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Application of polysulfone (PSf)– and polyether ether ketone (PEEK)–tungstophosphoric acid (TPA) composite membranes for water electrolysis
Journal of Membrane Science 322 (2008) 154–161
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Application of polysulfone (PSf)– and polyether ether ketone (PEEK)–tungstophosphoric acid (TPA) composite membranes for water electrolysis In-Young Jang a , Oh-Hwan Kweon a , Kyoung-Eon Kim a , Gab-Jin Hwang b , Sang-Bong Moon c , An-Soo Kang a,∗ a
Department of Chemical Engineering, Myongji University, San 38-2, Nam-dong, Cheoin-gu, Yongin-si 449-728, Republic of Korea Department of Hydrogen & Fuel Cell Research, Korea Institute of Energy Research, 71-2 Jang-dong, Yuseong-gu, Daejeon 305-343, Republic of Korea c Elchem Tech Co., Ltd., New T Castle 1001, Gasandong 429-1, Geumchun-gu, Seoul 153-803, Republic of Korea b
a r t i c l e
i n f o
Article history: Received 8 August 2007 Received in revised form 11 April 2008 Accepted 18 May 2008 Available online 25 May 2008 Keywords: Cation exchange membrane Non-equilibrium impregnation–reduction method Membrane electrode assembly (MEA) Polymer electrolyte membrane electrolysis (PEME)
a b s t r a c t The ion exchange membrane using polysulfone (PSf) and polyether ether ketone (PEEK) as a basic material was prepared to apply in the polymer electrolyte membrane electrolysis (PEME). The sulfonated block copolymer of PSf and poly(phenylene sulfide sulfone) (SPSf-co-PPSS) and the sulfonated PEEK (SPEEK) were blended with tungstophosphoric acid (TPA) to avoid water swelling at elevated temperatures led to decrease in mechanical strength. These prepared ion exchange membranes showed some interesting characteristics including physicochemical stabilities, mechanical and membrane properties. The prepared ion exchange membrane was utilized to prepare the membrane electrode assembly (MEA). MEA consisted of Pt/PEM/Pt was prepared by equilibrium and non-equilibrium impregnation–reduction (I–R) methods. The prepared MEA by non-equilibrium I–R method was used in the PEME unit cell. The cell voltages of the MEA using SPSf-co-PPSS/TPA and SPEEK/TPA membranes were 1.83 V and 1.90 V at 1 A/cm2 and 80 ◦ C, with platinum loadings of 1.12 and 1.01 mg/cm2 , respectively. © 2008 Elsevier B.V. All rights reserved.
1. Introduction The change toward new energy system with hydrogen as an energy carrier would be giving an immense impact in the human society [1]. Polymer electrolyte membrane electrolysis (PEME) is one of the highly efficient energy technologies for hydrogen production from water, which is abundant resource. The advantages of PEME over alkaline electrolysis include lower parasitic energy loss and higher ecological purity hydrogen output. Therefore PEME is a simple, sustainable and cost-effective technology for hydrogen generation. Recently the commercially available Nafion membrane, which is a perfluorosulfonic acid solid polymer electrolyte membrane from DuPont Co., is successfully used to polymer electrolyte [2]. However the current industry standards for perfluorosulfonic polymers have been limited by their high cost and loss of membrane characteristics such as proton conductivity at temperatures above 80 ◦ C. The aromatic polymers such as polyether ether ketone (PEEK), poly-
∗ Corresponding author. Tel.: +82 31 330 6386; fax: +82 31 33 337 1920. E-mail address: [email protected] (A.-S. Kang). 0376-7388/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2008.05.028
sulfone (PSf), polyimides, polybenzimidazoles, polyoxadiazole and polyphosphazenes expect satisfactory chemical and electrochemical properties and lower production costs [3]. In addition the blend of tungstophosphoric acid (TPA) and sulfonated copolymers using those aromatic polymers would be giving a significant influence on PEME cell performance at high temperatures. The membrane electrode assembly (MEA) fabricated using the conventional hot-pressing method are usually inclined to delaminate both the anode and cathode interfaces after long-term operation led to degrade the PEME cell performance. In this work to achieve a better interfacial contact between the electro-catalysts and the PEM, MEA (Pt/PEM/Pt) is prepared by an equilibrium and non-equilibrium I–R method. MEA prepared using these methods has a few microns thick, hydrophilic and porous [4]. In addition, MEA prepared using these methods exhibits excellent adhesion and durability, because the metal clusters are embedded in the membrane surfaces. In this paper the cation exchange membrane was prepared by blending TPA and sulfonated copolymers using polysulfone and PEEK. The prepared membrane was measured for its properties (electrochemical performances and thermo-mechanical stability). Thus MEA was prepared from the prepared cation exchange
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membranes using two I–R methods and Pt-loading content, and current–cell voltage in PEME cell was investigated.
2. Experimental 2.1. Preparation of membrane 2.1.1. SPEEK/TPA composite membrane Polyether ether ketone (PEEK, Victrex 450G, Mn = 100,000) was dried at 100 ◦ C in a vacuum oven for 12 h for a pre-treatment. An amount of 20 g PEEK is dissolved in 400 ml of sulfuric acid (95 wt.%) with stirring at room temperature for 24 h. After a certain time, the solution precipitated with a large excess of ice water to obtain the sulfonated polymer. The obtained sulfonated polymer were washed repeatedly with deionized water, and dried under vacuum at 100 ◦ C for 12 h to obtain the sulfonated PEEK (SPEEK). The obtained SPEEK was dissolved in 1-methyl-2-pyrrolidinone (NMP, Lancaster, 99%), and the various weights (0.33–60 wt.%) of powdered tungstophosphoric acid (TPA, H3 PW12 O40 ·29H2 O) was added to the solution, and then the mixture was stirred for 16 h. The mixed solution was cast onto a glass plate with a doctor’s knife, and dried in series of at room temperature for 12 h, at 60 ◦ C for 4 h and at 120 ◦ C for 12 h to obtain the cation exchange membrane. The obtained membrane which was blended with SPEEK and TPA is named as SPEEK/TPA.
2.1.2. SPSf-co-PPSS/TPA composite membrane The SPSf-co-PPSS/TPA composite membrane was prepared by two-step synthesis with polysulfone (PSf, UDEL type, Aldrich Chem. Co., Mn = 26,000). Firstly, in order to obtain the PSf-co-PPSS block copolymer (BPSf), 18.9532 g of 4,4 -dichlorodiphenylsulfone (DCDPS, Fluka Chem. Co., 95%), 4.6824 g of sodium sulfide hydrate (Na2 S·nH2 O, Janssen, 60–62%) and 6.1212 g of lithium acetate dehydrate (CH3 COOLi·2H2 O, Kanto Chem. Co., 98%) were dissolved in 120 ml of NMP, and then 26.0054 g of PSf was added thereto [5]. The mixture was stirred at 160 ◦ C for 3 h with nitrogen gas. Secondly, the obtained BPSf was dissolved in 1,1,2,2-tetrachloroethane (TCE, Samchun Chem.) and the various weights (1.4–22 wt.%) of powdered TPA was added to the solution. Then, the chlorosulfonic acid (CSA, Aldrich Chem.) was added to those solutions in the molar ratio of 3:1 against BPSf at room temperature for 1 h to obtain the sulfonated block copolymer (SBPSf/TPA). The obtained SBPSf/TPA was dissolved in NMP with stirring for 16 h. The mixed solution was cast onto a glass plate with a doctor’s knife, and dried in series of at room temperature for 12 h, at 60 ◦ C for 4 h and at 120 ◦ C for 12 h to obtain the cation exchange membrane. The obtained membrane is named as SPSf-co-PPSS/TPA.
2.2. Characteristics of the prepared ion exchange membrane 2.2.1. FT-IR spectra and EDX The fourier transform infrared (FT-IR) spectra (KBr discs with polymer or ATR with membrane) were recorded with a Bomem MB104 spectrometer and Pt concentration profiles were measured with EDX (Energy Dispersive X-ray, Horiba EMAX) with Pt/Nafion 117/Pt MEA.
2.2.2. Thermal stability A TGA 2950 thermo-gravimetric analysis (TGA) instrument (TA Company, New Castle, Delaware) was used to study the thermal stability behavior of PEM sample. Each sample was heated in nitrogen from 25 to 700 ◦ C with a scanning rate of 10 ◦ C/min.
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2.2.3. Mechanical properties and oxidative stabilities The tensile strength–elongation tests were carried out according to the standard method (ASTM D 882) using a Lloyd universal testing machine (model LR5K, UK). The test specimens were cut into strips (70-mm long and 25-mm wide) and the thickness of each strip was measured using digital vernier calipers. The relative humidity, gauge length and crosshead speed were set as 50%, 30 mm, and 5 mm/min, respectively. All specimens were drawn at ambient temperature. Replicate measurements are performed and the results are quoted as average values. The tensile strength and breaking elongation of the prepared ion exchange membranes are directly obtained from the tensile tests. The energy-to-break point, which is the fracture energy per unit volume of the sample and corresponds to the toughness of the prepared ion exchange membranes, was obtained from the tensile stress–elongation curves of each sample. A membrane sample was weighed and soaked in Fenton’s reagent (2 ppm FeSO4 in 3% H2 O2 ) at 80 ◦ C with continuous stirring. The anti-oxidative stability was quantified by the time elapsed between immersion in Fenton’s reagent and membrane breakage or fracture. 2.3. Membrane property of the prepared ion exchange membrane 2.3.1. Water content (WU ) Measurement of water content (WU ) was determined from the difference in weight between the dried and the swollen membranes. The weight of the dried membrane (Wdry ) was measured and then the membrane soaked in water until the weight remained constant. The membrane was removed from the water, and wiped with blotting paper, and then weighed again to obtain the wet weight (Wwet ). The WU was calculated with the following formula: WU =
Wwet − Wdry Wdry
× 100
(1)
2.3.2. Ion exchange capacity (IEC) After immersion in pure water for 1 day, a membrane sample was immersed for 1 day in a large volume of 1 M HCl aqueous solution to transform the membrane into the H+ form. The membrane was then washed with deionized water to remove excess HCl, and equilibrated with deionized water for an additional 4 h with frequent changes of the deionized water to remove the last traces of acid. The membrane was then equilibrated with exactly 50 ml of 0.01 M NaOH aq. solution for 24 h. IEC was quantified by measuring the reduction in alkalinity using back-titration. The IEC of the PEM was calculated using the following equation: IEC =
NNaOH fNaOH VNaOH /1000 − (VNaOH /Vsample )NHCl fHCl VHcl /1000 g dry membrane ×1000
(2)
where f is the factor of the solution, N is the normal concentration of the solution, and V is the volume of the acid–base solution. 2.3.3. Proton conductivity The proton conductivity of the polymer membranes was measured by ac impedance spectroscopy with a Solartron 1260 analyzer across 13-mm diameter samples clamped between two blocking platinum electrodes. The sample discs were hydrated by soaking them in water overnight and they were wet when placed into the measurement cell. The conductivity, , of the samples was calculated from the impedance data, using the relation, = d/RS, where d and S are the thickness and surface area of the sample, respectively, and R was derived from the low intersect of the high frequency semi-circle on a complex impedance plane with the Re(Z) axis. The
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impedance data were corrected for the contribution from the empty and short-circuited cell. 2.4. Membrane electrode assembly Pt(NH3 )4 Cl2 and NaBH4 were used as the electro-catalytic material and reducing agent, respectively. The prepared ion exchange membrane was fixed in the plating cell [6] with the top surface facing the impregnation solution of 5 mmol Pt(NH3 )4 Cl2 in methanol–water mixture for 24 and 1 h in accordance with equilibrium and non-equilibrium impregnation–reduction (I–R) methods. After impregnation the platinum solution was removed from the plating cell and was replaced with reducing solution of NaBH4 ranging in concentration from 0.3 to 1.0 mol/L. The other side of membrane was impregnated and reduced under the conditions described above. After the reduction step was completed, the prepared MEA were soaked in 0.5 M H2 SO4 for 2 h to exchange Na+ for H+ and then were immersed in deionized water for 2 h before re-drying at 80 ◦ C and 12 h. 2.5. Evaluation of MEA and cell performance The characteristics and performance of the prepared MEA were analyzed and evaluated by scanning electron microscope (SEM), cyclic voltammetry (CV) and I–V curves. The cross-sectional morphologies and platinum distribution of the prepared MEA were analyzed by Hitachi S-3500N SEM. Cyclic voltammetry was performed with EG&G PAR potentiostat–galvanostat model 273A and a conventional three-electrode cell with the prepared MEA as the working electrode, SCE as the reference and Pt wire as the counter electrode. The potential ranged from 0 to 1.5 V vs. NHE and the scan rate was 10 mV/s. The CVs were measured at the working electrode in nitrogen-purged 0.5 M H2 SO4 electrolyte at 25 ◦ C. Electrochemical surface area (ESA) was calculated from the charge passed (QH = 210 C/cm2 ) by a full monolayer of adsorbed hydrogen atoms. The prepared MEA was tested by using a single cell with an active area of 9 cm2 at 80 ◦ C and under ambient pressure for application in PEME. 3. Results and discussion 3.1. Characteristics of the prepared ion exchange membrane 3.1.1. FT-IR study Fig. 1 shows the FT-IR spectra of SPEEK/TPA(16%) and SPSf-coPPSS/TPA(4.3%) membrane.
Fig. 1. FT-IR spectra of SPEEK/TPA(16%) and SPSf-co-PPSS/TPA(4.3%).
Fig. 2. TGA curves of SPEEK/TPA(16%) and SPSf-co-PPSS/TPA(4.3%).
The symmetric stretching vibration due to the ion exchange group (SO3 H) in the prepared ion exchange membranes (SPEEK/TPA and SPSf-co-PPSS/TPA) was evident at 1026 cm−1 . In general, by addition of TPA, terminal oxygen interacts with the protonated-water dimmer in the secondary structure and the result for a shift is evident at from 1007 to 980 cm−1 in the vibrational band of the terminal oxygen in the primary structure [7]. The symmetric stretching bands of corner-shared octahedral (W–Oc –W) and edge-shared octahedral (W–Oe –W) were evident at 880–890 cm−1 and 814–895 cm−1 , respectively. These bands are due to the columbic interaction between the hydroxyl groups of the PEEK donor and the salt of TPA in the hybrid membrane. But, as for the SPSf-co-PPSS/TPA membrane, the intensity of the W Ot band at 976 cm−1 decreased and proton which had been hydrogenbonded to oxygen atom of water molecule ((H2 O)2 H+ ) migrated onto the bridging O2 of the heteropoly anion (PW12 O4 O3− ) band, and a new band at 1080 cm−1 appeared compared to pure SPSfco-PPSS [8–10]. This result indicates that the TPA particles interact with the sulfonic acid moiety as opposed to some other functional unit in the backbone. From the results, it was confirmed that the prepared ion exchange membranes had a sulfonic group for the cation exchange membrane as shown in Fig. 1. 3.1.2. Thermal stability Fig. 2 illustrates the thermo-gravimetric analysis of the prepared ion exchange membranes in the temperature range from 25 to 600 ◦ C. As for the SPEEK/TPA(16%) membrane, the weight was lost from about 300 ◦ C, and was rapidly lost with an increase of the temperature. As for the SPSf-co-PPSS/TPA(4.3%) membrane, the weight was lost from about 100 ◦ C, and was lost by three steps with an increase of the temperature, as shown in Fig. 2. It seems that the weight losses of SPEEK/TPA and SPSf-co-PPSS/TPA membrane which were due to the splitting-off of sulfonic acid groups occurred about 300 and 100 ◦ C, respectively, and the thermal degradation which was due to main chain decomposition [11] occurred about 500 ◦ C. The thermal properties of the SPPEK/TPA and SPSf-co-PPSS/TPA membrane are satisfactory for the pertain requirement of PEME cell which operates above 80 ◦ C. 3.1.3. Mechanical properties and oxidative stabilities Fig. 3 shows the influence of TPA concentration in the SPEEK and SPSf-co-PPSS composite matrix of 65% sulfonation degree (SD) on the tensile strength–elongation. Tests carried out at room temperature and 40% RH.
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Fig. 3. Influence of TPA concentration in the SPEEK and SPSf-co-PPSS composite matrix of 65% sulfonation degree (SD) on the tensile strength–elongation.
In general, elastic ductile polymer films exhibit two characteristic regions of deformation in their stress–elongation curves [12]: below yield points, the stress increases rapidly with increasing elongation and the steep initial slope can be observed in the elastic moduli; and, above yield points, the stress slowly decreases with elongation until breaking failure occurs, indicating that the increase in TPA concentration induces a transition from brittle to ductile behavior. With increasing TPA content, the tensile strength decreased and the elongation at the break point decreased, in the SPEEK/TPA membrane, as shown in Fig. 3(a). The stress–elongation curves from 0.33 to 60% of TPA content was similar to BPO4 /SPEEK membrane which was reported Reyna-Valencia et al. [13]. With increasing TPA content, the tensile strength increased and the elongation at the break point decreased, in the SPSf-co-PPSS/TPA membrane, as shown in Fig. 3(b). It seems that the increase of tensile strength occurred by specific hydrogen bonding interactions between the TPA and the sulfonated matrix, producing a mechanism for reinforcement [7]. The oxidative stability of the membranes was quantified by measuring the time elapsed between immersions of the membranes in oxidant until they broke into pieces. As shown in Table 1, the SPSf-co-PPSS/TPA membrane was higher than that of the SPEEK/TPA membrane, possibly due to the relatively poor watersolubility and block copolymerization of PSf and PPSS. Moreover, the anti-oxidative stabilities shown in Table 1 are much better than literature values reported for SPEEK membranes [14]. 3.2. Membrane properties of the prepared ion exchange membrane 3.2.1. Water content and IEC The water content and water state in sulfonated polymers are very important, as they directly affect proton transport across the membranes. And the water content strongly depends on the
sulfonic acid content and also is related to the IEC. Accordingly, proper water content should be maintained in sulfonated polymer membranes to guarantee high proton conductivity. Fig. 4 shows the influence of TPA concentration in the SPEEK/TPA and SPSf-co-PPSS/TPA membrane on the water content and IEC. The water content of the each membrane decreased with an increase of the TPA content. Water uptake is interpreted as not only due to the incomplete removal of the hydrogen-bonded water molecules in the composite membranes, but also to the decrease in number of water absorption sites, i.e. sulfonic acid, due to the relatively stronger interaction between the sulfonic acid and HPA [7,15]. As for the SPEEK/TPA membrane, IEC rapidly increased until about 16% of TPA, and then slowly decreased with an increase of TPA content as in the case of Baradie et al. [15]. As for the SPSf-co-PPSS/TPA membrane, IEC increased with an increase of TPA content. In each prepared membrane, IEC showed the almost same trend with TPA content, and increased with a decrease of the water content, as shown in Fig. 4. This means that the increase of IEC leads to the decrease of water content. From the results, it would be expected that the TPA content was suitable about 16 and 4.3% for SPEEK/TPA membrane and SPSf-co-PPSS/TPA membrane, respectively, in the viewpoint of IEC. 3.2.2. Proton conductivity One of the important determinants of the suitability of ionomer membranes for PEME is proton conductivity. Generally, protons can be transported along hydrogen-bonded ionic channels and with cationic mixtures, such as H3 O+ , H5 O2 + and H9 O4 + in the aqueous media. Free protons move through a localized ionic network within fully water-swollen polymer membranes. TPA agent is one of the most attractive inorganic modifiers because this material in crystalline form leads to be highly conductive and thermally stable Keggin units [7].
Table 1 The physical and electrochemical property of SPEEK/TPA and SPSf-co-PPSS/TPA composite membrane Property Thickness (m) Basis Weight (g/m2 ) Conductivity (S/cm) at 25 ◦ C IEC (mequiv./g) Tensile strength (MPa) Elongation at break (%) Water content (%) Thickness change (%) Linear expansion (%) Oxidative stability (min)
Nafion 117 183 360 0.059 0.89 43 225 38 14 15 –
SPEEK/TPA(16%)
SPSf-co-PSS/TPA(4.3%)
Test method
180 177 0.038 1.94 9.5 250 32.35 31 15 339
170 132 0.044 1.97 12.2 147 23.9 5.6 18 379
– – Impedance – ASTM D 882 ASTM D 570 ASTM D 756 (at 80 ◦ C) –
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Fig. 4. Influence of TPA concentration in the SPEEK/TPA and SPSf-co-PPSS/TPA membrane on the water content and IEC.
Fig. 5 shows the influence of TPA concentration in the SPEEK/TPA and SPSf-co-PPSS/TPA membrane on the proton conductivity at each temperature. As for the SPEEK/TPA membrane, as shown in Fig. 5(a), the proton conductivity measured at 25 and 60 ◦ C decreased below about 4% of TPA, and increased from 4 to 16% of TPA, and then slowly decreased with an increase of TPA content. The proton conductivity measured at 80 ◦ C rapidly increased below about 16% of TPA, and slowly decreased with an increase of TPA content. The initial decrease phenomena of proton conductivity measured at 25 and 60 ◦ C could not clearly explain, yet, it would be effect by the interaction between ion exchange group and TPA at these temperatures. So, it needs the investigation to solve this phenomenon for the further study. The maximum proton conductivity at each temperature had at about 16% of TPA and the values were 3.78 × 10−2 , 4.85 × 10−2 and 5.94 × 10−2 S/cm at 25, 60 and 80 ◦ C, respectively. This maximum proton conductivity at about 16% of TPA matched with IEC which had a maximum value at same TPA content, as well as. This means that higher IEC in the ion exchange membrane lead to high proton conductivity. As for the SPSf-co-PPSS/TPA membrane, as shown in Fig. 5(b), the proton conductivity measured at each temperature increased with an increase of TPA content, and then decreased above 4.3% of TPA. The maximum proton conductivity at each temperature had at 4.3% of TPA and the values were 4.36 × 10−2 , 5.5 × 10−2 and 6.29 × 10−2 S/cm at 25, 60 and 80 ◦ C, respectively. It appears that TPA, being a stronger acid, systematically yields a higher proton
conductivity, as well as better water retention at higher temperature [16]. The maximum proton conductivity at 4.3% of TPA matched with IEC which had a maximum value at same TPA content, as well as. The maximum proton conductivity of SPSf-coPPSS/TPA membrane at 80 ◦ C had higher than that of SPEEK/TPA membrane. The membrane prepared without addition of TPA agent hydrolyzed above 80 ◦ C but that with addition of TPA agent did not hydrolyze [17]. Table 1 illustrates other physical and electrochemical properties of the prepared ion exchange membrane and commercial membrane. The properties of the prepared ion exchange membrane were quite similar to Nafion 117, but some mechanical property (tensile strength) was worse than that of Nafion 117. 3.3. Water electrolysis with MEA 3.3.1. SEM and EDX property Fig. 6 shows cross-sectional SEM images and EDX profiles of MEA (Pt/Nafion 117/Pt) prepared by equilibrium and non-equilibrium I–R method. Two different impregnation–reduction plating procedures were applied to prepare a MEA. The principal difference between two chemical techniques is the impregnation step of platinic solution [18]. At the onset of the reduction step, the concentration distributions of Pt ions in impregnated membrane were quite different. One had the non-equilibrium state in the membrane; the other was carried to equilibrium state due to the long impregnation time.
Fig. 5. Influence of TPA concentration in the SPEEK/TPA and SPSf-co-PPSS/TPA membrane on the proton conductivity at each temperature.
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Fig. 6. Cross-sectional SEM images and EDX profiles of MEA (Pt/Nafion 117/Pt) prepared by equilibrium and non-equilibrium I–R method.
The cross-sectional images showed the almost same crosssection of MEA and Pt layer prepared by equilibrium and nonequilibrium I–R method. In general, the Pt layer formed extensively on the membrane surface and Pt particles distributed into the membrane inside, as for the MEA prepared by equilibrium I–R method. However, the Pt layer formed intensively on the membrane surface and Pt particles did not distribute into the membrane inside, as for the MEA prepared by non-equilibrium I–R method [6].
As shown in Fig. 6, Pt particles distribution in MEA analyzed by EDX profiles showed the same trends compared to the above explanation for the equilibrium and non-equilibrium I–R method. 3.3.2. Single cell performance in polymer electrolyte membrane electrolysis cell (PEMEC) Fig. 7 shows the effect of the reduction time and reducing agent concentration on the PEME cell performance using the MEA (Pt/Nafion 117/Pt, Pt/SPEEK–TPA(16%)/Pt, Pt/SPSf-co-
Fig. 7. Effect of the reduction time and reducing agent concentration on the PEME cell performance using the MEA (Pt/Nafion 117/Pt, Pt/SPEEK–TPA(16%)/Pt, Pt/SPSf-coPPSS–TPA(4.3%)/Pt) prepared by equilibrium I–R method.
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Fig. 8. Effect of the reduction time and reducing agent concentration on the PEME cell performance using the MEA (Pt/Nafion 117/Pt, Pt/SPEEK–TPA(16%)/Pt, Pt/SPSf-coPPSS–TPA(4.3%)/Pt) prepared by non-equilibrium I–R method.
PPSS–TPA(4.3%)/Pt) prepared by equilibrium I–R method. Fig. 8 shows the effect of the reduction time and reducing agent concentration on the PEME cell performance using the MEA (Pt/Nafion 117/Pt, Pt/SPEEK–TPA(16%)/Pt, Pt/SPSf-co-PPSS–TPA(4.3%)/Pt) prepared by non-equilibrium I–R method. The PEME cell was operated at 80 ◦ C and atmosphere pressure in each case. To determine the optimum reduction time and reducing agent (NaBH4 ) concentration, MEA using Nafion 117 was prepared by two I–R methods. As shown in Figs. 7 and 8, the cell voltage of PEME using MEA (Pt/Nafion 117/Pt) increased with an increase of the current density. In the equilibrium I–R method as shown in Fig. 7(a) and (b), the cell voltage had lower value at 60 min of the reduction time and 0.8 mol of reducing agent concentration led to increase in the PEME efficiency. In the non-equilibrium I–R method as shown in Fig. 8(a) and (b), the cell voltage had lower value at 90 min of the reduction time and 0.8 mol of reducing agent concentration, and the lower cell voltage led to increase in the voltage efficiency. From these results, it is expected that reducing agent concentration was suitable at 0.8 mol and the reduction time was suitable at 60 and 90 min for equilibrium and non-equilibrium I–R method, respectively, from the viewpoint of PEME low cell voltage, at each current density. Two MEAs (Pt/SPEEK–TPA(16%)/Pt, Pt/SPSf-co-PPSS–TPA (4.3%)/Pt) were prepared at 0.8 mol of reducing agent concentration and at 60 and 90 min of the reduction time for equilibrium and non-equilibrium I–R method, respectively.
The PEME cell voltage using MEA (Pt/SPEEK–TPA(16%)/Pt, Pt/SPSf-co-PPSS–TPA(4.3%)/Pt) prepared by non-equilibrium I–R method had lower value than that using MEA prepared by equilibrium I–R method at each current density, as shown in Figs. 7 and 8. The Pt particle distribution in the equilibrium I–R method was denser and extends deeper into the membrane than the nonequilibrium I–R method, as shown Fig. 6. Using the non-equilibrium I–R method, Pt clusters formed on the interface region where they were accessible to the reactant and were in intimate contact with the polymer electrolyte. Particles deposited deep within the membrane that do not come in contact with the current collector are useless for electrolysis. To establish good contact with the current collector, it is necessary for a MEA (Pt/PEM/Pt) to have a Pt layer exposed at the surface. From the above result, it is expected that the PEME cell performance using MEA prepared by non-equilibrium I–R method was better than that using MEA prepared by equilibrium I–R method. Table 2 illustrates the additional cell performance characteristics for Pt/SPEEK–TPA(16%)/Pt and Pt/SPSf-co-PPSS–TPA(4.3%)/Ptbased PEME cells. For comparison, the cell performance using Pt/Nafion 117/Pt under the same condition is also shown. The cell performance using Pt/SPEEK–TPA(16%)/Pt and Pt/SPSf-co-PPSS–TPA(4.3%)/Pt were quite similar to that using Pt/Nafion 117/Pt. 3.3.3. Cyclic voltammograms Fig. 9 shows the cyclic voltammograms of the MEA prepared by non-equilibrium I–R method with Nafion 117, SPSf-coPPSS/TPA(4.3%) and SPEEK/TPA(16%) membrane. The electrochemical surface areas (ESAs) were obtained from the coulombic charge for the oxidation of the atomic hydrogen adsorbed on the electrode. The upper potential limit of the integration of the current–potential curve was taken as the point where the double layer current is no longer constant, approximately at E = 0.4 V vs. NHE. The lower potential limit of the integration was determined by the local maximum at E = 0.05 V vs. NHE occurring Table 2 The electrochemical property of MEA prepared by non-equilibrium I–R method (Pt(NH3 )4 Cl2 concentration = 5 mmol/L, NaBH4 = 0.8 mol/L, impregnation time = 60 min, reduction time = 90 min, cell temperature = 80 ◦ C)
Fig. 9. Cyclic voltammograms of the MEA prepared by non-equilibrium I–R method with Nafion 117, SPSf-co-PPSS/TPA(4.3%) and SPEEK/TPA(16%) membrane; geometrical area: 1.0 cm2 , scan rate: 10 mV/s.
Pt loading (mg/cm2 ) ESA (m2 /g) Cell voltage (V, at 1 A/cm2 ) Voltage efficiency (%, εH )
Nafion 117
SPEEK/TPA(16%)
SPSf-coPPSS/TPA(4.3%)
1.21 22.48 1.85 80.0
1.01 10.11 1.90 77.9
1.12 16.93 1.83 80.9
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between the hydrogen evolution and hydrogen adsorption. ESAs of SPSf-co-PPSS/TPA(4.3%) and SPEEK/TPA(16%) were 16.93 and 10.11 m2 /g, respectively. 4. Conclusions Although membranes prepared with the SPEEK and SPSf-coPPSS hydrolyzed above 80 ◦ C, but that with addition of TPA agent did not hydrolyze. In addition, electrochemical and mechanical properties were improved with addition of TPA agent. And the SPEEK/TPA and SPSf-co-PPSS/TPA membrane exhibited good antioxidative stability. The SPSf-co-PPSS/TPA membrane with 4.3% of TPA showed best properties; proton conductivity (6.3 × 10−2 S/cm, at 80 ◦ C), water content (23.9%), ion exchange capacity (1.97 mequiv./g dry memb.), tensile strength (12.2 MPa) and elongation (147%). The optimal conditions for the preparation of MEA using the non-equilibrium I–R method were as following: 0.5 mmol/L Pt solution, an impregnation time of 1 h, 0.8 mol/L NaBH4 and a reduction time of 90 min. The cell performance values during water electrolysis were the following: the cell voltages of MEA using SPSf-co-PPSS/TPA(4.3%) and SPEEK/TPA(16%) were 1.83 and 1.90 V at 1 A/cm2 and 80 ◦ C; platinum loadings was 1.12 and 1.01 mg/cm2 , respectively. ESAs of SPSf-co-PPSS/TPA(4.3%) and SPEEK/TPA(16%) were 16.93 and 10.11 m2 /g, respectively. Acknowledgements This paper was performed for the Hydrogen Energy R&D Center, one of the 21st Century Frontier R&D Program, funded by the Ministry of Science and Technology of Korea. References [1] W. Vielstich, A. Lamm, H.A. Gasteiger, Handbook of Fuel Cells: Fundamentals Technology and Applications, vol. 2, John Wiley & Sons, Ltd., Chichester, 2003.
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[2] J.M. Bae, I. Honma, Properties of selected sulfonated polymers as protonconducting electrolytes for polymer electrolyte fuel cells, Solid State Ionics 147 (2002) 189–194. [3] F. Lufrano, G. Squadrito, A. Patti, E. Passalacqua, Sulfonated polysulfone as promising membranes for polymer electrolyte fuel cells, J. Appl. Polym. Sci. 77 (2000) 1250–1256. [4] N. Fujiwara, K. Yasuda, T. Ioroi, Z. Siroma, Y. Miyazaki, Preparation of platinum–ruthenium onto solid polymer electrolyte membrane and the application to a DMFC anode, Electrochim. Acta 47 (2002) 4079–4084. [5] G.-J. Hwang, H. Ohya, Preparation of anion exchange membrane based on block copolymers. Part I. Amination of the chloromethylated copolymers, J. Membr. Sci. 140 (1998) 195–203. [6] R. Liu, W.H. Her, P.S. Fedkiw, In situ electrode formation on a nafion membrane by chemical platinization, J. Electrochem. Soc. 139 (1992) 15–23. [7] Y.S. Kim, F. Wang, M. Hickner, T.A. Zawodzinski, J.E. McGrath, Fabrication and characterization of heteropolyacid/directly polymerized sulfonated poly(arylene ether sulfone) copolymer composite membranes for higher temperature fuel cell applications, J. Membr. Sci. 212 (2003) 263–282. [8] K.Y. Lee, N. Mizuno, T. Okuhara, M. Misono, Catalysis by heteropoly compounds. VIII. An infrared study of ethanol and diethyl ether in the pseudoliquid phase of 12-tunstophsophoric acid, Bull. Chem. Soc. Jpn. 62 (1989) 1731–1739. [9] L. Li, Y. Wang, Proton conducting composite membranes from sulfonated polyethersulfone cardo and phosphotungstic acid for fuel cell application, J. Power Sources 162 (2006) 541–546. [10] V. Ramani, H.R. Kunz, J.M. Fenton, Stabilized heteropolyacid/nafion composite membranes for elevated temperature/low relative humidity PEFC operation, Electrochim. Acta 50 (2005) 1181–1187. [11] H.B. Park, H.S. Shin, Y.M. Lee, J.W. Rhim, Annealing effect of sulfonated polysulfone ionomer membranes on proton conductivity and methanol transport, J. Membr. Sci. 247 (2005) 103–110. [12] N.E. Dowling, Mechanical Behaviour of Materials, Prentice-Hall, New Jersey, 1993. [13] A. Reyna-Valencia, S. Kaliaguine, M. Bousmina, Tensile mechanical properties of sulfonated poly(ether ether ketone) (SPEEK) and BPO4 /SPEEK membranes, J. Appl. Polym. Sci. 98 (2005) 2380–2393. [14] L. Zhang, S. Mukerjee, Investigation of durability issues of selected no fluorinated proton exchange membranes for fuel cell application, J. Electrochem. Soc. 153 (2006) A1062–A1072. [15] B. Baradie, C. Poinsignon, J.Y. Sanchez, Y. Piffard, G. Vitter, N. Bestaoui, D. Foscallo, A. Denoyelle, D. Delabouglise, M. Vaujany, Thermostable ionomeric filled membrane for H2 /O2 fuel cell, J. Power Sources 74 (1998) 8–16. [16] J. Kerres, C.M. Tang, C. Graf, Improvement of properties of poly(ether ketone) ionomer membranes by bending and cross-linking, Ind. Eng. Chem. Res. 43 (2004) 4571–4579. [17] S.M.J. Zaidi, S.D. Mikhailenko, G.P. Robertson, M.D. Guiver, Proton conducting composite membrane from polyether ether ketone and hetero-polyacids for fuel cell applications, J. Membr. Sci. 173 (2000) 17–34. [18] O. Enea, D. Duprez, R. Amadelli, Gas phase electrocatalysis on metal/nafion membranes, Catal. Today 25 (1995) 271–276.
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Journal of Membrane Science journal homepage: www.elsevier.com/locate/memsci
Application of polysulfone (PSf)– and polyether ether ketone (PEEK)–tungstophosphoric acid (TPA) composite membranes for water electrolysis In-Young Jang a , Oh-Hwan Kweon a , Kyoung-Eon Kim a , Gab-Jin Hwang b , Sang-Bong Moon c , An-Soo Kang a,∗ a
Department of Chemical Engineering, Myongji University, San 38-2, Nam-dong, Cheoin-gu, Yongin-si 449-728, Republic of Korea Department of Hydrogen & Fuel Cell Research, Korea Institute of Energy Research, 71-2 Jang-dong, Yuseong-gu, Daejeon 305-343, Republic of Korea c Elchem Tech Co., Ltd., New T Castle 1001, Gasandong 429-1, Geumchun-gu, Seoul 153-803, Republic of Korea b
a r t i c l e
i n f o
Article history: Received 8 August 2007 Received in revised form 11 April 2008 Accepted 18 May 2008 Available online 25 May 2008 Keywords: Cation exchange membrane Non-equilibrium impregnation–reduction method Membrane electrode assembly (MEA) Polymer electrolyte membrane electrolysis (PEME)
a b s t r a c t The ion exchange membrane using polysulfone (PSf) and polyether ether ketone (PEEK) as a basic material was prepared to apply in the polymer electrolyte membrane electrolysis (PEME). The sulfonated block copolymer of PSf and poly(phenylene sulfide sulfone) (SPSf-co-PPSS) and the sulfonated PEEK (SPEEK) were blended with tungstophosphoric acid (TPA) to avoid water swelling at elevated temperatures led to decrease in mechanical strength. These prepared ion exchange membranes showed some interesting characteristics including physicochemical stabilities, mechanical and membrane properties. The prepared ion exchange membrane was utilized to prepare the membrane electrode assembly (MEA). MEA consisted of Pt/PEM/Pt was prepared by equilibrium and non-equilibrium impregnation–reduction (I–R) methods. The prepared MEA by non-equilibrium I–R method was used in the PEME unit cell. The cell voltages of the MEA using SPSf-co-PPSS/TPA and SPEEK/TPA membranes were 1.83 V and 1.90 V at 1 A/cm2 and 80 ◦ C, with platinum loadings of 1.12 and 1.01 mg/cm2 , respectively. © 2008 Elsevier B.V. All rights reserved.
1. Introduction The change toward new energy system with hydrogen as an energy carrier would be giving an immense impact in the human society [1]. Polymer electrolyte membrane electrolysis (PEME) is one of the highly efficient energy technologies for hydrogen production from water, which is abundant resource. The advantages of PEME over alkaline electrolysis include lower parasitic energy loss and higher ecological purity hydrogen output. Therefore PEME is a simple, sustainable and cost-effective technology for hydrogen generation. Recently the commercially available Nafion membrane, which is a perfluorosulfonic acid solid polymer electrolyte membrane from DuPont Co., is successfully used to polymer electrolyte [2]. However the current industry standards for perfluorosulfonic polymers have been limited by their high cost and loss of membrane characteristics such as proton conductivity at temperatures above 80 ◦ C. The aromatic polymers such as polyether ether ketone (PEEK), poly-
∗ Corresponding author. Tel.: +82 31 330 6386; fax: +82 31 33 337 1920. E-mail address: [email protected] (A.-S. Kang). 0376-7388/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2008.05.028
sulfone (PSf), polyimides, polybenzimidazoles, polyoxadiazole and polyphosphazenes expect satisfactory chemical and electrochemical properties and lower production costs [3]. In addition the blend of tungstophosphoric acid (TPA) and sulfonated copolymers using those aromatic polymers would be giving a significant influence on PEME cell performance at high temperatures. The membrane electrode assembly (MEA) fabricated using the conventional hot-pressing method are usually inclined to delaminate both the anode and cathode interfaces after long-term operation led to degrade the PEME cell performance. In this work to achieve a better interfacial contact between the electro-catalysts and the PEM, MEA (Pt/PEM/Pt) is prepared by an equilibrium and non-equilibrium I–R method. MEA prepared using these methods has a few microns thick, hydrophilic and porous [4]. In addition, MEA prepared using these methods exhibits excellent adhesion and durability, because the metal clusters are embedded in the membrane surfaces. In this paper the cation exchange membrane was prepared by blending TPA and sulfonated copolymers using polysulfone and PEEK. The prepared membrane was measured for its properties (electrochemical performances and thermo-mechanical stability). Thus MEA was prepared from the prepared cation exchange
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membranes using two I–R methods and Pt-loading content, and current–cell voltage in PEME cell was investigated.
2. Experimental 2.1. Preparation of membrane 2.1.1. SPEEK/TPA composite membrane Polyether ether ketone (PEEK, Victrex 450G, Mn = 100,000) was dried at 100 ◦ C in a vacuum oven for 12 h for a pre-treatment. An amount of 20 g PEEK is dissolved in 400 ml of sulfuric acid (95 wt.%) with stirring at room temperature for 24 h. After a certain time, the solution precipitated with a large excess of ice water to obtain the sulfonated polymer. The obtained sulfonated polymer were washed repeatedly with deionized water, and dried under vacuum at 100 ◦ C for 12 h to obtain the sulfonated PEEK (SPEEK). The obtained SPEEK was dissolved in 1-methyl-2-pyrrolidinone (NMP, Lancaster, 99%), and the various weights (0.33–60 wt.%) of powdered tungstophosphoric acid (TPA, H3 PW12 O40 ·29H2 O) was added to the solution, and then the mixture was stirred for 16 h. The mixed solution was cast onto a glass plate with a doctor’s knife, and dried in series of at room temperature for 12 h, at 60 ◦ C for 4 h and at 120 ◦ C for 12 h to obtain the cation exchange membrane. The obtained membrane which was blended with SPEEK and TPA is named as SPEEK/TPA.
2.1.2. SPSf-co-PPSS/TPA composite membrane The SPSf-co-PPSS/TPA composite membrane was prepared by two-step synthesis with polysulfone (PSf, UDEL type, Aldrich Chem. Co., Mn = 26,000). Firstly, in order to obtain the PSf-co-PPSS block copolymer (BPSf), 18.9532 g of 4,4 -dichlorodiphenylsulfone (DCDPS, Fluka Chem. Co., 95%), 4.6824 g of sodium sulfide hydrate (Na2 S·nH2 O, Janssen, 60–62%) and 6.1212 g of lithium acetate dehydrate (CH3 COOLi·2H2 O, Kanto Chem. Co., 98%) were dissolved in 120 ml of NMP, and then 26.0054 g of PSf was added thereto [5]. The mixture was stirred at 160 ◦ C for 3 h with nitrogen gas. Secondly, the obtained BPSf was dissolved in 1,1,2,2-tetrachloroethane (TCE, Samchun Chem.) and the various weights (1.4–22 wt.%) of powdered TPA was added to the solution. Then, the chlorosulfonic acid (CSA, Aldrich Chem.) was added to those solutions in the molar ratio of 3:1 against BPSf at room temperature for 1 h to obtain the sulfonated block copolymer (SBPSf/TPA). The obtained SBPSf/TPA was dissolved in NMP with stirring for 16 h. The mixed solution was cast onto a glass plate with a doctor’s knife, and dried in series of at room temperature for 12 h, at 60 ◦ C for 4 h and at 120 ◦ C for 12 h to obtain the cation exchange membrane. The obtained membrane is named as SPSf-co-PPSS/TPA.
2.2. Characteristics of the prepared ion exchange membrane 2.2.1. FT-IR spectra and EDX The fourier transform infrared (FT-IR) spectra (KBr discs with polymer or ATR with membrane) were recorded with a Bomem MB104 spectrometer and Pt concentration profiles were measured with EDX (Energy Dispersive X-ray, Horiba EMAX) with Pt/Nafion 117/Pt MEA.
2.2.2. Thermal stability A TGA 2950 thermo-gravimetric analysis (TGA) instrument (TA Company, New Castle, Delaware) was used to study the thermal stability behavior of PEM sample. Each sample was heated in nitrogen from 25 to 700 ◦ C with a scanning rate of 10 ◦ C/min.
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2.2.3. Mechanical properties and oxidative stabilities The tensile strength–elongation tests were carried out according to the standard method (ASTM D 882) using a Lloyd universal testing machine (model LR5K, UK). The test specimens were cut into strips (70-mm long and 25-mm wide) and the thickness of each strip was measured using digital vernier calipers. The relative humidity, gauge length and crosshead speed were set as 50%, 30 mm, and 5 mm/min, respectively. All specimens were drawn at ambient temperature. Replicate measurements are performed and the results are quoted as average values. The tensile strength and breaking elongation of the prepared ion exchange membranes are directly obtained from the tensile tests. The energy-to-break point, which is the fracture energy per unit volume of the sample and corresponds to the toughness of the prepared ion exchange membranes, was obtained from the tensile stress–elongation curves of each sample. A membrane sample was weighed and soaked in Fenton’s reagent (2 ppm FeSO4 in 3% H2 O2 ) at 80 ◦ C with continuous stirring. The anti-oxidative stability was quantified by the time elapsed between immersion in Fenton’s reagent and membrane breakage or fracture. 2.3. Membrane property of the prepared ion exchange membrane 2.3.1. Water content (WU ) Measurement of water content (WU ) was determined from the difference in weight between the dried and the swollen membranes. The weight of the dried membrane (Wdry ) was measured and then the membrane soaked in water until the weight remained constant. The membrane was removed from the water, and wiped with blotting paper, and then weighed again to obtain the wet weight (Wwet ). The WU was calculated with the following formula: WU =
Wwet − Wdry Wdry
× 100
(1)
2.3.2. Ion exchange capacity (IEC) After immersion in pure water for 1 day, a membrane sample was immersed for 1 day in a large volume of 1 M HCl aqueous solution to transform the membrane into the H+ form. The membrane was then washed with deionized water to remove excess HCl, and equilibrated with deionized water for an additional 4 h with frequent changes of the deionized water to remove the last traces of acid. The membrane was then equilibrated with exactly 50 ml of 0.01 M NaOH aq. solution for 24 h. IEC was quantified by measuring the reduction in alkalinity using back-titration. The IEC of the PEM was calculated using the following equation: IEC =
NNaOH fNaOH VNaOH /1000 − (VNaOH /Vsample )NHCl fHCl VHcl /1000 g dry membrane ×1000
(2)
where f is the factor of the solution, N is the normal concentration of the solution, and V is the volume of the acid–base solution. 2.3.3. Proton conductivity The proton conductivity of the polymer membranes was measured by ac impedance spectroscopy with a Solartron 1260 analyzer across 13-mm diameter samples clamped between two blocking platinum electrodes. The sample discs were hydrated by soaking them in water overnight and they were wet when placed into the measurement cell. The conductivity, , of the samples was calculated from the impedance data, using the relation, = d/RS, where d and S are the thickness and surface area of the sample, respectively, and R was derived from the low intersect of the high frequency semi-circle on a complex impedance plane with the Re(Z) axis. The
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impedance data were corrected for the contribution from the empty and short-circuited cell. 2.4. Membrane electrode assembly Pt(NH3 )4 Cl2 and NaBH4 were used as the electro-catalytic material and reducing agent, respectively. The prepared ion exchange membrane was fixed in the plating cell [6] with the top surface facing the impregnation solution of 5 mmol Pt(NH3 )4 Cl2 in methanol–water mixture for 24 and 1 h in accordance with equilibrium and non-equilibrium impregnation–reduction (I–R) methods. After impregnation the platinum solution was removed from the plating cell and was replaced with reducing solution of NaBH4 ranging in concentration from 0.3 to 1.0 mol/L. The other side of membrane was impregnated and reduced under the conditions described above. After the reduction step was completed, the prepared MEA were soaked in 0.5 M H2 SO4 for 2 h to exchange Na+ for H+ and then were immersed in deionized water for 2 h before re-drying at 80 ◦ C and 12 h. 2.5. Evaluation of MEA and cell performance The characteristics and performance of the prepared MEA were analyzed and evaluated by scanning electron microscope (SEM), cyclic voltammetry (CV) and I–V curves. The cross-sectional morphologies and platinum distribution of the prepared MEA were analyzed by Hitachi S-3500N SEM. Cyclic voltammetry was performed with EG&G PAR potentiostat–galvanostat model 273A and a conventional three-electrode cell with the prepared MEA as the working electrode, SCE as the reference and Pt wire as the counter electrode. The potential ranged from 0 to 1.5 V vs. NHE and the scan rate was 10 mV/s. The CVs were measured at the working electrode in nitrogen-purged 0.5 M H2 SO4 electrolyte at 25 ◦ C. Electrochemical surface area (ESA) was calculated from the charge passed (QH = 210 C/cm2 ) by a full monolayer of adsorbed hydrogen atoms. The prepared MEA was tested by using a single cell with an active area of 9 cm2 at 80 ◦ C and under ambient pressure for application in PEME. 3. Results and discussion 3.1. Characteristics of the prepared ion exchange membrane 3.1.1. FT-IR study Fig. 1 shows the FT-IR spectra of SPEEK/TPA(16%) and SPSf-coPPSS/TPA(4.3%) membrane.
Fig. 1. FT-IR spectra of SPEEK/TPA(16%) and SPSf-co-PPSS/TPA(4.3%).
Fig. 2. TGA curves of SPEEK/TPA(16%) and SPSf-co-PPSS/TPA(4.3%).
The symmetric stretching vibration due to the ion exchange group (SO3 H) in the prepared ion exchange membranes (SPEEK/TPA and SPSf-co-PPSS/TPA) was evident at 1026 cm−1 . In general, by addition of TPA, terminal oxygen interacts with the protonated-water dimmer in the secondary structure and the result for a shift is evident at from 1007 to 980 cm−1 in the vibrational band of the terminal oxygen in the primary structure [7]. The symmetric stretching bands of corner-shared octahedral (W–Oc –W) and edge-shared octahedral (W–Oe –W) were evident at 880–890 cm−1 and 814–895 cm−1 , respectively. These bands are due to the columbic interaction between the hydroxyl groups of the PEEK donor and the salt of TPA in the hybrid membrane. But, as for the SPSf-co-PPSS/TPA membrane, the intensity of the W Ot band at 976 cm−1 decreased and proton which had been hydrogenbonded to oxygen atom of water molecule ((H2 O)2 H+ ) migrated onto the bridging O2 of the heteropoly anion (PW12 O4 O3− ) band, and a new band at 1080 cm−1 appeared compared to pure SPSfco-PPSS [8–10]. This result indicates that the TPA particles interact with the sulfonic acid moiety as opposed to some other functional unit in the backbone. From the results, it was confirmed that the prepared ion exchange membranes had a sulfonic group for the cation exchange membrane as shown in Fig. 1. 3.1.2. Thermal stability Fig. 2 illustrates the thermo-gravimetric analysis of the prepared ion exchange membranes in the temperature range from 25 to 600 ◦ C. As for the SPEEK/TPA(16%) membrane, the weight was lost from about 300 ◦ C, and was rapidly lost with an increase of the temperature. As for the SPSf-co-PPSS/TPA(4.3%) membrane, the weight was lost from about 100 ◦ C, and was lost by three steps with an increase of the temperature, as shown in Fig. 2. It seems that the weight losses of SPEEK/TPA and SPSf-co-PPSS/TPA membrane which were due to the splitting-off of sulfonic acid groups occurred about 300 and 100 ◦ C, respectively, and the thermal degradation which was due to main chain decomposition [11] occurred about 500 ◦ C. The thermal properties of the SPPEK/TPA and SPSf-co-PPSS/TPA membrane are satisfactory for the pertain requirement of PEME cell which operates above 80 ◦ C. 3.1.3. Mechanical properties and oxidative stabilities Fig. 3 shows the influence of TPA concentration in the SPEEK and SPSf-co-PPSS composite matrix of 65% sulfonation degree (SD) on the tensile strength–elongation. Tests carried out at room temperature and 40% RH.
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Fig. 3. Influence of TPA concentration in the SPEEK and SPSf-co-PPSS composite matrix of 65% sulfonation degree (SD) on the tensile strength–elongation.
In general, elastic ductile polymer films exhibit two characteristic regions of deformation in their stress–elongation curves [12]: below yield points, the stress increases rapidly with increasing elongation and the steep initial slope can be observed in the elastic moduli; and, above yield points, the stress slowly decreases with elongation until breaking failure occurs, indicating that the increase in TPA concentration induces a transition from brittle to ductile behavior. With increasing TPA content, the tensile strength decreased and the elongation at the break point decreased, in the SPEEK/TPA membrane, as shown in Fig. 3(a). The stress–elongation curves from 0.33 to 60% of TPA content was similar to BPO4 /SPEEK membrane which was reported Reyna-Valencia et al. [13]. With increasing TPA content, the tensile strength increased and the elongation at the break point decreased, in the SPSf-co-PPSS/TPA membrane, as shown in Fig. 3(b). It seems that the increase of tensile strength occurred by specific hydrogen bonding interactions between the TPA and the sulfonated matrix, producing a mechanism for reinforcement [7]. The oxidative stability of the membranes was quantified by measuring the time elapsed between immersions of the membranes in oxidant until they broke into pieces. As shown in Table 1, the SPSf-co-PPSS/TPA membrane was higher than that of the SPEEK/TPA membrane, possibly due to the relatively poor watersolubility and block copolymerization of PSf and PPSS. Moreover, the anti-oxidative stabilities shown in Table 1 are much better than literature values reported for SPEEK membranes [14]. 3.2. Membrane properties of the prepared ion exchange membrane 3.2.1. Water content and IEC The water content and water state in sulfonated polymers are very important, as they directly affect proton transport across the membranes. And the water content strongly depends on the
sulfonic acid content and also is related to the IEC. Accordingly, proper water content should be maintained in sulfonated polymer membranes to guarantee high proton conductivity. Fig. 4 shows the influence of TPA concentration in the SPEEK/TPA and SPSf-co-PPSS/TPA membrane on the water content and IEC. The water content of the each membrane decreased with an increase of the TPA content. Water uptake is interpreted as not only due to the incomplete removal of the hydrogen-bonded water molecules in the composite membranes, but also to the decrease in number of water absorption sites, i.e. sulfonic acid, due to the relatively stronger interaction between the sulfonic acid and HPA [7,15]. As for the SPEEK/TPA membrane, IEC rapidly increased until about 16% of TPA, and then slowly decreased with an increase of TPA content as in the case of Baradie et al. [15]. As for the SPSf-co-PPSS/TPA membrane, IEC increased with an increase of TPA content. In each prepared membrane, IEC showed the almost same trend with TPA content, and increased with a decrease of the water content, as shown in Fig. 4. This means that the increase of IEC leads to the decrease of water content. From the results, it would be expected that the TPA content was suitable about 16 and 4.3% for SPEEK/TPA membrane and SPSf-co-PPSS/TPA membrane, respectively, in the viewpoint of IEC. 3.2.2. Proton conductivity One of the important determinants of the suitability of ionomer membranes for PEME is proton conductivity. Generally, protons can be transported along hydrogen-bonded ionic channels and with cationic mixtures, such as H3 O+ , H5 O2 + and H9 O4 + in the aqueous media. Free protons move through a localized ionic network within fully water-swollen polymer membranes. TPA agent is one of the most attractive inorganic modifiers because this material in crystalline form leads to be highly conductive and thermally stable Keggin units [7].
Table 1 The physical and electrochemical property of SPEEK/TPA and SPSf-co-PPSS/TPA composite membrane Property Thickness (m) Basis Weight (g/m2 ) Conductivity (S/cm) at 25 ◦ C IEC (mequiv./g) Tensile strength (MPa) Elongation at break (%) Water content (%) Thickness change (%) Linear expansion (%) Oxidative stability (min)
Nafion 117 183 360 0.059 0.89 43 225 38 14 15 –
SPEEK/TPA(16%)
SPSf-co-PSS/TPA(4.3%)
Test method
180 177 0.038 1.94 9.5 250 32.35 31 15 339
170 132 0.044 1.97 12.2 147 23.9 5.6 18 379
– – Impedance – ASTM D 882 ASTM D 570 ASTM D 756 (at 80 ◦ C) –
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Fig. 4. Influence of TPA concentration in the SPEEK/TPA and SPSf-co-PPSS/TPA membrane on the water content and IEC.
Fig. 5 shows the influence of TPA concentration in the SPEEK/TPA and SPSf-co-PPSS/TPA membrane on the proton conductivity at each temperature. As for the SPEEK/TPA membrane, as shown in Fig. 5(a), the proton conductivity measured at 25 and 60 ◦ C decreased below about 4% of TPA, and increased from 4 to 16% of TPA, and then slowly decreased with an increase of TPA content. The proton conductivity measured at 80 ◦ C rapidly increased below about 16% of TPA, and slowly decreased with an increase of TPA content. The initial decrease phenomena of proton conductivity measured at 25 and 60 ◦ C could not clearly explain, yet, it would be effect by the interaction between ion exchange group and TPA at these temperatures. So, it needs the investigation to solve this phenomenon for the further study. The maximum proton conductivity at each temperature had at about 16% of TPA and the values were 3.78 × 10−2 , 4.85 × 10−2 and 5.94 × 10−2 S/cm at 25, 60 and 80 ◦ C, respectively. This maximum proton conductivity at about 16% of TPA matched with IEC which had a maximum value at same TPA content, as well as. This means that higher IEC in the ion exchange membrane lead to high proton conductivity. As for the SPSf-co-PPSS/TPA membrane, as shown in Fig. 5(b), the proton conductivity measured at each temperature increased with an increase of TPA content, and then decreased above 4.3% of TPA. The maximum proton conductivity at each temperature had at 4.3% of TPA and the values were 4.36 × 10−2 , 5.5 × 10−2 and 6.29 × 10−2 S/cm at 25, 60 and 80 ◦ C, respectively. It appears that TPA, being a stronger acid, systematically yields a higher proton
conductivity, as well as better water retention at higher temperature [16]. The maximum proton conductivity at 4.3% of TPA matched with IEC which had a maximum value at same TPA content, as well as. The maximum proton conductivity of SPSf-coPPSS/TPA membrane at 80 ◦ C had higher than that of SPEEK/TPA membrane. The membrane prepared without addition of TPA agent hydrolyzed above 80 ◦ C but that with addition of TPA agent did not hydrolyze [17]. Table 1 illustrates other physical and electrochemical properties of the prepared ion exchange membrane and commercial membrane. The properties of the prepared ion exchange membrane were quite similar to Nafion 117, but some mechanical property (tensile strength) was worse than that of Nafion 117. 3.3. Water electrolysis with MEA 3.3.1. SEM and EDX property Fig. 6 shows cross-sectional SEM images and EDX profiles of MEA (Pt/Nafion 117/Pt) prepared by equilibrium and non-equilibrium I–R method. Two different impregnation–reduction plating procedures were applied to prepare a MEA. The principal difference between two chemical techniques is the impregnation step of platinic solution [18]. At the onset of the reduction step, the concentration distributions of Pt ions in impregnated membrane were quite different. One had the non-equilibrium state in the membrane; the other was carried to equilibrium state due to the long impregnation time.
Fig. 5. Influence of TPA concentration in the SPEEK/TPA and SPSf-co-PPSS/TPA membrane on the proton conductivity at each temperature.
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Fig. 6. Cross-sectional SEM images and EDX profiles of MEA (Pt/Nafion 117/Pt) prepared by equilibrium and non-equilibrium I–R method.
The cross-sectional images showed the almost same crosssection of MEA and Pt layer prepared by equilibrium and nonequilibrium I–R method. In general, the Pt layer formed extensively on the membrane surface and Pt particles distributed into the membrane inside, as for the MEA prepared by equilibrium I–R method. However, the Pt layer formed intensively on the membrane surface and Pt particles did not distribute into the membrane inside, as for the MEA prepared by non-equilibrium I–R method [6].
As shown in Fig. 6, Pt particles distribution in MEA analyzed by EDX profiles showed the same trends compared to the above explanation for the equilibrium and non-equilibrium I–R method. 3.3.2. Single cell performance in polymer electrolyte membrane electrolysis cell (PEMEC) Fig. 7 shows the effect of the reduction time and reducing agent concentration on the PEME cell performance using the MEA (Pt/Nafion 117/Pt, Pt/SPEEK–TPA(16%)/Pt, Pt/SPSf-co-
Fig. 7. Effect of the reduction time and reducing agent concentration on the PEME cell performance using the MEA (Pt/Nafion 117/Pt, Pt/SPEEK–TPA(16%)/Pt, Pt/SPSf-coPPSS–TPA(4.3%)/Pt) prepared by equilibrium I–R method.
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Fig. 8. Effect of the reduction time and reducing agent concentration on the PEME cell performance using the MEA (Pt/Nafion 117/Pt, Pt/SPEEK–TPA(16%)/Pt, Pt/SPSf-coPPSS–TPA(4.3%)/Pt) prepared by non-equilibrium I–R method.
PPSS–TPA(4.3%)/Pt) prepared by equilibrium I–R method. Fig. 8 shows the effect of the reduction time and reducing agent concentration on the PEME cell performance using the MEA (Pt/Nafion 117/Pt, Pt/SPEEK–TPA(16%)/Pt, Pt/SPSf-co-PPSS–TPA(4.3%)/Pt) prepared by non-equilibrium I–R method. The PEME cell was operated at 80 ◦ C and atmosphere pressure in each case. To determine the optimum reduction time and reducing agent (NaBH4 ) concentration, MEA using Nafion 117 was prepared by two I–R methods. As shown in Figs. 7 and 8, the cell voltage of PEME using MEA (Pt/Nafion 117/Pt) increased with an increase of the current density. In the equilibrium I–R method as shown in Fig. 7(a) and (b), the cell voltage had lower value at 60 min of the reduction time and 0.8 mol of reducing agent concentration led to increase in the PEME efficiency. In the non-equilibrium I–R method as shown in Fig. 8(a) and (b), the cell voltage had lower value at 90 min of the reduction time and 0.8 mol of reducing agent concentration, and the lower cell voltage led to increase in the voltage efficiency. From these results, it is expected that reducing agent concentration was suitable at 0.8 mol and the reduction time was suitable at 60 and 90 min for equilibrium and non-equilibrium I–R method, respectively, from the viewpoint of PEME low cell voltage, at each current density. Two MEAs (Pt/SPEEK–TPA(16%)/Pt, Pt/SPSf-co-PPSS–TPA (4.3%)/Pt) were prepared at 0.8 mol of reducing agent concentration and at 60 and 90 min of the reduction time for equilibrium and non-equilibrium I–R method, respectively.
The PEME cell voltage using MEA (Pt/SPEEK–TPA(16%)/Pt, Pt/SPSf-co-PPSS–TPA(4.3%)/Pt) prepared by non-equilibrium I–R method had lower value than that using MEA prepared by equilibrium I–R method at each current density, as shown in Figs. 7 and 8. The Pt particle distribution in the equilibrium I–R method was denser and extends deeper into the membrane than the nonequilibrium I–R method, as shown Fig. 6. Using the non-equilibrium I–R method, Pt clusters formed on the interface region where they were accessible to the reactant and were in intimate contact with the polymer electrolyte. Particles deposited deep within the membrane that do not come in contact with the current collector are useless for electrolysis. To establish good contact with the current collector, it is necessary for a MEA (Pt/PEM/Pt) to have a Pt layer exposed at the surface. From the above result, it is expected that the PEME cell performance using MEA prepared by non-equilibrium I–R method was better than that using MEA prepared by equilibrium I–R method. Table 2 illustrates the additional cell performance characteristics for Pt/SPEEK–TPA(16%)/Pt and Pt/SPSf-co-PPSS–TPA(4.3%)/Ptbased PEME cells. For comparison, the cell performance using Pt/Nafion 117/Pt under the same condition is also shown. The cell performance using Pt/SPEEK–TPA(16%)/Pt and Pt/SPSf-co-PPSS–TPA(4.3%)/Pt were quite similar to that using Pt/Nafion 117/Pt. 3.3.3. Cyclic voltammograms Fig. 9 shows the cyclic voltammograms of the MEA prepared by non-equilibrium I–R method with Nafion 117, SPSf-coPPSS/TPA(4.3%) and SPEEK/TPA(16%) membrane. The electrochemical surface areas (ESAs) were obtained from the coulombic charge for the oxidation of the atomic hydrogen adsorbed on the electrode. The upper potential limit of the integration of the current–potential curve was taken as the point where the double layer current is no longer constant, approximately at E = 0.4 V vs. NHE. The lower potential limit of the integration was determined by the local maximum at E = 0.05 V vs. NHE occurring Table 2 The electrochemical property of MEA prepared by non-equilibrium I–R method (Pt(NH3 )4 Cl2 concentration = 5 mmol/L, NaBH4 = 0.8 mol/L, impregnation time = 60 min, reduction time = 90 min, cell temperature = 80 ◦ C)
Fig. 9. Cyclic voltammograms of the MEA prepared by non-equilibrium I–R method with Nafion 117, SPSf-co-PPSS/TPA(4.3%) and SPEEK/TPA(16%) membrane; geometrical area: 1.0 cm2 , scan rate: 10 mV/s.
Pt loading (mg/cm2 ) ESA (m2 /g) Cell voltage (V, at 1 A/cm2 ) Voltage efficiency (%, εH )
Nafion 117
SPEEK/TPA(16%)
SPSf-coPPSS/TPA(4.3%)
1.21 22.48 1.85 80.0
1.01 10.11 1.90 77.9
1.12 16.93 1.83 80.9
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between the hydrogen evolution and hydrogen adsorption. ESAs of SPSf-co-PPSS/TPA(4.3%) and SPEEK/TPA(16%) were 16.93 and 10.11 m2 /g, respectively. 4. Conclusions Although membranes prepared with the SPEEK and SPSf-coPPSS hydrolyzed above 80 ◦ C, but that with addition of TPA agent did not hydrolyze. In addition, electrochemical and mechanical properties were improved with addition of TPA agent. And the SPEEK/TPA and SPSf-co-PPSS/TPA membrane exhibited good antioxidative stability. The SPSf-co-PPSS/TPA membrane with 4.3% of TPA showed best properties; proton conductivity (6.3 × 10−2 S/cm, at 80 ◦ C), water content (23.9%), ion exchange capacity (1.97 mequiv./g dry memb.), tensile strength (12.2 MPa) and elongation (147%). The optimal conditions for the preparation of MEA using the non-equilibrium I–R method were as following: 0.5 mmol/L Pt solution, an impregnation time of 1 h, 0.8 mol/L NaBH4 and a reduction time of 90 min. The cell performance values during water electrolysis were the following: the cell voltages of MEA using SPSf-co-PPSS/TPA(4.3%) and SPEEK/TPA(16%) were 1.83 and 1.90 V at 1 A/cm2 and 80 ◦ C; platinum loadings was 1.12 and 1.01 mg/cm2 , respectively. ESAs of SPSf-co-PPSS/TPA(4.3%) and SPEEK/TPA(16%) were 16.93 and 10.11 m2 /g, respectively. Acknowledgements This paper was performed for the Hydrogen Energy R&D Center, one of the 21st Century Frontier R&D Program, funded by the Ministry of Science and Technology of Korea. References [1] W. Vielstich, A. Lamm, H.A. Gasteiger, Handbook of Fuel Cells: Fundamentals Technology and Applications, vol. 2, John Wiley & Sons, Ltd., Chichester, 2003.
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