Electrochimica Acta, Vol. 43, No. 24, pp. 3703±3709, 1998 # 1998 Published by Elsevier Science Ltd. All rights reserved Printed in Great Britain S0013-4686(98)00128-5 0013±4686/98 $19.00 + 0.00
Measurements of proton conductivity in the active layer of PEM fuel cell gas diusion electrodes C. Boyer*, S. Gamburzev, O. Velev, S. Srinivasan and A. J. Appleby Center for Electrochemical Systems and Hydrogen Research, Texas Engineering Experiment Station, Texas A and M University System, College Station, TX 77843-3402, U.S.A. (Received 2 January 1998; in revised form 2 January 1998) AbstractÐThis paper reports further studies to understand and optimize the Membrane and Electrode Assembly (MEA) structure in Polymer Electrolyte Membrane Fuel Cells (PEMFCs). The eective proton conductivity in the active catalyst layer was measured as a function of its composition, which consisted of platinum catalyst on carbon support (E-Tek) and Na®on1 polymer electrolyte (DuPont de Nemours). The conductivity was calculated from the resistance added to a standard MEA by the addition of an inactive composite layer in the electrolyte path between the anode and cathode. The speci®c conductivity of the active layer was found to be proportional to the volume fraction of Na®on1 in the composite mixture, following the relationship eff 0:078"Nafion 0:004 S cmÿ1 . Modeling studies showed that this ionic conducH tivity limits the utilized active layer thickness to 20±25 mm. # 1998 Elsevier Science Ltd. All rights reserved Key words: fuel cell, conductivity, electrodes, Na®on, polymer electrolyte.
f* keff H
LIST OF SYMBOLS b c* iH n rA t A Dgas water F JA L NA R V a eA
Tafel slope a reference concentration proton current density normal vector to an interfacial surface between two phases in a composite reaction rate of species A thickness of a layer a unit area of surface on an interface the diusion coecient for water vapor in air Faraday's constant ¯ux of species A through an interfacial surface mass per area loading ¯ux of species A through a unit area internal cell resistance a unit volume of space in a composite material diusion mass transfer coecient, see Ref.  volume fraction occupied by material A in a composite
*Author to whom correspondence should be addressed. Tel.: +1 409 845 1326; Fax: +1 409 845 9287; E-mail: [email protected]
mA r x F DiH overbar
a reference potential eective speci®c proton conductivity of a composite material chemical potential of species A density osmotic drag coecient, see Ref.  electric potential tortuosity vector for proton ¯ux through a composite material overbar denotes a volume averaged quantity
INTRODUCTION High performance PEMFCs require high surface area of active catalyst per geometric area of electrode, especially in the cathode where the oxygen reduction reaction is kinetically slow. Increasing the thickness of the catalyst layer increases catalyst surface area, but mass transfer rates progressively become more rate limiting. The ionic conductivity of the active layer is clearly one of the mass transfer rates which limit thickness. This experimental study was conducted to measure the ionic conductivity of the active layer as a function of the weight percent platinum on carbon support and the amount of
C. Boyer et al.
per¯uorosulfonic ionomer (PFSI) electrolyte in the active layer so that the MEA structure could be optimized to achieve the highest power density with the highest platinum utilization. Modeling has determined that the ionic conductivity in the active layer is an essential parameter of electrode performance [1±4]. However, few papers have been published to date that study the proton conductivity in the active layer. Springer et al.  reported an approximate measurement of the conductivity of Na®on1 mixed with silica. Uchida et al. [5, 6] reported the aects of Flemion1 (another per¯uorosulfonic ionomer) content on the speci®c pore volume and related it to overall fuel cell performance. Parthasarathy et al.  reported the ionic conductivity of a recast Na®on1 ®lm. Several authors have used the speci®c conductivity of the Na®on1 membrane and an approximation of the volume fraction to estimate the ionic conductivity [2±4]. Common methods for measuring the resistance of the electrolyte are not practical for the study of the proton conductivity in the active layer. Impedance measurements cannot be used because the resistance of the ionic phase is masked by that of the more conductive electronic phase and by the capacitance eects of the electronic/ionic interface. Calculating the ionic conductivity from potential vs current (E± I) curves is not possible because the eects of ionic resistance cannot be distinguished from the eects of reactant mass transfer. Theoretical predictions of the conductivity of the active layer are dicult because the microscopic composite geometry and proton paths are not clearly de®ned. In this study, a new MEA structure was developed which contained an inactive composite layer placed between the anode and cathode as part of the electrolyte pathway. The cell performance of MEAs containing inactive layers were compared to a standard MEA to determine the resistance of the inactive layer. From the resistance and the layer thickness, speci®c conductivity was found. Proton conductivity as a function of the active layer composition (catalyst type and Na®on1 content) was examined. THEORY A practical model for transport in a composite is best described in terms of an empirical equation based on the principles of volume averaging, which expresses the super®cial ¯ux through the composite in terms of the transport properties of each individual phase and what is known about the geometrical structure of the composite . The transport of protons through the electrode layer occurs in the polymer electrolyte phase, which is distributed as recast material around the carbon support particles. The development of equations describing the ¯ux through the composite layer starts with the transport equations for the bulk polymer electrolyte
phase derived by Fuller . The proton current is driven by a potential gradient and diusion induced convection caused by a gradient in the chemical potential of the water, as shown in equation (1) where x represents the moles of water ``dragged'' along with each proton, and k is the proton conductivity. iH ÿkrF ÿ
kx rmwater : F
The water ¯ux is driven by a drag from the moving protons and by a gradient in the chemical potential as shown in equation (2) where a is the diusion mass transfer coecient. kx kx2 Nwater ÿ rF ÿ a 2 rmwater :
2 F F To describe the proton ¯ux in the composite active layer, the material balance is volume averaged over a representative region,
1 DiviH dV 0: V Region The volume averaged ¯ux can be expressed as the average ¯ux in the electrolyte phase and the ¯ux out of the electrolyte phase boundary caused by the reaction at the catalyst/electrolyte interface,
1 DiviH r dA:
3 V CatSurf H The same treatment can be applied to the water ¯ux with the addition of another ¯ux term at the interface between the polymer phase and the open pore phase,
1 DivNwater rwater dA V CatSurf
1 Jwater n dA:
4 V PoreSurf No reaction takes place in the inactive composite layers studied here. The layer is either electrically separated from the anode and cathode by membranes on each side or the layer does not contain platinum catalyst. This means the right hand side of equation (3) and equation (4) are set to zero. It is assumed that under the proper operating conditions there is no water concentration gradient through the inactive layers. The lack of a signi®cant water gradient can be rationalized by showing that the water content in the polymer phase is balanced by transport in the gas phase pores. The ¯ux of water in the gas phase due to a concentration gradiÿ6 ent, c*Dgas mol cmÿ1 sÿ1, is faster than water12.3 10 the ¯ux of water in the electrolyte phase caused by the ``drag'' from the proton current, kelectrolyte
x=F f*12:1 10ÿ7 mol cmÿ1 sÿ1 . Since H the gas channels provide a source and sink for water such that the water content at the boundaries is constant, the water content throughout the inac-
Proton conductivity of PEM fuel cells tive layer will also remain constant. This assumption means that only the ®rst part of equation (1) is necessary to describe proton transport in the inactive layer. Under the constant water content condition, the averaged proton ¯ux can be expressed by equation (5) iH ÿeH kbulk H r F ÿ DiH
where DiH is the tortuosity vector representing the dierence between a fractional ¯ux of the bulk phase and the true behavior of the microscopic ¯ux consisting of discontinuities and non-linearities in the proton path caused by the composite structure. The tortuosity vector is dependent on composite properties such as phase fractions, interfacial surface areas, and geometric and physiochemical properties of the phases which are determined by the composition and by the electrode fabrication technique. It is commonly assumed that the tortuosity vector is a linear function of the potential gradient, so that the term may be included with the phase fraction dependence expressed as an eective transport coecient as written in equation (6). iH keff H r F:
Although the eective conductivity parameter has no physical meaning on the microscopic scale, it is a parameter that can be measured experimentally and can be used in a macroscopic model as done in this study.
EXPERIMENTAL The conductivity of the active layer composite was investigated in two types of MEA structures. The ®rst type of MEA contained an inactive layer consisting of Na®on1 960 (DuPont de Nemours) and XC-72 Vulcan furnace black (Cabot, without catalyst) placed between the cathode and a Na®on1 112 membrane (Fig. 1(a)). The second structure contained a copy of the active layer (Na®on1 solution mixed with carbon supported catalyst from ETek) sandwiched between two Na®on1 112 membranes (Fig. 1(b)). Although the second structure was more complicated to fabricate, the inactive layer was electronically isolated such that it could contain platinum on carbon and remain inactive. Electrodes with active areas of 5 cm2 were fabricated by the paint, roll, and hot press method previously described in the literature . The inactive layer of the ®rst type of MEA structure was painted on top of the cathode catalyst layer before hot pressing the membrane between the anode and cathode. The inactive layer in the second MEA structure was painted directly onto the second side of a membrane after the cathode was hot pressed to the ®rst side. Then, the second membrane and the anode were added. To assure that electrodes in dierent MEAs would exhibit similar performance, they were cut from the same sheet of electrode material. After conditioning the MEAs for several hours, polarization curves were taken using an HP data acquisition system. The cells were operated at 508C with saturated hydrogen and oxygen at 2.4 atma (20 psig). Within a test set, each MEA had identical
Fig. 1. Experimental MEA structures with added electrolyte composite layer, (a) structure #1 with inactive layer between the cathode and membrane, (b) structure #2 with inactive catalyst layer between two membranes.
C. Boyer et al.
high frequency resistance as determined from impedance measurements. From this, it was assumed that the contact and membrane resistances were equivalent. Cyclic voltammograms were obtained to compare the cathode active areas calculated from the area under the hydrogen desorption peaks. Results showed that the eective platinum surface areas were similar and not aected by the addition of the inactive layer. Attempts were made to measure the resistance of the inactive layer under hydrogen evolution conditions, but problems were experienced in maintaining a water saturated electrolyte. After the electrochemical tests, cross sections of the MEAs were observed with a scanning electron microscope (SEM) to determine the thickness of each composite layer. An ohmic resistance term, R, was found for each MEA by ®tting the low current density region (5 to 800 mA cmÿ2) of the polarization curve to the standard electrode equation. Ecell E0 ÿ b logi ÿ Ri:
In the low current density, upper potential region, the kinetic rate followed the Tafel equation with a slope of 60 mV decÿ1 and all mass transfer losses were estimated by a linear dependence on the current. The resistance term assigned to each MEA included the membrane ionic resistance, the ionic resistance of the electrodes, the electronic resistance
of the cell, contact resistance, some reactant mass transfer resistance, and the resistance of the inactive layer. Since all of the MEA components and test ®xtures were the same except for the inactive layer, it was assumed that the dierence of one MEA's resistance compared to another was due to changes in the inactive layer. The conductivity of a given inactive layer composition was calculated from the slope of the resistance vs the loading of the inactive layer in a set of MEAs. MEAs prepared in this laboratory have a typical performance reproducibility better than 20 mV at cell potentials of 0.6 V, but a higher deviation was expected in measurements of the new MEA structures used here due to the addition of more interfaces. For this reason, multiple tests were performed to help obtain a more accurate value of the speci®c conductivity. The speci®c conductivity in terms of volume was determined from the speci®c resistance in terms of mass and the density of the composite. The density of each composite mixture was calculated from the loading and thickness. mg ÿ1
cmÿ1 : rL cm2
Fig. 2. Current vs potential curves for the series of MEAs with dierent loadings of an inactive layer consisting of 33 wt% Na®on on carbon.
Proton conductivity of PEM fuel cells RESULTS AND DISCUSSION A set of MEAs with the ®rst type of structure (Fig. 1(a)) were tested with varying loading (i.e. thickness) of an inactive layer with a composition of 33 wt% Na®on1 and 67 wt% carbon. Figure 2 contains the E±I curves and shows the drop in performance due to an increased resistance with increased loading of the inactive layer. By plotting the cell resistance vs the inactive layer loading (Fig. 3), the slope yields a speci®c resistance of 0.150 2 0.008 O cm4 mgÿ1 of composite material. The density of the composite mixtures were calculated from the loading used to fabricate the layer and the cross section thickness measured from SEM observations. The thickness of some layers deviated by as much as 30% due to the size of the ®bers in the weave of the cloth used as the gas diusion and current collection layer. The empirical formula, X ÿ1 xi rcomposite
9 ri i where rPt=21 000 mg cmÿ3, rNa®on=2000 mg cmÿ3, and rcarbon=420 mg cmÿ3, ®t the data collected for the materials used and for the method of electrode fabrication. The speci®c conductivity was calculated from the speci®c mass resistance and the composite density. mg O cm2 ÿ1 keff r :
10 R H cm3 mg
The speci®c conductivity of the inactive layer described above was calculated to be 0.013 S cmÿ1. Another series of MEAs was tested with inactive layers containing a higher Na®on1 content (60 wt%) and a speci®c conductivity of 0.018 S cmÿ1 was obtained. These values agree with the measurement of Springer et al.  who obtained 0.01 S cmÿ1 for a composite containing 25 wt% Na®on1 on silica. The second type of MEA structure, with the composite layer between two PEM layers, was fabricated to determine whether the presence of platinum had an eect on conductivity and to eliminate the eects of any carbon electroactivity in the inactive layer. A composite layer consisting of 60 wt% platinum on carbon with 33 wt% Na®on1 was found to have a resistance of 0.040 2 0.002 O cm4 mgÿ1 of composite material, i.e., 0.100 O cm4 mgÿ1 of platinum. Two series of tests were conducted with the second type of MEA structure to examine how composition aects the density and conductivity of the composite layer. The ®rst series examined composite layers with 60 wt% platinum on carbon in which the weight fraction of Na®on1 was varied between 17 and 66 wt%. As expected, the speci®c mass resistance decreased with increasing Na®on1 content due to an increase in the number of proton pathways and to an increase in density, which resulted in a thinner layer for an equivalent composite mass loading. The second series examined composite layers with 33 wt% Na®on1 and used 20, 40 and
Fig. 3. A plot of the cell resistance vs the inactive layer consisting of 33 wt% on carbon. The slope yields the speci®c mass resistance.
C. Boyer et al.
60 wt% platinum on Vulcan XC-72 carbon. At constant composite loading (mg/cm2), the active layer thickness was then inversely proportional to the weight fraction of platinum on carbon. Since the weight fraction of Na®on1 was ®xed in the inactive layer, lower platinum weight fractions also had lower Na®on1 volume fractions. For these reasons, the speci®c mass resistance increased with a decrease in the weight fraction of platinum. Most composites with a mixed conductive phase have an eective conductivity which follows the empirical equation, keff en kbulk
where n is some number between 1.2 and 4.5 [10, 11]. Several published PEMFC models have set n equal to 1.5 [2±4]. Using the bulk conductivity measured by Parthsarathy , kbulk 0:1 S cmÿ1 , H the closest ®t of the data in this experiment to equation (11) occurs with n equal to unity as seen in Fig. 4. By comparing the results of the ®rst type of MEA structure with the second type of MEA structure, it was concluded that the presence of platinum did not have an aect on the ionic conductivity. The higher than expected conductivity observed at low electrolyte fractions may be explained by some transport of protons along anion impurities on the carbon surface. This would
account for the non-zero intercept of a more exact 0:078e 0:004 S cmÿ1 . ®t of the data, keff H The macro-homogeneous model can be used to optimize the MEA structure using the conductivity information obtained above. Assuming fast oxygen transport and the kinetic parameters from Parthasarathy , Fig. 5 shows the increase in current which may be obtained by increasing the cathode platinum loading for 60 and 20 wt% platinum on carbon with a 20% volume fraction of Na®on1 in the active layer. Each curve shows that the active layer has a critical thickness of about 20±25 mm; further increases in thickness do not improve performance. Low percentage platinum catalysts are better at low overall platinum loading (mg cmÿ2) because of the high surface area per gram of platinum of these materials. The higher percentage platinum catalysts give increased performance at high loading because the thickness of the active layer is then minimized per unit catalyst area and results in a low IR drop in the active layer. The optimum Na®on1 content will depend on its eects on oxygen mass transport, water transport, and platinum coverage as well as proton conductivity. Uchida et al. [5, 6] studied cell performance with hydrogen and oxygen vs Flemion1 (Asahi Glass Co.) content (Flemion1 is similar to Na®on1). It was shown that increasing the polymer
Fig. 4. Plot showing the eective speci®c conductivity vs the volume fraction of Na®on in the composite layer for each series of experiments. The line shows the ®t using the bulk conductivity value, ke=ekbulk where kbulk=0.1 S cmÿ1.
Proton conductivity of PEM fuel cells
Fig. 5. Model predictions of the current density obtained at 0.7 V cell potential (H2/O2, T = 508C, P = atm) for varying platinum loading in the cathode. The loading in which the layer reaches a thickness of 20 mm is marked on the graph.
content decreased the speci®c volume of larger pores in the 0.04±1.0 mm range, thereby restricting the mass transport of oxygen. To complete optimization studies on the MEA cathode, further research must study the eects of Na®on1 content on other transport properties.
ACKNOWLEDGEMENTS This work was supported by DOE Contract number DE-AC08-96NV11985 and the Department of Chemical Engineering at Texas A and M University, College Station. REFERENCES
CONCLUSION Optimizing fuel cell electrodes requires a knowledge of the ionic conductivity in the active layer. The eective speci®c conductivity of Na®on1 960 dispersed with platinum supported catalyst was found to be keff 0:078eNafion 0:004 S cmÿ1 , or H approximately proportional to the volume fraction of the polymer phase, keff kbulk e . Modeling H H Nafion studies show that a cathode layer with 20% Na®on1 by volume has a conductivity that limits the eective thickness to 20±25 mm, which corresponds to a platinum loading of 0.2 mg cmÿ2 for 20% platinum on carbon and 1.0 mg cmÿ2 for 60% platinum on carbon. Increasing the thickness will oer only small increases in performance compared to the amount of platinum added. Oxygen mass transfer resistance will further reduce the eective thickness, and this subject requires further study. A study of the eects of Na®on1 content on the available platinum surface area is also desirable.
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