Performance Of Sr-and Y-based Perovskite Cobaltites As Cathode Materials For Intermediate-temperature Solid Oxide Fuel Cells

Solid oxide fuel cell?SOFC?is an efficient and clean energy power generation technology that converts chemical energy directly into electrical energy.To reduce the operating temperature of SOFC from high temperature?1000°C?to medium temperature?600–800°C,IT?,developing low-cost,high-life IT-SOFC is the development direction of SOFC.However,the decrease in operating temperature also leads to a sharp increase in electrode polarization resistance,and the cathode polarization loss is the main factor that restricts the efficiency of IT-SOFC operation.Therefore,it is of great significance to develop high-performance new cathode materials.In this paper,perovskite structure SrCoO₃–??SCO?and layered perovskite structure YBaCo₂O₅₊??YBCO?oxides which with excellent mixed ionic-electronic conduction characteristic and high catalytic activity toward oxygen reduction reaction?ORR?were selected as research objects.In order to solve the problems of unstable phase structure and high thermal expansion coefficient?TEC?of these two materials,three strategies of A and B site cation doping and composite material preparation were adopted to optimize their structure and properties.The main research contents and experimental results of this paper are as follows:Aiming at the problem of unstable structure of the perovskite structure SCO oxide due to the high temperature thermal reduction of Co⁴⁺ions and the excessive lattice oxygen loss,we adopt the strategy of replacing the low-valence Sr²⁺ions at A-site with high-valence Y³⁺ions,and prepared the Sr1-xY_xCoO3-??SYC,x=0.05,0.10,0.15 and0.20,denote as SYC05,SYC10,SYC15 and SYC20?cathode materials by sol-gel method,and also discussed the influence of Y³⁺doping on the structure and properties of SCO.Phase structure analysis show that SYC05 and SYC10 samples crystallized well with pure cubic phase,while SYC15 and SYC20 samples contained heterogeneous peaks of oxygen ordered superstructure.In situ HT-XRD results show that SYC05samples are partially decomposed into Sr₆Co₅O₁₅ and Co₃O₄ at temperatures above700°C,while SYC10 samples maintain good structural stability below 850°C.The SYC10 sample has good chemical compatibility with LSGM and GDC electrolytes below 950°C.The Y doping greatly improves the conductivity of the material compared to undoped SCO.The conductivity of the SYC10 exceeds 350 S cm^-1 over the temperature range of 300-850°C.SYC10 also exhibit excellent electrochemical performance,at 700,750 and 800°C,the ASR of LSGM-supported symmetrical cell are 0.080,0.042 and 0.022?cm²,respectively,and the maximum power density of the single cell reached 353.3,498.2 and 643.1 mW cm^-2,respectively.The above studies show that 10 mol%Y doping effectively improves the stability of the high temperature phase structure of SCO oxide and obtains excellent electrochemical performance.However,the TEC of SYC10 sample is as high as 23.3×10^-66 K^-1,which is much higher than that of commonly used SOFC electrolyte materials,and hinders its practical application as a cathode material in SOFC.To solve the problem of excessive TEC of SYC10 material,we adopted the strategy of introducing GDC electrolyte material with low TEC and high ionic conduction characteristics to make composite cathode,and prepared SYC10-xGDC?x=20,30,40,50 wt.%?composite cathodes,to seek the optimal ratio of SYC10 cathode material and GDC electrolyte.The results show that the TEC of SYC10-xGDC composite cathode materials decreases with the increase of GDC content,among which the average TEC of SYC10-50GDC with GDC content of 50 wt.%is the smallest,i.e.18.5×10^-66 K^-1,which is reduced by 20.6%as compared to SYC10 single-phase cathode material.The introduction of GDC is also helpful to improve the catalytic performance of cathode material.The best composition of composite cathode SYC10-40GDC,at 700°C,its ASR is only 0.044?cm²,which is 45%lower than the ASR of the SYC10 cathode material at the same temperature.To solve the problem that the phase instability and high TEC in YBCO at elevated temperature,from the point of view of increasing the bond energy of Co-O bond and reducing the amount of spin state transition of Co³⁺ion,we adopt a single and double doping strategy of doping Fe or/and Cu at Co site,and prepared YBaCoCuO₅₊??YBCC?,YBaCoFeO₅₊??YBCF?and YBaCo_2/3Fe_2/3Cu_2/3O₅₊??YBCFC?cathode materials by sol-gel method.The as-prepared YBCC and YBCFC samples crystallized well,forming a tetragonal single-phase double perovskite structure,while the hexagonal phase?-Fe2O3 impurity appeared in the YBCF sample.YBCC,YBCFC and YBCF samples maintain structural stability below 800°C.Both YBCC and YBCFC have good chemical compatibility with LSGM electrolytes below 950°C.Cu or Fe-doped YBCC,YBCF and YBCFC samples did not undergo structural phase transitions in the temperature range of 30-850°C,and the average TEC was lower than that of undoped YBCO.The reason is that the doping of Cu and Fe reduces the oxygen loss of the material at high temperature,thereby reducing the influence of oxygen vacancies on the chemical expansion of the material at high temperatures.According to the results of first-principles calculation,the[LnO?]layer is the easiest to form oxygen vacancies in the doped samples,and all doped samples have higher oxygen vacancy formation energy than undoped YBCO,thus reducing the oxygen loss of the material at high temperatures.The above studies show that the doping of Cu or/and Fe reduces the TEC of the material and improves the structural stability.Among the doped samples,Cu-doped YBCC has the best catalytic performance.However,due to the decrease in Co content,its electrochemical performance is reduced compared to YBCO.For example,at 700°C,the ASR between YBCC and LSGM electrolyte is 0.138?cm²,which is higher than the ASR?0.125?cm²?of YBCO under the same conditions.For the degradation of electrochemical performance caused by doping at Co site of YBCO,we adopt another doping strategy.From the viewpoint of increasing the lattice oxygen content of the material,the method was used by partly replacing the Ba²⁺ion with Ca²⁺cation with a smaller ionic radius,to reduce the adverse effects of the difference between the Y³⁺ion and the Ba²⁺ion radius on the oxygen vacancy concentration and structural stability,thus improving the thermal expansion compatibility and structural stability,and ensuring the materials with high electrochemical performance?which associated with the content of Co and the concentration of oxygen vacancies?.To this end,we prepared YBa_1-xCa_x CoO₅₊??YBCa,x=0.05,0.10 and 0.15,denote as YBCa05,YBCa10?YBCa15?cathode materials by solid state reaction.The as-prepared YBCa samples all crystallized well with perovskite phase,which have a 3?2a_p×3?2a_p×2a_p supercell structure with a space group of P4/nmm.The YBCa materials maintain structural stability below 800°C,and have good chemical compatibility with the LSGM electrolyte below 950°C.Compared with YBCO,the Ca-doped YBCa samples have increased lattice oxygen content,improved thermal expansion compatibility,conductivity and electrochemical performance.YBCa15 cathode material has the lowest TEC and best electrochemical performance,which is a promising IT-SOFC cathode material.