IntroductionSolid oxide fuel cell (SOFC) is one of the strong competitors of energy technology. SOFC has the characteristics of wide fuel selection, less pollution emission, high safety, and low noise as an excellent energy conversion and utilization device. However, the existing commercial SOFC still has some problems like high working temperature and large attenuation rate due to the limitation of material performance, restricting the application. Among the components of SOFC, the performance of air pole material is a key factor affecting SOFC. LSC material is one of the main research directions of solid oxide fuel cells (SOFC) air polar materials with the superior electrochemical properties, but its major disadvantage is a great thermal expansion coefficient (TEC). The negative thermal expansion (NTE) material provides an effective way to reduce the TEC of the SOFC air pole material. NTE material is named because of the characteristics of thermal contraction in the specific temperature interval. Theoretically, it can reduce the TEC of composite material and facilitate the thermal match between SOFC air electrode material and electrolyte material. Some NTE materials are a perovskite structure with a higher conductivity than Gd_0.2Ce_0.8O_2-δ (GDC) of fluorite structure. These advantages make the theoretical cell power density of NTE composite air electrode material above the GDC composite cathode In this paper, La_0.6Sr_0.4CoO_3-δ (LSC) material was used as a matrix, Sm_0.85Zn_0.15MnO₃ (SZM) material with the superior negative thermal expansion characteristics was selected as a composite phase, and a series of LS-xSZM (x = 0-40%) materials were designed to achieve the thermal matching of composite materials.MethodsLSC and SZM powders were synthesized by a solid-liquid composite method. First, the corresponding quality of acetate raw materials was accurately weighed according to the stoichiometric ratio, and then the appropriate citric acid (CA) was weighed as a complexing agent according to the total metal particle weight and citric acid (CA) amount of 5:1. All the raw materials were mixed with an appropriate amount of deionized water and ground in a ball mill at a speed of 300 r/ min for more than 12 h. The turbid liquid in the ball mill was taken out and placed in a beaker, and the precursor powder was obtained after drying in the oven at 250 ℃ for 6 h, and then the LSC powder was obtained via calcination in a muffle furnace at 950 ℃ for 2 h, and the SZM powder was obtained via calcination at 1 300 ℃ for 2 h. The two powders were mixed with ethanol and ground in the ball mill for 6 h. The cathode slurry was prepared by mixing powders, and then full cells and symmetrical cells were made by screen printing for subsequent tests.Results and discussionThe X-ray diffraction patterns show that the SZM is chemically compatible with LSC. The thermal expansion test indicates that SZM has an excellent thermal offset ability, and the average thermal expansion coefficient of the composite material from room temperature to 800 ℃ at x of 30% is 13×10^-6 K^-1, which is similar to that of the barrier material. The area specific resistance of the LSC-30% SZM composite material at 800-500 ℃ decreases from 0.012-1.240 Ω·cm² in the case of LSC to 0.008-0.620 Ω·cm², indicating a good interfacial contact. The power density of the button cell at 750 ℃ is 1 702 mW·cm^-2, while the pure phase LSC at the same temperature is 1 209 mW·cm^-2, which is increased by 41%. The polarization resistance Rp reduces from 0.65 Ω·cm² to 0.50 Ω·cm². The voltage drop of thermal cycling and constant current discharge is below 1%. In the 200 h constant current discharge test at 300 mA·cm^-2, the voltage of the battery remains stable after the initial increase, and the whole process only reduces to 10 mV, which is decreased by 1.2%, showing a good long-term stability. No delamination occurs in the SEM images of the battery cross-section after the test. The comprehensive analysis of a better interfacial contact due to a good thermal matching of the composites ultimately achieves a simultaneous increase in the power density and stability of the cell.ConclusionsA perovskite structural oxide material of Sm_0.85Zn_0.15MnO₃ (SZM) with negative thermal expansion characteristics was synthesized by a solid-liquid recombination method. The XRD analysis indicated that SZM was chemical compatible with LSC. The pure phase SZM had special thermal shrinkage characteristics, while the thermal expansion characteristics of the composite materials decreased. The TEC of the pure phase LSC was 20 ×10^-6 K^-1, while the TEC of the 30% SZM composite LSC was only 13 ×10^-6 K^-1. The ASR of the composite material at 750 ℃ decreased from 0.02 Ω·cm² to 0.013 Ω·cm², while that at 550 ℃ decreased from 1.2 Ω·cm² to 0.6 Ω·cm², having an excellent performance. The battery of LSC-30% SZM material was improved at 750 ℃ from 1 209 mW·cm^-2 of pure phase LSC to 1 702 mW·cm^-2 of LSC-30% SZM, which was increased by 41%. In the thermal cycle test, the variation of battery OCV was within 1%, and the voltage decay rate of long-term constant current discharge at 200 h was 1.2%. The microscopic section analysis indicated that there was little interface stratification phenomenon. LSC-30% SZM composite material had good long-term thermal stability and structural stability.