Investigation On Critical Materials For Intermediate-temperature Solid Oxide Fuel Cell Operating On Methane Fuel

Solid oxide fuel cell(SOFC)is a power generation device that directly converts chemical energy stored in fuels into electricity by electrochemical reaction.Compared to conventional power generation device,SOFC has the advantages of high energy conversion efficiency,fuel flexibility and low environmental pollutant and so on.Traditional SOFC with nickel-based anode has good performance when hydrogen is employed as the fuel,but the storage and transportation of hydrogen is still facing great challenge.Direct operation of SOFC on hydrocarbons has many important advantages.However,the nickel-based anode will produce serious carbon deposition,which will lead to cell performance degradation.Therefore,considerable efforts have been devoted to the modification of the Ni-based anodes to suppress the coke formation toward hydrocarbon fuels.Traditional SOFC is operated at high temperature above 850°C,which leads to several issues,such as high cost,elecctrode sintering and poor durability of cell.Therefore,many studies have been conducted to reduce the operating temperature,which is the main development tendency of SOFC.However,when the operating temperature reduces,the cathode polarization resistance increases.Therefore,developing structurally stable,high-performance cathode materials to reduce polarization resistance is crucial.On the other hand,the proton-conducting SOFC(H+-SOFC)has a lower activation energy of proton transportation than that of oxygen ion transportation;a low activation energy implies that the proton conductivity of these materials has less temperature dependence.Moreover,water is formed at the cathode rather than the anode,which avoids the dilution of the fuel and decreases the electrode polarization arising from mass transfer at the anode,the proton-conducting electrolyte is more suitable for low operating temperature of SOFC.Therefore,it is critical to develop proper cathodes for the H⁺-SOFC.In addition,when using hydrocarbon fuel,the cathode for H⁺-SOFC is in a high concentration CO_2-H₂O atmosphere,which requires that the cathode has the highest tolerance to CO₂.In this research,we focused on developing electrode materials for intermediate-temperature H⁺-SOFC when operating on methane fuel.BaZr_0.1Ce_0.7Y_0.1Yb_0.1O_3-?(BZCYYb),which has highly proton-conductive and structurally stable in the intermediate-temperature range,had been regarded as an electrolyte in this research.First,we synthesized the SrCoO_3-?based materials and investigated its oxygen reduction activity and CO₂tolerance.Second,a Ni-Fe alloy composite catalyst was prepared,and evaluated its catalytic activity in the partial oxidation of CH₄.The catalyst was applied on Ni-YSZ anode for methane SOFCs.The catalyst-modified cells showed much higher performances and durability.The perovskite SrCoO_3-?is a promising parent compound owing to its excellent catalytic activity for the oxygen reduction reaction.However,for application as a cathode in SOFC,SrCoO_3-?is impeded by its low structural stability and poor CO₂-tolerant.An effective way to increase its stability is to partially substitute the B-site ions with suitable dopants while retaining its catalytic activity and desirable conductivity.1.Research on B-site doping on SrCoO_3-?base cathode materialIn this paper,SrCoO_3-?based cubic perovskite were synthesized and investigated as cathodes for BZCYYb electrolyte based-SOFC.(1)Doping 20 mol%Fe into SrCoO_3-?formed a stabilized perovskite SCF.SrCo_0.8Fe_0.2O_3-?(SCF)was investigated as a cathode based on the BZCYYb electrolyte for SOFC.The chemical compatibility between SCF and BZCYYb was investigated,the impurity Sr Zr O_3-?was detected in the calcined SCF-BZCYYb mixture powder.SCF has poor chemical compatibility with BZCYYb.Although Fe doping improves the structure stability of SrCoO_3-?,but the stability of SCF is poor.SCF exhibits higher conductivity than SrCoO_3-?,the conduction mechanism changes after Fe doping.The average thermal expansion coefficient(TEC)of SCF is26.01×10^-6K^-1.SCF has poor CO₂-tolerance.The results indicate that SCF is not an appropriate cathode for SOFC with the BZCYYb electrolyte.(2)Because of the structural instability of SCF,by doping 5 mol%Zr into SCF to promote its structural stability and CO₂tolerance.When the doping amount of Zr increases,the conductivity decreases.SCF doped with 5mol%Zr,which can promote the structural stability without significantly sacrificing the conductivity.SCFZ showed good chemical compatibility with the electrolyte BZCYYb.Although Zr-doping decreases the electrical conductivity of SCFZ,it is still significantly high in the intermediate temperature range,the electrical conductivities are 202-345 S cm^-1from 700to 400°C.The ionic radius of Zr⁴⁺is larger than those of the Fe³⁺/Fe⁴⁺,resulting in larger unit-cell volume of the SCFZ than that of SCF.A larger volume is commonly associated with higher TEC.So,the average TEC of SCFZ is larger than that of SCF.The structural stability of SCFZ powders exposed to CO₂and H₂O was investigated by treating the samples in 3%H₂O-5%CO₂-O₂atmosphere over 24 h at 700°C,no carbonate phase was detected in the XRD spectra of SCFZ.CO₂-TPD also proves that SCFZ has the high tolerance to CO₂.The degradation of cathode performance was evaluated at 700°C as a function of time using a symmetrical cell SCFZ|BZCYYb|SCFZ in a gas mixture of air-10 vol%CO₂,the results suggested that SCFZ showed a significantly slower performance degradation compared to SCF and BSCF.The area specific resistance(ASR)of symmetrical cell is 0.07?cm²at 700°C,which indicates that SCFZ has high oxygen reduction activity.The NiO-BZCYYb|BZCYYb|SCFZ-BZCYYb cell demonstrated a peak output of 712 m W cm^-2at 700°C and a long-term stability over at least 300 h,which indicates that SCFZ is a potential cathode candidate for intermediate temperature SOFC with BZCYYb electrolyte.(3)Fe,Zr and Y were co-doped into SrCoO_3-?to form a structurally stable cubic perovskite material SCFZY.SCFZY cathode is chemical compatible with the BZCYYb electrolyte under the working conditions.The electrical conductivity values of SCFZY is higher than that of un-doped SrCoO_3-?.The average TEC of SCFZY is 24.89×10^-6K^-1.The structure of SCFZY did not change and no second impurity phase formed after exposed to 3%H₂O-5%CO₂-O₂atmospheres at 700,650 and 600°C for 24 h,which indicate that the SCFZY is stable at H₂O and low concentration CO₂atmosphere.The CO₂-TPD tests indicate that SCFZY has outstanding CO₂resistance.The Rp of symmetrical cell SCFZY|BZCYYb|SCFZY was tested in air containing 10 vol%CO₂at 700°C.SCFZY has a high CO₂tolerance and shows a significantly slower performance degradation compared to SCFZ and BSCF.The NiO-BZCYYb|BZCYYb|SCFZY-BZCYYb single cell outputs a peak power density of 679 m W cm^-2and a voltage variation over500 h.The results indicate that SCFZY is a potential cathode candidate for intermediate temperature SOFC with BZCYYb electrolyte.2.Research on coking resistance of composite anode materialLa_0.7Sr_0.3Fe_0.8Ni_0.2O_3-?(LSFN)was prepared by a sol-gel method and spray-coated over a NiO-YSZ anode as the catalyst layer.Under a reducing atmosphere,LSFN decomposed into Fe0.₆₄Ni_0.36,Sr La Fe O₄,and La₂O₃with the in situ exsolved Fe-Ni alloy uniformly anchored on the oxide backbone.The catalytic activity of the alloy composite for the partial oxidation of methane(POM)was tested,and the electrochemical performance and discharge stability of the catalyst-modified SOFCs were investigated using methane based-fuels.This work provides theoretical basis and reference of material design for the application of intermediate-temperature solid oxide fuel cell operating on methane fuel.