Performance Investigation and Optimization of a SOEC Stack Operated Under Industrially Relevant Conditions

Abstract

Solid oxide electrolysis cells (SOEC) are a highly efficient technology for producing green hydrogen using only steam and electricity from renewable sources. However, a variety of different applications and associated operating conditions can cause performance deterioration and microstructural degradation of SOECs. Hereby, various factors such as inlet gas composition, operating temperature or supplied current profile can have a significant different impact on the performance and long-term stability of SOECs. In order to investigate the influence of different industrially relevant operating conditions on the performance of SOECs, a series of experiments was performed on a ten-layer electrolyte-supported stack with an active area of 127 cm2/cell. With the aim of creating a test and results matrix that includes all relevant operating parameters, their influence on performance and optimization potential. As the first dimension of the test matrix, the performance of the stack was investigated at different steam/hydrogen-ratios of between 1 – 9 of the inlet gas composition, thus focusing on high-efficient electrolysis operation at low hydrogen partial pressures at gas inlet. Another dimension was added to the test matrix by varying the operating temperature of the stack between 785 – 835 °C since operation at lower temperatures can hold potential advantages because the total energy demand for maintaining the operating temperature can be significantly reduced. Furthermore, different combinations of anodic and cathodic volume flow were fed to the stack. Hereby, the steam flow was lowered in order to achieve higher steam conversion rates at lower power input. Additionally, the performance of the stack was investigated at varying air volume flows. During the experiments polarization curves and electrochemical impedance spectroscopy (EIS) in combination with analysis via distribution of relaxation times (DRT) were used for characterizing the performance of the single cells within the stack at current densities up to 0.8 A/cm2. The monitoring of the temperature distribution within the stack was realized through thermocouples integrated at the in- and outlet of the gas flow channels as well as through three additional thermocouples mounted within the stack. This study presents correlations between the performance of a SOEC stack and specific industrially relevant operating conditions and proposes optimal operating conditions for different applications with a specific focus on high efficiency as well as low degradation potential. Figure 1