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Chapter 5. Aspects of Metal-YSZ Electrode Kinetics 147 just taken at 850 °C which clearly contradicts the temperature-activated nature of the electrochemical reactions. The LSRP was increasing at 1000 °C – after another 28 hours the LSRP had returned to the value from before the oxidation (5 kΩ cm). The electrode was oxidized again for 30 h at 1000 °C (Figure 5-25). The LSRP reduced by a factor of 3 for 3% H2O/H2 and began to increase. The gas composition was changed to 50% H2O/H2. The LSRP was ~0.3 kΩ cm and continued to increase. Examining the Ni wire electrode in the SEM after testing revealed a porous surface (Figure 5-26). After the second oxidation, the electrode was tested in a variety of H2/H2O and CO/CO2 atmospheres for 110 h more at 1000 °C followed by 130 h more at 850 °C before cooling down. The porosity is the result of the oxidation and it is believed to have smoothened by sintering over time. The smoothening would explain the increasing LSRP after oxidation events – immediately after oxidation, the NiO of the electrode surface is reduced back to Ni metal producing a rough microstructure with high surface area, and the high temperature causes the surface area to slowly decrease. The formation of the NiO during oxidation and/or the reduction back to Ni must have occurred non-homogenously to yield the rough microstructure with high surface area and longer TPB than before oxidation. At 850 °C, the new high-surface- area microstructure was preserved, at least for the few hours of measurements. At 1000 °C, the microstructure smoothened and LSRP increased dramatically. Upon oxidizing the electrode again at the higher temperature, a new higher surface area microstructure was again formed yielding a longer TPB, which also began to smoothen. The final result is the TPB shown in the SEM micrograph. If the TPB length was enhanced like this, the data normalization procedure does not account for this and is not exactly correct, but is still useful to compare the degree of activation that occurred, as discussed at the end of section 5.3.2. It is interesting that oxidizing the Ni electrode greatly enhances the electrochemical activity. However, such treatment on a Ni-YSZ cermet may not be practical because the Ni is structural, and the microstructural changes and expansion resulting from oxidizing Ni to NiO and reducing back again to Ni (―redox cycles‖) are known to severely damage the structure, and in the case of Ni-YSZ electrode supported cells, redox cycles can crack the adhered electrolyte layer as well, destroying the cell [64, 65]. In addition, cermets are in fact prepared as NiO-YSZ composites and the NiO is most often reduced to Ni in situ at the start of cell operation, so the enhanced microstructure is most likely already present (and because the Ni and YSZ are intermixed fine particles, the YSZ prevents the Ni from sintering too much even at high temperatures). However, at low temperatures around 650 °C where Ni is less prone to rapid oxidation, the redox expansion-contraction is less severe and the Ni-YSZ cermet electrode was found to remain intact as well as activate somewhat [66], though to a much lesser extent than observed here but most likely due to similar microstructural enhancements as observed here.PDF Image | Electrolysis of CO2 and H2O
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