| An approach to make a balance of long lifetime and low cost in reversible solid oxide cells(RSOCs)devices is to decrease the operating temperature to intermediate temperature(IT,500-800?),which can suppress the degradation and extend the selection range of interconnecting materials.However,the reducing temperature is accompanied by the increasing of the polarization resistances of the oxygen electrode,which will reduce the activity of oxygen reduction reaction(ORR)in SOFC and oxygen evolution reaction(OER)in SOEC.Consequently,how to design high performance electrocatalysts which can accelerate the kinetics of the ORR and OER has become a hot research topic.In this paper,the current hot spots in research of mixed ionic-electronic conductors(MIEC)materials and composite hetero-structure oxygen electrode materials were studied,including perovskite structure Nd1-xSrxCoO3-?(NSC 113),Ruddlesden Popper(RP)structure Nd2-xSrxCoO4-?(NSC214),double perovskite structure REBaCo2O5+? and composite hetero-structure materials NSC113-214 and NSC113-GDC.The performance of these materials was evaluated for the first time when they were used as oxygen electrode in IT-RSOCs,and the origin of the enhancement of ORR and OER activity was also discussed.Perovskite structure Nd1-xSrxCoO3-?(x = 0.3,0.4,0.5,0.6 and 0.7)materials,which exhibited an orthorhombic structure with the space group Pnma,were prepared via the glycine nitrate process(GNP).The lattice volume of these materials increases with increasing Sr content,x.Nd0.5Sr0.5CoO3-?(NSC0505)has a good chemical compatibility with the GDC electrolyte.The weight loss of the samples increases with increasing Sr content,which may result from electronic compensation or ionic compensation.The average thermal expansion coefficients ofNd1-xSrxCoO3-?(x= 0.3,0.4,0.5,0.6,and 0.7)are 15.7×10-6,16.2×10-6,17.7×10-6,18.4×10-6 and 18.5×10-6K-1 between 100 and 900?,respectively.The optimal sintering temperature for NSC0505 is 1100?.In addition,NSC0505 has the lowest total cathodic polarization resistance(Rp)with regard to Nd1-xSrxCoO3-?,which can be ascribed to a higher electrical conductivity and lower activation energies.At 750?,for x=0.3,0.4,0.5,0.6,and 0.7,the Rp is 0.25,0.157,0.099,0.101 and 0.475 ?·cm2 respectively.The result of single cell test for NSC0505 shows that,the area-specific polarization resistance(ASR)values in SOFC mode and SOEC mode are 1.92 and 1.55 ?·cm2 respectively.In SOFC mode,the maximum of the power density at 800 ? is 205 mW·cm-2,which is much higher than that of the traditional material LSM(110mW·cm-2).In SOEC mode,for a voltage of 1.2V the power density value at 800 ?is 435.6 mW·cm-2.Powders of Nd2-xSrxCoO4-?(x = 0.8,1.0 and 1.2),which exhibited a tetragonal structure with space group I4/mmm,were prepared via citrate-nitrate combustion method.The lattice volume of these materials increases with the increasing Sr content.Nd0.8Sr1.2CoO4-?(NSC0812)has a good chemical compatibility with the GDC electrolyte.The average thermal expansion coefficients for Nd2-xSrxCoO4-?(x=0.8,1.0 and 1.2)are 13.97×10-6,15.2×10-6 and 17.1 ×10-6 K-1 between 100 and 900? respectively.The activation energies for Nd2-xSrxCoO4-?(x = 0.8,1.0 and 1.2)are 0.27,0.19 and 0.2 eV,respectively.The total cathodic polarization resistance(Rp)of these materials decreases with increasing Sr content,and Nd0.8Sr1.2CoO4-? has the lowest Rp.At 800? the Rp of NSC0812 is 0.193 ?·cm2.The result of single cell tests for Nd2-xSrxCoO4-?(x = 0.8,1.0 and 1.2)shows that,in SOFC mode,the ASR values are 6.22,6.72 and 4.44 ?·cm2 respectively,and the maximum of the power densities at 800 ? are 40.2,37.2 and 58.8 mW·cm-2,respectively.In SOEC mode,for a voltage of 1.2 V,the power density values at 800? are 219.6,208.8 and 247.2 mW·cm-2 respectively,and the corresponding ASR values are 2,2.46 and 1.8?·cm2 respectively.Powders of REBaCo2O5+?(RE= Pr,Nd and Gd)were prepared via the EDTA-citrate process.PrBaCo2O5+?(PBCO)and NdBaCo2O5+?(NBCO)crystallize in a tetragonal structure with the space group P4/mmm while GdBaCo2O5+?(GBCO)crystallizes in an orthorhombic structure with the space group Pmmm.REBaCo2O5+? materials have a good chemical compatibility with the GDC electrolyte.The average thermal expansion coefficients for REBaCo2O5+?(RE= Pr,Nd and Gd)are 21.8×10-6,19.96×10-6 and 19.3×10-6 K-1 between 100 and 900?,respectively.The electrical conductivities of the samples are 645 S·cm-1(PBCO),361 S·cm-1(NBCO)and 318 S·cm-1(GBCO)at 600?.The electrical conductivity decreases as the ionic radii of RE decreases,which is related to an increasing concentration of oxygen vacancies.In these oxides,NBCO has the lowest Rp,and the Rp values of REBaCo2O5+?(RE= Pr,Nd and Gd)at 800? are 0.13,0.07 and 0.14 ?·cm2,respectively.For the single RSOC tests,in SOFC mode,the ASR values for REBaCo2O5+?(RE= Pr,Nd and Gd)at 800? are 1.27,0.55 and 0.62 ?·cm2 respectively,and the maximum of the power densities are 212,425 and 320 mW·cm-2,respectively.In SOEC mode,for a voltage of 1.2V,the power density values at 800 ? are 0.65,1.58 and 0.4 W·cm-2 respectively,and the corresponding ASR values are 0.796,0.5 and 1.02?·cm2 respectively.In conclusion,the catalytic activity of these three oxygen electrode materials is in the order of REBaCo2O5+?>Nd0.5Sr0.5Co03-?>Nd2-xSrxCoO4-?.This can be ascribed to that more oxygen defects exist in double perovskite material(REBaCo2O5+?),which can enhance the IT performance significantly.In addition,comparing to RP material(Nd2-xSrxCoO4-?),the performance of the perovskite material(Nd0.5Sr0.5CoO3-?)is higher due to its higher conductivity.With regard to the performance evaluation in SOFC model,REBaCo2O5+? and Nd0.5Sr0.5CoO3-? show a better performance than traditional LSM,suggesting that MIEC materials have the potential as IT-RSOC oxygen electrode materials.Composite hetero-structure Nd0.5Sr0.5CoO3-?Nd0.8Sr1.2CoO4-?(NSC113-214,perovskite-like structures)and Nd0.5Sr0.5CoO33-?Gd0.2Ce0.8O1.9(NSC113-GDC,perovskite-fluorite structures)materials were prepared and evaluated as oxygen electrode for IT-RSOCs.The electrochemical performance and surface exchange kinetics of the two hetero-structure electrodes were investigated.NSC 113-214 and NSC 113-GDC composite materials were demonstrated to have a faster oxygen exchange kinetics compared to the single-phase materials at 600?.In comparison to NSC113-214,the total Rp of NSC113-214 and NSC 113-GDC composite materials could reduce by more than half,Furthermore,the NSC113-GDC composite oxygen electrode has a lower Rp than NSC113-214,which is mainly due to more oxygen vacancies in GDC materials.Interestingly,the XPS results related with Osurf/OLa were opposite for the two materials,indicating that the governing mechanism to enhance ORR performance were different.As for perovskite-like composite materials,the main causes to enhance the ORR performance are fast oxygen transport conductivity and the changed electronic structure.Whereas,the governing mechanism for perovskite-fluorite structures composite materials is related to atomic reconstruction and the extending of active areas in three-phase boundary(3PB). |
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