zirconium phosphate composite membranes for operation of PEMFCs above 100 °C

Electrochimica Acta 47 (2002) 1023– 1033 www.elsevier.com/locate/electacta

Nafion® 115/zirconium phosphate composite membranes for operation of PEMFCs above 100 °C P. Costamagna a,b,*, C. Yang a,c, A.B. Bocarsly b, S. Srinivasan a a

Center for Energy and En6ironmental Studies, Princeton Uni6ersity, Princeton, NJ 08540, USA b Department of Chemistry, Princeton Uni6ersity, Princeton, NJ 08540, USA c Department of Mechanical and Aerospace Engineering, Princeton Uni6ersity, Princeton, NJ 08540, USA Received 5 April 2001; received in revised form 19 September 2001

Abstract Composite Nafion/zirconium phosphate membranes were investigated for high temperature operation of proton exchange membrane fuel cells (PEMFCs). The composite membranes were prepared via impregnation of Nafion films (either commercial Nafion 115 or recast Nafion) with zirconyl chloride and 1 M phosphoric acid at 80 °C. An MEA employing a composite membrane prepared starting from commercial Nafion 115 gave a H2/O2 PEMFC performance of about 1000 mA/cm2 at 0.45 V at a temperature of 130 °C and a pressure of 3 bar; this result compares very favorably with the performance of an MEA based on commercial unmodified Nafion, which gave only 250 mA/cm2 at 0.45 V when operated under the same conditions of temperature and pressure. Similar experiments performed with recast Nafion and recast Nafion/zirconium phosphate composites confirmed an analogous improvement of performance of the composite membranes over the unimpregnated ones. In this case, the composite recast Nafion/zirconium phosphate gave about 1500 mA/cm2 at 0.45 V at a temperature of 130 °C and a pressure of 3 bar. The composite membranes showed stable behavior during time when maintained at 130 °C, while irreversible degradation affected Nafion under the same conditions. © 2002 Elsevier Science Ltd. All rights reserved. Keywords: Fuel cells; Proton conducting membranes; Organic/inorganic composites; Nafion; Zirconium phosphate

1. Introduction Operation of Proton Exchange Membrane Fuel Cells (PEMFC) at temperatures above 100 °C is receiving worldwide attention in order to alleviate the problem of anode electrocatalyst poisoning in the presence of CO impurities in the H2 feed gas. However, high temperature operation of PEMFCs is not practical due to the low proton conductivity of the state-of-the-art membrane under these conditions. The proton conduction mechanism of the perfluorosulfonic acid (PFSA) membranes relies on the presence of water; but because of the unfavorable equilibrium and high evaporation rate at temperatures above the boiling point of water, there is a dramatic decrease of water content, proton conduc* Corresponding author. Present address: DICHEP, Universita` di Genova, Via Opera Pia 15, 16145 Genova, Italy. Tel.: +39-0103536505; fax: + 39-010-3532586. E-mail address: [email protected] (P. Costamagna).

tivity and consequently fuel cell performance. The boiling point of water can be raised by increasing the operating pressure, but pressures above 3 bar (corresponding to a boiling point of about 134 °C) are undesirable for a PEMFC from an efficiency perspective due to the energy penalty associated with compressing the reactant gases. In this respect, modification of the electrolyte membrane to permit operation at higher temperatures and lower water vapor pressure seems to be a promising route. For this purpose, different approaches can be followed, such as: (i) modifying PFSA membranes in order to improve their water retention properties at temperatures above 100 °C; (ii) modifying PFSA membranes to attain proton conduction independent of water; and (iii) selecting new electrolytes based on solid state proton conducting materials. One of the early research studies for high temperature operation of PEMFCs was the impregnation of Nafion 117 with heteropolyacids [1], in order to im-

0013-4686/02/$ - see front matter © 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 0 1 3 - 4 6 8 6 ( 0 1 ) 0 0 8 2 9 - 5

P. Costamagna et al. / Electrochimica Acta 47 (2002) 1023–1033

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ductivity of 10 − 2 S/cm at room temperature under conditions of full humidification [2]. The proton conductivities of some other compounds of this type are reported in Table 1 (materials A–H). Table 1 also reports some data about materials that have been considered to be suitable for the other two approaches previously mentioned. Indeed, materials I and J appear interesting for option (ii), since, in these cases, imidazole and pyrazole substitute for the solvating water of the proton conducting polymeric matrix, and proton conduction can take place at high temperatures in absence of water. Finally materials K–O are currently under study for option (iii), since they are inorganic proton conductors which work in absence of water. More detailed considerations about the advantages and the limitations of each approach (i)–(iii) will be ana-

prove the water retention properties at high temperatures (approach (i) above). These membranes have shown interesting results for short term operation of PEMFCs at 110 –115 °C and 1 bar, but, unfortunately, they tend to lose the heteropolyacids by dissolution in water present in the fuel cell environment. In this paper we still follow approach (i), but we impregnate the nanopores of a Nafion membrane with a highly hygroscopic insoluble solid to minimize the problems of dissolution/leeching. We have chosen zirconium phosphate as the impregnating material since it shows the above mentioned characteristics; another interesting feature of zirconium phosphate and some of its derivatives (Table 1) is that they show proton conductivity properties. For example, zirconium phosphate glasses prepared through a sol– gel route have a proton con-

Table 1 Proton conductivity of materials potentially interesting for high temperature PEMFC operation Material

Proton conductivity (S/cm)

A

a-Zr(O3PCH2OH)1.27(O3PC6H4SO3H)0.73·nH2O

B

g-Zr(PO4)(H2PO4)0.54(HO3PC6H4SO3H)0.46·nH2O

Operating conditions

References

8×10−3 10−5

100 °C, 60% RH 180 °C, 0% RH

[14], [15]

5×10−2

100 °C, 95% RH

[16]

−7

C

Zr(O3PC2H5)1.15Y0.85

6×10 2×10−3 3.5×10−6

100 °C, 0% RH 100 °C, 60% RH 170 °C, 0% RH

[17]

D

Zr(O3PCH2OH)1.27Y0.73·nH2O

10−5 10−2

100 °C, 0% RH 100 °C, 60% RH

[17]

E

a-Zr(O3PC6H4SO3H)·3.6H2O

2.1×10−2

105 °C, 85% RH

[18]

F

a-Zr(O3POH)·H2O

5×10−6 1×10−4

100 °C, 60% RH 100 °C, 95% RH

[19]

G

(P2O5)4(ZrO2)3 glass

H

P2O5ZrO2SiO2 glass

I

Imidazole intercalated into sulfonated polyetherketone membrane

10−2 2×10−2

120 °C, 0% RH 200 °C, 0% RH

[21]

J

Pyrazole intercalated into sulfonated polyetherketone membrane

4×10−3 8×10−3

120 °C, 0% RH 200 °C, 0% RH

[21]

K

CsDSO4

5×10−6 5×10−2 Superprotonic transition 140 °C

140 °C, 0% RH 152 °C, 0% RH

[22]

L

b-Cs3(HSO4)2(Hx (P, S)O4)

3×10−5 10−2 1.6×10−2 Superprotonic transition 125 °C

90 °C, 0% RH 150 °C, 0% RH 200 °C, 0% RH

[23]

M

a-Cs3(HSO4)2(H2PO4)

2.5×10−3 Superprotonic transition 140 °C

140 °C, 0% RH

[24]

N

CsHSO4

2.5×10−7 1×10−2 1.6×10−2 Superprotonic transition 130 °C

130 °C, 0% RH 150 °C, 0% RH 200 °C, 0% RH

[25]

O

Ba2YSnO5.5

10−4 10−5

120 °C, 0% RH 200 °C, 0% RH

[26]

10−2 5×10−3

90 °C, 50% RH

[4]

90 °C, 50% RH

[20]

P. Costamagna et al. / Electrochimica Acta 47 (2002) 1023–1033

lyzed in detail in another paper [3]. As mentioned above, the present paper focuses on the PEMFC results obtained with the use of a composite membrane of Nafion 115 with zirconium phosphate impregnated into its nanoporous structure.

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midity environment over a wide range of temperatures. The measurements were taken using a two-probe ac impedance method along the length of the film at frequencies between about 1 and 100 kHz, using a lock-in amplifier (PAR model 5210) and a potentiostat (PAR model 273A).

2. Experimental

2.3. Membrane–electrode assembly (MEA) 2.1. Preparation of membranes The composite membranes were prepared using Nafion 115 films (Du Pont) or recast Nafion films as the base material. The recast Nafion films were prepared from a Du Pont Nafion solution with nominal equivalent weight (EW) of 1100. A 1:2 ratio of 5% Nafion solution and isopropyl alcohol were mixed; the solution then was recast in glass trays of the desired dimensions overnight at 80 °C, and finally heated up to 160 °C for 30 min. Both the as-purchased and recast Nafion films were purified by boiling in 3% hydrogen peroxide for 1 h. Then they were rinsed with boiling water, treated in boiling 1 M sulfuric acid for 1 h and finally rinsed again in boiling water several times. Using the procedure described by Grot and Rajendran [4], zirconium phosphate was incorporated into Nafion 115 via an ion exchange reaction involving Zr4 + ions followed by precipitation of zirconium phosphate by immersion of the membrane in H3PO4. For this purpose, the membranes were first swollen in a boiling methanol –water solution (1:1 vol) and dipped in a 1 M solution of zirconyl chloride (Aldrich) for several hours at 80 °C. The membranes were then rinsed in cold water to remove excess solution and finally placed in 1 M phosphoric acid overnight at 80 °C. As a result, an insoluble zirconium phosphate salt was formed in-situ and entrapped in the nanopores of the Nafion membrane. The membranes were then repeatedly rinsed in distilled water to remove excess acid.

2.2. Physico-chemical characterization of membranes After keeping the membranes in an oven at 80 °C for 20 min, they were let stabilize at ambient conditions for 5 min, and then weight and thickness of the dry membranes were measured. Titration experiments were performed as well, in order to evaluate the membrane equivalent weight (weight of Nafion/mol acid) as described by Chen et al. [5]. The composite membranes were also characterized by an electron microprobe elemental analysis carried out using a Cameca SX50 experimental microbeam. Furthermore, the proton conductivity of the composite membranes was measured at atmospheric pressure and 100% relative hu-

Commercially available gas-diffusion electrodes (20% Pt-on-carbon, 0.4 mg Pt/cm2, from E-TEK) were impregnated with 5 wt% solubilized Nafion (Aldrich) and then dried at 80 °C for 1 h. The geometrical area of the electrodes was 5 cm2. A membrane was sandwiched between two electrodes, and the resulting membrane– electrode assembly (MEA) was then pressed for 2 min at 130 °C at 1 metric ton force.

2.4. Performance e6aluation in PEMFCs The MEAs were coupled with gas-sealing gaskets and placed in a single cell test fixture. H2 and O2 (high purity gases) were fed to the single cell at controlled flowrate, humidity, temperature and pressure using a Globe-Tech test station. In order to humidify the gases prior to entry to the fuel cell, H2 and O2 were bubbled through water in stainless steel bottles, the temperatures of which were individually controlled. After installing the single cell in the test station, performance evaluation studies were carried out using an electronic load (Amrel). The reactant gases were passed to the fuel cell maintaining the temperatures of the humidification bottles at higher temperatures than the fuel cell; these temperatures were slowly raised to the desired value to begin PEMFC performance evaluation. The cell was initially tested at 1 bar and a temperature of 80 °C with Tanode = 99 °C and Tcathode = 88 °C. The cell voltage was often cycled between 1 and 0.3 V. Cyclic voltammetry was performed on both electrodes in the range of potentials from 100 mV to 1 V, at several sweep rates between 1 and 100 mV/s. During these measurements, N2 instead of oxygen was passed to the test electrode and H2 to the counter electrode. The latter served as the reference electrode. The fuel cell performance was measured at 80 °C and atmospheric pressure as well as in the range of temperatures between 80 and 140 °C, at 3 atm pressure (Table 2). For a fuel cell temperature of 120 °C and above, the humidifiers were kept at 130 °C (slightly below the boiling point) to maintain an adequate partial pressure of reactant gases. All the experimental results were obtained under steady-state conditions.

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Table 2 Temperatures of the anodic humidifier (Ta), of the fuel cell (Tfc) and of the cathodic humidifier (Tc) and operating pressure (p) applied in the experimental tests Operating condition Ta (°C)

Tfc (°C)

Tc (°C)

p (psig)

1 2 3 4 5

80 80 120 130 140

88 88 130 130 130

0 30 30 30 30

99 99 130 130 130

3. Results and discussion

3.1. Membrane characterization The cross-sections of several Nafion and composite Nafion/zirconium phosphate membranes were analyzed using electron microprobe elemental analysis to determine the presence and distribution of phosphorous and zirconium. A micrograph of the cross-section of two membranes (Fig. 1) reveals that the control Nafion contains negligible traces of these elements, while the composite membranes have a uniform distribution throughout the cross-section. Analogous investigations

on different zones of the same membranes and on different membranes confirmed this result. Since the microprobe measurement is a qualitative technique, no conclusions could be drawn about the concentration of zirconium phosphate in the membrane. An average value was obtained by weighing the dry membrane before and after impregnation and the results are reported in Table 3, together with the values of thickness and EW. From Table 3, it can be seen that the thickness of the composite membranes increased of about 30% after impregnation, while the weight increased of about 23 and 36% when starting from the Nafion 115 and the recast Nafion membrane, respectively. The EW of the recast Nafion membrane was slightly lower than with the unmodified commercial Nafion 115, which is in agreement with the findings of other authors [5]. Also, the EW of the composite membranes diminished of about 30% if compared to the starting unmodified Nafion films; the reason why the Nafion 115/zirconium phosphate has a lower EW than the recast Nafion/zirconium phosphate film is not fully clear at present and will be the subject of future investigation. Results of XRD analysis carried out on the same type of membranes are reported elsewhere [6]. Analysis of the peaks of the composite membranes shows reflec-

Fig. 1. WDS data of phosphorous (left) and zirconium (right) distribution in: (a) unimpregnated Nafion 115; (b) Nafion 115/23 wt% zirconium phosphate composite membrane. Table 3 Thickness, equivalent weight and weight of dry Nafion membranes before and after the impregnation with zirconium phosphate

Nafion 115 Nafion 115/ZrP Recast Nafion Recast Nafion/ZrP

Thickness (mm)

Equivalent weight (g/mol)

Weight (g/cm2)

125 165 70 90

1004 683 989 720

0.028 0.034 0.018 0.025

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Fig. 2. Proton conductivity of (“) Nafion 115 and ( ) Nafion 115/23 wt% zirconium phosphate composite membrane.

tions that are attributed to Nafion, and several other that are attributed to the presence of zirconium phosphate. On the basis of the XRD data, the zirconium phosphate particles were evaluated to be 119 1 nm in size, i.e. slightly larger than the pore sizes in Nafion under complete hydration. The membranes were also characterized using AC impedance spectroscopy to determine their proton conductivities as a function of temperature (Fig. 2). Water dissociates the protons from the sulfonic acid groups and its high dielectric constant enables the separation of charge between proton and sulfonate group, resulting in highly mobile protons. The overall mechanism for conduction involves an activation barrier and thus the relationship between the ionic conductivity and temperature can be expressed by the Arrhenius law: | =A exp(− EA/RT) where |, A, EA, R, and T denote the ionic conductivity, frequency factor, activation energy for conduction, gas constant and temperature, respectively. The interpolation of the experimental data reported in Fig. 2 allows an evaluation an activation energy of 9.34 and 9.82 kJ/mol for pure Nafion and composite Nafion/zirconium phosphate, respectively. Such values are quite close to values reported in the literature for pure Nafion [7]. Because the values of proton conductivity and activation energy are quite similar for pure Nafion and composite Nafion/zirconium phosphate membranes, we can hypothesize that the presence of zirconium phosphate does not significantly change the

proton conduction mechanism and the proton conduction properties of Nafion in well hydrated membranes.

3.2. Performance in PEMFCs Cyclic voltammetry experiments on the PEMFC with a composite Nafion 115/23% zirconium phosphate membrane shows the characteristic behavior (Fig. 3) for a PEMFC with unmodified Nafion. This confirms the compatibility of traditional PEMFC electrocatalysts towards composite Nafion/zirconium phosphate membranes, which is one of the biggest issues with innovative PEMFC membranes [3]. The electrochemically active surface areas were evaluated as the coulombic charge for the oxidation of the atomic hydrogen adsorbed on the electrode (i.e. the area under the cathodic peak minus the double layer charge) [8]. By assuming a coulombic charge of 220 mC/cm2 for a smooth platinum surface, the roughness factor is evaluated to be 65.291 cm2 per cm2 of geometric area. The same result approximately holds also for the PEMFCs based on bare Nafion membrane reported above, and is in agreement with literature values [8]. By repeating the cyclic voltammetry on the same MEA at very low scan rate (2 mV/s), the cross-over current was found to be approximately 0.2 mA/cm2, which is significantly lower than with non-impregnated Nafion 115 membranes we measured under the same conditions (that gave a crossover current of about 2 mA/cm2). This can be explained by an increased hydraulic resistance to gas transport caused by the presence of the solid salt granules in the nanopores of the composite membrane.

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The performances of the MEAs were characterized from the cell potential vs. current density plots. The results are reported below; all the experimental data are presented together with interpolations based on the literature equation [9]: E =E0 −b log (i )− Ri,

(1)

where: E0 = Er +b log (i0),

(2)

where E0 is a fitting parameter whose physical meaning is related to the open circuit potential, Er is the reversible cell potential (from the Nernst equation), i0 and b are the Tafel parameters for oxygen reduction, and R represents the resistance which causes a linear variation of E with i. Contributions to R are from the ionic resistance of the electrolyte, from the mass transport resistance within the electrodes and from the electronic resistance of the electrodes and their contacts with the current collectors [10]. Indeed, there are some cases where Eq. (1) does not provide a perfect fitting of the experimental data; this occurs when the experimental data show a departure from linearity at high current densities, typically due to mass transfer limitations. This is somewhat visible with the data taken from the Nafion 115/zirconium phosphate (23%) composite membranes reported in Fig. 7; however, since the departure from linearity is not dramatic, Eq. (1) has been used to fit all the data reported in the paper. Table 4 reports the values of the fitting parameters E0, b and R as evaluated from least squares fits of potential vs. current density plots recorded in the conditions listed in Table 2.

Even if some of the results obtained for i0 appear difficult to explain (as already discussed by other authors [10]), some considerations can be made on the basis of the data of E0, R and b reported in Table 4: (i) E 0 is much lower with the Nafion recast than with the Nafion 115 membrane. This can be elucidated considering that the Nafion recast membrane is much thinner than Nafion 115, and thus higher cross-over could take place affecting E0 (which is closely related to the cell open circuit voltage, which generally tends to decrease in presence of cross-over effects). This consideration is also confirmed by the fact that impregnation of the Nafion membranes with zirconium phosphate improves the value of E0: the presence of zirconium phosphate in the membrane nanopores significantly decreases crossover effects, as already discussed; (ii) the value of R at high temperature (operating conditions 4 and 5) decreases from Nafion 115 to 115/zirconium phosphate and from recast Nafion to Recast Nafion/zirconium phosphate. This is evidence of the increased water retention properties of the latter membranes compared to the previous ones. On the other hand, at low temperatures (operating conditions 2 and 3) R slightly increases with the impregnated membranes if compared to the non-impregnated ones; and (iii) the Tafel slope b, again, decreases from Nafion 115 to 115/zirconium phosphate to recast Nafion to Recast Nafion/zirconium phosphate. This means that the increased water retention properties affect in a positive way not only the membrane resistance, but also the electro-kinetics of the electrode; indeed, adequate hydration of the ion conducting phase in the electrode is necessary for good electrode performance.

Fig. 3. Cyclic voltammetry of an MEA employing a Nafion 115/23 wt% zirconium phosphate composite membrane at 80 °C and 1 bar.

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Table 4 Values of fitting parameters obtained by interpolation of potential vs. current density experimental curves recorded in the conditions listed in Table 2 Membrane

Operating condition

E0 (mV)

b (mV/dec.)

i0×103 (mA/cm2)

R (V cm2)

Nafion 115

2 3 4 5

1012 982 1000 938

84.7 82.0 93.4 –

3.7 4.6 13.7 –

0.17 0.24 1.3 9.8

Nafion 115/zirconium phosphate (23%)

2 3 4 (a) 4 (c) 5

1019 958 951 941 941

79.0 66.4 59.6 61.5 55.1

Nafion recast

1 2 3 4 5

949 974 930 917 903

56.8 74.0 64.0 57.5 26.2

Nafion recast/zirconium phosphate (36%)

2 3 4 5

1007 951 948 947

69.2 52.1 56.8 60.3

3.0 0.6 0.18 0.16 0.06 0.08 0.5 0.15 0.03 4.5×10−11 0.9 0.05 0.05 0.17

0.27 0.27 0.34 0.37 0.79 0.26 0.13 0.21 0.4 1.8 0.16 0.2 0.26 0.45

For Nafion 115/zirconium phosphate (23%) in the operating conditions 4, two sets of fitting parameters are reported, corresponding to the curves indicated as (a) and (c) in Fig. 7.

More detailed considerations about the results obtained for each type of membrane are reported in the remaining part of this section. The results in Fig. 4 define a set of reference data for an MEA based on unmodified Nafion 115. The data, obtained at an absolute pressure of 3 atm, show a decrease of performance by increasing the operating temperature from 80 to 140 °C, which is mainly due to a sharp increase of the resistance R (see Table 4). One of the reasons of this behavior is the decrease of reactant concentration in the feed gases when increasing the humidifying temperature. When the total pressure is maintained at 3 atm, the saturation vapor pressure of water vapor in the reactant gaseous streams increases from 0.47 atm at 80 °C to 2.7 atm at 130 °C, thereby reducing the partial pressure of hydrogen at the anode and oxygen at the cathode. This increases mass transport overpotentials at both electrodes, which is one of the causes which contribute to increasing R. Another effect which contributes to augmenting R is the increase of the ionic resistance of the electrolyte. The latter phenomenon occurs as well, in particular when the cell temperature is brought above 120 °C; under all these conditions, the humidifier temperatures are always kept at 130 °C (as already discussed in Section 2.4), and thus no further decrease of reactant concentration occurs. The loss of performance at temperatures higher than 120 °C is due to the operation of the fuel cell at reduced relative humidity resulting in loss of hydration water from the MEA. This loss of water is also accompanied by degradation of the MEA;

after operation of the cell at 130 and 140 °C, the initial cell performance cannot be recovered when the cell is brought back to 80 °C and rehydrated. This decrease of membrane performance is most likely attributed to the high temperature heat treatment around the glass transition temperature, which leads to membrane changes similar to those of the S-form (shrunken) of Nafion [11]. The results (Fig. 5) for an MEA based on a composite Nafion/zirconium phosphate membrane and of a comparison between composite and plain Nafion membranes (Fig. 6) demonstrate that the performance of the PEMFCs with the composite membranes decreases with increasing temperature as with the PEMFC with un-

Fig. 4. H2/O2 PEMFC performance of an MEA employing a bare Nafion 115 membrane at different operating temperatures and 3 atm.

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Fig. 5. H2/O2 PEMFC performance of an MEA employing a Nafion 115/23 wt% zirconium phosphate composite membrane at 3 atm and different operating temperatures (Ta/Tfc/Tc indicated in the figure label).

Fig. 6. Comparison between H2/O2 PEMFC performance of MEAs employing Nafion 115 and 115/23 wt% zirconium phosphate composite membrane at 3 atm and different operating temperatures (Ta/Tfc/Tc indicated in the figure label).

Fig. 7. H2/O2 PEMFC performance of MEAs employing a Nafion 115/23 wt% zirconium phosphate composite membrane at 3 atm and different operating temperatures (Ta/Tfc/Tc indicated in the figure label). The letters a, b, c indicate the temporal sequence of the experiments.

modified Nafion. However, there is a significantly higher performance of the composite membrane compared to the unmodified Nafion at high operating temperatures (130 and 140 °C), while the performance is similar at lower temperatures (80 °C). The most striking differences occur at high temperature in the linear region of the voltage vs. current curves; indeed, in the operating condition 5 the value of R decreases from 9.8 to 0.79 when zirconium phosphate is incorporated into the membrane. Even if the semi-exponential parts appear very similar, an analysis of the Tafel slope b (Table 4) reveals a decrease for the composite membrane if compared to the unimpregnated one; this decrease becomes remarkable at high temperatures, and this evidences that the increased water retention properties affect in a positive way not only the membrane resistance, but also the electro-kinetics of the electrode. The composite membranes show good performance stability over time when operated at temperatures as high as 140 °C. The data in Fig. 7 show an MEA with Nafion 115/23 wt% zirconium phosphate composite membrane. The cell was operated at 130 °C for 1 h, and then the cell temperature was raised to 140 °C. The membrane resistance increased very quickly, and then was unchanging. After 1 h of operation under the latter conditions (during which several i– 6 curves were recorded, showing unchanged performance after the first 15 min), the cell temperature was set back to 130 °C, and it was found that the cell recovered almost completely its initial performance. So, unlike the unmodified membrane, these composite membranes were only slightly damaged by the high temperature conditions (as a comparison, with Nafion 115 the performance reported in Fig. 4 for operation at 130/140/130 remained unchanged even when setting the operating conditions back to 130/130/130 for several hours). In the composite membranes, the presence of zirconium phosphate particles prevented significant damage when the temperature was increased to the glass transition. In addition, when brought back to 130 °C, the membranes reabsorbed water from the vapor phase that was lost at 140 °C. This demonstrates that the zirconium phosphate in the nanopores of the Nafion does not simply reduce the rate of evaporation from the membrane, but also causes a change in the thermodynamic water uptake properties. Some experiments were also carried out with composite membranes prepared starting from recast Nafion films prepared following the procedure described in Section 2.1. These films are thinner than commercial Nafion 115 (70 m vs. 125 mm), and have a lower EW (Table 3). These effects lead to better properties in terms of water management and water retention, and thus better performance at high temperature. Thus, the impregnation of recast films with zirconium phosphate appears to be a promising way to obtain a further

P. Costamagna et al. / Electrochimica Acta 47 (2002) 1023–1033

Fig. 8. H2/O2 PEMFC performance of an MEA employing a recast Nafion/36 wt% zirconium phosphate composite membrane at 3 atm and different operating temperatures (Ta/Tfc/Tc indicated in the figure label).

Fig. 9. Comparison between H2/O2 PEMFC performance of MEAs employing recast Nafion and recast Nafion/36 wt% zirconium phosphate composite membrane at 3 atm and various operating temperatures (Ta/Tfc/Tc indicated in the figure label).

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improvement of the PEMFC performance at 130– 140 °C. Results of H2/O2 PEMFC performance with an MEA employing a recast Nafion/36 wt% zirconium phosphate composite membrane (Fig. 8) show a satisfactory performance up to temperatures as high as 140°. The composite recast Nafion/zirconium phosphate membrane displays a remarkable improvement over the plain recast Nafion (Fig. 9). In particular, as shown in Fig. 10, the performance of the MEA with the composite recast membrane at 130 °C and 3 atm is equivalent to that of the unmodified recast film at 80 °C and 1 atm. The data presented demonstrate that the introduction of zirconium phosphate into the nanopores of a Nafion membrane improve its proton conductivity even under dehydrating conditions. It is reasonable to correlate this effect to a change in the thermodynamic properties of the membrane, and in particular of its level of hydration under different conditions. An explanation of this phenomenon lays in the hygroscopicity of the zirconium phosphate particles entrapped in the membrane pores and the displacement of unassociated bulk water. Another proposed explanation for the improved water retention is that the presence of the solid zirconium phosphate leads to smaller dimensions of the free spaces in the nanopores, inducing capillary condensation effects. Theoretically, the average pores of Nafion membranes themselves should be small enough (2–3 nm) [12] to induce a significant depression of the vapor pressure due to capillarity. However, the pore size distribution in Nafion membranes is very broad [13], and a small fraction of pores is larger than 10 nm. Capillary depression of the water vapor pressure is not appreciable in pores larger than 10 nm, and thus at high temperatures the largest pores dehydrate first, interrupting the continuity of the proton conduction paths through the membrane. The presence of zirconium phosphate is likely to reduce the free space in the larger nanopores, promoting capillary condensation and thus water retention and proton conductivity. These two effects could lead to a substantial reduction in the membrane water chemical potential— increasing the level of hydration at lower relative humidity, and improving proton conduction.

4. Conclusions

Fig. 10. Comparison between H2/O2 PEMFC performance of MEAs employing ( ) recast Nafion, and ( ) recast Nafion/36 wt% zirconium phosphate composite. Operating conditions of temperature (Ta/Tfc/Tc) and pressure indicated in the figure label.

We have demonstrated that it is possible to operate a PEMFC at temperatures equal to or higher than 130 °C. The main problem for operation at these high temperatures is the water loss from the perfluorosulfonic acid electrolyte membrane. Impregnation with a hygroscopic insoluble salt, such as zirconium phosphate, gives significant improvement in performance under dehydrating conditions; this result has been

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demonstrated with composite electrolytes prepared starting from both commercial Nafion and recast Nafion films. A Nafion 115/23 wt% zirconium phosphate MEA operated with H2/O2 at 130 °C and 3 atm pressure showed a fourfold increase in current density at 0.4 V over an MEA based on commercial Nafion and operated under the same conditions. The PEMFCs analyzed in this paper show a decrease of performance when increasing the cell temperature above that of the humidifiers (i.e. when relative humidity drops below 100%). This performance decline is also seen with PEMFCs based on our composite Nafion/zirconium phosphate membranes, although the effect is less pronounced than with unmodified commercial Nafion films. We believe that this limitation affects, in general, all the PEMFCs based on electrolytes whose proton conduction mechanism is based on the presence of water. The relative humidity in the fuel cell is an important operating parameter and decreases dramatically when the cell temperature increases above the humidification bottle temperatures [3]. In turn, the temperature of the humidifiers is limited by the temperature of boiling water, and cannot be increased without increasing the operating pressure above the desirable limits for PEMFC operation. Further improvements can be made by optimizing the structure, chemical and physical properties of these composite membranes. But still, fundamental limitations remain so that dramatic improvements may only be realized with new proton conducting materials with water-independent conduction mechanisms. However, the selection of a suitable material is difficult, because many requirements, besides a good proton conductivity level, have to be met. Compatibility with the electrocatalyst and the fuel cell environment is also a very difficult requirement [3].

Acknowledgements The authors wish to thank Dr Jen Willson for her assistance in acquiring SEM and FTIR-ATR data and K. Adjemian, Dr J. Ogden, Professor A. Bocarsly, Dr S.J. Lee and for valuable discussions and cooperation. Financial support from the US Department of Energy (Grant N°DE-PS02-98EE50493) is also acknowledged.

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