Samson Yuxiu Lai

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.

Operando and In situ X-ray Spectroscopies of Degradation in La0.6Sr0.4Co0.2Fe0.8O3−δ Thin Film Cathodes in Fuel Cells

Information from ex situ characterization can fall short in describing complex materials systems simultaneously exposed to multiple external stimuli. Operando X-ray absorption spectroscopy (XAS) was used to probe the local atomistic and electronic structure of specific elements in a La0.6Sr0.4Co0.2Fe0.8O(3-δ) (LSCF) thin film cathode exposed to air contaminated with H2O and CO2 under operating conditions. While impedance spectroscopy showed that the polarization resistance of the LSCF cathode increased upon exposure to both contaminants at 750 °C, XAS near-edge and extended fine structure showed that the degree of oxidation for Fe and Co decreases with increasing temperature. Synchrotron-based X-ray photoelectron spectroscopy tracked the formation and removal of a carbonate species, a Co phase, and different oxygen moieties as functions of temperature and gas. The combined information provides insight into the fundamental mechanism by which H2O and CO2 cause degradation in the cathode of solid oxide fuel cells.

Electrostatic Force Microscopic Characterization of Early Stage Carbon Deposition on Nickel Anodes in Solid Oxide Fuel Cells

Carbon deposition on nickel anodes degrades the performance of solid oxide fuel cells that utilize hydrocarbon fuels. Nickel anodes with BaO nanoclusters deposited on the surface exhibit improved performance by delaying carbon deposition (i.e., coking). The goal of this research was to visualize early stage deposition of carbon on nickel surface and to identify the role BaO nanoclusters play in coking resistance. Electrostatic force microscopy was employed to spatially map carbon deposition on nickel foils patterned with BaO nanoclusters. Image analysis reveals that upon propane exposure initial carbon deposition occurs on the Ni surface at a distance from the BaO features. With continued exposure, carbon deposits penetrate into the BaO-modified regions. After extended exposure, carbon accumulates on and covers BaO. The morphology and spatial distribution of deposited carbon was found to be sensitive to experimental conditions.

Thermally sprayed high-performance porous metal-supported solid oxide fuel cells with nanostructured La0.6Sr0.4Co0.2Fe0.8O3−δ cathodes

While porous metal-supported solid oxide fuel cells (PMS-SOFCs) have potential to dramatically reduce the cost while enhancing the durability of SOFC technology, the available fabrication processes are still cumbersome and unsuitable for commercial applications. Here, we report our findings in exploring low-cost, additive, thermal spray processes suitable for large-scale manufacturing of PMS-SOFCs. The additive fabrication process starts with a porous metal support, on which a porous nickel-based anode layer and a dense electrolyte membrane are sequentially deposited. Then, a nanostructured La0.6Sr0.4Co0.2Fe0.8O3−δ (LSCF) cathode layer is applied using a liquid precursor high velocity oxygen fuel flame (LP-HVOF) spraying process. The polarization resistance of the LSCF cathode is reduced to 0.15 Ω cm2 at 600 °C and 0.025 Ω cm2 at 750 °C. The PMS-SOFCs display excellent performance, demonstrating peak power densities of 0.23, 0.65, 1.1, and 1.5 W cm−2 at 500, 600, 700, and 750 °C, respectively, while maintaining impressive stability (no observable change for more than 600 h at 650 °C). Our results suggest that thermal spraying has potential to be a low-cost and flexible process suitable for large-scale fabrication of commercial PMS-SOFCs.

Shock compression induced devitrification of amorphous Ce3Al melt-spun ribbons

Shock compression experiments were performed on amorphous Ce3Al melt-spun ribbons using the 50 J laser shock loading system at the Omega laser facility. A multi-layered sample of 2mm total thickness with 1 mm × 1.5 mm width and 40 µm thick ribbons sandwiched with 6 µm epoxy was used as the target in order to study the effects of varying pressures (due to attenuation) on the transformation behavior. The shock-induced changes were characterized post-mortem via synchrotron XRD analysis. Comparisons of the initial amorphous, thermally devitrified (at 500°C), and shock compressed samples indicate devitrification under high pressure shock compression into the thermodynamically stable hexagonal α-Ce3Al intermetallic phase, similar to the thermally devitrified samples.