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Chapter 4. Co-electrolysis of CO2 and H2O in Solid Oxide Cells 99 that was subsequently used for the electrolysis durability test. Because the free energies of the CO2 and H2O dissociation (to yield CO and H2 respectively) reactions are nearly equivalent at this temperature ([49] and see section 3.2.2), the open-circuit voltage (OCV) for the three 50%- reactants compositions are nearly all equivalent, as can be seen in the figure. The cell exhibited a high performance for co-electrolysis of CO2/H2O mixtures, comparable to the performance for steam electrolysis which is known to be high [50]. As has been reported for similar cells [15, 20, 26, 27, 31-34], the cell exhibited slightly higher performance for H2 oxidation and H2O reduction than for CO and CO2 respectively. For the 25%-each composition, co-electrolysis performance was between that of CO2 and H2O electrolysis and closer to H2O. This suggests that besides the CO2 and H2O electrolysis reactions, the reverse WGS reaction occurs. The role of the reverse WGS is further studied, using different gas composition variations, in ref. [34]. The degree to which reverse WGS is responsible for CO production cannot be determined from these results. True quantification of the relative amounts of the reactions taking place would require 18O isotope tracing or a similar method. However, whether part of the syngas is produced by the WGS equilibrium has little effect on the energy demand of syngas production in the cell, because the cell’s internal resistance can supply the heat needed for either the electrochemical or catalytic reactions. From Figure 4-3 it is also observed that at higher current densities the resistance increases for the three 50%-reactants i-V curves. It appears that a limiting current density is approached. This effect is often explained as a gas diffusion limitation of the reactants inside the electrodes [33, 51]. However, it is possible that the increased resistance may arise from effects related to the test set-up instead of the gas transport in the electrode microstructure— gas conversion resistance [52, 53], Rconv, and/or gas diffusion resistance due to a volume of stagnant gas outside the porous electrode structure [53, 54], Rdiff. Rconv and Rdiff are both minimal when the reactant/product ratio is 1. Therefore, when the gasses are supplied at a reactant/product ratio of 1 (the 50%-reactants tests), Rconv and Rdiff are smallest at OCV, and reactant utilization resulting from polarization will reduce this ratio and thereby increase these types of resistance. On the other hand, when the gasses are supplied at a reactant/product ratio of 9 (45% CO2 + 45% H2O + 10% H2), reactant utilization will bring the reactant/product ratio closer to 1, reducing Rconv and Rdiff—hence the relative lack of curvature in that i-V curve. Further study of these effects is important if high reactant utilization is necessary. The IS measurements at OCV for the same gas compositions (Figure 4-4) agree with the i-V curves—the resistance for reduction and oxidation of the co-electrolysis gas composition is higher than for H2O/H2 and lower than for CO2/CO, and closer to that of H2O/H2. It can be seen that the serial resistance, RS, which is almost entirely the ohmic resistance of the electrolyte, contributes 40-50% of the total resistance at OCV at this temperature. From the Nyquist plot, itPDF Image | Electrolysis of CO2 and H2O
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