Adsorption Behavior of Perfluorinated Sulfonic Acid Ionomer on Highly

Article pubs.acs.org/JPCC

Adsorption Behavior of Perfluorinated Sulfonic Acid Ionomer on Highly Graphitized Carbon Nanofibers and Their Thermal Stabilities Shuang Ma Andersen,*,† Maryam Borghei,‡ Rajnish Dhiman,† Virginia Ruiz,§ Esko Kauppinen,‡ and Eivind Skou† †

Department of Chemical Engineering, Biotechnology and Environmental Technology, University of Southern Denmark, Niels Bohrs Allé 1, DK-5230 Odense M, Denmark ‡ NanoMaterials Group, Department of Applied Physics, Aalto University, Helsinki, Finland § New Materials Department, IK4-CIDETEC, Paseo Miramón 196, E-20009 Donostia-San Sebastián, Spain S Supporting Information *

ABSTRACT: A systematic adsorption study of perfluorinated sulfonic acid Nafion ionomer on ribbon-type highly graphitized carbon nanofibers (CNFs) was carried out using fluorine-19 nuclear magnetic resonance spectroscopy. On the basis of the values obtained for the equilibrium constant (Keq , derived from Langmuir isotherm), the ionomer has varying affinities for CNFs (Keq between 5 and 22) as compared to Vulcan (Keq = 18), depending on surface treatments. However, the interactions are most likely governed by different adsorption mechanisms depending on hydrophilicity/hydrophobicity of the adsorbent carbon. The ionomer is probably adsorbed via the polar sulfonic group on hydrophilic Vulcan, whereas it is adsorbed primarily via hydrophobic −CF2− backbone on the highly hydrophobic pristine CNFs. Ionomer adsorption behavior is gradually altered from apolar to polar group adsorption for the acidmodified CNFs of decreasing hydrophobicity. This is indicated by the initial decrease and then increase in the value of Keq with the increasing strength of the acid treatment. The corresponding carbon−ionomer composite also showed varying thermal stability depending on Nafion orientation. The specific maximum surface coverage (ΓSmax) of the CNFs is 1 order of magnitude higher than that of Vulcan. The large discrepancy is due to the fact that the ionomers are inaccessible to the internal surface area of Vulcan with high microporosity.

1. INTRODUCTION

In order to extend the electrochemical reaction zone, impregnation or immersion of Nafion solution in catalyst layer is a routinely applied procedure in PEMFC electrode preparation.7 The optimal electrode morphology includes a fine dispersion of Nafion thin film over the catalyst and a porous structure.8 The thickness of the film should be low enough to guarantee not only a wide protonic network but also good electronic conductivity through tunneling effect.9 The structure of Nafion in solution and confined in a thin film has shown significant differences from the corresponding bulk material and the reason is still not well understood. Solution-casted or spin-coated thin films10,11 on compact artificial substrates such as SiO2 or Au were studied with ellipsometry, reflectivity, TEM, or AFM to obtain fundamental polymer structure information in combination with conductivity and/or water uptake measurements. The work provided key insight into the polymer morphology and laid the foundation for simulations.12 However, the relative affinity between the

Nafion is by far the most widely used protonic conductive polymer in proton exchange membrane fuel cells (PEMFCs).1 It is a copolymer of tetrafluoroethylene and perfluoro(3,6dioxa-4-methyl-7-octene)sulfonic acid, first produced by Du Pont de Nemours and Co.

where x = 5−13.5, y = ca. 1000, and z = 1, 2, 3, .... Nafion presents several attractive features such as high protonic conductivity, good mechanical and chemical stability, low electronic conductivity, and low gas permeability. The structure of Nafion in bulk membrane (ten to hundreds of micrometers thick) has been both extensively and intensively studied over the past three decades.2−6 © 2014 American Chemical Society

Received: January 30, 2014 Revised: April 26, 2014 Published: April 28, 2014 10814

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points for catalyst loading,21 which consequently modifies the wetting property and surface structure of the nanocarbon. Moreover, the treatment also influences Nafion adsorption, which later may affect the electrode structure and the catalyst stability. In our previous work, we explored the interaction between multiwalled carbon nanotubes (MWCNTs) and Nafion.13 Although both MWCNTs and CNFs bear high content of graphitic constituent, there are significant differences in their nano structure, surface area, and porosity, which account for differences in fundamental adsorption and interaction behavior with the protonic conductive ionomer. In this work, highly graphitized carbon nanofibers were stepwise oxidized. The interactions of CNFs with Nafion ionomer were studied with adsorption isotherm in aqueous solution based on fluorine-19 nuclear magnetic resonance spectroscopy (19F-NMR). The equilibrium constant (Keq ) and surface coverage (Γmax) of the adsorption were monitored in relation to the wetting property of the substrate. Different adsorption mechanisms were proposed. Thermal stability of the polymer carbon composites was analyzed in relation to the adsorption mechanisms.

polymer and the substrate, and the surface coverage of the substrate, were not explored. Besides, most catalyst supports for PEMFC applications are highly porous. The porosity of the substrate may play a vital role in the adsorption process, as indicated by our previous study.13 It has been demonstrated by several groups that the interaction between Nafion ionomer and catalyst substrate has a great impact on the catalyst performance and durability. This is due to the fact that optimization of a fuel cell electrode involves not only catalyst and support but also the protonconductive ionomer phase, the interactions between the phases, and the resulting electrode structure, since the electrochemical reactions only take place at the three-phase boundary (TPB).14 In addition, strong affinity between binder and catalyst materials has been shown to prevent catalyst coalescence and detachment.15,16 Carbon nanofibers (CNFs) are attractive materials for PEMFCs. During the past decade, CNFs have been studied as catalyst supports giving place to improved performance and durability of PEMFC compared to the classical carbon blacks.17−19 The highly tailored graphitic nanofiber structure was found to enhance the activity of the catalyst due to specific crystallographic orientations of Pt crystal.20 The interaction between the surface of CNF and the catalyst also was shown to enhanced catalyst stability and durability.17 In contrast to carbon black, the negligible micropore content in CNFs is a natural advantage, as it allowed higher catalyst utilization21 and enhanced mass transport.22 Furthermore, high resistance to carbon corrosion was a fundamental benefit of utilizing highly graphitized carbon nanofibers rather than traditional carbon blacks23 for catalyst support that operates at an elevated temperature, low pH, and highly oxidizing environment. At the same time, as confirmed in fuel cell practices, the ionomer content and especially morphology in a CNF-based electrode are crucial for the cell performance. Li et al.24 reported that a membrane electrode assembly (MEA) containing Pt supported on stacked-cup CNF with 50 wt % Nafion ionomer displayed higher PEMFC performance than the commercial CB (E-TEK)-based MEA with 30 wt % Nafion ionomer content. The improved performance was attributed to the high aspect ratio (length to diameter ratio) of CNFs which allowed formation of continuously conducting networks in the ionomer matrix. The optimal ionomer content of around 50% was also reported by Kanninen et al.25 for a direct methanol fuel cell (DMFC) with CNF supported PtRu anode. However, Jung et al.26 reported that the optimum ionomer content was 5 wt % in a 47.5 wt % Pt/CNF-based catalyst ink. The discrepancy is most likely due to the differences in MEA preparation, catalyst loading, intrinsic properties of the CNFs used in each study, and the associated ionomer−catalyst/ support interaction. Jung et al. performed the impregnation by applying the ionomer solution on the catalyst layer, which was already painted on the membrane. However, other groups allowed thorough mixing of the ionomer with the catalyst in the ink before MEA preparation. Besides, high metal catalyst loading is more sensitive to high ionomer content due to the surface coverage. Moreover, those studies were carried out based on different types of CNFs, which bear distinct differences in not only nanostructure but also specific surface area,27 which will certainly lead to different adsorption with the ionomer phase. Due to the high crystallinity, functionalization of CNF with polar groups is a necessary pretreatment to create anchoring

2. EXPERIMENTAL SECTION Highly graphitized vapor grown carbon nanofibers were kindly provided by Showa Denko, under the trade name of VGCF. The powder resistivity is below 0.012 Ω·cm according to the supplier. Surface modifications were carried out by means of exposure to mixtures of nitric and sulfuric acid with various concentration ratios and reflux times. The pristine carbon nanomaterial is labeled as CNF, and the corresponding functionalized products are named as CNFF1, CNFF2, and CNFF3. Acid-treated Vulcan (Vulcan Fun.) and pristine Vulcan were also studied for comparison. Full treatment procedures can be found in Table 2. A detailed procedure for Nafion ionomer adsorption on different carbon substrates can be found in our previous publications.13,28 Briefly, a thorough mixing/adsorption between carbon powder and Nafion solution (Sigma-Aldrich) was provided by 15 min ultrasonication and 24 h mechanical shaking. Then the samples were centrifuged for 1 h with 12 000 rpm at 4 °C. Finally, 600 μL of well-separated clean solution was carefully transferred into a 5 mm NMR glass tube. 20 μL heavy water (Sigma-Aldrich 99.9%) was added to provide a deuterium lock, and 20 μL 1% trifluoroacetic acid (99.9% Aldrich) was added for internal reference. All NMR experiments were performed in a Bruker 400 MHz with an autosampler. Signal analysis was done using Mestre Nova. Data fitting was assisted by OriginPro 9.1. One-step contact angle measurement was performed as follows: about 20 mg of CNF powder was placed in a jade mortar and was carefully ground with 200 μL of ethanol by using a pestle. The final carbon paste was transferred onto a piece of PTFE tape (Klingerflon) supported by glass plate, and then dry naturally in a fume hood. The coating was repeated until a complete layer of CNF was formed. The sample was directly used for water droplet contact angle measurement without additional treatment. During the measurement, a 20 μL Milli-Q (Millipore, MA) water droplet was carefully placed on the sample surface by using an autopipet and a snapshot was taken with an Olympus E3 camera equipped with a 55 mm lens on a bellows unit. The image analysis was assisted by the free software MB-Ruler. 10815

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Figure 1. Nafion ionomer adsorption isotherms for CNF and Vulcan samples.

3. RESULTS 3.1. Nafion Ionomer Adsorption. The relative interaction strength and surface coverage were quantified by monitoring the adsorption equilibrium constant (Keq ) and the maximum surface coverage (Γmax) based on Langmuir isotherm. Strong binding between ionomer and catalyst support was reflected by a large value of Keq . This may indicate improved durability because the binder can effectively prevent Pt particle from migration and coalescence. Γmax value may predict the relative ratio between ionomer and support, providing information for the electrode structure development and optimization. Results of perfluorinated sulfonic acid Nafion ionomer interaction with carbon nanofibers (CNFs) and amorphous carbons (Vulcan and functionalized Vulcan) are summarized in Figure 1. The corresponding adsorption equilibrium constant (Keq ) and maximum surface coverage (Γmax) for primary adsorption are listed in Table 1. As illustrated in Figure 1, differences in the interaction between Nafion ionomer and carbon nanofibers were observed. The adsorption can be fairly well simulated with Langmuir isotherms at low concentration (up to monolayer level) as

High-resolution transmission electron microscopy (HRTEM) was performed using a double-aberration-corrected JEOL 2200FS (JEOL, Japan) microscope equipped with a field emission gun (FEG) operated at 200 kV. Electron microscopy (SEM) studies were carried out with a field emission gun using JEOL JSM-7500FA equipped with an energy-dispersive X-ray spectrometer (EDXS). Brunauer−Emmett−Teller (BET) surface area and porosity measurements were performed by N2 adsorption−desorption isotherm at 77 K with Micromeritics physisorption determination. Surface studies were carried out by X-ray photoelectron spectroscopy (XPS) using a spectrometer from SPECS. The X-ray source used in XPS studies was Al Kα (1486.7 eV), and the XPS data was analyzed using CasaXPS. Raman spectroscopy was performed using a Horiba Jobin-Yvon LabRam 300 Raman microscope equipped with a CCD detector and an external cavity stabilized single-mode diode laser at 532 nm. Thermogravimetry was performed using NETZSCH STA-449-F3 with Proteus-61 data analyzing software. All the carbon samples were heated between 50 and 1000 °C with a heating rate of 20 °C/s under a gas mixture of oxygen at 10 mL/min and nitrogen at 40 mL/min. 10816

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The oxygen content was seen to increase with the strength of the chemical treatment for both CNF samples and Vulcan. CNFF2 showed slightly higher oxygen content than CNFF1. This implied that longer treatment time seems more efficient in functionalizing CNF surface than using higher acid concentration. This is also confirmed by XPS, contact angle, Raman spectroscopy, and DTG (see later). CNFF3 showed highest oxygen content among CNFs due to a combination of the longer treatment time and higher concentration of the acids. Trace amount of Fe was also detectable. The metal impurity was coming from the chemical vapor deposition process used to synthesize CNFs. Its content was just above the detection limit of the technique, and the amount was seen to decrease with acid treatment. The sulfur content in Vulcan was also seen to decrease as a result of the functionalization. 3.4. XPS Characterization. X-ray photoemission spectroscopy was also used to analyze the surface functional groups. Carbon and oxygen content and deconvolution of C 1s, O 1s regions are summarized in Table 4. Both element content and carbon deconvolution patterns indicate that pristine CNF showed a higher carbon or C−C bonding content than Vulcan. With increasing acid oxidation strength, lower graphitic content, higher oxygen content, and higher defect content were detected on the surface. However, these changes are more pronounced for Vulcan, showing CNFs are more resistant to oxidation than Vulcan. O 1s deconvoluted spectra for CNFs and Vulcan samples are shown in Figure 3 and Table 4. The rest of the spectra (C 1s) are available in the Supporting Information. An interesting trend shows that CNFs prefer double oxygen bond in the forms of carboxyl (O−CO) or carbonyl (CO) rather than single oxygen bond of ether (C−O−C) or hydroxyl (C−O) as a result of the chemical oxidation. This interesting tendency became more evident with the increasing strength of the treatment. It is probably due to the fact that the highly crystalline graphitic content of CNFs is resistant to chemical modifications and that the oxygen−carbon double bond is an easier product, since lower bonding energy is required. Amorphous carbon black tends to have the opposite preference: single-bonded oxide is more dominating than double bonded as shown in the case of Vulcan Fun. 3.5. Wetting Properties. As shown in Figure 4, nice truncated water spheres are readily formed on the CNF thin films. The contact angle was measured by fitting a tangent to the three-phase point, where liquid surface touches the solid surface. The corresponding contact angle values are presented in the figure. The pristine carbon nanofibers showed the highest contact angle value of 155°, indicating very little wetting with water and strong hydrophobicity. With increasing strength of the acid treatment, the contact angle was seen to decrease, indicating better wetting by water and increasing hydrophilicity probably due to the increasing content of surface oxygen functional groups, as observed by EDXS and XPS also in agreement with others.29 The “thin film” method was also applied to Vulcan samples. The surfaces were completely wetted by water, even when a compact tablet was used). This indicates that pristine Vulcan is much more hydrophilic than CNFs, as also reported in the literature,30 with a contact angle of 65°. Furthermore, as shown by the sedimentation experiment (mixing a few micrograms of Vulcan powder with distilled water and equilibrium for month), functionalized Vulcan is even more hydrophilic than the

Table 1. Summary of Primary Adsorption Parameters for the Langmuir Isotherms eq constant Keq CNF CNFF1 CNFF2 CNFF3 Vulcan Vulcan Fun.

22.49 12.44 4.62 21.07 18.19 24.23

max surface coverage Γmax (g ionomer/g adsorbent) 7.57 6.21 8.37 3.09 1.05 4.23

× × × × × ×

10−2 10−2 10−2 10−2 10−1 10−2

specific max surface coverage ΓSmax (g ionomer/ m2 adsorbent) 6.88 5.65 6.98 2.58 9.63 2.02

× × × × × ×

10−3 10−3 10−3 10−3 10−4 10−4

plotted in the figures with solid lines. Important parameters were extracted from the model to quantify the interaction behavior (Table 1). As indicated by Keq and Γmax, there are significant differences in the interaction between ionomer in aqueous phase and nanocarbons subjected to different chemical oxidation treatment. Therefore, surface characterization of the nanocarbons was carried out for further interpretation of the adsorption results. 3.2. HR-TEM, BET, and Porosity Measurements. HRTEM images of highly graphitized carbon nanofibers and amorphous Vulcan before and after the acid treatments are presented in Figure 2. A summary of the treatment procedure and the corresponding physical properties of the nanocarbons can be found in Table 2. In general, the as-received CNFs were highly graphitized at elevated temperatures (above 2800 °C) with the average diameter of about 150 nm and ∼10 μm length. The pristine CNFs (Figure 2a,b) showed a well-organized structure almost free of amorphous carbon on the surface. With increasing strength of chemical oxidation from CNFF1 to CNFF3, more unstructured carbonaceous features were visible on the surface of the carbon nanofiber, as shown in Figure 2 (c, d, and e). Under the most severe oxidation conditions, even broken fibers were detected as shown in Figure 2f. Vulcan appeared as turbostratic aggregate of near-spherical particles of diameter ∼35 nm, as shown in Figure 2g. After acid treatment, the carbon black showed even more unstructured surface, as well as expansion of the aggregate with diameter ∼40 nm, as illustrated in Figure 2h. For the CNFs, the specific surface area increased slightly with the strength of the acid treatment. As indicated by porosity data, the increase of the specific surface area was probably due to the increased amount of mesopores. The increase of mesoporosity took place probably at the cost of the graphitic structure of the CNF, as indicated by Raman spectroscopy and TG (see section 3.5 and 3.6). Apart from the nearly negligible microporosity for pristine CNF, none of the acid-modified CNF samples showed detectable micropores. Furthermore, mesoporosity of CNFs was 1 order of magnitude lower than that of the Vulcan samples regardless of the treatment. Hence we can conclude that CNFs have a much denser, compact carbon nanostructure with smooth crystalline surface. In the case of Vulcan, functionalization led to almost 50% reduction of specific surface area. This was probably due to the functional groups from the chemical oxidation or the expansion of the carbon structure effectively blocking the micropores; this happened simultaneously with a reduction of mesopores. 3.3. EDXS Characterization. EDXS elemental acquisition of the carbon samples is summarized in Table 3. 10817

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Figure 2. TEM images of (a) pristine CNF at low magnification; HR-TEM images of (b) pristine CNF, (c) CNFF1, (d) CNFF2, (e) CNFF3, (f) CNFF3 broken tip, (g) Vulcan, and (h) Vulcan Fun.

As indicated by the ID/IG ratio, CNFs are much more

pristine Vulcan (Figure 5). Hydrophilicity is indicative of the relative amount of surface polar groups in the carbons. 3.6. Raman Spectroscopy. The crystallinity of the carbon materials was examined with Raman spectroscopy. First-order Raman spectra and intensity ratio between defect band and graphitic band (ID/IG) are summarized in Figure 6.

graphitic compared to Vulcan even after severe chemical oxidation. The percentage of defective over graphitic content in CNFs increases only slightly with increasing strength of the acid treatments, in agreement with the observations from HR10818

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Also in Figure 7, DTGs of Vulcan and functionalized Vulcan are presented. Comparatively, the Vulcan samples decompose at lower temperature (80−130 °C lower than CNF) and over a wider temperature range. This is probably due to their amorphous structure and broader range of crystallinity. Acidtreated Vulcan showed slightly lower decomposition temperature. For carbon−ionomer composites (carbon homogeneously mixed with ionomer in aqueous phase), similar thermal decomposition studies were carried out and are summarized in Figure 8. In general, two decomposition regions were detected. Weight loss in the low-temperature region, between 300 and 500 °C, is due to combustion of the ionomer. Weight loss between 550 and 850 °C is due to pyrolysis of carbon. The DPT of CNFs in the composites is decreased by 30−95 °C comparing to CNFs without ionomer. Among CNF−ionomer composites, differences of more than 40 °C (757−798 °C) in the decomposition peak temperatures were noted, which decrease in the order CNFF3 > CNFF2 > CNFF1 > CNF, exactly the opposite trend of ionomer-free CNFs. This indicates ionomer orientation preference due to different adsorption mechanisms, as discussed in detail later. Vulcan−ionomer composites show 34−50 °C lower decomposition temperatures than the ionomer-free counterparts. The thermal stability also shows reverse order, with functionalized Vulcan−ionomer composite decomposing at 12 °C higher than the Vulcan− ionomer composite.

Table 2. CNF Sample Specifications item CNF CNFF1 CNFF2 CNFF3 Vulcan Vulcan Fun. a

treatment procedure pristine 2 M HNO3, 4 M H2SO4; 2 h, 120 °C 2 M HNO3, 1 M H2SO4; 6 h, 120 °C 2 M HNO3, 4 M H2SO4; 6 h, 120 °C pristine 2 M HNO3, 1 M H2SO4; 4 h, 120 °C

BET (m2/g)

micropore (cm3/g)

mesopore (cm3/g)

11 11

1.02 × 10−4 BDLa

0.038 0.040

12

BDL

0.044

12

BDL

0.046

3.69 × 10−2 3.54 × 10−3

0.32 0.19

210 109

Below detection limit.

Table 3. EDXS Elemental Content of the Carbon Samples C (wt %) CNF CNFF1 CNFF2 CNFF3 Vulcan Vulcan Fun.

97.97 97.97 97.56 96.61 95.62 93.71

± ± ± ± ± ±

0.15 0.51 0.36 0.28 1.06 0.49

O (wt %) 1.97 2.00 2.42 3.37 3.46 5.68

± ± ± ± ± ±

0.13 0.51 0.27 0.28 0.63 0.55

Fe (wt %)

S (wt %)

± ± ± ±

na na na na 0.91 ± 0.45 0.61 ± 0.09

0.05 0.03 0.02 0.02 na na

0.04 0.05 0.03 0.02

TEM images. Quality degradation was also detected for acidtreated Vulcan, leading to larger defect content. 3.7. DTG of Carbon and Carbon−Ionomer Composite. First-order derivative of thermal weight losses of carbon samples are presented in Figure 7. One distinct weight loss peak appears between 650 and 900 °C for each CNF. Hardly any weight loss was detected at lower temperature range. This indicates a highly graphitic surface structure and nearly free of amorphous carbon. The decomposition peak temperature (DPT) (as indicated in the figure) decreases with increasing strength of chemical oxidation. This is probably due to the increasing amount of surface oxygen-containing groups and defects that catalyze the combustion reaction, rendering the carbon surface easier to be oxidized.31 A DPT difference of close to 50 °C (852−806 °C) was found among CNFs, according to the following trend: CNF > CNFF1 > CNFF2 > CNFF3.

4. DISCUSSION There is a clear change of physical properties of the ribbon-type carbon nanofibers upon oxidative treatment. However, due to their high crystallinity (graphitized at 2800 °C), the quality of the nanofibers after different oxidations was slightly altered compared to carbon black and even to the multiwalled carbon nanotube.13 The corresponding adsorption equilibrium constant (Keq ) of the various CNFs interacting with Nafion ionomer is between 5 and 22. This relative small change reflects inertness of the CNFs toward various chemical treatments. However, it is interesting to observe that when the strength of acid oxidation treatment increases (CNF< CNFF1< CNFF2 < CNFF3), Keq initially decreases (from 22 to 12 and to 5) and then increases (to 21), rather than a monotonous changing

Table 4. XPS Carbon and Oxygen Content and Deconvolution of C 1s and O 1s Peaks at eV element C% O% C 1s deconvolution C−C defects C−O O−CO carbonate π−π* total COa O 1s deconvolution CO, O−CO C−O, C−O−C a

CNF

CNFF1

CNFF2

CNFF3

Vulcan

Vulcan Fun.

98.7 1.4

98.5 1.5

98.2 1.8

98.1 1.9

97.0 3.0

93.9 6.1

∼284.5 ∼285.5 ∼286.6 ∼288.0 ∼289.0 ∼291.5

77.5 7.8 4.8 3.3 3.5 3.2 11.5

76.4 8.0 4.7 4.2 3.6 3.0 12.5

75.2 9.5 4.3 5.9 2.9 2.3 13.1

73.8 10.6 4.1 5.8 3.6 2.0 13.6

76.0 7.1 5.2 4.0 3.2 4.5 12.4

68.7 11.6 7.1 4.4 3.4 4.8 14.9

∼531.9 ∼533.6

47.9 52.1

57.9 42.1

61.5 38.5

69.4 30.7

51.2 48.8

48.5 51.5

Sum of C−O, OCO, and carbonate. 10819

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Figure 3. Core level high-resolution O 1s XPS spectra of CNF and Vulcan samples.

Figure 5. Pristine Vulcan and Vulcan Fun. mixed with water and left to sediment over a month. Figure 4. Contact angle measurements on CNFs.

matrix, Nafion in solution is a colloidal dispersion of rodlike particles surrounded with the ionic charges.3,32 The ionomer can be considered as a surfactant of amphiphilic nature. The linear fluorocarbon backbone, similar to poly(tetrafluoroethylene) (PTFE), is hydrophobic and the sulfonic acid group, located at the end of perfluorovinyl ether pendant

trend. This indicates a gradual transition of adsorption mechanisms. In contrast to low water containing bulk membrane of connected spherical domains of water embedded in polymer 10820

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Figure 6. Raman spectra of carbon materials. Figure 9. Nafion adsorption on surfaces of different wetting properties and porosity.

adsorption equilibrium constant decreases from about 22 to 12. When the CNF surface is further oxidized to CNFF2, there are three possibilities of the slightly higher content of oxygencontaining groups: they serve as (1) barrier/interference for hydrophobic−hydrophobic adsorption; (2) interaction point for a weak hydrophilic−hydrophilic adsorption; and (3) combination of (1) and (2). In all three cases, the adsorption equilibrium constant is further decreased, from about 12 to 5. With even higher degree of oxidation, in CNFF3, even higher content of oxygen-containing groups is achieved on the CNF surface; besides, increased porosity and defects were observed (sections 3.2 and 3.4). There is a stronger interaction between ionomer and CNFs based on hydrophilic adsorption, as indicated by a higher value of the equilibrium constant around 21. Moreover, the adsorption studies between Nafion and nanocarbons in aqueous solution mimic the electrode preparation procedure based on the actual fuel cell materials. Not only wetting properties but also the surface structures are involved in the adsorption process. Though the functionalized CNF (CNFF3) is hydrophobic (contact angle above 130°), the increment of porosity (Table 2) and more non-oxygencontaining edge and defect sites (Table 4) are also seen to play a role in the process leading to the hydrophilic adsorption. Considering DTG analysis, when the ionomer and CNF interact via a hydrophobic−hydrophobic adsorption mechanism (Figure 9a), the resulting composite has a surface structure saturated with dangling, oxygen-containing sulfonic groups, which can be more easily oxidized during combustion and hence can be decomposed at lower temperature.35,36 Conversely, for interaction via hydrophilic−hydrophilic adsorption (Figure 9b,c), the resulting composite is covered with an inert fluorocarbon shell that can protect the bulk material, preventing decomposition until higher temperature. This could explain the opposite trends in decomposition temperatures noted for CNFs subjected to various chemical oxidation treatments and for the corresponding CNF−ionomer composites (section 3.7). As for carbon black samples, Keq indicates that functionalized Vulcan has stronger interaction with the ionomer than pristine Vulcan. Hence, functionalized Vulcan is more intensely covered with the ionomer via hydrophilic− hydrophilic interaction, exposing an inert fluorocarbon shell at the surface. This again would explain the higher decomposition temperature for functionalized Vulcan−ionomer composite than that of the pristine Vulcan−ionomer composite.

Figure 7. DTG of carbon samples.

Figure 8. DTG of carbon−ionomer composites.

side chain, is hydrophilic and has high ion exchange capacity. Therefore, depending on the surface properties of the substrate, the ionomer may adsorb differently as illustrated in Figure 9a,b, in agreement with other works.12,29,33,34 The highly graphitized carbon nanofibers possess high aspect ratio of edge to basal carbon atoms, are free from carbonaceous impurities, and are highly hydrophobic with low porosity. Therefore, when the Nafion ionomer interacts with CNF, it is more likely that the hydrophobic fluorocarbon backbone is preferentially adsorbs on its surface. When slight chemical oxidation is carried out, as for CNFF1, the original crystallinity is mildly compromised and the strength of hydrophobic− hydrophobic interaction is reduced. Consequently, the 10821

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used. The interaction among phases and how this is affected by surface functionalization of the carbon support are crucial factors to enhance the three-phase boundary and hence the electrode performance in PEMFCs.

As a general trend, the maximum surface coverage (Γmax) of CNFs was seen to be decreased with increasing strength of chemical treatment, indicating that there was less space available for Nafion ionomer adsorption despite the negligible changes in specific surface area determined by BET measurements. This implies some sort of spatial adsorption hindrance or lower packing efficiency of the ionomer due to the chemical treatment. Furthermore, the specific maximum surface coverage (ΓSmax) for CNF is 1 order of magnitude higher than that of carbon black and multiwalled carbon nanotubes.13 This is due to the large difference in porosity: Vulcan exhibits large pore volume (Table 2) and is known for its microsize porosity (less than 2 nm);37 CNFs possess negligible micropore and low mesopore volume, as shown by our porosity measurements. In Nafion solution, the ionomer has a colloidal structure formed of polymer rodlike particles with the ionic groups located at the polymer−solvent interface. The diameter of the rod is around 5 nm in water and the length is larger than 30 nm.38,39 These facts imply that, due to spatial limitations, Nafion ionomer cannot access the micropores, as illustrated schematically in Figure 9c. This would explain the lower value of specific maximum surface coverage for Vulcan. A future way to improve the catalyst layer structure is to use the hydrophilic interaction between Nafion and the catalyst support: the orientation of the sulfonic group can guarantee a good proton supply toward catalytic sites;12 and depending on the mixing ratio, the consequent fluorocarbon backbone may lead to hydrophobic interphase, resulting in electrode structure with better water management properties and reduced mass transport resistance. The hydrophilic interaction can be formed from a rather hydrophilic catalyst/substrate surface and/or combined with a certain surface porosity. So, there might be a dilemma between the wetting properties and stability of the catalyst support: amorphous carbon is rather hydrophilic but undesirable due to corrosion issues;40 similar problem is also experienced with nanocarbons after severe acid treatment.41 Therefore, tuning of wetting property should be carried out in a more nondestructive manner. Polar functional groups introduced through noncovalent π−π interaction might be beneficial to serve both aforementioned purposes, besides the enhanced catalyst performance.42

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ASSOCIATED CONTENT

S Supporting Information *

Carbon 1s XPS spectra of the carbon samples. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the financial support from European Commission, INTERREG IVA, Southern Denmark-SchleswigK.E.R.N, Project no. 111-1.2-12, the Danish PSO project Catbooster 2011-1-10669, the Danish PSO project DuraPEM III 2013-1-12064, and the Academy of Finland (M.B. and V.R., Academy Research Fellowship). This work was carried out using the Aalto Nanomicroscopy Center premises. Sinh Hy Nguyen and Chao Pan from DTU are acknowledged for performing BET and porosity measurements.

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5. CONCLUSIONS Adsorption interaction between highly graphitized carbon nanofibers with different degrees of functionalization and perfluorinated sulfonic acid ionomer was studied by 19F-NMR. The results were interpreted with the aid of HR-TEM, BET surface area, porosity, EDXS, XPS, Raman, contact angle, and DTG. Fair interaction between CNFs samples and Nafion ionomer was noted, with adsorption equilibrium constant (Keq ) between 5 and 22. With increasing strength of acid treatment on the carbon nanomaterials, the interaction was found to initially decrease and then increase, which indicates a gradual transition from hydrophobic adsorption mechanism to hydrophilic adsorption. This, in turn, has an influence on the thermal stability of the carbon−ionomer composites. It leads to a shift of the DPT for highly functionalized carbon−ionomer composite to higher temperature. The specific surface coverage (ΓSmax) of CNFs is 1 order of magnitude higher than that of Vulcan due to the more compact and micropore-free surface. These results may contribute optimization of electrode structure when carbon nanomaterials other than Vulcan are 10822

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