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Chapter 5. Aspects of Metal-YSZ Electrode Kinetics 134 At overpotentials below the unstable regimes, the shapes of the curves in H2/H2O atmospheres are generally consistent with those performed on simplified Ni geometries in H2/H2O atmospheres in literature [7, 18, 20, 24]. It seems likely that the large hysteresis in the cathodic curve measured at 1000 °C in 50% H2O/H2 is related to the unstable anodic curve that had taken place just beforehand. In the log current plot, some linear regions indicative of Tafel kinetics might be present, but they are not easy to identify. Such linear regions seem to be present at the start of the polarization curves, when sweeping in the anodic direction below the unstable regime. After reaching +300 mV the electrodes seem to be activated during the downwards sweep, resulting in hysteresis. In cathodic sweep that followed, linear regions seem to sometimes be present but often only during the downwards sweep back to OCV. For CO oxidation at 1000 °C there is clearly no linear region, whereas for CO2 reduction there does seem to be one. Polarization curves performed on the Ni electrode later in the test at 850 °C in 50% H2O/H2 and 50% CO2/CO are shown in Figure 5-14. The possible linear regions are more evident in 50% CO2/CO, especially in the cathodic sweep, however for 50% H2O/H2 there is again curvature throughout the anodic sweep and only at the end of the cathodic sweep returning to OCV can a possible linear region be identified. However, the general slopes are nearly the same as for CO/CO2. Assuming Tafel kinetics describe the curves, the anodic and cathodic charge transfer coefficients corresponding to the labeled slopes are approximately 1.40 and 0.56 respectively, indicative of a multi-step reaction as rate limiting. The anodic slope of the 50% H2O/H2 curve at 1000 °C, however, is much steeper and would give a very high anodic transfer coefficient. It is possible that the reactions are single-step charge-transfer limited at some potentials but the interface seems to be dynamic even ignoring the unstable anodic regime. Adsorbed intermediate species might accumulate on the Ni or YSZ surface at some potentials. This will be further discussed in the next section. Sweeps with lower limits of +200 and –200 mV did not reveal clearer linear regions; the continuous curvature for H2 oxidation on Ni was still present. Polarization curves for all of the metals at 850 °C are shown in Figure 5-16 and Figure 5-17. Although one curve is shown at each condition, multiple were performed at different polarization ranges up to the maximum range shown (+300 mV to –500 mV). The shapes of the sweeps were generally reproducible at consecutively larger overpotentials, usually with slight activation after each sweep that reached high positive or negative overpotentials (see also section 5.3.4.3). In some cases, hysteresis was observed. In the cases of hysteresis, nearly the same hysteresis curve was followed when scanning to lower overpotentials (e.g. +200 mV and – 300 mV). For example, the large hysteresis observed for Pt in 50% H2O/H2 in the cathodic direction was also reproducible through sweeps with lower cathodic overpotential limits, as well as the hysteresis observed for anodic polarization of Ni in CO/CO2. The magnitude and shapes of the curves were also generally reproducible between different electrodes of the same metals.PDF Image | Electrolysis of CO2 and H2O
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