Recently, researches on fuel cells, rechargeable batteries, capacitors, and polymer actuators applicable to the fields of electrical, electronic, mechanical, or bio industries have been under way for alternative energy sources and energy storage/output mediums in order to make provision against depletion of fossil fuels. Fuel cells such as a proton exchange membrane fuel cell (PEMFC) and a direct methanol fuel cell (DMFC) and an ionic polymer-metal composite (IPMC) actuator are composed of a unit assembly consisting of a polymer electrolyte layer and a pair of metallic catalyst layers or electrode layers. In addition, a rechargeable battery and a capacitor may comprise polymer electrolytes in each of electrolyte layers therein.
Polymer electrolyte membranes of the fuel cells such as PEMFC and DMFC are required to have a range of characteristics such as a low level of fuel permeability, a high level of mechanical strength and dimensional stability, strong adhesion to a catalyst layer, a high level of proton conductivity, and the like. The polymer electrolyte membranes mounted on the IPMC actuator are also required to show the characteristics of the electrolyte membrane that are necessary for the polymer fuel cells.
Despite many researches made on the polymer fuel cells and actuators, their commercialization still needs improvement of their performance and reductions in their production costs. This is because their preparation needs to use electrodes of noble metals such as platinum (Pt) and gold (Au) and expensive polymer electrolytes such as Nafion commercially available from DuPont Inc. (U.S.A.), which entails high production cost.
In order to reduce the production cost of the polymer electrolytes, it is required that new polymer electrolytes be synthesized and modified to have an enhanced level of performance comparable to that of the conventional polymer electrolytes such as Nafion, thereby replacing the expensive Nafion.
Various polymer electrolytes such as sulfonated aromatic poly(ether ether ketone (s-PEEK) (see: M. Gil et al., J. Mem. Sci., 234, 2004, 75-81), sulfonated poly(vinyl alcohol) (s-PVA) (see: J. W. Rhim et al., J. Mem. Sci., 238, 2004, 143-151), and sulfonated polystyrene (s-PS) (see: M. Luqman, J. W. Lee, K. K. Moon, and Y. T. Yoo, J. Ind. Eng. Chem., 17, 2011, 49-55) were reported as novel alternative polymer electrolytes. Poly(vinyl phosphate-b-styrene) (PVPP-b-PS) (see: G. H. Li et al., Solid State Ionics, 177, 2006, 1083-1090) and sulfonated poly(styrene-b-ethylene-co-butylene-b-styrene) (s-SEBS) (see: X. L. Wang, Mater. Lett., 61, 2007, 5117-5120) were reported as a block copolymer based polymer electrolyte. In recent years, Kraton Inc. (U.S.A.) synthesized a sulfonated styrene penta-block copolymer having a well-controlled molecular structure and making an improvement in the performance of s-SEBS, poly((t-butyl-styrene)-b-(ethylene-r-propylene)-b-(styrene-r-styrene sulfonate)-b-(ethylene-r-propylene)-b-(t-butyl-styrene)) (tBS-EP-SS-EP-tBS) by using an anionic polymerization, and commercialized the same. Furthermore, researches have been reported on modifying novel alternative polymer electrolytes by using composite techniques and blending techniques.
However, such polymer electrolytes have yet to achieve satisfactory properties such as ionic conductivity and the like.