H3O+/NH4 + -β"-alumina was synthesized using K+-β"-alumina and two types of ion-exchange mediums, NH4NO3 aqueous solutions and molten NH4NO3 salts, by an ion-exchange reaction in an autoclave and a heating mantle, respectively. In the autoclave reaction, the potassium concentration of the K+-β"-alumina was varied among the [K2O] : [Al2O3] composition ratios of 1 : 4-1 : 8, and the concentrations of the NH4NO3 solution was chosen to be either 1M and 2M, depending on the experiment. Each ion-exchange reaction was carried out at 130, 150 or 180 oC for 2, 4, 6, 8 or 24 h under high pressure. In the heating mantle reaction, the ion-exchange reaction was performed at 180 oC or 200 oC for 2-48 h in molten NH4NO3 salts under atmospheric pressure. After repeating the preceding reaction 5 times with 5 hour re-reactions followed by washing, the highest ion-exchange rate achieved was 90% in the autoclave, and 98% in the heating mantle. The phase stability and phase transformation were analyzed by X-ray diffraction(XRD) and the ion-exchange rate was measured using an inductively coupled plasma(ICP).

Keywords: K+- β"-alumina, ion-exchange reaction, H3O+/NH4 + -β"-alumina. Introduction Fuel cell technology has been developed especially in proton exchange membrane fuel cell(PEMFC), which use a Nafion polymer electrolyte. However, the working temperature of commercial PEMFCs is limited to ~80 oC because of the properties of the polymer electrolyte at high temperatures, such as a low thermal resistance. In the PEMFC system, some problems arise due to the low working temperature. First, electrolytes can be destroyed because of swelling or shrinkage which is caused by water in the liquid and gas states below 100 oC. It is also very difficult to control the input temperature of the fuel gas supplied. Second, the Pt catalyst in the anode can be poisoned by a very small amount of CO easily at low working temperatures. In general, the available level of CO is 20 ppm at 80 oC, but the amount of CO needed to poison the catalyst can be improved by increasing the working temperature above 100 oC.(1000 ppm at 130 oC and 30,000 ppm at 200 oC) [1]. It is known that H3O+/NH4 + -β"-alumina, which is a proton conductive ceramic,