Proton conducting behavior of a novel polymeric gel membrane bad on poly(ethylene oxide)-grafted-poly(methacrylate)
Jinli Qiao,Nobuko Yoshimoto,Masayuki Morita *
Department of Applied Chemistry and Chemical Engineering,Faculty of Engineering,Yamaguchi University,
2-16-1Tokiwadai,Ube 755-8611,Japan
Received 16May 2001;received in revid form 10September 2001;accepted 25September 2001
Abstract
A novel proton conducting polymeric gel membrane that consists of poly(ethylene oxide)-grafted-poly(methacrylate)(PEO-PMA)with poly(ethylene glycol)dimethyl ether (PEGDE)as a plasticizer doped with aqueous phosphoric acid (H 3PO 4)has been prepared and its physicochemical properties were studied in detail.The ionic conductivity was dependent much on the concentration of H 3PO 4,the immersion time,and content of the plasticizer.This type of proton conducting polymeric gels shares not only good mechanical properties but also thermal stability.Maximum conductivities up to 2:6Â10À2S c
m À1at room temperature (258C)and 2:8Â10À2S cm À1at 708C were obtained for the composition of the polymer matrix to the plasticizer as 35/65(in mass)after the H 3PO 4doping from the aqueous solution with 2.93mol l À1.FT-IR spectra showed that the high proton conductivities are attributed to the prence of excess free H 3PO 4in the polymeric gel in addition to the hydrogen-bonded H 3PO 4to the polymer matrix.#2002Elvier Science B.V .All rights rerved.
Keywords:Polymeric gel;Proton conduction;Plasticizer;H 3PO 4;FT-IR
1.Introduction
Solid polymer electrolyte fuel cells (SPEFCs)bad on proton conducting polymers have been identi®ed as promis-ing power sources for electric vehicles due to its high conductivity and non-polluting characteristics [1±3].They overcome many drawbacks,such as leakage and dif®culty in electrolyte management,that fuel cells with conventional liquid electrolytes posss.In SPEFCs,ion exchange mem-branes play a vital role on obtaining not only the high ionic conductivity but also thermal and mechanical stabilities.Na®on 1,as a typical candidate,is known to be the most successfully ud ionic conductor owing to its high proton conductivity (10À2S cm À1or higher)and chemical stability,but its excessive cost (US$750m À2)and unstable properties at high temperature s
till hinder the complete application of this technology [4].Whereas proton conducting polymer composites consisting of neutral polymers doped with inor-ganic acids [5]have high conductivities even below the glass transition temperature,T g [6].This type of polymer electrolytes,contrary to Na®on 1-type ones,posss a lot of merits such as low cost (US$100m À2)and high
conductivity both in hydrated and dehydrated states (up to 0.1S cm À1).The low cost materials have many potential applications in a variety of electrochemical devices includ-ing fuel cells,electrode coatings,actuators,arti®cial mus-cles,and nsors [7].Hence,the development of cheaper and better proton conducting polymer electrolyte membranes than tho of conventional materials is urgently needed.In this study,we developed a novel proton conducting polymeric gel membrane that consists of poly(ethylene oxide)-grafted-poly(methacrylate)(PEO-PMA)matrix with poly(ethylene glycol)dimethyl ether (PEGDE)as a plasti-cizer doped with aqueous H 3PO 4solution.We have pre-viously reported that this type of plasticized polymer system containing rare earth salts exhibited high ionic conductivity of 10À4to 10À5S cm À1at room temperature [8,9].Due to the hydrophilic characteristics of poly(ethylene oxide)(PEO),it is also possible for PEO-PMA to cast a good proton conductor from strong acid solutions.Experiments show that the proton conducting polymeric gel membrane developed in the prent study shares a number of attractive properties,su
执行工作ch as a wider optical transparency range,ea of preparation,good mechanical properties,thermal stabi-lity,and high proton conductivity at room temperature.Therefore,it is believed that this type of proton-conducting polymeric gel membranes could have a number of electro-chemical
applications.
Journal of Power Sources 105(2002)45±51
*
Corresponding author.Tel.: 81-836-85-9211;fax: 81-836-85-9201.E-mail address :morita@po.yamaguchi-u.ac.jp (M.Morita).
0378-7753/02/$±e front matter #2002Elvier Science B.V .All rights rerved.PII:S 0378-7753(01)00963-6
2.Experimental
2.1.Preparation of the polymeric gel electrolyte,PEO-PMA/PEGDE/H 3PO 4
Poly(ethylene oxide)monomethacrylate (PEM)and poly(ethylene oxide)di-methacrylate (PED)(Shin-Naka-mura Chemical Co.Ltd.)were ud as prepolymers (macro-mers)for the matrix formation [9].Poly(ethylene glycol)dimethylether (PEGDE,M W ca :400)(Toho Chemical Co.Ltd.)was added into the mixed solution of the macro-mers (PEM:PED 3:1by molar ratio).This was followed by the addition of 2,2-dimethoxy-2-phenylacetophenone (99%purity)as an initiator and mixing the entire solution to homogeneity.The resulting mixture onto an aluminum plate was then expod to UV light for polymerization at room temperature to yield cross-linked polymer matrix,PEO-PMA (Fig.1).The en
明知做戏tire operations were carried out under a dry Ar atmosphere.Then the prepared polymeric gel membranes were swollen in H 3PO 4aqueous solutions with various concentrations and immersion times.In this stage,H 3PO 4was doped in the polymeric membrane (PEO-PMA/PEGDE/H 3PO 4).The surface moisture on the ®lms was removed carefully by wiping before measurements.Finally,the ®lms were dried at 508C under a vacuum up to constant weight,and then the absorption level of H 3PO 4was determined by measuring the change in mass before and after the immersion.In this paper,H 3PO 4uptake will be reported as the mass ratio of H 3PO 4to the net polymer mass.To discuss conveniently,however,we will u the molar concentration of aqueous H 3PO 4solutions as a parameter in¯uencing the polymer properties.
2.2.Conductivity measurements
PEO-PMA/PEGDE/H 3PO 4complexes were presd into round disks (13mm 1,60±110m m thick)at ®rst,then sandwiched between two platinum electrodes packed in a aled stainless steel cell for conductivity measurements.The ionic conductivity was determined by a complex impe-dance method done in a frequency range from 100kHz to 10Hz using an NF Electronics S-5720C impendence analyzer controlled by a personal computer.Temperature dependence of the conductivity was measured using a model SB-9EYELA bath under controlled temperature range of 2
0±708C.The atmospheric humidity in the cell will be equilibrated with the water in the gel membrane during the measurements.
2.3.Characterization of PEO-PMA/PEGDE/H 3PO 4complexes
PEO-PMA/PEGDE/H 3PO 4complexes were identi®ed by FT-IR spectroscopy (Model FT-IR-4200spectrometer,Shi-madzu Co.Ltd.).The gel ®lms were ground into powder,and the KBr mixtures were presd into a round disc.FT-IR spectra were recorded with a wavenumber resolution of 2cm À1in the wavenumber range from 4000to 400cm À1.3.Results and discussion
3.1.FT-IR spectrum characteristics of PEO-PMA/PEGDE/H 3PO 4complexes
The polymeric ®lm (PEO-PMA/PEGDE)was obtained in a lf-standing form with suf®cient ¯exibility even after the immersion in aqueous H 3PO 4solutions [8,9].An important characteristic of any electrolyte in relation to the ionic conductivity is,in fact,the number of charge carriers.In order to make clear any interactions between the dopant H 3PO 4and the PEO-PMA matrix,FT-IR spectra were registered in the wavenumber region between 3500and 400cm À1,which covers the whole range of all of the IR vibration characteristics.Fig.2prents the FT-IR spectra of polymeric gels with and without H 3PO 4doping that were prepared by immersing the polymeric ®lms with different conc
entrations of H 3PO 4aqueous solutions.The absorption peaks at 2900,1725and 1115cm À1are ascribed to the stretching vibrations of C±H,C=O,and C±O bonds in the polymer matrix,respectively [10].When the polymeric gel ®lms were immerd in aqueous solution of 0.43mol l À1H 3PO 4,the absorption peak at 1040cm À1split into two peaks at 1025and 1002cm À1.The latter peak comes from the stretching vibration characteristics of P±O(H)bond [10,11].The intensity of vibration characteristics of P±O(H)bonds incread greatly with an increa in H 3PO 4concentration.Then the absorption band at 1115cm À1that comes from the ester group of the polymer matrix
was
Fig.1.A scheme for preparation of cross-linked polymer matrix with a plasticizer (PEGDE).
46J.Qiao et al./Journal of Power Sources 105(2002)45±51
地奥司明片的作用
masked by stronger mode due to excess H 3PO 4when its concentration in the solution was up to 5.95mol l À1.The absorption peak at 1725cm À1due to n (C=O)was weakened and a broad peak centered at 2500±3500cm À1appeared with increasing the H 3PO 4concentration.This suggests that the hydrogen bond interaction occurs between O atoms of C=O in the polymer matrix and phosphoric acids.In addi-tion,a new absorption peak at 493cm À1and a shoulder peak at 890cm À1appeared and became evident as H 3PO 4con-centration was over 0.85mol l À1.This result,as well as the fact that the maximal position of the P±O(H)vibrational band of H 3PO 4was kept at 1002cm À1over the whole concentration range of H 3PO 4,indicates that the excess free H 3PO 4is prent in the polymeric gels with very weak interaction to the host polymer [10±12].
3.2.Dependence of ionic conductivity on the composition Fig.3shows the ionic conductivity of the PEO-PMA/PEGDE/H 3PO 4complexes prepared from various H 3PO 4concentrations as a function of the immersion time.It can be en that the speci®c conductivity of the PEO-PMA/PEGDE/H 3PO
4complex at room temperature depended on the immersion time.It ®rst incread to a maximum at 2±3h as the samples were soaked in aqueous H 3PO 4solu-tions,then decread with longer immersion time.Similar trends were obtained for the concentration dependence,where the speci®c conductivity of the samples ®rst incread to a maximum of 2:1Â10À2S cm À1as H 3PO 4concentra-tion was incread from 0.43to 2.93mol l À1,then decread
with H 3PO 4concentration (e Fig.4).Here,the most of the ionic conductivity would be bad on the proton conduction,judging from its magnitude and dependence on the acid concentration in the immersing solution.Due to the hydro-philic characteristics,the polymer could be swollen well in water.Hence,it allows the excess of H 3PO 4to remain ®xed in the interspaces of the gel framework together with water,resulting in the higher proton conductivity.The decreasing trend of the conductivity with longer immersion time (Fig.3)or higher H 3PO 4concentration (Fig.4)is a common phe-nomenon obrved in polymer electrolytes and can be explained as the result of weak ionic mobility since the degree of freedom of the ion transport is reduced as so much H 3PO 4enters into the polymeric gel [13,14].In addition,due to a lower dielectric constant of PEGDE (e r <8),it can increa the ionic association at high H 3PO 4concentration.Thus,the formation of ion-pairs and higher ion-multiples may occur in PEGDE and the associated species lowers the conductivity [15,16].
We noted that the polymeric gel membranes prented in this work all manifest the good mechanical properties and chemical stabilities even soaked in high H 3PO 4concentra-tions.It was found that membrane samples tended to shrink when they are taken out from the aqueous acid solutions and dried naturally in the air for 1day or long.Meantime,the color of the membranes changed from colorless transparent to yellow with increasing the H 3PO 4content.Table 1sum-marizes the amounts of H 3PO 4uptake and water content after the immersion of the polymeric gel membrane in aqueous solutions with different concentrations of H 3PO 4.As mentioned above,the H 3PO 4content depended not only on the acid concentration in the solution but also on the immersion time.In this table,the values obtained for the polymers immerd for 3h are compared each other.When the dried membrane samples were re-introduced into their corresponding original acid solutions,both the membranes dimension and their speci®c conductivity soon recovered.And the yellow color of the membrane samples (which is a mark of the prence of free H 3PO 4in the polymeric gel [12,17])at high H 3PO 4concentrations disappeared to recover their original colorless transparency again.
This
Fig.2.FT-IR spectra registered for PEO-PMA/H 3PO 4-bad gels.The concentration of H 3PO 4from bottom to top (mol l À1):(a)0.00;(b)0.43;(c)0.85;(d)1.70;(e)2.93;(f)3.57and (g)4.69,respectively.
大象无形Table 1
Gel composition of H 3PO 4-doped polymer complex (PEO-PMA/PEGDE/H 3PO 4)a Concentration of H 3PO 4(mol l À1)Maximum H 3PO 4uptake per gram of polymer (g)Average of
糖果子弹maximum water uptake per gram of polymer (g)
Color after drying
0.430.13None 0.850.34None 1.700.71 4.41
Yellowish 2.93 1.02Yellowish 3.57 1.16Yellowish 4.69
2.22
Yellowish
a萃取分液
遁迹Gel composition:PEO-PMA/PEGDE 38/62(in mass).
J.Qiao et al./Journal of Power Sources 105(2002)45±5147
obrvation also suggests that the proton conducting mem-branes developed in this study posss good reproducibility of conductivity and lifetime of usage.
The effect of the plasticizer content on the ionic con-ductivity is prented in Fig.5,where the PEGDE content ranged from 38to 65mass%.A higher s -value was obrved for the membrane with higher PEGDE content.At the PEGDE content of 65%,the room temperature conductivity rais 6.3-fold as that at the lowest PEGDE content of 38%.Evidently,this can be attributed to an increa in the number of charge carriers due to PEGDE as the entrapped solvent.The low permittivity and protophilic nature of PEGDE allows it to promote the ion transport through the protona-tion by H 3PO 4.Thus,the higher the content of PEGDE,the higher the proton conductivity value was [13,18].PEGDE was also noted to have a great in¯uence on the mechanical stability of the polymeric gel membranes.It was found that the polymeric membranes became much brittle to broken easily when the content of PEGDE was lower than 35%,and very soft to be dif®cult in forming ®lms and ha
rd to handle as the PEGDE content was higher than 65%.Therefore,it
can
Fig.3.Conductivity changes of gel electrolytes with immersion time under different H 3PO 4concentrations (mol l À1):(1)0.43;(2)0.85;(3)1.70;(4)2.93;(5)3.57;(6)4.69.(*)PEO-PMA/PEGDE 50/50(in mass);(*)PEO-PMA/PEGDE 38/62(in
mass).
Fig.4.Conductivity changes with the H 3PO 4concentrations,at immersion time of 2h.(*)PEO-PMA/PEGDE 50:50(in mass);(*)PEO-PMA/PEGDE 38/62(in mass).
48J.Qiao et al./Journal of Power Sources 105(2002)45±51
be concluded that PEGDE is both functioned as a plasticizer to stabilize the polymer membrane and as a complexing reagent to make a high s -value.In this work,the ratio of 38/62(in mass)of the matrix PEO-PMA to the plasticizer PEGDE was the optimum proportion both for obtaining high conductivity and good mechanical properties.3.3.Temperature dependence of the conductivity Fig.6shows the temperature dependence of the ionic conductivity for polymeric gel membranes with different H 3PO 4concentrations.It can be en that all the samples ®tted well with Arrhenius relations when the H 3PO 4con-centration of the soaked solution was lower than 3mol l À1.The plot for the sample with low H 3PO 4concentration
(0.43mol l À1)showed a light curvature.In Table 2,the maximum conductivity (s max )and the corresponding tem-perature (T max )obtained for each H 3PO 4concentration are summarized together with the apparent activation energy (E a )for ionic conduction that were determined from the
slopes of the Arrhenius plots.The maximum conductivity of 2:8Â10À2S cm À1was obtained at 708C for the gel swollen with H 3PO 4concentration of 2.93mol l À1.With the increa in the H 3PO 4concentration,the activation energy for the conduction decread gradually.This is in agreement with the fact that the density of ions in the gel increas with increasing the H 3PO 4concentration (Table 1),thus the energy barrier to the proton transport decreas,which would lead to a decrea in the activation energy [19].The values of the activation energy obtained from the Arrhenius plots were generally lower than 10kJ mol À1,which indicate that the proton transport mainly occurs via a Grotthus-type mechanism [20],in which the proton trans-port proceeds through the hydrogen bond.However,consid-ering that the conductivity measured for the prent system is much higher than that of the conventional polymeric
gel
Fig.5.Ionic conductivity at room temperature (258C)as a function of the plasticizer (PEGDE)
content.
Fig.6.Arrhenius plots for the ionic conductivity of PEO-PMA-bad gel electrolytes.Gel composition:PEO-PMA/PEGDE 38/62(in mass);concentrations of H 3PO 4(mol l À1):(*)0.43,(*)0.85,(^)1.70,(^)2.93,(!)3.57,(S )4.63.Table 2
Summary of the thermo-conductance analysis for PEO-PMA/PEGDE/H 3PO 4complexes a Concentration of H 3PO 4(mol l À1)T max (8C)s max (S cm À1)E a (kJ mol À1)0.43709.4Â10À3 6.70.85709.8Â10À3 6.11.7070 2.2Â10À2 4.12.9370 2.8Â10À2 4.73.5750 1.9Â10À2±4.69
30
2.2
Â10À2
±
a
Gel composition:PEO-PMA/PEGDE 38/62(in
mass).
Fig.7.Isotherms of the ionic conductivity of PEO-PAM/PEGDE/H 3PO 4-bad gel electrolytes as a function of H 3PO 4concentration in the solution.Gel composition:PEO-PMA/PEGDE 38/62(in mass);temperature (K):(*)293,(*)303,(^)313,(^)323,(~)333.
复方黄黛片
J.Qiao et al./Journal of Power Sources 105(2002)45±5149
