Performance of nanostructured metal oxane derived ceramic membranes for fuel cell applications
Tsui, Eliza M.
Wiesner, Mark R.
Doctor of Philosophy
An iron-based ceramic material is shown to be a practical candidate as an electrolyte material for proton exchange membrane in fuel cells. These membranes have comparable conductivity to the NafionRTM membrane with the advantages of lower permeability of methanol, less sensitivity to humidity, good chemical stability in fuel cell environment and lower material costs. Iron oxide nanoparticles (ferroxane) and aluminum oxide nanoparticles (alumoxane) were prepared as a pre-cursor materials for membrane fabrication. The structures of ferroxane and alumoxane derived ceramics were characterized with FTIR, SEM, TEM, and nitrogen adsorption-desorption. Protonic conductivity of the sintered membranes was studied by electrochemical impedance spectroscopy (EIS) to determine their feasibility in fuel cell applications. Ferroxane derived ceramics fired at 300°C has high proton conductivity and low dependence of humidity (ranging from 1.29 x 10-2 to 2.65 x 10-2 S·cm-1 at relative humidities of 33% to 100%). The values are comparable to, but on the low end of, the reported conductivities of Nafion. Aluminum-based ceramic material (alumoxane) has a lower conductivity at 2.23 x 10-4 to 3.83 x 10-4 S·cm -1 from 33% RH to 100% RH. The conductivity study as a function of operating temperature indicated the proton transfer for sintered ferroxane-derived membrane likely occurs via a Grotthus mechanism. The results of H2/air fuel cell indicated sintered ferroxane electrolyte could be operated at low temperature. The fuel cell exhibited steady performance with increasing power density over time. The sintered ferroxane-derived membrane with PVA sintered at 500°C has a power density of 5.21 mW·cm-2 and a current density of 16.5 mA·cm-2 measured at room temperature. The methanol permeabilities of sintered ferroxane and alumoxane derived ceramics were lower than that of Nafion and were 1.23 x 10-7 and 1.65 x 10-7 cm2·s-1 respectively. However, the open circuit voltage of ferroxane in DMFC was not improved in comparison to Nafion. Ferroxane-derived ceramic electrolyte sintered at 300°C in methanol/air fuel cell measured at 20°C had a power density of 7.7 muW·cm-2 with 2 M methanol solution. The power density increased to 30 muW·cm-2 with increasing methanol concentration to 18.5 M.
Chemical engineering; Environmental engineering; Engineering; Materials science