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Chapter 4. Co-electrolysis of CO2 and H2O in Solid Oxide Cells 101 is not clear how much each electrode contributes to the polarization resistance, RP, and what electrode processes might be contributing. The impedance data, measured at systematically varied temperature and gas compositions at each electrode, were transformed to the DRT and compared to resolve these processes. Figure 4-5 shows the DRT for three different gas atmospheres at 850 °C. Transforming to the DRT enhances the frequency-domain resolution of the impedance response. By varying the gas composition supplied to the Ni/YSZ electrode, only the Ni/YSZ electrode impedance processes should be affected, and the same goes for the LSM/YSZ electrode. Five rate-limiting processes are visible in the DRT plot—2 LSM/YSZ electrode processes, 1 Ni/YSZ process, and 2 low-frequency processes mainly at the Ni/YSZ side of the cell, which were independent of temperature (not shown) and correspond to gas conversion and gas diffusion impedance [52-54]. The electrode process with the highest characteristic frequency, which is independent of any gas changes, is ascribed to the LSM/YSZ electrode based on the similarities of the summit frequency and gas-independence as found using simplified cells composed of only LSM/YSZ (symmetrical cells) in another study [42, 43]). The summit frequencies are in approximate agreement with the five cell processes found by the equivalent circuit method for this type of cell (described earlier). The DRT therefore enables identification of the rate-limiting processes that contribute to the total impedance of a cell without relying on a model. Modeling would of course still be very useful to study the nature of the processes. The breakdown of the total cell impedance into these processes could at this point be quantified by modeling the DRT (e.g. applying a measurement model by fitting distribution functions to the transformed data to quantify the peaks, or deriving and applying an electrochemical model). However for this study the DRT is used as is, to study the degradation mechanism of the cell by simply identifying which processes are affected by long-term testing. 4.3.2. Durability During the galvanostatic segments, the cell voltage increased at a faster rate at higher current densities (or correspondingly, higher cell voltages) (Figure 4-6). During the low current density (–0.25 A/cm2) segment, the cell voltage increased initially during the first 100 hours and then leveled off to a slow degradation rate of 0.005-0.008 mV/h during the last 100 hours. The minor passivation and activation of the cell during the first 200 hours was much less pronounced compared with what has been observed for other H2O [17, 21, 37], CO2 [27], and co-electrolysis tests [34]. If the previously observed passivation/activation is rooted in impurities, this test illustrates the inconsistency of impurity-related passivation phenomena since the same gasses, same cell type and nearly identical test set-up were used in this test as in some of the others (excluding the tests that used glass sealing, which was believed to contribute to degradation by supplying silica impurities). During the higher current density segments, it is clear that thePDF Image | Electrolysis of CO2 and H2O
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