ABSTRACT
Pinhole-free ionic conductors are critical to achieve optimal performance in thin film-solid oxide fuel cells (TF-SOFCs). However, nanoscale defects, especially pinholes, can induce current leakage and contribute to cell failure by creating electrical short circuits. This study introduced a novel methodology for detecting pinholes in yttria-stabilized zirconia (YSZ) thin-film solid oxide electrolytes. The approach utilized selective adsorption of silver (Ag) nanoparticles generated via a spark discharge generator (SDG). Analytical techniques, including focused ion beam (FIB), scanning electron microscopy (SEM), and transmission electron microscopy (TEM), were employed to investigate interactions between Ag nanoparticles and nanoscale defects. Results showed that nanoparticle-based diagnostic methods were efficacious for defect characterization, offering a solution for enhancing the quality of thin-film electrolytes.
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KEYWORDS: Yttria-stabilized zirconia, Solid oxide electrolyte, Thin film solid oxide fuel cell, Pinhole, Spark discharge, Ag nanoparticle
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KEYWORDS: 이트리아 안정화 지르코니아, 고체 산화물 전해질, 박막 고체산화물 연료전지, 핀홀, 스파크 방전, 은 나노입자
1. Introduction
Unlike conventional solid oxide fuel cells (SOFCs), which operate above 800
oC, thin film (TF)-SOFCs aim to reduce operating temperatures and maximize performance by fabricating electrodes and electrolytes with nanoscale to microscale thicknesses. While TF-SOFCs adhere to the fundamental operational principles of conventional SOFCs, their thin-film architecture significantly reduces ion and electron transport distances and enhances ionic conductivity, thereby enabling high power densities even at temperatures below 500
oC [
1,
2]. Lowering the operating temperature offers several advantages, including the minimization of interfacial diffusion between electrodes and the electrolyte, mitigation of material degradation rates, improvement of thermal cycling stability, and simplification of component integration, all of which contribute to a substantial reduction in overall fuel cell system costs [
3,
4].
To implement TF-SOFCs, microfabrication processes based on micro electro mechanical systems (MEMS) are commonly employed, and the integration of MEMS technology facilitates the miniaturization and weight reduction of fuel cells, enabling their application in wearable devices, biosensors, and implantable medical devices [
5]. Additionally, MEMS deposition techniques, such as atomic layer deposition (ALD), pulsed laser deposition (PLD), and sputtering, are typically utilized for nanoscale thin-film deposition [
6,
7]. For TF-SOFCs, to prevent electrical short circuits, the electrolyte layer must be entirely dense and free of pinholes, necessitating high fracture toughness to ensure a mechanically robust electrolyte membrane [
8]. However, achieving a pinhole-free electrolyte layer remains a significant challenge, as the size of pinholes or cracks is within the sub-micrometer range in the nanometer-thick electrolyte layer, making the precise visualization of the exact locations of these defects equally critical.
Pinhole defects in yttria-stabilized zirconia (YSZ) electrolytes can establish electron leakage pathways that reduce ionic conduction efficiency, induce electrical short-circuits between the anode and cathode, and potentially trigger device failures, ultimately degrading the performance of fuel cells and systems [
9,
10]. Conventional methods for detecting defects, such as gas leakage measurements or electrolyte surface observations, enable the identification of defect size, shape, and location. However, it is often more efficient to utilize nanoscale precision methods to detect submicron pinhole defects [
8,
11-
14].
This study addresses this challenge by proposing a novel technique for defect detection using Ag nanoparticles synthesized via a spark discharge generator (SDG). By leveraging the unique electrostatic and morphological properties of Ag nanoparticles, this approach aims to provide a robust and scalable solution for defect characterization. The use of nanoparticles for defect detection is particularly promising due to their size-dependent properties and ability to interact with surface imperfections. In addition, detection using Ag nanoparticles can be applied to the fuel cell manufacturing process as a fast and non-destructive method compared to existing detection methods, and it enables efficient quality control and early defect response. This study explores the interplay between Ag nanoparticles and YSZ thin films, focusing on their selective adsorption to defect sites and the mechanisms governing their aggregation.
2. Experimental
The SDG used for Ag nanoparticle synthesis consisted of a stainless-steel chamber equipped with electrode holders and gas flow systems, as shown in
Fig. 1(a). Pure Ag plates served as the electrodes, and high-voltage DC power supplies provided a voltage of 3-6 kV to the pin electrode through an external circuit at a spark frequency of 200 Hz. Carrier gas flow, regulated by mass flow controllers, consisted of an Ar/H
2 mixture introduced into the generator chamber. To prevent oxidation, the electrostatic precipitator was placed in a nitrogen-filled chamber. After high voltage is applied, a strong electric field is formed in the gap between the Ag electrodes, causing a spark discharge. In this process, a high-temperature plasma is generated for a very short time, which acts to evaporate part of the Ag electrode surface. The evaporated Ag atoms exist in a high-energy state in the plasma, and some are ionized. The evaporated and ionized Ag atoms are rapidly recondensed as the temperature decreases due to the cooling effect of the carrier gas. Through this recondensation process, Ag nanoparticles with a size of several tens of nanometers are formed. The SDG allowed for precise control of nanoparticle size distribution, producing particles in the range of 10-50 nm. Further details on the SDG setup can be found in previous studies [
15-
17].
Fig. 1(b) shows Ag particle distribution varied with the applied voltages on a nanoporous Pt electrode. The spark discharge (SD) voltage is the main determinant of SD energy. As the voltage increases, the SD energy and duration increase, causing more atoms or ions to evaporate. As a result, the agglomeration phenomenon within the particles is promoted, increasing the size and number of nanoparticles produced. SD is affected by the carrier gas. The composition of the carrier gas directly affects the temperature, density, and chemical stability of the plasma. For example, when Ar or He is used as a carrier gas, the SD voltage decreases compared to when air is used. Ar and He have different thermal conductivity and ionization energy, so even under the same voltage conditions, the plasma characteristics and the cooling rate of Ag atoms differ. He has high thermal conductivity, which induces relatively rapid cooling, which can be advantageous for the formation of smaller particles, while Ar tends to maintain a higher temperature, which can increase the particle size. This is caused by the difference in coefficients that vary depending on each material. The gap between the electrodes also has an effect [
18-
20]. The electrode gap also significantly affects the SD voltage, with narrower electrode spacing resulting in lower SD voltage. The relationship between SD voltage, carrier gas pressure, and electrode spacing can be explained by Paschen's law.
Fig. 1 (a) Schematic of spark discharge generator for creating Ag nanoparticles, and (b) Ag particle distribution on porous Pt electrode according to the applied voltage
Additionally, the SD voltage is affected by various parameters such as the volumetric flow rate of the carrier gas and the velocity around the electrodes [
21-
23]. Additionally, the production rate of SDG is determined by the mass concentration and flow rate of the particles [
22,
24].
Commercial anode aluminum oxide (AAO) substrates with a thickness of 100 µm and a pore diameter of 80 nm (Synkera Technology Inc.) were used as the base for thin-film YSZ electrolyte deposition. A 150 nm-thick Pt anode was sputtered onto the AAO substrate under DC power of 200 W at an Ar pressure of 12 Pa without substrate heating. Subsequently, YSZ electrolytes of 500 nm in thickness were sputtered onto the Pt anode at 200 W under an Ar/O2 atmosphere with a pressure of 0.67 Pa at room temperature. Finally, a Pt cathode was deposited on the YSZ layer using the same sputtering conditions as the Pt anode.
The cross-sectional microstructures of the YSZ electrolyte were examined by focused ion beam (FIB: Quanta 3D FEG, operating voltage: 5 kV), and scanning electron microscope (SEM: Zeiss Sigma, operating voltage: 2 kV) with energy dispersive X-ray spectroscopy (EDX) to visualize the distribution of Ag nanoparticles and their selective adsorption at defect sites.
Fig. 2 presents FIB-SEM image revealing a nanoscale pinhole defect in the YSZ electrolyte layer. Such pinholes, often caused by issues during thin-film deposition or thermal stresses, can lead to electrical shorting, gas leakage, and reduced cell efficiency.
Fig. 2 FIB cross-sectional SEM view near nanoscale pinholes
3. Results and Discussion
During the SDG process, Ag particles are generated and subsequently evaporated by the high-temperature plasma environment. These vaporized Ag species undergo ionization before cooling, leading to their recondensation and aggregation into nanoparticulate forms. These nanoparticles acquire a negative charge and can interact with the surface of YSZ electrolytes, which possess surface defects that increase surface energy, thereby promoting interactions with external particles. The negatively charged Ag nanoparticles and the high surface energy of the defect sites on the YSZ surface lead to the development of electrical and mechanical stresses between the two surfaces. These forces induce the selective adsorption of Ag nanoparticles onto the defect sites of the YSZ electrolyte. This interaction mechanism is depicted in
Fig. 3 [
25-
29].
Fig. 3 Schematic of the equipotential surface near a YSZ electrolyte pinhole
Under certain conditions, Ag nanoparticles may undergo electrochemical reactions on the surface of YSZ electrolytes, which in turn alter the nanoparticles' surface properties and facilitate agglomeration. Peterson et al. [
30] reported that external agents such as chloride ions present in solution modify the surface charge distribution of Ag nanoparticles, thereby diminishing the electrostatic repulsion and enhancing the interparticle interactions that promote agglomeration. This mechanism can similarly operate at defect sites on the YSZ electrolyte, where the localized high surface energy—relative to the surrounding regions—significantly increases the likelihood of selective adsorption of externally introduced metal nanoparticles.
Fig. 4(a) illustrates the directly observed morphology of Ag nanoparticles agglomerated at these defect sites. Moreover, He et al. [
31] confirmed that high-energy defect sites within the electrode induce the selective adsorption and subsequent agglomeration of nanoparticles, a phenomenon that can be extended to describe the strong local interactions and rearrangement processes occurring at defect sites on YSZ electrolytes.
Fig. 4 (a) Agglomerated Ag nanoparticles near pinholes, (b) EDX point spectrum, and (c) EDX compositional line mapping on the cross-sectional SEM view of Ag nanoparticle
The energy-dispersive X-ray spectroscopy (EDX) point spectrum shown in
Fig. 4(b) quantitatively confirms that the nanoparticles present at the defect site are predominantly composed of Ag, thereby further elucidating the selective adsorption phenomenon at these sites. Moreover,
Fig. 4(c) visually presents the compositional distribution along the cross-section of the sample, clearly demonstrating a concentrated detection of Ag at the defect areas. In this manner, following the preferential adsorption of Ag nanoparticles onto the defect sites of the YSZ electrolyte, the particles gradually approach one another and agglomerate into larger aggregates through interparticle interactions—such as van der Waals forces and electrostatic attractions—as time progresses. This process is in accordance with the thermodynamic principle whereby metal nanoparticles adsorb onto high-energy defect sites to reduce the overall free energy of the system. Thus, when the agglomeration of Ag nanoparticles observed in
Fig. 4(a) is integrated with the EDX analysis results presented in
Figs. 4(b) and 4(c), it scientifically confirms that such agglomeration is accelerated at high-energy defect sites [
30-
33].
4. Conclusion
The proposed method leverages the interplay between the electric field and the surface energy of Ag nanoparticles to delineate a mechanism for their selective adsorption and agglomeration on the surface of YSZ electrolytes. In SDG, charged Ag nanoparticles are driven toward regions of high electric field intensity and are selectively adsorbed at defect sites, such as pinholes and crack boundaries. This selective adsorption enhances the electrical and mechanical attractive interactions between the particles, leading to their gradual agglomeration into larger aggregates over time. Consequently, this agglomeration phenomenon enables the effective detection of defect sites on the YSZ electrolyte surface. Since agglomeration is further promoted at these high-energy defect regions, analyzing the distribution and size of the aggregates facilitates precise determination of the location and characteristics of the defects.
Future investigations will focus on experimentally validating these mechanisms and developing methods to control the adsorption and agglomeration of Ag nanoparticles, thereby contributing to enhanced fuel cell efficiency. Such research constitutes a pre-detection technique that verifies manufacturing integrity and is essential for sustaining and improving the overall performance of fuel cells.
ACKNOWLEDGMENTS
This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea Government (MSIT) (No. 2020R1C1C1014787).
NOMENCLATURE
Thin Film-solid Oxide Fuel Cell
Oxygen Reduction Reaction
Yttria-stabilized Zirconia
Spark Discharge Generator
Energy Dispersive X-ray Spectroscopy
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Biography
- DoYoon Kim
Ph.D. candidate in the Department of Automotive Engineering, Yeungnam University. His research interests are hydrogen fuel cells and characterization of lithium ion batteries.
- Ikwhang Chang
Associate Professor in the Department of Mechanical Engineering, Wonkwang University, South Korea. His research interests are fuel cells, thin film, and battery.
- Jong Dae Baek
Associate Professor in the Department of Automotive Engineering, Yeungnam University, South Korea. His research interests include low-temperature SOFCs, thin film mechanics, nano/micro fabrications, and energy conversion devices.
