Effect of Bi2O3 as a Sintering Aid in the Anode for Electrolyte Supported SOFC
Article information
Abstract
Electrolyte-supported solid oxide fuel cells (Es-SOFCs) utilize dense electrolytes and require high sintering temperatures to enhance the interfacial properties between the electrolyte and anode. In this study, we propose the addition of a Bi2O3 sintering aid to the anode to reduce the sintering temperature of the Yttria-stabilized zirconia with 8 mol% (YSZ) electrolyte-based Es-SOFCs. The anodes of the cells consisted of NiO and GDC in a 6:4 weight ratio, with various quantities of Bi2O3. The electrochemical performance of the cells was methodically examined using current-voltage curves and constant current bias. Incorporating Bi2O3 into the anode enhanced its durability by reducing the degradation rate after 240 h of operation. Among these, the full cell with an anode containing 5 mol% Bi2O3 exhibits a better peak power density (PPD) of 0.49 W/cm2 and a PPD degradation rate of 17.6% at 800°C, compared to the cell without Bi2O3, which shows a PPD of 0.47 W/cm2 and a PPD degradation rate of 25.6%. Our results indicate that the addition of Bi2O3 as a sintering aid to an anode is an appropriate method for achieving reliable long-term stability of Es-SOFCs.
INTRODUCTION
Solid oxide fuel cells (SOFCs) are recognized for their environmental benefits and high efficiency in converting chemical energy into electricity using various fuels, such as hydrogen, methane, and biomass [1–4]. Thus, SOFCs are poised to take the central stage as a solution to environmental pollution and meet the demand for clean energy. Among the three primary types of SOFCs, electrolyte-supported SOFCs (Es-SOFCs) stand out for their robust mechanical strength, ease of handling, and the straightforward design of their metallic interconnects [5,6]. Yttria-stabilized zirconia with 8 mol% (YSZ) is one of the most widely used electrolyte materials for Es-SOFCs and has been widely investigated [7–9]. Es-SOFCs typically feature thick and dense electrolytes, which limit their applications owing to their low electrochemical performance and the need for extremely high sintering temperatures [10]. The anode is usually composed of electrolyte materials and NiO. High temperatures are generally required to achieve good adhesion properties on the surface of a dense electrolyte because typical ceramic electrolyte materials are refractory [11–14]. Thus, poor sintering activity of the anode remains a significant challenge for Es-SOFCs.
Sintering aids such as Fe2O3, CuO, ZnO, and Bi2O3 have been extensively investigated [15–18]. Utilizing these sintering aids in the cell fabrication process reduces the sintering temperature and time and can lower the costs of cell production while enhancing the performance and durability. However, sintering aids are typically used primarily to densify the electrolyte at relatively lower temperatures compared to the usual sintering temperatures of >1400°C [19,20]. Gadolinium-doped ceria (GDC) electrolyte materials are well-known as good anode components because of their high conductivity and mixed electron-ion conductivity under reducing conditions, along with their comparatively better sintering properties than YSZ [21,22]. Furthermore, several studies confirmed that the GDC electrolyte’s densification temperature can be effectively reduced to low temperatures of <1200°C with a sintering aid [23–25]. F. Li reported that the addition of Bi2O3 was shown to better mechanical, microstructure properties, and reduce sintering temperature [26]. Thus, we proposed that GDC-NiO anode with a sintering aid of Bi2O3, which has a low melting temperature of 817°C, could be a good composition for an anode to reduce the sintering temperature for Es-SOFC. The incorporation of sintering aids into the anode can produce a liquid phase owing to its low melting temperature, which bridges the particles and fills the voids around the grain boundaries, thereby significantly reducing the sintering temperature [27,28]. The low sintering process by the suggested new anode composition can effectively prevent delamination and warping of the cell during the sintering process of the anode of Es-SOFC, which is sintered at high temperatures in the fuel cell, and this effect plays a more critical role in the large-area cell manufacturing process.
In this study, we investigated the impact of applying Bi2O3 sintering aids that can sinter the GDC-NiO anode layer at significantly lower temperatures for electrolyte-supported SOFC. The full cell was prepared by screen printing using a dense YSZ electrolyte with 250 μm thickness. The final cells were fabricated as cells composed of multiple layers of LSCF-GDC | GYBC | YSZ | NiO-GDC-(Bi2O3), which serve various Bi2O3 loading amounts. The addition of Bi2O3 induces anode densification and effectively prevents performance degradation at an operating temperature of 800°C for 240 h. After stabilization for 60 h, the full cell containing 10 mol% Bi2O3 relative to GDC content effectively enhanced the durability with a degradation rate of 2%/kh under a constant bias of 150 mA/cm2. Our findings indicate that Bi2O3 sintered as an anode for Es-SOFCs is highly promising for the development of reliable and durable SOFCs.
EXPERIMENTAL
Cell fabrication
The electrolyte cells of YSZ (8 mol% Y2O3 stabilized ZrO2) were prepared by cutting into 20 mm diameter using water-jet from commercial dense YSZ square sheet (Kceracell) with a thickness of 250 μm. Both surfaces of the electrolyte cells were cleaned by ethyl alcohol. Each anode material maintained a consistent weight ratio of NiO (Kceracell, BET 8.8 m2/g) to GDC (Kceracell, 20 mol% Gd-doped ceria, BET 12.5 m2/g) of 6:4 wt%, whereas the Bi2O3 (Sigma Aldrich, 99.999%) loading was prepared as varied from 0–10 mol% as a sintering aid. The Bi2O3 loadings considered were at 0, 2.5, 5, 7.5, and 10 mol% relative to the GDC contained in the anode. The anode pastes were prepared by mixing the anode powder with binder paste (Kceracell) in a 7:3 weight ratio using a Resodyn Acustic Mixer (LabRAM II) at 50 G for 5 min. Both commercial slurry products of the GYBC (Gd0.135Y0.015Bi0.02Ce0.83O1.915, BET 15 m2/g) and LSCF-GDC (La0.6Sr0.4Co0.2Fe0.8O3-δ-Gd0.1Ce0.9O1.95, BET 7.5 m2/g) were purchased from Kceracell Co., Ltd, and used to buffer layer and cathode, respectively. GYBC, which needs a low sintering temperature of 1100°C, was considered for the buffer layer to minimize the sintering impact on the anode during the sintering process of the cathode side. Three pastes of anode, cathode, and buffer layers were prepared to obtain a uniform quality by mixing at 2200 rpm for 3 min, followed by defoaming at 2000 rpm for 2 min using Thinky Mixer (ARE-310).
All layers of SOFC cells are prepared by screen printing using each mask with 40, 5, and 40 μm depth and 8, 10, and 8 mm diameter for an anode, buffer layer, and cathode, respectively. The anode layer was coated on a clean surface of YSZ and then sintered at 1200°C for 2 h with a 2°C ramp rate of heating/cooling. The buffer layer was coated on YSZ, on the opposite side of the anode, following a dry process at 60°C for half an hour in a convection oven. The cathode layer was coated on top of the dried buffer layer and then co-sintered at 1100°C for 2 h with a 3°C ramp rate of heating/cooling. Finally, the current collectors were attached to each electrode using a homemade NiO sheet for an anode and an LSC (LaSr0.6Co0.4O3-δ) sheet for a cathode.
Characterization and electrochemical performance test
A single cell with LSCF-GDC | GYBC | YSZ | NiO-GDC-(Bi2O3) was electrochemically tested using the current-voltage (I-V) and constant bias methods using an electrochemical workstation. All cells were sealed onto single-cell measurement equipment (Nano Ionics) using sealants (Schott 311, Aremco 552) and heated to 800°C with appropriate heating steps as described by the sealant manufacturer. Air and H2 were used as the oxidant and fuel, respectively, during the operation, and both gas flow rates were 100 mL/min. The operating stability of fuel cells was carried out at a constant current density of 150 mA/cm2 at 800°C. I–V measurement was examined from open-circuit voltage to 0.4 V with 10 mA/cm2 intervals. The microstructures and morphologies of the anodes were examined using field-emission scanning electron microscopy (FE-SEM, Hitachi SU-8230).
RESULTS AND DISCUSSION
Effect of Bi2O3 on the thickness and microstructure of NiO-GDC anode
In this study, the samples were labelled B0, B2.5, B5, B7.5, and B10, corresponding to the Bi2O3 loading. For instance, B10 indicates that 10 mol% Bi2O3 was added relative to the GDC contained in the anode. Fig. 1a illustrates the relationship between the material weight loading and Bi2O3 addition to the anode across all samples. As the Bi2O3 loading increased, the weight proportions of NiO and GDC decreased. The anode thickness of all the cells was measured using a micrometer before and after sintering. The anode thickness of the B0 cell decreased from 25 μm to 18 μm after sintering at 1200°C for 2 h. Cells with added Bi2O3 had the same initial anode thickness as the B0 cell, but after sintering, they exhibited varied anode thicknesses of 17, 14, 10, and 10 μm for B2.5, B5, B7.5, and B10, respectively, as depicted in Fig. 1b. The use of Bi2O3 as a sintering aid produced dense anodes, suggesting that the significantly enhanced sintering activity could be attributed to the presence of Bi2O3. Since the resultant Bi2O3 has a low melting temperature of ~825°C, a liquid phase is probably formed on the surface of NiO-GDC mixture grains when sintered at 1200°C, which enables the shrinkage and sintering by the effect of liquid phase sintering. The decreasing anode thickness correlates with an increase in Bi2O3 loading; however, the thickness remains unchanged when the Bi2O3 loading increases from 7.5 mol% to 10 mol%, suggesting that the optimal Bi2O3 loading limit to densify is approximately 7.5 mol%. Increasing the Bi2O3 loading further could lead to the creation of more pores, which results in reduced density because of the volatile nature of Bi2O3 during the high-temperature sintering process [29,30].
Bi2O3 loading in anode: (a) material composition and (b) electrode thickness.
Fig. 2 presents cross-sectional SEM images of the NiO-GDC anode after the electrochemical measurements. The interface between the anode and electrolyte of Ni-GDC-(Bi2O3) layers can be clearly observed to be attached indicating the successful sintering process of the anode at 1200°C. The sintered Ni-GDC anode nanoparticles exhibited strong interconnectivity alongside the formation of a porous microstructure. The reduction in the anode’s thickness, despite the constant diameter of 8 mm before and after sintering, can be attributed to the presence of Bi2O3. Therefore, the densification of the anode is likely due to the Bi2O3 as sintering aid. SEM images revealed that the microstructure of the anode containing Bi2O3 was denser than that of the anode lacking Bi2O3; however, no significant changes were observed. This result indicates that the volatilization of occupied Bi2O3 particle inside anode layer generated pore during the sintering process. Furthermore, the low weight percentage of NiO-GDC in the anode due to the addition of Bi2O3 likely affects the relationship between porosity and thickness. The micromorphology of the anode without Bi2O3 appears to be a framework composed of softly rounded particles arranged in a three-dimensional distribution. However, for the anodes containing Bi2O3, the emergence of a sharp surface is contingent on the amount of Bi2O3 loaded, suggesting that Bi2O3, which melts during the sintering process, adequately coats the NiO-GDC surface and persists even after the reduction of NiO to Ni. This unique shape of particles can increase the pores and the electrode surface to improve the triple-phase boundary. The addition of Bi2O3 potentially improved the mechanical strength by forming bonds with the NiO-GDC matrix.
Cross-section SEM image of the coin cells included by Bi2O3 in GDC-NiO anode: (a) 0%, (b) 5%, (c) 10%, (d) 15%, (e) 20% versus GDC weight. The scale bar is 2μm.
Effect of Bi2O3 included in the anode on the performance and degradation of the full cell
Fig. 3 shows the electrochemical performance of LSCF-GDC | GYBC | YSZ | NiO-GDC-(Bi2O3) cells fueled with 3% humidified hydrogen at 800°C. The measured open circuit voltage (OCV) value of the cells was 1.1 V, regardless of the presence of Bi2O3. The OCV remained consistent after 240 h of operation under 150 mA/cm2 bias, indicating that no significant mechanical damage, such as cracking, occurred in any cell during the measurement period. The initial current-voltage (I–V) curves of the cells overlapped (Fig. 3a), indicating similar electrochemical activities among the cells. The initial peak power density (PPD) values of the cells are also comparable, with values of 0.476, 0.48, 0.491, 0.468, and 0.478 W/cm2 for B0, B2.5, B5, B7.5, and B10, respectively, at 800°C. However, a distinct deviation in the PPD was observed in the final I-V measurement after 240 h of operation under 150 mA/cm2 bias among the cells, which differed only in the anode, as shown in Fig. 3b. The final PPD of the cells were 0.354, 0.374, 0.404, 0.396, and 0.397 W/cm2 for B0, B2.5, B5, B7.5, and B10, respectively, after 240 h of operation. The change in the PPD caused by the anode indicates that it plays a crucial role in electrolyte-supported cells, in contrast to the minimal impedance impact of anode in the anode-supported cells. To examine the impact of Bi2O3 on the cell performance, the PPD of all cells, for both the initial and final I–V curves, is re-plotted in Fig. 3c. The initial PPD of the cells was almost constant regardless of the presence of Bi2O3 in the anode. These findings indicate that a suitable quantity of Bi2O3 does not significantly influence the electrochemical reactions at the triple-phase boundary within the anode, even though molten Bi2O3 may cover parts of the Ni-GDC surface. The largest PPD drop of 0.12 W/cm2 appears for the B0 cell and the gap can be reduced by increasing Bi2O3 loading up to at least 5 mol%. The B5, B7.5, and B10 cells exhibit a comparable PPD degradation rate of approximately 16.7%, which was lower compared to that of the B0 cell at 25.6% and the B2.5 cell at 21.9%.
The electrochemical performance of coin cells with various Bi2O3 loading in the anode: (a) I–V curve and power density at initial operation, (b) I–V curve and power density after 240 h of operation under 150 mA/cm2 bias, and (c) comparison of the peak power density.
Fig. 4a shows the changes in PPD over time. The difference in PPD between the cells widened with time, although the initial PPD values were comparable. The cells containing Bi2O3 demonstrated improved performance compared to those without Bi2O3. Within the range of B2.5–B10 cells, B5 cell exhibited the highest PPD throughout the measurement period. The order of PPD was B5 > B7.5 ≈ B10 > B2.5 > B0. The PPD degradation rate was re-plotted into four regimes to delineate the operating time for each 60 h period, as shown in Fig. 4b. The PPD degradation of all cells occurred critically, with an average rate of 200 %/kh during the initial regime of 0 to 60 h. This rate decreases to approximately 100 %/kh in the second regime and stabilizes at approximately 30 %/kh in the third and fourth regimes. This result indicates that the initial set point, following rapid degradation in the first regime, serves as a more reliable baseline for calculating the degradation rate during prolonged operation. The cells without Bi2O3 exhibited the highest PPD degradation rate in the initial regime, whereas the PPD degradation rates in the cells containing Bi2O3 were more evenly distributed across both regimes. Consequently, Bi2O3 effectively mitigated the anode-induced PPD degradation and enhanced cell longevity.
The degradation of peak power density: (a) the changes of peak power density over time, (b) degradation rate in the regime of 60-hour intervals.
Fig. 5a shows the long-term stability of the fuel cells equipped with the NiO-GDC-Bi2O3 anode layer. The measurement is conducted at a temperature of 800°C and a constant current density of 150 mA/cm2. All the cells exhibited a two-phase degradation profile: rapid voltage degradation over 60 h, followed by slower degradation for up to 240 h. These results suggest that a period of approximately 60 h is required for stabilization to ensure long-term operation of the electrolyte-supported cell. Fig. 5b shows the voltage degradation of all cells over 240 h, which was less than 4%. The degradation profile of the cells without Bi2O3 exhibited a significantly steeper slope than that of the cells with Bi2O3. However, both demonstrate a similar pattern: a substantial voltage drop during the first 60 h, followed by a gradual decline thereafter. In addition, the magnitude of the voltage drop appeared to decrease at higher Bi2O3 loadings. The voltage degradation rates of the cells were recalculated for an extended period of 1000 h, as listed in Table 1. Over a significant degradation period from the beginning to 60 h, the voltage degradation rate of cells without Bi2O3 was 46 %/kh, while the rate for cells with Bi2O3 was reduced by half to approximately 18–24 %/kh. After 60 h, the voltage degradation of B0, B2.5, B5, B7.5, and B10 significantly decreased to 3.2, 6.2, 5.3, 3.8, and 2 %/kh, respectively, over a measurement period of 60 to 240 h. Considering both voltage degradation rate and power density, thus, the heavy loading of Bi2O3 may be a viable strategy for enhancing the long-term operation of electrolyte-supported SOFCs.
The degradation of voltage under a current density of 150 mA/cm2 : (a) The profiles of operational cell voltages for the cell with various Bi2O3 loading, and (b) their rate of voltage degradation.
The voltage degradation rate of cells during the operational time
CONCLUSIONS
Electrolyte-supported SOFCs incorporating a Bi2O3-containing anode were successfully assembled using screen printing and sintered at 1200°C, a temperature lower than what is typically employed. The cell containing Bi2O3 exhibited enhanced durability and superior electrochemical performance throughout the operation. In the measured samples, the fuel cell featuring an anode with 5 mol% Bi2O3 reached the highest PPD of 0.49 W/cm2 and exhibited a better PPD degradation rate of 17.6% at 800°C, compared to the cell without Bi2O3 exhibiting PPD of 0.47 W/cm2 and PPD degradation rate of 25.6%. Furthermore, the degradation of the B10 cell under a constant bias of 150 mA/cm2 during the stabilized regime (60 to 240 h) exhibited the lowest degradation rate of 2 %/kh, which was 37% less than that of the B0 cell with 3.2 %/kh. As a result, the NiO-GDC anode, enhanced with Bi2O3 as a sintering aid is a promising option for improving the durability of Es-SOFCs.
Notes
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.
ACKNOWLEDGEMENTS
This work was supported by the Korea Electric Power Corporation(R22EA08).