In situ modification of Nafion(R) membranes with phospho-silicate for improved water retention and proton conduction
Proton-conducting hybrids from Nafion® and phospho-silicate networks were prepared by solvent-directed infiltration and copolymerization of 2-(methacryloyloxy)ethyl phosphate (EGMP) and 3-[(methacryloyloxy)propyl]trimethoxysilane (MEMO) at different ratios in Nafion®. Fourier transform infrared (FTIR) spectroscopy and 29Si and 31P nuclear magnetic resonance (NMR) spectroscopy confirm the presence of hydrophilic –POH groups and Si–O–P and Si–O–Si bond formation. The hybrid membrane samples show phase-separated morphology. Scanning electron microscopic (SEM) images confirm uniform distribution of 40–60 nm sized phospho-silicate nanoparticles in the membranes. The use of ethanol as solvent, directed the deposition of nanoparticles to the hydrophilic ionic cluster of Nafion®. The water uptake of all the hybridized membranes is higher than that of the unmodified Nafion® membrane due to the presence of strong hydrogen-bonded water molecules within the phospho-silicate inorganics. In general, the proton conductivity of hybridized membranes below 100 °C is lower than that of the unmodified membrane due to restricted mobility as a result of decrease in free volume and disruption of arrangement of hydrophilic domain thereby disrupting the proton movement path. However, above 100 °C and in anhydrous conditions, the hybrid membranes show increased proton conductivity and thermal stability than the blank Nafion®. Among the hybrid membranes, the sample with higher phosphate content exhibits higher water uptake and better proton conductivity.