Efficient 3-D coral La2-xPrxNiO4+δ SOFC cathodes: a compromise in electrochemical performance and chemical stability

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Lnn+1NinO3n+1 -type materials: state of the art

Nickelates belonging to the Ruddlesden-Popper (RP) series have recently attracted much attention as promising cathode materials for the next generation of IT- SOFCs [67-69]. RP phases adopt general formula An+1MnO3n+1 (typically n = 1, 2 and 3, A = rare earth, M = metal) [70]. The layered crystal structure of the RP phases, (AO)(AMO3)n, is illustrated in Fig. 7. It consists of n consecutive (AMO3)n perovskite layers, alternating with one AO rock-salt layer along the crystallographic c-axis. The well-known perovskite (AMO3) and K2NiF4-type structure correspond to n = ∞ and n = l, respectively. The A and M site cations are typically comprised of rare/alkaline earth metals (La, Pr, Nd) and transition metals (Ni, Co, Cu), respectively. Such layered MIECs display diverse defect chemistry, allowing non-stoichiometric defect structures (oxygen hypo- and hyper-stoichiometry, which can also be defined as oxygen deficiency and excess, respectively) due to their oxygen content through tailoring A- and M-site. This feature makes these oxides very attractive for the fine-tuning of their electrical and electrochemical properties [68, 69, 71-81]. For the case of oxygen hyper-stoichiometry, particularly, the excess oxygen occupies interstitial sites within the AO rock-salt layers.
Fig. 7 Schematic crystal structures of the n = 1, 2 and 3 members of the Ruddlesden–Popper (RP) type An+1MnO3n+1. The unit cell of each structure is presented by a black rectangle [82].

K2NiF4-type materials (Ln2NiO4+δ, Ln = La, Pr and Nd)

In earlier studies, the Ln2NiO4+δ (Ln = La, Pr and Nd) materials have been extensively studied for their application as a SOFC cathode [45, 82-85]. They have been widely synthesized by various techniques such as conventional ceramic processes (solid state and sol-gel methods) [65, 68, 69, 86-88], pulsed laser deposition (PLD) [89-91], metallorganic chemical vapor deposition [72, 92, 93]. As mentioned above, the crystal structure of Ln2NiO4+δ is consisted of a LnNiO3 perovskite block intergrown with LnO rock-salt blocks, as shown in Fig. 7 and Fig. 8 (for La2NiO4+δ). In general, two different crystallographic structures: tetragonal and orthorhombic (including different space groups in the latter case), have been reported in literature. Phase transitions between the two structures were evidenced in a whole range of temperature for La2NiO4+δ [94, 95]. A low temperature transition occurs at ~80 K from a low temperature tetragonal (LTT) phase (P42/ncm space group) to a low temperature orthorhombic (LTO) one (Bmab or Fmmm space group) [96-98]. With increasing temperature up to ~ 423 K, the LTO phase transforms to a high temperature tetragonal (HTT) one (I4/mmm space group) [95]. Similar phase transition has been also reported for Pr2NiO4+δ and Nd2NiO4+δ [99-101]. These transition temperatures are highly dependent on the oxygen stoichiometry of the material [94, 95]. It has been well established that Ln2NiO4+δ (Ln = La, Pr and Nd) materials with K2NiF4 structure exhibit a large range of oxygen over-stoichiometry [100, 102-104], and the additional oxygens are mainly responsible of the high ionic conductivity [66, 85]. This is also dependent upon the size of rare-earth ion as well as the synthesis method for a given composition [105, 106]. In general, oxygen non-stoichiometry « δ” is determined by iodometric titration and / or thermo-gravimetric analysis under an atmosphere partially or completely reductive. Table 1 shows the « δ » for Ln2NiO4+δ compounds, prepared by different methods.

Oxygen diffusion and ionic conductivity

The rate of oxygen reduction decreases with decreasing the SOFC operating temperature. The ion conduction (and the O2- ions transfer at the interface cathode electrolyte) must be improved in order to optimize the performance. The ionically conductive O2- is linked to the presence of structural defects which may be of two types: i) oxygen vacancy, as in the perovskites sub-stoichiometric oxygen or ii) interstitial oxygen associated with over-stoichiometric oxygen, in the oxides as K2NiF4 type.
The oxygen diffusion (D*) and surface exchange (k*) coefficients of these nickelates are among the highest values available in the literature, especially at intermediate temperatures (about one order of magnitude larger than that of conventional perovskites in an intermediate temperature range, 600 < T (°C) < 800). Table 2 shows oxygen diffusion (D*) and surface exchange coefficients (k*) at 700 °C obtained by different researchers. The diffusion coefficients (D*) of the materials A2NiO4+δ are higher than those of perovskites. The ionic conductivity of these materials is directly related to the D* value, hence the ionic conductivity is the highest for Pr2NiO4+δ [107]. The favoured mechanism is probably of intersticialcy type, involving both apical and interstitial oxygens [112, 113], as shown in Fig. 8. Compared to the perovskite materials, these materials exhibit strong anisotropic ionic transport properties [81, 111, 114].
The corresponding k* values at 700 °C are 1.6×10-7, 1×10-6 and 3×10-7 cm.s-1 for La2NiO4+δ, Pr2NiO4+δ and Nd2NiO4+δ respectively [85]. The surface exchange coefficient “k*” of La2NiO4+, is improved when the nickel is substituted with cobalt. Recently, Vibhu et al. have studied the thermal variation of the oxygen diffusion coefficients, D*, and surface exchange coefficients, k*, in La2−xPrxNiO4+δ (x = 0.0, 0.5, 1.0, 1.5 and 2.0) as shown in Fig. 9
[65]. The value of D* increases with increasing Pr content. The D* values of mixed phases (x = 0.5, 1.0 and 1.5) are in the range of those reported for Pr2NiO4+δ (2.5 × 10−8 cm2 s−1) and La2NiO4+δ (1.5 × 10−8 cm2 s−1) at 600 °C [85]. As a comparison, the D* values for LSCF are much smaller than the ones of the nickelates (Fig. 9a) [85]. Regarding the k* values (Fig. 9b), except at 500 °C, they are slightly higher at high temperature than those reported for La2NiO4+δ and Pr2NiO4+δ [85].

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Electronic Conductivity

The total electrical conductivity of oxides, measured in air, comprises an electronic contribution and a very much lower ionic contribution. The electrical conductivity of Ln2NiO4+δ is characterized by a semiconductor-type electronic conductivity, which occurs via hopping of p-type charge carriers between mixed-valence nickel cations [122-125]. A maximum in conductivity is observed between 600 and 700 K, above which a smooth apparent change from semiconducting to a metallic-like behavior is observed [123, 126, 127]. Bassat et al. [123] presented convincing evidence for various La deficient La2−xNiO4±δ phases that the increase in resistivity with increasing temperature, previously attributed to a metal-insulator transition at T > 600 K, is actually due to oxygen loss and the associated decrease in the number of charge carriers (Ni3+). The maximum reported conductivity values for polycrystalline bulk ceramics, films (maximum ≈ 80 S/cm) [68, 105, 122-128], and on single crystal samples along the a-b plane (maximum ≈ 200 S/cm) [124, 129] are acceptable for using such materials as SOFC cathodes.
E. Boehm et al have shown the conductivity variation vs. temperature for Ln2NiO4+δ (Ln = La, Nd and Pr) compounds, as shown in Fig. 10 [85]. Note that the electrical conductivity of praseodymium nickelate (Pr) is greater than those of the corresponding neodymium (Nd) and lanthanum (La) one in the whole temperature range. In the same figure, it is evidenced that the conductivity of the compounds in the Nd2-xNiO4+δ series (x = 0, 0.05, 0.10) at 700 º C varies within a relatively wide range on the order of 35 to 100 S cm-1.

Table of contents :

Chapter 1: Literature survey
1. Solid oxide fuel cell (SOFC)
1.1. Electrolyte
1.2. Anode
1.3. Cathode
2. Lnn+1NinO3n+1 -type materials: state of the art
2.1. K2NiF4-type materials (Ln2NiO4+δ, Ln = La, Pr and Nd)
2.1.1. Oxygen diffusion and ionic conductivity
2.1.2. Electronic conductivity
2.1.3. Current development on Ln2NiO4+δ (Ln = La, Pr and Nd) for SOFC application
2.1.4. Chemical compatibility and stability
2.2. Higher order RP phases Lan+1NinO3n+1(n =2 and 3)
2.2.1. Current development on higher order RP phases for SOFC application
Chapter 2: An innovative architectural design to enhance the electrochemical performance of La2NiO4+δ cathodes for solid oxide fuel cell applications
1. Introduction
2. Experimental Section
2.1. Films preparation
2.2. Characterization
3. Results and discussion
3.1. Microstructural control of La2NiO4+ coating by ESD
3.1.1. Influence of the deposition time
3.1.2. Influence of the nozzle-to-substrate distance
3.1.3. Influence of the substrate temperature
3.1.4. Influence of the flow rate
3.1.5. Influence of the nature of the solvent
3.1.6. Architectural design of the LNO cathode
3.2. Electrochemical properties of the selected LNO cathodes
3.3. Stability and reactivity
4. Conclusions
Supporting information
Chapter 3: Efficient 3-D coral La2-xPrxNiO4+δ SOFC cathodes: a compromise in electrochemical performance and chemical stability
1. Introduction
2. Experimental
2.1. Film preparation and powder synthesis
2.2. Symmetric and single cell preparation
2.3. Physico-chemical characterization
2.4. Electrochemical measurements
2.4.1. Symmetrical cells characterization
2.4.2. Single cells characterization
3. Results and discussion
3.1. Structural properties of La2-xPrxNiO4+δ (0 ≤ x ≤ 2) films
3.2. Microstructural properties of the La2-xPrxNiO4+δ (0 ≤ x ≤ 2) films
3.3. Compositional properties of La2-xPrxNiO4+δ (0 ≤ x ≤ 2) films
3.4. Effect of praseodymium content on the stability and compatibility of La2-xPrxNiO4+δ with CGO
3.5. Electrochemical properties
3.5.1. Symmetrical cells performance
3.5.2. LaPrNiO4+δ single cell performance
4. Conclusions
Supporting information
Chapter 4: La4Ni3O10-δ as an efficient solid oxide fuel cell cathode: electrochemical properties versus microstructure
1. Introduction
2. Experimental
3. Results and discussion
3.1. Structural characterization and elemental analysis
3.2. Microstructural characterization
3.2.1. Influence of the deposition time
3.2.2. Influence of the precursor solution concentration
3.2.3. Influence of the solvent composition
3.2.4. Influence of the substrate temperature
3.2.5. Effect of the nozzle to substrate distance
3.2.6. Selected La4Ni3O10-δ films microstructures for electrochemistry
3.3. Electrochemical properties
4. Conclusions
Chapter 5: Influence of CGO addition on electrochemical properties of nickelates based cathodes
Part I: Design of interfaces in efficient Ln2NiO4+δ (Ln = La, Pr) cathode for SOFCs application
1. Introduction
2. Experimental
2.1. Materials and solution preparation
2.2. Cathode preparation and characterization
3. Results and discussion
3.1. Structural characterization of the LnNO films
3.2. Microstructural characterization of the LnNO films with different architectures
3.3. Electrochemical properties and stability
4. Conclusions
Supporting information
Part II: Lan+1NinO3n+1 (n = 2 and 3) phases and composites for solid oxide fuel cell cathodes: facile synthesis and electrochemical properties
1. Introduction
2. Experimental
2.1. Synthesis process and characterization
3. Results and discussion
3.1. Structural characterization
3.2. Oxygen content analysis by TGA
3.3. Thermal expansion
3.4. Microstructural characterization
3.5. Electrode performances
3.6. Chemical stability and compatibility with CGO
4. Conclusions
Supporting information
Conclusions and perspectives


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