THERMO-ELECTROCHEMICAL PROPERTIES OF POLYMER ELECTROLYTE MEMBRANE DERIVED FROM POLYISOPRENE/POLYSTYRENE BLEND
Apart from the very high cost of the state-of-the-art Nafion® membranes that are derived from polytetrafluoroethylene, another serious factor that hinders their full commercialization is the loss of conductivity at elevated temperature (above 120°C) under fuel cell operation. These factors have challenged researchers to seek alternative membranes from other polymeric materials. On this premise, this study seeks to derive a possible alternative from polyisoprene and polystyrene (PIPS) blend via sulfonation process, and to investigate its thermo-electrochemical properties. The PIPS blend (70:30) was sulfonated with chlorosulfonic acid for 1–18 h, and the ion exchange capacity (IEC) and degree of sulfonation (DS) were determined from the sulfur content. Thermal properties were examined by differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA), proton conductivity by AC impedance spectroscopy, and the open circuit voltage and power density in a single fuel cell at catalyst loadings of 15, 30 and 40 wt%. From the study carried out, results show that the membrane derived from the sulfonated PIPS (SPIPS) demonstrates thermal stability with a glass transition temperature of 150°C and a degradation temperature of 400 °C, making it suitable for fuel cell application, particularly proton exchange membrane fuel cell and direct methanol fuel cell that operate up to 120°C. An optimum proton conductivity of 4.09 × 10⁻² S/cm is achieved with the membrane of optimum degree of sulfonation (3.969%), which is of the same order as that of Nafion®. Other electrochemical testing shows that the synthesized membrane achieves an optimum open circuit voltage (OCV) of 645.91 mV and power density of 62.0 mW/cm² using 40% catalyst loading. These characteristics, therefore, place the PIPS blend as a viable alternative polymer electrolyte membrane material for fuel cell applications.
Keywords: Conductivity, current density, degree of sulfonation, power density, thermo-electrochemical, voltage.
*Correspondence: christopher.idibie@unidel.edu.ng
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