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Heater-assisted intense pulsed light irradiation for lanthanum strontium cobaltite thin film electrode fabrication
Thin Solid Films 697 (2020) 137778
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Heater-assisted intense pulsed light irradiation for lanthanum strontium cobaltite thin film electrode fabrication Jun-Sik Parka,1, Hojae Leea,1, Suhaeng Heoa, Young Beom Kima,b, a b
T
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Department of Mechanical Convergence Engineering, Hanyang University, Seoul 133-791, Korea Institute of Nano Science and Technology, Hanyang University, Seoul 133-791, Korea
A R T I C LE I N FO
A B S T R A C T
Keywords: Ceramic electrode Strontium-doped lanthanum cobaltite Metal organic chemical solution deposition Flash light sintering Solid oxide fuel cell
This study describes the bottom-heater-assisted flash light irradiation method for fabrication of lanthanum strontium cobaltite (La 0.6Sr0.4 CoO3 − δ , LSCO) thin films. Unlike the conventional sintering process, flash light irradiation proceeds instantly under ambient conditions. Since the flash light irradiation process involves the photothermal effect, heat energy supplied by a bottom heater can compensate for decreased irradiation energy. The substrate temperature was varied from room temperature to 200 °C and 300 °C, and the radiation energy requirement for obtaining an optimized LSCO film was decreased with increasing temperature. In addition, the instantaneous temperature difference due to highly intense energy between the top surface and the substrate can be relieved. With the innovative sintering system, an electrolyte-supported solid oxide fuel cell could be fabricated by the flash light irradiation method, and electrochemical characterization of the fuel cells was conducted.
1. Introduction Among renewable energy conversion systems, solid oxide fuel cells (SOFCs) have received considerable attention because of their efficiency and practical applications [1–6]. SOFCs have been investigated extensively for solid-state energy conversion systems, and considerable progress has been made in their development. Generally, to gain a reasonable power output from a solid-state SOFC, a high-operating temperature regime (~800 °C) is required to ensure high ionic conductivity and occurrence of the oxygen reduction reaction (ORR) [7,8]. However, such high temperatures (600 °C~) introduce challenges by limiting the choice of materials that can be used; this is the main reason for their high cost. To avoid the high cost of materials, the operating temperature should be decreased [9–11]. Recent studies have decreased it to a low-temperature regime (~500 °C), though lowering the operation temperature is accompanied by high ohmic and polarization losses. To minimize these losses, many studies have decreased the thickness of the electrolyte to micrometer or even nanometer scale [12–16]. Ohmic loss can be proportionally decreased by reducing the lengths of ionic conducting paths. Another approach to minimizing the losses is to introduce an additional functional layer between the electrolyte and cathode [17–20]. This additional layer can facilitate the oxygen kinetics and eventually improve the ORR rate. Another study increased the electrochemically active area by controlling the cathode
surface structure [21]. The output power can proportionally increase as a function of increased area. Although there are diverse approaches to minimizing losses under a low-temperature regime, changing the cathode material to one with a high catalytic activity is a critical strategy in terms of cell performance. Noble metals such as platinum are an example of an electrode material with a high catalytic activity that can improve the reaction rate for effective operation in the low-temperature regime. However, the cost of a noble metal is extremely high, and such an approach typically involves a vacuum process. Based on its electrochemical performance in the low-temperature regime, lanthanum strontium cobaltite (LSCO) could be an alternative to platinum. As a low-temperature cathode material with reasonable electrical performance and catalytic activity, LSCO could replace noble metals. Moreover, LSCO is a mixed ionic electronic conductor (MIEC) that can be utilized in SOFCs on account of the possibility of both ionic and electronic conduction. Compared to cathodes with negligible ionic conductivity, a much improved cell performance can be achieved using an MIEC because the ORR sites are not confined to triple phase boundaries (TPB) [22]. There are many researches to replace Co with materials such as Fe in LSCO. This is mainly to reduce the difference of thermal expansion coefficient (TEC) with electrolyte material. However, the conductivity is very low compared to reducing the difference of TEC. Therefore, LSCO with high conductivity is still widely used
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Corresponding author at: Mechanical Engineering, 222 Wangshimni-ro, Seongdong-gu, Seoul, 133791, Korea. E-mail address: [email protected] (Y.B. Kim). 1 The authors contributed equally to this work. https://doi.org/10.1016/j.tsf.2019.137778 Received 14 May 2019; Received in revised form 24 December 2019; Accepted 27 December 2019 Available online 28 December 2019 0040-6090/ © 2020 Elsevier B.V. All rights reserved.
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Fig. 1. (A) Spectrum of flash light irradiation from the xenon lamp. (B) UV–Vis spectra of LSCO gel film.
2. Experimental details
[23]. Moreover, instead of using costly vacuum deposition techniques, non-vacuum processes such as wet chemical processes can be employed [24,25]. Wet chemical processes have many benefits over vacuum processes, including simplicity, versatility, and low cost. Therefore, films prepared via this method are advantageous for industrial applications. However, these processes require a high-temperature (600 °C~) heat-treatment step as part of the sintering process to achieve desirable material properties. Heat treatment is typically performed using a conventional method such as furnace heating, which is still costly in terms of time and energy. Therefore, intense pulsed light irradiation can be used as an alternative to conventional post heattreatment [26–31]. By employing a xenon arc lamp for the spectral range 400–950 nm [31,32], white flash light irradiation can be used to sinter the specimen instantly. In addition, the flash light irradiating process consumes less energy than the conventional method and is pollution free. The flash light irradiation technique is an ultra-fast and environmentally friendly process, and it can be applied in printed electronics technology, which includes flexible displays, wearable electronics, flexible solar cells, and organic light emitting diodes (OLED) [33,34]. In this study, we employed the wet chemical deposition method for coating an LSCO film and introduced the irradiation process for effective sintering. LSCO can be sintered using intense white flash light, but irradiation involves certain potential risks. Substrates can be exposed to high flash light energies, rendering the possibility of them being shattered due to thermal shock from excessive energy and temperature difference between the top surface and substrate. For this reason, we attempted to relieve some of the energy by reducing the input power of the intense pulsed light radiation. Considering that the photothermal effect is one of the factors influencing the white flash light sintering process [35,36], the decrease in amount of energy was compensated for by the heat energy obtained from employing a bottom heater. With this additional heat energy, the overall input irradiation energy required for obtaining an optimized LSCO film was clearly decreased. Here, a cathode of electrolyte-supported SOFC was fabricated by the heat-assisted intense pulsed light irradiation method. The possibility of excess energy being supplied to the electrolyte substrate is reduced, and an LSCO cathode can be obtained. Generally, the bottom heat can decrease the required overall energy irradiated by flash light. The fabricated SOFC shows reasonable performance in the low-temperature regime. The significance of this work is verification of the feasibility of the heater-assisted intense pulsed light irradiation method.
For preparation of La 0.6Sr0.4 CoO3 − δ thin films, an LSCO solution was synthesized using metal-organic precursors. Lanthanum nitrate hexahydrate [La(NO3)3 · 6H2O] (Aldrich, USA), strontium acetate [Sr (CH3COO)2] (Fluka, USA), and cobalt acetate tetrahydrate [Co (CH3COO)2 · 4H2O] (Aldrich, USA) were introduced for solution preparation. The total concentration of the prepared solution was 0.4 M, and the amount of each precursor was gravimetrically determined. The cobalt precursor was dissolved in 6 M diluted acetic acid, and the corresponding amounts of lanthanum and strontium precursors were dissolved in deionized water and acetic acid, respectively. Additional amounts of water and acid were added to obtain the desired concentration, and the solutions were mixed together and stirred on a hotplate for 10 h at room temperature. The mixed solution was filtered several times using a nylon filter membrane (0.2 µm mesh) to eliminate external impurities and precipitates for proper film deposition. A silicon (100) wafer was used as the substrate, and the prepared solution was spin coated onto the wafer. The wafer was spun at 3500 rpm for 40 s, and the coated films were dried in an oven (at 200 °C) for 5 min before sintering. The sintering process was performed using a heater-assisted flashlight sintering system consisting of a power supply, beam guide, aluminum reflector, xenon lamp (PerkinElmer Corp., UK), simmer pulse controller, and bottom heater. The spectrum of the lamp spanned wavelengths of 350–980 nm, as depicted in Fig. 1A, which includes the visible, near-ultraviolet, and near-infrared regions. Fig. 1B shows that the light absorption wavelength band of LSCO gel and the emission wavelength band of xenon lamp are almost identical. Therefore, it can be seen that when flash light sintering, the LSCO film itself absorbs light. The lamp used in this study is 200 mm long, 12 mm in diameter, and the beam size is irradiated proportionally. The flashlight radiation originated from the arc plasma of the lamp, and the irradiation parameters are energy density per pulse (J/ cm2), number of pulses (#), irradiation time per pulse (on-time, ms), and time interval between pulses (off-time, ms). In addition, other factors that can affect the process include bottom heating temperature for additional thermal energy and relieving thermal shock during the process. Also, we could utilize pulse as control cycle. In the abovementioned flashlight sintering, it includes the pulse condition adjustment within one cycle that makes up the pulses and further adjusts the interval and the total energy between cycles. By adjusting the super cycle, which includes all of the cycles, it is possible to gradually reach the desired sintering temperature. The bottom temperature due to the 2
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heater is another parameter to consider. The process of flashlight sintering can be divided into two steps: a preheating step and the main sintering step. The preheating step eliminates the organics remaining in the film, as was demonstrated in our previous study [37]. The main sintering step of the LSCO film immediately followed the preheating step. The preheating irradiation step involves 30 pulses (on-time: 10 ms, off-time: 100 ms), while the main sintering irradiation step involves five pulses (on-time: 10 ms, off-time: 10 ms). In the main sintering step, the energy density per pulse was varied, but, the energy density per pulse typically was higher than that in the preheating step. The total energy density was measured using a power meter (Nova II, People Laser Tech Inc., South Korea). For fabrication of the SOFC, a polycrystalline yttria-stabilized zirconia (YSZ) pellet (8 mol % yttria, 100 × 100 × 0.3 mm, MTI Korea Corp., South Korea) was used as the electrolyte, and the prepared solution was coated onto the pellet as the cathode via spin coating. After the simple heat treatment and flashlight sintering process, the opposite side of the pellet was sputtered with platinum as the anode. A DC magnetron sputtering system (Daeki Hi-Tech, South Korea) was used to apply 100 W of DC power and 0.25 A of constant current under a regulated background pressure. The deposition time at the anode was precisely controlled to match the thickness of the cathode. A JSM-6701F field emission scanning electron microscope (JEOL Ltd.) was used to analyze the surface morphology of the LSCO films. The microstructures of the deposited films were investigated using the images recorded by an in-lens detector with an acceleration voltage of 14 kV. For the surface porosity evaluation of the fabricated films, surface images were also imported to MATLAB for further analysis calculating ratio of the number of void pixels and image pixels. Additionally, a D8-Advance, Bruker Co., CuKα irradiation [λ = 1.54 Å] with 2θ scan range of 20–80° was used for X-ray diffraction (XRD) measurements to analyze the crystalline phases of the flash light-irradiated LSCO films. The peaks observed in the diffraction patterns were indexed from the PDF2 XRD database, Card No. 48–0121 [38]. For measurement of the conductive properties of the flash lightirradiated films, the DC four-probe method was employed at room temperature using a Keithley 2400 (Keithley Instrument Inc.). The performance of the cell was characterized with a GAMRY potentiostat (FAS2, Gamry Instruments, Inc.) and through electrochemical impedance spectroscopy (EIS) (frequency range: 1 MHz-1 Hz); the electrochemical impedance was displayed in a Nyquist plot for further investigation of the electrochemical reactions.
Fig. 2. (A) Sheet resistance of sintered LSCO film as a function of sintering temperature. (B) Sheet resistance of sintered LSCO film as a function of irradiating energy density.
value at 80 J/cm2. Above 80 J/cm2, the sheet resistances in the plots start to increase. This result suggests that excessive energy can deteriorate the conductive properties of LSCO. Two-step irradiation involves the main sintering process, which is performed immediately after the preheating step. The effect of the preheating process has been studied previously [37]. This process can remove the residual organics in the film as these organics can cause secondary contamination and eventually deteriorate the film properties. The lowest sheet resistance in this plot is 837 Ω/square. Comparing with the thermal sintered film in Fig. 2A indicates that these results are slightly lower than the optimal value. One-step irradiation involves only the main sintering process, without the preheating step, and this plot shows a higher sheet resistance than that corresponding to the twostep process. The sheet resistance of one-step irradiation is more than 10 times higher, which implies that preheating is essential for optimization of the LSCO film. Considering that flash light irradiation includes the photothermal effect, addition of heat energy can be an interesting approach. The bottom heat supplied can compensate for the reduced flash light energy supplied to the film. Meanwhile, the organics unreacted with flash light irradiation at the film and substrate interface can cause a rupture by the fast rise in temperature escape. Bottom heat could alleviate this thermal issue. Fig. 3 shows the sheet resistance change with total energy
3. Results and discussion For reference, the thermally sintered LSCO film is presented in Fig. 2A. For the LSCO film, sheet resistance is shown as a function of sintering temperature for a ramping rate of 1 °C/min and dwell time of 1 h. The sheet resistance of the thermal sintered film was measured via the DC four-probe method at room temperature. The resistance is unclear at temperatures below 500 °C, which signifies that the sintering process has not commenced yet. Actually, at this temperature, the remnant organics in the film start to decompose, and the sintering process does not occur. The sheet resistance starts to decrease sharply at 500 °C, which indicates that sintering has occurred. The plot becomes flat at 600 °C, with the lowest resistance (1623 Ω/square), and it starts to increase after 700 °C. It is speculated that this phenomenon is due to structural rupture caused by constrained sintering of LSCO. The flash light-sintered LSCO shows a similar sheet resistance plot under room temperature conditions. Measured by the four-probe method, the sheet resistance of the flash light sintered film has obviously decreased, as depicted in Fig. 2B. All the plots shown in the figure exhibit decreased sheet resistance soon after the total density reaches 60 J/cm2 (12 J/cm2 per pulse). Below 60 J/cm2, the resistance is extremely high. The sheet resistance starts to drop sharply with increasing energy density until 70 J/cm2 and shows the lowest 3
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Fig. 3. Sheet resistance of LSCO film as a function of irradiating energy with bottom heat. (a) Sample irradiated without heater. Samples irradiated with heater (b) at 200 °C and (c) at 300 °C.
Fig. 4. X-ray diffraction patterns of LSCO film coated on silicon wafers. (A) 60 J/cm2 (main sintering) at room temperature without a bottom heater and (B) with a bottom heater (200 °C), (C) (300 °C).
density, and each plot corresponds to a particular bottom heater temperature. The plots corresponding to room temperature, 200 °C, and 300 °C are shown in Fig. 3. It is possible to determine the total energy density required to optimize LSCO as a function of bottom heater temperature. At room temperature (Fig. 3a), the lowest sheet resistance appears at 837 Ω/square at 80 J/cm2, while that at 200 °C (Fig. 3b) is 585 Ω/square at 65 J/cm2, and that at 300 °C (Fig. 3c) is 659 Ω/square at 50 J/cm2. This result implies that the energy from the bottom heater can support the flash light irradiation energy and can be used to intentionally decrease the irradiation energy supplied to the substrate. From the point of view of the substrate, controlling the irradiation energy could be important because certain substrates are vulnerable to flash light irradiation energy. This is crucial, so decreasing the irradiation energy can be an important approach. The flash light irradiation energy can instantly increase the surface temperature of the film, so vulnerability of deposited substrates is not unexpected. Excessive energy irradiated onto a deposited film could stress the interface of substrate and film and eventually induce its deterioration or even a structural rupture. For this reason, the irradiation energy supplied to these deposited films should be decreased. As shown in Fig. 3, it is possible to determine whether each plot has a “sintered” section. Low sheet resistances are observed in the ranges of 70–85 J/cm2 (Fig. 3a) at room temperature, 60–75 J/cm2 at 200 °C (Fig. 3b), and 45–60 J/cm2 at 300 °C (Fig. 3c). For 60 J/cm2, at both 200 °C and 300 °C, sintering occurred, though it was not observed at room temperature. This result shows that, even though the same amount of energy is applied, the sintering process depends on the bottom temperature condition. Therefore, even though less energy is irradiated, the heat energy of the bottom heater can make up for the deficiency. However, when excess energy is supplied by the bottom heater, excessive sintering conditions can occur in the LSCO film (in terms of transferred energy over time), resulting in an increase of sheet resistance. For instance, at 70 J/cm2 of irradiation energy, both room temperature (Fig. 3a) and 200 °C (Fig. 3b) reveal low sheet resistances. However, at 300 °C (Fig. 3c), the sheet resistance was increased even with the proper irradiation condition for lower bottom heater cases. This result supports the concept of additional energy from bottom heater. For further investigation of the effectiveness of bottom heat, XRD analysis was carried out. Fig. 4 presents the XRD pattern of each sample irradiated at 60 J/cm2. No crystalline phase diffraction peak was detected for a sample without bottom heat, as shown in Fig. 4A. According
to Fig. 3, it is possible to determine whether the plot indicates infinite sheet resistance at 60 J/cm2, which signifies that the sintering process has not commenced yet. In contrast, the patterns of Fig. 4B (200 °C) and 4C (300 °C) reveal noticeable diffraction peaks, and it is evident that these labeled peaks with orientations correspond to the perovskite structure of LSCO (referred from the PDF2 database). This trend is consistent with Fig. 3, which indicates a low sheet resistance in the sintered section. These diffraction patterns support the idea of bottom heating with flash light irradiation. In addition, a secondary phase is not observed in the diffraction patterns in Figs. 4B and 4C, according to the PDF2 database. Thus, there is no change in crystallinity upon heater-assisted flash light irradiation. To investigate the surface morphology, Fig. 5 shows the SEM surface images of the heater only (200, 300 °C), main sintering only, main sintering with preheating, and heater with both preheating and main sintering (200 °C and 300 °C). With the heater only at 200 °C and 300 °C (Fig. 5A, 5B), it is possible that some protruding grains exist on the surface. These grains are attributed to a sudden rise in the temperature of the film, which in turn is attributed to remnant organics. When flash light is irradiated with the heater, as shown in Fig. 5C and 5D, these unreacted organics in the interface are rapidly decomposed, leaving their traces in the form of pores. The porosity of the film could be controlled by the flash light irradiation process and time in terms of organic removal. In the flash light irradiation process to remove organics, increasing the temperature rapidly within a short time by reducing the interval between pulses can increase the porosity. Higher temperature is associated with more pores, as seen in Fig. 5A and 5B. The pores could decrease film density and conductive properties by acting as a hindrance. It might be possible to achieve denser morphology by modifying the concentration of the LSCO solution or the flash light irradiation conditions; however, considering that LSCO is the cathode of SOFCs, the pores could be exploited advantageously. The flash light-irradiated only sample (Fig. 5E, 5F) shows a surface with a wrinkle-structured morphology, expected due to the stress of the cathode against a substrate undergoes thermal expansion. By comparing this sample with the main sintered only sample (Fig. 5E) and the preheating sample (Fig. 5F), no porosity is observed in the former. This signifies that the remnant organics are not completely decomposed, which can explain the difference in sheet resistance observed in Fig. 2B. The trace of decomposed organics can be observed in Fig. 5F along with the existence of pores. With preheating, most of the residual organics remaining in the film are decomposed. 4
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Fig. 5. Surface FE-SEM images of heater only, (A) 200 °C, and (B) 300 °C; main sintering [60 J/cm2] (preheating included) at (C) 200 °C and (D) 300 °C bottom heat. Flash light-irradiated only [60 J/cm2] sample (E) without preheating and (F) with preheating.
As mentioned above, thin film LSCO can serve as the cathode of lowtemperature SOFCs. Furthermore, 80 J/cm2 of flash light irradiation energy could be applied at room temperature, as can be inferred from Fig. 3. However, as the YSZ substrate is vulnerable to high irradiation energy, there is the possibility of deteriorating or even shattering it. For this reason, compensating the total applied flash light irradiation energy by introducing a bottom heater could be a good strategy to broaden the application of this technique. In this study, the temperature of the bottom heater was 300 °C, and the applied flash light irradiation energy to LSCO was 50 J/cm2. In addition, a cell thermally sintered using a furnace was also included for comparison. The thicknesses of both cells were carefully controlled to the same value, and the sintering temperature of the thermal sintered cell was 650 °C. This temperature and other details were referred from our previous work. Fig. 6 shows cross-sectional and surface SEM images of LSCO coated on YSZ. The resistivities of flash light-irradiated LSCO with the assistance of a heater and of a thermal sintered LSCO film were 9.95 × 10−4 Ω cm and 9.84 × 10−4 Ω cm , respectively. These are reasonable values in comparison to other LSCO thin films, so it is possible to conclude that the LSCO films are completely sintered. Both the surfaces of the coated LSCO, acting as the cathode of SOFCs, are porous, as revealed in Fig. 6C, 6D. The surfaces of thermally sintered and flash light-irradiated films with heater assistance display similar
morphologies. The porosity could be evaluated using MATLAB by converting the images to binary form and dividing the total number of void pixels by the total number of pixels. The surface porosity of the thermally sintered film is 21.76%, while that of the flash light-irradiated sample with heater assistance is 15.49%. The electrochemical performances of the fuel cell samples were characterized at the operating temperature of 450 °C, and the cells were measured for more than 10 h. The open circuit voltages of the cells were above 1.00 V. According to the I-V curves displayed in Fig. 7, the thermally sintered film shows a slightly higher peak power density (5.84 mW/cm2) than the flash light-irradiated film with heater assistance (5.56 mW/cm2). Rather than aiming at high energy density outputs, this study aims to fabricate and verify electrodes using a flashlight sintering process. Therefore, a stable commercial thick YSZ electrolyte supported substrate was used to obtain reliable results. This is the reason why the performance is not very high given the operating temperature. In the future, we plan to apply the flash light sintering method to anode support cells or other structures other than electrolyte support cells. It is also better to apply to electrolyte and ceria based or composite layer structure. Considering that the rate of the hydrogen oxidation reaction (HOR) on the platinum anode is faster than that of the ORR on the LSCO cathode [22,39], the observed difference in peak power density probably originated from the cathode. This difference 5
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Fig. 6. Cross-section and surface SEM images of (A), (C) thermal sintered LSCO film and (B), (D) flashlight irradiated film with bottom heater on YSZ. (E), (F) Corresponding binary converted images for quantification of porosity.
electrolyte, and electrically connected catalytic electrode. Considering that the entire surface area of the MIEC cathode acts as a reaction site, increased surface area introduced by porosity increases the number of sites and directly increases the performance of the fuel cell. However, considering that LSCO is an MIEC material, the electrolyte is not the crucial factor, so the number of reaction sites can be increased [42–44]. The porosity of thermally sintered film is slightly greater than that of the flash light film irradiated with heater assistance, so the results can be considered reasonable (Table 1). For further analysis, Fig. 8 shows the EIS Nyquist plots of the thermally sintered cell and the flash light-irradiated cell with heater assistance. All the plots in Fig. 8A were measured at 450 °C with different operating bias voltages (0.3 V, 0.5 V, and 0.7 V). In the figure, the semicircle in the low-frequency range increases as a function of operating bias voltage. This result implies that the semicircle in the lowfrequency range corresponds to the electrode polarization resistance [45]. Considering that the impedance of the anode is negligible due to an extremely high HOR rate, it is possible to conclude that this visible semicircle is the cathode polarization resistance. The resistance of the anode is invisibly small and is expected to be merged with the semicircle of the cathode [46,47]. Fig. 8B shows the EIS comparison of the
Fig. 7. I–V curves and power densities of flash light irradiation with heater and thermally sintered cells. All measurements were performed at 450 °C.
can be attributed to the surface morphology of the cathode, especially the porosity, which can play a crucial role in the electrochemical reaction [40,41]. The fuel cell reaction sites are the contacts between gas, 6
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Table 1 Summary of resistance, porosity and cell power density of each sample. Samples
Resistance (Ω · cm)
Porosity (%)
Cell power density (mW/cm2)
Flashlight irradiated with bottom heater Thermal sintered
9.95 X 10–4 9.84 X 10–4
15.49 21.76
5.56 5.84
flash light irradiation was performed for effective sintering. The initial low-energy preheating step was used to eliminate the remnant organics, while the main heating step required high energy for further sintering. Preheating was able to remove the remnant organics in the film, but the main heating step had a few potential risks. Some substrates are vulnerable to high radiation energy and could deteriorate. For this reason, a bottom heating system was employed to relieve some of the flash light irradiation energy. The bottom heat could decrease the flash light energy applied to the sample through the heat energy supplied from the bottom. In addition, fuel cell samples were fabricated and demonstrated reasonable performance with this fabrication method. The significance of this work is the demonstration of the feasibility of the bottom-heatassisted flash light irradiation method. This study also shows the possibility of extending the application of the method to possible substrates that are vulnerable to high flash light irradiation energy. CRediT authorship contribution statement Jun-Sik Park: Conceptualization, Investigation, Writing - original draft. Hojae Lee: Validation, Investigation, Writing - review & editing. Suhaeng Heo: Investigation. Young Beom Kim: Funding acquisition, Project administration, Validation. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Fig. 8. (A) An EIS Nyquist plot of a flash light-irradiated cell measured at 450 °C with different operating bias voltages. (B) EIS comparison of a flash light-irradiated cell and a thermally sintered cell.
Acknowledgments Y.B.K. gratefully acknowledges financial support from the Korea Institute of Energy Technology Evaluation and Planning (KETEP) from the Ministry of Trade, Industry & Energy, Republic of Korea (No. 201700000003242) and from the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (2012R1A6A1029029 and 2017R1D1A1A09000586).
thermally sintered and flash light-irradiated (with heater assistance) films with bias voltage of 0.5 V. Each cathode polarization resistance is plotted, and it is confirmed that the width of the semicircle of the thermally sintered sample is smaller than that of the irradiated film. This result corresponds well with the I–V curve of Fig. 7, which is explained in terms of the difference in porosity. As mentioned above, electrochemical reaction sites such as TPB have a direct correlation with cathodic interface impedance. The cathode polarization resistance seems to be influenced by the porosity of the cathode surface. In this study, we demonstrated that flash light irradiation energy is effectively decreased with a heater-assisted system by compensating total energy applied to deposited film. Therefore, the thermal shock issue could be resolved, and the decreased irradiation energy required for the sintering process could eventually relieve the severe strain present on the substrate. The results of this study are expected to increase the feasibility of fuel cell commercialization, with implications for fabricating other energy conversion and storage devices using ceramic thin films.
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4. Conclusions Flash light irradiation is an innovative alternative to sintering using a furnace, a more traditional approach that requires significant processing time. The flash light irradiation system was therefore employed as a replacement for the conventional furnace method. The wet chemical method was applied for LSCO thin film deposition, and two-step 7
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lower‐temperature solid oxide fuel cell cathodes via nanotailoring of co‐assembled composite structures, Angew. Chem. Int. Ed. 53 (2014) 13463–13467. Y.B. Kim, J.H. Shim, T.M. Gür, F.B. Prinz, Epitaxial and polycrystalline gadoliniadoped ceria cathode interlayers for low temperature solid oxide fuel cells, J. Electrochem. Soc. 158 (2011) B1453–B1457. T. Van Gestel, D. Sebold, H.P. Buchkremer, D. Stöver, Assembly of 8YSZ nanoparticles into gas-tight 1–2 μm thick 8YSZ electrolyte layers using wet coating methods, J. Eur. Ceram. Soc. 32 (2012) 9–26. T. Mahata, S. Nair, R. Lenka, P. Sinha, Fabrication of Ni-YSZ anode supported tubular SOFC through iso-pressing and co-firing route, Int. J. Hydrogen Energy 37 (2012) 3874–3882. J.H. Shim, S. Kang, S.-W. Cha, W. Lee, Y.B. Kim, J.S. Park, T.M. Gür, F.B. Prinz, C.C. Chao, J. An, Atomic layer deposition of thin-film ceramic electrolytes for highperformance fuel cells, J. Mater. Chem. A 1 (2013) 12695–12705. C.-C. Chao, Y.B. Kim, F.B. Prinz, Surface modification of yttria-stabilized zirconia electrolyte by atomic layer deposition, Nano Lett. 9 (2009) 3626–3628. C.-C. Chao, C.-M. Hsu, Y. Cui, F.B. Prinz, Improved solid oxide fuel cell performance with nanostructured electrolytes, ACS Nano 5 (2011) 5692–5696. J. Bae, D. Lee, S. Hong, H. Yang, Y.-B. Kim, Three-dimensional hexagonal GDC interlayer for area enhancement of low-temperature solid oxide fuel cells, Surf. Coat. Technol. 279 (2015) 54–59. Y.B. Kim, J.S. Park, T.M. Gür, F.B. Prinz, Oxygen activation over engineered surface grains on YDC/YSZ interlayered composite electrolyte for LT-SOFC, J. Power Sources 196 (2011) 10550–10555. J. Bae, S. Hong, B. Koo, J. An, F.B. Prinz, Y.-B. Kim, Influence of the grain size of samaria-doped ceria cathodic interlayer for enhanced surface oxygen kinetics of low-temperature solid oxide fuel cell, J. Eur. Ceram. Soc. 34 (2014) 3763–3768. Y. Jee, G.Y. Cho, J. An, H.-.R. Kim, J.-W. Son, J.-H. Lee, F.B. Prinz, M.H. Lee, S.W. Cha, High performance Bi-layered electrolytes via atomic layer deposition for solid oxide fuel cells, J. Power Sources 253 (2014) 114–122. J. An, Y.-B. Kim, J. Park, T.M. Gür, F.B. Prinz, Three-dimensional nanostructured bilayer solid oxide fuel cell with 1.3 W/cm2 at 450 C, Nano Lett. 13 (2013) 4551–4555. R. O'hayre, S.-W. Cha, F.B. Prinz, W. Colella, Fuel Cell Fundamentals, John Wiley & Sons, 2016. F. Zhao, R.R. Peng, C.R. Xia, A La0.6Sr0.4CoO3-delta-based electrode wit high durability for intermediate temperature solid oxide fuel cells, Mater. Res. Bull. 43 (2008) 370–376. E.O. Oh, C.M. Whang, Y.R. Lee, S.Y. Park, D.H. Prasad, K.J. Yoon, J.W. Son, J.H. Lee, H.W. Lee, Extremely thin bilayer electrolyte for solid oxide fuel cells (SOFCs) fabricated by chemical solution deposition (CSD), Adv. Mater. 24 (2012) 3373–3377. E.-O. Oh, C.-M. Whang, Y.-R. Lee, J.-H. Lee, K.J. Yoon, B.-K. Kim, J.-.W. Son, J.H. Lee, H.-W. Lee, Thin film yttria-stabilized zirconia electrolyte for intermediatetemperature solid oxide fuel cells (IT-SOFCs) by chemical solution deposition, J. Eur. Ceram. Soc. 32 (2012) 1733–1741. C.-J. Moon, I. Kim, S.-J. Joo, W.-H. Chung, T.-M. Lee, H.-S. Kim, Flash light sintering of ag mesh films for printed transparent conducting electrode, Thin Solid Films 629 (2017) 60–68. Y. Myeong-Hyeon, J. Sung-Jun, K. Hak-Sung, Multi-pulse flash light sintering of bimodal Cu nanoparticle-ink for highly conductive printed Cu electrodes, Nanotechnology 28 (2017) 205205. H. Hyun-Jun, L. Soo-Chul, O. Kyung-Chul, P. Jin-Seong, K. Hak-Sung, Photonic
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sintering via flash white light combined with deep UV and NIR for SrTiO 3 thin film vibration touch panel applications, Nanotechnology 27 (2016) 505209. H.-J. Hwang, K.-H. Oh, H.-S. Kim, All-photonic drying and sintering process via flash white light combined with deep-UV and near-infrared irradiation for highly conductive copper nano-ink, Sci. Rep. 6 (2016) 19696. D. Angmo, T.T. Larsen-Olsen, M. Jørgensen, R.R. Søndergaard, F.C. Krebs, Roll-toRoll inkjet printing and photonic sintering of electrodes for ITO free polymer solar cell modules and facile product integration, Adv. Energy Mater. 3 (2013) 172–175. S.-H. Park, H.-S. Kim, Flash light sintering of nickel nanoparticles for printed electronics, Thin Solid Films 550 (2014) 575–581. H.-J. Hwang, W.-H. Chung, H.-S. Kim, In situ monitoring of flash-light sintering of copper nanoparticle ink for printed electronics, Nanotechnology 23 (2012) 485205. Y.-T. Hwang, W.-H. Chung, Y.-R. Jang, H.-S. Kim, Intensive plasmonic flash light sintering of copper nanoinks using a band-pass light filter for highly electrically conductive electrodes in printed electronics, ACS Appl. Mater. Interfaces 8 (2016) 8591–8599. J. Sung-Jun, H. Hyun-Jun, K. Hak-Sung, Highly conductive copper nano/microparticles ink via flash light sintering for printed electronics, Nanotechnology 25 (2014) 265601. T.-H. Yoo, S.-.J. Kwon, H.-S. Kim, J.-M. Hong, J.A. Lim, Y.-W. Song, Sub-second photo-annealing of solution-processed metal oxide thin-film transistors via irradiation of intensely pulsed white light, RSC Adv. 4 (2014) 19375–19379. S.-H. Park, H.-S. Kim, Environmentally benign and facile reduction of graphene oxide by flash light irradiation, Nanotechnology 26 (2015) 205601. J.-S. Park, D.-J. Kim, W.-H. Chung, Y. Lim, H.-S. Kim, Y.-B. Kim, cool sintering of wet processed yttria-stabilized zirconia ceramic electrolyte thin films, Sci. Rep. 7 (2017) 12458. PDF2 XRD database, Card No. 48-0121. A.L. Dicks, D.A. Rand, Fuel Cell Systems Explained, John Wiley & Sons, 2018. J.-S. Park, J. Bae, Y.-B. Kim, Performance stability of strontium-doped lanthanum cobaltite ceramic cathode synthesized by a wet chemical method, Ceram. Int. 42 (2016) 12853–12859. D. Gostovic, z.J. Smith, D. Kundinger, K. Jones, E. Wachsman, Three-dimensional reconstruction of porous LSCF cathodes, Electrochem. Solid-State Lett. 10 (2007) B214–B217. P. Hjalmarsson, M. Søgaard, M. Mogensen, Electrochemical performance and degradation of (La0. 6Sr0. 4) 0.99 CoO3− δ as porous SOFC-cathode, Solid State Ion. 179 (2008) 1422–1426. J. Hayd, L. Dieterle, U. Guntow, D. Gerthsen, E. Ivers-Tiffée, Nanoscaled La0. 6Sr0. 4CoO3− δ as intermediate temperature solid oxide fuel cell cathode: microstructure and electrochemical performance, J. Power Sources 196 (2011) 7263–7270. A. Egger, E. Bucher, M. Yang, W. Sitte, Comparison of oxygen exchange kinetics of the IT-SOFC cathode materials La0. 5Sr0. 5CoO3− δ and La0. 6Sr0. 4CoO3− δ, Solid State Ion. 225 (2012) 55–60. J.R. Macdonald, E. Barsoukov, Impedance Spectroscopy: Theory, Experiment, and Applications, 2nd Edition, Wiley, 2005. Y.-B. Kim, Geometry-controlled triple phase boundary study for low-temperature solid oxide fuel cells reaction kinetics, J. Nanosci. Nanotechnol 13 (2013) 7895–7901. S. De Souza, S.J. Visco, L.C. De Jonghe, Thin-film solid oxide fuel cell with high performance at low-temperature, Solid State Ion. 98 (1997) 57–61.
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Thin Solid Films journal homepage: www.elsevier.com/locate/tsf
Heater-assisted intense pulsed light irradiation for lanthanum strontium cobaltite thin film electrode fabrication Jun-Sik Parka,1, Hojae Leea,1, Suhaeng Heoa, Young Beom Kima,b, a b
T
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Department of Mechanical Convergence Engineering, Hanyang University, Seoul 133-791, Korea Institute of Nano Science and Technology, Hanyang University, Seoul 133-791, Korea
A R T I C LE I N FO
A B S T R A C T
Keywords: Ceramic electrode Strontium-doped lanthanum cobaltite Metal organic chemical solution deposition Flash light sintering Solid oxide fuel cell
This study describes the bottom-heater-assisted flash light irradiation method for fabrication of lanthanum strontium cobaltite (La 0.6Sr0.4 CoO3 − δ , LSCO) thin films. Unlike the conventional sintering process, flash light irradiation proceeds instantly under ambient conditions. Since the flash light irradiation process involves the photothermal effect, heat energy supplied by a bottom heater can compensate for decreased irradiation energy. The substrate temperature was varied from room temperature to 200 °C and 300 °C, and the radiation energy requirement for obtaining an optimized LSCO film was decreased with increasing temperature. In addition, the instantaneous temperature difference due to highly intense energy between the top surface and the substrate can be relieved. With the innovative sintering system, an electrolyte-supported solid oxide fuel cell could be fabricated by the flash light irradiation method, and electrochemical characterization of the fuel cells was conducted.
1. Introduction Among renewable energy conversion systems, solid oxide fuel cells (SOFCs) have received considerable attention because of their efficiency and practical applications [1–6]. SOFCs have been investigated extensively for solid-state energy conversion systems, and considerable progress has been made in their development. Generally, to gain a reasonable power output from a solid-state SOFC, a high-operating temperature regime (~800 °C) is required to ensure high ionic conductivity and occurrence of the oxygen reduction reaction (ORR) [7,8]. However, such high temperatures (600 °C~) introduce challenges by limiting the choice of materials that can be used; this is the main reason for their high cost. To avoid the high cost of materials, the operating temperature should be decreased [9–11]. Recent studies have decreased it to a low-temperature regime (~500 °C), though lowering the operation temperature is accompanied by high ohmic and polarization losses. To minimize these losses, many studies have decreased the thickness of the electrolyte to micrometer or even nanometer scale [12–16]. Ohmic loss can be proportionally decreased by reducing the lengths of ionic conducting paths. Another approach to minimizing the losses is to introduce an additional functional layer between the electrolyte and cathode [17–20]. This additional layer can facilitate the oxygen kinetics and eventually improve the ORR rate. Another study increased the electrochemically active area by controlling the cathode
surface structure [21]. The output power can proportionally increase as a function of increased area. Although there are diverse approaches to minimizing losses under a low-temperature regime, changing the cathode material to one with a high catalytic activity is a critical strategy in terms of cell performance. Noble metals such as platinum are an example of an electrode material with a high catalytic activity that can improve the reaction rate for effective operation in the low-temperature regime. However, the cost of a noble metal is extremely high, and such an approach typically involves a vacuum process. Based on its electrochemical performance in the low-temperature regime, lanthanum strontium cobaltite (LSCO) could be an alternative to platinum. As a low-temperature cathode material with reasonable electrical performance and catalytic activity, LSCO could replace noble metals. Moreover, LSCO is a mixed ionic electronic conductor (MIEC) that can be utilized in SOFCs on account of the possibility of both ionic and electronic conduction. Compared to cathodes with negligible ionic conductivity, a much improved cell performance can be achieved using an MIEC because the ORR sites are not confined to triple phase boundaries (TPB) [22]. There are many researches to replace Co with materials such as Fe in LSCO. This is mainly to reduce the difference of thermal expansion coefficient (TEC) with electrolyte material. However, the conductivity is very low compared to reducing the difference of TEC. Therefore, LSCO with high conductivity is still widely used
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Corresponding author at: Mechanical Engineering, 222 Wangshimni-ro, Seongdong-gu, Seoul, 133791, Korea. E-mail address: [email protected] (Y.B. Kim). 1 The authors contributed equally to this work. https://doi.org/10.1016/j.tsf.2019.137778 Received 14 May 2019; Received in revised form 24 December 2019; Accepted 27 December 2019 Available online 28 December 2019 0040-6090/ © 2020 Elsevier B.V. All rights reserved.
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Fig. 1. (A) Spectrum of flash light irradiation from the xenon lamp. (B) UV–Vis spectra of LSCO gel film.
2. Experimental details
[23]. Moreover, instead of using costly vacuum deposition techniques, non-vacuum processes such as wet chemical processes can be employed [24,25]. Wet chemical processes have many benefits over vacuum processes, including simplicity, versatility, and low cost. Therefore, films prepared via this method are advantageous for industrial applications. However, these processes require a high-temperature (600 °C~) heat-treatment step as part of the sintering process to achieve desirable material properties. Heat treatment is typically performed using a conventional method such as furnace heating, which is still costly in terms of time and energy. Therefore, intense pulsed light irradiation can be used as an alternative to conventional post heattreatment [26–31]. By employing a xenon arc lamp for the spectral range 400–950 nm [31,32], white flash light irradiation can be used to sinter the specimen instantly. In addition, the flash light irradiating process consumes less energy than the conventional method and is pollution free. The flash light irradiation technique is an ultra-fast and environmentally friendly process, and it can be applied in printed electronics technology, which includes flexible displays, wearable electronics, flexible solar cells, and organic light emitting diodes (OLED) [33,34]. In this study, we employed the wet chemical deposition method for coating an LSCO film and introduced the irradiation process for effective sintering. LSCO can be sintered using intense white flash light, but irradiation involves certain potential risks. Substrates can be exposed to high flash light energies, rendering the possibility of them being shattered due to thermal shock from excessive energy and temperature difference between the top surface and substrate. For this reason, we attempted to relieve some of the energy by reducing the input power of the intense pulsed light radiation. Considering that the photothermal effect is one of the factors influencing the white flash light sintering process [35,36], the decrease in amount of energy was compensated for by the heat energy obtained from employing a bottom heater. With this additional heat energy, the overall input irradiation energy required for obtaining an optimized LSCO film was clearly decreased. Here, a cathode of electrolyte-supported SOFC was fabricated by the heat-assisted intense pulsed light irradiation method. The possibility of excess energy being supplied to the electrolyte substrate is reduced, and an LSCO cathode can be obtained. Generally, the bottom heat can decrease the required overall energy irradiated by flash light. The fabricated SOFC shows reasonable performance in the low-temperature regime. The significance of this work is verification of the feasibility of the heater-assisted intense pulsed light irradiation method.
For preparation of La 0.6Sr0.4 CoO3 − δ thin films, an LSCO solution was synthesized using metal-organic precursors. Lanthanum nitrate hexahydrate [La(NO3)3 · 6H2O] (Aldrich, USA), strontium acetate [Sr (CH3COO)2] (Fluka, USA), and cobalt acetate tetrahydrate [Co (CH3COO)2 · 4H2O] (Aldrich, USA) were introduced for solution preparation. The total concentration of the prepared solution was 0.4 M, and the amount of each precursor was gravimetrically determined. The cobalt precursor was dissolved in 6 M diluted acetic acid, and the corresponding amounts of lanthanum and strontium precursors were dissolved in deionized water and acetic acid, respectively. Additional amounts of water and acid were added to obtain the desired concentration, and the solutions were mixed together and stirred on a hotplate for 10 h at room temperature. The mixed solution was filtered several times using a nylon filter membrane (0.2 µm mesh) to eliminate external impurities and precipitates for proper film deposition. A silicon (100) wafer was used as the substrate, and the prepared solution was spin coated onto the wafer. The wafer was spun at 3500 rpm for 40 s, and the coated films were dried in an oven (at 200 °C) for 5 min before sintering. The sintering process was performed using a heater-assisted flashlight sintering system consisting of a power supply, beam guide, aluminum reflector, xenon lamp (PerkinElmer Corp., UK), simmer pulse controller, and bottom heater. The spectrum of the lamp spanned wavelengths of 350–980 nm, as depicted in Fig. 1A, which includes the visible, near-ultraviolet, and near-infrared regions. Fig. 1B shows that the light absorption wavelength band of LSCO gel and the emission wavelength band of xenon lamp are almost identical. Therefore, it can be seen that when flash light sintering, the LSCO film itself absorbs light. The lamp used in this study is 200 mm long, 12 mm in diameter, and the beam size is irradiated proportionally. The flashlight radiation originated from the arc plasma of the lamp, and the irradiation parameters are energy density per pulse (J/ cm2), number of pulses (#), irradiation time per pulse (on-time, ms), and time interval between pulses (off-time, ms). In addition, other factors that can affect the process include bottom heating temperature for additional thermal energy and relieving thermal shock during the process. Also, we could utilize pulse as control cycle. In the abovementioned flashlight sintering, it includes the pulse condition adjustment within one cycle that makes up the pulses and further adjusts the interval and the total energy between cycles. By adjusting the super cycle, which includes all of the cycles, it is possible to gradually reach the desired sintering temperature. The bottom temperature due to the 2
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heater is another parameter to consider. The process of flashlight sintering can be divided into two steps: a preheating step and the main sintering step. The preheating step eliminates the organics remaining in the film, as was demonstrated in our previous study [37]. The main sintering step of the LSCO film immediately followed the preheating step. The preheating irradiation step involves 30 pulses (on-time: 10 ms, off-time: 100 ms), while the main sintering irradiation step involves five pulses (on-time: 10 ms, off-time: 10 ms). In the main sintering step, the energy density per pulse was varied, but, the energy density per pulse typically was higher than that in the preheating step. The total energy density was measured using a power meter (Nova II, People Laser Tech Inc., South Korea). For fabrication of the SOFC, a polycrystalline yttria-stabilized zirconia (YSZ) pellet (8 mol % yttria, 100 × 100 × 0.3 mm, MTI Korea Corp., South Korea) was used as the electrolyte, and the prepared solution was coated onto the pellet as the cathode via spin coating. After the simple heat treatment and flashlight sintering process, the opposite side of the pellet was sputtered with platinum as the anode. A DC magnetron sputtering system (Daeki Hi-Tech, South Korea) was used to apply 100 W of DC power and 0.25 A of constant current under a regulated background pressure. The deposition time at the anode was precisely controlled to match the thickness of the cathode. A JSM-6701F field emission scanning electron microscope (JEOL Ltd.) was used to analyze the surface morphology of the LSCO films. The microstructures of the deposited films were investigated using the images recorded by an in-lens detector with an acceleration voltage of 14 kV. For the surface porosity evaluation of the fabricated films, surface images were also imported to MATLAB for further analysis calculating ratio of the number of void pixels and image pixels. Additionally, a D8-Advance, Bruker Co., CuKα irradiation [λ = 1.54 Å] with 2θ scan range of 20–80° was used for X-ray diffraction (XRD) measurements to analyze the crystalline phases of the flash light-irradiated LSCO films. The peaks observed in the diffraction patterns were indexed from the PDF2 XRD database, Card No. 48–0121 [38]. For measurement of the conductive properties of the flash lightirradiated films, the DC four-probe method was employed at room temperature using a Keithley 2400 (Keithley Instrument Inc.). The performance of the cell was characterized with a GAMRY potentiostat (FAS2, Gamry Instruments, Inc.) and through electrochemical impedance spectroscopy (EIS) (frequency range: 1 MHz-1 Hz); the electrochemical impedance was displayed in a Nyquist plot for further investigation of the electrochemical reactions.
Fig. 2. (A) Sheet resistance of sintered LSCO film as a function of sintering temperature. (B) Sheet resistance of sintered LSCO film as a function of irradiating energy density.
value at 80 J/cm2. Above 80 J/cm2, the sheet resistances in the plots start to increase. This result suggests that excessive energy can deteriorate the conductive properties of LSCO. Two-step irradiation involves the main sintering process, which is performed immediately after the preheating step. The effect of the preheating process has been studied previously [37]. This process can remove the residual organics in the film as these organics can cause secondary contamination and eventually deteriorate the film properties. The lowest sheet resistance in this plot is 837 Ω/square. Comparing with the thermal sintered film in Fig. 2A indicates that these results are slightly lower than the optimal value. One-step irradiation involves only the main sintering process, without the preheating step, and this plot shows a higher sheet resistance than that corresponding to the twostep process. The sheet resistance of one-step irradiation is more than 10 times higher, which implies that preheating is essential for optimization of the LSCO film. Considering that flash light irradiation includes the photothermal effect, addition of heat energy can be an interesting approach. The bottom heat supplied can compensate for the reduced flash light energy supplied to the film. Meanwhile, the organics unreacted with flash light irradiation at the film and substrate interface can cause a rupture by the fast rise in temperature escape. Bottom heat could alleviate this thermal issue. Fig. 3 shows the sheet resistance change with total energy
3. Results and discussion For reference, the thermally sintered LSCO film is presented in Fig. 2A. For the LSCO film, sheet resistance is shown as a function of sintering temperature for a ramping rate of 1 °C/min and dwell time of 1 h. The sheet resistance of the thermal sintered film was measured via the DC four-probe method at room temperature. The resistance is unclear at temperatures below 500 °C, which signifies that the sintering process has not commenced yet. Actually, at this temperature, the remnant organics in the film start to decompose, and the sintering process does not occur. The sheet resistance starts to decrease sharply at 500 °C, which indicates that sintering has occurred. The plot becomes flat at 600 °C, with the lowest resistance (1623 Ω/square), and it starts to increase after 700 °C. It is speculated that this phenomenon is due to structural rupture caused by constrained sintering of LSCO. The flash light-sintered LSCO shows a similar sheet resistance plot under room temperature conditions. Measured by the four-probe method, the sheet resistance of the flash light sintered film has obviously decreased, as depicted in Fig. 2B. All the plots shown in the figure exhibit decreased sheet resistance soon after the total density reaches 60 J/cm2 (12 J/cm2 per pulse). Below 60 J/cm2, the resistance is extremely high. The sheet resistance starts to drop sharply with increasing energy density until 70 J/cm2 and shows the lowest 3
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Fig. 3. Sheet resistance of LSCO film as a function of irradiating energy with bottom heat. (a) Sample irradiated without heater. Samples irradiated with heater (b) at 200 °C and (c) at 300 °C.
Fig. 4. X-ray diffraction patterns of LSCO film coated on silicon wafers. (A) 60 J/cm2 (main sintering) at room temperature without a bottom heater and (B) with a bottom heater (200 °C), (C) (300 °C).
density, and each plot corresponds to a particular bottom heater temperature. The plots corresponding to room temperature, 200 °C, and 300 °C are shown in Fig. 3. It is possible to determine the total energy density required to optimize LSCO as a function of bottom heater temperature. At room temperature (Fig. 3a), the lowest sheet resistance appears at 837 Ω/square at 80 J/cm2, while that at 200 °C (Fig. 3b) is 585 Ω/square at 65 J/cm2, and that at 300 °C (Fig. 3c) is 659 Ω/square at 50 J/cm2. This result implies that the energy from the bottom heater can support the flash light irradiation energy and can be used to intentionally decrease the irradiation energy supplied to the substrate. From the point of view of the substrate, controlling the irradiation energy could be important because certain substrates are vulnerable to flash light irradiation energy. This is crucial, so decreasing the irradiation energy can be an important approach. The flash light irradiation energy can instantly increase the surface temperature of the film, so vulnerability of deposited substrates is not unexpected. Excessive energy irradiated onto a deposited film could stress the interface of substrate and film and eventually induce its deterioration or even a structural rupture. For this reason, the irradiation energy supplied to these deposited films should be decreased. As shown in Fig. 3, it is possible to determine whether each plot has a “sintered” section. Low sheet resistances are observed in the ranges of 70–85 J/cm2 (Fig. 3a) at room temperature, 60–75 J/cm2 at 200 °C (Fig. 3b), and 45–60 J/cm2 at 300 °C (Fig. 3c). For 60 J/cm2, at both 200 °C and 300 °C, sintering occurred, though it was not observed at room temperature. This result shows that, even though the same amount of energy is applied, the sintering process depends on the bottom temperature condition. Therefore, even though less energy is irradiated, the heat energy of the bottom heater can make up for the deficiency. However, when excess energy is supplied by the bottom heater, excessive sintering conditions can occur in the LSCO film (in terms of transferred energy over time), resulting in an increase of sheet resistance. For instance, at 70 J/cm2 of irradiation energy, both room temperature (Fig. 3a) and 200 °C (Fig. 3b) reveal low sheet resistances. However, at 300 °C (Fig. 3c), the sheet resistance was increased even with the proper irradiation condition for lower bottom heater cases. This result supports the concept of additional energy from bottom heater. For further investigation of the effectiveness of bottom heat, XRD analysis was carried out. Fig. 4 presents the XRD pattern of each sample irradiated at 60 J/cm2. No crystalline phase diffraction peak was detected for a sample without bottom heat, as shown in Fig. 4A. According
to Fig. 3, it is possible to determine whether the plot indicates infinite sheet resistance at 60 J/cm2, which signifies that the sintering process has not commenced yet. In contrast, the patterns of Fig. 4B (200 °C) and 4C (300 °C) reveal noticeable diffraction peaks, and it is evident that these labeled peaks with orientations correspond to the perovskite structure of LSCO (referred from the PDF2 database). This trend is consistent with Fig. 3, which indicates a low sheet resistance in the sintered section. These diffraction patterns support the idea of bottom heating with flash light irradiation. In addition, a secondary phase is not observed in the diffraction patterns in Figs. 4B and 4C, according to the PDF2 database. Thus, there is no change in crystallinity upon heater-assisted flash light irradiation. To investigate the surface morphology, Fig. 5 shows the SEM surface images of the heater only (200, 300 °C), main sintering only, main sintering with preheating, and heater with both preheating and main sintering (200 °C and 300 °C). With the heater only at 200 °C and 300 °C (Fig. 5A, 5B), it is possible that some protruding grains exist on the surface. These grains are attributed to a sudden rise in the temperature of the film, which in turn is attributed to remnant organics. When flash light is irradiated with the heater, as shown in Fig. 5C and 5D, these unreacted organics in the interface are rapidly decomposed, leaving their traces in the form of pores. The porosity of the film could be controlled by the flash light irradiation process and time in terms of organic removal. In the flash light irradiation process to remove organics, increasing the temperature rapidly within a short time by reducing the interval between pulses can increase the porosity. Higher temperature is associated with more pores, as seen in Fig. 5A and 5B. The pores could decrease film density and conductive properties by acting as a hindrance. It might be possible to achieve denser morphology by modifying the concentration of the LSCO solution or the flash light irradiation conditions; however, considering that LSCO is the cathode of SOFCs, the pores could be exploited advantageously. The flash light-irradiated only sample (Fig. 5E, 5F) shows a surface with a wrinkle-structured morphology, expected due to the stress of the cathode against a substrate undergoes thermal expansion. By comparing this sample with the main sintered only sample (Fig. 5E) and the preheating sample (Fig. 5F), no porosity is observed in the former. This signifies that the remnant organics are not completely decomposed, which can explain the difference in sheet resistance observed in Fig. 2B. The trace of decomposed organics can be observed in Fig. 5F along with the existence of pores. With preheating, most of the residual organics remaining in the film are decomposed. 4
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Fig. 5. Surface FE-SEM images of heater only, (A) 200 °C, and (B) 300 °C; main sintering [60 J/cm2] (preheating included) at (C) 200 °C and (D) 300 °C bottom heat. Flash light-irradiated only [60 J/cm2] sample (E) without preheating and (F) with preheating.
As mentioned above, thin film LSCO can serve as the cathode of lowtemperature SOFCs. Furthermore, 80 J/cm2 of flash light irradiation energy could be applied at room temperature, as can be inferred from Fig. 3. However, as the YSZ substrate is vulnerable to high irradiation energy, there is the possibility of deteriorating or even shattering it. For this reason, compensating the total applied flash light irradiation energy by introducing a bottom heater could be a good strategy to broaden the application of this technique. In this study, the temperature of the bottom heater was 300 °C, and the applied flash light irradiation energy to LSCO was 50 J/cm2. In addition, a cell thermally sintered using a furnace was also included for comparison. The thicknesses of both cells were carefully controlled to the same value, and the sintering temperature of the thermal sintered cell was 650 °C. This temperature and other details were referred from our previous work. Fig. 6 shows cross-sectional and surface SEM images of LSCO coated on YSZ. The resistivities of flash light-irradiated LSCO with the assistance of a heater and of a thermal sintered LSCO film were 9.95 × 10−4 Ω cm and 9.84 × 10−4 Ω cm , respectively. These are reasonable values in comparison to other LSCO thin films, so it is possible to conclude that the LSCO films are completely sintered. Both the surfaces of the coated LSCO, acting as the cathode of SOFCs, are porous, as revealed in Fig. 6C, 6D. The surfaces of thermally sintered and flash light-irradiated films with heater assistance display similar
morphologies. The porosity could be evaluated using MATLAB by converting the images to binary form and dividing the total number of void pixels by the total number of pixels. The surface porosity of the thermally sintered film is 21.76%, while that of the flash light-irradiated sample with heater assistance is 15.49%. The electrochemical performances of the fuel cell samples were characterized at the operating temperature of 450 °C, and the cells were measured for more than 10 h. The open circuit voltages of the cells were above 1.00 V. According to the I-V curves displayed in Fig. 7, the thermally sintered film shows a slightly higher peak power density (5.84 mW/cm2) than the flash light-irradiated film with heater assistance (5.56 mW/cm2). Rather than aiming at high energy density outputs, this study aims to fabricate and verify electrodes using a flashlight sintering process. Therefore, a stable commercial thick YSZ electrolyte supported substrate was used to obtain reliable results. This is the reason why the performance is not very high given the operating temperature. In the future, we plan to apply the flash light sintering method to anode support cells or other structures other than electrolyte support cells. It is also better to apply to electrolyte and ceria based or composite layer structure. Considering that the rate of the hydrogen oxidation reaction (HOR) on the platinum anode is faster than that of the ORR on the LSCO cathode [22,39], the observed difference in peak power density probably originated from the cathode. This difference 5
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Fig. 6. Cross-section and surface SEM images of (A), (C) thermal sintered LSCO film and (B), (D) flashlight irradiated film with bottom heater on YSZ. (E), (F) Corresponding binary converted images for quantification of porosity.
electrolyte, and electrically connected catalytic electrode. Considering that the entire surface area of the MIEC cathode acts as a reaction site, increased surface area introduced by porosity increases the number of sites and directly increases the performance of the fuel cell. However, considering that LSCO is an MIEC material, the electrolyte is not the crucial factor, so the number of reaction sites can be increased [42–44]. The porosity of thermally sintered film is slightly greater than that of the flash light film irradiated with heater assistance, so the results can be considered reasonable (Table 1). For further analysis, Fig. 8 shows the EIS Nyquist plots of the thermally sintered cell and the flash light-irradiated cell with heater assistance. All the plots in Fig. 8A were measured at 450 °C with different operating bias voltages (0.3 V, 0.5 V, and 0.7 V). In the figure, the semicircle in the low-frequency range increases as a function of operating bias voltage. This result implies that the semicircle in the lowfrequency range corresponds to the electrode polarization resistance [45]. Considering that the impedance of the anode is negligible due to an extremely high HOR rate, it is possible to conclude that this visible semicircle is the cathode polarization resistance. The resistance of the anode is invisibly small and is expected to be merged with the semicircle of the cathode [46,47]. Fig. 8B shows the EIS comparison of the
Fig. 7. I–V curves and power densities of flash light irradiation with heater and thermally sintered cells. All measurements were performed at 450 °C.
can be attributed to the surface morphology of the cathode, especially the porosity, which can play a crucial role in the electrochemical reaction [40,41]. The fuel cell reaction sites are the contacts between gas, 6
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Table 1 Summary of resistance, porosity and cell power density of each sample. Samples
Resistance (Ω · cm)
Porosity (%)
Cell power density (mW/cm2)
Flashlight irradiated with bottom heater Thermal sintered
9.95 X 10–4 9.84 X 10–4
15.49 21.76
5.56 5.84
flash light irradiation was performed for effective sintering. The initial low-energy preheating step was used to eliminate the remnant organics, while the main heating step required high energy for further sintering. Preheating was able to remove the remnant organics in the film, but the main heating step had a few potential risks. Some substrates are vulnerable to high radiation energy and could deteriorate. For this reason, a bottom heating system was employed to relieve some of the flash light irradiation energy. The bottom heat could decrease the flash light energy applied to the sample through the heat energy supplied from the bottom. In addition, fuel cell samples were fabricated and demonstrated reasonable performance with this fabrication method. The significance of this work is the demonstration of the feasibility of the bottom-heatassisted flash light irradiation method. This study also shows the possibility of extending the application of the method to possible substrates that are vulnerable to high flash light irradiation energy. CRediT authorship contribution statement Jun-Sik Park: Conceptualization, Investigation, Writing - original draft. Hojae Lee: Validation, Investigation, Writing - review & editing. Suhaeng Heo: Investigation. Young Beom Kim: Funding acquisition, Project administration, Validation. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Fig. 8. (A) An EIS Nyquist plot of a flash light-irradiated cell measured at 450 °C with different operating bias voltages. (B) EIS comparison of a flash light-irradiated cell and a thermally sintered cell.
Acknowledgments Y.B.K. gratefully acknowledges financial support from the Korea Institute of Energy Technology Evaluation and Planning (KETEP) from the Ministry of Trade, Industry & Energy, Republic of Korea (No. 201700000003242) and from the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (2012R1A6A1029029 and 2017R1D1A1A09000586).
thermally sintered and flash light-irradiated (with heater assistance) films with bias voltage of 0.5 V. Each cathode polarization resistance is plotted, and it is confirmed that the width of the semicircle of the thermally sintered sample is smaller than that of the irradiated film. This result corresponds well with the I–V curve of Fig. 7, which is explained in terms of the difference in porosity. As mentioned above, electrochemical reaction sites such as TPB have a direct correlation with cathodic interface impedance. The cathode polarization resistance seems to be influenced by the porosity of the cathode surface. In this study, we demonstrated that flash light irradiation energy is effectively decreased with a heater-assisted system by compensating total energy applied to deposited film. Therefore, the thermal shock issue could be resolved, and the decreased irradiation energy required for the sintering process could eventually relieve the severe strain present on the substrate. The results of this study are expected to increase the feasibility of fuel cell commercialization, with implications for fabricating other energy conversion and storage devices using ceramic thin films.
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