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Chapter 4. Co-electrolysis of CO2 and H2O in Solid Oxide Cells 102 degradation rate is significantly faster, on the order of 0.1 mV/h, although the degradation rate was not allowed to fully level off. The impedance spectra measured before and after each segment (Figure 4-7) show an increasing serial resistance (RS), especially after the final segment at –1 A/cm2. The high- frequency contributions to the impedance also appear to have enlarged with the consecutively higher current densities. As expected, the low frequency gas concentration impedance arcs do not appear to have noticeably changed. However, not much more can be learned from the Nyquist plot (Figure 4-7). The DRT can be used to reveal more information about which processes were affected during each segment of long-term operation. The DRT does not show RS (nor changes in RS). The DRT and the Nyquist plot are therefore complementary for analyzing impedance data. The DRTs of impedance spectra measured before and after the segments (Figure 4-8) clearly show that the impedance contributions related to gas concentration have negligibly changed. The DRTs show that after the –0.25 A/cm2 segment, the resistance of the Ni/YSZ electrode process has grown, whereas the lower-frequency LSM/YSZ (LSMLF) peak has not been affected. At first glance the high-frequency LSM/YSZ (LSMHF) peak appears to have grown slightly—however since the DRT result is the sum of all the peaks, the growth of the nearby Ni/YSZ peak can visually boost the sum near the LSMHF peak (the inset of Figure 4-8 illustrates this with example Gaussian peaks that might fit the DRT data). After the –0.5 A/cm2 segment and especially after the –1 A/cm2 segment, however, it is clear that the LSM/YSZ electrode has degraded as well; an enlarged Ni/YSZ peak cannot account for the large increases near LSMLF and LSMHF even if the enlarged Ni/YSZ peak is represented by a Gaussian (or any type of) distribution with the maximum width that can fit under the DRT sum. This analysis suggests different degradation mechanisms for high and low current density operation. At low current density, degradation at the Ni/YSZ electrode dominates, as has been reported for various electrolysis durability tests (discussed earlier), whereas at higher current densities, both serial resistance and LSM/YSZ electrode degradation play a significant role. The Ni/YSZ electrode degradation seems to be related to impurities that accumulate at the TPB, as discussed earlier [17, 21, 27, 34, 38]. Although Ni is a good catalyst for coking reactions, carbon deposition is not thermodynamically favored at any of the conditions that the cell was tested in, and a prior study of pure CO2 electrolysis [27], which would provide more favorable conditions for carbon formation, determined that carbon formation is not a source of degradation. Since higher current induces higher potentials across each component of the cell, the different degradation mechanisms might be related to the distribution of the potentials and the magnitude of the potential across each component (the electrodes and electrolyte). When this type of cell has been tested as a fuel cell under similar conditions, far less degradation at high current density has been observed [55], suggesting that the degradation observed here may be specific to electrolysis operation. One possibility is a build-up of oxygen partial pressure in andPDF Image | Electrolysis of CO2 and H2O
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