Abstract
The present invention relates to a composite of a cobalt-based perovskite material with a negative thermal expansion material, and a preparation method of the same, and a solid oxide fuel cell (SOFC) comprising the same, and belongs to the technical field of fuel cells. In the present invention, a negative thermal expansion material is introduced into a cobalt-based perovskite oxide to successfully prepare an SOFC cathode material with excellent electrochemical performance and low thermal expansivity. The composite electrode achieves prominent mechanical tolerance in SOFC, which can moderate a volume change during the whole calcination process and enable a smooth transition to a high-temperature stage. The composite electrode has a thermal expansion coefficient (TEC) only of 12.9×10.sup.−6 K.sup.−1, which is perfectly matched with that of an SDC electrolyte. In addition, the composite shows excellent oxygen reduction reaction (ORR) activity, high TEC, and extremely-excellent anti-CO.sub.2 poisoning performance.
Claims
1. A preparation method of a composite of a cobalt-based perovskite material with a negative thermal expansion material, wherein the composite of the cobalt-based perovskite material with the negative thermal expansion material is obtained by combining the negative thermal expansion material with the cobalt-based perovskite material; the negative thermal expansion material is Y.sub.2W.sub.3O.sub.12, and the cobalt-based perovskite material is SrNb.sub.0.1Co.sub.0.9O.sub.3−δ; and the preparation method comprises the following steps: mixing the negative thermal expansion material and the cobalt-based perovskite material to obtain a precursor material, and subjecting the precursor material to calcination; wherein a content of the negative thermal expansion material in the precursor material is 5 wt % to 40 wt %; and the calcination is conducted at 600° C. to 1,000° C. for 1 h to 6 h.
2. The preparation method of the composite of the cobalt-based perovskite material with the negative thermal expansion material according to claim 1, wherein the content of the negative thermal expansion material in the precursor material is 10 wt % to 20 wt %.
3. The preparation method of the composite of the cobalt-based perovskite material with the negative thermal expansion material according to claim 1, wherein the calcination is conducted at 650° C. to 800° C. for 2 h.
4. The preparation method of the composite of the cobalt-based perovskite material with the negative thermal expansion material according to claim 1, wherein the SrNb.sub.0.1Co.sub.0.9O.sub.3−δ is prepared by taking Sr(NO.sub.3).sub.2, C.sub.10H.sub.5NbO.sub.20, and Co(NO.sub.3).sub.2.6H.sub.2O according to a predetermined molecular stoichiometric ratio to prepare a solid precursor by a citrate-ethylenediamine tetraacetic acid complexing method, and subjecting the solid precursor to calcination.
5. The preparation method of the composite of the cobalt-based perovskite material with the negative thermal expansion material according to claim 1, wherein the Y.sub.2W.sub.3O.sub.12 is prepared by taking Y.sub.2O.sub.3 powder and WO.sub.3 powder as raw materials according to a predetermined stoichiometric ratio, mixing the raw materials to obtain a resulting mixture, and subjecting the resulting mixture to ball-milling and calcination.
6. A composite of a cobalt-based perovskite material with a negative thermal expansion material which is directly prepared by the preparation method according to claim 1.
7. The composite of the cobalt-based perovskite material with the negative thermal expansion material according to claim 6, used to manufacture a solid oxide fuel cell.
8. A fuel cell, comprising the composite of the cobalt-based perovskite material with the negative thermal expansion material according to claim 6.
Description
BRIEF DESCRIPTION OF DRAWINGS
[0028] FIG. 1 shows X-ray diffractometry (XRD) patterns of SNC+ZWOx (x=10 wt %, 20 wt %, and 40 wt %)/SNC+YWOx (x=10 wt %, 20 wt %, and 40 wt %) calcined at 800° C. for 2 h.
[0029] FIG. 2 shows XRD patterns of SNC+ZWO20/SNC+YWO20 samples calcined at 400° C. to 1,200° C. for 2 h.
[0030] FIG. 3 shows scanning electron microscopy (SEM) images of surfaces of SNC+ZWOx (x=0 wt %, 10 wt %, 20 wt %, and 40 wt %) powders calcined in air at 800° C. for 2 h, where (a and b) x=0, (c and d) x=10, (e and f) x=20, and (g and h) x=40.
[0031] FIG. 4 shows SEM images of surfaces of SNC+YWOx composite cathodes calcined in air at 800° C. for 2 h (x=0 wt %, 10 wt %, 20 wt %, and 40 wt %), where (a and b) x=0, (c and d) x=10, (e and f) x=20, and (g and h) x=40.
[0032] FIG. 5 shows thermal expansion curves of dense strip samples of SNC+ZWOx (x=0 wt %, 10 wt %, and 20 wt %)/SNC+YWOx (x=0 wt %, 10 wt %, and 20 wt %) in a temperature range of 200° C. to 900° C.
[0033] FIG. 6 shows thermogravimetric analysis (TGA) results, where (a) shows TGA curves of SNC, ZWO, SNC/ZWO20, and SNC+ZWO20 samples in air, and (b) shows TGA curves of SNC, YWO, SNC/YWO20, and SNC+YWO20 samples in air.
[0034] FIG. 7 shows ASRArrhenius curves of SNC+YWO20 cathodes calcined at 800° C., 900° C., and 1,000° C.
[0035] FIG. 8 shows Arrhenius curves illustrating ASR changes of SNC+ZWOx (x=10, 20, and 40) cathodes in a test temperature range of 500° C. to 750° C.
[0036] FIG. 9 shows Arrhenius curves illustrating ASR changes of SNC+YWOx (x=10, 20, and 40) cathodes in a test temperature range of 500° C. to 750° C.
[0037] FIG. 10 shows electrochemical impedance spectroscopy (EIS) spectra of SNC+YWOx (x=0, 10, 20, 30, and 40) composite electrode-based symmetric cells at 600° C., where the inset graph shows EIS spectra for x=0, 10, and 20, and the effect of ohmic resistance has been removed.
[0038] FIG. 11 shows ASR curves of cells, where (a) shows ASR values of SNC+YWOx symmetric cells at 120 min after the symmetric cells are placed under 10% CO.sub.2 for 60 min at 600° C. and then CO.sub.2 is removed; and (b) shows a relative change rate of an ASR value of each sample relative to initial ASR0.
[0039] FIG. 12 shows EIS analysis of cells, where (a) shows EIS spectra of SNC+YWO20 and SNC after 60 min of carbon dioxide treatment; and (b) shows distribution of relaxation times (DRT) analysis of the EIS spectra of SNC+YWO20 before and after 60 min of CO.sub.2 treatment.
[0040] FIG. 13 shows results of phase structure analysis, where (a) shows XRD patterns of samples c-SYNC (red curve), SNC (blue curve), and YWO (gray curve) that are calcined at 800° C. for 2 h; and (b) shows enlarged parts of the XRD patterns of SNC and c-SYNC powders, with 20=300 to 35°.
[0041] FIG. 14 shows Rietveld refinement results of a c-SYNC powder, including measured data and simulation and difference curves.
[0042] FIG. 15 shows X-ray photoelectron spectroscopy (XPS) spectra of (a) Co2p orbit and (b) Nb3d orbit of c-SYNC and SNC/YWO.
[0043] FIG. 16 shows XPS spectra of Sr3d orbit of c-SYNC and SNC/YWO.
[0044] FIG. 17 is a schematic diagram of a c-SYNC cathode.
[0045] FIG. 18 shows a high-resolution transmission electron microscopy (HRTEM) image (a1) of a c-SYNC powder and an energy-dispersive X-ray spectrometry (EDS) spectrum (a2) of a corresponding component, a corresponding EDX spectrum (b) of the c-SYNC powder, and HRTEM and selected-area electron diffraction (SAED) images of the c-SYNC powder (c-g): where (c, f) show transmission electron microscopy (TEM) images of the c-SYNC powder, (d) shows an HRTEM image of YWO, (e) shows an HRTEM image of SrWO.sub.4, and (g1-g4) show SEAD and HRTEM images of SYNC and fast Fourier transformation (FFT) plots corresponding to [101] space axis (g1, g3) and [110] space axis (g2, g4).
[0046] FIG. 19 shows high-temperature in-situ XRD patterns of a c-SYNC sample in air during a heating and cooling cycle from room temperature (RT) to 750° C.
[0047] FIG. 20 shows O1sXPS curves of SNC, SNC/YWO, and c-SYNC powders.
[0048] FIG. 21 shows thermal expansion curves, where (a) shows thermal expansion curves of uncalcined SNC and SNC/YWO strip samples in air from 100° C. to 1,200° C.; and (b) shows thermal expansion curves of SNC and c-SYNC dense strip samples from RT to 900° C.
[0049] FIG. 22 shows EIS spectra of c-SYNC cathodes (calcined at 800° C.) with a partial oxygen pressure (pO.sub.2) ranging from 0.1 atm to 1 atm at 600° C. using N.sub.2 as an equilibrium gas.
[0050] FIG. 23 shows EIS spectra of c-SYNC and SNC (calcined at 800° C.) at different temperatures 550° C. (a), 600° C. (b), and 650° C. (c).
[0051] FIG. 24 shows Arrhenius curves of ASR of c-SYNC and SNC cathodes at different temperatures.
[0052] FIG. 25 shows DRT analysis of EIS results of c-SYNC samples at varying pO.sub.2 (a) and at varying measurement temperatures (500° C. to 700° C.) (b).
[0053] FIG. 26 shows R1 (a) and R2 (b) values of fitted EIS for c-SYNC and SNC samples at 550° C., 600° C., and 650° C., with an equivalent circuit diagram in the inset graph.
[0054] FIG. 27 shows the distribution of comparative TEC and ASR values of various excellent cathode materials at 600° C.
[0055] FIG. 28 shows ASR values of c-SYNC and SNC cathodes with different thicknesses in air at 600° C. (a); the ASR stability of c-SYNC and SNC cathodes in air at 600° C. during a period of 200 h (b), where the two cathodes have a thickness of 40 μm, and (c) EIS spectra of SNC and c-SYNC before and after a stability test.
[0056] FIG. 29 shows SEM images of cross sections along a thickness and surface of a c-SYNC cathode calcined at 800° C. for 2 h in air, where (a and b) thickness=10 μm, (c and d) thickness=20 μm, (e and f) thickness=30 μm, and (g and h) thickness=40 μm.
[0057] FIG. 30 shows I-V and I-P curves of an anode-supported c-SYNC single cell with a thickness of 40 μm under H.sub.2 (a); I-V and corresponding I-P curves of SNC cathodes with thicknesses of 40 μm (b) and 10 μm (c) under H.sub.2; and a result of a single-cell durability test conducted at 600° C. for 200 h with H.sub.2 and air as a fuel and an oxidant (d), where the inset graphs are SEM cross-sectional images of c-SYNC and SNC cathodes, respectively.
[0058] FIG. 31 shows ASR values of SNC and c-SYNC cathodes during 40 temperature cycles from 600° C. and 300° C. that last for 90 h (a); EIS values of c-SYNC (b) and SNC (c) after 1, 10, 20, 30, and 40 cycles; and a durability-enhancing mechanism proposed through in-situ formation of c-SYNCs (d).
DESCRIPTION OF EMBODIMENTS
Material Synthesis
[0059] 1. Preparation of SNC materials: Sr(NO.sub.3).sub.2, C.sub.10H.sub.5NbO.sub.20, and Co(NO.sub.3).sub.2.6H.sub.2O are mixed according to a predetermined stoichiometric ratio to obtain a resulting solution, and then the resulting solution is mixed with EDTA-NH.sub.3 and citric acid to obtain a solution with a pH of about 6 to 7, and the solution is subjected to evaporation for 5 h to obtain an SNC powder precursor (Evaluation of the CO.sub.2 Poisoning Effect on a Highly Active Cathode SrSc.sub.0.175Nb.sub.0.025Co.sub.0.8O.sub.3−δ in the Oxygen Reduction Reaction [J]. Acs Applied Materials & Interfaces, 2016, 8 (5): 3003). The solid precursor of SNC is calcined in air at 1,000° C. for 5 h to obtain a final powder. The SNC materials can also be prepared by a solid-phase method, and a specific preparation process can be seen in existing relevant technical documents, such as Wei, Zhou, and, et al. Structural, electrical and electrochemical characterizations of SrNb.sub.0.1Co.sub.0.9O.sub.3−δ as a cathode of solid oxide fuel cells operating below 600° C. [J]. International Journal of Hydrogen Energy, 2010.
[0060] 2. Preparation of negative thermal expansion materials: YWO is prepared as follows: Y.sub.2O.sub.3 and WO.sub.3 powders are mixed, and the resulting mixture is subjected to ball-milling at a rotational speed of 400 rpm with ethanol as a solvent, then dried, and calcined in air at 1,100° C. for 5 h to obtain an YWO powder. Similarly, ZWO is prepared by the same method with ZrO and WO.sub.3 powders as raw materials.
[0061] 3. Preparation of SOFC composite cathode materials: An appropriate amount of YWO (or ZWO) is mixed with SNC, and the resulting mixture is calcined in air at 800° C. for 2 h to obtain a c-SYNC composite powder.
XRD Characterization
[0062] ZrW.sub.2O.sub.8 (ZWO) is a phase-forming powder obtained by weighing and mixing corresponding oxides according to a predetermined stoichiometric ratio, and calcining the resulting mixture at 1,150° C. for 20 h (high-temperature solid phase process), and Y.sub.2W.sub.3O.sub.12 (YWO) is also a phase-forming powder obtained by calcining at 1,200° C. for 20 h (high-temperature solid phase process). The host material SrNb.sub.0.1Co.sub.0.9O.sub.3−δ (SNC) is synthesized by the EDTA-CA method, and calcination is conducted at 1,000° C. for 5 h to form a phase. SNC is mixed with ZWO or YWO according to different ratios to prepare composite cathode materials (which are represented by SNC+ZWOx (x=10 wt %, 20 wt %, and 40 wt %) and SNC+YWOx (x=10 wt %, 20 wt %, and 40 wt %)), and the resulting mixture is mechanically mixed by ball-milling, dried, and calcined at 800° C. for 2 h to allow the two phases to be fully combined. Then an XRD test is conducted to determine changes of phase structures, as shown in FIG. 1. From the comparison in the figure, it can be concluded that SNC undergoes an obvious phase reaction with both ZWO and YWO at a high temperature, the phase reaction intensifies with the increase in a proportion of the NTE material, and SrWO.sub.4 and CoWO.sub.4 are generated. Only the SrWO.sub.4 phase is found in the composite of YWO with SNC, but both the SrWO.sub.4 and CoWO.sub.4 phases are found in the composite of ZWO with SNC, and an impurity phase generated by the phase reaction has a very strong peak intensity, indicating that the phase reaction is intense, and the difference in phase structure may lead to differences in the oxygen reduction activity and performance of the two.
[0063] In order to study the impact of temperature on the phase reactions in the composites, an SNC+ZWOx composite and an SNC+YWOx composite (x=20 wt %) are taken and each calcined for 2 h at different temperatures, and then the changes of phase structures are studied by XRD characterization. FIG. 2 shows XRD patterns of SNC+ZWO20/SNC+YWO20 samples calcined at 400° C. to 1,200° C. for 2 h, and it can be seen from curves in the figure that there is almost no phase reaction between the host material and the NTE material at a temperature of 800° C. or lower. However, a preparation process of an SOFC cathode material must involve high-temperature calcination, and thus high-temperature phase reactions are inevitable. When SrWO.sub.4 generated at 800° C. is at 1,000° C., a corresponding peak gradually weakens, indicating that the component is decomposed at the decomposition temperature, and then new impurity phases are generated in both SNC+ZWO20 and SNC+YWO20: tetragonal crystal Sr(CoW).sub.5O.sub.3, P4 mm space group structure (PDF #74-2464), and cubic crystal Sr.sub.2CoWO.sub.6, Fm-3m space group structure (PDF #74-2470). However, it can be seen through comparison that, in SNC+ZWO20, at a high temperature, the phase reaction between SNC and ZWO is intense, the SNC peak is significantly weakened, and the main phase is difficult to maintain and is mostly replaced by an impurity phase, and in SNC+YWO20, at a high temperature, the phase reaction is moderate and the main phase peak still exists. It can be inferred that the ORR activity of the two composites may be affected at high temperatures, and thus reducing a calcination temperature for electrode preparation can avoid this problem.
SEM Characterization
[0064] FIG. 3 and FIG. 4 show SEM images of composite cathode materials of SNC+ZWOx and SNC+YWOx in different mixing ratios after calcination, respectively. Areas (a) and (b) in FIG. 4 are topographic images of pure SNC, which is a smooth large-granular powder. It can be seen from FIG. 1 that, after ZWO is added, SNC and ZWO are well combined to form a powder with fine-grained morphology, and with the increase of a ZWO content, this granular powder occupies the total field of view, indicating that as the phase reaction proceeds, an amount of a second phase generated increases, and when the ZWO content increases to x=20 wt %, the second phase SrWO.sub.4 is precipitated from the powder phase and distributed on the surface of the material in a flocculent form. In the experiment of combining SNC with YWO, a similar phenomenon can also be observed. However, compared with ZWO, the phase reaction between SNC and YWO is not so intense, such that a small number of SrWO.sub.4 impurity particles are generated and distributed on the surface of the host material, and the uniform fine-grained powder is still dominant, and when the YWO content reaches x=40 wt %, the morphology does not change much, which further proves that the phase reaction of ZWO and YWO with the host material SNC will make a composite change from smooth particles to fine-grained structures, and the generated impurity phases are distributed on the surface of the host material. Due to the intense phase reaction, when the ZWO content increases, a large amount of impurities adhere to the surface of the material, which is rare in YWO composites.
TEC and TGA Analysis
[0065] In order to study TECs of SNC+ZWOx and SNC+YWOx, 0.6 g of each of samples with x=0 wt %, 10 wt %, and 20 wt % is taken and pressed into a strip-shaped body, the body is calcined at 1,200° C. for 5 h to be densified, and then a TEC thereof is tested during a heating process from 200° C. to 900° C. in an air atmosphere. TEC test results of the SNC+ZWOx and SNC+YWOx are shown in FIG. 5. It can be observed from the curves in the figure that a TEC value of SNC is 22.7×10.sup.−6 K.sup.−1 in a temperature range of 200° C. to 900° C., and in the same temperature range and at the same combining ratio with the NTE material, an SNC+ZWO composite has a lower TEC value than an SNC+YWO composite, which is related to the low TEC value of ZWO itself (ZWO has a TEC of about −9×10.sup.−6 K.sup.−1, and YWO has a TEC of about −7×10.sup.−6 K.sup.−1). The occurrence of thermal expansion can be explained as the lattice expansion caused by reduction of Co.sup.4+ into Co.sup.3+ when a cathode is heated to an operating temperature of SOFC and reduction of Co.sup.3+ into Co.sup.2+ at a higher temperature. It can be seen from FIG. 5 that the TEC of SNC suddenly increases at 600° C., which is caused by the reduction of transition metal Co, and the introduction of ZWO alleviates this situation, when the temperature exceeds 600° C., the TEC of SNC+ZWO10 still shows a stable linear relationship with the temperature, but this rule is not applicable for SNC+ZWO20 in which the TEC value increases significantly after the temperature reaches 700° C. This may be because the intense phase reaction between ZWO and SNC observed above leads to the generation of SrWO.sub.4 and changes the host structure and material morphology. In addition, it can be known that SNC+ZWO10 and SNC+ZWO20 respectively have TEC values of 14.0×10.sup.−6 K.sup.−1 and 9.24×10.sup.−6 K.sup.−1 in a temperature range of 200° C. to 900° C. In contrast to SNC+ZWOx, FIG. 5 shows test results of SNC+YWOx (x=0 wt %, 10 wt %, and 20 wt %), and it can be seen from curves in the figure that SNC+YWO10 and SNC+YWO20 respectively have TEC values of 16.6×10.sup.−6 K.sup.−1 and 12.9×10.sup.−6 K.sup.−1 in a temperature range of 200° C. to 900° C. Although a composite of YWO with SNC is weaker than ZWO in terms of TEC regulation, it is found that, when x=20, a TEC of SNC+YWO20 is almost the same as a TEC of an electrolyte material.
[0066] In order to further prove a change trend of thermal expansion, TGA is conducted by heating from RT to 1,000° C. in an air atmosphere, and results are shown in FIG. 6. In the figure, SNC/YWO20 is a mixture of SNC and YWO20 without calcination treatment, and the same is true for SNC/YWO20. It is found that ZWO and YWO respectively have weight losses of 0.46% and 0.35%, which do not change in a high temperature range, and thus the weight loss of ZWO and YWO has no impact on a composite. Moreover, it can be observed from FIG. 6 that the weight loss of SNC+ZWO20 is smaller than the weight losses of SNC and SNC/ZWO20, especially when the temperature is 500° C. or higher, and SNC+YWO20 also has a similar phenomenon, and the weight loss of SNC+ZWO20 is smaller than the weight loss of SNC+YWO20. The weight loss at a temperature higher than 400° C. should be attributed to the loss of lattice oxygen caused by the decrease of a valence state of Co, such as the reduction of Co.sup.4+ into Co.sup.3+ and Co.sup.3+ into Co.sup.2+, and thus the composite can inhibit the decrease of a valence state of Co in perovskite at a high temperature to inhibit the increase of TEC, which is also consistent with the change rule of thermal expansion mentioned above.
Electrochemical Resistance Analysis
[0067] In order to study the ORR catalytic activity of the composite cathode materials, an EIS test is conducted. Therefore, SNC+YWO20|SDC|SNC+YWO20 symmetric cells calcined at 800° C., 900° C., and 1,000° C. are first investigated under open-circuit conditions from 500° C. to 750° C. in air.
[0068] A preparation process of the symmetric cell is as follows: an electrode powder (c-SYNC or SNC) is mixed with isopropyl alcohol (IPA), ethylene glycol (EG), and glycerol to obtain a resulting mixture, the resulting mixture is subjected to ball-milling for 30 min to obtain an electrode slurry, then the slurry is sprayed on both sides of an SDC disc, and the resulting product is calcined at 800° C. for 2 h to obtain a symmetric cell. A thickness of the electrode is controlled by adjusting a spraying time.
[0069] FIG. 7 shows Arrhenius curves of ASRs of SNC+YWO20 electrodes at different calcination temperatures, where Ea of samples calcined at 800° C. is much lower than Ea of samples calcined at 900° C. and 1,000° C. It indicates that a high calcination temperature will lead to low ORR activity, which is attributed to complex phase reactions occurring at 800° C. or above according to XRD results. Therefore, a calcination temperature of 800° C. is adopted to prepare all subsequent electrodes for further study.
[0070] Table 1 shows ASR values of several composites at different temperatures. Through resistance curves, the ASR values of components and relationships thereof with the temperature can be visually compared, and the smaller the resistance value, the higher the ORR electrical activity.
[0071] Table 1 shows ASR values of SNC+ZWOx (x=10, 20, and 40) and SNC+YWOx (x=10, 20, and 40) cathodes at 550° C. to 700° C.
TABLE-US-00001 SNC + ZWOx SNC + YWOx Temperature/° C. SNC x = 10 x = 20 x = 40 x = 10 x = 20 x = 40 700 0.018 0.016 0.040 4.239 0.009 0.011 0.286 650 0.040 0.033 0.092 10.101 0.021 0.023 0.662 600 0.089 0.081 0.225 26.240 0.052 0.059 1.662 550 0.237 0.238 0.643 75.160 0.133 0.154 4.733 500 0.787 0.862 2.144 247.566 0.442 0.502 15.597
[0072] It can be seen from FIG. 7 that, taking the resistance at 600° C. as an example, the SNC host material has an ASR value of 0.09 Ωcm.sup.2, and after 10% ZWO is introduced into SNC, the ASR value becomes 0.08 Ωcm.sup.2, indicating that a small amount of ZWO will not change the ASR value significantly. As a proportion of ZWO continues to increase and reaches x=20%, the ASR value of SNC+ZWO20 increases to 0.23 Ωcm.sup.2, indicating a significant increase in resistance, and when the proportion increases to x=40%, the ASR value of SNC+ZWO20 becomes 26.24 Ωcm.sup.2, which is more than 10 times the ASR value when x=20% and is greatly different from the original value, and this is because the unfavorable phase reactions between SNC and ZWO severely destroy the host structure and cause ORR catalytic activity decay.
[0073] In contrast to SNC+ZWOx, ASR curves of SNC+YWOx are shown in FIG. 9. When x=10 and x=20, corresponding SNC+YWO10 and SNC+YWO20 composite cathodes respectively have ASR values of 0.052 Ωcm.sup.2 and 0.059 Ωcm.sup.2 at 600° C. It is found through comparison that, after a predetermined amount of YWO is added, the ORR catalytic activity of the composite SNC+YWOx is significantly reduced compared with the original SNC, indicating that the phase reaction between the two does not hinder the oxygen reduction catalytic process of the composite, but promotes the process, and this may be because the phase reaction changes the phase structure of the SNC host material. When the YWO content continues to increase and reaches x=40, the ASR value of SNC+YWO40 becomes 1.66 Ωcm.sup.2, which is also much larger than the original value, indicating that the SrWO.sub.4 impurity generated by the phase reaction observed also hinders the oxygen reduction catalytic reaction of the cathode to some extent.
[0074] In order to further analyze the change of the ORR catalytic activity of SNC+YWOx with the increase of the YWO proportion, the EIS spectra of SNC+YWOx symmetric cells at 600° C. are analyzed, as shown in FIG. 10. In the EIS spectra, an intersection of a high frequency (HF) band and a real axis reflects an ohmic resistance of an electrode, which generally represents a contact surface between the electrode and an electrolyte and an ohmic resistance generated in the electrolyte. An intercept of an HF band and a low frequency (LF) band on the real axis is a polarization resistance of an electrode, that is, the ASR mentioned above, where the HF band represents a charge transfer process of the electrode, and the LF band represents a resistance generated by a surface diffusion process of oxygen. It can be seen from the figure that, with x=20 as a critical value, when x is less than or equal to 20, SNC+YWOx can form a composite conducive to ORR activity, which can further reduce the electrochemical resistance value and improve the catalytic activity on the basis of excellent ORR of SNC; and when x is larger than 20, the resistance value of SNC+YWOx increases greatly, and a specific cause for this phenomenon needs to be determined from different rate control steps of the catalytic process. It can be simply determined from FIG. 10 that, compared with the resistance value of SNC, the resistance values of SNC+YWO10 and SNC+YWO20 in the HF and LF bands are significantly reduced, and the resistance values in the LF band are reduced more, indicating that the composite products promote the bulk phase charge transfer process and the formed fine-grained structure greatly facilitates the surface diffusion process of oxygen. Moreover, the resistance values of SNC+YWO30 and SNC+YWO40 are significantly larger than that of other components, and it can be concluded through analysis that corresponding resistance values in HF and LF bands are increased. This may be because the severe phase reaction destroys the host perovskite component of the composite, and a large amount of impurities produced by the phase reaction are precipitated and attached to the surface, which greatly reduces the surface diffusion process of oxygen, thereby drastically weakening the ORR activity.
Impact of CO.SUB.2 .on Electrochemical Resistance
[0075] Many SOFC cathodes with high activity are susceptible to CO.sub.2 poisoning. Therefore, a CO.sub.2 poisoning test is conducted at 600° C., and then EIS is used to evaluate the tolerance of the SNC+YWOx (x=0, 10, 20, 30, and 40) cathodes in a CO.sub.2-containing atmosphere. As shown in (a) of FIG. 11, at 600° C., SNC+YWOx (x=0, 10, 20, 30, and 40)-based symmetric cells are placed in air introduced with 10 vol. % CO.sub.2, then the CO.sub.2 is removed, and the ASR is continuously monitored for 120 min in the air. Since a too-high YWO concentration will greatly increase an ASR value of an SNC+YWOx composite, after CO.sub.2 is introduced, a relative change rate of the ASR value relative to an initial value under the same conditions is calculated (as shown in (b) of FIG. 11). It can be clearly seen that, with the increase of a YWO content, the relative change rate of ASR gradually decreases under CO.sub.2-containing test conditions, and this is because the tolerance of the cathode to CO.sub.2 poisoning is improved due to the introduction of SrWO.sub.4 and YWO covering the surface. As shown in (a) of FIG. 12, after 10 vol. % CO.sub.2 is introduced at 600° C. and ASR is continuously monitored for 60 min, the ASR (about 1.75 Ωcm.sup.2) of SNC+YWO20 is less than half of the resistance of the SNC cathode (about 4.13 Ωcm.sup.2). A corresponding ASR growth rate of the SNC+YWO20 cathode is 0.028 Ωcm.sup.2 min.sup.−1, and a corresponding ASR growth rate of the SNC cathode is 0.067 Ωcm.sup.2 min-. It is generally believed that the mechanism of CO.sub.2 poisoning is as follows: the competitive adsorption of CO.sub.2 for active sites inhibits the surface migration process of oxygen that determines the ORR rate. Moreover, the evidence that the LF peak of SNC+YWO20 becomes larger after CO.sub.2 treatment in (b) of FIG. 12 is obtained from the above results. Therefore, the prominent CO.sub.2 tolerance observed in SNC+YWO20 can be attributed to the high acidity of the YWO additive and the coverage of SrWO.sub.4 on the cathode surface. Therefore, the SNC+YWO20 composite will hinder CO.sub.2 adsorption and protect SNC from CO.sub.2 poisoning. In addition, after switching back to air, both samples can be recovered.
[0076] It can be seen from the above tests that a negative thermal expansion material is introduced into a perovskite oxide to prepare an SOFC cathode material. Two common isotropic NTE materials (ZWO and YWO) are selected and combined with SNC perovskite cathodes with high ORR activity and large TEC, and the phase structure, material morphology, thermal expansion change, and electrochemical resistance are investigated. (1) The phase reaction between ZWO and SNC is intenser than that between YWO and SNC, and with the increase of the NTE material content, the phase reaction intensifies in both the composite SNC+ZWOx and the composite SNC+YWOx, and substances such as SrWO.sub.4 are generated. (2) The phase reaction of ZWO and YWO with SNC can make the surface morphology of the host material change from smooth particles to fine particles, and impurities such as SrWO.sub.4 will be precipitated and attached to the surface of the material. (3) With the increase of the NTE content, the TEC value of the composite decreases significantly, and the effect of ZWO to reduce TEC becomes obvious. The TEC value of SNC+YWO20 is 9.24×10.sup.−6 K.sup.−1, which is far lower than the TEC value of an electrolyte material; and the TEC value of the SNC+YWO20 composite is 12.9×10.sup.−6 K.sup.−1, which is perfectly matched with the TEC value of SDC. (4) In an electrochemical resistance test, the resistance of SNC+ZWOx is high due to the phase reaction, and gradually increases with the increase of the ZWO proportion. The ASR values of SNC+YWO10 and SNC+YWO20 at 600° C. are 0.052 Ωcm.sup.2 and 0.059 Ωcm.sup.2, respectively, and the two corresponding HF and LF processes are favorable for ORR. (5) SNC+YWOx shows high ORR activity, large TEC, and prominent anti-CO.sub.2 poisoning performance. After 10 vol. % CO.sub.2 is introduced at 600° C. and ASR is continuously monitored for 60 min, an ASR value (about 1.75 Ωcm.sub.2) of SNC+YWO20 is less than half of a resistance value of the SNC cathode (about 4.13 Ωcm.sup.2).
Phase Structure Analysis
[0077] A Y.sub.2W.sub.3O.sub.12 oxide (YWO) is adopted as an NTE candidate material and combined with a SrNb.sub.0.1Co.sub.0.9O.sub.3−δ (SNC) cathode to offset a mismatched TEC value with perovskite. By calcining a physical mixture of SNC and YWO at a high temperature, an interfacial phase reaction occurs between SNC and YWO, and the phase structure among the host material SNC, the mixture of SNC and YWO, and the composite obtained by mixing SNC and YWO and calcining the resulting mixture at a high temperature is further explored. Due to the large difference in TEC between YWO and SNC, if the two phases are bonded through weak physical contact, the two phases can be easily debonded during a thermal cycling process. The chemical reaction between the two phases will enhance the bonding between the two phases, thus ensuring prominent mechanical integrity. The potential phase reaction between the two phases is investigated by calcining YWO and SNC.
[0078] (a) of FIG. 13 shows XRD patterns of pure SNC, pure YWO, and a mixture of 20 wt % SNC and 80 wt % YWO (c-SYNC) subjected to thermal treatment at 800° C. for 2 h. After calcination, the formation of a second phase SrWO.sub.4 is detected (JCPDF85-0587). It means that Sr precipitated from SNC can react with W in YWO to form SrWO.sub.4, such that A-site cation deficiencies appear at the A-site of SNC perovskite in the composite. Through careful observation, it is found that a peak corresponding to the [110] crystal zone axis at about (2θ=33°) in the XRD pattern of the SNC phase (as shown in (b) of FIG. 13) indicates a shift to a low angle, indicating that the lattice volume expansion is due to the generation of cation deficiencies at the A-site in the perovskite lattice. Moreover, the SNC almost retains the original primary P4/mmm structure. Sr.sub.x(Nb.sub.0.1Co.sub.0.9Y.sub.y)O.sub.3−δ (SYNC) can be formed in-situ by doping Y at the B-site during a thermal treatment process, and this conclusion can be supported by refinement results, as shown in FIG. 14. The refinement results show that the SNC has a P4/mmm space group, with a lattice size of a=b=3.8795 Å and c=7.7771 Å (detailed results of refinement are shown in Table 2). The above results confirm that Y with a large ionic radius is doped at the B-site of SNC, and a Sr precipitate causes deficiencies at the A-site, which in turn leads to lattice expansion. After the thermal treatment, the SNC/YWO mechanical mixture becomes a mixture of SYNC, YWO, and SrWO.sub.4 newly formed, which is defined as c-SYNC.
[0079] Table 2 shows rietveld refinement data for c-SYNC powder.
TABLE-US-00002 Rietveld refinement value Rexp Rwp Rp GOF DW 4.13 6.47 4.52 1.57 0.71 Name SYNC YWO SrWO.sub.4 Bragg radius 3.594 3.404 2.295 Space group P4/mmm Pnca I41/a Range 0.000580004111 4.35759031E−007 7.83812735E-006 Cell quality 409.985 3685.421 1341.874 Cell volume 117.05189 1305.69679 349.53399 Wt %-Rietveld 82.822 6.239 10.939 Crystallite size Lorentzian lattice size 9685.7 78.0 111.8 (nm) Strain Strain length 0.1646415 0.3784114 0.1425521 Crystal linear absorption 782.345 597.232 792.908 coefficient (cm.sup.−1) Crystal density (g cm.sup.−3) 5.816 4.687 6.375 Lattice parameter a 3.8795299 10.0224150 5.4058441 b / 13.4390213 / c 7.7771452 9.6939844 11.9608591
[0080] The A-site cation deficiencies and B-site low-valent dopants of SNC can introduce more oxygen vacancies, which are beneficial to the ORR. To elucidate the chemical state of the B-site element after Y doping, the valence states of Co and Nb in the c-SYNC sample and the valence states of precursors thereof (SNC/YWO, a mixture of SNC and YWO before thermal treatment) are subjected to comparative analysis according to XPS. It can be observed from (a) of FIG. 15 and (b) of FIG. 15 that SNC/YWO and c-SYNC have the same Co2p and Nb3d peak profiles, indicating that the chemical states of Co and Nb hardly change. Therefore, the substitution of Y.sup.3+ will not affect the valence states of Co and Nb cations.
[0081] An XPS spectrum of Sr 3d in the c-SYNC sample is shown in FIG. 16, and it can be seen from the figure that differentiated Sr 3d.sub.5/2 and 3d.sub.3/2 peaks correspond to two different kinds of Sr, namely, lattice Sr (low binding energy) and surface Sr (high binding energy). After calcination, a ratio of surface Sr to lattice Sr increases significantly. This increase can be attributed to the formation of surface SrWO.sub.4, which is consistent with the XRD results.
[0082] According to the above analysis, it can be inferred that a phase reaction will occur between SNC and YWO through a cation exchange mechanism at 800° C. As shown in FIG. 17, Sr cations at the A-site of SNC react with YWO to form SrWO.sub.4 on the surface of the material, and then Y diffuses to the B-site of SNC to finally form a stable SYNC perovskite structure, which also includes some substances such as YWO and SrWO.sub.4.
[0083] A crystal structure of c-SYNC can also be verified by HRTEM and EDS coupled with HRTEM (as shown in (a) of FIG. 18), where W and Y appear together with Sr, Nb, and Co at 7.5 μm. It reveals the existence of YWO and the strong combination of SYNC and YWO particles. From the element distribution analysis of selected areas ((b) of FIG. 18), Y and W are uniformly distributed in a particle area in which Sr, Co, and Nb are concentrated. The above results once again prove that SYNC and SrWO.sub.4 are formed through surface cation exchange and cover the surface of SNC particles. SrWO.sub.4 can be found from the image of a single c-SYNC composite particle in d of (c) of FIG. 18. The existence of YWO can also be confirmed by a lattice spacing d (200)=5.06 Å in (d) of FIG. 18. The 4.91 Å lattice spacing measured in the enlarged view ((e) of FIG. 18) corresponds to the [101] axis of SrWO.sub.4. The existence of SYNC can be fully proved by the d (101)=3.47 Å and d (110)=2.74 Å lattice spacings observed in (f) of FIG. 18 and (g) of FIG. 18 and corresponding FFT and SAED.
Phase Stability and Thermal Stability Analysis
[0084] Well chemical and thermal matching is very important for SOFC operation, especially for the tolerance of cathode materials. The crystal structure stability of the c-SYNC powder is first investigated by in-situ XRD characterization at RT to 750° C. (FIG. 19). The sample is kept at each temperature point for 5 h during a heating and cooling process, the phase structure of c-SYNC does not change, and no new phase is formed. Previously, XRD is conducted at RT after the SNC/YWO composite is calcined at 400° C. to 1,200° C. for 2 h, and it is found that SrWO.sub.4 is formed in the c-SYNC composite at 800° C., and is converted into cubic Sr.sub.2CoWO.sub.6 at 1,000° C. or above (PDF #74-2470). Moreover, in the previous TGA results of c-SYNC, a weight loss of c-SYNC is smaller than weight losses of SNC and SNC/YWO, especially at 500° C. or above, which is attributed to the generation of A-site cation deficiencies in the c-SYNC perovskite that inhibits the reduction of Co valence in the perovskite at a high temperature. In addition, reactive oxygen species (ROS) (namely, highly-oxidative O.sub.2.sup.2−/O) may be formed on the surface of a catalyst. FIG. 20 shows XPS spectra of SNC, SNC/YWO, and SYNC at O 1s sequentially from bottom to top, where there are four distinct characteristic peaks, namely, lattice oxygen (O.sup.2−) at about 529.0 eV, ROS (O.sub.2.sup.2−/O.sup.−) at about 530.1 eV, hydroxyl/adsorbed surface oxygen (OH/O.sub.2) at about 531.2 eV, and adsorbed H.sub.2O molecules at about 532.1 eV. After YWO is introduced to form a composite, a proportion of —OH/O.sub.2 first increases and then decreases during a calcination process to form c-SYNC. Moreover, a proportion of O.sub.2.sup.2−/O.sup.− in c-SYNC increases during calcination. Compared with the SNC powder, c-SYNC has a higher concentration of adsorbed oxygen (O.sub.ad, including O.sup.2−, O.sup.−, and OH), as shown by an O.sub.ad/O.sub.lattice ratio, and thus it can be concluded that c-SYNC has more oxygen vacancies on the surface.
[0085] TEC is a key characteristic of SOFC cathodes, which reflects the thermal stability of the cathode materials. A TEC of a cathode must be matched with a TEC of an electrolyte to reduce the risk of debonding between the cathode and the electrolyte during thermal cycling. In general, classical cobalt-based perovskite cathodes have high ORR activity, but show unsatisfactory thermal stability due to their high TEC values, such as Ba.sub.0.5Sr.sub.0.5Co.sub.0.8Fe.sub.0.2O.sub.3 (24×10.sup.−6 K.sup.−1) and La.sub.0.6Sr.sub.0.4CoO.sub.3 (21×10.sup.−6 K.sup.−1). This can be explained by the ionic radius expansion caused by the reduction of Co.sup.4+ into Co.sup.3+ and then the further reduction of Co.sup.3+ into Co.sup.2+ when the cathode is heated to an operating temperature of SOFC (usually above 500° C.). It is simple to lower TEC by mixing an NTE material with SOFC to improve the thermal stability. For the precursor SNC/YWO, thermal expansion curves from RT to 1,200° C. during a combining process of YWO with SNC are determined, which covers a temperature range of calcination, as shown in (a) of FIG. 21. It can be found that a volume shrinkage of SNC is much larger than a volume shrinkage of SNC/YWO, indicating that YWO can inhibit the shrinkage of SNC particles to alleviate the adverse effect of vigorous calcination of SNC. It should be noted that the shrinkage of SNC/YWO also increases to some extent when the temperature is higher than 800° C., which may be due to the reduction of a mass fraction of YWO caused by the phase reaction of YWO and SNC at 600° C. or above. After the calcination at 1,200° C., TEC curves of the prepared c-SYNC and SNC are measured once again, as shown in (b) of FIG. 21, and the same test is also conducted above. Results in different temperature ranges are summarized in Table 3. The reduction in TEC of c-SYNC measured at RT to 900° C. (12.9×10.sup.−6 K.sup.−1) is perfectly matched with TEC of an SDC electrolyte (12.3×10.sup.−6 K.sup.−1) and is much smaller than TEC of pure SNC (22.7×10.sup.−6 K.sup.−1) and TEC of other cobalt-containing perovskite oxides. By comparing TECs measured at 550° C. to 900° C., TEC of c-SYNC is significantly lower than TEC of SNC (10.8×10.sup.−6 K.sup.−1 and 26.5×10.sup.−6 K.sup.−1, respectively). This result is consistent with the reduction of Co.sup.3+ into Co.sup.2+ illustrated by TGA and the phase reaction between SNC and YWO at 600° C. or above demonstrated by XRD results. Therefore, it can be concluded that c-SYNC is a promising durable candidate cathode material, because this material enables a large reduction in TEC, especially at a typical operating temperature range of SOFC from 500° C. to 800° C.
[0086] Table 3 shows TEC values of SNC and c-SYNC samples in air.
TABLE-US-00003 RT-900° C. RT-550° C. 550-900° C. SNC SYNC SNC SYNC SNC SYNC TEC/10.sup.−6 K.sup.−1 22.7 12.9 16.0 14.4 26.5 10.8
Electrochemical Resistance Analysis
[0087] The ORR activity of a cathode can be reflected by ASR measured by a difference between two intercepts on the real axis in EIS. Therefore, c-SYNC|SDC|c-SYNC symmetric cells are investigated under open-circuit conditions from 500° C. to 750° C. in air. As mentioned above, the electrodes are all calcined at 800° C. The polarization resistance of c-SYNC electrodes is tested at different partial oxygen pressures, as shown in FIG. 22. At a high oxygen pressure, the total polarization resistance is low, for example, the ASR is 0.024 Ωcm.sup.2 at pO.sub.2=1 atm; and the ASR is 0.084 Ωcm.sup.2 at pO.sub.2=0.1 atm. As shown in FIG. 23 and FIG. 24, a total resistance of the c-SYNC cathode is also lower than a total resistance of the SNC cathode at 550° C. to 650° C. For example, the ASR value of c-SYNC at 600° C. is 0.063 Ωcm.sup.2, which is much lower than the ASR value of SNC (0.098 Ωcm.sup.2). Therefore, it can be concluded that YWO can be mixed with SNC to form an A-site deficient perovskite, which is favorable for the ORR activity. Furthermore, the surface coverage of the composite (rich in YWO and SrWO.sub.4) has no impact on the ORR activity.
[0088] DRT is used to determine rate-controlling steps of an electrode oxygen reduction process. (a) of FIG. 25 shows fitted curves at HF peak and LF peak, which are usually represented as an electron transfer process and an oxygen surface process, respectively. The change in pO.sub.2 does not seem to affect the HF peak, but heavily affects the LF peak; and the LF peak shifts to an LF as pO.sub.2 decreases. This means that, when the frequency of the LF peak is in a range of 10.sup.1 Hz to 10.sup.2 Hz, the adsorption/desorption and surface transport of surface oxygen are considered as rate-controlling steps. In addition, the decrease in temperature significantly increases the LF peak in (b) of FIG. 25, which can be attributed to the oxygen adsorption/desorption process.
[0089] In addition, an equivalent circuit model with L1-R.sub.ohm-(R1-CPE1)-(R2-CPE2) is proposed to fit the EIS spectrum. An inductive element L1 is related to an external circuit, and R.sub.ohm is mainly related to an ohmic resistance of a cell. R1 and R2 represent resistance values from HF and LF processes, respectively. (a) of FIG. 26 and (b) of FIG. 26 respectively show R1 and R2 fitting results of SNC and c-SYNC samples at 550° C., 600° C., and 650° C. It is found that a value of R2 is higher than a value of R1, indicating that the oxygen surface exchange may be a rate-controlling step in this case, which is perfectly consistent with the DRT analysis. Compared with SNC, large reductions in R1 and R2 are observed in the c-SYNC samples at different temperatures. These observations show that the introduction of YWO to form A-site deficiencies in the c-SYNC composite may be an efficient way to facilitate bulk electron transfer and surface oxygen processes.
[0090] FIG. 27 shows the distribution of comparative TEC and ASR values of various excellent cathode materials at 600° C. The electrochemical resistance values (at 600° C.) and TEC values of various excellent SOFC cathodes with high ORR catalytic activity are summarized.
[0091] Specific reference materials may include: cobalt-rich materials: Ba.sub.0.5Sr.sub.0.5Co.sub.0.8Fe.sub.0.2O.sub.3−δ, La.sub.0.4Ba.sub.0.6CoO.sub.3−δ, La.sub.0.3Ba.sub.0.7Co.sub.0.6Fe.sub.0.4O.sub.3−δ, La.sub.0.6Sr.sub.0.4CoO.sub.3−δ, La.sub.0.8Sr.sub.0.2Co.sub.0.8Fe.sub.0.2O.sub.3−δ, SrCo.sub.0.8Nb.sub.0.1Ta.sub.0.1O.sub.3−δ, SrCo.sub.0.8Fe.sub.0.2O.sub.3−δ, and Sm.sub.0.5Sr.sub.0.5CoO.sub.3−δ; iron-rich materials: Ba.sub.0.5Sr.sub.0.5Co.sub.0.2Fe.sub.0.8O.sub.3−δ, La.sub.0.4Sr.sub.0.6Co.sub.0.2Fe.sub.0.8O.sub.3−δ, Sm.sub.0.6Sr.sub.0.4Fe.sub.0.8Co.sub.0.3O.sub.3−δ, Gd.sub.0.6Sr.sub.0.4Fe.sub.0.8Co.sub.0.2O.sub.3−δ, Ba.sub.0.5Sr.sub.0.5Zn.sub.0.2Fe.sub.0.8O.sub.3−δ, Ba.sub.0.5Sr.sub.0.5Cu.sub.0.2Fe.sub.0.8O.sub.3−δ, Sm.sub.0.5Sr.sub.0.5Cu.sub.0.2Fe.sub.0.8O.sub.3−δ, SrNb.sub.0.2Fe.sub.0.8O.sub.3−δ, and Sm.sub.0.5Sr.sub.0.5FeO.sub.3−δ; and double perovskites (DPs): PrBaCo.sub.2O.sub.5+δ, PrBa.sub.0.5Sr.sub.0.5Co.sub.2O.sub.5+δ, PrBa.sub.0.5Sr.sub.0.5CoCuO.sub.5+δ (650° C.), NdBaCo.sub.2O.sub.5+δ, NdBa.sub.0.5Sr.sub.0.5Co.sub.2O.sub.5+δ, SmBaCo.sub.2O.sub.5+δ, SmBa.sub.0.5Sr.sub.0.5Co.sub.2O.sub.5+δ, GdBaCo.sub.2O.sub.5+δ, GdBa.sub.0.5Sr.sub.0.5Co.sub.2O.sub.5+δ, and YBaCo.sub.2O.sub.5+δ.
[0092] Compared with other materials, c-SYNC exhibits excellent ORR activity and the lowest TEC value. For example, Sm.sub.0.5Sr.sub.0.5CoO.sub.3−δ shows high ORR activity (0.87 Ωcm.sup.2), but has a TEC of 22.3×10.sup.−6 K.sup.−1; and Sm.sub.0.5Sr.sub.0.5FeO.sub.3−δ has a small TEC (18.3×10.sup.−6 K.sup.−1), but shows low ORR activity (2 Ωcm.sup.2). Therefore, it confirms that a simple NTE material synthesis strategy can effectively reduce the TEC and enhance the ORR activity of a cathode, without doping less active Cu or Fe in perovskite, which reduces the TEC at the expense of the ORR activity.
[0093] Compared with the thermal matching of the SNC cathode, the c-SYNC cathode shows better thermal compatibility with an electrolyte layer, which can further improve the tolerance of a cell under operating conditions. The improved tolerance can be attributed to the optimization of thermal expansion of c-SYNC. Another advantage of the improved compatibility is that c-SYNC can be used to make thick cathodes for SOFCs. A thick cathode can increase the ORR catalytic active sites, shows high tolerance to cathode poisoning, and can reduce the risk of cathode spalling (which may lead to SNC cathode failure). Consequently, the impact of cathode thickness on the ORR activity of symmetric cells (FIG. 28) is investigated to demonstrate this speculation. The cathode thickness varies from 5 μm to 40 μm, with an increment of 5 μm, which is manually controlled by a spraying time during a coating process. A thickness of a coating is verified by SEM images of symmetric cells (FIG. 29). In (a) of FIG. 28, the ASR value of c-SYNC decreases from 0.084 Ωcm.sup.2 to 0.059 Ωcm.sup.2 when the cathode thickness increases from 5 μm to 40 μm at 600° C. Conversely, the ASR of SNC increases from 0.12 Ωcm.sup.2 to 0.23 Ωcm.sup.2 under corresponding conditions, indicating that the ORR activity of c-SYNC can be effectively improved by increasing the electrode thickness. It can be seen from (b) of FIG. 28 that no obvious decrease is observed during a durability test of a 40 μm-thick cathode of c-SYNC at 600° C.; and when a test time at 600° C. exceeds 200 h, the ASR value increases from 0.073 Ωcm.sup.2 to 0.080 Ωcm.sup.2 ((c) of FIG. 28). The above results show that there is prominent compatibility at an interface between the cathode and the electrolyte layer. In contrast, the optimal thickness of SNC is 10 μm. Because the ASR value increases from 0.23 Ωcm.sup.2 to 0.35 Ωcm.sup.2 during a stabilization period, a small amount of layering occurs at the 40 μm-thick SNC cathode interface, which leads to the increase in ASR. Therefore, the attempt to fabricate a thicker SNC cathode (40 μm) is not suitable for applications of SOFC cathodes.
Single Cell Performance and Stability Test
[0094] Correspondingly, the electrochemical performance of c-SYNC and SNC cathodes is further evaluated by fabricating single cells supported by a YSZ-Ni anode with an YSZ (8 μm)/SDC (5 μm) bilayer electrolyte. In the fabrication of single cells, anode-supported half-cells (NiO+YSZ/YSZ/SDC) are first fabricated by tape casting, and a c-SYNC (or SNC) cathode slurry is then sprayed on a center of an SDC surface (with a circle area of 0.45 cm.sup.2), and then calcined in air at 800° C. for 2 h.
[0095] As shown in (a) of FIG. 30, the maximum power densities of the 40 μm-thick c-SYNC cathode at 750° C., 700° C., and 650° C. are 1,690 mW cm.sup.−2, 1,139 mW cm.sup.−2, and 817 mW cm.sup.−2, respectively, which exceed a power density of SNC. (For SNC samples, a 40 μm-thick cathode has a power density of 890 mW cm.sup.−2 at 750° C. ((b) of FIG. 30); and a 10 μm-thick cathode has a power density of 1,088 mW cm.sup.−2 at 700° C. ((c) of FIG. 30). As shown in (d) of FIG. 30, at 600° C., when c-SYNC and SNC are continuously operated for 200 h at constant currents of 450 mA cm.sup.−2 and 360 mA cm.sup.−2, respectively, the durability of a single cell of the c-SYNC cathode (40 μm) is also better than that of SNC (40 μm). SEM images of cross sections of the c-SYNC and SNC cathodes are shown in (d) of FIG. 30. The morphologies of cathodes of the two materials are significantly different. An electrode surface for c-SYNC has a fine porous honeycomb structure, indicating superior three-phase interface and mechanical stability.
[0096] To further evaluate the effectiveness in enhancing thermal cycling stability by reducing TEC, the change in ASR of SNC and c-SYNC symmetric cells when subjected to rigorous heating-cooling cycles. Because the risk of layering at an electrolyte-electrode interface can be reduced by optimizing the TEC matching of a c-SYNC cathode, a thickness of an SNC cathode is set to 10 μm and a thickness of c-SYNC is set to 40 μm. A rate of programmed temperature from 300° C. to 600° C. is set to 30° C. min.sup.−1, and a cell is stabilized for 10 min, tested for resistance, then immediately cooled to 300° C. (at an average rate of 7.5° C. min.sup.−1), and stabilized for 10 min to complete a cycle. It can be seen from (a) of FIG. 31 that, after 40 cycles, the ASR of c-SYNC only increases from 0.075 Ωcm.sup.2 to 0.081 Ωcm.sup.2, but the ASR of the SNC cathode increases from 0.095 Ωcm.sup.2 to 0.12 Ωcm.sup.2. The changes in EIS values of c-SYNC and SNC after thermal cycling as shown in (b) of FIG. 31 and (c) of FIG. 31 also confirm that c-SYNC has better tolerance than SNC. It can be found that the Rn of SNC increases, indicating that the interfacial resistance between the electrolyte and the cathode increases after cycling. As shown in (d) of FIG. 31, the improved tolerance can alleviate the problem of interfacial layering. By forming a composite with YWO, a more compatible TEC is produced for the entire cathode layer deposited and calcined, thereby reducing a stress at a cathode-cathode interface. The frangible SNC-electrolyte interfaces (sintering necks) can be replaced by the formed c-SYNC-electrolyte, which has a more compatible TEC and thus can reduce a thermal strain between c-SYNC and an electrolyte.
