Lowered Sintering Temperature on Synthesis of La 9

Solid oxide fuel cell (SOFC) is the device that can convert chemical energy into electricity with highest efficiency among other fuel cell. La 9.33 Si 6 O 26 (LSO) is the potential electrolyte at intermediate operation temperature SOFC. Low ionic conductivity of lanthanum silicate-based electrolyte will lead into bad electrical performance on lanthanum manganite-based anode. In this study, LSO was combine with La 0.8 Sr 0.2 Ga 0.8 Mg 0.2 O 2.55 (LSGM) electrolyte by using conventional solid state reaction to enhance the electrical performance of LSO on LCM cathode. However, pre-requisite high sintering temperature on preparation of LSO-LSGM composite will lead into phase transition phase of LSGM that may affect in decreasing the electrical performance. This study resulted that lowered sintering temperature from its ideal temperature still give an improved electrical performance of LCM/LSO-LSGM/LCM symmetrical cell. The ASR value is 0.14 Ω.cm 2 which much lower than its analogous symmetrical cell, LSM/LSO/LSM that was reported before.


Introduction
In the recent years, electricity consumption is noted a tremendous value as 20,201.301TWh all over the world.Besides, the emission of carbon dioxide had reached 32,294 Mt in 2015 by fuel usage [1].This issue has led an attraction of evolutionary electrical generator that reflects an eco-friendly fueled device in which converting chemical energy into electricity such as solid oxide fuel cell (SOFC).SOFC can generate the electricity up to 10MW [2] which has the highest efficiency 40-60% among any other fuel cells [3].SOFC itself consists of electrolyte and electrode components which was fabricated from many rare-earth elements.In order to obtain a good electrical output, SOFC must have been operated at high temperature.However, much higher temperature will lead the cell into incompatibility in physicochemimechanical properties that drives the cell toward electrical deficiency and even destruction due to chemical instability of components based on high-temperature reaction occurred.
On the other hand, Shi et.al.[5] reported high temperature sintering on the preparation of LSO-SDC electrolyte composite as 1823K.While Noviyanti et.al.[4] also showed high sintering temperature on LSO-YSZ electrolyte as 1673K.LSO is melting at above 2273K based on its phase diagram [11], while LSGM starts to melt at ~1773K [12].In order to fabricate a good combination of LSO-LSGM, the solid state reaction must take place above 1673K based on Tamman's rules for ideal sintering temperature [13].However, this temperature will lead the phase conversion of LSGM at high temperature [14] which lead a tendency of the lowering electrical performances of individual components.Hence, lowering the sintering temperature below 1673K was designated to decrease the effect of this phase transition.This research aimed to synthesize the electrolyte composite of LSO-LSGM at below its Tamman's temperature and study the electrical performance of LSO-LSGM electrolyte composite on LCM cathode.

LSO-LSGM composite preparation
LSGM electrolyte was obtained from Sigma-Aldrich (99.999%, trace metal).Both materials with ratio 51:49 (LSO-LSGM) were mixed by using mechanical milling using zirconium ball for 24h.Then it was sintered at 1273K for 36h using gradual steps sintering temperatures as 573K, 1073K, and 1273K by milling at each step.

Characterization of LSO-LSGM composite and LCM/LSO-LSGM/LCM performance analysis
The result was recorded using Rigaku SmartLab X-Ray Diffraction with Cu-Kα radiation, λ= 0.15418nm, T=298K, v=0.02° min -1 .The actual composition was studied using SEM-EDS analysis (Hitachi-EDAX Team).This mixed material then was uni-axially pressed into a pellet (area=1,76cm 2 , and thickness = 0.15cm).LCM cathode was synthesized from La2O3 (Aldrich, 99.999%, trace metal), CaCO3 (Merck, 99.9%), and MnO2 (Merck, 99.9%) using conventional solid state reaction [17].The electrolyte pellet was re-sintered at 1523K for densification.It was furthermore coated with LCM cathode slurry at both side by cellulose acetate in DMF as a binder.The sintering process was prerequisite to eliminate the binder at 1123K for 1h.The prototype of symmetrical cell LCM/LSO-LSGM/LCM that was attained from this process then was tested for electrical performance using EIS method at ambient air atmosphere by using LCR Meter GW Instek 8105G in frequency range of 20Hz -5MHz.In order to get ASR value, the resistance that was obtained from EIS measurement and fitted with ZView® then calculated.

Preparation of LSO-LSGM electrolyte composite
LSO as the precursor was successfully synthesized using hydrothermal method and then the structure was analyzed using XRD.The XRD pattern (Fig. 1) showed a good refinement of the LSO phase within 7 days synthesis time.The data analysis was using Highscoreplus©.The XRD pattern as shown in Fig. 1 was pointing a characteristic peak at 2θ = 30.84°(2 1 1) and 2θ = 31,07° (1 1 2) with the relative intensity as 100% and 71% respectively.All the peak was matched with the standard ICSD No.98-015-5624 with Gof (Goodness of fit) as 2.1522.The impurities phase as La2SiO5 that was found in [16] and another phase like La2Si2O7 in Sakao et.al.[6] were not found in the LSO pattern.This phenomena justified the formation of LSO crystallite phase.LSO itself has p63/m space group (a=b=9,7271(8) c=7,1863 (6)) that was accorded to LSO synthesized by Noviyanti et.al.[16] with cell unit volume as 588.87Å 3 .Fig. 1.XRD pattern of La9,33Si6O26 (LSO) LSO-LSGM electrolyte composite was successfully synthesized using conventional solid state method at 1473K.Fig. 2 depicted distinguishable peak from each component phase of LSO and LSGM respectively while the inset represented an actual XRD pattern.The diffraction peaks do not show another peak impurity that might emerge from both materials.The diffraction denotes two main peak in which correlating with LSO at 2θ = 30.3396(2 1 1) and with LSGM at 2θ = 32.1447(0 1 1).The crystallinity from both peaks can be determined through a ratio calculation of peak height (h) per total peak intensity (I) [18] as shown in Fig. 3.The percentage of crystallinity for both peaks were calculated as 65.86% and 70.70%.This value confirmed the good crystallinity for electrolyte composition (51:49) which was obtained at lowered sintering temperature.The LSO peak shifted toward lower degree rather than its ICSD standard from 2θ = 30.71 to 30.34 which indicated unit cell volume expansion (Table 1.) while the LSGM peak shifted toward higher degree from 2θ = 31.80 to 32.14 that was indicating shrinkage of unit cell volume as shown in Table 1.The shifting was depicted in a zoomed diffraction peak as shown in Fig. 4. The shrinkage of LSGM peak was estimated from phase transition from orthorhombic to cubic that was occurred at 1073K [5].In this study, LSGM phase was found in cubic crystal structure with P m -3 m space group in accordance with Philippeau et.al.SEM-EDS analysis at cross-sectional LSO-LSGM electrolyte pellet was carried out to study the actual composition of LSO-LSGM in the composite.Fig. 5 showed relatively a dense LSO-LSGM electrolyte with low porosity.The porous that was formed reflects a small diameter as 0.67±0.25µmat 15 points based on the length calculation technique by using parallel dimension tool in CorelDraw ©2018.This porous might refer as discontinued macroporous which lead to a relatively dense electrolyte.

Fig. 6. EDS Spectrum of LSO-LSGM electrolyte composite
The expected theoretical composition in this study was 0.5110g (LSO): 0.4927 (LSGM).This value then was calculated to each percentage of sample components in LSO-LSGM as shown in Table 2.The percentage signified an error between theoretical and practical value as average 0.19.This is confirming that LSO-LSGM composite was formed as expected in such composition.This value took no significant effect to the composition, as diffraction data showed no additional peak of impurity and EDS spectrum (Fig. 6) showed every single component in LSO-LSGM.

Electrical Performance of LCM/LSO-LSGM/LCM
Area specific resistance (ASR) is value of total resistans (Rtot = R1, R2, and R3) with geometrical factordependent (A) at which being obtained from value fitting of Nyquist plot using ZView.The ASR value was calculated using an equation ASRelektrode = Rtot • A/2 ; Rtot = R1 + R2 + R3 [22].The Nyquist plot itself was obtained from EIS measurement.Fig. 7 depicted Nyquist diagram for symmetrical cell LCM/LSO-LSGM/LCM.From the diagram, increasing in temperature led to shrinking in semi-circle which indicated the decreasing of total polarization resistance (Rtot = Rs+Rp) as expected.At lower frequency, Rp was pointed as a small 'tail' that may refer to the second semicircle as the existence of an electrode on the electrolyte [23].That was confirmed that polarization resistance could occur at this symmetrical cell that was containing LCM cathode.Rp was pointed by R1, R2, and R3 symbols as at its equivalent circuit in Fig. 7 and noted in Table 3. Ohmic resistance (Rs) refers to activity of ionic conductivity that was possessed by the bulk phase of electrolyte.The lower values indicated higher ionic activity on electrolyte.LSGM as the reinforcement in LSO-LSGM composite was estimated play a great role to lower the resistance rather than LSO.The conductivity of LSGM is 0.166 S.cm -1 (1073K) [7] which is much larger than LSO, 0.002 S.cm -1 at 1073K [6].ASR value denoted higher value than its ohmic resistances at all temperature as shown in Table 3.This behavior indicated the dominant ionic conduction activity of electrolyte bulk phase rather than electronic conductivity and electrochemical activity at cathode.The diffusion transfers resistance (R3) gave a great contribution on ASR value.R3 shows diffusion transfer of oxide ion (O 2-) at triple phase boundary at electrolyte/electrode interface.This might correlate with low reactivity of lanthanum silicate apatite and lanthanum gallate-based electrolyte with lanthanum manganite-based cathode [10].However, the Mn diffusion on LSO electrolyte and Si diffusion on LCM electrode [9] had a probability to block reaction site on TPB at which lowering the electrochemical activity and increasing the value of ASR.
On the other side, Cao and Jiang [24] reported ASR value of analogous symmetrical cell with the cell in this study, LSM/LSO/LSM as 1.74 Ω.cm 2 (1173 K) which is 12.43 times larger than was found in this experiment.It can be rationalized that the electrical conductivity might take effect on this change.The conductivity of La0.7Ca0.3MnO3cathode was reported as 264 S.cm -1 at 1223 K [8] which much larger than conventional cathode, La0.7Sr0.3MnO3(200 S.cm -1 at 1273 K) [25].This behavior could confirm that the lowered sintering temperature on LSO-LSGM electrolyte composite preparation might have been taken no significant effect on electrical performance and it was predicted that lowered sintering temperature could resist the phase transition of LSGM.

Conclusion
LSO-LSGM electrolyte composite was successfully synthesized using conventional solid state method.The lowered sintering temperature was expected to sustain a phase stability of LSGM as the reinforcement material at elevated temperature.The electrical performance of LSO-LSGM electrolyte composite on LCM cathode in LSM/LSO-LSGM/LCM showed a better value than its analogous symmetrical cell LSM/LSO/LSM.ASR value of LCM/LSO-LSGM/LCM denoted as 0.14 Ω.cm 2 at 1173K.The value showed that the lowered sintering temperature of LSO-LSGM composite formation still give an improved electrical performance of LSO-LSGM composite on LCM cathode.

Acknowledgement
This research was supported by Physics-Inorganics Chemistry Laboratory and PSTNT-BATAN.

Figure 4 .Fig. 5
Figure 4.The zoomed 2θ degree that showed the shifting of LSO and LSGM peak before its individual standard

Table 2 .
Percentage of EDS composition in LSO-LSGM electrolyte composite

Table 3 .
ASR value of symmetrical cell LCM/LSO-LSGM/LCM at various temperature