Investigation Of Sr₂Fe_1.5Mo_0.5O_6-?and LiFeO₂ Based Anode Materials For Intermediate Temperature Solid Oxide Fuel Cell

Solid oxide fuel cell?SOFC?is one kind energy producing technology which is highly efficient and environmentally friendly.In recent years,the focus of SOFC development has been on lowering the operating temperature and developing novel intermediate temperature electrode materials.Lowering operating temperature could enlarge the application areas of SOFC,hence promoting its commercialization.Therefore,anode materials with high conductivity and catalytic activity have attracted great attentions.In this research,the electrochemical performance of perovskite Sr₂Fe_1.5Mo_0.5O_6-?and LiFeO₂ based anode materials were mainly improved by doping on Fe-site.The crystal structure,element valences and chemical stability were also characterized in details.On this base,the performance of prepared materials and application in many other fields were investigated.Initially,the Ni-substituted Sr₂Fe_1.5-xNi_xMo_0.5O_6-?materials were prepared using the combustion method.XRD results showed that Sr₂Fe_1.5-xNi_xMo_0.5O_6-?crystallized in a single-phase perovskite structure when nickel doping was under 20 mol%.Temperature programmed reduction?TPR?results indicated that the stable temperature for samples under reduction conditions decreased with Ni content.And metallic nickel particles were detected on the grain surface with higher Ni doping level.XPS analysis suggested when x=0.1,the ratio of Fe³⁺/Mo⁵⁺became the largest,which leads to the highest electronic conductivity of20.6 S cm^-1.We inferred that Ni enhanced the strong hybridization of Fe-O so as to retain a metallic character.The maximum power density of 530 mW cm^-2 was achieved with Sr₂Fe_1.4Ni_0.1Mo_0.5O_6-?anode at 800 ^oC,which is twice that of the un-doped material.Then,the effect of Sr deficiency on Sr_2-x-x Fe_1.4Ni_0.1Mo_0.5O_6-?anode performance was investigated.XRD refinements indicated that Sr deficiency could enhance coulombic cation repulsion,resulting in the enlargement of lattice.TGA results showed that more oxygen vacancies were introduced and the chemical stability was enhanced by Sr deficiency.With increase of Sr deficiency,the electrical conductivity in 5%H₂/Ar reached a peak value of26.6 S cm^-1 for Sr_1.95Fe_1.4Ni_0.1Mo_0.5O_6-?.While further increase of Sr deficiency reduced conductivity by affecting the mobility of electronic charge carriers.The cell with LSGM electrolyte and LSCF cathode demonstrated a maximum power density of 606 mW cm^-2 at800 ^oC operating in H₂.In addition,the H₂ production rate was 512 mL cm^-2 h^-1 at 850 ^oC when this cell was applied in SOEC.All the analysis suggests that the introduction of A-site deficiency coupled with reducible transition metal catalyst is an effective approach to improve the anode performance.Additionally,the novel intermediate temperature materials LiFe_1-xM_xO₂?M=Al,Mg?were synthesized by solid state reaction.At 700 ^oC,the chemical stability of doped LiFeO₂increased under reducing atmosphere.Meanwhile some Fe metal in-situ exsolved from the lattice,which was one kind of excellent anode catalysts.Adequate oxygen vacancies were introduced by Mg doping and it is conducive to increase the conductivity.Among all doped materials,LiFe_0.98Mg_0.02O₂ exhibited the maximum value of 11.5 S cm^-1 at 700 ^oC.All the cells were fabricated with LSGM electrolyte and LSCF cathode for testing.The performance of LiFe_0.9Al_0.1O₂ and LiFe_0.98Mg_0.02O₂ anodes in H₂ at 700 ^oC were 245 mW cm^-22 and 225mW cm^-2 respectively.No obvious degradation were observed after 18 h stable performance at 650 ^oC.The LiFe_0.9Al_0.1O₂ and LiFe_0.98Mg_0.02O₂ anode performance with carbon/carbonates fuel were also explored,giving maximum power densities of 162 mW cm^-2 and 140 mW cm^-2.These results indicate that LiFe_1-xM_xO₂?M=Al,Mg?materials could be applied as anode materials for fuel cells.Finally,the influence of Ni doped LiFeO₂ on crystal structure,reduction property and electrochemical performance were studied.XRD patterns showed that the unit cell parameter decreased with Ni content increasing and there was a phase transition from cubic to rhombohedral.After reducing at 700 ^oC,Ni-Fe metals exsolved out of the lattice and significantly improved the electronic conductivity.The nano-scaled electrodes were prepared by impregnating LiFe_1-xNi_xO₂ into the porous LSGM scaffold in order to solve the mismatch in TEC.After that,the fabricated cell was tested with carbon/carbonates fuel,displaying a maximum power density of 180 mW cm^-2 at 700 ^oC for LiFe_0.5Ni_0.5O₂ anode.The cell output almost doubles that of LiFeO₂ anode.EIS analysis manifested that Ni doping could effectively decrease the polarization resistance by accelerating the anode kinetic reaction.The cell showed stable performance at 650 ^oC with no element diffusion under a constant voltage of 0.7 V for 12 h.Therefore,LiFe_1-xM_xO₂?M=Al,Mg,Ni?are promising candidates as the anodes for intermediate temperature SOFC.