In 2018, a 5/15 kWDC reversible solid oxide cell system was developed and successfully operated by Forschungszentrum Jülich. Based on the knowledge gained with this first system, an optimized system in the power class of 10/40 kWAC was developed afterwards in 2019 that uses the well-established Integrated Module. This represents the main components of the system. The basic system layout was retained in general from the previous system and adjusted in accordance with the higher power level. During the experimental evaluation in fuel cell mode, the system could provide an electrical output power from 1.7 to 13 kWAC. The maximum system efficiency of 63.3 % based on the lower heating value (LHV) could be reached at a system power of 10.4 kWAC. In electrolysis mode, a maximum efficiency of 71.1 % (LHV) was achieved with an electrical power input of - 49.6 kWAC. At this operating point, about 11.7 Nm³ h-1 of hydrogen are generated. The stack temperature distribution and the efficiency losses are also analyzed for the electrolysis mode. Finally, the potential for the efficiency optimization through higher heat integration in this mode is experimentally evaluated and discussed.

Solid oxide cells are capable of efficiently converting various chemical energy carriers to electricity and vice versa. The urgent challenge nowadays is the faster degradation rate compared with other fuel cell/electrolyzer technologies. To understand the degradation mechanisms, simulation of a solid oxide cell is helpful. Since most previous research developed models using commercial software, such as COMSOL and ANSYS Fluent, a gap for knowledge transfer is being gradually formed between academia and industry due to licensing issues. This paper introduces a multiphysics model, developed by a computational code, openFuelCell2. The code is implemented with an open-source library, OpenFOAM. It accounts for momentum transfer, mass transfer, electrochemical reactions and metal interconnect oxidation. The model can precisely predict I–V curves under different temperatures, fuel humidity and operation modes. Comparison between OpenFOAM and COMSOL simulations shows good agreement. The metal interconnect oxidation is modeled, which can predict the thickness of the oxide scale under different protective coatings. Simulations are conducted by assuming an ultra-thin film resistance on the rib surface. It is revealed that coatings fabricated by atmospheric plasma spraying can efficiently prevent metal interconnect oxidation, with a contribution of only 0.53 % to the total degradation rate.

In the scope of clean energy, the solid oxide cell (SOC) offers a promising solution for different applications. The electrolyte, the air electrode and the fuel electrode of an SOC are typically based on yttrium stabilized zirconia (YSZ), lanthanum strontium cobaltite (LSC) or manganite (LSM) and nickel cermet, respectively. It typically operates in a temperature range of 600 – 1000 oC and can be operated as a fuel cell (SOFC), an electrolysis cell (SOEC) or reversibly (rSOC). An SOC-stacks consists of several repeating units, each of which is assembled by various components. Among them, the sealants are important to maintain the reliability and long-term operations of a stack. Several issues lead to high stress in the sealants, including the mechanical load, mismatch of thermal expansion coefficients, and thermal gradients 1. It was indicated that a small temperature gradient near the manifolds results in excessively high tensile stress. This may lead to glass ceramics sealing failure, which is critical in SOC designs 2. Therefore, the authors carry out the elastic stress simulations in an operating SOC-stack due to non-uniform temperature distributions. A previously introduced CFD model 3 is further developed to address coupled chemical and electrochemical reactions in rSOC-stacks. This model has been implemented in the open-source library, OpenFOAM. It considers the major multiphysical phenomena e.g. heat, mass, momentum and charge transport as well as heat radiation and electrochemical reactions and employs a volume-averaged approach 4. The computational requirements can be greatly reduced, which offers the possibility to simulate large stacks in reduced computing time. The temperature distributions derived from the stack simulation are used as inputs for thermal stress analysis, by mapping the interconnect temperatures to the detailed repeating unit. An open-source, finite element method (FEM) based software, Calculix, is applied to simulate the stress distributions. It is assumed all layers are tightly connected without slipping. The SOC-stack design considered in the multiphysical simulation as well as the FEM stress analysis is an in-house design, Mark-F20. It consists of 18 repeating units, each with an active area of approximately 320 cm2. The stack operates in the furnace condition, at a temperature of 700 oC, with CH4 mixture being supplied to the anode in the SOFC-mode (i=0.5 A/cm², U=0.8 V). The flow is in a counter-flow regime. A comparison of temperature distributions in interconnect is conducted between simulation and experiment. A maximum deviation of 10 K can be observed near the fuel inlet. It is acceptable providing the overall temperature variations of 80 K and the uncertainties in the measurements. This low-temperature zone results from the fast endothermic methane steam reforming reaction. The zone near the fuel outlet is cooled down by the inlet airflow on the cathode side. The stress-free temperature is 800 oC. It can be found that the maximum stress appears around the fuel inlet manifold with a magnitude of 100 MPa. The stress is also large near the air inlet manifold with a magnitude of 70 MPa. These areas show higher possibilities of mechanical failure. Consequentially, the stack model that has been developed enables to investigate the heat distribution in SOC-stacks. By comparing temperature distribution and the IV-curve predicted by the model with experimental measurements, good agreements were found. In the future, the sealant failure will be investigated 2 and attention has to be paid to future stack designs. References (1) Blum, L.; Groß, S. M.; Malzbender, J.; Pabst, U.; Peksen, M.; Peters, R.; Vinke, I. C. Investigation of Solid Oxide Fuel Cell Sealing Behavior under Stack Relevant Conditions at Forschungszentrum Jülich. Journal of Power Sources 2011, 196 (17), 7175–7181. (2) Bremm, S.; Dölling, S.; Becker, W.; Blum, L.; Peters, Ro.; Malzbender, J.; Stolten, D. A Methodological Contribution to Failure Prediction of Glass Ceramics Sealings in High-Temperature Solid Oxide Fuel Cell Stacks. Journal of Power Sources 2021, 507, 230301. (3) Zhang, S.; Peters, R.; Varghese, B. A.; Deja, R.; Kruse, N.; Beale, S. B.; Blum, L.; Peters, R. Modeling of Reversible Solid Oxide Cell Stacks with an Open-Source Library. ECS Trans. 2021, 103 (1), 569. (4) Beale, S. B.; Zhubrin, S. V. A Distributed Resistance Analogy for Solid Oxide Fuel Cells. Numerical Heat Transfer, Part B: Fundamentals 2005, 47, 573–591. Figure 1

This study presents a novel model for investigating the microstructural evolution of nickel (Ni), yttria-stabilized zirconia (YSZ), and gas phases in a solid oxide cell (SOC), and its effects on cell performance. The triple-phase-boundary (TPB), which is the interface between the three phases, plays a crucial role in the electrochemical reaction of the SOC. However, during operation, nickel particles coarsen or migrate, leading to the redistribution of the TPB. To study this phenomenon, a phase field method was utilized to simulate the fuel electrode's detailed structure, and an approach was developed to track the TPB lines (TPBl) and voxels (TPBv). The study then employed the open-source computational fluid dynamics library, OpenFOAM, to simulate the half-cell performance. The results provide a detailed understanding of the dynamics of the TPB and its impact on multiphysical transport phenomena.

Forschungszentrum Jülich has been operating an rSOC system in the 10/40 kWAC power class since 2021. This system uses four 20-layer sub-stacks in the mark H20 stack design. During the test campaign, a power range from 1.7 to 13 kWAC could be shown in fuel cell mode. The highest efficiency in fuel cell mode of 63.3 % was achieved at a power output of 10.4 kWAC, related to the lower heating value (LHV) of hydrogen. With a power input of -49.6 kWAC, the highest efficiency of 71.1% (LHV) was achieved in electrolysis mode. At this point, 11.7 Nm³ h-1 of hydrogen were produced. The following manuscript shows the layout and the experimental results of the rSOC demonstration system.