Redox cycling of Ni–YSZ anodes for solid oxide fuel cells: Influence of tortuosity, constriction and percolation factors on the effective transport properties

• Published in: Journal of power sources Vol. 242; pp. 179 - 194
• Main Authors: Holzer, L., Iwanschitz, B., Hocker, Th, Keller, L., Pecho, O., Sartoris, G., Gasser, Ph, Muench, B.
• Format: Journal Article
• Language: English
• Published: Amsterdam Elsevier B.V 15.11.2013; Elsevier
• Subjects: 3D image analysis; Anodes; Applied sciences; Cermet; Constrictions; Cycles; Degradation; Direct energy conversion and energy accumulation; Effective transport property; Electrical engineering. Electrical power engineering; Electrical power engineering; Electrochemical conversion: primary and secondary batteries, fuel cells; Energy; Energy. Thermal use of fuels; Equipments for energy generation and conversion: thermal, electrical, mechanical energy, etc; Exact sciences and technology; FIB-tomography; Fuel cells; Materials; Metals. Metallurgy; Microstructure degradation; Nickel; Percolation; Powder metallurgy. Composite materials; Production techniques; Sintered metals and alloys. Pseudo alloys. Cermets; Tortuosity; Yttria stabilized zirconia; 3D image analysis; Effective transport property; Cermet; Microstructure degradation; FIB-tomography; Transport properties; Anode; Tortuosity; Electrode material; Solid oxide fuel cell; Degradation; Tomography; Percolation; Electrochemical reaction; Electrical characteristic; Damaging; Cycling; Oxidation reduction; Transition metal; Secondary cell; Stabilized zirconia; Thermal properties; Image analysis; Yttrium Oxides; Tridimensional image; Nickel; Microstructure; Effective medium model; Contact thermal resistance
• ISSN: 0378-7753; 1873-2755
• DOI: 10.1016/j.jpowsour.2013.05.047

A methodology based on FIB-tomography and image analysis (IA) is proposed which allows quantification of all relevant morphological features that are necessary to predict effective transport properties in porous SOFC electrodes. These morphological features are constrictivity, tortuosity, percolation factor and phase volume fraction. An M-factor can then be calculated which represents the ratio of effective over intrinsic conductivities. The methodology is used to describe effects of microstructure degradation in Ni–YSZ anodes which are caused by redox cycling at 950 °C. The so calculated M-factors predict that because of redox cycling the effective electronic conductivity of nickel decreases from 3 to 1.2% which is mainly due to changes of percolation and constriction factors. Based on these results the effective electrical conductivity of nickel is predicted to be 685 S/cm before redox and 243 S/cm after 8 redox cycles. The predictions fit well with the experimental measurements that reveal 600 S/cm before and 200 S/cm after redox cycling at 950 °C. For YSZ the M-factors obtained with 3D-analysis predict that the degradation causes a drop of the effective ionic conductivity from 7 to 0.6%, which is due to a change of the bottleneck dimensions. This finding contradicts the frequent interpretation of YSZ as a ‘rigid backbone’ that is not affected by microstructure degradation. Finally, the effective bulk gas diffusivity increases from 2 to 11% due to an increase of porosity associated with swelling of the anode. •Redox degradation of Ni–YSZ anode is described by 3D-imaging.•The effective electrical and ionic conductivities are predicted based on 3D-topology.•Electronic transport decreases due to changes of percolation and constrictivity in Ni.•Ionic transport is limited by constrictivity (narrow bottlenecks) in YSZ.