Yanxiang Zhang

Hierarchically Oriented Macroporous Anode-Supported Solid Oxide Fuel Cell with Thin Ceria Electrolyte Film

Application of anode-supported solid oxide fuel cell (SOFC) with ceria based electrolyte has often been limited by high cost of electrolyte film fabrication and high electrode polarization. In this study, dense Gd0.1Ce0.9O2 (GDC) thin film electrolytes have been fabricated on hierarchically oriented macroporous NiO-GDC anodes by a combination of freeze-drying tape-casting of the NiO-GDC anode, drop-coating GDC slurry on NiO-GDC anode, and co-firing the electrolyte/anode bilayers. Using 3D X-ray microscopy and subsequent analysis, it has been determined that the NiO-GDC anode substrates have a porosity of around 42% and channel size from around 10 μm at the electrolyte side to around 20 μm at the other side of the NiO-GDC (away from the electrolyte), indicating a hierarchically oriented macroporous NiO-GDC microstructure. Such NiO-GDC microstructure shows a tortuosity factor of ∼1.3 along the thickness direction, expecting to facilitate gas diffusion in the anode during fuel cell operation. SOFCs with such Ni-GDC anode, GDC film (30 μm) electrolyte, and La0.6Sr0.4Co0.2Fe0.8O3-GDC (LSCF-GDC) cathode show significantly enhanced cell power output of 1.021 W cm(-2) at 600 °C using H2 as fuel and ambient air as oxidant. Electrochemical Impedance Spectroscopy (EIS) analysis indicates a decrease in both activation and concentration polarizations. This study has demonstrated that freeze-drying tape-casting is a very promising approach to fabricate hierarchically oriented porous substrate for SOFC and other applications.

Releasing metal catalysts via phase transition: (NiO)0.05-(SrTi0.8Nb0.2O3)0.95 as a redox stable anode material for solid oxide fuel cells.

Donor-doped perovskite-type SrTiO3 experiences stoichiometric changes at high temperatures in different Po2 involving the formation of Sr or Ti-rich impurities. NiO is incorporated into the stoichiometric strontium titanate, SrTi0.8Nb0.2O3-δ (STN), to form an A-site deficient perovskite material, (NiO)0.05-(SrTi0.8Nb0.2O3)0.95 (Ni-STN), for balancing the phase transition. Metallic Ni nanoparticles can be released upon reduction instead of forming undesired secondary phases. This material design introduces a simple catalytic modification method with good compositional control of the ceramic backbones, by which transport property and durability of solid oxide fuel cell anodes are largely determined. Using Ni-STN as anodes for solid oxide fuel cells, enhanced catalytic activity and remarkable stability in redox cycling have been achieved. Electrolyte-supported cells with the cell configuration of Ni-STN-SDC anode, La0.8Sr0.2Ga0.87Mg0.13O3 (LSGM) electrolyte, and La0.6Sr0.4Co0.2Fe0.8O3 (LSCF) cathode produce peak power densities of 612, 794, and 922 mW cm(-2) at 800, 850, and 900 °C, respectively, using H2 as the fuel and air as the oxidant. Minor degradation in fuel cell performance resulted from redox cycling can be recovered upon operating the fuel cells in H2. Such property makes Ni-STN a promising regenerative anode candidate for solid oxide fuel cells.

Functional significance of conserved glycine 127 in a human dual-specificity protein tyrosine phosphatase.

Using site-directed mutagenesis and steady-state kinetic measurements, the functional role of the conserved glycine 127 in a human vaccinia H1-related phosphatase (VHR) was investigated. The mutations of Gly127 to Ala and Pro resulted in a significant decrease in kcat/Km, and increase in Ki for arsenate, indicating that flexibility at the Gly127 site has a large effect on substrate binding and catalytic activity. No substantial decrease in kcat/Km and increase in Ki values were observed for G127 deletion mutant. This showed the conformational flexibility of the PTP loop also affected the enzymatic activity of VHR. Our data suggest that the flexibility of the PTP loop in VHR is probably controlled by Gly127, and that even subtle changes in the loop flexibility may interfere with substrate binding and enzymatic reaction.

A highly active, CO2-tolerant electrode for the oxygen reduction reaction

One challenge facing the development of high-performance cathodes for solid oxide fuel cells (SOFC) is the fast degradation rate of cathodes due to poisoning by contaminants commonly encountered in ambient air such as CO2. Here we report a double perovskite PrBa0.8Ca0.2Co2O5+δ (PBCC) cathode with excellent ORR activity and remarkable CO2 tolerance under realistic operation conditions. When tested in a symmetrical cell in air with ∼1 vol% CO2 at 750 °C, the PBCC electrode shows an area specific resistance of ∼0.024 Ω cm2, which increases to 0.028 Ω cm2 after 1000 h operation. The degradation rate is ∼1/24 of that of the state-of-the-art La0.6Sr0.4Co0.2Fe0.8O3 (LSCF) cathode under the same conditions. Impedance spectroscopy and in situ surface enhanced Raman spectroscopy analyses indicate that the surface of the PBCC electrode is much more active for oxygen exchange and more robust against CO2 than that of LSCF, as confirmed by density functional theory calculations. The fast ORR kinetics and excellent durability of PBCC in air with CO2 highlight the potential of PBCC as a highly promising material for devices involving oxygen electrochemistry such as solid oxide fuel cells, electrolysis cells, or gas separation membranes.

CHAPTER 3:Cathode Material Development

Various cathode materials and cathode with different structures have been developed for SOFCs based on oxygen ion and proton conductors. Perovskite manganates such as strontium-doped lanthanum manganates are the conventional cathode materials widely used for oxygen-ion conducting SOFCs due to its stability at high temperature (800–1000 °C) and compatibility with the electrolyte. Their application is limited at intermediate temperature (<750 °C) because of the negligible oxygen ion conductivity and relatively low catalytic activity. Cobalt and iron are thus used to replace manganese in order to increase the catalytic activity as well as the oxygen ion conductivity so that the reaction zone is extended from the electrode-electrolyte physical interface to the electrode bulk. The reaction zone can be further extended by tailoring the cathode structures such as cooperating electrolytes to the cathode materials and forming nanostructures with ion impregnation. These materials and structures developed for the oxygen-ion conducting SOFCs are also applicable to the proton-conducting SOFCs, suggesting complicated cathode reaction steps, which might consist of proton and oxygen ion transportation. In addition, mixed electronic-proton conductor such as BaCe0.5Fe0.5O3 is developed. Proton conductors are also cooperated to the catalysts to enhance the cathode activity by extending the three-phase boundaries where proton, electron and oxygen molecular meet.