Catalysts and materials development for fuel cell power generation

Abstract

Catalytic processing of fuels was explored in this thesis for both low-temperature polymer electrolyte membrane (PEM) fuel cell as well as high-temperature solid oxide fuel cell (SOFC) applications. Novel catalysts were developed to generate hydrogen for PEM applications from the oxidative steam reforming of methanol. The activity of lanthanum nickel perovskite (LaNiO3) was examined in both dilute fuel and full fuel conditions. Autothermal operation was successfully achieved with higher hydrogen selectivity than conventional Pd-based catalysts. The selected complex oxide catalyst was applied as a thin film onto a 0.2 [mu]m-thick Pd membrane. Pure hydrogen effluent was obtained from the resulting microreactor as desired for PEM applications. SOFC systems would be of interest for portable power generation if the thermal cycling and slow start-up issues could be addressed. One potential solution is the development of Si-supported ultrathin electrolyte structures (~100 nm-thick) of low thermal mass. Due to the low maximum fabrication temperature (< 600°C), electrodes cannot be applied by traditional ceramic processing techniques. Alternative wet-chemical approaches were explored for the electrode deposition. In particular, ceria sol-gel and yttria-stabilized zirconia (YSZ) colloid were developed as inorganic binders for cathode application at temperatures below 600°C. The YSZ sol provided adhesion strength for La0.8Sr0.2Fe0.8Co0.2O3 (LSCF) in excess of 1000 psi. However, the low-temperature calcination process did not provide the LSCF cathode with sufficiently high electrical conductivity. As an alternative, porous Pt thin films with excellent conductivity were developed as the cathode for micro-SOFC applications. To reduce the stack cost, improve the lifetime, and minimize the coking problem of hydrocarbon-based SOFC systems, it is important to reduce the operating temperature from 1000°C to 800°C. Novel anode systems were examined for their ability to process dry methane at the lower operating temperature. Specifically, three different anode formulations were developed for anode-supported SOFC architectures with 10-40 [mu]m-thick YSZ electrolytes. These included ceramic nanocomposite anodes, CeO2/LaCrO3 and Sm-CeO2/La-CaTiO 3. The former gave rise to Cr(VI) formation due to the intimate mixing of the different ceramic nanoparticles. The latter was limited in applicability due to its low electrical conductivity. Thus, 2 a novel cermet system, Ni-Sn/YSZ, was investigated as the anode. Unlike Ni/YSZ, it did not lead to the formation of crystalline carbon, and successfully sustained 1.5 h of methane exposure at 800°C without mechanical damage to the YSZ electrolyte. Power densities comparable to the best existing direct hydrocarbon SOFC systems were achieved by the Ni-Sn/YSZ cermet.