中温固体氧化物燃料电池Pr1.2Sr0.8NiO4阴极材料的制备、结构和性能

杨俊芳 程继贵,* 樊玉萌 王 睿 高建峰

(1合肥工业大学材料科学与工程学院,合肥230009;2中国科学技术大学材料科学与工程系,合肥230026)

中温固体氧化物燃料电池Pr1.2Sr0.8NiO4阴极材料的制备、结构和性能

杨俊芳1程继贵1,*樊玉萌1王 睿1高建峰2

(1合肥工业大学材料科学与工程学院,合肥230009;2中国科学技术大学材料科学与工程系,合肥230026)

以相应的氧化物粉末和盐为原料,通过甘氨酸-硝酸盐法合成出了中温固体氧化物燃料电池(IT-SOFC) Pr1.2Sr0.8NiO4(PSNO)阴极原料粉体,并制备出了烧结体试样.采用X射线衍射(XRD)分析对所合成粉体的相组成进行了分析,分别采用热膨胀仪和四端子法对PSNO烧结体试样的热膨胀系数和电导率进行了测定,同时对该阴极材料与Sm0.2Ce0.8O1.9(SCO)电解质材料的电化学阻抗谱(EIS)进行了测试分析.以SCO作电解质,分别以NiO/SCO和PSNO作阳极和阴极材料,制备出固体氧化物燃料单电池,并对其性能进行测试.实验结果表明,通过甘氨酸-硝酸盐法,在1050°C以上煅烧前驱体,可以获得具有K2NiF4结构的PSNO粉体.所制备的PSNO烧结体试样在200-800°C间的热膨胀系数约为12×10-6K-1,在450°C下的电导率约为155 S·cm-1,在400-800°C,平均电导活化能为0.034 eV.电化学阻抗谱分析结果表明,在700°C下PSNO阴极和SCO电解质间的比表面阻抗(ASR)为0.37 Ω·cm2,而Ni-SCO/SCO/PSNO单电池的比表面阻抗为0.61 Ω·cm2;所制备的SOFC单电池在800°C下的输出功率为288 mW·cm-2,开路电压为0.75 V.本研究的初步结果表明PSNO材料是一种综合性能较为优良的新型中温固体氧化物燃料电池阴极材料.

中温固体氧化物燃料电池;PSNO阴极材料;甘氨酸-硝酸盐法;热膨胀系数;电化学性能

1 Introduction

Solid oxide fuel cell(SOFC)is one of the modern,environmentally friendly and most efficient power generation devices with low greenhouse gas emission and is attracting a lot of attentions.1At present,much work has been done on the development of intermediate temperature solid oxide fuel cells (IT-SOFCs)to accelerate the commercialization of SOFCs. The lower operating temperature would solve many problems associated with the high temperature operation including electrode sintering and interfacial diffusion between electrolyte and electrodes.As the operation temperature is lowered,however,the cathodic processes at the interface between the cathode and the electrolyte become predominant,limiting SOFC performance.Thus much work has gone into finding suitable materials for low temperature cathodes.2-4

Up to now,cathode materials for SOFC are normally based on mixed oxygen ionic and electronic conducting oxides (MIEC),such as perovskite-type oxides AMO3.5,6Relatively, A2MO4oxides with K2NiF4type structure are less intensively investigated.The ideal K2NiF4-type structure represents a combination of AMO3perovskite and AO rock-salt layers arranging one upon the other.7Its thermodynamic stability is higher and the thermal expansion coefficient(TEC)is generally lower than that of the perovskite-type oxides.8Furthermore,the K2NiF4-type oxides exhibit high electrical conductivity as well as high oxygen diffusion coefficient and surface exchange coefficient,which are about one order of magnitude higher than those of the best perovskite materials.9,10It has reported that partial substitution on the A site of the A2BO4compounds by some alkaline-earth metallic ions can increase oxygen vacancy concentration in the structure and enhance electrical conductivity and oxygen diffusion coefficient of the materials.But by now,most researches on A2BO4-type oxides have focused on La and Nd at A-site,and there are few reports about other rare earths like Pr.11,12Recently,the structure and properties of Pr2-xSrxNiO4(x=0.3,0.5,0.8)materials have been investigated. It was shown that Pr1.2Sr0.8NiO4(PSNO)had the best performances.13Therefore,in this work,PSNO materials were prepared,and their physical and electrochemical properties for used as cathodes of IT-SOFC were investigated.Single cells based on the PSNO cathode and Sm0.2Ce0.8O1.9(SCO)electrolyte were also constructed and tested.

2 Experimental

2.1 Sample preparation

PSNO powders were synthesized by a glycine-nitrate process,in which Pr6O11(Shanghai Yuelong Co.,AR),Ni(NO3)2· 6H2O(Xilong Chemical Industry Incorporated Co.,AR)and SrCO3(Sinopharm Chemical Reagent Co.,AR)powders,and HNO3(Xilong Chemical Industry Incorporated Co.,AR)and C2H5NO2(Sinopharm Chemical Reagent Co.,AR)were used as raw materials.Stoichiometric amount of the oxides and salts were dissolved into nitric acid and glycine was added into the solution at a molar ratio of 1:1.1 of metalic ions/glycine to form a transparent and homogeneous solution,which was subsequently heated and the viscosity of the solution increased gradually.As temperature rose,a self sustaining combustion finally occurred and precursor powders were obtained and calcined at different temperatures.SCO electrolyte powders were synthesized using the gel-casting process.14

The synthesized PSNO powders were pressed into rectangular bars under a pressure of 100 MPa and then sintered at 1200-1300°C for 4 h for test of TEC and electrical conductivity.SCO green disks were prepared by pressing the gelcast SCO powders under a pressure of 100 MPa and sintering the green compacts at 1450°C for 5 h.PSNO/SCO/PSNO symmetrical cells were prepared by screen-printing PSNO pastes onto the two sides of the sintered SCO disks and then co-firing at 1250°C for 4 h.

Commercial NiO(Tianjin Dibo Business Co.,AR)powders and SCO powders were mechanically mixed(w(NiO):w(SCO)=60: 40),the mixed NiO-SCO powders were pre-pressed in a rigid die at 100 MPa.Then SCO powder was dispersed uniformly onto the NiO-SCO substrate and co-pressed at 200 MPa to form green NiO/SCO bilayer compacts,which were co-sintered at 1450°C for 5 h in air.PSNO paste was subsequently screen-printed onto the electrolyte side of the sintered bilayers. Single Ni-SCO/SCO/PSNO cells were finally obtained by sintering the sandwich structure at 1250°C for 4 h.The active area of the cathode is about 0.24 cm2.

2.2 Characterization

Phase structure of the PSNO powders calcined at different temperatures was characterized by X-ray diffraction(XRD,D/ Max-Rb,Japan).Average linear TEC of the sintered PSNO samples were tested using a dilatometer(PCY-Ⅲ,Xiantan Huafeng,China).Electrical conductivity of the sintered PSNO samples was measured by the four-probe method(U3606A,Agilent Technology,USA).Electrochemical impedance spectroscopy(EIS)measurements of the symmetrical PSNO/SCO/ PSNO cell and the single Ni-SCO/SCO/PSNO cell were carried out using an electrochemical workstation(CHI604A, Chenhua,China)under open-circuit conditions with a frequency range from 0.01 Hz to 1 MHz.The spectra were simulated by the Zsimpwin software.Single cell performance was tested using a house-made fuel cell testing device,with humidified hydrogen(3%H2O)as the fuel and air as the oxidant.The voltage-current(V-I)and power-current(P-I)curves of the cells were also tested.Microstructure of the single cell after the electrochemical test was observed by scanning electron microscope(SEM,XT30ESEM-TMP,Holland).

3 Results and discussion

3.1 XRD analysis

Fig.1 shows XRD patterns of the precursor and the PSNO powders calcined at different temperatures.There exist some peaks of miscellaneous phases in the precursor powders,indicating that the chemicals did not react completely during the self-sustaining combustion.When the precursors were calcined at 1050°C,all the peaks were well indexed as K2NiF4-type structure(PDF#24-1222).The samples retained the K2NiF4-type structure when the calcining temperature increases to 1150°C.But the diffraction peaks become sharper and narrower than those of 1050°C,which indicates the grain growth. Grain size of the PSNO powder was calculated according to the Scherrer formula to be 28.91,35.10,43.27 nm,when the precursors were calcined at 950,1050,1150°C,respectively.

3.2 Thermal expansion analysis

Fig.1 XRD patterns of the precursor and PSNO powders calcined at different temperatures

Fig.2 shows TEC of the sintered PSNO specimen as a function of testing temperature.Thermal expansion depends on within-lattice electrostatic forces,which in turn depend on the concentration of positive and negative charges and their distances within the lattice,and are influenced by sintering temperature of the samples.Therefore,the samples sintered at different temperatures show different TEC values.Average TEC values(200-800°C)of the PSNO samples are 12.04×10-6, 12.70×10-6,and 10.69×10-6K-1for samples sintered at 1200, 1250,and 1300°C,respectively.These TEC values are closed to that of SCO electrolyte(12.62×10-6K-1),15which shows that the PSNO materials have good thermal compatibility with SCO electrolyte.

3.3 Electrical conductivity of the PSNO samples

Electrical conductivity(σ)of the sintered PSNO samples was measured in air by the four-probe method at temperature of 400-800°C.Fig.3 shows electrical conductivities of the PSNO samples sintered at different temperatures.As sintering temperature rises form 1200 to 1250°C,the samples become dense and the grain grows,which are beneficial to the transfer of ions and electrons.Therefore,the electrical conductivity increases.However,when the sintering temperature continuously increases to 1300°C,electrical conductivity of the PSNOsample becomes lower than that sintered at 1250°C.This may be caused by the decreases of the grain boundaries.The Arrhenius plots of ln(σT)and 1/T for the sample sintered at 1250°C were also given in the inset of Fig.3.Average conduction activation energy(Ea)was calculated from the ln(σT)and 1/T,and the value is about 0.034 eV at 400-800°C,which is lower than the similar value of MIEC materials(Ea=0.091 eV).16It can also be seen from Fig.3 that as testing temperature increases the conductivities of the PSNO samples first increase and then decrease,indicating the ionic-electronic conduction behavior of the PSNO materials.

Fig.2 TEC of the PSNO samples sintered at different temperatures

Fig.3 Electrical conductivity of the Pr1.2Sr0.8NiO4samples sintered at different temperatures

Fig.4 Electrochemical impendence spectra of the cells based on the PSNO cathode at 700°C(a)symmetrical PSNO/SCO/PSNO cell,(b)single cell Ni-SCO/SCO/PSNO; In Fig.4(b),the equivalent circuit was given,where L is the inductance, C is the capacitance,Q is the constant phase element,and R is the resistance.

3.4 Electrochemical impedance spectroscopy

Fig.4 shows electrochemical impedance spectra of the symmetrical PSNO/SCO/PSNO cells and the single Ni-SCO/SCO/ PSNO cells at 700°C.The high-frequency intercept with the real axis represents the electrolyte ohmic losses,because the electrode ohmic resistance and the contact resistance are negligibly small.17The real axis intercept of the impedance plot is considered to be the area specific resistance(ASR)in the cell, which is caused by the adsorption/desorption of the molecular oxygen and migration/diffusion of the oxygen ions.18,19Fig.4 (a)shows the ASR value of the PSNO cathode on SCO electrolyte,which is 0.37 Ω·cm2at 700°C.This value is lower than that of Nd1.6Sr0.4NiO4cathode on Ce0.9Gd0.1O1.9(CGO)electrolyte(0.93 Ω·cm2).20The lower ASR makes the PSNOmaterials have high electrocatalytic activity for oxidation-reduction reactions and the excellent cathode performance.Fig.4(b)shows that the ASR of the single Ni-SCO/SCO/PSNO cell is 0.61 Ω· cm2at 700°C.

3.5 Single cell performance

Fig.5 V-I and P-I curves of the single Ni-SCO/SCO/PSNO cell at different test temperatures

Fig.6 Cross-section image of the single Ni-SCO/SCO/PSNO cell

Fig.5 shows V-I and P-I curves of the single Ni-SCO/ SCO/PSNO SOFC cell with PSNO as cathode using humidified hydrogen(3%H2O)as fuel and air as oxidant in temperature of 500-800°C.The single cell produces a maximum power density of 288 mW·cm-2and an open circuit voltages (OCV)of 0.75 V at 800°C.Fig.6 shows microstructure of the single Ni-SCO/SCO/PSNO cell after performance test.The SCO electrolyte film is about 25 μm thickness and is sandwiched between a porous cathode layer and a porous anode layer.The SCO layer seems uniform and almost fully dense and no cross-film cracks or pinholes are observed.Good adhesion can also be seen at the cathode/electrolyte and anode/electrolyte interfaces.

4 Conclusions

PSNO powders with K2NiF4-type structure were prepared by the glycine-nitrate process.Average TEC of the sintered PSNO samples is about 12×10-6K-1.The PSNO materials have an electrical conductivity of about 155 S·cm-1at 450°C and an average conduction activation energy of 0.034 eV at 400-800°C.Electrochemical impendence spectroscopy measurement shows that the ASR for the PSNO cathode material on SCO electrolyte is 0.37 Ω·cm2and the ASR of the single Ni-SCO/SCO/PNCO cell is 0.61 Ω·cm2at 700°C.Single Ni-SCO/SCO/PSNO cell with a 25 μm thickness electrolyte and the PSNO cathode produces a maximum power density of about 290 mW·cm-2and an open circuit voltages of about 0.75 V at 800°C.The preliminary test results have shown that the PSNO materials can be a potential cathode material for IT-SOFC.

(1)Ardigò,M.R.;Perron,A.;Combemale,L.;Heintz,O.;Caboche, G.;Chevalier,S.J.Power Sources 2011,196,2037.

(2)Tu,H.Y.;Stimming,U.J.Power Sources 2004,127,284.

(3) Chiba,R.;Yoshimura,F.;Sakurai,Y.Solid State Ionics 1999, 124,281.

(4) Liu,Y.;Rauch,W.;Zha,S.W.;Liu,M.L.Solid State Ionics 2004,166,261.

(5) Santillán,M.J.;Caneiro,A.;Quaranta,N.;Boccaccini,A.R. J.Eur.Ceram.Soc.2009,29,1125.

(6) Kim,Y.M.;Kim-Lohsoontorn,P.;Baek,S.W.;Bae,J.Int.J. Hydrog.Energy 2011,36,3138.

(7)Amow,G.;Whitfield,P.S.;Davidson,I.J.;Hammond,R.P.; Munnings,C.N.;Skinner,S.J.Ceram.Int.2004,30,1635.

(8)Al Daroukh,M.;Vashook,V.V.;Ullmann,H.;Tietz,F.;Arual Raj,I.Solid State Ionics 2003,158,141.

(9) Skinner,S.J.;Kilner,J.A.Solid State Ionics 2000,135,709.

(10) Boehm,E.;Bassat,J.M.;Steil,M.C.;Dordor,P.;Mauvy,F.; Grenier,J.C.Solid State Sci.2003,5,973.

(11) Caronna,T.;Fontana,F.;Sora,I.N.;Pelosato,R.;Viganò,L. Solid State Ionics 2010,181,1355.

(12) Ding,X.F.;Kong,X.;Jiang,J.G.;Cui,C.Int.J.Hydrog. Energy 2009,34,6869.

(13) Huang,X.Q.;Zhang,F.M.J.Phys.Chem.Solids 2009,70,665.

(14) Cheng,J.G.;Jiang,Q.M.;He,H.G.;Yang,J.F.;Wang,Y.F.; Gao,J.F.Mater.Chem.Phys.2011,125,704.

(15) Zheng,Y.F.;He,S.C.;Ge,L.;Zhou,M.;Chen,H.;Guo,L.C. Int.J.Hydrog.Energy 2011,36,5128.

(16)Nie,H.W.;Wen,T.L.;Wang,S.R.;Wang,Y.S.;Guth,U.; Vashook,V.Solid State Ionics 2006,177,1929.

(17) Lv,H.;Wu,Y.J.;Huang,B.;Zhao,B.Y.;Hu,K.A.Solid State Ionics 2006,177,901.

(18) Horita,T.;Yamaji,K.;Sakai,N.;Yokokawa,H.;Weber,A.; Ivers-Tiffée,E.Electrochim.Acta 2001,46,1837.

(19) Khandale,A.P.;Bhoga,S.S.J.Power Sources 2010,195,7974.

(20) Sun,L.P.;Li,Q.;Zhao,H.;Huo,L.H.;Grenier,J.C.J.Power Sources 2008,183,43.

July 29,2011;Revised:October 28,2011;Published on Web:November 16,2011.

Preparation,Structure and Properties of Pr1.2Sr0.8NiO4Cathode Materials for Intermediate-Temperature Solid Oxide Fuel Cells

YANG Jun-Fang1CHENG Ji-Gui1,*FAN Yu-Meng1WANG Rui1GAO Jian-Feng2
(1School of Materials Science and Engineering,Hefei University of Technology,Hefei 230009,P.R.China;
2Department of Material Science and Engineering,University of Science and Technology of China,Hefei 230026,P.R.China)

Pr1.2Sr0.8NiO4(PSNO)cathode material for an intermediate-temperature solid oxide fuel cell (IT-SOFC)was synthesized by a glycine-nitrate process.The phase structure of the synthesized powders was characterized by X-ray diffraction(XRD)analysis.The thermal expansion coefficient(TEC)and the electrical conductivity of the sintered PSNO samples were measured.Electrochemical impedance spectroscopy(EIS)measurements of the PSNO materials were carried out using an electrochemical workstation.Single cells based on the Sm0.2Ce0.8O1.9(SCO)electrolyte were also assembled and tested. The results show that PSNO materials with a K2NiF4-type structure can be obtained by calcining the precursors at temperatures higher than 1050°C.The sintered PSNO samples have an average TEC of about 12×10-6K-1within 200-800°C,an electrical conductivity of 155 S·cm-1at 450°C and an average conduction activation energy of 0.034 eV at 400-800°C.Electrochemical impedance spectroscopy(EIS) shows that the area specific resistance(ASR)of the PSNO cathode on the SCO electrolyte is 0.37 Ω·cm2and the ASR of the single Ni-SCO/SCO/PSNO cell is 0.61 Ω·cm2at 700°C.The single Ni-SCO/SCO/ PSNO cell produces a power density of 288 mW·cm-2and an open circuit voltage of 0.75 V at 800°C. Preliminary work showed that the PSNO materials may be a potential cathode material for use in IT-SOFC.

Intermediate temperature solid oxide fuel cell;PSNO cathode material;Glycinenitrate process;Thermal expansion coefficient;Electrochemical performance

10.3866/PKU.WHXB201111161

*Corresponding author.Email:jgcheng63@sina.com.Tel:+86-551-2901793.

The project was supported by the Natural Science Foundation of Anhui Province,China(070414186),Program of Science and Technology of Anhui Province,China(2008AKKG0332),Nippon Sheet Glass Foundation for Materials Science and Engineering,China(070304B2),and Open Project Program of Key Laboratory of Low Dimensional Materials&Application Technology(Xiangtan University),Ministry of Education of China (DWKF0802).

安徽省自然科学基金(070414186),安徽省科学攻关项目(2008AKKG0332),材料科学与工程日本玻璃片基金(070304B2)和低维材料及其应用技术教育部重点实验室开放基金(DWKF0802)资助

O646;TM911.4