Calcium oxide as synergistic catalyst for oxygen reduction reaction on strontium-doped lanthanum ferrite for SOFC

doi:10.21203/rs.3.rs-3226045/v1

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Calcium oxide as synergistic catalyst for oxygen reduction reaction on strontium-doped lanthanum ferrite for SOFC

https://doi.org/10.21203/rs.3.rs-3226045/v1

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CaO is demonstrated as a synergistic electro-catalyst for oxygen reduction reaction on strontium-doped lanthanum ferrite (La_0.8Sr_0.2FeO_3-δ, LSF) cathodes for intermediate-temperature solid oxide fuel cells based on ytrria-stabilized zirconia (YSZ) electrolytes. With 3.01wt% CaO, the interfacial polarization resistance at 750°C is reduced from 0.83 to 0.42 Ω cm² for LSF-SDC (samaria doped ceria) composite electrodes on the YSZ electrolytes. With 3.2wt% CaO, the peak power density at 800°C is increased from 0.45 to 0.58 Wcm^-2 for anode supported single cells consisting of Ni-YSZ anodes, YSZ electrolytes and LSF-SDC cathodes. The CaO particles can greatly enhance the kinetics of oxygen reduction reaction on LSF such as improving the oxygen surface exchange coefficient at 750°C from 1.9×10^− 5 to 54×10^− 5 cm s^-1 as demonstrated by the electrochemical conductivity relaxation measurement. Specifically, CaO particles infiltrated to the LSF electrodes reduce the interfacial polarization resistance associated with the low-frequency response, which is often attributed to the surface reaction steps of the cathode reaction.

Solid oxide fuel cell

Cathode

Calcium oxide

Oxygen reduction reaction

Strontium-doped lanthanum ferrite

Solid oxide fuel cells (SOFCs) are high efficient electrochemical energy conversion devices and are currently extensively studied to meet global challenges for a sustainable supply of clean and economical energy [1, 2]. It is well known to the SOFC community that losses associated with the electrode processes can be minimized through engineering and optimization of the electrode microstructures [3, 4]. The nano-scale and nano-structured electrodes achieved by the impregnation/infiltration method attract increasing attention due to their high electrochemical activities resulted from the enlarged surface area for catalytic reaction and/or enriched three-phase boundaries (TPBs), where the electrode catalyst, electrolyte and gas phases meet [5–7]. In addition to the improved electrochemical performance, the infiltrated cathodes have advantages such as lower fabricating temperatures and higher mechanical stability upon thermal cycling [3, 8, 9]. Infiltrating oxygen ion conductors such as gadolinia doped ceria (GDC) and yttria-stabilized bismuth oxide (YSB) can promote the ionic conductivity of the electrode, thus enlarging the TPBs length and consequently reducing the interfacial polarization resistance. For example, infiltrating YSB nanoparticles can reduce the interfacial polarization resistance from 85.2 to 1.08 Ω cm² at 600°C for La_0.8Sr_0.2MnO_3−δ (LSM) cathode [10]. When a La_0.6Sr_0.4Co_0.2Fe_0.8O_3−δ (LSCF) electrode is infiltrated with 1.5 mg cm^− 2 GDC, the cathode interfacial resistance reduction is also significant, from 0.22 to 0.06 Ω cm² at 750°C [11]. Infiltrating catalyst nanoparticles such as Co₃O₄ can also reduce the resistance by improving the catalytic activity for the electrochemical reactions. When a LSM-YSZ (yttria-stabilized zirconia) electrode is infiltrated with Co₃O₄ nanoparticles, the interfacial resistance is reduced from 2.4 to 1.6 Ω cm² at 700°C [12]. BaCO₃ as a non-electric/ionic conductor has also been reported to have synergistic catalytic activity in SOFC cathode oxygen reduction reaction [13–15]. The catalytic activity and conductivity have been assumed to the major factors for the performance promotion [16–18]. However many researchers have pointed out that the catalytic activity and conductivity do not explain the electrode promotion for the different types of infiltrated catalytic nanoparticles [19, 12].

CaO is an oxide often encountered in the SOFC technology. It is used in the electrolyte to stabilize the ZrO₂ matrix [20], in electrode materials such as CaO-doped LaMnO₃ [21], and in interconnector materials such as CaO-doped LaCrO₃ [22]. In addition, CaO is a common bulk contaminant which segregates to the external surface at the fabricating and operating temperatures. Furthermore, CaO appears to be the most potent dopant for suppressing grain growth [23].

CaO itself has not been reported as a catalyst for the electrochemical reactions. Its conductivity is negligible compared with the typical electrode materials. It is interesting to note that several reports have demonstrated that CaO has some effects on the electrode performance. Wagner et al. [24] have shown that CaO particles can increase the oxygen surface exchange kinetics of SrTiO₃ by nearly one order of magnitude. Ridder et al. [25] have suggested that the presence of CaO on YSZ surface decreases the oxygen surface exchange kinetics by reducing the area available for oxygen exchange reaction. Gorte et al. [19] have demonstrated that infiltrating CaO to La_0.8Sr_0.2FeO_3−δ (LSF)–YSZ electrodes can reduce the interfacial polarization resistance, from 2.60 to 0.70 Ω cm² at 700°C when the LSF-YSZ electrode is fabricated at 1100°C. They have suggested that the reduced resistance is related to structural changes rather than the improved catalytic properties or conductivity [26].

In this study the impact of CaO infiltrated LSF is systematically investigated. The infiltrated CaO particles can substantially increase the oxygen chemical surface exchange coefficient, indicating that CaO has significant synergistic effect on the oxygen reduction reaction. The effect is further demonstrated with reduced resistance for the low-frequency arcs of the impedance spectra measured with symmetrical cells and increased cell output power density with anode supported cells consisting of Ni-YSZ anodes, YSZ electrolytes and LSF-SDC cathodes.

2.1 Powder preparation

La_0.8Sr_0.2FeO_3−δ (LSF) powder was made using the glycine-nitrate method [27] with raw materials of La(NO₃)₃, Sr(NO₃)₂ and Fe(NO₃)₃ (Sinopharm Chemical Reagent Co. Ltd). YSZ (Y_0.15Zr_0.85O_1.925), SDC (Sm_0.2Ce_0.8O_1.9) and NiO were also prepared with the glycine-nitrate process and finally calcined at 850°C for 4 h [28, 29]. To test the chemical compatibility, LSF powders were mixed with Ca(NO₃)₂ at the weight ratio of 1:1 and sintered at 800°C in air for 10 h.

2.2 Cell fabrications

Symmetrical cells, which consisted of dense YSZ electrolyte substrates and porous LSF or LSF-SDC electrodes impregnated with CaO particles, were fabricated for the electrochemical tests. LSF and LSF-SDC (weight ratio 6:4) slurries were prepared by mixing LSF and LSF-SDC powders with an organic binder (α-terpineol as solvent and ethyl cellulose as the binder). Ca(NO₃)₂ were dissolved in a mixture of distilled water and ethanol (volume ratio of 2:1) at a concentration of 0.3 mol L^− 1 to prepare Ca(NO₃)₂ solution. The infiltration was carried out by placing a drop of the solution on the top of the porous LSF or LSF-SDC frame, letting the solution soak into the porous structure, drying, and heating at 800°C in air for 1 h to form CaO particles.

Dense bar samples were prepared for the electrical conductivity relaxation (ECR) measurement. The LSF powders were ground, pressed into a rectangular bar under 300 MPa, and sintered at 1450°C for 5 h in air to form dense LSF bars. The sintered bars had dimensions of about 40.00×5.42×0.90 mm³. Their density was 97% of the theoretical density as determined using the Archimedes method. To fabricate anode-supported button cells, a NiO-YSZ anode with YSZ electrolyte layer (~ 13µm) was fabricated using suspension coating [30] followed by sintered at 1400°C for 5 h. Cathodes were fabricated onto the electrolytes with the same process as that for the symmetrical cells.

2.3 Characterizations

The phase structure was characterized by X-ray diffraction (Rigaku TTR-III) analysis using Cu K radiation (D/Max-gA, 2kV) under the speed of 3 degrees per minute. The morphology was observed using a scanning electron microscope (SEM, FEI XL30) and transition electron microscope (TEM, JEOL JEM-2010). Electrochemical measurements were carried out with a Zahner Im6e electrochemical workstation under open circuit conditions. The impedance of a symmetric cell was measured in ambient air, and the frequency typically ranged from 10^− 1 to 10⁶ Hz with amplitude of 10 mV. AC impedance plots were fitted using Zview software according to an equivalent circuit with a standard deviation below 5%. The bar conductivity was measured by a standard, four-probe method using a digital multimeter (Keithley 2001) with the atmosphere changed from Po₂ = 0.01 bar (O₂ + N₂) to Po₂ = 0.1 bar (O₂ + N₂) as described previously [31].

3.1 Chemical compatibility

Figure 1a shows the XRD pattern of the perovskite structured LSF, which is obtained by heating the combustion derived powder at 700 ºC. Figure 1b is the pattern for the powder prepared by heating Ca(NO₃)₂ at 800 ºC for 2 h. It is cubic structured CaO with space group Fm-3m(225), indicating that Ca(NO₃)₂ decomposes to CaO at 800 ºC. Figure 1c is the pattern for a LSF-CaO composite, which is obtained by co-heating the LSF-Ca(NO₃)₂ mixture at 800 ºC for 10 h. All the peaks can be attributed to either LSF or CaO, indicating that the decomposition of Ca(NO₃)₂ to CaO is not affected by LSF. No extra peaks or notable peak shits are observed, indicating the apparent absence of solid-state reaction between CaO and LSF at 800 ºC, i.e. CaO is chemically compatible with LSF under the operating conditions of intermediate-temperature SOFCs.

3.2 Oxygen surface exchange kinetics enhanced with CaO

Figure 2a shows the surface image of a bare LSF bar. The bar is completely dense and the relative density is 97%, fully meeting the requirement of ECR test [32]. The average grain size is about 1.6 µm. Figure 2b shows the surface modified with 0.3 mgcm^-2 CaO particles. The microstructure is characterized by partly covered CaO particles, which are much smaller than the LSF grains. After several times infiltrating treatment, the surface is covered with 0.45 and 0.7 mgcm-2 CaO particles as shown in Fig.c-d respectively. It could be seen that the CaO particle size is still about 200–300 nm, but particles are connected with each other due to higher loading weight.

Figure 3a shows the normalized conductivity at 750 ºC as a function of the elapsed time. The equilibrium time is about 9500 s for the bare bar, while it decreases to 4000 s, 2000 s and 1500 s, when the surface is coated with 0.3, 0.45 and 0.7 mgcm^-2 CaO particles. The relaxation process is performed by increasing the oxygen partial pressure, which ensures oxygen incorporation, i.e. oxygen reduction reaction. In this reaction, after oxygen is incorporated into the first crystal layer through the surface steps, further transport occurs via chemical diffusion. The decrease in the equilibrium time should be attributed to the acceleration of the surface reaction because surface modification doesn’t change the bulk diffusion properties.

The ionic transport properties within LSF bulk should not be affected by the surface properties, i.e, the deposited CaO on LSF surface would not be expected to change D_chem. Therefore, the change in relaxation time with different CaO loading seems to be due to the variation on surface exchange rate of CaO coated LSF since the gas phase conditions are kept the same. Low relaxation time means high surface exchange rate and fitting the relaxation profile can obtain the surface reaction kinetics, i.e. the oxygen chemical exchange coefficient K_chem values [33]. Figure 3b compares the oxygen surface exchange coefficients. The CaO particles can increase the K_chem value by a factor of about 10. For example, at 750°C, K_chem is 1.9×10^− 5 cm s^-1 for the bare LSF while it increases to 17×10^− 5 cm s^-1 when 0.3 mgcm^-2 CaO particles are deposited. It further increases to 23×10^− 5 cm s^-1 with 0.45 mgcm^-2, and 54×10^− 5 cm s^-1 with 0.7 mgcm^-2 CaO particles.

The considerable increase in K_chem value demonstrates that CaO is an excellent synergistic catalyst for the oxygen reduction reaction on LSF. The kinetics of the oxygen exchange reaction may be coupled to molecular diffusion and surface transport processes, as well as to bulk ionic and electronic transport [17]. In our experiment, the driving force condition is kept the same and the bulk transport properties are identical. Therefore, the variation in oxygen surface exchange kinetics for LSF is likely to be due to the surface conditions. Comparing to LSF with no CaO coating, coating CaO to LSF increases the surface exchange rate. The increase in surface exchange rate for LSF coated with CaO is unlikely to be attributed to the surface exchange reaction on the CaO surface. On the other hand, the increase in surface exchange rate for LSF coated with CaO can neither be attributed to the LSF surface since the effective surface area exposed to the gas phase should have been reduced when CaO is deposited on LSF surface. Therefore, the increase in surface exchange rate for LSF coated with CaO may be originated from the reaction occurring at the LSF/CaO interface where gas is available.

3.3 LSF electrode infiltrated with CaO particle

Figure 4a shows the cross-sectional micro-view of the LSF/YSZ interface. The bare LSF electrode is a porous structure consisting of particles about 1 µm in size, Fig. 4b. After the infiltration process is conducted, much smaller CaO particles are clearly seen to be deposited on the large LSF particles, Fig. 4c. The average size of the fine CaO particle is about 100 nm, which is further confirmed with TEM analysis (Fig. 4d). The microstructure analysis also indicates strong bonding between the CaO particles and LSF grains.

Figure 5a-d present the impedance spectra for the symmetrical cells. Generally, the total spectrum size is increased with infiltration of the CaO particles. But the impedance spectrum shape does not change, implying that CaO does not change the oxygen reduction process. An equivalent circuit consisting of two arcs is used to fit the spectrum, Fig. 6e. In the circuit, L is the inductance arising from the apparatus, R_b is attributed to the resistance of the electrolyte and the lead wires, and (RCPE) in series represents the arcs present at different frequency ranges, where R is the corresponding resistance and CPE is the corresponding constant phase element for each arc. The expression for CPE is CPE = Y₀(jw)ⁿ, where Y₀ is the admittance, w is the angular frequency, and n is a frequency exponent (0 < n < 1). The subscripts H and L refer to the high ((RCPE)_H) and low ((RCPE)_L) frequency arcs. As shown in Fig. 5a-d, L and R_b are plotted from 0 in order to clearly compare the interfacial polarization performance. The fitting lines match well with the experimental data.

Figure 6a compares the area specific interfacial polarization resistances (ASRs) for the symmetrical cells. It could be found that infiltrating CaO does not reduce the resistance. On the contrary, the resistance is increased by the presence of CaO particles. For example, ASR at 750 ºC is 2.11 Ω cm² for the bare LSF electrode. It increases to 2.93 Ω cm² when 1.3wt% CaO is infiltrated and further to 6.25 Ω cm² with the CaO loading of 3.4wt%. In addition, the activation energy is slightly increased. The reduced electrode performance seems to contradict with the enhanced oxygen surface exchange kinetics as determined from the ECR measurement.

The effect of CaO loading on the low frequency resistance, R_L, is illustrated in Fig. 6b. R_L at 750°C is 1.51 Ω cm² for the bare LSF electrode. It is reduced to 0.64 and 1.16 Ω cm² with 1.3wt% and 3.4wt% CaO, respectively. The low frequency arc is generally attributed to the oxygen dissociative adsorption process, i.e. the electrode surface catalytic reaction [34, 35]. The reduced R_L demonstrates enhanced surface reaction kinetics, which is consistent with the increased K_chem.

Different with the low frequency response, the high frequency behavior is characterized by an increase in the resistance, Fig. 6c. For example, R_H at 750°C is 0.55 Ω cm² for the bare LSF. After infiltrated with 1.3wt% CaO, it increases to 1.89 Ω cm² and further to 5.76 Ω cm² with 3.4 wt% CaO. The high frequency response usually corresponds to oxygen ion transfer process [36, 37], which is the incorporation of O^2- from the electrode into the electrolyte. The oxygen ions could be transferred along surfaces or inside the bulk of the electrode material LSF to the electrolyte YSZ, where they are fully and formally incorporated as electrolytic O^2- [4, 38]. Because the CaO particles are deposited onto the LSF surface, it is very likely that the transport pathway along the LSF surface is blocked by these particles. Another evidence for the reduced oxygen ion incorporation rate can be found from the equivalent capacitance of (R_HCPE_H). Smaller capacitance means less transfer paths for the oxygen ion transportation [39]. The CPE_H value at 750°C is about 8.12×10^− 4 µF cm^-2 for the bare LSF. It decreases to 6.89×10^− 4 Fcm^-2 when 1.3 wt% CaO is impregnated, and further to 6.55×10^− 4 µFcm^-2 for 3.4wt% CaO.

3.4 LSF-SDC composite electrode infiltrated with CaO particle

Figure 7 shows ASRs for the LSF-SDC composite electrodes infiltrated with various amount of CaO particles. Comparing with the LSF electrode, the performance is significantly improved by adding 40 wt.% SDC. For example, the resistance at 750°C is 0.83 Ω cm² for the bare LSF-SDC composite, while it is 2.11 Ω cm² for the bare LSF electrode. The improved performance is usually attributed to SDC's high ionic conductivity, which can facilitate the oxygen ion transport kinetics in the electrodes as well as extend the reaction sites by enlarging the TPB length. The infiltration effect on the LSF-SDC composite electrodes is quite different with the LSF electrodes; infiltrating CaO leads to a significant decrease in ASR value in the testing temperature range. When 2.08wt% CaO is infiltrated, at 750°C, it decreases to 0.48 Ω cm². The lowest resistance is 0.42 Ω cm² at the loading of 3.01wt%.

Figure 8a gives the high-frequency resistance for the LSF-SDC composite electrodes. R_H is derived from the impedance spectra analysis using the equivalent circuit as shown in Fig. 5e. R_H is reduced by infiltrating CaO. At 750°C, it decreases from 0.58 Ω cm² to the lowest 0.29 Ω cm² with 3.01wt% CaO. Since the fine CaO particles are deposited on surface of larger LSF and SDC particles, the bulk oxygen ion transport properties should not be affected while the surface oxygen ion transfer kinetics could be changed. As demonstrated in Fig. 6, CaO particles have negative effects on the oxygen ion transfer via the LSF surface. Therefore, the reduced R_H, which corresponds to enhanced oxygen ion transfer kinetics, is possibly caused by enhanced oxygen ion transfer via the SDC surface. Figure 8b shows the CaO effect on the low-frequency resistance. Similar to the LSF electrodes, the LSF-SDC composite electrode with CaO has an impressive decrease; from 0.35 to 0.13 Ω cm² at 750°C with 3.01 wt% CaO. The reduced R_L is caused by the enlarged K_chem. Gorte et al. have shown that infiltrating CaO into YSZ-LSF composite could improve the electrode performance. They have attributed the improved performance to structural change as caused by the infiltration process. In this work, the microstructures are not changed by infiltrating CaO, Fig. 9. Consequently, the improved performance should be attributed to the synergetic effect on the electrode reaction rather than the microstructure change.

3.5 Performance of anode-supported single cell

Figure 10a shows the cell voltage and power density at 800°C as a function of current density for single cells consisting of Ni-YSZ anodes, YSZ electrolytes and LSF-SDC cathodes. The peak power density for the cell with the bare LSF-SDC cathode is 0.45 Wcm^− 2. It increases to 0.53 Wcm^− 2 and 0.58 Wcm^− 2 when the cathode is infiltrated with 2.1wt% and 3.2wt% CaO, respectively. The improved power density must be caused by the CaO particles since these cells have the same anodes and electrolytes. The impedance spectra measured under open-circuit conditions are also shown in Fig. 10a. These cells have almost the same electrolyte resistances, about 0.24 Ω cm². The interfacial polarization resistance is decreased by the CaO particles. The resistance is 0.51 Ω cm² for the single cell with bare LSF-SDC cathode. It decreases to 0.34 and 0.25 Ω cm² when 2.1wt% and 3.2wt% CaO is infiltrated, respectively. Shown in Fig. 10b is the performance at 700–800°C for the single cell with 3.2wt% CaO infiltrated cathode. The peak power densities at 700°C, 750°C and 800°C are 0.18 Wcm^− 2, 0.34 Wcm^− 2 and 0.58 Wcm^− 2, respectively. It should be noted that the cell performance could be further increased by using doped ceria interlayer between the LSF cathode and YSZ electrolyte as demonstrated by Simner et al. [27].

CaO, which does not have any catalytic activities, is demonstrated as an excellent synergistic catalyst for oxygen reduction reaction on LSF electrocatalyst. The synergistic effect is clearly shown with the increase in oxygen surface exchange coefficient by a factor up to 10. It is further shown by the reduction in the interfacial polarization resistance associated with the low-frequency response. However, negative effect is observed for the high-frequency response of the LSF electrode, which is possibly caused by increased resistance for ionic transfer through the surface path. The negative effect could be completely removed by applying composite electrode. As a result, LSF-SDC electrodes infiltrated with CaO particles have shown improved electrochemical performance, i.e. reduced interfacial polarization resistance and improved cell power density.

Ethical Approval

No experiments about human or animals were conducted.

Competing interests

The authors declare that the authors have no competing interests as defined by Springer, or other interests that might be perceived to influence the results and/or discussion reported in this paper.

Authors' contributions

Materials preparation and data collection: Wenming Xu, Chengyu Li and Xiang Pan. Original draft preparation: Wenming Xu. Review and editing: Tao Hong and Yingwei Lu. All authors read and approved the manuscript.

Funding

We acknowledge for the support by National Natural Science Foundation of China (51802065), and China Postdoctoral Science Foundation (2018M630702).

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Springer Nature or its licensor (e.g. a society or other partner) holds exclusive rights to this article under a publishing agreement with the author(s) or other rightsholder(s); author self-archiving of the accepted manuscript version of this article is solely governed by the terms of such publishing agreement and applicable law.

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Calcium oxide as synergistic catalyst for oxygen reduction reaction on strontium-doped lanthanum ferrite for SOFC

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Abstract

Figures

1 Introduction

2 Experimental

2.1 Powder preparation

2.2 Cell fabrications

2.3 Characterizations

3 Results and discussing

3.1 Chemical compatibility

3.2 Oxygen surface exchange kinetics enhanced with CaO

3.3 LSF electrode infiltrated with CaO particle

3.4 LSF-SDC composite electrode infiltrated with CaO particle

3.5 Performance of anode-supported single cell

4 Conclusions

Declarations

References

Additional Declarations

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Version 1