(329g) Microstructural Engineering of Apatite Membranes to Enhance Proton Conductivity
At elevated temperatures, it has been shown that protons are quite mobile along the c-axis of apatite crystals. However, proton mobility in traditional sintered apatite ceramics is limited by grain boundary resistance. Here we report an approach to significantly enhance proton transport through apatite membranes by eliminating grain boundaries and aligning the c-axes of crystal domains through the membrane thickness. A novel multistage synthesis is used to achieve the desired membrane structure. First, apatite seed crystals are deposited onto a substrate electrochemically. Next, hydrothermal growth onto the seed crystals is carried out under conditions that promote c-axis growth normal to the substrate. Finally, a second hydrothermal growth step is carried out under conditions that promote a-axis growth and the formation of a fully dense film. By engineering the membrane micro-structure and composition, we have been able to demonstrate more than four orders of magnitude enhancement in measured proton conductivity when compared to traditional sintered apatite ceramics. The resulting membranes are promising for application in intermediate temperature fuel cells and other electrochemical devices.