MgAl layered double hydroxide treated in a ͑CH 3 COO͒ 2 Mg aqueous solution ͑LDH-Ac͒ and composite membranes consisting of sulfonated poly͑etheretherketone͒ ͑SPEEK͒ and the LDH-Ac in various volume ratios of 0-50 vol % were prepared. Their proton conductivities were examined at 40-150°C under 100 or 35% relative humidity ͑RH͒. The composite membrane with 12 vol % LDH-Ac showed a relatively high conductivity of 8.0 ϫ 10 −3 S cm −1 at 150°C under 100% RH and a better dry resistance than the SPEEK membrane. The SPEEK/LDH-Ac composites were promising membranes for use above 100°C.The polymer electrolyte membrane fuel cell ͑PEMFC͒, which uses a polymer as its electrolyte, is one of the most promising candidate devices for high efficiency energy conversion systems and is expected to be used as the automotive and stationary power sources owing to its operating temperature below 100°C. However, PEMFC has the following problems: ͑i͒ the poisoning of the platinum-based catalyst in electrodes by CO included in reformed fuel gas, ͑ii͒ the decrease in the effective area of electrodes by liquid H 2 O, and ͑iii͒ the high cost of perfluorosulfonic acid ͑PFSA͒ membranes and platinum-based catalysts. The first two problems are solved by operating PEMFC at intermediate temperatures ͑100-200°C͒. However, PFSA membranes cannot be used at these temperatures owing to their heat resistance limitation. Thus, the development of polymer electrolyte membranes ͑PEMs͒ with a high proton conductivity and a high chemical stability at intermediate temperatures is desired. [1][2][3][4][5] Sulfonated hydrocarbon polymers have attracted attention as PEMs for use at intermediate temperatures owing to their high chemical stability. 6-11 Among these polymers, sulfonated poly͑etheretherketone͒ ͑SPEEK͒ is promising for PEMFC application in terms of its chemical stability and proton conductivity. The proton conductivity of SPEEK membranes increases by increasing the degree of the sulfonation ͑DS͒, accompanied with an increase in water uptake ͑WU͒. However, a high WU leads to excessive swelling of the membranes, consequently reducing their chemical stability. Therefore, it is necessary to use a SPEEK with a high DS as a PEM for an improved chemical stability. The chemical stability of PEMs is improved by the addition of an inorganic compound to a polymer matrix. 7,12,13 This improvement in chemical stability has been suggested to be attributed to hydrogen bonds formed between an inorganic compound and an organic polymer, as has been reported for the SPEEK/Sn͑HPO 4 ͒ 2 ·nH 2 O ͑SnP͒ membrane systems. [12][13][14] Layered compounds with interlayer water, such as tungsten trioxide dihydrate ͑WO 3 ·2H 2 O͒ and layered metal phosphate ͑MP͒ hydrate ͓M͑HPO 4 ͒ 2 ·nH 2 O ͑MP:M = Sn, Zr, Ti͔͒, exhibit proton conductivities. 15-20 WO 3 ·2H 2 O exhibits proton conductivity that remains unchanged even with decreasing water vapor pressure until interlayer water has been completely removed. 15 The MPs also show a relatively high proton conductivity at intermediate temp...
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