David Kennouche

Three-dimensional microstructure of high-performance pulsed-laser deposited Ni–YSZ SOFC anodes

The Ni-yttria-stabilized zirconia (YSZ) anode functional layer in solid oxide fuel cells produced by pulsed laser-deposition was studied using three-dimensional tomography. Anode feature sizes of ~130 nm were quite small relative to typical anodes, but errors arising in imaging and segmentation were shown using a sensitivity analysis to be acceptable. Electrochemical characterization showed that these cells achieved a relatively high maximum power density of 1.4 W cm(-2) with low cell resistance at an operating temperature of 600 °C. The tomographic data showed anode three-phase boundary density of ~56 μm(-2), more than 10 times the value observed in conventional Ni-YSZ anodes. Anode polarization resistance values, predicted by combining the structural data and literature values of three-phase boundary resistance in an electrochemical model, were consistent with measured electrochemical impedance spectra, explaining the excellent intermediate-temperature performance of these cells.

Solid oxide cells with zirconia/ceria Bi-Layer electrolytes fabricated by reduced temperature firing

Anode-supported solid oxide cells (SOCs) with thin bi-layer Y0.16Zr0.92O2−δ (YSZ)/Gd0.1Ce0.9O1.95 (GDC) electrolytes were prepared by a reduced-temperature (1250 °C) co-firing process enabled by the addition of a Fe2O3 sintering aid. The Fe2O3 amounts in the layers affected the formation of voids at the GDC/YSZ interface; the case with 1 mol% Fe2O3 in the YSZ layer and 2 mol% Fe2O3 in the GDC layer yielded minimal interfacial voids, presumably because of optimized shrinkage matching between the electrolyte layers during co-firing. The best cells yield fuel cell power density at 0.7 V in air and humidified hydrogen of 1.74 W cm−2 (800 °C) and 1.0 W cm−2 (700 °C). Under electrolysis conditions, i.e., air and 50 vol% H2O–50 vol% H2, the best cell area specific resistance is 0.12 Ω cm2 at 800 °C and 0.27 Ω cm2 at 700 °C. This excellent cell performance was explained by a number of factors related to the reduced firing temperature: (1) low electrolyte resistance due to minimization of YSZ/GDC interdiffusion; (2) minimal zirconate phase formation between the YSZ and the La0.6Sr0.4Fe0.8Co0.2O3 (LSFC) cathode because of the dense GDC barrier layer; (3) high three phase boundary density in the Ni–YSZ anode functional layer; and (4) good pore connectivity in the Ni–YSZ support. Preliminary life testing under fuel cell and electrolysis operation shows promising cell stability.

Effect of Ni content on the morphological evolution of Ni-YSZ solid oxide fuel cell electrodes

The coarsening of Ni in Ni–yttria-stabilized zirconia (YSZ) anodes is a potential cause of long term solid oxide fuel cells (SOFC) performance degradation. The specifics of the Ni-YSZ structure—including Ni/YSZ ratio, porosity, and particle size distributions—are normally selected to minimize anode polarization resistance, but they also impact long-term stability. A better understanding of how these factors influence long-term stability is important for designing more durable anodes. The effect of structural details, e.g., Ni-YSZ ratio, on Ni coarsening has not been quantified. Furthermore, prior measurements have been done by comparing evolved structures with control samples, such that sample-to-sample variations introduce errors. Here, we report a four dimensional (three spatial dimensions and time) study of Ni coarsening in Ni-YSZ anode functional layers with different Ni/YSZ ratios, using synchrotron x-ray nano-tomography. The continuous structural evolution was observed and analyzed at sub-100 nm res...

Imaging of Fuel Cell and Battery Electrodes Using Focused Ion Beam Scanning Electron Microscopy

This paper reviews results on the use of Focused Ion Beam – Scanning Electron Microscopy (FIB-SEM) for three-dimensional (3D) tomographic studies of fuel cell and battery electrode morphology. Electrodes are typically complex twoor three-phase (including porosity) structures where each phase transports a different species, and electrochemical reactions occur at the interfaces between phases. Thus, the electrode morphology plays an important role in determining the transport and reaction rates, and hence the electrochemical performance. 3D imaging has been used to understand electrochemical processes by determining macrohomogeneous parameters (surface areas, tortuosities, etc) and by utilizing measured 3D structures in 3D electrochemical simulations. Electrode electrochemical degradation has also been studied by relating observed morphological changes to degradation during accelerated testing – mechanistic models are developed to predict long-term performance behaviour.