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Chapter 5. Aspects of Metal-YSZ Electrode Kinetics 111 the different metals revealed more information about the reactions than a single material would have. A comparative characterization of the electrode kinetics of several metals under identical conditions is also useful for addressing recent studies that speculated/concluded that all metals share the same reaction mechanism. In this study the metal electrodes were also subjected to some extreme conditions that cause temporary phase changes, including oxidation by exposure to oxygen followed by re-reduction to metal by hydrogen, and strong cathodic polarization that is known to partially reduce the zirconia. 5.1.1. Prior Work A number of studies, using a variety of cell geometries, have been carried out with the intention of elucidating the reaction mechanisms. Point-contact electrodes are often a metal wire, either bent or melted to a ball-shaped end. Primarily Ni [3-15], but also sometimes Pt [6, 10, 12, 13], Au [6, 10], and Ag [6], have been used. Others have investigated Ni [16-22] and Pt [23] pattern electrodes and the differences between multiple geometries including patterns, meshes, and porous Ni electrodes (both all-metal porous electrodes and Ni-YSZ cermets) [16, 24-27]. Many different metals (Au, Co, Fe, Mn, Ni, Pd, Pr, Pt, Rh, and Ru) were included in one study that used porous metal electrodes [28]. Most other studies that have included different metals have only done so to assess their relative performance for a given reaction – they did not conduct a systematic test to try to determine the reaction mechanisms as is done in the present study. The reaction mechanisms have proven very difficult to determine. A number of possibilities have been proposed to explain experimental findings. The proposed mechanisms were recently reviewed in ref. [1]. The only general agreement is that the electrode kinetics are not limited by a single charge-transfer rate-limiting step; the Butler-Volmer equation which is most often used in electrochemistry cannot be used to describe this system [1, 3, 7, 24, 29]. Some studies have derived impedance models from chemical rate equations or created physical models based on first principles and fit them to data [2, 9, 12, 30, 31]. However, these fundamental models are usually applied to a single dataset, often with measurements taken in a limited range of conditions. None of the existing models appear able to describe all (or at least a wider set) of data measured at different conditions and by different studies. One reason that it may be difficult to put together a clear picture is the lack of consistency in the reported data: contradictory results have been reported for a given geometry and for different geometries. For example, the number of impedance processes that have been identified for simple Ni point electrodes (as well as for Ni pattern electrodes [22]) at similar conditions varies from 1 to 3. The polarization resistance normalized to the three-phase boundary (TPB; the Ni/YSZ/gas interface) length at the same conditions has been reported to vary by 2 orders of magnitude [1, 11, 22]. In some cases Ni and Pt were found to have nearly the same electrochemical activity [6] while in others they were very different [23]. Finally,PDF Image | Electrolysis of CO2 and H2O
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