PEMFC Component Characterization and Its ...

ECS Transactions, 3 (1) 753-763 (2006) 10.1149/1.2356195, copyright The Electrochemical Society

PEMFC Component Characterization and Its Relationship to Mass-Transport Overpotentials during Long-Term Testing D. L. Wooda,b, J. R. Daveya, P. Atanassovb, and R. L. Borupa a

Los Alamos National Laboratory, Materials Physics Applications, Los Alamos, New Mexico 87545, USA b Department of Chemical & Nuclear Engineering, University of New Mexico, Albuquerque, New Mexico 87131, USA Individual overpotential contributions to PEM Fuel Cell performance are examined to determine how these change during long operating times. The cathode mass-transport overpotential and its relationship to O2 diffusion mechanisms and liquid-water transport is examined in detail. A method, governing equation, and durability case study is presented here for estimating both overpotential contributions as a function of testing time, as well as the breakdown of mass-transport overpotential for the cathode Gas Diffusion Layer (GDL) and catalyst layer. The importance of correlating component characterization with this durability data is also shown using high-resolution scanning electron microscopy (HRSEM) and pore-size distribution (PSD) measurements obtained with Hg and H2O porosimetry. Introduction Stack component durability is difficult to quantify and improve not only because of the quantity and duration of testing time required, but also because the fuel-cell stack is a system of components (electrocatalysts, membranes, gas diffusion media, bipolar plates, and gaskets) for which the degradation mechanisms, component interactions, and effects of operating conditions are not fully understood. Individual components must be well characterized during durability or accelerated testing to determine and quantify degradation mechanisms that occur over long periods (1-3). The focus of the research presented here is to correlate the long-term performance characteristics of the Gas Diffusion Layer (GDL) and Membrane Electrode Assembly (MEA) with mass-transport phenomena in a PEMFC operating environment. Mass-transport performance losses that occur in the GDL consist mainly of liquidphase water blockage of the macroporosity in both the graphite-fiber matrix of the substrate and the carbon-particle matrix of the bilayer, or “microporous layer” (MPL), coating. This effect is commonly known as GDL flooding, and it results in inadequate removal of product water and inadequate supply of reactant gases to the electrode layers, a phenomenon seen primarily at the cathode. Effective diffusivities of the reactants are reduced, which occurs on the scale of 0.1–100 µm in the GDL pores. Similar masstransport resistances occur within the electrode layers of the MEA, but on a smaller scale of ~1-100 nm. To reach the Pt nanoparticle interfaces, reactant molecules must diffuse through porous voids between carbon-support particles and ionomer, mesopores within carbon-support aggregates, liquid water pockets, and water-saturated regions of the ionomer network, all of which comprise the operating environment of state-of-the-art electrocatalyst layers. These many mass-transport steps are closely tied to the material

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ECS Transactions, 3 (1) 753-763 (2006)

selection and processing conditions of the GDL and MEA components, as well as the operating conditions (both short-term and long-term) of the cell or stack. Experimental Cell Construction The primary cell discussed here (referred to as “G-2”) contained a PRIMEA® Series 5620 MEA (W.L. Gore & Assoc., Elkton, MD), which had 35-µm reinforced perfluorosulfonic acid (PFSA) Gore-SELECT® ionomer membrane. Catalyst loadings were 0.40/0.60 mg-Pt/cm2 anode/cathode. SIGRACET® GDL 24BC (SGL Technologies GmbH, Meitingen, Germany) was used for the GDL at both electrodes, which had polytetrafluoroethylene (PTFE) loadings of 5/23 wt% in the bulk fiber-matrix and MPL, respectively. Dual-material gaskets were used at each side – 0.30-mm (12-mil) thick MYLAR®/PORON® (polyester/polyurethane) – and the cell was compressed to 100 in-lb torque. The active area of cell G-2 was 50.0 cm2. A nearly identical cell (referred to as “G-7”), the only difference being the GDL PTFE contents, was constructed to obtain the oxygen reduction reaction (ORR) kinetic parameters for use in extracting the mass-transport overpotentials during long-term testing of cell G-2. The rationale for using a separate cell for these O2 cathode measurements was to prevent damage to the membrane (pinhole formation or thinning) or cathode catalyst layer (electrolyte degradation or Pt oxidation) of cell G-2, even if these were only minor, short-lived effects. Just one set of the H2/O2 polarization curves were measured after the break-in period and 35 hours of constant current operation. Durability Testing and Cell Impedance Long-term testing was completed in constant-current mode at 80°C using standard 50-cm2 hardware (graphite flow-field plates with a single serpentine flow channel, goldplated Cu current-collector plates, and anodized Al end plates) and a single-cell test station (Fuel Cell Technology Corp., Albuquerque, NM) at 0.90 A/cm2. Gas flow rates were controlled to 415×10-6 (1.0 A/cm2 × 1.2 equivalent stoich.) and 1650×10-6 (1.0 A/cm2 × 2.0 equivalent stoich.) standard cubic meters per minute for hydrogen and air, respectively. Both reactant gas streams were pre-humidified with sparging bottles at a temperature of 73°C, which corresponds to a relative humidity (RH) of 75% for each stream. Humidifier efficiencies of these test stands have been shown to be near 100% for operating temperatures near 80°C. Backpressure of each stream was held at 103×103 Pa gauge, or 15.0 psig, or 1.78 atm absolute (Los Alamos, NM atmospheric pressure is ~11.1 psia). Cell impedance, often referred to as high-frequency resistance (HFR) was measured at 8 kHz (to eliminate capacitance and mass-transfer impedance, leaving only the total contribution of bulk and contact resistances) every 5-20 hours. This measurement was performed by superimposing an 8-kHz sine wave signal (with voltage amplitude of 10 mV) over the DC cell voltage measured in constant-current mode. The cell current density was chosen automatically by the LabVIEW® control software and was set to about 0.5 A/cm2 for the impedance measurement. The AC signal was generated from a DAQCardTM (National Instruments) and sent to the cell via the electronic load. Impedance was then obtained from the return signal via a shunt resistor.



Polarization Measurements After ~100 hours of operation, a set of three air polarization curves were taken for studying the mass-transport changes as a function of durability testing time. The data was collected in “constant-flow” mode, which means the stoichiometric flow rates of the reactant gases were not controlled (i.e., they were a function of the current density over the polarization range). Voltage was scanned from open circuit (typically 1.00-1.02 V) down to 0.4 V. All conditions were the same as given in the previous sub-section, except for the reactant flow rates, which were 623×10-6 (1.5 A/cm2 × 1.2 equivalent stoich.) and 2470×10-6 (1.5 A/cm2 × 2.0 equivalent stoich.) standard cubic meters per minute for hydrogen and air, respectively. Identical conditions were used to acquire the H2/O2 polarization curves, with the exception of the gas flow rates, which were 1040×10-6 (1.0 A/cm2 × 3.0 equivalent stoich.) and 520×10-6 (1.0 A/cm2 × 3.0 equivalent stoich.) standard cubic meters per minute of hydrogen and oxygen, respectively. Cyclic Voltammetry Electrochemical surface area (ECSA) of the cathode catalyst layer was measured by in-situ hydrogen adsorption-desorption (HAD) cyclic voltammetry (CV) approximately every 100 operating hours. Details of this procedure have been discussed elsewhere (4-6). For this set of experiments, a series of 12 cathode voltammograms were measured in sequence within a scan range of 0.1 V to 1.0 V at a scan rate of 10 mV/s. The 12th scans were used for integration of the desorption charges at each 100-hr interval. Actual ECSA data is not shown in this paper, but it is discussed in context of the following derivation and is required for calculation of the individual overpotentials. Pore-Size Distribution Measurements Water (intrusion) porosimetry was used to characterize the amount of hydrophobic pore volume in the GDL materials used in this study. To also obtain hydrophilic pore volume, water porosimetry was used in conjunction with Hg porosimetry (7). The Hg porosimetry volume represents the total amount of pore volume (hydrophobic + hydrophilic) because Hg is non-wetting for both types of pores, while the water porosimetry volume represents only the hydrophobic pore volume (i.e., post-equilibration with water). The difference between Hg intrusion volume and water intrusion volume is equal to the hydrophilic pore volume, yielding the relative fractions of each (such as in a GDL or MEA). Maximum pressures used in this work were typically 120–250 MPa (18,000–36,000 psi) and 69 MPa (10,000 psi) for Hg and water, respectively. Scanning Electron Microscopy A ~4×8 mm sample was cut from the middle of the cathode side GDL 24BC for surface examination with a Hitachi S-5200 Nano SEM coupled with a Princeton GammaTech (PGT) energy dispersive spectrometer (EDS). This scanning electron microscope (SEM) provides the highest resolution at low electron-beam voltages (1-2 kV) available today. A guaranteed spacial resolution of 1.7 nm can be obtained at 1 kV with no special sample preparation. In this case sample mounting of the GDL material was done using double-sided carbon tape affixed to an aluminum boat.



Theory and Overpotential Contributions The following discussion and derivation are based on the derivations presented by Gasteiger et. al. (8) and Williams et. al. (9). The work of Gasteiger et. al. was done with the purpose of precise estimation of PEMFC ORR kinetic parameters and individual contributions to electrochemical efficiency measured directly from a single cell. A derivation with similar intent was presented by Williams et. al. as a standardized method for analyzing contributions to performance losses within the three different regions of a PEMFC polarization curve. A follow-up to these two methods will be discussed here, combining and extending the approaches with the added objective of better understanding of long-term fuel cell test data. Nomenclature will be used similar to that by Gasteiger et. al (9). Beginning with the general expression for the contributions to total overpotential (ηTotal) experienced by an operating PEMFC, the following equation is obtained:

Vcell = E eq (T , p H 2 , pO2 ) − η Total

[1]

where the individual overpotentials are expressed as

ηTotal = ηORR + η HOR + ηΩ, HFR + ηtx , elec + ηtx ,GDL

[2]

For the purposes of this analysis method, ηHOR will be assumed to include all sources of anode activation and concentration polarization and to be approximately zero. It is actually slightly greater than zero, but safe to assume negligible, due to the fast HOR kinetics and high effective binary diffusivity with water vapor. In fact, the overall contribution of the anode overpotential to Vcell is typically less than 50 mV for a well designed cell/stack, even operating at high current density (>1.0 A/cm2). Quantifying the performance constituents of a PEMFC operating at 0.65 V (~53% electrochemical efficiency) based on this assumed maximum anode overpotential yields a maximum corresponding error of 7-8% in analysis of total cathode overpotential when neglecting the anode contribution. Therefore, the ηtx,elec and ηtx,GDL terms may also be considered within acceptable accuracy to be only O2 transport losses within the cathode catalyst layer and GDL, respectively (the flow fields are also assumed to cause no transport resistance). The ηORR term represents all kinetic and catalytic sources of the cathode activation polarization, and ηΩ,HFR is the total Ohmic polarization of the single cell (sum of the contributions of anode, cathode, and membrane bulk and contact resistances to total overpotential). The primary goal of this work is to study the trends of ηtx,elec and ηtx,GDL as a function of operating time and conditions without having to acquire a set of polarization curves with both air and O2 for each characterization point (the regular intervals during longterm testing where the single-cell performance is quantified). The first step is to correct the cell voltage for the “iR” losses, those losses due to electronic and protonic conduction through and across the interfaces of the various PEMFC component materials. Combining Equations 1 and 2 and rearranging gives

ViR − free = Vcell + ηΩ, HFR = Vcell + iRΩ, HFR = Eeq (T , pH 2 , pO2 ) − ηORR − ηtx , elec − ηtx ,GDL

[3]



where RΩ,HFR is the cell electrochemical impedance. For thinner membranes (≤50 µm), this value is typically about 55–70 mΩ·cm2 under fully humidified operating conditions (high inlet RHs, lower reactant stoichiometry, higher gas pressures, or moderate current density) and about 75–300 mΩ·cm2 under sub-saturated conditions (lower inlet RHs, higher reactant stoichiometry, lower gas pressures, and lower current density). In order to properly evaluate the air polarization curves taken under the same conditions as those during steady-state durability testing (such as constant current at specified RHs and current density) or accelerated durability testing (such as potential or drive cycling at specified RHs, loads, or voltage ranges), effects of these conditions must be accounted for on the kinetic parameter analysis. At the low current densities of ~5– 200 mA/cm2 required for kinetic parameter analysis, these flow conditions correspond to 9/15 anode/cathode stoich. for 0.2 A/cm2 up to 360/600 anode/cathode stoich. for 0.005 A/cm2 for the cell G-2 polarization flows. When the inlet streams are sub-saturated (75% RH for each reactant stream in this case), harsh conditions may be imposed on PFSA ionomers. The first main effect of this operating environment is handled through the iR correction in Equation 3, as HFR is high in this situation (>>100 mΩ·cm2). The second main effect, as will be shown later, is a substantial cathode catalyst-layer mass-transport resistance in the kinetic region of the polarization curve at low cathode RH and high air stoich. Under these conditions, the O2 solubility in under hydrated PFSA is significantly reduced due to the much lower equilibrium water content within its structure. In other words, O2 solubility in hydrated PFSA is proportional to the water content; in fact, the solubility of O2 in hydrated PFSA is ~10× that in liquid water (10). Dry, recast PFSA ionomer within the composite electrode structure may also contort the void structure, which could increase the tortuosity factor of the catalyst layer pore volume. The result would be lower effective binary O2 diffusion coefficients and constricted access to the triple-phase reaction sites, further increasing the catalyst-layer masstransport resistance. Equation 3 is then rewritten as follows: ViR* − free = ViR − free + ηtx , elec = Eeq (T , pH 2 , pO2 ) − ηORR − ηtx ,GDL

[4]

where V*iR-free represents the iR-corrected cell voltage plus cathode catalyst-layer masstransport overpotential effects of O2 diffusion resistance. The separation of ηtx,elec will be handled later. When the current density is lower and the stoichiometric flow rates are sufficiently high, as should be the case when obtaining kinetic parameters, ηtx,GDL approaches zero. The remaining ηORR term is treated through a Tafel kinetic approximation as discussed by Gasteiger et. al. (8) according to the following expression: ⎡ ieff η ORR = b log ⎢ ⎢⎣10 Lcath APt ,elec i0 ( pO

⎤ ⎥ ) ⎥⎦ 2

[5]

where b is the Tafel slope in mV/decade of current density. This term accounts for the driving force required to make the ORR kinetics proceed per order of magnitude of current density. Lcath is the cathode areal loading of supported Pt catalyst in mg-Pt/cm2geometric, APt,elec is the mass-specific surface area of supported Pt available for the ORR in m2-Pt/g-Pt, and i0 is the ORR exchange current density as a function of temperature and O2 concentration. The H2-crossover-corrected current density, ieff (in A/cm2),



accounts for the kinetic and activity losses associated with H2 that has diffused across the membrane from the anode side (values extracted from HAD CV data) and is described as ieff = i + i x −over

[6]

Equations 5 and 6, as written, ensure that the ηORR term properly accounts for both the intrinsic catalyst activity and reaction kinetics portions of the ORR activation overpotential. Substituting Equations 5 and 6 and ηtx,GDL = 0 into Equation 4 gives

(

ViR* − free = −bideal log ieff + Eeq (T , pH 2 , pO2 ) + bideal log 10 Lcath APt , eleci0,ideal ( pO2 )

)

[7]

where bideal and i0,ideal are the Tafel slope and exchange current density when ViR-free is corrected for the significant O2 diffusion barrier in the under hydrated cathode catalyst layer (the concept on which V*iR-free is based). These “ideal” values are simply those that were obtained for an identical set of experiments with saturated, pure O2 as the cathode inlet gas (measured with cell G-7). Typical Tafel slope values at these conditions for 20 wt% Pt/C (Vulcan XC72) at a loading of 0.4 mg-Pt/cm2 with a PFSA membrane (such as Aciplex S-1004 or Nafion 115) range between 59 and 64 mV/decade. Corresponding exchange current densities range between 0.8×10-9 and 17×10-9 (8). Since it is difficult to directly quantify ηtx,elec and, hence, V*iR-free, an indirect method for obtaining it will now be shown. Operating with saturated O2 means that ηtx,elec ≈ 0 and ViR-free ≈ V*iR-free (see Eqns. 4 and 7), and the iR-corrected cell voltage is equal to the right-hand side of Equation 7. Performing a Tafel analysis on this polarization data would yield the ideal values of b and i0. However, the effects of ηtx,elec on the physical processes represented by b and i0 when operating with sub-saturated air (even at high stoichiometric ratios) should yield exaggerated Tafel parameters when performing the same analysis without accounting for ηtx,elec. This point is described mathematically as follows: * ViR − free = −b* log ieff + B* = Eeq (T , pH 2 , pO2 ) − ηORR

[8]

* η ORR = η ORR + η tx ,elec

[9]

where

and

(

B * = E eq (T , p H 2 , pO2 ) + b * log 10 Lcath APt ,elec i0* ( pO2 )

)

[10]

Now b* and i0* are the Tafel slope and exchange current density without correcting ViR-free for the O2 electrode concentration overpotential. These alternate values still include the physically unfavorable consequence of the mass-transport overpotential within the electrode reaction zone, but are not themselves true kinetic parameters. The term B* in Equations 8 and 10 represents the y-intercept value at log(1) when constructing a Tafel plot in the form of y = -b·ln(x) + B. The final form of this modified Tafel equation is simply a distillation of the relation shown in Equation 8:



ViR − free = −b* log ieff + B*

[11]

When calculating b* and extrapolating this slope to the temperature and reactantconcentration dependent Eeq, the value for i0* is obtained. An expression for the reversible equilibrium potential as a function of temperature, hydrogen partial pressure, and oxygen partial pressure is available from Bernardi and Verbrugge (11). Expressions for normalizing exchange current density to oxygen partial pressure as a function of ORR order is also given by Gasteiger et. al. and will not be discussed here (8). Results and Discussion

Figure 1 shows a semi-log plot of ViR-free vs. log(ieff) for beginning-of-life (BOL) polarization curves with H2 and air (cell G-2), from which b* and i0* are obtained. Also shown is the data for the H2/O2 polarization measurements of cell G-7, from which the values of bideal (true Tafel slope) and i0,ideal (true exchange current density) were obtained. For this MEA type (Gore Series 5620), catalyst type (Gore proprietary), and catalyst loading (0.60 mg-Pt/cm2), the true Tafel slope and exchange current density were 78 mV/decade and 1.9×10-8 A/cm2-Pt (normalized to O2 partial pressure and ORR order), respectively. These values compare fairly well with the summary data provided by Gasteiger et. al., although the Tafel slope is about 10 mV/decade higher than those published for catalyst loadings in the range of 0.30-0.45 mg-Pt/cm2 (8). Because of this higher Tafel slope, the associated exchange current density is also somewhat higher (~212×) than the normalized summary values calculated by Gasteiger et. al. There are several possible reasons for this discrepancy: 1) the partial pressure of the humidified, pure O2 stream was 144 kPa and lower than that used by many other researchers; 2) the reactant utilizations of ~25-60% for both gases are relatively high for this type of experiment and could have added to the total polarization in the Ohmic and concentration regions; 3) the carbon-supported weight-fraction of Pt, which is unknown in this case, may have been higher than the typically used amounts of 20% and 47%; 4) 1.05

b ideal = 78 mV/decade i 0,ideal (normalized) = 1.9×10-8 A/cm2-Pt

iR -Corrected Voltage (V)

0.95

0.85

0.75

0.65

i0

*

b * = 105 mV/decade (normalized) = 1.6×10-7 A/cm2-Pt

0.55

0.45 0.001

0.01

0.1

1

10

i eff (A/cm2)

Figure 1. Tafel plots of iR-corrected voltage as a function of H2-crossover-corrected current density (ieff) for cell G-2 (H2/air) and cell G-7 (H2/O2).



the cathode electrode/membrane interface formed with the Gore-SELECT membrane may influence the ORR kinetics in a different manner than the same interface with a nonreinforced PFSA membrane; and 5) the proprietary electrode formulation may be significantly different than commonly used formulations based on Nafion® solutions. Nonetheless, these parameters are believed to be accurate and are used in the remaining equations and overpotential calculations. It is also assumed the ORR kinetic parameters do not change significantly over the durability time scales considered in our work (~5003000 hr); however, this assumption should be verified in future experiments where the O2 polarization data is also measured periodically and not just at the onset of testing. Figure 1 also shows that the sub-saturated H2/air conditions caused a significant increase to the Tafel slope (b*), suggesting an effective dampening of the ORR kinetics. The normalized i0* was found to be 1.6×10-7 A/cm2-Pt, 8.4× higher than the true exchange current density. Once these two values are known, Equation 8 can be solved for ηORR*. Then Equations 4 and 9 can be combined and rearranged to determine ηtx,GDL, a mathematically and physically accurate representation of the concentration overpotential in the cathode GDL as i approaches ilim (i >> 0.2 A/cm2): * ηtx ,GDL = Eeq (T , pH , pO ) − ViR − free − ηORR 2

[12]

2

Since ηORR* is only a means by which to obtain the actual overpotentials of interest, it will now be shown how ηtx,elec and ηORR are estimated based on the preceding derivation. Substituting Equations 7, 8, and 10 into Equation 4 and solving for ηtx,elec yields

ηtx , elec

⎡ (10 Lcath APt , eleci0,ideal )bideal = (b − bideal ) ⋅ log ieff + log ⎢ * b* ⎢⎣ (10 Lcath APt , eleci0 ) *

1.0

⎤ ⎥ ⎥⎦

[13]

0.45 0.40

0.9

0.35 ⇒Tcell = 80°C ⇒Gas Pressure = 15 / 15 psig (A/C) ⇒RH = 75% / 75% (A/C) ⇒Flow Rates = 1.2 / 2.0 2 (A/C) × 1.5 A/cm equiv.

Vi R-free (V)

0.8 Time, Vi R-free

0.30 0.25

0.7

0.6

0.20 Catalyst-Layer Mass-Transport Improvement from More Hydrated Electrode Ionomer

0.15

Time, ηtx,elec

0.10 0.05

0.5 Time, ηtx,GDL 0.4 0.00

η (V)

Time, ηORR

0.25

0.50

Black Symbols = 0 hr (Beginning of Life) Gray Symbols = 536 hr Durability Testing White Symbols = 1014 hr Durability Testing

0.75

1.00

1.25

0.00

-0.05 1.50

Current Density (A/cm2)

Figure 2. Cathode catalyst-layer and GDL mass-transport overpotentials, ORR overpotential, and iR-corrected cell voltage as a function of current density at different durability testing times for cell G-2.



Figure 2 shows results of all calculations for each overpotential as a function of current density at 0 hr, 536 hr, and 1014 hr of durability testing. Increases in mass-transport overpotential for the cathode GDL (ηtx,GDL) and ORR overpotential (ηORR) were mostly offset by improvements in the mass-transport overpotential of the cathode catalyst layer (ηtx,elec) and reductions in the high-frequency resistance. As a result, the iR-corrected polarization curves remained mostly unchanged in the higher current-density region and actually improved in the range of ~0.05–0.6 A/cm2 (see Fig. 2). The increases in ηORR may be due primarily to reduction in cathode ECSA, which decreased from 79.4 to 36.3 m2/g-Pt over 1014 operating hours, and seemed to be leveling off between ~500-1000 hr. In contrast, ηtx,GDL values exhibited a roughly proportional increase with operating time and were highest in the high current-density range. This finding suggests that the GDL mass-transport losses at the cathode would continue to worsen indefinitely, in addition to being irreversible (i.e., hydrophobicity degradation, C-surface structural changes, etc.). The mass-transport overpotential of the cathode catalyst-layer (ηtx,elec) became almost negligible at the end of 1000 hrs of operation, even at high current density, suggesting little O2 diffusion resistance through the ionomer and adjacent void volume. High constant current-density operation of 0.90 A/cm2 could have played a role by keeping the cathode ionomer mostly in contact with liquid water. Material Characterization

Pore-size distribution (PSD) is an important physical property that can be used to characterize the mass-transport behavior of a GDL or electrode layer (from a material standpoint), both for transport of gaseous phase reactants to the catalyst sites and liquid water removal from the catalyst layer. Figure 3 shows PSD data for two different SIGRACET® GDL types, GDL 24BC (used in cell G-2) with 5 wt% bulk PTFE and GDL 24DC with 20 wt% bulk PTFE (with respect to the fiber matrix). Both GDLs contained the same loading of 23 wt% PTFE in the MPL. There is a substantial difference between total and hydrophobic PSDs for each of the mean pore size, mode pore size, and pore-size range. This data (see Fig. 3) shows that the state-of-the-art bilayer GDL contains three distinct regions, which agrees well with intuition: 1) a larger, predominantly hydrophilic pore-region corresponding to the fibrous substrate, which contains a great deal of fiber surface not coated by PTFE; 2) a smaller, predominantly hydrophobic pore region corresponding to the MPL, which contains a high loading of PTFE and is closely intermingled with the carbon particles; 3) an intermediate region between ~100 nm and ~10 µm that contains a similar amount of hydrophobic and hydrophilic pore volume, i.e. where the MPL penetrates into the fiber matrix. Figure 3 also shows that GDL 24DC with 4× the loading of bulk PTFE has substantially less total pore volume than GDL 24BC, as evidenced by the lower peak in the mode/mean pore-size region. This observation suggests the PTFE for the 20-wt% case fills in the substrate pore volume, which the total intrusion volumes also support (2.03 cm3/g for GDL 24BC and 1.62 cm3/g for GDL 24DC). The hydrophobic intrusion volumes were also in accordance and were found to be 1.76 cm3/g (87% of the total) and 1.28 cm3/g (79% of the total) for GDL 24BC and GDL 24DC, respectively. Despite the much higher total PTFE content of GDL 24DC, nearly the entire hydrophobic PSD is shifted lower as well. This situation is explained by the non-uniform PTFE distribution with respect to the carbon phase. Higher PTFE loadings tend to result in thicker films on the carbon phases in the smaller pore regions and leave many of the large pores unchanged. All of these properties must be balanced with the long-term operating



0.12

0.10

3

Total Pore Volume (cm /g)

0.10

3

GDL 24BC (Hg) GDL 24DC (Hg) GDL 24BC (Water) GDL 24DC (Water)

0.08

0.08

0.06

0.06

0.04

0.04

0.02

0.02

0.00 0.001

0.01

0.1

1

10

100

Hydrophobic Pore Volume (cm /g)

0.12

0.00 1000

Pore Diameter (µm)

Figure 3. Comparison of Hg and H2O porosimetry for fresh SIGRACET® GDL 24BC (5 wt% substrate PTFE) and GDL 24DC (20 wt% substrate PTFE).

conditions the cell/stack will be exposed to. The delicate balance between hydrophobic and hydrophilic pores for both the GDLs and catalyst layers must not be altered over time and is the main emphasis of Figure 3. SEM micrograph comparisons between the fiber matrix of fresh GDL 24BC and that taken from cell G-2 after the ~1050-hr durability test are shown in Figure 4. Noticeable changes in the microstructure of the PTFE particles are observed. The durability tested GDL is shown on the right in Figure 4, and within the white circle, a loss of definition can be seen delineating the individual PTFE particles (typically shaped like and ellipsoid and 200-500 nm diameter). Many features of this type have been observed with SEM of durability tested GDLs, and it is thought that they are related to changes in GDL water management characteristics. Ultimately, it is desired to bring post-mortem characterization of this type together with the PSD measurements and mass-transport overpotential characterization discussed above.

Figure 4. Comparison of PTFE-fiber matrix before and after long-term testing for ~1000 hr (non-MPL side of GDL 24BC taken from cell G-2).



Conclusions

A comprehensive method for separating the cathode catalyst-layer and GDL masstransport overpotentials was derived. The derivation was applied to periodic polarization data from a ~1050-hr durability test and was shown to provide accurate breakdowns of the sources of performance losses. Increases in mass-transport overpotential for the cathode GDL (ηtx,GDL) and ORR overpotential (ηORR) were mostly offset by improvements in the mass-transport overpotential of the cathode catalyst layer (ηtx,elec) and reductions in the high-frequency resistance. The mass-transport overpotential of the cathode catalyst-layer (ηtx,elec) became almost negligible at the end of 1000 hrs of operation (

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