Yttrium-Doped Hydroxyapatite Membranes with High Proton Conductivity

Fully dense yttrium-doped hydroxyapatite membranes were synthesized having aligned crystal domains that span the membrane thickness. The membranes were grown by multistage hydrothermal crystallization onto a palladium substrate electrochemically seeded with hydroxyapatite nano-crystals. Synthesis conditions were chosen to promote the alignment of the crystallographic c-axis normal to the palladium substrate to promote proton transport through the membrane. The palladium substrate was used as an electrode for electrochemical characterization of the membrane, with platinum as the counter electrode. The measured proton conductivity of the novel membranes approaches 10(-2) S/cm at 700 degrees C, a value higher by a factor of >100 than previously recorded for any apatite ceramic at similar conditions. The enhancement in proton conductivity is effects of yttrium doping and the novel membrane microstructure that eliminates grain boundary resistance to proton transport. With a hydrogen atmosphere at the palladium electrode and dry air at the counter electrode, open circuit voltages of 0.9-1.0 V were measured. The high open circuit voltage indicates that there is minimal electronic conductivity through the membrane and that the membrane is a barrier maintaining separation of hydrogen and air. The high proton conductivity and good barrier properties suggest that the novel membranes may be effective in fuel cells and other electrochemical devices.