Optimized Sol−Gel Routes to Synthesize Yttria-Stabilized ZirconiaThin Films as Solid Electrolytes for Solid Oxide Fuel CellsEmilie Courtin,*,† Philippe Boy,† Clement Rouhet,† Luc Bianchi,‡ Eric Bruneton,§ Nathalie Poirot,∥
Christel Laberty-Robert,⊥ and Clement Sanchez⊥
†Laboratoire Sol−Gel et Simulation, ‡Laboratoire Projection THermique, and §Laboratoire Microstructure et Comportement, CEA,DAM Le Ripault, F-37260, Monts, France∥Groupe de Recherche en Materiaux microelectronique Acoustique Nanotechnologie, UMR 7347, IUT de Blois, F-41029 BloisCedex, France⊥Laboratoire de Chimie de la Matiere Condensee de Paris, Universite Paris 6, UMR-7574, College de France, F-75231 Paris Cedex05, France
*S Supporting Information
ABSTRACT: Sol−gel strategies are carefully evaluated andcompared to synthesize electrolyte materials for IntermediateTemperature SOFCs (Solid Oxide Fuel Cells). Robust 10−20μm thick cubic Yttria-Stabilized Zirconia (YSZ),(Y2O3)0.08(ZrO2)0.92, films have been prepared by usingnanocomposite stable sols composed with “sol−gel made”YSZ binders and YSZ powder. Two different strategies havebeen investigated for the synthesis of 8YSZ binders: a mixedalkoxide-nitrate route in alcoholic media, and an environment-friendly hydrothermal route, in water media. Ionic conductivities of ∼2.5 × 10−2 S cm−1 are obtained at 800 °C for powdersmade by the hydrothermal and the alkoxide routes. Films synthesized from composite sols that contain commercial YSZ powderand the organic binder or the water-based binder, have been deposited on YSZ-NiO cermets and integrated in an entire singlefuel cell. The two optimized routes yield to gastight YSZ electrolytes. In particular with the organic-based binder, a maximumpower density of 250 mW cm−2 at 850 °C has been achieved. The proposed route can be extended to other materials fordifferent applications such as ferroelectric and dielectric materials.
Solid Oxide Fuel Cells (SOFC) are of particular interestbecause of their high energy conversion efficiency, lowpollution emission, and ability to work with various fuels.1
Yttria-Stabilized Zirconia (YSZ), (Y2O3)0.08(ZrO2)0.92, is themost widely used material for electrolyte because of its highionic conductivity and good chemical and mechanical stability.2
However, SOFCs originally worked at high temperatures, above850 °C, which could lead to a rapid degradation of the stack,and expensive interconnect materials are generally needed.1,3
To decrease this operating temperature to 800 °C and evenlower, new electrolyte materials have to be designed andprocessed. The most common challenges targeted are theincrease of the ionic conductivity and the decrease of theelectrolyte thickness to reduce the ohmic loss.4,5 Sol−gel routesare highly versatile therefore providing interesting strategies toprocess thin films or powder that can be used as electrolytes inSOFCs. However, one step processed sol−gel films are usuallyvery thin (≤1 μm) and therefore their use as electronicceramics or solid electrolytes need multiple depositions andnumerous adequate thermal treatments that generate additionalcost and difficulties (porosity decohesion, cracks and failures
associated to thermo-mechanical strains) to obtain coatingswith optimized properties.6 To synthesize thicker films (10−20μm) in one deposition step by dip-coating or spin-coating,strategies using composite sols, as the one proposed by Barrowet al. for piezoelectric materials, have been developed.7 Itconsists in mixing with a limiting amount of organic matter, apowder (commercially available or “sol−gel made”) and acolloidal sol (called the binder) made with the samecomposition.8 The YSZ binder synthesis is a key issue in thisprocess. This binder must be stable with time, and itsstoichiometry perfectly controlled. The grain size should besmall (<10 nm) to facilitate the densification of films preparedfrom the composite sol made of YSZ powder (generally ∼100nm size) dispersed in the binder.9 Crystallization temperatureof these oxides should be as low as possible to avoid any phasechanges, which could lead to thermo-mechanical strains duringhigh temperature heat treatment.10 Finally, the organic content
Received: July 11, 2012Revised: November 9, 2012Published: November 14, 2012
should be limited to avoid the formation of porosity duringheat treatment.The sol−gel strategies, presented in this study, allow a good
control of solution homogeneity.11 Furthermore, YSZ powderwith well-defined size and shape has been synthesized.In the present work, two sols have been prepared to
synthesize YSZ powders and films: a first one in alcoholicmedia, using a zirconium alkoxide precursor, and a second one,environmentally friendly, made via a hydrothermal synthesis, inwater.The alkoxide method to synthesize Yttria-doped Zirconia
nanoparticles was adapted from the preparation of non-aggregated nanocrystalline zirconia particles.12 Hydrolysis andco-condensation of Zirconium alkoxides and Yttrium nitrateprecursors in propanol lead to the formation of mixed metaloxo-polymers which transform into an oxide network uponheating. As transition metal alkoxides are highly reactive, therate of both hydrolysis and condensation reactions is fast, andcomplexing ligands such as acetylacetone (acac) are required todecrease the condensation rate and prevent the precipitation ofhydroxides.13−15 The ligand also acts as a surface protectingagent and gives rise to the formation of well-defined and non-aggregated nanoparticles,12.16,17 P-toluene sulfonic acid (PTSA)is generally used to allow nucleation and very oftencrystallization of the nanoparticles in relatively mild conditions(80 °C).14 Following this sol−gel strategy a good control of thegrain growth can be obtained until 450 °C.15 The control of thegrain size at higher temperature is also of particular interest forthe preparation of fine powders by sol−gel route.The hydrothermal synthesis can also be used to form directly
crystallized particles in solution. Following a previous work,18
the synthesis of YSZ nanoparticles was achieved undercontrolled pressure and temperature, using urea as a catalyst.This optimized process presents two main advantages: it allowsthe synthesis of highly stable colloidal solutions without the useof organic compounds and the nanoparticle size is easily tunedby modifying both the temperature and the concentration ofspecies in solution.In this article, the objective was to synthesize stable sols
containing YSZ nanoparticles with a controlled stoichiometry,crystallinity, and particle size through various approaches(alkoxide and hydrothermal syntheses). A particular attentionconcerns the synthesis of nanocrystallites that will be able to actas an efficient mortar. These nanoparticles will be locatedaround the bigger commercial particles dispersed in thecomposite sol, that constitute the main bricks of the finalceramic.Different characterization methods, X-ray diffraction (XRD),
Raman spectroscopy, Transmission Electron Microscopy(TEM), simultaneous Differential Thermal Analysis andThermo Gravimetric Analysis (DTA/TGA) were carried outto understand the formation of YSZ nanoparticles (crystal-lization, grain growth, decomposition). A particular interestconcerns the tuning of their morphology through varioussynthesis parameters (acid, precursors, time). The electro-
chemical properties of powders made from these solutions arealso discussed.Finally, the optimized composite sols prepared with
commercial powder and either the alkoxide or the hydro-thermal binder were deposited on YSZ−NiO supports tomeasure the electrolytes properties in a fuel cell, and theirperformances have been compared.
■ EXPERIMENTAL SECTION1. Binders and Powders Syntheses. 1.1. Alkoxide Synthesis.
The zirconium alkoxide precursor is zirconium(IV) propoxide(Zr(OPr)4), 70 wt % solution in 1-propanol (Aldrich). Acetylacetone(98%, Fluka) is added as a complexing agent to limit the strongreactivity of Zr(OPr)4 with moisture. A mixture of PTSA (98.5%,Sigma−Aldrich) or HCl (37%, Prolabo) and water is then addedbefore yttrium precursor (nitrate or acetylacetonate). When PTSA isused as the acid, it is necessary to heat up the sol at 80 °C to removethe zirconium bonded complexing ligands, so that Zr−O−Zr bondscan form. In all syntheses, hydrolysis rate (h = [H2O]/[Zr]) andcomplexing ratio (x = [Acac]/[Zr]) are kept constant: h = 2.8 and x =0.8. Complexing ratio has to be high enough to allow the formation ofa new precursor, Zr(OPrn)3(acac), which is less hydrolyzable. Tostudy the influence of precursors and acid nature, four YSZ solutionsare prepared with either Yttrium(III) acetylacetonate hydrate (99.95%,Aldrich) or Yttrium(III) nitrate hexahydrate (99.8%, Aldrich)dissolved in 1-propanol (99.9%, Sigma Aldrich) to obtain(Y2O3)0.08(ZrO2)0.92 sols. To study the effect of Yttria addition onthe sol properties, a solution containing only zirconia is also prepared(ZrO2-5). All solutions are stable for months at room temperature,that is to say no gel formation is observed. 0.2 mol/L sols wereprepared. Precursors used for the synthesis of the different sols arelisted in Table 1.
1.2. Hydrothermal Synthesis. A so-called YSZ-6 sol is prepared viaa hydrothermal process. ZrOCl2, 8H2O (99.5%, Merck) and YCl3, 6H2O (99.99%, Aldrich) precursors are dissolved in water to obtain a(Y2O3)0.08(ZrO2)0.92 oxide. Urea (Prolabo, VWR) was then addedfollowing the ratio [urea]/[Zr] = 1.4. The mixture was poured in anautoclave. Two experiments are made to study the effect oftemperature, pressure, and time. In the first experiment, the autoclaveis heated to 200 °C under 21 bar during 8 h. In the second one, theautoclave is heated to 180 °C under 18 bar during 4 h. The as-synthesized white precipitates are centrifuged and cleaned with severaldialyses to remove the residual salts (Cl−). These precipitates are thendiluted in water and peptised with hydrochloric acid up to pH 2 tostabilize the particles. Sols with a concentration of 8 wt % ofnanoparticles dispersed in water are obtained. These sols are stableover months.
2. Composite Sol Synthesis. The composite sol consists inmixing a commercial YSZ powder (TZ−8YS, Tosoh Corporation)with a YSZ sol, called binder, 0.5 wt % of Triton X (Aldrich), used as adispersant, and 5 wt % of Ethylene Glycol (VWR) to increase thecomposite sol viscosity. The selected commercial powder is generallyused for the preparation of YSZ films or pellets by classical method,like screen-printing or tape-casting. Powder content and YSZ solconcentration are chosen to obtain a single layer in the appropriatethickness range (10−20 μm). The composite sol with the organicbased binder is prepared from a binder concentrated at 0.2 mol·L−1
and 56 wt % of commercial powder.8 The composite sol synthesized inwater is made of YSZ crystallized nanoparticles (10 wt %) andcommercial powder (90 wt %) dispersed in water. The film thickness
Table 1. Precursors of the Different Sols Synthesized Following the Alkoxide Route
is then simply controlled by the amount of water in solution. Forexample, to synthesize a 10 μm thick film, a solution containing 65 wt% of water is used. Mixtures are stirred under constant agitation.Composite sols compositions to prepare 10 μm thick electrolytes aresummarized in Table 2.
3. Cell Preparation. YSZ layers are deposited on non-sinteredYSZ−NiO commercial anode supports (60 mm diameter, HCStarck).The anode layer is deposited on these supports by screen-printing anddried at 200 °C before depositing the electrolyte. YSZ composite solsare then deposited by dip-coating and heat-treated at 700 °C for 2 hand then up to 1400 °C during 12 h with a heating rate of 120 °C/h.Indeed, it has been demonstrated, in a previous work, that eventhough, on dilatometric analyses, the composite sols sinteringtemperature seems to be lowered compared to the Tosoh; this lastone still controls the sintering process.8 Finally, a 30 μm thick YSZ/LSM counter-electrode was screen-printed from a commercial ink onthe electrolytes to test the entire cell.
4. Characterization Techniques. Sols are characterized bysimultaneous Differential Thermal Analysis (DTA) and Thermogravimetric Analysis (TGA) using a SETARAM TAG 24 and a 10 °C/min heating rate. Sols are dried at 120 °C or calcined at 600 or 900 °Cduring 5 h, with a heating flow of 120 °C/h. Powders are characterizedby XRD with a PANALYTICAL X’PERT PRO diffractometer usingthe CuKα1 radiation. The crystallites size is determined by theDebye−Scherrer formula.19 XRDs are also performed under temper-ature variation with a θ−2θ Bruker D8 Advance diffractometer, aGe(III) Johansson type monochromator, and a linear Lynxeye FourAnton PAAR HTK1200 detector. Fourier Transform Infrared (FTIR)spectrum is obtained with a Nicolet Magma-IR550 spectrometer.Powders are pelleted with dried KBr and analyzed in transmissionmode. Raman spectra are performed with a Renishaw Invia Reflexspectrometer and a Leica DM2500 microscope. The chosen excitationlaser radiation is the 457 nm line. The morphology of calcinatednanocrystallites is examined by Transmission Electron Microscopy(TEM) using a JEOL 2100F−UHR apparatus. The amount of yttriumin the crystallites is measured using Energy Dispersive Spectrometry(EDS). A CM20 FEI microscope is also used for some TEM images.To characterize electrical properties by impedance spectroscopy, 20mm diameter YSZ pellets are prepared by uniaxial pressing the powderwith a pressure of 2 tons. The compacts are then sintered in air at 1400°C for 10 h with a 120 °C/h heating rate. Before pressing, YSZpowders are calcinated at 200 and 700 °C and milled after eachcalcinations step in a planetary ball milling with YSZ balls in a YSZ jarfor 45 min. Electrochemical measurements on symmetrical Platinum/YSZ/Platinum cells (electrode surface of nearly 3.1 cm2) are carried onusing alternating current (AC) impedance spectroscopy (SOLAR-TRON 1260). Platinum ink used was the “Platinum Ink 6926”purchased at Metalor. Measurements are performed in static airbetween 700 and 900 °C. First, the signal was varied from 30 to 50 mV
Table 2. Composition of the Composite Sols (in Weight %)
composite sol with organicbinder
binder (0.2 mol·L−1) inpropanol
38.95
Tosoh 56ethylene glycol 5Triton X 0.05
composite sol in water nanoparticles 3Tosoh 26.95water 65ethylene glycol 5Triton X 0.05
Figure 1. High resolution TEM images of YSZ-1 powder dried at (a) 120 °C, (b) 600 °C, (c) 900 °C and ZrO2-5 dried at 600 °C (d). Insets are theSelected Area Electron Diffraction (SAED) patterns including a lot of small crystals.
to check the linearity domain. Measurements are made at 50 mV, atopen circuit voltage in the 10−1−106 Hz frequency range. Single cellsare installed in a home-made assembly previously described.8 Theexperimental device allows a feeding of the cell with water andhydrogen, diluted with nitrogen, on the anode side and an air supplyon the cathode side. The presence of hydrogen caused the reduction ofnickel oxide in the cermet at the beginning of the test (nickel in itsoxidized state is present in the cell). For electrical connectors, specificwires, in which there is no current flow, are dedicated to voltagemeasurement. The anodic side is sealed with Schott glass, whereas thecathode output sealing is not ensured since the enhancement ofunused oxygen is not under investigation. A load (200 g/cm2) isapplied to the cell to ensure a proper electrical contact. The system israised to 850 °C at a rate of 0.5 °C/min under nitrogen atmosphere.Nonhumidified hydrogen is then gradually introduced to ensure thecermet reduction. At first, before the cell characterization, a rapid testat 850 °C is performed to validate the system (integrity of the cell, fullcermet reduction, and so forth).
■ RESULTS AND DISCUSSIONS1. Influence of Experimental Parameters in the
Alkoxide Synthesis. 1.1. Influence of Yttrium Doping.TEM observations are performed on sols dried at 120, 600, and900 °C. Only images of YSZ-1 are presented in Figure 1 asresults are similar for all YSZ-1, YSZ-2, YSZ-3, and YSZ-4powders.As observed on the SAED, after drying at 120 °C for solvent
release, powders are amorphous even if crystallization startedunder the electron beam. At this temperature, crystallinedomains are around 5 ± 1 nm. After heating the powder at 600°C, no grain growth is observed.To study the influence of yttrium doping on the grain
growth, comparison is made with ZrO2-5, synthesized as YSZ-1but with no yttrium. At 600 °C, SAED show that powders are
crystallized. Pure ZrO2 crystallites contain a mixture ofmonoclinic (P21/c) and tetragonal (P42/nmc) phases. Theirsize is three times bigger than for YSZ-1 at the sametemperature: 15 ± 1 nm.For YSZ powder, grain growth is only observed on TEM
images performed at 900 °C (15 ± 1 nm). It indicates thatyttrium doping seems to limit the grain growth up to 600 °C.
1.2. Influence of Yttrium Precursors and Acids. Yttriumprecursors and/or acid have been systematically varied. All thesols appear stable at room temperature. Their pH value isbetween 2 and 2.5. As described above, no influence of theyttrium precursor and/or acid is observed on the grain size andgrowth.To understand the various degradation mechanisms
occurring under heat treatment up to 800 °C DTA/TGAanalyses are performed on the sols dried at 120 °C (Figure 2).As a general observation, a more important weight loss is
observed for syntheses made with PTSA (52−58%) than forsyntheses made with HCl (∼ 40%). This is due to PTSAorganic residue decomposition which is more important thanHCl decomposition. Several phenomena are observed onDTA/TGA curves (Figure 2). The first one, appearing on allcurves, corresponds to a weight loss of approximately 6%, fromroom temperature to 200 °C. This loss corresponds to theevaporation of residual free organics (propanol) and water. Theexothermic peak between 250 and 350 °C, associated with aweight loss of approximately 8%, is linked to the decompositionof acetylacetonate.15 For sols YSZ-2 and YSZ-4, the weight lossobserved between 450 and 500 °C, associated with anexothermic peak, corresponds to PTSA decomposition.12
Only one main exothermic sharp phenomenon is observedhere at around 580 °C because of precursors and residues of
Figure 2. DTA/TGA performed under air on the as-synthesized sols dried at 120 °C for the various compositions YSZ-1, YSZ-2, YSZ-3, and YSZ-4.
PTSA decomposition, overlapped with the particle crystal-lization. For sols synthesized with HCl (YSZ-1 and YSZ-3), theexothermic peaks are much larger and appear at lowertemperatures (between 350 and 530 °C). This broad peakcorresponds to a superposition of different phenomena(precursor and organic compounds decomposition andcrystallization) showing a less homogeneous compositionthan the ones synthesized with PTSA as confirmed by EDSanalyses performed on the powders dried at 120 °C. It showsthat composition of powders prepared with HCl is not ashomogeneous as the ones synthesized with PTSA. However,when powders are dried at higher temperature (600 °C), theircomposition gets homogenized, and the right proportion ofyttrium (14.8 at. %) is obtained.20 Moreover, crystallization isdelayed when PTSA is used, as confirmed by additional XRDperformed at different temperatures (Figure 3). Indeed, it startsat 500 °C when PTSA is used whereas it starts at 400 °C whenthe acid used is HCl. Indeed, Scolan et al.16 have shown thatPTSA is associated to the nanoparticles and probably located inthe solvation shell, which may stabilize the structure and sodelay crystallization. HCl does not play this role. It is used hereto acidify the solution and allow an easier acac decomposition.Finally, it has to be noted that the nature of yttrium precursorsdoes not impact the crystallization temperature.2. Control of the Particle Stoichiometry. Comparison
between Alkoxide and Hydrothermal Syntheses. XRDsare performed on nanoparticles prepared by hydrothermalsyntheses and dried at 100 °C. No microstructure or sizedifferences have been observed when changing the synthesis
temperature, pressure, and time, as shown in Figure 4.Crystallized nanoparticles of 8 ± 1 nm are obtained afterevaporation of the solvent (100 °C). For the second part of thestudy, the optimized YSZ-6 synthesis is selected. This synthesiscorresponds to the lowest temperature and in the shortest time(180 °C, 4 h). XRDs are also performed on colloids obtainedby the alkoxide sols (YSZ-1, YSZ-2, YSZ-3, and YSZ-4) dried at600 °C, over the crystallization temperature determinedpreviously (Figure 3). Crystallite size is estimated at 6 ± 1nm whatever the synthesis route.Because of the very small particles size and so the bands
width, peaks characterizing tetragonal (P42/nmc) and cubic(Fm3m) phases might overlap. It could explain the differenceobserved on the splitting of (311) and (200) XRD peaksbetween the hydrothermal and the alkoxyde syntheses (Figure4b). Raman spectra are then performed on these variouspowders to determine their crystallization phase (Figure 5).It shows that nanoparticles synthesized via the hydrothermal
synthesis are crystallized in the cubic phase (Fm3m) character-istic of the good stoichiometry (Y2O3)0.08(ZrO2)0.92. On thecontrary, Raman spectra performed on powders dried fromalkoxide sols show peaks at 260, 330, and 475 cm−1 that arecharacteristic of the tetragonal phase,21 which is observed for aY2O3 doping below 7%. It can indicate that yttrium is not fullyincorporated in the structure. IR spectra performed on powdersdried at 600 °C are presented in Figure 6. They show carbonatebands at 1400 and 1550 cm−1 for all syntheses.22 Yttriumcarbonates are formed, which can confirm that yttria is nottotally incorporated into the YSZ structure. Yttrium carbonates
Figure 3. In-situ XRD patterns performed under various temperatures on YSZ-1, YSZ-2, YSZ-3, and YSZ-4 powders.
are reported to be stable only up to 700 °C.23 Indeed, IRspectrum recorded at 900 °C only shows the YSZ metal−oxygen band at 460 cm−1. To avoid carbonate formation,synthesis should be performed under an inert atmosphere.3. Deposition of the Binder. Influence of the
Crystallized Particles Morphology. First, the binders havebeen deposited on YSZ−NiO cermets to study their ability to
form homogeneous and adherent thin films. Whatever thesynthesis and the deposition technique, dip-coating or spin-coating, very thin films are obtained (<200 nm). To process 10μm thick electrolytes, it is necessary to deposit several layerswhich usually leads to cracks after heat-treatment.For nanoparticles prepared with the hydrothermal route, a
comparison has been made between thin YSZ films and thinZrO2 films (Figure 7).
For pure ZrO2, films are homogeneous and crack-free, incontrast to YSZ ((Y2O3)0.08(ZrO2)0.92) films. This difference inmicrostructure might be due to the difference of particlemorphology. Indeed, YSZ particles exhibit a cubic shape whichprobably does not help their arrangement in the films. This iswhy composite sols, prepared from the YSZ alkoxide orhydrothermal binders and commercial powder, have beenpreferred to process 10 μm thick films in one step.
4. Electrical Properties. 4.1. Impedance Spectroscopy.To study the influence of synthesis methods on electrochemicalproperties, impedance spectroscopy measurements are per-
Figure 4. XRD patterns performed on nanoparticles prepared viahydrothermal synthesis at (a) 180 °C, 18 bar, 4 h and (b) 200 °C, 21bar, 8 h and on colloids prepared via the alkoxide synthesis (YSZ-1,YSZ-2, YSZ-3, and YSZ-4).
Figure 5. Raman spectra performed on YSZ powders calcined at 600°C.
Figure 6. IR spectra performed on the YSZ powders calcined at 600and 900 °C for the various compositions YSZ-1, YSZ-3, and YSZ-4.
Figure 7. SEM images of (a) YSZ films and (b) ZrO2 films synthesizedvia the hydrothermal route and sintered at 1400 °C and thecorresponding TEM images of (c) YSZ particles and (d) ZrO2particles dried at 120 °C.
formed on pellets made from powders YSZ-1, YSZ-2, YSZ-4(Table 3). The results are compared with a pellet made fromthe powder synthesized via the hydrothermal route (YSZ-6).Performances are also compared with a pellet made fromcommercial powder (Tosoh, Japan) synthesized in the sameconditions.Ionic conductivities obtained with YSZ-1 and YSZ-4 are
similar (Table 3), showing that for the alkoxide synthesis,electrical performances are similar when Yttrium nitrate is usedas a precursor, whatever the nature of the acid. At 800 °C, theionic conductivity for both materials is comprised between 2.5± 0.1 S cm−1 and 2.9 ± 0.1 × 10−2 S cm−1. These values andtheir activation energy are close to those from literature.24
Moreover, even if their conductivity is lower than thecommercial powder, their activation energy is close. Thismight be due to a lower density for pellets prepared with YSZ-1and YSZ-4 compared to the density of the pellet made with thecommercial powder (Table 3). XRD patterns performed on thepellets and cell parameters are provided on SupportingInformation, Figure 1 and Table 1. It indicates that even ifsols with HCl are less homogeneous, composition getshomogenized at high temperatures and so electrical propertiesare not affected. When Yttrium acetylacetonate is used as astarting material (YSZ-2), activation energy is much higher (1.3± 0.1 eV) than with the Yttrium nitrate precursor. This materialhas similar conductivity than YSZ-1 and YSZ-4 at 900 °C (6.5± 0.1 × 10−2 S cm−1), but it decreases at 800 and 700 °Cmaking it less appropriate for SOFCs working at intermediatetemperatures. This might be due to an inhomogeneouscomposition, observed by EDS, in the powder prepared withyttrium acetylacetonate. For both alkoxide (YSZ-1 and YSZ-4)and hydrothermal syntheses (YSZ-6), ionic conductivities andactivation energies are similar. It indicates that these sols can beused as binders and/or dried to prepare powders for thesynthesis of composite sols. To synthesize the composite solwith the alkoxide binder, the sol YSZ-1 (made with yttriumnitrate and hydrochloric acid) was used as it contains a lowerorganic amount, as shown in Figure 2. Its maximum weight loss(40%) corresponds to the lowest among all the sols prepared inalcoholic media. Ionic conductivities and activation energies, inthe temperature range of 700 to 900 °C, for both pelletsobtained from composite sols made of commercial powder andthe YSZ-1 alkoxide or the YSZ-6 hydrothermal binder, arecompared in Figure 8.Ionic conductivities of both pellets obtained from composite
sols, prepared with commercial powder, are similar: 3.5 ± 0.1 ×10−2 S cm−1 at 800 °C (Table 3). This value is comprised
between the ionic conductivity of pellets obtained from theTosoh powder and the binders. These good properties allowthe composite sols to be used to synthesize electrolyte materialsfor SOFCs. These electrical properties are even superior to theones reported in the literature for 8YSZ (3 × 10−2 S cm−1 at800 °C).25 Moreover, their activation energies are similar (∼1.0± 0.1 eV).26
4.2. Cell Tests. Two composite sols (one prepared with thealkoxide binder in alcoholic media and the other one preparedwith the hydrothermal binder in water) were dip-coated on a500 μm thick YSZ−NiO commercial cermet and co-sintered at1400 °C to synthesize thin films. SEM images of both as-prepared electrolytes are presented in Figure 9.According to SEM images, electrolytes seem to be dense with
only few closed pores. Cells performances are evaluated at 850°C and results are reported in Figure 10.Both cells present an Open Circuit Voltage (OCV) of
approximately 1.2 V. This result indicates that both electrolytesare gas-tight. Better performances are obtained with the oneprepared in alcoholic media, with the “alkoxide binder”. Amaximum power density of 250 mW cm−2 is obtained. For thecell prepared with the electrolyte synthesized from thecomposite sol with the “hydrothermal binder”, the maximumpower density is 180 mW cm−2 at 850 °C. The difference inperformance cannot be attributed to the difference in ionicconductivities as the values measured are comparable in theuncertainty of the measure. This difference can be attributed tovarious factors: (a) electrode microstructure, (b) cathode/electrolyte interfaces. Recently, cell performances (OCV = 1.2V, Pmax = 280 mW cm−2) have been improved by decreasing
Table 3. Ionic Conductivities at 700, 800 and 900 °C and Activation Energies for Pellets Made from YSZ-1, YSZ-2, YSZ-4, YSZ-6 Powders, Commercial Powder, and Composite Solsa
conductivity (S cm−1)
temperature(°C)
pellets fromYSZ-1
pellets fromYSZ-2
pellets fromYSZ-4
pellets fromYSZ-6
pellets fromcommercial powder
pellets made from composite solwith organic binder
grain size (nm) 5 ± 1 5 ± 1 5 ± 1 8 ± 1 50 ± 1aConductivity uncertainty is 0.1 × 10−3 S cm−1, activation energy uncertainty is 0.1 eV, density uncertainty is 0.1 g cm−3.
Figure 8. Conductivity measurements on pellets obtained from YSZ-1(purple diamonds), YSZ-6 (blue triangles), composite sols in water(blue stars) and alcoholic media (brown squares), and commercialpowder (Tosoh red circles).
the thickness of the “alkoxide route” made electrolyte to 8 μm.Even higher results could be obtained by better controlling theYSZ/LSM electrode microstructure27 or by using aLa1−xSrxCo1−yFeyO3−δ (LSCF) cathode with Gadolinia DopedCeria (GDC) interlayer.28
These syntheses are interesting for Solid Oxide Fuel Cellsand High Temperature Electrolyzer Cells applications as theyallow the preparation of films with a controlled thickness andmicrostructure. Moreover, broader interests can be found in thepreparation of various oxide materials and coatings such as forprotection layers, conductor, piezoelectric films.29 Variousmicrostructures could also be obtained: from a dense film, aspresented in this study, to a porous coating with the addition ofpore formers.
■ CONCLUSION
Various routes have been explored to elaborate YSZ sols usedas “binders” for the synthesis of sol−gel made solid electrolytes.The influence of acid and precursors nature has been studiedfor the synthesis of YSZ solutions via an alkoxide sol−gel route.It has been observed that when using PTSA as an acid, insteadof HCl, YSZ crystallization is delayed although better crystallitehomogeneity is obtained at lower temperatures. However, thecomposition gets homogenized at higher temperature. Alimitation of the grain growth is observed up to 600 °C withall the YSZ syntheses.Powders processed from the alkoxide sols have been
compared with powders synthesized via an optimized hydro-thermal route. After synthesis, the last ones are alreadycrystallized in the cubic phase whereas crystallization of powderprepared via the alkoxide synthesis takes place at highertemperature (>400 °C). Moreover, these alkoxide derivedcolloids crystallize in a mixture of cubic and tetragonal phasesbecause of the presence of yttrium carbonates.Impedance spectroscopy measurements have been per-
formed on pellets made from the as-prepared powders.Powders prepared via the alkoxide route with HCl and yttriumnitrate exhibit the same ionic conductivity as those obtained viathe hydrothermal synthesis (∼2.5 S cm−1 at 800 °C).To synthesize single cells with YSZ electrolytes, composite
sols made of commercial powder dispersed in the two types ofbinders have been prepared and deposited by dip-coating on anon-sintered YSZ−NiO cermet. According to the electro-chemical tests performed on these single cells, both electrolytesare gastight, and a maximum power voltage of 250 mW cm−2 at850 °C is obtained for the cell made with the electrolyteprepared with the “alkoxide binder”.Higher power density could be probably achieved after an
optimization of the interface between the electrode and theelectrolyte. To improve the cell performance, further experi-ments are currently devoted to the improvement of theelectrode microstructure and optimization of electrolytethickness.These syntheses can also be used to synthesize various oxide
materials. It allows the preparation of coatings with a controlledmicrostructure and thickness: from a few hundred nanometers,when only the binder is deposited, to several tens ofmicrometers with a composite sol.
■ ASSOCIATED CONTENT
*S Supporting InformationXRD diffraction patterns performed on pellets prepared fromYSZ-1, YSZ-2, YSZ-3, and YSZ-4 pellets; cell parametersobtained for pellets prepared from YSZ-1, YSZ-2, YSZ-3, andYSZ4 powders; and Nyquist diagram for YSZ binder YSZ-1, at700, 800 and 900°C. This material is available free of charge viathe Internet at http://pubs.acs.org.
Author ContributionsAll authors have given approval to the final version of themanuscript.
NotesThe authors declare no competing financial interest.
Figure 9. SEM images (fractures) of the tested electrolytes preparedwith a composite sol made with (a) an alkoxide binder in alcoholicmedia, and (b) a hydrothermal binder in water.
Figure 10. Polarization curves at 850 °C. QH2 =18 mL/min cm2.
The Region Centre provided financial support for this work.The authors would like to thank Th. Piquero (CEA, DAM LeRipault), J. Vulliet (CEA, DAM Le Ripault) for electrochemicalcharacterizations, V. Frotte (CEA, DAM Le Ripault) for TD/TG Analyses and the Laboratoire Sciences des ProcedesCeramiques et de Traitements de Surface (CNRS−SPCTS) forXRD under varying temperature, D. Jalabert for TEM images atCME, Orleans.
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