SOFCs Components: cathodes

Transcription

1 Laurea Magistrale in Scienza dei Materiali Materiali Inorganici Funzionali SOFCs Components: cathodes Prof. Antonella Glisenti - Dip. Scienze Chimiche - Università degli Studi di Padova

2 Cathode: requirements Functions: To provide reaction sites for the electrochemical reduction of the oxidant Requirements: Stability chemical, morphological, dimensional stability at the oxidant atmosphere (inlet and outlet) and at the operating and fabrication temperatures (no disruptive phase transformation) Electronic (Mixed) conductivity in the oxidation atmosphere (at the operating temperature) to minimize ohmic losses (constant with P O2 changes) Compatibility chemical compatibility with the other cell components Thermal expansion must match (from RT to the operating and fabrication temperatures, thermal cycling) that of other components; thermal coefficient stable in the oxidant atmosphere Porosity high porosity to allow gas transport to the reaction sites Catalytic activity High catalytic activity to low polarization for electrochemical reduction of oxidant

3 Cathodes Doped La manganites: strontium-doped LaMnO 3 (LSM - SOFC OT ~1000 C) Chemical stability and relatively low interactions with electrolyte. With ceria-based electrolytes other cathode materials are considered (e.g. (La,Sr)(Co,Fe)O 3 or LSCF). Adequate electronic and ionic conductivity: ionic and electronic resistance can become a significant factor, especially in cell designs that incorporate long current paths through the cathode. For lowertemperature cells other materials (LSF) are considered. Relatively high activity. Manageable interactions with ceramic interconnects (LaCrO 3 ) Thermal expansion coefficients that closely match those of YSZ.

4 Mechanisms of (a) oxygen permeation thorough ceramic membranes and (b) oxygen activation at a MIEC cathode.

5 Cathodes: the structure types The three oxide structure types that have been studied as cathode materials: Perovskite: ABO 3 (La 1-x Sr x MnO 3 (LSM), La 1-x Sr x FeO 3 (LSF), Sm x Sr 1-x CoO 3 (SSC), Sm x Sr 1-x Co 1-x Fe x O 3 (SSCF), Ba 0.5 Sr 0.5 Co 0.8 Fe 0.2 O 3-x (BSCF), and the composites SSC-La 0.8 Sr 0.2 Ga 0.8 Mg 0.2-x Co x O 3 (LSGMC) or SSC- Gd 0.1 Ce 0.9 O 1.95 (CGO). K 2 NiF 4 Ruddlesden-Popper phases (La 2-x LN x NiO 4, La 2-x Sr x NiO 4+x, La 2 Ni 1-x Cu x O 4, La 2 Ni 1-x CoO 4, (La,Sr) n+1 (Fe,Co) n O 3n+1 with n=2 and n=3) Ordered Double Perovskites (AA B 2 O 5+x - A = RE, A A = alkali-earth metal, B = Co, Cu, Fe, Ni, ) YBaCo 2 O 5+x, LaBaCuFeO 5+x, LaBaCuCoO 5+x SmBa 0.5 Sr 0.5 Co 2 O 5+x, NdBaCo 2-x Ni x O 5+x REBaB 2 O 5 (RE = Pr, Gd; B = Mn, Co).

6 Lanthanum manganite properties Cubic perovskite structure: MnO 6 framework of corner-shared octahedra that contains La cations within 12-coordinate sites Orthorhombic to rhombohedral at 600 C (but also lower T vs δ) Mn(III)/Mn(IV) = orthorhombic/rhombohedral Melts at about 1880 C Ideal perovskite structure of LaMnO 3 Phase diagram of a Mn 2 O 3 -La 2 O 3 system

7 Properties of LaMnO 3 Lanthanum manganite properties

8 Lanthanum manganite defects LaMnO 3 in reducing atmospheres becomes oxygen deficient and dissociates into La 2 O 3 and MnO (dissociation is reversible) LaMnO 3 can also exhibit La deficiency or excess: with lanthanum excess La 2 O 3 may be present which tends to be hydrated to La(OH) 3 (disintegration of the sintered LaMnO 3 structure); LaMnO 3 can have up to about 10% La deficiency without second phase formation; above this level Mn 3 O 4 is present. LaMnO 3 can have oxygen excess, stoichiometry or deficiency depending on the preparation conditions (firing atmosphere, temperature, time) Oxygen excess 6Mn x Mn + 3/2O 2 = V La + V Mn + 6Mn Mn + 3Ox O Oxygen deficiency 2Mn Mn + Ox O = 2Mnx Mn + V O + ½ O 2 Oxygen content of undoped LaMnO 3 as a function of oxygen partial pressure and temperature

9 Lanthanum manganite defects Doped-LaMnO 3 : the level of oxygen excess decreases with increasing dopant content (LaMnO 3 containing 20mol% Ca has no oxygen excess even at high oxygen activities) Oxygen content of La 0.9 Sr 0.1 MnO 3 as a function of oxygen partial pressure and temperature Oxygen content of La 1-x Sr x MnO 3 as a function of oxygen partial pressure at 1000 C

10 Doped LaMnO 3 : electrical conductivity LaMnO 3 = p-type conductivity (cation vacancies) M-doped LaMnO 3 M = Ba, Ca, Cr, Pb, Mg, Ni, K, Rb, Na, Sr, Ti, Y. Sr and Ca: high electrical conductivity in oxidizing atmospheres and relatively good match of the thermal expansion coefficient Sr doping enhances the electronic conductivity of LaMnO 3 by increasing the Mn(IV) content (Sr > mol%, Ca > 60 mol% = metallic-type conduction) LaMnO 3 La 3+ 1-x Sr2+ x Mn3+ 1-x Mn4+ x O 3 Conductivity data for doped LaMnO 3 Electrical conductivity of undoped an Srdoped LaMnO 3

11 Doped LaMnO 3 : electrical conductivity vs Oxygen partial pressure Decomposition Decomposition Conductivity as a function of oxygen partial pressure for undoped LaMnO 3 at various temperatures Conductivity as a function of oxygen partial pressure for Sr doped LaMnO 3 at various temperatures

12 Lanthanum manganite stability CRITICAL OXYGEN PARTIAL PRESSURE (COPP) = Oxygen partial pressure before LaMnO 3 dissociates into multiple phases (at 1000 C = atm) COPP depend on temperature (> T > pressure) COPP depend on dopants (> dopant concentration > pressure) LaMnO 3 decomposes to La 2 O 3 + MnO; at lower temperature ( C) the material tend to transform to other phases (La 2 MnO 4, La 8 Mn 8 O 23, La 4 Mn 4 O 11 ) COPP of La 1-x Sr x MnO 3

13 LaMnO 3 /YSZ: chemical interaction La, Mn unidirectional diffusion into YSZ Mn = mobile species at high temperatures; it can easily diffuse into the electrolyte changing the electrical characteristics or the structure of cathode and electrolyte (fabrication temperature < 1400 C; operating temperature < C). La deficiency reduces the interaction with YSZ Temperature T < C no interaction T > 1200 C = formation of La 2 Zr 2 O 7 (5 μm thick layer formed at the interface upon treatment at 1450 C for 48 h Nonstoichiometry La y MnO 3-δ ; y > 0.86 formation of La 2 Zr 2 O 7 ; y < 0.86: Mn dissolution in YSZ Diagram showing reaction products formed from La 1-x Sr x MnO 3 and YSZ at different temperatures

14 Doped-LaMnO 3 /YSZ: chemical interaction The dopants in LaMnO 3 suppress the Mn migration; Dopant amount Substitution of La with a low dopant concentration reduces the La 2 Zr 2 O 7 formation. A high dopant content results in the formation of other phases (SrZrO 3, CaZrO 3 ) Diagram showing reaction products formed from Sr and Ca doped-lamno 3 and YSZ at 1400 C

15 LaMnO 3 : thermal expansion Undoped LaMnO 3 : thermal expansion coefficient = 11.2±0.3 x 10-6 cm/cmk; La deficiency = < thermal expansion; Oxygen nonstoichiometry = lower thermal expansion Dopant effect Sr = > thermal expansion coefficient. La for smaller cations (Ca, Y) = < thermal expansion coefficient LaMnO 3 /LaCoO 3 /LaCrO 3, La 0.5 Sr 0.5 MnO 3 /LaCoO 3 solid solutions: > thermal expansion coefficients Thermal expansion coefficients of La 1- x Sr x MnO 3 compounds Thermal expansion coefficients of Doped-LaMnO 3 Thermal expansion coefficients of La 0.5 Sr 0.5 Mn 1-x Co x O 3 compounds

16 LaCoO 3 Undoped LaCoO 3 : rhombohedral from RT to 1000 C; the rhombohedral structure may transform to a cubic phase at a temperature dependent on dopant content T 800 C = stoichiometric 1000 C oxygen stoichiometry ranges from to (P O2 = 10-2 to 10-4 atm) Doped LaCoO 3 : > dopant amount, > T, < P O2 = > oxygen deficiency (La 0.7 Sr 0.3 CoO 3-δ oxygen from to at P O2 = 1 to 10-5 atm ) Oxygen deficiency: Oxygen nonstoichiometry is inversely proportional to the 0.45th power of oxygen partial pressure (for undoped LaCoO 3 ), to the 0.13th power of oxygen partial pressure (for La 0.7 Sr 0.3 CoO 3 )

17 LaCoO 3 Stability: LaCoO 3 = less stable toward reduction when compared with LaMnO 3 (at 1000 C LaCoO 3 dissociates into other phases at the COPP = 10-7 atm); doping reduces the stability decreasing the COPP); stable in oxidizing conditions. Conductivity: intrinsic p-type conductivity; electrical conductivity enhanced by substituting a lower-valence ion (Sr, Ca) on the La site (La 0.8 Sr 0.2 CoO 3-δ = 1200 Ω -1 cm -1, La 0.8 Ca 0.2 CoO 3-δ = 800 Ω -1 cm -1 at 1000 C): max conductivity at 30 mol% for Ca and 40 mol% for Sr. Undoped cobaltite = semiconducting-metallic conduction transition at about 800 C: Sr and Ca lower this transition temperature; for Sr > 30 mol% metallic behaviour fro RT to 1000 C. Chemical interaction: LaCoO 3 /YSZ = La 2 Zr 2 O 7 at T > 1100 C; dopants may produce other phases (SrZrO 3 ) Thermal expansion thermal expansion coefficient x 10-6 cm/cmk; can be modified by Sr, Ca, Mn, Ni doping (17 x 10-6 cm/cmk for LaCo 0.6 Ni 0.4 O 3 ) but is usually higher than that of YSZ

18 Ruddlesden-Popper phases A 2 BO 4+δ : ABO 3 perovskite and AO rock-salt layers arranged one upon the other in the c-direction. This structure allows for the accommodation of oxygen overstoichiometry as oxygen interstitial species with negative charges, which are balanced through the oxidation of the B site cations Good: electronic conductivity La 2 NiO 4 Exhibits a broad metal-insulating transition from K with a maximum electrical conductivity of 100 S7cm (due to the mixed valence of B site metal) oxygen ionic transport properties (due to the oxygen overstoichiometry) electrocatalysis for the oxygen reduction, moderate thermal expansion properties

19 The degree of hyperstoichiometry, δ, 1. Can have a profound effect on the structural and physical properties 2. Is influenced by synthesis conditions. 3. Hyperstoichiometry can also be influenced by varying the identity of the rare earth or transition-metal cations: δ was observed to increase with the substitution of the larger Lanthanum ion with the progressively smaller praseodymium and neodymium ions δ was observed to increase with successive B-site doping with higher-valence ions such as iron and cobalt

20 Oxygen ion conduction mechanism Calculated E a for oxygen migration in La 2 NiO 4 A Apical site E Equatorial site. Vacancy (i. iv.) and interstitial (v.) Red spheres = O 2-, Blue spheres = Ni 2+, Yellow spheres = La 3+, transparent red cube = oxygen vacancy green sphere = interstitial oxygen. Vacancy (i. iv.) and interstitial (v.) Red spheres = O 2-, Blue spheres = Ni 2+, Yellow spheres = La 3+, transparent red cube = oxygen vacancy green sphere = interstitial oxygen.

21 Oxygen ion conduction mechanism Along c axis In the ab plane Vacancy (i. iv.) and interstitial (v.) Red spheres = O 2-, Blue spheres = Ni 2+, Yellow spheres = La 3+, transparent red cube = oxygen vacancy green sphere = interstitial oxygen.

22 The La 2 Ni 1 x Co x O 4+δ system as SOFC cathode material with CGO and LSGM as electrolytes La 2 Ni 1 x Co x O 4+δ (x = 0-1) synthesised via a Pechini route. Nitrate salts of the metal ions were dissolved in water to which a 1:1 molar ratio of ethylene glycol and citric acid was added. The molar ratios of ethylene glycol and citric acid to the nitrate salts were 4:1. The solutions were stirred and heated until majority of the water evaporated to leave behind a gel. The resulting foam-like residues were ground and prefired at 1023 K for 4 h in air to remove unwanted organic residues. Pellets of25-mm diameter of each composition were made from the treated powder and single-phase materials were obtained by firing under various temperatures.

23 The La 2 Ni 1 x Co x O 4+δ system as SOFC cathode material with CGO and LSGM as electrolytes Hyperstoichiometric oxygen δ vs cobalt-content (x) in La2Ni1xCoxNiO4+δ Slight δ increase with increasing Co As cobalt is introduced, the system tends toward insulating behavior until x=0.7 For x > 0.7, the conductivity is observed to increase slightly until x=1.0. RP electronic conductivity dominated by TM-O-TM interactions. > Co: = decrease of the Ni(Co) O covalent interactions = decrease in the electronic conductivity) ionic radii electronegativity? Slight increase = deviation of Ni(Co) O Ni(Co) bond angle from 180 Electrical conductivity vs T for La2Ni1xCoxNiO4+δ

24 The La 2 Ni 1 x Co x O 4+δ system as SOFC cathode material with CGO and LSGM as electrolytes Log ASR vs 1/T for La2Ni1-xCoxNiO4+δ (x=0.1) on CGO10 and LSGM The electrode performance on LSGM is consistently better than CGO TEC: CGO x 10-6 LSGM = 12 x 10-6 La 2 NiO 4+δ = 13.7 x 10-6 Log ASR vs 1000/T for La2Ni1-xCoxNiO4+δ (x=0.0, 0.1, 0.3, and 0.5) on LSGM

25 Evaluation of the La 2 Ni 1 x Cu x O 4+δ system as SOFC cathode material with 8YSZ and LSGM as electrolytes La 2 Ni 1 x Cu x O 4+δ (x = 0, 0.2, 0.4, 0.6 and 1) synthesised via a nitrate citrate route. Stoichiometric amounts of La 2 O 3, Ni(NO 3 ) 2 6H 2 O and CuO were dissolved in nitric acid. Citric acid was added in a large excess with continuous stirring. The obtained solution was dehydrated and slowly heated until selfcombustion of the precipitate. The obtained precursors were calcined at 600 C for 2 h and finally fired in air at 950 C for 8 h.

26 Overall cationic composition by ion-coupled plasma (ICP-AES) oxygen content by iodometric titration XRD/Rietveld analyses Powder pellets (10 mm in diameter and 1.4 mm thick) for: Thermal expansion coefficients and electrical conductivity measurements (polyethylene glycol as a binder) Electrical conductivity by the DC four-probe method. Ac impedance spectroscopy measurements on symmetrical cells of La 2 Ni 1 x Cu x O 4+δ /electrolyte/la 2 Ni 1 x Cu x O 4+δ. The dense electrolytes were obtained by pressing the commercial powders of 8YSZ or LSGM (La 0.9 Sr 0.1 Ga 0.8 Mg 0.2 O 3 δ ) and sintering in air at 1500 C. Cathode inks were obtained by mixing each prepared powder with terpineol (15 wt.%), and the milled ink was symmetrically painted onto the surface of the sintered pellets. The cell was then calcined at 1000 C for 4 h to obtain a good adherence between the components. The cell was placed into a ceramic support and Pt plates were used as current collectors. All the impedance measurements were performed in the C temperature range in air atmosphere.

27 Evaluation of the La 2 Ni 1 x Cu x O 4+δ system as SOFC cathode material with 8YSZ and LSGM as electrolytes The replacement of Ni 2+ (0.69 Å) by Cu 2+ (0.73 Å) = expansion of the [Ni(Cu)O 2 ] 2 layer which increases the strain between the perovskite and the NaCl layer, promoting the rotation of the CuO 6 octahedra (a) La 2 Ni 0.8 Cu 0.2 O 4+δ, (b) La 2 CuO 4+δ ; containing corrugated [Ni(Cu)O 2 ] 2 layers exhibiting Cu O1 Cu angles of (7). Interstitial oxygen atoms are not shown. The insertion of the O4 additional oxygen atoms into the lattice causes: 1. a displacement of the axial O2 from its normal location to a new axial O3 position. 2. a large vacancy concentration at the O2 positions.

28 δ values (iodometric titration): 0.16 (x = 0) 0.10 (x = 0.2) 0.06 (x = 0.4) 0.04 (x = 0.6) 0.02 (x =1) Electrical conductivity for the system La 2 Ni 1 x Cu x O 4+δ (0 x 1), collected in air The electrode performance is strongly correlated to its microstructure. > calcination or sintering temperature > particle coarsening < specific surface area According to the conventionally held triple phase boundary (TPB) model, the coarsening of particles lowers the number of TPB points and hence a high polarisation resistance is observed. SEM images for as-prepared La 2 Ni 1 x Cu x O 4+δ a) x=0; b) x=0.4: c) x=0.6 and d) x=1.

29 8YSZ. Arrhenius plots of the ASR values under air for the system La 2 Ni 1 x Cu x O 4+δ (0 x 1). All the Arrhenius plots show a single slope, (same reaction mechanisms controlling the overall electrode behaviour in the temperature range) The incorporation of Cu in the structure does not induce significant variations in the E a values The lowest E a value was exhibited by the x=0.4 sample (0.9 ev), also showing a higher conductivity. LSGM

30 Nyquist plot of the symmetrical cell La 2 Ni 0.6 Cu 0.4 O 4+δ /electrolyt e/la 2 Ni 0.6 Cu 0.4 O 4+δ at 750 C in air with LSGM and 8YSZ as the electrolytes. Thermal expansion coefficients (TEC, 10 6 K 1 ) for LaNi 1 x Cu x O 4+δ (0 x 1) The total conductivity is not the only factor that affects the ASR value 1. the matching between the thermal expansion behaviour of the electrode and the electrolyte 2. cathode microstructure 3. tendency of the La 2 Ni 1 x Cu x O 4+δ samples (X > 0.4) to react with 8YSZ forming the insulating phase La 2 Zr 2 O 7 when heating the sample Comparative ASR data for x = 0.2 and x = 1 samples on both 8YSZ and LSGM electrolytes.

31 High Order (n > 1) Ruddlesden-Popper phases Ruddlesden-Popper structure (La n+1 Ni n O 3n+1 ; n=1, 2, and 3) The relatively open structural framework afforded by the rocksalt intergrowth allows for the accommodation of hyperstoichiometric oxide-ions in the rocksalt layer as interstitials.

32 La n+1 Ni n O 3n+1 : Effect of increasing n Electrical conductivity vs T for La 2 NiO 4.15, La 3 Ni 2 O 6.95, and La 4 Ni 3 O 9.78 in air La 4 Ni 3 O 9.78 = lowest ASR values E a =1.36 ev La 3 Ni 2 O 6.95 E a = 1.24 La 2 NiO 4.15 E a = 1.27 ev, > n > amount of Ni(III) > vacancies: δ = for n = 2, δ = for n = 3 > n > electrical conductivity due to the increasing number of Ni-O-Ni interactions in the perovskite layers responsible of the electronic conduction pathways Log ASR vs 1,000/T for La 2 NiO 4.15, La 3 Ni 2 O 6.95, and La 4 Ni 3 O 9.78 on LSGM in air

33 Effect of composition and preparation procedure La 3 Ni 2 O 7 (sx) is nonmetallic, (dρ/dt < 0) La 4 Ni 3 O 10 (dx) and LaNiO 3 exhibit metallic resistivity (dρ/dt > 0). route δ τ (Ni 3+ ) Formulation La 3 Ni 2 O 7-δ Nitrate Citrate La 3 Ni 2 O 6.93 La 3 Ni 2 O 7.03 La 4 Ni 3 O 7-δ Nitrate Citrate La 4 Ni 3 O 9.75 La 4 Ni 3 O The preparation method has a strong influence on the oxygen stoichiometry of these compounds. The samples obtained by citrate route are always more oxidized than those prepared by other methods and have a higher Ni 3+ content. These observations are very important since they lead to variations in the physicochemical properties of the different samples as has been shown by electrical resistivity.

34 BSCF XRD pattern of BSCF Well developed crystallization In contrast to the dense electrolyte film, electrodes are highly porous. Electrolyte GDC BSCF has a much coarser structure than the (though the former was sintered at lower temperature 950 versus 1300 C): BSCF powder exhibits a much higher sinterability than the NiO GDC powder. BSCF Ni-GDC anode

35 BSCF Open-circuit voltage (OCV) of 0.903V at 600 C. The OCV value displays a large deviation from the theoretical value, i.e V at 600 C, due to the electronic conductivity of doped-ceria materials induced by the reduction of Ce4+ to Ce3+ in reducing atmospheres Cell voltages (solid symbols) and power densities (open symbols) as function of current density of anode-supported cell, consisting of GDC electrolyte, BSCF cathode and Ni GDC anode Limiting corrent density due to the diffusion polarization (with different hydrogen and air flows: polarization id due to hydrogen diffusion limitation) This loss in OCV can be partly alleviated as the temperature decreases. At 500 and 400 C, the cell produces an OCV of and 1.032V, respectively. These voltages are much closer to the theoretical value and suggests that the electronic conductivity of the GDC electrolyte is insignificant at these low temperatures.

36 Ba 0.5 Sr 0.5 Co 0.8 Fe 0.2 O 3-δ (BSCF) based systems Comparison of maximum power densities of thin-film GDC or SDC electrolyte cells with various cathodes. Comparison of total interfacial resistance of thin-film GDC electrolyte cells with various cathodes.

37 Layered Double Perovskites AA B 2 O 6-δ, where A is normally Ba, A is a lanthanide, and B is a first row transition metal. The most widely studied: LnBaCo 2 O 5+δ (Ln = Pr, Nd, Sm, Gd) Unit cell of NdBaCo 2 O 6 δ (for orthorhombic structures, the O(3) site splits into O(3) and O(4)). The key features:. A and A are ordered in alternate layers. High electronic conductivity (above the metal-insulator transition temperature - around 350 C). Excellent oxide ion conductivity

38 BSCF Fabrication of the anode-supported thin-film GDCelectrolyte 1. The anode powder was prepared by ball-milling NiO powder and nanosized Gd0.1Ce0.9O1.95 powder in a composition (65:35 wt.). The resulting NiO GDC mixture was then uniaxially pressed. 2. To prepare the electrolyte film on the anode: a suspension of GDC in alcohol was sprayed onto the pre-pressed green NiO GDC substrate After the solvent was evaporated, the GDC powder and the anode substrate were co-pressed to form a green bilayer. 3. The bilayer was co-sintered at 1300 C for 4 h in a furnace to become a dense electrolyte film 4.The cathode material Ba0.5Sr0.5Co0.8Fe0.2O3 δ (BSCF) was synthesized by a sol gel process (metal nitrates, EDTA and citric acid). Gelation of the solution on the evaporation of water. The gel was then heattreated at 135 C for 5 h to produce a primary powder, which was subsequently calcined at 800 C for 2 h to form the final powder. 4. To prepare the cathode, BSCF powder was mixed with polyethylene glycol 400 and the resultant paste was painted on the electrolyte. The electrode was then sintered at 950 C for 2 h. To minimize the contact resistance between the cathode and the Pt mesh, Pt paste was painted on the cathode surface to serve as a current-collector. The assembly was then sintered at 900 Cfor 30 min.

39 LnBaCo 2 O 5+δ Structures of simple cubic perovskite and doublelayered perovskite. These materials possess an ordered structure in which lanthanide and alkali-earth ions occupy the A-site sub-lattice and oxygen vacanciesare localized into layers. The ideal structure of this family of compounds can be represented by the stacking sequence LnO δ CoO 2 BaO CoO 2 Transformation of a simple cubic perovskite with randomly occupied A-sites into a layered crystal with alternating lanthanide and alkaliearth planes reduces the strength of oxygen binding and provides disorder-free channels for ionic motion, thereby theoretically increasing the oxygen diffusivity

40 LnBaCo 2 O 5+δ XRD at RT r (pm) Gd 93.8 Sm 96.4 Nd 99.5 Pr La < Ln 3+ ionic radius < lattice parameters The size of dopant Ln 3+ is essential for a stable layered crystalline structure. (Phase transition for Ln = Y and La) Annealing in air Annealing in O 2 Annealing in N 2

41 LnBaCo 2 O 5+δ α oxygen β Oxygen (Co 3+ /Co 2+ ) r (pm) Gd 93.8 Sm 96.4 Nd 99.5 Pr La > Ln 3+ ionic radius > oxygen content and the nominal oxidation state of the cobalt ions in the oxide > oxygen mobility in the LnO δ layer. < Ln 3+ ionic radius > Ln-O attraction < Co reducibility < oxygen mobility in the LnO δ layer.

42 LnBaCo 2 O 5+δ Nd Sm Nd Sm Pr Temperature dependence of ionic conductivity of LnBaCo 2 O 5+δ (sx) air (dx) nitrogen > Ln 3+ ionic radius > Co reducibility > electrical conductivity > Ln 3+ ionic radius > B-O-B length < electrical conductivity. r (pm) Gd 93.8 Sm 96.4 Nd 99.5 Pr La 101.6

43 Sm-doped BSCF Conductivity vs. T Sm doping: 1. Sm introduced in the A-site = > concentration of electronic charge carriers because of the reduction of Co 4+, Fe > Sm > oxygen vacancy 3. > Sm = > Fe 3+, Co 3+ = increasing size of BO 6 octahedra = > ion diffusion SEM cross-section of the half-cell (BSSCF/SDC); top view of the BSSCF Illustration of the single cell.

44 Sm-doped BSCF Resistance vs. T In this system, the resistance of the oxygen adsorption/decomposition and mass transfer is the main part of the total resistance, so reducing the resistance of oxygen diffusion effectively is important for the reduction of total resistance Total resistance (R el ), charge transfer resistance (R 2 ) and oxygen diffusion resistance (W 1 ) of BSCF and BSSCF. For the BSSCF material, the doped of Sm 3+ enhances the average valence of A-sites, inducing the reduction of Co 4+ and Fe 4+ to maintain the charge balance. This benefits the diffusion of oxygen in BSSCF and lowers the resistance to oxygen diffusion, and therefore reduces the total resistance Fe help in increasing electronic conductivity

45 BSCF: The effect of CO 2 on the cell performance and microstructure Single cell performance in the presence of various CO 2 concentrations in the oxidant gas line (a) Powder XRD pattern of BSCF as synthesized. (b) after recovering for 100 min with pure He, (c) for 100 min with pure CO 2 (d) for 4320 min with CO 2 EDXS Elemental distributions at the permeate side when permeation was stopped after a treatment (a) for 100 min with pure CO 2, (b) after recovering for 100 min with pure helium, (c) for 4320 min with CO 2.

46 BSCF Arrhenius plots for Ba 0.5 Sr 0.5 Co 0.8 Fe 0.2 O 3 - po 2 =0.5 bar. a D* solid symbols and D VO open symbols; b k. Data for La 0.5 Sr 0.5 CoO 3 - and La 0.8 Sr 0.2 CoO 3 - po 2 =1 atm are shown for comparison. Normalized 18 O concentration profile c * r (x) after isotope exchange at 700 C and po 2 =0.5 bar for 600 s. Line=fit according to Eq. 1. Inset: Sketch of the diffusion experiment.