Preparation And Performance Of CeO₂ Based Composite Electrolyte Materials For IT-SOFCs

A fuel cell is a device that can convert chemical energy into electricity directly.The fuel cell is also called a cell because it is composed of electrolyte,anode and cathode,which are the same for a normal cell.The fuel cell generates electricity is the fourth generation technique after the hydraulic power,thermal power and the atomic energy.Theoretically,as long as continue to supply the fuel,the fuel cell can continue to generate electricity then.It generates electricity from fuels such as hydrogen,natural gas and other hydrocarbons.It is different from traditional cells that can only reserve energy;fuel cell can convert energy continuously.Because it is not limited by the carnot cycle,fuel cell has advantages of higher energy conversion efficiency and lower polluted gases emission than the traditional generator.With the natural resource exhaustion and environment deterioration,developing efficient and environmental friendly energy techniques is necessary recently.Since fuel cell just matches such requirements,it attracts the interest all over the world.As the fourth generation fuel cell,SOFC(Solid Oxide Fuel Cell)has many outstanding advantages.First,with all solid components,SOFC eliminates problems liquid electrolyte fuel cell faces,such as corrosion and leakage of liquid electrolytes.Second,operating at high temperatures,its electrode reaction is so fast that it is unnecessary to use noble metals as electrodes.Thus the cost of cells can be minimized.At the same time,the high quality heat it emits can be fully used.The overall energy conversion efficiency of the thermal-electric system can be added up to 80%.The most outstanding advantage of SOFC is that it can use a large of different fuels, from the hydrogen,carbon monoxide to the natural gas,even other combustive gases.The main difficulty the SOFC faces currently is the problem caused by high temperature and the ceramic components' match.As the central component,the electrolyte affects the performance of a SOFC directly.The electrolyte of a SOFC must have adequate oxygen ion conductivity,stability at both oxidizing and reducing atmospheres,enough hardness and toughness and low costs.Today's demonstration SOFCs utilize yttria stabilized zirconia(YSZ)containing typically 8mol%Y,as the electrolyte,in general,they must operate at high temperature because of the low ionic conductivity of YSZ electrolyte at lower operating temperatures, such high operating temperatures cause many serious problems such as physical and chemical degradation of the SOFCs component materials. Therefore,it is important to reduce the operation temperature of SOFCs,it is desirable to develop the new electrolyte operating at the intermediate-Temperature(IT)and the electrode materials that match with it mutually,one path is to make the electrolyte of YSZ into thin film, another is to look for a new electrolyte that work in intermediate -temperature area(600℃-800℃).Because the thin film technique cost is high,craft complicacy,So the second path most contain attraction,Recently, intermediate-temperatures(600℃-800℃)SOFC(IT-SOFC)has received much interest because a replacement of YSZ by a reduced-temperature oxygen-ion conductor in SOFCs would greatly reduce material and fabrication problems and improved cell reliability during prolonged operation.So exploring new type of electrolyte and electrode and their synthesis method is very important for developing IT-SOFC.Now,Ceria-based electrolytes are noteworthy as candidates for electrolyte materials,because this material contains the electronics conductivity,so make open circuit voltage(OCV)and exportation power of the fuel cell lowered.We can make it into composite material that increases the various functions of CeO₂.The electrolytes materials Ce_0.85Sm_0.15O_1.925(SDC)and La_0.9Sr_0.1Ga(0.8)Mg_0.2O_2.85(LSGM)were synthesized by means of glycine-nitrate process(GNP)respectively,then the composite electrolytes were prepared by mixing SDC and LSGM(the weight ratio between SDC powder and LSGM powder were 9:1,8:2,5:5,respectively).Its electrical properties were investigated by impedance spectroscopy in air.It shows that when the composition was 90 wt.%SDC and 10wt.%LSGM(SL91), the electrolyte has higher electrical conductivity as compared to SDC in the temperature of 350-800℃.Electrolyte-supported solid oxide fuel cells (SOFC)were fabricated with NiO/SDC as the anode and Sm_0.5Sr_0.5CoO₃ as the cathode.The V-I characteristics of single cell shows that the maximum output power density was 0.25W/cm² at 800℃and maximum current density was 1.06A/cm² for the cell with SL91 as electrolyte.The open circuit voltage(OCV)of the cell using the composite electrolytes was higher than the cell using single SDC as electrolyte at the working temperature(600-800℃).The temperature and the oxygen partial pressure dependencies of the electronic conductivity of SDC-LSGM composite electrolytes were measured by direct current polarization method using Hebb-Wagner ion blocking cell at 700-800℃in the oxygen partial pressure range of 10^-6-10^-20MPa.The slopes of logσ_e-logPo₂ plot for all the composite electrolytes agree with the theoretically predicted value of-1/4 at some intermediate oxygen partial pressures and -1/6 at low oxygen partial pressure.The electronic conductivity of SDC-LSGM composite electrolytes decreased with increasing LSGM content.The total conductivity of the sintered pellets was measured in air by the standard four-point method.The ionic transport number t_i of all the samples increased with increasing LSGM content.We can confirm that to some extent,doping LSGM to SDC can block off the electronic current caused by the Ce⁴⁺/Ce³⁺reduction process.CeO₂ is present in the form ofpolycrystals,the grain boundary is a crucial part of the microstructure.The grain boundary has a blocking effect to the ionic transport across the electrolyte.To a large extent,the blocking behavior of grain boundaries is attributed to the presence of thin siliceous films.The presence of SiO₂ impurity is ubiquitous in precursor materials.The SiO₂ contamination can also be introduced from furnace refractories during high temperature sintering. Therefore,it is difficult to eliminate the negative effect of grain boundaries to the total conductivity.As a result,many attempts have been made to reduce the grain boundaries effect.It has been reported that the addition of transition metal oxide(Fe₂O₃)had a positive effect on both grain boundaries and total conductivities.Fe₂O₃ could be used as a grain boundary scavenger for ceria-based electrolytes.The composite electrolytes materials Ce_0.85Sm_0.15O_1.925-Fe₂O₃ were synthesized by means of glycine-nitrate process(GNP).The effect of different composite proportion and sintering temperature on the electrical properties of Ce_0.85Sm_0.15O_1.925electrolyte were investigated in detail,and microcosmic mechanisms of the difference of the electrical properties were discussed.The results show that under different sintering temperature,most of the Fe³⁺was exist as Fe₂O₃ in the grain boundary.Fe₂O₃ promoted grain growth of Sm doped ceria during the sintering process.Fe₂O₃ have an obvious scavenging effect on SiO₂ impurity after sintered at 1350℃for 10 h,and the electrical conductivity was increased with increasing Fe₂O₃ content.The grain boundary resistance of the Ce_0.85Sm_0.15O_2-δ-Fe₂O₃ composite electrolyte sintered at 1450℃for 10h was decreased becasued of the larger grain size.Among the composite proportion, Ce_0.85Sm_0.15O_1.925-0.5mol%FeO_1.5exhibited a highest conductivity of 0.0597S·cm^-1at 800℃.Electrolyte-supported solid oxide fuel cells(SOFC) were fabricated with NiO/SDC as the anode and Sm_0.5Sr_0.5CoO₃ as the cathode.The V-I characteristics of single cell shows that the maximum output power density was 0.398W/cm² at 800℃and maximum current density was 1.06A/cm² for the cell with Ce_0.85Sm_0.15O_1.925-0.5mol%FeO_1.5 sintered at 1450℃as electrolyte,which was 0.168 W/cm² higher than the single cell output power density the pure SDC as electrolyte can achieve.Another transition metal oxide NiO was added to Ce_0.85Sm_0.15O_1.925.The composite electrolytes materials were synthesized by means of glycine-nitrate process(GNP).The effect of different composite proportion and sintering temperature on the electrical properties of Ce_0.85Sm_0.15O_1.925electrolyte were investigated in detail,and microcosmic mechanisms of the difference of the electrical properties were discussed.Electrolyte-supported solid oxide fuel cells(SOFC)were fabricated with NiO/SDC as the anode and Sm_0.5Sr_0.5CoO₃ as the cathode. The results show that under different sintering temperature,most of the Ni²⁺ was exist as NiO in the grain boundary.NiO promoted grain growth of Sm doped ceria during the sintering process.Becaused of the higher melting point of NiO,NiO didn't show an obvious effect on promoting grain growth of Sm doped ceria during the sintering process.The grain boundary conductivity was a little higher than Ce_0.85Sm_0.15O_1.925without NiO.The V-I characteristics of single cell show that the power density of the cell using SDC-NiO as composite electrolyte was higher than the cell using SDC as electrolyte,the maximum output power density was 0.303W/cm² at 800℃for the cell with Ce_0.85Sm_0.15O_1.925-0.5mol%NiO sintered at 1450℃as electrolyte.