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Chapter 5. Aspects of Metal-YSZ Electrode Kinetics 165 H2O/H2 to 50% H2O/H2 change in capacitance, the impedance spectra measured under bias shown in Figure 5-18 exhibit a similar increase in capacitance between OCV and +150 mV anodic bias and similar magnitudes. As discussed in ref. [30], at high temperature H2O was found to adsorb onto Pt by H2O OH + H and hydroxyl coverage is promoted on the surface [84, 85]. It is also sensible to assume that in higher pH2O there would be more OH adsorbed on the surface whereas in higher pH2 there would be more H adsorbed on the surface. Since Ni, Cu and Pt share a similar response to pH2O and only Pd is different, we propose that hydroxyl species adsorbed on the metal surface are the source of activation for Ni, Cu, and Pt. The apparent higher activity of Pd in the presence of more H2 can probably be explained by the Pd–H system. It is well-known that hydrogen is soluble in Pd. In fact Pd-hydride has been considered as a means of hydrogen storage. At standard temperature and pressure, Pd can absorb up to 900 times its own volume of hydrogen. On a H+ conducting electrolyte, a dense Pd film electrode was observed to have higher performance for H2 oxidation than a porous Pd electrode [86] – the H2 transports through the Pd film and the entire film surface is reactive. This might explain some of the anomalous behavior of Pd in H2/H2O. However, it does not explain the higher electrolysis performance for CO2 than for CO oxidation. Recently, Vogler et al [31] showed via modeling that, besides a hydroxyl transfer mechanism between Ni and YSZ, a double hydrogen transfer mechanism might be able to explain the pH2O dependency of the polarization resistance, and that the double hydrogen transfer mechanism seems to be more consistent with the lack of pH2 dependence. The adsorption of hydroxyl species on metal surfaces at high temperature does not appear to be well known in surface science, whereas dissociative adsorption is well known and known to be fast and can result in high Had coverage [87]. In addition to the uncertainty of which species accumulates, it is also not known which surface, the Ni or YSZ (or both), hosts the accumulated species. Clearly, more modeling efforts will be needed to determine which adsorbed species are part of the rate-limiting step. Nearly linear polarization curves, which are indicative of a very high exchange current density, were obtained after strong cathodic polarization, most likely due to the longer TPB that resulted from either the fine microstructure that formed or the removed impurities that no longer blocked the TPB. This type of polarization curves are typically measured only on fine cermet electrodes. Before strong cathodic polarization, many i-V curves could possibly be described by Tafel kinetics. The impedance spectra also indicate that the strong cathodic polarization seems to have eliminated one or more rate limiting processes. Finally, it is worth considering the magnitude of the length-specific polarization resistance values measured here and measurements reported in literature and how it relates to porous cermet electrodes. A few recent studies have provided estimates of the TPB length per unit volume of a porous Ni-YSZ cermet electrode, of around 2-6 μm/μm3 [88-91]. It is also known that a good Ni-YSZ cermet electrode can have an OCV polarization resistance of about 0.1 Ω cm2 at 850 °C in H2/H2O and that the physical thickness of the electrode that contains activePDF Image | Electrolysis of CO2 and H2O
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