• the low-temperature systems, namely PEMFCs (proton exchange membrane fuel cells) used mainly for transportation applications,

• the high-temperature systems, namely solid oxide fuel cells (SOFCs), used for stationary applications and combined heat production.

• lifetime of the cells to be increased due to lowered reactivity of materials,

• thermal cycling to be facilitated,

• the use of inexpensive stainless steel or metal alloys as interconnects to be possible (Fergus, 2005),

• manufacturing costs to be substantially lowered by simplifying the production processes (Menzler et al., 2010).

• First, new cell architectures and morphologies of electrodes have been developed: for instance thin electrolyte membranes for lessening the ohmic losses, composite electrodes with gradients of composition and/or porosity, improved electrode/electrolyte interfaces (Menzler et al., 2010; Tietz, 2008).

• Second, the search for innovative materials has been intensified concerned with electrolytes as well as with electrodes, especially cathode materials (Sun; Orera, 2010; et al., 2010; Tarancon et al., 2010).

13.2.1 The oxygen reduction reaction mechanism

The electrochemical ORR taking place at the cathode of a SOFC can be summarized as follows:

Using the Kröger–Vink notation, it can be also written as

This reaction requires the simultaneous presence of three species, that is, gas/electrons from the cathode/electrolyte, which is only met at the so-called TPB. As illustrated in Fig. 13.2, the agreed macroscopic reaction pathways for oxygen reduction processes on porous cathode/solid electrolyte are (Pizzini, 1973):

When the cathode is an electronic conducting material, this reaction can be achieved only in the vicinity of the TPB as summarized by Fleig (2003). As a consequence, the oxygen reduction is restricted to a nearly one-dimensional (1D) region and the performance of such electrodes is thereby poor (Srinivasan et al., 1967; Siebert et al., 1995). This is the case of the most classical and early cathode material, the strontium lanthanum manganite perovskite-type oxide (La_{1 − x} Sr_xMnO₃, so-called LSM); its properties are described in Section 13.3 (Jorgensen and Mogensen, 2001).

It is obvious that the performance of such electrodes is highly dependent on their morphology and microstructure, which clearly affect the size and distribution of the TPBs.

To reduce the cathodic overpotential, it is necessary to operate at high temperature, typically T > 800°C. Another option is to employ designed composite electrodes with appropriate composition and porosity gradients that can provide both ionic and electronic conductivities, and thus a significant extension of the three-phase area, as long as open porosity and continuous paths of the electron-conducting and ion-conducting phases are guaranteed (Kenjo and Nishiya, 1992; Holtappels and Bagger, 2002; Menzler et al., 2010).

A more sophisticated approach is to choose an electrode material that has both intrinsic electronic and ionic conductivity (mixed ionic and electronic conducting, MIEC). Instead of multiplying the TPB ‘reaction lines’, the catalytically active area is extended to the entire surface of the cathode (the so-called DIB, double interface boundary or IDE, internal diffusion electrode) as schematized in Fig. 13.3. In such a way, one can expect electrode polarization losses to be significantly lowered.

From a general viewpoint, MIEC oxides can be formulated as (A_{1 − x} A’_x)_m(Mⁿ⁺M^{(n + 1)+})_p O_{z ± δ}; such oxides exhibit simultaneously:

Many different materials have been studied as SOFC cathode, most of them belonging to the perovskite or perovskite-related oxide families (Chroneos et al., 2010b; Sun; Orera, 2010; et al., 2010; Tarancon et al., 2010).

13.2.2 Basic requirements for SOFC cathode materials

Whatever the cathode active material – single phase (MIEC oxide) or composite (electronic conducting oxide mixed with ionic conducting oxide) – various requirements must be met:

In order to select materials able to fulfil the SOFC cathode functionality, one has first to determine some basic intrinsic properties of the compounds, usually bulk properties.

13.3.1 The structure of lanthanum manganite-related oxides

The lanthanum manganite LaMnO₃ belongs to the perovskite family of compounds formulated AMO₃ (Mitchell, 2002); they crystallize with a structure (usually of cubic symmetry) made of MO₆ octahedra sharing corners along the three directions of the space; M is usually a 3d metal cation. The largest cation, A (rare- or alkaline-earth), is located in the central cage of the 3D network of octahedral in a cuboctahedron of 12 oxygen atoms (Fig. 13.4). This site can be partially vacant (as in the tungsten bronzes Na_xWO₃ or the titanate La_2/3TiO₃), whereas the M one is very seldom deficient. Conversely, the structure can quite easily show oxygen deficiency in compounds formulated as AMO_{3 − δ} (Grenier et al., 1981; Hagenmuller et al., 1991; Mitchell, 2002). The amount of vacancies can reach values as high as δ = 1 and are ordered in such a way that new structural types are formed – as for instance in LaNiO₂ or YBa₂Cu₃O₆ (Hewat et al., 1987). For small values of δ (typically lower than 0.15), the vacancies are considered as statistically disordered, almost without significant interactions and with no long-range ordering. The presence of these oxygen vacancies induces an ionic conductivity thanks to the mobility of the O^2 − oxide ions in the incomplete network.

13.3.2 Early cathode materials: manganites and composite electrodes

In LaMnO₃, substituting the lanthanum with a lower-valence cation such as Sr^2 + (up to 20–30%) leads to the formation of Mn^4 +. The mixed valence of the manganese considerably enhances the p-type electrical conductivity of the La_{1 − x}Sr_xMnO₃ manganites as well as its electrocatalytic properties (Anderson, 1992). These compounds show a noteworthy stability in air and their apparent oxygen overstoichiometry, improperly formulated (La, Sr)MnO_{3 + δ}, has given rise to many controversies (Mizusaki et al., 2000; Miyoshi et al., 2002). Actually, there is almost no occurrence of oxygen interstitials at high temperature and they are preferably expressed as cation-deficient compounds (La, Sr)_{1 − y}Mn_{1 − y}O₃, which makes these compounds almost pure electronic conducting oxides. Indeed, the oxygen diffusivity measured for LSM is very low, which involves very small ionic conductivity values, even at high temperatures (Yasuda et al., 1996). However, the good performance of this compound at high temperature was assigned to the fact that the ORR activity is enhanced under high polarization (‘cathodic biasing’) which leads to the creation of oxygen vacancies able to promote ionic transfer at the interface between cathode and electrolyte (Wang and Jiang, 2006; La O’ et al., 2009).

As LSM had acceptable performance only at temperatures higher than 900°C, the early air electrodes that have been industrialized for high-temperature SOFCs (HT-SOFCs) were formed as composite electrodes of LSM (a highly electronic conducting oxide) mixed together with a highly ionic conducting oxide – usually the electrolyte, yttrium-doped zirconia (YSZ) or gadolinium-doped ceria (GDC) – in order to increase the active surface area. Typically, composites LSM/YSZ with 50–70 wt% of YSZ seem to be the most efficient in terms of electronic conductivity and ionic conduction (Kim et al., 2001; Yang et al., 2003). They are used for operating at temperatures lower than 800°C (Sun et al., 2010).

13.3.3 From high-temperature to low-temperature SOFC cathode compounds: oxygen-deficient perovskites

As mentioned previously, to be able to operate at lower temperatures, an alternative approach is to use MIEC perovskite oxides. These compounds are obtained either by replacing La^3 + (or another lanthanide) by large amounts of Sr^2 + or Ba^2 +, or by substituting Mn by other metallic cations such as Fe, Co, Ni or Cu with the aim of creating oxygen vacancies. As a consequence, basic properties such as electronic conductivity, TEC, and chemical and thermal stability are significantly affected. Useful references can be found in Sun et al. (2010, p. 6806). The most relevant results are summarized in the following.

Replacing strontium for lanthanum in the perovskite LaCoO3 leads to a mixed-valence Co^{3 + 4 +} (and high electronic conductivity), as well as to the occurrence of oxygen vacancies, resulting in enhanced mixed electronic and ionic conducting as well as electrochemical properties with respect to LSM (Gödickemeier et al., 1996; Kawada et al., 1999). Although these materials have been much studied and even compositions such as La_0.6Sr_0.4CoO_{3 − δ} commercialized, they show some drawbacks:

In order to reduce the thermal expansion mismatch while maintaining good electrochemical performance, the best compromise is to substitute iron for cobalt, an optimized composition being La₀.₆Sr₀.₄Fe₀.₈Co₀.₂O_{3 − δ} (Sahibzada et al., 1997). With regard to electrocatalytic activity, a rough classification of cathode materials has been stated to follow the order: cobaltites > manganites > ferrites > chromites (Takeda et al., 1987; Menzler et al., 2010), which makes possible the design of many different compositions. For instance, ferro-nickelates appear quite attractive as some compositions, for example La(Ni,Fe)O₃, do not contain strontium, which is interesting with respect to chromium poisoning caused by interconnects (Basu et al., 2003; Millar et al., 2008). Replacing strontium by barium also results in excellent electrochemical properties, but with a large TEC and a high sensitivity to CO₂ from oxidant gas leading to rapid performance loss (Yan et al., 2006; Bucher et al., 2008).

In order to overcome these drawbacks, new cathode materials were developed during the past decade. Instead of oxygen-deficient oxides, compounds with oxygen interstitials were sought.

13.4.1 Double perovskites

As previously mentioned, the perovskite compounds AMO_{3 − y} may exhibit large amounts of oxygen vacancies. For y values equal to 0.50, due to large electrostatic interactions, the vacancies order. According to the involved cations (their size, valence state, electronic configuration, etc.) various structural types can be formed. For instance, A₂M₂O₅ compounds may crystallize with structures as different as the brownmillerite, the so-called 112 double perovskite or even other structural types (Mitchell, 2002). Some of these materials exhibit good performance as SOFC cathode materials.

13.4.2 Brownmillerite-type phases

Another kind of ion vacancy ordering in oxygen-deficient perovskites may lead to the formation of superstructures whose typical example is the structure of the brownmillerite. It belongs to the family of compounds formulated as A_nM_nO_{3n – 1} (n = 2 to ∞). Their structures consist of (n – 1) alternating perovskite-type layers of corner-sharing MO₆ octahedra and one layer of MO₄ tetrahedra rows (Grenier et al., 1981; Berastegui et al., 1999). In the structure of the brownmillerite A₂M₂O₅ (n = 2, Fig. 13.7), the oxygen vacancies are ordered along (010) planes, forming a one-dimensional (1D) diffusion pathway for oxygen ion migration in the tetrahedral layer. Its orthorhombic unit cell is related to the basic cubic perovskite cell by the following relations: . This kind of material has attracted much attention, due to its ability to accommodate additional oxygen atoms in the lattice, which in turn results in fast oxygen ionic conductivity (Kendall et al., 1995). Some brownmillerite structure compounds have been found to exhibit relatively high ionic-electronic mixed conductivity, promising oxygen permeability and compatible thermal expansion coefficients (TECs) with the usual solid electrolytes (≈ 11.3−13.6 × 10^− 6 K^− 1) (Kharton et al., 2003; Shaula et al., 2006; Vente et al., 2006). These properties might be of interest for applications as SOFC cathode. For the first time, the oxygen electrode properties of the brownmillerite-type Ca₂Fe_{2 – x}Co_xO₅ compounds have been recently reported; these materials show good electrochemical properties, the lowest polarization resistance being 0.23 Ω cm² at 700°C in air. These first results evidence that brownmillerite-type oxides are likely to be new promising cathode materials for IT-SOFCs. They emphasize again that reduced dimensionality could be interesting for improving the oxygen diffusivity.

13.4.3 Ruddlesden-Popper-type oxides: A₂MO₄ compounds

Oxides belonging to the so-called Ruddlesden–Popper (RP) series are formulated as A_{n + 1}M_nO_{3n + 1} (n ≥ 2) (Ruddlesden and Popper, 1958). Their structures consist of n AMO₃ perovskite layers separated by one AO rocksalt layer, the value of n determining the number of perovskite layers. Figure 13.8 represents the n = 1, 2, 3 and ∞ terms. With respect to the characteristics of their atomic arrangement, these compounds can show various kinds of oxygen non-stoichiometry (oxygen deficiency or additional oxygen atoms), which makes them very attractive with regard to their transport properties (electronic and ionic conductivities).

Commonly, A is a rare earth/alkaline earth cation and M is, a transition metal cation, which makes it possible to obtain a large variety of compounds with fitted properties.

The most widely studied systems are the n = 1 phases, which crystallize with the well-known K₂NiF₄ structure; they are considered as very promising cathode materials.

Some preliminary studies have been devoted to n > 1 terms, however, they are not considered in this review.

13.5.1 Electrical conductivity

Figure 13.13 shows the thermal variation of the electrical conductivity of some perovskite-related oxides interesting as SOFC cathode materials and having the mixed valence of metallic cations (Dailly, 2008; Kim et al., 2008). Most of the compounds exhibit a semiconducting-type behaviour, that is, an Arrhenius-type thermally activated conductivity, at temperatures lower than 400°C. At higher temperatures, the materials lose some oxygen (as previously quoted), which leads to decrease of the M^3 + or M^4 + content and correlatively the carrier density (i.e., electron holes). These competing phenomena result in the observed maximum of σ_e and in its decrease at high temperature. In the operating temperature domain, the values of conductivity range between 50 and 1000 S cm^− 1, which meets the requirement for a SOFC cathode.

13.5.2 Oxygen diffusivity: effect of dimensionality

Oxygen diffusivity in oxides may arise from various processes depending on the oxygen exchange type:

Compared to the perovskite compounds, the oxygen transport in these materials is supposed to be quite different with respect to the oxygen non-stoichiometry type which involves oxygen interstitials instead of oxygen vacancies. Indeed, the fact that electrochemical oxygen intercalation is possible at room temperature in these compounds suggests that inserted oxygen atoms are likely to be highly mobile in such networks. Early calculation of the migration energy of oxide ions in interstitial sites of La₂CuO₄ by Allan and Mackrodt (1991) gave a rather small value of the activation energy, E_a ≈ 0.55 eV. Using the model of Cottrell, the value of the diffusion coefficient (D_O) was determined to be about 10^− 9− 10^− 11 cm² s^− 1, at room temperature, which is somewhat higher than the value usually found for O^2 − ion diffusion in oxides (D_O ≈ 10^− 15 cm² s^− 1) (Arrouy et al., 1991; Grenier et al., 1996).

Numerous data concerned with the measure of the oxygen diffusion coefficient D_O as well as that of the exchange coefficient k_O, at high temperature, are available. Authors have used various techniques and the most reliable results seem to be obtained by the isotopic ¹⁸O/¹⁶O exchange technique; results are reported in Figs 13.14 and 13.15 for polycrystalline samples

(Yasuda et al., 1996; De Souza et al., 2000; Skinner and Kilner, 2000; Boehm et al., 2003, 2005; Tarancon et al., 2010). These values agree with those determined by other techniques. It can be noticed that:

On the basis of the Nernst-Einstein relation, the oxygen ionic conductivity of some perovskites and Ln₂NiO_{4 + δ} compounds has been calculated and plotted in Fig. 13.16 (Boehm et al., 2005). It is of the same order of magnitude as that of 8YSZ, the stabilized zirconia (σ ≈ 10^− 2Ω^− 1cm^− 1 at 700°C).

From a general viewpoint, the activation energies are lower for the Ln₂NiO_{4 + δ} compounds, which means that the lower the temperature, the better the expected performances of these 214 materials.

Experiments have also been performed on single crystals of both cuprates (La₂CuO_{4 + δ}) and nickelates (La₂NiO_{4 + δ}) as well as on thin-films using the IEDP-SIMS method (Claus et al., 1996; Bassat et al., 2004; Burriel et al., 2008). All data clearly show anisotropy of the oxygen diffusivity (Fig. 13.17). The values of D_O* as well as of the activation energy measured in the a-b planes of crystals are close to those of ceramic samples, the D_O* along the c-direction being about one to two orders of magnitude smaller. However, the values of the activation energy i the c-direction appear questionable and the experiments need to be repeated. This structural anisotropy seems also to induce anisotropy in the catalytic activity of these SOFC cathode materials, which depends on the crystalline surface as shown by Yamada et al. (2010).

Several simulations have been performed in order to explain these results. Early simulations were carried out on cuprates in relation to their superconducting properties. For the oxygen-deficient cuprates (strontium substituted), it was shown that the oxygen defects are located at the equatorial positions (i.e., in the MO₂ plane, Fig. 13.9). On the other hand, for oxygen-excess materials, it was established, especially using molecular dynamics, that interstitial oxygen atoms are mobile within the (a–b) La₂O₂ layer (Allan and Mackrodt, 1991; Mazo and Savvin, 2004; Savvin et al., 2008). Simulations (using atomistic computer simulation techniques, DFT, molecular dynamics, etc.) were also performed on Ln₂NiO_{4 + δ} compounds (Read et al., 1999; Minervini et al., 2000; Cleave et al., 2008); they definitively showed:

Recent molecular dynamics simulations with La-NiO_{4 + δ} and Pr-NiO_{4 + δ} as well as neutron scattering studies and synchrotron diffraction data in doped Pr₂NiO_{4 + δ} by Yashima et al. have nicely confirmed the large anisotropy of both the electronic conduction taking place in the NiO₂ plane and the ionic diffusion in the Ln₂O₂ layer (Yashima et al., 2008, 2010; Chroneos et al., 2010a; Parfitt et al., 2010).

It is now clearly established that the oxygen diffusion mechanism involves both the interstitial oxygen atoms as well as the apical oxygen O_ap atoms (Figs 13.9 and 13.23) with an activation energy of about 0.85–0.90 eV.

Finally, with regard to the values of D* and k*, it appears that a 2D character of the oxygen diffusion (instead of a 3D character as in usual perovskite-type compounds) seems to be a preponderant factor for improving the basic properties required for a SOFC cathode material. Most of these materials have been integrated in single cells for tests.

13.5.3 Comparative electrochemical performance of 2D non-stoichiometric oxides and fuel cell tests

The electrochemical performance of these compounds has been evaluated, first, through electrochemical measurements by EIS of the ASR of porous electrodes deposited on usual electrolytes (Zr_xY_{1 − x}O_{2 − δ} (YSZ), Ce_{1 – x}Gd_xO_{2 − δ} (CGO)), then, for the best materials, through single-cell tests. Numerous electrochemical studies and single-cell tests have been reported in the literature but their comparison is difficult as the results strongly depend on the method used to fabricate the cell (Menzler et al., 2010).

The most widely utilized cathode material La_xSr_{1 – x}MnO₃ (LSM) shows very good performance around 800°C, but due to its poor ionic conductivity, overpotential is dramatically increased at lower temperatures (Brandon et al., 2003). The replacement of Mn by Co or Fe leading to MIEC perovskites is worthwhile in terms of activation of the oxygen reduction. One can mention for instance La_xSr_{1 – x}Co_{1 – y}Fe_yO_{3 – δ} (LSCF) (Takeda et al., 1987; Riza et al., 2001; Tietz et al., 2006) and Sm₀.₅Sr₀.₅CoO_{3 – δ} (SSC) (Xia et al., 2002) which have been commercialized.

Recent studies devoted to the new double perovskites LnBaCo.O_{5 + δ} (Ln = Nd, Pr, Gd, Sm) report cathodic ASRs that meet the targets, that is, lower than 0.25 Q cm² at operating temperatures such as 625°C for GdBaCo₂O_{5 + δ} or even as low as 550°C for PrBaCo.O_{5 + δ} deposited on CGO (Tarancon et al., 2007, 2008; Kim et al., 2007). The samarium compound, SBCO, deposited on Sm₀.₂Ce₀.₈O₁.₉ (SDC) or La₀.₉Sr_0.1Ga₀.₈Mg₀.₂O_{3 – δ} (LSGM) exhibits ASR values of 0.098 and 0.054 Ω cm², respectively, at 750°C and maximum power densities of single cells, at 800°C, of 641 and 777 mWcm^− 2, respectively (Zhou et al., 2008).

Although these cobalt-based cathodes exhibit good performance, they suffer from the problems previously mentioned (high reactivity with electrolytes, high thermal expansion coefficients (TECs), as well as the high cost of cobalt element (Sun et al., 2010). Developing cobalt-free cathodes with good catalytic activity at reduced temperatures for IT-SOFCs is clearly significant. Several cobalt-free oxides with the simple perovskite structure, such as La_xSr_{1 – x}FeO_{3 − δ} (LSF) (Simner et al., 2002) or LNF (Chiba et al., 1999; Komatsu et al., 2006) have been proposed as cathode materials for IT-SOFC cathodes. In the same way, double perovskites such as LnBaFe₂O_{5 + δ} exhibit interesting properties. By EIS, in a symmetrical cell with Sm_0.2Ce_0.8O_1.9 (SDC) electrolyte, the observed ASR of GdBaFe₂O_{5 + δ} was 0.15 Ω cm² at 700°C and 0.39 Ω cm² at 650°C; a maximal power density of 861 mWcm^− 2 was achieved at 700°C (Ding and Xue, 2010).

The 214 nickelates have been intensively studied in recent years with the same strategy to avoid the presence of cobalt in the hope that they may be a promising alternative to the traditional perovskite cathode materials. Electrochemical studies have been performed using all the classical electrolytes, Zr_xY_{1 – x}O_{2 – x/2} (YSZ), Ce_{1 − x}GdxO₂ – _x/2 (CGO), La_{1 − x}Sr_xGa_{1 − y}MgO_{3 − δ} (LSGM) and the apatite La₉Sr(SiO₄)O₂, with Ln₂NiO_{4 + δ} (Ln = La, Nd or Pr), being the porous cathode material.

Although these nickelates showed promising basic properties in terms of D* and k* coefficients compared to those of perovskite compounds, the first measured ASR values were not as small as expected. Using YSZ as electrolyte, Zhao et al. (2008) and Escudero et al. (2007) found values of 4 Ω cm² at 708°C and 2.5 Ω cm² at 800°C, respectively. These ASRs were much improved when making graded electrodes (0.11 Ω cm² at 800°C (Rieu et al., 2010) and thanks to the addition of a doped ceria interfacial layer (Sayers et al., 2011). Power densities up to 2.2 Wcm^− 2 were measured at 800°C by Laberty et al. when using a composite electrode made of 1/1 nickelate/Sm_0.2Ce₀.₈O_1.9 (SDC), even though the kinetics of ORR was claimed to be rather slow (Laberty et al., 2007).

Earlier experiments on the neodymium nickelate Nd₂NiO_{4 + δ} reported an ASR value of about 1 Ω cm² at 700°C (Mauvy et al., 2003). Later, Lalanne et al. (2008) obtained with the selected neodymium-deficient Nd_1.95NiO_{4 + δ} composition, screen-printed on anode-supported SOFC half-cell (HTceramix® commercial cell based on co-tape cast 8YSZ/Ni–8YSZ), current density up to 1.31 Acm^− 2, at 0.70 V – that is, a power density of 1W cm^− 2 at 800°C. As shown by Haanappel et al. (2009), again the use of an interlayer of CGO improved the performance and durability of the cell. The feasibility to produce cathode-supported SOFCs on porous neodymium nickelate substrates, that is, cathode tubes of 4.9 mm inner diameter, the electrolyte layers being applied by dip-coating, was demonstrated by Luebbe et al. (2009).

Up to now, only few data have been reported with the praseodymium nickelate which seems to show the best results, especially at low temperatures of about 600°C. A polarization resistance value as low as 0.08 Ω cm² has been measured on a symmetrical cell configuration (with commercial YSZ as electrolyte) by Ferchaud et al. (2011); the electrodes made of powder, with average particle size of 0.4 μm, were sintered at 1130°C on a barrier layer made of gadolinium-doped ceria (CGO) that was shown to be better than the yttrium-doped one (YDC). The best cell performance – that is, a maximum power output density of approximately 400 mW cm^− 2 at 600°C – was obtained for a cell combined with a thin screen-printed Co-doped GCO layer.

In general, substitution of an alien cation for nickel does not seem to significantly improve the electrochemical properties of these nickelates (Kharton et al., 2004a, 2004b); it usually increases the Ni^3 + content and consequently the electrical conductivity but lessens the additional oxygen interstitials (S decreases, i.e. the ionic conductivity decreases) (Sun et al., 2008).

13.6.1 Nickelates as PCFC cathodes

A specific feature of protonic-conducting H⁺-SOFCs is that water vapour is produced at the cathode side, due to the fact that the electrolyte is a proton-conducting oxide. Such a feature involves some additional requirements for the cathode material with respect to the usual ones for oxide conducting O^2 −-SOFC application, the first one being their chemical stability in moist air.

Additionally, according to Tao et al. (2000) suitable cathode compounds must exhibit both high electronic and oxygen-ion or/and proton conductivities as well as good catalytic activity for the following electrode reaction:

[13.1]

The reaction [13.1] can be decomposed into two steps:

[13.2]

[13.3]

When the cathode material is an oxygen-ion mixed conducting oxide (Fig. 13.18a), reaction [13.2] may occur all over its surface, but water is formed only at the TPB near the electrolyte surface and has to flow out through the electrode.

Using a protonic mixed conducting oxide (Fig. 13.18b) allows the water to be produced not only at the TPB sites, but over the whole cathode surface. From a practical viewpoint, the last system would obviously exhibit faster electrode processes.

Nickelates were recently studied as cathodes for protonic fuel cells. They were found to be stable in moist atmosphere: their electrical conductivity is unchanged and, according to TGA experiments and IR spectroscopy, a small quantity of water is inserted (e.g. less than 0.03 mole H₂O/mole for La₂NiO_{4 + δ}) (Dailly et al., 2010). Their electrochemical characteristics were determined on symmetrical half-cells under zero dc conditions and under dc polarization (three-electrode cell): Pr₂NiO_{4 + δ} shows the lowest ASR at 600°C.

A single cell was fabricated starting from an anode-supported cell made of Ni cermet as anode, BaCe_0.9Y_0.1O_{2 95} as electrolyte and Pr₂NiO_{4 + δ} as cathode; a power density of 130 mW cm^− 2 was obtained at 923 K (Taillades et al., 2010). Improving the microstructure of the cathode recently made it possible to reach values of up to 180 mW/cm² (Grimaud et al., 2010).

Again these materials showing oxygen over stoichiometry appear to be very efficient as H⁺-SOFC cathode; however, their mixed protonic-electronic conductivity has not yet been evidenced.

13.6.2 Nickelates as high-temperature steam electrolyzer anodes

Hydrogen being considered as a promising energetic vector for the future (Winter, 2009), its massive production using clean processes has become crucial. HTSE is one of the most promising ways to achieve this target (Dönitz, 1984). It supposes the components constituting the solid oxide electrolysis cell (SOEC based on oxide-conducting electrolyte) where the HTSE reactions occur, to be optimized, especially the oxygen electrode (anode) where oxygen is produced according to the following reaction:

Most current studies concerned with HTSE consider reversible solid oxide cells (fuel cell/electrolysis cell), constituted of a hydrogen electrode (a nickel-YSZ cermet), YSZ electrolyte and perovskite-type oxygen electrode (usually LSM) (Hauch et al., 2008). Recently, it has been reported that alternative oxygen electrode materials with Ln.NiO_{4 + δ} (Ln = La, Nd,) compositions show excellent performances (Pérez-Coll et al., 2009; Chauveau et al., 2010). For instance, Chauveau et al. tested, in both fuel cell (SOFC) and electrolysis (HTSE) modes, a TZ3Y-supported cell (TZ3Y as electrolyte) with a Ni–CGO as hydrogen electrode and Ln₂NiO_{4 + δ} (Ln = La, Nd) as oxygen electrode; they found ASR values to be slightly lower in the electrolysis mode. Compared to a commercial electrolyte-supported cell containing the same hydrogen electrode and electrolyte membrane but with a conventional LSM-based oxygen electrode, improved performances have been evidenced for both nickelates over the temperature range 750–850°C. For instance, at 750°C, the current densities at 1.3 V (the so-called thermoneutral potential), are 1.8 and 4.4 times larger than the one observed for LSM (Fig. 13.19). Such results show that these materials are very promising for this application, which can be explained by the beneficial effect of using mixed conducting electrode materials, allowing the oxygen formation reaction to be delocalized over the entire electrode surface.

13.6.3 K₂NiF₄-type oxides: flexible oxides as oxygen electrodes

Previous examples pointed to an interest in using K₂NiF₄-type compounds, especially Ln₂NiO₄ nickelates as oxygen electrodes in various systems, such as cathode for either O^2 −-SOFC or H⁺-SOFC and as anode for HTSE.

Therefore, it is interesting to analyse why these materials are so flexible and efficient.

First, one should recall some basic features of these compounds:

It follows that, whatever the operating conditions, that is, the local oxygen pressure, additional oxygen atoms O_i will be more or less present in the Ln₂O₂ layers (Fig. 13.23).

As a consequence of the application as oxygen electrode, when the material operates as a SOFC cathode (i.e., in oxygen depletion conditions), the material is able to lose some oxygen atoms. But as δ decreases, stresses are generated and act to prevent a further reduction of the material, keeping the mixed valence of the nickel and additional mobile oxygen atoms. On the other hand, under anodic polarization in HTSE (i.e., under highly oxidizing conditions), oxygen can enter into the network, and the nickel is oxidized (the electrical conductivity is enhanced) until a mechanical limit in terms of size fitting of the layers is reached (t ≈ 1). The basic properties of the materials remain mobility of oxygen and mixed valence of the cations.

Thanks to these peculiar structural features, it appears that these compounds are genuinely flexible to be used in different systems, which is illustrated by the good performances reported in recent years. In particular, these materials seem to be especially interesting for electrolysis anode applications as, contrary to oxygen-deficient perovskites that completely oxidize in such conditions, they are never saturated in oxygen, which preserves their oxygen mobility and, therefore, their mixed ionic and electronic conduction properties.

• their electronic conductivity results from the presence of a mixed valence of the metallic cations;

• their ionic conductivity results from the presence of additional oxide ions instead of oxygen vacancies as in the usual cathode materials;

• their structures show a reduced dimensionality, two-dimensional for the A₂MO₄-type compounds and almost two-dimensional for the double perovskites.

Adler, S. Mechanism and kinetics of oxygen reduction on porous La₁._xSr_xCoO_3-δ electrodes. Solid State Ionics. 1998; 111(1–2):125–134.

Adler, S., Lane, J., Steele, B. Electrode kinetics of porous mixed-conduct-ing oxygen electrodes. J Electrochem Soc. 1996; 143(11):3554–3564.

Aguadero, A., Alonso, J., Martinez-Lope, M., Fernandez-Diaz, M., Escudero, M., Daza, L. In situ high temperature neutron powder diffraction study of oxygen-rich La₂NiO_4+δ in air: Correlation with the electrical behaviour. J Mater Chem. 2006; 16(33):3402–3408.

Allan, N., Mackrodt, W. Oxygen ion migration in La₂CuO₄. Philos Mag A. 1991; 64(5):1129–1132.

Anderson, H.U. Review of p-type doped perovskite materials for SOFC and other applications. Solid State Ionics. 1992; 52(1–3):33–41.

Arrouy, F., Wattiaux, A., Cros, C., Demazeau, G., Grenier, J.-C., Pouchard, M., Etourneau, J. Superconducting behaviour of the T/O structural type phase La_2-xNd_xCuO_4 + δ (0<x<0.60) under oxidizing conditions. Physica C. 1991; 175(3–4):342–346.

Bassat, J., Odier, P., Villesuzanne, A., Marin, C., Pouchard, M. Anisotropic ionic transport properties in La₂NiO_4 + δ single crystals. Solid State Ionics. 2004; 167(3–4):341–347.

Basu, R., Tietz, F., Teller, O., Wessel, E., Buchkremer, H., Stöver, D. LaNi_0.6Fe_0.4O₃ as a cathode contact material for solid oxide fuel cells. J Solid State Electrochem. 2003; 7(7):416–420.

Berastegui, P., Eriksson, S.G., Hull, S. A neutron diffraction study of the temperature dependence of Ca₂Fe₂O₅. Mater Res Bull. 1999; 34(2):303–314.

Boehm, E., Bassat, J.-M., Dordor, P., Mauvy, F., Grenier, J.-C., Stevens, P. Oxygen diffusion and transport properties in non-stoichiometric Ln_2-xNiO_4 + δ oxides. Solid State Ionics. 2005; 176(37–38):2717–2725.

Boehm, E., Bassat, J.-M., Steil, M., Dordor, P., Mauvy, F., Grenier, J.-C. Oxygen transport properties of La₂Ni_1-xCu_xO_4 + δ mixed conducting oxides. Solid State Sci. 2003; 5(7):973–981.

Brandon, N., Skinner, S., Steele, B. Recent advances in materials for fuel cells. Annu Rev Mater Res. 2003; 33:183–213.

Bucher, E., Egger, A., Caraman, G., Sitte, W. Stability of the SOFC cathode material (Ba, Sr)(Co, Fe)O3-δ in CO. containing atmospheres. J Electrochem Soc. 2008; 155(11):B1218–B1224.

Burriel, M., Garcia, G., Santiso, J., Kilner, J., Chater, R., Skinner, S. Anisotropic oxygen diffusion properties in epitaxial thin films of La.NiO_4 + δ. J Mater Chem. 2008; 18(4):416–422.

Chauveau, F., Mougin, J., Bassat, J., Mauvy, F., Grenier, J. ‘A new anode material for solid oxide electrolyser: The neodymium nickelate Nd₂NiO_4 + δ. J Power Sources. 2010; 195(3):744–749.

Chen, C.H., Bouwmeester, H.J.M., van Doorn, R.H.E., Kruidhof, H., Burggraaf, A.J. Oxygen permeation of La2₀.₃Sr₀.₇CoO_3-δ. Solid State Ionics. 1997; 98(1–2):7–13.

Chen, C., Nasrallah, M., Anderson, H., Alim, M. Immittance response of La₀.₆Sr_0.4Co_0.2Fe_0.8O₃ based electrochemical cells. J Electrochem Soc. 1995; 142:491–496.

Chiba, R., Yoshimura, F., Sakurai, Y. An investigation of LaNi_1-xFe_xO₃ as a cathode material for solid oxide fuel cells. Solid State Ionics. 1999; 124(3–4):281–288.

Choi, M.-B., Jeon, S.-Y., Lee, J.-S., Hwang, H.-J., Song, S.-J. Chemical diffusivity and ionic conductivity of GdBaCo₂ O_5 + δ. J Power Sources. 2010; 195(4):1059–1064.

Chroneos, A., Parfitt, D., Kilner, J., Grimes, R. Anisotropic oxygen diffusion in tetragonal LaiNiO_4 + δ: Molecular dynamics calculations. J Mater Chem. 2010; 20(2):266–270.

Chroneos, A., Vovk, R., Goulatis, I., Goulatis, L. Oxygen transport in perovskite and related oxides: A brief review. J Alloy Comp. 2010; 494(1–2):190–195.

Claus, J., Borchardt, G., Weber, S., Hiver, J.-M., Scherrer, S. Combination of EBSP measurements and SIMS to study crystallographic orientation dependence of diffusivities in a polycrystalline material: Oxygen tracer diffusion in La_2–xSr_xCuO_4 + δ. Mater Sci Eng B. 1996; 38(3):251–257.

Cleave, A., Kilner, J., Skinner, S., Murphy, S., Grimes, R. Atomistic computer simulation of oxygen ion conduction mechanisms in La₂NiO₄. Solid State Ionics. 2008; 179(21–26):823–826.

Dailly, J. 2008, ‘Synthèse et caractérisation de nouveaux matériaux de cathode pour piles a combustible a conduction protonique PCFC (Protonic Ceramic Fuel Cell)’. Ph.D. thesis, University of Bordeaux.

Dailly, J., Fourcade, S., Largeteau, A., Mauvy, F., Grenier, J., Marrony, M. Perovskite and A₂MO₄-type oxides as new cathode materials for protonic solid oxide fuel cells. Electrochim Acta. 2010; 55(20):5847–5853.

De Souza, R.A., Kilner, J.A., Walker, J.F. A SIMS study of oxygen tracer diffusion and surface exchange in La₀.₈Sr₀.₂MnO_4 + δ. Mater Lett. 2000; 43(1–2):43–52.

Demourgues, A., Weill, F., Darriet, B., Wattiaux, A., Grenier, J., Gravereau, P., Pouchard, M. Additional oxygen ordering in “La₂NiO₄.₂₅ “ (La₈NiO₁₇,). I. Electron and neutron diffraction study; 2 Structural features. J Solid State Chem. 1993; 106(2):317–338.

Demourgues, A., Weill, F., Grenier, J., Wattiaux, A., Pouchard, M. Electron microscopy study of electrochemically prepared La₂NiO_4 + δ (0.17 ≤ S ≤ 0.26). Phys C Supercond Appl. 1992; 192(3–4):425–434.

Ding, H., Xue, X. Cobalt-free layered perovskite GdBaFe₂O_5+x as a novel cathode for intermediate temperature solid oxide fuel cells. J Power Sources. 2010; 195(15):4718–4721.

Dönitz, W. Economics and potential application of electrolytic hydrogen in the next decades. Int J Hydrog Energy. 1984; 9(10):817–821.

Er-Rakho, L., Michel, C., Lacorre, P., Raveau, B. YBaCuFeO_5 + δ: A novel oxygen-deficient perovskite with a layer structure. J Solid State Chem. 1988; 73(2):531–535.

Erdle, E., Dönitz, W., Schamm, R., Koch, A. Reversibility and polarization behaviour of high temperature solid oxide electrochemical cells. Int J Hydrog Energy. 1992; 17(10):817–819.

Escudero, M., Aguadero, A., Alonso, J., Daza, L. A kinetic study of oxygen reduction reaction on La₂NiO₄ cathodes by means of impedance spectroscopy. J Electroanal Chem. 2007; 611(1–2):107–116.

Ezin, A.N., Kurumchin, E.K., Murygin, I.V., Tsidilkovski, V.I., Vdovin, G.K. The types of surface exchange and diffusion of oxygen in La2₀.₇Sr₀.₃CoO_3 − δ. Solid State Ionics. 1998; 112(1–2):117–122.

Ferchaud, C., Grenier, J.-C., Zhang-Steenwinkel, Y., Van Tuel, M., Van Berkel, F., Bassat, J.-M. High performance praseodymium nickelate oxide cathode for low temperature solid oxide fuel cell. J Power Sources. 2011; 196(4):1872–1879.

Fergus, J. Metallic interconnects for solid oxide fuel cells. Mater Sci Eng A. 2005; 397(1–2):271–283.

Fleig, J. Solid oxide fuel cell cathodes: Polarization mechanisms and modeling of the electrochemical performance. Annu Rev Mater Res. 2003; 33(1):361–382.

Flynn, J.H., Dickens, B. Steady-state parameter-jump methods and relaxation methods in thermogravimetry. Thermochim Acta. 1976; 15(1):1–16.

Frayret, C., Villesuzanne, A., Pouchard, M. Application of density functional theory to the modeling of the mixed ionic and electronic conductor La₂NiO_4 + δ: Lattice relaxation, oxygen mobility, and energetics of Frenkel defects. Chem Mater. 2005; 17(26):6538–6544.

Frontera, C., Caneiro, A., Carrillo, A., Oro-Solé, J., Garcia-Munoz, J.L. Tailoring oxygen content on PrBaCo₂O_5 + δ layered cobaltites. Chem Mater. 2005; 17(22):5439–5445.

Gödickemeier, M., Sasaki, K., Gauckler, L.J., Riess, I. Perovskite cathodes for solid oxide fuel cells based on ceria electrolytes. Solid State Ionics. 1996; 86–88(Part 2):691–701.

Grenier, J., Mauvy, F., Lalanne, C., Bassat, J.-M., Chauveau, F., Mougin, J., Dailly, J., Marrony, M., A₂MO_4 + δ oxides: Flexible materials for solid oxide cells. 11th International Symposium on Solid Oxide Fuel Cells (SOFC-XI)-216th ECS Meeting, Vienna; Vol. 25, 2009:2537–2546.

Grenier, J.-C., Pouchard, M., Hagenmuller, P. Vacancy ordering in oxygen-deficient perovskite-related ferrites. Struct Bond. 1981; 47:1–25.

Grenier, J.-C., Pouchard, M., Wattiaux, A. Electrochemical synthesis: Oxygen intercalation. Curr Opin Solid State Mater Sci. 1996; 1(2):233–240.

Grenier, J.-C., Wattiaux, A., Doumerc, J.-P., Dordor, P., Fournes, L., Chaminade, J.-P., Pouchard, M. Electrochemical oxygen intercalation into oxide networks. J Solid State Chem. 1992; 96(1):20–30.

Grimaud, A., Mauvy, F., Bassat, J.-M., Grenier, J.-C., Dailly, J., Marrony, M., Electrochemical study of rare earth oxides used as cathode materials for SOFC-H⁺ applications. Proceedings of E-MRS 2010 Spring Meeting, 9–11 June, Strasbourg, France, 2010.

Haanappel, V., Lalanne, C., Mai, A., Tietz, F. Characterization of anode-supported solid oxide fuel cells with Nd₂NiO₄ cathodes. J Fuel Cell Sci Technol. 2009; 6(4):0410161–0410166.

Hagenmuller, P., Pouchard, M., Grenier, J.C. Nonstoichiometry in perovskite-type oxides. Influence of the electronic configuration on formation of extended defects. C-MRS Int. Symp. Proc. (Ed. by Kong, Meiying;Huang, Liji). 1991; 1:25–37.

Hauch, A., Ebbesen, S.D., Jensen, S.H., Mogensen, M. Highly efficient high temperature electrolysis. J Mater Chem. 2008; 18:2331–2340.

Hewat, A.W., Capponi, J.J., Chaillout, C., Marezio, M., Hewat, E.A. Structures of superconducting Ba₂YCu₃O_7-δ and semiconducting Ba₂YCu₃O₆ between 25°C and 750°C. Solid State Commun. 1987; 64(3):301–307.

Holtappels, P., Bagger, C. Fabrication and performance of advanced multilayer SOFC cathodes. J Eur Ceram Soc. 2002; 22(1):41–48.

Huang, Q.-A., Hui, R., Wang, B., Zhang, J. A review of AC impedance modeling and validation in SOFC diagnosis. Electrochim Acta. 2007; 52(28):8144–8164.

Ivers-Tiffée, E., Weber, A., Herbstritt, D. Materials and technologies for SOFC-components. J Eur Ceram Soc. 2001; 21(10–11):1805–1811.

Jiang, S. Development of lanthanum strontium manganite perovskite cathode materials of solid oxide fuel cells: A review. J Mater Sci. 2008; 43(21):6799–6833.

Jorgensen, M., Mogensen, M. Impedance of solid oxide fuel cell LSM/YSZ composite cathodes. J Electrochem Soc. 2001; 148(5):A433–A442.

Kawada, T., Masuda, K., Suzuki, J., Kaimai, A., Kawamura, K., Nigara, Y., Mizusaki, J., Yugami, H., Arashi, H., Sakai, N., Yokokawa, H. Oxygen isotope exchange with a dense La₀.₆Sr₀.₄CoO_3-δ electrode on a Ce₀.₉Ca₀.₁O₁.₉ electrolyte. Solid State Ionics. 1999; 121(1–4):271–279.

Kendall, K.R., Navas, C., Thomas, J.K., Loye, H.-C.Z. Recent developments in perovskite-based oxide ion conductors. Solid State Ionics. 1995; 82(3–4):215–223.

Kenjo, T., Nishiya, M. LaMnO₃ air cathodes containing ZrO₂ electrolyte for high temperature solid oxide fuel cells. Solid State Ionics. 1992; 57(3–4):295–302.

Kharton, V.V., Marozau, I.P., Vyshatko, N.P., Shaula, A.L., Viskup, A.P., Naumovich, E.N., Marques, F.M.B. Oxygen ionic conduction in brownmillerite CaAl₀.₅Fe₀.₅O₂._5 + δ. Mater Res Bull. 2003; 38(5):773–782.

Kharton, V., Tsipis, E., Yaremchenko, A., Frade, J. Surface-limited oxygen transport and electrode properties of La₂Ni₀₈Cu₀₂O_4 + δ. Solid State Ionics. 2004; 166(3–4):327–337.

Kharton, V.V., Viskup, A.P., Kovalevsky, A.V., Naumovich, E.N., Marques, F.M.B. Ionic transport in oxygen-hyperstoichiometric phases with K₂NiF₄-type structure. Solid State Ionics. 2001; 143(3–4):337–353.

Kharton, V., Yaremchenko, A., Shaula, A., Patrakeev, M., Naumovich, E., Logvinovich, D., Frade, J., Marques, F. Transport properties and stability of Ni-containing mixed conductors with perovskite and K₂NiF₄-type structure. J Solid State Chem. 2004; 177(1):26–37.

Kilner, J.A., Shaw, C.K.M. Mass transport in La2Ni1, xCoxO4+8 oxides with the K2NiF4 structure. Solid State Ionics. 2002; 154–155:523–527.

Kim, J.-D., Kim, G.-D., Moon, J.-W., Park, Y.-I., Lee, W.-H., Kobayashi, K., Nagai, M., Kim, C.-E. Characterization of LSM-YSZ composite electrode by ac impedance spectroscopy. Solid State Ionics. 2001; 143(3–4):379–389.

Kim, J.-H., Prado, F., Manthiram, A. Characterization of GdBa_1-xSr_xCo₂O_5 + δ (0 < x < 1.0) double perovskites as cathodes for solid oxide fuel cells. J Electrochem Soc. 2008; 155(10):B1023–B1028.

Kim, G., Wang, S., Jacobson, A., Reimus, L., Brodersen, P., Mims, C. Rapid oxygen ion diffusion and surface exchange kinetics in PrBaCo₂O_5+x with a perovskite related structure and ordered a cations. J Mater Chem. 2007; 17(24):2500–2505.

Komatsu, T., Arai, H., Chiba, R., Nozawa, K., Arakawa, M., Sato, K. Cr poisoning suppression in solid oxide fuel cells using LaNi(Fe)O₃ electrodes. Electrochem Solid State Lett. 2006; 9(1):A9–A12.

La O’, G., Savinell, R., Shao-Horn, Y. Activity enhancement of dense strontium-doped lanthanum manganite thin films under cathodic polarization: A combined AES and XPS study. J Electrochem Soc. 2009; 156(6):B771–B781.

Laberty, C., Zhao, F., Swider-Lyons, K., Virkar, A. High-performance solid oxide fuel cell cathodes with lanthanum-nickelate-based composites. Electrochem Solid State Lett. 2007; 10(10):B170–B174.

Lalanne, C., Prosperi, G., Bassat, J.-M., Mauvy, F., Fourcade, S., Stevens, P., Zahid, M., Diethelm, S., Van herle, J., Grenier, J.-C. Neodymium-deficient nickelate oxide Nd_1.95NiO_4 + δ as cathode material for anode-supported intermediate temperature solid oxide fuel cells. J Power Sources. 2008; 185(2):1218–1224.

Lane, J., Benson, S., Waller, D., Kilner, J. Oxygen transport in La₀.₆Sr₀.4Co₀.₂Fe₀.₈O_3-δ. Solid State Ionics. 1999; 121(1):201–208.

Lane, J., Kilner, J. Measuring oxygen diffusion and oxygen surface exchange by conductivity relaxation. Solid State Ionics. 2000; 136–137:997–1001.

Luebbe, H., Van herle, J., Hofmann, H., Bowen, P., Aschauer, U., Schuler, A., Snijkers, F., Schindler, H.-J., Vogt, U., Lalanne, C. Cathode-supported micro-tubular SOFCs based on Nd _1.95NiO_4 + δ. Solid State Ionics. 2009; 180(11–13):805–811.

Maignan, A., Martin, C., Pelloquin, D., Nguyen, N., Raveau, B. Structural and magnetic studies of ordered oxygen-deficient perovskites LnBaCo₂O_5 + δ, closely related to the “112” structure. J Solid State Chem. 1999; 142(2):247–260.

Manning, P., Sirman, J., Kilner, J. Oxygen self-diffusion and surface exchange studies of oxide electrolytes having the fluorite structure. Solid State Ionics. 1996; 93(1–2):125–132.

Mauvy, F., Bassat, J.M., Boehm, E., Manaud, J.P., Dordor, P., Grenier, J.C. Oxygen electrode reaction on Nd₂NiO_4 + δ cathode materials: Impedance spectroscopy study. Solid State Ionics. 2003; 158(1–2):17–28.

Mauvy, F., Bassat, J., Boehm, E., Dordor, P., Grenier, J., Loup, J. Chemical oxygen diffusion coefficient measurement by conductivity relaxation-correlation between tracer diffusion coefficient and chemical diffusion coefficient. J Eur Ceram Soc. 2004; 24(6):1265–1269.

Mazo, G., Savvin, S. The molecular dynamics study of oxygen mobility in La₂._xSr_xCuO_4-δ. Solid State Ionics. 2004; 175(1–4):371–374.

Menzler, N., Tietz, F., Uhlenbruck, S., Buchkremer, H., Stöver, D. Materials and manufacturing technologies for solid oxide fuel cells. J Mater Sci. 2010; 45(12):3109–3135.

Millar, L., Taherparvar, H., Filkin, N., Slater, P., Yeomans, J. Interaction of (La_1-xSr_x)_1-yMnO₃-Zr_1-zY_zO_2-δ cathodes and LaNi₀.₆Fe₀.₄O₃ current collecting layers for solid oxide fuel cell application. Solid State Ionics. 2008; 179(19–20):732–739.

Minervini, L., Grimes, R., Kilner, J., Sickafus, K. Oxygen migration in La₂NiO_4 + δ. J Mater Chem. 2000; 10(10):2349–2354.

Mitchell, R. Perovskites, Modern and Ancient. Canada: Almaz Press Inc.; 2002.

Miyoshi, S., Hong, J.-O., Yashiro, K., Kaimai, A., Nigara, Y., Kawamura, K., Kawada, T., Mizusaki, J. Lattice creation and annihilation of LaMnO_3 + δ caused by nonstoichiometry change. Solid State Ionics. 2002; 154–155:257–263.

Mizusaki, J., Mori, N., Takai, H., Yonemura, Y., Minamiue, H., Tagawa, H., Dokiya, M., Inaba, H., Naraya, K., Sasamoto, T., Hashimoto, T. Oxygen nonstoichiometry and defect equilibrium in the perovskite-type oxides La₁._xSr_xMnO_3 + δ. Solid State Ionics. 2000; 129(1–4):163–177.

Nakamura, T., Yashiro, K., Sato, K., Mizusaki, J. Defect chemical and statistical thermodynamic studies on oxygen nonstoichiometric Nd_2-xSr_xNiO_4 + δ. Solid State Ionics. 2009; 180:1406–1413.

Nakamura, T., Yashiro, K., Sato, K., Mizusaki, J. Structural analysis of La_2-xSr_xNiO_4 + δ by high temperature X-ray diffraction. Solid State Ionics. 2010; 181(5–7):292–299.

Orera, A., Slater, P.R. New chemical systems for solid oxide fuel cells. Chem Mater. 2010; 22(3):675–690.

Parfitt, D., Chroneos, A., Kilner, J., Grimes, R. Molecular dynamics study of oxygen diffusion in Pr₂NiO_4 + δ. Phys Chem Chem Phys. 2010; 12(25):6834–6836.

Pizzini, S. Fast Ion Transport in Solids. New York: North-Holland Publishing Company; 1973.

Pérez-Coll, D., Aguadero, A., Escudero, M., Daza, L. Effect of DC current polarization on the electrochemical behaviour of La₂NiO_4 + δ and La₂Ni₂O_7 + δ⁻ based systems. J Power Sources. 2009; 192(1):2–13.

Read, M., Islam, M., King, F., Hancock, F. Defect chemistry of La₂Ni_1-xM_xO₄ (M = Mn, Fe, Co, Cu): Relevance to catalytic behavior. J Phys Chem B. 1999; 103(9):1558–1562.

Rieu, M., Sayers, R., Laguna-Bercero, M., Skinner, S., Lenormand, P., Ansart, F. Investigation of graded La2NiO_4 + δ cathodes to improve SOFC electrochemical performance. J Electrochem Soc. 2010; 157(4):B477–B480.

Riza, F., Ftikos, C., Tietz, F., Fischer, W. Preparation and characterization of Ln_0.8Sr_0.2Fe_0.8Co_0.2O_3-x (Ln=La, Pr, Nd, Sm, Eu, Gd). J Eur Ceram Soc. 2001; 21(10–11):1769–1773.

Ruddlesden, S., Popper, P. The compound Sr₃Ti₂O₇ and its structure. Acta Crystallogr. 1958; 11:54–55.

Ruiz-Trejo, E., Sirman, J., Baikov, Y., Kilner, J. Oxygen ion diffusivity, surface exchange and ionic conductivity in single crystal Gadolinia doped Ceria. Solid State Ionics. 1998; 113–115:565–569.

Sahibzada, M., Steele, B., Zheng, K., Rudkin, R., Metcalfe, I. Development of solid oxide fuel cells based on a Ce(Gd)O_2-x electrolyte film for intermediate temperature operation. Catal Today. 1997; 38(4):459–466.

Savvin, S., Mazo, G., Ivanov-Schitz, A. Simulation of ion transport in layered cuprates La_2-xS_xCuO_4-δ. Crystallogr Rep. 2008; 53(2):291–301.

Sayers, R., Rieu, M., Lenormand, P., Ansart, F., Kilner, J., Skinner, S. Development of lanthanum nickelate as a cathode for use in intermediate temperature solid oxide fuel cells. Solid State Ionics. 2011; 192:531–534.

Shaula, A., Pivak, Y., Waerenborgh, J., Gaczynski, P., Yaremchenko, A., Kharton, V. Ionic conductivity of brownmillerite-type calcium ferrite under oxidizing conditions. Solid State Ionics. 2006; 177(33–34):2923–2930.

Siebert, E., Hammouche, A., Kleitz, M. Impedance spectroscopy analysis of La_1-xSr_xMnO₃-yttria-stabilized zirconia electrode kinetics. Electrochim Acta. 1995; 40(11):1741–1753.

Simner, S., Bonnett, J., Canfield, N., Meinhardt, K., Sprenkle, V., Stevenson, J. Optimized lanthanum ferrite-based cathodes for anode-supported SOFCs. Electrochem Solid State Lett. 2002; 5(7):A173–A175.

Singhal, S. Advances in solid oxide fuel cell technology. Solid State Ionics. 2000; 135(1–4):305–313.

Skinner, S. Characterisation of La₂NiO_4 + δ using in situ high temperature neutron powder diffraction. Solid State Sci. 2003; 5(3):419–426.

Skinner, S., Amow, G. Structural observations on La2(Ni, Co)O4±δ phases determined from in situ neutron powder diffraction. J Solid State Chem. 2007; 180(7):1977–1983.

Skinner, S.J., Kilner, J.A. Oxygen diffusion and surface exchange in La_2-xSr_xNiO_4 + δ. Solid State Ionics. 2000; 135(1–4):709–712.

Srinivasan, S., Hurwitz, H., Bockris, J. Fundamental equations of electrochemical kinetics at porous gas-diffusion electrodes. J Chem Phys. 1967; 46(8):3108–3122.

Steele, B. Fast Ion Transport in Solids. North-Holland: Elsevier; 1973.

Steele, B., Heinzel, A. Materials for fuel-cell technologies. Nature. 2001; 414(6861):345–352.

Steele, B.C.H. Survey of materials selection for ceramic fuel cells II. Cathodes and anodes. Solid State Ionics. 1996; 86–88(Part 2):1223–1234.

Sun, C., Hui, R., Roller, J. Cathode materials for solid oxide fuel cells: A review. J Solid State Electr. 2010; 14:1125–1144.

Sun, L.-P., Li, Q., Zhao, H., Huo, L.-H., Grenier, J.-C. Preparation and electrochemical properties of Sr-doped Nd₂NiO_4 + δ cathode materials for intermediate-temperature solid oxide fuel cells. J Power Sources. 2008; 183(1):43–48.

Taillades, G., Dailly, J., Taillades-Jacquin, M., Mauvy, F., Essouhmi, A., Marrony, M., Lalanne, C., Fourcade, S., Jones, D., Grenier, J.-C., Rozière, J. Intermediate temperature anode-supported fuel cell based on BaCe_0.9Y_0.1O₃ electrolyte with novel Pr₂NiO₄ cathode. Fuel Cells. 2010; 10(1):166–173.

Takeda, Y., Kanno, R., Noda, M., Tomida, Y., Yamamoto, O. Cathodic polarization phenomena of perovskite oxide electrodes with stabilized zirconia. J Electrochem Soc. 1987; 134(11):2656–2661.

Takeda, Y., Ueno, H., Imanishi, N., Yamamoto, O., Sammes, N., Phillipps, M.B. Gd_{1- x}Sr_xCoO₃ for the electrode of solid oxide fuel cells. Solid State Ionics. 1996; 86–88(Part 2):1187–1190.

Tao, S., Wu, Q., Peng, D., Meng, G. Electrode materials for intermediate temperature proton-conducting fuel cells. J Appl Electrochem. 2000; 30(2):153–157.

Tarancon, A., Burriel, M., Santiso, J., Skinner, S., Kilner, J. Advances in layered oxide cathodes for intermediate temperature solid oxide fuel cells. J Mater Chem. 2010; 20(19):3799–3813.

Tarancon, A., Pena-Martinez, J., Marrero-Lopez, D., Morata, A., Ruiz-Morales, J., Nunez, P. Stability, chemical compatibility and electrochemical performance of GdBaCo₂O_5+x layered perovskite as a cathode for intermediate temperature solid oxide fuel cells. Solid State Ionics. 2008; 179(40):2372–2378.

Tarancon, A., Skinner, S., Chater, R., Hernandez-Ramirez, F., Kilner, J. Layered perovskites as promising cathodes for intermediate temperature solid oxide fuel cells. J Mater Chem. 2007; 17(30):3175–3181.

Taskin, A., Ando, Y. Electron-hole asymmetry in GdBaCo₂O_5+x: Evidence for spin blockade of electron transport in a correlated electron system. Phys Rev Lett. 2005; 95(17):1–4.

Taskin, A., Lavrov, A., Ando, Y. Achieving fast oxygen diffusion in perovskites by cation ordering. Appl Phys Lett. 2005; 86(9):1–3.

Taskin, A., Lavrov, A., Ando, Y. Fast oxygen diffusion in A-site ordered perovskites. Progr Solid State Chem. 2007; 35(2–4 SPEC. ISS.):481–490.

Tietz, F., Solid oxide fuel cells. Encyclopedia of Materials: Science and Technology. 2nd ed, 2008:1–8.

Tietz, F., Buchkremer, H.P., Stöver, D. Components manufacturing for solid oxide fuel cells. Solid State Ionics. 2002; 152–153:373–381.

Tietz, F., Haanappel, V., Mai, A., Mertens, J., Stöver, D. Performance of LSCF cathodes in cell tests. J Power Sources. 2006; 156(1 SPEC. ISS.):20–22.

Vente, J., McIntosh, S., Haije, W., Bouwmeester, H. Properties and performance of Ba_xSr₁._xCo₀.₈Fe₀.₂O_3-δ materials for oxygen transport membranes. J Solid State Electrochem. 2006; 10(8):581–588.

Wang, W., Jiang, S.P. A mechanistic study on the activation process of (La, Sr)MnO3 electrodes of solid oxide fuel cells. Solid State Ionics. 2006; 177(15–16):1361–1369.

Wattiaux, A., Park, J.-C., Grenier, J.-C., Pouchard, M. Une nouvelle voie d’accès aux oxydes supraconducteurs: l’oxydation électrochimique de La₂CuO₄. C R Acad Sci Paris. 1990; 310(II):1047–1052.

Winter, C.-J. Hydrogen energy – abundant, efficient, clean: A debate over the energy-system-of-change. Int J Hydrogen Energ. 2009; 34(14, Suppl. 1):S1–S52.

Xia, C., Rauch, W., Chen, F., Liu, M. Sm₀.₅Sr₀.₅CoO₃ cathodes for low-temperature SOFCs. Solid State Ionics. 2002; 149(1–2):11–19.

Yamada, A., Saka, K., Uehara, M., Taminato, S., Kanno, R., Mauvy, F., Grenier, C. Anisotropic catalytic activity of the orientation controlled Nd2NiO_4 + δ/ YSZ hetero-epitaxial system for SOFC cathode. Electrochem Commun. 2010; 12(12):1690–1693.

Yan, A., Cheng, M., Dong, Y., Yang, W., Maragou, V., Song, S., Tsiakaras, P. Investigation of a Ba₀.₅Sr₀.₅Co₀.₈Fe₀.₂O_3-δ based cathode IT-SOFC. I.The effect of CO₂ on the cell performance. Appl Catal B Environ. 2006; 66(1–2):64–71.

Yang, C.-C., Wei, W.-C., Roosen, A. Electrical conductivity and microstructures of La₀.₆₅Sr₀.₃MnO₃-8mol yttria-stabilized zirconia. Mater Chem Phys. 2003; 81(1):134–142.

Yashima, M., Enoki, M., Wakita, T., Ali, R., Matsushita, Y., Izumi, F., Ishihara, T. Structural disorder and diffusional pathway of oxide ions in a doped Pr₂NiO_4 + δ⁻based mixed conductor. J Am Chem Soc. 2008; 130(9):2762–2763.

Yashima, M., Sirikanda, N., Ishihara, T. Crystal structure, diffusion path, and oxygen permeability of a Pr₂NiO₄-based mixed conductor (Pr_0.9La_0.1)₂Ni_0.74 Cu₀.₂₁Ga₀.₀₅O_4 + δ. J Am Chem Soc. 2010; 132(7):2385–2392.

Yasuda, I., Hishinuma, M. Electrical conductivity and chemical diffusion coefficient of strontium-doped lanthanum manganites. J Solid State Chem. 1996; 123(2):382–390.

Yasuda, I., Ogasawara, K., Hishinuma, M., Kawada, T., Dokiya, M. Oxygen tracer diffusion coefficient of (La, Sr)MnO_{3 ± δ}. Solid State Ionics. 1996; 86–88(Part 2):1197–1201.

Zhao, H., Mauvy, F., Lalanne, C., Bassat, J.-M., Fourcade, S., Grenier, J.-C. New cathode materials for ITSOFC: Phase stability, oxygen exchange and cathode properties of La_2-xNiO_4 + δ. Solid State Ionics. 2008; 179(35–36):2000–2005.

Zhou, Q., He, T., Ji, Y. SmBaCo₂O_5+x double-perovskite structure cathode material for intermediate-temperature solid-oxide fuel cells. J Power Sources. 2008; 185(2):754–758.