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Chapter 5. Aspects of Metal-YSZ Electrode Kinetics 154 was a potential-dependent process and since it no longer dominates the polarization resistance, the polarization resistance is now nearly independent of potential. The activation of all of the electrodes is evident in the polarization curves. CO oxidation curves for all of the metals reach several times higher current density (Pt in fact has improved by more than an order of magintude), as well as all cathodic curves (both H2O reduction and CO2 reduction). At high anodic overpotentials of H2 oxidation for Cu and Ni, however (Figure 5-33a+b), the earlier curves meet the new curves, and for Pd they nearly meet. The relative performance of the metals for oxidation and reduction of H2/H2O/CO/CO2, at 200-300 mV anodic and cathodic overpotentials, is shown in Figure 5-34 and summarized in Table 5-6 in comparison with Figure 5-17. The relative performance has not changed. With the ―straightened‖ curves, Ni is more active at the full range of anodic overpotentials whereas before Pd was more active at lower overpotentials. Pd is still by far the most active for H2O and CO2 reduction. The relative activity of Pt seems to have somewhat increased (but not enough to move up in ranking). One of the Ni electrodes that had been oxidized (and had subsequently lost its resulting improved performance due to 1000 °C heat treatment for many hours, see section 5.3.5.1) was also subjected to SCP under the same conditions but to a more negative limit of –2000 mV. The polarization curves measured before and after are shown in Figure 5-35. This stronger cathodic polarization activated the electrode even more – the maximum currents are much higher and the polarization curves are even more linear. Explaining what occurred during the SCPs must address at least the temporary decrease of RS and the more lasting decrease of LSRP (generally, that the electrodes activated), and ideally the change in the shape of the impedance spectra and the change in the shape of the polarization curves as well. The test was not continued for long after the SCP to see if the LSRP approached its earlier values – in the literature it was observed that some Ni point electrodes relaxed (LSRP increased to near the value before the SCP) after the activation over the course of 2 weeks in 3% H2O/H2 at 700 °C whereas others were more permanently activated, with LSRP still lower by more than a factor of 5-10 after 1 month [14]. —Changes in RS. Electrochemical reduction (as well as reduction by very dry H2 at high temperature) of yttria- or scandia-stabilized zirconia removes oxygen from the crystal lattice and visibly blackens the zirconia [68-73]. The mechanism by which the zirconia is reduced is a valence change either of Zr4+ to Zr3+ or of a point defect (a dissolved chemical impurity or a structural defect such as an oxygen vacancy). The outcome of reduction is (a) a variation in the stoichiometry due to oxygen release, e.g. ZrO2-x, (b) a change in the ionic and electronic conductivities of the stabilized zirconia, and (c) a coloration due to trapped electrons which can be excited to higher energy levels [74]. The electrons are trapped at oxygen vacancies [75].PDF Image | Electrolysis of CO2 and H2O
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