Investigation of Cathode Kinetics in SOFC: Model Thin Film SrTi_(1-x)Fe_xO_(3-δ) Mixed Conducting Oxides

To understand the kinetics controlling the SOFC cathode processes, a model mixed conducting perovskite materials system, SrTi_(1-x)Fe_xO_(3-δ), was selected, offering the ability to systematically control both the levels of electronic and ionic electrical conductivity as well as the energy band structure. This, in combination with considerably simplified electrode geometry, served to demonstrate that the rate of oxygen exchange at the surface of SrTi_(1-x)Fe_xO_(3-δ) is only weakly correlated with either high electronic or ionic conductivity, in apparent contradiction with common expectations. On the other hand, evidence was found suggesting the importance of minority electronic species in determining the rate of oxygen exchange. Furthermore, the enrichment of Sr to the surface of the electrodes was found to reduce the oxygen exchange rate constant; this effect becoming more evident with increasing values of x. The observed trends are discussed in relation to the cathodic behavior of MIEC electrodes

Enhanced oxygen exchange of perovskite oxide surfaces through strain-driven chemical stabilization

Surface cation segregation and phase separation, of strontium in particular, have been suggested to be the key reason behind the chemical instability of perovskite oxide surfaces and the corresponding performance degradation of solid oxide electrochemical cell electrodes. However, there is no well-established solution for effectively suppressing Sr-related surface instabilities. Here, we control the degree of Sr-excess at the surface of SrTi0.5Fe0.5O3-δ thin films, a model mixed conducting perovskite O2-electrode, through lattice strain, which significantly improves the electrode surface reactivity. Combined theoretical and experimental analyses reveal that Sr cations are intrinsically under a compressive state in the SrTi0.5Fe0.5O3-δ lattice and that the Sr–O bonds are weakened by the local pressure around the Sr cation, which is the key origin of surface Sr enrichment. Based on these findings, we successfully demonstrate that when a large-sized isovalent dopant is added, Sr-excess can be remarkably alleviated, improving the chemical stability of the resulting perovskite O2-electrodes Please click Additional Files below to see the full abstract

High electrochemical activity of the oxide phase in model ceria–Pt and ceria–Ni composite anodes

Fuel cells, and in particular solid-oxide fuel cells (SOFCs), enable high-efficiency conversion of chemical fuels into useful electrical energy and, as such, are expected to play a major role in a sustainable-energy future. A key step in the fuel-cell energy-conversion process is the electro-oxidation of the fuel at the anode. There has been increasing evidence in recent years that the presence of CeO_2-based oxides (ceria) in the anodes of SOFCs with oxygen-ion-conducting electrolytes significantly lowers the activation overpotential for hydrogen oxidation. Most of these studies, however, employ porous, composite electrode structures with ill-defined geometry and uncontrolled interfacial properties. Accordingly, the means by which electrocatalysis is enhanced has remained unclear. Here we demonstrate unambiguously, through the use of ceria–metal structures with well-defined geometries and interfaces, that the near-equilibrium H_2 oxidation reaction pathway is dominated by electrocatalysis at the oxide/gas interface with minimal contributions from the oxide/metal/gas triple-phase boundaries, even for structures with reaction-site densities approaching those of commercial SOFCs. This insight points towards ceria nanostructuring as a route to enhanced activity, rather than the traditional paradigm of metal-catalyst nanostructuring

Robust nanostructures with exceptionally high electrochemical reaction activity for high temperature fuel cell electrodes

Metal nanoparticles are of significant importance for chemical and electrochemical transformations due to their high surface-to-volume ratio and possible unique catalytic properties. However, the poor thermal stability of nano-sized particles typically limits their use to low temperature conditions (<500 °C). Furthermore, for electrocatalytic applications they must be placed in simultaneous contact with percolating ionic and electronic current transport pathways. These factors have limited the application of nanoscale metal catalysts (diameter <5 nm) in solid oxide fuel cell (SOFC) electrodes. Here we overcome these challenges of thermal stability and microstructural design by stabilizing metal nanoparticles on a scaffold of Sm_(0.2)Ce_(0.8)O_(2−δ) (SDC) films with highly porous and vertically-oriented morphology, where the oxide serves as a support, as a mixed conducting transport layer for fuel electro-oxidation reactions, and as an inherently active partner in catalysis. The SDC films are grown on single crystal YSZ electrolyte substrates by means of pulsed-laser deposition, and the metals (11 μg cm^(−2) of Pt, Ni, Co, or Pd) are subsequently applied by D.C. sputtering. The resulting structures are examined by TEM, SIMS, and electron diffraction, and metal nanoparticles are found to be stabilized on the porous SDC structure even after exposure to 650 °C under humidified H_2 for 100 h. A.C. impedance spectroscopy of the metal-decorated porous SDC films reveals exceptionally high electrochemical reaction activity toward hydrogen electro-oxidation, as well as, in the particular case of Pt, coking resistance when CH_4 is supplied as the fuel. The implications of these results for scalable and high performance thin-film-based SOFCs at reduced operating temperature are discussed

Investigation of surface Sr segregation in model thin film solid oxide fuel cell perovskite electrodes

While SOFC perovskite oxide cathodes have been the subject of numerous studies, the critical factors governing their kinetic behavior have remained poorly understood. This has been due to a number of factors including the morphological complexity of the electrode and the electrode- electrolyte interface as well as the evolution of the surface chemistry with varying operating conditions. In this work, the surface chemical composition of dense thin film SrTi_(1−x)Fe_xO_(3-δ) electrodes, with considerably simplified and well-defined electrode geometry, was investigated by means of XPS, focusing on surface cation segregation. An appreciable degree of Sr-excess was found at the surface of STF specimens over the wide composition range studied. The detailed nature of the Sr-excess is discussed by means of depth and take-off angle dependent XPS spectra, in combination with chemical and thermal treatments. Furthermore, the degree of surface segregation was successfully controlled by etching the films, and/or preparing intentionally Sr deficient films. Electrochemical Impedance Spectroscopy studies, under circumstances where surface chemistry was controlled, were used to examine and characterize the blocking effect of Sr segregation on the surface oxygen exchange rate

Effect of grain boundaries on ion migration in stabilized δ-Bi2O3 thin- film electrolyte

Solid electrolytes with high oxygen-ion conductivity are of significant interest for many applications. Over the past several decades, numerous studies have been conducted on the effect of grain boundaries on the process of increasing the ionic conductivity of solid electrolytes. Given that nanocrystalline thin- or thick-films have been investigated in relation to lowering the operating temperature of solid electrolytes to less than 650 °C, more rigorous and quantitative assessments are necessary to determine how the ion transport characteristics are affected by the numerous interfaces formed in nano-grains devices. Please click Additional Files below to see the full abstract

Surface Sr segregation behaviors in a model thin film perovskite cathode for solid oxide fuel cells

Surface cation segregation, strontium (Sr) in particular, has been considered as one of crucial barriers to achieving a fast surface oxygen exchange rate of perovskite oxide electrodes for solid oxide fuel cells (SOFCs). However, the major driving force for the segregation phenomenon still remains unknown, and thus it is also unknown how to maximize the cathode performance. In this work, we fabricated epitaxial thin films of SrTi1-xFexO3-δ (STF) via pulsed laser deposition (PLD) and quantitatively characterized their microstructures, surface chemical compositions and oxygen exchange rates by a range of analysis tools, in this case HR-TEM, HR-XRD, angle resolved X-ray photoelectron spectroscopy (AR-XPS) and electrical conductivity relaxation (ECR). The use of well-defined epitaxial thin films not only guarantees high precision, reproducibility and reliability of the surface properties, but also enables us to control the degree of misfit strain by varying the choice of the substrate and the target composition. This, in combination with density functional theory (DTF) simulation, enabled to reveal a close relationship between the degree of surface Sr segregation and the misfit strain and thereby to identify the governing factors for the Sr segregation phenomenon

Determination of optical and microstructural parameters of ceria films

Light-matter interactions are of tremendous importance in a wide range of fields from solar energy conversion to photonics. Here the optical dispersion behavior of undoped and 20 mol. % Sm doped ceria thin films, both dense and porous, were evaluated by UV-Vis optical transmission measurements, with the objective of determining both intrinsic and microstructural properties of the films. Films, ranging from 14 to 2300 nm in thickness, were grown on single crystal YSZ(100) and MgO(100) using pulsed laser deposition (both dense and porous films) and chemical vapor deposition (porous films only). The transmittance spectra were analyzed using an in-house developed methodology combining full spectrum fitting and envelope treatment. The index of refraction of ceria was found to fall between 2.65 at a wavelength of 400 nm and 2.25 at 800 nm, typical of literature values, and was relatively unchanged by doping. Reliable determination of film thickness, porosity, and roughness was possible for films with thickness ranging from 500 to 2500 nm. Physically meaningful microstructural parameters were extracted even for films so thin as to show no interference fringes at all

High electrode activity of nanostructured, columnar ceria films for solid oxide fuel cells

Highly porous oxide structures are of significant importance for a wide variety of applications in fuel cells, chemical sensors, and catalysis, due to their high surface-to-volume ratio, gas permeability, and possible unique chemical or catalytic properties. Here we fabricated and characterized Sm_(0.2)Ce_(0.8)O_(1.9−δ) films with highly porous and vertically oriented morphology as a high performance solid oxide fuel cell anode as well as a model system for exploring the impact of electrode architecture on the electrochemical reaction impedance for hydrogen oxidation. Films are grown on single crystal YSZ substrates by means of pulsed laser deposition. Resulting structures are examined by SEM and BET, and are robust up to post-deposition processing temperatures as high as 900 °C. Electrochemical properties are investigated by impedance spectroscopy under H_2–H_2O–Ar atmospheres in the temperature regime 450–650 °C. Quantitative connections between architecture and reaction impedance and the role of ceria nanostructuring for achieving enhanced electrode activity are presented. At 650 °C, _pH_2O = 0.02 atm, and _pH_2 = 0.98 atm, the interfacial reaction resistance attains an unprecedented value of 0.21 to 0.23 Ω cm^2 for porous films 4.40 μm in thickness

Surface modification through oxide ALD to improve oxygen exchange rate on perovskite surface

Segregation and phase separation on perovskite oxide (ABO3) surface have been considered as a key detrimental factor to the performance of energy conversion devices such as solid oxide/electrolysis cells. Recently, the overcoat of less reducible cations has been suggested as a way to suppress the surface Sr segregation on Sr-containing perovskite oxides. However, the detailed requirements of the coating layer to sufficiently stabilize the perovskite surface hasn’t been systematically investigated yet. In this wok, we fabricate La0.6Sr0.4CoO3 (LSC) thin-film model electrode via pulse layer deposition and observe how the degree of Sr segregation varies with the type and thickness of the overcoat layer. Al2O3 and HfO2 with different thickness are coated on LSC via ALD, and the oxygen exchange rate of both bare and ALD-coated samples is measured via electrical conductivity relaxation. It is found that both Al2O3 and HfO2 layers suppress the Sr segregation only within a narrow thickness range, i.e., 1-2 nm for Al2O3 and 0.2 – 0.4 nm for HfO2, respectively. These observations are discussed with solubility and diffusivity of Al and Hf in the host oxide lattice, providing a critical guideline of a new surface modification method to stabilize the perovskite surface at high temperatures. Please click Additional Files below to see the full abstract