Grain Controlled Interlayer of Solid Oxide Fuel Cells Via Nitrogen Reactive Sputtering
Authors: Jeong Davin , Kim Hyeontaek , Park Yongchan , Hong Soonwook 
Published: December 2024

Renewable energy conversion systems have been extensively studied and utilized due to increased environmental pollutions caused by fossil fuels. Among the various renewable energy sources, such as wind, solar, geothermal, and hydrogen, hydrogen is well known for its high energy density, non-toxicity, and easy transportation. Therefore, hydrogen production and utilization processes have been studied to enhance their efficiencies and lower costs for decades. To convert hydrogen into electricity, various types of fuel cells are frequently employed by chemical reactions between hydrogen and oxygen. Solid oxide fuel cells (SOFCs), one of the most promising energy conversion devices among the fuel cells, guarantee high conversion efficiency which is temperature dependent, thus the SOFCs are usually operate in high temperature regime (800 °C–1000 °C) to maximize their output. These high operating temperatures, however, induce thermal issues such as catalyst degradation, sealing, and cell durability which are needed to be concerned. Therefore, many researchers tried to mitigate these issues by lowering the working temperature (400 °C–600 °C). This strategy was successful, however, as abovementioned, the SOFCs are temperature dependent so that relatively low temperatures delay oxygen reduction reaction (ORR) resulted in increased polarization losses. To facilitate the SOFCs with high efficiency at this temperature regime, many studies inserted oxygen ion conducting materials between electrodes and electrolyte to promote transfer of oxygen ions. Further, researchers tried to control surface morphology of ion conductors to have large grain boundary density which is another approach to reduce polarization losses. In this study, yttria-stabilized zirconia (YSZ) was introduced between cathode and electrolyte via sputtering technique. During the sputtering processes with both argon and oxygen gases, nitrogen gas was further supplied to induce collision of YSZ atoms so that YSZ atoms with low kinetic energy will reach on substrates resulted in smaller grain sizes of YSZ interlayers.
Silicon and YSZ substrates were utilized to analyze properties of YSZ interlayers and electrochemical characteristics of SOFCs, respectively. The YSZ interlayers were fabricated by sputtering of ZrY alloy target with argon, oxygen, and/ or nitrogen gases. While maintaining sputtering chamber pressure of 0.67 Pa, argon and oxygen gases were supplied with fixed amount of 40 sccm and 5 sccm, respectively. Furthermore, nitrogen gas was also introduced to sputtering chamber by 4 sccm differences from 0 to 16 sccm, hereafter, fabricated YSZ interlayers are denoted as N0, N4, N8, N12, and N16 with the meaning of applied nitrogen amount. All YSZ interlayers were deposited with RF power of 70 W and sputtering times were controlled to fabricate about 70 nm. To analyze the properties of fabricated YSZ interlayers, scanning electron microscopy (SEM), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS) were conducted. Electrochemical characteristics of SOFCs were evaluated by linear sweep voltammetry and electrochemical impedance spectroscopy.
SEM images observed all the YSZ interlayers were about 70 nm thicknesses with various surface morphologies. This phenomenon is attributed to different adatom mobility of atoms due to flow rates of nitrogen during sputtering process. Further, XRD results shown that nitrogen influenced grain development of YSZ interlayers resulted in weaker and broaden peaks compared to N0. To calculate grain sizes of the YSZ interlayers, Williamson and Hall equation was utilized. Average grain sizes of YSZ interlayers were decreased from N0 to N12, and increased at nitrogen flow rate of 16 sccm. The electrochemical performances indicated that the smallest grain sizes of YSZ interlayers had the lowest polarization losses which means the highest ORR kinetics were found at the largest grain boundary density thus, the highest peak power density was observed.
All the YSZ interlayers were fabricated by sputtering technique with constant flow rates of argon, oxygen, while nitrogen flow rate was varied. The SEM images and XRD patterns revealed that nitrogen successfully modified surface morphologies in terms of various grain sizes. The EIS analysis confirmed the smallest grain sizes of N12 had the lowest polarization losses resulted in the highest peak power density. As a result, we successfully modified surface morphologies of YSZ interlayers by nitrogen reactive sputtering. We expect that this one-step novel strategy can be utilized in other thin film fabrication fields by significantly reducing process times.
Acknowledgements
This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (No.2022R1C1C1006824) and the Basic Research Program through the National Research Foundation of Korea (NRF) funded by the MSIT (No.2022R1A4A3023960).