Oxygen diffusion and surface exchange kinetics for the mixed-conducting oxide La0.6Sr0.4Co0.8Fe0.2O3-δ 196 D O I: 1 0. 15 82 6/ ch im te ch .2 01 8. 5. 04 .0 4 Porotnikova N. M., Antonova E. P., Khodimchuk A. V., Tropin E. S., Farlenkov A. S., Ananyev M. V. Chimica Techno Acta. 2018. Vol. 5, No. 4. P. 196–204. ISSN 2409–5613 N. M. Porotnikovaab*, E. P. Antonovaab, A. V. Khodimchukab, E. S. Tropinab, A. S. Farlenkovab, M. V. Ananyevab a Institute of High Temperature Electrochemistry, Ural Branch of Russian Academy of Sciences, 20 Akademicheskaya St., Ekaterinburg, 620137, Russian Federation b Ural Federal University, 19 Mira St., Ekaterinburg, 620002, Russian Federation *E-mail: n.porotnikova@mail.ru Oxygen diffusion and surface exchange kinetics for the mixed‑conducting oxide La0.6Sr0.4Co0.8Fe0.2O3–δ Studies of oxygen surface exchange kinetics for La0.6Sr0.4Co0.8Fe0.2O3–δ oxide were performed using the technique of isotopic exchange of molecular oxygen with analysis of gas phase isotopic composition in a static circulation system at the temperatures of 600–800 °С in the oxygen pressure range of 0.27– 2.13 kPa. The values of interphase exchange rate and oxygen diffusion coefficient were determined. The effective activation energies for oxygen exchange and diffusion processes as well as the exponents in the dependence of these values versus oxygen pressure in the double logarithmic coordinates were calculated. The process of oxygen dissociative adsorption at the surface of La0.6Sr0.4Co0.8Fe0.2O3–δ oxide was found to be the rate-determining stage. Keywords: oxygen isotope exchange; oxygen diffusion; lanthanum-strontium cobaltite-ferrite; rate-determining stage Received: 14.11.2018. Accepted: 11.12.2018. Published: 31.12.2018. © Porotnikova N. M., Antonova E. P., Khodimchuk A. V., Tropin E. S., Farlenkov A. S., Ananyev M. V., 2018 Introduction Over the last decades, complex oxides with the perovskite structure based on the lanthanum cobaltite attract much attention as  potential cathode materials for  solid oxide electrochemical devices [1–4]. Variation of cations concentration in  these oxides results in  the physical- chemical properties changes in the oxide- based materials [5, 6]. Complex oxides with a  common formula La1−xSrxCo1− yFeyO3−δ are among the most perspective cathode materials. A sufficient amount of information available in the literature has focused on the study of phase equilibria and physicochemical properties of oxides in  this system [7–13]. In order to  create the effectively operating cathodes based on lanthanum-strontium cobaltite-ferrites, it is vital to understand in detail the oxy- gen exchange and diffusion mechanisms in these materials. The main purpose of  the pre- sent work was to  study oxygen dif- fusion and surface exchange kinetics in La0.6Sr0.4Co0.8Fe0.2O3–δ oxide. 197 Experimental The La0.6Sr0.4Co0.8Fe0.2O3–δ oxide was prepared using the citrate-nitrate technolo- gy. Lanthanum oxide (LaO-D, 99,99635 %), strontium carbonate SrCO3 (ACS), cobalt nitrate Co(NO3)2·6H2O (chemically pure), iron citrate C6H5FeO7·H2O (Fluka Analyti- cal) were used as initial reagents. The syn- thesis was performed at the temperature of 1100 °С for 5 hours. In order to perform the oxygen isotope exchange measurements, dense ceramic was fabricated. The obtained powders were ground and compacted into the form of disks using 1 % water solution of poly- vinyl alcohol that served as  a  bounding agent. The final sintering was performed at the temperature of 1250 °С for 5 hours in  air. A relative density of  the obtained ceramics was about ~ 92 %. Finally, the sin- tered dense specimens were polished using diamond pastes, such as АСМ 7 / 5 NVMC (Federal Standard 25593–83) and АСМ 1 / 0 NOM (Federal Standard 16377–71). The phase composition of  the La0.6Sr0.4Co0.8Fe0.2O3–δ sample was deter- mined before and after the isotope experi- ments using a Rigaku D / MAX-2200V dif- fractometer in the Cu Ka emission at room temperature. According to the X-ray analy- sis, the sample was single phase after syn- thesis (R3c space group, cell parameters a = 5.4270(4) Å, c = 13.239(2) Å) and after completing measurements at high temper- atures and low oxygen pressures (Fig. 1), which confirms its stability during long- term tests. The analysis of the particle size distribu- tion for powder materials was performed by the laser scattering method using a Mal- vern Mastersizer 2000. To grind the ag- glomerated particles, the slurry was mixed using a stirrer with the rate of ~2000 rpm, as well as subjected to an ultrasound treat- ment. Fig. 2 illustrates the volume fraction versus particle size distribution function. The microphotographs of the ceramics cross-section were made by  a  scanning electron microscope Tescan MIRA 3. Fig. 3 presents the microphotograph obtained in a beam of back-scattered electrons. The contrast of the images is mainly due to the chemical composition of the studied ma- terial surface. As can be seen from Fig. 4, there are small inclusions of  cobalt-rich phase at the grains boundaries (probably cobalt oxide); however, the fraction is in- significant, it is less than 0.5 %. Fig. 1. XRD patterns for La0.6Sr0.4Co0.8Fe0.2O3–δ oxide before and after isotope experiments Fig. 2. Particle size distribution function for the La0.6Sr0.4Co0.8Fe0.2O3–δ powder 15 30 45 60 75 In te ns it y, a .u . 2Θ before experiment aster experiment 0,1 1 10 100 1000 0 1 2 3 4 5 6 7 8 vo lu m e fr ac ti on , % particle size, µm 198 The oxygen exchange kinetics between the gas phase and oxide was studied by the oxygen isotope exchange method with gas phase equilibration in  the experimental rig [12]. Enriched oxygen 18O, whose frac- tion was 83.6 %, served as a match mark. During the experiment, the changes in  concentrations of  three mass num- bers  — С32, С34, С36  — were recorded depending on  time using a  quadrupolar mass-spectrometer Agilent 5973N. The detailed description of  the experiment methodology, evaluation of  the inter- phase exchange rate detection accuracy (rH, atom·cm –2 · s–1) and oxygen diffusion coefficient (D, cm2·s–1) has been reported elsewhere [14, 15]. The oxygen interphase exchange rate is numerically equal to the number of oxygen atoms, which exchange at  the surface of  a  unit area per unit of time. The isotope exchange method is one of the few direct methods of oxygen exchange kinetics study, which advan- tage is a possibility to obtain information Fig. 4. EDX mapping for La0.6Sr0.4Co0.8Fe0.2O3–δ ceramics Fig. 3. SEM image of cross-section of La0.6Sr0.4Co0.8Fe0.2O3–δ ceramics in BSE mode 199 on  the oxygen redistribution between a  solid oxide and gaseous phase in  the adsorption-desorption equilibrium. This allows obtaining high accuracy for the ki- netics characteristic values. To compare the obtained values of the interphase ex- change rate with the literature data the following translation formula was used: k r M NH r A � �( ) , 3 � � (1) where rH is the oxygen interphase exchange rate (atom∙cm–2 ∙ s–1), k is the oxygen ex- change coefficient (cm ∙ s–1), Mr is the mo- lecular mass, δ is the oxygen non-stoichi- ometry, NA is the Avogadro constant, ρ is the sample crystallographic density. Results and discussion The oxygen exchange kinetics for the La0.6Sr0.4Co0.8Fe0.2O3–δ oxide was studied at the temperatures of 600–800 °С and the oxygen pressure interval 0.27–2.13 kPa. During the experiment, the changes in ion- ic current, which correspond to the weight of 32, 34 and 36, versus the exchange time were recorded. Then the obtained values were recalculated into the mass numbers concentrations (see Fig. 5). Typical time dependence for  the 18О oxygen isotope fraction changes in a particular experimen- tal condition is presented in  Fig. 6. The obtained data can be described using the model developed by Ezin et al. [16] on the basis of the solution suggested by Klier et al. [17]. Fig. 7 illustrates the dependencies of  the interphase exchange rate versus oxygen pressure in  logarithmic coordi- nates at  different temperatures for  the La0.6Sr0.4Co0.8Fe0.2O3–δ oxide. The values of exchange rate increase noticeably as the oxygen pressure and temperature rise. The dependence of  interphase exchange rate versus oxygen pressure exhibited a form of the exponential function: r PH O n~ 2 . The exponent values that were calculated from the line slopes decreased from 1.03 ± 0.04 down to  0.52 ± 0.03 as  the temperature rose. Boreskov et al. [18] proved that the oxygen exchange occurs at  the elevated temperatures according to the oxygen dis- sociative adsorption-desorption mecha- nism for a number of simple oxide systems. In particular, the oxides with equilibrium 0 5000 10000 15000 20000 0,0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 co nc en tr at io n С 3 2, C 34 , C 3 6 time, s C 32 C 34 C 36 Fig. 5. Experimental concentrations of C32, C34, C36 in the gas phase versus time (T = 800 °С, РO2 = 0.53 kPa) Fig. 6. Fraction of 18O-isotope in the gas phase vs. time in the typical isotope exchange experiment at Т = 800 °С, РO2 = 0.53 kPa. Points are experimental data, and the line is the fitting result 200 oxygen concentration do not obey the first type of the exchange mechanism, accord- ing to which oxygen atom from the surface does not involved. The isotope exchange proceeds mainly according to the second and third exchange types with the partici- pation of one or two oxygen atoms from the surface. Recently, for the complex oxide systems [15, 19–23] we have demonstrated that the exponents in the equations for the oxygen interphase exchange versus oxygen pressure varies depending on the process which is a rate-determining stage. Often the exchange at high temperature occurs according to the oxygen dissociative ad- sorption mechanism. Depending on  the relation of three exchange types contribu- tion, the oxygen exchange mechanism dif- fers for oxides. Fig. 8 demonstrates the dependence of the tracer diffusion coefficients of oxy- gen versus oxygen pressure at  various temperatures. The value of the oxygen dif- fusion coefficient is almost independent of oxygen pressure. Earlier [15] we dem- onstrated the influence of the oxygen non- stoichiometry on the value of the oxygen tracer diffusion coefficient, for  example in lanthanum strontium cobaltites. Almost constant value of oxygen tracer diffusion coefficient in  the Fe-doped lanthanum strontium cobaltite is likely associated with the insignificant changes in the oxygen va- cancies concentration within the oxygen pressures range of 0.27 ≤ Ро2 ≤ 2.13 kPa, which was also confirmed earlier [24–26]. Figs. 9, 10 illustrate the temperature dependencies of  oxygen exchange coef- ficient and oxygen diffusion coefficient, respectively, in  comparison with liter- ary data for different oxides [22, 26–29]. The values of  activation energy for  the exchange and diffusion processes are listed in  Table 1. It should be noted that relatively high values of oxygen diffusion and exchange coefficients for the studied La0.6Sr0.4Co0.8Fe0.2O3–δ oxide are comparable with the values for barium praseodymium cobaltite [29] and are greater by the value of  magnitude than those for  lanthanum strontium manganite [22]. Therefore, it may be assumed that La0.6Sr0.4Co0.8Fe0.2O3–δ is a perspective oxide for the SOFC cath- ode materials, because of  its high values of  exponents in  the equation for  oxygen exchange reaction and good stability in the reducing atmosphere. The substitution of iron for cobalt results in the insignifi- Fig. 7. The oxygen interphase exchange for La0.6Sr0.4Co0.8Fe0.2O3–δ plotted as a function of oxygen partial pressure at different temperatures, Рoо2 = 101.3 kPa (n ~ tgβ) Fig. 8. Oxygen diffusion coefficients versus oxygen partial pressure at different temperatures for  the La0.6Sr0.4Co0.8Fe0.2O3–δ oxide -2,6 -2,4 -2,2 -2,0 -1,8 -1,6 15,2 15,4 15,6 15,8 16,0 16,2 16,4 16,6 16,8 600°C, n=1.03 700°C, n=0.88 800°C, n=0.52 lo g( r H , at om c m 2 c –1 ) log(Po2/P°o2) -2,6 -2,4 -2,2 -2,0 -1,8 -1,6 -9,0 -8,5 -8,0 -7,5 -7,0 -6,5 600°C 700°C 800°C log(Po2/P°o2) lo g( D , c m 2 s –1 ) 201 cant decrease in the oxygen exchange rate; however, its introduction into the cobalt sublattice increases the stability of  oxide in the reducing atmospheres [6]. Conclusions The oxygen exchange kinetics for the La0.6Sr0.4Co0.8Fe0.2O3–δ oxide was studied using the isotope exchange method with gas phase equilibration. The values of the interphase exchange rate and oxygen dif- fusion coefficient in La0.6Sr0.4Co0.8Fe0.2O3–δ were calculated. The values of effective ac- tivation energies for the oxygen interphase exchange and diffusion were calculated. The exponents in the equations for the oxygen interphase exchange rate were found within the range of  0.52–1.03 at the temperatures of 600–800 °С in the oxygen pressures range of 0.27–2.13 kPa. Table 1 The apparent activation energy values for the oxygen surface exchange and oxygen diffusion processes in the mixed conducting oxides Oxide Oxygen pressure, kPa ∆Т, °С Activation energy, eV SourceExchange Diffusion La0.6Sr0.4Co0.8Fe0.2O3–δ 2.13 600–800 0.57 ± 0.05 0.92 ± 0.05 This work La0.6Sr0.4CoO3±δ 0.67 600–850 0.11 1.08 [26] La0.6Sr0.4MnO3±δ 0.67 700–850 0.71 1.42 [22] Pr2NiO4±δ 0.67 600–700 700–800 2.0 1.4 2.0 [27] La2NiO4±δ 1.33 600–800 1.38 1.41 [28] PrBaCo2O6–δ 1.33 600–800 0.76 0.75 [29] Fig. 9. Temperature dependences of the oxygen exchange coefficient for various mixed conducting oxides Fig. 10. Temperature dependences of the oxygen tracer diffusion coefficient for various mixed conducting oxides 0,9 1,0 1,1 1,2 -9 -8 -7 -6 La 0.6 Sr 0.4 Co 0.8 Fe 0.2 O 3±δ La 0.6 Sr 0.4 CoO 3±δ [26] La 0.6 Sr 0.4 MnO 3±δ [22] Pr 2 NiO 4±δ [27] La 2 NiO 4±δ [28] PrBaCo 2 O 6-δ [29] lo g (k ,c m s- 1 ) 1000/T, К–1 0,9 1,0 1,1 1,2 -12 -11 -10 -9 -8 -7 La 0.6 Sr 0.4 Co 0.8 Fe 0.2 O 3±δ La 0.6 Sr 0.4 CoO 3±δ [26] La 0.6 Sr 0.4 MnO 3±δ [22] Pr 2 NiO 4±δ [27] La 2 NiO 4±δ [28] PrBaCo 2 O 6-δ [29] 1000/T, К–1 lo g (D ,c m 2 s- 1 ) 202 Based on  these results, it can be con- cluded that the exchange in  the case of La0.6Sr0.4Co0.8Fe0.2O3–δ occurs according to the mechanism of the molecular oxygen dissociative adsorption at the oxide surface. The obtained results demonstrate that La0.6Sr0.4Co0.8Fe0.2O3–δ possesses high values of  oxygen interphase exchange rate and oxygen diffusion coefficient as compared to other oxide materials with mixed con- ductivity. 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