Proton transport properties at high temperature of modified sulfonated polyether-ether ketone membrane for direct methanol fuel cells application

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1 ISBN Proton transport properties at high temperature of modified sulfonated polyether-ether ketone membrane for direct methanol fuel cells application Sri Handayani 1, Eniya Listiani Dewi 1 Department of Chemical Engineering, Institute Technology Indonesia, Jl. Raya Puspiptek Serpong, Tangerang 1532, Indonesia. sri_anny@yahoo.com 2 Agency for the Assesment and Aplication of Technology, Center of Materials Technology BPPT Gd. II, Lt. 22, Jl. M.H. Thamrin 8, Jakarta 134, Indonesia. Abstract Experimental methods for studying proton conductivity and methanol permeability of sulfonated polyether-ether ketone () modified membrane containing hydrophilic particles (zeolite and silica) over wide range of temperature have been synthesized and characterized. The proton conductivity was investigated by standard bridge impedance spectroscopy at various temperatures (25 14 o C). At temperature 14 o C, the addition of modified contains zeolite and silica have conductivities (.72 and.76 S/cm) as higher as 13% than unmodified (.67 S/cm). The activation energy of modified membrane (6 and 8 kj/mol) has lower than unmodified membrane (14 kj/mol). The addition of inorganic filler, onto could lowering methanol permeability and showed high mechanical stability at high temperature rather than membrane. Thus modified membranes can be said as promising candidates for high temperature direct methanol fuel cell. Keywords: High Temperature, Sulfonated Polyether-Ether Ketone, Zeolite, Silica, Fuel cell Introduction The direct methanol fuel cell (DMFC) has been receiving great attention as clean and efficient alternative technology for transport apllication. It is can be operated both in low temperature (less than 8 o C) and high temperature (more than 1 o C). Recently, the DMFC that can work at high temperature provide faster reaction kinetic in electrodes and reduce Pt-based catalyst poisoning by CO (Savadogo, 24). But at high temperature, it will decrease the membrane performance i.e. low proton conductivity is affected by humidification, high methanol crossover as methanol vapor easy to diffuse in membrane and high durability. Requirement of electrolyte membrane for DMFC s application which at high temperature operation, it has low methanol diffusion coefficient (less than cm 2 /s at temperature 25 o C), high proton conductivity (more than.8 S/cm), high chemical and mechanical durability especially at temperature higher than 8 o C and low cost (Neburchilov et al., 27). Various efforts to increase proton conductivity such as decreasing methanol permeability and thermal stability in membrane are modified membrane with adding inorganic particle or to find polymer which have thermal stability. Modified Nafion is adding inorganic particle in Nafion (Antonucci et al., 1999; Arimura et al., 1999; Domitrova et al., 22; Arico et al., 23; Eniya, 25; Sancho et al., 27). One of alternative polymer which has thermal stability is aromatic polymer such as polysulfone (Lufrano et al., 2), polybenzimidazole (Jones and Roziere, 21), and polyether-ether ketone (Lei et al., 23). The polyether-ether ketone (PEEK) was selected as based polymer membrane due to good thermal stability and mechanical strength (Savadogo, 24). In order to produce hydrophilicity, polymer of polyether-ether ketone must be sulfonated using concentrated sulfonic acid to form sulfonated polyether-ether ketone (). Some researches added various filler as BPO 4, SiO 2, TiO 2, and ZrO 2 to improve properties of elecrolyte membrane such as (Mikhailenko et al., 21; Nunes et al., 22; Othman et al., 25). In above research, the proton transport properties of modified membrane especially for 68% of sulfonation degree at high temperature are not has been discussed. Here in we describes the preparation of modified membranes with sulfonation degree of 68% in easy method, in order to increase the proton conductivity and reduce methanol 152

2 permeability. The influence of addition particles on the membrane properties are studied using proton conductivity and methanol permeability analysis at various temperature. Materials and Methods Modified membrane preparation Polyteher-ether ketone (grade 45-P Victrex Inc.) was sulfonated with concentrated sulfuric acid. Detail procedure sulfonated polyetherether ketone () was as described in previous publications (Handayani et al., 27). PEEK powder (5 g) was added into concentrated sulfuric acid 1 ml under vigorous stirring for 3 hours at temperature 5 o C. To terminate sulfonation reaction, the polymer solution precipitated into a large excess of ice water. The polymer was washed repeatedly with deionized water until the rinse water was at ph 6-7. The recovered were dried at in an oven overnight. The sulfonation process at 5 o C produced 68% sulfonation degree of membrane. The modified and unmodified membrane was prepared by solvent casting. The polymer was first dissolved with n-methyl pyrrolidone and mixed with added particles (silica made in Trisindo Sejati Corp. Indonesia, H-Yzeolite made in Japan) and stirred for 7 h. Solution were cast onto glass plates and then dried at 6 o C for 72 h. Analysis Water uptake was measured base on water weight absorbing per dry weight membrane. Weighed films were immersed in deionized water at room temperature for 24 h. The membranes were saturated with water until no further weight gain was observed. The water on the surface of immersed membranes was removed using tissue paper before weighing. Proton conductivity was measured using standard bridge LCR (Impedance Capacitance Resistance), impedance spectroscopy (HIOKI LCR HiTESTER) with various frequencies from 3 khz to 1 khz and 2 mv oscillating voltage. Legend 1. Teflon cover 2. Thumbscrew 3. Teflon block 4. Open area to allow equilibration with environment 5. Membrane sample 6. Gold foil electrode 7. Gold wire Fig. 1 Conductivity cell The conductance of each membrane was measured at 25, 5, 9 and 14 o C under fully hydrated condition. A conductivity cell was made up of two gold foils carrying the current and two gold wires sensing the potential drop, which was apart 1 cm as shown in Figure 1. The fully hydrated membrane with deionized water during 24 h was cut in 1 cm wide, 4 cm long prior to mounting on the cell. After mounting sample onto two gold foils on the lower compartment, upper compartment was covered, and then the upper and lower compartments were clamped as described by authors (Zawodzinski et al., 1991; Sancho et al., 27). The proton conductivity (σ) of the membrane can be calculated using Eq. (1), L σ = G (1) W. d where G, L, W and d are conductance (S), the length between the electrode (cm), wide (cm) and thickness of the membrane samples (cm), respectively. Methanol permeability was determined using a test cell. The membrane was equilibrated in pure water for 5 hour before clamped vertical at the diffusion cell. Magnetic stirrers were used on each compartment to ensure uniformity. A solution containing 2 M methanol in water was placed on cell 1 (C 1 ) and water was placed on cell B (C 2 ) (Handayani et al., 27). The densities of the initial and final solutions were measured by a means of a pycnometer. The duration of measurement was 4 hour. The methanol permeability (DK) was calculated as in Eq. (2) (Caretta et al., 2), C2 ( t) xv2xl DK = (2) AxC ( t t ) 1 where C 1 and C 2 are the concentration of methanol in cells 1 and 2, V 2 are the volume of cell 2, A and l are the surface area and thickness of membrane, and D and K are the methanol diffusivity and partition coefficient, respectively. The methanol permeability of each membrane was measured at 25, 5, 9, and 14 o C. Results and Discussion The effects of high temperature on proton conductivity of membranes Figure 2 is proton conductivity as a function of temperature at 1% relative humidity (RH) of unmodified, modified and membrane. At room temperature, proton conductivity (σ) of modified membrane higher than unmodified (.18 S/cm) but still lower than membrane. For all membranes, proton conductivities were increased at higher temperature. 153

3 Proton conductivity (S/cm) Fig Temperature ( o C) Influence of temperature in proton Conductivity: ( ) ; ( ) ; ( ) +Z; ( ) +Si The addition zeolite and silica on membrane caused the increasing of water uptake at room temperature and high temperature as shown in Table 1. It is indicates that additives is a hygroscopic (easy to absorb water) which could maintain water management at high temperature by the formation of silanol group in acidy silica (Arico et al., 23). Table 1 Properties of proton transport and water uptake of membranes Type +Z +Si σ 25 o C (S/cm) σ 9 o C (S/cm) water uptake, 25 o C (wt.%) water uptake, 9 o C (wt.%) And also for the acidic aluminasilicate of zeolite, the adsorption is caused by water bonding (Demuth et al., 21). The membranes that have high absorbance of water would facilitated proton transport, thus proton conductivity could reach up. According to showed proton conductivity higher than unmodified and modified membrane, in the range of temperature up to until 9 o C and could not determine at more than 9 o C. On the other hand unmodified and modified membranes were more stable at high temperature, up to 14 o C. Proton conductivity in, +Z and +Si increases with temperature mainly due to higher proton mobility and bigger quantity of adsorbed water. Figure 3 showed an Arrhenius plot of the proton conductivity (σ) as a function of temperature at 1% RH for membranes and membrane. From the Arrhenius Equation (3), the apparent activation energy of membrane can be calculated (Zhou et al., 23), ln(σ T) = ln σ o E a,c / (RT ) (3) where R is gas constant, σ o is a constant with respect to T, and E a,c is the activation energy of proton conduction. The apparent activation energy of proton migration is reported in Table 2. ln[t σ (K S cm -1 )] Fig T -1 (K -1 ) Arrhenius plot showing dependency of proton conductivity: ( ) ; ( ) ; ( ) +Z; ( ) +Si The activation energy of membrane is 7 kj/mol, which is slightly different, with literature value of 4 kj/mol (Sancho et al., 27). The activation energy of modified membrane (silica and zeolite) is close to membrane. But activation energy for unmodified membrane (14 kj/mol) was higher than modified membrane (6 and 8 kj/mol. As showed from Table 2, water uptake membrane affected lowering activation energy. Table 2 Activation energy of membrane Membrane +Z +Si Temperature range ( o C) E a, c (proton migration) (kj mol -1 ) E a,p (methanol permeation) (kj mol -1 ) However, the addition of zeolite and silica make the membrane capable to adsorb more water as a consequence of higher proton conductivity. The effect of high temperature on methanol permeability of membranes The methanol resistance of membranes at high temperature, is conducted by measuring methanol permeability at temperature o C. Figure 4 shows the methanol permeability of membranes as a function of temperature. As shown at Figure 4, the slope of is larger 3 times than based membrane. In order to know methanol 154

4 permeability in the same condition of, membrane was analyzed at 25 o C until 9 o C. It showed that had reached 2.3 times and this membrane was broken after applying temperature 14 o C. Furthermore, the methanol permeability of unmodified membrane was reached 4 times when analyzed from 25 o C until 14 o C. Whereas the +Z and +Si membrane was reached about 3 times for the same condition. The addition zeolite and silica in will retain increasing methanol permeability at high temperature. Methanol Permeability (cm 2 /s) 3.5E-5 3.E-5 2.5E-5 2.E-5 1.5E-5 1.E-5 5.E-6.E T -1 (K -1 ) Fig. 4 Arrhenius plot showing the temperature dependency of methanol permeability: ( ) Nafion- 117; ( ); ( ) +Z; ( ) +Si The dependency of methanol permeability upon temperature is of the Arrhenius type as from Eq. (4). ln(dk) = ln (DK ) o E a,p / (RT ) (4) where DK is the methanol permeability, (DK ) o is the pre-exponential factor, E a,p is the activation energy obtained by correlating diffusivity-temperature data with Arrhenius approach. The apparent activation energies of methanol permeation reported in Table 2. The activation energy of modified membrane is close to membrane (12 kj/mol), with exception of unmodified membrane, for which a considerably higher energy is noted. Selectivity of membrane Selectivity is a ratio between proton conductivity with methanol permeability. Membrane with higher selectivity would perform better in the fuel cell. In Figure 5 shows selectivity of membranes at temperature from 25 o C to 14 o C. The membrane which is at present utilized in DMFC is also included for comparison. Selectivity at 9 o C of modified is higher than which exhibits about 3 times. This is very attractive results from the perspective of potential application in high temperature DMFC, because of low methanol crossover would be accompanied by proportionate improvement of the fuel cell efficiency and its power density. Selectivity (S/cm)/(cm 2 /s) Fig. 5 Conclusions Temperature ( o C) +Z +Si Selectivity of membrane as a function temperature In summary, proton conductivity increased by the additional of zeolite and silica onto sulfonated polyether-ether ketone () which determined at high temperature. The proton conductivity of modified membranes can reaches about.7 S/cm above 14 o C at RH 1%. At high temperature of all modified membranes retaining better performance of methanol permeability, activation energy of proton migration and selectivity value to. Thus electrolyte membrane based with DS 68% can be used to apply at high temperature (>8 o C) direct methanol fuel cell with better electrical performance than with Nafion membranes, respectively. Acknowledgments This work was conducted as a part of the Ph.D. program at University of Indonesia. This program was funded by Technological and Professional Skills Development Sector Project (TPSDP) Batch III, the Ministry of National Education of Indonesia, ADB Loan No.: 1792-INO to Chemical Engineering Department of Institute Technology Indonesia. A part of this work was supported by Center of Materials Technology of Agency for the Assesment & Application of Technology, Indonesia. References Antonucci, P. L., A. S. Arico, P. Creti, E. Ramunni and V. Antonucci; Investigation of a Direct Methanol Fuel Cell Based on a Composite Nafion-Silika Electrolyte for High Temperature Operation, Solid State Ionics, 125, (1999) Arico, A.S., V. Baglio, A. Di Blasi, P. Creti, P.L. Antonucci and V. Antonucci; Influence of the Acid-Base Characteristics of Inorganic Fillers on 155

5 the High Temperature Performance of Composite Membranes in Direct Methanol Fuel Cells, Solid State Ionics, 161, (23) Arimura, T., D. Ostrovskii, T. Okada and G. Xie; The Effect of additives on the Ionic Conductivity Performances of Perfluoroalkyl Sulfonated Ionomer Membranes, Solid State Ionics, 118, 1 1 (1999) Carbone, A., R. Pedicini, G. Portale, A. Longo, L.D. D Ilario and E. Passalacqua; Sulphonated poly(ether ether ketone) membranes for fuel cell application: Thermal and structural characterisation, J. Power Source, 163, (26) Caretta, N., V. Tricoli and V. Picchioni; Ionomeric membranes based on partially sulfonated poly(styrene): synthesis, proton conduction and methanol permeation, J. Membr. Sci. 166, (2) Domitrova, P., K.A. Friedriech, U. Stimming and B. Vogt; Modified Nafion-based membranes for use in direct methanol fuel cells, Solid State Ionics, 15, (22) Eniya, L. D.; The Effect of Moisture Absorbing Particle on Methanol Permeability of Modified Nafion Membrane in Fuel Cells, The 3 rd Regional Symposium on membrane Technology for Industry and Enviromental Protection, ITB- Bandung, pp. 1 6, (25) Handayani, S., W.P. Widodo, L.D.Eniya and W.S. Roekmijati; Synthesis and characterization of electrolyte membrane of sulfonated polyetherether ketone, Indonesian J. of Materials Sci. 8- Vol 2: (27) Jones, D.J. and J. Roziere; Recent advances in the functionalisation of polybenzimidazole and polyetherketone for fuel cell applications, J. Membr. Sci., 185, (21) Lei Li, Jun Zhang and Yuxin Wang; Sulfonated Poly(Ether Ether Ketone) Membranes for Direct Methanol Fuel Cells, J. Membr. Sci., 226, (23) Lufrano, F., G. Squadrito, A. Patti and E.Passalacqua; Sulfonated Polysulfone as Promising Membranes for Polymer Electrolyte Fuel Cells, J. Appl. Polym. Sci., 77, (2) Mikhailenko, S.D., S.M.J. Zaidi and S. Kaliaguine; Sulfonated polyether ether ketone based composite polymer electrolyte membranes, Catalysis Today, 67, (21) Neburchilov, V., J. Martin, H. Wang and J. Zhang; Review of Polymer Electrolyte Membranes for Direct Methanol Fuel Cells, J. Power Sources, 169, (27) Nunes, S.P., B. Ruffmann, E. Riwokowski, S. Vetter, and K. Richau; Inorganic Modification of Proton Conductive Polymer Membranes for Direct Methanol Fuel Cells, J. Membr. Sci., 23, (22) Othman, M.H.D., S.L. Ho, A. Mustafa and A.F. Ismail; Organic/Inorganic Hybrid Membrane for Direct Methanol Fuel Cell Aplication, The 3 rd Regional Symposium on membrane Technology for Industry and Enviromental Protection, ITB-Bandung, Indonesia, (25) Sancho, T., J. Soler, and M.P. Pina; Conductivity in zeolite-polymer composite membranes for PEMFCs, J. Power Sources, 169, (27) Savadogo, O.; Emerging Membranes for Electrochemical Systems Part II. High Temperature Composite Membranes for Polymer Electrolyte Fuel Cell (PEFC) Aplications, J. Power Source, 127, (24) Zawodzinski, T.A., M. Neeman, L.O. Sillerud and S. Gottesfeld; Determination of Water Diffusion Coefficients in Perfluorosulfonate Ionomeric Membranes, J. Phys. Chem. 95, (1991) Zhou, X., J. Weston, E. Chelkova, A. Michel, Hofmann, C.M. Ambler, et al.; High Temperature Transport Properties of Polyphosphazene Membranes for Direct Methanol Fuel Cells, Electrochemica Acta, 48, (23) 156

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