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The surface oxygen exchange rates for NdBaCo 2 O 5 + d (NBCO), PrBaCo 2 O 5 + d (PBCO), and GdBaCo 2 O 5 + d (GBCO) were measured between 573 K and 973 K using an isotopic pulse exchange technique. Impedance spectroscopy was utilized to compare these surface rates with electrode performance. Both experiments showed NBCO to be the most active material, with both PBCO and GBCO approximately equal but with lower surface exchange rates and higher electrode area specific resistance. A discontinuity in both data sets was linked to a crystal phase transition in GBCO. It is proposed that the electrode performance is limited by the oxygen exchange rate.

The effect of adding transition-metal catalysts to the surfaces of Solid-Oxide-Fuel-Cell (SOFC) cathodes was examined. Monolayer amounts of Pd, Ni, and Co were deposited by atomic layer deposition (ALD) to cathodes prepared by impregnation of La0.8Sr0.2FeO3 (LSF), La0.8Sr0.2CoO3 (LSCo), and La0.6Sr0.4 Co0.2Fe0.8O3 (LSCF) into YSZ scaffolds. Incorporating the catalysts by ALD allowed modification of the surface composition without changing the cathode surface area or structure. X-ray Photoelectron Spectroscopy and High Sensitivity-Low Energy Ion Scattering were used to confirm ALD growth rates that were measured gravimetrically. Electrochemical impedance spectroscopy indicated that the addition of Pd had a small negative effect on all of the cathodes. Ni and Co exhibited only a small effect on LSCo and LSCF cathodes but both decreased the cathode impedance for LSF. It is suggested that an essential feature in cathode promotion by Co and Ni is that these cations are incorporated into the perovskite surface and thereby increase the concentration of oxygen vacancies, while Pd only blocked adsorption sites.

The structure and oxygen stoichiometry of Ba 0.5 Sr 0.5 Co 0.5 Fe 0.5 O 3-δ (BSCF) between 793 K and 1100 K at an oxygen partial pressure of 0.1 atm were determined by in-situ neutron diffraction. Additional analysis at room temperature was conducted by synchrotron X-ray diffraction. BSCF has the cubic perovskite structure, space group Pm 3 ¯ m , for all investigated temperatures. The oxygen stoichiometry ranged from 2.26 (2) at 1100 K to 2.36 (2) at 793 K. The formation of secondary phases during long equilibration periods is suggested to be responsible for the discrepancy between oxygen stoichiometry values determined by neutron diffraction and those determined by thermogravimetry for this class of materials.

Neutron powder diffraction was used to characterize the layered perovskite PrBaCo 2 O 5 + δ (PBCO) under in-situ conditions from 573 to 852 °C and at p O 2 = 10 − 1 –10 − 4 atm. The data were fit to a tetragonal model in space group P4/mmm . The oxygen vacancies were found to be localized within the Pr layer, with total oxygen stoichiometry between 5.57(1) and 5.17(2). This was verified by coulometric titration. The location of these vacancies and the anisotropic displacement of the surrounding oxygen anion sites indicate that ion transport occurs via a hopping mechanism between O sites in the Pr layer and the nearest neighbor sites in the Co layer. While the direct hopping distance between these sites was found to be greater for PBCO than previously reported for NdBaCo 2 O 5 + δ , the larger radius of Pr 3 + stabilizes a higher concentration of mobile anions within the Ln layer.

Reversible proton conducting solid oxide cells (SOCs) off a highly efficient route to matching supply from intermittent, renewable resources, with power demand by consumers. The cells would store excess electrical energy as chemical fuel during times of peak production, and operate in reverse during times of peak demand. In this study we examine the operation of anode supported proton conducting SOCs in electrolysis mode. The required overpotential for a given current density decreases with increasing humidity at the anode and increasing temperature. All of the V–I curves show distinct curvature. The electrode polarization resistance increases and electrolyte ohmic resistance decreases with increasing current density. This is accompanied by a deviation below the theoretical rate of hydrogen production. We interpret these changes as resulting from deviation away from pure proton conduction in the cell with increasing polarization.

La0.6Sr0.4CoxFe1−xO3−δ (LSCF), La0.6Sr0.4Cu0.2Fe0.8O3−δ, Ba0.5Sr0.4Co0.8Fe0.2O3−δ and LaFeO3−δ nanoparticles were synthesized by a reverse micelle procedure. Controlling the size of the micelles through the water:oil phase ratio enabled synthesis of phase pure perovskite particles with average sizes from 14 nm to 50 nm. Small amounts of an impurity phase, likely cobalt oxide, were detected in the XRD spectrum of high cobalt content samples of LSCF (x = 0.8). La0.6Sr0.4Co0.2Fe0.8O3−δ nanoparticles were utilized to coat the surface of a dense thin-film La0.6Sr0.4Co0.2Fe0.8O3−δ solid oxide fuel cell cathode. The polarization resistance of the nanoparticle coated electrode, measured at open circuit in air at 973 K, was 20% lower than an equivalent un-coated electrode.

There is growing interest in developing oxide materials for direct hydrocarbon solid oxide fuel cell anodes. In addition to electronic and ionic conductivities, the electrocatalytic activity of these materials is a critical requirement for a high performance anode. In this paper, we present evidence for the important role of variable lattice oxygen stoichiometry and anode geometry in dictating the activity and reaction mechanism of La 0.75 Sr 0.25 Cr 1 − x Mn x O 3 − δ -based anodes for CH 4 oxidation. Total oxidation of CH 4 is favored by low oxygen vacancy concentration and availability of reducible B-site cations. The non-linear dependence of electrode polarization resistance with current density is attributed to dynamic changes in lattice oxygen vacancy concentration. The relatively high open circuit potential of porous anodes compared with thin films is attributed to an increase in secondary reactions of the fuel within the porous anode.

Electrical conductivity relaxation (ECR) was utilized to determine the oxygen surface exchange coefficient, k chem , and bulk diffusion coefficient, D chem , for La 0.6 Sr 0.4 Co 0.2 Fe 0.8 O 3 − δ (LSCF 6428) solid oxide fuel cell (SOFC) cathodes. Measurements were performed at 1073 K in the 100% to 3.3% p O 2 range. Both k chem and D chem were nearly one order of magnitude larger than previously reported values. This discrepancy is attributed to the low reliability, yet commonly utilized, two-parameter fit procedure that seeks to simultaneously determine k chem and D chem from a single measurement. The value of k chem is shown to vary by almost one order of magnitude depending on the number of terms chosen in the data fit, while the fit quality, S R , remained constant throughout. Polished LSCF 6428 samples were also individually surface-doped with La, Sr, Co, and Fe, as well as with LSCF 6428 nanoparticles (25–50 nm) to investigate trends in k chem and D chem . Fe surface doping caused a decrease in k chem , while La, Sr, Co and nanoparticle doping did not affect the oxygen surface exchange or bulk diffusion coefficients.

Cobalt doping between 2 and 10 at.% was utilized to lower the required sintering temperature of materials in the series BaCe 0.5 Zr 0.4 (Y,Yb) 0.1 − y Co y O 3 − δ to between 1373 and 1698 K. The required sintering temperature decreased with increasing Co content; however, significant electronic conductivity was observed in both oxidizing and reducing environments for materials with 10 at.% Co. This was accompanied by a loss of chemical stability in H 2 O/H 2 and CO 2 environments. BaCe 0.5 Zr 0.4 Yb 0.07 Co 0.03 O 3 − δ was stable in these environments and provided the highest proton conductivity of the materials tested, 1.98 × 10 − 3 S/cm at 923 K in humidified H 2 . Measurements in a hydrogen concentration cell indicated that the total ionic transference number for this material was between 0.86 and 1.00 with proton transference number between 0.84 and 0.75 at 773 and 973 K respectively. Under oxidizing conditions, the ionic transference number decreased to below 0.10. The grain boundary resistance dominated the total conductivity at low temperatures but was found to decrease with increased sintering temperature due to grain growth.

High temperature X-ray diffraction (HT-XRD), temperature programmed desorption (TPD), thermogravimetric analysis–differential thermal analysis (TGA/DTA) and neutron diffraction were combined to determine the structure and oxygen stoichiometry of SrCo0.8Fe0.2O3−δ (SCF) and Ba0.5Sr0.5Co0.8Fe0.2O3−δ (BSCF) up to 1273 K in the pO2 range of 1 to 10−5 atm. Formation of the vacancy-ordered brownmillerite phase, SrCo0.8Fe0.2O2.5, was observed as a region of zero oxygen release in the TPD measurements and confirmed by HT-XRD and TGA/DTA. No ordering was observed in the BSCF system by any of the techniques utilized in this work. The oxygen vacancy concentration of BSCF was found to be considerably higher than that of SCF and always higher than that of the ordered brownmillerite phase of SCF, δ=0.5. The combination of a high vacancy concentration and absence of ordering leads to higher oxygen permeation fluxes through BSCF membranes in comparison to SCF.

This paper reviews recent work on SOFC anode fabrication at the University of Pennsylvania. In this work, anode fabrication is based on the preparation of a porous YSZ matrix, into which electronic and catalytic components are added by impregnation of the appropriate metal salts. First, the methods used to prepare porous YSZ are described, along with a description of the structures that are obtained. Next, it is demonstrated that cell performance is strongly affected by the methods used to impregnate and pretreat ceria that is added to the porous YSZ. Third, the role of carbonaceous deposits within the anode is discussed. These deposits can lead to improved electronic conductivity that results in improved performance. Finally, the effect of precious-metal dopants, added to ceria to improve the catalytic properties of the anode, is discussed. Pd, Pt, and Rh are shown to give large increases in the performance of the cells, particularly in CH4.

Cathodic and anodic overpotentials were measured using current interruption and AC impedance spectroscopy for two separate solid oxide fuel cells (SOFCs). The fuel cells used yttria-stabilized zirconia (YSZ) as the electrolyte, strontium-doped lanthanum manganite (LSM) as the cathode, and a porous YSZ layer impregnated with copper and ceria as the anode. The Cu/CeO2/YSZ anode is active for the direct conversion of hydrocarbon fuels. Overpotentials measured using both current interruption and impedance spectroscopy for the fuel cell operating at 700 °C on both hydrogen and n-butane fuels are reported. In addition to providing the first electrode overpotential measurements for direct conversion fuel cells with Cu-based anodes, the results demonstrate that there may be significant uncertainties in measurements of electrode overpotentials for systems where there is a large difference between the characteristic frequencies of the anode and cathode processes and/or complex electrode kinetics.