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Article

BaCO3 Nanoparticles-Modified Composite Cathode with Improved Electrochemical Oxygen Reduction Kinetics for High-Performing Ceramic Fuel Cells

by
Halefom G. Desta
1,2,3,
Quan Yang
3,
Dong Tian
3,
Shiyue Zhu
3,
Xiaoyong Lu
3,
Kai Song
3,
Yang Yang
4,
Yonghong Chen
3,
Baihai Li
2 and
Bin Lin
1,2,3,*
1
School of Mechanical and Electrical Engineering, School of Materials and Energy, University of Electronic Science and Technology of China, Chengdu 611731, China
2
Yangtze Delta Region Institute (Huzhou), University of Electronic Science and Technology of China, Huzhou 313001, China
3
Anhui Key Laboratory of Low-Temperature Co-Fired Material, Huainan Engineering Research Center for Fuel Cells, Huainan Normal University, Huainan 232001, China
4
School of Materials Science and Physics, China University of Mining and Technology, Xuzhou 221116, China
*
Author to whom correspondence should be addressed.
Catalysts 2022, 12(9), 1046; https://doi.org/10.3390/catal12091046
Submission received: 21 August 2022 / Revised: 7 September 2022 / Accepted: 8 September 2022 / Published: 14 September 2022
(This article belongs to the Special Issue Featured Papers in Electrochemistry and Electrocatalysis in China)
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Abstract

:
The effects of the electrochemical oxygen reduction reaction (ORR) on the surface of single-phase perovskite cathodes are well understood, but its potential for use in a complex system consisting of different material types is unexplored. Herein, we report how BaCO3 nanoparticles-modified La0.6Sr0.4Co0.2Fe0.8O3-δ-Gd0.2Ce0.8O2-δ (LSCF–GDC)-composite cathodes improved the electrochemical oxygen reduction kinetics for high-performing ceramic fuel cells. Both X-ray diffraction (XRD) and thermogravimetric analysis (TGA) studies reveal that BaCO3 is stable, and that it does not show any solid-state reaction with LSCF–GDC at SOFCs’ required operating temperature. The electrochemical conductivity relaxation (ECR) study reveals that during the infiltration of BaCO3 nanoparticles into LSCF–GDC, the surface exchange kinetics (Kchem) are enhanced up to a factor of 26.73. The maximum power density of the NiO-YSZ anode-support cell is increased from 1.08 to 1.48 W/cm2 via surface modification at 750 °C. The modified cathode also shows an ultralow polarization resistance (Rp) of 0.027 Ω.cm2, which is ~4.4 times lower than that of the bare cathode (~0.12 Ω.cm2) at 750 °C. Such enhancement can be attributed to the accelerated oxygen surface exchange process, possibly through promoting the dissociation of oxygen molecules via the infiltration of BaCO3 nanoparticles. The density functional theory (DFT) illustrates the interaction mechanism between oxygen molecules and the BaCO3 surface.

1. Introduction

Solid oxide fuel cells (SOFCs) are among the cleanest and most highly efficient forms of chemical energy storage, which can convert a variety of fuels into electrical energy. But regardless of their many advantages, SOFCs only exhibit sufficiently high performance at high temperatures (>800 °C) [1,2,3,4]. Thus, the practical application of SOFC technology is hindered by its high operating temperature requirement, which induces various issues such as high manufacturing costs, materials degradation, and slow startup/shutdown cycles. Currently, researchers are mainly focused on mitigating these problems by reducing the SOFCs’ operating temperature to a temperature below 800 °C [5,6,7,8]. Unfortunately, reducing the operating temperature typically leads to sluggish oxygen reduction reaction (ORR) kinetics, thereby hindering the overall SOFC output performance [9,10,11,12]. Therefore, the availability of cathode materials with high ORR activity at reduced temperatures is critical for promoting the commercialization of SOFC technology.
Recently, perovskite-type oxides (La, Sr) (Co, Fe) O3 (LSCF) have been widely studied as the state-of-the-art cathode materials for the operation of SOFCs at reduced temperatures, due to their high electrical and satisfactory ionic conductivity properties. Considering the high oxygen-ion-conducting and adequate thermal expansion coefficient (TEC), the incorporation of rare-earth-doped ceria into cathode materials leads to further enhanced three-phase boundary (TPB) pathways for the ORR. Among the incorporated doped ceria, the La0.6Sr0.4Co0.2Fe0.8O3-δ-Gd0.2Ce0.8O2-δ (LSCF–GDC) composite cathode is one of the most extensively studied candidates for IT-SOFCs applications [7,13,14,15,16,17]. However, the surface oxygen exchange kinetics are still slow at reduced temperatures. In this regard, it is desirable to enhance the surface oxygen exchange kinetics at reduced temperatures.
Modifying the surface of cathode materials is an efficient technique for enhancing the electrochemical performance of SOFCs at reduced operating temperatures, without changing the pre-existing merits of the backbone materials [18]. Recently, wide efforts have been devoted to surface modification of cathode materials via solution-based infiltration for enhancing the performance of SOFCs [19,20,21,22]. Several suitable catalysts/materials have been investigated for surface modifications of LSCF|LSCF–GDC cathode materials to enhance the ORR activity, including mixed ionic-electronic conductors (MIECs) [23,24,25], noble metals [26,27], and transition metal oxides [28,29,30,31].
More recent studies have also shown that the catalytic activity of cathode materials was significantly improved by the infiltration of non-ionic/electronic conducting phases, such as alkali earth metal compounds, into the surface. Additionally, alkali earth metal compounds are low-cost and widely available [32,33,34]. For example, Lu Zhang et al. [35] reported a peak power density of 0.835 W/cm2 at 650 °C for a NiO-SDC anode-support single with a 2.47 wt.% CaO-modified LSCF-SDC cathode, which is much higher than that of the pristine cathode (0.622 W/cm2). Similarly, Tao Hong et al. [36] reported a peak power density of 0.73 W/cm2 at 700 °C on a surface-modified LSCF cathode with BaCO3. Moreover, our recent work also shows that the performance of NiO-YSZ-anode-supported cells based on the LSF-GDC composite cathode is greatly enhanced from 0.712 to 0.993 W/cm2 at 750 °C via the impregnation of BaCO3 nanoparticles [37]. However, to the best of our knowledge, the origin of the catalytic mechanisms of BaCO3 for ORR is still not well understood. In this context, this work deals with the infiltration of BaCO3 nanoparticles into the surface of the LSCF–GDC composite cathode to investigate the catalytic mechanism of BaCO3 for ORR at reduced temperatures.
Herein, we report on BaCO3 nanoparticles-modified La0.6Sr0.4Co0.2Fe0.8O3-δ-Gd0.2Ce0.8O2-δ (LSCF–GDC) composite cathodes with improved electrochemical oxygen reduction kinetics for high-performing ceramic fuel cells. BaCO3 nanoparticles were prepared via solution-based infiltration of barium acetate (Ba-(OAc)2) as a precursor into the LSCF–GDC surface and followed by heating at 750 °C for 2 h (Figure 1). Furthermore, using the first-principle method, the interaction mechanism between the oxygen molecules and the BaCO3 surface is clarified. As a result, the modified cathodes can significantly enhance the performance of single-cell and symmetrical cells at reduced temperatures.

2. Results and Discussion

Figure 2a shows the thermogravimetric analysis (TGA) of Ba-(OAc)2 conducted in the air from 25 to 750 °C. The thermal decomposition temperature of Ba-(OAc)2 starts at ~435 °C and ends up at ~520 °C. The total weight loss is ~22.58%, which can be associated with the evaporation of moisture (H2O) and carbon dioxide (CO2). The weight loss percentage is nearly close to the theoretical value of 22.75% for the Ba-(OAc)2 thermal decomposition [34,38]. No extra weight change is observed from 520 to 750 °C, demonstrating that Ba-(OAc)2 is completely decomposed at ~520 °C, and kept stable at this operating temperature.
Figure 2b displays the XRD pattern of as-prepared pure powders and the chemical compatibility of composite powders. Figure 2b(i) shows the XRD peaks of the GDC powder calcined at 700 °C and exhibiting cubic fluorite phase with space group Fm-3m. The single-phase LCSF was obtained after calcination at 1000 °C and exhibited an orthorhombic phase perovskite structure with space group R-3c, as displayed in Figure 2b(ii). Meanwhile, for the chemical compatibility study, LSCF–GDC was mixed at a 50:50, weight ratio, followed by co-heating at 1000 °C for 5 h. The XRD peaks of LSCF + GDC composite powders exhibited two main structure phases corresponding to GDC and LSCF, as shown in Figure 2b(iii). The results confirmed that LSCF is chemically compatible with GDC; since no additional peaks appearing between LSCF and GDC were detected. The XRD patterns of the Ba-(OAc)2, which was thermally heated at 750 °C, for 2 h to form an orthorhombic structured BaCO3 with space group Pnma (PDF card number: 41-0373), confirmed the successful formation of BaCO3, as displayed in Figure 2b(iv). Moreover, we also studied the chemical compatibility of composite powder BaCO3|(LSCF–GDC) (50:50 weight ratio) after being co-heated at 750 °C for 5 h in air. All diffraction peaks are associated with either of the starting powders BaCO3|(LSCF–GDC) without any impurities as presented in Figure 2b(v). These results demonstrate that Ba-(OAc)2 decomposes to BaCO3 in the presence of LSCF–GDC and shows good chemical compatibility between the BaCO3 and (LSCF–GDC) at the operating conduction.
The microstructure of the single cell based on the BaCO3 modified LSCF-GDC cathode is presented in Figure 3. The dense YSZ electrolyte, ~19 μm in thickness, is well-adhered to both the GDC buffer layer and the NiO-YSZ anode without any connected pores or cracks, as displayed in Figure 3a. The thicknesses of the buffer layer and anode are aapproximeee~2 and~40 μm, respectively. The microstructure of the targeted cathode adhered well to the buffer layer, thereby preventing direct contact between the LSCF–GDC and YSZ. As shown in Figure 3b, BaCO3 nanoparticles are homogeneously distributed on the surface of the cathode without changing the porosity of the pre-existing cathode. The presence of BaCO3 on the surface of the LSCF–GDC cathode is also analyzed using energy-dispersive spectroscopy (EDS), as shown in Figure 4. All elements, including La, Sr, Co, Fe, Ce, Gd, Ba, C, and O, are evenly distributed within the surface-modified LSCF–GDC cathode. Hence, Ba and C are incorporated on the surface of the cathode, confirming that the BaCO3 has been successfully incorporated into the surface of the LSCF–GDC cathode after infiltration of the Ba-(OAc)2 solutions.
Figure 5 shows the normalized conductivity relaxation transients of bare LSCF–GDC and 0.065 mg/cm2 BaCO3 infiltrated LSCF–GDC at 750 °C, upon the step-change atmosphere from P(O2) = 0.20 atm (O2 + N2) to P(O2) = 1.0 atm (O2), via controlling a mixture of O2 and N2. The change in ECR, indicates the change in the oxygen concentration of the sample, after shifting the partial P (O2). The elapsed time to reach re-equilibrium for the bare LSCF–GDC is about 9000 s, while for the modified LSCF–GDC, is reduced to 350 s. The decreased re-equilibrium time confirmed an enhanced surface oxygen exchange process since the oxygen bulk-diffusion properties of the sample could remain unchanged by the surface modification. The surface oxygen reduction kinetics (Kchem) were derived from the ECR curve and the data were fitted using MATLAB software. The derived Kchem value of the bare is 3.03 × 10−5 cm·s−1 at 750 °C, which is comparable with previously reported values of LSCF [32,33]. The Kchem value of the modified LSCF–GDC is 8.1 × 10−4 cm·s−1, which is a roughly 26.73-fold improvement. This result is consistent with the theoretical results which suggest that the infiltration of BaCO3 improves the dissociation of oxygen molecules, thereby accelerating the oxygen reduction kinetics (Kchem).
The polarization resistance (Rp) of the cathodes is studied using a symmetrical cell configuration, i.e., LSCF–GDC|GDC|YSZ|GDC|LSCF–GDC. An electrical equivalent circuit model, L-Ro-(RHQH)-(RLQL) was fitted with the experimental EIS data, where L, R, and Q, represent the inductance, resistance, and constant phase, respectively, and where the subscripts H and L correspond to the high- and low-frequency arc resistances, respectively. The intercept of the Nyquist plots on the real axis at high frequency corresponds to ohmic resistance, while the intercept at low frequency corresponds to the total resistance. The difference between the ohmic resistance and total resistance is equivalent to the polarization resistance (Rp) of the cathode. For a simple comparison of the cathode performance, the ohmic resistance was deducted from the spectra. The calculated Rp values of the modified cathode are 0.027, 0.037, 0.043, and 0.12 Ω.cm2 for 8.26, 5.64, 11 wt.% BaCO3 and bare LSCF–GDC at 750 °C, respectively, as shown in Figure 6a. The Rp value of the bare LSCF–GDC cathode reported in this study is comparable with the value reported in the literature [39,40]. As shown in Figure 6c, the modified cathode shows a significant decrease in Rp values over the temperature ranges of 600 to 750 °C, compared to the bare cathode. The significant decrease in Rp can be attributed to the infiltration of BaCO3 nanoparticles, which improves the dissociation of oxygen molecules. The lowest Rp values are obtained with moderate loading of 8.26 wt.% BaCO3 at different temperatures. For example, the Rp value with 8.26 wt.% BaCO3 loading decreases from 0.12 to 0.027 Ω.cm2 (~4.4 times reduced) at 750 °C, and from 0.27 to 0.09 Ω.cm2 (~3 times) at 700 °C. However, when the loading of BaCO3 nanoparticles is further increased to 11 wt.%, which leads to an increase in the Rp from 0.027 to 0.043 Ω·cm2, this can be associated with the blocking of the active pore of the cathode with BaCO3, thereby leading to a delay of the diffusion of oxygen to the active area. Figure 6d shows the temperature dependence of the Rp values for different BaCO3 loadings and the corresponding activation energy (Ea) calculated from the slope of the Arrhenius plots. The activation energy of the moderate loading of 8.26 wt.% BaCO3 is 1.23 eV, which is lower than the activation energy of the bare LSCF–GDC cathode (1.48 eV), demonstrating that the oxygen reduction process is effectively enhanced after being modified with BaCO3 nanoparticles.
To evaluate the performance of cathodes in SOFCs, a NiO-YSZ-based anode-supported cell was fabricated using a YSZ|GDC bilayer electrolyte, which prevents the formation of the insulating phase by blocking direct contact between LSCF–GDC and YSZ electrolytes. Figure 7 displays the typical I–V and I–P curves of single-cells measured between 600 °C and 750 °C, where wet H2 (~3% H2O) as fuel was provided to the anode side and the ambient air at the cathode side. The OCV values of both cells were between ~1.00–1.02 V, which is nearly close to the theoretical values obtained from the Nernst equation. The maximum power densities (MPD) achieved for BaCO3-modified LSCF–GDC cathode-based single cells are 1.48, 1.26, 0.94, and 0.61 W/cm2 at 750, 700, 650, and 600 °C, respectively, as shown in Figure 7a. However, the maximum power densities of bare LSCF–GDC cathode-based single cells are 1.08, 0.840, 0.6, and 0.4 W/cm2 at the corresponding temperature, as displayed in Figure 7b, which is comparable to the highest reported values of the NiO-YSZ-based anode-supported cell with a LSCF–GDC cathode [41,42]. As expected, the cell with the BaCO3 modified cathode exhibited superior performance compared to the bare cathode-based cell, as shown in Figure 7c. For example, the maximum power density of the cell based on the BaCO3 modified cathodes is 1.48 W/cm2 at 750 °C, whereas the maximum power density of the cell based on the bare cathode is, 1.08 W/cm2, implying a 37% increase in the maximum power densities. Since both cells have the same electrolytes and anodes, such a noticeable enhancement of cell performance at reduced temperatures is predominantly attributed to the improved cathode performance.
Table 1 shows a comparison of the performances of anode-support single cells of SOFCs with different cathode materials reported in the literature. The performances listed are compared using the improving factor to minimize the effect of different electrolytes and fabrication processes. The improving factor of LSCF–GDC modified with BaCO3 (56.6%) is lower compared to the reported values of LSCF–GDC modified with the active catalyst of La0.6Sr0.4CoO3-δ (85.7%). However, generally speaking, the improvement factor of the present work is higher than those of the majority of the materials mentioned in Table 1. This high improvement factor suggests that BaCO3 effectively enhanced the ORR at reduced temperatures for SOFCs.
Figure 8 shows the EIS plots of the single cells operated with wet H2 (~3% H2O) as fuel and flowing air as the oxidant. The polarization resistance (Rp) of a single cell mainly comes from the cathode and anode, but the Rp value of the anode is negligible when wet H2 (~3% H2O) is used as fuel [46,47]. Therefore, the Rp values of the single cell predominantly correspond to the cathode side. Figure 8b shows the Rp values of the bare cathode are 1.00, 0.44, 0.19, and 0.1 Ω·cm2 at 600, 650, 700, and 750 °C, respectively. In contrast, the Rp values of the 8.26 wt.% BaCO3 nanoparticles-modified cathode are 0.38, 0.137, 0.093, and 0.054 Ω·cm2 at 600, 650, 700, and 750 °C, respectively, as shown in Figure 8a. The corresponding values of ohmic resistance (Ro), polarization resistance (Rp), and total resistance (Rt) of the single cell-modified and bare cathodes as a function of temperature are shown in Figure 8c,d, respectively. The ohmic resistance is the same for both cells, indicating that the main difference in the Rt value is predominantly associated with the cathode performance. This significant decrease in Rp can principally be attributed to the infiltration of BaCO3 nanoparticles. The EIS result achieved with the single cell is slightly different from the symmetrical cell values. The slight difference may be associated with different working conditions of the single cell and symmetrical cells, and similar phenomena have also been observed by other researchers [48,49].
Recently, several efforts have been devoted to the understanding of surface engineering of electrode materials to enhance their performance [50,51]. The first-principles method based on density functional theory (DFT) is used to verify surface engineering via understanding the mechanism by which BaCO3 promotes ORR performance on SOFC electrode materials. Because the chemical formulas of La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF) and Gd0.2Ce0.8O2-δ (GDC) are complex, to simplify the calculation, LaFeO3 and CeO2 have been selected to represent LSCF and GDC, respectively. According to the XRD patterns shown in Figure 2, the LaFeO3 crystal face (110), the CeO2 crystal face (111), and the BaCO3 crystal face (111) were constructed as a substrate for oxygen molecule adsorption. The bond length of the free oxygen molecule (RO-O) is about 1.21 Å. In Figure 9a,b, when an oxygen molecule is placed on the surface of LaFeO3 and CeO2 for structural optimization, the RO-O values are 1.36 Å and 1.56 Å, respectively. Although here, the RO-O has increased, it still maintains the integrity of the oxygen molecule. Additionally, the distances between the O2 molecule and the LaFeO3/CeO2 substrates (d) are 3.04 Å and 2.48 Å, respectively. Interestingly, when oxygen is adsorbed on the BaCO3 crystal face (111), the RO-O increases to 4.05 Å, which is much larger than the 1.56 Å of LaFeO3 and 1.36 Å of CeO2, indicating that oxygen molecules dissociate spontaneously on the BaCO3 surface, as shown in Figure 9c. During this process, an oxygen molecule dissociates into two oxygen ions on the BaCO3 crystal face (111). Moreover, Bader Charge analysis showed that oxygen molecules adsorbed on the BaCO3 surface, 0.87 electrons transfer from the substrate to the oxygen ions during the adsorption process, which is larger than the 0.55 e and 0.54 e transferred from LaFeO3 and CeO2, respectively, suggests that a greater charge transfer contributes to the dissociation of oxygen molecules. As shown in Figure 9d, the charge density differences show that there is a yellow area around the oxygen, providing evidence that oxygen gets electrons from the BaCO3 substrate, which is consistent with the Bader Charge analysis data. Therefore, as shown in Table 2 the calculated result shows that the introduction of BaCO3 greatly promotes the dissociation of oxygen molecules, which induces the oxygen molecule to spontaneously dissociate into oxygen ions on its surface, thereby improving the ORR rate of the LSCF–GDC cathode material. This theoretical calculation explains why the introduction of BaCO3 would improve the performance of SOFC cathode materials.

3. Experimental Section

3.1. Sample Preparation

The La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF) and Gd0.2Ce0.8O2-δ (GDC) powders were synthesized using a modified Pechini method [52,53]. A Stoichiometric of La (NO3)3·6H2O, Sr (NO3)3, Co (NO3)3·6H2O, and Fe (NO3)3·9H2O were dissolved in distilled water. Subsequently, citric acid as a chelating agent was added at a 1.5:1 ratio of citric acid to total metal ions. Ammonia solution was added to adjust the pH value of the solutions to ~7.0. The precursor solution was heated in a hotplate oven until the self-combustion was completed. The GDC powder was also synthesized using a similar procedure to the LSCF powders. To synthesize the desired Gd0.2Ce0.8O2-δ (GDC), first, Gd2O3 was dissolved in the appropriate nitric acid (HNO3) and Ce(NO3)3·6H2O was dissolved in distilled water, followed by mixing both of the solutions. Finally, the resulting precursors were calcined at 700 and 1000 °C for 3 h to get the desired GDC and LSCF powders, respectively. The LSCF powder was homogeneously mixed with GDC powders (60:40 wt.%) in weight ratio, using a ball milling technique to prepare LSCF–GDC composite powders.

3.2. Cell Fabrication and Electrochemical Testing

For electrochemical impedance spectrum (EIS) studies, symmetrical cells with the configuration of LSCF–GDC|GDC buffer layer |YSZ|GDC buffer layer |LSCF–GDC were fabricated. The dense YSZ electrolyte was prepared via uniaxial pressing of the 8%, YSZ powder under 250 MPa and sintered at 1450 °C, for 10 h. The GDC and LSCF–GDC powders were homogeneously mixed with 5 wt.% and 10 wt.% ethyl cellulose in terpineol to prepare buffer layer and cathode slurries, respectively. The resulting buffer layer (GDC) slurries were coated onto the YSZ pellets and then, as-fabricated symmetric cells were sintered at 1300 °C, for 3 h. Finally, as-prepared cathode slurries were coated on the sintered GDC and co-sintered at 1000 °C, for 3 h.
The fuel cell performance was measured using an anode-supported cell of NiO-YSZ|YSZ|GDC|LSCF–GDC configuration. Anode mixtures containing NiO, YSZ, and starch (as pore former) were first mixed with a 6:4:2 weight ratio, respectively. The prepared anode precursors were pressed at 220 MPa and pre-fired at 1000 °C, for 3 h to obtain green pellets. The electrolyte (YSZ) slurry was also prepared using a similar process as the GDC buffer layer slurries as described earlier for the fabrication of symmetrical cells. The electrolyte slurries were spin-coated on the sintered anode pellet and co-sintered at 1400 °C, for 10 h. The GDC buffer layer slurries were coated onto the dense YSZ electrolyte and followed by sintering at 1300 °C. Finally, cathode slurries were coated on the GDC buffer layer and co-sintered at 1000 °C, to prepare a single with an area of cathode 0.2 cm2.
The surface of the prepared single and symmetrical cells was modified via BaCO3 nanoparticles using the solution infiltration process. An appropriate amount of Ba-(OAc)2 was dissolved in the mixed solvent distilled water and ethanol at the volume ratio of 1:1, to prepare a 0.3 M Ba-(OAc)2 precursor solution. As-prepared Ba-(OAc)2 solution was infiltrated into the porous LSCF–GDC electrode, then calcined at 750 °C for 2 h. The BaCO3 loading was calculated by determining the weight difference of the cathode before and after each infiltration cycle. The electrochemical measurements were examined using equipment from an IM6 electrochemical workstation (ZAHNER, Ursensollen, Germany). The EIS measurement was conducted over the frequency range of 0.01 Hz to 1 MHz with a signal amplitude of 5 mV. The performances of fuel cells were studied using an anode-supported single cell from 600 to 750 °C, where wet H2 (~3% H2O) as fuel was supplied to the anode side and the cathode side was exposed to the flowing air.

3.3. Characterization

Crystal structures of as-synthesized and chemical compatibility were examined by X-ray diffraction (XRD) with Cu-Ka radiation. The energy-dispersive X-ray spectroscopy (EDS) and field emission scanning electron microscope (FESEM, ZEISS Gemini300, Jena, Germany) were performed to examine the sample’s elemental composition and microstructure, respectively. Thermogravimetric analysis (TGA) was carried out from 25 to 750 °C in the air to determine the thermal decomposition temperature of Ba-(OAc)2. The electrochemical conductivity relaxation (ECR) was measured using a four-probe DC method, to analyze the chemical surface exchange (Kchem) of the sample. An appropriate weight of LSCF–GDC powders pressed was at 220 MPa, followed by sintering at 1200 °C for 5 h, to prepare a rectangular bar (~97%) (1.1 mm × 5 mm × 10 mm) with a density of about 97%. Furthermore, Ba-(OAc)2 was infiltrated onto surfaces of the sintered LSCF–GDC bar and then heated at 750 °C for 2 h. The ECR was measured when the partial pressure of oxygen (P (O2) changed from 0.2 to 1 atm, with a flow rate of 100 mL.min−1, until a new equilibrium was achieved.

3.4. Computational Methods

Vienna ab initio simulation package (VASP) [54,55,56] based on DFT [57,58] was used for the first-principles calculations. Generalized-gradient-approximation (GGA) [59] was selected as the exchange-correlation function, and projected-augmented-wave (PAW) [60] was used as the pseudopotential to describe the interactions between ion and valence electrons. The k-point grid division of the Brillouin area adopts the Monkhorst–Pack [61] method. The k-point mesh was selected as 3 × 3 × 1, and the cut-off energy was 400 eV. The convergence standard of the interaction force between atoms is 0.01 eV. Å−1 and the convergence standard of energy is 10−5 eV. A vacuum layer of 15 Å was set along the perpendicular direction above the atomic slab to have avoided the influence of periodic conditions. The VESTA [62] software was used to display the structure.

4. Conclusions

In summary, this work demonstrates that BaCO3 nanoparticles are successfully integrated into state-of-the-art LSCF–GDC cathode through the use of a solution-based infiltration of Ba-(OAc)2, which promotes the remarkably enhanced electrochemical performance of SOFCs. The modified cathode has a polarization resistance (Rp) as low as 0.027 Ω cm2 at 750 °C in a symmetrical cell and has a maximum power density of 1.48 W/cm2 in a NiO-YSZ anode-support cell. This improvement can be attributed to the enhanced dissociation of an oxygen molecule by infiltration of the BaCO3 nanoparticles, which accelerate the ORR kinetics values (Kchem). These explanations were confirmed by the DFT study, which shows that oxygen molecules dissociate spontaneously to form oxygen ions on the BaCO3 surface, thereby improving the ORR rate of the cathode material. These results demonstrate that BaCO3 nanoparticles have great potential to serve as an efficient solution for the surface modification of cathode materials for IT-SOFCs.

Author Contributions

Conceptualization, B.L. (Bin Lin), and H.G.D.; methodology, H.G.D., and Q.Y.; software, B.L. (Baihai Li) and Y.Y.; validation, B.L. (Baihai Li); formal analysis, H.G.D., S.Z. and K.S.; resources, D.T., X.L., Y.C. and B.L. (Bin Lin); data curation, H.G.D., and B.L. (Bin Lin); writing—original draft preparation, H.G.D.; writing—review and editing, B.L. (Bin Lin); visualization, H.G.D., and B.L. (Bin Lin); supervision, B.L. (Bin Lin) and D.T.; funding acquisition, B.L. (Bin Lin). All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Ministry of Science and Technology of China (2021YFE0100200), Pakistan Science Foundation (PSF) Project (PSF/CRP/18thProtocol (01)), the Fundamental Research Funds for the Central Universities of the University of Electronic Science and Technology of China (A03018023601020), the Foundation of Yangtze Delta Region Institute (Huzhou) of UESTC, China (U04210055), Research and Development Project of Anhui Province (201904a07020002), the Anhui Provincial Department of Education (gxbjZD2021074) and the Huainan Science and Technology Bureau (2018A374).

Data Availability Statement

Not applicable.

Conflicts of Interest

The authors declare no conflict of interest.

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Catalysts 12 01046 g001 550
Figure 1. Schematic figure of the Barium Acetate (Ba-(OAc)2) infiltration into the surface of LSCF–GDC composite cathode.
Figure 1. Schematic figure of the Barium Acetate (Ba-(OAc)2) infiltration into the surface of LSCF–GDC composite cathode.
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Catalysts 12 01046 g002 550
Figure 2. (a) TG curve for Ba-(OAc)2 measured in air, (b) XRD patterns of GDC, LSCF, and chemical compatibility composite powders.
Figure 2. (a) TG curve for Ba-(OAc)2 measured in air, (b) XRD patterns of GDC, LSCF, and chemical compatibility composite powders.
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Catalysts 12 01046 g003 550
Figure 3. SEM images of (a) anode-supported single-cell, and (b) BaCO3 nanoparticle infiltrated LSCF–GDC cathode.
Figure 3. SEM images of (a) anode-supported single-cell, and (b) BaCO3 nanoparticle infiltrated LSCF–GDC cathode.
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Catalysts 12 01046 g004 550
Figure 4. Energy-dispersive spectroscopy (EDS) elements mapping result of BaCO3 infiltrated LSCF–GDC.
Figure 4. Energy-dispersive spectroscopy (EDS) elements mapping result of BaCO3 infiltrated LSCF–GDC.
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Catalysts 12 01046 g005 550
Figure 5. The electrochemical conductivity relaxation (ECR) curves bare LSCF–GDC and 0.065 mg/cm2 BaCO3 modified LSCF–GDC after a sudden shift of the P (O2) from 0.2 to 1 atm at 750 °C.
Figure 5. The electrochemical conductivity relaxation (ECR) curves bare LSCF–GDC and 0.065 mg/cm2 BaCO3 modified LSCF–GDC after a sudden shift of the P (O2) from 0.2 to 1 atm at 750 °C.
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Catalysts 12 01046 g006 550
Figure 6. EIS spectra of the LSCF−GDC cathode with different loadings of BaCO3; (a) 750 °C, (b) 700 °C, (c) Rp of the cathodes from 600−750 °C, and (d) corresponding Arrhenius plots.
Figure 6. EIS spectra of the LSCF−GDC cathode with different loadings of BaCO3; (a) 750 °C, (b) 700 °C, (c) Rp of the cathodes from 600−750 °C, and (d) corresponding Arrhenius plots.
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Figure 7. Power density curves of NiO-YSZ supported cell with (a) LSCF–GDC cathodes modified with 8.26 wt.% BaCO3, (b) bare LSCF–GDC cathode, and (c) comparative maximum power density.
Figure 7. Power density curves of NiO-YSZ supported cell with (a) LSCF–GDC cathodes modified with 8.26 wt.% BaCO3, (b) bare LSCF–GDC cathode, and (c) comparative maximum power density.
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Figure 8. EIS plots of the single cell (a) BaCO3 nanoparticles modified LSCF–GDC, (b) bare cathode, and corresponding values of Ro, Rp, and RT of the impedance spectra of single cells with (c) modified cathodes, and (d) bare cathode.
Figure 8. EIS plots of the single cell (a) BaCO3 nanoparticles modified LSCF–GDC, (b) bare cathode, and corresponding values of Ro, Rp, and RT of the impedance spectra of single cells with (c) modified cathodes, and (d) bare cathode.
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Figure 9. The structure of oxygen molecules when stably adsorbed on (a) the LaFeO3 crystal face (110) and (b) the CeO2 crystal face (111). (c) The structure of the oxygen molecules dissociated on the BaCO3 crystal face (111). (d) Differential charge densities of oxygen molecule dissociation on the BaCO3 crystal face (111).
Figure 9. The structure of oxygen molecules when stably adsorbed on (a) the LaFeO3 crystal face (110) and (b) the CeO2 crystal face (111). (c) The structure of the oxygen molecules dissociated on the BaCO3 crystal face (111). (d) Differential charge densities of oxygen molecule dissociation on the BaCO3 crystal face (111).
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Table
Table 1. Comparative performance of anode-support single cells of SOFCs with different cathodes reported in the literature.
Table 1. Comparative performance of anode-support single cells of SOFCs with different cathodes reported in the literature.
Half CellCathode + Infiltrated MaterialMPD (W/cm2)
at 650 °C		Improving Factor (%)Ref.
NiO-YSZ|YSZ|GDCLSCF–GDC + BaCO3Bare: 0.6
Infiltrated: 0.94		56.6This work
NiO-SDC|SDCLSCF + CuO Bare: 0.54
Infiltrated: 0.72		25[31]
NiO-SDC|SDCLSCF-SDC + CaO Bare: 0.62
Infiltrated: 0.83		34.24[35]
NiO-SDC|SDCLSCF + CaO Bare: 0.6
Infiltrated: 0.80		33.3[35]
NiO-YSZ|YSZ|GDCLSCF + CeO2 (nanofiber)Bare: 0.75
Infiltrated: 0.981		30[43]
NiO-YSZ|YSZ|GDC(La0.6Sr0.4)0.95Co0.2Fe0.8O3-δ +Pr0.8Ce0.2O2-δBare: 0.75
Infiltrated: 1.07		42.47[44]
NiO-YSZ|YSZ|GDC(La0.6Sr0.4)0.95Co0.2Fe0.8O3-δ + PrO2-δBare: 0.751
Infiltrated: 1.108		47.53[44]
Ni-GDC|GDC(LSCF–GDC) + La0.6Sr0.4CoO3-δBare: 0.7
Infiltrated: 1.3		85.7[45]
Table
Table 2. The distance of the O2 molecule and substrates (d), the bond length of the O2 molecule (RO-O), and the charge of each O atom and O2 molecule.
Table 2. The distance of the O2 molecule and substrates (d), the bond length of the O2 molecule (RO-O), and the charge of each O atom and O2 molecule.
d (Å)RO-O (Å)O1 (e)O2 (e)Sum (e)
LaFeO33.041.56−0.08−0.47−0.55
CeO22.481.36−0.14−0.40−0.54
BaCO32.614.05−0.41−0.46−0.87
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Desta, H.G.; Yang, Q.; Tian, D.; Zhu, S.; Lu, X.; Song, K.; Yang, Y.; Chen, Y.; Li, B.; Lin, B. BaCO3 Nanoparticles-Modified Composite Cathode with Improved Electrochemical Oxygen Reduction Kinetics for High-Performing Ceramic Fuel Cells. Catalysts 2022, 12, 1046. https://doi.org/10.3390/catal12091046

AMA Style

Desta HG, Yang Q, Tian D, Zhu S, Lu X, Song K, Yang Y, Chen Y, Li B, Lin B. BaCO3 Nanoparticles-Modified Composite Cathode with Improved Electrochemical Oxygen Reduction Kinetics for High-Performing Ceramic Fuel Cells. Catalysts. 2022; 12(9):1046. https://doi.org/10.3390/catal12091046

Chicago/Turabian Style

Desta, Halefom G., Quan Yang, Dong Tian, Shiyue Zhu, Xiaoyong Lu, Kai Song, Yang Yang, Yonghong Chen, Baihai Li, and Bin Lin. 2022. "BaCO3 Nanoparticles-Modified Composite Cathode with Improved Electrochemical Oxygen Reduction Kinetics for High-Performing Ceramic Fuel Cells" Catalysts 12, no. 9: 1046. https://doi.org/10.3390/catal12091046

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