20126
J. Phys. Chem. 1996, 100, 20126-20133
Nonaqueous H3PO4-Doped Gel Electrolytes D. Raducha, W. Wieczorek, Z. Florjan´ czyk,† and J. R. Stevens* Department of Physics, UniVersity of Guelph, N1G 2W1 Guelph, Ontario, Canada ReceiVed: August 9, 1996X
Nonaqueous protonic gels incorporating H3PO4 have been synthesized using propylene carbonate (PC), dimethylformamide (DMF), 1-methyl-2-pyrrolidone, ethylene glycol, and several polyglycols as possible entrapped solvents. The properties of these various gels have been studied with the purpose of optimizing the protonic conductivity and understanding the conduction mechanism. Two gel electrolytes, DMF in a network based on glycidyl methacrylate and PC in a network based on methyl methacrylate, were selected for in-depth study because of higher conductivity. The operating temperature range for these electrolytes is from -50 to +100 °C. These were compared with their H3PO4 solution analogues in order to understand the role of the gel network, if any, in the conduction process. Possible conduction mechanisms in these protonic gels are discussed.
Introduction In spite of a growing interest in ionically conducting solid materials, there is still limited data available on proton conducting systems.1-4 This is contrary to previous expectations, based on aqueous protonic electrolytes, that a variety of proton conducting solids would be found which exhibit superior conductivities in comparison with alkali-metal-based systems. Recent review papers indicate that there are serious limitations regarding the availability of fast proton solid conductors, especially at ambient temperatures.1-4 Ambient temperature proton conducting systems studied so far either are not very stable1 (e.g., heteropolyacids) or their conductivity depends on the amount of water present (e.g., hydrated Nafion or DOW membranes). The presence of water can be a serious limitation for a variety of applications in which water-sensitive materials are used.1 There has recently been widespread interest in the development of proton conducting polymeric electrolytes which can be used at ambient and moderate temperatures.5-12 These are systems in which polar polymers with basic sites on a main polymer chain form compounds with strong acids such as H2SO4 or H3PO4. The polymeric films formed must be chemically and mechanically stable. Properties of these systems have recently been reviewed by Lassegues.6 Ambient temperature conductivities obtained for some of these proton polymeric electrolytes were higher than 10-3 S/cm.5,6,11,12 The high conductivities, measured for poly(acrylamide) (PAAM) systems with high H2SO4 or H3PO4 concentration cast from water solutions,5,6 are higher than those obtained for films cast from an organic solution of poly(ethylene oxide) (PEO) and H3PO47 but still lower than that of the pure acid. The C-O bond in ethers and alcohols is broken by strong acids; such degradation is accelerated by traces of water. PAAM is easily hydrolyzed in acidic solution to give acids and imides. Under laboratory conductions all necessary precautions have been made to keep these electrolytes “dry”.5 However, it is not clear what the effect of residual traces of water on conductivity will be as well as the long time stability of these electrolytes in large area applications. * To whom correspondence should be addressed. † Department of Chemistry, Warsaw University of Technology, ul. Noakowskiego 3, 00-664 Warszawa, Poland. X Abstract published in AdVance ACS Abstracts, November 15, 1996.
S0022-3654(96)02436-7 CCC: $12.00
It has been found that the conductivity of various polymer electrolytes can be enhanced by plasticization with low or medium molecular weight plasticizers13 (e.g., propylene carbonate). Highly plasticized, cross-linked gel type polymeric electrolytes doped with alkali metal salts often exhibit conductivities higher than 10-3 S/cm even at subambient temperatures. In these systems a solution of alkali metal salt in a nonaqueous solvent is trapped in a polymer matrix which keeps the whole electrolyte mechanically stable. In these gel electrolytes ionic transport predominantly occurs in a liquid electrolyte phase.13 The electrochemistry of nonaqueous alkali metal salt electrolytes is well developed because of the possible application of these systems in ambient temperature alkali metal batteries.14 However, little is known about the properties of nonaqueous concentrated solutions of strong acids which potentially have high protonic conductivity. Most of the studies of liquid proton conducting electrolytes described in the literature are devoted to aqueous electrolytes or to very dilute solutions (
