Kirk Gerdes

Enhancing SOFC cathode performance by surface modification through infiltration

Solid oxide fuel cells (SOFCs) have the potential to be one of the cleanest and most efficient energy technologies for direct conversion of chemical fuels to electricity. Economically competitive SOFC systems appear poised for commercialization, but widespread market penetration will require continuous innovation of materials and fabrication processes to enhance system lifetime and reduce cost. One early technical opportunity is minimization of resistance to the oxygen reduction reaction (ORR) at the cathode, which contributes the most to performance degradation and efficiency loss in the existing SOFCs, especially at temperatures <700 °C. Detailed study over the past 15 years has revealed the positive impact of catalyst infiltration on SOFC cathode performance, both in power density and durability metrics. However, realizable performance improvements rely upon strongly-coupled relationships in materials and morphology between the infiltrate and the backbone, and therefore efficacious systems cannot be simply generated with a set of simple heuristics. This article reviews recent progress in enhancing SOFC cathode performance by surface modification through a solution-based infiltration process, focusing on two backbone architectures – inherently functional and skeletal – infiltrated using wet-chemistry processes. An efficient cathode consists of a porous mixed-conducting backbone and an active coating catalyst; the porous backbone provides excellent ionic and electronic conductivity, while the infiltrated surface coating possesses high catalytic activity and stability. As available, performance comparisons are emphasized and reaction schematics for specific infiltrate/backbone systems are summarized. While significant progress has been achieved in enhancing surface catalytic activity and durability, the detailed mechanisms of performance enhancement are insufficiently understood to obtain critical insights and a scientific basis for rational design of more efficient catalysts and novel electrode architectures. Recent progress in characterization of surfaces and interfaces is briefly discussed with challenges and perspectives in surface modification of SOFC electrodes. Surface modification through infiltration is expected to play an increasingly important role in current and next-generation commercial SOFC development, and this review illustrates the sophisticated technical considerations required to inform judicious selection of an infiltrate for a given SOFC system.

Single crystalline La0.5Sr0.5MnO3 microcubes as cathode of solid oxide fuel cell

The efficiency of solid oxide fuel cells (SOFCs) is heavily dependent on the electrocatalytic activity of the cathode toward the oxygen reduction reaction (ORR). In order to achieve better cathode performance, single crystalline La0.5Sr0.5MnO3 (LSM) microcubes with the {200} facets have been synthesized by the hydrothermal method. It is found that the LSM microcubes exhibit lower polarization resistance than the conventional polycrystalline La0.8Sr0.2MnO3 powder in air from 700 °C to 900 °C. The ORR activation energy of the LSM microcubes is lower than that of the conventional powder. The ORR kinetics for the microcubes is limited by the charge transfer step while that for the conventional powder is dominated by the oxygen adsorption and dissociation on the cathode surface.

Effect of Sr-Doped LaCoO3 and LaZrO3 Infiltration on the Performance of SDC-LSCF Cathode

The effects on performance of commercially produced solid oxide fuel cell (SOFC) were evaluated for two types of cathode infiltration: a mixed ionic-electronic conductor and an electronic insulator. The bi-layered cathode backbone consists of a thin, dual-phased functional layer containing Sm 2 O 3 -doped CeO 2 (SDC) and La 0.6 Sr 0.4 Co 0.2 Fe 0.8 O 3―δ (LSCF) and a thick current collecting layer of LSCF. The backbone was infiltrated with either La 0.6 Sr 0.4 CoO 3 (LSCo) or La 1.97 Sr 0.03 Zr 2 O 7 (LSZ) using nitrate solution precursors, followed by calcination at 850 or 950°C, respectively. LSCo infiltration decreased the measured full cell overpotential by 28―40% after 6 wt % loading, and no further effect was observed with higher loading. The LSCo-infiltrated cells demonstrated stable performance for over 200 h of operation at 0.25 A/cm 2 and 750°C. Conversely, cathode infiltration with electrically insulating LSZ pyrochlore had a negative influence on the cathode performance that became substantial with increased infiltrate loading. Characteristic wetting behavior of the two infiltrates on the composite backbone affects the dependency of infiltrate amounts on cathode performance. The results imply that the composite cathode reaction is primarily under the control of the surface exchange reaction rate. This comparative study demonstrates that the cathode performance of a commercially produced SOFC can be influenced by applying infiltrates in a simple process, and indicates that the infiltrates electrocatalytic activity and conductivity must be carefully considered when infiltrating a standard SDC-LSCF cathode.

Phase-field modeling of three-phase electrode microstructures in solid oxide fuel cells

A phase-field model for describing three-phase electrode microstructure (i.e., electrode-phase, electrolyte-phase, and pore-phase) in solid oxide fuel cells is proposed using the diffuse-interface theory. Conserved composition and non-conserved grain orientation order parameters are simultaneously used to describe the coupled phase coarsening and grain growth in the three-phase electrode. The microstructural evolution simulated by the phase-field approach demonstrates the significant dependence of morphological microstructure and output statistic material features on the prescribed kinetic parameters and three-phase volume fractions. The triple-phase boundary fraction is found to have a major degradation in the early evolution.

Phase field modeling of microstructure evolution of electrocatalyst-infiltrated solid oxide fuel cell cathodes

A phase field model is developed to examine microstructural evolution of an infiltrated solid oxide fuel cell cathode. It is employed to generate the three-phase backbone microstructures and morphology of infiltrate nano-particles [La1−xSrxMnO3 (LSM)]. Two-phase Y2O3 + ZrO2 and LSM backbones composed of 0.5–1 μm particles are first generated and then seeded with infiltrate, and evolution is compared for starting infiltrate particle diameters of 5 nm and 10 nm. The computed lifetime triple phase boundary (3PB) density of the infiltrated cathode is then compared to the cathode backbone. Results indicate that initial coarsening of infiltrate nano-particles is the primary evolution process, and infiltrate coarsening is the majority contributor to 3PB reduction. However, at all times, the infiltrated cathode possesses significantly greater 3PB length than even the uncoarsened backbone. Infiltrate particle size effects indicate that the smaller particle size produces greater 3PB length for the same infiltration...

Overview of SOFC Anode Interactions with Coal Gas Impurities

An overview of the results of SOFC anode interactions with phosphorus, arsenic, selenium, sulfur, antimony, and hydrogen chloride as single contaminants or in combinations is discussed. Tests were performed using both anode- and electrolyte-supported cells in synthetic and actual coal gas for periods greater than 1000 hours. Post-test analyses were performed to identify reaction products formed and their distribution, and compared to phases expected from thermochemical modeling. The ultimate purpose of this work is to establish maximum permissible concentrations for impurities in coal gas, to aid in the selection of appropriate coal gas clean-up technologies.

Interdiffusion across solid electrolyte-electrode interface

A phase-field model is developed for studying the cation interdiffusion across electrolyte-electrode interfaces in solid oxide fuel cell (SOFC) that can be contributing to long timescale performance degradation. Demonstrated on an interface between an 8%molY2O3-stabilized ZrO2 and a La0.65Sr0.3MnO3−x typically used in SOFC, time-dependent evolution of the cation interdiffusion profiles are predicted by linking the phase-field model to a diffusion equation. The simulated interdiffusion profiles agree with independent experimental data in both time and space domains at different temperatures.

Direct foamed and nanocatalyst impregnated SOFC cathodes

A binder system containing polyurethane precursors was used to in situ foam (direct foam) an (La 0.6 Sr 0.4 ) 0.98 (Co 0.2 Fe 0.8 )O 3−δ (LSCF) cathode composition on an yttrium-stabilised zirconia (YSZ) electrolyte coated with a porous ∼10 µm thick cathode active layer. The YSZ electrolyte was ∼110 μm thick, and a fuel cell was created by application of a Ni/(Ce 0.9 Gd 0.1 )O 2 cermet as the baseline anode. Cells possessing the foamed LSCF cathode were compared to cells constructed via standard methods in terms of resultant microstructure, electrochemical performance, and introceptive character. The foamed cathode tended to possess a high level of tortuous porosity which was ellipsoidal and interconnected in character. Both the standard and foamed cathode structures were subjected to an infiltration process, and the resultant microstructure was examined. The impregnation efficiency of the foamed cathode was at least ∼10% greater per deposition than that of an unfoamed porous LSCF cathode. The SOFC with the Pt nanocatalyst impregnated foamed cathode demonstrated a maximum power density of 593 mW/cm 2 utilising wet H 2 fuel, which is 52% higher than an SOFC with the baseline Pt-impregnated LSCF cathode (∼390 mW/cm 2 ) at 800°C. The cathode compositional and microstructural alterations obtainable by foaming led to the elevated power performance, which was shown to be quite high relative to standard SOFCs with a thick YSZ electrolyte.