The effect of the number of sulfonic acids in polysiloxanes as fillers for polymer electrolyte membranes in energy storage applications

A series of sulfonic acid-functionalized polysiloxanes (MASFPs) that can be used to reduce the cost and improve the properties of proton exchange membranes (PEMs) by acting as fillers have been synthesized. In this paper, we describe the synthesis of MASFPs with varying numbers of sulfonic acid groups with the aid of mercaptoethanol. An array of analytical techniques were employed to characterize the synthesized MASPs. The broad peak observed in the X-ray diffraction pattern of MASFPs between 2θ = 15 to 25° signifies their amorphous characteristics. Analyses by infrared spectroscopy also identify MASFPs that contain stretching vibrations for –SO3H, –CH2, and Si–O– group. The thermogravimetric analysis demonstrates the thermal stability of these MASFPs. The surface morphology analysis of the MASFPs using FESEM and HRTEM revealed the presence of spherical particles that exhibited significant aggregation and distortion. By employing X-ray photoelectron spectroscopy (XPS) to determine the atomic weight percentage of sulfur present in the MASFPs, the number of sulfonic acids contained within was verified. The sulfur content of MASFPs is determined by XPS to be as follows: MASFP-4 > MASFP-3 > MASFP-2 > MASFP-1. The presence of Q4, Q3, T3, and T2 signals, as determined by 29Si MAS NMR analysis, confirms the polysiloxane formation. Preliminary studies of the composite membrane prepared using the MASFPs with perfluorosulfonic acid (PFSA:MASFP in 75:25 ratio) exhibited superior ionic conductivity (0.102 S/cm) and ion-exchange capacity (1.16 meq/g) in comparison to the commercial Nafion membrane. Overall, using a mercaptoethanol-assisted process to synthesize sulfonic acid functionalized polysiloxanes is an excellent way to increase the incorporation of sulfonic acids and it can be applied to the synthesis of various other inorganic nanofillers.