Guntae Kim

Rapid oxygen ion diffusion and surface exchange kinetics in PrBaCo2O5+x with a perovskite related structure and ordered A cations

As part of an investigation of new cathode materials for intermediate temperature solid oxide fuel cells, we have investigated particular perovskite oxides with ordered A cations which, in turn, localize the oxygen vacancies into layers. The oxygen exchange kinetics of polycrystalline samples of the oxygen-deficient double perovskite PrBaCo2O5+x (PBCO) have been determined by electrical conductivity relaxation (ECR) and by oxygen-isotope exchange and depth profiling (IEDP). The ECR and IEDP measurements reveal that PBCO has high electronic conductivity and rapid oxygen ion diffusion and surface exchange kinetics. Both techniques demonstrate that the oxygen kinetics in this structure type are significantly faster than in corresponding disordered perovskites.

Layered oxygen-deficient double perovskite as an efficient and stable anode for direct hydrocarbon solid oxide fuel cells

Different layered perovskite-related oxides are known to exhibit important electronic, magnetic and electrochemical properties. Owing to their excellent mixed-ionic and electronic conductivity and fast oxygen kinetics, cation layered double perovskite oxides such as PrBaCo2O5 in particular have exhibited excellent properties as solid oxide fuel cell oxygen electrodes. Here, we show for the first time that related layered materials can be used as high-performance fuel electrodes. Good redox stability with tolerance to coking and sulphur contamination from hydrocarbon fuels is demonstrated for the layered perovskite anode PrBaMn2O5+δ (PBMO). The PBMO anode is fabricated by in situ annealing of Pr0.5Ba0.5MnO3-δ in fuel conditions and actual fuel cell operation is demonstrated. At 800 °C, layered PBMO shows high electrical conductivity of 8.16 S cm(-1) in 5% H2 and demonstrates peak power densities of 1.7 and 1.3 W cm(-2) at 850 °C using humidified hydrogen and propane fuels, respectively.

Highly efficient and robust cathode materials for low-temperature solid oxide fuel cells: PrBa0.5Sr0.5Co2−xFexO5+δ

Solid oxide fuel cells (SOFC) are the cleanest, most efficient, and cost-effective option for direct conversion to electricity of a wide variety of fuels. While significant progress has been made in anode materials with enhanced tolerance to coking and contaminant poisoning, cathodic polarization still contributes considerably to energy loss, more so at lower operating temperatures. Here we report a synergistic effect of co-doping in a cation-ordered double-perovskite material, PrBa0.5Sr0.5Co2−xFexO5+δ, which has created pore channels that dramatically enhance oxygen ion diffusion and surface oxygen exchange while maintaining excellent compatibility and stability under operating conditions. Test cells based on these cathode materials demonstrate peak power densities ~2.2 W cm−2 at 600°C, representing an important step toward commercially viable SOFC technologies.

Investigation of the Structural and Catalytic Requirements for High-Performance SOFC Anodes Formed by Infiltration of LSCM

Composites formed by infiltration of 45 wt % La 0.8 Sr 0.2 Cr 0 .5 Ma 0.05 O 3 (LSCM) into a 65% porous yttria-stabilized zirconia (YSZ) scaffold were investigated in order to understand the reasons this material is able to provide excellent anode performance in solid oxide fuel cells (SOFCs). Scanning electron microscopy showed that the LSCM forms a film over the YSZ after calcination at 1473 K but that this film undergoes cracking to expose a long three-phase boundary after reduction at 1073 K. Coulometric titration demonstrated that the reduction of LSCM and La 0.8 Sr 0.2 MnO 3 occurred over a similar range of P(O 2 ) and that reduction is the likely cause for film cracking. To achieve low anode impedances in humidified H 2 at 973 K, it was necessary to add a catalyst. The addition of 0.5-1 wt % Pd, Rh, or Ni was sufficient to increase the maximum power density of SOFCs with 60 μm thick YSZ electrolytes to >500 mW/cm 2 in humidified H 2 at 973 K. The addition of either 1 wt % Fe or 5 wt % ceria also improved power densities but to a lesser extent. Finally, the use of Pt paste as the current collector increased performance to a similar extent as intentionally adding catalyst, showing the importance of using inert materials in electrode testing.

SOFC Anodes Based on Infiltration of La0.3Sr0.7TiO3

Composites formed by infiltration of 45 wt % La0.3Sr0.7TiO3 LST into 65% porous yttria-stabilized zirconia YSZ were examined for application as solid oxide fuel cell SOFC anodes. Although LST does not react with YSZ, the structure of the LST deposits was strongly affected by the calcination temperature. At 1373 K, the LST formed loosely packed, 0.1 m particles that filled the YSZ pores. The conductivity of this composite depended strongly on the pretreatment conditions but was greater than 0.4 S/cm after heating to 1173 K in humidified 3% H2O H2. Following calcination at 1573 K, the LST had sintered significantly, decreasing the conductivity of the composite by a factor of approximately 5. The addition of a catalyst was critical for achieving reasonable electrochemical performance, with the addition of 0.5 wt % Pd and 5 wt % ceria increasing the power density of otherwise identical cells from less than 20 to 780 mW/cm2 for operation in humidified 3% H2O H2 at 1073 K. Electrodes prepared from LST deposits calcined at 1373 K were found to exhibit a much better performance than those prepared from LST deposits calcined at 1573 K, demonstrating that the structure of the composite is critical for achieving high performance.

Oxygen exchange kinetics of epitaxial PrBaCo2O5+δ thin films

The oxygen exchange kinetics of thin films of the oxygen-deficient double perovskite PrBaCo2O5+δ (PBCO) have been determined by electrical conductivity relaxation (ECR) and by oxygen-isotope exchange and depth profiling (IEDP). Microstructural studies indicate that the PBCO films, prepared by pulsed laser deposition, have excellent single-crystal quality and epitaxial nature. The ECR and IEDP measurements reveal that the PBCO films have high electronic conductivity and rapid surface exchange kinetics, although the ECR data indicate the presence of two distinct kinetic pathways. The rapid surface kinetics compared with those of other perovskites suggest the application of PBCO as a cathode material in intermediate-temperature solid oxide fuel cells.

Development of Double-Perovskite Compounds as Cathode Materials for Low-Temperature Solid Oxide Fuel Cells

A class of double-perovskite compounds display fast oxygen ion diffusion and high catalytic activity toward oxygen reduction while maintaining excellent compatibility with the electrolyte. The astoundingly extended stability of NdBa(1-x)Ca(x)Co2O(5+δ) (NBCaCO) under both air and CO2-containing atmosphere is reported along with excellent electrochemical performance by only Ca doping into the A site of NdBaCo2O(5+δ) (NBCO). The enhanced stability can be ascribed to both the increased electron affinity of mobile oxygen species with Ca, determined through density functional theory calculations and the increased redox stability from the coulometric titration.

Etched Graphite with Internally Grown Si Nanowires from Pores as an Anode for High Density Li-Ion Batteries

A novel architecture consisting of Si nanowires internally grown from porous graphite is synthesized by etching of graphite with a lamellar structure via a VLS (vapor-liquid-solid) process. This strategy gives the high electrode density of 1.5 g/cm(3), which is comparable with practical anode of the Li-ion battery. Our product demonstrates a high volumetric capacity density of 1363 mAh/cm(3) with 91% Coulombic efficiency and high rate capability of 568 mAh/cm(3) even at a 5C rate. This good electrochemical performance allows porous graphite to offer free space to accommodate the volume change of Si nanowires during cycling and the electron transport to efficiently be improved between active materials.

Triple-conducting layered perovskites as cathode materials for proton-conducting solid oxide fuel cells.

We report on an excellent anode-supported H(+) -SOFC material system using a triple conducting (H(+) /O(2-) /e(-) ) oxide (TCO) as a cathode material for H(+) -SOFCs. Generally, mixed ionic (O(2-) ) and electronic conductors (MIECs) have been selected as the cathode material of H(+) -SOFCs. In an H(+) -SOFC system, however, MIEC cathodes limit the electrochemically active sites to the interface between the proton conducting electrolyte and the cathode. New approaches to the tailoring of cathode materials for H(+) -SOFCs should therefore be considered. TCOs can effectively extend the electrochemically active sites from the interface between the cathode and the electrolyte to the entire surface of the cathode. The electrochemical performance of NBSCF/BZCYYb/BZCYYb-NiO shows excellent long term stability for 500 h at 1023 K with high power density of 1.61 W cm(-2) .

SOFC Anodes Based on LST–YSZ Composites and on Y0.04Ce0.48Zr0.48O2

The properties of solid oxide fuel cell (SOFC) anode functional layers prepared by impregnation of 1 wt % Pd and 10 wt % ceria into porous scaffolds of either Y 0.04 Ce 0.48 Zr 0.48 O 2 (CZY) or composites of La 0.3 Sr 0.7 TiO 3 (LST) and yttria-stabilized zirconia (YSZ) were examined to determine whether these scaffold materials would have sufficient electronic and ionic conductivity. Laminated tapes were cofired to produce 50 μm YSZ electrolytes and 50 μm scaffolds, supported on LSF-YSZ cathodes. The electronic conductivities of LST-YSZ composites were a function of the porosity and the weight fraction of LST but could be sufficient for use in thin functional layers. However, anodes made with LST-YSZ composites had higher nonohmic losses than cells made with YSZ scaffolds. With CZY scaffolds, some migration of Ce into the YSZ electrolyte was observed after cofiring. While CZY exhibited electronic conductivity, the loss in ionic conductivity compared to YSZ again resulted in higher nonohmic losses. The implications of these results for producing better ceramic anodes are discussed.