Characterization of 3D Interconnected Microstructural Network in Mixed Ionic and Electronic Conducting Ceramic Composites

The microstructure and connectivity of the ionic and electronic conductive phases in composite ceramic membranes are directly related to device performance. Transmission electron microscopy (TEM) including chemical mapping combined with X-ray nanotomography (XNT) have been used to characterize the composition and 3-D microstructure of a MIEC composite model system consisting of a Ce0.8Gd0.2O2 (GDC) oxygen ion conductive phase and a CoFe2O4 (CFO) electronic conductive phase. The microstructural data is discussed, including the composition and distribution of an emergent phase which takes the form of isolated and distinct regions. Performance implications are considered with regards to the design of new material systems which evolve under non-equilibrium operating conditions

Accessible triple-phase boundary length: A performance metric to account for transport pathways in heterogeneous electrochemical materials

The performance of materials for electrochemical energy conversion and storage depends upon the number of electrocatalytic sites available for reaction and their accessibility by the transport of reactants and products. For solid oxide fuel/electrolysis cell materials, standard 3-D measurements such as connected triple-phase boundary (TPB) length and effective transport properties partially inform on how local geometry and network topology causes variability in TPB accessibility. A new measurement, the accessible TPB, is proposed to quantify these effects in detail and characterize material performance. The approach probes the reticulated pathways to each TPB using an analytical electrochemical fin model applied to a 3-D discrete representation of the heterogeneous structure provided by skeleton based partitioning. The method is tested on artificial and real structures imaged by 3-D x-ray and electron microscopy. The accessible TPB is not uniform and the pattern varies depending upon the structure. Connected TPBs can be even passivated. The sensitivity to manipulations of the local 3-D geometry and topology that standard measurements cannot capture is demonstrated. The clear presence of preferential pathways showcases a non-uniform utilization of the 3-D structure that potentially affects the performance and the resilience to alterations due to degradation phenomena. The concepts presented also apply to electrochemical energy storage and conversion devices such as other types of fuel cells, electrolyzers, batteries and capacitors. (C) 2016 Elsevier B.V. All rights reserved

Evolution of 3-D Transport Pathways and Triple-Phase Boundaries in the Ni-YSZ Hydrogen Electrode upon Fuel Cell or Electrolysis Cell Operation

The degradation of the Ni-YSZ electrode was investigated after SOFC or SOEC operation for up to 4,700 h and 10,700 h, respectively. 3-D FIB-SEM serial sectioning combined with 3-D EDS elemental mapping allowed the measurement of structural, material and electrochemical properties. Ni coarsening was observed during SOFC and SOEC operation. In SOFC, the comparison of EIS measurements and Electrochemical Fin theory simulations suggested that degradation was mainly due to a decrease in the connected TPB length. However, longer aging time should be investigated. In SOEC, large microstructural alterations were observed with the depletion of Ni close to the YSZ electrolyte. The average connected TPB length was not significantly affected possibly because of finer Ni, but local variations were observed. Accessible TPB length calculations indicate that the access of TPBs by electrons and ions is less efficient, which is likely due to the modified spatial distribution of the TPBs

Characterization of Cracks and their Effects on the Effective Transport Pathways in Ni-YSZ Anodes after Reoxidation Using X-Ray Nanotomography

Reduction-oxidation cycling of Ni-based electrodes for solid oxide fuel/electrolysis cells irreversibly alters their microstructure and can cause the fracture of the electrolyte. Non-destructive 3-D imaging enables tracking of microstructural changes that occur during cycling. Despite recent advances, the understanding of how local 3-D geometrical features in the heterogeneous electrode material contribute to the material degradation remains incomplete. Absorption contrast X-ray nanotomography (XNT) of a same Ni(O)-yttria-stabilized zirconia (YSZ) sample was performed at the Ni K-edge white-line peak (8348 eV), before and after exposure to air at 800°C during 45 minutes. A complimentary XNT at 8376 eV confirmed a degree of oxidation in the range of 98%. The morphology of the Ni(O) phase was as expected completely different after re-oxidation. The spatial resolution better than 20 nm enabled the detection of cracks in the brittle YSZ phase above this dimension. The detrimental effects of the cracks on the effective 3-D transport pathways in the Ni-YSZ anode under polarization was investigated using a skeleton-based discrete representation of the imaged volume and an analytical electrochemical fin model. Topological properties, effective ionic conductivity and polarization resistance were calculated before and after oxidation. For the latter estimate, the effect of the cracked YSZ network was considered alone so far; that of the spatial redistribution of triple-phase boundaries induced by re-oxidation will be included in the future. Cracks in the brittle YSZ phase induced an increase in the effective ionic resistivity and in the polarization resistance in the range of 25 ± 9% and 12 ± 5%, respectively

Three-dimensional microstructural mapping of poisoning phases in the Neodymium Nickelate solid oxide fuel cell cathode

Nd-Nickelate (NNO), Nd1.95NiO4+delta, an alternative solid oxide fuel cell cathode material, has been imaged and mapped in 3D using synchrotron-based x-ray nanotomography. The NNO cathode material, which suffered from silicon contamination during fabrication, was found to contain the desired NNO, plus two distinct poisoning phases. The likely composition and description of the poisoning phases are presented, as well as a detailed description of the microstructural mapping and material characterization. The insulating poisoning phases are likely to have deleterious implications for the cell, as they have a tendency to form a coating layer on the NNO surface, significantly decreasing its active area. Additional losses may be expected due to the poisoning phases inhibiting ionic and electronic transport pathways, increasing cathode polarization resistance. (C) 2013 Elsevier B.V. All rights reserved