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Solid acid composites CsH 2 PO 4 /NaH 2 PO 4 /SiO 2 with different mole ratios of CsH 2 PO 4 and NaH 2 PO 4 to SiO 2 were synthesized and characterized. Preliminary infrared measurements of CsH 2 PO 4 and its composites indicated that hydrogen bonds breaking and formation were detected between 1710 to 2710 cm -1 , while the rotation of phosphate tetrahedral anions occurred between 900 and 1200 cm -1 . The superprotonic transition of CsH 2 PO 4 /NaH 2 PO 4 /SiO 2 composite was identified at superprotonic temperatures between 230 and 260 °C, under atmospheric pressure. This study reveals higher conductivity values for composites with higher CsH 2 PO 4 (CDP) content. Solid acid composite CDP 613 appeared as the composite with the highest conductivity that is 7.2 x 10 -3 S cm -1 at 230 °C. Thermal stability of the solid acid composites such as temperature of dehydration, melting and decomposition were investigated. The addition of NaH 2 PO 4 lowers the dehydration temperature of the solid acid composites.Keywords: solid acid, conductivity, thermal analysis, caesium dihydrogen phosphate, fuel cell Abstrak Asid pepejal komposit CsH 2 PO 4 /NaH 2 PO 4 /SiO 2 dengan nisbah mol CsH 2 PO 4 kepada SiO 2 dan NaH 2 PO 4 kepada SiO 2 yang berbeza telah disintesis dan dicirikan dalam ujikaji ini. Pencirian awal sinar inframerah menunjukkan bahawa pemecahan dan pembentukan ikatan hidrogen dikesan antara 1710 cm -1 dan 2710 cm -1 , manakala putaran anion tetrahedron fosfat berlaku diantara 900 cm -1 dan 1200 cm -1 . Fasa peralihan berkonduktiviti tinggi bagi asid pepejal komposit CsH 2 PO 4 /NaH 2 PO 4 /SiO 2 telah dikenal pasti antara suhu 230 hingga 260 °C, di bawah tekanan atmosfera. Nilai kekonduksian proton adalah lebih tinggi bagi komposit yang mempunyai kandungan CsH 2 PO 4 (CDP) yang lebih tinggi. Asid pepejal komposit CDP 613 telah muncul sebagai komposit dengan kekonduksian tertinggi iaitu 7.2 x 10 -3 S cm -1 pada suhu 230 °C. Kestabilan terma asid pepejal komposit seperti suhu dehidrasi, takat lebur dan penguraian telah dikenal pasti melalui analisis termogravimetri dan kalorimeter imbasan perbezaan. Penambahan NaH 2 PO 4 merendahkan suhu dehidrasi asid pepejal komposit.Kata kunci: asid pepejal, kekonduksian, analisa terma, sesium dihidrogen fosfat, sel bahanapi Introduction Solid acids emerged as a potential solid electrolyte for fuel cell applications due to its high proton conductivity, also known as superprotonic conductivity as the conductivity values increases by 2 to 3 orders of magnitude at intermediate temperatures between 100 to 250 °C, accompanied by phase transition [1]. Solid acids are defined by the basic chemical formula M a H b (XO 4 ) c where M is a monovalent or divalent metal cation, XO 4 is a tetrahedral oxyanion, and a, b, c are integers [2]. Solid acids caesium dihydrogen phosphate, CsH 2 PO 4 (CDP) and caesium

Solid acid composites of CsH 2 PO 4 /NaH 2 PO 4 /SiO 2 (CDP/SDP/SiO 2) were synthesised with varied SDP to CDP and SiO 2 to CDP mole fractions. The composites were arranged for optimisation via the Taguchi method with L9 orthogonal array, which manipulates four factors: the amount of SDP, amount of SiO 2 , amount of pressure used for pellet production and operating temperature. Responses were recorded as the averaged conductivity value for each experimental set. The composite powders were uniaxially pressed to form pellets of 1 mm thickness for conductivity measurements. Conductivity value was averaged from three readings and used in the signal-to-noise (S/N) ratio formula. The S/N ratio was set such that larger is better and calculated to maximise the response required for the analysis. Operating temperature has the largest effect on the conductivity value of solid acid composites, whereas the amount of SiO 2 has the smallest effect. The amount of SDP in the composite has a larger effect than the pressure applied during pellet production prior to the conductivity measurements. The optimised composite consists of 0.2 mole fraction of SDP to CDP and 0.3 mole fraction of SiO 2 to CDP pressed at 3 tonnes cm-2 to produce pellets and tested at the temperature of 250 °C. The optimisation was validated by using analysis of variance from the Design-Expert ® software.