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Chapter 5. Aspects of Metal-YSZ Electrode Kinetics 163 quantity of material (and corresponding quantity of impurities) as a result of fine microstructure, the reaction sites at the TPB would become relatively less blocked by impurities as they segregate during heat treatment. With different starting purity of the materials, different studies would observe very different performance for the same metals even if they used the exact same electrode geometry or microstructure because their cells have different available TPB lengths. The effect of impurities on increasing the polarization resistance was clearly demonstrated for Ni point electrodes by several authors [11, 14, 36, 37, 39, 43, 79]. This can explain items 1 and 2 above. How could the higher activation energies (item 3 above) be explained by the TPB being simply partly blocked? There appears to be a correlation between LSRP and activation energy for Ni electrodes [1, 11] [38]. A possible explanation is that with fewer available reaction sites due to impurities, a different rate-limiting step with a higher activation energy dominates, such as diffusion around or through the impurities. This would also explain the different number of rate-limiting processes (item 4 above) reported on Ni electrodes and assuming that strong cathodic polarization reduces impurities as proposed this would explain the apparent disappearance of rate-limiting processes as a result of strong cathodic polarization observed here (Figure 5-32). Microstructural differences alone might also be able to explain some of these inconsistencies. In the study mentioned earlier which correlated electrocatalytic activity of various metals with the metal oxide formation enthalpy [28], it was noted that the grain sizes of the porous electrodes tested varied by more than a factor of 2. However, Pd was found to be more than an order of magnitude less active for H2 oxidation than Ni, which cannot reconcile the results with those found here. Some of the inconsistencies seem to be more related to microstructure than impurities. In terms of the different relative activities of Ni in H2/H2O versus CO/CO2 (item 5), after the strong cathodic polarization the activity was still around an order of magnitude lower for CO/CO2 than for H2/H2O for all metals (Figure 5-30 and Figure 5-33). The strong cathodic polarization, which likely reduces impurities, did not help reduce the gap in performance between H2/H2O and CO/CO2. A fine three-dimensional microstructure in a porous cermet might provide enough reaction sites that the relatively slower adsorption and surface diffusion of CO/CO2 intermediates than H2/H2O intermediates has less effect. The gas partial pressure dependencies (namely the pH2O dependency) and the bias dependency of the polarization resistance of Ni (items 6 and 7 above) seem to be higher for coarser microstructures [25, 26]. Bieberle and Gauckler [26] observed stronger activation with bias for smaller-TPB Ni electrodes and this seems to be independent of purity because they used high-purity single- crystal YSZ for all cells. The results of Brown et al [25] along with their tabulation of prior work show an apparent correlation between fineness of microstructure and pH2O dependence of the polarization resistance for various Ni electrode geometries, ranging from 0.5-0.6 for wire, paste, felt, and pattern electrodes to 0.4 for the coarse cermet they tested and only 0.2-0.3 for finePDF Image | Electrolysis of CO2 and H2O
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