Synthesis and characterization of sulfonated poly(vinylidene fluoride-co-hexafluoropropylene)/sAl₂O₃ composites as a novel-alternative electrolyte membranes of Nafion

Abstract

Fuel cell membrane of Nafion is commonly used as the electrolyte material of fuel cell which has good mechanical properties, but the hydrophobicity reduce its proton conductivity. Thus, the other polymer is used for the electrolyte membrane. A polymer modification of sulfonated poly (vinylidene fluoride-co-hexafluoropropylene) (PVDF-co-HFP) by attaching sulfonate group and oxide compounds such as sAl₂O₃ would enhance the proton conductivity. The research is aimed to modify PVDF-co-HFP via the addition of sAl₂O₃ or pristine Al₂O₃ and studying the effect of sulfonated Al₂O₃ on the conductivity. The research includes Al₂O₃ particles synthesis, the sulfonation of Al₂O₃, the membrane preparation, the sulfonation of membrane, and the characterisation of membrane using FTIR, SEM-EDX, and Four-point Probe Electrical Device Analysis. SEM-EDX analysis explained that the 6% addition of Al₂O₃ showed denser cross-sectioned membrane rather than the 9% addition of Al₂O₃. The highest conductivity achieved can be revealed at the 6% sAl₂O₃ addition on PVDF-co-HFP membrane as 2.27 × 10⁻³ S cm⁻¹ which is higher than a previous study.

Electrochemistry, Inorganic chemistry, Materials chemistry, Proton conductivity, PVDF-co-HFP, sAl₂O₃

1. Introduction

Energy-source device such as fuel cell can be engineered as an alternative to alleviate greenhouse effect from carbon emission in the entire world [1]. Polymer electrolytes have been extensively researched due to the possibility of their commercial use in a variety of electrochemical devices such as electrochemical cells, rechargeable batteries, and sensors [2]. Proton exchange membrane fuel cell (PEMFC) is one of the energy sources which applies the electrochemical principle to generate high-efficient electricity (40–60%), operate at low temperature (50–100 °C), be clean, and eco-friendly [3].

PEMFC neccesitates an electrolyte membrane, as in the part of it, to exchange proton (H⁺) in the terms of electrons production over the circuit. Nafion, as the conventional membrane, is the co-polymer perfluorosufonate which has considerable proton conductivity, and good mechanical and thermal stability [4]. However, the conductivity of Nafion can diminish in low humidity at high temperature (80 °C). Dehydration of the membrane can cause shrinking-up, lowering the conductivity, thus reducing interface contact between the electrodes and the membrane, beside creating crossover of gaseous reactants to allow them escape to the bulk of membrane [5]. Thus, Nafion membrane is non-favourable and an alternative to this issue must be manifested to substitute Nafion. Another membrane requires good thermal and mechanical properties to lessen the dehydration issue.

Polyvinylidene fluoride (PVDF) is a semi-crystalline polymer with excellent mechanical and thermal properties [6]. The semi-crystalline properties, however, can inhibit proton diffusion. PVDF can be modified by combining with another polymer with low crystallinity such as HFP (hexafluoropropylene). PVDF-co-HFP has lower crystallinity than PVDF owing to an amorphicity of HFP, this amorphous character will allow collected water to flow freely within the structure [7]. On the other hand, the hydrophobicity of PVDF-co-HFP, due to high surface tension between water and membrane surface [6], will also clog proton diffusion through membrane and resulting in decreasing of proton conductivity [8].

Metal oxide compounds such as silicate, titanate, and zirconate addition to the membrane can enhance its performance as it shown in water-treatment plant [9]. The hydrophilicity of these oxides can help membrane sustain the solvation of water molecule in the membrane under the operating process as it increase mechanical strength of the membrane [10]. PVDF-co-HFP composite membranes with ZrO₂, TiO₂, and SiO₂ signified the highest proton conductivity of 9.3 × 10⁻⁵ S cm⁻¹ [11]; maximum of 1×10⁻³ S cm⁻¹ [[12], [13]]; and 1.16 × 10⁻³ S cm⁻¹, respectively [14]. An addition of acidic species like sulfonate compounds can also improve the hydrophilicity and conductivity. Sulfonated polystyrene ethylene butylene polystyrene (SPSEBS) membranes with TiO₂ and sTiO₂ oxide had reached the conductivity of 1.08 × 10⁻² S cm⁻¹ and 3.57 × 10⁻² S cm⁻¹ which much higher that its pure SPSEBS, 1.52 × 10⁻² S cm⁻¹ [15]. A sulfonated TiO₂ in PVDF-co-HFP membrane showed high conductivity of 3.6×10⁻³ S cm⁻¹ [16]. The other study suggested that an addition of non-sulfonated Al₂O₃ into PVDF-co-HFP membrane can produce the highest conductivity of 0.98 × 10⁻³ S cm⁻¹ [11]. Based on those studies, we were trying to directly add a sulfonate group into Al₂O₃ and compare it with the PVDF-co-HPF with non-sulfonated Al₂O₃. In this study, we experimented an addition of metal oxide compound (Al₂O₃ with the concentration of 3, 6, and 9% (w/v)) on PVDF-co-HFP membranes in order to study the effect of sulfonation of Al₂O₃ on the PVDF-co-HFP membrane comparing to the PVDF-co-HFP with pure Al₂O₃. Al₂O₃ is expected to enhance the hydrophilicity of the membrane by water absorption and increase its proton conductivity [17].

2. Experimental method

The materials needed for the experiment are aquabidest, alumina (Al₂O₃, J.T. Baker), oxalic acid (H₂C₂O_4, Merck), sulfonic acid (H₂SO₄, 97%, Smart Lab Indonesia), dimethyl acetamide (DMAc/C₄H₉NO, 99%, Merck), methanol (CH₃OH, Merck), sodium hydroxide (NaOH, Merck), sodium chloride (NaCl, Merck), Polyvinylidene fluoride –co-hexafluoropropylene polymer (PVDF-co-HFP). The laboratory equipment and instruments applied in this study include ball milling, FT-IR (PerkinElmer Spectrum 100), magnetic stirrer (Heldolph, MR 3001 K), analytical balance (Mettler Toledo, AB 104-S), Particle Size Analyser (PSA) (Beckman Coulter, LS 13 320), four line probe, SEM-EDX (Hitachi SU 3500), dan ultrasonic bath (GB 0102).

2.1. The preparation of Al₂O₃ particle

Al₂O₃ powder was obtained from Sigma. The powder was reduced into small particles using planetary ball mill using ZrO₂ balls in a Polytetrafluoroethylene (PTFE) container. The mass ratio of balls and Al₂O₃ during the milling process is 10:1 for 32 h with 300 rpms. The reduced-size Al₂O₃ was then analysed using PSA/particle size analyser.

2.2. The sulfonating of Al₂O₃

An amount of 20 mL methanol as the solvent was added to 1.0000 g of Al₂O₃, subsequently 0.5M sulfuric acid was added as the source of –SO₃H functional groups. The mixture was stirred for 12 h until the white-opaque solution formed. It was then heated at 100 °C for 24 h to form white precipitate.

2.3. The synthesis of PVDF-co-HFP/Al₂O₃ and PVDF-co-HFP/sAl₂O₃ membranes

An amount of 1.8 g PVDF-co-HFP was diluted in 10.0 mL DMAc to constitute a solution of PVDF-co-HFP 18% (w/v), DMAc can help shrink up the membrane porosity. Al₂O₃/sAl₂O_3, thereupon, was put into PVDF-co-HFP solution with variation of 3, 6, and 9% concentration and stirred for 24 h until the Al₂O₃/sAl₂O₃ dispersed in the PVDF-co-HFP membrane. To reduce the suspension, increase the dispersion and homogeneity, the mixture was sonicated using 40 kHz frequency for 4 h. Thereafter, the polymer mixture was cast onto glass mold to form a layered-sheet membrane. All these membrane were analysed using FTIR and SEM-EDX with the magnification of 1000×.

2.4. The sulfonating of PVDF-co-HFP/Al₂O₃ and PVDF-co-HFP/sAl₂O₃ membranes

PVDF-co-HFP, PVDF-co-HFP/Al₂O₃, and PVDF-co-HFP/sAl₂O₃ membranes were soaked, respectively, in H₂SO₄ (97%) solvent at 60 °C for 6 h. The membranes were then washed out using aquabidest to a neutral pH and dried at 60 °C for at least an hour, please be awared of membranes humidity.

2.5. The determination of sulfonation degree (SD)

The Sulfonation degree was determined by using a titration method. All membranes were soaked in a 0.50 M NaCl solution for 24 h. The 24-hours-soaked solution were then titrated using 0.01 M NaOH. The SD can be calculated using Eq. (1):

$$SD=\frac{\frac{MwPVDF-co-HFP}{1000}x{C}_{NaOH}x{V}_{NaOH}x100}{W_{dry}-(\frac{Mwsulfon}{1000}x{C}_{NaOH}x{V}_{NaOH})}$$ (1)

Where:

• SD = sulfonation degree (%)

• Mw of PVDF-co-HFP = 214 g/mol

• V_NaOH = volume of NaOH (mL)

• Mw of sulfonate group = 81 g/mol

• C_NaOH = concentration of NaOH (M)

• W_dry = Dried weight of membrane (g)

2.6. Proton conductivity measurement

All 4-cm²-cut membranes with measured thickness were put onto the four line probe plate with the probe width of 0.2 cm. The electrical current was applied in the magnitude of 1–5 × 10⁻⁷ A to obtain the output voltage. The electrical conductivity of membranes were calculated by using Eq. (2).

$$\sigma =\frac{(\text{i x d})}{\text{V x l x T}}$$ (2)

Where:

• σ= proton conductivity (S.cm⁻¹)

• T = membrane thicness (cm)

• i= current (A)

• d = probe width (cm)

• V= output voltage (V)

• l = membrane length (cm)

3. Results and discussion

3.1. Analysis of Al₂O₃ particle size

The PSA analysis of commercial Al₂O₃ signified an average particles size of 112,5 μm as shown in Figure 1. A milling using planetary ball mill is required to reduce the size to enlarge the particle surface area, thus the water-particle contact can be considerably amplified to maintain the membranes being hydrated during the operating process [10] (see Figure 2). The particle size of Al₂O₃ can decrease up to 100 times smaller as the milling process was deployed.

Figure 1.

Particle size distribution of commercial Al₂O₃.

Figure 2.

Particle size distribution of 32-hours-ball-milled Al₂O₃.

The ball-milled Al₂O₃ was re-analysed using PSA and showed an average particles size of 1.26μm with the mode of 1.92μm, and the smallest size of 0.48μm. Agglomeration of the particles during milling process can induce the size inhomogeneity of Al₂O₃ [18]. Diameter size of ball-milled particles will conform equally to <80% of the particles volume [19]. We may perceive a disparate distribution in between commercial Al₂O₃ and the ball-milled Al₂O₃. The commercial Al₂O₃ showed a single peak in range of 2.21–300μm which indicates a uniform distribution of the particles, while the ball-milled one depicted a double peak in the range of 0.21–2.92μm which suggested that the ball-milled process can reduce the particle size, yet proposed a polarization in particle distribution at which the size was split into 2 different size distribution of 0.21–1.05μm and 1.05–2.92μm.

3.2. FTIR analysis of Al₂O₃ and sulfonated Al₂O₃

FTIR spectrum of Al₂O₃ and sAl₂O₃ can be seen in Figure 3. The spectrum pointed peaks of 1393, 1027, and 974 cm⁻¹ which indicate the sulfonate group bond. The band of 1393 cm⁻¹ which has medium-shoulder intensity and 1027 cm⁻¹, strong-sharp intensity at which they signified the peaks of both asymmetry and symmetry strain of the S = O bond, and at 974 cm⁻¹ with strong-sharp intensity is the peak of the SO bond strain. The band of 1635 cm⁻¹ which has moderate-sharp intensity and 3376 cm⁻¹ with strong-width intensity respectively is the bending vibration distribution and strain of the –OH group found on the surface of Al₂O₃. Al–O bond strain of Al₂O₃ has been also seen at 511 cm⁻¹ with weak-shoulder intensity. These bands indicated that the sulfonation attachment on Al₂O₃ has successfully occurred.

Figure 3.

FTIR spectrum of Al₂O₃ (blue) and sulfonated Al₂O₃ (pink).

3.3. FTIR analysis of PVDF-co-HFP and sulfonated PVDF-co-HFP membranes

FTIR analysis results of PVDF-co-HFP/Al₂O₃ membrane and sPVDF-co-HFP/Al₂O₃ membrane are shown in Figure 4. The band of 1395 cm⁻¹ which has strong-sharp intensity and 1256 cm⁻¹ with strong shoulder-intensity are asymmetric and symmetry strains of the S = O bond. While the band of 890 cm⁻¹ with low-shoulder intensity is the peak of the S–O bond strain. In addition, there are asymmetrical stretch peaks and C–F bond symmetries at 1216 cm⁻¹ with shoulder-strong intensity and 946 cm⁻¹ wide-strong bands, respectively. The band of 2906 cm⁻¹ with shoulder-strong intensity is the peak of the C–H bond streching, while the band at 1454 cm⁻¹ weak-shoulder is the bending vibration of the CH₂ bond. A peak of 3422 cm⁻¹ with shoulder-strong intensity constitutes a strain of the –OH group of water contained in the membrane. The peak at 635 cm⁻¹ with medium-wide intensity is the Al–O bond streching. This spectrum shows that the sulfonate group has been affixed to the PVDF-co-HFP/Al₂O₃ membrane.

Figure 4.

FTIR spectrum of PVDF-co-HFP/Al₂O₃ (blue) and sulfonated PVDF-co-HFP/Al₂O₃ (orange).

3.4. Scanning electron microscope-energy dispersive X-Ray (SEM-EDX)

Figure 5 depicts the surface of PVDF-co-HFP membranes with the various addition of Al₂O₃ and sAl₂O₃ at which the subtle conformity of the suface was displayed. A single PVDF-co-HFP membrane has more porous rather than when Al₂O₃ was added into PVDF-co-HFP membrane [20]. The discrepancies occurred when the less amount of porous membranes emerge at which the Al₂O₃ and sAl₂O₃ were added to the membrane in sPVDF-co-HFP/9%sAl₂O₃ and sPVDF-co-HFP/9%Al₂O₃ samples, respectively as shown in Figures 5a and 5c. The agglomeration of Al₂O₃ during the sonication process accounts for the formation of porous which led to inequality of particle dispersion over the suface membrane.

Figure 5.

(a) surface of sPVDF-co-HFP/6%sAl₂O₃, (b) cross-section of sPVDF-co-HFP/6%sAl₂O₃, (c) surface of sPVDF-co-HFP/9%Al₂O₃, and (d) cross-section of sPVDF-co-HFP/9%Al₂O₃.

Figures 5c and 5e displayed the cross-section of sPVDF-co-HFP/6%Al₂O₃ and sPVDF-co-HFP/9%Al₂O₃ membranes, respectively. Figure 5c of sPVDF-co-HFP/6%Al₂O₃ showed a denser structure than sPVDF-co-HFP/9%Al₂O₃. At high concentration of Al₂O₃, the agglomerates take more control to the porosity of the membranes resulted in changes in membrane morphologies. Moreover, the sulfonated Al₂O₃ tend to densify the membrane comparing to non-sulfonated Al₂O₃.

A 4-cm²-cut sPVDF-co-HFP/6%sAl₂O₃ membrane was analysed using EDX to examine the elemental compositions in the membrane. We considered using this analysis to confirm if the sulfonated Al₂O₃ has been successfully incorporated throughout the membrane. Figure 6 presents EDX qualitative analysis of sPVDF-co-HFP/6%sAl₂O₃ membrane which indicated the contents of fluor, carbon, aluminium, oxygen, and sulfur. The carbon and fluor are the most abundant elements in the membrane as 49.83% and 41.23% that constitute the main elements of PVDF-co-HFP. The oxygen made up 5.22%, while aluminium and sulfur account for 1.95% and 1.77% of the membrane, respectively which signified the composition of the membrane constituents from Al₂O₃ and –SO₃H groups. The EDX spectrum confirmed that the Al₂O₃ and sulfonated group have been successfully incorporated into the membrane.

Figure 6.

EDX spectrum of sulfonated PVDF-co-HFP/6%sAl₂O₃ membrane.

3.5. Sulfonation degree (SD)

Figure 7 reveals the result of sulfonation degree determination, it can be seen that the sPVDF-co-HFP membrane has the lowest sulfonation degree compared to the sPVDF-co-HFP membrane with the addition of Al₂O₃ or sAl₂O₃, then it can be concluded that increasing oxide compounds will increase the degree of sulfonation. Hydrophilic oxide compounds such as Al₂O₃ can help in the sulfonation process, because the hydrophilic sulfonate group will more easily diffuse and bind to the membrane [21].

Figure 7.

The sulfonation degree of sPVDF-co-HFP membrane with various concentration of Al₂O₃ and sAl₂O₃.

Membrane with the addition of sAl₂O₃ has a higher degree of sulfonation compared to Al₂O₃ without disulfonation beforehand, this can be due to the sulfonate group on sAl₂O₃ contributing H + so that the value of the sulfonation degree is higher than Al₂O₃. However, the membrane with the addition of 6% sAl₂O₃ has a lower value compared to the degree of sulfonation on the 6% Al₂O₃ membrane, this can occur because the 6% sAl₂O₃ membrane has been saturated or cannot react again during sulfonation for 6 h so that no sulfonate groups are present to bound to the membrane.

Based on Figure 7, at a concentration of 9% both oxide compounds have a low degree of sulfonation when compared to a concentration of 6%. The addition of oxide compounds with concentrations exceeding their optimum conditions can affect the freedom of movement of diffused sulfonate groups in the membrane due to the presence of aggregated Al₂O₃ and sAl₂O₃ particles on the surface [22].

The SD which indicated less than 15%, can be influenced by several factors, including sulfonating agents, reaction temperatures, polymer concentrations, and reaction times [23]. Besides the concentration of oxide compounds can affect the value of the degree of sulfonation as discussed earlier.

3.6. Proton conductivity

The proton conductivity value was determined by means of a four line probe, by flowing a predetermined current and then measuring the voltage from the membrane. It can be seen in Figure 8 that PVDF-co-HFP membrane added with sAl₂O₃ (orange) and Al₂O₃ (purple) has a decreasing conductivity value with increasing concentration. This can be due to the higher content of oxide compounds in the membrane will form two phases, first the oxide compound is in the membrane and second is located on a surface rich in oxide compounds so that the diffusion of protons on the membrane will slow down [24], this is also explained from the conductivity value of the membrane 9% with Al₂O₃ which is considerably small of 0.221 × 10⁻³ S cm⁻¹.

Figure 8.

The conductivity values of PVDF-co-HFP and sulfonated PVDF-co-HFP with various concentration of Al₂O₃ and sAl₂O₃.

The proton value of PVDF-co-HFP membrane added by sAl₂O₃ is higher than that added by Al₂O₃, because the membrane with sAl₂O₃ has sulfonate groups in Al₂O₃ so that it can increase proton conductivity in the membrane even though the results have not exceeded the pure PVDF-co-HFP membrane (1.06×10⁻³ S cm⁻¹) [25]. The conductivity value of the proton membrane without sulfonation has a lower value than the sulfonated membrane because the sulfonate group plays an important role in the proton transfer process.

Figure 8 shows that the sPVDF-co-HFP membrane with the addition of Al₂O₃ (green) and sAl₂O₃ (blue) at various concentrations has an increasing conductivity value as the concentration of oxide compounds increases. However, the 3% Al₂O₃ membrane has a smaller conductivity value than the pure sPVDF-co-HFP membrane and its conductivity is adjacent to the 3% sAl₂O₃ membrane that is 1.110×10⁻³ S cm⁻¹ for PVDF-co-HFP/3% sAl₂O₃ and 1.16×10⁻³ S cm⁻¹ for sPVDF-co-HFP/3% Al₂O₃ membranes. This can occur because the value of the degree of sulfonation produced is almost the same, the value of sulfonation of the sPVDF-co-HFP/3% Al₂O₃ membrane is 6.30% while the pure sPVDF-co-HFP membrane is 6.24%. Thus, the number of sulfonate groups in the two membranes is almost comparable, and the decrease in conductivity can be caused by the presence of Al₂O₃ particles on the surface. Al₂O₃ can slow down the proton diffusion process in the membrane so that the conductivity value of pure membrane protons is higher.

In addition to the sPVDF-co-HFP/3%-Al₂O₃ membrane, the sPVDF-co-HFP/9%-sAl₂O₃ membrane has a small conductivity value when compared to the sPVDF-co-HFP/6%-sAl₂O₃ membrane. This result can be influenced because the sulfonation degree of 6% sAl₂O₃ has the highest sulfonation degree compared to other concentrations, so it can be concluded that the sPVDF-co-HFP/6%-sAl₂O₃ membrane is the optimum sAl₂O₃ concentration that can be added to the PVDF-co- membrane The HFP supported by the high degree of sulfonation and proton conductivity was 10.76% and 2.27×10⁻³ S cm⁻¹, respectively.

When comparing the value of the degree of sulfonation with the conductivity value of the proton membrane PVDF-co-HFP/9% Al₂O₃ has a value that is not comparable. The 9% Al₂O₃ membrane has a low degree of sulfonation compared to 6% Al₂O₃, while for the proton conductivity value 9% Al₂O₃ has a value higher than 6% Al₂O₃. The conductivity value is influenced not only by sulfonate groups in the membrane but also by hydrated membranes. The addition of composites can assist membranes in holding water, because hydrogen bonding occurs between composites and water molecules, in addition to that the composite functions to stabilize proton displacement so that it protects the proton exchange process in the hydronium ion with the –SO₃H group. The addition of Al₂O₃ can accelerate the process of transfer of protons because water that functions as a proton binder, can be held or guarded by Al₂O₃ so that it facilitates the process of proton transfer [26, 27].

The diffusion of protons in the membrane is very dependent on water and the number of sulfonate groups, but can also be influenced by the crystallinity and membrane pores. Decreased crystallinity in the membrane will increase conductivity, the addition of oxide compounds in the membrane can reduce crystallinity [28]. Crystalline membranes can inhibit the proton diffusion process and reduce water contact on the membrane. Porous membranes can cause gas reactant crossovers and proton diffusion in the membrane to slow down [29]. However, porous membranes can increase water contact on the membrane because water particles can be retained in the pore so that water absorption and moisture membrane increases.

The highest proton conductivity value produced is 2.27×10⁻³ S cm⁻¹ almost close to the conductivity value of Nafion NAFNR-212 which is 6.08 × 10⁻³ S cm⁻¹ [30] and PVDF-co-HFP/sTiO₂ which showed the conductivity of 4.30×10⁻³ S cm⁻¹ [31]. Although the conductivity cannot match Nafion NAFNR-212, the addition of sAl₂O₃ or Al₂O₃ concentration can increase the conductivity value of sPVDF-co-HFP membrane.

4. Conclusion

Al₂O₃ particles can impove proton conductivity. The 6% sulfonated Al₂O₃ can achieve the most optimum conductivity value of 2.270 × 10⁻³ S cm⁻¹. The optimum sulfonation degree that can be obtained is 10.94%. The particles size can be reduced more to nanoparticle size to increase the effect on proton conductivity of for PVDF-co-HFP/sAl₂O₃ composites membranes.

Declarations

Author contribution statement

Iman Rahayu: Conceived and designed the experiments.

Akmalia Risalatul Umma: Performed the experiments; Wrote the paper.

Juliandri: Conceived and designed the experiments; Analyzed and interpreted the data.

Yoga Trianzar Malik: Analyzed and interpreted the data; Wrote the paper.

Muhammad Nasir: Contributed reagents, materials, analysis tools or data.

Funding statement

This work was supported by a grant from Directorate General for Higher Education, Ministry of Research and Technology of the Republic Indonesia (Kementrian Riset dan Teknologi Republik Indonesia) for Research Grant of PDUPT No. 2787/UN6.D/LT/2018.

Competing interest statement

The authors declare no conflict of interest.

Additional information

No additional information is available for this paper.