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Chapter 5. Aspects of Metal-YSZ Electrode Kinetics 131 observed here, so this is a likely possibility. Possible reasons will be discussed in section 5.4. In 50% CO2/CO, the order is Ni > Cu > Pd/Pt. In CO/CO2, Cu is more active than Pd and Pt. Cu is known to be a good catalyst for CO/CO2 reactions such as methanol synthesis by CO2 hydrogenation (CO2 + 3H2 CH3OH + H2O, see Chapter 3). Pt has the lowest performance in 20% CO2/CO, which might be due to the blocking of Pt electrodes by CO, as proposed to explain the poor performance of Nernst probe oxygen sensors with Pt electrodes in dry CO/CO2 atmospheres [53]. SOFCs with Pt electrodes were also shown to have poor performance in dry CO/CO2 compared with H2/H2O atmospheres [34]. In regards to the magnitudes of LSRP found in this study, for Ni at 1000 °C in 3% H2O/H2 the LSRP is around 5 kΩ cm, which is within the wide range of ~0.2-20 kΩ cm found in the literature for these same conditions for Ni point electrodes [1, 11]. At 800 °C in 3% H2O/H2, the LSRP for Ni is approximately 60 kΩ cm which is within the wide range reported for point and porous electrodes at similar conditions [1, 11]. 5.3.4. Effects of Applied Potentials 5.3.4.1. DC Polarization Curves Cyclic polarization curves (also known as cyclic voltammograms, or potential sweeps) were performed on all of the electrodes in 50% H2O/H2 and 50% CO2/CO at 1-2 mV/s. For Ni, the polarization curves show higher activity for H2 oxidation than for H2O reduction (Figure 5-13). High anodic potentials sometimes resulted in instability in both H2/H2O and CO/CO2 which became apparently more regular oscillations in H2/H2O (Figure 5-13). Such oscillations were observed by Schmidt et al [15] for Ni point electrodes in H2/H2O (also in very different conditions – air – oscillations were observed for Pt [41] and Pd [42]). The instabilities appear to begin at approximately +190 mV for 50% H2O/H2 at 1000 °C and +150 mV for 50% CO2/CO at 1000 °C, which is –690 mV vs air for both. In 3% H2O/H2, the instabilities appear to be absent, even at +300 mV (–760 mV vs air). At 850 °C in 50% H2O/H2, the instabilities begin at about +250 mV (–672 mV vs air). The instabilities seem to consistently begin when the local pO2 becomes higher than a certain value (around 10-12 and 10- 13 atm at 1000 °C and 850 °C respectively), independent of H2/H2O and CO/CO2 atmospheres. It is near these values that Ni oxidizes to NiO, according to thermodynamics [54]. Therefore, because the instability is observed in both H2/H2O and CO/CO2 atmospheres, this is clear evidence that at least part of the instability is caused by Ni oxidizing to NiO (after which it would be reduced again by incoming H2 or CO). Both H2 oxidation and CO oxidation are significantly activated by the instability phenomena, evident in the hysteresis in the polarizationPDF Image | Electrolysis of CO2 and H2O
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