High-Pressure Solid Oxide Electrolysis Cell (SOEC) for Enhanced P2X Efficiency
Authors: Li Yaqi , Cui Xiaoti , Jensen Søren Højgaard , Blaabjerg Frede 
Published: December 2024

As green energy emerges as a pivotal solution for replacing traditional fossil fuels, efficient power storage methods become crucial. Wind and solar energy sources have the potential to meet global energy demands multiple times over. However, their intermittent nature necessitates a fundamental reassessment and redesign of our energy infrastructure. Power-to-X (P2X) technologies offer a promising avenue for converting renewable electricity into other usable forms. Hydrogen, produced via electrolysis and stored, is one such example. High-temperature solid oxide cells (SOC), which can function as both electrolysis cells (SOEC) and fuel cells (SOFC), represent a highly efficient technology for this purpose. By 2030, SOEC are anticipated to play a pivotal role in electrolysis, driven by their high electrical efficiency, reduced material costs, and dual functionality as fuel cells. However, SOEC face challenges such as limited stack lifetime and restricted cell size. A novel operational approach for SOEC, termed 'AC:DC', has emerged as a promising solution to overcome these challenges. The AC:DC method involves superimposing an alternating current (AC) on a direct current (DC), facilitating intermittent operation in fuel cell mode. Tests conducted on SOECs have demonstrated that AC:DC can prolong lifespan by mitigating temperature fluctuations and enhancing impurity tolerance through electrochemical oxidation/reduction processes. Increasing the operational pressure of SOEC from ambient pressure to 5 bar can result in a 50% reduction in capital expenses (CAPEX) for balance-of-plant equipment such as heat exchangers, pipes, and compressors. Moreover, operating at high pressure enables greater hydrogen production capacity for SOEC. In summary, our objective is to develop a setup for conducting SOEC cell testing under elevated pressure (up to 5 bar) using AC:DC operation. This includes presenting the preliminary design of the setup and conducting experimental investigations. Additionally, we discuss the dynamic SOEC operations during testing from a system perspective.

To conduct dynamic operational tests under elevated pressure, a pressure vessel must be designed and integrated into the system. The gas inlet is consisted of air, H2O, H2 and N2. The flow rate of H2 can be adjusted from 0-50 L/h, while air and N2 can be adjusted from 0-300 L/h to clean and provide a safety atmosphere inside the pressure vessel. The normal H2O flow rate is set to 20 μL/min. Heat up process for the furnace is set to 1.5 °C/min, from room temperature (25 °C) to 700 °C, which is the working temperature for SEOC, due to rapid heat ramp will cause uneven distribution on the SOEC, resulting in the waste of energy as well as the degradation of SOEC. The furnace structure, consists of layers starting with an Inconel housing bearing a nickel catalyst welded on top. H2O is supplied to the anode through a central hole, while unreacted gas combusts out the housing sides. The catalyst undergoes grinding between cell exchanges to ensure optimal contact and reaction.

The SOEC cell used for the test is a commercial cell based on the basic materials of nickel/YSZ for cathode, YSZ for electrolyte, LSC for air electrode. The measurements are under safety gas atmosphere. The fuel electrode needs to be heated up under N2 atmosphere to prevent it from oxidation. The current collector, a gold mesh, is positioned over the exposed air electrode. Alumina silica felt serves as the insulation material. Thus far, the development of a vessel for elevated pressure testing of SOEC has been successfully completed. To assess the feasibility of the setup, 5% H2 was applied to a cell used for validating and operated it as a SOFC. Open-circuit-voltage (OCV) measurements were conducted at various temperatures to assess the feasibility of the setup within the pressure vessel. At 700 °C, the OCV reached 1.12 V, which corresponds to the standard value. After validation of the system, the water pump and power were connected and ran the cell as SOEC mode. The IV curve of the SOEC is obtained under 700 °C, as shown in Fig.1.

In conclusion, a high-pressure vessel for SOEC is successfully designed, aimed at enhancing SOEC efficiency and extending its lifetime. Electrochemical impedance spectroscopy will be performed in the future work, combining with an optimization of different frequency of AC: DC profile, and validating by long-term test and post-mortem analysis. Therefore, the degradation mechanisms, such as nickel migration and poisoning from impurities will be fully discussed. Moreover, conducting tests on large stacks with thermocouples inserted throughout the stack volume could help elucidate the limitations of electrothermally balanced operation concerning stack size.

Figure 1