Preparation And Study Of Core-shell Structure Cathode For Intermediate Temperature Solid Oxide Fuel Cell

Solid oxide fuel cell(SOFC)is one of the high-efficiency and clean electrochemical devices whose development and application would benefit both the energy and environment.For SOFC's soon commercialization and industrialization,it's key to control its operation and maintenance' costs in a reasonable range.Hence,lowering the operation temperature of SOFC from high temperature(HT,800-1000 oC)to intermediate temperature(IT,600-800 oC)draws the attention of researchers at home and abroad.However,the cathode's performance always degrades with the lowering temperature.Therefore,the development of high-performance cathode materials used for IT-SOFC is of great significant.Simple perovskite materials such as Sr-doped La1-x Srx Mn O3-?(LSM),Sr,Fe-doped La1-x Srx Co1-y Fey O3-?(LSCF)Ba1-x Srx Co1-y Fey O3-?(BSCF)are all considered as IT-SOFC' cathode.Besides,double perovskite materials(DP,AA'B2O6)and Ruddlesden-Popper perovskite related materials(RP,An+1Bn O3n+1)are attracting ever-growing attentions due to their excellent ORR catalytic activity.Among them,Pr Ba0.5Sr0.5Co1.5Fe0.5O5+?(PBSCF)double perovskite cathode is highly valued due to its great oxygen ion conductivity.It turns out that good electrochemical performance can be achieved by PBSCF cathode at the intermediate temperature range.However,the existence of alkaline earth metal,Ba and Sr,is a big challenge for PBSCF cathode's long-term stability due to the CO2 gas contained in air.To cope with this problem,we choose the alkaline-earth-metal-free material La2 Ni O4+?(LN)as a dense coating cover the whole PBSCF scaffold as an PBSCF-LN core-shell structure cathode.Comprehensive characterizations and tests are carried out on this core-shell structure cathode,and all the results are thoroughly analyzed.The following conclusions can be drawn:(1)LN nitrates solution is infiltrated onto PBSCF scaffold and fired to form certain LN phase,and so prepared PBSCF-LN core-shell structure cathode' electrochemical performance is evaluated by EIS tests.Compare to pristine PBSCF cathode,PBSCF-LN cathode has better ORR catalytic activity.LN's high oxygen surface exchange coefficient k*(~10-6 cm s-1,590 oC)enable its enhancement of oxygen surface process of cathode,and thus enhancement of the cathode electrochemical performance;(2)EIS tests are carried out on pristine PBSCF cathode and PBSCF-LN cathode under a 10% CO2 atmosphere,the changing trends of these two cathodes' RP values as the function of time are collected.PBSCF cathode's performance degrades dramatically during the 20 h tests,and its performance hardly recovers even when CO2 is removed.PBSCF-LN cathode' performance degrades much slower,and it almost recovers to its initial value when CO2 is removed.The performance degradation caused by CO2 is irreversible for PBSCF cathode,and it's reversible for PBSCF-LN cathode;(3)The LN coating' s thickness can be controlled by LN nitrates solution's concentration,and the electrochemical performance of PBSCF-LN cathodes with different LN coating are all tested and analyzed.As the oxygen bulk diffusion coefficient D* of LN is quite low(~10-8 cm2 s-1,600 oC),the best performance turns out to be the cathode with the thinnest LN coating(10nm~);The current polarization tests show that the LN-coated PBSCF cathodes have better electrode stability than the uncoated ones.Post-test cathode samples' XPS characterizations show that the existence of LN coating can reduce the surface Sr's generation,which also enable the PBSCF-LN cathode's better polarization stability;TEM characterization carried out on post-test PBSCF-LN cathode shows that,the Sr O phase has generated at the PBSCF/LN interface,besides,an interplanar distance as 2.79 ? is found which could be the expanded(110)lattice plane in LN caused by partial Sr replacement for La in LN lattice.And the very replacement enable more oxygen vacancies' generation,thus more oxygen ions paths were realized and compensate the cathode's polarization degradation to some extent;(4)Cr-poisoning effect on pure double-perovskite PBSCF cathode and PBSCF-LN cathode is investigated.During the Cr poisoning test in the presence of SUS430 interconnect,the electrochemical performance of pristine PBSCF cathode shows severe degradation,PBSCF-LN cathode,by contrast,shows remarkable tolerance towards chromium.The Ba Cr O4 deposits are identified on the PBSCF cathode surface.The reaction between volatile Cr species and PBSCF cathode can be prevented by a dense LN coating.Thus,a more stable electrochemical performance is achieved by PBSCF-LN cathode during the Cr poisoning test.