Phase Separation and Development of Proton Transport Pathways in

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C: Physical Processes in Nanomaterials and Nanostructures

Phase Separation and Development of Proton Transport Pathways in Metal Oxide Nanoparticles/Nafion Composite Membranes During Water Uptake Chongshan Yin, Jingjing Li, Yawei Zhou, Haining Zhang, Pengfei Fang, and Chunqing He J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b02535 • Publication Date (Web): 09 Apr 2018 Downloaded from http://pubs.acs.org on April 9, 2018

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The Journal of Physical Chemistry

Phase Separation and Development of Proton Transport Pathways in Metal Oxide Nanoparticles/Nafion Composite Membranes During Water Uptake 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 Nafion has been extensively used as proton exchange membrane in the promising energy source of polymer-electrolyte fuel cell. However, the phase morphology in Nafion, which basically governs its unique proton transport capability, is very sensitive to the ambient environment. In this work, free volumes as well as the phase separations in pristine and hybrid Nafion membranes (Nafion-TiO2 , Nafion-SiO2 ) during water uptake were studied by the positron annihilation lifetime spectroscopy measurement. Because of the o-Ps annihilation in ionic-water clusters and Nafion skeleton regions, it was attempted to resolve two individual o-Ps lifetime components from the positron annihilation lifetime spectra. Results showed that the development of phases in Nafion mem-

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branes as a function of relative humidity was well correlated to a bimodal o-Ps lifetime distribution. Accordingly, the obviously enhanced proton conductivities in Nafion-TiO2 and Nafion-SiO2 hybrid membranes beyond 6 wt% water content were attributed to the formation of additional ionic-water cluster phases around the hydrophilic SiO2 /TiO2 additives in the hybrid Nafion membranes, which reduced the tortuosity of the overall proton transport pathways.

1. INTRODUCTION Recently, polymer-electrolyte fuel cells (PEFCs) show extensive application prospects as the alternative energy sources, because of their high energy conversion efficiency, high power density, low pollution, and fast start-up. Nafion membranes (the perfluorinated sulfonic-acid (PFSA) based membranes) have been widely used as proton exchange membranes (PEMs) in PEFCs due to their remarkable proton conductivity and chemical stability under various humidity and temperature conditions. 1–3 Nafion has a bicontinuous nanostructure, in which perfluoroether side-chains with a pendant sulfonic acid group (-SO3 H) are distributed along the polytetrafluoroethylene (PTFE) Nafion backbones disorderly. 2–4 During water uptake, the formation of ionic-water clusters in Nafion is preferentially around the hydrophilic sulfonic acid groups instead of the hydrophobic polymer backbones, thus results in a natural phase separation in Nafion. This phase separation is closely associated with the unique proton transport capability of Nafion, because the major proton transportation in hydrated Nafion is commonly considered happening in the interconnected ionic-water clusters (water channels) through the proton hopping and the proton vehicular transport. 5–9 Various significant efforts such as doping hydrophilic metal oxide nanoparticles, for instance, TiO2 , SiO2 , ZrO2 , Al2 O3 and Fe3 O4 into Nafion membranes have been attempted to enhance the water retention ability of the membranes. 10–16 Nevertheless, it’s known that the phase morphology in Nafion membranes is very sensitive to the ambient environment, 10,17 and Nafion suffers a significant degradation in proton conductivity at both high temperature (above 80 ◦ C) 2


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and low humidity. 2 Therefore, it is very important to study the association between proton conductivity and development of phase separation in pristine Nafion and metal oxide nanoparticles/Nafion hybrid membranes during water uptake. Nevertheless, it is exceedingly difficult to observe the microstructure evolution in polymers as a function of environmental condition. Positron annihilation technique (PAT) is a powerful and nondestructive tool, and it is capable of the characterization of nanometer-sized free volume holes in polymers 10,18–25 as well as many types of defects 26,27 and pores 28–31 in condensed matters. After entering polymer, some positrons may annihilate via free positron with a typical lifetime of ∼300 ps, while part of the positrons may annihilate after forming a hydrogen-like bound state, i.e. the positronium (Ps) atoms of an electron and a positron. There are two spin states of Ps atoms: spin antiparallel para-positronium (p-Ps) and spin parallel ortho-positronium (o-Ps). The typical p-Ps lifetime is ∼125 ps, and the theoretical lifetime of o-Ps via 3γ annihilation is 142 ns in a vacuum. Meanwhile, the o-Ps atoms are preferentially trapped in free volume holes in polymers, and tend to annihilate via 2γ pick-off annihilation with much shorter lifetimes of several nanoseconds. The so-called pick-off annihilation of o-Ps means that the positron in o-Ps atom "picks up" and annihilates with an electron with opposite spin from the molecules on the free volume walls. 32,33 The actuallymeasured lifetimes (τ o−P s ) of o-Ps atoms trapped in free volumes are basically decided by the size of those free volume holes. A semiempirical relationship between the measured τ o−P s and the free volume size, in a spherical approximation was developed based on the quantum mechanics of positronium trapping and annihilation in the free volume holes. 32–34 Thus, the PAL has been widely used as a sensitive probe for the characterization of free volumes in a variety of polymers. 2,33,35–43 In our previous work, one o-Ps component was used to evaluate the average size of free volumes from the overall Nafion membranes during water uptake. In this work, the development of phases in Nafion membranes at high humidities was investigated and well correlated to a bimodal o-Ps lifetime distribution. Accordingly, the proton conductivities in Nafion-TiO2 and Nafion-SiO2 hybrid membranes at various humidities were

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studied and their enhanced proton conductivities were well interpreted for the formation of extra ionic-water cluster phases around the hydrophilic SiO2 /TiO2 additives in the hybrid Nafion membranes at high humidities.

2. EXPERIMENTAL SECTION Metal oxide nanoparticles/Nafion hybrid membranes (Nafion-SiO2 and Nafion-TiO2 membranes) and pristine Nafion membranes were prepared by a self-assembly technique in situ. The content of TiO2 or SiO2 in hybrid Nafion membranes was ∼5 wt% regarding to Nafion, and the thickness of the membranes was 78±5 µm. Details of membranes preparation can be found in our recent study. 10 Water uptake of Nafion membranes was determined by the weight of hydrated and dry Nafion membranes as the following equation,

W U (%) = 100 ×

(M wet − M dry ) M dry

(1)

where Mdry is the weight of Nafion membranes which have been completely dried in a vacuum at 60 ◦ C for 12 hours, and Mwet is the weight of fully hydrated (immersed in water) Nafion membranes whose surface-adsorbed water has been carefully wiped off with tissue paper. Ion exchange capacity (IEC) of Nafion membranes was measured by a back-titration method. A pH meter (Sartorius, PB-10) was used to monitor the pH value of solutions. The mechanical properties of prepared membranes were examined for the membrane specimens with 50 mm in length, at room temperature and humidity. Dumbbell type specimens were placed between the grip of the testing machine, and deformed under tension at a tensile rate of 2 mm min−1 . All water uptake, IEC and mechanical tests were conducted three times for each membrane, and the results are presented by the average values. The membrane swelling behavior (geometrical expansion) was calculated from the volumes of water saturated (Vwet ) and completely dried (Vdry ) Nafion membranes according to the calculation of Vwet /Vdry . The thickness of membranes was measured using a thickness 4


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gauge with a resolution of 1.0 µm, and was reported as an average of three measurement points for each strip-shaped membrane. The length and width of the membranes were measured with a caliper of 0.02 mm resolution. Positron annihilation characteristics and proton conductivities of membranes were measured under various ambient conditions. Details of these experiments can be found in our published work. 10

3. RESULTS AND DISCUSSION 3.1. Water uptake and swelling behavior of Nafion-SiO2 and Nafion-TiO2 composite membranes. Table 1 summarizes water uptake (WU), ion exchange capacity (IEC), the number of water molecules per sulfonic acid groups (λ) and tensile strength at break of pristine and hybrid Nafion membranes. λ is calculated from the WU and IEC of membranes by using the following equation,

λ=

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

(2)

where Mwater is molecular weight of water molecular (18 g mol−1 ). Apparently, both the Nafion-TiO2 and Nafion-SiO2 hybrid membranes show an enhancement in water uptake and λ, mainly due to the hydrophilic surface properties as well as the hygroscopic nature of the SiO2 or TiO2 nanoparticles. 10 The slight increment in break strength of hybrid Nafion membranes is likely attributed to those in situ formed nanoparticles, which show a short diameter of 3.0∼6.0 nm and are largely covered by Nafion matrices. 44 Thus, these nanoparticles ideally have impacts on the improper packing of Nafion chains as well as the mechanical property of Nafion membranes. Meanwhile, no significant difference in IEC is found between pristine and hybrid Nafion membranes. The preservation of swelling behavior of Nafion membranes was also investigated. Firstly, the thickness, length, and width of membranes were measured after the membranes were immersed in deionized water at room temperature for 48 hours. After that, 5



Table 1 Water uptake, IEC , λ, break strength and volume swelling (geometrical expansion) of composite Nafion membranes. sample

water IEC λ break volume − uptake (meq/g) (H2 O/SO3 ) strength swelling (wt%) (M P a) (V /V0 ) pristineN af ion 24.3 ± 1.3 0.88 ± 0.02 15.3 ± 0.5 14.3 ± 1.1 1.45/1.49/1.47 ± 0.04 N af ion − SiO2 26.7 ± 2.0 0.85 ± 0.02 17.5 ± 0.9 15.6 ± 0.9 1.47/1.51/1.49 ± 0.04 N af ion − T iO2 27.2 ± 1.6 0.87 ± 0.02 17.4 ± 0.6 15.1 ± 1.3 1.45/1.48/1.46 ± 0.04

membranes were dried at 60 ◦ C for 6 hours, and kept under ∼0% RH for 24 hours before the thickness, length, and width of membranes were measured again. This cycle was repeated three times for each membrane, and the measured values of all three cycles are reported in Table 1. Obviously, the swelling behaviors of all membranes show good reproducibility. It’s noticed that those incorporated metal oxide nanoparticles hardly effected the swelling ratio of Nafion membranes, indicating of almost no influence on the flexibility of Nafion skeleton from those metal oxide nanoparticles. 3.2. Positronium annihilation in different phases in Nafion membranes as a function of ambient humidity. Generally, one o-Ps lifetime component is basically sufficient and has been widely used for the characterization of the average size of free volumes in polymers. 18,45 In our previous paper, 10 one o-Ps lifetime component was used to evaluate the mean free volume sizes of the overall Nafion membranes, and it’s found that the distribution of o-Ps lifetime in Nafion membranes became wider and wider during water uptake, particularly beyond 6 wt% water content, indicating a phase separation of Nafion matrix and ionic-water clusters in Nafion membranes. Meanwhile, the most possible o-Ps lifetimes, i.e. the peaks of o-Ps lifetime distributions, all shifted to around the o-Ps lifetime in water (∼1.8 ns). 46,47 These results mean that it may be possible to decompose the positron annihilation lifetime spectra of Nafion membranes at high humidities with two o-Ps lifetime components. This consideration will be interesting and meaningful for studying the development of phases (Nafion matrix and ionic-water clusters) and proton conducting pathways

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in Nafion membranes during water uptake. Thus, all positron annihilation lifetime spectra have been further analyzed in this work, and the previous single o-Ps lifetime was tried to be resolved into two individual o-Ps lifetime components with fixing one o-Ps lifetime to ∼1.8 ns (o-Ps lifetime in water) by the PATFIT program. Interestingly, two independent o-Ps lifetime components with good fitting were found for Nafion membranes with relatively high water contents, and the spectra fitting variances (χ2 ) given by the PATFIT program are shown in Table 2. Apparently, all fitting variances are close to 1, indicating the good reliability of all the spectral fitting. With increasing humidity, the values of χ22

o−P s

of all of

those three kinds of Nafion membranes are approaching more close to 1. At high humidities, χ22

o−P s

is outstanding and is even better than χ21

humidities, it’s rational to find that χ22

o−P s

o−P s .

Simultaneously, at relatively lower

is relatively worse (but acceptable), because the

phase separation in membranes is not obvious. Further, at much lower humidities, it was rather difficult to decompose the spectra with a good fitting with 2 o-Ps lifetime components.

Table 2 The fitting variances (χ2 ) of positron lifetime spectra with one (χ21 o−P s ) and two (χ22 o−P s ) o-Ps lifetime components given by the PATFIT program. sample humidity /% RH χ21 o−P s χ22 o−P s 74 0.959 0.956 pristine N af ion pristine N af ion 76 1.014 1.098 pristine N af ion 86 1.043 0.965 pristine N af ion 98 1.049 0.999 N af ion − SiO2 61 1.203 1.147 N af ion − SiO2 72 1.095 1.125 N af ion − SiO2 81 1.168 1.108 N af ion − SiO2 100 1.010 0.955 N af ion − T iO2 40 1.104 1.146 N af ion − T iO2 55 1.048 1.134 N af ion − T iO2 61 1.036 1.026 N af ion − T iO2 71 1.021 1.127 N af ion − T iO2 83 1.044 1.009

The variations of the o-Ps lifetimes (τ o−P s ) and the mean void radius derived from the

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Pristine Nafion

4.0

0.42

3.2

0.36

2.4 Fixed 1.8 ns

Radius (nm)

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

o-Ps Lifetime (ns)


0.28

1.6

0

20

40

60

80

100

Relative Humidity (%)

Figure 1: The variations in o-Ps lifetime (τ o−P s ) and the mean void radius derived from the Tao-Eldrup model of pristine Nafion membranes as a function of humidity. Tao-Eldrup model 32,34 in pristine Nafion membranes as a function of humidity are shown in Figure 1. Below ∼70% RH, only one average o-Ps lifetime (τ o−P s ) can be derived from the positron annihilation lifetime spectra, and it basically remains unchanged indicating the mean size of free volumes measured in Nafion membranes essentially remains constant. 10 Meanwhile, beyond ∼70% RH, it’s interesting to find two independent o-Ps lifetimes, indicating two types of free volumes with significantly different sizes in Nafion membranes. When analyzing PALS results in a multiphasic system such as Nafion, one of the most important issues is the possible location of positronium formation/annihilation or distribution of the free volumes. 48 At low humidity, it’s naturally that most o-Ps atoms annihilate in the Nafion matrix. With increasing water uptake, more and more o-Ps atoms tend to form and annihilate in the enlarged ionic-water clusters, especially beyond the water content of ∼6 wt%. 10 As shown in Fig. 1, beyond ∼70% RH, the shorter o-Ps lifetime τ 1o−P s is fixed to 1.8 ns (a typical o-Ps lifetime in water) 46,47 and corresponds to the o-Ps annihilation in ionicwater clusters. Simultaneously, since the o-Ps lifetime in the amorphous region of pristine 8


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polytetrafluoroethylene is about 3.8∼4.2 ns at room temperature, 49 the longer τ 2o−P s ranging from ∼3.5 ns to ∼4.0 ns is expected to represent the o-Ps annihilation in the (including the polytetrafluoroethylene Nafion backbones and the amorphous region between the Nafion backbones and ionic-water clusters). 3,49,50 This is supported by the fact that the τ 2o−P s shows a moderate increment with increasing humidity, because the adsorbed water molecules acting as plasticizers of Nafion chains result in the swelling of overall Nafion membranes as well as the expansion of free volumes in Nafion matrices.

Nafion-SiO

2

0.42

4.0

3.2

0.36

2.4 Fixed 1.8 ns

Radius (nm)

o-Ps Lifetime (ns)

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


0.28

1.6 0

20

40

60

80

100


Figure 2: The variations in o-Ps lifetime (τ o−P s ) and the mean void radius derived from the Tao-Eldrup model of Nafion-SiO2 composite membranes as a function of humidity. It should be noted that, even under a very low humidity condition (such as 10% RH), there are a small quantity of ionic-water groups/clusters in Nafion, however, few o-Ps atoms may be formed and annihilate in the ionic-water groups/clusters. Actually, it’s extremely difficult, if not impossible, to precisely extract the information of one specific lifetime component with very low-intensity from a positron annihilation lifetime spectrum. Therefore, only with a distinct phase separation in Nafion membranes, the o-Ps formation probability in the ionicwater clusters is relatively high, and two discrete o-Ps lifetimes can be accurately derived by 9


4.0

Nafion-TiO

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0.42

2

3.2 0.36 2.4 Fixed 1.8 ns

Radius (nm)

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

o-Ps Lifetime (ns)


0.28

1.6

0

20

40

60

80

100


Figure 3: The variations in o-Ps lifetime (τ o−P s ) and the mean void radius derived from the Tao-Eldrup model of Nafion-TiO2 composite membranes as a function of humidity. the analysis programs with fixing the shorter o-Ps lifetime to that in water. This nature of the PALS measurement is also responsible for the lower error of τ 2o−P s at higher humidities due to the enhanced phase separation in Nafion membranes. Figure 2 and Figure 3 exhibit the τ o−P s and the mean void radius in Nafion-SiO2 and Nafion-TiO2 composite membranes at different humidities, respectively. Similar to pristine Nafion membrane, two discrete o-Ps lifetimes were resolved in Nafion-SiO2 and Nafion-TiO2 hybrid membranes at high humidities. At higher humidities, the error of τ 2o−P s becomes lower, indicating that the phase separation in Nafion-SiO2 and Nafion-TiO2 membranes becomes more significant. On the contrary, the larger and larger error bars of τ 2o−P s at lower humidities suggest that it becomes more difficult to resolve two o-Ps lifetime components from the spectra measured at low humidities because of less phase separation in membranes. From Figs. 2 and 3, two discrete o-Ps lifetime components could not be obtained from spectra of Nafion-SiO2 and Nafion-TiO2 membranes measured below ∼55% RH and ∼40% RH, respectively, due to no significant phase separation in membranes at low humidities. 10


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Actually, at the relative humidity of ∼70% RH, ∼55% RH and ∼40% RH, water contents in pristine Nafion, Nafion-SiO2 and Nafion-TiO2 membranes are all around ∼6.0 wt%. 10 The present results further confirmed that the distinct phase separation can be formed in Nafion membranes beyond a critical water content of ∼6.0 wt%, regardless of metal oxide additives. Certainly, the nature of the distinct phase separation in Nafion is definitely related to the formation of well-connected water channel network. 3.3. o-Ps lifetime distributions and phase developments in Nafion membranes at different relative humidities. As mentioned above, positronium may be formed and annihilate in different phases in Nafion, i.e. either in the Nafion matrix phase or in the ionic-water cluster phase, and the o-Ps lifetime distributions definitely represent the free volume elements in both phases. Thus, it has been attempted to obtain two o-Ps lifetime distributions from each of the positron annihilation lifetime spectra by the LT program, 51 which were found to be only valid for the spectra of Nafion membranes with relatively high water contents. The o-Ps lifetime distributions in pristine Nafion membranes at different humidities are shown in Figure 4. Obviously, with increasing humidity from 74% RH to 98% RH, intensity of τ 1o−P s (I1o−P s , o-Ps formation probability in ionic-water clusters) increases from 0.96% to 2.61%, while the intensity of τ 2o−P s (I2o−P s , o-Ps formation probability in Nafion matrix) decreases from 3.22% to 2.33%. These variations in I1o−P s and I2o−P s vividly describe the enlargement of ionic-water cluster phase in Nafion membranes during water uptake. At a high humidity of 98% RH, the water content (WU) in Nafion membranes reaches ∼23 wt%. The water volume fraction (Cw ) in Nafion can be calculated using the following Equation,

Cw =

W U × ρ0 Vwater = (V0 ) (1 + W U ) × ρw

(3)

where Vwater and V0 are volume of the region occupied by ionic-water cluster phase and the overall Nafion membranes, respectively. ρw hereby denotes the density of water (1.0 g cm−3 ), and ρ0 is the density of Nafion membranes which is vary with water content and is

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Pristine Nafion

2.61%

98% RH

2.33%

1.74%

Probability Density Function (a.u.)

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2.86%

1.21%

86% RH

3.07%

76% RH

water content = ~6 wt%

0.96%

3.22%

74% RH

3.66%

58% RH 3.48%

31% RH 3.24%

21% RH 3.26%

low water content

3% RH

0

2

4

6

8

10

o-Ps Lifetime (ns)

Figure 4: The variations in o-Ps intensity and distribution of pristine Nafion membranes as a function of humidity.

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available in a published review. 2 With a water content of ∼23 wt%, ρ0 is ∼1.75 g cm−3 . Accordingly, at 98% RH, Cw in Nafion turns out to be ∼33 v%, indicating that the majority region in Nafion is determined by the Nafion skeletons. However, at 98% RH, the I1o−P s (2.61%) is higher than I2o−P s (2.33%), because the o-Ps atoms prefer to annihilate in the ion rich region (such as the ionic-water clusters in Nafion). 10,52 It’s known that the o-Ps formation probability in liquid water is ∼27 %, 47 while that in Nafion matrix (completely dried Nafion, at 2% RH ) is only 3.26 %. The high o-Ps formation probability in ionic-water clusters is also responsible for the increment of overall o-Ps intensity from 3.26% to 4.94% with increasing relative humidity from 3% RH to 98% RH. Figure 5 and Figure 6 display the o-Ps lifetime distributions in Nafion-SiO2 and NafionTiO2 hybrid membranes at different humidities, respectively. With increasing humidity, o-Ps lifetime distributions in hybrid Nafion membranes show similar trends. At 3% RH, o-Ps intensities in Nafion-SiO2 and Nafion-TiO2 composite membranes are similar, and are obviously lower than that of pristine Nafion. The lower o-Ps intensities indicating a decline in the number of free volumes in the matrices of hybrid Nafion membranes, because the SiO2 or TiO2 nanoparticles have influenced the improper packing of Nafion chains, as mentioned above. Furthermore, fewer free volumes in membranes suggest a more dense structure of the Nafion-SiO2 and Nafion-TiO2 hybrid membranes, which agrees well with the slight increment in break strength of hybrid Nafion membranes (as shown in Table 1). Under the same humidity conditions, it’s noted that the relative intensity of τ 1o−P s (I1o−P s /(I1o−P s +I2o−P s )) of hybrid Nafion membranes is significantly higher than that in the pristine Nafion membranes, indicating the formation of additional ionic-water cluster phases around the hydrophilic SiO2 /TiO2 additives in the hybrid Nafion membranes. Particularly, it’s known that the water contents in pristine Nafion, Nafion-SiO2 and Nafion-TiO2 membranes all reach ∼6 wt% (Cw = ∼11 v%) at the relative humidity of ∼70% RH, ∼55% RH and ∼40% RH, respectively. 10 Thus, under these conditions, it’s rational to find that the I1o−P s in all membranes are very close to each others (0.96 %, 0.91 % and 0.93 % for pristine Nafion, Nafion-SiO2 and Nafion-

13



Nafion-SiO2


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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2.34%

1.83%

99% RH

1.71%

2.09%

81% RH

2.12%

1.31%

72% RH

0.91%


2.25%

61% RH

2.19%

55% RH 2.12%

50% RH 2.66%

20% RH 2.63%

low water content

3% RH

0

2

4

6

8

10

o-Ps Lifetime (ns)

Figure 5: The variations in o-Ps intensity and distribution of Nafion-SiO2 composite Nafion membranes as a function of humidity.

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Nafion-TiO2

1.70%

1.95%


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


1.93%

1.60%

71% RH

2.23%

1.39%

61% RH

2.19%

1.22%

0.93%

83% RH

55% RH


2.32%

40% RH

2.58%

30% RH 2.70%

20% RH 2.61%

low water content

3% RH

0

2

4

6

8

10

o-Ps Lifetime (ns)

Figure 6: The variations in o-Ps intensity and distribution of Nafion-TiO2 composite Nafion membranes as a function of humidity.

15



TiO2 membranes, respectively). Hence, the bimodal o-Ps lifetime distributions with good fitting of positron annihilation lifetime spectra are closely correlated to the phase separation in all those three kinds of Nafion membranes beyond the critical water content of ∼6 wt%. 3.4.

Variations in phase morphology and proton conductivity of pristine

Nafion and metal oxide nanoparticles/Nafion hybrid membranes as a function

RH 45 %

RH 55 %

RH 62 %

(a)

-20 3% RH

-400

-10

-200

0

40

60 )

0.06

0.04

-1

(b)

Nafion-TiO2

(S cm

Nafion-SiO2

1.2

~ 61% RH

Pristine Nafion

-5

-1

)

Z' (K

80

s )

20

600

~ 40% RH

0.8 water content = ~ 6 wt%

0.02

0.4

~ 74% RH

0.00

0.0 0

20

40

2

0

300

cm

0

60

80

100

Diffusion Coefficient (10

Z'' (K

)

85 %

10 0%

-30

RH

RH

of relative humidity. Proton conductivities of Nafion-SiO2 and Nafion-TiO2 hybrid mem-

Proton Conductivity

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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Figure 7: (a) Nyquist plots from AC impedance data of pristine Nafion at room temperature and varying humidity between 45% RH and 100% RH. (b) Proton conductivity and proton diffusion coefficient of pristine and metal oxide nanoparticles/Nafion hybrid membranes as a function of humidity.

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Page 17 of 31

branes were measured at different humidities and results are shown in Figure 7. For the purpose of comparison, proton conductivities of pristine Nafion membranes were measured again and shown in Fig. 7. Figure 7(a) shows the Nyquist plots of pristine Nafion membranes as a function of ambient humidity, at room temperature. It’s found that the bulk and grain boundary resistances of membranes, which are mainly determined by the semicircle in the high-frequency region of Nyquist plots, reduce at higher humidities. Proton conductivities (σ) and corresponding diffusion coefficients (D) derived from the Nernst-Einstein equation of Nafion membranes as a function of ambient humidity are displayed in Figure 7(b). With increasing humidity, proton conductivities/diffusivities of all membranes increase due to the higher water uptake. Simultaneously, when compared with pristine Nafion membranes, the

(S cm

-1

)

Nafion-TiO2

0.06

= 0.043; W

= 5.5 wt%;

= 0.126

= 0.041; W

= 5.4 wt%;

= 0.121

= 0.036; W

= 5.0 wt%;

= 0.075

c

Nafion-SiO2 Pristine Nafion

c

~ 6 wt%

0.04

(s cm

-1

)

c

0.02

-1.2

-1.4

Log

Proton Conductivity

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


-1.6 -0.6

0.0

0.6

1.2

Log(W-W ) c

0.00

6

12

18

24

30

Water Content (wt%)

Figure 8: Proton conductivities of composite Nafion membranes as a function of water content. The dash 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 dash lines. The inset is double-logarithmic plot of (σ) versus (W - W c ), and the solid lines are linear fitting with data. hybrid Nafion-SiO2 and Nafion-TiO2 membranes show evidently higher proton conductivities/diffusivities at various humidities, owing to their improved water retention capacities. As shown by the dash line in Fig. 7b, with the water content of ∼6 wt%, all membranes 17



Page 18 of 31

exhibit similar proton conductivities/diffusivities, regardless of the SiO2 or TiO2 additives. 10 Commonly, there are two percolation behaviors of proton conductivity in Nafion membranes during water uptake. The first one was found at an extremely low water content (∼0.1 v%), which is suggested mainly depending on the proton migration within Nafion membranes via the vehicular transport. 2 With higher water content (∼6 wt %), 10 the percolation of ionic-water clusters results in another percolation behavior of proton conductivity. In this study, the emphasis is focused on the later percolation of proton conductivity. As depicted by the dash lines in Figure 8, the variations in σ of all membranes as a function of water uptake (WU) obey the power law, 53

σ ∝ σ0 (W U − W Uc )β

(4)

where σ 0 denotes the σ of hydrated Nafion membranes, WU c represents the water content threshold of the percolation of ionic-water clusters, and the scaling exponent β provides information on morphological features or the tortuosity of proton transport pathways. The inset of Fig. 8 depicts a good linear relation between the logarithmic σ and the logarithmic (WU - WU c ). Below the water content of 6 wt%, no evident difference in proton conductivity is found in pristine Nafion and hybrid Nafion membranes, which suggests that the hydrophilic additives of SiO2 /TiO2 nanoparticles in Nafion are beneficial to proton transportation by retaining more water molecules at low humidities, however, have no significant influence on the proton transport pathways in Nafion membranes with low water content. Beyond ∼6 wt% water content, markedly enhanced proton conductivities in Nafion-SiO2 and Nafion-TiO2 composite membranes were found. And it’s noted that the hybrid Nafion membranes show obviously higher value of β, indicating that the proton pathways in Nafion-SiO2 and Nafion-TiO2 membranes are less tortuose than those in pristine ones. Accordingly, results strongly suggested the formation of additional water channels in the hybrid membranes beyond ∼6 wt% water

18


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(a)

(b)

Figure 9: The schematic proton transportation of metal oxide nanoparticles/Nafion hybrid membranes (a) around 6 wt% and (b) beyond ∼6 wt% water content.

19


-1

(S cm

0.20

(a)

Page 20 of 31

Pristine Nafion Nafion-SiO2 Nafion-TiO2

0.10 -1

)

~ 0.193 S/cm

(S cm

Proton Conductivity

0.15

0.18 ~ 0.162 S/cm

0.16

0.05

70

80

90

100

Ambient Temperature ( C)

0.00

20

40

60

80

100

(b)

Pristine Nafion

-1.8

Nafion-SiO2

-1

)

Nafion-TiO2

-2.4

Activation Energy (KJ mol

(S cm

-1

)

Ambient Temperature ( C)

Ln(Proton Conductivity)

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

)


-3.0

-3.6 2.6

20

15

10

5

2.8

3.0 -1

1000 T

3.2

3.4

-1

(K )

Figure 10: (a) Proton conductivities of pristine Nafion, Nafion-SiO2 and Nafion-TiO2 membranes with increasing temperature, and (b) activation energy values of membranes according to Arrhenius equation.

20


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content, which are likely situated around the hydrophilic additives of SiO2 /TiO2 and lead to less tortuosity of the overall proton transport pathways in hybrid Nafion membranes, as schematically displayed in Figure 9. Proton conductivities of hydrated Nafion membranes at various temperatures (from 25 ◦ C to 110 ◦ C) were measured in a closed chamber under ∼100% RH, as shown in Figure 10(a). The increment in proton conductivities of all membranes at higher temperatures is attributed to thermal induced enhancement in the diffusivity of protons as well as the higher water content in membranes. In Fig. 10(b), the values of apparent activation energy (derived from a semi-empirical Arrhenius equation 54 ) of hybrid Nafion membranes are significantly higher than that of the pristine ones. To some degree, this relatively high apparent activation energy of hybrid Nafion membranes should responsible for their enhanced proton conductivities. However, all membranes show poor proton conductivities over 100 ◦ C, because of the rapid evaporation of water molecules in membranes. 2,10,55 Nevertheless, it’s found that proton conductivities in pristine Nafion membranes approach their maximum (0.162 S cm−1 ) at ∼85 ◦ C, while those in Nafion-SiO2 and Nafion-TiO2 membranes keep growing to ∼0.193 S cm−1 until the ambient temperature reaches ∼95 ◦ C. These better thermal stability of proton conductivity in hybrid Nafion membranes might result from the additives of the hydrophilic SiO2 or TiO2 nanoparticles which can help the retention of water molecules around them. At the same time, as suggested by their lower o-Ps intensities, the low abundance of free volumes in hybrid Nafion membranes can also help preventing the rapid evaporation water molecules by reducing the diffusion rate of water molecules in the membranes.

4. CONCLUSION Pristine Nafion membranes and Nafion-SiO2 and Nafion-TiO2 hybrid Nafion membranes were prepared. Since the free volumes in ionic-water cluster phase and Nafion matrix phase are significantly different in size, two average o-Ps lifetime distributions with good fittings of

21



positron annihilation lifetime spectra (for Nafion membranes with relatively high water content) enable one to find out two o-Ps lifetime components (bimodal lifetime distributions), which respectively represent the free volumes in the ionic-water cluster phase and the Nafion matrix phase. The variations in the intensities of these two o-Ps lifetime components vividly demonstrated the enlargement of ionic-water cluster phase in Nafion membranes during water uptake. At high humidities, the formation of extra ionic-water cluster phases around TiO2 or SiO2 nanoparticles in Nafion matrix provided additional proton transport pathways and reduced the overall tortuosity of proton transport network in the Nafion-TiO2 or Nafion-SiO2 hybrid membranes, which is responsible for their obviously enhanced proton conductivity. Moreover, Nafion-SiO2 and Nafion-TiO2 membranes showed better thermal stability of proton conductivity, and their proton conductivities reached 0.193 S cm−1 at a relatively high temperature of ∼95 ◦ C. The present results demonstrate that PALS is a very useful technique for the characterization of phase separation in perfluorinated sulfonic-acid (PFSA) based membranes.

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.

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Graphical TOC Entry Distinct phase separation

Formation and growth of ionic-water clusters

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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430x210mm (300 x 300 DPI)


Phase Separation and Development of Proton Transport Pathways in

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