- Email: [email protected]
Nafion nanocomposite membranes for proton exchange membrane fuel cells
Journal of Membrane Science 377 (2011) 89–98
Contents lists available at ScienceDirect
Journal of Membrane Science journal homepage: www.elsevier.com/locate/memsci
Cesium hydrogen salt of heteropolyacids/Nafion nanocomposite membranes for proton exchange membrane fuel cells Mehdi Amirinejad a , Sayed Siavash Madaeni a,∗ , Ezzat Rafiee b , Sedigheh Amirinejad a a b
Membrane Research Center, Department of Chemical Engineering, Faculty of Engineering, Razi University, Kermanshah, Iran Department of Inorganic Chemistry, Faculty of Chemistry, Razi University, Kermanshah, Iran
a r t i c l e
i n f o
Article history: Received 2 February 2011 Received in revised form 27 March 2011 Accepted 7 April 2011 Available online 15 April 2011 Keywords: Heteropolyacid Cesium salt Nanocomposite membrane Nafion Proton exchange membrane fuel cell Stability
a b s t r a c t Cesium hydrogen salt of heteropolyacids (CsHPs) including Cs2.5 H0.5 PMo12 O40 (CsPMo) and Cs2.5 H0.5 PW12 O40 (CsPW) are incorporated into Nafion to produce stable nanocomposite membranes at the moderate temperature/low relative humidity (RH). The addition of hygroscopic and conductive CsHP enhances the water content but limits the activity of the sulfonic group of the Nafion. As a result, ion exchange capacity (IEC) is decreased. The increase in conductivity versus RH is higher. The conductivity at anhydrous and high-temperature condition is higher due to the additional water retention or additional surface functional sites. The results of oxidative stability of membranes show that the CsPW/Nafion composite membrane has superior stability against oxidative agents due to the CsPW in lowering H2 O2 diffusion. The nanocomposite membranes have better performance in the PEM fuel cell test at temperatures 60, 80 and 100 ◦ C (35% RH, ambient pressure) than plain Nafion membrane. The stability of single cells under a constant load demonstrates that the decay rate for plain Nafion membrane is rapid due to the dehydration. The covering effect for the CsPW particle is stronger than the CsPMo particle results in higher water uptake, IEC, conductivity and fuel cell performance and lower voltage decay for the CsPMo/Nafion membrane rather than the CsPW/Nafion membrane. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Heteropolyacids (HPAs) are nano-sized metal–oxygen anion clusters with unique variety in structure and strong acidity [1–3]. Their acid–base and redox properties can be varied over a wide range by changing the chemical composition [4]. The Keggin-type HPAs have received the most attention due to the ease of preparation and strong acidity. Solid HPAs possess a discrete ionic structure comprising fairly mobile basic structural heteropolyanions and countercations (H+ , H3 O+ , H5 O2 + , etc.). This unique structure exhibits extremely high proton mobility. The Keggin heteropolyanion typically represented by the formula XM12 O40 x−8 where X is the central atom (P5+ , Si4+ , etc.), x is its oxidation state, and M is the metal ion (Mo6+ or W6+ ). The M6+ ions can be substituted by many other metal ions, e.g., V5+ , Co2+ , Zn2+ , etc. The structure of heteropolyanion itself i.e. the metal oxide cluster molecule is called the primary structure. The secondary structure is the three-dimensional arrangement of the primary structure and counter cations, which corresponds to the primary particles (microcrystallites) [5]. The tertiary structure represents the manner in which the secondary structure assembles into solid
∗ Corresponding author. Tel.: +98 831 4274530; fax: +98 831 4274542. E-mail address: [email protected] (S.S. Madaeni). 0376-7388/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2011.04.014
particles which correspond to the secondary particles and relates to properties such as particle size, surface area, and pore structure [6]. Heteropolyanions are larger than the inorganic acids, so the strength of bonding between the proton and heteropoly anions should be lower, which implies that the dissociation constants should be higher as compared to the usual acids. Furthermore, greater degrees of delocalization of the charge of the anion would lower the effective negative charge on its individual basic proton accepting centers and, thus, weaken the attraction of the proton to the anion [5]. Although the HPAs such as phosphotungstic acid (H3 PW12 O40 , hereafter PWA), phosphomolybdic acid (H3 PMo12 O40 , hereafter PMA) and silicotungstic acid (H4 SiW12 O40 , hereafter SWA) in the solid state are pure Brønsted acids and stronger acids than the conventional solid acids such as SiO2 , Al2 O3 , H3 PO4 , and HX and HY zeolites [7], but the acid forms have low surface acidity (the number of acidic sites on the surface) because of the low surface area (∼5 m2 g−1 ) and are highly soluble in water. Partial substitution of the proton by a large monovalent ion such as Cs+ results in unique changes in the surface area (surface acidity) as well as the insolubility in water [8]. Peculiar changes in the activity for Csx H3−x PM12 O40 (M = W, Mo) are observed for the Cs salt maxima at x = 2.5 and has been interpreted by the change in the surface acidity [9]. For instance, the surface area, particle size and crystallite size
90
M. Amirinejad et al. / Journal of Membrane Science 377 (2011) 89–98
of Cs2.5 H0.5 PW12 O40 (Hereafter CsPW) are 135 m2 g−1 , 6.9 nm and 11.3 nm, respectively [10]. HPAs have been incorporated into the membranes to improve their structure and functionality and produce composite membranes. The HPAs increase the binding energy of water and/or proton conductivity. HPA-incorporated composite solid electrolyte for application in the fuel cell has been given more attention in recent years [11–51]. Table 1 lists recent literature review on this subject. The matrices have been reported are Nafion and Nafion composites [11–23], sulfonated polymers such as sulfonated poly(ether ether ketone) [24–29] and sulfonated polysulfone [36], hydrophilic polymers such as chitosan [39,40] and polyvinyl alcohol [42], high temperature and high mechanical resistances polymers such as polybenzimidazole [30,31] and poly(vinylidene) fluoride [34] and inorganic matrices such as P2 O5 –SiO2 [43–45] and MCM-41 [47,49]. The most HPAs reported are Keggin types like PWA, PMA and SWA. The modified HPAs such as supported PWA and PMA by SiO2 and Al2 O3 and their salt have been developed. Perfluorosulfonic acid based membranes show excellent thermal, chemical and mechanical stability in operating proton exchange membrane fuel cells (PEMFCs) at 80 ◦ C but instability is compromised at high temperature/low humidity operations. Perfluorosulfonic acid-based organic/inorganic composite membranes with various HPA additives have been investigated as alternate materials for high temperature/low humidity PEMFC operation [11–19]. The size, uniform dispersion, loading content and acid sites amount of inorganic additives can influence the ion exchange capacity and proton transport of per-fluorinated ionomer composite membrane [52]. In recent publication [21], Nafion/CsPW nanocomposite membranes for PEMFCs have been proposed. The nanocomposite membrane showed better performance than plain Nafion membrane especially at high temperature/low relative humidity (RH). Cs2.5 H0.5 PMo12 O40 (hereafter CsPMo) is more active in some reactions than CsPW such as benzylation of benzene [9]. The composite membranes using CsPMo for application in fuel cells have been reported [32,42]. Li et al. [32] investigated CsPMo/PBI/H3 PO4 composite membrane for use in PEMFCs and found that this type of composite showed higher conductivity and superior performance in PEMFCs compared to PBI/H3 PO4 membrane at high temperature/low RH. Helen et al. [42] tested cesium salt of HPAs incorporated into PVA/PAM blend membranes for direct methanol fuel cell (DMFC) and found that CsPMo/PVA/PAM membrane demonstrated higher conductivity. This study was conducted to investigate the effect of replacement of W in CsPW with Mo on the characteristics of CsPMo/Nafion nanocomposite membranes. The CsPW/Nafion nanocomposite (with similar composition) and recast Nafion (without any additive, hereafter R-Nafion) membranes were prepared at similar conditions for comparison. Thermal analysis (TGA/DTG and DSC), physico-chemical characterizations including FT-IR, water uptake, ion exchange capacity, oxidative stability, proton conductivity and PEMFC performance were conducted to investigate the stability of nanocomposite membranes at moderate temperature and low RH.
hydrate (H3 PMo12 O40 ·xH2 O) and cesium carbonate, acetonitrile and n-butylamine were obtained from Merck. 2.2. Preparation of cesium hydrogen salt of HPAs (CsPW and CsPMo) Cesium hydrogen salt of heteropolyacids (hereafter CsHPs) including CsPW and CsPMo were prepared by precipitation titration [53]. The aqueous solutions of HPA (0.08 M) and Cs2 CO3 (0.1 M) were prepared. Cs2 CO3 solution was added to 20 mL of HPA solution dropwise with stirring in room temperature. The colloidal solution was stirred overnight and then liquid phase was evaporated at 45 ◦ C. The powders of CsPW and CsPMo were obtained by heating at 300 ◦ C for 1 h. Finally, prepared powders were grinded and dried in vacuum oven at 80 ◦ C and stored in a dry atmosphere before use. 2.3. Measurement of acidity of CsHPs The acidity of CsHP powders was measured based on potentiometric titration according to the last published paper [54]. Around 0.05 g CsHP was dispersed in 90 mL acetonitrile and stirred for 3 h. The solution was titrated with n-butylamine in acetonitrile solution (0.05 N). The potential variations (mV) versus meq g−1 of amine were recorded by Hanna 302 pH meter using a double junction electrode. 2.4. Preparation of membranes Membranes were prepared by recast procedure. The solvents of Nafion solution were exchanged gradually by high boiling point solvent (NMP) at 80 ◦ C. CsHP (10 wt.% based on solid Nafion) was dispersed in NMP with stirring to avoid particle aggregation and then added to the solution and mixed for 24 h. The concentrated and viscose solution was poured onto a Petri dish. Membranes preliminarily were dried at 50 ◦ C for 12 h. The solvent evaporated completely by the vacuum at 90 ◦ C (12 h). Dried membranes were peeled off from the surface and their organic and inorganic impurities were removed by boiling in 3% H2 O2 (w/v) for 90 min. Membranes boiled in H2 SO4 0.5 M for 90 min for protonation and finally washed in boiling deionized water for 90 min. Recast Nafion membranes without inorganic filler were also prepared by the same procedure. 2.5. Fourier transform infrared (FT-IR) The infrared spectra on powders were recorded in KBr pellets using PERKIN ELMER Spectrum 2000. The spectrum obtained after multiple scans was a plot of transmittance percent against wavenumber. 2.6. Thermal analyses
2. Experimental
Thermal gravimetric analysis (TGA) of membranes was performed on TA instruments (TGA-Q50) in nitrogen atmosphere from 25 ◦ C to 600 ◦ C with the heating rate of 10 ◦ C min−1 . Differential scanning calorimetry (DSC) of membranes was studied using TA instruments (DSC-Q200) with nitrogen at scan rates of 10 ◦ C min−1 at two heating cycles and second cycle is reported.
2.1. Materials
2.7. Water uptake and ion exchange capacity (IEC)
Nafion 5% solution (EC-NS-05) was purchased from ElectroChem. 1-Methyl-2-pyrrolidone (hereafter NMP) was supplied by Sigma–Aldrich. Sodium chloride, sodium hydroxide, hydrogen peroxide and sulfuric acid were obtained from Aldrich. Phosphotungstic acid hydrate (H3 PW12 O40 ·xH2 O), phosphomolybdic acid
Water uptake (wu) of membranes at room temperature was determined by the following formula: wu (%) =
Wwet − Wdry Wdry
× 100
(1)
M. Amirinejad et al. / Journal of Membrane Science 377 (2011) 89–98
91
Table 1 Recent literature reviews on the application of heteropolyacids in the solid electrolyte of fuel cells. Entry
Matrix
Heteropolyacid
1
Nafion
• PWA and SWA • PWA • Modified PWA by large ions like Cs+ , NH4+ , Rb+ and Tl+ • Supported PWA and PMA by SiO2 , Al2 O3 , ZrO2 or TiO2 • Cs2.5 H0.5 PW12 O40 • Cs2.5 H0.5 PW12 O40 • PMA • PWA • PWA or SWA • SWA supported on SiO2 –Al2 O3 • ␥-K8 SiW10 O36 • Cs2.5 H0.5 PW12 O40 • Cs2.5 H0.5 PW12 O40 supported Pt • PWA or SWA • PWA or SWA • Cs2.5 H0.5 PMo12 O40 • H8 SiW11 O39 • PWA • PWA • PWA • PWA • PWA
2 3 4
Nafion/poly(tetraflouroethylene) (PTFE) Nafion/poly(phenylene oxide) (PPO) Sulfonated polyether ether ketone (SPEEK)
5
Polybenzimidazole (PBI)
6 7 8 9 10 11 12
Polybenzimidazole (PBI)/H3 PO4 Poly(ethylene glycol) (PEG) Poly(vinylidene fluoride) (PVdF) Sulfonated polyimides (SPI) Sulfonated polysulfone (SPS) Sulfonated polyethersulfone (SPES) Sulfonated poly(arylene ether ketone)/polyaniline (SPAES/PANI) Chitosan Poly(vinylidenedifluoride/hexafluoropropylene) (PVdF/HFP) Poly(vinyl alcohol)/polyacrylamide (PVA/PAM) P2 O5 –SiO2 Organically modified silicate MCM-41 3-Glycidoxypropyltrimethoxysilane (GPTMS) Y-zeolite Zirconia bridged hydrocarbon
13 14 15 16 17 18 19 20 21
Additional inorganic SiO2
SiO2 CsHSO4 SiO2
CsHSO4
Al2 O3
• PWA or PMA • PWA, ␣-H3 P2 W18 O62 , H6 P2 W21 O71 and H6 As2 W21 O69 • Cs2.5 H0.5 PW12 O40 and Cs2.5 H0.5 PMo12 O40 • PWA • PWA or SWA • PWA or PMA • PWA • PWA or PMA • PWA
where Wwet is the membrane weight after cleaning and when the surface water was removed and Wdry is the membrane weight after drying at 70 ◦ C. IEC was performed by classical titration. Dried membranes were immersed in NaCl (0.1 M) solution and the H+ was titrated by NaOH 0.01 N aqueous solution. The IEC was measured at least two times and the average is reported.
2.8. Oxidative stability and durability test of membranes The oxidative stability of the membrane samples was measured in Fenton test solution (3% H2 O2 solution containing 2 ppm FeSO4 ) at 80 ◦ C. The membranes were immersed in a shaking flask containing the Fenton’s reagent. The oxidative stability was evaluated by the elapsed time when the membranes start to be brittle and the retained weights of membranes for 1 h. The durability test was done by recording the voltage decay of the membrane electrode assembly (MEA) including prepared membranes in the fuel cell (H2 /O2 ) by fixation of current at 600 mA cm−2 and operating condition of 80 ◦ C, 35% RH and ambient pressure for 24 h.
2.9. Proton conductivity The proton conductivity of the membranes was measured by four-point probe technique [55]. The membrane sample (1 cm × 3 cm) was sandwiched between two Teflon parts attached with two platinum foils and two platinum wires. The impedance was measured using Autolab PGSTAT302N electrochemistry workstation in the frequency range of 100 kHz to 10 Hz. The proton conductivity (, S cm−1 ) of membranes was calculated using the
Reference [11,12] [13–17] [18] [19,20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34] [35] [36] [37] [38] [39,40] [41]
TiO2 or ZrO2
[42] [43–45] [46] [47,48] [49] [50] [51]
following equation: =
L RS
(2)
where L, R and S are the thickness (cm), the impedance () and the surface area (cm2 ) of the membranes, respectively. The conductivity values were measured at different RHs and temperature conditions. The conductivity at 25 ◦ C as a function of RH was carried out by keeping the samples (24 h) inside a chamber containing saturated salt solutions [56,57]: CaCl2 for 29% RH; Ca(NO3 )2 for 51% RH; NaNO3 for 74% RH and KNO3 for 92% RH. The RH was verified by a hygrometer before measurement. Conductivity measurements in 100% RH are carried out with the cell immersed in liquid water at the desired temperature for 12 h. The conductivity at high temperatures and 100% RH was determined by saturated water vapor. The conductivity measurements in anhydrous conditions were performed by placing the cell inside a dry controlled temperature chamber. For all conditions, the cell was allowed to reach steady state before taking any measurement. 2.10. MEA fabrication and PEMFC performance A fuel cell test station (FCT-2000, ElectroChem) connected with a 5 cm2 cell hardware (FC05-01SP, ElectroChem) was used to measure cell polarization. Commercial gas diffusion electrodes (20% Pt/C, E-Tek) with 0.4 mg Pt cm−2 loading were impregnated with 0.8 mg cm−2 of Nafion solution. The MEA was fabricated by hotpressing at 140 ◦ C and 10 atm for 90 s. H2 and O2 with the pressure of 1 atm, desired humidity and the flow rates a bit higher than stoichiometric flows were used as the anode and the cathode, respectively. The humidity of reactant gases was set at 35% to study the fuel cell performance at low RH.
92
M. Amirinejad et al. / Journal of Membrane Science 377 (2011) 89–98
Fig. 1. FT-IR of the prepared CsPW and CsPMo.
3. Results and discussion 3.1. FT-IR The FT-IR measurement of the prepared CsPW and CsPMo heteropolyacids is represented in Fig. 1. The main bands are assigned and tabulated in Table 2. The primary Keggin structure (PWA and PMA) remains unaltered in the cesium salt forms [58,59]. For CsPW/CsPMo, the bands at 1080/1060, 985/959, 891/865, 815/760 and 588/591 cm−1 are assigned to stretching vibrations of P–O, M O, M–O–M corners shared bonds, M–O–M edges shared bonds and to the bending vibration of O–P–O, respectively [40]. Absorption band at 1614/1610 indicates the presence of water ion (H5 O2 + ) in the structure of CsHPs and is assigned to ␦(H2 O) vibration [60]. The changes in the frequency characteristics of Mo–O–Mo edge and ␦(H2 O) in CsPW may be assigned to the interaction between [PM12 O40 ]3− anion and Cs+ cation [60]. Fig. 2. Thermal gravimetric response of the prepared membranes: (a) TGA and (b) DTG.
3.2. TGA/DTG Fig. 2 compares the TGA–DTG responses of the prepared membranes. Three main weight losses corresponding to major peaks in the DTG curves can be observed for membranes. Preliminary mass loss during the heating process up to 270 ◦ C is attributed to the evaporation of absorbed water molecules [61]. The first peak at 270 ◦ C is associated with the degradation of sulfonic acid groups [62]. The desulfonation rate of the CsPW/Nafion and CsPMo/Nafion nanocomposite membranes due to catalytic property of CsPW and CsPMo [10] or the interfacial interaction between CsHP and Nafion is faster than the R-Nafion membrane. The second and third weight losses are attributed to decomposition of side-chain and the PTFE backbone, respectively [14,21]. The onset temperatures for the second and third weight losses in the nanocomposite membranes are lower with faster mass losses than the plain Nafion. This effect indi-
Table 2 Properties of the prepared CsPW and CsPMo heteropolyacids. Entry 1 2 3
a
Property −1
Acidity (meq g ) Surface area (m2 g−1 ) a FTIR assignments (cm−1 ) P–O M O M–O–M (corner-sharing) M–O–M (edge-sharing) O–P–O ␦(H2 O) Data obtained from Ref. [66].
CsPW
CsPMo
0.4 >100
0.2 <50
1080 985 891 815 588 1614
1060 959 865 760 591 1610
cates that the presence of CsHP may accelerate the decomposition of CF2 backbone. 3.3. Water uptake Water content in the fuel cell is important because it affects the proton conductivity and overall system performance [63]. The water uptake of prepared membranes is represented in Table 3. Water molecules in the secondary structures connect the individual heteropoly anions through weak hydrogen bonds. In the case of [PW12 O40 ]3− , its radius is only 0.5–0.6 nm, while the spacing between ions is 23 nm, leaving therefore a considerable space between ions [64]. This space may hold water and increase water content through the nanocomposite membrane. Table 3 reveals that the water uptake of CsPMo/Nafion membrane is higher than CsPW/Nafion membrane while the acidity of CsPW is a bit higher than CsPMo (refer to Table 2). This phenomenon may be explained based on the physical effect of CsHP on Nafion cluster. Schematically, the cluster-network model [65] and the size of clusters and channels of Nafion are illustrated in Fig. 3a. The particle and crystallite sizes of CsHP [66] are not smaller than Nafion clusters to make an embedded type of nanocomposite [67] and these particles may settle on the clusters and shield the Nafion sulfonic groups (Fig. 3b). Therefore, although the addition of CsHP inorganic to Nafion matrix enhances the water content, but may limit the activity of hydrophilic sulfonic group of the Nafion cluster. The surface area of CsPW is higher than CsPMo (Table 2) and with regard
M. Amirinejad et al. / Journal of Membrane Science 377 (2011) 89–98
93
Table 3 Physico-chemical properties of prepared membranes. Entry
Membrane
Thickness (m)
Water uptake (%)
IEC (meq g−1 )
H (J gNafion −1 )
Ea (kJ mol−1 )
1 2 3
R-Nafion CsPW/Nafion CsPMo/Nafion
62 65 67
33 62 76
0.84 0.72 0.79
136 229 291
11.55 10.20 9.97
to the same structural porosity for two HPAs, the particle size of CsPMo is bigger than CsPW. Based on this investigation, the property of covering effect of CsPW particle is stronger than that for CsPMo particle (Fig. 3c). The water uptake of nanocomposite membranes is associated with two factors included sulfonic group of Nafion and high hygroscopic nature of CsHP nanoclusters. With consideration of covering effect, the higher water uptake of CsPMo/Nafion membrane rather than CsPW/Nafion membrane could be interpreted.
3.4. IEC The IEC of nanocomposite membranes is slightly lower than the plain Nafion (Table 3.). This effect is due to covering the Nafion active sites (sulfonic groups) and decreasing the effective number of replaceable ion exchange sites [68] by the salts. The greater IEC
Oxidative stability Retained weight (%)
Elapsed time (h)
92 98 95
8.5 >12 10
of the CsPMo/Nafion membrane than the CsPW/Nafion membrane may be associated with the free sulfonic group of Nafion matrix in the CsPMo than the CsPW.
3.5. DSC The strong endothermic peaks around temperatures 125–185 ◦ C for membranes in the DSC traces (Fig. 4) may be associated with the structural changes within the ionic clusters of membranes [69]. The enthalpy change corresponding to these peaks (H, Table 3) may be related to the degree of hydration of Nafion-based membranes which allows high mobility of the chain segments [69]. In other word, H values increase with increasing water content in the membrane. Sulfonic groups of the Nafion host polymer and the hygroscopic nature of CsPW particles can serve to absorb more water through the nanocomposite membrane; so the H values
Fig. 3. The schematic drawing of Nafion cluster-network model (a); shielding the active sites of Nafion clusters by CsHP particles (b); the effect of particle size on covering property (c) (CsPW has lower particle diameter than CsPMo and has higher covering effect).
94
M. Amirinejad et al. / Journal of Membrane Science 377 (2011) 89–98
Fig. 4. DSC of the prepared membranes.
of nanocomposite membranes are higher than plain Nafion (see Table 3). The settlement of CsHP on Nafion clusters may decrease the mobility of chain segments and reduction in the enthalpy change. As a result, the H of CsPW/Nafion membrane is lower than CsPMo/Nafion membrane.
Fig. 6. The conductivity values of membranes at high temperatures in anhydrous condition.
3.6. Proton conductivity For good proton conductivity, solid electrolyte should have fixed charged sites surrounded by water molecules which facilitate the transport of protons [68]. Solid HPAs possess extremely high proton mobility [4]. Unlike the rigid network structure of zeolites, HPA Keggin anions are quite mobile [4]. Proton transport in Nafion nanocomposite membranes is the result of a complex process dominated by the surface and chemical properties of both Nafion and additive [70]. Fig. 5a reveals the conductivity values at 25 ◦ C against the RH for the prepared membranes. As seen, by increasing RH of membranes, the conductivity is increasing. This effect may be interpreted by the absorption of more water by the Nafion clusters, resulting in increase of the clusters diameter and the exchange sites per clusters and promoting the conductivity [71]. The hygroscopic and conductive CsHPs may improve the water content and support this behavior. Therefore, the conductivity of nanocomposite membranes is more sensitive to humidity. The Arrhenius plot for the prepared membranes at 100% RH is presented in Fig. 5b. The conductivity of nanocomposite membranes is greater than plain Nafion. The conductivity of CsPMo/Nafion membrane is higher than CsPW/Nafion membrane. In solid HPA with crystalline water, the proton migration can be ascribed to the vehicle mechanism [72]. Based on the Arrhe-
SO3-
SO
3 -
-
SO 3
-
SO 3
H+
Proton Pathway
-
SO 3
CsHP
CsHP
SO3-
SO 3
SO
3
Fig. 5. Conductivity curves for the prepared membranes: (a) conductivity vs. RH at 25 ◦ C, (b) Arrhenius plot at 100% RH.
-
-
SO 3
H+
Fig. 7. The mechanism of proton transport in CsHP/Nafion nanocomposite membranes in anhyrous condition; Nafion backbone is not shown.
M. Amirinejad et al. / Journal of Membrane Science 377 (2011) 89–98
95
Fig. 8. The PEM fuel cell responses of membranes with H2 /O2 reactant gases as at ambient pressure and low RH (35%): (a) (V–I) curves at 60 ◦ C; (b) (P–I) curves at 60 ◦ C; (c) (V–I) curves at 80 ◦ C; (d) (P–I) curves at 80 ◦ C; (e) (V–I) curves at 100 ◦ C; (f) (P–I) curves at 100 ◦ C.
nius plot, the activation energy of the membranes can be evaluated (refer to Table 3). The activation energy may be used to investigate the mechanism of proton conduction [73]. The ions transportation of Nafion membrane is governed by Grotthus and vehicle mechanisms [74]. In the nanocomposite membranes, the CsHP can interact with the sulfonic groups of Nafion and the water hydration molecules [75], forming a network of hydrogen bonds. The free proton may be formed by the reaction of anion moiety of CsHP with the water
molecule as described by Ukshe et al. [76] (M = W, Mo): (PM12 O40 )3− + nH2 O → [(PM12 O40 )(OH)x ](3+x)− + xH+
(3)
The mechanism of proton conductivity in a given RH may be related to dynamic equilibrium between different proton moieties: H5 O2 + ↔ H3 O+ + H2 O ↔ H+ + 2H2 O
(4)
The higher conductivity of nanocomposite membranes than plain Nafion at anhydrous and high-temperature (100, 110 and
96
M. Amirinejad et al. / Journal of Membrane Science 377 (2011) 89–98
Table 4 Maximum power density of membranes obtained from the PEMFC performance at 35% RH.
1 2 3
Membrane
R-Nafion CsPW/Nafion CsPMo/Nafion
Maximum power density (mW cm−2 ) [corresponding current density (mA cm−2 )] Temperature 60 ◦ C
80 ◦ C
100 ◦ C
160 (500) 238 (900) 304 (800)
266 (700) 351 (900) 420 (1000)
90 (300) 145 (500) 180 (500)
120 ◦ C) condition (Fig. 6) may be associated with the additional water retention of CsHP salt particles within the membrane. The hydrophobic Cs+ cation interacts strongly with the anions, and maintains a rigid structure, which resists solvation in water at high temperatures. Water associated with the cation is held strongly until the decomposition temperature is reached [64]. The conductivity of two nanocomposite membranes at 120 ◦ C tends to be similar while the conductivity of Nafion membrane in this state is near zero. The CsHP may provide the additional surface functional sites to facilitate proton transport in anhydrous condition. This mechanism can be illustrated in Fig. 7. The proton transport can occur from the −SO3 H as proton donor to the −SO3 − as acceptor [77] on the CsHP surface. In the crystalline HPA, the proton transport occurs on the surface [4].
0.8 0.7 Voltage decay = -5.2 mV h 0.6
Cell voltage (V)
Entry
Voltage decay = -8.3 mV h 0.5 0.4 0.3 0.2
Voltage decay = -20.0 mV h
R-Nafion CsPW/Nafion CsPMo/Nafion
0.1 0.0 0
2
3.7. Oxidative stability
4
6
8
10
12
14
16
18
20
22
24
time (h)
The oxidative stability of membranes was investigated using Fenton’s reagent to evaluate the stability of membranes against the attack of radical species (HO• and HOO• ) [78]. The elapsed time when the membranes start to be brittle and the retained weights of membranes for 1 h are represented in Table 3. The results show that the CsPW/Nafion composite membrane has superior stability against oxidative agents. The reason may be the role of CsPW in the cover of sulfonic acid against the diffusion of H2 O2 .
Fig. 9. The durability of membranes at 80 ◦ C, low RH (35%) and ambient pressure under the constant load of 600 mA cm−2 for 24 h.
3.8. PEMFC performance
The addition of CsHP inorganics to the Nafion matrix enhances the water content, but limits the activity of hydrophilic sulfonic group of the Nafion due to covering the Nafion clusters and shielding the Nafion sulfonic groups by the CsHP. The covering effect for the CsPW particle is stronger than that for the CsPMo particle resulted in higher water uptake of CsPMo/Nafion membrane rather than CsPW/Nafion membrane. The increase in conductivity values versus the RH for the prepared membranes at 25 ◦ C may be associated with the absorption of more water by Nafion clusters which may increase the cluster diameter and the exchange site numbers per clusters. The hygroscopic and conductive CsHPs improved the water content. The conductivity of CsPMo/Nafion membrane was higher than CsPW/Nafion membrane. The higher conductivity of nanocomposite membranes than plain Nafion at anhydrous and high-temperatures condition may be related to the additional water retention of CsHP salt particles within the membrane. The CsHP may provide the additional surface functional sites to facilitate proton transport in anhydrous condition. The nanocomposite membranes have better performance in the PEMFC test at temperatures of 60, 80 and 100 ◦ C (35% RH, ambient pressure) than plain Nafion membrane. The performance of CsPMo/Nafion membrane is higher than CsPW/Nafion membrane due to the difference in their water uptake and proton conductivity. The stability of single cells under a constant load of 600 mA cm−2 at 80 ◦ C, low 35% RH and 1 atm H2 /O2 reactant gases demonstrates that the decay rate for R-Nafion membrane is rapid due to the dehydration in the period of time. The voltage decay for nanocomposite
The PEMFC test of membranes at temperatures of 60, 80 and 100 ◦ C (35% RH, ambient pressure) is summarized in Fig. 8. By comparing the performance of two nanocomposite membranes with the plain Nafion membrane, we may evaluate the effect of CsHP additives on the fuel cell performance. The nanocomposite membranes have better performance than plain Nafion membrane for this low RH at all temperatures. This effect may be associated with the hydrophilic CsHP particles that retained more water content to keep the membrane in a hydrated state [79]. The performance of CsPMo/Nafion membrane is higher than CsPW/Nafion membrane due to the difference in their water uptake and proton conductivity. The maximum power densities at different temperatures for the MEAs prepared by membranes are tabulated in Table 4. The maximum power density of 420 mW cm−2 at 1000 mA cm−2 was achieved for the MEA prepared by CsPMo/Nafion membrane at 80 ◦ C. 3.9. Durability test The durability test is an evaluation of membrane degradation and the voltage decay of the MEA incorporated membrane in the PEMFC versus time. Fig. 9 shows the stability of single cells with Nafion/CsHP nanocomposite membranes and recast Nafion membranes under a constant load of 600 mA cm−2 at 80 ◦ C, low 35% RH and 1 atm H2 /O2 reactant gases. This figure demonstrates that the decay rate for R-Nafion membrane is rapid due to the dehydra-
tion in the period of time. The voltage decay for nanocomposite membranes especially CsPMo/Nafion is low due to water retention property of the CsHP. 4. Conclusion
M. Amirinejad et al. / Journal of Membrane Science 377 (2011) 89–98
membranes especially CsPMo/Nafion is low due to water retention property of the CsHP.
[26]
Acknowledgment [27]
The authors wish to thank Iranian Renewable Energies Organization (SUNA) for financial support.
[28]
References [29] [1] I.V. Kozhevnikov, Catalysts for fine chemical synthesis, in: Catalysis by Polyoxometalates, vol. 2, John Wiley & Sons, Chichester, 2002. [2] L.C.W. Baker, D.C. Glick, Present general status of understanding of heteropoly electrolytes and a tracing of some major highlights in the history of their elucidation, Chemical Review 98 (1998) 3–49. [3] P.Y. Vuillaume, A. Mokrini, A. Siu, K. Théberge, L. Robitaille, Heteropolyacid/saponite-like clay complexes and their blends in amphiphilic SEBS, European Polymer Journal 45 (2009) 1641–1651. [4] I.V. Kozhevnikov, Catalysis by heteropoly acids and multicomponent polyoxometalates in liquid-phase reactions, Chemical Review 98 (1998) 171–198. [5] T. Okuhara, Water-tolerant solid acid catalysts, Chemical Review 102 (2002) 3641–3666. [6] M. Misono, Unique acid catalysis of heteropoly compounds (heteropolyoxometalates) in the solid state, Chemical Communications (2001) 1141–1152. ´ M. Davidovic, ´ Ph. Colomban, I. Holclajtner[7] U.B. Mioˇc, M.R. Todorovic, ´ Heteropoly compounds – from proton conductors to biomedical Antunovic, agents, Solid State Ionics 176 (2005) 3005–3017. [8] T. Okuhara, New catalytic functions of heteropoly compounds as solid acids, Catalysis Today 73 (2002) 167–176. [9] N. Mizuno, M. Misono, Heterogeneous catalysis, Chemical Reviews 98 (1998) 199–217. [10] T. Okuhara, H. Watanabe, T. Nishimura, K. Inumaru, M. Misono, Microstructure of cesium hydrogen salts of 12-tungstophosphoric acid relevant to novel acid catalysis, Chemistry of Materials 12 (2000) 2230–2238. [11] H. Tian, O. Savadogo, Silicotungstic acid Nafion composite membrane for proton-exchange membrane fuel cell operation at high temperature, Journal of New Materials for Electrochemical Systems 9 (2006) 61–71. [12] V. Ramani, H.R. Kunz, J.M. Fenton, Effect of particle size reduction on the conductivity of Nafion/phosphotungstic acid composite membranes, Journal of Membrane Science 266 (2005) 110–114. [13] Z.-G. Shao, P. Joghee, I.-M. Hsing, Preparation and characterization of hybrid Nafion–silica membrane doped with phosphotungstic acid for high temperature operation of proton exchange membrane fuel cells, Journal of Membrane Science 229 (2004) 43–51. [14] Z.-G. Shao, H. Xu, M. Li, I.-M. Hsing, Hybrid Nafion–inorganic oxides membrane doped with heteropolyacids for high temperature operation of proton exchange membrane fuel cell, Solid State Ionics 177 (2006) 779–785. [15] W. Xu, T. Lu, C. Liu, W. Xing, Low methanol permeable composite Nafion/silica/PWA membranes for low temperature direct methanol fuel cells, Electrochimica Acta 50 (2005) 3280–3285. [16] A. Mahreni, A.B. Mohamad, A.A.H. Kadhum, W.R.W. Daud, S.E. Iyuke, Nafion/silicon oxide/phosphotungstic acid nanocomposite membrane with enhanced proton conductivity, Journal of Membrane Science 327 (2009) 32–40. [17] X.-M. Yan, P. Mei, Y. Mi, L. Gao, S. Qin, Proton exchange membrane with hydrophilic capillaries for elevated temperature PEM fuel cells, Electrochemistry Communications 11 (2009) 71–74. [18] V. Ramani, H.R. Kunz, J.M. Fenton, Stabilized heteropolyacid/Nafion composite membranes for elevated temperature/low relative humidity PEFC operation, Electrochimica Acta 50 (2005) 1181–1187. [19] V. Ramani, H.R. Kunz, J.M. Fenton, Metal dioxide supported heteropolyacid/Nafion composite membranes for elevated temperature/low relative humidity PEFC operation, Journal of Membrane Science 279 (2006) 506–512. [20] A. Saccà, A. Carbone, R. Pedicini, M. Marrony, R. Barrera, M. Elomaa, E. Passalacqua, Phosphotungstic acid supported on a nanopowdered ZrO2 as a filler in Nafion-based membranes for polymer electrolyte fuel cells, Fuel Cells 08 (2008) 225–235. [21] M. Amirinejad, S.S. Madaeni, M.A. Navarra, E. Rafiee, B. Scrosati, Preparation and characterization of phosphotungstic acid-derived salt/Nafion nanocomposite membranes for proton exchange membrane fuel cells, Journal of Power Sources 196 (2011) 988–998. [22] M. Li, Z.-G. Shao, H. Zhang, Y. Zhang, X. Zhu, B. Yi, Self-humidifying Cs2.5 H0.5 PW12 O40 /Nafion/PTFE composite membrane for proton exchange membrane fuel cells, Electrochemical and Solid-State Letters 9 (2006) A92–A95. [23] J. Sauk, J. Byun, H. Kim, Composite Nafion/polyphenylene oxide (PPO) membranes with phosphomolybdic acid (PMA) for direct methanol fuel cells, Journal of Power Sources 143 (2005) 136–141. [24] I. Colicchio, F. Wen, H. Keul, U. Simon, M. Moeller, Sulfonated poly(ether ether ketone)–silica membranes doped with phosphotungstic acid. Morphology and proton conductivity, Journal of Membrane Science 326 (2009) 45–57. [25] S.Y. Oh, T. Yoshida, G. Kawamura, H. Muto, M. Sakai, A. Matsuda, Proton conductivity and fuel cell property of composite electrolyte consisting of Cs-
[30]
[31]
[32]
[33]
[34]
[35]
[36]
[37]
[38]
[39]
[40]
[41]
[42]
[43]
[44]
[45]
[46]
[47]
[48]
[49]
[50]
97
substituted heteropoly acids and sulfonated poly(ether–ether ketone), Journal of Power Sources 195 (2010) 5822–5828. A.F. Ismail, N.H. Othman, A. Mustafa, Sulfonated polyether ether ketone composite membrane using tungstosilicic acid supported on silica–aluminium oxide for direct methanol fuel cell (DMFC), Journal of Membrane Science 329 (2009) 18–29. M.L. Ponce, L.A.S. de, A. Prado, V. Silva, S.P. Nunes, Membranes for direct methanol fuel cell based on modified heteropolyacids, Desalination 162 (2004) 383–391. H. Dogan, T.Y. Inan, E. Unveren, M. Kaya, Effect of cesium salt of tungstophosphoric acid (Cs-TPA) on the properties of sulfonated polyether ether ketone (SPEEK) composite membranes for fuel cell applications, International Journal of Hydrogen Energy 35 (2010) 7784–7795. Y. Zhang, H. Zhang, C. Bi, X. Zhu, An inorganic/organic self-humidifying composite membranes for proton exchange membrane fuel cell application, Electrochimica Acta 53 (2008) 4096–4103. R. He, Q. Li, G. Xiao, N.J. Bjerrum, Proton conductivity of phosphoric acid doped polybenzimidazole and its composites with inorganic proton conductors, Journal of Membrane Science 226 (2003) 169–184. S.-Y. Oh, T. Yoshida, G. Kawamura, H. Muto, M. Sakai, A. Matsuda, Inorganic–organic composite electrolytes consisting of polybenzimidazole and Cs-substituted heteropoly acids and their application for medium temperature fuel cells, Journal of Materials Chemistry 20 (2010) 6359–6366. M.-Q. Li, Z.-G. Shao, K. Scott, A high conductivity Cs2.5 H0.5 PMo12 O40 /polybenzimidazole (PBI)/H3 PO4 composite membrane for proton-exchange membrane fuel cells operating at high temperature, Journal of Power Sources 183 (2008) 69–75. D.R. Vernon, F. Meng, S.F. Dec, D.L. Williamson, J.A. Turner, A.M. Herring, Synthesis, characterization, and conductivity measurements of hybrid membranes containing a mono-lacunary heteropolyacid for PEM fuel cell applications, Journal of Power Sources 139 (2005) 141–151. A. Martinelli, A. Matic, P. Jacobsson, L. Börjesson, M.A. Navarra, D. Munaò, S. Panero, B. Scrosati, Structural analysis of PVA-based proton conducting membranes, Solid State Ionics 178 (2007) 527–531. W. Jang, S. Choi, S. Lee, Y. Shul, H. Han, Characterizations and stability of polyimideephosphotungstic acid composite electrolyte membranes for fuel cell, Polymer Degradation and Stability 92 (2007) 1289–1296. B. Smitha, S. Sridhar, A.A. Khan, Proton conducting composite membranes from polysulfone and heteropolyacid for fuel cell applications, Journal of Polymer Science: Part B: Polymer Physics 43 (2005) 1538–1547. L. Li, Y. Wang, Proton conducting composite membranes from sulfonated polyethersulfone Cardo and phosphotungstic acid for fuel cell application, Journal of Power Sources 162 (2006) 541–546. C. Zhao, H. Lin, Q. Zhang, H. Na, Layer-by-layer self-assembly of polyaniline on sulfonated poly(arylene ether ketone) membrane with high proton conductivity and low methanol crossover, International Journal of Hydrogen Energy 35 (2010) 10482–10488. Z. Cui, C. Liu, T. Lu, W. Xing, Polyelectrolyte complexes of chitosan and phosphotungstic acid as proton-conducting membranes for direct methanol fuel cells, Journal of Power Sources 167 (2007) 94–99. Z. Cui, W. Xing, C. Liu, J. Liao, H. Zhang, Chitosan/heteropolyacid composite membranes for direct methanol fuel cell, Journal of Power Sources 188 (2009) 24–29. J.L. Malers, M.-A. Sweikart, J.L. Horan, J.A. Turner, A.M. Herring, Studies of heteropoly acid/polyvinylidenedifluoride–hexafluoroproylene composite membranes and implication for the use of heteropoly acids as the proton conducting component in a fuel cell membrane, Journal of Power Sources 172 (2007) 83–88. M. Helen, B. Viswanathan, S. Srinivasa Murthy, Poly(vinyl alcohol)–polyacrylamide blends with cesium salts of heteropolyacid as a polymer electrolyte for direct methanol fuel cell applications, Journal of Applied Polymer Science 116 (2010) 3437–3447. T. Uma, M. Nogami, Development of new glass composite membranes and their properties for low temperature H2 /O2 fuel cells, A European Journal of Chemical Physics and Physical Chemistry 8 (2007) 2227–2234. T. Uma, M. Nogami, Synthesis and characterization of P2 O5 –SiO2 –X (X = phosphotungstic acid) glasses as electrolyte for low temperature H2 /O2 fuel cell application, Journal of Membrane Science 280 (2006) 744–751. T. Uma, M. Nogami, Properties of PWA/ZrO2 -doped phosphosilicate glass composite membranes for low-temperature H2 /O2 fuel cell applications, Journal of Membrane Science 323 (2008) 11–16. ˇ ˘ U. Lavrenˇciˇc Stangar, N. Groselj, B. Orel, A. Schmitz, Ph. Colomban, Protonconducting sol–gel hybrids containing heteropoly acids, Solid State Ionics 145 (2001) 109–118. M.I. Ahmad, S.M.J. Zaidi, S. Ahmed, Proton conducting composites of heteropolyacids loaded onto MCM-41, Journal of Power Sources 157 (2006) 35–44. S. Lu, D. Wang, S.P. Jiang, Y. Xiang, J. Lu, J. Zeng, HPW/MCM-41 phosphotungstic acid/mesoporous silica composites as novel proton-exchange membranes for elevated-temperature fuel cells, Advanced Materials 22 (2010) 971–976. T. Inoue, T. Uma, M. Nogami, Performance of H2 /O2 fuel cell using membrane electrolyte of phosphotungstic acid-modified 3-glycidoxypropyltrimethoxysilanes, Journal of Membrane Science 323 (2008) 148–152. M.I. Ahmad, S.M.J. Zaidi, S.U. Rahman, S. Ahmed, Synthesis and proton conductivity of heteropolyacids loaded Y-zeolite as solid proton conductors for fuel cell applications, Microporous and Mesoporous Materials 91 (2006) 296–304.
98
M. Amirinejad et al. / Journal of Membrane Science 377 (2011) 89–98
[51] J.-D. Kim, I. Honma, Synthesis and proton conducting properties of zirconia bridged hydrocarbon/phosphotungstic acid hybrid materials, Electrochimica Acta 49 (2004) 3179–3183. [52] G. Alberti, M. Casciola, Composite membranes for medium-temperature PEM fuel cells, Annual Review of Materials Science 33 (2003) 129–154. [53] E. Rafiee, S. Eavani, F. Khajooei Nejad, M. Joshaghani, Cs2.5 H0.5 PW12 O40 catalyzed diastereoselective synthesis of -amino ketones via three component Mannich-type reaction in water, Tetrahedron 66 (2010) 6858–6863. [54] E. Rafiee, M. Joshaghani, S. Eavani, S. Rashidzadeh, A revision for the synthesis of -enaminones in solvent free conditions: efficacy of different supported heteropoly acids as active and reusable catalysts, Green Chemistry 10 (2008) 982–989. [55] K.-S. Lee, M.-H. Jeong, J.-P. Lee, J.-S. Lee, End-group cross-linked poly(arylene ether) for proton exchange membranes, Macromolecules 42 (2009) 584–590. [56] D.R. Lide, Handbook of Chemistry and Physics, 87th ed., CRC Press, 2006–2007. [57] R.-Q. Fu, D. Julius, L. Hong, J.-Y. Lee, PPO-based acid–base polymer blend membranes for direct methanol fuel cells, Journal of Membrane Science 322 (2008) 331–338. [58] L. Wang, B.L. Yi, H.M. Zhang, D.M. Xing, Cs2.5 H0.5 PWO40 /SiO2 as addition selfhumidifying composite membrane for proton exchange membrane fuel cells, Electrochimica Acta 52 (2007) 5479–5483. [59] L.R. Pizzio, M.N. Blanco, Isoamyl acetate production catalyzed by H3 PW12 O40 on their partially substituted Cs or K salts, Applied Catalysis A: General 255 (2003) 265–277. [60] A. Zurowski, A. Kolary-Zurowska, S. Dsoke, P.J. Barczuk, R. Marassi, P.J. Kulesza, Activation of carbon-supported platinum nanoparticles by zeolite-type cesium salts of polyoxometallates of molybdenum and tungsten towards more efficient electrocatalytic oxidation of methanol and ethanol, Journal of Electroanalytical Chemistry 649 (2010) 238–247. [61] T. Li, Y. Yang, A novel inorganic/organic composite membrane tailored by various organic silane coupling agents for use in direct methanol fuel cells, Journal of Power Sources 187 (2009) 332–340. [62] J.Y. Xi, Z.H. Wu, X.P. Qiu, L.Q. Chen, Nafion/SiO2 hybrid membrane for vanadium redox flow battery, Journal of Power Sources 166 (2007) 531–536. [63] M.A. Hickner, H. Ghassemi, Y.S. Kim, B.R. Einsla, J.E. McGrath, Alternative polymer systems for proton exchange membranes (PEMs), Chemical Reviews 104 (2004) 4587–4612. [64] A. Corma, Inorganic solid acids and their use in acid-catalyzed hydrocarbon reactions, Chemical Review 95 (1995) 559–614. [65] W.Y. Hsu, T.D. Gierke, Ion transport and clustering in Nafion perfluorinated membranes, Journal of Membrane Science 13 (1983) 307–326.
[66] Y. Izumi, M. Ogawa, K. Urabe, Alkali metal salts and ammonium salts of Keggintype heteropolyacids as solid acid catalysts for liquid-phase Friedel-Crafts reactions, Applied Catalysis A: General 132 (1995) 127–140. [67] B.A. Rozenberg, R. Tenn, Polymer-assisted fabrication of nanoparticles and nanocomposites, Progress in Polymer Science 33 (2008) 40–112. [68] L. Barbora, S. Acharya, R. Singh, K. Scott, A. Verma, A novel composite Nafion membrane for direct alcohol fuel cells, Journal of Membrane Science 326 (2009) 721–726. [69] M.A. Navarra, F. Croce, B. Scrosati, New, high temperature superacid zirconiadoped Nafion composite membranes, Journal of Materials Chemistry 17 (2007) 3210–3215. [70] W.H.J. Hogarth, J.C. Diniz da Costa, G.Q.(Max) Lu, Solid acid membranes for high temperature (>140 ◦ C) proton exchange membrane fuel cells, Journal of Power Sources 142 (2005) 223–237. [71] K.A. Mauritz, R.B. Moore, State of understanding of Nafion, Chemical Review 104 (2004) 4535–4585. [72] K.D. Kreuer, M. Hampele, K. Dolde, A. Rabenau, Proton transport in some heteropolyacidhydrates a single crystal PFG-NMR and conductivity study, Solid State Ionics 28–30 (1988) 589–593. [73] B. Smitha, S. Sridhar, A.A. Khan, Polyelectrolyte complexes of chitosan and poly(acrylic acid) as proton exchange membranes for fuel cells, Macromolecules 37 (2004) 2233–2239. [74] K.D. Kreuer, On the development of proton conducting polymer membranes for hydrogen and methanol fuel cells, Journal of Membrane Science 185 (2001) 29–39. [75] E. Fontananova, F. Trotta, J.C. Jansen, E. Drioli, Preparation and characterization of new non-fluorinated polymeric and composite membranes for PEMFCs, Journal of Membrane Science 348 (2010) 326–336. [76] E.A. Ukshe, L.S. Leonova, A.I. Korosteleva, Proton conduction in heteropoly compounds, Solid State Ionics 36 (1989) 219–223. [77] M. Yamada, I. Honma, Heteropolyacid-encapsulated self-assembled materials for anhydrous proton-conducting electrolytes, Journal of Physical Chemistry B 110 (2006) 20486–20490. [78] R. Gosalawit, S. Chirachanchai, S. Shishatskiy, S.P. Nunes, Sulfonated montmorillonite/sulfonated poly(ether ether ketone) (SMMT/SPEEK) nanocomposite membrane for direct methanol fuel cells (DMFCs), Journal of Membrane Science 323 (2008) 337–346. [79] S. Vengatesan, H.-J. Kim, S.-Y. Lee, E.A. Cho, H.Y. Ha, I.-H. Oh, T.-H. Lim, Operation of a proton exchange membrane fuel cell under non-humidified conditions using a membrane–electrode assemblies with composite membrane and electrode, Journal of Power Sources 167 (2007) 325–329.
Contents lists available at ScienceDirect
Journal of Membrane Science journal homepage: www.elsevier.com/locate/memsci
Cesium hydrogen salt of heteropolyacids/Nafion nanocomposite membranes for proton exchange membrane fuel cells Mehdi Amirinejad a , Sayed Siavash Madaeni a,∗ , Ezzat Rafiee b , Sedigheh Amirinejad a a b
Membrane Research Center, Department of Chemical Engineering, Faculty of Engineering, Razi University, Kermanshah, Iran Department of Inorganic Chemistry, Faculty of Chemistry, Razi University, Kermanshah, Iran
a r t i c l e
i n f o
Article history: Received 2 February 2011 Received in revised form 27 March 2011 Accepted 7 April 2011 Available online 15 April 2011 Keywords: Heteropolyacid Cesium salt Nanocomposite membrane Nafion Proton exchange membrane fuel cell Stability
a b s t r a c t Cesium hydrogen salt of heteropolyacids (CsHPs) including Cs2.5 H0.5 PMo12 O40 (CsPMo) and Cs2.5 H0.5 PW12 O40 (CsPW) are incorporated into Nafion to produce stable nanocomposite membranes at the moderate temperature/low relative humidity (RH). The addition of hygroscopic and conductive CsHP enhances the water content but limits the activity of the sulfonic group of the Nafion. As a result, ion exchange capacity (IEC) is decreased. The increase in conductivity versus RH is higher. The conductivity at anhydrous and high-temperature condition is higher due to the additional water retention or additional surface functional sites. The results of oxidative stability of membranes show that the CsPW/Nafion composite membrane has superior stability against oxidative agents due to the CsPW in lowering H2 O2 diffusion. The nanocomposite membranes have better performance in the PEM fuel cell test at temperatures 60, 80 and 100 ◦ C (35% RH, ambient pressure) than plain Nafion membrane. The stability of single cells under a constant load demonstrates that the decay rate for plain Nafion membrane is rapid due to the dehydration. The covering effect for the CsPW particle is stronger than the CsPMo particle results in higher water uptake, IEC, conductivity and fuel cell performance and lower voltage decay for the CsPMo/Nafion membrane rather than the CsPW/Nafion membrane. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Heteropolyacids (HPAs) are nano-sized metal–oxygen anion clusters with unique variety in structure and strong acidity [1–3]. Their acid–base and redox properties can be varied over a wide range by changing the chemical composition [4]. The Keggin-type HPAs have received the most attention due to the ease of preparation and strong acidity. Solid HPAs possess a discrete ionic structure comprising fairly mobile basic structural heteropolyanions and countercations (H+ , H3 O+ , H5 O2 + , etc.). This unique structure exhibits extremely high proton mobility. The Keggin heteropolyanion typically represented by the formula XM12 O40 x−8 where X is the central atom (P5+ , Si4+ , etc.), x is its oxidation state, and M is the metal ion (Mo6+ or W6+ ). The M6+ ions can be substituted by many other metal ions, e.g., V5+ , Co2+ , Zn2+ , etc. The structure of heteropolyanion itself i.e. the metal oxide cluster molecule is called the primary structure. The secondary structure is the three-dimensional arrangement of the primary structure and counter cations, which corresponds to the primary particles (microcrystallites) [5]. The tertiary structure represents the manner in which the secondary structure assembles into solid
∗ Corresponding author. Tel.: +98 831 4274530; fax: +98 831 4274542. E-mail address: [email protected] (S.S. Madaeni). 0376-7388/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2011.04.014
particles which correspond to the secondary particles and relates to properties such as particle size, surface area, and pore structure [6]. Heteropolyanions are larger than the inorganic acids, so the strength of bonding between the proton and heteropoly anions should be lower, which implies that the dissociation constants should be higher as compared to the usual acids. Furthermore, greater degrees of delocalization of the charge of the anion would lower the effective negative charge on its individual basic proton accepting centers and, thus, weaken the attraction of the proton to the anion [5]. Although the HPAs such as phosphotungstic acid (H3 PW12 O40 , hereafter PWA), phosphomolybdic acid (H3 PMo12 O40 , hereafter PMA) and silicotungstic acid (H4 SiW12 O40 , hereafter SWA) in the solid state are pure Brønsted acids and stronger acids than the conventional solid acids such as SiO2 , Al2 O3 , H3 PO4 , and HX and HY zeolites [7], but the acid forms have low surface acidity (the number of acidic sites on the surface) because of the low surface area (∼5 m2 g−1 ) and are highly soluble in water. Partial substitution of the proton by a large monovalent ion such as Cs+ results in unique changes in the surface area (surface acidity) as well as the insolubility in water [8]. Peculiar changes in the activity for Csx H3−x PM12 O40 (M = W, Mo) are observed for the Cs salt maxima at x = 2.5 and has been interpreted by the change in the surface acidity [9]. For instance, the surface area, particle size and crystallite size
90
M. Amirinejad et al. / Journal of Membrane Science 377 (2011) 89–98
of Cs2.5 H0.5 PW12 O40 (Hereafter CsPW) are 135 m2 g−1 , 6.9 nm and 11.3 nm, respectively [10]. HPAs have been incorporated into the membranes to improve their structure and functionality and produce composite membranes. The HPAs increase the binding energy of water and/or proton conductivity. HPA-incorporated composite solid electrolyte for application in the fuel cell has been given more attention in recent years [11–51]. Table 1 lists recent literature review on this subject. The matrices have been reported are Nafion and Nafion composites [11–23], sulfonated polymers such as sulfonated poly(ether ether ketone) [24–29] and sulfonated polysulfone [36], hydrophilic polymers such as chitosan [39,40] and polyvinyl alcohol [42], high temperature and high mechanical resistances polymers such as polybenzimidazole [30,31] and poly(vinylidene) fluoride [34] and inorganic matrices such as P2 O5 –SiO2 [43–45] and MCM-41 [47,49]. The most HPAs reported are Keggin types like PWA, PMA and SWA. The modified HPAs such as supported PWA and PMA by SiO2 and Al2 O3 and their salt have been developed. Perfluorosulfonic acid based membranes show excellent thermal, chemical and mechanical stability in operating proton exchange membrane fuel cells (PEMFCs) at 80 ◦ C but instability is compromised at high temperature/low humidity operations. Perfluorosulfonic acid-based organic/inorganic composite membranes with various HPA additives have been investigated as alternate materials for high temperature/low humidity PEMFC operation [11–19]. The size, uniform dispersion, loading content and acid sites amount of inorganic additives can influence the ion exchange capacity and proton transport of per-fluorinated ionomer composite membrane [52]. In recent publication [21], Nafion/CsPW nanocomposite membranes for PEMFCs have been proposed. The nanocomposite membrane showed better performance than plain Nafion membrane especially at high temperature/low relative humidity (RH). Cs2.5 H0.5 PMo12 O40 (hereafter CsPMo) is more active in some reactions than CsPW such as benzylation of benzene [9]. The composite membranes using CsPMo for application in fuel cells have been reported [32,42]. Li et al. [32] investigated CsPMo/PBI/H3 PO4 composite membrane for use in PEMFCs and found that this type of composite showed higher conductivity and superior performance in PEMFCs compared to PBI/H3 PO4 membrane at high temperature/low RH. Helen et al. [42] tested cesium salt of HPAs incorporated into PVA/PAM blend membranes for direct methanol fuel cell (DMFC) and found that CsPMo/PVA/PAM membrane demonstrated higher conductivity. This study was conducted to investigate the effect of replacement of W in CsPW with Mo on the characteristics of CsPMo/Nafion nanocomposite membranes. The CsPW/Nafion nanocomposite (with similar composition) and recast Nafion (without any additive, hereafter R-Nafion) membranes were prepared at similar conditions for comparison. Thermal analysis (TGA/DTG and DSC), physico-chemical characterizations including FT-IR, water uptake, ion exchange capacity, oxidative stability, proton conductivity and PEMFC performance were conducted to investigate the stability of nanocomposite membranes at moderate temperature and low RH.
hydrate (H3 PMo12 O40 ·xH2 O) and cesium carbonate, acetonitrile and n-butylamine were obtained from Merck. 2.2. Preparation of cesium hydrogen salt of HPAs (CsPW and CsPMo) Cesium hydrogen salt of heteropolyacids (hereafter CsHPs) including CsPW and CsPMo were prepared by precipitation titration [53]. The aqueous solutions of HPA (0.08 M) and Cs2 CO3 (0.1 M) were prepared. Cs2 CO3 solution was added to 20 mL of HPA solution dropwise with stirring in room temperature. The colloidal solution was stirred overnight and then liquid phase was evaporated at 45 ◦ C. The powders of CsPW and CsPMo were obtained by heating at 300 ◦ C for 1 h. Finally, prepared powders were grinded and dried in vacuum oven at 80 ◦ C and stored in a dry atmosphere before use. 2.3. Measurement of acidity of CsHPs The acidity of CsHP powders was measured based on potentiometric titration according to the last published paper [54]. Around 0.05 g CsHP was dispersed in 90 mL acetonitrile and stirred for 3 h. The solution was titrated with n-butylamine in acetonitrile solution (0.05 N). The potential variations (mV) versus meq g−1 of amine were recorded by Hanna 302 pH meter using a double junction electrode. 2.4. Preparation of membranes Membranes were prepared by recast procedure. The solvents of Nafion solution were exchanged gradually by high boiling point solvent (NMP) at 80 ◦ C. CsHP (10 wt.% based on solid Nafion) was dispersed in NMP with stirring to avoid particle aggregation and then added to the solution and mixed for 24 h. The concentrated and viscose solution was poured onto a Petri dish. Membranes preliminarily were dried at 50 ◦ C for 12 h. The solvent evaporated completely by the vacuum at 90 ◦ C (12 h). Dried membranes were peeled off from the surface and their organic and inorganic impurities were removed by boiling in 3% H2 O2 (w/v) for 90 min. Membranes boiled in H2 SO4 0.5 M for 90 min for protonation and finally washed in boiling deionized water for 90 min. Recast Nafion membranes without inorganic filler were also prepared by the same procedure. 2.5. Fourier transform infrared (FT-IR) The infrared spectra on powders were recorded in KBr pellets using PERKIN ELMER Spectrum 2000. The spectrum obtained after multiple scans was a plot of transmittance percent against wavenumber. 2.6. Thermal analyses
2. Experimental
Thermal gravimetric analysis (TGA) of membranes was performed on TA instruments (TGA-Q50) in nitrogen atmosphere from 25 ◦ C to 600 ◦ C with the heating rate of 10 ◦ C min−1 . Differential scanning calorimetry (DSC) of membranes was studied using TA instruments (DSC-Q200) with nitrogen at scan rates of 10 ◦ C min−1 at two heating cycles and second cycle is reported.
2.1. Materials
2.7. Water uptake and ion exchange capacity (IEC)
Nafion 5% solution (EC-NS-05) was purchased from ElectroChem. 1-Methyl-2-pyrrolidone (hereafter NMP) was supplied by Sigma–Aldrich. Sodium chloride, sodium hydroxide, hydrogen peroxide and sulfuric acid were obtained from Aldrich. Phosphotungstic acid hydrate (H3 PW12 O40 ·xH2 O), phosphomolybdic acid
Water uptake (wu) of membranes at room temperature was determined by the following formula: wu (%) =
Wwet − Wdry Wdry
× 100
(1)
M. Amirinejad et al. / Journal of Membrane Science 377 (2011) 89–98
91
Table 1 Recent literature reviews on the application of heteropolyacids in the solid electrolyte of fuel cells. Entry
Matrix
Heteropolyacid
1
Nafion
• PWA and SWA • PWA • Modified PWA by large ions like Cs+ , NH4+ , Rb+ and Tl+ • Supported PWA and PMA by SiO2 , Al2 O3 , ZrO2 or TiO2 • Cs2.5 H0.5 PW12 O40 • Cs2.5 H0.5 PW12 O40 • PMA • PWA • PWA or SWA • SWA supported on SiO2 –Al2 O3 • ␥-K8 SiW10 O36 • Cs2.5 H0.5 PW12 O40 • Cs2.5 H0.5 PW12 O40 supported Pt • PWA or SWA • PWA or SWA • Cs2.5 H0.5 PMo12 O40 • H8 SiW11 O39 • PWA • PWA • PWA • PWA • PWA
2 3 4
Nafion/poly(tetraflouroethylene) (PTFE) Nafion/poly(phenylene oxide) (PPO) Sulfonated polyether ether ketone (SPEEK)
5
Polybenzimidazole (PBI)
6 7 8 9 10 11 12
Polybenzimidazole (PBI)/H3 PO4 Poly(ethylene glycol) (PEG) Poly(vinylidene fluoride) (PVdF) Sulfonated polyimides (SPI) Sulfonated polysulfone (SPS) Sulfonated polyethersulfone (SPES) Sulfonated poly(arylene ether ketone)/polyaniline (SPAES/PANI) Chitosan Poly(vinylidenedifluoride/hexafluoropropylene) (PVdF/HFP) Poly(vinyl alcohol)/polyacrylamide (PVA/PAM) P2 O5 –SiO2 Organically modified silicate MCM-41 3-Glycidoxypropyltrimethoxysilane (GPTMS) Y-zeolite Zirconia bridged hydrocarbon
13 14 15 16 17 18 19 20 21
Additional inorganic SiO2
SiO2 CsHSO4 SiO2
CsHSO4
Al2 O3
• PWA or PMA • PWA, ␣-H3 P2 W18 O62 , H6 P2 W21 O71 and H6 As2 W21 O69 • Cs2.5 H0.5 PW12 O40 and Cs2.5 H0.5 PMo12 O40 • PWA • PWA or SWA • PWA or PMA • PWA • PWA or PMA • PWA
where Wwet is the membrane weight after cleaning and when the surface water was removed and Wdry is the membrane weight after drying at 70 ◦ C. IEC was performed by classical titration. Dried membranes were immersed in NaCl (0.1 M) solution and the H+ was titrated by NaOH 0.01 N aqueous solution. The IEC was measured at least two times and the average is reported.
2.8. Oxidative stability and durability test of membranes The oxidative stability of the membrane samples was measured in Fenton test solution (3% H2 O2 solution containing 2 ppm FeSO4 ) at 80 ◦ C. The membranes were immersed in a shaking flask containing the Fenton’s reagent. The oxidative stability was evaluated by the elapsed time when the membranes start to be brittle and the retained weights of membranes for 1 h. The durability test was done by recording the voltage decay of the membrane electrode assembly (MEA) including prepared membranes in the fuel cell (H2 /O2 ) by fixation of current at 600 mA cm−2 and operating condition of 80 ◦ C, 35% RH and ambient pressure for 24 h.
2.9. Proton conductivity The proton conductivity of the membranes was measured by four-point probe technique [55]. The membrane sample (1 cm × 3 cm) was sandwiched between two Teflon parts attached with two platinum foils and two platinum wires. The impedance was measured using Autolab PGSTAT302N electrochemistry workstation in the frequency range of 100 kHz to 10 Hz. The proton conductivity (, S cm−1 ) of membranes was calculated using the
Reference [11,12] [13–17] [18] [19,20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34] [35] [36] [37] [38] [39,40] [41]
TiO2 or ZrO2
[42] [43–45] [46] [47,48] [49] [50] [51]
following equation: =
L RS
(2)
where L, R and S are the thickness (cm), the impedance () and the surface area (cm2 ) of the membranes, respectively. The conductivity values were measured at different RHs and temperature conditions. The conductivity at 25 ◦ C as a function of RH was carried out by keeping the samples (24 h) inside a chamber containing saturated salt solutions [56,57]: CaCl2 for 29% RH; Ca(NO3 )2 for 51% RH; NaNO3 for 74% RH and KNO3 for 92% RH. The RH was verified by a hygrometer before measurement. Conductivity measurements in 100% RH are carried out with the cell immersed in liquid water at the desired temperature for 12 h. The conductivity at high temperatures and 100% RH was determined by saturated water vapor. The conductivity measurements in anhydrous conditions were performed by placing the cell inside a dry controlled temperature chamber. For all conditions, the cell was allowed to reach steady state before taking any measurement. 2.10. MEA fabrication and PEMFC performance A fuel cell test station (FCT-2000, ElectroChem) connected with a 5 cm2 cell hardware (FC05-01SP, ElectroChem) was used to measure cell polarization. Commercial gas diffusion electrodes (20% Pt/C, E-Tek) with 0.4 mg Pt cm−2 loading were impregnated with 0.8 mg cm−2 of Nafion solution. The MEA was fabricated by hotpressing at 140 ◦ C and 10 atm for 90 s. H2 and O2 with the pressure of 1 atm, desired humidity and the flow rates a bit higher than stoichiometric flows were used as the anode and the cathode, respectively. The humidity of reactant gases was set at 35% to study the fuel cell performance at low RH.
92
M. Amirinejad et al. / Journal of Membrane Science 377 (2011) 89–98
Fig. 1. FT-IR of the prepared CsPW and CsPMo.
3. Results and discussion 3.1. FT-IR The FT-IR measurement of the prepared CsPW and CsPMo heteropolyacids is represented in Fig. 1. The main bands are assigned and tabulated in Table 2. The primary Keggin structure (PWA and PMA) remains unaltered in the cesium salt forms [58,59]. For CsPW/CsPMo, the bands at 1080/1060, 985/959, 891/865, 815/760 and 588/591 cm−1 are assigned to stretching vibrations of P–O, M O, M–O–M corners shared bonds, M–O–M edges shared bonds and to the bending vibration of O–P–O, respectively [40]. Absorption band at 1614/1610 indicates the presence of water ion (H5 O2 + ) in the structure of CsHPs and is assigned to ␦(H2 O) vibration [60]. The changes in the frequency characteristics of Mo–O–Mo edge and ␦(H2 O) in CsPW may be assigned to the interaction between [PM12 O40 ]3− anion and Cs+ cation [60]. Fig. 2. Thermal gravimetric response of the prepared membranes: (a) TGA and (b) DTG.
3.2. TGA/DTG Fig. 2 compares the TGA–DTG responses of the prepared membranes. Three main weight losses corresponding to major peaks in the DTG curves can be observed for membranes. Preliminary mass loss during the heating process up to 270 ◦ C is attributed to the evaporation of absorbed water molecules [61]. The first peak at 270 ◦ C is associated with the degradation of sulfonic acid groups [62]. The desulfonation rate of the CsPW/Nafion and CsPMo/Nafion nanocomposite membranes due to catalytic property of CsPW and CsPMo [10] or the interfacial interaction between CsHP and Nafion is faster than the R-Nafion membrane. The second and third weight losses are attributed to decomposition of side-chain and the PTFE backbone, respectively [14,21]. The onset temperatures for the second and third weight losses in the nanocomposite membranes are lower with faster mass losses than the plain Nafion. This effect indi-
Table 2 Properties of the prepared CsPW and CsPMo heteropolyacids. Entry 1 2 3
a
Property −1
Acidity (meq g ) Surface area (m2 g−1 ) a FTIR assignments (cm−1 ) P–O M O M–O–M (corner-sharing) M–O–M (edge-sharing) O–P–O ␦(H2 O) Data obtained from Ref. [66].
CsPW
CsPMo
0.4 >100
0.2 <50
1080 985 891 815 588 1614
1060 959 865 760 591 1610
cates that the presence of CsHP may accelerate the decomposition of CF2 backbone. 3.3. Water uptake Water content in the fuel cell is important because it affects the proton conductivity and overall system performance [63]. The water uptake of prepared membranes is represented in Table 3. Water molecules in the secondary structures connect the individual heteropoly anions through weak hydrogen bonds. In the case of [PW12 O40 ]3− , its radius is only 0.5–0.6 nm, while the spacing between ions is 23 nm, leaving therefore a considerable space between ions [64]. This space may hold water and increase water content through the nanocomposite membrane. Table 3 reveals that the water uptake of CsPMo/Nafion membrane is higher than CsPW/Nafion membrane while the acidity of CsPW is a bit higher than CsPMo (refer to Table 2). This phenomenon may be explained based on the physical effect of CsHP on Nafion cluster. Schematically, the cluster-network model [65] and the size of clusters and channels of Nafion are illustrated in Fig. 3a. The particle and crystallite sizes of CsHP [66] are not smaller than Nafion clusters to make an embedded type of nanocomposite [67] and these particles may settle on the clusters and shield the Nafion sulfonic groups (Fig. 3b). Therefore, although the addition of CsHP inorganic to Nafion matrix enhances the water content, but may limit the activity of hydrophilic sulfonic group of the Nafion cluster. The surface area of CsPW is higher than CsPMo (Table 2) and with regard
M. Amirinejad et al. / Journal of Membrane Science 377 (2011) 89–98
93
Table 3 Physico-chemical properties of prepared membranes. Entry
Membrane
Thickness (m)
Water uptake (%)
IEC (meq g−1 )
H (J gNafion −1 )
Ea (kJ mol−1 )
1 2 3
R-Nafion CsPW/Nafion CsPMo/Nafion
62 65 67
33 62 76
0.84 0.72 0.79
136 229 291
11.55 10.20 9.97
to the same structural porosity for two HPAs, the particle size of CsPMo is bigger than CsPW. Based on this investigation, the property of covering effect of CsPW particle is stronger than that for CsPMo particle (Fig. 3c). The water uptake of nanocomposite membranes is associated with two factors included sulfonic group of Nafion and high hygroscopic nature of CsHP nanoclusters. With consideration of covering effect, the higher water uptake of CsPMo/Nafion membrane rather than CsPW/Nafion membrane could be interpreted.
3.4. IEC The IEC of nanocomposite membranes is slightly lower than the plain Nafion (Table 3.). This effect is due to covering the Nafion active sites (sulfonic groups) and decreasing the effective number of replaceable ion exchange sites [68] by the salts. The greater IEC
Oxidative stability Retained weight (%)
Elapsed time (h)
92 98 95
8.5 >12 10
of the CsPMo/Nafion membrane than the CsPW/Nafion membrane may be associated with the free sulfonic group of Nafion matrix in the CsPMo than the CsPW.
3.5. DSC The strong endothermic peaks around temperatures 125–185 ◦ C for membranes in the DSC traces (Fig. 4) may be associated with the structural changes within the ionic clusters of membranes [69]. The enthalpy change corresponding to these peaks (H, Table 3) may be related to the degree of hydration of Nafion-based membranes which allows high mobility of the chain segments [69]. In other word, H values increase with increasing water content in the membrane. Sulfonic groups of the Nafion host polymer and the hygroscopic nature of CsPW particles can serve to absorb more water through the nanocomposite membrane; so the H values
Fig. 3. The schematic drawing of Nafion cluster-network model (a); shielding the active sites of Nafion clusters by CsHP particles (b); the effect of particle size on covering property (c) (CsPW has lower particle diameter than CsPMo and has higher covering effect).
94
M. Amirinejad et al. / Journal of Membrane Science 377 (2011) 89–98
Fig. 4. DSC of the prepared membranes.
of nanocomposite membranes are higher than plain Nafion (see Table 3). The settlement of CsHP on Nafion clusters may decrease the mobility of chain segments and reduction in the enthalpy change. As a result, the H of CsPW/Nafion membrane is lower than CsPMo/Nafion membrane.
Fig. 6. The conductivity values of membranes at high temperatures in anhydrous condition.
3.6. Proton conductivity For good proton conductivity, solid electrolyte should have fixed charged sites surrounded by water molecules which facilitate the transport of protons [68]. Solid HPAs possess extremely high proton mobility [4]. Unlike the rigid network structure of zeolites, HPA Keggin anions are quite mobile [4]. Proton transport in Nafion nanocomposite membranes is the result of a complex process dominated by the surface and chemical properties of both Nafion and additive [70]. Fig. 5a reveals the conductivity values at 25 ◦ C against the RH for the prepared membranes. As seen, by increasing RH of membranes, the conductivity is increasing. This effect may be interpreted by the absorption of more water by the Nafion clusters, resulting in increase of the clusters diameter and the exchange sites per clusters and promoting the conductivity [71]. The hygroscopic and conductive CsHPs may improve the water content and support this behavior. Therefore, the conductivity of nanocomposite membranes is more sensitive to humidity. The Arrhenius plot for the prepared membranes at 100% RH is presented in Fig. 5b. The conductivity of nanocomposite membranes is greater than plain Nafion. The conductivity of CsPMo/Nafion membrane is higher than CsPW/Nafion membrane. In solid HPA with crystalline water, the proton migration can be ascribed to the vehicle mechanism [72]. Based on the Arrhe-
SO3-
SO
3 -
-
SO 3
-
SO 3
H+
Proton Pathway
-
SO 3
CsHP
CsHP
SO3-
SO 3
SO
3
Fig. 5. Conductivity curves for the prepared membranes: (a) conductivity vs. RH at 25 ◦ C, (b) Arrhenius plot at 100% RH.
-
-
SO 3
H+
Fig. 7. The mechanism of proton transport in CsHP/Nafion nanocomposite membranes in anhyrous condition; Nafion backbone is not shown.
M. Amirinejad et al. / Journal of Membrane Science 377 (2011) 89–98
95
Fig. 8. The PEM fuel cell responses of membranes with H2 /O2 reactant gases as at ambient pressure and low RH (35%): (a) (V–I) curves at 60 ◦ C; (b) (P–I) curves at 60 ◦ C; (c) (V–I) curves at 80 ◦ C; (d) (P–I) curves at 80 ◦ C; (e) (V–I) curves at 100 ◦ C; (f) (P–I) curves at 100 ◦ C.
nius plot, the activation energy of the membranes can be evaluated (refer to Table 3). The activation energy may be used to investigate the mechanism of proton conduction [73]. The ions transportation of Nafion membrane is governed by Grotthus and vehicle mechanisms [74]. In the nanocomposite membranes, the CsHP can interact with the sulfonic groups of Nafion and the water hydration molecules [75], forming a network of hydrogen bonds. The free proton may be formed by the reaction of anion moiety of CsHP with the water
molecule as described by Ukshe et al. [76] (M = W, Mo): (PM12 O40 )3− + nH2 O → [(PM12 O40 )(OH)x ](3+x)− + xH+
(3)
The mechanism of proton conductivity in a given RH may be related to dynamic equilibrium between different proton moieties: H5 O2 + ↔ H3 O+ + H2 O ↔ H+ + 2H2 O
(4)
The higher conductivity of nanocomposite membranes than plain Nafion at anhydrous and high-temperature (100, 110 and
96
M. Amirinejad et al. / Journal of Membrane Science 377 (2011) 89–98
Table 4 Maximum power density of membranes obtained from the PEMFC performance at 35% RH.
1 2 3
Membrane
R-Nafion CsPW/Nafion CsPMo/Nafion
Maximum power density (mW cm−2 ) [corresponding current density (mA cm−2 )] Temperature 60 ◦ C
80 ◦ C
100 ◦ C
160 (500) 238 (900) 304 (800)
266 (700) 351 (900) 420 (1000)
90 (300) 145 (500) 180 (500)
120 ◦ C) condition (Fig. 6) may be associated with the additional water retention of CsHP salt particles within the membrane. The hydrophobic Cs+ cation interacts strongly with the anions, and maintains a rigid structure, which resists solvation in water at high temperatures. Water associated with the cation is held strongly until the decomposition temperature is reached [64]. The conductivity of two nanocomposite membranes at 120 ◦ C tends to be similar while the conductivity of Nafion membrane in this state is near zero. The CsHP may provide the additional surface functional sites to facilitate proton transport in anhydrous condition. This mechanism can be illustrated in Fig. 7. The proton transport can occur from the −SO3 H as proton donor to the −SO3 − as acceptor [77] on the CsHP surface. In the crystalline HPA, the proton transport occurs on the surface [4].
0.8 0.7 Voltage decay = -5.2 mV h 0.6
Cell voltage (V)
Entry
Voltage decay = -8.3 mV h 0.5 0.4 0.3 0.2
Voltage decay = -20.0 mV h
R-Nafion CsPW/Nafion CsPMo/Nafion
0.1 0.0 0
2
3.7. Oxidative stability
4
6
8
10
12
14
16
18
20
22
24
time (h)
The oxidative stability of membranes was investigated using Fenton’s reagent to evaluate the stability of membranes against the attack of radical species (HO• and HOO• ) [78]. The elapsed time when the membranes start to be brittle and the retained weights of membranes for 1 h are represented in Table 3. The results show that the CsPW/Nafion composite membrane has superior stability against oxidative agents. The reason may be the role of CsPW in the cover of sulfonic acid against the diffusion of H2 O2 .
Fig. 9. The durability of membranes at 80 ◦ C, low RH (35%) and ambient pressure under the constant load of 600 mA cm−2 for 24 h.
3.8. PEMFC performance
The addition of CsHP inorganics to the Nafion matrix enhances the water content, but limits the activity of hydrophilic sulfonic group of the Nafion due to covering the Nafion clusters and shielding the Nafion sulfonic groups by the CsHP. The covering effect for the CsPW particle is stronger than that for the CsPMo particle resulted in higher water uptake of CsPMo/Nafion membrane rather than CsPW/Nafion membrane. The increase in conductivity values versus the RH for the prepared membranes at 25 ◦ C may be associated with the absorption of more water by Nafion clusters which may increase the cluster diameter and the exchange site numbers per clusters. The hygroscopic and conductive CsHPs improved the water content. The conductivity of CsPMo/Nafion membrane was higher than CsPW/Nafion membrane. The higher conductivity of nanocomposite membranes than plain Nafion at anhydrous and high-temperatures condition may be related to the additional water retention of CsHP salt particles within the membrane. The CsHP may provide the additional surface functional sites to facilitate proton transport in anhydrous condition. The nanocomposite membranes have better performance in the PEMFC test at temperatures of 60, 80 and 100 ◦ C (35% RH, ambient pressure) than plain Nafion membrane. The performance of CsPMo/Nafion membrane is higher than CsPW/Nafion membrane due to the difference in their water uptake and proton conductivity. The stability of single cells under a constant load of 600 mA cm−2 at 80 ◦ C, low 35% RH and 1 atm H2 /O2 reactant gases demonstrates that the decay rate for R-Nafion membrane is rapid due to the dehydration in the period of time. The voltage decay for nanocomposite
The PEMFC test of membranes at temperatures of 60, 80 and 100 ◦ C (35% RH, ambient pressure) is summarized in Fig. 8. By comparing the performance of two nanocomposite membranes with the plain Nafion membrane, we may evaluate the effect of CsHP additives on the fuel cell performance. The nanocomposite membranes have better performance than plain Nafion membrane for this low RH at all temperatures. This effect may be associated with the hydrophilic CsHP particles that retained more water content to keep the membrane in a hydrated state [79]. The performance of CsPMo/Nafion membrane is higher than CsPW/Nafion membrane due to the difference in their water uptake and proton conductivity. The maximum power densities at different temperatures for the MEAs prepared by membranes are tabulated in Table 4. The maximum power density of 420 mW cm−2 at 1000 mA cm−2 was achieved for the MEA prepared by CsPMo/Nafion membrane at 80 ◦ C. 3.9. Durability test The durability test is an evaluation of membrane degradation and the voltage decay of the MEA incorporated membrane in the PEMFC versus time. Fig. 9 shows the stability of single cells with Nafion/CsHP nanocomposite membranes and recast Nafion membranes under a constant load of 600 mA cm−2 at 80 ◦ C, low 35% RH and 1 atm H2 /O2 reactant gases. This figure demonstrates that the decay rate for R-Nafion membrane is rapid due to the dehydra-
tion in the period of time. The voltage decay for nanocomposite membranes especially CsPMo/Nafion is low due to water retention property of the CsHP. 4. Conclusion
M. Amirinejad et al. / Journal of Membrane Science 377 (2011) 89–98
membranes especially CsPMo/Nafion is low due to water retention property of the CsHP.
[26]
Acknowledgment [27]
The authors wish to thank Iranian Renewable Energies Organization (SUNA) for financial support.
[28]
References [29] [1] I.V. Kozhevnikov, Catalysts for fine chemical synthesis, in: Catalysis by Polyoxometalates, vol. 2, John Wiley & Sons, Chichester, 2002. [2] L.C.W. Baker, D.C. Glick, Present general status of understanding of heteropoly electrolytes and a tracing of some major highlights in the history of their elucidation, Chemical Review 98 (1998) 3–49. [3] P.Y. Vuillaume, A. Mokrini, A. Siu, K. Théberge, L. Robitaille, Heteropolyacid/saponite-like clay complexes and their blends in amphiphilic SEBS, European Polymer Journal 45 (2009) 1641–1651. [4] I.V. Kozhevnikov, Catalysis by heteropoly acids and multicomponent polyoxometalates in liquid-phase reactions, Chemical Review 98 (1998) 171–198. [5] T. Okuhara, Water-tolerant solid acid catalysts, Chemical Review 102 (2002) 3641–3666. [6] M. Misono, Unique acid catalysis of heteropoly compounds (heteropolyoxometalates) in the solid state, Chemical Communications (2001) 1141–1152. ´ M. Davidovic, ´ Ph. Colomban, I. Holclajtner[7] U.B. Mioˇc, M.R. Todorovic, ´ Heteropoly compounds – from proton conductors to biomedical Antunovic, agents, Solid State Ionics 176 (2005) 3005–3017. [8] T. Okuhara, New catalytic functions of heteropoly compounds as solid acids, Catalysis Today 73 (2002) 167–176. [9] N. Mizuno, M. Misono, Heterogeneous catalysis, Chemical Reviews 98 (1998) 199–217. [10] T. Okuhara, H. Watanabe, T. Nishimura, K. Inumaru, M. Misono, Microstructure of cesium hydrogen salts of 12-tungstophosphoric acid relevant to novel acid catalysis, Chemistry of Materials 12 (2000) 2230–2238. [11] H. Tian, O. Savadogo, Silicotungstic acid Nafion composite membrane for proton-exchange membrane fuel cell operation at high temperature, Journal of New Materials for Electrochemical Systems 9 (2006) 61–71. [12] V. Ramani, H.R. Kunz, J.M. Fenton, Effect of particle size reduction on the conductivity of Nafion/phosphotungstic acid composite membranes, Journal of Membrane Science 266 (2005) 110–114. [13] Z.-G. Shao, P. Joghee, I.-M. Hsing, Preparation and characterization of hybrid Nafion–silica membrane doped with phosphotungstic acid for high temperature operation of proton exchange membrane fuel cells, Journal of Membrane Science 229 (2004) 43–51. [14] Z.-G. Shao, H. Xu, M. Li, I.-M. Hsing, Hybrid Nafion–inorganic oxides membrane doped with heteropolyacids for high temperature operation of proton exchange membrane fuel cell, Solid State Ionics 177 (2006) 779–785. [15] W. Xu, T. Lu, C. Liu, W. Xing, Low methanol permeable composite Nafion/silica/PWA membranes for low temperature direct methanol fuel cells, Electrochimica Acta 50 (2005) 3280–3285. [16] A. Mahreni, A.B. Mohamad, A.A.H. Kadhum, W.R.W. Daud, S.E. Iyuke, Nafion/silicon oxide/phosphotungstic acid nanocomposite membrane with enhanced proton conductivity, Journal of Membrane Science 327 (2009) 32–40. [17] X.-M. Yan, P. Mei, Y. Mi, L. Gao, S. Qin, Proton exchange membrane with hydrophilic capillaries for elevated temperature PEM fuel cells, Electrochemistry Communications 11 (2009) 71–74. [18] V. Ramani, H.R. Kunz, J.M. Fenton, Stabilized heteropolyacid/Nafion composite membranes for elevated temperature/low relative humidity PEFC operation, Electrochimica Acta 50 (2005) 1181–1187. [19] V. Ramani, H.R. Kunz, J.M. Fenton, Metal dioxide supported heteropolyacid/Nafion composite membranes for elevated temperature/low relative humidity PEFC operation, Journal of Membrane Science 279 (2006) 506–512. [20] A. Saccà, A. Carbone, R. Pedicini, M. Marrony, R. Barrera, M. Elomaa, E. Passalacqua, Phosphotungstic acid supported on a nanopowdered ZrO2 as a filler in Nafion-based membranes for polymer electrolyte fuel cells, Fuel Cells 08 (2008) 225–235. [21] M. Amirinejad, S.S. Madaeni, M.A. Navarra, E. Rafiee, B. Scrosati, Preparation and characterization of phosphotungstic acid-derived salt/Nafion nanocomposite membranes for proton exchange membrane fuel cells, Journal of Power Sources 196 (2011) 988–998. [22] M. Li, Z.-G. Shao, H. Zhang, Y. Zhang, X. Zhu, B. Yi, Self-humidifying Cs2.5 H0.5 PW12 O40 /Nafion/PTFE composite membrane for proton exchange membrane fuel cells, Electrochemical and Solid-State Letters 9 (2006) A92–A95. [23] J. Sauk, J. Byun, H. Kim, Composite Nafion/polyphenylene oxide (PPO) membranes with phosphomolybdic acid (PMA) for direct methanol fuel cells, Journal of Power Sources 143 (2005) 136–141. [24] I. Colicchio, F. Wen, H. Keul, U. Simon, M. Moeller, Sulfonated poly(ether ether ketone)–silica membranes doped with phosphotungstic acid. Morphology and proton conductivity, Journal of Membrane Science 326 (2009) 45–57. [25] S.Y. Oh, T. Yoshida, G. Kawamura, H. Muto, M. Sakai, A. Matsuda, Proton conductivity and fuel cell property of composite electrolyte consisting of Cs-
[30]
[31]
[32]
[33]
[34]
[35]
[36]
[37]
[38]
[39]
[40]
[41]
[42]
[43]
[44]
[45]
[46]
[47]
[48]
[49]
[50]
97
substituted heteropoly acids and sulfonated poly(ether–ether ketone), Journal of Power Sources 195 (2010) 5822–5828. A.F. Ismail, N.H. Othman, A. Mustafa, Sulfonated polyether ether ketone composite membrane using tungstosilicic acid supported on silica–aluminium oxide for direct methanol fuel cell (DMFC), Journal of Membrane Science 329 (2009) 18–29. M.L. Ponce, L.A.S. de, A. Prado, V. Silva, S.P. Nunes, Membranes for direct methanol fuel cell based on modified heteropolyacids, Desalination 162 (2004) 383–391. H. Dogan, T.Y. Inan, E. Unveren, M. Kaya, Effect of cesium salt of tungstophosphoric acid (Cs-TPA) on the properties of sulfonated polyether ether ketone (SPEEK) composite membranes for fuel cell applications, International Journal of Hydrogen Energy 35 (2010) 7784–7795. Y. Zhang, H. Zhang, C. Bi, X. Zhu, An inorganic/organic self-humidifying composite membranes for proton exchange membrane fuel cell application, Electrochimica Acta 53 (2008) 4096–4103. R. He, Q. Li, G. Xiao, N.J. Bjerrum, Proton conductivity of phosphoric acid doped polybenzimidazole and its composites with inorganic proton conductors, Journal of Membrane Science 226 (2003) 169–184. S.-Y. Oh, T. Yoshida, G. Kawamura, H. Muto, M. Sakai, A. Matsuda, Inorganic–organic composite electrolytes consisting of polybenzimidazole and Cs-substituted heteropoly acids and their application for medium temperature fuel cells, Journal of Materials Chemistry 20 (2010) 6359–6366. M.-Q. Li, Z.-G. Shao, K. Scott, A high conductivity Cs2.5 H0.5 PMo12 O40 /polybenzimidazole (PBI)/H3 PO4 composite membrane for proton-exchange membrane fuel cells operating at high temperature, Journal of Power Sources 183 (2008) 69–75. D.R. Vernon, F. Meng, S.F. Dec, D.L. Williamson, J.A. Turner, A.M. Herring, Synthesis, characterization, and conductivity measurements of hybrid membranes containing a mono-lacunary heteropolyacid for PEM fuel cell applications, Journal of Power Sources 139 (2005) 141–151. A. Martinelli, A. Matic, P. Jacobsson, L. Börjesson, M.A. Navarra, D. Munaò, S. Panero, B. Scrosati, Structural analysis of PVA-based proton conducting membranes, Solid State Ionics 178 (2007) 527–531. W. Jang, S. Choi, S. Lee, Y. Shul, H. Han, Characterizations and stability of polyimideephosphotungstic acid composite electrolyte membranes for fuel cell, Polymer Degradation and Stability 92 (2007) 1289–1296. B. Smitha, S. Sridhar, A.A. Khan, Proton conducting composite membranes from polysulfone and heteropolyacid for fuel cell applications, Journal of Polymer Science: Part B: Polymer Physics 43 (2005) 1538–1547. L. Li, Y. Wang, Proton conducting composite membranes from sulfonated polyethersulfone Cardo and phosphotungstic acid for fuel cell application, Journal of Power Sources 162 (2006) 541–546. C. Zhao, H. Lin, Q. Zhang, H. Na, Layer-by-layer self-assembly of polyaniline on sulfonated poly(arylene ether ketone) membrane with high proton conductivity and low methanol crossover, International Journal of Hydrogen Energy 35 (2010) 10482–10488. Z. Cui, C. Liu, T. Lu, W. Xing, Polyelectrolyte complexes of chitosan and phosphotungstic acid as proton-conducting membranes for direct methanol fuel cells, Journal of Power Sources 167 (2007) 94–99. Z. Cui, W. Xing, C. Liu, J. Liao, H. Zhang, Chitosan/heteropolyacid composite membranes for direct methanol fuel cell, Journal of Power Sources 188 (2009) 24–29. J.L. Malers, M.-A. Sweikart, J.L. Horan, J.A. Turner, A.M. Herring, Studies of heteropoly acid/polyvinylidenedifluoride–hexafluoroproylene composite membranes and implication for the use of heteropoly acids as the proton conducting component in a fuel cell membrane, Journal of Power Sources 172 (2007) 83–88. M. Helen, B. Viswanathan, S. Srinivasa Murthy, Poly(vinyl alcohol)–polyacrylamide blends with cesium salts of heteropolyacid as a polymer electrolyte for direct methanol fuel cell applications, Journal of Applied Polymer Science 116 (2010) 3437–3447. T. Uma, M. Nogami, Development of new glass composite membranes and their properties for low temperature H2 /O2 fuel cells, A European Journal of Chemical Physics and Physical Chemistry 8 (2007) 2227–2234. T. Uma, M. Nogami, Synthesis and characterization of P2 O5 –SiO2 –X (X = phosphotungstic acid) glasses as electrolyte for low temperature H2 /O2 fuel cell application, Journal of Membrane Science 280 (2006) 744–751. T. Uma, M. Nogami, Properties of PWA/ZrO2 -doped phosphosilicate glass composite membranes for low-temperature H2 /O2 fuel cell applications, Journal of Membrane Science 323 (2008) 11–16. ˇ ˘ U. Lavrenˇciˇc Stangar, N. Groselj, B. Orel, A. Schmitz, Ph. Colomban, Protonconducting sol–gel hybrids containing heteropoly acids, Solid State Ionics 145 (2001) 109–118. M.I. Ahmad, S.M.J. Zaidi, S. Ahmed, Proton conducting composites of heteropolyacids loaded onto MCM-41, Journal of Power Sources 157 (2006) 35–44. S. Lu, D. Wang, S.P. Jiang, Y. Xiang, J. Lu, J. Zeng, HPW/MCM-41 phosphotungstic acid/mesoporous silica composites as novel proton-exchange membranes for elevated-temperature fuel cells, Advanced Materials 22 (2010) 971–976. T. Inoue, T. Uma, M. Nogami, Performance of H2 /O2 fuel cell using membrane electrolyte of phosphotungstic acid-modified 3-glycidoxypropyltrimethoxysilanes, Journal of Membrane Science 323 (2008) 148–152. M.I. Ahmad, S.M.J. Zaidi, S.U. Rahman, S. Ahmed, Synthesis and proton conductivity of heteropolyacids loaded Y-zeolite as solid proton conductors for fuel cell applications, Microporous and Mesoporous Materials 91 (2006) 296–304.
98
M. Amirinejad et al. / Journal of Membrane Science 377 (2011) 89–98
[51] J.-D. Kim, I. Honma, Synthesis and proton conducting properties of zirconia bridged hydrocarbon/phosphotungstic acid hybrid materials, Electrochimica Acta 49 (2004) 3179–3183. [52] G. Alberti, M. Casciola, Composite membranes for medium-temperature PEM fuel cells, Annual Review of Materials Science 33 (2003) 129–154. [53] E. Rafiee, S. Eavani, F. Khajooei Nejad, M. Joshaghani, Cs2.5 H0.5 PW12 O40 catalyzed diastereoselective synthesis of -amino ketones via three component Mannich-type reaction in water, Tetrahedron 66 (2010) 6858–6863. [54] E. Rafiee, M. Joshaghani, S. Eavani, S. Rashidzadeh, A revision for the synthesis of -enaminones in solvent free conditions: efficacy of different supported heteropoly acids as active and reusable catalysts, Green Chemistry 10 (2008) 982–989. [55] K.-S. Lee, M.-H. Jeong, J.-P. Lee, J.-S. Lee, End-group cross-linked poly(arylene ether) for proton exchange membranes, Macromolecules 42 (2009) 584–590. [56] D.R. Lide, Handbook of Chemistry and Physics, 87th ed., CRC Press, 2006–2007. [57] R.-Q. Fu, D. Julius, L. Hong, J.-Y. Lee, PPO-based acid–base polymer blend membranes for direct methanol fuel cells, Journal of Membrane Science 322 (2008) 331–338. [58] L. Wang, B.L. Yi, H.M. Zhang, D.M. Xing, Cs2.5 H0.5 PWO40 /SiO2 as addition selfhumidifying composite membrane for proton exchange membrane fuel cells, Electrochimica Acta 52 (2007) 5479–5483. [59] L.R. Pizzio, M.N. Blanco, Isoamyl acetate production catalyzed by H3 PW12 O40 on their partially substituted Cs or K salts, Applied Catalysis A: General 255 (2003) 265–277. [60] A. Zurowski, A. Kolary-Zurowska, S. Dsoke, P.J. Barczuk, R. Marassi, P.J. Kulesza, Activation of carbon-supported platinum nanoparticles by zeolite-type cesium salts of polyoxometallates of molybdenum and tungsten towards more efficient electrocatalytic oxidation of methanol and ethanol, Journal of Electroanalytical Chemistry 649 (2010) 238–247. [61] T. Li, Y. Yang, A novel inorganic/organic composite membrane tailored by various organic silane coupling agents for use in direct methanol fuel cells, Journal of Power Sources 187 (2009) 332–340. [62] J.Y. Xi, Z.H. Wu, X.P. Qiu, L.Q. Chen, Nafion/SiO2 hybrid membrane for vanadium redox flow battery, Journal of Power Sources 166 (2007) 531–536. [63] M.A. Hickner, H. Ghassemi, Y.S. Kim, B.R. Einsla, J.E. McGrath, Alternative polymer systems for proton exchange membranes (PEMs), Chemical Reviews 104 (2004) 4587–4612. [64] A. Corma, Inorganic solid acids and their use in acid-catalyzed hydrocarbon reactions, Chemical Review 95 (1995) 559–614. [65] W.Y. Hsu, T.D. Gierke, Ion transport and clustering in Nafion perfluorinated membranes, Journal of Membrane Science 13 (1983) 307–326.
[66] Y. Izumi, M. Ogawa, K. Urabe, Alkali metal salts and ammonium salts of Keggintype heteropolyacids as solid acid catalysts for liquid-phase Friedel-Crafts reactions, Applied Catalysis A: General 132 (1995) 127–140. [67] B.A. Rozenberg, R. Tenn, Polymer-assisted fabrication of nanoparticles and nanocomposites, Progress in Polymer Science 33 (2008) 40–112. [68] L. Barbora, S. Acharya, R. Singh, K. Scott, A. Verma, A novel composite Nafion membrane for direct alcohol fuel cells, Journal of Membrane Science 326 (2009) 721–726. [69] M.A. Navarra, F. Croce, B. Scrosati, New, high temperature superacid zirconiadoped Nafion composite membranes, Journal of Materials Chemistry 17 (2007) 3210–3215. [70] W.H.J. Hogarth, J.C. Diniz da Costa, G.Q.(Max) Lu, Solid acid membranes for high temperature (>140 ◦ C) proton exchange membrane fuel cells, Journal of Power Sources 142 (2005) 223–237. [71] K.A. Mauritz, R.B. Moore, State of understanding of Nafion, Chemical Review 104 (2004) 4535–4585. [72] K.D. Kreuer, M. Hampele, K. Dolde, A. Rabenau, Proton transport in some heteropolyacidhydrates a single crystal PFG-NMR and conductivity study, Solid State Ionics 28–30 (1988) 589–593. [73] B. Smitha, S. Sridhar, A.A. Khan, Polyelectrolyte complexes of chitosan and poly(acrylic acid) as proton exchange membranes for fuel cells, Macromolecules 37 (2004) 2233–2239. [74] K.D. Kreuer, On the development of proton conducting polymer membranes for hydrogen and methanol fuel cells, Journal of Membrane Science 185 (2001) 29–39. [75] E. Fontananova, F. Trotta, J.C. Jansen, E. Drioli, Preparation and characterization of new non-fluorinated polymeric and composite membranes for PEMFCs, Journal of Membrane Science 348 (2010) 326–336. [76] E.A. Ukshe, L.S. Leonova, A.I. Korosteleva, Proton conduction in heteropoly compounds, Solid State Ionics 36 (1989) 219–223. [77] M. Yamada, I. Honma, Heteropolyacid-encapsulated self-assembled materials for anhydrous proton-conducting electrolytes, Journal of Physical Chemistry B 110 (2006) 20486–20490. [78] R. Gosalawit, S. Chirachanchai, S. Shishatskiy, S.P. Nunes, Sulfonated montmorillonite/sulfonated poly(ether ether ketone) (SMMT/SPEEK) nanocomposite membrane for direct methanol fuel cells (DMFCs), Journal of Membrane Science 323 (2008) 337–346. [79] S. Vengatesan, H.-J. Kim, S.-Y. Lee, E.A. Cho, H.Y. Ha, I.-H. Oh, T.-H. Lim, Operation of a proton exchange membrane fuel cell under non-humidified conditions using a membrane–electrode assemblies with composite membrane and electrode, Journal of Power Sources 167 (2007) 325–329.