Effect of the addition of hydrated titanium oxide on proton conductivity for aromatic polymer electrolyte membrane

Solid State Ionics 277 (2015) 72–76

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Effect of the addition of hydrated titanium oxide on proton conductivity for aromatic polymer electrolyte membrane Atsuhiko Onuma a,b,⁎, Jun Kawaji a, Shuichi Suzuki a, Makoto Morishima a, Yoshiyuki Takamori a, Naoki Asano c, Kiyoharu Tadanaga d a

Research & Development group, Hitachi, Ltd., 7-1-1 Omika-cho, Hitachi, Ibaraki 319-1292, Japan Graduate School of Chemical Sciences and Engineering, Hokkaido University, Kita 13, Nishi 8, Kita-ku, Sapporo 060-8628, Japan Tsukuba Research Laboratory, Hitachi Chemical Co., Ltd., 48, Wadai, Tsukuba-shi, Ibaraki 300-4247, Japan d Division of Materials Chemistry, Faculty of Engineering, Hokkaido University, Kita 13, Nishi 8, Kita-ku, Sapporo 060-8628, Japan b c

a r t i c l e

i n f o

Article history: Received 18 December 2014 Accepted 31 March 2015 Available online 21 May 2015 Keywords: Polymer electrolyte fuel cell Polymer electrolyte membrane Micro-phase-separated structure Titanium oxide

a b s t r a c t To improve the proton conductivities of a polymer electrolyte membrane which is composed of multi-block copolymers, in low humidity conditions, the effect of adding hydrated titanium oxide into a membrane was investigated. Membranes containing hydrated titanium oxide were prepared by using tetraethoxy titanium. Membranes containing hydrated titanium oxide showed higher proton conductivity than the membrane not containing the additives regardless of humidity. This result showed that adding hydrated titanium oxide improved proton conductivity. By contrast, all the polymer electrolyte membranes showed almost the same water content rate. The content rates of hydrated titanium oxide in membranes were lower than 3 wt%. Thus, adding hydrated titanium oxide was not considered to affect the water content rate of membranes. This result showed that proton conductivity was improved by adding hydrated titanium oxide without increasing water content rate. From the scanning transmission electron microscope observation, micro-phase-separated structures were observed clearly in membranes containing hydrated titanium oxide. It seemed that hydrated titanium oxide enhanced the hydrophilic sengments aggregation during the film–forming process. Probably, the welldefined hydrophilic domains functioned as proton paths and enhanced the proton conductivity of the membrane. © 2015 Elsevier B.V. All rights reserved.

1. Introduction Polymer electrolyte fuel cells (PEFCs) are clean and highly efficient energy conversion systems that are anticipated for use in alternative portable, automotive, and stationary power sources [1–3]. Membrane electrode assembly (MEA) is a main component of PEFC, and it is composed of a polymer electrolyte membrane (PEM) sandwiched between an anode and a cathode. Perfluorosulfonic acid (PFSA) membranes are commonly used because of their high proton conductivity and chemical stability. Aromatic polymer electrolyte membranes have also been widely studied for PEM [4–16]. For example, sulfonated polyether sulfone (S-PES), sulfonated polyether ether ketone (S-PEEK), sulfonated polyphenylene (S-PP), and sulfonated polyimide (S-PI) have been investigated because these materials show good thermal stability and

⁎ Corresponding author at: Research & Development group, Hitachi, Ltd., 7-1-1 Omikacho, Ibaraki 319-1292, Japan. Tel.: +81 294 52 5111; fax: +81 294 52 7624. E-mail address: [email protected] (A. Onuma).

http://dx.doi.org/10.1016/j.ssi.2015.03.038 0167-2738/© 2015 Elsevier B.V. All rights reserved.

mechanical strength. Recently, the number of peripheral equipment such as humidifiers needs to be decreased to reduce the production cost of the PEFC system. PEMs should be used in low humidity conditions for the PEFC system. Therefore, PEMs should be developed that show high proton conductivity in low humidity. Although proton conductivity is increased by increasing the ion exchange capacity of the PEM, the water content rate is also increased and the mechanical characteristics worsen. To improve proton conductivity of the PEM, many researchers have studied PEMs that have micro-phaseseparated structures [12,13,15,16]. They reported that micro-phaseseparated structures were formed in a PEM composed of a multi-block copolymer. Additionally, these PEMs had higher proton conductivities than PEMs composed of random copolymers. Hydrophilic domains are considered to function as a proton path, which improves the proton conductivity, and thus, the micro-structure of PEMs needs to be controlled to improve the proton conductivity of the PEM. Microstructures of PEMs are affected by not only block length of the multiblock copolymer but also the film–forming method [13,17]. It is considered that the factors that affect proton conductivity of the PEM, such as the film–forming method, must be controlled to obtain the PEM that

A. Onuma et al. / Solid State Ionics 277 (2015) 72–76

73

Fig. 1. Chemical structures of the copolymer.

shows high proton conductivity. One method that controls the microstructure of PEM is to add additives during the film–forming process. Moreover, putting additives into a PEM is considered to effectively improve the proton conductivities of PEMs. Many researchers have studied PEMs containing metal oxides that have hygroscopic properties. Hagihara et al. and Uchida et al. prepared PEMs containing TiO2 and SiO2, respectively [18,19]. Some other reports have investigated aromatic polymer electrolyte membranes containing additives [20–22]. These PEMs are composed of random copolymers and zirconium oxide, phosphated zirconia, and heteropoly acid used as additives. However, there have been few reports about additive-containing PEMs that are composed of a multi-block copolymer. In this study, the effect of adding hydrated titanium oxide into PEMs composed of multi-block copolymers was investigated in order to improve proton conductivities in low humidity conditions. Tetraethoxy titanium (TET) was used as a precursor of hydrated titanium oxide. The effect of the addition of tetraethoxy titanium on the micro-structures of PEM was also investigated.

and dried at 90 °C for 20 min. Second, the PEMs were dried at 200 °C for 2 h. Third, the obtained membranes were removed from glass substrates and washed with deionized water. Fourth, membranes were dipped in 1 mol/L NaOH for 30 min and washed with deionized water. Fifth, membranes were dried at 120 °C for 5 h again. Finally, membranes were acidified with 1 mol/L H2SO4 for 1 h and then washed with deionized water, and PEMs were obtained (Table 2). To evaluate the crystal structure, hydrated titanium oxide powder was synthesized using the same process for preparing PEMs. In this process, 10 wt% TET solution was first mixed with deionized water. Second, the solution was dried at 90 °C for 20 min. Third, the obtained powder was dried at 200 °C for 2 h and washed with deionized water. Fourth, the powder was dipped in 1 mol/L NaOH for 30 min and washed with deionized water. Fifth, the powder was dried at 120 °C for 5 h again. Finally, the powder was acidified with 1 mol/L H2SO4 for 1 h and washed with deionized water.

2. Experimental

Content rates of hydrated titanium oxide in PEMs were analyzed using a thermo gravimetric analyzer (TG, EXSTER TG/DTA6200, Seiko Instruments Inc.). In this process, PEMs were heated to 500 °C, polymer in the PEM was burned, and the weight of residual solid was measured. The content rate of titanium oxide (C) was calculated as:

2.1. Preparation of polymer electrolyte membranes The molecular structure of the copolymer is shown in Fig. 1. Multiblock copolymer based on poly (arylene ether sulfone) was synthesized in a process similar to the one described in the literature [11]. At first, hydrophilic and hydrophobic segments were synthesized. Multi-block copolymer was copolymerized by hydrophilic and hydrophobic segments. Table 1 lists descriptions of the copolymer used in this study. The molecular weights of the hydrophobic segments and copolymer were evaluated using gel permeation chromatography with a polystyrene standard. The morecular weight of hydrophilic segment was measured before hydrophilic and hydrophobic segments were combined. Hydrated titanium oxide was obtained using a hydrolysis reaction of tetraethoxy titanium which is described in the literature [23]. PEMs' preparation processes are shown in Fig. 2. The PEM without additives was obtained by process 1 (Fig. 2(a)), in which the polymer was dissolved in N-Methyl-2-pyrrolidone (NMP) and its varnish was obtained. PEMs with additives were obtained by processes 2 and 3 (Fig. 2(b) and (c)). In process 2 (Fig. 2(b)), 10 wt% tetraethoxy titanium solution, which was diluted with ethanol, was mixed with NMP, and their mixed solution was obtained. The polymer was dissolved in the mixed solution, and its varnish was obtained. In process 3 (Fig. 2(c)), multi-block copolymer was first dipped in 10 wt% TET solution for 1 h. After multi-block copolymer was separated from TET solution and was washed with ethanol, multi-block copolymer with additive was obtained. Then, the polymer was dried and mixed with NMP, and varnish was obtained. After varnish was obtained, all PEMs were prepared using the same process. The varnish was first cast on glass substrates with an applicator

2.2. Characterization of PEMs and hydrated titanium oxide powder

C ¼ Wres =Wdry1  100 ½%  … (a) Process 1

ð1Þ

(b) Process 2

Polymer

(c) Process 3

10 wt% TET solution

NMP

Polymer 10 wt% TET solution

NMP

Mixing, stirring

Mixing, stirring Polymer

Vanish

Washing, drying Polymer with additives NMP

Mixing, stirring Film forming

Mixing, stirring Vanish Washing

Vanish

Film forming

Membrane-1

Film forming Washing Washing

Membrane-2,-3

Membrane-4 Fig. 2. Forming processes of membranes.

Table 2 Film forming conditions of membranes. Table 1 Characteristics of the copolymer. Type of polymer

Hydrophobic segment

Copolymer

Mn × 104

Mw / Mn

Mn × 104

Mw / Mn

Multi-block

0.5

1.8

6.3

3.2

IEC (meq/g) 2.2

Membrane

Type of polymer

Process

Content rate of added TET calculated as TiO2 (wt%)

Membrane-1 Membrane-2 Membrane-3 Membrane-4

Multi-block Multi-block Multi-block Multi-block

1 2 2 3

0 1.3 2.6 –

74

A. Onuma et al. / Solid State Ionics 277 (2015) 72–76

σ ¼ B=ðR  D  EÞ ½S=cm …

Intensity (a.u.)

where Wres (g) is the weight of residual solid that was burned at 500 °C more than 30 min and Wdry1 (g) is the weight of the PEM that was dried at 120 °C more than 30 min. The proton conductivity of the PEM in a plane direction was measured by electrochemical impedance spectroscopy using an impedance/ gain-phase analyzer (Solartron, SI1280 B). To measure the proton conductivity of PEMs in various humidity conditions at 80 °C, a humidity–temperature oven (BEL JAPAN Inc., MSB-AD-V-FC) was used. The proton conductivity (σ) was calculated as: ð2Þ

where B (cm) is the distance between electrodes, R(Ω) is the measured resistance, and D (cm) and E (cm) are the width and thickness of the PEM. Water content rate of the PEMs and hydrated titanium oxide powder (RW) was determined gravimetrically in a humidity–temperature oven (BEL JAPAN Inc., MSB-AD-V-FC) and calculated as:

15

25

35

45

55

65

75

85

2θ (deg.) Fig. 3. XRD pattern of the synthesized titanium oxide powder.

  RW ¼ Wwet− Wdry2 =Wdry2  100 ½%  …

ð3Þ

0.6

3. Result and discussion

Membrane-1

Proton conductivity (S/cm)

where Wwet corresponds to the weights of the PEM in a wet condition and Wdry2 corresponds to the weights of PEM that was dried at 85 °C for more than 3 h in vacuum. The micro-structure of PEMs were examined by using a scanning transmission electron microscope (STEM, Hitachi High-Technologies Corporation, HD-2700). Prior to the STEM observation, PEMs were dipped in a 1 mol/L CsCl aqueous solution to substitute the protons with cesium cations. The crystal structure of hydrated titanium oxide powder was analyzed using X-ray diffraction (XRD, Rigaku Corporation, ATX-G).

0.5

Membrane-2 Membrane-3

0.4

Membrane-4

0.3 0.2 0.1

3.1. Content rate of hydrated titanium oxide in PEMs

Table 3 Content rates of titanium oxide in membranes.

0 20

40

60

80

100

Relative humidity (%RH) Fig. 4. Proton conductivities of PEMs as a function of relative humidity at 80 °C.

80 Membrane-1

70

Water content (wt%)

Content rates of titanium oxide in PEM are shown in Table 3. PEMs-2, -3, and -4 contained additives and the content rates were lower than 3 wt%. Although content rates of titanium oxide in each PEM were similar to the input, there were some differences between the input and content. In Membrane-2, the content rate of titanium oxide was larger than the input. Although titanium oxide was calculated as TiO2 in this study, hydrated titanium oxide can exist in PEM as Ti(OH)4 or TiO2 nH2O, which have larger formula weights than TiO2. In Membrane-3, the content rate of titanium oxide was smaller than the input. It was considered that hydrated titanium oxide can be dissolved in 1 mol/L H2SO4 during the film–forming process. An XRD pattern of hydrated titanium oxide synthesized in this study is shown in Fig. 3. No clear diffraction peak was observed, indicating that hydrated titanium oxide was amorphous. Although rutile can be stable in an acid solution, hydrated titanium oxide cannot be stable according to the Pourbaix diagram [24]. It was considered that hydrated titanium oxide in PEMs was amorphous and was partly dissolved in 1 mol/L H2SO4.

Membrane-2

60

Membrane-3

50

Membrane-4

40 30 20 10

Membrane

Type of polymer

Content rate of added TET calculated as TiO2 (wt%)

Content rate of titanium oxide in membrane (wt%)

Membrane-1 Membrane-2 Membrane-3 Membrane-4

Multi-block Multi-block Multi-block Multi-block

0 1.3 2.6 –

– 1.4 2.1 1

0 20

40

60

80

100

Relative humidity (%RH) Fig. 5. Water content rates of PEMs as a function of relative humidity at 80 °C.

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75

Fig. 6. Dark field STEM images of Cs+ exchanged Membrane-1 in parts of (a) clear micro-phase-separated structure, and (b) unclear micro-phase-separated structure.

3.2. Effect of adding hydrated titanium oxide on proton conductivity The relationship between relative humidity and proton conductivity is shown in Fig. 4. Regardless of humidity, PEMs containing hydrated titanium oxide (Membranes-2, -3, and -4) showed higher proton conductivity than Membrane-1 which did not contain the additives. This result showed that adding hydrated titanium oxide improved proton conductivity. There were no big differences among proton conductivities of PEMs containing hydrated titanium oxide. Thus effects of the content rate and the adding process of hydrated titanium oxide into proton conductivity were not confirmed. The relationship between relative humidity and water content rate is shown in Fig. 5. All the PEMs showed almost the same water content rate. This result showed that water content rates of PEMs were not affected by adding hydrated titanium oxide. In this study, the content rates of

titanium oxide in PEMs were lower than 3 wt%, meaning that adding hydrated titanium oxide was not considered to affect the water content rate of PEMs. Therefore, adding hydrated titanium oxide improved proton conductivities of PEMs without increasing the water content rate. Many reports have recently shown that the micro-structures of PEM affect the proton conductivity [12,13,15,16]. Dark field STEM images of Cs+ exchanged Membrane-1 are shown in Fig. 6. Bright and dark phases correspond to hydrophilic and hydrophobic domains, respectively. In Membrane-1, which did not contain hydrated titanium oxide, a clear micro-phase-separated structure was observed in some places (Fig. 6(a)). By contrast, other places that did not have clear a microphase-separated structure were also observed (Fig. 6(b)). Dark field STEM images of Cs+ exchanged membranes containing hydrated titanium oxide are shown in Fig. 7. There was no big difference among microstructures of these PEMs and micro-phase-separated structures were

Fig. 7. Dark field STEM images of Cs+ exchanged membranes containing hydrated titanium oxide. (a) Membrane-2, (b) Membrane-3, and (c) Membrane-4.

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Fig. 8. Schematic illustration of the membrane structure composed of a multi-block copolymer with hydrated titanium oxide.

observed in all PEMs. There are some reports showing that the proton conductivity of a PEM with a micro-phase-separated structure is higher than that of a PEM without the micro-phase-separated structure. [13, 16]. Hydrophilic domains were considered to function as a proton path. Micro-phase-separated structures were observed in PEMs containing hydrated titanium oxide. Thus, PEMs with hydrated titanium oxide showed higher proton conductivity than the PEM without it. In accordance with the method of D. W. Shin et al. [17], the micro-phaseseparated structures were considered to be formed by the heating process. After the heating process, micro-phase-separated structures were clearly observed in the PEM that did not show a clear micro-phaseseparated structure before the process. Therefore, membrane-1 was considered to be still in the state of micro-phase-separated structure formation, and the forming had not finished. The schematic illustration of the forming process of PEM containing hydrated titanium oxide is shown in Fig. 8. T. Yonemoto et al. have reported that TET reacts with H2O and hydrated titanium oxide is formed [23]. At first, TET solution was considered to react with H2O that was adsorbed at a hydrophilic segment in a multi-block copolymer. Hydrated titanium oxide was deposited at a hydrophilic segment. Hydrated titanium oxide was considered to enhance the hydrophilic sengments aggregation, and clear phase separated structures were observed in PEMs containing hydrated titanium oxide. 4. Conclusion The effect of adding hydrated titanium oxide into polymer electrolyte membranes composed of multi-block copolymers was investigated in order to improve proton conductivities in low humidity conditions. Regardless of humidity, PEMs that contained hydrated titanium oxide showed higher proton conductivity than PEMs that did not. This result showed that adding hydrated titanium oxide improved proton conductivity. By contrast, all the PEMs showed almost the same water content rate. In this study, the content rates of titanium oxide in PEMs were lower than 3 wt%, and thus, adding hydrated titanium oxide was not considered to affect the water content rate of PEMs. These results indicated that the addition of hydrated titanium oxide into PEMs increased proton conductivity but not the water content rate. Micro-phase-

separated structures were clearly observed in PEMs containing hydrated titanium oxide. Hydrated titanium oxide was considered to enhance the hydrophilic sengments aggregation during the film-forming process. Probably, the well-defined hydrophilic domains functioned as proton paths and enhanced the proton conductivity of the PEM.

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