Application Of Negative Thermal Expansion Sm_0.85Zn_0.15MnO₃ In O-SOFC Cathode

As an all-solid-state electrochemical power generation device with low pollution and high conversion efficiency,solid oxide fuel cell（SOFC）has been widely studied.At present,the commonly used high-performance cathode material for SOFC is mainly mixed ionic-electronic conductor（MIEC）with high thermal expansion coefficient（TEC）,leading to a serious thermal mismatch between cathode and electrolyte.This mismatch degrades the long-term stability and thus hinders the practical application of SOFC.To solve this problem,the method of adding negative thermal expansion（NTE）oxide in conventional cathodes is adopted here.It aims to improve the thermal matching of cathode/electrolyte interface by reducing TEC of composite cathodes,and thereby to further improve the electrochemical performance and long-term stability of cells.First,NTE oxide Sm_0.85Zn_0.15Mn O₃（SZM）is introduced into the highly catalytically active Fe-based Ba_0.5Sr_0.5Fe_0.8Cu_0.1Ti_0.1O_3-δ（BSFCT）MIEC cathode.XRD results indicate that BSFCT,SZM and electrolyte Ce_0.8Sm_0.2O_2-δ（SDC）oxides have good chemical compatibility up to 1173 K.The average linear TEC of dense BSFCT-x SZM（x=0,10,20 and 30 wt.%）decreases markedly from 29.2×10^-6 K^-1 for x=0 wt.%to 15.6×10^-6 K^-1 for x=30 wt.%.The electrochemical performance of single cells with configuration of Ni O-BZCY|SDC|BSFCT-x SZM was tested in the773-973 K.The best performance is obtained for single cell with x=20 wt.%.The corresponding peak power density（PPDs）is 136-918 m W·cm^-2,which is 249%-64%higher than that(39-559 m W·cm^-2)with single BSFCT cathode.It should be caused by the balance between thermal matching of cathode/electrolyte layers and ORR activity of composite cathode since the former improves while the latter decreases the cell performance with increasing x.Due to the existence of electron blocking layer at anode/electrolyte interface,the open circuit voltage of all cells is higher than 1.0 V,which is significantly higher than that of traditional O-SOFC（<0.9 V）.Secondly,in order to further verify the universality of NTE addition in MIEC cathode,SZM oxide was also introduced into the highly catalytically active Co-based Ba_0.5Sr_0.5Co_0.75Fe_0.2Nb_0.05O_3-δ（BSCFN）MIEC cathode.XRD results show that BSCFN,SZM and SDC oxides have also good chemical compatibility.The average TEC of dense BSCFN-x SZM（x=0,10,20 and 30 wt.%）at 373-1023 K decreases slightly from 18.6×10^-6 K^-1 for x=0 wt.%to 14.7×10^-6 K^-1 for x=30 wt.%.The electrochemical performance of single cells in the Ni O-BZCY|SDC|BSCFN-x SZM configuration was tested in the 773-973 K.The performance of single cell with BSCFN-20SZM is also the best.The corresponding PPDs is 241-1122 m W·cm^-2,which is about 16%higher than that(208-931 m W·cm^-2)with single BSCFN cathode.Comparing to apparently improvement（249%-64%）observed in cells with BSFCT-x SZM cathode,much smaller improvement（～16%）can be obtained with BSCFN-x SZM one.It should be caused the fact that the TEC difference between BSCFN and SDC is apparently smaller than that between BSFCT and SDC.Hence,the performance promotion caused by the thermal matching improvement is relatively small.In addition,the long-term stability of single cells with BSCFN and BSCFN-20SZM cathodes was comparatively tested for 100 hours.It shows that the PPD of single cells with BSCFN-20SZM cathode degrades by 21%,which is apparently smaller than that（36%）obtained in cell with BSCFN one.That is,SZM addition can indeed improve the long-term stability of single cells.In conclusion,the addition of NTE oxide SZM in cathodes can not only enhance the electrochemical performance by improving the thermal matching between cathode and electrolyte with reduced cathode TEC,but also significantly improve the long-term stability of single cells.The larger TEC of MIEC cathode,the more improvement of performance can be obtained by NTE addition.In all,the appropriate addition of NTE SZM to cathodes is indeed an effective strategy to improve the overall performance of O-SOFC.