Optimization of ESC Performance for Co-Electrolysis Operation

Abstract

Production of synthetic hydrocarbon fuels from co-electrolysis of steam/CO2 mixtures using renewable energy has been already presented as a promising solution to diminish problems from energy and environmental point of view. The possibility to recycle CO2 into chemicals and value-added fuels helps to reduce the accumulation of atmospheric CO2 and realize the carbon neutral cycling of fuels. The solid oxide cells have great potential within this field and can been used for converting of steam/CO2 mixtures to the syngas for following Fischer-Tropsch conversion by excessive electricity at high temperatures above 750°C. It presents an opportunity for recycling of CO2 into a useful liquid fuel. While the co-electrolysis is more complicated than steam electrolysis, the electrochemical performance electrolyte supported cell with G3 electrodes were investigated over a wide range of operating conditions (SOEC as well as SOFC), varying the ratio of H2O/H2 and CO2/CO, the operating temperature (750–850°C) and current. Comparable activity for operation in the SOEC and SOFC modes was observed and electrodes exhibited similar polarization losses during operation in steam electrolysis and steam/CO2 co-electrolysis. During electrolysis and co-electrolysis operation anomalous increase of polarization resistance during first 50 h on both sides has been observed. The raise of polarization resistance was reversible and diminished after switching to SOFC operation mode. Operating temperature variation during co-electrolysis helped to get insight into relationship between observed performance and electrodes microstructure. Additional experiments and electrochemical testing combined with detailed microstructural analysis have been carried out to better understand the observed phenomena. Obtained performance for single cell has been reproduced also on stack level. To improve electrochemical performance the additional modification of both electrodes, especially of their microstructure, is done. Durability of solid oxide electrolyte supported electrolysis cells (standard G3 and modified) tested at different current densities (from -300 mA/cm2 up to -500 mA/cm2) for high temperature steam or steam/CO2 electrolysis is investigated under stack relevant operating conditions (up to 5,000 h) and showed power degradation rate of ca. 1%/1,000 h for steam electrolysis. The changes in polarization resistance of single cells under different operating conditions as well as during durability tests are performed and discussed on basis of analysis of impedance spectra. Microstructure observations at the interfaces in both electrodes were carried out after long-term tests to understand the reasons for degradation. The cell with optimized fuel and air electrodes was successfully tested for ~2400 hours at 803°C under co-electrolysis conditions (H2/CO2/H2O/CO mixture as fuel, j = -400 mA/cm2). The calculated linear power degradation rate over all durability test (~2400 h) is about 0.4%/1,000 h. Further development of ESC for co-electrolysis and its verification in stack is topic of ongoing activities. Fig. 1 Power density vs. time for long-term test with modified G3 cell based on 165µm 10Sc1CeSZ electrolyte at 803°C in air : (H2/CO2/H2O/CO) mixture (j = - 400 mA/cm2). As reference point the power density in SOFC mode (air : H2/H2O) mixture (j = 300 mA/cm2) is also represented. Figure 1