Development of fuel cell and electrochemical devices using inorganic proton-conducting materials

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The sol-gel technique is widely used to prepare new functional glasses with new compositions. Our recent researches on the preparation of glasses exhibiting high proton-conductivity and their applications to the electro-devices were ingestigated. P_2O_5-SiO_2 glasses containing some metal oxides were prepared by hydrolysis of their metal alkoxides, followed by heating up to ~600℃. The obtained glasses are porous with pores smaller than few nm in diameter and contain water molecules in pores. The electrical conduction in the glass is related to the dissociation of proton from the hydroxyl bonds and the proton hopping through water molecules. PMA (phosphomolybdic acid) and PWA (phosphotungstic acid) are considered as the most dopants in the sol-gel matrix. A new class of heteropolyacid glass materials yield a high proton conductivities (10-2 S/cm) at room temperature. Though, they exhibit highly attractive fuel cell performance (~22.2 mW/cm^2) and high stability at 30℃ and relative humidity 30 %.Proton conducting phosphate hydrogels are easily derived from the reaction of water with some kinds of metaphosphate glass powders. The hydration breaks the long-chain phosphate structure in the glasses and protons break the cross-links between the chains, resulting in connection to the non-bridging oxygens. Subsequently, water molecules are hydrogen-bonded around the protons. The hydrogels show high conductivities of ~5 mS/cm at room temperature. The high conductivity is suggested to be due to the fast proton transfer promoted by coexistence of large amounts of acidic POH groups and water molecules. A fuel cell prepared using the hydrogel electrolyte generates the high power density of ~160 mW/cm^2. The optimization of the electrode structure is now being investigated. An electric double-layer capacitor (EDC) with capacitance of ~3 F/g can be also prepared by using the hydrogel, which shows low a self-discharge rate and a fast charge/discharge capability.