Enhancement in Proton Conductivity and Thermal Stability in Nafion

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Applications of Polymer, Composite, and Coating Materials

Enhancement in Proton Conductivity and Thermal Stability in Nafion Membranes Induced by Incorporation of Sulfonated Carbon Nanotubes Chongshan Yin, Jingjing Li, Yawei Zhou, Haining Zhang, Pengfei Fang, and Chunqing He ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b01513 • Publication Date (Web): 05 Apr 2018 Downloaded from http://pubs.acs.org on April 9, 2018

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ACS Applied Materials & Interfaces

Enhancement in Proton Conductivity and Thermal Stability in Nafion Membranes Induced by Incorporation of Sulfonated Carbon Nanotubes Chongshan Yin,† Jingjing Li,† Yawei Zhou,† Haining Zhang,‡ Pengfei Fang,† and Chunqing He∗,† †Key Laboratory of Nuclear Solid State Physics Hubei Province, School of Physics and Technology, Wuhan University, Wuhan 430072, China. ‡State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, Wuhan 430070, China. E-mail: [email protected] Abstract Proton exchange membrane fuel cell (PEMFC) is one of the most promising green power sources, in which perfluorinated sulfonic-acid (PFSA) ionomers based membranes (e.g. Nafion) are widely used. However, the widespread application of PEMFCs is greatly limited by the sharp degradation in electrochemical properties of the proton exchange membranes under high temperature and low humidity conditions. In this work, the high-performance sulfonated carbon nanotubes/Nafion composite membranes (SuCNTs/Nafion) for the PEMFCs were prepared, and the mechanism of the microstructures on the macroscopic properties of membranes was intensively studied. Microstructure evolution in Nafion membranes during water uptake was investigated by positron

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annihilation lifetime spectroscopy, and results strongly showed that the Su-CNTs or CNTs in Nafion composite membranes significantly reinforced Nafion matrices, which influenced the development of ionic-water clusters in them. Proton conductivities in Su-CNTs/Nafion composite membranes were remarkably enhanced due to the mass formation of proton conducting pathways (water channels) along the Su-CNTs. In particular, these pathways along Su-CNTs in Su-CNTs/Nafion membranes interconnected the isolated ionic-water clusters at low humidity, and resulted in less tortuosity of the water channel network for proton transportation at high humidity. At a high temperature of 135 ◦ C, Su-CNTs/Nafion membranes maintained high proton conductivity because the reinforcement of Su-CNTs on Nafion matrices reduced the evaporation of water molecules from membranes as well as the hydrophilic Su-CNTs were helpful for binding water molecules.

Keywords: Nafion; free volume; proton conductivity; proton pathway; tortuosity; low humidity; thermal stability

INTRODUCTION Nowadays, proton exchange membrane fuel cell (PEMFC) is considered as one of the most preferential alternative power sources due to its excellent properties, such as stable operation, high electrochemical efficiency, high power density, fast start-up, low environmental pollution, etc. 1–5 Proton exchange membrane (PEM), which provides proton conduction between fuel and oxidant like hydrogen and oxygen, 6,7 is one of the key components of PEMFCs. With good physical and chemical stabilities, Nafion is regarded as one of the most promising PEMs. Nafion is a random copolymer, in which perfluoroether side chains terminated with sulfonic acid groups (-SO3 H) are randomly distributed along the perfluoroethylene backbones. 3,8 The dissimilar nature of the hydrophilic sulfonic acid groups and the hydrophobic backbones results in natural phase separation in Nafion membranes at various humidities (ionic-water clusters and backbones). Proton migration in Nafion happens mainly in the 2


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water channels (interconnected ionic-water clusters) via either the proton hopping aided by water molecules, or the vehicular transport by forming a hydrated ion (H3 O+ ). 9–11 The tortuosity of the water channels for proton transportation in Nafion shows directly influence on the overall macroscopic membrane proton conductivity. 2 Meanwhile, experimental results revealed that the formation of water channels in Nafion membranes is sensitive to the ambient relative humidity (RH) and temperature. 12–15 To break through this barrier, several strategies have been proposed. One possible approach is to design alternative materials based on those engineering polymers, for example, polybenzimidazole, poly ether sulfone and poly etheretherketone. 5,16–18 The second research attempt is modifying the microstructure of PEMs with kinds of organic/inorganic components/nanoparticles, and many composite PEMs with improved electrochemical properties have been synthesized. 19–28 These hybrid nanocomposites, made of two or more kinds of nanomaterials, usually demonstrate combined attraction and properties of each component. Among numerous inorganic additives, carbon nanotubes (CNTs) are one of the most preferred options due to their high aspect ratios (100∼1000), high specific surface areas, and their advanced mechanical properties. 24 Further, kinds of high-performance PEMs doped with chemically functionalized CNTs have been reported. 20–28 Generally, sulfonic acid groups are known to be helpful and efficiency for proton transportation in PEMs. 25–28 However, most of those studies are mainly focusing on the performance of the membranes, and there are few studies on sulfonated carbon nanotubes/Nafion hybrid membranes under different humidity and temperature conditions. Thus, the mechanism of the microstructures on the macroscopic properties of sulfonated carbon nanotubes/Nafion membranes needs intensive studies, which are very meaningful. Macroscopic physical properties such as proton conductivity, water content and mechanical property of polymer membranes may change at different relative humidities because of water adsorption. The impacts of microstructure evolution in Nafion membranes on their physical properties at various humidities are of interest and importance. However, microstructure (especially the atom-sized free volumes) evolutions in polymers as a func-

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tion of ambient conditions are extremely difficult to be observed with conventional techniques, such as transmission electron microscope (TEM) or atomic force microscope (AFM). For several decades, positron annihilation technique has been widely used for characterization of open-volumes, 29 vacancy-type defects, 30–32 and pores 33–37 in various materials. In a positron annihilation lifetime spectroscopy (PALS) experiment on polymers or other insulators, the formation of positronium (Ps) atoms in samples is of great importance. Ps is a hydrogen-like bound state of an electron and a positron with two spin states: spin parallel ortho-positronium(o-Ps) and spin antiparallel para-positronium (p-Ps). The lifetime of p-Ps is extremely short (∼125 ps), while the intrinsic lifetime of o-Ps is as long as 142 ns in a vacuum (via 3γ annihilation). Meanwhile, when o-Ps is localized in hole free volumes in polymers, the lifetime of o-Ps is shortened down to several nanoseconds (via 2γ pickup annihilation). The so-called o-Ps pickup annihilation means that the positron in o-Ps atom "picks up" and annihilates with an electron of the molecules (from the free volume walls), having an opposite spin to that of the positron. 38 Without formation of positronium atoms, free positrons may directly annihilate with electrons in a typical lifetime of ∼300 ps in condensed matters. Hence, normally, there are three annihilation components of positrons annihilate in polymers, namely p-Ps, free positron and o-Ps, which can be respectively characterized by their lifetimes τ p−P s , τ f ree , τ o−P s and their corresponding intensities. The exact value of τ o−P s is greatly depending on the free volume hole size, and the average free volume hole size in polymers can be derived from τ o−P s according to the Tao-Eldrup model. 38,39 Recently, PALS is widely used to characterize the free volumes in polymers. 14,15,40–49 In this work, composite Nafion membranes doped with carbon nanotubes (CNTs) and sulfonated ones (Su-CNTs) were synthesized via a self-assembly sol-gel process. Ion exchange capacity (IEC), water uptake, mechanical strength and surface morphology of these membranes were measured. The microstructure evolution in the membranes was investigated, which is correlated to the macroscopic properties, such as mechanical strength, water uptake and proton conductivity, of the membranes under various environmental conditions.

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EXPERIMENTAL SECTION 2.1. Surface modification of CNTs. Surface modification of CNTs was achieved through chemical functionalization. 20,27,28 Firstly, CNTs (100 mg, outer diameter: 10-20 nm, average length: 0.5-2 µm) were added into a mixture of nitric acid (25 ml, volume concentration: 70 vol%) and sulfuric acid (75 ml, volume concentration: 98 vol%), then the mixture was stirred continuously at 25 ◦ C for 24 hours. Nextly, the suspension was diluted by deionized water, then filtered and washed with deionized water several times until the PH of the filtrate was 7.0. The resulted acid CNTs (Co-CNTs) were dried under vacuum at 80 ◦ C for 12 hours. Then 50 mg of Co-CNTs were dispersed in 65 ml thionyl chloride (SOCl2 ) solution and the suspension was sonicated for 2 hours and magnetically stirred at 60 ◦ C for 12 hours. The resulted CNTs (COCl-CNTs) was filtered and dried under vacuum at 80 ◦ C for 12 hours. After that, definite amounts of sulfanilic acid and the COCl-CNTs were dissolved in deionized water with magnetic stirring at 80 ◦ C for 24 hours. Subsequently, the suspension was firstly filtered and washed several times with sodium carbonate solution, and then filtered, washed several times with deionized water until the PH of the filtrate was 7.0. Finally, the resulted sulfonated CNTs (Su-CNTs) was dried under vacuum at 80◦ C for 24 hours. Fig. 1 depicts the schematic outline of Su-CNTs preparation. 2.2. Preparation of Su-CNTs/Nafion composite membranes. Firstly, a certain amount of Su-CNTs were suspended in tetrahydrofuran (THF) at 25 ◦ C, stirred for 1 hour, and sonicated for 2 hours. Subsequently, the Su-CNTs suspension was blended with Nafion solution at 25 ◦ C, sonicated and stirred for 3 hours. Then, the solution was evaporated into one-third of its original volume after an 8 hours ultrasonic treatment at 50 ◦ C. After the solution was cooled to room temperature, it was casted on a flat-bottomed quartz dish and evaporated at 25◦ C for 8 hours. Finally, the resulted membranes were thermal treated in a vacuum at 140 ◦ C for 2 hours. Before measurements, the resulted Su-CNTs/Nafion hybrid membranes (the content of Su-CNTs is 5 wt% regarding to Nafion) were thoroughly rinsed through a standard procedure, as shown in Supporting Information. Pristine Nafion 5



Mixed Acid

MWCNT

Magnetic Stirring Room Temperature

SOCl2 Solution

Co-CNT

COOH Sonication Magnetic Stirring

NH2

60 ć CoCl-CNT

COCL

SO3H

Magnetic Stirring 80 ć

Su-CNT

=O

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

C NH

Figure 1:

SO3H

H2O

Schematic outline of Su-CNTs preparation.

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membranes and CNTs/Nafion membranes (the content of CNTs is 5 wt% regarding to Nafion) were also prepared. The thickness of the obtained membranes was 45±5 µm. 2.3. Characterizations. Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), and simultaneous thermal gravimetry (STG) measurements were used to characterize the physical properties of Nafion membranes. Positron annihilation lifetime measurements were used for characterization of microstructure evolution in Nafion membranes as a function of relative humidity. Proton conductivities of the membranes were measured by an AC impedance technique under various humidity and temperature conditions. Details of experiments and characterizations can be found in the Supporting Information and our published work. 13

RESULTS AND DISCUSSION Synthesis of Su-CNTs/Nafion composite membranes. FTIR spectra of the sulfonated CNTs (Su-CNTs), acid CNTs (Co-CNTs) and pure CNTs are shown in Fig. 2. The broad asymmetrical stretching bands observed at around 3450 cm−1 for all samples are mainly due to the trace of water in the KBr powder. 50 In addition, the O-H stretching band arising from carboxylic acid groups (-COOH) as well as the hydroxyl groups (-OH) also overlaps in the range from 3200 to 3550 cm−1 , which results in that the intensity of the stretching band at around 3450 cm−1 of Co-CNTs and Su-CNTs is higher than that of CNTs. The peaks around 1705 cm−1 of Su-CNTs and Co-CNTs refer to the carboxylic acid groups. 51 For Su-CNTs, bands appear at 3327 cm−1 and 1625 cm−1 , indicating the formation of the amide bond (CO-NH-) after the functionalization of CNTs with sulfanilic acid. Bands at 1437 cm−1 , 1574 cm−1 and 3036 cm−1 of Su-CNTs are typical of the benzene ring in sulfanilic acid. Further, the -SO3 H stretching at 1238 cm−1 and 1085 cm−1 can be clearly seen for Su-CNTs. These results demonstrate that the sulfanilic acid has been successfully grafted on the CNTs via an amide functionality. As shown in Fig. S1 in the Supporting Information, the Su-CNTs and

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Co-CNTs are better dispersed in water than the pure CNTs, indicating the functionalized CNTs are more hydrophilic than the pristine one.

Figure 2:

FTIR spectra of Su-CNTs, Co-CNTs and pure CNTs with baseline correction.

As depicted in Fig. S2(a) and Fig. S2(b), it seems that the diameter of Su-CNTs is thicker than that of the pure CNTs, which may be attributed to the sulfanilic acid groups, hydroxyl groups and carboxylic acid groups grafted on the surface of Su-CNTs. In order to prevent a short circuit between electrodes of a fuel cell, Nafion composite membranes should not be electrically conductive, and the CNTs or Su-CNTs in Nafion composite membranes should not protrude out of the membrane surface. The SEM images of pristine Nafion membranes (Fig. S2(c)) and Su-CNTs/Nafion membranes (Fig. S2(d)) both exhibit a smooth surface morphology. A two-terminal direct current (DC) impedance method was used to measure the electronic conductivity of completely dried (kept in the nitrogen atmosphere for several days) and fully water hydrated Nafion membranes at room temperature, and results are listed in Table 1. It is found that the electronic conductivities of all membranes are very low, and electronic conductivities of both CNTs/Nafion and Su-CNTs/Nafion membranes are 8


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essentially the same as that of the pristine Nafion membranes, indicating that the doped SuCNTs or CNTs (the content of Su-CNTs or CNTs is 5 wt% regarding to Nafion) in composite membranes were completely embedded in the Nafion matrices and did not lead to electronic short circuits. Table 1 Electrical conductivity (EC), water uptake, IEC , λ and break strength of composite Nafion membranes. EC W ater IEC λ Break dried / hydrated U ptake (meq g −1 ) (H2 O/SO3− ) Strength (S cm−1 ) (wt%) (M P a) −6 −3 25.6 0.88 16.2 14.0 P ristine N af ion 1.1 × 10 /1.3 × 10 CN T s/N af ion 1.3 × 10−6 /1.7 × 10−3 20.7 0.82 13.2 18.2 −6 −3 Su − CN T s/N af ion 1.1 × 10 /1.1 × 10 21.9 0.95 12.8 19.7 Sample

Water uptake of membranes at different humidities. Water uptake (WU), ion exchange capacity (IEC), the number of water molecules per sulfonic acid groups (λ) and the tensile strength at break of composite Nafion membranes were investigated at room temperature. The results are also listed in Table 1. λ is obtained using the following equation, λ=

WU mol H2 O − = mol SO3 (IEC × Mwater )

(1)

where Mwater is molecular weight of water molecular (18 g mol−1 ). In Su-CNTs/Nafion composite membranes, a slight increment observed in the IEC values suggests that the addition of 5 wt% Su-CNTs in Nafion membranes can improve the concentration of conducting protons. 27 From the increment in IEC of Su-CNTs/Nafion membranes, it’s easy to estimate the IEC of Su-CNTs to be ∼2.35 meq g−1 . The relatively lower IEC value of CNTs/Nafion membranes is likely due to the incorporation of 5 wt% CNTs, which have no ion-exchange capacity. The tensile strengths at break of the CNTs/Nafion and Su-CNTs/Nafion composite membranes are much higher than that of pristine Nafion membranes due to the reinforcement of doped CNTs or Su-CNTs with unique mechanical property. 14,21,22 In comparison to CNTs/Nafion membranes, the higher break strength of Su-CNTs/Nafion membranes is 9



attributed to a strong molecular interaction between Nafion backbones and Su-CNTs. 27 As shown in Table 1, the water uptakes at room temperature of both Su-CNTs/Nafion and CNTs/Nafion composite membranes are lower than that of pristine Nafion membranes, because of the strong reinforcement of CNTs or Su-CNTs on Nafion membranes, which suppressed the swelling of Nafion matrices such that the development of ionic-water clusters was retarded. 52 Water uptakes of membranes at different relative humidities were also investigated at 25 ◦ C, as shown in Fig. S3. Apparently, water uptake of pristine Nafion is always higher than those of the other two kinds of composites membranes at any desired humidity. Although water content is very important in protons conduction, the relatively low water contents of Su-CNTs/Nafion and CNTs/Nafion membranes may also have some advantages. From the point of view of direct methanol fuel cell, the lower water content in PEMs may result in lower methanol crossover. 24 Furthermore, a dynamic vapor sorption (DVS) of Nafion membranes was measured to investigate the dynamics of water sorption in Nafion membranes at 99% RH and 25 ◦ C, as displayed in Fig. S4. The characteristic time constants (t0 ) for the membranes saturated with water, obtained from the fitting of experimental data, are often used to describe the water transport and sorption processes in membranes. 2 Details of the derivation of t0 from experimental data can be found in the Supporting Information. It’s noted that the t0 of Su-CNTs/Nafion (1.46 hour) and CNTs/Nafion (1.55 hour) composite membranes are nearly twice of that of pristine Nafion membranes (0.79 hour), indicating the water diffusion in them is less efficient than in pristine one. Commonly, the lower diffusion coefficient of water molecules in perfluorinated sulfonic-acid (PFSA) system, for instance Nafion, can be caused by the stiffer backbones exhibiting slower water-uptake responses. 1 Water molecules diffusion in polymers is strongly associated with nanodomain swelling, microstructure and mechanical property in them. 2,3,53 Hence, in Su-CNTs/Nafion and CNTs/Nafion composite membranes, the apparently longer t0 can be attributed to the strong restriction on the rearrangement of the perfluoroethylene Nafion backbones from the doped Su-CNTs and CNTs, which suppressed the swelling of composite membranes. Besides,

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the similar t0 values of Su-CNTs/Nafion and CNTs/Nafion composite membranes mean that the hydrophilic Su-CNTs do not contribute much to the improvement of water sorption rate. Thus, the diffusion of water molecules in composite membranes occurs mainly among the Nafion matrices, however, the microstructures and mechanical properties of hybrid membranes seem to govern the water diffusion in them. Microstructure evolution in composite Nafion membranes during water uptake. Normally, the variations in free volume size in Nafion membranes can be used to evaluate the microstructure evolution in them. 14,15,54 The o-Ps lifetime (τ o−P s ) and the corresponding free volume radius (R) in the three kinds of Nafion membranes as a function of ambient humidity are shown in Fig. 3. In pristine Nafion membranes, free volume size firstly increases gradually as a function of ambient humidity, and beyond ∼80% RH it decreases rapidly. Below ∼80% RH, despite the absorbed water molecules can fill the free volumes in Nafion membranes, 12,14 the significant expansion of free volumes is resulted from the absorbed water molecules acting as plasticizers of Nafion backbone. 1,13,15 Beyond ∼80% RH, the water content of Nafion membranes reaches a relatively high level (∼6 wt%, as showed in Fig. S3), thus plenty of positronium atoms annihilate in the water (ionic-water clusters phase) with a short lifetime of ∼1.8 ns, 43 thus the observed mean o-Ps lifetime decreases rapidly. One should keep in mind that the measured free volume sizes are the average values of those in the overall Nafion membranes, i.e. free volumes in the Nafion matrix (either with or without water molecules) and those in the ionic-water clusters. 13 Meanwhile, a rapid increment in the intensity of o-Ps (Io−P s ) beyond ∼80% RH (as shown in Fig. S5) further confirmed that a growing number of o-Ps atoms tend to form and annihilate in the enlarged ionic-water clusters phase, since o-Ps intensity in liquid water (∼27 %) is much higher than that in pristine Nafion matrix (∼10 %). 43 Surprisingly, the trends of τ o−P s /R in both CNTs/Nafion and Su-CNTs/Nafion membranes during water uptake are quite different from that in pristine Nafion membranes. A rapid decrement in free volume size of both CNTs/Nafion and Su-CNTs/Nafion membranes

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0.382

3.2

0.376

3.1

0.370

(ns)

3.3

o-Ps

CNTs/Nafion

3.3

0.382

3.2

0.376

Free Volume Radius (nm)

Pristine Nafion

o-Ps Lifetime

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Su-CNTs/Nafion

3.2

0.376

3.0

0.363

2.8

0.349

0

20

40

60

80

100

Relative Humidity (%)

Figure 3: The o-Ps lifetime τ o−P s and the corresponding free volume radius (R) in Nafion membranes as a function of relative humidity.

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is observed from ∼0% RH to ∼40% RH. As mentioned above, numerous CNTs or Su-CNTs in Nafion can reinforce Nafion matrices because they act as cross-linkings and decline the mobility of Nafion chains. 21,22,55 Hence, during water uptake, the CNTs and Su-CNTs in composite membranes restricted the rearrangement of Nafion backbones and suppressed the expansion of free volume holes. Meanwhile, the preexisting free volumes in composite membranes were continuously filled by absorbed water molecules with increasing humidity. Consequently, the measured free volume hole size apparently decreased. Simultaneously, it’s noticeable that τ o−P s in Su-CNTs/Nafion membranes decreased by 0.35 ns, while that in CNTs/Nafion membranes only decreased by 0.05 ns from ∼0% RH to ∼40% RH. This is not surprising because, besides the restriction of Su-CNTs on Nafion backbones, the strong interactions among sulfonic acid groups (-SO3 − ) of Nafion and the sulfanilic acid groups grafted on the surface of Su-CNTs further suppressed the rearrangement of Nafion backbones in Su-CNTs/Nafion membranes. 40 The higher break strength of Su-CNTs/Nafion membranes is also in favor of this explanation. Beyond 40% RH, the significant increments in τ o−P s /R in both CNTs/Nafion and Su-CNTs/Nafion membranes are attributed to the expansion of free volumes caused by the growth of ionic-water clusters, 13 which can generate great stress (≥2.3 MPa) toward the membranes thus lead to rearrangement of Nafion backbones in spite of the reinforcement of CNTs or Su-CNTs. 56 A similar dramatic expansion of free volumes in pristine Nafion membranes caused by the ionic-water clustering was also found from ∼60% RH to ∼80% RH. Enhanced Proton conductivities of Su-CNTs/Nafion membranes at room temperature. Fig. 4(a) shows the Nyquist plots of Su-CNTs/Nafion membranes at different humidities, and the bulk and grain boundary resistances in membranes are mainly determined by the semicircle in the high-frequency region of Nyquist plots. 57 Obviously, the internal resistances of Su-CNTs/Nafion membranes decrease at higher ambient humidity. The environmental effects of relative humidity on proton conductivities (σ) and their corresponding diffusion coefficients (D) derived from the Nernst-Einstein equation of the three

13



Z'' (K

RH 32 %

RH 38 %

RH

)

-30

49 %

95 %

61 %

RH

RH

(a)

-20

2% RH

-700

-10

-350

0

40

-1

(b)

Pristine Nafion

0.08

1.5

CNTs/Nafion Su-CNTs/Nafion

-5

-1

) s )

Z' (K

60

0.06

1.0 0.04

2

20

1200

cm

0

600

-1

~ 0.03 S cm

0.5 0.02

-1

0.01 S cm

0.0

0.00 0

25

50

75

Diffusion Coefficient (10

0

Proton Conductivity (S cm )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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100

Relativity Humidity (%)

Figure 4: (a) Nyquist plots from AC impedance data of Su-CNTs/Nafion composite membranes at room temperature and varying humidity between 2% RH and 95% RH. (b) Proton conductivity and proton diffusion coefficient of membranes as a function of relative humidity.

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kinds of Nafion membranes are illustrated in Fig. 4(b). With increasing relative humidity, proton conductivities/diffusivities of all membranes increase significantly due to the higher water uptake, because proton migration in Nafion occurs mainly among the ionic-water clusters phase via proton hopping (also known as Grotthuss or diffusion) and/or the vehicular transport aided by water molecules. 3,9–11 Particularly, in comparison with the electronic conductivity of present Nafion membranes (as shown in Table 1), the measured proton conductivities of all membranes are much higher. Therefore, one should keep in mind that the electronic conductivity in these three kinds of Nafion membranes are negligible. In pristine Nafion membranes, only slight increment in proton conductivity/diffusivity of membranes can be found below ∼50% RH. However, further increasing humidity until ∼80% RH leads to dramatic improvement in proton conductivity/diffusivity. Beyond ∼80% RH, proton conductivity/diffusivity increases gradually. The significant variation in proton conductivities/diffusivities in Nafion membranes can be explained by the growth and interconnection of ionic-water clusters during water uptake. 4,13,58 Because of higher water uptake in the Nafion membranes, the enlarged ionic-water clusters in Nafion tend to interconnect with each other, such that water channels can be formed, which are greatly beneficial to proton conductivity. Accordingly, the results reveal that, at ∼80% RH, water channel network is basically formed in present pristine Nafion membranes and proton transportation is efficient among them. However, proton conductivities in CNTs/Nafion composite membranes are found lower than that of pristine Nafion membranes at various relative humidities above ∼40% RH, which further confirmed that the doped CNTs or Su-CNTs in composite membranes did not lead to electronic short circuits. The low proton conductivities in CNTs/Nafion composite membranes can be attributed to the lower swelling ratio of CNTs/Nafion membranes because of the reinforcement of hydrophobic CNTs on Nafion backbones. Thus, the growth of the ionic-water clusters is suppressed, such that the formation of interconnected water channels for proton conduction is retarded. At a high humidity of ∼99% RH, proton conductivities and

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diffusion coefficients in CNTs/Nafion composite membranes (0.054 S cm−1 and 1.07×10−5 cm2 s−1 ) and in pristine Nafion membranes (0.055 S cm−1 and 1.09×10−5 cm2 s−1 ) are essentially the same. The present results indicate that pure CNTs may suppress the formation of water channels in CNTs/Nafion membranes during water uptake, and they have no direct influence on the proton transport pathways in hydrated Nafion membranes. Interestingly, the proton conductivities of Su-CNTs/Nafion composite membranes are significantly higher than that of other membranes at various humidities beyond ∼20% RH, despite the fact that the water contents of Su-CNTs/Nafion membranes are essentially the same with that of CNTs/Nafion membranes, and are even lower than that of the pristine ones. The significantly enhanced proton conductivity in Su-CNTs/Nafion composite membranes can be interpreted for that the sulfonic acid groups grafted on the Su-CNTs, acting as active sites for proton transportation, help to improve proton conduction. In particular, because of numerous sulfonated acid groups on the Su-CNTs, water molecules aggregate to form ionic-water clusters along the Su-CNTs in the membranes, such that plenty of "short" proton migration paths can be formed. As a result, "highways" for proton conduction are formed along the Su-CNTs in the membranes. 24–27 In other words, the sulfonic acid groups along Su-CNTs paved many new "short" paths for proton transportation such that the proton conductivity is significantly enhanced. Simultaneously, with increasing ambient humidity from ∼0% RH to ∼50% RH, proton conductivity in Su-CNTs/Nafion membranes shows a steady increment from 1.7×10−4 S cm−1 to ∼0.03 S cm−1 , while those in pristine and CNTs/Nafion membranes only increase slightly. Under the condition of ∼50% RH, the proton conductivity of ∼0.03 S cm−1 in Su-CNTs/Nafion membranes is 3 times higher than those in pristine Nafion and CNTs/Nafion membranes (≤0.01 S cm−1 ), indicating that the transportation of protons in Su-CNTs/Nafion membranes could be more efficient than other membranes at low humidities. Commonly, working at low humidity is one possible approach to prolong the operation life of PEMs, because dynamic humidity always results in degradation of PEMs. 6 Hence, the new property of high proton conductivity at low humidity

16


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of Su-CNTs/Nafion membranes may be beneficial for the development of practical PEMFCs with long operation life. As mentioned above, beyond ∼80% RH, a slight increment in proton conductivity is found in both pristine Nafion and CNTs/Nafion composite membranes. Surprisingly, proton conductivities of Su-CNTs/Nafion membranes keep increasing with increasing the humidity higher than ∼80% RH. In our previous work, it’s found that the critical value of ∼4 H2 O molecules per sulfonate group (λ = 4) is enough for efficient proton transportation in pristine Nafion membranes because the formation of water channel network is basically completed. 13 At a relative humidity of ∼80% RH, derived from present water uptake and IEC results, λ of pristine Nafion, CNTs/Nafion and Su-CNTs/Nafion membranes are 3.8, 3.7 and 3.4, respectively. The λ values for the present pristine Nafion and CNTs/Nafion membranes agree well with previous results, 13 but it is likely that there are still some potential sulfonic acid groups in Su-CNTs/Nafion membranes available for the formation of ion-water clusters even at a relatively high humidity of ∼80% RH. Thus, it is rational that further enhancement in proton conductivity can be achieved beyond ∼80% RH for Su-CNTs/Nafion membranes. Generally, during water uptake, a percolation behavior of proton conductivity happens in Nafion at extremely low water uptake (≤0.1 v%), 2 which is related to the proton transportation between isolated ionic-water groups. Simultaneously, due to the percolation of ionic-water clusters, Nafion membranes show another percolation behavior of proton conductivity at a higher water uptake. 13,59,60 In our previous work, the later percolation of proton conductivity in Nafion was observed between water uptakes of ∼4.5 wt% (λ = 3) and ∼6 wt% (λ = 4). 13 The impacts of water content on proton conductivity in various membranes are depicted in Fig. 5. Obviously, proton conductivities in these membranes during water uptake all first increase slightly, then followed by a dramatic increment, and finally increase moderately. As shown by the solid lines in Fig. 5, proton conductivities (σ)

17


; W

= 3.7 wt%;

= 0.101

; W

= 4.8 wt%;

= 0.071

c

1.8

-1

-1

2

cm = 0.047 S cm

-1

c

0.06

1.2 = 0.046 S cm

(s cm

-1

)

0.04

0.02

-1

; W

c

= 4.6 wt%;

= 0.072

-1.00

0.6

-1.25

-1.50 -0.6

0.0

0.6

1.2

Diffusion Coefficient (10

-5

0.08

= 0.061 S cm

Su-CNTs/Nafion

Log

-1

)

CNTs/Nafion

s

Pristine Nafion

Proton Conductivity (S cm

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 33

)


Log(W-W )

0.00

c

5

10

15

20

0.0

Water Content (wt%)

Figure 5: Proton conductivities of pristine and composite Nafion membranes as a function of water content. The solid lines are shown for the proton conductivity (σ) in membranes versus water content (W ) based on the power law equation. The σ 0 , W c and β in the power law equation of different membranes are displayed near the corresponding lines. The insert shows the double-logarithmic plot of (σ) versus (W - W c ), and the solid lines are linear fittings of data.

18


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in these present Nafion membranes during water uptake obey the power law, 61

σ ∝ σ0 (W − Wc )β

(2)

where W is the water content in composite membranes, σ 0 represents the σ in hydrated Nafion membranes, W c is the critical water content of the percolation threshold of ionicwater clusters, and β is a scaling exponent describing the variation in proton conductivity near W c , which provides information on morphological features or at least the tortuosity of proton transport pathways. 2 The logarithmic σ as a function of logarithmic (W - W c ) is replotted in the insert of Fig. 5. The critical water content W c in Su-CNTs/Nafion composite membranes (3.7 wt%, λ = 2.2) is significantly lower than that of pristine Nafion (4.8 wt %, λ = 3.0) and CNTs/Nafion membranes (4.6 wt%, λ = 3.1). Above W c , percolation of ionic-water clusters in the membranes occurred, resulting in formation of interconnected water channels. Generally, the proton conduction in Nafion depends on not only the formation of water clusters, but also other multiple sequential and interrelated factors, for instance, the interconnectivity of ionic-water clusters and the tortuosity of water channels. Giving the water content lower than the W c of pristine Nafion membranes, the Su-CNTs may interconnect the isolated ionic-water clusters in Su-CNTs/Nafion membranes. As a result, a lower water content of 3.7 wt% is feasible for the formation of interconnected ionic-water clusters in Su-CNTs/Nafion membranes. At mesoscale, the tortuosity of the water channel network impacts overall macroscopic proton conductivity. 2 At a high humidity, because of the formation of lots of short water channels along the Su-CNTs, the tortuosity of water channels in Su-CNTs/Nafion composite membranes can be lower than that of pristine Nafion and CNTs/Nafion membranes. The lower value of W c and remarkably enhanced proton conductivities in Su-CNTs/Nafion membranes strongly showed that the ionic-water channels are less tortuose than those in pristine Nafion and CNTs/Nafion membranes.

19



Figure 6: The schematic illustration of proton transportation in water hydrated (a) pristine Nafion membranes with tortuose proton pathways and (b) Su-CNTs/Nafion membranes with proton "highways" along Su-CNTs.

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Further, the scaling exponent value β of CNTs/Nafion membranes (0.072) is almost the same as that of pristine Nafion membranes (0.071). However, β of Su-CNTs/Nafion composite membranes is 0.101, which is ∼40 % higher than that of other membranes. In the PFSA system including Nafion, the change of the scaling exponent value β suggests a distinct variation in the mesostructures of ionic-water clusters or the tortuosity of proton transport pathways. 2 The essentially same β values for pristine Nafion and CNTs/Nafion membranes are due to the fact that additional CNTs in Nafion almost have no impact on the formation of proton transport path, which is also responsible for the same critical water content of ionic-water clusters percolation threshold in them. While, the larger value of β for SuCNTs/Nafion membranes further confirmed that plenty of proton migration paths with less tortuosity were formed along the Su-CNTs in the Su-CNTs/Nafion composite membranes, as mentioned above. The formation of "short" proton migration paths along the Su-CNTs in Su-CNTs/Nafion composite membranes is schematically displayed in Fig. 6. Nevertheless, the water channel orientation is also very important for practical PEMs, which is out of the present scope and will be further studied for the Su-CNTs/Nafion composite membranes. Enhanced proton conductivities of Su-CNTs/Nafion membranes at high temperatures. The proton conductivities of composite Nafion membranes in a wide range of temperatures (from 25 ◦ C to 140 ◦ C) without external humidification were investigated, as shown in Fig. 7(a). Proton conductivities of all membranes increase with increasing temperature from 25 ◦ C to 110 ◦ C (from 25 ◦ C to 135 ◦ C for Su-CNTs/Nafion membranes) because of the increment in diffusivity of protons in Nafion membranes. Furthermore, with increasing the temperature, water content as well as free volume size in membranes may increase because higher temperature condition leads to expansion and higher swelling ratio of Nafion membranes. 62 However, proton conductivity of Nafion suffers at elevated temperatures over 100 ◦ C, 4 because of the rapid evaporation of water molecules in membranes. Thus, it is rational to observe that the proton conductivities of pristine Nafion and CNTs/Nafion membranes dropped remarkably at the temperatures over 120 ◦ C. This poor performance of

21


-1

Proton Conductivity (S cm

(a)

Pristine Nafion CNTs/Nafion

0.25

Su-CNTs/Nafion

0.20 0.15 0.10 0.05 20

40

60

80

100

120

140

-1

)

Temperature (°C)

Ln (Proton Conductivity) (S cm

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 33

)


Pristine Nafion

(b)

CNTs/Nafion

-1.5

Su-CNTs/Nafion

-2.0

-2.5

-3.0

Ea =

10.3 KJ mol

Ea =

10.6 KJ mol

-1

-1

-1

Ea =

2.4

2.6

7.8

KJ mol

2.8

3.0 -1

1000 T

-1

(K

3.2

3.4

)

Figure 7: (a) Proton conductivities of pristine Nafion, Su-CNTs/Nafion and CNTs/Nafion membranes at different temperatures, and (b) activation energy values of membranes according to Arrhenius equation. At a high temperature around 135 ◦ C, the arrows in Fig. 7(a) indicate the enhancement and the failure of proton conductivity in the Su-CNTs/Nafion membranes and the CNTs/Nafion, pristine Nafion membranes, respectively.

22


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Nafion membranes at high temperatures is intrinsic and can cause a failure of traditional fuel cell. These results are consistent with the published literatures. 2,4 In addition, the decrement in proton conductivity of Nafion membranes at high temperatures can also be due to the thermally induced discontinuity of proton conducting network, which is resulted from the distorted cross-linking geometry of polymer matrix through structural reconfiguration process at high temperature. 63 Evidently, the proton conductivities of the Su-CNTs/Nafion membranes at various temperatures are considerably higher than that of other membranes. As mentioned above, this high proton conductivity is attributed to the formation of new proton migration paths with less tortuosity along the Su-CNTs in the Su-CNTs/Nafion membranes. Basically, the temperature dependence of proton conductivity is fulfilled for a semi-empirical Arrhenius equation, 64 σ = σ0 exp

−Ea RT

(3)

where σ is the proton conductivity in membranes (in S cm−1 ), σ 0 is a pre-exponential factor, Ea is the apparent activation energy of proton migration (in kJ mol−1 ), R is the universal gas constant (8.31 J mol−1 K−1 ) and T is the absolute temperature (in K). The Ea s of proton conduction in different membranes are derived by fitting the data lnσ versus T−1 , and the derived Ea s are displayed in Fig. 7(b). The Ea s of pristine and CNTs/Nafion membranes are almost the same, while the Ea of Su-CNTs/Nafion membranes is lower than those of pristine and CNTs/Nafion membranes. The relatively lower Ea of Su-CNTs/Nafion membranes is probably associated with the less tortuosity of their proton transport pathways, which is responsible for the high conductivities of Su-CNTs/Nafion membranes at various humidities and temperatures. Surprisingly, the proton conductivity of Su-CNTs/Nafion composite membranes maintains a satisfactory value (∼0.28 S cm−1 and the corresponding D = 7.5×10−5 cm2 s−1 ) even at a high temperature of 135 ◦ C, which means the heat resistance of Su-CNTs/Nafion membranes can be better than the other two kinds of membranes. To elucidate this, a thermal 23



100

Pristine Nafion CNTs/Nafion Su-CNTs/Nafion

75 50 25

100 Mass (%)

Mass (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 33

99 98 97 45

0 0

90

135

Temperature ( C)

150

300

450

Temperature ( C)

Figure 8: Thermal gravity analysis of pristine Nafion, Su-CNTs/Nafion and CNTs/Nafion membranes.

24


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gravimetry measurement was used to explore the weight loss of membranes with increasing temperature and the results are shown in Fig. 8. Prior to the measurements, the membranes were first kept at 75% RH and room temperature for several days. As displayed in the insert of Fig. 8, with increasing temperature from room temperature to 160 ◦ C, the weight losses of Su-CNTs/Nafion and CNTs/Nafion composite membranes are much lower that of pristine Nafion. Below 160 ◦ C, the weight loss of Nafion is mainly attributed to the evaporation of water molecules, thus the lower weight losses of Su-CNTs/Nafion and CNTs/Nafion membranes indicate that their water retention abilities are better than that of pristine Nafion at a relatively high temperature. These improved water retention abilities of Su-CNTs/Nafion and CNTs/Nafion membranes might be attributed to their lower membrane swelling ratio and slower water diffusion, which is helpful for preventing rapid evaporation of water molecules from membranes. Nevertheless, it’s noted that the proton conductivity of CNTs/Nafion membranes dropped as that of pristine Nafion membranes beyond 120 ◦ C, despite of the enhanced water retention ability of CNTs/Nafion membranes. It could be interpreted as that during the structural reconfiguration process in CNTs/Nafion membranes at high temperature, lots of discontinuity of proton conducting networks were formed due to the reinforcement on the Nafion backbones from the numerous hydrophobic CNTs, which thus decreased overall macroscopic proton conductivity. 2 However, the better water retention ability in Su-CNTs/Nafion membranes can be attributed to the abundant hydrophilicity chemical groups (including the sulfanilic acid groups and the residual hydrophilic carboxylic acid groups and hydroxyl groups) on the Su-CNTs, which are in favor of binding water molecules in the interconnected ionic-water clusters, i.e. many proton paths along the Su-CNTs are preserved at high temperature.

25



CONCLUSIONS In this study, sulfonated carbon nanotubes (Su-CNTs) were synthesized, and Su-CNTs/Nafion composite membranes had been prepared by a self-assembly sol-gel process. Significant reduction of Nafion backbone mobility and reinforcement of Nafion matrices through the incorporation of CNTs and Su-CNTs in membranes had been found by PALS and mechanical property measurements. Particularly, at various humidities and temperatures, the SuCNTs/Nafion composite membranes showed significantly higher proton conductivities than pristine Nafion and CNTs/Nafion membranes, because plenty of "short" proton conducting pathways were formed along the Su-CNTs. At a relatively low humidity of ∼50% RH, proton conductivity in Su-CNTs/Nafion membranes reached ∼0.03 S cm−1 , while those in pristine Nafion and CNTs/Nafion membranes were less than 0.01 S cm−1 . Further, because of the enhanced water retention ability at a high temperature and the formation of water channels along Su-CNTs, the Su-CNTs/Nafion membranes maintained a high proton conductivity at a considerably high temperature of 135 ◦ C, where pristine and CNTs/Nafion membranes failed to be effective for PEMs. Compared with many other inorganic additives, Su-CNTs with advanced mechanical properties in Nafion membranes have a strong restriction on the rearrangement of Nafion backbones, which not only is in favor of the mechanical strength of membranes, but also can prolong the operation life and increase the operation temperature of the present membranes by preventing the rapid water evaporation as well as the degradation of membranes. Therefore, the macroscopic properties were successfully elucidated from the point of view of the microstructures of Su-CNTs/Nafion membranes, whose excellent proton conductivity and stability at both low humidities and high temperatures show potential applications of them in industrial fuel cell system.

26


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Acknowledgement The work is supported by the National Natural Science Foundation of China (NSFC) under Grants Nos.11375132 and 11575130. The author (C. Yin) appreciates Mr Zheng Wang and Miss Qing Liu for their helpful assistance in experiments.

Supporting Information Available • The contents of Supporting Information including: images of Su-CNTs and hybrid Nafion membranes, water uptakes of membranes, intensities of o-Ps lifetime in membranes, and details of experiments.

27



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Graphical TOC Entry Pristine Nafion

Sulfonated Carbon Nanotubes/Nafion

(H2O)nH+ (H2O)nH+

(H2O)nH+ SO O3-

(H2O)nH+

(H2O)nH+

33


Enhancement in Proton Conductivity and Thermal Stability in Nafion

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