Nanoengineering of cathode layers for solid oxide fuel cells to achieve superior power densities

Solid oxide fuel cells (SOFCs) offer high efficiency in converting chemical energy to electricity and can utilize various fuels, making them attractive alternatives to fossil fuels. However, widespread commercialization is hindered by high costs. Improving power density is a key strategy to reduce system size and cost. Advanced thin-film techniques, such as pulsed laser deposition (PLD), offer a route to nanoengineered cathode materials with superior performance compared to conventional screen-printed cathodes. These techniques allow for tailoring material properties at the nanoscale to optimize low area-specific resistance (ASR) and high oxygen exchange properties. While previous studies have shown the potential of nanoscale cathodes using techniques like sol-gel, long-term stability remains a crucial challenge. Combining conventional cathode materials (like LSC or LSCF) with good ionic conductors (like rare-earth-doped ceria) in nanocomposite structures can enhance performance by increasing interfacial area density and facilitating oxide ion transfer. However, challenges remain in maintaining nanostructures at high operating temperatures due to grain coarsening and surface segregation, and optimizing current collectors for good electrical contact with nanometer-sized grains. This study aims to address these challenges by developing advanced cathode materials with a novel cell architecture incorporating nanoengineered cathode layers, systematically examining the roles of each layer on electrochemical performance, and ultimately achieving superior power densities.

Significant research focuses on improving SOFC durability and performance. Studies have explored advanced thin-film techniques like PLD to create nanoengineered cathode materials with superior oxygen exchange and low ASR compared to conventional screen-printing methods. The use of nanoscale LSC cathodes has also been reported with record-low ASR values. However, most studies focus on initial performance rather than long-term stability, which is crucial for practical applications. Combining cathode materials like LSC or LSCF with ionic conductors like rare-earth-doped ceria in nanocomposites is a promising approach to enhancing performance, but only limited nanocomposites like LSC-GDC and SSC-SDC have been successfully grown via PLD. Challenges include maintaining nanostructures at high temperatures and addressing the poor lateral conductivity of thin-film cathodes, which require careful consideration of current collectors for optimal electrical contact.

This study employs pulsed laser deposition (PLD) to fabricate nanoengineered cathode layers. A self-assembled LSCF-GDC nanocomposite thin film (~300 nm) is first deposited on a GDC substrate. The PLD parameters are carefully chosen to achieve a nanoscale-level distribution of LSCF and GDC phases, resulting in a highly ordered nanostructure with alternating nanostripes of each phase (2-5 nm width). This structure significantly increases the cathode/electrolyte interfacial area density. The nanocomposite’s crystalline phases are confirmed using STEM-HAADF, STEM-EDX, and SAED. A nanoporous LSC thin film (~1 µm) is then deposited on top of the nanocomposite layer under conditions optimized for porosity. The porous structure facilitates gas permeability for the oxygen reduction reaction (ORR). The microstructure of the LSC thin film before and after high-temperature annealing is characterized using SEM to evaluate its stability and porosity. Symmetrical cells with various cathode configurations are fabricated on GDC electrolytes, using unsintered LSC paste as the current collector, and electrochemical impedance spectroscopy (EIS) is employed to determine ASR values. This allows for evaluating the contribution of each cathode layer (LSCF-GDC nanocomposite and nanoporous LSC) to overall performance. Anode-supported cells are then fabricated using conventional methods (extrusion and screen printing for the anode support, AFL, and YSZ electrolyte), with the nanoengineered cathode layers deposited via PLD. A dense GDC interlayer is also incorporated to prevent chemical interdiffusion and ensure a suitable surface for subsequent depositions. The microstructure of the complete anode-supported cell is characterized using SEM. I-V and I-P curves are measured for different anode-supported cell configurations at various temperatures, using unsintered LSC paste and Pt paste as current collectors. EIS is used to extract ohmic and polarization resistance values. Long-term stability is assessed by measuring voltage at a constant current over an extended period. The impact of the current collector material on performance is also investigated, showing a significant improvement with Pt paste at higher temperatures.

The study successfully demonstrated the fabrication of a highly ordered, self-assembled LSCF-GDC nanocomposite with alternating nanostripes of LSCF and GDC phases, significantly increasing the interfacial area density. The nanoporous LSC layer maintained its porosity even after high-temperature annealing, facilitating gas permeability. Symmetrical cell measurements revealed that the combination of the LSCF-GDC nanocomposite and nanoporous LSC layers resulted in the lowest ASR values, superior to cells with only one of the layers or conventional cathodes reported in the literature. In anode-supported cells, the nanoengineered cathode architecture achieved high current densities of 2.2 and 4.7 A/cm² at 0.7 V and 650 °C and 700 °C, respectively, and power densities of 1.5 and 3.3 W/cm². This represents a significant improvement compared to literature values for anode-supported and metal-supported cells using LSCF-GDC or SSC-SDC composite cathodes. The use of Pt paste as a current collector resulted in improved high-temperature performance compared to LSC paste, highlighting the importance of current collector selection for thin-film cathodes. Long-term stability tests showed an initial voltage drop, followed by relative stability at 0.8 V over 250 h at 700 °C, indicating room for improvement in long-term performance.

The results demonstrate the significant enhancement in SOFC performance achievable through nanoengineering of cathode layers. The self-assembled LSCF-GDC nanocomposite effectively functions as a transition layer, increasing the interfacial area density and facilitating oxygen ion transfer. The nanoporous LSC top layer provides high surface area for the ORR, enhancing the oxygen exchange kinetics. The combination of these two layers leads to exceptionally low ASR values and high power densities. The successful integration of the nanoengineered cathode into conventional anode-supported cells further emphasizes the practical implications of this approach. The choice of current collector material significantly influences performance evaluation, particularly at high temperatures, indicating that the selection of appropriate current collectors is vital for evaluating thin film cathodes accurately. While the observed long-term stability shows a need for further optimization, the achieved performance improvements are substantial and suggest the potential for developing high-performance and compact SOFC devices.

This research successfully demonstrated the development of high-performance nanoengineered cathodes for SOFCs through the combination of a self-assembled LSCF-GDC nanocomposite and a nanoporous LSC layer. The nanoengineered architecture resulted in superior electrochemical performance with record-low ASR values and high current and power densities. The findings highlight the significance of nanoengineering in enhancing SOFC performance and pave the way for developing next-generation SOFCs with improved efficiency and reduced cost. Future work will focus on optimizing the cell architecture, further improving long-term stability, and exploring other materials and fabrication methods.

The study acknowledges the initial voltage drop observed in long-term stability tests, indicating a need for optimization of long-term durability. The use of unsintered LSC paste as a current collector may have limited the accurate assessment of performance at high temperatures, highlighting the importance of current collector selection. Further research is needed to fully understand the long-term degradation mechanisms and to develop strategies for mitigating the initial performance drop.