46784-AC10
Synthesis and Characterization of Novel Hybrid Organic-Inorganic Proton Conducting Membranes
A key component of a fuel cell is the proton-conducting membrane that separates fuels while conducting protons from the anode to the cathode. We proposed a novel design for proton-conducting membranes, based on hybrid colloidal materials. The results of this work, supported by PRF, have been summarized in 1 published and 1 submitted manuscript.
Next, we have modified the surface of silica spheres with sulfonic acid groups with the goal of introducing the proton conductivity into the colloidal membrane. To obtain preliminary conductivity data, both unmodified and sulfonated silica spheres were either pressed into disk-shaped pellets using a hydraulic press, or self-assembled into close-packed membranes. We used electrochemical impedance spectroscopy to measure the proton conductivity of the above materials. The proton conductivity of unmodified pellets at 100 °C in dry air is below 10-7 S/cm. The conductivity increases as a result of the surface sulfonation ca. and reaches ~10-6 S/cm. The proton conductivity of the sulfonated pellets increases exponentially with temperature in dry air, suggesting proton transport via structural diffusion under these conditions. At 100 °C in dry air the proton conductivity of unmodified self-assembled colloidal membranes was also very low. In contrast, under these conditions the proton conductivity of the sulfonated self-assembled colloidal membranes was ca. 10-5 S/cm, one order of magnitude higher compared to the unmodified colloidal membranes. We believe that these results strongly suggest that the self-assembled colloidal membranes contain a much higher number of interconnected nanopores, leading to more efficient proton transport. We also found that at 100 °C and 100% R.H. the proton conductivity of the sulfonated self-assembled colloidal membranes was ca. 0.01 S/cm, three orders of magnitude higher compared to the unmodified colloidal membranes and two orders of magnitude higher than that of the sulfonated disordered pellets.
Most recently, we prepared sintered self-assembled nanoporous silica colloidal crystals modified with poly(3-sulfopropyl-methacrylate) and poly(stryrenesulfonic acid) brushes covalently attached to the nanopore surface. The resulting robust membranes possess temperature and humidity-dependent proton conductivity of ~2 × 10-2 S cm-1 at 30 °C and 94% R.H., ~1 × 10-2 S cm-1 at 85 °C and 60% R.H., and water uptake of ca. 20 wt% at room temperature. We also prepared proton conductive membranes by self-assembly of silica nanospheres modified with poly(3-sulfopropylmethacrylate) and poly(stryrenesulfonic acid) brushes of different thickness. These membranes showed a slightly higher proton conductivity and water uptake but poor mechanical properties.
Our future work will focus on the preparation of a new class of proton-conducting membrane materials, namely, nanoporous colloidal membranes whose proton conductivity will result from the nanopore surface modification with polymer molecules carrying acid functionalities. The proposed design has a number of unique features that distinguish it from polyelectrolyte or composite organic-inorganic membranes, relying on the polymer phase for both the mechanical stability and the proton conductivity. In our design the highly ordered silica colloidal crystal film serves as a matrix containing a continuous network of nanopores and providing mechanical stability and additional water retention. The matrix eliminates the need to simultaneously optimize the mechanical and proton-conducting properties of the polymer and thus allows introducing and investigating unprecedented polymer brush architectures prepared by surface-initiated living polymerization inside the inorganic nanopores. In addition, the inorganic matrix allows achieving high degrees of acid functionalization of the polymer without the danger for the membrane to become water soluble, and maintaining mechanical stability under oxidative conditions and high temperature. Furthermore, the inorganic matrix itself has several unique characteristics, e. g., it is formed via self-assembly, can be made only a few micrometers thin, can be easily integrated with the electrodes in a fuel cell assembly, can be prepared using proton-conducting inorganic oxide spheres and even using nanoporous spheres to further extend the working temperature and humidity range of the membranes.
