Enhancing the Mechanical Strength of Electrolyte-Supported Solid Oxide Cells with Thin and Dense Doped-Ceria Interlayers.

The penetration of fuel cells and electrolyzers in energy systems calls for their scale-up to the gigawatt (GW) level. High temperature solid oxide cells (SOC) offer unrivaled efficiencies in both electrolysis and fuel cell operation. However, they are made of ceramics and are brittle by nature. Consequently, a high mechanical strength to avoid failure during stacking is essential to achieve a high manufacturing yield. Here, we show that without changing the materials of the state-of-the-art cells, thin and dense ceria interlayers enable comparable power densities and durability in fuel cell operation. The sole tuning of the morphology and processing of the interlayers reduce the residual stress in the cell significantly which increases its mechanical strength by up to 78%. These results promise performance gains of similar magnitude by enabling a substantial decrease of the electrolyte thickness while maintaining robustness. This stress engineering approach presents a way to increase the volumetric power density and material efficiency of SOC systems.

Intercalation of Thin-Film Gd-Doped Ceria Barrier Layers in Electrolyte-Supported Solid Oxide Cells: Physicochemical Aspects.

To minimize alteration of the La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF)/Gd0.2Ce0.8O2-δ(CGO20)/Y0.06Zr0.94O2-δ(3YSZ) interface via strontium zirconate formation in solid oxide cells, electron beam physical vapor deposition was employed to manufacture dense, thin gadolinium-doped ceria (CGO) interlayers. CGO layers with thicknesses of 0.15, 0.3, and 0.5 µm were integrated in state-of-the-art 5 × 5 cm2-large electrolyte-supported cells, and their performance characteristics and degradation behavior were investigated. Electrochemical impedance spectroscopy measurements are correlated with a postmortem scanning electron microscopy/energy-dispersive X-ray spectroscopy analysis to show that 0.15 µm-thick layers lead to the formation of a continuous Sr-containing secondary phase at the CGO/YSZ interface, likely related to the formation of a SrO-ZrO2 phase. Major performance losses were confirmed by an increase in both Ohmic and polarization resistance with an increase in the frequency region ∼103 Hz. Cells with 0.3 µm- and 0.5 µm-thick CGO layers showed similar high performance and low degradation rates over a testing period of ∼800 h. The YSZ/CGO interface of the cells with a 0.3 µm-thick CGO layer showed the formation of a discontinuous Sr-containing secondary phase; however, performance losses were still successfully prevented. Furthermore, it is observed that 0.5 µm-thick CGO layers were sufficient to suppress the formation of the Sr-containing secondary phase.

Performance and Limitations of Nickel-Doped Chromite Anodes in Electrolyte-Supported Solid Oxide Fuel Cells.

Ni-doped chromite anodes were integrated into electrolyte-supported cells (ESC) with 5×5 cm2 size and investigated in fuel cell mode with H2 /H2 O fuel gas. Both a stoichiometric and a nominally A-site deficient chromite anode material showed promising performance at 860 °C approaching the ones of state-of-the-art Ni/Gd-doped ceria (CGO) anodes. While the difference in polarization resistance was small, an increased ohmic resistance of the perovskite anodes was observed, which is related to their limited electronic conductivity. Increasing the chromite electrode thickness was shown to enhance performance and stability considerably. Degradation increased with current density, suggesting its dependency on the electrode potential, and could be reversed by redox cycling. Sulfur poisoning with 20 ppm hydrogen sulfide led to rapid voltage drops for the chromite anodes. It is discussed that Ni nanoparticle exsolution facilitates hydrogen dissociation to the extent that it is not rate-limiting at the investigated temperature unless an insufficiently thick electrode thickness is employed or sulfur impurities are present in the feed gas.

Evaluation of the Effect of Sulfur on the Performance of Nickel/Gadolinium-Doped Ceria Based Solid Oxide Fuel Cell Anodes.

The focus of this study is the measurement and understanding of the sulfur poisoning phenomena of Ni/gadolinium-doped ceria (CGO) based solid oxide fuel cells (SOFC). Cells with Ni/CGO10 and NiCu5/CGO40 anodes were characterized by using impedance spectroscopy at different temperatures and H2 /H2 O fuel ratios. The short-term sulfur poisoning behavior was investigated systematically at temperatures of 800-950 °C, current densities of 0-0.75 A cm-2 , and H2 S concentrations of 1-20 ppm. A sulfur poisoning mitigation effect was observed at high current loads and temperatures. The poisoning behavior was reversible for short exposure times. It was observed that the sulfur-affected processes exhibited significantly different relaxation times that depend on the Gd content in the CGO phase. Moreover, it was demonstrated that the capacitance of Ni/CGO10 anodes is strongly dependent on the temperature and gas-phase composition, which reflects a changing Ce3+ /Ce4+ ratio.

AFM as an analysis tool for high-capacity sulfur cathodes for Li-S batteries.

In this work, material-sensitive atomic force microscopy (AFM) techniques were used to analyse the cathodes of lithium-sulfur batteries. A comparison of their nanoscale electrical, electrochemical, and morphological properties was performed with samples prepared by either suspension-spraying or doctor-blade coating with different binders. Morphological studies of the cathodes before and after the electrochemical tests were performed by using AFM and scanning electron microscopy (SEM). The cathodes that contained polyvinylidene fluoride (PVDF) and were prepared by spray-coating exhibited a superior stability of the morphology and the electric network associated with the capacity and cycling stability of these batteries. A reduction of the conductive area determined by conductive AFM was found to correlate to the battery capacity loss for all cathodes. X-ray diffraction (XRD) measurements of Li2S exposed to ambient air showed that insulating Li2S hydrolyses to insulating LiOH. This validates the significance of electrical ex-situ AFM analysis after cycling. Conductive tapping mode AFM indicated the existence of large carbon-coated sulfur particles. Based on the analytical findings, the first results of an optimized cathode showed a much improved discharge capacity of 800 mA·g(sulfur)(-1) after 43 cycles.