A c-axis-oriented HAP thin film synthesized by seeded growth on a palladium hydrogen membrane substrate. An exemplary synthetic process includes electrochemical seeding on the substrate, and secondary and tertiary hydrothermal treatments under conditions that favor growth along c-axes and a-axes in sequence. By adjusting corresponding synthetic conditions, an HAP this film can be grown to a controllable thickness with a dense coverage on the underlying substrate. The thin films have relatively high proton conductivity under hydrogen atmosphere and high temperature conditions. The c-axis oriented films may be integrated into fuel cells for application in the intermediate temperature range of 200-600.degree. C. The electrochemical-hydrothermal deposition technique may be applied to create other oriented crystal materials having optimized properties, useful for separations and catalysis as well as electronic and electrochemical applications, electrochemical membrane reactors, and in chemical sensors. Additional high-density and gas-tight HAP film compositions may be deposited using a two-step deposition method that includes an electrochemical deposition method followed by a hydrothermal deposition method. The two-step method uses a single hydrothermal deposition solution composition. The method may be used to deposit HAP films including but not limited to at least doped HAP films, and more particularly including carbonated HAP films. In addition, the high-density and gas-tight HAP films may be used in proton exchange membrane fuel cells.
FEDERALLY SPONSORED RESEARCH
This invention was made with government support under Contract Nos. DE-FG02-05ER15722 and DE-FC03-92SF19460 awarded by the United States Department of Energy, and under Contract No. CMMI-0856128 awarded by the National Science Foundation (NSF). The government has certain rights in the invention.