Journal of The Electrochemical Society, 156 共1兲 B152-B159 共2009兲
B152
0013-4651/2008/156共1兲/B152/8/$23.00 © The Electrochemical Society
Analysis of the Performance of Nafion-Based Hydrogen–Oxygen
Fuel Cells
S. R. Narayanan,*,z Thomas I. Valdez,* and Samad Firdosy*
National Aeronautics and Space Administration (NASA)-Jet Propulsion Laboratory, California Institute
of Technology, Pasadena, California 91109, USA
The present study aims at understanding the effect of materials and operating conditions on the performance of in-house-prepared
and vendor-supplied Nafion-based hydrogen–oxygen fuel cells. Eight different membrane electrode assemblies 共MEAs兲 with
different membrane thicknesses, two different equivalent weights for the membrane material, and made by three different MEA
fabrication techniques, were investigated. The electrical performance and internal resistance of the cells were measured as a
function of temperature and reactant pressures. The test results have been analyzed in terms of various polarization phenomena.
The values for io,c and ␣cn were generally in agreement with reports, but were found to have a range depending on the analysis
conditions. While mass-transfer limitations were not observed in any of the cases, dry-out of the anode catalyst layer and
“back-diffusion” were found to limit the maximum current densities, especially with the thicker membranes. The in-housedeveloped MEA fabrication process provides a lower internal resistance and higher performance during short-term testing when
compared to the MEAs from the two industrial vendors studied. These results have led to the demonstration of a high-performance
MEA for hydrogen–oxygen fuel cells based on Nafion 1035.
© 2008 The Electrochemical Society. 关DOI: 10.1149/1.3008015兴 All rights reserved.
Manuscript submitted June 3, 2008; revised manuscript received October 6, 2008. Published November 14, 2008.
The National Aeronautics and Space Administration 共NASA兲 is
interested in advancing hydrogen–oxygen fuel cell power systems
for future space applications.1 Polymer electrolyte membrane 共PEM兲
fuel cells are expected to provide improved power density, efficiency, longevity, safety, and load-following ability over the stateof-practice alkaline fuel cells. PEM hydrogen–oxygen fuel cells
were successfully used to power NASA’s Gemini missions in the
1960s. The increased capability required for future NASA missions
to the Moon and beyond, and the recent advances in PEM fuel cells,
have prompted further development of this technology for future
space applications.2-5 There has been an explosion of literature in
the area of hydrogen–air fuel cells for terrestrial transportation applications. However, fundamental studies on cells using pure oxygen
and unsupported platinum catalysts for operation in space, is
sparse.6-9 The present study aims at understanding the performance
of Jet Propulsion Laboratory 共JPL兲-prepared and vendor-supplied
Nafion-based hydrogen–oxygen fuel cells. Specifically, the effect of
material properties and operating conditions on the performance has
been studied. The results and analysis presented here aim at providing the directions for further development and optimization of the
PEM fuel cell technology for space applications.
Experimental
Eight different types of cells based on Nafion as the polymer
electrolyte were investigated. Each cell, commonly termed a
membrane-electrode assembly 共MEA兲, consisted of an electrolyte
membrane sandwiched between two catalyzed electrodes. These
cells were fabricated with membranes of different thicknesses,
equivalent weights of Nafion, and by different manufacturing techniques. The nominal values of membrane thickness in these cells
were 51, 89, 127 and 178 ␮m. Two commonly available equivalent
weights 共EWs兲 for Nafion, 1000 and 1100 g equivalent−1, were selected for the tests. All MEAs had an active electrode area of 25 cm2
and the catalyst loading was 4 mg/cm2 of platinum black on each
electrode. MEAs prepared in-house were compared with those obtained from two industrial vendors. These vendors are hereinafter
identified as vendor 1 and vendor 2. The MEAs were prepared inhouse using a process that was different from that used by the vendors. In the MEAs fabricated in-house, a wet-proofed Toray gasdiffusion backing was first coated with a catalyst layer and then
bonded to a Nafion membrane by a hot-pressing operation.10 Vendor
1 supplied MEAs with a cold-pressed wet-proofed carbon felt back-
* Electrochemical Society Active Member.
z
E-mail: [email protected]
ing layer; vendor 2 supplied MEAs with detached Toray backing
layers. These MEAs from at least three different sources allowed the
effect of MEA configuration and manufacturing technique to be
compared. Table I identifies the MEAs and the material variables
investigated.
Parametric tests were conducted using an in-house-developed
fuel cell test station capable of automated data acquisition and control. The electrical performance 共current density vs cell voltage兲 of
the MEAs was tested at three temperatures 共30, 50, and 70°C兲 and at
three values of reactant pressure 共1.5, 2.5, and 3 atm兲. The hydrogen
and oxygen streams were maintained at the same pressure for each
experiment. Dry ultrapure hydrogen and oxygen were used in the
tests without any humidification. The flow of reactants to both sides
was set up to be “dead-ended.” The hydrogen and oxygen streams
were purged every 10 min for about 5 s at a flow rate of approximately 1 L/min to remove accumulated water. The test cell hardware, purchased from Lynntech Inc., accommodated MEAs with an
active area of 25 cm2 and had serpentine flow fields on the anode
and cathode plates. The temperature of the test cell was controlled
using a heated water loop. The internal resistance of the cell was
measured at 1 kHz using an Agilent 4263B LCR meter. Cyclic voltammograms on the fuel cell were obtained by flooding one electrode with water and allowing hydrogen to flow over the other electrode. The flooded electrode became the working electrode, while
the hydrogen electrode functioned as a pseudoreference and a
counter electrode.7 A PAR 273A potentiostat was used for measuring
Table I. MEAs and variables investigated in the test experiments
MEA variables investigated
Membrane/MEA
Manufacturing
technique
Manufacturer
EW
Thickness
Nafion 105
JPL
冑
Nafion 115
JPL
冑
Nafion 1035
JPL
冑
Nafion 115
Vendor 1
冑
Nafion 115
Vendor 2
冑
Nafion 117
Vendor 2
冑
Nafion 1135
Vendor 2
冑
Nafion 112
Vendor 2
冑
冑
冑
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Journal of The Electrochemical Society, 156 共1兲 B152-B159 共2009兲
1.2
known that cathode processes dominate the charge-transfer overpotential because the exchange current density for the oxygen reduction reaction is several orders of magnitude lower than that for the
hydrogen oxidation reaction. When the charge-transfer overpotential
is much larger than 25 mV the Butler–Volmer equation can be approximated by the Tafel equation. A simplified expression that accounts for the losses arising from charge-transfer overpotential
dominated by the cathode processes is as follows
Nafion 115 JPL 3 atm H2/O2
1
Voltage (V)
0.8
0.6
o
70 C
o
30oC
50 C
0.4
Vth − Vmeas + iARhf = ␩ct = −
0.2
0
0
500
1000
1500
2000
Current Density, mA/cm2
Figure 1. 共Color online兲 Effect of temperature on the performance of Nafion
115-JPL at reactant pressure of 3 atm.
the cyclic voltammetric response. The electrochemically active area
of the electrodes was determined from the area under the peaks for
hydrogen electrosorption/desorption in the range of 0.0–0.4 V vs the
pseudo hydrogen reference electrode.
Results and Discussion
The operating variables, material properties, and manufacturing
techniques affected the electrical performance of the cells. An analysis of these effects is presented in the following.
Effect of operating variables.— Temperature.— Increasing the
operating temperature from 30 to 70°C increased the cell voltage
for all the MEA types and at all the reactant pressures tested. On
average, the increases in cell voltage were 20 mV at 40 mA/cm2,
54 mV at 400 mA/cm2, and 77 mV at 800 mA/cm2. Increasing the
temperature also lowered the high-frequency cell resistance. Representative results are shown in Fig. 1 and 2.
With an increase in temperature, the exchange current densities
for the electrode reactions and the diffusion coefficients for the reactants and products are expected to increase.11 It is also wellknown that the water content of the membrane and the ionomer
phase in the electrodes depend on temperature.12 Thus, when the
relative humidity is constant, increasing the temperature decreases
the resistances associated with ionic transport, charge transfer, and
reactant transport.
Changes to the charge-transfer overpotential can be investigated
by examining the current–voltage curves at a low current density
where the mass-transport limitations are not significant. It is well0.009
Nafion 115 JPL MEA, 3 atm H2/O2
High Frequency Resistance, Ohm
0.008
30oC
0.007
o
50 C
0.006
o
70 C
0.005
0.004
0.003
0.002
0.001
0
0
200
400
600
800
1000
1200
1400
1600
B153
1800
2
Current Density, mA/cm
Figure 2. 共Color online兲 Effect of temperature on the high-frequency resistance of Nafion-115JPL MEA operated at a reactant pressure of 3 atm.
冉
i
RT
ln
c
␣cnF
io,c exp共−Eact
/RT兲
冊
关1兴
In Eq. 1, Vth is the thermodynamic potential for the hydrogen–
oxygen fuel cell reaction, Vmeas is the measured cell voltage, Rhf is
the high-frequency resistance, A is the electrode area, ␩ct is the
charge-transfer overpotential, ␣c is transfer coefficient, i is the measured specific current density 共based on geometric area兲, io,c is the
exchange current density, Eact is the activation energy for the electron transfer reaction, R is the gas constant, T is the cell temperature
in Kelvin, F is the Faraday constant, and the subscript “c” refers to
the cathodic process.
To treat the experimental data according to Eq. 1, several criteria
must be met:
共i兲 The “state of the electrode” must be unperturbed in the experiment. For example, if the electrochemically active surface area
changed, this would distort the results. In our study, the internal
resistance of the cell served as a good indicator of the state of the
electrodes and the membrane.
共ii兲 Data close to open-circuit conditions 共below 20 mA/cm2兲
must not be considered for the analysis because the diffusion of
hydrogen across the membrane is a significant fraction of the applied current density.13
共iii兲 Vth, the theoretical cell voltage must be calculated for the
specific temperature and pressure conditions.
共iv兲 The ohmic resistance R f must be measured for every value of
current density and then applied as a correction to the cell voltage.
The state of the electrode requires special attention. The results
of resistance measurements in Fig. 2 show that the internal resistance increases rapidly beyond a certain current density, indicative
of the effect of membrane properties. For the MEA that uses Nafion
115 共127 ␮m兲, this current density is about 350–400 mA/cm2. It is
shown herein later that this rapid increase in resistance is associated
with significant change in water content at the anode.12,14,15 Such
changes in water content and resistance would result in changes in
the utilization of the electrochemically active area. The current density at which the increase in internal resistance becomes significant
depends on the membrane properties. The upper limit in current
density for the application of Eq. 1, as determined from the increase
in resistance, was 670, 580, 440, and 340 mA/cm2 for the 51, 89,
127, and 178 ␮m MEAs, respectively. Thus, the practical application of Eq. 1 becomes restricted to only a small portion of the
current–voltage curve.
Applying the above criteria, the apparent kinetic parameters io,c
and ␣cn have been evaluated from the straight line plots of log i vs
␩ct, satisfying a regression coefficient greater than 0.995. These calculations were carried out for all the MEAs as a function of temperature and pressure. An example of this type of analysis is provided in Fig. 3. The results from such analysis are summarized in
Table II.
The ␣cn values ranged from 0.74 to 1.1 over the complete range
of operating conditions and the MEA types studied. Even when
analyzing data for a single MEA, variation was observed with the
values of io,c and the product ␣cn. For example, even if a couple of
points on the curve were deleted or added, the values of io,c and ␣cn
were affected by over 10%. This is understood as follows.
The cell voltage regime that was chosen for analysis in this study
was between 1.0 and 0.8 V. It is well-known that when the potential
is changed from 1.0 to 0.8 V the surface coverage of the oxide decreases on platinum catalysts, affecting the Tafel slope and the ex-
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Journal of The Electrochemical Society, 156 共1兲 B152-B159 共2009兲
B154
Figure 3. 共Color online兲 Overpotential and its dependence of current density
at 30, 50, and 70°C for Nafion 115-JPL MEA at 3 atm pressure.
change current density significantly. Other researchers report values
of ␣cn as low as 0.6 in the oxide-covered region and as high as 1.0
for the oxide-free region.6 At temperatures of 30°C, the cell voltages
were generally lower and thus the data points sampled for the Tafel
analysis were in the vicinity of 0.8 V. These cell voltages correspond to a fairly high cathode overpotential of approximately
0.43 V. This explains the lower values of ␣cn 共0.70–0.75兲 observed
at the lower temperatures. At 70°C, the cell voltages sampled in the
analysis were closer to 1.0 V. This cell voltage corresponds to cathode overpotential values of about 0.23 V. Consequently the values
of ␣cn were closer to the reported values of 1.0. The io,c values for
all the MEAs under the operating conditions studied were found to
be in the range of 1.3–6.3 ⫻ 10−6 A/cm2. Because the ␣cn values
were used in calculating the values of io,c, variations observed in io,c
and ␣cn are related; when higher values of ␣cn were used, the calculated values of io,c from the observed intercept values were low.
Consequently, it was found that the io,c, value at the standard reduction potential for the oxygen electrode determined by Tafel analysis
of the potential region of 1.0–0.8 V 共where the surface oxide coverage is changing兲 is prone to considerable variation. Thus, unique
values of the intrinsic io,c and ␣cn that are corrected for all such
effects remain undeterminable from real operating fuel cells, and
researchers have to be content with “apparent” values of kinetic
parameters that reflect variations on the electrode. These circum-
Table II. Effect of temperature on the properties of the Nafion
115 MEA operated at 3 atm of reactant pressure. Values are normalized for geometric area and are for an electrode loaded with
4 mgÕcm2 of platinum black catalyst.
Temperature
Property
30°C
50°C
70°C
1.246
1.229
1.214
Vth, volt
Apparent exchange 4.3–5.1 ⫻ 10−6 2.1–6.3 ⫻ 10−6 1.3–4.7 ⫻ 10−6
current density, io,c,
A/cm2 at Vth
0.74–0.75
0.84–0.94
1.0–1.1
Transfer coefficient
parameter 共␣cn兲
0.17
0.13
0.12
High-frequency
resistance at
20 mA/cm2 ⍀ cm2
0.090
0.140
0.200
Current density
共A/cm2兲 at 0.9 V
共IR corrected兲
stances have led to the choice of current density at a fixed value of
cell voltage as a more practical metric for comparing performance.17
The values of io,c, ␣cn, and the current density at 0.9 V for the
Nafion 115-JPL MEA are summarized in Table II. The electrochemically active area was determined from cyclic voltammetry to be
about 700 cm2 for every cm2 of geometric area for the catalyst loading of 4 mg/cm2. Thus, the values of io,c normalized for the electrochemically active area for the JPL MEAs is determined to be in the
range of 1.8–9.4 ⫻ 10−9 A/cm2. These values are consistent with
data reported for low surface area platinum microelectrodes in contact with Nafion membranes.17,18
After membrane resistance effects have been accounted for, the
increase in cell voltage with temperature at about 0.9 V is usually
attributed to the improved kinetics of the oxygen reduction reaction.
However, the apparent values of io,c shown in Table II did not show
a clear increasing trend with temperature. This lack of trend further
emphasizes the influence of various changes at the electrode on the
calculated values of the kinetic parameters. Therefore, the activation
energy, Eact, was calculated from the temperature dependence of the
current density at various cell voltages after correcting for the resistance changes 共Eq. 1兲. Please note that under these conditions, the
activation energy calculated from the slopes of the log I vs 1/T plots
must be corrected for ␣cnF␩ct, and the need for this is obvious when
Eq. 1 is rearranged as follows
冉 冊 冉
i = io,c exp
c
− Eact
− ␩ct␣cnF
exp
RT
RT
冊
关2兴
By evaluating the Arrhenius relationship shown in Eq. 2 and assuming the values of ␣cn and ␩ct to be 1.0 and 0.330 V, respectively, for
a cell voltage of 0.9 V, the activation energy is calculated to be in
the range of 40–55 kJ/mole at all the reactant pressures studied.
These values are consistent with the observations for unsupported
platinum catalysts.19
Results in Fig. 2 show that the high-frequency resistance of the
cell decreased from 7 to 5 m⍀ when the temperature increased from
30 to 70°C. This decrease in internal resistance is consistent with
the reported increase in conductivity of Nafion 1100 EW membrane
with an increase in temperature.20 However, as the current density
increases the resistance begins to increase. The increase in cell resistance with current density is a consequence of the changes in
water distribution at the anode.15 In the present study, the anode feed
was not externally humidified. Hence, water removed from the anode by electro-osmotic processes is replenished only through “backdiffusion” of water from the cathode; when the water content in the
anode decreases below a certain value, the ohmic resistance of the
catalyst layer sharply increases. This will render the outside of the
anode catalyst layer facing the reactant chamber drier than the side
facing the membrane. Consequently, the effective thickness of the
reaction layer decreases. While the overall increase in cell resistance
is only a couple of m⍀, the reduction in proton conductivity in the
anode catalyst layer leads to a drastic reduction in performance.
Results in Fig. 4 show that the drop-off in cell performance below
0.8 V correlates with the rapid increase in cell resistance. However,
results in Fig. 5 show that the usual correction of the cell voltage
with the simple product term iARhf is inadequate and the limiting
current behavior at high current densities persists even after the
ohmic resistance correction is applied. Such a drop-off in cell performance at high current densities is often attributed to masstransport limitations, but in a case where the anode reaction layer
dry-out occurs, a similar limiting current would be observed. To
avoid this sharp drop-off, and maintain high current densities, external humidification of the hydrogen stream will be necessary. These
conclusions are consistent with reported observations12,21,22 and are
analyzed further in the following sections under “Effect of Membrane Thickness.”
Reactant pressure.— Figure 6 shows that the performance of the
Nafion115-JPL MEA at 70°C increases upon raising the pressure
from 1.5 to 3 atm. Similar curves are obtained at 50 and 30°C. The
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Journal of The Electrochemical Society, 156 共1兲 B152-B159 共2009兲
1.2
0.007
Nafion 115 JPL 70oC. 3 atm H2/O2
High Frequency Resistance, Ohm
Cell Voltage, Volt
1
0.8
0.6
0.4
0.2
0
0.004
0.0045
0.005
0.0055
B155
0.006
Nafion 115 JPL, 70oC
H2/O2
0.006
2.5 atm
1.5 atm
0.005
3 atm
0.004
0.003
0.002
0.001
0
0.0065
0
Resistance, Ohm
200
400
600
800
1000
1200
1400
1600
1800
Current Density, mA/cm2
Figure 4. 共Color online兲 Cell voltage as a function of high-frequency resistance for Nafion 115 JPL MEA at 70°C and 3 atm reactant pressure.
Figure 7. 共Color online兲 Effect of reactant pressure on the cell resistance at
70°C for Nafion 115 JPL MEA.
magnitude of the increase in cell voltage resulting from increasing
the operating pressure from 1.5 to 3 atm for all MEAs tested is 25,
37, and 52 mV at 40, 400, and 800 mA/cm2, respectively, for a cell
operating at 70°C. The changes in cell voltage arising from changes
in pressure are usually attributed to the change in the thermodynamic potential, the effect of pressure on the exchange current density, and a reduction in mass-transport resistance.23,24 Attainment of
thermodynamic potentials is precluded by the irreversibility of the
oxygen electrode and the crossover of gases. However, the effect of
temperature and pressure on the thermodynamic potential is significant, and its impact is to shift the cell voltage through by a constant
value. The changes in the thermodynamic potential of the electrode
can be predicted by Eq. 3
冉
0.5
pO
· p H2
RT
2
⌬V = Vth − E =
ln
2F
p H2O
o
0.007
Nafion 115 JPL, 70oC, 20 psig H2/O2
0.006
cell resistance
cell voltage, V
0.9
cell voltage
corrected for
resistance
0.8
0.7
0.005
0.004
0.003
Observed
cell voltage
0.6
0.5
0.002
cell resistance, Ohm
1
0.001
0.4
1
10
100
0
10000
1000
current density, mA/cm2
Figure 5. 共Color online兲 Dependence of cell voltage, resistance-corrected
cell voltage, and cell resistance as a function of current density.
关3兴
where pO2, pH2, and pH2O are the partial pressures of the reactants.
The variation predicted by Eq. 3 for an increase in total pressure
from 1 to 3 atm is about 16 mV at 30°C and 40 mV at 70°C. The
observed increases in cell voltage at low current densities are consistent with the calculated values for an increase in thermodynamic
potential. In general, the high-frequency cell resistance did not show
any discernible dependence on pressure 共see Fig. 7兲. When backdiffusion through the membrane is adequate to keep the anode hydrated, as in the case of the Nafion 112 MEAs, the plots of the
current–voltage curves corrected for the ohmic resistance of the
membrane show that no mass-transfer limitations were observed
even until 2700 mA/cm2 共Fig. 8 and 9兲. Thus, ohmic limitations are
found to govern the performance even at current densities as high as
2700 mA/cm2. When localized anode dry-out gives rise to a limiting
current, as is observed with thicker membranes, the accurate estimation of the mass-transfer effects in the MEA become more difficult
to separate. If electrochemical impedance spectroscopy measure-
1.2
1.2
3 atm
1.5 atm
2.5 atm
0.4
cell voltage, V
1
0.8
0.6
0.01
Nafion 112 MEA
H2/O2 3 atm 70oC
Nafion 115 JPL 70oC
H2/O2
1
Cell Voltage, Volt
冊
0.009
Data corrected for membrane
resistance
0.008
0.007
0.8
0.006
As Measured
0.6
0.005
High Frequency Resistance
0.4
0.2
0.2
0
0
0.004
0.003
cell resistance, Ohm
1.1
0.002
0.001
0
200
400
600
800
1000
1200
1400
1600
1800
Current Density, mA/cm2
Figure 6. 共Color online兲 Effect of reactant pressure on performance of
Nafion 115 JPL MEA at 70°C.
1
10
100
1000
0
10000
current density, mA/cm2
Figure 8. 共Color online兲 Performance of Nafion 112 MEA showing no masstransfer limitation until 2700 mA/cm2.
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Journal of The Electrochemical Society, 156 共1兲 B152-B159 共2009兲
B156
1.2
Nafion MEAs, Vendor 2, 2.5 atm, H2/O2 , 50oC
Nafion 112 MEA, 70oC
Balanced Pressure H2/O2
1
0.9
1
3 atm
0.8
0.7
cell voltage, V
Resistance corrected cell voltage, Volt
1.1
2.5 atm
1.5 atm
0.6
0.5
0.8
0.6
Nafion 117
0.4
Nafion 115
Nafion 1135
Nafion 112
0.4
0.2
0.3
0.2
0
0
500
1000
1500
2000
2500
3000
0
500
1000
current density, mA/cm2
1500
2000
2500
current density, mA/cm2
Figure 9. 共Color online兲 Polarization curve for Nafion 112 at 70°C corrected
for ohmic resistance showing absence of mass-transport limitations.
Figure 11. 共Color online兲 Effect of membrane thickness on the performance
of cells based on Nafion 1100 membrane.
ments can be carried out at such high current densities, then such a
separation can be accomplished.
and the results shown here are generally consistent with the predictions in the literature.16-18 While the amount of water transported
from the anode to the cathode by electro-osmotic drag is determined
by the operating current density, the back-diffusion of water is governed by thickness and temperature. These competing transport
mechanisms lead to a back-diffusion-limited current that corre-
0.012
1200
current density, mA/cm2 at 0.7 V
Nafion Membrane, 20 psig H2/O2, Vendor 2
800
600
70oC
400
o
50 C
o
30 C
200
0
50
100
150
200
Membrane thickness, microns
Figure 12. 共Color online兲 Dependence of the current density at 0.7 V on
membrane thickness at various temperatures for Nafion 1100-based MEAs.
0.014
Nafion 1100 based MEAs from Vendor 2 at 50oC
Nafion MEAs (Vendor 2), 2.5 atm H2/O2, 50oC
0.01
0.008
0.006
Resistance = (4E-05)*(Thickness) + 0.0025
R2 = 0.9994
0.004
1000
0
High Frequency Resistance, Ohms
High Frequency Resistance at 20 mA/cm2,Ohm
Effect of membrane thickness.— At very low current densities,
the membrane and catalyst layer stay fully hydrated. The highfrequency resistance measured under these conditions correlated
well with the thickness of the membrane 共Fig. 10兲. In Fig. 10, the
intercept value of 0.0025 ⍀ corresponds to the sum of the resistance
contributions from the catalyst layers, electrode structures, and contact resistances.
The effect of membrane thickness on performance was found to
be significant. Results in Fig. 11 and 12 show that at 50°C, with the
decrease in membrane thickness from 178 to 51 ␮m, the current
density increases by almost 90% at 0.8 V, and 120% at 0.7 V. This
effect of membrane thickness on the current density is similar at the
various temperatures studied. These results suggest that ohmic contributions largely govern the performance below 0.9 V. Similar
trends have been reported by others on MEAs using supported platinum catalysts.19,20 Thus, achieving performance improvements has
generally focused on using thinner membranes.
The results in Fig. 13 show that the high-frequency resistance
increases rapidly with current density as the thickness is increased;
results very similar to this has been reported by others.25 This resistance increase is a direct consequence of the reduced rates of backdiffusion of water from the cathode to the anode as the thickness is
increased. The effect of membrane thickness on water transport
characteristics has been studied and modeled extensively by others,
0.002
0.012
Nafion 117
0.01
Nafion 115
0.008
Nafion 1135
0.006
Nafion 112
0.004
0.002
0
0
0
50
100
150
200
Membrane thickness, microns
Figure 10. 共Color online兲 Dependence of the high-frequency resistance of
MEAs based on Nafion 1100 membrane at 70°C.
0
500
1000
1500
2000
2500
current density, mA/cm2
Figure 13. Dependence of resistance of various Nafion 1100-based MEAs as
a function of current density at 50°C and 2.5 atm.
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Journal of The Electrochemical Society, 156 共1兲 B152-B159 共2009兲
1.2
Nafion 1100 MEAs
3.5
70 oC, 2.5 atm H2/O2 Nafion
115 Membrane
1
3
Cell Voltage, Volts
"back diffusion" limiting current density,
mA/cm2
4
B157
70oC
2.5
2
1.5
30oC
1
0.8
JPL
0.6
Vendor 1
Vendor 2
0.4
0.2
0.5
0
0
0
20
40
60
80
100
0
120
200
400
sponds to the dry-out of the anode. Consequently, the current density
at which the steep rise in cell resistance is observed decreases as the
membrane thickness increases. With thicker MEAs, this backdiffusion-limiting current is low, and for very thin membranes 共as
for example, Nafion 112兲, this limiting current will be hard to observe before other limitations are reached.
A simple one-dimensional analysis of the transport processes in a
hydrogen–oxygen cell with an “unhumidified” anode operated under
dead-ended conditions is provided in the Appendix. This analysis
leads to the conclusion that the back-diffusion-limiting current Ibdl
observed at any temperature is directly related to the membrane
properties according to the following
D wC wF
␥ . ␦mem
800
1000
1200
1400
1600
1800
关4兴
where ␦mem is the thickness, Dw, is the diffusion coefficient of water
in the membrane, Cw is equilibrium water content, and ␥ is the
electro-osmotic drag coefficient.
The plot of limiting current density as a reciprocal of the membrane thickness shown in Fig. 14 is consistent with the foregoing
explanation. The strong effect of temperature in enhancing the backdiffusion rates is evident as higher limiting currents are observed at
higher cell temperatures. This increase is through the effect of temperature on the water content and also on the diffusion coefficient of
water. Assuming a drag coefficient of 2.5, the diffusion coefficient of
water in Nafion 1100 at 30°C was calculated to be 2
⫻ 10−5 cm2 s−1. This value for diffusion coefficient is comparable
to the self-diffusion coefficient 1 ⫻ 10−5 cm2 s−1 of water reported
for a fully hydrated Nafion 117.26 These results lend support to the
simplified model for understanding the limiting current behavior observed in the absence of external humidification at the anode.
Effect of MEA fabrication technique.— MEAs fabricated by
three different methods were tested. Figure 15 shows that a much
higher performance is obtained by the in-house process with the
same Nafion 115 membrane.
Vendor 1 used a screen-printing technique to apply the catalyst
layer on the membrane and lightly bonded the gas-diffusion layers
共GDLs兲 to the membrane by a cold-pressing technique, while vendor
2 used a similar process to coat the catalyst but followed it by
hot-pressing, and supplied backing layers that were to be assembled
before cell testing. The JPL process involves coating the Toray paper
electrodes with the catalyst layer and then hot-pressing these onto
the membrane, making the backing layer an integral part of the
MEA. Thus, there were significant differences in the methods of
fabrication that could potentially affect the properties of the catalyst
Figure 15. 共Color online兲 Effect of MEA fabrication procedure on the performance of the MEA.
layer. The exact compositions of catalyst and ionomer in the catalyst
layer are not available from the vendors, as these are considered
proprietary.
It was found that the MEAs prepared by the JPL process exhibited a lower internal resistance compared to the MEAs from the two
vendors 共Fig. 16兲. We measured the thickness of the membrane section of the three types of MEAs and found that the JPL MEAs were
about 15% thinner. The JPL hot-pressing technique could explain
the thinner membrane section. Further, the catalyst coated GDLs
that were hot-pressed onto the membrane in fabrication of the JPL
MEAs resulted in a more intimate electrical contact between the
GDL and the catalyst layer. These factors could explain largely the
lower ohmic resistance and the improved performance observed in
the JPL MEAs. While the active area measurements by cyclic voltammetry yielded a similar utilization of the catalyst in all the
MEAs, the actual distribution of the ionic phase in the catalyst layer
could be different for the MEAs; no definitive information is available at this time on the distribution of the ionic phase.
Effect of EW.— The performance of MEAs prepared from
Nafion 105 共EW 1000兲 and Nafion 115 共EW 1100兲 was investigated.
The performance at lower current densities was higher with Nafion
105 by about 15–20 mV. The Nafion 105 MEA also had a highfrequency resistance that was about 10% lower than the 1100 EW
membrane. This is consistent with reports of lower resistance for the
0.01
Nafion 115 MEAs 70 oC, 2.5 atm H2/O2
0.009
Resistance at 1 kHz, Ohm
Figure 14. 共Color online兲 Effect of membrane thickness on the limiting
current.
Ibdl =
600
current density, mA/cm2
1/Thickness, cm-1
Vendor 1
0.008
0.007
Vendor 2
0.006
0.005
JPL
0.004
0.003
0.002
0.001
0
0
200
400
600
800
1000
1200
1400
1600
1800
current density, mA/cm2
Figure 16. 共Color online兲 High-frequency cell resistance for MEAs prepared
by different manufacturing processes using Nafion 115. Testing conducted at
70°C and 2.5 atm.
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Journal of The Electrochemical Society, 156 共1兲 B152-B159 共2009兲
B158
Table III. Comparison of apparent kinetic parameters at 70°C
for Nafion 1000 and Nafion 1100-based JPL MEAs.
Property
Apparent exchange
current density io,c,
A/cm2 at Vth
Transfer coefficient
parameter 共␣cn兲
High-frequency
resistance at
20 mA/cm2 ⍀ cm2
Current density兲 at
0.9 V 共IR corrected兲 A/cm2
1000
1100
3.2–4.6 ⫻ 10−5
1.3–4.7 ⫻ 10−6
0.76–0.84
0.98–1.04
0.11
0.12
0.240
0.200
Anode
C1
δanode
Membrane
I1
I2
Cw
Cathode
Co
1000 EW membrane. The kinetic parameters at 70°C and 2.5 atm
are compared for the 1000 and 1100 EW membrane in Table III.
The apparent exchange current density values are slightly higher
for the 1000 EW membrane. The higher exchange current density is
consistent with several differences in the properties of Nafion 1000
and 1100 EW. Because the Nafion 1000 EW membrane has a higher
water content and is also mechanically more yielding compared to
the Nafion 1100 EW membrane, the interfacial area achieved
through the bonding process could be higher. The solubility of oxygen in the Nafion 1000 EW membrane is higher compared to the
Nafion 1100 EW membrane. The slightly lower ␣cn values observed
with the Nafion 1000 EW membrane could not be explained. However, the lower resistance makes the Nafion 1000 EW preferable for
achieving higher cell voltages over a wide range of current densities.
To derive the performance benefits of employing a lower thickness and the lower internal resistance with the 1000 EW membrane,
a Nafion 1035-based MEA has been prepared and tested at JPL.
Results in Fig. 17 show that the Nafion 1035 MEA has the highest
performance of all the MEAs tested and is much higher than the
Nafion 115 MEA. An average cell voltage of 0.89 V was achieved at
200 mA/cm2 at 70°C and at a reactant pressure of 3 atm. In the
fully hydrated state, the internal resistance value for a 25 cm2 cell
was found to be 3 m⍀ for the Nafion 1035 MEA.
Conclusions
The experimental study has obtained data and understanding on
MEAs operating on pure hydrogen and oxygen. The Tafel analysis
of cell voltage–current curves on MEAs has highlighted the challenges of obtaining well-defined kinetic parameters. The values for
io,c and ␣cn determined from the study were generally in agreement
with the literature reports, but were found to have a range depending
on the analysis conditions. For the entire set of MEA experiments,
1.2
JPL MEAs 70oC
3 atm H2/O2 humified flow 3-stoic
1
δmem
Figure A-1. Schematic of MEA for analysis purposes.
values of io,c for the platinum black catalyst were in the range of
1.8–9.4 ⫻ 10−9 A/cm2 when normalized for electrochemically active area; the value of ␣cn was found to be in the range of 0.70–1.1;
the activation energy was found to be in the range of
40–55 kJ/mole. The observed effect of reactant pressure on the cell
performance was consistent with the predictions of thermodynamics.
Measurement of high-frequency resistance as a function of current
density was found to be very useful in understanding the performance changes. Dry-out of the anode catalyst layer was found to
limit the maximum current densities, especially with the thicker
membranes. Even for the very thin membranes, where the backdiffusion rates of water were high, membrane resistance governed
the performance at least until 2.7 A/cm2, and no mass-transfer limitation was observed. A simplified one-dimensional model for the
MEA could explain the role of back-diffusion of water in limiting
the maximum attainable current density. The model explained the
major effects observed in the study and was consistent with the other
observations and detailed models in the literature. Also, we found
that the Nafion 1000 EW provided an improvement in performance
over Nafion 1100 EW because of the lower overall internal resistance of the cells prepared with the former membrane. The process
of fabricating MEAs developed at JPL also appeared to provide a
lower internal resistance and hence a higher performance as compared to the products provided by at least two other vendors. By
combining the benefits of a lower membrane thickness, lower EW
for the membrane, and JPL’s method of fabrication, we were able to
demonstrate a very high-performance MEA based on the Nafion
1035 for hydrogen–oxygen fuel cells. To meet the NASA goal for
high-efficiency MEAs, we continue to investigate approaches to
raise the cell voltage from 0.89 to 0.92 V at 200 mA/cm2, at the cell
temperature of 70°C, and reactant pressure of 3 atm.
cell voltage, V
Acknowledgment
0.8
0.4
The work presented here was carried out at the Jet Propulsion
Laboratory, California Institute of Technology, under a contract from
the National Aeronautics and Space Administration. The authors
thank Mark Hoberecht and Kenneth Burke for their support and
guidance during various parts of this research.
0.2
NASA-Jet Propulsion Laboratory assisted in meeting the publication
costs of this article.
Nafion 1035
0.6
Nafion 115
0
0
500
1000
1500
2000
2500
current density, mA/cm2
Figure 17. 共Color online兲 Comparison of the performance of Nafion 115 and
Nafion 1035 MEAs fabricated at JPL.
Appendix
Analysis of Back-Diffusion-Limited Current Density
One-dimensional analysis of back-diffusion flux in an “unhumidified” dead-ended
PEM cell is provided below. The MEA is considered a three-layer sandwich consisting
of the anode, cathode, and membrane as shown in Fig. A-1 below.
Because water is transported from the anode to the cathode by electro-osmotic drag,
and water is produced by electrochemical reaction at the cathode, the water content at
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Journal of The Electrochemical Society, 156 共1兲 B152-B159 共2009兲
the cathode is high. The equilibrium water content at the cathode is represented as Cw.
Within the anode structure, the water content is governed by the balance of the flux
toward the membrane due to electro-osmotic drag and the back-diffusion of water from
the membrane into the anode. The water content at the anode at any particular current
density I is related to the electro-osmotic and diffusion properties by
␥·I
F
=
koDw兵C1共I兲 − Co共I兲其
␦anode共I兲
关A-1兴
In Eq. A-1, ␥ is the electro-osmotic coefficient, I is the current density, F is the Faraday
constant, Dw is the diffusion coefficient of water in the membrane and ionomer phase,
C1 is the concentration of water at the membrane anode interface, Co is the concentration at the outer reaction edge of the anode 共see schematic兲, and ko is the utilization
factor 共varying from 0 to 1, and unitless兲 that accounts for the fraction of the electrodes
that sustains the water transport; the value of ko is a function of the ionomer content in
the electrode. The concentrations in the anode would be expected to change as a function of the current density.
Under steady-state conditions the fluxes in the electrode and at interface of the
anode and the membrane must be balanced. Therefore
␥·I
F
=
Dw兵Cw共I兲 − C1共I兲其
␦mem共I兲
关A-2兴
where Cw is the concentration at the cathode end of the membrane and ␦mem is the
thickness of the membrane electrolyte. ko is not applicable for Eq. A-2 as this refers to
the bulk of the membrane.
When the concentration at the outer edge of the anode drops to zero this is the
beginning of a dry-out situation. This would correspond to a loss of conductivity in the
anode; hence, an expression for the change in the reaction layer thickness ␦anode, can be
obtained from Eq. A-1 and A-2 as follows
␦anode共I兲 =
koDwF关Cw共I兲 − Co共I兲兴
␥I
− ko␦mem
关A-3兴
As the current increases, the concentration of water at the outer edge of the anode
catalyst layer, Co共I兲, will tend to zero, giving rise to Eq. A-4
␦anode共I兲 =
k oD wC wF
␥I
− ko␦mem
关A-4兴
Further, increases in current density cause the thickness of the reaction layer in the
anode, ␦anode, to shrink. When ␦anode approaches zero, the potential drop across the
anode will increase substantially, resulting in a limiting current behavior. This limiting
value of current Ibdl also can be termed as the back-diffusion-limited current density for
an unhumidified cell, and can be calculated by setting ␦anode to zero in Eq. A-4, as
Ibdl =
D wC wF
␥ . ␦mem
关A-5兴
B159
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