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Performance of passive direct methanol fuel cell with poly(vinyl alcohol)-based polymer electrolyte membranes
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Performance of passive direct methanol fuel cell with poly(vinyl alcohol)-based polymer electrolyte membranes Mitsuru Higa*, Kentaro Hatemura, Mikinori Sugita, Shin-ichi Maesowa, Megumi Nishimura, Nobutaka Endo Graduate School of Science and Engineering, Yamaguchi University, 2-16-1 Tokiwadai, Ube-city, Yamaguchi 755-8611, Japan
article info
abstract
Article history:
Polymer electrolyte membranes (PEMs) were prepared from poly(vinyl alcohol) (PVA) and
Received 7 March 2011
a modified PVA polyanion containing 2 mol% of 2-methyl-1-propanesulfonic acid (AMPS)
Received in revised form
groups as a copolymer. The effect of the AMPS content and the crosslinking conditions on
1 October 2011
the properties of the membranes was investigated in PEMs with various AMPS contents
Accepted 4 October 2011
prepared under various crosslinking conditions. The proton conductivity and the perme-
Available online 5 November 2011
ability of methanol through the PEMs increased with increasing AMPS content, CAMPS, and
Keywords:
swelling. The permeability coefficient of methanol through the PEM prepared under the
with decreasing annealing temperature, Ta, because of the increase in the degree of Polymer electrolyte membrane
conditions of CAMPS ¼ 2.0 mol% and Ta 190 C was approximately 30 times lower than that
Poly(vinyl alcohol)
of Nafion 117 under the same measurement conditions. A maximum proton permse-
Direct methanol fuel cell
lectivity of 96 103 S cm3 s, which is defined as the ratio of the proton conductivity to the
Proton conductivity
permeability of methanol, was obtained for this PEM. The permselectivity value is about
Methanol concentration
three times higher than that of Nafion 117. A passive air-breathing-type DMFC test cell
Maximum power density
constructed using the PEM delivered 2.4 mW cm2 of maximum power density, Pmax, at 2 M methanol concentration, which is smaller than the value obtained with Nafion 117. However, at high methanol concentrations (>10 M), the Pmax of the PEM decreases slightly to 1.6 mW cm2 (at a methanol concentration of 20 M), whereas the Pmax of Nafion 117 falls to almost zero. Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
Direct methanol fuel cells (DMFCs) using polymer electrolyte membranes (PEMs) represent one of the most attractive power sources because of their stable operation at relatively low temperatures, high energy density using highly concentrated methanol solutions as fuel, and their simplicity [1,2]. In particular, a passive air-breathing DMFC system consisting of an anode that absorbs methanol from the built-in reservoir and a cathode that “breathes” from the ambient atmosphere
works without any auxiliary devices such as heat exchangers, humidifiers, fuel pumps, gas blowers/compressors, etc. [3]. Commercially available perfluorinated sulfonated membranes, such as Nafion, developed by Dupont, have been studied as PEMs for DMFCs because of their generally high proton conductivity, excellent mechanical properties, and excellent chemical stability [4e7]. However, their high methanol permeability is one of the critical drawbacks in their application to PEMs for DMFC systems. In one reported method for reducing methanol crossover, Lin et al. [4]
* Corresponding author. Tel.: þ81 836859203; fax: þ81 836859201. E-mail address: [email protected] (M. Higa). 0360-3199/$ e see front matter Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2011.10.012
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prestretched a uniaxially recast Nafion membrane in order to reduce its methanol permeability. A membrane-electrode assembly (MEA) constructed using the prestretched Nafion membrane shows higher DMFC performance than that employing Nafion 117 for methanol feed concentrations within 10 M [5]. In other methods, some researchers employed a methanol barrier layer, such as a hydrophobic porous carbon plate [8], flexible graphite material [9], poly(dimethylsiloxane) membrane [10], and porous poly(tetrafluoroethylene) plate [11], between the fuel reservoir and anode current collector in a passive DMFC. Recently, there have been many attempts at using non-fluorinated membrane materials as PEMs for DMFCs, not only for lower fuel crossover, but also to provide better environmental adaptability and lowered production costs [12e15]. Poly(vinyl alcohol) (PVA) is one of the candidate polymers for use as a PEM in the DMFC system. PVA has excellent methanol barrier properties because of the dense structure afforded by strong intra- and inter-molecular hydrogen bonding [16e18]. Because PVA is a semicrystalline polymer, heat-treatment above the glass-transition temperature [19,20], as well as repeated freezeethaw cycles [19,21,22], increase the crystallinity of the polymer, which acts as a physical network [22]. PVA chains can also be chemically crosslinked using crosslinking agents such as glutaraldehyde (GA). Hence, the degree of swelling of a PEM, which plays an important role in its proton conductivity and methanol permeability, can be easily controlled by varying the annealing and/or chemical crosslinking conditions. There have been many reports on PVAbased PEMs [23e32]. In a previous study [31], we prepared a PVA-based PEM with an inter-penetrating network (IPN) structure from PVA and a modified PVA polyanion containing 2-methyl-1-propanesulfonic acid (AMPS) groups as a copolymer by varying the AMPS content and chemical crosslinking conditions, to obtain PEMs with high proton conductivity, high methanol barrier properties, and high mechanical strength. This paper describes the preparation of PVA-based PEMs with various AMPS contents prepared by varying the physical crosslinking conditions to investigate the effect of the AMPS content and the crosslinking conditions on the properties of the PEM for use in a DMFC. In addition, we examine and compare the DMFC performance of the test cell constructed with the PVA-based PEMs with that using the Nafion 117 membrane at high methanol concentrations. Hence, fuel cell tests are carried out using a passive air-breathing-type DMFC test cell with a membrane-electrode assembly (MEA) prepared from the PEMs, using various methanol concentrations in the feed solution.
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and was obtained from Wako Pure Chemical Industries. Sulfuric acid and methanol were of analytical grade (Nacalai Tesque, Japan). Pt/C (46.4% Pt on high surface area carbon, Tanaka Precious Metals, Japan) was used as the cathode electrocatalyst and PteRu/C (52.9% Pt:Ru on Vulcan XC-72, Tanaka Precious Metals, Japan) was used as the anode electrocatalyst. Nafion (1100 EW, 5 wt% solution in lower aliphatic alcohols and 15e20% water) was purchased from Aldrich Co.
2.2.
Preparation of precursor membranes for PEMs
An aqueous solution of a mixture of PVA and AP-2 was cast on a plastic plate and dried over a hot stage (Nissin Co., Japan, NH-45N) at 50 C for 24 h to afford a self-standing precursor membrane for fabricating the PEMs (thickness ca. 0.1 mm). The molar percentage of the proton-exchange groups in the obtained precursor membrane, CAMPS, was controlled by changing the PVA/AP-2 ratio in the casting solution.
2.3.
Crosslinking of the precursor membranes
To obtain the PEMs, the precursor membranes were physically crosslinked by annealing them, followed by chemical crosslinking with GA solutions as follows. Square samples of the precursor membranes (5.0 cm 5.0 cm) were annealed at a predetermined temperature for 30 min, under vacuum, to increase the degree of crystallinity of PVA and the AP-2 segments. Parts (a) and (b) of Fig. 1 show schematic diagram of the membrane structure of a PVA-based PEM without and with the annealing process, respectively. The crystalline regions of the PEM will increase after the annealing process, and act as physical crosslinking points between the PVA and AP-2 chains. After annealing, the membranes were soaked in saturated aqueous Na2SO4 solutions for 24 h at 25 C and then soaked in a crosslinking solution, which consisted of 0.01 vol% GA, 0.1 M H2SO4 as an acid catalyst and saturated aqueous Na2SO4, for 24 h at 25 C.
2.4. water
Measurement of the degree of swelling in deionized
The degree of swelling of the PEMs in deionized water was measured as follows: the PEM was weighed in the dry state and then immersed in deionized water at 25 C for 1 week. The PEM was removed from the water, tapped with filter paper to remove excess surface water, and then weighed in the wet state. The degree of swelling in deionized water, DS(0), was calculated from the weights in the wet state, WW, and in the dry state, WD, as DS(0) ¼ WW/WD.
2.
Experimental
2.5.
2.1.
Materials
The relationship between the crystallinity of the PVA precursor membranes and the annealing temperature was investigated using differential scanning calorimetric (DSC) experiments, which were performed with a DSC apparatus (Mac Science Ltd., MTC 1000S and DSC3100S). In a typical experiment, 5e10 mg of a predried membrane sample was placed in an aluminum pan and heated at a scanning speed of 10 C/min up to 240 C. The melting endotherm, DH, was
Poly(vinyl alcohol) (PVA, 100% hydrolyzed, average Mw 198,000) and poly(vinyl alcohol)-co-2-methyl-1-propanesulfonic acid (AP-2, 100% hydrolyzed) were obtained from Kuraray Co., Ltd., Japan. AP-2 is a modified PVA that contains 2 mol% of 2-methyl1-propanesulfonic acid (AMPS) groups as a copolymer. Glutaraldehyde (GA) (25 wt% solution in water) was of analytical grade
Thermal analysis
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Fig. 1 e Schematic diagram of the change in the crystalline region (Zone A) of PVA-based PEM induced by annealing process. (a) Without annealing, (b) with annealing.
determined and the degree of crystallinity, X, was determined by dividing DH by the heat of crystallization of 100% crystalline PVA, DHc 37.3 cal/g, as determined in Ref. [33].
IEC ¼
CKþ 100 Wd 1000
2.8. 2.6.
Measurement of proton conductivity
Mechanical properties
The mechanical properties of the PEMs in the wet state were measured as follows. The PEMs were equilibrated in deionized water for at least 24 h prior to the experiments. Specimens with a nominal 20-mm gauge length and ca. 0.1-mm thickness were punched out from the PEM with a dumbbell-type punch. The mechanical strength was determined with a table modeltesting machine (Shimadzu CO., Japan EZ-Test 50 N). For the static stressestrain curves, the specimens were deformed under tension at a constant strain rate of 20 mm/min at 25 C, and stressestrain data were determined to failure.
Each PEM sample was immersed in 0.10 M HCl for 1 h. The PEM was removed from the solution, tapped with filter paper to remove excess HCl solution on the surface, and then sandwiched with two stainless steel electrodes. The proton conductivity of the PEMs at 25 C was determined by an AC impedance technique using an electrochemical impedance analyzer (Hioki, Japan 3532-80), where the AC frequency was scanned from 100 kHz to 4 Hz at a voltage amplitude of 0.05 V. The proton conductivity was calculated from the following equation: s¼
2.7.
(1)
Measurement of ion-exchange capacity (IEC)
IEC is expressed as milliequivalents per gram of membrane (meq/g-dry-PEM) and was determined as follows: a sample membrane was immersed in 0.10 M KCl solution for 3 h before measuring the IEC. The membrane was rinsed with deionized water to remove non-exchanged KCl electrolyte adsorbed on the membranes and was then immersed in 50 cm3 of 0.20 M NaNO3 for 12 h, under stirring, to achieve the complete exchange of Kþ ions in the membrane with Naþ ions in the solution. The concentration of Kþ ions in the solution, CKþ, was determined using an ion chromatograph (Japan Dionex Co. Japan ICS-1500). The membrane was dried under vacuum for 24 h and was weighed in the dry state, Wd. The IEC of the precursor membrane was calculated using the following equation:
d RS
(2)
where s is the proton conductivity and d, R, and S are the thickness, measured impedance, and the surface area of the PEM, respectively. The PVA-based PEMs are more rigid than Nafion 117 even in the wet state, and there is a minor degree of ruggedness on the surface of the laboratory-made PEMs. Hence, the electrical resistance on the interface between the electrode and the surface of a water-equilibrated PEM during the conductivity measurements will not be negligible. Therefore, we measured the conductivity after immersing the PEMs in 0.10 M HCl, CHCl. The contribution of Cl ions in a sample PEM on the value of the proton conductivity will be negligible under the measurement conditions that the charge density, Cx, of the proton-exchange groups in the PEMs, which is defined as the ion-exchange capacity divided by the water content, is higher than 0.3 M. This is because that: the conductivity of an ion is the product of the mobility and
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concentration of the ions. The mobility of Cl ions in an aqueous solution is about 1/5 of that of Hþ ions. The Donnan equilibrium theory gives the concentration ratio of Hþ ions to Cl ions in a PEM, CH =CCl , as CH ¼ K2 CCl
(3)
Cx K¼ 2CHCl
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 Cx Cx þ1 z 2CHCl CHCl
ðwhen Cx > CHCl Þ
(4)
where K is the Donnan equilibrium constant. Therefore, the contribution of Cl ions in a PEM to the proton conductivity measurement is less than 1/45 of that of Hþ ions when Cx of a sample PEM is more than 0.3 M.
2.9.
Measurement of methanol permeability
Permeation experiments were performed using an acrylic plastic cell comprising two parts separated by a sample PEM. The PEMs were equilibrated in a 10 M methanol aqueous solution for at least 24 h prior to the experiments. One chamber (chamber I) of the cell was filled with 10 M methanol solution, and the other (chamber II) was filled with deionized water. The volumes of chambers I and II were 400 and 100 cm3, respectively. The effective membrane area of the apparatus was 2.54 cm2. Methanol diffused from chamber I to chamber II, based on the concentration gradient between the two chambers, which were well stirred during the measurements. The concentration of methanol in chamber II (CII) was measured using high-performance liquid chromatography (HPLC). The HPLC measurements were conducted at 40 C, at a 0.4 mL/min flow rate of ultrapure water, using a pump (Jasco, Japan PU-980 Intelligent HPLC Pump), HPLC column (Wako Co. Japan Sil-II 3C18 AR), a refractive index monitor (Jasco RI-2031 Plus Intelligent RI Detector) and an auto sampler (Jasco AS2055 Plus Intelligent Sampler). The methanol permeability, P, through the PEM was then determined from the slope of the timeeconcentration curve of methanol in chamber II as expressed in the following equation: VII d DCII P¼ I Dt C0 CII0 S
(5)
where CI0 and CII0 are the initial methanol concentrations in chambers I and II, respectively. VII is the volume of chamber II and S and d are the area and thickness of the sample PEM, respectively. DCII =Dt is the initial slope of the timeeconcentration curve for chamber II.
2.10.
2.11. Fabrication of MEA and evaluation of single-cell performance Laboratory-made electrodes prepared in the following manner were used to fabricate the membrane-electrode assembly (MEA). Catalyst slurry was prepared using a combination of PteRu/C and 5 wt% Nafion solution as the anode ink. The catalyst ink was coated on carbon cloth (EC-CC1-060, ElectroChem. Inc., Japan) and carbon paper (MFG-070, Mitsubishi Rayon Co., Ltd., Japan) by a casting method to give a metal loading of 2.2 mg cm2 on the anode. The catalyst for the cathode was 46.4% Pt/C on the carbon cloth and carbon paper with a catalyst loading of 1.4 mg cm2. The binder loadings on the anode and cathode were 0.92 mg cm2 and 0.46 mg cm2, respectively. The electrodes were simply placed on either side of a PEM sample, and no special treatment such as hot-pressing of the electrodes and the PEM was used to optimize the electrode/membrane interface. The active electrode area for a single-cell test was 5.0 cm2. For comparison, experiments were also carried out with MEA fabricated from Nafion 117 under similar experimental conditions. Single-cell performance was measured using a passive airbreathing DMFC test cell with a predetermined concentration of methanol solution. Methanol was allowed to diffuse into the anode catalyst layer from the built-in reservoir of 4.5 mL capacity, while oxygen from the surrounding air diffused into the cathode catalyst layer through the opening of the cathode fixture. The single-cell performance experiments were carried out at room temperature (298 K) and at atmospheric pressure. Polarization curves were obtained using a potentiostat (HA301, Hokuto Denko Corp., Japan) equipped with a function generator (HB-104, Hokuto Denko Corp., Japan) and recorder (WX1000, Graphtec Corp., Japan).
3.
Results and discussion
3.1.
Thermal analysis
The degrees of crystallinity (X) of the precursor membranes were determined using DSC analysis, and are reported in Table 1. The degree of crystallinity of the precursor membranes increases with increasing annealing temperature. Clearly, the annealing process can lead to significant crystallization, as the values of X reached levels as high as 33%. The crystallites acted as physical crosslinks forming a physical network [22]; hence, when the precursor membranes were
Degree of swelling of a PEM in methanol solution
The degree of swelling of a PEM in methanol solution was measured as follows: a PEM was weighed in the dry state and then immersed in an aqueous solution of methanol at a predetermined concentration at 25 C, for 1 week. The PEM was removed from the solution, tapped with filter paper to remove excess solution on the surface, and then weighed in the wet state. The degree of swelling in methanol solution, DSðCMeoH Þ ¼ WW =WD , was calculated from the weights in the wet state, WW, and in the dry state, WD.
Table 1 e Relationship between the degree of crystallinity, X, and annealing temperature, Ta, of the precursor membranes. Precursor membrane PM-1 PM-2 PM-3 PM-4 CAMPS 1.8 mol%.
Ta [ C]
X [e]
50 180 185 190
24 28 32 33
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immersed in an aqueous solution of Na2SO4 at 25 C for pretreatment during the chemical crosslinking process, the membranes did not dissolve, but, rather, became swollen in the solution.
3.2. Ion-exchange capacity of the PEMs as a function of AMPS content Fig. 2 shows the ion-exchange capacity of the PEMs as a function of the AMPS content, CAMPS. The IEC values of all of the PEMs are almost proportional to CAMPS, with very little inter-membrane variation in the values, independent of the annealing temperature. This indicates that the thermal decomposition of the proton-exchange groups of the PEMs induced by the annealing process is negligible. The IEC of the PEMs with a CAMPS value of 2.0 mol% is 0.30 meq/g-drymembrane, which is almost one-third the value for Nafion 117 (0.91 meq/g-dry-membrane).
3.3.
Degree of swelling of the PEMs in deionized water
Fig. 3 shows the degree of swelling of the PEMs in deionized water, DS(0), as a function of the AMPS content, CAMPS. DS(0) increases slightly with increasing CAMPS because the osmotic pressure in the PEMs increases with an increase in the number of charged groups in the PEMs. DS(0) decreases with increasing annealing temperature, Ta. In general, the degree of swelling of a PEM decreases as the degree of crosslinking increases. Hence, the decrease in DS(0) with increasing Ta implies that the number of physical crosslinking points of PVA and AP-2 increases with increasing Ta.
3.4.
Mechanical properties of the PEMs
Parts (a) and (b) of Fig. 4 show the tensile strength at the breaking point and the Young’s modulus of the PEMs as a function of the CAMPS value. Both the tensile strength and Young’s modulus of the PEMs decrease with increasing CAMPS
Fig. 3 e The degree of swelling of the PEMs in deionized water, DS(0), as a function of AMPS content, CAMPS. Markers in this figure are the same as those in Fig. 2.
because the water content of the PEMs increases with increasing CAMPS. The tensile strength and the Young’s modulus both increase with increasing Ta. This means that the increase in the number of physical crosslinking points gives the PEMs high mechanical strength. The values of the tensile strength and Young’s modulus of the Nafion 117 membrane under the same conditions are 44 and 77 MPa, respectively. Hence, the PVA-based PEMs annealed at 190 C have approximately half the mechanical strength of Nafion 117.
3.5. Proton conductivity of the PEMs as a function of AMPS content Fig. 5 shows the proton conductivity of the PEMs as a function of CAMPS. The proton conductivity increases with increasing CAMPS on a curve, independent of the values of Ta, because the number of proton-exchange groups increases with increasing CAMPS. The proton conductivity through Nafion 117 under the same conditions is 0.90 mS cm1. Hence, the proton conductivity of the PVA-based PEM prepared under the conditions of CAMPS 2.0 mol% and Ta 190 C is about one-seventh that of Nafion 117. The charge densities, Cx, of the PEMs estimated from the IEC and water content of the PEM are about 0.6 M. Hence, the contribution of Cl ions in the PEMs on the value of the proton conductivity will be negligible in the measurement of the PEMs.
3.6. Methanol permeability of the PEMs as a function of AMPS content
Fig. 2 e Ion-exchange capacity of PEMs as a function of AMPS content. Annealing temperature: solid circles: 180 C; open circles: 185 C; solid triangles: 190 C.
Fig. 6 shows the methanol permeability coefficient of the PEMs as a function of CAMPS. The permeability coefficient of methanol through the PEM annealed at 180 C increases with increasing CAMPS. The permeability coefficient increases more steeply at CAMPS 1.5 mol% because of the somewhat steep increase in DS(0), as shown in Fig. 3. The PEMs annealed at temperatures above 185 C have a lower methanol
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reducing the efficiency of the fuel cell. Hence, in order to evaluate the performance of a PEM for DMFC, the proton permselectivity is defined as the division of the proton conductivity by the methanol permeability coefficient: s fh P
(6)
The higher the proton permselectivity of a PEM, the more suitable it is for DMFC. Fig. 7 shows the proton permselectivity of the PEMs as a function of CAMPS. The proton permselectivity of the PEMs annealed above 185 C increases with increasing CAMPS. The permselectivity of the samples annealed at 180 C is lower than that of the other PEMs and has a maximum value at CAMPS 1.5 mol%. The increase in the profile of the proton permselectivity in the initial stage is a result of the proton conductivity increasing more steeply than the methanol permeability. At the last stage of the profile, the permselectivity decreases with increasing CAMPS because the methanol permeability increases steeply with increasing CAMPS, as shown in Fig. 6. The proton permselectivity through Nafion 117 measured under the same conditions is 24 103 S cm3 s. Hence, the proton permselectivity of all of the PEMs was higher than that of Nafion 117. The PEM prepared under the conditions of CAMPS 2.0 mol% and Ta ¼ 190 C had a permselectivity that was ca. three times higher than that of Nafion 117. In a previous study [31], the PEM crosslinked with a high concentration of GA, CGA 0.35 vol%, had a higher proton selectivity than that crosslinked with CGA 0.01 vol%. However, the former is too brittle for use in the preparation of an MEA. Hence, the PEMs in this study were chemically crosslinked with a low concentration of GA solution (CGA 0.01 vol%).
3.8. Degree of swelling of the PEMs and Nafion 117 in methanol solution Fig. 4 e Mechanical properties of the PEMs as a function of AMPS content, CAMPS. (a) Tensile strength at breaking point and (b) Young’s modulus. Markers in this figure are the same as those in Fig. 2.
Fig. 8 shows the degree of swelling of the PEMs and Nafion 117 as a function of methanol concentration. The degree of
permeability than the samples annealed at 180 C because the DS(0) value of the PEMs annealed at 185 and 190 C are almost the same, and lower than that of the PEMs annealed at 180 C. The permeability coefficient through the PEM prepared under the conditions of CAMPS 2.0 mol% and Ta ¼ 190 C is about 30 times lower than that through Nafion 117 under the same measurement conditions (P ¼ 3.0 106 cm2 s1) (Fig. 6).
3.7. Proton permselectivity of the PEMs as a function of AMPS content PEMs for DMFC applications should concomitantly possess high proton conductivity and a low permeability coefficient for methanol. Methanol migration from the anode to the cathode through the PEM, referred to as methanol crossover, increases the over-potential of the cathode as a result of poisoning of the cathode catalyst, thereby significantly
Fig. 5 e Proton conductivity of PEMs at 25 C as a function of AMPS content, CAMPS. Markers in this figure are the same as those in Fig. 2.
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Fig. 6 e Methanol permeability coefficient of PEMs at 25 C as a function of AMPS content, CAMPS. Methanol concentration: 10 M. Markers in this figure are the same as those in Fig. 2. The permeability coefficient of Nafion 117 is 3.0 3 10L6 cm2sL1.
swelling of Nafion 117 increases with increasing methanol concentration, whereas that of the PVA-based PEMs decreases because methanol is a poor solvent for PVA. The permeability coefficient of methanol through a PEM increases as the degree of swelling increases [31]. Hence, the PVA-based PEMs will have better DMFC performance than Nafion 117 at high methanol concentrations.
3.9.
Single-cell performance in DMFC
Fig. 9 shows the single-cell performance of the passive airbreathing DMFC cell using one of the PVA-based PEMs
Fig. 7 e Proton permselectivity of the PEMs at 25 C as a function of AMPS content, CAMPS. Markers in this figure are the same as those in Fig. 2. The broken line shows the data for Nafion 117 measured under the same conditions.
(CAMPS ¼ 2.0 mol% and Ta ¼ 190 C). For comparison, the single-cell performance of Nafion 117 as a proton-conducting electrolyte under similar conditions is also shown. The thickness of the PVA-based PEMs used in the cell performance test is 95 mm; hence, the thickness is ca. half that of Nafion 117 (thickness 180 mm). The polarization and power density curves were obtained at a methanol concentration of 2 M. The test cell employing the PVA-based PEM (CAMPS ¼ 2.0 mol% and Ta ¼ 190 C) delivers a maximum power density (Pmax) of 2.4 mW cm2, whereas the cell employing Nafion 117 has a power density of 7.9 mW cm2. The opencircuit voltages (OCV) for the cell with the PVA-based PEM and that with Nafion 117 are 0.54 and 0.55 V, respectively. In other studies on passive-type DMFCs using PVA-based PEMs presented for comparison, Lin et al. [26] reported a Pmax of 5.0 mW cm2 using hybrid membranes composed of PVA and phosphotungstic acid (PVA/PWA) at room temperature, using a 2.0 M methanol solution fed by a small pump at a flow rate of 3 mL min1. Helen et al. [28] reported a Pmax of 6.0 mW cm2 with a hybrid membrane based on the cesium salt of a heteropolyacid, zirconium phosphate and PVA (PVAeZrPeCs2STA) at room temperature, where 4 M methanol solution was fed by diffusion from the reservoir. Fig. 10 shows the OCV and Pmax of the test cell employing the PVA-based PEM, as well as that of the cell with Nafion117, as a function of methanol concentration in the feed solution. The OCV of the PVA-based PEM decreases gradually with increasing methanol concentration; however, that of Nafion 117 decreases drastically in the concentration range over 10 M. This is attributed to the fact that the poisoning of the laboratory-made cathode Pt electrode in the MEA with Nafion 117 becomes remarkable at high methanol concentrations, though the thickness of Nafion 117 is about twice as large as that of the PVA-based PEM. This is because Nafion 117 has a higher methanol permeability coefficient than the PVA-based PEMs, as shown in Fig. 6. Moreover, the increase in the degree of swelling shown in Fig. 8 indicates that Nafion 117 will have a much higher methanol permeability at high methanol concentrations.
Fig. 8 e The degree of swelling of the PEMs and Nafion 117 in methanol solution, DS(CMeOH), as a function of methanol concentration. Markers in this figure are the same as those in Fig. 2. Data for Nafion 117 are shown by cross makers.
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The Pmax of Nafion 117 is higher than that of the PVAbased PEM at low methanol concentrations because Nafion 117 has a higher proton conductivity than the PVA-based PEM. However, the Pmax of Nafion 117 decreases drastically with increasing methanol concentration and approaches zero at concentrations above 10 M. In contrast, the Pmax of the PVAbased PEM maintains an almost constant value. Hence, at high methanol concentrations, the test cell using the PVAbased PEM has a higher performance than the test cell using Nafion 117. Park et al. [6] reported that the power density of a single cell with Nafion 115 operating at 60 C with a 10 M methanol feed concentration was ca. 20 mW cm2, which is only 15% of the power density achieved with a 1 M methanol feed (ca. 137 mW cm2). Lin et al. [4] reported that the DMFC test cells using prestretched recast Nafion (draw ratio of 4, three-layer stack, with a total thickness of 180 mm) and Nafion 117 had respective power densities of 40 and 27 mW cm2 at 10 M, which are 50% and 45% of those at the respective 1 M values (ca. 80 and 60 mW cm2, respectively). In this study, the power density of the MEA with Nafion 117 is 3.5 mW cm2 with a 10 M methanol feed, which is 44% of that obtained with 2 M methanol (7.9 mW cm2). Some researchers have reported the DMFC performance at methanol feed concentrations greater than 10 M using passive-type cells with Nafion 117 and a methanol barrier layer between the fuel reservoir and the anode current collector [8e11]. However, to our knowledge, there are no reports of the performance of passive-type test cells at high methanol concentrations (>10 M) using an MEA without any methanol barrier layer. In this study, the power density of the MEA with Nafion 117 decreases to zero at methanol feed concentrations greater than 12 M. This may be because the methanol barrier property and/or anti-poisoning property of our laboratory-made electrodes were lower than those of commercially available electrodes for DMFC such as those of Johnson Matthey, UK. Hence, the comparative effect
Fig. 9 e Cell voltage (circles) and power density (triangles) of the DMFC test cell. Cell temperature: 25 C; [MeOH]: 2 M. Solid symbols: PVA-based PEM (CAMPS [ 2.0 mol% and Ta 190 C), open symbols: Nafion 117.
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of the decrease in power density caused by methanol crossover on the cell performances of the Nafion and the PVAbased PEMs was more pronounced in our MEA with the laboratory-made electrodes. DMFC cell performance using an MEA with commercially available electrodes may not show the drastic decrease in power density at high methanol concentrations which is shown in our data. The PVA-based PEMs in this study were prepared from the random type polyanion: poly(vinyl alcohol-co-2-methyl-1-propanesulfonic acid). The PEMs are brittle in a completely dry state (after drying them under vacuum for 24 h). The MEA constructed with the PEMs has low-efficient properties for use in DMFC electrodes because of the low proton conductivity as shown in Fig. 5. However, the MEA works at high methanol concentrations in spite of the low proton conductivity. The IEC of the PVA-based PEM used in the performance test is about onethird that of Nafion 117. The proton conductivity of PVAbased PEMs will increase with increasing IEC. PVA-based PEMs prepared from block type polyanion: poly(vinyl alcohol-b-styrene sulfonic acid) [34] have higher IEC than the
Fig. 10 e OCV and Pmax of the cell with the PVA-based PEM and the cell with Nafion 117. Solid circles, PVA-based PEM (CAMPS [ 2.0 mol% and Ta 190 C); open circles: Nafion 117.
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PVA-based PEMs in this study. The block type PVA-based PEMs have almost the same permselectivity for cations as a commercially available cation-exchange membrane. The block type PEMs have higher mechanical strength than the PVA-based PEMs in this study. Hence, PVA-based PEMs having a higher IEC will be potential candidates for DMFC applications at high methanol concentrations.
[4]
[5]
[6]
4.
Conclusions
We have prepared PEMs with an inter-penetrating network structure of poly(vinyl alcohol) and poly(vinyl alcohol-co-2methyl-1-propanesulfonic acid) by varying the content of the proton-exchange groups, CAMPS, and annealing temperature, Ta. The proton conductivity and permeability of methanol through the PEMs increase with increasing CAMPS, and with decreasing Ta, because of the increase in the degree of swelling. The permeability coefficient of methanol through the PVA-based PEM prepared under the conditions of CAMPS ¼ 2.0 mol% and Ta ¼ 190 C is ca. 30 times lower than that of Nafion 117 under the same measurement conditions. The PEM also has an approximately three times higher proton permselectivity than Nafion 117. The PVA-based PEM (CAMPS ¼ 2.0 mol% and Ta ¼ 190 C) delivers a maximum power density, Pmax, of 2.4 mW cm2 at 2 M methanol concentration. This value is smaller than that of Nafion 117 and that of other PVA-based membranes (PVA/ PWA [20] and PVAeZrPeCs2STA [23]). However, the PVAbased PEM in this study shows a small decrease in Pmax at high methanol concentrations, whereas the Pmax of Nafion 117 is almost zero at methanol concentrations above 10 M. Hence, the PVA-based PEMs are potential candidates for DMFC applications at high methanol concentrations.
[7]
[8]
[9]
[10]
[11]
[12]
[13]
[14]
[15]
[16] [17]
Acknowledgements This work was supported by a Grant-in-Aid for Scientific Research (C) (No. 19560849) from the Japan Society for the Promotion of Science, Electric Technology Research Foundation of Chugoku, the Mazda Foundation, the UBE Foundation, and the Salt Science Research Foundation, No. 0612, No. 0709 and No. 0810.
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various structural conditions. Int J Hydrogen Energy 2011;36: 2237e49. Lin J, Wu P-H, Wycisk R, Pintauro PN, Shi Z. Properties of water in prestretched recast Nafion. Macromolecules 2008; 41:4284e9. Lin J, Wu P-H, Wycisk R, Trivisonno A, Pintauro PN. Direct methanol fuel cell operation with pre-stretched recast Nafion. J Power Sources 2008;183:491e7. Park Y-C, Peck D-H, Kim S-K, Lim S, Jung D-H, Lee D-Y. Operation of a direct methanol fuel cell stack by self-heating at low temperatures. Int J Hydrogen Energy 2011;36:5655e65. Yang WW, Zhao TS, Wu QX. Modeling of a passive DMFC operating with neat methanol. Int J Hydrogen Energy 2011;36: 6899e913. Abdelkareem MA, Nakagawa N. DMFC employing a porous plate for an efficient operation at high methanol concentrations. J Power Sources 2006;162:114e23. Zhang HF, Hsing IM. Flexible graphite-based integrated anode plate for direct methanol fuel cells at high methanol feed concentration. J Power Sources 2007;167:450e4. Eccarius S, Krause F, Beard K, Agert C. Passively operated vapor-fed direct methanol fuel cells for portable applications. J Power Sources 2008;182:565e79. Li X, Faghri A, Xu C. Water management of the DMFC passively fed with a high concentration methanol solution. Int J Hydrogen Energy 2010;35:8690e8. Ismail AF, Othman NH, Mustafa A. Sulfonated polyether ether ketone composite membrane using tungstosilicic acid supported on silicaealuminium oxide for direct methanol fuel cell (DMFC). J Membr Sci 2009;329:18e29. Jaafar J, Ismail AF, Matsuura T. Preparation and barrier properties of SPEEK/Cloisite 15A/TAP nanocomposite membrane for DMFC application. J Membr Sci 2009;345:119e27. Chen K, Hu Z, Endo N, Fang J, Higa M, Okamoto K. Sulfonated polyimides bearing benzimidazole groups for direct methanol fuel cell applications. J Membr Sci 2010;351:214e21. Wu H, Hou W, Wang J, Xiao L, Jiang Z. Preparation and properties of hybrid direct methanol fuel cell membranes by embedding organophosphorylated titania submicrospheres into a chitosan polymer matrix. J Power Sources 2010;195: 4104e13. Pivovar BS, Wang Y, Cussler EL. Pervaporation membranes in direct methanol fuel cells. J Membr Sci 1999;154:155e62. Chiang WY, Hu CM. Separation of liquid mixtures by using polymer membranes. I. Waterealcohol separation by pervaporation through PVA-g-MMA/MA membrane. J Appl Polym Sci 1991;43:2005e12. Rhim JW, Yeom CK, Kim SW. Modification of poly(vinyl alcohol) membranes using sulfur-succinic acid and its application to pervaporation separation of waterealcohol mixtures. J Appl Polym Sci 1998;68:1717e23. Hassan CM, Peppas NA. Structure and applications of poly(vinyl alcohol) hydrogels produced by conventional crosslinking or by freezing/thawing methods. Adv Polym Sci 2000;153:37e65. Hasimi A, Stavropoulou A, Papadokostaki KG, Sanopoulou M. Transport of water in polyvinyl alcohol films: effect of thermal treatment and chemical crosslinking. Euro Polym J; 2008:4098e107. Mallapragada SK, Peppas NA. Dissolution mechanism of semicrystalline poly(vinyl alcohol) in water. J Polym Sci, Part B: Polym Phys 1996;34:1339e46. Hickey AS, Peppas NA. Mesh size and diffusive characteristics of semicrystalline poly(vinyl alcohol) membranes prepared by freezing/thawing techniques. J Membr Sci 1995;107:229e37. Rhim JW, Park HB, Lee CS, Jun JH, Kim DS, Lee YM. Crosslinked poly(vinyl alcohol) membranes containing
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sulfonic acid group: proton and methanol transport through membranes. J Membr Sci 2004;238:143e51. Kang MS, Kim JH, Won J, Moon SH, Kang YS. Highly charged proton exchange membranes prepared by using water soluble polymer blends for fuel cells. J Membr Sci 2005;247: 127e35. Qiao J, Hamaya T, Okada T. New highly proton-conducting membrane poly(vinylpyrrolidone)(PVP) modified poly(vinyl alcohol)/2-acrylamido-2-methyl -1-propanesulfonic acid (PVAePAMPS) for low temperature direct methanol fuel cells (DMFCs). Polymer 2005;46:10809e16. Lin CW, Thangamuthu R, Yang CJ. Proton-conducting membranes with high selectivity from phosphotungstic acid-doped poly(vinyl alcohol) for DMFC applications. J Membr Sci 2005;253:23e31. Kim DS, Guiver MD, Nam SY, Yun TI, Seo MY, Kim SJ, et al. Preparation of ion-exchange membranes for fuel cell based on crosslinked poly(vinyl alcohol) with poly(styrene sulfonic acid-co-maleic acid). J Membr Sci 2006;281: 156e62. Helen M, Viswanathan B, Murthy SS. Fabrication and properties of hybrid membranes based on salts of heteropolyacid, zirconium phosphate and polyvinyl alcohol. J Power Sources 2006;163:433e9. Yang T. Preliminary study of SPEEK/PVA blend membranes for DMFC applications. Int J Hydrogen Energy 2008;33: 6772e9. Yang T. Poly(vinyl alcohol)/sulfated b-cyclodextrin for direct methanol fuel cell applications. Int J Hydrogen Energy 2009; 34:6917e24. Higa M, Sugita M, Maesowa S, Endo N. Poly(vinyl alcohol)based polymer electrolyte membranes for direct methanol fuel cells. Electrochim Acta 2010;55:1445e9. Yang C-C. Fabrication and characterization of poly(vinyl alcohol)/ montmorillonite/poly(styrene sulfonic acid) proton-conducting composite membranes for direct methanol fuel cells. Int J Hydrogen Energy 2011;34:4419e31. Cha W-I, Hyon S-H, Ikada Y. Gel spinning of poly(vinyl alcohol) from dimethyl sulfoxide/water mixture. J Polym Sci, Part B: Polym Phys 1994;32:297e304.
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[34] Higa M, Nishimura Kinoshita K, Jikihara A. Characterization of cation-exchange membranes prepared from poly(vinyl alcohol) and poly(vinyl alcohol-b-styrene sulfonic acid). Int J Hydrogen Energy; 2011. doi:10.1016/j.ijhydene.2011.06.003.
Symbols CAMPS: AMPS content in the membrane, mol% CGA: Concentration of GA solution, vol% CKþ : Concentration of Kþ ions in the solution in the IEC measurements, mol dm3 CH : Concentration of Hþ ions in a PEM in the proton conductivity measurements, mol dm3 CCl : Concentration of Cl ions in a PEM in the proton conductivity measurements, mol dm3 CHCl : Concentration of HCl in the solution in the proton conductivity measurements, mol dm3 CI0 : Initial concentration in chamber I, mol dm3 CII0 : Initial concentration in chamber II, mol dm3 CMeOH : Methanol concentration, mol dm3 Cx: Charge density of the proton-exchange groups of a PEM, mol dm3 DC/Dt: The initial slope of the timeeconcentration curve for chamber II, mol dm3 s1 d: Membrane thickness, cm DS(0): Degree of swelling in deionized water DSðCMeOH Þ: Degree of swelling in methanol solution of CMeOH K: The Donnan equilibrium constant P: Methanol permeability coefficient, cm2 s1 Pmax: Maximum power density, mW/cm2 R: Measured impedance, U S: Membrane surface area, cm2 Ta: Annealing temperature, C VII: Volume of chamber II, cm3 WW: Weight in wet state, g WD: Weight in dry state, g Greek letters s: Proton conductivity, mS cm1 4: Methanol permeability coefficient, Ss cm3
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Performance of passive direct methanol fuel cell with poly(vinyl alcohol)-based polymer electrolyte membranes Mitsuru Higa*, Kentaro Hatemura, Mikinori Sugita, Shin-ichi Maesowa, Megumi Nishimura, Nobutaka Endo Graduate School of Science and Engineering, Yamaguchi University, 2-16-1 Tokiwadai, Ube-city, Yamaguchi 755-8611, Japan
article info
abstract
Article history:
Polymer electrolyte membranes (PEMs) were prepared from poly(vinyl alcohol) (PVA) and
Received 7 March 2011
a modified PVA polyanion containing 2 mol% of 2-methyl-1-propanesulfonic acid (AMPS)
Received in revised form
groups as a copolymer. The effect of the AMPS content and the crosslinking conditions on
1 October 2011
the properties of the membranes was investigated in PEMs with various AMPS contents
Accepted 4 October 2011
prepared under various crosslinking conditions. The proton conductivity and the perme-
Available online 5 November 2011
ability of methanol through the PEMs increased with increasing AMPS content, CAMPS, and
Keywords:
swelling. The permeability coefficient of methanol through the PEM prepared under the
with decreasing annealing temperature, Ta, because of the increase in the degree of Polymer electrolyte membrane
conditions of CAMPS ¼ 2.0 mol% and Ta 190 C was approximately 30 times lower than that
Poly(vinyl alcohol)
of Nafion 117 under the same measurement conditions. A maximum proton permse-
Direct methanol fuel cell
lectivity of 96 103 S cm3 s, which is defined as the ratio of the proton conductivity to the
Proton conductivity
permeability of methanol, was obtained for this PEM. The permselectivity value is about
Methanol concentration
three times higher than that of Nafion 117. A passive air-breathing-type DMFC test cell
Maximum power density
constructed using the PEM delivered 2.4 mW cm2 of maximum power density, Pmax, at 2 M methanol concentration, which is smaller than the value obtained with Nafion 117. However, at high methanol concentrations (>10 M), the Pmax of the PEM decreases slightly to 1.6 mW cm2 (at a methanol concentration of 20 M), whereas the Pmax of Nafion 117 falls to almost zero. Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
Direct methanol fuel cells (DMFCs) using polymer electrolyte membranes (PEMs) represent one of the most attractive power sources because of their stable operation at relatively low temperatures, high energy density using highly concentrated methanol solutions as fuel, and their simplicity [1,2]. In particular, a passive air-breathing DMFC system consisting of an anode that absorbs methanol from the built-in reservoir and a cathode that “breathes” from the ambient atmosphere
works without any auxiliary devices such as heat exchangers, humidifiers, fuel pumps, gas blowers/compressors, etc. [3]. Commercially available perfluorinated sulfonated membranes, such as Nafion, developed by Dupont, have been studied as PEMs for DMFCs because of their generally high proton conductivity, excellent mechanical properties, and excellent chemical stability [4e7]. However, their high methanol permeability is one of the critical drawbacks in their application to PEMs for DMFC systems. In one reported method for reducing methanol crossover, Lin et al. [4]
* Corresponding author. Tel.: þ81 836859203; fax: þ81 836859201. E-mail address: [email protected] (M. Higa). 0360-3199/$ e see front matter Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2011.10.012
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prestretched a uniaxially recast Nafion membrane in order to reduce its methanol permeability. A membrane-electrode assembly (MEA) constructed using the prestretched Nafion membrane shows higher DMFC performance than that employing Nafion 117 for methanol feed concentrations within 10 M [5]. In other methods, some researchers employed a methanol barrier layer, such as a hydrophobic porous carbon plate [8], flexible graphite material [9], poly(dimethylsiloxane) membrane [10], and porous poly(tetrafluoroethylene) plate [11], between the fuel reservoir and anode current collector in a passive DMFC. Recently, there have been many attempts at using non-fluorinated membrane materials as PEMs for DMFCs, not only for lower fuel crossover, but also to provide better environmental adaptability and lowered production costs [12e15]. Poly(vinyl alcohol) (PVA) is one of the candidate polymers for use as a PEM in the DMFC system. PVA has excellent methanol barrier properties because of the dense structure afforded by strong intra- and inter-molecular hydrogen bonding [16e18]. Because PVA is a semicrystalline polymer, heat-treatment above the glass-transition temperature [19,20], as well as repeated freezeethaw cycles [19,21,22], increase the crystallinity of the polymer, which acts as a physical network [22]. PVA chains can also be chemically crosslinked using crosslinking agents such as glutaraldehyde (GA). Hence, the degree of swelling of a PEM, which plays an important role in its proton conductivity and methanol permeability, can be easily controlled by varying the annealing and/or chemical crosslinking conditions. There have been many reports on PVAbased PEMs [23e32]. In a previous study [31], we prepared a PVA-based PEM with an inter-penetrating network (IPN) structure from PVA and a modified PVA polyanion containing 2-methyl-1-propanesulfonic acid (AMPS) groups as a copolymer by varying the AMPS content and chemical crosslinking conditions, to obtain PEMs with high proton conductivity, high methanol barrier properties, and high mechanical strength. This paper describes the preparation of PVA-based PEMs with various AMPS contents prepared by varying the physical crosslinking conditions to investigate the effect of the AMPS content and the crosslinking conditions on the properties of the PEM for use in a DMFC. In addition, we examine and compare the DMFC performance of the test cell constructed with the PVA-based PEMs with that using the Nafion 117 membrane at high methanol concentrations. Hence, fuel cell tests are carried out using a passive air-breathing-type DMFC test cell with a membrane-electrode assembly (MEA) prepared from the PEMs, using various methanol concentrations in the feed solution.
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and was obtained from Wako Pure Chemical Industries. Sulfuric acid and methanol were of analytical grade (Nacalai Tesque, Japan). Pt/C (46.4% Pt on high surface area carbon, Tanaka Precious Metals, Japan) was used as the cathode electrocatalyst and PteRu/C (52.9% Pt:Ru on Vulcan XC-72, Tanaka Precious Metals, Japan) was used as the anode electrocatalyst. Nafion (1100 EW, 5 wt% solution in lower aliphatic alcohols and 15e20% water) was purchased from Aldrich Co.
2.2.
Preparation of precursor membranes for PEMs
An aqueous solution of a mixture of PVA and AP-2 was cast on a plastic plate and dried over a hot stage (Nissin Co., Japan, NH-45N) at 50 C for 24 h to afford a self-standing precursor membrane for fabricating the PEMs (thickness ca. 0.1 mm). The molar percentage of the proton-exchange groups in the obtained precursor membrane, CAMPS, was controlled by changing the PVA/AP-2 ratio in the casting solution.
2.3.
Crosslinking of the precursor membranes
To obtain the PEMs, the precursor membranes were physically crosslinked by annealing them, followed by chemical crosslinking with GA solutions as follows. Square samples of the precursor membranes (5.0 cm 5.0 cm) were annealed at a predetermined temperature for 30 min, under vacuum, to increase the degree of crystallinity of PVA and the AP-2 segments. Parts (a) and (b) of Fig. 1 show schematic diagram of the membrane structure of a PVA-based PEM without and with the annealing process, respectively. The crystalline regions of the PEM will increase after the annealing process, and act as physical crosslinking points between the PVA and AP-2 chains. After annealing, the membranes were soaked in saturated aqueous Na2SO4 solutions for 24 h at 25 C and then soaked in a crosslinking solution, which consisted of 0.01 vol% GA, 0.1 M H2SO4 as an acid catalyst and saturated aqueous Na2SO4, for 24 h at 25 C.
2.4. water
Measurement of the degree of swelling in deionized
The degree of swelling of the PEMs in deionized water was measured as follows: the PEM was weighed in the dry state and then immersed in deionized water at 25 C for 1 week. The PEM was removed from the water, tapped with filter paper to remove excess surface water, and then weighed in the wet state. The degree of swelling in deionized water, DS(0), was calculated from the weights in the wet state, WW, and in the dry state, WD, as DS(0) ¼ WW/WD.
2.
Experimental
2.5.
2.1.
Materials
The relationship between the crystallinity of the PVA precursor membranes and the annealing temperature was investigated using differential scanning calorimetric (DSC) experiments, which were performed with a DSC apparatus (Mac Science Ltd., MTC 1000S and DSC3100S). In a typical experiment, 5e10 mg of a predried membrane sample was placed in an aluminum pan and heated at a scanning speed of 10 C/min up to 240 C. The melting endotherm, DH, was
Poly(vinyl alcohol) (PVA, 100% hydrolyzed, average Mw 198,000) and poly(vinyl alcohol)-co-2-methyl-1-propanesulfonic acid (AP-2, 100% hydrolyzed) were obtained from Kuraray Co., Ltd., Japan. AP-2 is a modified PVA that contains 2 mol% of 2-methyl1-propanesulfonic acid (AMPS) groups as a copolymer. Glutaraldehyde (GA) (25 wt% solution in water) was of analytical grade
Thermal analysis
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Fig. 1 e Schematic diagram of the change in the crystalline region (Zone A) of PVA-based PEM induced by annealing process. (a) Without annealing, (b) with annealing.
determined and the degree of crystallinity, X, was determined by dividing DH by the heat of crystallization of 100% crystalline PVA, DHc 37.3 cal/g, as determined in Ref. [33].
IEC ¼
CKþ 100 Wd 1000
2.8. 2.6.
Measurement of proton conductivity
Mechanical properties
The mechanical properties of the PEMs in the wet state were measured as follows. The PEMs were equilibrated in deionized water for at least 24 h prior to the experiments. Specimens with a nominal 20-mm gauge length and ca. 0.1-mm thickness were punched out from the PEM with a dumbbell-type punch. The mechanical strength was determined with a table modeltesting machine (Shimadzu CO., Japan EZ-Test 50 N). For the static stressestrain curves, the specimens were deformed under tension at a constant strain rate of 20 mm/min at 25 C, and stressestrain data were determined to failure.
Each PEM sample was immersed in 0.10 M HCl for 1 h. The PEM was removed from the solution, tapped with filter paper to remove excess HCl solution on the surface, and then sandwiched with two stainless steel electrodes. The proton conductivity of the PEMs at 25 C was determined by an AC impedance technique using an electrochemical impedance analyzer (Hioki, Japan 3532-80), where the AC frequency was scanned from 100 kHz to 4 Hz at a voltage amplitude of 0.05 V. The proton conductivity was calculated from the following equation: s¼
2.7.
(1)
Measurement of ion-exchange capacity (IEC)
IEC is expressed as milliequivalents per gram of membrane (meq/g-dry-PEM) and was determined as follows: a sample membrane was immersed in 0.10 M KCl solution for 3 h before measuring the IEC. The membrane was rinsed with deionized water to remove non-exchanged KCl electrolyte adsorbed on the membranes and was then immersed in 50 cm3 of 0.20 M NaNO3 for 12 h, under stirring, to achieve the complete exchange of Kþ ions in the membrane with Naþ ions in the solution. The concentration of Kþ ions in the solution, CKþ, was determined using an ion chromatograph (Japan Dionex Co. Japan ICS-1500). The membrane was dried under vacuum for 24 h and was weighed in the dry state, Wd. The IEC of the precursor membrane was calculated using the following equation:
d RS
(2)
where s is the proton conductivity and d, R, and S are the thickness, measured impedance, and the surface area of the PEM, respectively. The PVA-based PEMs are more rigid than Nafion 117 even in the wet state, and there is a minor degree of ruggedness on the surface of the laboratory-made PEMs. Hence, the electrical resistance on the interface between the electrode and the surface of a water-equilibrated PEM during the conductivity measurements will not be negligible. Therefore, we measured the conductivity after immersing the PEMs in 0.10 M HCl, CHCl. The contribution of Cl ions in a sample PEM on the value of the proton conductivity will be negligible under the measurement conditions that the charge density, Cx, of the proton-exchange groups in the PEMs, which is defined as the ion-exchange capacity divided by the water content, is higher than 0.3 M. This is because that: the conductivity of an ion is the product of the mobility and
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concentration of the ions. The mobility of Cl ions in an aqueous solution is about 1/5 of that of Hþ ions. The Donnan equilibrium theory gives the concentration ratio of Hþ ions to Cl ions in a PEM, CH =CCl , as CH ¼ K2 CCl
(3)
Cx K¼ 2CHCl
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 Cx Cx þ1 z 2CHCl CHCl
ðwhen Cx > CHCl Þ
(4)
where K is the Donnan equilibrium constant. Therefore, the contribution of Cl ions in a PEM to the proton conductivity measurement is less than 1/45 of that of Hþ ions when Cx of a sample PEM is more than 0.3 M.
2.9.
Measurement of methanol permeability
Permeation experiments were performed using an acrylic plastic cell comprising two parts separated by a sample PEM. The PEMs were equilibrated in a 10 M methanol aqueous solution for at least 24 h prior to the experiments. One chamber (chamber I) of the cell was filled with 10 M methanol solution, and the other (chamber II) was filled with deionized water. The volumes of chambers I and II were 400 and 100 cm3, respectively. The effective membrane area of the apparatus was 2.54 cm2. Methanol diffused from chamber I to chamber II, based on the concentration gradient between the two chambers, which were well stirred during the measurements. The concentration of methanol in chamber II (CII) was measured using high-performance liquid chromatography (HPLC). The HPLC measurements were conducted at 40 C, at a 0.4 mL/min flow rate of ultrapure water, using a pump (Jasco, Japan PU-980 Intelligent HPLC Pump), HPLC column (Wako Co. Japan Sil-II 3C18 AR), a refractive index monitor (Jasco RI-2031 Plus Intelligent RI Detector) and an auto sampler (Jasco AS2055 Plus Intelligent Sampler). The methanol permeability, P, through the PEM was then determined from the slope of the timeeconcentration curve of methanol in chamber II as expressed in the following equation: VII d DCII P¼ I Dt C0 CII0 S
(5)
where CI0 and CII0 are the initial methanol concentrations in chambers I and II, respectively. VII is the volume of chamber II and S and d are the area and thickness of the sample PEM, respectively. DCII =Dt is the initial slope of the timeeconcentration curve for chamber II.
2.10.
2.11. Fabrication of MEA and evaluation of single-cell performance Laboratory-made electrodes prepared in the following manner were used to fabricate the membrane-electrode assembly (MEA). Catalyst slurry was prepared using a combination of PteRu/C and 5 wt% Nafion solution as the anode ink. The catalyst ink was coated on carbon cloth (EC-CC1-060, ElectroChem. Inc., Japan) and carbon paper (MFG-070, Mitsubishi Rayon Co., Ltd., Japan) by a casting method to give a metal loading of 2.2 mg cm2 on the anode. The catalyst for the cathode was 46.4% Pt/C on the carbon cloth and carbon paper with a catalyst loading of 1.4 mg cm2. The binder loadings on the anode and cathode were 0.92 mg cm2 and 0.46 mg cm2, respectively. The electrodes were simply placed on either side of a PEM sample, and no special treatment such as hot-pressing of the electrodes and the PEM was used to optimize the electrode/membrane interface. The active electrode area for a single-cell test was 5.0 cm2. For comparison, experiments were also carried out with MEA fabricated from Nafion 117 under similar experimental conditions. Single-cell performance was measured using a passive airbreathing DMFC test cell with a predetermined concentration of methanol solution. Methanol was allowed to diffuse into the anode catalyst layer from the built-in reservoir of 4.5 mL capacity, while oxygen from the surrounding air diffused into the cathode catalyst layer through the opening of the cathode fixture. The single-cell performance experiments were carried out at room temperature (298 K) and at atmospheric pressure. Polarization curves were obtained using a potentiostat (HA301, Hokuto Denko Corp., Japan) equipped with a function generator (HB-104, Hokuto Denko Corp., Japan) and recorder (WX1000, Graphtec Corp., Japan).
3.
Results and discussion
3.1.
Thermal analysis
The degrees of crystallinity (X) of the precursor membranes were determined using DSC analysis, and are reported in Table 1. The degree of crystallinity of the precursor membranes increases with increasing annealing temperature. Clearly, the annealing process can lead to significant crystallization, as the values of X reached levels as high as 33%. The crystallites acted as physical crosslinks forming a physical network [22]; hence, when the precursor membranes were
Degree of swelling of a PEM in methanol solution
The degree of swelling of a PEM in methanol solution was measured as follows: a PEM was weighed in the dry state and then immersed in an aqueous solution of methanol at a predetermined concentration at 25 C, for 1 week. The PEM was removed from the solution, tapped with filter paper to remove excess solution on the surface, and then weighed in the wet state. The degree of swelling in methanol solution, DSðCMeoH Þ ¼ WW =WD , was calculated from the weights in the wet state, WW, and in the dry state, WD.
Table 1 e Relationship between the degree of crystallinity, X, and annealing temperature, Ta, of the precursor membranes. Precursor membrane PM-1 PM-2 PM-3 PM-4 CAMPS 1.8 mol%.
Ta [ C]
X [e]
50 180 185 190
24 28 32 33
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immersed in an aqueous solution of Na2SO4 at 25 C for pretreatment during the chemical crosslinking process, the membranes did not dissolve, but, rather, became swollen in the solution.
3.2. Ion-exchange capacity of the PEMs as a function of AMPS content Fig. 2 shows the ion-exchange capacity of the PEMs as a function of the AMPS content, CAMPS. The IEC values of all of the PEMs are almost proportional to CAMPS, with very little inter-membrane variation in the values, independent of the annealing temperature. This indicates that the thermal decomposition of the proton-exchange groups of the PEMs induced by the annealing process is negligible. The IEC of the PEMs with a CAMPS value of 2.0 mol% is 0.30 meq/g-drymembrane, which is almost one-third the value for Nafion 117 (0.91 meq/g-dry-membrane).
3.3.
Degree of swelling of the PEMs in deionized water
Fig. 3 shows the degree of swelling of the PEMs in deionized water, DS(0), as a function of the AMPS content, CAMPS. DS(0) increases slightly with increasing CAMPS because the osmotic pressure in the PEMs increases with an increase in the number of charged groups in the PEMs. DS(0) decreases with increasing annealing temperature, Ta. In general, the degree of swelling of a PEM decreases as the degree of crosslinking increases. Hence, the decrease in DS(0) with increasing Ta implies that the number of physical crosslinking points of PVA and AP-2 increases with increasing Ta.
3.4.
Mechanical properties of the PEMs
Parts (a) and (b) of Fig. 4 show the tensile strength at the breaking point and the Young’s modulus of the PEMs as a function of the CAMPS value. Both the tensile strength and Young’s modulus of the PEMs decrease with increasing CAMPS
Fig. 3 e The degree of swelling of the PEMs in deionized water, DS(0), as a function of AMPS content, CAMPS. Markers in this figure are the same as those in Fig. 2.
because the water content of the PEMs increases with increasing CAMPS. The tensile strength and the Young’s modulus both increase with increasing Ta. This means that the increase in the number of physical crosslinking points gives the PEMs high mechanical strength. The values of the tensile strength and Young’s modulus of the Nafion 117 membrane under the same conditions are 44 and 77 MPa, respectively. Hence, the PVA-based PEMs annealed at 190 C have approximately half the mechanical strength of Nafion 117.
3.5. Proton conductivity of the PEMs as a function of AMPS content Fig. 5 shows the proton conductivity of the PEMs as a function of CAMPS. The proton conductivity increases with increasing CAMPS on a curve, independent of the values of Ta, because the number of proton-exchange groups increases with increasing CAMPS. The proton conductivity through Nafion 117 under the same conditions is 0.90 mS cm1. Hence, the proton conductivity of the PVA-based PEM prepared under the conditions of CAMPS 2.0 mol% and Ta 190 C is about one-seventh that of Nafion 117. The charge densities, Cx, of the PEMs estimated from the IEC and water content of the PEM are about 0.6 M. Hence, the contribution of Cl ions in the PEMs on the value of the proton conductivity will be negligible in the measurement of the PEMs.
3.6. Methanol permeability of the PEMs as a function of AMPS content
Fig. 2 e Ion-exchange capacity of PEMs as a function of AMPS content. Annealing temperature: solid circles: 180 C; open circles: 185 C; solid triangles: 190 C.
Fig. 6 shows the methanol permeability coefficient of the PEMs as a function of CAMPS. The permeability coefficient of methanol through the PEM annealed at 180 C increases with increasing CAMPS. The permeability coefficient increases more steeply at CAMPS 1.5 mol% because of the somewhat steep increase in DS(0), as shown in Fig. 3. The PEMs annealed at temperatures above 185 C have a lower methanol
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reducing the efficiency of the fuel cell. Hence, in order to evaluate the performance of a PEM for DMFC, the proton permselectivity is defined as the division of the proton conductivity by the methanol permeability coefficient: s fh P
(6)
The higher the proton permselectivity of a PEM, the more suitable it is for DMFC. Fig. 7 shows the proton permselectivity of the PEMs as a function of CAMPS. The proton permselectivity of the PEMs annealed above 185 C increases with increasing CAMPS. The permselectivity of the samples annealed at 180 C is lower than that of the other PEMs and has a maximum value at CAMPS 1.5 mol%. The increase in the profile of the proton permselectivity in the initial stage is a result of the proton conductivity increasing more steeply than the methanol permeability. At the last stage of the profile, the permselectivity decreases with increasing CAMPS because the methanol permeability increases steeply with increasing CAMPS, as shown in Fig. 6. The proton permselectivity through Nafion 117 measured under the same conditions is 24 103 S cm3 s. Hence, the proton permselectivity of all of the PEMs was higher than that of Nafion 117. The PEM prepared under the conditions of CAMPS 2.0 mol% and Ta ¼ 190 C had a permselectivity that was ca. three times higher than that of Nafion 117. In a previous study [31], the PEM crosslinked with a high concentration of GA, CGA 0.35 vol%, had a higher proton selectivity than that crosslinked with CGA 0.01 vol%. However, the former is too brittle for use in the preparation of an MEA. Hence, the PEMs in this study were chemically crosslinked with a low concentration of GA solution (CGA 0.01 vol%).
3.8. Degree of swelling of the PEMs and Nafion 117 in methanol solution Fig. 4 e Mechanical properties of the PEMs as a function of AMPS content, CAMPS. (a) Tensile strength at breaking point and (b) Young’s modulus. Markers in this figure are the same as those in Fig. 2.
Fig. 8 shows the degree of swelling of the PEMs and Nafion 117 as a function of methanol concentration. The degree of
permeability than the samples annealed at 180 C because the DS(0) value of the PEMs annealed at 185 and 190 C are almost the same, and lower than that of the PEMs annealed at 180 C. The permeability coefficient through the PEM prepared under the conditions of CAMPS 2.0 mol% and Ta ¼ 190 C is about 30 times lower than that through Nafion 117 under the same measurement conditions (P ¼ 3.0 106 cm2 s1) (Fig. 6).
3.7. Proton permselectivity of the PEMs as a function of AMPS content PEMs for DMFC applications should concomitantly possess high proton conductivity and a low permeability coefficient for methanol. Methanol migration from the anode to the cathode through the PEM, referred to as methanol crossover, increases the over-potential of the cathode as a result of poisoning of the cathode catalyst, thereby significantly
Fig. 5 e Proton conductivity of PEMs at 25 C as a function of AMPS content, CAMPS. Markers in this figure are the same as those in Fig. 2.
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Fig. 6 e Methanol permeability coefficient of PEMs at 25 C as a function of AMPS content, CAMPS. Methanol concentration: 10 M. Markers in this figure are the same as those in Fig. 2. The permeability coefficient of Nafion 117 is 3.0 3 10L6 cm2sL1.
swelling of Nafion 117 increases with increasing methanol concentration, whereas that of the PVA-based PEMs decreases because methanol is a poor solvent for PVA. The permeability coefficient of methanol through a PEM increases as the degree of swelling increases [31]. Hence, the PVA-based PEMs will have better DMFC performance than Nafion 117 at high methanol concentrations.
3.9.
Single-cell performance in DMFC
Fig. 9 shows the single-cell performance of the passive airbreathing DMFC cell using one of the PVA-based PEMs
Fig. 7 e Proton permselectivity of the PEMs at 25 C as a function of AMPS content, CAMPS. Markers in this figure are the same as those in Fig. 2. The broken line shows the data for Nafion 117 measured under the same conditions.
(CAMPS ¼ 2.0 mol% and Ta ¼ 190 C). For comparison, the single-cell performance of Nafion 117 as a proton-conducting electrolyte under similar conditions is also shown. The thickness of the PVA-based PEMs used in the cell performance test is 95 mm; hence, the thickness is ca. half that of Nafion 117 (thickness 180 mm). The polarization and power density curves were obtained at a methanol concentration of 2 M. The test cell employing the PVA-based PEM (CAMPS ¼ 2.0 mol% and Ta ¼ 190 C) delivers a maximum power density (Pmax) of 2.4 mW cm2, whereas the cell employing Nafion 117 has a power density of 7.9 mW cm2. The opencircuit voltages (OCV) for the cell with the PVA-based PEM and that with Nafion 117 are 0.54 and 0.55 V, respectively. In other studies on passive-type DMFCs using PVA-based PEMs presented for comparison, Lin et al. [26] reported a Pmax of 5.0 mW cm2 using hybrid membranes composed of PVA and phosphotungstic acid (PVA/PWA) at room temperature, using a 2.0 M methanol solution fed by a small pump at a flow rate of 3 mL min1. Helen et al. [28] reported a Pmax of 6.0 mW cm2 with a hybrid membrane based on the cesium salt of a heteropolyacid, zirconium phosphate and PVA (PVAeZrPeCs2STA) at room temperature, where 4 M methanol solution was fed by diffusion from the reservoir. Fig. 10 shows the OCV and Pmax of the test cell employing the PVA-based PEM, as well as that of the cell with Nafion117, as a function of methanol concentration in the feed solution. The OCV of the PVA-based PEM decreases gradually with increasing methanol concentration; however, that of Nafion 117 decreases drastically in the concentration range over 10 M. This is attributed to the fact that the poisoning of the laboratory-made cathode Pt electrode in the MEA with Nafion 117 becomes remarkable at high methanol concentrations, though the thickness of Nafion 117 is about twice as large as that of the PVA-based PEM. This is because Nafion 117 has a higher methanol permeability coefficient than the PVA-based PEMs, as shown in Fig. 6. Moreover, the increase in the degree of swelling shown in Fig. 8 indicates that Nafion 117 will have a much higher methanol permeability at high methanol concentrations.
Fig. 8 e The degree of swelling of the PEMs and Nafion 117 in methanol solution, DS(CMeOH), as a function of methanol concentration. Markers in this figure are the same as those in Fig. 2. Data for Nafion 117 are shown by cross makers.
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The Pmax of Nafion 117 is higher than that of the PVAbased PEM at low methanol concentrations because Nafion 117 has a higher proton conductivity than the PVA-based PEM. However, the Pmax of Nafion 117 decreases drastically with increasing methanol concentration and approaches zero at concentrations above 10 M. In contrast, the Pmax of the PVAbased PEM maintains an almost constant value. Hence, at high methanol concentrations, the test cell using the PVAbased PEM has a higher performance than the test cell using Nafion 117. Park et al. [6] reported that the power density of a single cell with Nafion 115 operating at 60 C with a 10 M methanol feed concentration was ca. 20 mW cm2, which is only 15% of the power density achieved with a 1 M methanol feed (ca. 137 mW cm2). Lin et al. [4] reported that the DMFC test cells using prestretched recast Nafion (draw ratio of 4, three-layer stack, with a total thickness of 180 mm) and Nafion 117 had respective power densities of 40 and 27 mW cm2 at 10 M, which are 50% and 45% of those at the respective 1 M values (ca. 80 and 60 mW cm2, respectively). In this study, the power density of the MEA with Nafion 117 is 3.5 mW cm2 with a 10 M methanol feed, which is 44% of that obtained with 2 M methanol (7.9 mW cm2). Some researchers have reported the DMFC performance at methanol feed concentrations greater than 10 M using passive-type cells with Nafion 117 and a methanol barrier layer between the fuel reservoir and the anode current collector [8e11]. However, to our knowledge, there are no reports of the performance of passive-type test cells at high methanol concentrations (>10 M) using an MEA without any methanol barrier layer. In this study, the power density of the MEA with Nafion 117 decreases to zero at methanol feed concentrations greater than 12 M. This may be because the methanol barrier property and/or anti-poisoning property of our laboratory-made electrodes were lower than those of commercially available electrodes for DMFC such as those of Johnson Matthey, UK. Hence, the comparative effect
Fig. 9 e Cell voltage (circles) and power density (triangles) of the DMFC test cell. Cell temperature: 25 C; [MeOH]: 2 M. Solid symbols: PVA-based PEM (CAMPS [ 2.0 mol% and Ta 190 C), open symbols: Nafion 117.
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of the decrease in power density caused by methanol crossover on the cell performances of the Nafion and the PVAbased PEMs was more pronounced in our MEA with the laboratory-made electrodes. DMFC cell performance using an MEA with commercially available electrodes may not show the drastic decrease in power density at high methanol concentrations which is shown in our data. The PVA-based PEMs in this study were prepared from the random type polyanion: poly(vinyl alcohol-co-2-methyl-1-propanesulfonic acid). The PEMs are brittle in a completely dry state (after drying them under vacuum for 24 h). The MEA constructed with the PEMs has low-efficient properties for use in DMFC electrodes because of the low proton conductivity as shown in Fig. 5. However, the MEA works at high methanol concentrations in spite of the low proton conductivity. The IEC of the PVA-based PEM used in the performance test is about onethird that of Nafion 117. The proton conductivity of PVAbased PEMs will increase with increasing IEC. PVA-based PEMs prepared from block type polyanion: poly(vinyl alcohol-b-styrene sulfonic acid) [34] have higher IEC than the
Fig. 10 e OCV and Pmax of the cell with the PVA-based PEM and the cell with Nafion 117. Solid circles, PVA-based PEM (CAMPS [ 2.0 mol% and Ta 190 C); open circles: Nafion 117.
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PVA-based PEMs in this study. The block type PVA-based PEMs have almost the same permselectivity for cations as a commercially available cation-exchange membrane. The block type PEMs have higher mechanical strength than the PVA-based PEMs in this study. Hence, PVA-based PEMs having a higher IEC will be potential candidates for DMFC applications at high methanol concentrations.
[4]
[5]
[6]
4.
Conclusions
We have prepared PEMs with an inter-penetrating network structure of poly(vinyl alcohol) and poly(vinyl alcohol-co-2methyl-1-propanesulfonic acid) by varying the content of the proton-exchange groups, CAMPS, and annealing temperature, Ta. The proton conductivity and permeability of methanol through the PEMs increase with increasing CAMPS, and with decreasing Ta, because of the increase in the degree of swelling. The permeability coefficient of methanol through the PVA-based PEM prepared under the conditions of CAMPS ¼ 2.0 mol% and Ta ¼ 190 C is ca. 30 times lower than that of Nafion 117 under the same measurement conditions. The PEM also has an approximately three times higher proton permselectivity than Nafion 117. The PVA-based PEM (CAMPS ¼ 2.0 mol% and Ta ¼ 190 C) delivers a maximum power density, Pmax, of 2.4 mW cm2 at 2 M methanol concentration. This value is smaller than that of Nafion 117 and that of other PVA-based membranes (PVA/ PWA [20] and PVAeZrPeCs2STA [23]). However, the PVAbased PEM in this study shows a small decrease in Pmax at high methanol concentrations, whereas the Pmax of Nafion 117 is almost zero at methanol concentrations above 10 M. Hence, the PVA-based PEMs are potential candidates for DMFC applications at high methanol concentrations.
[7]
[8]
[9]
[10]
[11]
[12]
[13]
[14]
[15]
[16] [17]
Acknowledgements This work was supported by a Grant-in-Aid for Scientific Research (C) (No. 19560849) from the Japan Society for the Promotion of Science, Electric Technology Research Foundation of Chugoku, the Mazda Foundation, the UBE Foundation, and the Salt Science Research Foundation, No. 0612, No. 0709 and No. 0810.
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Symbols CAMPS: AMPS content in the membrane, mol% CGA: Concentration of GA solution, vol% CKþ : Concentration of Kþ ions in the solution in the IEC measurements, mol dm3 CH : Concentration of Hþ ions in a PEM in the proton conductivity measurements, mol dm3 CCl : Concentration of Cl ions in a PEM in the proton conductivity measurements, mol dm3 CHCl : Concentration of HCl in the solution in the proton conductivity measurements, mol dm3 CI0 : Initial concentration in chamber I, mol dm3 CII0 : Initial concentration in chamber II, mol dm3 CMeOH : Methanol concentration, mol dm3 Cx: Charge density of the proton-exchange groups of a PEM, mol dm3 DC/Dt: The initial slope of the timeeconcentration curve for chamber II, mol dm3 s1 d: Membrane thickness, cm DS(0): Degree of swelling in deionized water DSðCMeOH Þ: Degree of swelling in methanol solution of CMeOH K: The Donnan equilibrium constant P: Methanol permeability coefficient, cm2 s1 Pmax: Maximum power density, mW/cm2 R: Measured impedance, U S: Membrane surface area, cm2 Ta: Annealing temperature, C VII: Volume of chamber II, cm3 WW: Weight in wet state, g WD: Weight in dry state, g Greek letters s: Proton conductivity, mS cm1 4: Methanol permeability coefficient, Ss cm3