Chem. Mater. 2003, 15, 2005-2010
2005
A Proton Conductor Based on a Polymeric Complex of Poly(Ethylene Oxide)-Modified Poly(Methacrylate) with Anhydrous H3PO4 Jinli Qiao, Nobuko Yoshimoto, Masashi Ishikawa, and Masayuki Morita* Department of Applied Chemistry and Chemical Engineering, Faculty of Engineering, Yamaguchi University, 2-16-1 Tokiwadai, Ube 755-8611, Japan Received November 5, 2002. Revised Manuscript Received February 25, 2003
Nonaqueous proton-conducting gel electrolyte has been prepared by incorporating anhydrous H3PO4 in poly(ethylene oxide)-modified poly(methacrylate) (PEO-PMA) plasticized by poly(ethylene glycol) dimethyl ether (PEGDE). Optically transparent polymeric gel membranes were obtained in a wide range of the component ratio. The sample membranes have been characterized by AC conductivity, FTIR, and DSC measurements. High conductivity of 1.3 × 10-4 S cm-1 was obtained at room temperature for the composition of the (PEOPMA)/PEGDE mass ratio 32:68. The ionic conductivity was enhanced by the addition of small amounts of the second plasticizer, dimethylformamide (DMF). The DC polarization experiments of the gel membranes with Pt blocking electrodes revealed that the charge transport in the gel was mainly ionic (tion > 0.99), where the type of the major mobile species was considered to be H+ cation with mobility µ ) 2.5 × 10-4 cm2 V-1 s-1. The FTIR spectroscopy proved that hydrogen-bonding and protonic interactions exist between the dopant H3PO4 and the PEO-PMA matrix and/or entrapped plasticizer in the gel. In all cases, the Arrhenius plots of the conductivity showed linear relations in the temperature range of 22-90 °C. The mechanism of the ion transport in the nonaqueous gel is briefly discussed.
Introduction Proton-conducting polymeric materials have aroused much interest in fundamental research for understanding ionic conduction properties, as well as the field of practical applications because of their possible use as thin film solid electrolytes in various electrochemical devices.1,2 Many efforts have been devoted to polyelectrolytes, or ionomers, such as Nafion because of their high proton conductivity (∼10-1 S cm-1) in the ambient temperature range,3-6 and to solid polymer complexes doped with inorganic or organic acids.7-17 In these * Corresponding author. Phone: +81-836-85-9211. Fax: +81-83685-9201. E-mail:
[email protected]. (1) Rikukawa, M.; Sanui, K. Prog. Polym. Sci. 2000, 25, 1463. (2) Kreuer, K. D. Solid State Ionics 1997, 97, 1. (3) Nouel, K. M.; Fedkiw, P. S. Electrochim. Acta 1998, 43, 2381. (4) Edmondson, C. A.; Stallworth, P. E.; Wintersgill, M. C.; Fontanella, J. J.; Dai, Y.; Greenbaum, S. G. Electrochim. Acta. 1998, 43, 1295. (5) Tricoli, V. J. Electrochem. Soc. 1998, 145, 3798. (6) Ludvigsson, M.; Lindgren, J.; Tegenfeldt, J. J. Electrochem. Soc. 2000, 147, 1303. (7) Wainright, J. S.; Wang, J.-T.; Weng, D.; Savinell, R. F.; Litt, M. J. Electrochem. Soc. 1995, 142, L121. (8) Wang, J.-T.; Wainright, J. S.; Savinell, R. F.; Litt, M. J. Appl. Electrochem. 1996, 26, 751. (9) Tsuruhara, K.; Rikukawa, M.; Sanui, K.; Ogata, N.; Nagasaki, Y.; Kato, M. Electrochim. Acta 2000, 45, 1391. (10) Glipa, X.; Haddad, M. E.; Jones, D. J.; Roziere, J. Solid State Ionics 1997, 97, 323. (11) Fontanella, J. J.; Wintersgill, M. C.; Wainright, J. S.; Savinell, R. F.; Litt, M. Electrochim. Acta 1998, 43, 1289. (12) Kawahara, M.; Mortia, J.; Rikukawa, M.; Sanui, K.; Ogata, N. Electrochim. Acta 2000, 45, 1395. (13) Miyatake, K.; Fukushima, K.; Takeoke, S.; Tsuchida, E. Chem. Mater. 1999, 11, 1171. (14) Samms, S. R.; Wasmus, S.; Savinell, R. F. J. Electrochem. Soc. 1996, 143, 1225.
polymer systems, the proton conduction is based on the migration of hydronium ions (H3O+) through the hydrophilic clusters of sulfonate aggregates,13 or hopping between the “gap” of the polymer matrix or the hydrophilic functional groups such as C-O, CdO, and C-N bonds,14,15 which require water supply for retaining their proton conductivity. However, the presence of water causes some problems in the devices containing moisture sensitive materials (eg., electrochromic windows). Also, degradation of the polymer will occur under strong acidic conditions with considerable amounts of water.18-20 Thus, polymers with polar units such as poly(oxyethylene) (or poly[ethylene oxide]),21 poly(iminoethylene),22 and poly(vinyl alcohol)23 have been used to replace water as a matrix for H2SO4 and H3PO4. Recently, it has been demonstrated that dissolution of H3PO4 in poly(methyl methacrylate) plasticized by dimethylformamide (PMMA/DMF) or poly(glycidyl methacrylate) with DMF or propylene carbonate (PGMA/ (15) Vargas, M. A.; Vargas, R. A.; Mellander, B.-E. Electrochim. Acta 1999, 44, 4227. (16) Vargas, M. A.; Vargas, R. A.; Mellander, B.-E. Electrochim. Acta. 2000, 45, 1399. (17) Reddy, D. S.; Reddy, M. J.; Rao, U. V. S. Mater. Sci. Eng. 2000, B78, 59. (18) Stevens, J. R.; Wieczorek, W.; Raducha, D.; Jeffrey, K. R. Solid State Ionics 1997, 97, 347. (19) Daniel, M. F.; Desbat, B.; Lassegues, J. C. Solid State Ionics 1988, 28-30, 632. (20) Lassegues, J. C.; Desbat, B.; Trinquet, O.; Cruege, F.; Poinsignon, C. Solid State Ionics 1989, 35, 17. (21) Donoso, P.; Gorecki, W.; Berthier, C. Solid State Ionics 1988, 28-30, 969. (22) Senadeera, G. K. R.; Careem, M. A.; Skaarup, S.; West, K. Solid State Ionics 1996, 85, 37. (23) Gupta, P. N.; Singh, K. P. Solid State Ionics 1996, 86-88, 319.
10.1021/cm021076s CCC: $25.00 © 2003 American Chemical Society Published on Web 04/22/2003
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DMF, PGMA/PC)18,24-32 produces so-called polymeric gel electrolytes with high proton conductivities around 10-4 S cm-1 at ambient temperatures. Because these kinds of polymeric gels are synthesized using anhydrous solvents under a dry atmosphere, the drawbacks mentioned above are avoided. In addition, the absence of water simplifies, in principle, the study of the conduction behavior of the system, and is likely to provide better electrochemical and thermal stability of the material. Here we report a novel nonaqueous polymeric gel electrolyte that consists of poly(ethylene oxide)-modified poly(methacrylate) (PEO-PMA) containing poly(ethylene glycol dimethyl ether) (PEGDE) as an organic plasticizer, in which anhydrous H3PO4 was chosen as a proton donor. A variety of experiments have been made for optimizing the gel composition to achieve high ionic conductivity as well as chemical stability. Possible mechanisms for the proton conduction in the gel are discussed based on experimental data such as AC conductivity, DSC profiles, FTIR spectra, and ionic transport number. The thermal stability of the gel membrane is also evaluated by the time dependence of the conductivity at elevated temperatures. Experimental Section Materials. Poly(ethylene oxide) monomethacrylate (PEM) and poly(ethylene oxide) dimethacrylate (PED) (Shin-Nakamura Chemical) were used as prepolymers for the matrix formation.33,34 The prepolymers PEM and PED, and plasticizing organic solvents PEGDE (MW ) ca. 400, Toho Chemical, reagent grade) and DMF (Futaba Chemicals, spectroscopic grade), were dehydrated twice by molecular sieves prior to use, and then kept in a glovebox filled with dry Ar. The dopant H3PO4 (anhydrous, Merck) was used as received. Sample Preparation. Nonaqueous polymeric gel membranes were prepared by photoinduced radical polymerization.33,34 Anhydrous H3PO4 was dissolved in PEGDE, or a mixture of PEGDE and DMF. Then, PEM/PED (3:1 by molar ratio) and a radical initiator, 2,2-dimethoxy-2-phenylacetophenone (99% purity), were added to the H3PO4/PEGDE (or H3PO4/PEGDE + DMF) solution, followed by continuous stirring until a homogeneous solution was obtained. The resulting mixture was then developed onto an Al plate and exposed to UV light for polymerization at room temperature, yielding H3PO4-doped poly(ethylene oxide)-modified poly(methacrylate) polymeric gel: (PEO-PMA)/PEGDE (or PEGDE + DMF)/H3PO4 (Figure 1). All steps of the preparation were carried out under a dry Ar atmosphere. (24) Raducha, D.; Wieczorek, W.; Florjanczyk, Z.; Stevens, J. R. J. Phys. Chem. 1996, 100, 20126. (25) Grillone, A. M.; Panero, S.; Retamal, B. A.; Scrosati, B. J. Electrochem. Soc. 1999, 146, 27. (26) Zukowska, G.; Rogowska, M.; Weczkowska, E.; Wieczorek, W. Solid State Ionics 1999, 119, 289. (27) Vaivars, G.; Azens, A.; Granqvist, C. G. Solid State Ionics 1999, 119, 269. (28) Ericson, H.; Svanberg, C.; Brodin, A.; Grillone, A. M.; Panero, S.; Scrosati, B.; Jacobsson, P. Electrochim. Acta 2000, 45, 1409. (29) Zukowska, G.; Monikowska, E. Z.; Langwald, N.; Folrjanczyk, Z.; Borkowska, R.; Kuzma, P.; Wieczorek, W.; Greenbaum, S.; Chung, S. H. J. New Mater. Electrochem. Syst. 2000, 3, 51. (30) Tanaka, R.; Yamamoto, H.; Shono, A.; Kubo, K.; Sakurai, M. Electrochim. Acta 2000, 45, 1385. (31) Zukowska, G.; Chojnacka, N.; Wieczorek, W. Chem. Mater. 2000, 12, 3578. (32) Wieczorek, W.; Zukowska, G.; Borkowska, R.; Chung, S. H.; Greenbaum, S. Electrochim. Acta 2001, 46, 1427. (33) Qiao, J. L.; Yoshimoto, N.; Morita, M. J. Power Sources 2002, 105, 45. (34) Qiao, J. L.; Yoshimoto, N.; Ishikawa, M.; Morita, M. Electrochim. Acta 2002, 47, 3441.
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Figure 1. Structure of cross-linked PEO-PMA containing PEGDE as a plasticizer.
Figure 2. Typical impedance spectra of PEO-PMA-based polymeric gel complexes with PEGDE plasticizer doped with different contents of H3PO4. Polymer composition (in mass): (PEO-PMA)/PEGDE, 38:62. H3PO4 content (mass%): 13 (a), 24 (b), 32 (c), and 46 (d). All spectra were taken at 22 °C. Measurements. The ionic conductivity of the gel membrane was measured by an AC impedance technique using an electrochemical impedance analyzer (S-5720C, NF Electronics), where the AC frequency was scanned from 100 kHz to 1 Hz. The sample membrane (13 mm diam., 0.6-1.1 mm thick) was sandwiched between two stainless steel blocking electrodes in a sealed Teflon cell case. The impedance was measured in the temperature range from 20 to 90 °C. The ionic and electronic transport numbers (tion, tele) were evaluated using Wagner’s polarization technique.17,35 The freshly prepared polymeric gel membranes, (PEO-PMA)/ PEGDE/H3PO4, were polarized in a configuration of Ag/ polymeric gel membrane/Pt under a DC bias with a step potential of 1.0 V at 22 ( 2 °C. The resultant current flows were monitored as a function of the polarization time, where a conventional potentio-/galvanostat system (HA-151, Hokuto Denko) was used as a DC source and an electrometer. Fourier-transformed infrared (FTIR) spectra were recorded on a spectrometer (FTIR-4200, Shimadzu) with a wavenumber resolution of 2 cm-1 in the range of 400-4000 cm-1 to characterize interactions between the polymer matrix or the plasticizing solvents with the dopant acid, H3PO4. The polymeric gel sample was diluted with KBr to yield a tablet for the IR measurement. Thermal analysis of the polymeric gel membrane was performed using DSC (DSC-50, Shimadzu). The measurement was made in the temperature range from -120 to 300 °C, with a temperature slope of 10 K min-1.
Results and Discussion Ionic Conductivity. Optically transparent and uniform polymeric gel membranes were obtained in a wide range of the component concentration, H3PO4 content of 13-52 mass%, and PEGDE content of 0-77 mass%. The membranes have sufficient mechanical strength to allow measurement of their electrochemical properties. Figure 2 shows typical Nyquist plots (complex-plane impedance diagrams) for (PEO-PMA)/PEGDE/H3PO4 (35) Wagner, J. B.; Wagner, C. J. Chem. Phys. 1957, 26, 1597.
Novel Nonaqueous Polymeric Gel Electrolyte
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Figure 4. Typical transient current for PEO-PMA-based polymeric gel complex with PGDE plasticizer. Polymer composition (in mass): (PEO-PMA)/PEGDE, 38:62. H3PO4 content: 41 mass%. Bias voltage: 1.0 V. Figure 3. Conductivity changes as a function of H3PO4 content for PEO-PMA-based polymeric gel complexes. (a) PEGDE as the plasticizer, and (PEO-PMA)/PEGDE (in mass), 38:62; (b) PEGDE + DMF as the plasticizer, and (PEO-PMA)/ PEGDE/DMF (in mass), 38:47:15.
gels with the (PEO-PMA)/PEGDE mass ratio of 38:62 containing different H3PO4 contents measured at 22 ( 2 °C. The frequency dependence of the impedance obtained for the sample with lower H3PO4 content showed two well-defined regions: an arc in the high frequency region which is attributable to the bulk properties of the samples, and a linear increase with decreasing the frequency in the low frequency limit that is associated with the diffusion process at the metal electrode/gel electrolyte interface. The frequency dependence approximating a semicircle that indicates an equivalent circuit corresponding to ion-blocking Pt electrodes gradually disappears with an increase in the H3PO4 content. This result coincides quite well with those reported for similar polymeric gel systems.13-15 The intercept of the semicircular arc with the Z′ (ReZ) axis was taken as the bulk resistance, R, of the gel electrolyte. Thus, the conductivity σ was obtained from eq 1
σ)
d RA
(1)
where d is the thickness and A is the contact area of the sample membrane. Variations in the conductivity of the (PEO-PMA)/ PEGDE/H3PO4 polymeric gel membranes are shown in Figure 3 as a function of H3PO4 content. The conductivity measured in this work was essentially ionic, as discussed in detail later. The conductivity for the sample containing 13 mass% of H3PO4 was about 3.3 × 10-6 S cm-1. The conductivity increased sharply to 1.3 × 10-4 S cm-1 when the gel contained 52 mass% of H3PO4 ([PEO-PMA]/PEGDE/H3PO4 ) 18:30:52 in mass). The increase in conductivity with the acid content may be mainly due to the increased numbers of the charge carriers in the gel. The presence of H3PO4 also decreased the viscosity of the polymeric gel complex, which makes the polymeric chain flexible, and consequently the carrier ions are easy to transport in the gel complex. The conductivity was enhanced by the addition of a small amount of the second plasticizing component DMF in the (PEO-PMA)/PEGDE/H3PO4 system. The improvement in the conductivity by addition of 7 mass% of DMF to (PEO-PMA)/PEGDE/H3PO4 was about
double: from 3.3 × 10-6 to 7.2 × 10-6 for the system containing 13 mass% of H3PO4, and from 1.1 × 10-4 to 2.1 × 10-4 for the sample containing 52 mass% of H3PO4 at 22 ( 2 °C. The addition of DMF reduces the viscosity of the gel and increases the dissociation of H3PO4 because of its high dielectric constant. As the conductivity of pure DMF and PEGDE is extremely low ( 0.99), where the major mobile species was consider to be H+ with mobility µ ) 2.5 × 10-4 cm2 V-1 s-1. Acknowledgment. This work was financially supported by the Grant-in-Aid for Scientific Research on Priority areas (B) No. 740 from the Ministry of Education, Culture, Sports, Science and Technology of Japan, and the Grant-in-Aid for Scientific Research (No. 14380226) from JSPS. CM021076S
