Current Sensing Atomic Force Microscopy Study of Thermal Aging of

Article pubs.acs.org/JPCC

Current Sensing Atomic Force Microscopy Study of Thermal Aging of Nafion Membranes Osung Kwon,† Yucong Liu,† Elaine Zhu,‡ Shijie Wu,§ and Da-Ming Zhu*,† †

Department of Physics, University of MissouriKansas City, Kansas City, Missouri 64110, United States Blue Valley High School, Stilwell, Kansas 66085, United States § Agilent Technologies, Inc., Chandler, Arizona 85282, United States ‡

ABSTRACT: We have investigated thermal aging in the ionic conductivity of Nafion membranes being annealed at elevated temperatures using current sensing atomic force microscopy (CSAFM), a unique technique that allows simultaneous imaging surface morphology and surface conductance of the materials. The local current flows on the membranes show an increase after initial annealing, followed by a gradual decrease as the annealing continues. The current distribution extracted from CSAFM images shows that the decrease in the conductance follows approximately an exponential decay which occurs uniformly across the membrane. Least-square fits to the measured conductance versus time t yield time constants at the annealing temperature. Considering aging is a thermally activated process, we used an Arrhenius equation to describe the relationship between the time constant and the annealing temperature, which gives activation energies for the membranes. The activation energy yields following this process are 54 and 83 kJ/mol for Nafion 115 and Nafion NR212, respectively. These results provide guidance for employing the materials for device applications.

1. INTRODUCTION Polymer electrolyte membrane (PEM) fuel cells have emerged among the most promising devices for environmentally friendly power generation and energy conversion/storage applications in recent years,1−3 but their durability and longevity under a wide variety of operation conditions remain to be further improved in order to be truly commercially viable.4−9 The key component which strongly affects the performance of a PEM fuel cell as well as its durability and longevity are an ion exchange membrane which provides channels for transporting ions across while separating the reaction gases and liquids from the opposite electrodes in the fuel cell.1−3 Currently the most widely used and that being considered as the state-of-the-art ion exchange membrane in fuel cell application is Nafion, which represents a family of comb-shaped ionomers which consist of a polytetrafluoroethylene (PTFE) hydrophobic backbone with perfluorinated pendant terminated by hydrophilic sulfonic acid head groups (−SO3H).5,10,11 The microscopic structure of a Nafion membrane has been actively studied but is still a subject under intensive debate.10−28 A commonly referenced model describes its structure as a PTFE backbone matrix embedded with ionic clusters, which are approximately spherical in shape with an inverted micelle structure formed by aggregated hydrophilic head groups in the membranes, connected by narrow hydrophilic channels allowing ionic conductivity.22,23 A recently proposed model based on computer simulation and reanalysis of the currently available small-angle X-ray and © 2017 American Chemical Society

neutron scattering data describes the ionic structure of Nafion membranes as parallel-aligned cylindrical hydrophilic channels of nanometer size with the directions predominantly perpendicular to the membrane’s surface.26 Protons or ions can transport along the hydrophilic channels, producing excellent ionic conductivity of the membranes. This model explains many important features of Nafion membranes including fast diffusion of water and protons through the membranes and their persistence at low temperatures.26 However, it remains a challenging task to reveal and to understand the nature and the characteristics of these ionic channels and to determine their structures and properties in Nafion membranes. More importantly, it is not clear at a microscopic scale how the ionic channels change under different conditions such as at an elevated temperature and over time, as the changes in the chemical composition and microstructure of Nafion membranes associated with the thermal aging processes have been identified using various techniques.4−8,10−28 In this work, we have studied the changes of proton channel network in Nafion membranes after being annealed at elevated temperature by systematically measuring their local ionic conductance distributions using current sensing atomic force Received: February 5, 2017 Revised: March 21, 2017 Published: March 22, 2017 7741

DOI: 10.1021/acs.jpcc.7b01111 J. Phys. Chem. C 2017, 121, 7741−7749

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The Journal of Physical Chemistry C microscopy (CSAFM). In one of our previous reports,29 we have demonstrated that after the initial annealing at a temperature close to but still below the glass transition temperatures of the membranes the alignment of proton channels on the membrane surface becomes mostly in parallel to the membrane surfaces, accompanied by an increase of proton conductivity of the membranes. Here, we show that, as the annealing continues, the proton conductivity starts to decrease. The current distribution extracted from the CSAFM images shows that the decrease in the conductance follows an approximately exponential decay which appears to occur uniformly across a membrane; the decay constant increases with the annealing temperature. Using an Arrhenius equation to describe the decay constant as a function of temperature, we have estimated the activation energies for Nafion 115 and Nafion NR 212 membranes. We discuss several possible mechanisms which lead to the degradation in the ionic conductivity and its relations to the changes of ionic channels due to the thermal annealing.

areas making contact with ionic channels exposed on the surface. The protons generated by the catalyst at the tip− membrane contacts transport through the proton channel network across the MEA, and a current is detected in CSAFM. The pathways of the current flowing across the proton channel network in the membrane are rather complicated. However, if each proton channel branches out or merges with others over a distance much shorter than the thickness of the membrane, the proton current flowing from the tip−membrane contact would spread out over a large portion of the network before reaching the opposite electrode, as illustrated in Figure 1. As mentioned above, according to a recently proposed model

2. METHODS AND MATERIALS Current sensing atomic force microscopy (CSAFM) is one of the major advances in the development of scanning probe microscopy in the past two decades.30−40 The technique allows for direct and simultaneous visualization and imaging of the morphology and local conductance of a sample surface. Using a metal film coated AFM tip and applying a voltage bias between the sample surface and the conducting tip, a current flow is generated between the AFM tip and the sample when the CSAFM operates in a contact mode. Under most conditions, the current flow can be used to construct spatially resolved surface conductance images. The current measurements with a resolution in the range of pico (10−12) to femto (10−15) -ampere have been achieved. Surface morphology and surface conductance images are formed as the conductive AFM tip roasting across a sample surface.29−36 It has been demonstrated that, in the study of proton exchange membranes, CSAFM imaging can provide valuable information related to the conductance of the individual channel and the configurations of the channels on the surface of the membrane.36−40 The experimental samples used in this work are Nafion 115 and Nafion NR212 membranes. The chemical compositions of the two types of membranes are supposedly identical. The differences between the two are their thickness, with Nafion 115 being 127 μm and Nafion NR212 being 51 μm in thickness. The former is extruded, while the latter is casted in the fabrication process.41 The membrane samples, used as received from the manufacturer without any further treatment, were hot pressed onto carbon cloth gas-diffusive electrode substrates to form half membrane−electrode assemblies (MEAs).42 The MEA was installed in the sample stage of an AFM; the bottom electrode was connected to a current sensing loop which connects to the probe tip of the AFM.43 In the measurements, a platinum-coated AFM tip was brought in contact with the exposed surface of the membrane in the MEA; the tip/MEA system (tip/platinum/membrane/ platinum−carbon cloth) forms a miniature fuel cell with the tip functioning as a nanometer-sized electrode. The tip/MEA was placed in an environmental chamber with controlled temperature and relative humidity. A bias voltage for initiating electrolysis of water is applied to the tip to produce hydrogens which subsequently get oxidized at the tip to generate protons. As the tip scans across the surface of a membrane, it passes over

Figure 1. Illustration of a current sensing AFM in the study of an ion exchange membrane. The interconnected ionic clusters (blue colored) in the membrane form ionic channels. The red colored clusters represent pathways for ionic current flowing in the ionic channel network of the membrane. Catalyst layer on the AFM tip and the catalyst on carbon cloth (black) are marked as yellow objects.

for Nafion membranes,26 the length of each proton channel is about 100 nm, which is much smaller than the typical thickness of a Nafion membrane.26 Therefore, between a fixed CSAFM conducting tip and the carbon electrode, the proton conductance across the proton channel network should be roughly a constant; the variation in proton current detected by the CSAFM reflects the changes in the contact conductance between the tip and proton conducting channels as the tip scans across the membrane. Hence, a current sensing image maps the contact probability of the tip and ionic clusters on the membrane surface which connects to the proton channel network in the membrane.36 If the contact area is much larger than the size of the individual ionic cluster, such a probability would follow a Poisson distribution. 44 For a Poisson distribution, the peak and the width of the distribution are dictated by a single variable; thus, their changes with annealing temperature and the time should be about the same.44 If the contact area is much larger than the size of the ionic clusters, the Poisson distribution would appear as a Gaussian distribution.44 In our experiments, several Nafion NR212 and Nafion 115 membrane samples were annealed over a temperature range from 80 to 120 °C. The pristine membrane samples were initially imaged with CSAFM to obtain surface morphology and proton conductance images. After that, the probe tip was retreated from the membrane surface, and the MEA was heated on the sample stage to an elevated temperature under a relative humidity of 50% for a predetermined time period. After the heating, the MEA was slowly cooled to room temperature; CSAFM imaging was conducted again on the membrane. The cooling process typically took several hours; after reaching 7742


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The Journal of Physical Chemistry C

Figure 2. Topography images (A1−E1) and simultaneously obtained current sensing images (A2−E2) on a Nafion NR212 membrane after it was annealed at 120 °C for different time periods: 3 h (Figures A#); 12 h (Figures B#); 15 h (Figures C#); 18 h (Figures D#); and 30 h (Figures E#). Figures A3−E3 are the ionic current distributions extracted from the corresponding current sensing images (A2−E2).

3. RESULTS AND DISCUSSION Topographic and current sensing images of the Nafion membranes were obtained simultaneously at different stages of the annealing. The conductance degradation that occurred on the membranes annealed at temperature of 80 °C or below is very slow; we did not see significant changes to the membranes annealed for about 5 days through CSAFM imaging. However, noticeable changes can be observed when the membranes were annealed at a temperature above 100 °C for a few hours. In Figures 2 and 3, we depict typical topography and current sensing images on Nafion NR212 and Nafion 115 membranes obtained simultaneously after they were annealed at elevated temperatures for different time periods. The topographic images in Figures 2A1−2E1 and in Figures 3A1−3E1 show that annealing at the temperatures used in this work has almost no effect on the membrane’s surface

room temperature the MEA was left for several hours under the same relative humidity before imaging was conducted, to ensure the membrane was in equilibrium with the environment. This heating−cooling−imaging process was repeated until the measurements on the aging process were completed. We find that at room temperature the degradation of the conductivity of the Nafion membrane is very slow; any changes to the membranes when they are sitting at room temperature do not contribute significantly to the thermal decay of the membranes that occurs at elevated temperatures. We assume that the time when the membranes were sitting at room temperature does not contribute to the decay of the membrane in comparison to that at elevated temperatures. Thus, the annealing time is the sum of all the time that the membranes were annealed at an elevated temperature, which we assign to be the degradation time t for the membranes. 7743


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Figure 3. Topography images (A1−E1) and simultaneously obtained current sensing images (A2−E2) on a Nafion 115 membrane after it was annealed at 110 °C for different time periods: 6 h (Figures A#); 14 h (Figures B#); 21 h (Figures C#); 33 h (Figures D#); and 37 h (Figures E#). Figures A3−E3 are the ionic current distributions extracted from the corresponding current sensing images (A2−E2).

morphology. However, the current sensing images in Figures 2A2−2E2 and in Figures 3A2−3E2 show the patterns of the

bright regions, which correspond to conductive regions on the membrane surfaces where ionic channels make contact with the 7744


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Figure 4. Peak current Ip, full width at half-maximum (fwhm), and the integrated current Ii of the current distributions for Nafion membranes annealed at 110 °C for different periods of time. Nafion 115 (a−c) and Nafion NR 212 (d−f). The lines are the least-squares fits to the data using equation 1.

CSAFM tip and change drastically with the annealing. In one of our previous reports,29 we have shown that in the initial stage of the annealing the bright areas in the current sensing images gradually evolve into chain-like filament structures (see, e.g., Figures 2B2, 2C2, 3B2, and 3C2), and the total current integrated over the current sensing images shows an increase with annealing time. Such a characteristic was interpreted as an indication that the configuration of the ionic domains on the membrane surface changes from cluster-like to chain-like structure, where the chains align themselves in parallel near the membrane surface and thus increase the contacts between the tip and the ionic channels.29 However, as annealing continues, typically as it passes beyond about 10 h, the conductivity of the membrane starts to decrease. The CSAFM images show that the configuration of the ionic channel network near the surface remains about the same, but overall the current detected by CSAFM decreases steadily with the annealing time. As can be seen in Figures 2A2−2E2 and 3A2−3E2 the maximum current measured shows a steady decrease as the annealing time increases. The decrease can be more clearly shown in the current distributions obtained from the current sensing images described below. CSAFM generates current sensing images of the membrane surface by plotting the ionic current detected at each pixel. The collection of the current detected at each pixel in a particular current sensing image provides a distribution which reflects the local conductance on the membrane surfaces. Figures 2A3−2E3 and Figures 3A3−3E3 are the current distributions extracted

from the corresponding CSAFM images displayed in Figures 2A2−2E2 and in Figures 3A2−3E2. Integrating the current at each pixel over an entire image gives the total current It which is proportional to the conductivity of the membranes across the imaged area. In this work, we found that, in most cases, the currents measured by the tip extracted from the current sensing images form a single peaked distribution, as shown in Figures 2 and 3. Such an observation is consistent with our discussion above about the statistics that current distribution would follow under the experimental condition. In a current distribution, the peak value corresponds to the product of the average number of clusters in contact with the tip and the average ionic conductance of individual clusters, while the full width at halfmaximum height (fwhm) of the distribution is roughly proportional to the variations of the size and the local density of the ionic clusters. Thus, the peak current Ip represents the most probable contact between the tip and ionic channels on the membrane surface; the width of the peak resembles randomness of the contacts across the membrane. In Figures 4 and 5, we plot the peak current Ip, the integrated current It, and the fwhm of the current distributions of the membranes annealed at different temperatures as a function of annealing time for the Nafion 115 and Nafion NR212 membranes. Indeed, they all decrease with annealing time; the decay accelerates at an elevated temperature, showing a characteristic behavior of thermal aging of materials. To further analyze the thermal aging behavior of the Nafion membranes, we assume that associated with the thermal aging of the 7745


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Figure 5. Peak current Ip, full width at half-maximum (fwhm), and the integrated current Ii of the current distributions for Nafion membranes annealed at 120 °C for different periods of time. Nafion 115 (a−c) and Nafion NR 212 (d−f). The lines are the least-squares fits to the data using equation 1.

membranes the quantities Ip and fwhm of the current distribution and It all follow a first-order kinetic equation

dI I =− dt τ

the results are listed in Table 1. The values of the activation energy obtained from the peak current, fwhm, and the integrated current are quite close for each of the Nafion membranes. The averaged value for Nafion 115 membranes is 54 kJ/mol, and that for the Nafion NR 212 membrane is 83 kJ/ mol. These values are consistent with those reported in the literature.47−49 Using these values for the activation energy to estimate the averaged lifetimes for Nafion 115 and Nafion NR 212 at room temperature yields the lifetimes in the range from about 6 months to about three years. The changes in the local conductance which is associated with the configuration of the proton channel network in the membranes at the annealing temperatures can be explained in terms of rapid increase of molecular mobility as the glass transition temperature of the membrane is approached. The glass transition temperature of Nafion membranes varies in the range of 100−150 °C, depending on the water uptake by the membrane.50−52 As the temperature is close to this temperature range, the mobility of molecules should increase rapidly, making the proton channels easier to rearrange themselves to form a stable state. The difference between activation energies for Nafion115 and Nafion NR212 probably can be related to the different processing for these two types of membranes: extrusion is used in processing Nafion 115 while casting for Nafion NR212 membranes.47,48 An extrusion process introduces additional stress to the membrane. It is known that stress tends to reduce the glass transition temperature of an amorphous polymer.53,54 Thus, the annealing temperature

(1)

where τ is the time constant of the degradation. Solving this equation by integrating the time t yields I = I0e−kt

(2)

where I0 is the initial value for I. Using this equation to fit the decay of quantities Ip, It, and fwhm of the current distribution versus annealing time t, we obtained the time constants τ for these quantities, which are plotted semilogarithmically in Figure 6 as a function of reciprocal temperature 1/T, where T is the absolute temperature. Thermal aging of the Nafion membranes could be attributed to a number of possibly structural and chemical changes in the membranes, but the macroscopic behavior of the thermal aging can always be described in terms of a kinetics model which considers the aging is related to a transformation process of the membrane over an activation energy barrier; the temperature dependence of the aging rate or equivalently the time constant τ follows a Arrhenius equation45,46 ⎛ E ⎞ ⎟ τ = A exp⎜ ⎝ RT ⎠

where E is the activation energy, and R is the universal gas constant. We fitted the data using this Arrhenius equation, and 7746


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Figure 6. Time constants obtained from the peak current Ip, fwhm, and the integrated current Ii of the current distributions extracted from the CSAFM images. Nafion 115 (a−c) and Nafion NR 212 (d−f). The lines are the Arrhenius fits to the time constat data. The fitting parameters are listed in Table 1. The lines are the least-squares fits to the data using equation 2.

Table 1. Parameters Obtained from Arrhenius Fits ln τ = ln A + (E/R)1/T to the Peak Current Ip, the Full Width at Half Maximum Height (fwhm), and the Integrated Current Ii of the Current Distribution Extracted from CSAFM Imagesa Nafion115 NafionNR 212 a

ln A E/R ln A E/R

peak current Ip

fwhm

integrated current Ii

−13.31 6152.4 −23.16 10022.0

−14.62 6776.9 −23.02 9956.5

−12.71 5905.3 −23.25 9969.4

explain the nature and the mechanism of thermal aging in Nafion membranes.

4. CONCLUSIONS In conclusion, we have investigated thermal aging in ionic conductance in Nafion 115 and Nafion NR212 membranes annealed at elevated temperatures using current sensing atomic force microscopy (CSAFM). The initial annealing increased the conductivity of the membrane, followed by an exponential decay as the annealing continues. Least-squares fit to the current distribution versus time t yields time constants at the annealing temperature. We obtained the activation energies of the membranes associated with the thermal aging by using an Arrhenius equation to describe the relationship between the time constant and the annealing temperatures. The activation energies obtained following this process are 54 and 83 kJ/mol for Nafion 115 and Nafion NR212, respectively, which will provide valuable information for the applications of these Nafion membranes.

R is the universal gas constant.

used was closer to the glass transition temperature for Nafion 115 than that for Nafion NR 212, which results in a smaller activation energy and a more rapid decay in the ionic conductivity for Nafion 115 membranes. The features revealed in CSAFM images show that the peak current Ip, the integrated current It, and the fwhm of the current distributions of the membranes all change in a synchronized manner, which indicate that conductance of the overall ionic channel network inside the membrane deteriorates upon annealing, but the configuration of the network probably remains about the same. A possible mechanism of the degradation in the conductance of the ionic channel network is that the hydrophilic regions inside the membranes shrink due to the loss of hydrophilic sulfonic acid head groups upon annealing at an elevated temperature (unpublished results). Certainly more detailed studies of the changes in chemical compositions and the microscopic structures of Nafion membranes due to thermal annealing are needed to completely

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: (816) 235-5326. Fax: (816) 235-5221. ORCID

Da-Ming Zhu: 0000-0002-7480-1578 Notes

The authors declare no competing financial interest. 7747


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ACKNOWLEDGMENTS This work is supported in part by a grant from National Science Foundation under project number 1308577. Da-Ming Zhu is supported by a “Funding for Excellency” grant from the University of Missouri-Kansas City.

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