| Solid acids CsH2PO4（CHP） and CsHSO4（CHS） are of interest in recent years, due to their high proton conductivity at intermediate temperature region（200400 °C）. This is a desirable operating temperature range for fuel cells for both energy conversion and chemical processes.In this work, CHP and CHS were prepared and their properties were studied by using AC impedance, XRD, FT-IR, TGA/DTA. CHP and CHS based inorganic-organic composite electrolytes were synthesized and characterized in different conditions for the intermediate temperature range.CHP undergoes a superionic phase transition at 233 °C, which is in an agreement with literature study. Proton conductivity increased by three orders of magnitude and reached 1.3 ×10-2 S cm-1 at the transition temperature. An endothermic peak at around 230 °C was found from DTA, which is a coincidence with the superionic phase transition temperature. Condensation of CHP into Cs2CO3 was observed under dry atmosphere after transition from TGA, whereas dehydration can be avoid under humidified air（p[H2O]> 0.2 atm）. The superionic phase transition of prepared CHP was investigated by AC impedance. Three types of impedance curves were obtained before, during and after superionic phase transition, respectively.CHP based inorganic-organic composites were fabricated from ball milling to improve the chemical properties of the electrolytes. These composites samples are prepared with benzimidazole（Ben）, imidazole（Im） and 1, 2, 4-triazole（Tz） in different mole ratios. Im and Tz resulted in an improvement of proton conductivity at low temperature range（below 200 °C）. Due to their low acidity, interaction between the acid and the base was better built than Ben. But thermal stability decreased at above 200 °C. More than one mechanism may lead to the decreasing of conductivity. Weak acid-base bond of the composite materials is broken at around 200 °C due to high pKa of CsH2PO4（pKa=7.2） Another probability might be inorganic-organic composites were only simply mixed. So high proton conductivity comes from phase change of Im and Tz. Conductivities decreased with the evaporation of organic base at high temperature region. A 12% of weight loss from TGA curve of CHP-Im（mole ratio: 70:30） is an evidence that Im is completely evaporate from the composite at 180 °C.Conductivity of prepared pure CHS increased four orders of magnitude at transition temperature. It was found to be 142 °C（141°C from literatures）. High proton conductivity achieved 2 ×10-3 S cm-1 over a wide temperature range from 140 to 200 °C under dry condition.As another solid acid, four CHS based inorganic-organic composites electrolytes formed with Im, Tz, cyanuric acid anhydrous（Cy）, melamine-cyanuric acid complex（MeCy） in mole ratio 80:20. High proton conductivities under dry condition were demonstrated by adding a small amount of Im or Tz. Proton conductivity of CHS-Im composite was observed 1 ×10-3 S cm-1, while for CHS-Tz is from 6 ×10-5 S cm-1 to 8 ×10-3 S cm-1, over a temperature range of 60-200 °C. In the case of Cy and MeCy, conductivities are barely improved.CHS based inorganic-organic composites electrolytes exhibit high conductivity over a larger temperature range while comparing with pure CHP based electrolytes. Better acid-base interaction established between CHS and N-heterocycle compounds, due to lower pKa of CHS than CHP. |
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