Strategy to Enhance Air Electrode Performance and Durability in Soe and Co-Soe Modes: Composite Electrode with a Gradually Changing Composition
Authors: Niemczyk Anna , Jagielski Stanisław , Machaj Krystian , Kluczowski Ryszard , Bąkała Maciej , Kupecki Jakub , Kosiorek Magdalena 
Published: August 2024

Among the key challenges of the Solid Oxide Cell (SOC) is stable, long-term operation in electrolysis (SOE, Solid Oxide Electrolysis), and co-electrolysis (co-SOE) modes. Many factors impact the SOC’s lifespan, i.e., the chemical composition of the fuel and air electrode, the type of the cell, and working conditions (e.g., current density, temperature). Therefore, the development of high-performance and durable cells seems to be crucial, as SOE/co-SOE systems may play an important role in the ongoing transformation toward a climate-neutral economy (low-emission hydrogen production; efficient utilization of CO2and renewable energy) [1,2].

One of the crucial degradation mechanisms of SOC in electrolysis/co-electrolysis mode is the delamination of the air electrode, caused by the accumulation of high oxygen partial pressure on the interphase of the electrolyte, the formation of additional phases, and/or reaction of voids/cracks in the electrolyte or buffer layer [2,3]. A strategy to mitigate the risk of air electrode delamination is to use composite electrode based on the buffer layer material (GDC- Gd0,1Ce0,9O2) and state-of-the-art electrode – LSC (La0,6Sr0,4CoO3-δ), as it will lower the effect of the thermal expansion mismatch between the electrode and buffer layer/electrolyte. Moreover, by manipulating the microstructure of the electrode, e.g., by introducing a pore-forming agent, it is possible to ensure fast and effective collection of oxygen generated during operation in the SOE/co-SOE mode on the air side (preventing potential differences leading to the stresses on the electrode-electrolyte interface).

In this study, the electrochemical performance of solid oxide cells with a modified, gradually changing composite LSC-GDC air electrode was verified in electrolysis and co-electrolysis modes. The following air electrode compositions were proposed for cell preparation: type 1: 1st layer 50LSC:50CGO, 2nd layer: 75LSC: 25GDC, 3rd layer: LSC, type 2: 1st layer 75LSC:25CGO, 2nd and 3rd layers: LSC with PMMA as a pore former; type 3: 1st layer 75LSC:25CGO, 2nd and 3rd layers: 90LSC:10GDC, and type 4: 1st layer 85LSC:15CGO, 2nd layer: 90LSC:10GDC; 3rd layer: LSC with PMMA as a pore former. The proposed air electrode was tested at the single-cell level (cells size of 50 mm x 50 mm), and the most promising solutions were implemented in the SOC stack (a stack consisting of 3-5 cells with a size of 110 mm x 110 mm). The electrochemical analysis of the developed cells includes measurements of the j-V dependences and EIS spectra under different modes (SOE and co-SOE), various temperatures, current densities, and different gas flows delivered at the air side of the cell. The microstructure, morphology, and composition of the prepared cells and samples after testing were analyzed using SEM, SEM-EDS, and, for selected samples, the FIB-SEM method. The obtained results showed that by a gradually changing the electrode composition, specifically by reducing the GDC content in the electrode, it is possible to obtain a stable electrode (a strategy to prevent air electrode delamination) with high catalytic activity in both electrolysis and co-electrolysis modes. The composite GDC-LSC electrodes with various compositions seem to be promising air electrodes for SOE and co-SOE systems. The study achieves relatively high current densities compared to the SoA cells, with only negligible degradation changes observed after long-term testing (>300 h in single cell configuration).

Acknowledgments

The presented research was financially supported by: the National Centre for Research and Development, Poland, within project no. LIDER/1/0003/L-12/20/NCBR/2021, which focuses on research related to composite electrodes. Anna Niemczyk acknowledges support from the Foundation for Polish Science (FNP) through a scholarship from the START program.

References

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