Synthesis of bismuth doped yttria stabilized zirconia electrolyte and study of ionic conductivity

x%molBi 3+ -doped yttria-stabilized zirconia with 4%molYO 1.5 (x=0-5,BiYSZ) are prepared via lamellar liquid crystal template method.The influence of different doping amount of Bi 3+ on crystal form,morphology,the ionic conductivity are investigated in detail.In addition,densification time of BiYSZ are determined.X-ray diffraction(XRD),Raman spectrum,Scanning Electron Microscope(SEM) results show that BiYSZ powder with stable tetragonal phase and spherical structure is nanometer level.Moreover,by using the Ac impedance spectroscopy to analyse conductivity of solid electrolytes,we find that Bi 3+ -doped yttria-stabilized zirconia with 4%molYO 1.5 improve the ionic conductivity.Comparing all the compositions,3mol%Bi 3+ doped yttria stabilized zirconia with 4%molY 3+ gives the highest ionic conductivity of 7.41×10 -5 S/cm -1 at 500℃,and sintering temperatures of BiYSZ reduce around 100℃ than Y 0.04 Zr 0.96 O 2 (4YSZ),ionic conductivity increases one order of magnitude.


Mongolia (2017MS0215) and Scientific research projects of the Inner Mongolian
Higher Educational System (NJZY7101).

1、introduction
Solid oxide fuel cells (SOFCs) have attracted attention as an ecofriendly and highly efficient power generation system [1] . Electrolyte is the core component of SOFC. The typical oxygen ion solid electrolyte has ZrO2-based, CeO2-based, Bi2O3-based, ABO3 perovskite type and so on.Yttria-stabilized zirconia (YSZ) is generally used as a solid electrolyte because of its high oxygen ion conductivity and chemical stability at high temperatures(over 800 ℃) [2,3] .However,ionic conductivity and sintering temperature of the sample need to be improved in order to popularization and application of solid oxide fuel cell at intermediate temperatures(400-650℃,IT-SOFC) [2,4] .
Powder preparation, solid forming and high temperature sintering are three important stages of electrolyte preparation process.Therefore, there are many factors affecting the ionic conductivity.Recent reports have suggested that co-doping strategy is an effective method for improving electrical conductivity of solid electrolytes [5,6] .
At present,most studies have doped commercial yttria-stabilized zirconia to change its electrical properties by regulating the crystal phase, morphology and particle size distribution.In addition,many groups have prepared different levels powder by different techniques to further decrease the ohmic resistance of solid electrolyte,for example,chemical coprecipitation method [7] ,sol-gel methods [8] and so on.Rudzani 3 sigwadi [9] prepared zirconia with relatively small particle size and small distance between lattice fringes(0.2928 nm and 0.2824 nm) by precipitation method,and the effect of calcination temperature on the crystallinity and particle size of zirconia particles was investigated.In this paper,lamellar liquid crystal template method is first adopted,which has the important advantage of mild reaction conditions,and enables synthesis of nanopowder at low temperature.
Bismuth oxides stabilized with lanthanide dopants in a fluorite structure (δ-Bi2O3) are known for superior ionic conductivity [10,11] ,but they reduce to metallic Bi under reducing atmosphere and undergo a transformation to a low ionic conductive phase at the operation temperatures [12,13] .So bismuth oxides cannot be utilized as stand-alone electrolytes [14] .Gil and others have proved the addition of Bi2O3 can reduce the sintering temperature of gadolinium-doped ceria(GDC) electrolytes by about 250-300℃ [15] .In another report,densification of the GDC electrolyte was achieved at a low sintering temperature of 1000-1200℃ by adding Bi2O3 as dopant [16,17] .Hirano [18] reported that doping with 1mol% Bi2O3 decreased the sintering temperature of scandia-stabilized zirconia ceramics by 300℃, while allowing its electrical conductivity at 1000℃and 800℃ to reach 0.33 and 0.12 S/cm,respectively.
As a sintering aid,Bi2O3 is also a very good oxygen ion conductor.Therefore,adding Bi 3+ to zirconia oxide may not only improve ionic conductivity, but also decrease sintering temperature.
Based on the above analysis,bismuth oxide doped yttrium oxide stabilized zirconia are synthetised by lamellar liquid crystal template method in this 4 work.,which aims to regulate the powder structure by template with SDS/C10H22O/H2O,further improve its electrical conductivity by doping ZrO2-based solid electrolyte.

Synthesis of powder and electrolyte
As raw Where D is grain size.B is Half-peak width of diffraction peak,θ is Bragg diffraction angle,K and λ is a constant of 0.89 and 0.154056 respectively.
Phase structures is further confirmed via using InVia Microscope Raman.The relative density of the sintered disk is determined via Archimedes' method using GP-120C Precision Ceramic Porosity Volume Density Test.Morphology of powders are observed by scanning electron microscope(SEM) with Hitachi, Su8220(Japan).
The electrical properties of the samples are measured using the AC impedance spectroscopy,which adopt a high temperature dielectric temperature spectrometer HDMS-1000 of Wuhan Bailibo Technology Co., Ltd,test conditions are as follows:T=300℃-500℃,50℃ intervals, the frequency range is 20 Hz-1MHz.The 6 impedance data is gained from intersection of AC impedance spectroscopy with x-axis, the conductivity is calculated from following equation [19] : Where L is the disk thickness.A is its superficial area,R is the resistance calculated from the impedance spectra.The activation energy (Ea) of the conductivity is determined by the Arrhenius law: Where 0 is the pre-exponential factor,T is testing temperature(K),k is the Boltzmann constant, Ea is activation energy.  atmosphere.All diffraction peaks of these patterns match with cubic and tetragonal stucture l(PDF#50-1089).Because the diffraction peaks of tetragonal and cubic phases have similar diffraction patterns,it is difficult to distinguish the cubic phase or the tetragonal phase accurately by XRD.However,Raman spectra can further determine crystal phase of the sample.As can be seen from the diagram,the diffraction peaks of synthesized powder correspond to the tetragonal phase [20] .The XRD and Raman results show that the phase structures of the sample is tetragonal phase, in other words, the addition of Bi (NO3)3 also stabilizes zirconia in the tetragonal phase.

Microstructure analysis
Scanning electron microscope images of BiYSZ nanopowders in 200k multiple are shown in Fig.4 a-f,which show the addition of Bi(NO3)3 don´t affect the morphology, size and dispersion of the samples.With the increase of the content of bismuth nitrate, BiYSZ powder is still monodisperse spherical structure with a diameter less than 15nm.Table2 shows particle size of BiYSZ obtained by different methods.A is 8 calculated by Scherre formula, B is gained by Jade6 software.The average particle sizes both tend to decrease first and then increase,and the total particle size are controled within 15nm. Fig.4 Table2 Fig.5 gives SEM images of 4YSZ(a=1200 ℃ )and 3Bi4YSZ(b=1100 ℃ ) after sintering.As can be seen from Fig.6(a),density of 4YSZ sample is still lacking.On the one hand,the equipment may limit the increase of temperature,on the other hand,multiple of the scanning equipment also may limit its clarity.While The sintering density decreases, and the grain and grain boundary melts,which leads to the spherical shape of the grain in Fig.6(b) .The resulting densities both are shown in Table 3.

Electrical conductivity
The corresponding equivalent circuit is shown in Fig.6.The typical AC impedance spectra of ionic conductor is consist of two semicircles and a diagonal line.High and low frequency semicircles correspond to grain impedance and grain boundary impedance respectively,and diagonal lines correspond to resistance and capacitance effect of electrode.Grain and grain boundary impedance of the sample can be obtained by the intersection of semicircle and spectrum x axis.
At the same time, Fig.6 gives the Ac impedance spectra or Nyquist plot of BiYSZ at 500℃.3Bi4YSZ is shown in the insert of Fig.6.The lines in Fig.6 are the experiment data and the different symbols with different colors are the fitting results.Due to the 9 relaxation properties of different materials and the relatively high measurement temperature, the high-frequency semicircle of 4YSZ sample represents its grain boundary resistance,and the grain resistance is calculated via the horizontal intercept of the high frequency curve [21] .In the 4YSZ sample, the total resistance mainly comes from the grain resistance.While in BiYSZ sample,with the increase of doping amount, the grain boundary semicircle become gradually intact, and the electrode line begins to appear,until the doping amount increases to 4mol%,the electrode line disappears.On the whole,grain resistance,grain boundary resistance and total resistance decrease rapidly first, then increase rapidly, finally decrease rapidly, and resistance is minimized at x=0.03,that is Rg=950Ω,Rg+Rgb=1420Ω.Comparing show that the doping amount is 3mol% is better,and the conductivity is 7.41×10 -5 by formula (2).