Effect of Bulk and Surface Treatments on the Surface Ionic Activity of

Journal of The Electrochemical Society, 154 共11兲 A1073-A1076 共2007兲

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0013-4651/2007/154共11兲/A1073/4/$20.00 © The Electrochemical Society

Effect of Bulk and Surface Treatments on the Surface Ionic Activity of Nafion Membranes Trung Van Nguyen,a,*,z Minh Vu Nguyen,a Karen J. Nordheden,a,* and Wensheng Heb,* a

Chemical and Petroleum Engineering Department, The University of Kansas, Lawrence, Kansas 66045, USA TVN Systems, Incorporated, Lawrence, Kansas 66046, USA

b

The ionic activity at the interfaces of the membrane and the ionic polymer phase in the catalyst layers can have a significant impact on the transport rates of protons and water in a proton exchange membrane fuel cell 共PEMFC兲. This study investigated the effects of some treatment processes on the surface ionic activity of extruded Nafion membranes and their performance in a PEMFC. The treatment processes included H2SO4 and H2O2 wash and plasma sputter and reactive ion etching. The membrane surface ionic activity was determined by the S:C ratio using X-ray photoelectron spectroscopy. The results showed that the surface of these membranes, as received and after H2SO4 wash, had lower ionic activity than that of its bulk 共S:C = 0.026 vs 0.053兲. Treatment with H2O2 had a significant impact on the surface ionic activity, lowering the surface S:C ratio further to 0.017. The Teflon-rich skin of extruded Nafion membranes could be removed by sputter etching with argon exposing a surface with higher ionic activity 共S:C = 0.047兲. However, reactive ion etching with SF6 and argon led to a further decrease in the surface ionic activity 共S:C = 0.011兲. Fuel cell results showed a strong correlation between the membrane surface ionic activity and the fuel cell performance. © 2007 The Electrochemical Society. 关DOI: 10.1149/1.2781247兴 All rights reserved. Manuscript submitted April 18, 2007; revised manuscript received August 6, 2007. Available electronically September 27, 2007.

The proton exchange membrane 共PEM兲 fuel cell has been recognized as a strong alternate energy conversion system and power source because of its high power density and efficiency, simplicity in design and operation, and zero emission if hydrogen is used as fuel. However, there are several aspects that need to be optimized to further increase the power output of this fuel cell, thus making it more competitive with conventional energy conversion devices. One area of interest to us is the reduction of the ionic resistance at the surface of the Nafion membrane. Nafion membrane, like other polymers, has a low surface energy, which causes poor adhesion. According to Mittal and Pizzi,1 polymer-to-polymer adhesion is more a result of mechanical interlocking than interfacial interaction, whereas polymer-to-metal adhesion is achieved mostly by interfacial interaction. In a PEM fuel cell, the power-generating component called the membrane-and-electrode assembly 共MEA兲 is prepared, mainly, by two methods. In one method, the catalyst layers composing of a Nafion ionic polymer phase and solid Pt on carbon support are hot pressed onto each side of the membrane to form a three-layer MEA. Any porous carbon diffusion layers used are placed in physical contact with the catalyst layers. In the other method, the catalyst layers are first applied on the porous carbon support layers and then hot pressed onto the membrane to form an integrated five-layer MEA. Regardless of the methods used to prepare the MEAs, both types of interfaces exist in the MEAs. The polymer-to-metal interface exists between the ionic polymer phase and the solid catalyst phase in the catalyst layers, and the polymer-to-polymer interface exists between the ionic polymer phase in the catalyst layers and the polymer electrolyte membrane. The surface ionic activity 共area concentration or number of the SO3−H+ group on the surface兲 of the membrane at the polymer-tocatalyst interface affects both the electrochemical reaction rates and the transport rates of the protons and water to and from the catalyst surface. The surface ionic activity at the interface of the polymer electrolyte membrane and ionic polymer phase in the catalyst layer affects the transport rates of the protons and water across this interface. Information on the effect of the membrane surface ionic activity on the polymer-to-catalyst interface can be found in Ref. 2. In this study, we investigated the effects of various membrane bulk and surface treatment processes on the surface ionic activity of the membrane and the subsequent effects of the membrane surface ionic activity at the polymer-to-polymer interface between the ionic poly-

* Electrochemical Society Active Member. z

E-mail: [email protected]

mer phase in the catalyst layer and the polymer electrolyte membrane on the performance of the MEA in a PEM fuel cell. Experimental The membranes used in this study were extruded Nafion 112 membranes obtained from Ion-Power, Inc. The membrane surface ionic activity was determined by measuring the sulfur-to-carbon 共S:C兲 ratio with X-ray photoelectron spectroscopy 共XPS兲. The XPS measurements were done with a Specs Sage 100 analyzer with the following settings: Base pressure of 2 ⫻ 10−8 Torr, X-ray angle of 45°, take-off angle of 90°, and no charge correction. The membrane bulk treatment processes investigated included 共i兲 the sulfuric acid wash treatment process typically used to remove the inorganic impurities in the membrane and to ensure that the membrane was fully converted into the H+-form, and 共ii兲 the dilute hydrogen peroxide 共H2O2兲 treatment process typically used to remove the organic impurities in the membrane.3-5 The membrane surface treatment processes investigated include the plasma reactive ion etching 共RIE兲 and plasma sputter etching 共SE兲 processes. Contact-mode atomic force microscopy 共AFM兲 was used to analyze the membrane topography before and after plasma surface treatment. The acid wash treatment used in this study consisted of the following steps. The membrane was placed in a 1 M sulfuric acid solution at room temperature. The solution was heated to 65°C, maintained at this temperature for 1 h, and then allowed to cool to room temperature. The membrane was removed and rinsed multiple times with deionized water. Next, the membrane was placed in deionized water at room temperature. The water was heated to 65°C, maintained at this temperature for 1 h, and then allowed to cool to room temperature. The membrane was removed and rinsed with deionized water until a pH of ⬃7 in the washed solution was obtained. For the dilute hydrogen peroxide treatment process, the following steps were used. The membrane was placed in a 3 wt % hydrogen peroxide solution, and the solution was heated to 65°C and maintained at this temperature for 1 h. Once the solution was allowed to cool to room temperature, the membrane was removed and washed with deionized water multiple times. For the study on the effect of combined acid and hydrogen peroxide treatment, the membrane was subjected to the hydrogen peroxide treatment first and then the acid treatment. The plasma treatment investigated consisted of two different processes, sputter etching 共SE兲 and reactive ion etching 共RIE兲. The SE

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Table I. Operating conditions of the SE and RIE processes. Etching Argon Etching flow rate SF6 flow rate RF power Gas pressure duration 共sccm兲共W兲共mTorr兲共min兲 method 共sccm兲 SE RIE

20 5

0 15

100 100

15 15

5 5

drogen gas flow rate was varied was also conducted to ensure that the anode had minimal effect on the overall fuel cell performance. The fuel cell temperature was held constant at 60°C unless otherwise stated. The outlet pressures for both the anode and cathode were at atmospheric pressures. The upstream pressures were not measured. Results and Discussion

process employed an inert gas, such as argon, to physically ion bombard the surface of the membrane. The RIE process relied on both the ion bombardment and reactions with a reactive gas such as SF6. A mixture of these two gases is often used to enhance the plasma etching rate.6 In this study, a Plasma-Therm 790 Series was used to etch Nafion membranes. For the SE process argon was used, and for the RIE process, a mixture of argon and SF6 was used. The operating conditions of the two processes are listed in Table I. For both processes, the substrate electrode was maintained at a constant temperature of 20°C. The etching time was selected to remove roughly 1 ␮m off the top surface of the membranes. An area of the membrane was masked off so that it could be used later to determine the etched depth. Both surfaces of the membranes were etched. Note that the removed layers 共2 ␮m兲 represent ⬍4% of the thickness 共51 ␮m兲 of a Nafion 112 membrane. Atomic force microscopy was also used to analyze the topography of the membrane surface before and after etching. For fuel cell testing, membrane-and-electrode-assemblies 共MEAs兲 with active areas of ⬃5 cm2 共2.2 ⫻ 2.2 cm兲 were prepared as follows. A catalyst ink consisting of a composition of Pt:C:Nafion:Teflon of 1:4:3:1 was prepared using 25 wt % Pt on carbon support catalyst from E-TEK, Inc., 5 wt % Nafion ionomer solution from Ion Power, Inc., and Teflon suspension from DuPont Company. Teflon was added to improve the catalyst layer ability to handle two-phase flow.7 The ink was applied onto a single-layer porous gas diffusion material 共Toray TGPH-060兲 and allowed to dry. The catalyst-coated gas diffusion layers were hot pressed onto the membrane at 130°C and 680 kPa. The catalyst loading for both electrodes was ⬃0.25 mg Pt/cm2 unless otherwise stated. To minimize variations in the fuel cell performance due to the electrode preparation, a single large electrode was coated with catalyst and then cut to smaller electrodes used in the MEAs. This was done to ensure that the electrodes used in these studies have uniform properties. It is well known in processing that variations within batches are often smaller than variations between batches. The hydrogen and airflow rates were adjusted according to the current density of the fuel cells. At the anode, neat hydrogen saturated with water vapor at 70°C was fed at a rate of three stoichiometries, and at the cathode, dry air was fed at a rate of about six stoichiometries. A higher anode humidification temperature was used to ensure that the anode gas stream was sufficiently humidified at high gas flow rates. To minimize the effect of condensation the following were used in the fuel cell setup: 共i兲 liquid water trap at the fuel cell inlet, 共ii兲 interdigitated flow field to remove liquid water from the gas diffusion layer 共GDL兲, and 共iii兲 high hydrogen gas flow rates. A preliminary test in which the hy-

Effect of bulk treatment on membrane surface ionic activity.— The XPS measurement results of extruded Nafion 112 membranes subjected to various bulk treatment processes are summarized in Table II. As shown in Table II, an as-received extruded Nafion membrane had lower surface ionic activity than that calculated from the chemical formula of Nafion 1100 polymer. The sulfur-to-carbon 共S:C兲 ratio on the surface of the as-received membrane was 0.024 as compared to 0.053 calculated from the chemical formula for Nafion 1100. Treatment with acid solution increased the surface ionic activity slightly from a sulfur-to-carbon ratio of 0.024 to 0.028, which could be considered to be within the experimental errors. The measurement errors for the instrument used in the XPS measurements were within 5–10%. Treatment with diluted 共3%兲 hydrogen peroxide solution, however, had a significant impact on the membrane surface ionic activity, reducing the already low sulfur-tocarbon ratio of the as-received material from 0.024 to 0.017. The impact observed was for the hydrogen peroxide conditions used in this study. The effects of longer treatment and higher hydrogen peroxide concentration have not been investigated. To determine the effects of the surface ionic activity of the polymer electrolyte membrane on the performance of a PEM fuel cell, MEAs made with membranes treated and untreated with diluted hydrogen peroxide solution were prepared and tested in a fuel cell at the conditions specified earlier. These two groups were selected because of their significantly different surface ionic activities. The results are shown in Fig. 1. Note that the fuel cell using the MEA made with a membrane treated with hydrogen peroxide showed much lower performance than the one using the MEA made with an untreated membrane. These results agree with recent findings that trace amounts of hydrogen peroxide could initiate rapid membrane degradation,8 which could affect both the polymer-to-catalyst interface and the polymerto-polymer interface in the MEA. The results shown above also agree with previous findings9-12 that extruded Nafion membranes exhibited Teflon-rich skins. These observations prompted us to believe that the Teflon-rich skin was an artifact of the extrusion process and that, if the top surface layers could be removed, then a new surface with higher ionic activity would result. Various plasma etching processes were then explored as a method of removing the Teflon-rich top layers from the extruded Nafion membranes. Effect of plasma etching processes on membrane surface ionic activity.— Two plasma etching processes were explored in this study, sputter etching and reactive ion etching. The results of these plasma etching processes on the surface ionic activity of extruded Nafion membranes are given in Table III.

Table II. Atomic existence ratio of extruded Nafion 112 membranes by XPS. Sample

Sulfur-to-carbon ratio

Unetched membrane, as-received Unetched membrane, treated with 1 M H2SO4 Unetched membrane, treated with 3% H2O2 and 1 M H2SO4 Bulk membrane based on chemical formula for Nafion 1100 Chemical formula − 共CF2CF2兲6 − 共CFCF2兲− of Nafion 1100

0.024 0.028 0.017 0.053

兩 O − CF2CF共CF3兲 − O − CF2CF2SO3H 共S to C ratio = 1 to 19 or 0.053兲



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Figure 2. Topography of unetched and etched Nafion 112 membranes.

Figure 1. Effect of H2O2 treatment on surface ionic activity and fuel cell performance.

The results in Table III showed that the reactive ion etching process, in addition to removing the top layers of the membranes, appeared to preferentially remove the ionic groups or side chains from the Teflon backbone leaving behind a surface that had even lower ionic activity with sulfur-to-carbon ratio of 0.011 as compared to 0.024 for unetched, untreated membranes. Note that the sulfur-tocarbon ratio of the membrane etched by the reactive ion etching process was even lower than that of the membrane treated with diluted hydrogen peroxide solution. The preferential removal of the ionic groups or side chains was attributed to its reaction with the fluorine ions in the plasma. Next, the results in Table III showed that the physical plasma etching process using argon removed the polymer material from the membrane surface with no preference for any specific groups. Once this Teflon-rich skin was removed, a new surface with a composition very close to that of the bulk composition was created with the sulfur-to-carbon ratio of 0.047 as compared to 0.053 calculated from the chemical formula of Nafion 1100. Even though the sulfur-to-carbon ratio of the surface of the membranes etched by the physical plasma etching process was not equal to the calculated value, it was very close, considering the experimental errors of the technique and the fact that the equivalent weights reported for these membranes are averaged values. Finally, the membrane surface sulfur-to-carbon ratio obtained from sputter etching represents a significant increase from that of as-received or untreated membranes. When the surfaces of these membranes were analyzed by atomic force microscopy 共AFM兲, it was observed that surfaces of the membranes etched by both the sputter and reactive ion etching processes were very similar showing surfaces with more consistency and higher roughness. The topographies of unetched and argon etched membranes obtained by AFM under contact mode are shown in Fig. 2. Note the irregular surface of the unetched membrane. The results

given in Ref. 2 showed that there was no correlation between the topography and surface ionic activity of the membranes. Once the surface ionic activities of these etched membranes were determined, MEAs with these membranes were prepared. These MEAs were tested in a PEM fuel cell under the conditions similar to those specified earlier. The fuel cell performance results are given in Fig. 3. The fuel cell results showed that the MEA made with the membrane etched by argon had the best fuel cell performance, followed by that with an unetched membrane and that with a membrane etched by reactive ion plasma which had the lowest surface ionic activity. These results along with the results given in Fig. 1 showed that there was a definite correlation between the surface ionic activity of the polymer electrolyte used in the MEA and the fuel cell performance; that is, fuel cells with MEAs made of membranes with higher surface ionic activities had higher fuel cell performance. Note further that the fuel cell performance increase was reflected in the lower voltage-current density slopes in the ohmic region of the polarization curves. These lower slopes could be attributed to the higher ionic conductivity at the polymer-to-polymer interfaces between the polymer electrolyte membrane and the ionic phase in the catalyst layers enabled by the higher surface ionic activities of the membranes. Finally, the results in Fig. 3 show that the effect of etching on the membrane thickness was insignificant because, while etching reduces the membrane thickness in all cases, in the sputter etching case the fuel cell performance increased and in the reactive ion etching case the performance decreased. Finally, to confirm the negative effect of diluted hydrogen peroxide treatment on the membrane surface ionic activity and its subsequent effect on the fuel cell performance, a membrane that was

Table III. Effect of plasma etching on extruded Nafion surface ionic activity.

Sample Unetched membrane, as received Etched membrane with argon and SF6 共RIE兲 Etched membrane with argon 共SE兲 Bulk membrane based on chemical formula for Nafion 1100

Sulfur-tocarbon ratio 0.024 0.011 0.047 0.053

Figure 3. Effect of plasma etching on membrane surface ionic activity and fuel cell performance.


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Figure 4. Fuel cell performance with etched Nafion 112 treated and untreated with H2O2. Catalyst loading for this case was 0.59 mg Pt/cm2.

physically etched by argon, a process that resulted in higher membrane surface ionic activities, was subsequently subjected to the hydrogen peroxide treatment. This membrane was used to prepare an MEA that was then tested in a PEM fuel cell. Figure 4 shows the fuel cell performance results for two MEAs, one with a membrane etched by argon and the other with a membrane etched with argon that was subsequently treated with diluted hydrogen peroxide solution. First, note that the testing conditions in this case were different from those used in the previous cases. Also, the catalyst loading in the electrodes was higher, 0.59 mg Pt/cm2 vs 0.25 mg Pt/cm2 in the previous cases shown in Fig. 1 and 3. This explains the higher performance observed in Fig. 4 for both the untreated and hydrogen peroxide treated MEAs. As before, the fuel cell with the MEA made with an argon-etched membrane performed much better than that with the MEA made with an argon-etched membrane that was subsequently treated with hydrogen peroxide. These results showed clearly that the hydrogen peroxide treatment on the polymer electrolyte membrane had a detrimental effect on the electrochemical performance of the fuel cell. This decrease in performance was attributed to the loss in ionic activity at the interfaces between the polymer electrolyte membrane and the ionic phase in the catalyst layers in the MEA leading to poor ionic conductivity and water transport across these interfaces. It was also noted in our study that MEAs made with membranes treated with hydrogen peroxide consistently had lower open-circuit voltages 共OCVs兲 than those with untreated membranes. It is possible that treatment with hydrogen peroxide solution affected both surface and bulk properties of the membrane allowing higher hydrogen crossover rate in the membrane, a process that was known to lead to lower OCVs in PEM fuel cells. This hypothesis could be validated by checking the equivalent weight of the membrane before and after treatment 共by titration兲 or/and subjecting a hydrogen peroxide treated membrane to sputter etching, which would remove the lower activity membrane surface but not affect its bulk properties. We will include this in our future studies. Conclusion Extruded Nafion 1100 membranes exhibited Teflon-rich skins. These Teflon-rich skins had sulfur-to-carbon ratios or surface ionic activity much lower 共S:C ratio of 0.024兲 than that calculated from the chemical formula of Nafion 1100 共S:C ratio of 0.053兲. Treatment with acid solutions to remove inorganic impurities and to protonate the membrane resulted in no changes in the membrane surface ionic activity. However, treatment with diluted hydrogen peroxide solutions led a significant reduction in the membrane surface ionic activity 共S:C ratio of 0.014兲.

Removal of the membrane top surface layers 共⬃1 ␮m兲 by physical plasma etching with argon resulted in surfaces with ionic activities very closed to that of the bulk membrane material, S:C ratios of 0.046 vs 0.053, respectively. The application of reactive ion etching with SF6 and argon to remove the top surface layers of the membrane led to additional damage to the membrane surface ionic activity resulting in a sulfur-to-carbon ratio at the surface that was even lower than that of diluted hydrogen peroxide treated membranes 共0.011 vs 0.017兲. Finally, there was a definite correlation between the membrane surface ionic activity and the fuel cell performance. MEAs made with membranes with higher surface ionic activities had polymer-topolymer interfaces between the polymer electrolyte membrane and the ionic phase in the catalyst layers that were more ionically conductive and had higher water transport properties. These resulted in lower ionic resistance in the fuel cell and better fuel cell performance. In our future work, we will look at cast Nafion membranes, which are believed to have higher surface ionic activity than extruded Nafion membranes. Acknowledgments The work done at the University of Kansas was supported partially by The University of Kansas Energy Research Center and the National Science Foundation under grant no. CTS-9909763. University of Kansas assisted in meeting the publication costs of this article.

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