Analysis of the Electrochemical Properties of LT-SOFCs According to the Thickness of PrOx Cathode Interlayer

1. Introduction

Due to recent climate changes, the concepts of eco-friendliness and sustainable development have become increasingly prominent. Therefore, research on eco-friendly power sources that can replace conventional internal combustion engines has intensified. In particular, Solid Oxide Fuel Cells (SOFCs) are gaining attention as eco-friendly power sources due to their excellent efficiency [1,2,3,4,5]. Generally, SOFCs require high operating temperatures, typically above 800°C, because the ionic conductivity of the electrolyte is very low at lower temperatures [1,6]. However, the high-temperature operating conditions raise concerns about the thermal stability of SOFCs. Therefore, research is consistently being conducted to reduce operating temperatures for thermal stability and to prevent performance degradation during low-temperature operation [7,8,9].

Among the various methods to improve the electrochemical performance of low-temperature SOFCs(LT-SOFCs), research on introducing a cathode interlayer has been continuously conducted [10,11,12]. Applying a cathode interlayer optimizes the cathode nanostructure, reducing resistance as oxygen ions (O^2-) move from the cathode to the electrolyte, facilitating electrochemical reactions more effectively [13]. Additionally, it promotes the oxygen reduction reaction, enhancing catalytic activity even at relatively low temperatures [14]. According to the study by Lee et al., the performance of LT-SOFCs was improved by applying Gadolinia doped ceria(Gd₂O₃ ,GDC,) as a cathode interlayer to reduce the activation energy of the ORR reaction at the interface between the cathode and the electrolyte [15].

Praseodymium oxide (PrO_x) is a rare earth oxide containing a mixed state of Pr ions with various oxidation states, such as Pr³⁺ and Pr⁴⁺ [16,17]. PrO_x is characterized by oxygen vacancies, which enhance its catalytic activity [18,19]. In PrO_x, Pr³⁺ promotes the formation of oxygen vacancies, thereby enhancing oxygen ion conductivity. Additionally, Pr⁴⁺ increases reversible redox activity, improving catalytic performance [20,21]. According to previous studies, PrO_x was applied to the cathode using the wet-chemical infiltration method to improve the charge transfer process of the existing cathode, thereby enhancing the electrochemical performance of SOFCs [22]. Furthermore, according to the study by Satoshi Okada et al., a symmetric cell was fabricated using PrO_x through a thin-film process, followed by surface modification with BaPrO₃ on both electrodes, after which electrochemical evaluations were conducted [23].

Meanwhile, powder-based fabrication methods, such as tape casting and infiltration, are widely used as fabrication techniques for manufacturing components of LT-SOFCs. [22,23,24,25,26,27,28]. However, such powder processes have difficulty achieving uniform thin film formation and precise nanoscale control [6,29]. Among these methods, the sputtering process is a physical vapor deposition technique that forms thin films by bombarding a target material with ions under high vacuum conditions [4,15]. Through the sputtering process, components of LT-SOFCs, including electrodes, can be fabricated at the nanoscale, and the microstructure can be controlled by adjusting temperature and pressure conditions [30,31]. The sputtering process not only enables mass production but also allows for the formation of uniform thin films. By optimizing the structure and thickness of the thin films, it enhances electrochemical performance [15,29].

However, research on the fabrication of PrO_x interlayers using thin-film processes and their application in LT-SOFCs remains limited. In this study, PrO_x is fabricated as a cathode interlayer using a thin-film process for LT-SOFCs. Furthermore, the electrochemical performance is evaluated based on the thickness of the cathode interlayer to determine the optimal thickness.

2. Experiments

The fabrication process of the PrO_x cathode interlayer was conducted using RF sputtering, as illustrated in Figure 1 (KVS-2000L, Korea Vacuum Tech, Korea). A silicon wafer was used as the deposition substrate to analyze the physical and chemical properties of PrO_x. For the PrO_x deposition process, a 2-inch diameter Pr₆O₁₁ target was used (VTM, Korea). Additionally, reactive sputtering was utilized during the sputtering process to ensure stable deposition of Pr₆O₁₁ [23,32]. To fabricate dense PrO_x thin films, the process pressure was uniformly set to 5 mTorr. For reactive sputtering, Ar and oxygen were supplied at 24 sccm and 6 sccm, respectively. The substrate rotated at 20 rpm, and PrO_x thin films of varying thicknesses were deposited based on deposition time. To analyze the thickness of the PrO_x films, Scanning Electron Microscopy (SEM, S-5200, Hitachi, Japan) was used. In addition, annealed for 1 hr and as-deposited PrO_x thin films were produced at 500 °C to analyze the crystal structure change caused by the operating temperature of LT-SOFCs. X-ray Diffraction (XRD, SmartLab, Rigaku, Japan) analysis was performed to analyze the crystallinity of the thin film. In addition, X-ray Photoelectron Spectroscopy (XPS, AXIS Supra+, Kratos, U.K.) was performed to analyze the chemical composition of PrO_x.

As shown in Figure 2, LT-SOFCs with a reaction area of 1.5 mm² were fabricated. Sc₂O₃-stabilized zirconia (ScSZ) pellets were used as electrolyte support. ScSZ is known for its high oxygen ion conductivity and excellent thermal stability [33]. LT-SOFCs were fabricated under the same process conditions as those used to prepare PrO_x samples on Si wafers. The cathode was deposited using a Pt target (VTM, Korea) at 50 mTorr with a supply of 30 sccm Ar gas at 100 W. Similarly, the anode was deposited using a Ni target (VTM, Korea) at 30 mTorr with 30 sccm Ar gas supply at 100 W.

To analyze the electrochemical properties of LT-SOFCs with interlayers of varying thickness, a custom-made SOFC test station, as shown in Figure 3, was utilized. The anode and cathode were connected using Ag wire and Ag paste (Silver Conductive Ink Liquid S-020, Alfa Aesar, US). Ceramic sealant (Ceramabond 571, Aremco, US) was used for sealing. LT-SOFC tests were conducted at 500°C. Hydrogen gas (H₂) was supplied at a flow rate of 50 sccm at 1.2 bar, while the cathode was exposed to ambient air. Electrochemical properties were measured using potentiostats (SP-300, Biologic, France). After performance measurements, electrochemical impedance spectroscopy (EIS) was performed by applying an alternating current perturbation of 10 mV at 0.5 V over a frequency range of 1 Hz to 1 MHz.

Table 1. Experimental Parameters of LT-SOFCs.

Parameters	Pr30	Pr55	Pr150
PrOx Thickness	30nm	55nm	150nm
PrOx Power	5mTorr, Ar 24sccm + O2 6sccm, RT, 40W
PrOx Deposition time	1hr 7min	2hr 15min	4hr 30min
Cathode (Pt)	50mTorr, Ar 30sccm, RT, 100W, 5min
Anode (Ni)	30mTorr, Ar 30sccm, RT, 100W, 25min

Table 1. Experimental Parameters of LT-SOFCs.

Parameters	Pr30	Pr55	Pr150
PrOx Thickness	30nm	55nm	150nm
PrOx Power	5mTorr, Ar 24sccm + O2 6sccm, RT, 40W
PrOx Deposition time	1hr 7min	2hr 15min	4hr 30min
Cathode (Pt)	50mTorr, Ar 30sccm, RT, 100W, 5min
Anode (Ni)	30mTorr, Ar 30sccm, RT, 100W, 25min

3. Results and Discussion

Figure 4 shows the surface of PrO_x thin films deposited with various thicknesses on a Si wafer. All films were observed to be deposited in a dense form. However, as the thickness of the PrO_x thin film decreased, a trend of decreasing grain size was observed. This trend of smaller grain sizes in thinner films is consistent with previous studies [31,34,35]. The decrease in grain size results in a relatively larger number of grain boundaries, which increases the likelihood of resistance during oxygen ion movement compared to larger grain sizes [36,37].

Figure 5 shows the cross-sections of PrO_x thin films with various thicknesses deposited on a Si wafer, along with the Pt thin films deposited on top of them. In Figure 5(a), the PrO_x thickness is approximately 30 nm; in Figure 5(b), it is about 55 nm; and in Figure 5(c), it is approximately 150 nm. The Pt layer in all samples showed a uniform thickness of approximately 150 nm. According to numerous previous studies, an increase in the thickness of the cathode interlayer tends to elongate the conduction path, leading to a decrease in oxygen ion conductivity. Consequently, this leads to an increase in ohmic resistance in LT-SOFCs [1].

Pr₆O₁₁ has a crystal structure based on a fluorite structure. Notably, it exhibits a mixed state of Pr³⁺ and Pr⁴⁺, which leads to the formation of oxygen vacancies, resulting in a defective-fluorite structure [32]. As shown in Figure 6, XRD analysis revealed that peaks were observed at (111), (200), (220), and (311) planes both in the as-deposited state and after annealing at 500°C for 1 hour. The XRD analysis confirmed that the peak positions remained the same before and after heat treatment, indicating that no phase change occurred in the material [16]. Meanwhile, in the case of annealing at 500°C, sharper peaks were observed compared to the as-deposited state. This indicates that crystallinity improved with heat treatment at 500°C. At the operating temperature of LT-SOFCs, this suggests reduced internal defects and micro strains, enhancing lattice stability and increasing pathways of ion conduction, thereby improving electrochemical performance [38].

XPS was used to investigate the composition ratio of Pr³⁺ and Pr⁴⁺ in both the as-deposited sample and the sample annealed at 500 °C [39]. As shown in Figure 7, deconvolution was performed on the Pr 3d spectrum to separate the superimposed peaks associated with various oxidation states (Pr³⁺, Pr⁴⁺) and bonding states [40]. The results showed the coexistence of Pr³⁺ and Pr⁴⁺ in both Figure 7 (a) and (b). Additionally, the areas of the main peaks obtained through deconvolution were calculated to determine the chemical composition ratio of the two oxidation states, as summarized in Table 2. For the case annealed at 500 °C, the Pr⁴⁺ ratio was 76.64 %, slightly higher than the 73.48 % observed in the as-deposited sample. This shows a similar trend to the findings of L. Grima et al., where heat treatment in an oxidizing atmosphere at high temperatures resulted in a higher Pr⁴⁺ content [41]. Although the XPS analysis of annealed PrO_x film showed a slight increase in Pr⁴⁺ over as-deposited, Pr³⁺ was still present. The relative increase in Pr⁴⁺ content means that Pr⁴⁺ can be more readily reduced to Pr³⁺, which improves the ability of oxygen to absorb and release [42]. However, the fact that the composition ratio of Pr³⁺ and Pr⁴⁺ is maintained even at the operating temperature of 500 °C suggests that the structure of the PrO_x catalyst remains stable under the operating conditions of LT-SOFCs.

The electrochemical performance evaluation of LT-SOFCs with cathode interlayers of various thicknesses was conducted at 500 °C. For comparison, a case without a PrO_x cathode interlayer was also included. As shown in Figure 8, the OCV of all LT-SOFCs conditions was approximately 1.0V. In the case of pure Pt without a cathode interlayer, the maximum power density reached 8.68 mW/cm². The Pr30 sample achieved 13.1 mW/cm², while the Pr55 sample showed a 26.5% improvement, reaching 17.9 mW/cm². However, the maximum power density of the Pr150 sample was 15.9 mW/cm², representing a 12.2% decrease compared to Pr55.

EIS was measured to analyze the electrochemical behavior of LT-SOFCs in greater detail. Ohmic resistance, which occurs when current flows through electrically conductive materials, corresponds to the x-intercept in the high-frequency region of the EIS graph shown in Figure 8 (b). The curve in the graph represents the faradaic resistance of the anode and cathode [1].

Figure 9 presents the EIS results at 0.5 V for LT-SOFCs with PrO_x cathode interlayers of various thicknesses. For Pr30, the ohmic resistance and activation resistance were 5.42 Ω·cm² and 3.05 Ω·cm², respectively. Comparing the resistances of Pr30 with those of Pure Pt (Ohmic: 5.07 Ω·cm², Activation: 13.5 Ω·cm²), the ohmic resistance showed a 6.37% increase, while the activation resistance decreased by approximately 3.4 times. This reduction of activation resistance leads to improved ORR performance, indicating an enhancement in the electrochemical reaction rate at the electrode. As in the preceding of Yuan Li et al., Pr₆O₁₁ was applied to LSFM electrodes in a filtration method to significantly improve the electrode performance of LT-SOFCs by increasing the catalytic activity of the electrode and reducing the activation resistance [43]. Compared to previous studies, the PrO_x cathode interlayer would have promoted the ORR kinetics in this study.

The ohmic resistance and faradaic resistance of Pr55 were measured as 6.12 Ω·cm² and 2.57 Ω·cm², respectively, while those of Pr150 were measured as 6.74 Ω·cm² and 2.45 Ω·cm². It was observed that as the thickness of the PrO_x thin film increased, the ohmic resistance increased slightly, while the activation resistance decreased. This increase in ohmic resistance is attributed to the longer ion conduction pathways through the thicker cathode interlayer. Meanwhile, the increasement of cathode interlayer thickness expands the Triple Phase Boundary (TPB) region, providing more active sites [4,28]. The TPB is the point where electrons, ions, and gases meet, serving as the primary location for electrochemical reactions [44,45]. The decrease in activation resistance is attributed to the presence of these additional active sites. Similar to the findings of previous study of Park et al., the application of a GDC interlayer resulted in an increase in ohmic resistance but a significant reduction in activation resistance due to improved ionic conductivity, demonstrating a comparable trend [46]. This is because the relatively thicker cathode interlayer promotes the ORR reaction to reduce activation resistance, thereby improving electrochemical performance [43]. However, the performance degradation due to ohmic resistance due to an increase in thickness should also be considered.

Meanwhile, in LT-SOFCs with a PrO_x cathode interlayer, Warburg impedance was commonly observed in the low-frequency region [47].Warburg impedance appears as a straight line with a slope of 45 degrees in the low-frequency region and represents the resistance arising from the diffusion of oxygen ions generated by the ORR reaction within the PrO_x layer [1,48]. Since oxygen ions neither move nor diffuse within the cathode in the case of Pure Pt, Warburg impedance was not observed. However, when a PrO_x cathode interlayer was applied, its excellent oxygen exchange capability rapidly facilitated the ORR reaction. Nonetheless, the inability to diffuse oxygen ions correspondingly as quickly resulted in the observed losses [22,32,49].

4. Conclusions

Throughout this study, it was demonstrated that stable PrO_x thin films can be deposited via reactive sputtering. SEM analysis confirmed the microstructure of the thin films. Considering the operating temperature of 500 °C for LT-SOFCs, the thin-film properties of PrO_x were analyzed using XRD, revealing improved crystallinity compared to the as-deposited state. XPS analysis showed a higher oxidation state (Pr⁴⁺) at 500 °C. This higher oxidization state provides a higher oxygen storage capacity and can easily cause redox reactions through high reduction potential. Additionally, electrochemical performance evaluation was conducted on LT-SOFCs fabricated with ScSZ electrolytes based on this process condition. It was observed that the application of the PrO_x interlayer and the increase in interlayer thickness led to an increase in ohmic resistance. However, the increment of TPB regions for oxygen reduction reactions increased, resulting in a significant reduction of activation resistance. In conclusion, the application of the PrO_x cathode interlayer effectively enhanced the performance of LT-SOFCs.