Browsing by Author "Heddrich, Marc P."

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Open Access
Experimental analysis of the co-electrolysis operation under pressurized conditions with a 10 layer SOC stack
(2020) Riedel, Marc; Heddrich, Marc P.; Friedrich, K. Andreas
This study examines the performance of a solid oxide cell (SOC) stack during co-electrolysis of CO2 and H2O at elevated pressures up to 8 bar. Steady-state and dynamically recorded U(i)-curves were performed in order to evaluate the performance over a wide temperature range and to quantify the area specific resistance (ASR) at different pressure levels. Furthermore, the outlet gas composition at various current densities was analyzed and compared with the thermodynamic equilibrium. The open circuit voltage (OCV) was found to increase with higher pressure due to well known thermodynamic relations. An increase of the limiting current density at elevated pressure was not observed for the investigated stack with electrolyte supported cells. The ASR of the stack was found to decrease slightly with higher pressure. It revealed an increase of the cell resistance with lower H/C ratios in the feed at lower temperatures, whereas the performance of the co-electrolysis was very similar to steam electrolysis for temperatures above 820 °C. Within an impedance study for steam, co- and CO2 electrolysis operation it was shown that pure CO2 electrolysis exhibits a higher pressure sensitivity compared to pure steam or co-electrolysis due to significantly increased activation and diffusion resistances.
Open Access
Investigation of the long‐term stability of solid oxide electrolysis stacks under pressurized conditions in exothermic steam and co‐electrolysis mode
(2020) Riedel, Marc; Heddrich, Marc P.; Friedrich, K. Andreas
In this study three identically constructed ten‐layer stacks with electrolyte supported cells were tested in exothermic steam and co‐electrolysis mode at elevated pressures of 1.4 and 8 bar. Investigations during constant‐current operation at a current density of -0.5 A cm-2 and a reactant conversion of 70% over 1,000-2,000 h were carried out. The inlet gas composition for steam electrolysis was 90/10 (H2O/H2) and 63.7/31.3/3.3/1.7 (H2O/CO2/H2/CO) for co‐electrolysis operation. All stacks showed highly similar resistances at the beginning of the tests indicating a high level of accuracy and repeatability during manufacturing. The stack operated in steam electrolysis mode at 1.4 bar showed comparably low degradation of 8 mV kh-1 cell-1, whereas the stack operated at 8 bar showed an approximately four times higher degradation. The third stack was operated in co‐electrolysis mode at 1.4 and 8 bar and showed noticeably higher degradation rates than during steam electrolysis mode. The predominant increase of the ohmic resistance during operation was identified to be mainly responsible for the observed degradation of all three stacks, whereas the increase of the polarization resistances played a subordinate role. Within the post‐test analysis, noticeably high nickel depletion was observed for the stack operated at the highest pressure in steam electrolysis mode. Furthermore, partial delamination of electrodes was observed. The degradation is discussed with relation to phenomena and experimental parameters during operation.