Rémi Costa

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

Summary 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.

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

Abstract 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/H2O 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 H2S 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.

Nanostructured Composite Cathodes by Suspension Plasma Spraying for SOFC Applications

In the metal supported SOFCs (MS-SOFCs), conventional sintering route for cathode layer appears problematic due to excessive oxidation of metal support in air at sintering temperatures and degradation of perovskites during sintering in low oxygen partial pressures. The novel process of suspension plasma spraying (SPS), capable of producing a wide variety of coating structures and compositions without any post-deposition sintering, is developed and evaluated in detail for the fabrication of cathode layers for MS-SOFCs. Composite cathodes of LSCF and CGO were developed and the influence of different cathode microstructures on fuel cell performance was examined. Four exemplary cathode coatings have been created, ranging from partially dense with small pores to highly porous layers preserving submicronic agglomerated structures. These cathode coatings have been tested on equal half-cells made of the same anode and electrolyte. The best performing microstructure had a power density of 798 mW/cm2 at 800°C. Microstructural differences could be attributed to different behavior and performance of the coatings.