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Chapter 5. Aspects of Metal-YSZ Electrode Kinetics 158 YSZ surface inside the interface. The glassy phases probably can be reduced in a similar range as their oxide constituents. Oxides of Al, Mg, and Ca, are reduced to metal at similar or slightly lower oxygen activities than Zr. In fact, according to the thermodynamic data these metals are liquid in these conditions. The SCP clearly provides the conditions to reduce the impurity phases (as well as the YSZ) and the reduced impurities may become mobile and rearrange such that when they re-oxidize upon releasing the strong cathodic bias, the TPB may be relatively unblocked and freshened. This mechanism seems very possible. The electrodes would likely slowly deactivate again after releasing the SCP. Changes in the microstructure of the metal-YSZ interface could occur. If the ZrO2 is reduced to Zr metal during the SCP (and possibly also the yttria to Y metal, which should occur at lower oxygen activity than 10-49 atm), the metal electrode might alloy with the metallic phases. Upon releasing the polarization, the Zr metal is favored to re-oxidize and a metal-Zr-O composite phase with a rough micro- or nanostructure could form. Sub- oxides of ZrO2, ZrO2-x, might also be reactive with the metal electrode. Both Zr metal and nano-crystallites of ZrO2-x [72] have been reportedly found along the grain boundaries of electrochemically reduced YSZ. Nano-sized grains of tetragonal YSZ were also found after re-oxidation of electrochemically reduced YSZ [75]. After tests that included SCP, most metal electrodes were strongly adhered to the YSZ surface (whereas usually the electrodes are easily lifted off), which supports the idea of alloying or reactivity between the reduced YSZ and the metal electrode. Indeed, SEM micrographs of the interface after pulling off the electrode revealed a complex microstructure composed of metal and YSZ inside the contact area and at the perimeter (Figure 5-37 and Figure 5-38). The metal wire also had a corresponding nanostructured surface where it had been in contact with the YSZ surface (Figure 5-38c), which is very different than the relatively smooth and flat surface observed when strong cathodic polarization had not been applied (Figure 5-7). The YSZ surface adjacent to the interface with an Ni electrode which had formed a belt of Ni particles earlier in the test (see Figure 5-15) exhibited Ni-YSZ nanostructures surrounding most of the Ni particles in the belt (Figure 5-38a+b+d). Probably the Ni particles were not connected to the bulk electrode, so the current path was through the reduced YSZ. Since RS decreased it is expected that the surface YSZ reduced outwards from the electrode. The YSZ probably also reduced downwards from the electrode, but if it had only reduced downwards RS would not increase until a large portion of the thickness of the YSZ pellet had been reduced, which is unlikely. Thus it seems likely that both reduction of impurities and reaction of the metal electrode with reduced zirconia, resulting in the formation of a fine microstructure, are responsible for the observed activation. In fact, both mechanisms would increase the TPB length, one un- blocking the TPB and the other roughening it. It would be interesting to see if exposure to high temperature coarsened the fine microstructure as was observed for the oxidized Ni electrodesPDF Image | Electrolysis of CO2 and H2O
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