This dissertation deals with the synthesis and characterization of ion conducting polymer electrolyte membranes based on semi-crystalline poly(ether ether ketone) (PEEK) functionalized with sulfonic acid groups. We have studied the impact of reaction conditions such as sulfonation temperature and duration, and the addition of inorganic filler particles on the properties of these membranes. The main objective of this work is to enhance the thermal and mechanical properties of the sulfonated PEEK (SPEEK) membranes, with the least possible compromise in the ionic conductivity. This gains importance in the context of fuel cells that use proton exchange membranes (PEM) as the electrolyte between the anode and the cathode. State-of-the art PEM fuel cells mostly use perfluoro-sulfonic acid (PFSA) membranes which perform better at temperatures below 80 °C and are susceptible to CO poisoning of the precious platinum-based catalysts. To overcome this problem, the operating temperature of the fuel cell has to be raised above 100 °C. As water is indispensable for the proton conduction in the PFSA membrane, such elevated temperatures above the boiling point of water seriously degrade the proton conduction and hence the overall performance of the fuel cell. These problems have imparted on the fuel cell community the need to develop low cost, non-fluorinated membranes that are less dependent on platinum group metal catalysts.Therefore, the major driving forces for the selection the hydrocarbon based PEEK polymer are its lower cost and relatively higher mechanical stability compared to standard PFSA-based polymers. On addition of hydrophilic inorganic fillers in the SPEEK polymer matrix, the thermal stability and the glass transition temperature (Tg) of the polymer are enhanced, which make these composite membranes highly favorable for applications in high-temperature PEM fuel cells. An iterative optimization process was followed to determine the conditions required to fabricate membranes with the desired properties, namely high proton conductivity, low swelling in water, and high thermo-mechanical stability. This optimization process involved a number of characterization techniques, such as proton nuclear magnetic resonance (1H NMR) spectroscopy to determine the degree of sulfonation (DS), thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) for measuring the thermal stability and Tg, respectively, and dynamic mechanical analysis (DMA) to study the mechanical properties.Structural changes in the polymer matrix were studied using Fourier-transform infrared (FTIR) spectroscopy, while the localized segmental motion and the corresponding dielectric transitions were studied by broadband dielectric spectroscopy (BDS). Proton conductivity values of the prepared membranes measured by electrochemical impedance spectroscopy (EIS) were observed to increase linearly with DS. The threshold DS value above which the membrane loses its mechanical stability, resulting in excessive swelling in water was determined to be 80%, whichcorresponds to 0.8 sulfonic acid groups per repeat unit of the PEEK polymer.To enhance the mechanical stability, hydrophilic inorganic fillers such as AlPO4 and silica functionalized with sulfonic acid groups were incorporated in the polymer matrix. Among the composite membranes, the SPEEK composite with 7 wt.% silica (DS = 56%) exhibited the best thermo-mechanical stability and proton conductivity. Compared to standard Nafion membrane, this composite membrane was thermally more stable up to 207 °C with a storage modulus value one order of magnitude higher. The observed Tg values of the composite membranes were also higher than that of Nafion, indicating a promising aspect for application in high-temperature PEM fuel cells. The observed lower conductivity values of the SPEEK membranes compared to Nafion could be attributed to the difference in the inherent microstructures of these two polymers. This difference was probed using small-angly X-ray spectroscopy (SAXS) which revealed a weaker hydrophilic/hydrophobic phase separation and a larger center-to-center distance between ionic clusters in the SPEEK polymer matrix than in Nafion. Although the addition of inorganic fillers slightly increased the ionic cluster density, the relatively narrower water channels in SPEEK with higher tortuosity combined with a large number of dead-ends significantly hinder the proton transport.Membrane-electrode assemblies (MEAs) with the best performing membrane were fabricated by catalyst-coated membrane (CCM) process and fuel cell tests were performed using hydrogen as the fuel and pure oxygen as the oxidant.Hydrogen crossover, in-situ impedance and polarization characteristics were studied during the operation of the fuel cell. Thus, a systematic development and characterization process was followed for the optimization of the membranes and it was demonstrated that by optimizing the reaction conditions and filler loading, it is possible to fabricate novel, low cost proton exchange membranes for high-temperature fuel cell applications.