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Chapter 5. Aspects of Metal-YSZ Electrode Kinetics 141 Under increasing anodic polarization, the spectra for Ni and Pd (and perhaps Cu as well, though it is less clear) transition into two distinct arcs. This is not observed under cathodic polarization for any of the metals in either atmosphere. Instead, under cathodic polarization, apparent Warburg-like diffusion limitations are sometimes seen (Figure 5-20b, Figure 5-21a, and Figure 5-22a+b). In some cases, the low-frequency impedance switches between inductive loops and diffusion-limitation type response at different cathodic overpotentials, e.g. for Pd in H2/H2O (Figure 5-21a) and Pt in CO/CO2 (Figure 5-22b). Under high anodic polarization for some metals, instability is seen in the measurements. This is expected for Ni, as observed in the polarization curves and described in the prior section 5.3.4.1. In the impedance measurements it is also observed for Cu and Pd. Ni oxidizes well before Cu and Pd, so the oxidation-reduction mechanism mentioned in the prior section 5.3.4.1 is not likely responsible for the instability for Cu and Pd. The hydroxide film oscillation mechanism mentioned in the prior section might be occurring. More tests would be necessary to try to determine the source of the instability. Low frequency inductive loops can arise in reactions involving adsorbed intermediates [12, 13, 59-62]. More specifically, these authors have shown via modeling that inductive effects may occur under applied potential due to the potential dependence of the adsorbed intermediates in the reaction mechanism: ―Inductive effects may occur whenever a stepwise electron transfer takes place towards adsorbed intermediates. Whether the inductive effect appears in the impedance diagram will depend on the value of the rate constants and hence on the potential dependence of the fraction of coverage of the adsorbed intermediates.‖ [60] On the other hand, an inductive loop might be the result of an increase of the electrode surface area by injection of electrons into the YSZ subsurface, resulting in enhanced electronic conductivity and spreading of the reaction zone during anodic polarization [60, 63]. However, if the YSZ surface has electronic conductivity and the effective contact area is larger, this should show up as a lower RS during polarization, and in the measurements presented here the RS varied only +/- 2% during anodic or cathodic bias relative to OCV and most often increased, not decreased. It is worth considering whether other activation phenomena (e.g. other changes in morphology) could also cause the inductive loops observed here. It is very important that measurements are made at steady-state, which was the reason for the 5-minute fixed DC bias (chronoamperometry) immediately before each impedance measurement as the same bias. As a test, some measurements were taken without chronoamperometry, and activation (or deactivation, depending on the metal and the applied potential) would sometimes take place during the impedance measurement, lowering impedance and causing a drift in the direction of the origin in Nyquist plots. Since the impedance was measured from high to low frequency and the majority of measurement time is spent on the low-frequency data points, this drift creates a shape similar to the inductive loops. Similarly, deactivation during measurement could result in the diffusion-limited shape. However, in these tests the resulting curve was not inductive – it did not extend into the positive imaginary regime – and it did not curl back upwards.PDF Image | Electrolysis of CO2 and H2O
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