Quantification of SOFC anode microstructure based on dual beam FIB-SEM technique

Rational SOFC material design: new advances and tools

Citrate–nitrate auto-combustion synthesis is used to prepare an iron, a cobalt and a cerium-perovskite. The influence of different synthesis conditions on the combustion process, phase composition, textural and morphological properties is studied in detail by X-ray diffraction, nitrogen adsorption and scanning electron microscopy. Results show that the combustion intensity increases from iron, to cerium, to cobalt-perovskite. Conversely, the combustion intensity decreases and thus the safety and the gain of the combustion process increase by using high fuel/oxidant ratios, low pH values or combustion reactors with high heat dispersion capacity. High fuel/oxidant ratios increase particle size and may enhance dopant segregation. Low citric acid/metal nitrates ratios may cause precipitation of the most insoluble compounds or segregation of the dopant. High citric acid/metal nitrates ratios increase the formation temperature of the perovskite-type structure. Low pH values are deleterious for the phase composition and/or for the morphology of the final product, although at high pH values dopant segregation may occur.

Citrate-nitrate auto-combustion synthesis of perovskite-type nanopowders: A systematic approach

Solid-state batteries have many enticing advantages in terms of safety and stability, but the solid electrolytes upon which these batteries are based typically lead to high cell resistance. Both components of the resistance (interfacial, due to poor contact with electrolytes, and bulk, due to a thick electrolyte) are a result of the rudimentary manufacturing capabilities that exist for solid-state electrolytes. In general, solid electrolytes are studied as flat pellets with planar interfaces, which minimizes interfacial contact area. Here, multiple ink formulations are developed that enable 3D printing of unique solid electrolyte microstructures with varying properties. These inks are used to 3D-print a variety of patterns, which are then sintered to reveal thin, nonplanar, intricate architectures composed only of Li7 La3 Zr2 O12 solid electrolyte. Using these 3D-printing ink formulations to further study and optimize electrolyte structure could lead to solid-state batteries with dramatically lower full cell resistance and higher energy and power density. In addition, the reported ink compositions could be used as a model recipe for other solid electrolyte or ceramic inks, perhaps enabling 3D printing in related fields.

3D-Printing Electrolytes for Solid-State Batteries.

The tortuosity of a structure plays a vital role in the transport of mass and charge in electrochemical devices. Concentration polarisation losses at high current densities are caused by mass transport limitations and are thus a function of microstructural characteristics. As tortuosity is notoriously difficult to ascertain, a wide and diverse range of methods have been developed to extract the tortuosity of a structure. These methods differ significantly in terms of calculation approach and data preparation techniques. Here, a review of tortuosity calculation procedures applied in the field of electrochemical devices is presented to better understand the resulting values presented in the literature. Visible differences between calculation methods are observed, especially when using porosity–tortuosity relationships and when comparing geometric and flux-based tortuosity calculation approaches.

https://www.tandfonline.com/doi/pdf/10.1080/09506608.2016.1249995

Tortuosity in electrochemical devices: a review of calculation approaches

Solid oxide fuel cells ! SOFCs" are efficient, environmentally friendly, and fuel-flexible electrochemical devices for the generation of electrical power and heat. 1 They consist of three basic layers: cathode, electrolyte, and anode. The cathode is a porous, conductive catalyst for the reduction of O2 and for the oxidation of fuel. Between the cathode and anode is the dense electrolyte. The circuit is completed via cathode and anode contacts to an external load. The basic chemical formula for the cathodic reduction reaction is 1 2 O2 +V o ·· +2 e! =O o x ! 1" Current SOFC performance is limited by cathode polarization, which increases with decreasing operational temperatures. 2,3 Cathode microstructure and morphology have a strong effect on this polarization. 2-4 In this initial study a dual-beam focused ion beam/scanning electron microscope ! FIB/SEM" was utilized to reconstruct an actual three-dimensional ! 3D" model of a La0.8Sr0.2Co0.2Fe0.8O3! ! ! LSCF" cathode and its interface with a dense yttrium-stabilized zirconia ! YSZ" electrolyte. This highresolution, 3D technique advances the understanding of the cathode microstructure’s effect on performance. The identification of critical microstructural properties such as surface area, tortuosity, and interfacial porosity may be correlated with the ionic, electronic, and catalytic processes for a better fundamental understanding of electrochemical performance. With this tool, SOFC material and microstructural design can be more effective in reducing cathodic polarization at lower operational temperatures. The semiconductor industry has used the FIB since the 1980s to deposit, etch, micromachine, and image specimens during different stages of circuit processing. 5,6 This technology was brought forward to reconstruct 3D, geometrically complex submicrometer structures. 7-11 With the advent of 3D modeling software, nanotomography utilizing the dual-beam FIB/SEM technique was used to quantify nanoceramic suspended powders. 10-12 This technique was applied to SOFC cermet anodes to quantify microstructural properties such as porosity, triple-phase-boundary ! TPB" length, and degree of anisotropy via tortuosity. 13 Such a technique has never before been applied to reconstruct a cathode and the cathode/ electrolyte interface.

D. P. Kundinger

Papers

Three-Dimensional Reconstruction of Porous LSCF Cathodes