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Evaluation of methanol crossover in proton-conducting polyphosphazene membranes
January 2002
Materials Letters 52 Ž2002. 192–196 www.elsevier.comrlocatermatlet
Evaluation of methanol crossover in proton-conducting polyphosphazene membranes Mark V. Fedkin a,b, Xiangyang Zhou a , Michael A. Hofmann c , Elena Chalkova a , Jamie A. Weston a , Harry R. Allcock c , Serguei N. Lvov a,b,) b
a The Energy Institute, The PennsylÕania State UniÕersity, 517 Deike Building, UniÕersity Park, PA 16802, USA Department of Energy and Geo-EnÕironmental Engineering, The PennsylÕania State UniÕersity, 517 Deike Building, UniÕersity Park, PA 16802, USA c Department of Chemistry, The PennsylÕania State UniÕersity, 517 Deike Building, UniÕersity Park, PA 16802, USA
Received 17 February 2000; accepted 12 April 2001
Abstract A diffusion cell was developed to evaluate the methanol crossover for a novel class of polyphosphazene electrolyte membranes. It was found that the methanol diffusion coefficients of phenyl phosphonic acid functionalized polywaryloxyphosphazenex membranes in an aqueous methanol solution Ž50% vrv. were ; 40 times lower than for Nafion 117, and ; 10–20 times lower than for sulfonated polyphosphazene membranes. q 2002 Elsevier Science B.V. All rights reserved. Keywords: Methanol crossover; Diffusion coefficients; Polyphosphazene electrolyte membranes; Polymer electrolyte fuel cells
1. Introduction Methanol crossover in polymer electrolyte membranes ŽPEMs. is one of the main issues impeding the development of direct-methanol fuel cells ŽDMFCs.. Methanol that is transported through the membrane is oxidized by oxygen at the cathode leading to a significant reduction in fuel cell performance w1–4x. The fact that Nafion membranes are subject to serious methanol crossover has stimulated
) Corresponding author. Center for Advanced Materials, The Pennsylvania State University, 517 Deike Building, University Park, PA 16802, USA. Tel.: q1-814-863-8377; fax: q1-814-8634718, q1-814-865-1573. E-mail address: [email protected] ŽS.N. Lvov..
a search for alternative polymers that are more resistant to methanol diffusion. Previous work in our program w5x and others w6x has shown that polyphosphazene-based membranes are a promising solution to this problem. Guo et al. w6x have studied many of the different properties associated with cross-linked PEMs made from sulfonated polyphosphazenes. Vapor-phase sorptionrdesorption experiments have shown that the methanol diffusivity in cross-linked, sulfonated polyphosphazene membranes was ; 100 times lower than in Nafion 117. Recently, a new class of polyphosphazene membranes, phenyl phosphonic acid functionalized polyaryloxyphosphazenes Žphosphonated polyphosphazenes., has been developed in our program w7x. In this study, the permeability or diffusivity of methanol in Nafion 117, sulfonated polyphosphazene and phosphonated poly-
00167-577Xr02r$ - see front matter q 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 5 7 7 X Ž 0 1 . 0 0 3 9 1 - 3
M.V. Fedkin et al.r Materials Letters 52 (2002) 192–196
phosphazene membranes in an aqueous methanol solution 50% Žvrv. was examined. The goal of this work was to compare the methanol permeation in the new phosphonated polyphosphazene membranes to Nafion 117 and sulfonated polyphosphazene membranes.
2. Experimental approach 2.1. Tested membranes Nafion 117, produced by E.I. du Pont de Nemours, was obtained from Aldrich. Samples of Nafion 117 were pretreated as described previously w8x. Sulfonated polyphosphazene membranes were produced using a procedure similar to the one reported by Guo et al. w6x and were cross-linked with 60 Co gammaradiation at a dosage of 15 Mrad. Methods for the preparation of phosphonated polyphosphazene membranes will appear in a separate publication w7x. Details about the membrane properties may be found in Table 1. According to Zawodzinski et al. w9x, the diffusion coefficient of Nafion 117 depends on the water content in the membrane. A similar behavior can be expected from polyphosphazenes, in which the water and methanol diffusion coefficients of sulfonated polyphosphazene membranes have been found to be comparable in magnitude w6x. In this study, for the purpose of comparison, all membranes were soaked in high-purity water for at least 7 days before testing. Ion exchange capacities ŽIEC. of the sulfonated polyphosphazene w6x and phosphonated polyphosphazene w7x membranes were measured as described elsewhere. The electrical conductivity of
193
protons in water-equilibrated membranes was determined using a four-electrode electrochemical impedance spectroscopy ŽEIS. method w10,11x. 2.2. Experimental method Diffusion coefficient measurements were performed using the experimental system shown in Fig. 1. The membrane was secured inside a stainless steel cell separating the internal volume of the cell into two chambers. Two Teflon gaskets were used to seal the gaps between the membrane and the stainless steel components. The cell was installed as shown in Fig. 1 with the membrane situated horizontally. The lower chamber was filled with an aqueous methanol solution Ž50% vrv., which was continuously renewed by pumping fresh solution through the chamber at a constant flow rate of 1.2 mlrmin. The upper chamber Žsampling chamber. was filled with pure water of constant volume for each test. During testing, the solution in the sampling chamber was stirred approximately every 30 min to minimize the formation of a concentration gradient near the membrane surface. The transport of methanol through a membrane from the lower chamber of the cell to the upper chamber takes place by molecular diffusion through the polymer structure. Diffusion due to a hydrostatic pressure gradient was eliminated by leveling the methanol solution outlet with the height of the water column in the sample chamber as depicted in Fig. 1. All tests were conducted at ambient temperature Ž228C. and atmospheric pressure and the methanol solution was pumped through the lower chamber for 3 1r2 h, after which the solution from the sample
Table 1 Membrane properties Membrane
IEC Žmeqrg.
Thickness Žmm.
Conductivity Ž10y2 Srcm.
Cross-linking ŽMrad.
Phosphonated polyphosphazene ŽSample 1. Phosphonated polyphosphazene ŽSample 2. Sulfonated polyphosphazene ŽSample 1. Sulfonated polyphosphazene ŽSample 2. Nafion
1.13 1.20 1.12 1.54 0.91
114 152 175 152 203
2.8 2.6 0.8 6.4 6.2
0 0 15 15 0
194
M.V. Fedkin et al.r Materials Letters 52 (2002) 192–196
Fig. 1. Schematic of the experimental system used to measure the diffusion coefficients of methanol in polymer membranes.
chamber was collected via syringe. The methanol concentration of the samples was determined by gas chromatography ŽGC.. A calibration line was obtained by plotting the methanol to ethanol peak area ratio versus volume percent methanol using a set of standards containing 1% ethanol and varying amounts of methanol. The unknown samples from the diffusion test were each treated with 1% ethanol and analyzed in the gas chromatograph. The ratios of the
methanol peak area to ethanol peak area of the diffusion samples were averaged over three separate runs in the GC and then substituted into the equation of the line for the calibration data to determine the average volume percent methanol. The molar flux of methanol through the membrane Ž J, molrcm2 s. was determined from the change in the methanol concentration of the sample solution that occurred per unit of time. The methanol
M.V. Fedkin et al.r Materials Letters 52 (2002) 192–196
concentration of the sample solution changes with time as follows w12x: dCsample dt
sJ
Vsample
,
dt
s 0.
Ž 2.
Fick’s law defines diffusivity as a proportionality coefficient between the flux and concentration gradient w12x:
J s yD
Csample y Cmethanol l
,
Ž 3.
where D is the diffusion coefficient and l is the thickness of the membrane. The methanol concentration gradient changes with time because Cmethanol is
s yD
Csample y Cmethanol
A
l
Vsample
dt
Ž 1.
where Csample is the methanol concentration in the sample due to diffusion; Vsample is the volume of the sample solution; A is the cross-section area of membrane; and t is the time interval during which diffusion occurs. For the present experimental procedure, Cmethanol is constant over time, therefore: dCmethanol
constant and Csample increases with time due to methanol influx. Combination of Eqs. Ž1. – Ž3. yields: dCsample
A
195
.
Ž 4.
Integration of this equation between the limits Csample s C1 at t s 0 and Csample s C2 at t s texp gives: ln
C2 y Cmethanol C1 y Cmethanol
s yD
Atexp lVsample
.
Ž 5.
By rearranging Eq. Ž5., the expression for the diffusion coefficient can be derived as: Ds
lVsample Atexp
ln
C1 y Cmethanol C2 y Cmethanol
.
Ž 6.
3. Results and discussion The diffusion coefficients calculated from the experimental data using Eq. Ž6. are shown in Fig. 2. Data obtained in repeated runs for Nafion 117 and for a number of sulfonated polyphosphazene and phosphonated polyphosphazene membranes are presented for comparison. The diffusion coefficient for
Fig. 2. Experimentally obtained methanol diffusion coefficients of the membranes in 50% methanol solution.
196
M.V. Fedkin et al.r Materials Letters 52 (2002) 192–196
Nafion 117 ŽIEC s 0.909 meqrg. was found to vary between 7.45 = 10y7 and 1.34 = 10y6 cm2rs over four separate runs. Thus, the uncertainty of the measurements was "25%. The values obtained for the methanol diffusion coefficient for Nafion 117 are in fairly good agreement with the methanol diffusivity of 6.5 = 10y6 cm2rs reported for a membrane soaked in aqueous methanol Ž1.0 M. at 308C w13x. Two sulfonated polyphosphazene membranes were tested with IEC of 1.12 and 1.54 meqrg. The methanol diffusion coefficients were found to be 3.09 = 10y7 and 5.58 = 10y7 cm2rs, respectively. The methanol diffusion coefficient for a nonsulfonated polywbisŽ3-methylphenoxy.phosphazenex membrane was found to be 2.15 = 10y8 cm2rs. Not surprisingly, the methanol diffusion coefficient increases with increasing IEC. Methanol diffusion coefficients for an ultraviolet ŽUV. cross-linked, sulfonated polyphosphazene membrane ŽIEC s 1.4 meqrg. that was determined by vapor-phase sorptionrdesorption experiments conducted at 30–458C have been reported to be between 1.62 = 10y8 and 8.50 =10y8 cm2rs w6x. Vapor-phase permeation measurements have been described as giving values that are not relevant for liquid feed fuel cells, primarily because of surface barrier layers w14x. The discrepancy between the results obtained by this method and by vapor-phase sorptionrdesorption experiments may be an indication of such surface barrier layer effects. Investigation of methanol diffusion in a system that utilizes liquid-membrane interfaces Žas in the present work., as opposed to vapor-membrane interfaces, should minimize these effects. Two phosphonated polyphosphazene membranes were also tested with ion exchange capacities of 1.13 and 1.20 meqrg. Methanol diffusion coefficients for these phosphonated polyphosphazene membranes were found to be 2.49 = 10y8 and 2.46 = 10y8 cm2rs, respectively. As shown in Fig. 2, these values are ; 40 times lower than for Nafion 117, and ; 10–20 times lower than for sulfonated polyphosphazene membranes. Thus, phosphonated polyphosphazene membranes appear to be very promising for DMFCs applications. At present, the optimum concentration of methanol used in Nafion-based DMFCs is about 10%. Higher methanol concentrations do not produce a higher power output mainly due to increased methanol permeability. The high resistance
of the phosphonated polyphosphazene membranes to methanol permeation, as indicated by the low diffusion coefficients, should allow for a significant increase in the methanol concentration of the fuel feed, and hence allow for increasing the power output. Acknowledgements The authors acknowledge the support of this work by the US Department of Energy ŽContract no. DEPS02-98EE504493, CARAT Program.. The authors thank D.D. Macdonald for helpful discussions during the course of this study. References w1x X. Ren, T.E. Springer, S. Gottesfeld, J. Electrochem. Soc. 147 Ž2000. 92. w2x E. Skou, P. Kauranen, J. Hentschel, Solid State Ionics 97 Ž1997. 333. w3x M.V. Verbrudge, J. Electrochem. Soc. 136 Ž1989. 417. w4x J.-T. Wang, S. Wasmus, R.F. Savinell, J. Electrochem. Soc. 143 Ž1996. 1233. w5x S.N. Lvov, X.Y. Zhou, M.V. Fedkin, H.R. Allcock, M.A. Hofmann, A.M. Cannon, E.C. Kellam III, R.V. Morford, Development of Proton Conducting Membranes for High Temperature Direct Methanol Fuel Cells, Int. Symp. on Energy Engineering in the 21st Century, Hong Kong, Jan. 9–13, 2000. w6x Q. Guo, P.N. Pintauro, H. Tang, S. O’Connor, J. Membr. Sci. 154 Ž1999. 175. w7x H.R. Allcock, M.A. Hofmann, E. Chalkova, S.N. Lvov, X.Y. Zhou, J.A. Weston—in press. w8x X. Ren, M.S. Wilson, S. Gottesfeld, J. Electrochem. Soc. 143 Ž1996. L12. w9x T.A. Zawodzinski Jr, M. Neeman, L.O. Sillerud, S. Gottesfeld, J. Phys. Chem. 95 Ž1991. 6040. w10x X.Y. Zhou, S.N. Lvov, M. Fedkin, H.R. Allcock, M.A. Hofmann, E. Chalkova, J.A. Weston, Study of Conductivity of Polyphosphazene Membranes in Liquid Phase, in press. w11x B.D. Cahan, J.S. Wainright, J. Electrochem. Soc. 140 Ž1993. L185. w12x H.J.V. Tyrell, K.R. Harris, Diffusion in Liquids, Butterworth, London, 1984. w13x X. Ren, T.A. Zawodzinski, F. Uribe, H. Dai, S. Gottesfeld, Methanol crossover in direct methanol fuel cells, in: S. Gottesfeld, G. Halpert, A. Landgrebe ŽEds.., Proton Conducting Membrane Fuel Cells I, PV 95-23, The Electrochemical Society Proceedings Series, 1995, pp. 284–298, Pennington, NJ. w14x A.R. Landgrebe, J. Milliken, P. Maupin, R. Carlin, Basic and Applied Research Needs for Polymer–Electrolyte Membrane Fuel Cells, Workshop Proceedings, Oct. 6–8, Baltimore, MD, 1999.
Materials Letters 52 Ž2002. 192–196 www.elsevier.comrlocatermatlet
Evaluation of methanol crossover in proton-conducting polyphosphazene membranes Mark V. Fedkin a,b, Xiangyang Zhou a , Michael A. Hofmann c , Elena Chalkova a , Jamie A. Weston a , Harry R. Allcock c , Serguei N. Lvov a,b,) b
a The Energy Institute, The PennsylÕania State UniÕersity, 517 Deike Building, UniÕersity Park, PA 16802, USA Department of Energy and Geo-EnÕironmental Engineering, The PennsylÕania State UniÕersity, 517 Deike Building, UniÕersity Park, PA 16802, USA c Department of Chemistry, The PennsylÕania State UniÕersity, 517 Deike Building, UniÕersity Park, PA 16802, USA
Received 17 February 2000; accepted 12 April 2001
Abstract A diffusion cell was developed to evaluate the methanol crossover for a novel class of polyphosphazene electrolyte membranes. It was found that the methanol diffusion coefficients of phenyl phosphonic acid functionalized polywaryloxyphosphazenex membranes in an aqueous methanol solution Ž50% vrv. were ; 40 times lower than for Nafion 117, and ; 10–20 times lower than for sulfonated polyphosphazene membranes. q 2002 Elsevier Science B.V. All rights reserved. Keywords: Methanol crossover; Diffusion coefficients; Polyphosphazene electrolyte membranes; Polymer electrolyte fuel cells
1. Introduction Methanol crossover in polymer electrolyte membranes ŽPEMs. is one of the main issues impeding the development of direct-methanol fuel cells ŽDMFCs.. Methanol that is transported through the membrane is oxidized by oxygen at the cathode leading to a significant reduction in fuel cell performance w1–4x. The fact that Nafion membranes are subject to serious methanol crossover has stimulated
) Corresponding author. Center for Advanced Materials, The Pennsylvania State University, 517 Deike Building, University Park, PA 16802, USA. Tel.: q1-814-863-8377; fax: q1-814-8634718, q1-814-865-1573. E-mail address: [email protected] ŽS.N. Lvov..
a search for alternative polymers that are more resistant to methanol diffusion. Previous work in our program w5x and others w6x has shown that polyphosphazene-based membranes are a promising solution to this problem. Guo et al. w6x have studied many of the different properties associated with cross-linked PEMs made from sulfonated polyphosphazenes. Vapor-phase sorptionrdesorption experiments have shown that the methanol diffusivity in cross-linked, sulfonated polyphosphazene membranes was ; 100 times lower than in Nafion 117. Recently, a new class of polyphosphazene membranes, phenyl phosphonic acid functionalized polyaryloxyphosphazenes Žphosphonated polyphosphazenes., has been developed in our program w7x. In this study, the permeability or diffusivity of methanol in Nafion 117, sulfonated polyphosphazene and phosphonated poly-
00167-577Xr02r$ - see front matter q 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 5 7 7 X Ž 0 1 . 0 0 3 9 1 - 3
M.V. Fedkin et al.r Materials Letters 52 (2002) 192–196
phosphazene membranes in an aqueous methanol solution 50% Žvrv. was examined. The goal of this work was to compare the methanol permeation in the new phosphonated polyphosphazene membranes to Nafion 117 and sulfonated polyphosphazene membranes.
2. Experimental approach 2.1. Tested membranes Nafion 117, produced by E.I. du Pont de Nemours, was obtained from Aldrich. Samples of Nafion 117 were pretreated as described previously w8x. Sulfonated polyphosphazene membranes were produced using a procedure similar to the one reported by Guo et al. w6x and were cross-linked with 60 Co gammaradiation at a dosage of 15 Mrad. Methods for the preparation of phosphonated polyphosphazene membranes will appear in a separate publication w7x. Details about the membrane properties may be found in Table 1. According to Zawodzinski et al. w9x, the diffusion coefficient of Nafion 117 depends on the water content in the membrane. A similar behavior can be expected from polyphosphazenes, in which the water and methanol diffusion coefficients of sulfonated polyphosphazene membranes have been found to be comparable in magnitude w6x. In this study, for the purpose of comparison, all membranes were soaked in high-purity water for at least 7 days before testing. Ion exchange capacities ŽIEC. of the sulfonated polyphosphazene w6x and phosphonated polyphosphazene w7x membranes were measured as described elsewhere. The electrical conductivity of
193
protons in water-equilibrated membranes was determined using a four-electrode electrochemical impedance spectroscopy ŽEIS. method w10,11x. 2.2. Experimental method Diffusion coefficient measurements were performed using the experimental system shown in Fig. 1. The membrane was secured inside a stainless steel cell separating the internal volume of the cell into two chambers. Two Teflon gaskets were used to seal the gaps between the membrane and the stainless steel components. The cell was installed as shown in Fig. 1 with the membrane situated horizontally. The lower chamber was filled with an aqueous methanol solution Ž50% vrv., which was continuously renewed by pumping fresh solution through the chamber at a constant flow rate of 1.2 mlrmin. The upper chamber Žsampling chamber. was filled with pure water of constant volume for each test. During testing, the solution in the sampling chamber was stirred approximately every 30 min to minimize the formation of a concentration gradient near the membrane surface. The transport of methanol through a membrane from the lower chamber of the cell to the upper chamber takes place by molecular diffusion through the polymer structure. Diffusion due to a hydrostatic pressure gradient was eliminated by leveling the methanol solution outlet with the height of the water column in the sample chamber as depicted in Fig. 1. All tests were conducted at ambient temperature Ž228C. and atmospheric pressure and the methanol solution was pumped through the lower chamber for 3 1r2 h, after which the solution from the sample
Table 1 Membrane properties Membrane
IEC Žmeqrg.
Thickness Žmm.
Conductivity Ž10y2 Srcm.
Cross-linking ŽMrad.
Phosphonated polyphosphazene ŽSample 1. Phosphonated polyphosphazene ŽSample 2. Sulfonated polyphosphazene ŽSample 1. Sulfonated polyphosphazene ŽSample 2. Nafion
1.13 1.20 1.12 1.54 0.91
114 152 175 152 203
2.8 2.6 0.8 6.4 6.2
0 0 15 15 0
194
M.V. Fedkin et al.r Materials Letters 52 (2002) 192–196
Fig. 1. Schematic of the experimental system used to measure the diffusion coefficients of methanol in polymer membranes.
chamber was collected via syringe. The methanol concentration of the samples was determined by gas chromatography ŽGC.. A calibration line was obtained by plotting the methanol to ethanol peak area ratio versus volume percent methanol using a set of standards containing 1% ethanol and varying amounts of methanol. The unknown samples from the diffusion test were each treated with 1% ethanol and analyzed in the gas chromatograph. The ratios of the
methanol peak area to ethanol peak area of the diffusion samples were averaged over three separate runs in the GC and then substituted into the equation of the line for the calibration data to determine the average volume percent methanol. The molar flux of methanol through the membrane Ž J, molrcm2 s. was determined from the change in the methanol concentration of the sample solution that occurred per unit of time. The methanol
M.V. Fedkin et al.r Materials Letters 52 (2002) 192–196
concentration of the sample solution changes with time as follows w12x: dCsample dt
sJ
Vsample
,
dt
s 0.
Ž 2.
Fick’s law defines diffusivity as a proportionality coefficient between the flux and concentration gradient w12x:
J s yD
Csample y Cmethanol l
,
Ž 3.
where D is the diffusion coefficient and l is the thickness of the membrane. The methanol concentration gradient changes with time because Cmethanol is
s yD
Csample y Cmethanol
A
l
Vsample
dt
Ž 1.
where Csample is the methanol concentration in the sample due to diffusion; Vsample is the volume of the sample solution; A is the cross-section area of membrane; and t is the time interval during which diffusion occurs. For the present experimental procedure, Cmethanol is constant over time, therefore: dCmethanol
constant and Csample increases with time due to methanol influx. Combination of Eqs. Ž1. – Ž3. yields: dCsample
A
195
.
Ž 4.
Integration of this equation between the limits Csample s C1 at t s 0 and Csample s C2 at t s texp gives: ln
C2 y Cmethanol C1 y Cmethanol
s yD
Atexp lVsample
.
Ž 5.
By rearranging Eq. Ž5., the expression for the diffusion coefficient can be derived as: Ds
lVsample Atexp
ln
C1 y Cmethanol C2 y Cmethanol
.
Ž 6.
3. Results and discussion The diffusion coefficients calculated from the experimental data using Eq. Ž6. are shown in Fig. 2. Data obtained in repeated runs for Nafion 117 and for a number of sulfonated polyphosphazene and phosphonated polyphosphazene membranes are presented for comparison. The diffusion coefficient for
Fig. 2. Experimentally obtained methanol diffusion coefficients of the membranes in 50% methanol solution.
196
M.V. Fedkin et al.r Materials Letters 52 (2002) 192–196
Nafion 117 ŽIEC s 0.909 meqrg. was found to vary between 7.45 = 10y7 and 1.34 = 10y6 cm2rs over four separate runs. Thus, the uncertainty of the measurements was "25%. The values obtained for the methanol diffusion coefficient for Nafion 117 are in fairly good agreement with the methanol diffusivity of 6.5 = 10y6 cm2rs reported for a membrane soaked in aqueous methanol Ž1.0 M. at 308C w13x. Two sulfonated polyphosphazene membranes were tested with IEC of 1.12 and 1.54 meqrg. The methanol diffusion coefficients were found to be 3.09 = 10y7 and 5.58 = 10y7 cm2rs, respectively. The methanol diffusion coefficient for a nonsulfonated polywbisŽ3-methylphenoxy.phosphazenex membrane was found to be 2.15 = 10y8 cm2rs. Not surprisingly, the methanol diffusion coefficient increases with increasing IEC. Methanol diffusion coefficients for an ultraviolet ŽUV. cross-linked, sulfonated polyphosphazene membrane ŽIEC s 1.4 meqrg. that was determined by vapor-phase sorptionrdesorption experiments conducted at 30–458C have been reported to be between 1.62 = 10y8 and 8.50 =10y8 cm2rs w6x. Vapor-phase permeation measurements have been described as giving values that are not relevant for liquid feed fuel cells, primarily because of surface barrier layers w14x. The discrepancy between the results obtained by this method and by vapor-phase sorptionrdesorption experiments may be an indication of such surface barrier layer effects. Investigation of methanol diffusion in a system that utilizes liquid-membrane interfaces Žas in the present work., as opposed to vapor-membrane interfaces, should minimize these effects. Two phosphonated polyphosphazene membranes were also tested with ion exchange capacities of 1.13 and 1.20 meqrg. Methanol diffusion coefficients for these phosphonated polyphosphazene membranes were found to be 2.49 = 10y8 and 2.46 = 10y8 cm2rs, respectively. As shown in Fig. 2, these values are ; 40 times lower than for Nafion 117, and ; 10–20 times lower than for sulfonated polyphosphazene membranes. Thus, phosphonated polyphosphazene membranes appear to be very promising for DMFCs applications. At present, the optimum concentration of methanol used in Nafion-based DMFCs is about 10%. Higher methanol concentrations do not produce a higher power output mainly due to increased methanol permeability. The high resistance
of the phosphonated polyphosphazene membranes to methanol permeation, as indicated by the low diffusion coefficients, should allow for a significant increase in the methanol concentration of the fuel feed, and hence allow for increasing the power output. Acknowledgements The authors acknowledge the support of this work by the US Department of Energy ŽContract no. DEPS02-98EE504493, CARAT Program.. The authors thank D.D. Macdonald for helpful discussions during the course of this study. References w1x X. Ren, T.E. Springer, S. Gottesfeld, J. Electrochem. Soc. 147 Ž2000. 92. w2x E. Skou, P. Kauranen, J. Hentschel, Solid State Ionics 97 Ž1997. 333. w3x M.V. Verbrudge, J. Electrochem. Soc. 136 Ž1989. 417. w4x J.-T. Wang, S. Wasmus, R.F. Savinell, J. Electrochem. Soc. 143 Ž1996. 1233. w5x S.N. Lvov, X.Y. Zhou, M.V. Fedkin, H.R. Allcock, M.A. Hofmann, A.M. Cannon, E.C. Kellam III, R.V. Morford, Development of Proton Conducting Membranes for High Temperature Direct Methanol Fuel Cells, Int. Symp. on Energy Engineering in the 21st Century, Hong Kong, Jan. 9–13, 2000. w6x Q. Guo, P.N. Pintauro, H. Tang, S. O’Connor, J. Membr. Sci. 154 Ž1999. 175. w7x H.R. Allcock, M.A. Hofmann, E. Chalkova, S.N. Lvov, X.Y. Zhou, J.A. Weston—in press. w8x X. Ren, M.S. Wilson, S. Gottesfeld, J. Electrochem. Soc. 143 Ž1996. L12. w9x T.A. Zawodzinski Jr, M. Neeman, L.O. Sillerud, S. Gottesfeld, J. Phys. Chem. 95 Ž1991. 6040. w10x X.Y. Zhou, S.N. Lvov, M. Fedkin, H.R. Allcock, M.A. Hofmann, E. Chalkova, J.A. Weston, Study of Conductivity of Polyphosphazene Membranes in Liquid Phase, in press. w11x B.D. Cahan, J.S. Wainright, J. Electrochem. Soc. 140 Ž1993. L185. w12x H.J.V. Tyrell, K.R. Harris, Diffusion in Liquids, Butterworth, London, 1984. w13x X. Ren, T.A. Zawodzinski, F. Uribe, H. Dai, S. Gottesfeld, Methanol crossover in direct methanol fuel cells, in: S. Gottesfeld, G. Halpert, A. Landgrebe ŽEds.., Proton Conducting Membrane Fuel Cells I, PV 95-23, The Electrochemical Society Proceedings Series, 1995, pp. 284–298, Pennington, NJ. w14x A.R. Landgrebe, J. Milliken, P. Maupin, R. Carlin, Basic and Applied Research Needs for Polymer–Electrolyte Membrane Fuel Cells, Workshop Proceedings, Oct. 6–8, Baltimore, MD, 1999.