturable coating pro-cess using slot-die technique is introduced, and the resulting issueconcerning high-current-density voltage loss is discussed. Finally, animproved process is demonstrated in mitigating the voltage loss. Thelikely origin of the voltage loss is discussed, and the key structuralfactors enabling a high performing interlayer/NSTF are suggested.

Experimental

Materials.— Fully assembled MEAs with NSTF PtCoMn cata-lysts (atomic ratio 68:28:3, respectively) on both cathode and anodewere provided by 3M company (St. Paul, MN, USA). The catalystsubstrates and membrane were laminated using 3M’s full-scale roll-good process.21 The anode and cathode Pt loadings were 0.10 and0.15 mgPt/cm2, respectively, unless stated otherwise. 35 μm thick850 equivalent weight (EW) 3M perfluorosulfonic acid (PFSA) mem-branes and 14 μm thick 3M 850 EW internally reinforced Goremembranes were used in this study. Non-platinized Vulcan carbon(XC-72) and platinized graphitized Vulcan carbon (20 wt% and50 wt% Pt/GrV) were received from Tanaka Kikinzoku Kogyo(Japan). The gas diffusion layers (GDL) were made from 200 μmthick Teflon-coated carbon fiber diffusion media (DM) with 30-μmmicroporous layers (MPL). All chemicals used were reagent grade.

Interlayer fabrications.— Three types of interlayers (ILs) werediscussed in this study: (1) IL coated on an MEA using a hand-spray

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Journal of The Electrochemical Society, 159 (11) F676-F682 (2012) F677

Figure 1. Schematic illustrations of standard NSTF (a) and NSTF with Pt/Cinterlayer (b) electrodes. MPL stands for microporous layer. The figure showsthat all individual whiskers are connected together with an open network of Ptand perylene red on the back side of the NSTF layer.

technique, (2) IL coated on a MEA using a slot-die technique, and(3) IL coated on a GDL using a slot-die technique. These are referredto as HS-MEA-IL, SD-MEA-IL and SD-GDL-IL interlayers, re-spectively. The lab-scale hand-sprayed interlayer was made by spraycoating a solution of water, alcohol, carbon particles, and ionomerdirectly on to a NSTF MEA as it was held flat on a heated (60◦C) vac-uum table. Following the coating step, the MEA was dried under aninfrared heater. The slot-die coated interlayers were applied either onthe MEA or gas diffusion layer. These are referred to as SD-MEA-ILand SD-GDL-IL, respectively. In the case of SD-MEA-IL, a solutionof water, alcohol, Pt/GrV, and ionomer was directly coated on to aNSTF MEA with the anode catalyst decal backing layer attached.The anode catalyst backing layer prevents the MEA from swellingin a lateral direction during the IL coating process. The solutionwet-lay-down thicknesses were about 50 μm. Dilution of Pt/GrVwith non-platinized GrV was used in order to increase the targetthickness while keeping the Pt loading low. The interlayer thicknesseswere estimated assuming 0.04 mgcarbon/cm2/μm, and were confirmedby scanning electron microscopy (SEM). In the case of SD-GDL-IL,a similar coating formulation and process was used but they werecoated on a GDL. The typical IL Pt loadings and thicknesses were inthe range of 0.010 to 0.025 mgPt/cm2 and 1 to 5 μm, respectively.

Measurements.— It was shown previously that NSTF electrodeswe have tested show very high fuel cell performance sensitivity to gasreactant humidification.15 Slight differences in mechanization amongtest stations and run-to-run variation led to greater scatter in perfor-mance, especially under dry conditions. This warranted the need tohave multiple replicates for experiments with NSTF electrodes in or-der to determine whether the observed result is statistically significant.Premature conclusions drawn from limited experimental data are of-ten incorrect. It was found that a short stack provides an excellentplatform in testing different NSTF MEAs as it ensures the same hu-midification across each cell within the same stack. In this study, fuelcell performance at high temperature and dry conditions was testedon a short stack platform unless stated otherwise. On the other hand,

fuel cell performances at low temperature and wet conditions weretested on a single cell platform due to ease of voltage control.

Fuel cell performance was evaluated in either a small-scale singlecell platform with an active area of 53 cm2 or a 29-cell short stackplatform with a large active area of 360 cm2. All MEAs were pre-conditioned using 3M’s recommended procedure.14 The procedureinvolves 40 repeated cycles of a voltage-cycling step at 75◦C in H2/airfollowed by a cool-down step to room temperature, while both anodeand cathode flow-field channels are filled with deionized water. Inthe 53-cm2 platform, the fuel cell polarization curves were measuredat two different operating conditions, in the order of anode/cathode:(1) 80◦C condition; H2/air; 80◦C; 60%/60% relative humidity (RH)in;150 kPaabs; stoichiometry = 2/2 and (2) 40◦C condition; H2/air; 40◦C;100%/50% RHin; 150 kPaabs; constant flows of 0.8/1.8 standard literper minute. In the short stack platform, the fuel cell polarization curveswere measured under a condition close to H2/air; 80◦C; 32%/32%RHin; 150 kPaabs; stoichiometry = 1.5/1.8.

Results and Discussion

The interlayer can be coated either on the MEA or on the GDLas shown in Figure 2. NSTF MEAs are fabricated in a way thatthey present a corrugated feature with the pitch-to-pitch distance of6 μm and the peak-to-valley distance of 3 μm. This is recommendedby 3M to increase catalyst utilization and to facilitate its roll-to-rollprocessing.21 Considering this geometry, it may be difficult to achievesufficient proton conduction between the two layers if the IL is notcoated directly on the MEA. In addition, if the interface (IL/NSTF orIL/MPL interface) is poor, liquid water may accumulate between thelayers and impose large gas transport resistance. Our general approachis to coat the IL directly on the MEA. On the other hand, coating theIL on the GDL may prove to be a more process-friendly choice dueto the ease of handling.

Hand-sprayed interlayer.— As shown earlier, the NSTF electrodeis very thin and hydrophilic, allowing liquid water to accumulate inthe electrode pores, hence hindering oxygen transport.18 AlthoughNSTF shows comparable fuel cell performance to conventional dis-persed Pt/C electrodes at normal operating temperature (>70◦C), itshows poor performance at temperatures less than about 60◦C ascompared with conventional dispersed-catalyst cathode due to elec-trode flooding.12 Note that 3M has shown that some anode GDLchoice and membrane thickness can improve the low-temperatureperformance.19,20

Figure 3 compares the polarization curves of NSTF and Pt/C elec-trodes taken at 40◦C. The current density at 0.3 V was only 0.25 A/cm2

for the NSTF. The low current density at low temperature results inthe need for increased time for the cell to heat itself and thus decreasesfuel cell efficiency during start-up while substantially increasing thestart-up time when the fuel cell stack has cooled down to ambient tem-peratures (40◦C and below). This could prevent the NSTF from beinga competitive catalyst. Figure 3 also shows that the low temperature

Figure 2. Schematic illustrations of hybrid NSTF MEAs at the cathode/GDLinterface when coated with the interlayer on a NSTF MEA (a) and on a GDLsubstrate (b).

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F678 Journal of The Electrochemical Society, 159 (11) F676-F682 (2012)

Figure 3. Comparison of polarization curves taken at 40◦C between con-ventional Pt/Vu electrode (0.1 mgPt/cm2, 3 μm thick) and NSTF electrodeswith and without HS-MEA-ILs. All MEAs were assembled with 35 μm mem-branes and were tested on a 53-cm2 small-scale platform. Averaged data of>4 samples with 95% confidence intervals of the means are shown.

current density can be enhanced by applying a carbon-only interlayer(CIL) or a platinized carbon interlayer (PtCIL) to the NSTF elec-trode. Because there is no Pt in the CIL, all current must be generatedwithin the NSTF layer. The improvement in current density impliesthat the CIL reduces oxygen transport resistance due to flooding by:(1) wicking liquid water from the NSTF layer, (2) preventing a liquidwater film at NSTF/MPL interface, and/or (3) introducing additionalsurface area for water evaporation. In the case of NSTF with PtCIL, inaddition to the above effects by CIL, the Pt within the interlayer itselfcan also generate current and heat if protons can be transported to thePt sites from the membrane. Note that although the cell temperature iskept at 40◦C, the electrode temperature could be higher depending onthe current density. Thus, an electrode with higher current increaseselectrode temperature, which in turn lowers the electrode liquid satu-ration. This in turn allows the electrode to generate more current.

Effects of thickness and ionomer content on the interlayers wereinvestigated on both CIL and PtCIL as shown in Figure 4. BecauseIL thicknesses were determined by the catalyst loading for PtCIL,the effect of Pt loading was also convoluted. Data labels shown inFigure 4a are Pt loadings in mgPt/cm2. It is obvious that when ahigh ionomer-to-carbon weight ratio (I/C = 0.6) was used in the ILs,both 40◦C (Figure 4a) and 80◦C (Figure 4b) performance degradedsubstantially. Considering how thin the NSTF layer is, this observedresult is probably due to ionomer seepage into the NSTF layer causinglow porosity and high oxygen transport resistance. Using the NSTFlayer properties provided previously15, only 21 μg/cm2 of ionomer isrequired to completely fill the NSTF electrode pores. This is equivalentto an I/C of 0.5 for a 1 μm thick IL (or I/C of 0.25 at 2 μm thick).Although some amount of ionomer associates with carbon particlesin the ink so not all of the ionomer enters the NSTF layer, it isobvious that one must be cognizant of the possibility of filling thisthin electrode with ionomer. Using scanning electron microscopy, theseepage of ionomer into the NSTF layer was observed on MEAs withhigh ionomer content in the coating ink, although the seepage was notalways obvious for MEAs with moderate ionomer content.

At lower I/C, both CIL and PtCIL showed improvements in their40◦C fuel cell performances. Note that the type of carbon used inthese two ILs was different. There appears to be a limitation on theuseful amount of IL that can be incorporated, as no improvement wasrealized for ILs thicker than a few microns. The limited utilization isprobably due to the low proton conduction in a carbon/ionomer layerwith an I/C as low as 0.2–0.3.22 In addition, at low I/C not all of thecarbon and Pt surface is covered with ionomer. Difference in surfaceinteraction will cause changes in the capillary pressure and how liquidwater distributes in the layer.23

Figure 4. Effects of interlayer thickness (and/or Pt loading) on fuel cell per-formances at 40◦C (a) and 80◦C (b). Current density at 0.3 V (a) and voltage at1.5 A/cm2 (b) are reported for 53-cm2 cells tested at 40◦C and 80◦C, respec-tively. Pt/GrV ILs were made from 20wt% Pt/GrV at 0.2 or 0.6 I/C ratio. Datalabels shown in the figure are Pt loadings in mgPt/cm2. Error bars representsingle standard deviations measured on >3 cells.

Due to the low Pt loading (0.01–0.06 vs. 0.15 mgPt/cm2 in theNSTF layer) and low proton conduction in the IL (low I/C ratio), thefuel cell high-current-density performance under normal operatingcondition (80◦C) is expected to be governed by the performance ofthe NSTF layer. In addition, considering the high porosity (∼60%)and small thickness (<10 μm) of the ILs, the gas transport resistanceis expected to be negligible. In contrast to this expectation, as shownin Figure 4b, the voltage measured at 1.5 A/cm2 decreased as ILthickness increased. The voltage loss can be attributed to increasingamount of ionomer seepage into the NSTF layer with increasing ILthickness.

To quantify the loss of high-current-density voltage when the HS-MEA-IL was applied, a 1 μm thick CIL was tested in a short stack(Figure 5). About 20 mV loss was realized at 1.5 A/cm2 under thisrelatively dry condition (32% RHin). This is significant consideringthe effect of such a thin layer. These results warrant further study toimprove performance and understanding of the IL/NSTF electrode.

Manufacturable interlayer coated on MEA.— The above-described HS-MEA-IL is not considered suitable for large-scale

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Journal of The Electrochemical Society, 159 (11) F676-F682 (2012) F679

Figure 5. Polarization curves of standard NSTF, and NSTF with 1 μm thickhand-sprayed CIL tested in a 29-cell short stack. NSTF: 0.2/0.2 mgPt/cm2

PtCoMn with 35 μm PFSA membrane. HS-MEA-IL: 1 μm thick Vulcan andionomer layer, I/C = 0.3. Averaged data of 14 samples with 95% confidenceintervals of the means are shown.

manufacturing for reasons of reproducibility and scalability. Herewe describe a slot-die technique where an ink is pushed out throughtwo stainless steel blades as the die moves above a substrate givinga constant wet laydown thickness of the ink across the coatingarea. The ink solvent may seep into the membrane and/or evapo-rates from the surface, immobilizing the coating particles within∼10 sec of coating. The coated MEA was then placed under an in-frared heater for complete drying. Figure 6 compares cross-sectionalSEM images of the ILs coated by hand-spray and slot-die techniqueswith a nominal thickness of 1 μm. It is obvious that the slot-die gavesuperior coating quality and reproducibility. The corrugated structureof the NSTF cathode appears flattened when coated with the slot-die.This is likely due to the greater time the slot-die-coated MEA waswetted by the solvent. The HS-MEA-IL dried within seconds whilethe SD-MEA-IL completely dried on the time scale of a few minutes.

The effects of solvent on the appearance and fuel cell performanceof the IL were investigated. Figure 7 compares the cross-sectionalSEM images of NSTF MEAs after they were wetted with differentsolvents (wet-lay-down thickness of 50 μm) then completely dried.The corrugated structure appeared unaffected after water coating(Figure 7c), but showed a pointed shape after ethanol coating(Figure 7d). This change recovered somewhat after the MEA wasequilibrated with water. A water-ethanol solvent system resulted ina structure change similar to what shown in Figure 7b. It was shownthat the degree of swelling and/or solubility of Nafion membraneis a strong function of solvent system with a water-alcohol systemshowing the largest swelling.24–26 When the IL is coated directly onthe MEA, it is conceivable that the solvent could cause membraneswelling or ionomer dissolution, and result in changes in the

Figure 6. Cross-sectional SEM images of MEA cathodes after they werecoated with 1 μm thick Pt/GrV interlayer using either (a) hand-spray or (b)slot-die technique.

Figure 7. Cross-sectional SEM images of MEA cathodes before (a), or afterthey were coated with 1 μm thick SD-MEA-IL (b), water (c), or ethanol (d).

catalyst/ionomer interface. Here, two solvent systems, i.e., high-swellsolvent (ethanol:n-propanol:water, 3:3:4 wt ratio) and low-swellsolvent (n-buthylacetate:n-propanol, 3:1 wt ratio), were chosen toinvestigate the effect of solvent on fuel cell performance. Figure 8shows the polarization curves of standard MEAs, high-swell solventcoated MEAs, and low-swell solvent coated MEAs. The solventmixtures show no significant effect on the fuel cell performance inour study, despite the apparent differences in the SEM images.

Figure 9 compares the fuel cell polarization curves of MEAs withand without a 5 μm thick SD-MEA-IL. Both MEAs gave similar per-formance at low current density, but MEA with SD-MEA-IL showed∼50 mV lower at 1.5 A/cm2. Despite the better coating quality of theSD-MEA-IL compared to the HS-MEA-IL, it showed an unacceptablevoltage loss at high current density. Note that among several iterationsthe only case for which the voltage loss at 1.5 A/cm2 became negli-gible was when the ink I/C ratio was reduced to 0.1 for a 1 μm thickSD-MEA-IL (not shown), in which case the low temperature benefitof the interlayer was too small to be of practical use.

These results reinforce the above-described hypothesis thationomer from the coating ink seeps into the NSTF catalyst layer,imposing a higher gas transport resistance. However, this ionomer

Figure 8. Polarization curves of standard NSTF MEA comparing with thosecoated with two different solvent systems. MEAs were tested in a short stack.MEAs were assembled with 14 μm reinforced membranes. Averaged data of9 samples with 95% confidence intervals of the means are shown.

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F680 Journal of The Electrochemical Society, 159 (11) F676-F682 (2012)

Figure 9. Polarization curves of standard NSTF, and NSTF with 5 μm thick(0.025 mgPt/cm2 and I/C of 0.6) slot-die coated IL tested in a 29-cell shortstack. MEAs were assembled with 14 μm membranes. Averaged data of7 samples with 95% confidence intervals of the means are shown.

seepage appeared too little to make conclusive observation using SEM.Significant effort to observe it using transmission electron microscopy(TEM) failed due to difficulty in preparing TEM sample without in-troducing artifacts and the low contrast of ionomer itself.

Manufacturable interlayer coated on GDL.— In order to avoidionomer interaction with the NSTF layer, IL was coated on the GDLand then laid on the MEA. Figure 10 shows a cross-sectional SEMimage of the NSTF MEA equipped with a SD-GDL-IL on the cathodeside. The surface of the SD-GDL-IL was flat when compared withthe NSTF electrode. The two appeared separated in Figure 10a due topenetration of epoxy adhesive used during sample preparation for theSEM. In a fuel cell both are pressed together. It is conceivable thatthe interface between the two layers can increase the thermal resis-tance across the electrode-channel, which in turn increases the elec-trode temperature. This hypothesis was tested by an ex-situ thermalresistance measurement.27 It was found that the thermal resistanceswere very similar among standard NSTF MEA and those with SD-MEA-IL or SD-GDL-IL. Therefore the thermal gradient through theelectrode/IL/MPL interface is similar among all configurations anddoes not cause noticeable effect.

When the IL was coated on the GDL, the I/C can be increased sinceit poses no risk of ionomer sinking in the NSTF layer. SD-GDL-ILwith I/C of 0.4 and 1.0 were evaluated in a fuel cell stack as shown inFigure 11. SD-GDL-IL with both I/C ratios showed no voltage losswhen compared to standard NSTF MEA. The superior performance ofSD-GDL-IL over SD-MEA-IL, despite the former’s higher I/C ratio,supports the hypothesis that ionomer seepage into the NSTF layer isthe cause of high-current-density voltage loss.

Figure 10. Cross-sectional SEM images of (a) a NSTF MEA equipped witha SD-GDL-IL and (b) a SD-GDL-IL at a higher magnification. Dashed line inthe figure indicates the border between the IL and cathode MPL.

Figure 11. Fuel cell performance of standard NSTF and NSTF with 5 μmthick (0.025 mgPt/cm2) SD-GDL-IL measured in a short stack. Averaged dataof 6 samples with 95% confidence intervals of the means are shown. MEAswere assembled with 14 μm reinforced membranes.

Figure 12 compares the polarization curves at 40◦C of standardNSTF MEAs and those with SD-GDL-IL and SD-MEA-IL. Againthe ILs enhanced the fuel cell performance remarkably. SD-MEA-ILshowed higher voltage at mid current densities probably due to itssuperior interface between the NSTF and IL (Figure 2). Proton con-duction across the interface is expected to be poor in the SD-GDL-IL.This is evident as its polarization curve showed an ohmic-resistance-like behavior across all current regions. The voltage gap betweenthe two narrowed as current was increased and more Pt area in theelectrodes was utilized. Although SD-GDL-IL may suffer from poorinterface between NSTF and the IL, its higher ionomer content im-proves the utilization of the IL and hence allows for higher currentdensity. These results showed that there is more room for improve-ment. Advances in processing to form intimate NSTF/IL interfacewithout excessive interaction of NSTF with ionomer is the key. Theapproaches may include: (1) use of ionomer that has strong bind-ing interaction with carbon (through solvent interaction, higher MWionomer, etc.) in the coating ink to reduce the amount of ionomerseepage into the NSTF layer, (2) use of alternative proton conductor

Figure 12. Polarization curves taken at 40◦C when the ILs were coated on theMEA (SD-MEA-IL) and on the GDL (SD-GDL-IL) using slot-die technique.The thicknesses and Pt loadings of the ILs were 5 μm and 0.025 mgPt/cm2,respectively. All MEAs were assembled with 14 μm membranes and weretested on a 53-cm2 small-scale platform. Averaged data of >4 samples with95% confidence intervals of the means are shown.

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(e.g., heteropoly acid, sulfonate-group-functionalized carbon) in theIL, (3) removal of NSTF electrode corrugated feature, and (4) use ofa dry coating process (e.g., freeze-dry).

Effect of ionomer distribution in the electrodes.— In electrodesmade from conventional dispersed catalysts, large-than-expected volt-age loss at high current density was observed when the Pt loading, i.e.,the available Pt surface area, was low.28–31 A recent review discussedthat whatever is restricting the high-current-density performance ishappening at or very close to the surface of each Pt nanoparticle, eithera kinetic phenomenon or as a very local oxygen transport restriction.29

None of the already-proposed complications to ORR kinetics such as adoubling of Tafel slope at low potentials where the Pt surface is oxide-free,32 can generate losses as large as those observed. To explain thelosses as a simple oxygen transport phenomenon, the assumption ofbulk diffusivities requires thicknesses of transport-restricting materi-als such as ionomer or water that are an order of magnitude thickerthan seems geometrically possible. Alternatively, one would have topropose oxygen diffusivities through nm-thick films which are ordersof magnitude smaller than those for bulk materials. In a more sophisti-cated model where Pt and carbon particles are encapsulated by a largerionomer film, so-called the agglomerate model, requires unreasonableassumptions inconsistent with general observations on real electrodesystem in order to explain experimental data.30,31,33 Because this kindof loss is happening at or very close to the Pt nanoparticle surface,the losses are progressively larger at lower Pt loadings (or Pt surfacearea). Therefore, proper electrode design with appropriate ionomerdistribution is crucial for low-Pt-loaded electrode.

It is obvious that this kind of local-current-density-dependentfalloff is not observed for NSTF electrode despite its lower Pt sur-face area than that of the dispersed catalyst (about a factor of 6 lowerat 0.15 mgPt/cm2). Recently Debe proposed that this was caused by thedifference in O2 physisorption attempts on the extended-surface-areacatalysts (NSTF) over the carbon-dispersed nanoparticle catalysts. Inthis model a pre-exponential collision frequency factor in the Butler-Volmer equation was added to capture this effect that depends onthe surface area per unit volume of the catalysts, which is higher forextended surface catalysts.34 In a separate study at GM, where a con-trolled averaged thickness (0 to 4 nm) of ionomer was applied to aNSTF cathode, large voltage loss at high current density was observedfor NSTF electrodes with ionomer averaged thickness larger than∼1 nm.35 The voltage loss was similar to what one would expect froma dispersed-catalyst electrode with similar Pt surface area suggest-ing an order of magnitude higher oxygen transport resistivity throughthese nm-thick films. This result supports the hypothesis that ionomeris responsible to the local-current-density-dependent falloff, and ex-plains why the standard NSTF cathode, with predominantly ionomer-free surface, does not show the same behavior as dispersed-catalystcathodes. Although this viewpoint of ours is driven by experimen-tal data, we realize that comparison between dispersed-catalyst andNSTF systems with a simple ionomer-coated Pt model may be anoversimplification and it may not properly represent real electrodes.

It is noteworthy that about one-fifth of the NSTF whiskers areembedded in the membrane in the NSTF MEA we tested and that amajority of the current is produced near the ionomer/air interface ofthe whiskers when operated at high current density in H2/air under rel-atively dry condition.15 Therefore, it is clear that the oxygen transportresistance through several nm thick ionomer at the membrane surfaceis not large enough to explain the loss described above. Because theproton conductivity on bare Pt surface in the NSTF electrode is lowerthan that in ionomer, one would expect improved fuel cell performancewith the addition of ionomer (by seepage in this study or intentionalcoating35). However, no improvement was observed in these studiessuggesting that the oxygen transport resistance due to the ionomer filmoverpowers the proton conduction benefit; this may be the case if theionomer thin film on Pt surface possesses similar proton conductivityto its bulk form. Indeed, one should not assume the same physicalproperties of a thin ionomer film supported on a foreign substrate asthose of a bulk membrane. Several ex-situ studies have shown sub-

stantially different proton-, water-, and oxygen-transport propertiesof supported ionomer thin films from those of bulk membranes.36–40

These studies prompt us to revisit the current electrode performancemodel that in many cases uses physical properties extrapolated frombulk properties, an assumption which is likely to be inaccurate. Thesediscrepancies also highlight our limited understanding of electrodeperformance for both dispersed-catalyst and NSTF electrodes withsmall Pt surface area. Furthermore, it is also conceivable that thesenanoscale properties can differ with ionomer and substrate types,opening a research avenue to engineer the electrode for better perfor-mance.

In a separate study, it was shown that fuel cell performance underrelatively dry condition of a NSTF cathode was enhanced significantlyby decorating 10 nm silica nanoparticles on the surface of Pt.18 Theresulting structure was speculated to enhance chemical interactionwith vapor water and promotes capillary condensation due to thepresence of a curved meniscus at the silica/Pt-alloy-whisker interface.This increases local water content and electrode proton conductivityand yet provides sufficient access for oxygen to the Pt surface. Theresult is exciting, as it opens a pathway to improving the performanceof ionomer-free electrodes in general.

As discussed above in this study, it is clear that the fuel cell per-formance of NSTF cathode is very sensitive to the distribution ofionomer. Even some cases where the maximum amount of ionomerfrom the interlayer-coating ink was much lower than that needed tofill the pores in the NSTF layer, noticeable voltage losses were ob-served. The proton conduction on bare Pt surface was estimated to belower than that of ionomer.15,41 Therefore, the added ionomer couldhave enhanced the overall performance, if the ionomer thin film on Ptsurface possesses similar proton conductivity to its bulk form. How-ever, in this study it appears that the NSTF fuel cell performance at80◦C was only adversely affected by virtually any amount of ionomerseepage. This poses great challenge in fabricating an IL using a viablemanufacturing method. It was shown that IL/NSTF with high 80◦Cperformance could be achieved by coating the IL on the GDL. How-ever, the poorer NSTF/IL interface in this configuration gave slightlylower voltage at mid current densities under 40◦C conditions. Theseresults showed that there is more room for improvement. Advancesin processing to form intimate NSTF/IL interface without excessiveinteraction of NSTF with ionomer is the key.

Summary

This study described development efforts in fabricating a highperforming hybrid NSTF MEA. Emphasis on developing lab-scalefabrication methods that can easily translate to roll-to-roll manufac-turing process was a key element of the hybrid electrode development.Slot-die coating gave high quality coating of the interlayer with uni-form thickness and reproducibility. The interlayers could be coateddirectly on the MEA or GDL. When the IL was coated directly onthe MEA, voltage at high current density dropped significantly. Thevoltage loss was a strong function of amount of applied ionomer. Thiswas surmised to be a result of ionomer seepage into the NSTF layerduring the coating process. The voltage loss could be eliminated if theIL was coated on the GDL and then placed onto the MEA. However,in this case the relatively poor interface between the NSTF and ILresulted in higher proton resistance. This study demonstrates the chal-lenges when developing a robust ultrathin electrode and highlights thesignificance of how catalyst and ionomer must be distributed withinthe electrode for optimum performance.

Acknowledgments

The authors thank Mark K. Debe, Edward M. Fischer, and AndrewSteinbach of 3M for providing NSTF MEAs and for useful discus-sions. Thanks are also extended to Frederick T. Wagner, ThomasRuscitti, Puneet Sinha, Vic Liu, Roland Koestner for insightful com-ments, and to Ron Stevens, Ted Gacek, Travis Downs, and Rob Mosesfor engineering support.

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F682 Journal of The Electrochemical Society, 159 (11) F676-F682 (2012)

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