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Cyclic electrophoretic deposition of electrolyte thin-films on the porous cathode substrate utilizing stable suspensions of nanopowders
SOSI-14190; No of Pages 7 Solid State Ionics xxx (2017) xxx–xxx
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Cyclic electrophoretic deposition of electrolyte thin-films on the porous cathode substrate utilizing stable suspensions of nanopowders E.G. Kalinina a,b, E.Yu. Pikalova b,c,⁎, A.A. Kolchugin b,c, S.M. Pikalov b,d, A.S. Kaigorodov a,b a
Institute of Electrophysics, UB RAS, 106 Amundsen St., 620016 Ekaterinburg, Russia Ural Federal University, 19 Mira St., 620002 Ekaterinburg, Russia Institute of High Temperature Electrochemistry, UB RAS, 20 Academicheskaya St., 620137 Ekaterinburg, Russia d Institute of Metallurgy, UB RAS, 101 Amundsen St., 620016 Ekaterinburg, Russia b c
a r t i c l e
i n f o
Article history: Received 29 July 2016 Received in revised form 28 December 2016 Accepted 23 January 2017 Available online xxxx Keywords: Self-stabilising suspension EPD Thin-film electrolyte SOFC Graded cathode substrate
a b s t r a c t In this study, nanopowder Се0.8(Sm0.75Sr0.2Ba0.05)0.2O2 − δ (CSSBO) with average diameter of particles of 15 nm and specific area of 53 m2/g prepared by the method of laser evaporation was used for a cyclic electrophoretic deposition (EPD) from a non-aqueous suspension on a functionally graded cathode substrate with a LaNi0.6Fe0.4O3 − δ (LNFO) collector layer and a La2NiO4 ± δ (LNO) functional layer. Thick LNFO support (1 mm) with 20% of pore former had a porosity of 45% after sintering at 1400°С. The values of gas-permeability which was measured by means of the computerised experimental set, varied from 31.60 × 10−3 to 25.71 × 10−3 μm2. A thin LNO functional layer was brush painted on the top of the support and pre-sintered at 1350°С, 2 h. Green CSSBO film with a thickness of 8 μm was stepwise deposited on the graded cathode substrate and sintered at 1400°С, 4 h. The critical load for fracture of the film (approximately 5 μm thick after sintering) was 111 ± 14 mN, conductivity was equal to 2.40 × 10−2 S/cm at 750 °C. © 2017 Elsevier B.V. All rights reserved.
1. Introduction The current approach to reduce a Solid Oxide Fuel Cell's (SOFC) operating temperature is motivated by a wider choice of low-cost materials for cells' functional layers and interconnectors, greater flexibility in cell design and the longer life-time of stacks due to reduced inter-diffusion and degradation of cell components [1,2]. However, the electrolyte conductivity decreases significantly when the operating temperature is lowered. This drawback can be overcome by lowering the electrolyte resistance either by developing alternative materials with higher ionic conductivity in an intermediate temperature range and/or decreasing the electrolyte thickness. The investigations of various multi-component solid state electrolytes based on CeO2 have shown that co-doping with rare earth and alkaline earth elements leads to an increased conductivity in the intermediate temperature range in comparison with single-doped solid solutions [3–6]. It was shown that in some cases co-doping suppresses Ce4+ → Ce3+ transition and expands the electrolytic domain, characterised by the critical oxygen partial pressure p*O2 below which the electronic conductivity exceeds the ionic conductivity, to lower values. In our recent study a high conductivity value in air of 6.25 × 10− 2 S/cm with the p* O2
⁎ Corresponding author. E-mail address: [email protected] (E.Y. Pikalova).
value of 2.76 × 10−23 atm at 750 °C was observed for Се0.8(Sm0.75Sr0.2Ba0.05)0.2O2 − δ (CSSBO) [7]. Electrophoretic deposition (EPD) is known to be a versatile and very cost-effective method for the formation of functional layers in SOFCs [8, 9]. Being a liquid-based method it is much more straightforward than gas-phase methods such as ion-plasma spraying, chemical vapour deposition and the method of laser sputtering in a vacuum, all of which require expensive equipment and careful monitoring of the technological environment [10]. Among the advantages of EPD are the short duration of film formation (approximately 1 μm/min), easy adaptation for various cell's design, no special requirements for binder burnout as the green coatings contain few or no organics and easy control of the film thickness through simple adjustment of the deposition time and applied potential [11]. Currently, there are some studies being made to obtain electrolyte films by the EPD method both on cathode [12–14] and anode substrates [11,15–19]. To deposit an electrolyte film on a nonconducting anode substrate such measures as the applying of a steel plate behind the substrate or modifying its surface with a conducting layer (graphite) are used. A conducting cathode support is highly preferable for the deposition but there is a problem with degradation of the electrode support's porous structure during the electrolyte film sintering. The solution is to choose cathode materials with the sinterability as low as possible and use a cathode support with a gradual reduction in porosity towards the electrolyte side. Both of these approaches were applied in the present study. La2NiO4 + δ (LNO) was
http://dx.doi.org/10.1016/j.ssi.2017.01.016 0167-2738/© 2017 Elsevier B.V. All rights reserved.
Please cite this article as: E.G. Kalinina, et al., Cyclic electrophoretic deposition of electrolyte thin-films on the porous cathode substrate utilizing stable suspensions of nanopo..., Solid State Ionics (2017), http://dx.doi.org/10.1016/j.ssi.2017.01.016
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the material of choice for the functional cathode layer due to its excellent electrochemical performance conditioned by the considerably high mobility of interstitial oxygen in its layered structure, a low value of the coefficient of thermal expansion (CTE ≈ 13.1 × 10− 6 К− 1), close to that for CeO2-based electrolytes, and decreased sinterability in comparison with the cobaltites of rare-earth and/or alkaline-in comparison elements [20–22]. The electronic conductivity of LNO is about of 60–80 S/cm in the intermediate temperature range and is insufficient to ensure uniform current distribution and low in-plane resistance for effective operation of the cathode. Thus, it is supposed to be used in multilayer electrodes in combination with an electronic conductor such as La4Ni3O10 − δ [23,24], La0.7Sr0.3CoO3 − δ and La0.7Sr0.3MnO3 − δ [25,26] as a current collector. In the present study LaNi0.6Fe0.4O3 − δ (LNFO) was chosen as a collector due to its higher electronic conductivity and lower sinterability as compared with the other possible candidates and a CTE value compatible with that of LNO (CTE ≈ 12.5 × 10− 6 К− 1) [27]. The technology to form a thin film electrolyte by the EPD method involves the preparation of a stable suspension of the ceramic powder which can be simplified by using weakly aggregated nanopowders fabricated by the laser evaporation method [28,29]. Similar weakly aggregated nanopowders of doped-ceria electrolytes and suspensions for EPD were obtained and investigated in [30–32]. In our recent paper [32] we analysed in detail the formation of a thin CSSBO electrolyte film on a dense LNO cathode support. According to our numerous experiments there are technological limitations to the thickness (approximately 2 μm) of the defect-free electrolyte film deposited by EPD during 1 cycle. In the present study deposition of the CSSBO electrolyte film was performed on a functionally graded cathode substrate with LNFO collector (1 mm) and thin La2NiO4 ± δ functional layer (5 μm after sintering) was used as a support for stepwise electrophoretic deposition of CSSBO electrolyte from a self-stabilised suspension. To ensure complete covering of the porous support surface a film with increased thickness was deposited by the cyclic EPD method with intermediate pre-sintering of the layers. Gas-permeability of the cell was inspected at every step of the cell formation. The microstructural, mechanical and electrical properties of the deposited film were investigated. 2. Experimental procedure 2.1. Electrolyte and cathode materials' preparation and characterisation Се0.8(Sm0.75Sr0.2Ba0.05)0.2O2 − δ (CSSBO) powder was initially prepared by a solid state reaction method as described elsewhere [30]. CSSBO nanopowder was obtained by the method of laser evaporation of a ceramic target using a LS-06 ytterbium fibre laser with a wavelength of 1.07 μm. A JEOL JEM 2100 transmission electron microscope (TEM) was used to characterise the CSSBO nanopowders. X-ray phase analysis was carried out on a D8 DISCOVER diffractometer in copper radiation with a graphite monochromator on a diffracted beam. Processing was carried out using Topas-3 software with the Rietveld algorithm of structural parameter refinement. The specific surface area of nanopowders was determined by a volume version of the Brunauer-Emmett-Teller (BET) method based on a low-temperature equilibrium sorption of nitrogen vapours by a Micromeritics TriStar 3000 vacuum sorption analyser. Particles of the nanopowder had a spherical shape with an average diameter of 15 nm and a specific surface area of 53 m2/g. XRD analysis confirmed that the CSSBO nanosized powder contained one crystalline phase – a solid solution based on a cubic form of CeO2 with the lattice parameter a = 5.440(3) Å and crystalline size dXRD = 14(1) nm. La2NiO4 + δ (LNO) was synthesised via a two-step ceramic technology (1150 °C for 2 h, 1250 °C for 5 h) as described elsewhere [24]. XRD analysis revealed in the case of LNO the formation of an orthorhombic structure (sp. gr. Fmmm, a = 5.448(1) Å, b = 5.477(1)
Å, c = 12.667(8) Å). The LNO powder was milled up to the specific surface area 1.7 and 2.6 m2/g. LaNi0.6Fe0.4O3 − δ (LNFO) was synthesised vie a modified Pechini method [24] and had a specific surface area equal to 1.2 m2/g after calcination at 1200 °C. XRD analysis revealed in the case of LNF the formation of a rhombohedral structure (sp. gr. R-3c a = 5.505(1) Å, b = 5.505(1) Å, c = 13.273(3) Å). 2.2. Fabrication of the functionally graded cathode substrate A functionally graded cathode substrate with LNFO collector layer and LNO functional layer was prepared in three stages. Thick supported layer LNFO (1 mm) with 20% of pore former (graphite, 6.9 m2/g) had a porosity of 45% after the sintering at 1400°С, 2 h. LNO layer (1.7 m2/g, 3– 5 μm) was brush painted on the top of the support and pre-sintered at 1100°С, 2 h. Second LNO layer (2.6 m2/g, 3–5 μm) was brush painted and sintered at 1350°С, 2 h. The values of gas-permeability of the cathode support were measured by means of computerised experimental set made in the Institute of Electrophysics of the Ural Branch of the Russian Academy of Sciences (UB RAS). The technique is based on Darcy's law – a constitutive equation that describes the gas flow rate through a porous medium and relies on measuring precisely the pressure drop at the inlet and outlet of the sample at a given value of the initial negative pressure. The gas permeability coefficient is determined by a law of evacuation decreasing with time due to gas leakage through the sample. Before every start the set is calibrated by means of measurement performed on a standard gas-tight sample. 2.3. Electrophoretic deposition EPD was carried out on the computerised equipment of laboratory design which provided constant voltage modes. The LNFO/LNO substrate of 12 mm2 in area served as a cathode, a disc (12 mm2) of stainless steel was used as an anode and the distance between the electrodes was 1 cm. Densification of the deposited films was performed in a Hermle Labnet Z383 centrifuge with a rotational velocity of 1000 rpm for 2 min. 2.4. Electrode substrate and thin film characterisation SEM images of the deposited film and microstructure of the electrode substrate were obtained using an electron microscope Mira 3 LMU. The roughness of the surface of the coating was determined by a Zygo NewView 500 device. Adhesion to the substrate was studied by the scratching method using a 600 Nanotest device with an acoustic emission module according to the following procedure: speed of the conical indenter (radius of curvature of the indenter point – 5 μm) along the surface of the sample – 5 μm/s, the length of the scratches – 500 μm, the speed of the load applying −5 μm/s, the maximal load – 500 mN. The critical load value was the load that was determined by the first response to the acoustic emission signal and this was averaged over the five measurements. The conductivity of compact LNFO, LNO and CSSBO samples was measured by the dc four-probe method in air in the temperature range of 500–850 °C with steps of 50 °C with isothermal exposure of 1 h within it. Evaluation of the thin film conductivity was made in the same temperature range during heating and cooling on the cell with Pt electrodes deposited on both sides of the three-layer (LNFO/LNO/ CSSBO) cell and sintered at 900 °C, 1 h. The measurements were carried out by the method of impedance spectroscopy (EIS) using a frequency response analyser FRA-1260 with an electrochemical interface EI-1287 (Solartron Instruments Inc.) in the frequency range of 0.1 Hz to 500 kHz at the amplitude of applied sinusoidal signal of 30 mV in air and in argon in the temperature range of 450–750 °C. Each measurement was finished by measuring a full dc resistance Rdc of the cell. The impedance spectra were analysed by using K-K Test and ZView software with an equivalent circuit presented in Fig. 6. Rhf, the serial resistance determined by extrapolation of the high-frequency region of an
Please cite this article as: E.G. Kalinina, et al., Cyclic electrophoretic deposition of electrolyte thin-films on the porous cathode substrate utilizing stable suspensions of nanopo..., Solid State Ionics (2017), http://dx.doi.org/10.1016/j.ssi.2017.01.016
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impedance spectrum to the intersection with the x-axis, was taken as the electrolyte resistance. 3. Results and discussion There are significant differences in engineering procedures for producing electrolyte- and electrode-supported fuel cells. In the first case the solid electrolyte is sintered at a high temperature to ensure its gas tightness while the electrodes are formed on the electrolyte at significantly lower temperatures to provide high porosity and electrochemical activity. In the second case the sintering temperature of the electrode support must be higher or, at least, the same as that for the electrolyte film. In the opposite case it is extremely difficult to prevent compaction of the electrode support structure and avoid gas-diffusion limitations during SOFC operation. But it should have the necessary level of mechanical strength, electrical conductivity and electrochemical activity. These requirements can be achieved by using a functionally graded electrode with a supporting (collector) layer possessing a high mechanical strength, porosity and electrical conductivity and a functional layer with a TEC value close to that of the electrolyte film, a high level of electrochemical activity and a level of porosity that is appropriate to the film electrolyte preparation. Results of testing electrical properties (a) and dependence of the porosity of the LNFO compacts on the sintering temperature (b) are shown
Fig. 1. Temperature dependence of the conductivity of the LNFO compacts with 20% of pore former sintered in the temperature range 1300–1450 °C (a) and porosity of the samples depending on the sintering temperature (b).
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in Fig. 1. The samples were sintered at temperatures in the range 1300– 1450 °C and their porosity decreased almost linearly from 58 to 38%. The temperature 1400 °C was chosen to fabricate the support with the necessary level of porosity (45%) and a high level of electronic conductivity (225 S/cm at 700 °C). Conductivity of the LNFO substrates was 2.5–3 times higher than for those made of LSM [33] and close to those reported in [34] where the authors have shown that the Pechini method provides a higher level of electrical conductivity of LNFO in comparison with other methods of synthesis at the same level of the samples' porosity. The thick LNFO collector layer, having a 45% degree of porosity, showed a high value of gas permeability equal to 30.38 × 10− 3 μm2 which decreased appreciably after deposition of the LNO functional layer (0.20 × 10−3 μm2). To deposit the thin electrolyte film on the cathode support by the EPD method we used a stable self-stabilising suspension of nanosized CSSBO powder in a mixed dispersion medium acetyl acetone/ isopropanol (50/50 vol% ratio) with concentration 10 g/l, obtained and studied earlier in [31,32]. In the suspension based on the mixed medium the natural pH value was 4 with a high positive value of ζ-potential (+ 31 mV). Before the deposition the suspension was ultrasonically treated. Fig. 2 shows a dependence of the average hydrodynamic size of aggregates deff on the duration time of the ultrasonic treatment. The average size of CSSBO aggregates decreased from 261 to 162 nm, reaching saturation level after 75 min of the treatment. Thus, the ultrasonic treatment did not lead to complete disaggregation; however, the hydrodynamic size of aggregates (effective diameter) reduced considerably after the treatment. Our numerous experiments reveal that a prolonged ultrasonic treatment (more than 25 min) results in longterm stability of the suspensions (for more than one month) and, decreasing the hydrodynamic size of aggregates, and ensures better quality and density of the EPD coatings formed [11,29]. To prevent cracking in the deposited film a modifier BMMA-5, which is a co-polymer of butyl methacrylate with the addition of 5 mol% of methacrylic acid, was added to the suspension. To ascertain the optimal concentration of the polymeric binder a set of experiments was conducted [32]. The density of the green coating in the presence of the modifier was 48–60%, which makes possible the subsequent sintering into a dense ceramic. To ensure the operational reliability of the electrolyte layer it is necessary to provide a thickness of approximately 5–10 μm which is difficult to implement with a single-step EPD process. As a result of extensive experimental study it was found that for our suspensions the maximal thickness of the electrolyte film which was possible to be obtained was approximately 3 μm. With any further increase in
Fig. 2. Dependence of the average hydrodynamic size of aggregates in the self-stabilising CSSBO suspension on the duration time of the continuous ultrasonic treatment.
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Fig. 3. Optical images of the surface of the graded cathode support prepared for EPD deposition (a), the surface after the EPD (b, c, e, g) and sintering at 1350°С, 1 h. (d, f) and sintering at 1400°С, 4 h (h).
Please cite this article as: E.G. Kalinina, et al., Cyclic electrophoretic deposition of electrolyte thin-films on the porous cathode substrate utilizing stable suspensions of nanopo..., Solid State Ionics (2017), http://dx.doi.org/10.1016/j.ssi.2017.01.016
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thickness surface cracking appeared upon drying due to increased internal stresses. In order to increase the thickness of the uniform CSSBO film we conducted a cyclic deposition. Fig. 3a shows the optical micrograph of the top surface of the porous cathode support prepared for the deposition. It was uniform with pores less than 1 μm. The three stages of the cyclic deposition were carried out in the mode selected according to our earlier study [32]: the voltage was set in a range of 80–100 V, the duration time of the deposition was 1 min. At the first stage, the film was deposited twice (Fig. 3b, c) with following sintering at 1350 °C, 1 h, with a heating rate of 1 °C/min and free cooling (Fig. 3d). Second and third stages comprise single deposition with intermediate sintering at 1350 °C (Fig. 3e, f, g). Throughout all the stages of the deposition the film formed was uniform and crack-free. As a result of the cyclic deposition a homogeneous electrolyte film with a total weight of 5.6 mg/cm2 and 8 μm thickness (green film) was obtained. The final sintering was carried out at 1400 °C, 4 h to ensure gas-tightness of the electrolyte film (Fig. 3h). The final temperature did not exceed the sintering temperature of the collector layer to safeguard its structure and gas permeability.
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After the final sintering the CSSBO film had uniform structure with few closed pores and was 3.5–5.1 μm thick with average grain size of 4.2 μm (Fig. 4a, c). The LNO layer is not clearly seen because LNO ceramic slurry deposited on the LNFO substrate with a very high porosity partly penetrated into its structure. It is demonstrated on the element map which shows the spatial distribution of elements in a sample (Fig. 4b). The density of the LNO functional layer, evaluated from experiments with compacts sintered at different temperatures, is supposed to be approximately 80% with a thickness after final sintering of 2–3 μm. At the same time, the collector layer LNFO kept well-developed pore structure after the sintering (Fig. 4d), which provides a good gas exchange. Since the LNO functional layer is quite dense, this ensures good contact on the electrode/electrolyte interphase providing the necessary adhesion of the electrolyte film and facilitating charge transfer. The gas-permeability of the cell measured after the final sintering was equal to zero. The surface roughness of the electrolyte film was 0.20 ± 0.05 μm. The critical load of the film destruction was equal to 111 ± 14 mN. The curve of the amplitude of the acoustic emission signal
Fig. 4. SEM images of the cross-section (a), the element map (b), the CSSBO electrolyte surface (c) and the cathode support back-surface (d) after the sintering at 1400°С, 4 h.
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shown in Fig. 5 has no separate peaks (as in the case of fragile coatings with low adhesion) and changes stepwise. Probably, with a constant increase in load, dissipation of fracture energy occurs in the form of plastic deformation of the coating without the formation of micro-cracks. This in turn indicates good performance properties of the gas-tight electrolyte film because its fracture occurs directly at the macroscopic level and at a sufficiently high mechanical load. This supposition is confirmed by the behaviour of the indenter with a constantly increasing load (Fig. 5b). Before the moment of destruction the indenter penetration depth d reached saturation in which there were no sharp changes in the depth of the indentation (peak around 110 mN seen in Fig. 5b was due to the roughness of the coating surface and disappeared entirely when averaging the data over the five measurements). Fig. 6(a, b) shows typical impedance spectra measured on the Pt/LNFO/LNO | CSSBO | Pt cell. All the measured spectra were asymmetric in shape, implying more than one process. Since we are dealing with a complex multilayer system, to identify the processes on the spectra and separate out the contributions of the electrode support and electrolyte we have changed the experimental conditions. It is clearly seen that changing the atmosphere from air to argon leads to dramatic changes in the spectrum shape as a whole (Fig. 6b) which supports our hypothesis that it is related to the electrode support response. Usually, for the description of spectra of LNO-based electrodes, the equivalent circuits containing two [33,34] or three distributed elements [35], composed of a constant phase element (Q) in parallel with a resistance (R) are used. The equivalent capacitance C and relaxation frequency f of an electrode process corresponding to a specific (RQ) can be applied as characteristic parameters to identify various electrode processes. The high frequency electrode processes with the effective capacitances of approximately 10−6–10−5 F cm−2 have been associated with the transport of oxygen ions across the interface between the electrode and electrolyte. In the middle frequency range the processes with the effective capacitances of 10−4–10−3 is assigned to the diffusion of oxygen ions into the electrode volume accompanied by a charge transfer, while in the low frequency range the processes with the effective capacitances of 10− 3–10− 1 are ascribed to the adsorption and dissociation of molecular oxygen on the surface of the electrode [35]. The spectra of the complex three-layer system collected in different atmospheres comprise three distributed elements with the effective capacitances close to those described above (the capacitance values are shown in Fig. 6). In this case Rhf, the serial resistance determined by extrapolation of the high-frequency region of the impedance spectrum to
the intersection with the x-axis, may be considered as the electrolyte resistance. With due account taken of inductance, the Rhf value slightly decreases (about 5–6%) (Fig. 6а). The conductivity data are presented in Fig. 7. The energy of activation of the total conductivity of the CSSBO film is equal to that of the CSSBO compact sample. However, the total conductivity values, calculated from the spectra for the CSSBO film deposited on the graded LNFO/LNO support are lower than those for the compact CSSBO sample (2.4 × 10−2 and 6.3 × 10−2 S/cm at 750 °C, respectively). The conductivity of the deposited film can be influenced by different factors: the size of the crystallites and lattice strain, which makes ionic diffusion migration barrier affecting oxygen vacancies and chemical bond strength, crystallites boundary conditions (boundary density and phase), crystalline and amorphous residual phases in the formed thin film etc. [36,37]. Internal nanopores appeared during the step deposition could also be the reason for the conductivity decreasing. The conductivity extracted from the impedance spectroscopy data can be also influenced by the polarisation of the thick cathode layer, which introduces remarkable gas-diffusion limitations which can be fairly high in the case of a cathode-supported cell [38,39]. After changing the atmosphere a remarkable increase in polarisation resistance was observed, especially
Fig. 5. Representative dependences: а) amplitude of the acoustic emission signal and b) depth of the indenter penetration at the applied load.
Fig. 7. Temperature dependences of the total conductivity of the CSSBO film deposited on the graded LNFO/LNO substrate, CSSBO, LNO, LNFO compact samples and the polarisation conductivity of Pt/LNFO/LNO|CSSBO|Pt cell measured in air.
Fig. 6. Impedance spectra of Pt/LNFO/LNO | CSSBO |Pt cell at 600 °C: a) measured and modified while taking into account an inductance and b) measured in air and in argon atmospheres.
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for the process with the effective capacitances of 10−4–10−3 assigned to the diffusion of oxygen ions through the electrode volume. It resulted in an increase of the Rhf value as well (Fig. 6b). Our future investigations will be directed towards clarification of factors influencing the conductivity of the electrolyte film deposited on the porous support and on the polarisation resistance of the electrode support such as porosity and thickness of the layers with search for methods to optimise the cell design. 4. Conclusions The paper presents the results of a study of the microstructure and the mechanical and electrical properties of the thin Се0.8(Sm0.75Sr0.2Ba0.05)0.2O2 − δ (CSSBO) electrolyte film deposited by the cyclic EPD method from a non-aqueous suspension on a functionally graded cathode substrate with a LaNi0.6Fe0.4O3 ± δ (LNFO) collector layer and a La2NiO4 ± δ (LNO) functional layer. The gas-permeability coefficient was measured at all stages of the cell manufacturing and was equal to 30.38 × 10−3 μm2 for the thick supported LNFO layer (1 mm) with subsequent further decreasing down to 0.10 × 10−3 μm2 after deposition and sintering of the functional LNO layer (3–5 μm thick). It was shown that after the CSSBO film deposition (8 μm of green thickness) and sintering at 1400 °C, 4 h (up to 5 μm after sintering) the gas-permeability of the cell was equal to zero. The film had a grained structure (average grain size 4.2 μm), excellent adhesive and mechanical properties and total conductivity of 2.4 × 10−2 S/cm at 750 °C. The cyclic EPD allows one to increase the thickness of the deposited film twice as compared with the one-stage EPD. Acknowledgements Authors also would like to express their appreciation to N.M. Bogdanovich, T.A. Dem'yanenko and S.M. Beresnev (IHTE, UB RAS, Russia) for fabrication of the graded electrode substrates and A.A. Pankratov (IHTE, UB RAS, Russia) for SEM investigations. This work is partly supported by a grant from the President of the Russian Federation (no. SP-536.2015.1), Russian Foundation for Basic Research (grant no. 1603-00025) and Act 211 Government of the Russian Federation, contract no. 02.A03.21.0006. References [1] W. Vielstich, A. Lamm, H.A. Gasteiger, Handbook of Fuel Cells: Fundamentals, Technology, Applications, 3 and 4, Wiley, USA, 2003 3826. [2] A.B. Stambouli, E.R. Traversa, Renew. Sust. Energ. Rev. 6 (2002) 433–455. [3] T. Mori, J. Drennan, Y. Wang, J.-H. Lee, J.-G. Li, T. Ikegami, J. Electrochem. Soc. 150 (2003) A665–A673.
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Cyclic electrophoretic deposition of electrolyte thin-films on the porous cathode substrate utilizing stable suspensions of nanopowders E.G. Kalinina a,b, E.Yu. Pikalova b,c,⁎, A.A. Kolchugin b,c, S.M. Pikalov b,d, A.S. Kaigorodov a,b a
Institute of Electrophysics, UB RAS, 106 Amundsen St., 620016 Ekaterinburg, Russia Ural Federal University, 19 Mira St., 620002 Ekaterinburg, Russia Institute of High Temperature Electrochemistry, UB RAS, 20 Academicheskaya St., 620137 Ekaterinburg, Russia d Institute of Metallurgy, UB RAS, 101 Amundsen St., 620016 Ekaterinburg, Russia b c
a r t i c l e
i n f o
Article history: Received 29 July 2016 Received in revised form 28 December 2016 Accepted 23 January 2017 Available online xxxx Keywords: Self-stabilising suspension EPD Thin-film electrolyte SOFC Graded cathode substrate
a b s t r a c t In this study, nanopowder Се0.8(Sm0.75Sr0.2Ba0.05)0.2O2 − δ (CSSBO) with average diameter of particles of 15 nm and specific area of 53 m2/g prepared by the method of laser evaporation was used for a cyclic electrophoretic deposition (EPD) from a non-aqueous suspension on a functionally graded cathode substrate with a LaNi0.6Fe0.4O3 − δ (LNFO) collector layer and a La2NiO4 ± δ (LNO) functional layer. Thick LNFO support (1 mm) with 20% of pore former had a porosity of 45% after sintering at 1400°С. The values of gas-permeability which was measured by means of the computerised experimental set, varied from 31.60 × 10−3 to 25.71 × 10−3 μm2. A thin LNO functional layer was brush painted on the top of the support and pre-sintered at 1350°С, 2 h. Green CSSBO film with a thickness of 8 μm was stepwise deposited on the graded cathode substrate and sintered at 1400°С, 4 h. The critical load for fracture of the film (approximately 5 μm thick after sintering) was 111 ± 14 mN, conductivity was equal to 2.40 × 10−2 S/cm at 750 °C. © 2017 Elsevier B.V. All rights reserved.
1. Introduction The current approach to reduce a Solid Oxide Fuel Cell's (SOFC) operating temperature is motivated by a wider choice of low-cost materials for cells' functional layers and interconnectors, greater flexibility in cell design and the longer life-time of stacks due to reduced inter-diffusion and degradation of cell components [1,2]. However, the electrolyte conductivity decreases significantly when the operating temperature is lowered. This drawback can be overcome by lowering the electrolyte resistance either by developing alternative materials with higher ionic conductivity in an intermediate temperature range and/or decreasing the electrolyte thickness. The investigations of various multi-component solid state electrolytes based on CeO2 have shown that co-doping with rare earth and alkaline earth elements leads to an increased conductivity in the intermediate temperature range in comparison with single-doped solid solutions [3–6]. It was shown that in some cases co-doping suppresses Ce4+ → Ce3+ transition and expands the electrolytic domain, characterised by the critical oxygen partial pressure p*O2 below which the electronic conductivity exceeds the ionic conductivity, to lower values. In our recent study a high conductivity value in air of 6.25 × 10− 2 S/cm with the p* O2
⁎ Corresponding author. E-mail address: [email protected] (E.Y. Pikalova).
value of 2.76 × 10−23 atm at 750 °C was observed for Се0.8(Sm0.75Sr0.2Ba0.05)0.2O2 − δ (CSSBO) [7]. Electrophoretic deposition (EPD) is known to be a versatile and very cost-effective method for the formation of functional layers in SOFCs [8, 9]. Being a liquid-based method it is much more straightforward than gas-phase methods such as ion-plasma spraying, chemical vapour deposition and the method of laser sputtering in a vacuum, all of which require expensive equipment and careful monitoring of the technological environment [10]. Among the advantages of EPD are the short duration of film formation (approximately 1 μm/min), easy adaptation for various cell's design, no special requirements for binder burnout as the green coatings contain few or no organics and easy control of the film thickness through simple adjustment of the deposition time and applied potential [11]. Currently, there are some studies being made to obtain electrolyte films by the EPD method both on cathode [12–14] and anode substrates [11,15–19]. To deposit an electrolyte film on a nonconducting anode substrate such measures as the applying of a steel plate behind the substrate or modifying its surface with a conducting layer (graphite) are used. A conducting cathode support is highly preferable for the deposition but there is a problem with degradation of the electrode support's porous structure during the electrolyte film sintering. The solution is to choose cathode materials with the sinterability as low as possible and use a cathode support with a gradual reduction in porosity towards the electrolyte side. Both of these approaches were applied in the present study. La2NiO4 + δ (LNO) was
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the material of choice for the functional cathode layer due to its excellent electrochemical performance conditioned by the considerably high mobility of interstitial oxygen in its layered structure, a low value of the coefficient of thermal expansion (CTE ≈ 13.1 × 10− 6 К− 1), close to that for CeO2-based electrolytes, and decreased sinterability in comparison with the cobaltites of rare-earth and/or alkaline-in comparison elements [20–22]. The electronic conductivity of LNO is about of 60–80 S/cm in the intermediate temperature range and is insufficient to ensure uniform current distribution and low in-plane resistance for effective operation of the cathode. Thus, it is supposed to be used in multilayer electrodes in combination with an electronic conductor such as La4Ni3O10 − δ [23,24], La0.7Sr0.3CoO3 − δ and La0.7Sr0.3MnO3 − δ [25,26] as a current collector. In the present study LaNi0.6Fe0.4O3 − δ (LNFO) was chosen as a collector due to its higher electronic conductivity and lower sinterability as compared with the other possible candidates and a CTE value compatible with that of LNO (CTE ≈ 12.5 × 10− 6 К− 1) [27]. The technology to form a thin film electrolyte by the EPD method involves the preparation of a stable suspension of the ceramic powder which can be simplified by using weakly aggregated nanopowders fabricated by the laser evaporation method [28,29]. Similar weakly aggregated nanopowders of doped-ceria electrolytes and suspensions for EPD were obtained and investigated in [30–32]. In our recent paper [32] we analysed in detail the formation of a thin CSSBO electrolyte film on a dense LNO cathode support. According to our numerous experiments there are technological limitations to the thickness (approximately 2 μm) of the defect-free electrolyte film deposited by EPD during 1 cycle. In the present study deposition of the CSSBO electrolyte film was performed on a functionally graded cathode substrate with LNFO collector (1 mm) and thin La2NiO4 ± δ functional layer (5 μm after sintering) was used as a support for stepwise electrophoretic deposition of CSSBO electrolyte from a self-stabilised suspension. To ensure complete covering of the porous support surface a film with increased thickness was deposited by the cyclic EPD method with intermediate pre-sintering of the layers. Gas-permeability of the cell was inspected at every step of the cell formation. The microstructural, mechanical and electrical properties of the deposited film were investigated. 2. Experimental procedure 2.1. Electrolyte and cathode materials' preparation and characterisation Се0.8(Sm0.75Sr0.2Ba0.05)0.2O2 − δ (CSSBO) powder was initially prepared by a solid state reaction method as described elsewhere [30]. CSSBO nanopowder was obtained by the method of laser evaporation of a ceramic target using a LS-06 ytterbium fibre laser with a wavelength of 1.07 μm. A JEOL JEM 2100 transmission electron microscope (TEM) was used to characterise the CSSBO nanopowders. X-ray phase analysis was carried out on a D8 DISCOVER diffractometer in copper radiation with a graphite monochromator on a diffracted beam. Processing was carried out using Topas-3 software with the Rietveld algorithm of structural parameter refinement. The specific surface area of nanopowders was determined by a volume version of the Brunauer-Emmett-Teller (BET) method based on a low-temperature equilibrium sorption of nitrogen vapours by a Micromeritics TriStar 3000 vacuum sorption analyser. Particles of the nanopowder had a spherical shape with an average diameter of 15 nm and a specific surface area of 53 m2/g. XRD analysis confirmed that the CSSBO nanosized powder contained one crystalline phase – a solid solution based on a cubic form of CeO2 with the lattice parameter a = 5.440(3) Å and crystalline size dXRD = 14(1) nm. La2NiO4 + δ (LNO) was synthesised via a two-step ceramic technology (1150 °C for 2 h, 1250 °C for 5 h) as described elsewhere [24]. XRD analysis revealed in the case of LNO the formation of an orthorhombic structure (sp. gr. Fmmm, a = 5.448(1) Å, b = 5.477(1)
Å, c = 12.667(8) Å). The LNO powder was milled up to the specific surface area 1.7 and 2.6 m2/g. LaNi0.6Fe0.4O3 − δ (LNFO) was synthesised vie a modified Pechini method [24] and had a specific surface area equal to 1.2 m2/g after calcination at 1200 °C. XRD analysis revealed in the case of LNF the formation of a rhombohedral structure (sp. gr. R-3c a = 5.505(1) Å, b = 5.505(1) Å, c = 13.273(3) Å). 2.2. Fabrication of the functionally graded cathode substrate A functionally graded cathode substrate with LNFO collector layer and LNO functional layer was prepared in three stages. Thick supported layer LNFO (1 mm) with 20% of pore former (graphite, 6.9 m2/g) had a porosity of 45% after the sintering at 1400°С, 2 h. LNO layer (1.7 m2/g, 3– 5 μm) was brush painted on the top of the support and pre-sintered at 1100°С, 2 h. Second LNO layer (2.6 m2/g, 3–5 μm) was brush painted and sintered at 1350°С, 2 h. The values of gas-permeability of the cathode support were measured by means of computerised experimental set made in the Institute of Electrophysics of the Ural Branch of the Russian Academy of Sciences (UB RAS). The technique is based on Darcy's law – a constitutive equation that describes the gas flow rate through a porous medium and relies on measuring precisely the pressure drop at the inlet and outlet of the sample at a given value of the initial negative pressure. The gas permeability coefficient is determined by a law of evacuation decreasing with time due to gas leakage through the sample. Before every start the set is calibrated by means of measurement performed on a standard gas-tight sample. 2.3. Electrophoretic deposition EPD was carried out on the computerised equipment of laboratory design which provided constant voltage modes. The LNFO/LNO substrate of 12 mm2 in area served as a cathode, a disc (12 mm2) of stainless steel was used as an anode and the distance between the electrodes was 1 cm. Densification of the deposited films was performed in a Hermle Labnet Z383 centrifuge with a rotational velocity of 1000 rpm for 2 min. 2.4. Electrode substrate and thin film characterisation SEM images of the deposited film and microstructure of the electrode substrate were obtained using an electron microscope Mira 3 LMU. The roughness of the surface of the coating was determined by a Zygo NewView 500 device. Adhesion to the substrate was studied by the scratching method using a 600 Nanotest device with an acoustic emission module according to the following procedure: speed of the conical indenter (radius of curvature of the indenter point – 5 μm) along the surface of the sample – 5 μm/s, the length of the scratches – 500 μm, the speed of the load applying −5 μm/s, the maximal load – 500 mN. The critical load value was the load that was determined by the first response to the acoustic emission signal and this was averaged over the five measurements. The conductivity of compact LNFO, LNO and CSSBO samples was measured by the dc four-probe method in air in the temperature range of 500–850 °C with steps of 50 °C with isothermal exposure of 1 h within it. Evaluation of the thin film conductivity was made in the same temperature range during heating and cooling on the cell with Pt electrodes deposited on both sides of the three-layer (LNFO/LNO/ CSSBO) cell and sintered at 900 °C, 1 h. The measurements were carried out by the method of impedance spectroscopy (EIS) using a frequency response analyser FRA-1260 with an electrochemical interface EI-1287 (Solartron Instruments Inc.) in the frequency range of 0.1 Hz to 500 kHz at the amplitude of applied sinusoidal signal of 30 mV in air and in argon in the temperature range of 450–750 °C. Each measurement was finished by measuring a full dc resistance Rdc of the cell. The impedance spectra were analysed by using K-K Test and ZView software with an equivalent circuit presented in Fig. 6. Rhf, the serial resistance determined by extrapolation of the high-frequency region of an
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impedance spectrum to the intersection with the x-axis, was taken as the electrolyte resistance. 3. Results and discussion There are significant differences in engineering procedures for producing electrolyte- and electrode-supported fuel cells. In the first case the solid electrolyte is sintered at a high temperature to ensure its gas tightness while the electrodes are formed on the electrolyte at significantly lower temperatures to provide high porosity and electrochemical activity. In the second case the sintering temperature of the electrode support must be higher or, at least, the same as that for the electrolyte film. In the opposite case it is extremely difficult to prevent compaction of the electrode support structure and avoid gas-diffusion limitations during SOFC operation. But it should have the necessary level of mechanical strength, electrical conductivity and electrochemical activity. These requirements can be achieved by using a functionally graded electrode with a supporting (collector) layer possessing a high mechanical strength, porosity and electrical conductivity and a functional layer with a TEC value close to that of the electrolyte film, a high level of electrochemical activity and a level of porosity that is appropriate to the film electrolyte preparation. Results of testing electrical properties (a) and dependence of the porosity of the LNFO compacts on the sintering temperature (b) are shown
Fig. 1. Temperature dependence of the conductivity of the LNFO compacts with 20% of pore former sintered in the temperature range 1300–1450 °C (a) and porosity of the samples depending on the sintering temperature (b).
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in Fig. 1. The samples were sintered at temperatures in the range 1300– 1450 °C and their porosity decreased almost linearly from 58 to 38%. The temperature 1400 °C was chosen to fabricate the support with the necessary level of porosity (45%) and a high level of electronic conductivity (225 S/cm at 700 °C). Conductivity of the LNFO substrates was 2.5–3 times higher than for those made of LSM [33] and close to those reported in [34] where the authors have shown that the Pechini method provides a higher level of electrical conductivity of LNFO in comparison with other methods of synthesis at the same level of the samples' porosity. The thick LNFO collector layer, having a 45% degree of porosity, showed a high value of gas permeability equal to 30.38 × 10− 3 μm2 which decreased appreciably after deposition of the LNO functional layer (0.20 × 10−3 μm2). To deposit the thin electrolyte film on the cathode support by the EPD method we used a stable self-stabilising suspension of nanosized CSSBO powder in a mixed dispersion medium acetyl acetone/ isopropanol (50/50 vol% ratio) with concentration 10 g/l, obtained and studied earlier in [31,32]. In the suspension based on the mixed medium the natural pH value was 4 with a high positive value of ζ-potential (+ 31 mV). Before the deposition the suspension was ultrasonically treated. Fig. 2 shows a dependence of the average hydrodynamic size of aggregates deff on the duration time of the ultrasonic treatment. The average size of CSSBO aggregates decreased from 261 to 162 nm, reaching saturation level after 75 min of the treatment. Thus, the ultrasonic treatment did not lead to complete disaggregation; however, the hydrodynamic size of aggregates (effective diameter) reduced considerably after the treatment. Our numerous experiments reveal that a prolonged ultrasonic treatment (more than 25 min) results in longterm stability of the suspensions (for more than one month) and, decreasing the hydrodynamic size of aggregates, and ensures better quality and density of the EPD coatings formed [11,29]. To prevent cracking in the deposited film a modifier BMMA-5, which is a co-polymer of butyl methacrylate with the addition of 5 mol% of methacrylic acid, was added to the suspension. To ascertain the optimal concentration of the polymeric binder a set of experiments was conducted [32]. The density of the green coating in the presence of the modifier was 48–60%, which makes possible the subsequent sintering into a dense ceramic. To ensure the operational reliability of the electrolyte layer it is necessary to provide a thickness of approximately 5–10 μm which is difficult to implement with a single-step EPD process. As a result of extensive experimental study it was found that for our suspensions the maximal thickness of the electrolyte film which was possible to be obtained was approximately 3 μm. With any further increase in
Fig. 2. Dependence of the average hydrodynamic size of aggregates in the self-stabilising CSSBO suspension on the duration time of the continuous ultrasonic treatment.
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Fig. 3. Optical images of the surface of the graded cathode support prepared for EPD deposition (a), the surface after the EPD (b, c, e, g) and sintering at 1350°С, 1 h. (d, f) and sintering at 1400°С, 4 h (h).
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thickness surface cracking appeared upon drying due to increased internal stresses. In order to increase the thickness of the uniform CSSBO film we conducted a cyclic deposition. Fig. 3a shows the optical micrograph of the top surface of the porous cathode support prepared for the deposition. It was uniform with pores less than 1 μm. The three stages of the cyclic deposition were carried out in the mode selected according to our earlier study [32]: the voltage was set in a range of 80–100 V, the duration time of the deposition was 1 min. At the first stage, the film was deposited twice (Fig. 3b, c) with following sintering at 1350 °C, 1 h, with a heating rate of 1 °C/min and free cooling (Fig. 3d). Second and third stages comprise single deposition with intermediate sintering at 1350 °C (Fig. 3e, f, g). Throughout all the stages of the deposition the film formed was uniform and crack-free. As a result of the cyclic deposition a homogeneous electrolyte film with a total weight of 5.6 mg/cm2 and 8 μm thickness (green film) was obtained. The final sintering was carried out at 1400 °C, 4 h to ensure gas-tightness of the electrolyte film (Fig. 3h). The final temperature did not exceed the sintering temperature of the collector layer to safeguard its structure and gas permeability.
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After the final sintering the CSSBO film had uniform structure with few closed pores and was 3.5–5.1 μm thick with average grain size of 4.2 μm (Fig. 4a, c). The LNO layer is not clearly seen because LNO ceramic slurry deposited on the LNFO substrate with a very high porosity partly penetrated into its structure. It is demonstrated on the element map which shows the spatial distribution of elements in a sample (Fig. 4b). The density of the LNO functional layer, evaluated from experiments with compacts sintered at different temperatures, is supposed to be approximately 80% with a thickness after final sintering of 2–3 μm. At the same time, the collector layer LNFO kept well-developed pore structure after the sintering (Fig. 4d), which provides a good gas exchange. Since the LNO functional layer is quite dense, this ensures good contact on the electrode/electrolyte interphase providing the necessary adhesion of the electrolyte film and facilitating charge transfer. The gas-permeability of the cell measured after the final sintering was equal to zero. The surface roughness of the electrolyte film was 0.20 ± 0.05 μm. The critical load of the film destruction was equal to 111 ± 14 mN. The curve of the amplitude of the acoustic emission signal
Fig. 4. SEM images of the cross-section (a), the element map (b), the CSSBO electrolyte surface (c) and the cathode support back-surface (d) after the sintering at 1400°С, 4 h.
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shown in Fig. 5 has no separate peaks (as in the case of fragile coatings with low adhesion) and changes stepwise. Probably, with a constant increase in load, dissipation of fracture energy occurs in the form of plastic deformation of the coating without the formation of micro-cracks. This in turn indicates good performance properties of the gas-tight electrolyte film because its fracture occurs directly at the macroscopic level and at a sufficiently high mechanical load. This supposition is confirmed by the behaviour of the indenter with a constantly increasing load (Fig. 5b). Before the moment of destruction the indenter penetration depth d reached saturation in which there were no sharp changes in the depth of the indentation (peak around 110 mN seen in Fig. 5b was due to the roughness of the coating surface and disappeared entirely when averaging the data over the five measurements). Fig. 6(a, b) shows typical impedance spectra measured on the Pt/LNFO/LNO | CSSBO | Pt cell. All the measured spectra were asymmetric in shape, implying more than one process. Since we are dealing with a complex multilayer system, to identify the processes on the spectra and separate out the contributions of the electrode support and electrolyte we have changed the experimental conditions. It is clearly seen that changing the atmosphere from air to argon leads to dramatic changes in the spectrum shape as a whole (Fig. 6b) which supports our hypothesis that it is related to the electrode support response. Usually, for the description of spectra of LNO-based electrodes, the equivalent circuits containing two [33,34] or three distributed elements [35], composed of a constant phase element (Q) in parallel with a resistance (R) are used. The equivalent capacitance C and relaxation frequency f of an electrode process corresponding to a specific (RQ) can be applied as characteristic parameters to identify various electrode processes. The high frequency electrode processes with the effective capacitances of approximately 10−6–10−5 F cm−2 have been associated with the transport of oxygen ions across the interface between the electrode and electrolyte. In the middle frequency range the processes with the effective capacitances of 10−4–10−3 is assigned to the diffusion of oxygen ions into the electrode volume accompanied by a charge transfer, while in the low frequency range the processes with the effective capacitances of 10− 3–10− 1 are ascribed to the adsorption and dissociation of molecular oxygen on the surface of the electrode [35]. The spectra of the complex three-layer system collected in different atmospheres comprise three distributed elements with the effective capacitances close to those described above (the capacitance values are shown in Fig. 6). In this case Rhf, the serial resistance determined by extrapolation of the high-frequency region of the impedance spectrum to
the intersection with the x-axis, may be considered as the electrolyte resistance. With due account taken of inductance, the Rhf value slightly decreases (about 5–6%) (Fig. 6а). The conductivity data are presented in Fig. 7. The energy of activation of the total conductivity of the CSSBO film is equal to that of the CSSBO compact sample. However, the total conductivity values, calculated from the spectra for the CSSBO film deposited on the graded LNFO/LNO support are lower than those for the compact CSSBO sample (2.4 × 10−2 and 6.3 × 10−2 S/cm at 750 °C, respectively). The conductivity of the deposited film can be influenced by different factors: the size of the crystallites and lattice strain, which makes ionic diffusion migration barrier affecting oxygen vacancies and chemical bond strength, crystallites boundary conditions (boundary density and phase), crystalline and amorphous residual phases in the formed thin film etc. [36,37]. Internal nanopores appeared during the step deposition could also be the reason for the conductivity decreasing. The conductivity extracted from the impedance spectroscopy data can be also influenced by the polarisation of the thick cathode layer, which introduces remarkable gas-diffusion limitations which can be fairly high in the case of a cathode-supported cell [38,39]. After changing the atmosphere a remarkable increase in polarisation resistance was observed, especially
Fig. 5. Representative dependences: а) amplitude of the acoustic emission signal and b) depth of the indenter penetration at the applied load.
Fig. 7. Temperature dependences of the total conductivity of the CSSBO film deposited on the graded LNFO/LNO substrate, CSSBO, LNO, LNFO compact samples and the polarisation conductivity of Pt/LNFO/LNO|CSSBO|Pt cell measured in air.
Fig. 6. Impedance spectra of Pt/LNFO/LNO | CSSBO |Pt cell at 600 °C: a) measured and modified while taking into account an inductance and b) measured in air and in argon atmospheres.
Please cite this article as: E.G. Kalinina, et al., Cyclic electrophoretic deposition of electrolyte thin-films on the porous cathode substrate utilizing stable suspensions of nanopo..., Solid State Ionics (2017), http://dx.doi.org/10.1016/j.ssi.2017.01.016
E.G. Kalinina et al. / Solid State Ionics xxx (2017) xxx–xxx
for the process with the effective capacitances of 10−4–10−3 assigned to the diffusion of oxygen ions through the electrode volume. It resulted in an increase of the Rhf value as well (Fig. 6b). Our future investigations will be directed towards clarification of factors influencing the conductivity of the electrolyte film deposited on the porous support and on the polarisation resistance of the electrode support such as porosity and thickness of the layers with search for methods to optimise the cell design. 4. Conclusions The paper presents the results of a study of the microstructure and the mechanical and electrical properties of the thin Се0.8(Sm0.75Sr0.2Ba0.05)0.2O2 − δ (CSSBO) electrolyte film deposited by the cyclic EPD method from a non-aqueous suspension on a functionally graded cathode substrate with a LaNi0.6Fe0.4O3 ± δ (LNFO) collector layer and a La2NiO4 ± δ (LNO) functional layer. The gas-permeability coefficient was measured at all stages of the cell manufacturing and was equal to 30.38 × 10−3 μm2 for the thick supported LNFO layer (1 mm) with subsequent further decreasing down to 0.10 × 10−3 μm2 after deposition and sintering of the functional LNO layer (3–5 μm thick). It was shown that after the CSSBO film deposition (8 μm of green thickness) and sintering at 1400 °C, 4 h (up to 5 μm after sintering) the gas-permeability of the cell was equal to zero. The film had a grained structure (average grain size 4.2 μm), excellent adhesive and mechanical properties and total conductivity of 2.4 × 10−2 S/cm at 750 °C. The cyclic EPD allows one to increase the thickness of the deposited film twice as compared with the one-stage EPD. Acknowledgements Authors also would like to express their appreciation to N.M. Bogdanovich, T.A. Dem'yanenko and S.M. Beresnev (IHTE, UB RAS, Russia) for fabrication of the graded electrode substrates and A.A. Pankratov (IHTE, UB RAS, Russia) for SEM investigations. This work is partly supported by a grant from the President of the Russian Federation (no. SP-536.2015.1), Russian Foundation for Basic Research (grant no. 1603-00025) and Act 211 Government of the Russian Federation, contract no. 02.A03.21.0006. References [1] W. Vielstich, A. Lamm, H.A. Gasteiger, Handbook of Fuel Cells: Fundamentals, Technology, Applications, 3 and 4, Wiley, USA, 2003 3826. [2] A.B. Stambouli, E.R. Traversa, Renew. Sust. Energ. Rev. 6 (2002) 433–455. [3] T. Mori, J. Drennan, Y. Wang, J.-H. Lee, J.-G. Li, T. Ikegami, J. Electrochem. Soc. 150 (2003) A665–A673.
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Please cite this article as: E.G. Kalinina, et al., Cyclic electrophoretic deposition of electrolyte thin-films on the porous cathode substrate utilizing stable suspensions of nanopo..., Solid State Ionics (2017), http://dx.doi.org/10.1016/j.ssi.2017.01.016