Development of a La0.2Sr0.25Ca0.45Ti0.95Ni0.05O3 Perovskite Fuel Electrode for Electrolyte Supported High Temperature Steam Electrolysis

Kurzfassung

Limiting climate change requires to decarbonize all kinds of industry sectors. This includes processes that cannot be electrified, but require CO2-free alternatives to fossil energy carriers, such as hydrogen. While producing green hydrogen is an energy-intensive procedure, solid oxide electrolysis cells (SOECs) offer high electrical efficiency due to favorable kinetics and thermodynamics as compared to low temperature electrolyzers if high temperature heat is available. The performance of state-of-the-art (SotA) nickel-cermet electrodes for the hydrogen evolution reaction (HER) is limited by their reaction sites at the triple-phase-boundaries between the electron-conducting nickel, the ion-conducting ceramic and the gas phase. The electrode degradation is driven by the metallic phase agglomerating under reducing conditions. From an economic point of view, nickel is a strategic raw material due to its limited accessibility and its rapid demand growth. It has highly desirable catalytic properties and a wide range of applications, e.g. in rapidly growing battery technologies. In addition, the high toxicity of its oxides makes it challenging to handle industrial quantities, especially in high temperature applications. The approach is therefore to make use of a perovskites that features mixed-ionic-electronic-conductivity (MIEC). These materials exhibit reactive sites for the HER all over their surface. Another advantage of MIEC ceramics is that there is no need for a percolating metallic phase that is prone to agglomeration. However, the exsolution of Nickel nanoparticles (NPs) from the matrix in hydrogen containing atmosphere (Fig. 1) still allows for the catalytic activity of the transition metal, but with a significantly reduced nickel content in the electrode. This contribution gives insights into the currently ongoing development of an all ceramic fuel electrode (FE) for electrolyte supported high temperature SOECs and its integration into a stack. Several perovskite materials were screen-printed onto 90 μm thick electrolytes of foil casted 3% yttria stabilized zirconia. As an oxygen electrode La0.58Sr0.4Fe0.8Co0.2O3-δ was used. These 5x5 cm² full cells with an active area of 1x1 cm² were electrochemically characterized in 500 h galvanostatic tests in electrolysis mode accompanied by electrochemical impedance spectroscopy (EIS). At 860 °C and a FE-atmosphere of 50 % H2O – 50 % H2, the (La,Sr,Ca)(Ti,Ni)O3 titanate was identified as the most promising material class. In several tests these FEs have shown degradation rates as low as 17.8 mV/kh and performances up to -880 mA/cm² at 1.3 V (Fig. 2). This is comparable with industrial cells containing SotA nickel-cermet electrodes that are tested simultaneously in the same operation conditions. Current challenges involve the investigation of how the material’s electrochemical mechanisms contribute to the full cell’s impedance and compare to the ones in the SotA-cells. Its oxygen capacitance seems to play a crucial role in that. Different approaches of changing the powder preparation, the sinter protocol, the cell conditioning or the operation conditions are considered to understand and control its impact. Therefore, the distribution of relaxation times calculated from EIS-data and post-mortem scanning electron microscopy accompanied by energy-dispersive x-ray spectroscopy are utilized. This understanding is especially relevant for the deployment of the electrode into a stack. To do so, the contacting materials (Pt) need to be replaced with a noble metal free current collector. Application of a nickel-based contact element was investigated and its impact in terms of performance and durability are assessed. The results are presented and critically discussed with the perspective of stack integration.