Three-Dimensionally Architected Solid Oxide Fuel Cells for Low-Temperature Operation

Abstract

Recently, while eco-friendly energy storage/conversion technologies have been attracting attention, researches have been reported to improve performance and provide multiple functions by applying three-dimensional nano-/micro-structures to energy devices. In general energy devices, the electrolyte and electrodes located on both sides have been designed to have a flat interface, but in the end, developed structures in consideration of three-dimensional efficiency should be required. Among them, in solid oxide fuel cells (SOFCs), which have advantages such as high efficiency and fuel flexibility, studies on the use of advanced cell structures were conducted to lower the operating temperature. Although micro-SOFC using etching or laser processing has been reported, there are limitations in practical operation due to poor material/structural stability and low pattern resolution. In this paper, we developed a novel ceramic micropatterning process by imprinting a pyramid-shaped polymer mold made by soft lithography on the anode support. Furthermore, we have implemented a three-dimensional SOFC with record-high cell performance through the combination of thin film process and large-area cell manufacturing technology. For microscale patterning of the anode, polymer-ceramic anode films with low elastic modulus was prepared by the tape casting process, and the anode support with pyramidal patterns was fabricated through the subsequent imprinting process. The imprinted pattern was maintained after the sintering process. Then, through RF sputtering and pulsed laser deposition (PLD) process, the nanoscale anode functional layer (nAFL), Y2O3-stabilized ZrO2 (YSZ), Gd0.1Ce0.9O1.95 (CGO) and La0.6Sr0.4CoO3- δ (LSC) layer were deposited to fabricate a full cell. To analyze the electrochemical effect of three-dimensional electrode/electrolyte interface, 3DA-cell with ceramic micropatterning process and Planar-cell with flat interface were compared. Interestingly, 3DA-cell showed higher performance than Planar-cell at all operating temperatures, and resulted in 55 % enhanced maximum power density (PDmax) and 19 %, 52 % decreased polarization, ohmic resistance at 500 °C, respectively. Notably, the CGO-based 3DA-cell using CGO as the main electrolyte material recorded a high PDmax of 0.931 W/cm2 at 500 °C. Therefore, to find the factors that improve the electrochemical performance, the microstructures of two different cells were analyzed by scanning electron microscope (SEM), transmission electron microscopy (TEM) and 3D reconstruction method. From the 3D reconstruction result, the nAFL/YSZ interface area, CGO/LSC interface area, and active TPB length of AFL of 3DA-cell increased by about 33 %. 23 %, and 64 % compared to Planar-cell, whereas pore volume ratio of AFL did not change. Therefore, it was confirmed that the increase of the electrode/electrolyte interface area by ceramic micropatterning had a direct effect on the improvement of cell performance, and the internal structure of AFL was not affected by the imprinting process. Furthermore, to demonstrate the versatility of ceramic micropatterning process, a 5 cm x 5 cm large-area three-dimensional thin film SOFC was fabricated and its performance was evaluated. As a result, we set a new milestone in SOFC technology by reporting a power of ~ 13 W at 500 °C, and confirmed the consistency of our process through a cell reproducibility test. Additionally, the stability of the three-dimensional structure was verified through the operation without notable degradation for 500 hours under the condition of 5 A and 8 A current load. This work exhibits an outstanding breakthrough to accomplish high-performance, high-stable large-area SOFCs, and can accelerate commercialization to the portable and transportation devices.