Comparison Between Microstructure Parameters and Electrochemical Performance of Ni-CGO Anodes in SOFC Subjected to Redox-Cycling
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The aim of this thesis was to create an understanding between the electrochemical performance of a porous Ni-CGO anode used in SOFC, and its microstructural parameters when subjected to redox-cycling. The anode samples were produced consisting of two different layers, and subjected to different number of redox-cycling. Their electrical resistance was measured using electrochemical impedance spectroscopy (EIS), and their parameters were obtained by analyzing cross-section images from the samples obtained by the focused ion beam (FIB) or from simulated 3D-models. A large increase of Ohmic resistance was observed after 3-4 redox-cycles, while there was only a slight increase of the polarization resistance. The high increase of the Ohmic resistance is believed to be due to the observed Ni-coarsening and the net increase of the anode volume due to oxidation of Ni to NiO and subsequently reduced back to Ni. Both the Ni-coarsening and the net volume expansion decrease the Ni connectivity which increases the Ohmic resistance. More electrons are also forced to go through the lower-conducting CGO phase, which increases the Ohmic resistance, though preventing a higher increase since it is connecting the isolated Ni grains together. The low increase of polarization resistance is believed to be due to the large pore-CGO interfacial area, which compensates for the decrease in triple-phase boundaries (TPB). The resolution of synthetic microstructures is not enough to capture constrictivity and tortuosity effects realistically. The dimensions of most of the bottle necks are below 100 nm and hence the size classes must be in the range of 10 nm, which is similar as the resolution of FIB-tomography.