Y0.07Sr0.895TiO3 (YST) and La0.2Sr0.7TiO3 (LST) samples sintered and reduced at different temperatures were pre-pared. The conductivity and chemical expansion of the samples were tested. It was found that the combination ofhigh conductivity and low chemical expansion therefore represents a conflict of material properties for YST andLST samples. The relationship between chemical expansion and sample cracking is discussed. The conductivity ofthe samples against redox cycles was also tested. Properties with respect to SOFC requirements were compared be-tween LST and YST samples. Suitable preparation conditions for YST and LST as SOFC anode materials wereoptimized.
Solid oxide fuel cells (SOFCs) are promising energy conversion de-vices that directly produce electrical power fromwide variety of fuels.Currently the state-of-the-art anode materials for SOFCs are Ni/YSZcermets, which are both excellent catalysts for fuel oxidation and effec-tive current collectors, andhave already beenwell developed andunder-stood. However, disadvantages like Ni agglomeration [1], coking [2],sulfur poisoning [3], and especially instability upon redox cycling (cyclicreduction and oxidation) [4,5], significantly prevented the commerciali-zation of SOFCs based onNi/YSZ cermet anodes. Searching for alternativematerials seemed to be more and more necessary in recently years.
Among the reported redox-stable materials as the alternatives forNi/YSZ cermets, lanthanum or yttrium-substituted SrTiO3 show bestprotential because of i) high electrical conductivity after heat treat-ment in reducing atmosphere [6,7], ii) matching thermal expansionto that of YSZ [7,8] and iii) good dimensional stability upon redox cy-cling [7–10]. During the last few years, they have attracted increasinginterests. Single cells based on these materials have been studied andreported extensively, some of them already showed quite promising re-sults (see Table 1). These cells can be fabricated by the extensively usedtape-casting or warm-pressing method as for Ni/YSZ based cells. Theonly difference is they may need a heat treatment in reducing atmo-sphere. The dimension of some fabricated cells reached 5×5 cm2 [9],quite close to the practical-using level, indicating a considerable poten-tial for further commercial application.
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Although the properties of lanthanum and yttrium-substitutedSrTiO3 are quite similar to each other, there are also obvious differences.Since the ionic radii of Y3+ (1.19 Å, twelvefold coordination) is muchsamller than that of Sr2+ (1.44 Å, twelvefold coordination), while thevalue of La3+ (1.36 Å, twelvefold coordination) is relatively more simi-lar [6], the doping limitation of Y3+ in SrTiO3 is only about 8 mol% [6],while that of La3+ can reach 40 mol% [7]. In this case, under the samereducing temperature lanthanum-substituted SrTiO3 can reach higherconductivity by increasing La doping level to 20 mol%–30 mol%, whilecomparitively the conductivity of yttrium-substituted SrTiO3 is a bit re-stricted. Fig. 1 shows the comparison of conductivity between denseSr0.895Y0.07TiO3 (YST) and La0.2Sr0.7TiO3 (LST) sintered in Ar/4% H2 atsame conditions, obviously LST has higher conductivity than that ofYST. At 800 °C, the values are 295 S/cm and 130 S/cm for LST and YSTsamples, respectively. However, also for lanthanum and yttrium-substituted SrTiO3 reduced at similar conditions, the chemical expan-sion, i.e. the dimensional change upon redox cycling, of lanthanum-substituted SrTiO3 is also higher than that of yttrium-substitutedSrTiO3 (about 0.5% at 1000 °C [13] and 0.1% at 800 °C [14], respectively).As a rough estimate, the target for the electronic conductivity for anodematerials is set at 100 S/cm for low ohmic resistance of the fuel cell.However, the actual requirement depends on the cell design, and par-ticularly on the length of the current path for current collection. Thusthis requirement may be relaxed to as low as 1 S/cm [15], which meansboth lanthanum and yttrium-substituted SrTiO3 are quite quanlified.While in order to guaranteemechanical stability, the chemical expansionof anode materials should be smaller than 0.2% for an anode supportedcell [4,8,16]. According to this standard, further works should be concen-trated on decreasing the chemical expansion of lanthanum-substitutedSrTiO3. Formerly, it is assumed that the combination of high conductivityand low chemical expansion therefore represents a conflict of material
Table 1Single cells reported in the literature based on La or Y-substituted SrTiO3 anode materials. All cell tests were performed with H2 as fuel and air as oxidant.
Cell type Dimension Anode Performance(maximum power density)
Redoxtesting
Reference
Anode-supported ~15 mm diameter,0.8 mm thickness
La0.2Sr0.8TiO3 support, NiO–Ce0.8Sm0.2O2/NiO–YSZ anode 0.85 W/cm2 at 800 °C 7 cycles [10]
Cathode-supported ~1 cm2, 0.4 mm thickness Ceria and Pd-infiltrated La0.3Sr0.7TiO3/YSZ 0.43 W/cm2 at 800 °C No test [11]anode-supported ~25 mm diameter,
0.2 mm thicknessCeria and Cu-infiltrated La0.2Sr0.7TiO3 0.5 W/cm2 at 750 °C No test [12]
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properties for donor-doped SrTiO3 with respect to SOFC requirements[17], which hints the chemical expansion of donor-doped SrTiO3 maybe possibly decreased by decreasing the conductivity of the material bycertain method, such like decreasing the reducing temperature of thesamples. Since the conductivity of lanthanum-substituted SrTiO3 is highenough, maybe suitable chemical expansion can be achieved by sacrific-ing conductivity of the material to certain degree. However, the abovediscussions have not been proved by systematical experiments.
It is well agreed that A-site deficient lanthanum or yttrium-substituted SrTiO3 such like YxSr1-1.5xTiO3 or LaxSr1-1.5xTiO3 is moresuitablematerial for SOFC anode than that of ion-stoichiometric samplesuch like YxSr1-xTiO3 or LaxSr1-xTiO3 because of their better conductivity[6,12,17,18]. In this study, A-site deficient Y0.07Sr0.895TiO3 (YST) andLa0.2Sr0.7TiO3 (LST) were chosen and compared based on the standardsof electrical conductivity and chemical expansion, suitable samples forSOFC anodes will be discussed.
2. Experimental
Titanium (IV) isopropoxide (97%), Sr(NO3)2 (99%) and Y(NO3)3 ⋅ 6H2O (99.9%), La2O3 (99.9%) were used as starting materials for the prep-aration of Sr0.895Y0.07TiO3 (YST) and La0.2Sr0.7TiO3 (LST) powders. The ti-tanium (IV) isopropoxide was dropped into distilled water that wasbeing stirred. The precipitatewasfiltered andwashed, and thendissolvedin HNO3 solution. For YST, Y(NO3)3 ⋅ 6 H2O was directly dissolved in dis-tilled water. For LST, La2O3 was dissolved in HNO3 solution. The Ti4+ Y3+
and La3+ concentration in solution was determined by thermal gravim-etry. Sr(NO3)2 was weighed directly. Corresponding amounts of the ni-trates were then combined to obtain the solution. After homogenization,
Fig. 1. Temperature dependence of the electrical conductivity in Ar/4% H2/3% H2O fordense samples of Sr0.895Y0.07TiO3 (YST) and La0.2Sr0.7TiO3 (LST) sintered in Ar/4% H2
at 1400 °C.
the solutionwas spray-pyrolyzed in a commercial spray dryer (Nubilosa,Konstanz, Germany). After spray pyrolysis the raw powder was heatedto 900 °C in air for 5 h. X-ray diffraction using a Siemens D5000 diffrac-tometer with Cu Kα radiation showed that the YST and LST powderswere pure perovskite after calcination.
The YST and LST powders was uniaxially pressed into rectangularbars (40×5× 3 mm3) and sintered in Ar/4% H2 at 1400 °C. The densebar samples were thusly prepared. They had relative densities ofabout 95% of the theoretical density. The powders were also coat-mixed and warm pressed [19] into plates with dimensions of70?×70?×2 mm3. After de-bindering and pre-sintering at 1250 °Cin air, the YST plates were then sintered in Ar/4% H2 at 1400 °C(YST1400), while the LST plates were sintered in air at 1400 °C, andthen reduced at 800 °C – 1200 °C (LST800–LST1200), or directly sinteredand reduced inAr/4%H2 at 1400 °C (LST1400). These plateswere porous,with the porosity of about 25%. Porous samples are more accordancewith practical SOFC anodes than that of the dense samples, and duringthe redox cycles, the samples can be oxidized and reduced more easily.Regardless of whether in Ar/4% H2 or air, these plates had all beenheat-treated at 1400 °C, because this is the approximate temperatureto densify YSZ electrolyte, therefore this temperature is also necessaryfor anode materials since they will be co-sintered with electrolytewhen fabricating single cells. The plates were then cut into bars(25×4×1.5 mm3), The thermal and chemical expansion behavior ofthe bars was measured using a push-rod dilatometer (Netzsch DIL402 C), and the electrical conductivity of the bars was measured by theDC four-probe method in the temperature range of 25–900 °C in Ar/4%H2 with 3% H2O or in air. The microstructure of the samples after chem-ical expansion testingwas investigated by scanning electronmicroscopy(SEM, HITACHI TM3000).
3. Results and discussion
Fig. 2 shows the conductivity of YST and LST porous samples re-duced at different temperatures. Obviously, the conductivity of LSTdecreases with decreasing temperature of reduction. At testing tem-perature of 800 °C, the conductivity of 1200 °C reduced LST(LST1200) is over 100 S/cm, while that of LST800 is just 0.3 S/cm. Ithas been discussed formerly [17,18] that the concentration of Ti3+
in donor doped SrTiO3 samples plays a key role for the conductivityof the samples. Certainly LST1200 has higher Ti3+ concentrationthan that of LST800 because of the higher reducing temperature,and consequently, LST1200 also has much higher conductivity thanthat of LST800. For LST samples reduced under 1000 °C, because thelow conductivity arouses the doubt of high ohmic resistance, theyare unlikely good candidates for SOFC anode materials. The conduc-tivities of LST1000, LST1100 and YST1400 are quite similar to eachother, which are 30–40 S/cm at 800 °C. Former research [9] showedthat the performance of single cells based on YST1400 anode materialcan reach 1 W/cm2 at 800 °C, indicating that if the conductivity is theonly considered parameter, high performances may also be expectedfor single cells based on LST1000 and LST1100 anodes. It should be
Fig. 2. Temperature dependence of the electrical conductivity in Ar/4% H2/3% H2O forporous samples of YST reduced at 1400 °C (YST1400) and LST sintered in air at1400 °C and reduced in Ar/4% H2 at 800 °C – 1200 °C (LST800 – LST1200).
110 Q. Ma, F. Tietz / Solid State Ionics 225 (2012) 108–112
mentioned that these samples were not in thermodynamic equilibri-um condition during conductivity testing. It is generally agreed thatduring the cooling procedure after sintering in reducing atmosphere,chemical defects (e.g. strontium or oxygen-ion vacancies) and the va-lence state (e.g. Ti3+/Ti4+) of the samples are just ‘frozen in’ [20,21].However, due to the slow equilibration kinetics, it is possible that theequilibrium will never be achieved in SOFC-operating conditions andthe conductivity of the samples can almost remain constant [22]. Inthis case, the discussion of the conductivity for the samples in ‘frozenin’ conditions is still meaningful.
The chemical expansions of LST1000–LST1400 and YST1400 samplesupon redox cycles between Ar/4%H2/3%H2O and air at 800 °C are shownin Fig. 3. Although 1400 °C reduced LST has the best conductivity in thewhole tested series (295 S/cm at 800 °C for dense samples, see Fig. 1),it also has the worst chemical expansion, which reached 0.7% duringthe first redox cycle and caused the fracture of the sample afterwards.The microstructure of LST1400 sample before and after redox cycles iscompared in Fig. 4 (See Fig. 4a and b). Clearly, the ceramic structure ofLST1400 was almost totally destroyed during the chemical expansiontesting. Sintered LST particles broke into numerous small pieces.Micro-cracks can easily be found everywhere. This result indicates thatas a potential substitution of Ni/YSZ cermets, LST cannot be reduced at
Fig. 3. Chemical expansion behavior of LST1000 - LST1400 and YST 1400 samples be-tween Ar/4%H2/3%H2O and air at 800 °C.
the temperature as high as that of to densify YSZ electrolyte, or itwill sig-nificantly lose its major advantage in redox stability field. For LST sam-ples sintered in air and reduced at lower temperatures, situations aremuch better. In the 3 redox cycles (6 h in air and 18 h in Ar/4%H2/3%H2O for each cycle) of chemical expansion testing, the dimensionalchange of LST1200 sample is from 0.10% to 0.20% for each cycle, muchlower than that of LST1400, while the conductivity of the sample is stillas high as over 100 S/cm at 800 °C (see Fig. 2). The chemical expansionfor LST1000 is even lower, which is from 0.03% to 0.05% for each redoxcycle, but the conductivity of the sample also significantly decreases(see Fig. 2).
The above results indicate the correctness of the former assumption inintroduction part: Material properties of high conductivity and lowchemical expansion for donor-doped SrTiO3 are not compatible. Sinceit's basically definite that the increasing conductivity of donor-doped SrTiO3 comes from increasing Ti3+ concentration, which isproduced by increasing reductive temperature, it's also quite possiblethat the higher chemical expansion of LST and YST samples reduced athigher temperature comes from higher Ti3+ concentration inside thesamples. An intuitionistic explanation seems to be the oxidation ofTi3+ to Ti4+ during redox cycling simultaneously leads to the dimen-sional change of the samples. But intriguingly, normally materialscontaining multivalent cations tend to expand during reducing andshrink during oxidizing, such like Ce1–xGdxO2 (Ce3+↔Ce4+) [23]and La1–xSrxCrO3 (Cr4+↔Cr3+) [24], because i) the ion-radius forcation in reducing status is larger, and ii) the oxygen vacancy producedin reductive atmosphere repulses the cations in neighbor [25]. Howev-er, the situation for LST and YST samples is totally in reverse. AlthoughTi3+ (0.67 Å, sixfold coordination) is larger than Ti4+ (0.61 Å, sixfoldcoordination), andmore oxygen vacancies are produced in reductive at-mosphere, LST and YST samples still tend to expand during oxidizing,and the expansion increases with original Ti3+ concentration insidethe samples. Similar phenomenon was also found in former reports,not only for donor-doped SrTiO3 [13,14], but also for some other Ti con-tented materials [25]. Yoed Tsur etc. reported [26] that for rare earthdoped BaTiO3, air fired samples also have larger lattice volumes than re-duced fired samples (both at 1400 °C), just like the situations in thisstudy. And the authors attributed this lattice volume expansion to B-site occupancy tendency of the rare earth elements for air fired samples[26]. Because rare earth ions are larger than Ti ion, B-site occupancywilllead to lattice parameter increase. However, this is hardly the case inthis study, because the chemical expansion for LST and YST samples inthis study occurred only at 800 °C. It is difficult to imagine that the Laor Y ion can change from A-site to B-site at such low temperature.Moreover, Hui etc. [6] and Fu etc. [27] both reported opposite resultsfor Y-doped SrTiO3. The lattice volumes of their reduced fired sampleswere larger than air fired samples (all at 1400 °C), indicating the ex-planation of ‘B-site occupancy tendency’ for rare earth doped BaTiO3
samples is even not suitable for 1400 °C sintered Y-doped SrTiO3.According to Wang etc. [28], in oxidizing atmosphere, La-dopedSrTiO3 will absorb oxygen, and form Sr-rich Ruddlesden-Popperphases like (SrO(Sr1− xLaxTiO3)n), and Sr-ion vacancy in perovskitestructure of Sr1− xLaxTiO3, which results in the increase of the dimen-sion of the samples. While in reducing atmosphere, this Sr-rich phaseand Sr-ion vacancywill disappear, leading to the decrease of the dimen-sion of the samples. This explanation is more reasonable than the for-mer one. However, Tietz etc. [25] reported a phenomenon of chemicalexpansion for TiO2 doped YSZ samples similar to the situation of LSTand YST in this study. At 1000 °C, the TiO2 doped YSZ samples also ex-panded when the atmosphere were changed from H2–Ar to air. Appar-ently, Wang et al.'s explanation cannot be used here. Tietz etc. [25]surmise this abnormal expansion comes from the electronic interac-tions between Ti3+ ions. Under reducing atmosphere, the generatedelectrons contributing to the Ti 3d band can be regarded as delocalized.In this case, at least among Ti3+ ions that are not more than three Ti–Tidistances away [29], small attractive forces between these Ti3+ ions are
Fig. 4. Microstructure of YST and LST samples: a) LST1400 before chemical expansion testing. b) LST1400 after chemical expansion testing. c) YST1400 after chemical expansiontesting. d) LST1200 after chemical expansion testing.
111Q. Ma, F. Tietz / Solid State Ionics 225 (2012) 108–112
formed because of the bandwidening. And the attractive forces result inthe lattice volume decrease under reducing atmosphere. This explana-tion may also be applicable for YST and LST samples in this study. How-ever, in-situ SEM and XRD tests are still needed to discuss the problemfurther.
The chemical expansion for YST1400 is 0.12% to 0.14% for eachredox cycle, slightly lower than that of LST1200 (see Fig. 3). But ifthe accumulated expansion in 3 redox cycles is considered, the chem-ical expansion for YST1400 (0.18%) is significantly lower than that ofLST1200 (0.30%). Consequences of this difference are shown in Fig. 4.Within the limitation of the SEM observation, no micro-cracks can befound in YST1400 samples after chemical expansion testing (seeFig. 4c). While for LST1200, small amount of slight micro-cracks areformed during the redox cycles (see Fig. 4d). Compared to YST1400,these micro-cracks for LST1200 samples may come from differentmechanical strength of the two materials, which needs further exper-iments to discuss, while it is more possible that the higher chemicalexpansion of LST1200 results in the micro-cracks. Although the situa-tion is not as severe as that of LST1400 samples (see Fig. 4b), thesemicro-cracks still will weaken the mechanical stability of the sample,indicating the balance between conductivity and redox stability forLST1200 samples inclining too much to the former. Although highconductivity and low chemical expansion cannot be reached simulta-neously, comparatively LST materials still have more operating spacefor the balance than that of YST. By adjusting the parameter of reduc-ing temperature, SOFCs based on LST materials can be emphasizedmore on performance or stability while keep the other endurable.Yet for YST samples, even reducing at the temperature of electrolytedensification (~1400 °C), the conductivity of the samples is barelysatisfactory. There is almost no space to sacrifice the conductivity toget better redox stability. But fortunately, YST1400 is already quitebalanced in both sides of conductivity and redox stability.
It is also widely concerned that the high conductivity of YST and LSTcomes from the ‘freezed’ ionic conditions during high temperature re-ducing, whichmeans the ionic conditions in YST and LST samples cannot
reach equilibrium condition during cooling, and still keep the status inhigh-temperature reducing to some extent. However, the equilibriumcondition between Ti3+ and Ti4+will be finally reached at the operatingconditions of SOFCs. This may cause performance decrease of the singlecells based on these materials to some extent. These worries are reason-able but can be explained. Firstly, the equilibrium conductivities of LSTand YST at 800 °C in reducing atmosphere are of course lower thanthose of just reduced at high temperatures, but the kinetics maybevery slow, even beyond the life time of the SOFC system. In this case, itis still meaningful to discuss the starting conductivity of the just reducedYST and LST samples. Secondly, according to the former research [14], at800 °C, even the conductivity of Y-doped SrTiO3 samples was as low as0.01 S/cm, it could still recover back to about 20 S/cm in a couple ofhours in reducing atmosphere (Ar/4% H2), indicating the equilibriumconductivity of the samples in the measuring conditions is still higherthan 20 S/cm.What'smore, if the ohmic resistance is the only consideredparameter,when the conductivity of an 1 mmthick YST or LST decreasedfrom 100 S/cm to 10 S/cm during operating or redox, the total area spe-cific resistance of the single cell will only increase for 0.009Ω cm2. This isstill acceptable. The conductivity changes of LST and YST samples uponredox cycles are shown in Fig. 5. After the same 3 redox cycles, the con-ductivity of LST800 , LST1000, and LST1200 samples changed from0.29 S/cm to 0.30 S/cm, 35 S/cm to 5 S/cm, and 130 S/cm to 11 S/cm, re-spectively. It seems that the higher original conductivity of the LST sam-ple is, the higher conductivity loss is during the redox cycles. Thismay beexplained as that the sample with higher original conductivity also hashigher Ti3+ concentration, and during redox cycles, also more Ti3+
ions are oxidized to Ti4+, which causes higher conductivity loss. Surelythis conductivity loss will lead to certain ohmic resistance increase dur-ing practical operating of SOFCs. But in practical, it is also unlikely thatthe redox conditions are as strong as inputting air into anode chamberfor several hours. When the time in air is decreased and the time in re-ductive atmosphere is increased, the conductivity loss for LST samplesduring redox cycles should be much smaller. Compared to LST samples,YST1400 sample has much lower conductivity loss during similar redox
Fig. 5. Electrical conductivity change upon 3 redox cycles for a): LST800, b) LST1000, c) LST1200 and d) YST1400.
112 Q. Ma, F. Tietz / Solid State Ionics 225 (2012) 108–112
cycles, which only decreased from 35 S/cm to 26 S/cm. This indicates theperformance of SOFCs based on YST samples maybe more stable duringredox cycles than that of LST based SOFCs, but the explanation still needsfurther experiments.
4. Conclusions
Material properties of high conductivity and low chemical expan-sion for donor-doped SrTiO3 are not compatible. Under same reduc-ing conditions, YST has better chemical expansion, while LST hasbetter conductivity. To decrease the chemical expansion for LST sam-ples, high conductivity must be sacrificed by decreasing the reducingtemperature. The conductivity of YST samples is more stable duringredox cycles than that of LST samples. Considering both conductivityand redox stability, YST sintered and reduced at 1400 °C or LST sin-tered at 1400 °C in air and reduced at around 1000–1200 °C are suit-able candidates for full ceramic based SOFC anodes. Abnormalexpansion was found for LST and YST samples during re-oxidation,the explanation still needs further experiments.
Acknowledgements
Financial support from the European Commission under contractno. SES6-CT-2006-020089 of the integrated Project “SOFC600” andcontract no. 256730 of the integrated Project “SCOTAS-SOFC” is grate-fully acknowledged.
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