The 1st International Seminar on Fundamental and Application ISFAChE 2010 of Chemical Engineering November 3-4, 2010, Bali-Indonesia

Conductivity Analysis of Modified Polyelectrolyte ABS using Sulfonated Poly-ether Ether Keton

Eniya Listiani Dewi 1a*, Liana Christiani 2a, and Sri Handayani 3b

a Center for Materials Technology, Agency for the Assessment and Application Technology

BPPT II, 22F, MH. Thamrin 8, Jakarta. bIndonesia Institute of Technology, Serpong, Tangerang

*Corresponding Author’s E-mail: eniyalist@webmail.bppt.go.id

Keywords: Proton Exchange Membrane; Ionic Conductivity ; PEM Fuel Cell; Sulfonated Polymer.

Abstract

This paper described the development of hydrocarbon polymeric membrane that used as polyelectrolyte for proton exchange membrane fuel cell (PEMFC). Nafion as the champion membrane for PEMFC have many disadvantages such as highly methanol crossover, degradation at above 100 °C, and also the high prices. Herein we are focusing to develop an anionic polyelectrolyte using sulfonation process at styrene-bond on polymeric acrylonitrile butadiene styrene (ABS) as previously reported. The sulfonated ABS membranes have shown an ionic conductivity of 10-4 S/cm in wet condition (100%RH). Due to highly hydrophilicity of sPEEK which is known as a good candidate to substitute nafion, the ABS was then added into PEEK to lowering the hydrophilicity to meet the requirement of MEA. This modified membrane of ABS and PEEK were analyzed using SEM, XRD and FTIR. The properties of ionic conductivity, swelling, methanol uptake of membrane as polyelectrolite were determined and calculated.

1. Introduction

Polymer electrolyte membrane (PEM) fuel cells with proton conductivity attract considerable attention as they have potential applications in transportation, stationary power, portable power and military use. There remain several challenges, both technical and economical, that need to be overcome prior to fuel cell commercialization [1-3]. PEMs in general have a functional group (usually sulfonic acid) attached to the polymer backbone. This group facilitates proton conduction. Upon hydration, the PEMs tend to show an increase in the proton conductivity. Some of the most promising PEMs are Nafion® (Dupont), Flemion® (Asahi Glass Company), Aciplex® (Asahi chemical Industry), Neosepta-F® (Tokuyama) and GoreSelect® (W.L. Gore and Associates). Although some of these membranes were originally developed for chlor-alkali electrolysis, they demonstrate good proton conductivities when used as electrolytes in a PEM fuel cell. The Nafion® based membrane shows complete domination in mechanical, physical and electrochemical properties. But there are some problems such as dehydration in high temperature and a high methanol permeability that caused the depolarization losses and conversion losses could affecting the performance of fuel cells [4-5]. Hence development of alternate membranes with desirable properties has been widely explored. Hydrocarbon membranes look promising in this scenario though durability remains a concern. Some of the hydrocarbon polymers studied is sulfonated polyether-ether ketone (sPEEK) [6] and acrylonitrile butadiene styrene (ABS) [7].

ABS membrane has been developed to replace Nafion® as electrolyte of PEM fuel cell. ABS copolymer was made from styrene and acrylonitrile polymerization with addition of polybutadiene which is long chain of polybutadiene. ABS has shiny and waterproof surface was given by styrene group. Butadiene is an elastic material which gives ABS constrain even at low temperature. In the other hand, acrylonitrile gives thermal and chemical resistant. Thus make ABS stronger than pure

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The 1st International Seminar on Fundamental and Application ISFAChE 2010 of Chemical Engineering November 3-4, 2010, Bali-Indonesia polystyrene [8-10]. Since ABS membrane is hydrophobic, it needs to be made hydrophilic so that it can be use as proton transfer media by sulfonated using sulfonic acid [11-12] or modifed with silica and zeolite [13].

The polyether-ether ketone (PEEK) was selected as based polymer membrane due to good thermal stability and mechanical strength. Poly (ether ether ketone) (PEEK) is an aromatic, semicrystalline polymer which shows mild solubility in organic solvents due to its crystallinity. In order to produce hydrophilicity, polymer of polyether-ether ketone must be sulfonated using concentrated sulfonic acid to form sulfonated polyether-ether ketone (sPEEK) [14-15]. A. Carbone, et al [16] showed the characteristics of S-PEEK polymers are strongly influenced from the sulphonation degree. By sulfonating PEEK, crystallinity is decreased and solubility is increased. SPEEK has been reported in the literature as a low cost alternate membrane for both PEMFC and DMFC applications [17-18].

In the other hand, SPEEK with higher sulfonation degree and proton conductive also have higher water uptake and methanol permeability. There have been several attempts to overcome the excessive swelling while maintaining high proton conductivity, for example, by synthesizing SPEEK with various hydrophobic block: hydrophilic block ratios [19], and by blending the SPEEK polymer with non-conductive engineering thermoplastics (e.g., SPEEK/PEI, SPEEK/PES, SPEEK/PBI, SPEEK/ABS) [20-21]. S. Handayani and E. L. Dewi [22] demonstrated blending SPEEK and ABS to reduce the methanol permeability in direct methanol fuel cell. SPEEK is expected to have not only higher proton conductivity but also has a low water uptake, so it has a good mechanical strength.

This paper was investigating proton conduction and water uptake properties of the blending SPEEK and ABS membrane to evaluate the potential for PEMFC application.

2. Experimental 2.1. Preparation of sulphonated PEEK 1

PEEK powder was dried in the oven at 60 °C for 3 h prior to sulfonation. An amount of 5 g of PEEK (Victrex, 450P) was dispersed in 100 mL of 95-97% sulphuric acid (pro analysis, Merck) maintaining under stirring for 3 h at two different reaction temperatures: 50°C. After this reaction time the sulphonated polymers were precipitated in cold water, washed thoroughly to remove excess acid until the pH of the wash solution was ±6 and completely dried. 2.2. Membranes preparation

For membrane 1 preparation 3 g of SPEEK was dissolve in 24 mL N-methyl prylolidone (NMP) (pro analysis, Merck). Membrane 2 and membrane 3 was made by dissolve 3 g the mixing of SPEEK and ABS (P.T. ARBE Styrindo & ABSii) (ratio = 95:5, 90:10, respectively) in 24 mL N-methyl prylolidone (NMP) for 7 hours under stirring, then they were filtered and placed in ultrasonicator to remove air bubble that may still contained in the solution. After membrane casting, they were left to dry on a glass plate at 50 °C. The thickness of the membranes are approximately 40-100 μm. 2.3. Characterization Method 2.3.1. FTIR

Fourier transform infrared spectroscopy (FT-IR) were measured in absorbance mode by using an FT-IR spectrometer in the range of wave numbers 600 – 4000 cm-1 to compare position of IR bands and to check the presence of functional groups and their interaction in composite membranes. Prior to FT-IR measurement, the samples were dried at 50°C for 5 h.

2.3.2. XRD

The crystal structure of particles and membranes were investigated using x-Ray Diffraction (XRD) Shimadzu XD-610.

2.3.3. SEM

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The 1st International Seminar on Fundamental and Application ISFAChE 2010 of Chemical Engineering November 3-4, 2010, Bali-Indonesia

The membrane morphology was investigated by field emission scanning electron microscopy in Philips XL-30 equipment. Samples were fractured in liquid nitrogen and sputtered with Au/Pd. 2.3.4. Proton conductivity The proton conductivity of samples in the lateral direction was measured with a measurement cell and Hioki Impedance Meter (LCR 3532-80). Two stainless steel electrodes connected to the FRA were pressed against the membrane to be tested. The measurement temperature was room temperature. The conductivity, σ, was calculated from the impedance data, using the relation σ = L / (Z * A), where L and A are the distance between the electrodes and the cross-section area of the membrane, respectively, and Z was derived from the low intersection of the high-frequency semi circle on a complex impedance plane with the Re (z) axis.

3. Results and Discussion 3.1. Sulfonation Process

SPEEK polymers 1 with 69 of DS values were obtained by adjusting the reaction temperature. The dissolvability and the swelling degree of SPEEK polymers were found to depend strongly on their DS. When DS was >38%, the SPEEK polymers were soluble at room temperature in strong polar aprotic solvents, including dimethylformadide (DMF), N,N-dimethylacetamide (DMAC), dimethyl sulfoxide (DMSO), and N-methyl-2-pyrrolidinone (NMP) [23]. Therefore, the SPEEK polymer with a relative high DS (DS = 69.5%) was used as the basic material of the membrane. It was prepared by sulfonating PEEK at 50°C for 3 h.

The effect of ABS on SPEEK is shown in table 1. Sulfonation degree decreases with increasing ABS composition. The more ABS added in SPEEK, the smaller the sulfonic group content in blending membrane. Which is mean ABS decrease the hydrophilicity.

Table 1: Sulfonation Degree at vairous wt.%ABS of ABS in sPEEK

Membrane Wt. % sPEEK Wt. % ABS Sulfonation

degree (%) 1 100 0 69.5 2 95 5 64.3 3 90 10 50.3

3.2. FTIR spectra

Fourier Transform Infra Red (FTIR) was used to detect the presence of sulfonic group. An FTIR spectrum of SPEEK membrane 1 is presented in figure 1. The spectrum of aromatic C = C vibration show strong peak at 1598.99 cm-1 and carbonyl C = O at 1647 cm-1. Where C = C bond and C = O are aromatic group which are contained in SPEEK. The intensity of O=S=O stretching (sulfonic group) show strong bands at 1030-1010 cm-1. The O-H stretching of sulfonic group results in strong peak at 3454 – 3415 cm-1. It proves that sulfonation was done successfully. The intensity of O-H stretching peak at 3450-3430 cm-1 is relatively increased because of protonation by sulfonic molecules. Hydrogen bonds seem to play an important role in the proton conductivity.

FTIR spectra of composite SPEEK/ABS membranes 2 and 3 are presented in Figure 2 and Figure 3. The presence of ABS is showed by the spectrums of C-H aliphatic (CH and CH2) bands at 3000–2850 cm-1, the intensity of C=N stretching at 3300-2900 cm-1. Characteristic for styrene aromatics vibration, which are showed by C-H aromatic stretching and C=C aromatic stretching aromatics results in strong peak at 1700-1600 cm-1. Based on figure 3, it shows that the absence of O-H stretching from sulfonic group affected in lower proton conductivity.

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The 1st International Seminar on Fundamental and Application ISFAChE 2010 of Chemical Engineering November 3-4, 2010, Bali-Indonesia

O=S=O

O – H

C = C

C = O

Figure 1: FT-IR spectra of SPEEK 1

C = CO=S=O

C ≡ NO – H

C – H

Figure 2: FT-IR spectra of composite SPEEK/5% wt.ABS 2

C – H

C ≡ N

O=S=O

Figure 3: FT-IR spectra of composite SPEEK/10% ABS 3

3.3. XRD

The nanostructure of membranes and its blends was investigated using X-ray diffraction scattering. The X-ray diffraction analysis SPEEK membrane 1 and also for SPEEK/ABS composite membranes (2-3) showed. All the blend and composite membranes show peaks at similar diffraction angles 2θ as shown in figure 4. By looking at the results, the SPEEK membrane 1 showed amourphus structure. ABS added in SPEEK make the membranes 2-3 have crystaline structure which decrease the water uptake.

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The 1st International Seminar on Fundamental and Application ISFAChE 2010 of Chemical Engineering November 3-4, 2010, Bali-Indonesia

(a)

0

20

40

60

80

100

120

140

160

0 5 10 15 20 25 30 35 40 45 50 55

2 theta (o)

inte

nsita

s

(b)

(c) Figure 4: X-ray diffraction profile: (a) Blank SPEEK 1, (b) composite SPEEK/5% wt. ABS 2,

(c) composite SPEEK/10% wt. ABS 3. 3.4. SEM

The basic homogeneous distribution of ABS within the SPEEK matrix can be observed from the SEM images of the SPEEK/ABS composite membranes 2-3 at magnification of up to 10000× (Figure 4) with the ABS content reaches 5%. Based on Figure 5 it was found that the addition of ABS bringing on pores on composite SPEEK/ABS. The porousity of blending polymer is correlated to the increasing of swelling and amorphous structure by XRD.

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The 1st International Seminar on Fundamental and Application ISFAChE 2010 of Chemical Engineering November 3-4, 2010, Bali-Indonesia

(a) (b)

Figure 5: SEM cross-sectional images of SPEEK membranes (a) Blank SPEEK (b)SPEEK + 5% ABS

3.5. Proton Conductivity

Table 2: Proton Conductivity of membranes

Membrane Wt. % sPEEK Wt.% ABS Sulfonation degree

(%) Proton Conductivity

(S/cm) 1 100 0 69.5 0,036 2 95 5 64.3 0,016 3 90 10 50.3 0,0049

The proton conductivity values of the composite membranes 2-3 containing different weight percentages of ABS are given in table 2. The proton conductivity of composite membranes decreases slightly as mass content of ABS increases. SPEEK membrane 1 conductivity reached 0.036 S. cm-1, with the adding of 5% and 10% wt. The 2-3 membranes the proton conductivity decrease to 0,016 S. cm-1 and 0,0049 S. cm-1 respectively. This is probably due to the hidrophobicity polymer structure of ABS that slowing up the transport proton so it will decrease the proton conductivity as well.

Figure 6: Conductivity of SPEEK and composite SPEEK-ABS

3.6. Swelling Behavior

The membranes swelled to different extents in water, 5% wt ethanol, and 3 mol. L-1 methanol as the ABS content decreased. The swelling degree of the SPEEK membrane 1, however, decreased

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The 1st International Seminar on Fundamental and Application ISFAChE 2010 of Chemical Engineering November 3-4, 2010, Bali-Indonesia markedly with the addition of ABS. Based on tabel 3, the blank SPEEK membrane 1 swelled 23%, whereas the membranes 2-3 swelled only 19% and 5% respectively at the room temperature.

Table 3: Swelling behavior of SPEEK-ABS

Membrane Wt. % sPEEK

Wt.% ABS DS (%) Water

uptake (%) Ethanol

uptake(%) Methanol

uptake (%) 1

100 0 69.5 23 27 30 2

95 5 64.3 20 19 29

3 90 10 50.3 11 5 13

This helps to keep good dimensional stability of the membranes. Generally, there are two reasons for the decrease in swelling of the membrane 1–3. First, the hydrophilicity of membrane 2 and 3 were lower than membrane 1 (DS = 69.5 %) which results from the lower DS (DS = 64.3% and 50.3%) of the ABS component. The increasing sulfonate group cause higher hydrophilicity. Second, the hydrogen bonds between sulfonic acid groups of SPEEK and ABS molecules will hinder the stretch of SPEEK molecular chains to some extent.

Figure 7: SPEEK and Composite SPEEK/ABS Membrane Swelling Behaviour

4. Conclusion Membrane 1–3 of sulfonated polyether-ether ketone (sPEEK) and ABS were synthesized and

characterized. Due to increasing of sulfonation degree, the ionic conductivity and swelling behaviour of membrane 1 was also increased in which determining at room temperature. The addition of ABS in SPEEK membrane that produce membrane 2 and 3 result in decreasing water, ethanol, dan methanol uptake that was correlated to lowering the hydrophilicity of membrane 1. Membrane 2 and 3 with higher levels of dimensional stability were prepared. The results of this study indicate a strong potential of these composite membranes as PEFC’s polyelectrolyte. Acknowledgement(s) This style file was provided by ISFACHE2010 (Asian Conference on Mixing 2010) as the 50 years Celebration of Institut Teknologi Sepuluh Nopember, Surabaya – Indonesia. References 1. C. Song (2002) Catalysis Today 77: 17–49. 2. H. Li, H. Wang, J. Zhang, D. Ghosh, Y. Yogendran, (2009) D. Bessarabov, Fuel Cell Buletin:

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