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Chapter 5. Aspects of Metal-YSZ Electrode Kinetics 132 curves that have instability (Figure 5-13). This observation is related to phenomena that will be discussed section 5.3.5.1 (related to Ni/NiO phase changes). This Ni-NiO transition was considered by Schmidt et al [15]. However CO/CO2 tests were not performed and it was noted that NiO formation should not begin quite so soon (but also noted that surfaces can oxidize sooner than bulk). Instead, the explanation proposed was that a passivating film of Ni(OH)2 was temporarily formed on the Ni, causing a slowing of the current, which was then reduced back to Ni by incoming H2, causing a jump in the measured current, resulting in the sawtooth oscillation. The Ni(OH)2 film is proposed to be passivating by assuming that the H2 oxidation reaction mechanism is proton diffusion through Ni and the diffusion path grows longer as the passivating film grows. In addition, it was proposed that the H2O formed at the TPB during anodic current reacts with impurities that are present at the TPB and mobilizes them (e.g. SiO2(s) +2H2O(g) Si(OH)4(g)), resulting in activation. Because the oscillations are more periodic in H2/H2O than in CO/CO2 (Figure 5-13), Ni- NiO may only explain part of the instability. A mechanism that involves Ni(OH)2 might still be possible. During high anodic polarization, H2 is consumed producing H2O, and the potentials vs air mentioned above correspond to high local pH2O/pH2 ratios of >95/1. Thermodynamics indicates that volatile Ni(OH)2 forms to an appreciable extent under such conditions [54], and literature studies of Ni/YSZ point electrodes and cermets have speculated that is the cause of observed morphological changes of Ni [8, 15, 55-57]. Alternatively, the formation of and surface transport of Ni2–OH complexes might explain observed morphological changes [58]. Indeed, in the present study the Ni morphology was changed by anodic polarizations – submicron-sized Ni particles were present on the YSZ surface as a belt around where the interface had been (Figure 5-15), as has been sometimes observed in literature for Ni point electrodes [8, 15, 25] and even in Ni cermets [57]. It is believed that as the hydroxylated Ni species move away from the TPB to a lower pH2O region, they reduce back to Ni metal and deposit as particles. Verification that the particle belt does not also form in CO/CO2 would further support this mechanism, however no tests with anodic polarizations only in CO/CO2 were conducted in this study. It seems plausible that the oscillations could be the result of the same mechanism that forms the particle belt and may not involve a passivating Ni(OH)2 film. Activation could occur if parts of the belt temporarily become connected, increasing the TPB length, and then slow deactivation could occur if the particles slowly agglomerate and disconnect (or even reincorporate into the bulk Ni electrode) due to the high temperature. Similar phenomena have been speculated to explain similar sawtooth oscillations in the study mentioned in section 5.3.1 about Pt migration in oxidizing atmosphere [41]. Alternatively, even if the particles never become connected, the presence of nanoparticles could improve adsorption or desorption kinetics and cause periodic activation if they were formed periodically, which could result from a number of accumulation mechanisms. Finally, it is also feasible thatPDF Image | Electrolysis of CO2 and H2O
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