Study on Preparation and Properties of La 0.6 Sr 0.4 Co 0.2 Fe 0.8 O 3-Δ - mGd 0.2 Ce 0.8 O 2 (M=0,30,40 and 50) Composite Cathode

. Solid Oxide Fuel Cells (SOFCs) can convert chemical energy into electrical energy, with high energy conversion rate, safe operation, no pollutant emissions, and is one of the most potential new energy power generation devices in the future. However, at low and medium temperature (below 800℃), the cathode impedance of SOFCs will increase sharply, resulting in adverse effects on cell performance. Finding cathode materials that can work at low and medium temperature is the key to realize large-scale commercial application of SOFCs. LSCF cathode material is one of the commonly used cathode materials for solid oxide fuel cells because of its excellent electronic and ionic conductivity. However, its cathode performance cannot meet the requirements of commercial applications. At high temperature, solid-state reactions are easy to occur between LSCF and commonly used electrolyte materials such as YSZ, SSZ and LSGM, resulting in the formation of an intermediate phase that is not conducive to Oxygen Reduction Reaction (ORR). In this paper, LSCF and Gd 0.2 Ce 0.8 O 2 (GDC) were combined by ball milling to prepare composite cathode. XRD test found that the two powders mixed at high temperature did not produce new substances, with good chemical compatibility. The electrical conductivity test showed that the electrical conductivity of the composite LSCF decreased significantly from 227 Sꞏcm -2 at 650℃ to 115, 90 and 77 Sꞏcm -2 at 550℃, respectively. The composite cathode with a composite ratio of 6:4 has the lowest polarization impedance, only 0.10 Ωꞏcm 2 at 800℃. The discharge test shows that the cathode with the mass ratio of 6:4 LSCF to GDC has the best performance, and the power density is 328 and 256 mW/cm 2 at 800 and 750℃, respectively. The performance of the cathode has good stability after working for 50h.


Introduction
Solid oxide fuel cells (SOFCs) are a new type of energy conversion device, which can directly act the electric potential generated by chemical reactions on the external circuit without complicated energy conversion steps, breaking through the limitations of Carnot cycle, and its energy conversion efficiency can reach more than 60% [1][2][3] . The cathode of SOFCs is the place where the ORR process occurs, which undertakes the role of transporting O 2 molecules and catalyzing their dissociation into O 2at the three-phase interface, then transporting O 2to the electrolyte layer, and the reaction rate of the ORR process directly affects the overall electrochemical performance of SOFC [4,5] . LSCF cathode material is one of the best cathode materials for solid oxide fuel cells because of its good electron and ion conductivity [6,7] . However, the performance of pure LSCF cathodes fails to meet the requirements of commercial applications. In addition, solid-state reaction is easy to occur between LSCF and common electrolyte materials such as YSZ, SSZ and LSGM at high temperature, resulting in the formation of mesophase which is not conducive to ORR, resulting in sharp deterioration of cell performance. Therefore, when choosing an LSCF cathode, Gd 0.2 Ce 0.8 O 2-δ (GDC) is usually chosen as the electrolyte or as an isolation layer between the LSCF cathode and the YSZ electrolyte [8] . The results show that the preparation of LSCF and electrolyte GDC into a composite cathode can not only strengthen its matching with the electrolyte, but also help to maintain the stability of the microstructure of the material and enhance its ionic conductivity, thereby expanding the TPB interface area and improving the oxygen reduction efficiency. According to M. Shah [9]

Material characterization and single cell performance test
The crystal structure of the sample was studied by X-ray diffraction (X 'Pert PRO MPD), the conductivity of the cathode material was tested by four probe method (Keithley 2400). Electrochemical impedance spectroscopy (EIS) was tested using Autolab (PGSTAT 302N), a fuel cell test system (MT2000) was used to test the discharge performance of a single cell.

Single cells fabrication
LSCF|GDC|YSZ|GDC|LSCF symmetric cells prepared by dry pressing method. First, YSZ electrolyte tablets were prepared by pressing mechanism and calcined at 1450 o C for 5h. Then, both sides of YSZ electrolyte tablets were coated with GDC barrier layer by spinning and calcined at 1400 o C for 2h. The LSCF slurry was coated on both sides of the GDC by screen printing method and calcined at 1100 o C for 2h. NiO-YSZ|YSZ anode-supported half cells prepared using extrusionimpregnation method. Firstly, NiO and YSZ were mixed at a mass ratio of 56:44, adding an appropriate amount of soluble starch and ethyl cellulose, adding an appropriate amount of deionized water and an appropriate amount of binder and lubricant to the mixed powder, stirring repeatedly to form a paste, then put into the vacuum mixing extrusion machine, control the appropriate temperature for extrusion molding, and sintering at 1100 o C for 5 h. Then, the anode functional layer was impregnated on the porous anode substrate, and was calcined at 1100 o C for 5 h after drying. The calcined substrate was impregnated with YSZ electrolyte layer, and after drying, it was calcined at 1400 o C for 2 h to obtain dense YSZ electrolyte layer. Then, the GDC barrier layer was coated on the YSZ electrolyte using the screen printing process to prevent the cathode material from reacting with the electrolyte. After the screen printing, it was sintered at 1400 o C for 5h. Finally, the LSCF cathode material was coated on the GDC barrier layer by screen printing process and sintered at 1100 o C for 2h to obtain the NiO-YSZ anode-supported flat tube solid oxide fuel cell.

Chemical compatibility analysis of composite cathode
After LSCF is compounded with GDC, it is necessary to prepare the cathode by high-temperature sintering (>1000°C), and it is also need to maintain a high temperature (>700°C) when the cell is operating. In order to ensure that the chemical reaction between LSCF and GDC will not occur at high temperature, resulting in the formation of heterophase which will affect the performance of the cathode, LSCF and GDC powder are mixed and ground evenly, and calcined at 1100 o C for 5 h in an air atmosphere. Then XRD test is conducted on the calcined mixed powder to analyze their chemical compatibility. The test results are shown in Figure 1. The results show that the diffraction peaks in the XRD diagram of the mixed powder after high temperature sintering correspond to the peak positions on the LSCF and GDC standard cards, and no new heterogeneous peaks are generated, indicating that LSCF and GDC powders have good chemical compatibility at high temperatures and can meet the requirements for the preparation of composite cathodes.   The conductivity of the composite powder with the mass ratio of LSCF and GDC of 7:3, 6:4 and 5:5 was tested, and the variation of the conductivity of the sample with temperature in the air atmosphere was obtained, as shown in Figure 2a. The conductivity of each sample increases first and then decreases with the increase of temperature, satisfying the characteristics of P-type semiconductor [10] . Pure LSCF has the highest conductivity, reaching 227.45 S cm -1 at 650 o C. As the mass ratio of GDC increases, the conductivity of the composite decreases gradually. As LSCF is an electronion mixed conductor with high conductivity, GDC is a pure ionic conductor with almost no electron conductivity, and its conductivity is much lower than that of pure LSCF, so the total conductivity of LSCF and GDC decreases, but its ionic conductivity increases, which is conducive to improving the catalytic performance of the cathode. Figure 2b shows the Arrhenius curve of electrical conductivity. The activation energies of the powder with the recombination ratio of LSCF and GDC at 7:3, 6:4 and 5:5 are 0.1001, 0.0901 and 0.1376 eV, respectively, which is smaller than that of pure LSCF, indicating that the catalytic activity of the composite material is enhanced.

Electrochemical impedance analysis of composite cathode
Electrochemical impedance (EIS) tests were carried out on composite cathodes of different composite proportions, and LR Ω (QR H )(QR I )(QR L ) model was used to fit the impedance data. The results of ohmic impedance and polarization impedance were shown in Figure 3a and b. The results show that the ohmic impedance of the cathode is minimum when the ratio of LSCF to GDC is 6:4. This is because as the proportion of GDC increases, the contact between the cathode and the impedance layer of GDC becomes closer, thus reducing the interface resistance between the composite cathode and the barrier layer, making it easier for oxygen ions to conduct from the cathode to the electrolyte. However, the interphase contact between LSCF particles and GDC particles increases with the increase of GDC mass fraction, and the interphase resistance increases. When GDC mass fraction is 0.5, the interphase resistance between powder particles is large, resulting in the overall ohmic impedance is large. When GDC mass fraction is 0.3, the interphase resistance between powder particles is small, but the interface resistance between cathode and barrier layer is large, which shows that the overall ohmic impedance is also large, the LSCF-40GDC has the smallest ohmic impedance. The polarization impedance of LSCF-40GDC is also the smallest. This is because the promotion effect of GDC on cathode ion conductivity on cathode impedance is superimposed with the inhibition effect of interface resistance between particles caused by GDC on cathode polarization impedance, resulting in the lowest polarization impedance value.

Discharge analysis of flat tube SOFC
In order to test the discharge performance of the composite cathode, cathode paste with different mass ratios of LSCF and GDC were screen-printed onto the flat tube SOFC. The effective area of the cathode is 3 cm 2 , and the discharge test is carried out in the range of 700 to 800°C with H 2 (180 mL/min) as fuel gas and air as oxidizing gas, and the results are shown in Figure 4. Compared to other cells, LSCF-40GDC single cell has higher power density and better performance. At 700, 750 and 800°C, the open-circuit voltages are 1.02, 1.0 and 0.95 V, and the power densities are 178, 256 and 328 mW/cm 2 , respectively. At the same time, the output power of the single cell decreases with the decrease of temperature. The EIS test results show that the cathode material is sensitive to the change of temperature, and the decrease of temperature will lead to the increase of electrode impedance, thus the power density of the cell is smaller at low temperature.   The flat tube using LSCF-40GDC composite cathode was subjected to constant current discharge to observe the stability of the flat tube SOFC during discharge. Figure 5a shows the trend of cell voltage over time when discharged at a current density of 0.1 A cm -2 at 800°C. The results showed that there was a slight increase in the cell voltage during the first few hours of constant current, which may be due to the NiO in the anode being fully reduced to improve the catalytic activity of the electrode.

Discharge stability test
In the subsequent constant current process, the voltage remained stable at about 0.6 V without attenuation. Figure 5b is the SEM image of the cross-section of the cell after constant current discharge. It can be found that the cathode, barrier layer and electrolyte-anode of the cell are closely connected. The test results show that the flat tube fuel cell with composite cathode has good stability, excellent electrochemical performance and long-term stable operation potential.

Conclusion
LSCF-mGDC (m= 0, 30, 40and 50) composite cathodes were prepared by mixing La 0.6 Sr 0.4 Co 0.2 Fe 0.8 O 3-δ (LSCF) with Gd 0.2 Ce 0.8 O 2 (GDC) by ball milling method. XRD results show that the two compounds have good chemical compatibility and will not react to produce new substances at high temperature. The conductivity test shows that the conductivity of the material decreases with the increase of the proportion of GDC. The EIS test shows that the polarization impedance of the cathode with the combination ratio of 6:4 is the smallest, only 0.10 Ωꞏcm 2 at 800 o C. The results show that LSCF-40GDC has the best discharge performance, and the maximum power is 178,256 and 328 mW/cm 2 at 700、 750 and 800 o C, respectively. The cell voltage was not attenuated within 50 h after 0.1 A cm -2 constant discharge. These results indicate that LSCF-40GDC composite cathode has excellent electrochemical performance and has the potential to be used as SOFC cathode.