La2NiO4+ δ with the K2NiF4-type structure is a mixed conductor with large oxygen excess. It is considered as a promising cathode material for solid oxide fuel cells (SOFC) due to a number of advantages such as relatively fast oxygen-ion transport via interstitial oxygen ions, moderate thermal expansion, and high electrocatalytic activity. Nevertheless, it has a few shortcomings such as relatively low total conductivity and chemical incompatibility with various well-known electrolytes [1-3].
The effective way to modify properties in order to improve the cathode characteristics of La2NiO4 is chemical doping. An acceptor-type strontium doping of La2NiO4 was extensively studied earlier [4-6]. The addition of strontium increases total conductivity of the oxides, although it significantly diminishes oxygen content . The thermal expansion coefficient (TEC) of La2−x Sr x NiO4 in the 0<x<0.6 range is almost constant and equals to 12-12.5×10−6 K−1 at 800∘C . Donor-type doping of La2NiO4 and La2−x Sr x NiO4 by iron exhibits an opposite effect: gradual decrease of electric conductivity along with the rise of interstitial oxygen concentration . The TEC values slightly increased with addition of iron .
Chemical compatibility of La2NiO4 with electrolytes such as YSZ, CGO, and LSGM was previously studied [2, 3]. Chemical reactions were observed in case of both YSZ and CGO electrolytes . It was also shown that La2NiO4 was not chemically compatible with LSGM electrolyte neither at fabrication conditions nor at operation ones .
Thus, the aims of this work are to modify properties of La2NiO4 by simultaneous doping with strontium and iron and to estimate the performance of the La1.5Sr0.5Ni1-yFeyO4 (y=0.1-0.4) cathodes applied to the LSGM electrolyte-based SOFC.
The series of the La1 .5Sr0.5Ni1-yFeyO4+δ complex oxides (y=0.1-0.4) was synthesized via citric-nitric technique as described elsewhere . The phase purity of the samples was confirmed by the XRPD analysis using Equinox 3000 (FWHM ∼ 0.05∘ at 2θ) instrument with Cu-K α radiation at room temperature (RT). Observed XRPD data were refined by the Le-Bail technique (profile-matching mode) using FullProf software. The absolute values of oxygen non-stoichiometry were obtained in the thermogravimetric analysis (TGA) setup (STA 409PC Netzsch GmbH) by reducing the samples at 1200∘C in H2/N2 gas mixture with flow rate 100 ml/min. Thermal expansion of La1.5Sr0.5Ni1− yFeyO4 +δ was investigated by high-temperature (HT) XRPD in air within 25-1100∘C temperature range using HTK 16N (Anton Paar) HT-chamber installed at Equinox 3000 diffractometer. Total conductivity of La1.5Sr0.5Ni1-yFeyO4+δ was measured by the 4-probe DC technique within 25-1100∘C temperature range in air. Chemical compatibility of the oxides with electrolyte materials La0.88Sr0.12Ga0.82Mg0.18O3−δ (LSGM) and Ce0.8Sm0.2O1.9 (SDC) was studied by mixing the cathode and electrolyte powders in a 1:1 weight ratio, following by calcination at 1250∘C for 1 h and XRPD examination.
The powder of La0.88Sr0.12Ga0.82Mg0.18O3−δ electrolyte was uniaxially pressed into the pellet of 0.7 mm thickness and sintered at 1400∘C in air for 20 h. In order to avoid possible chemical interaction between the La1.5Sr0.5Ni1-yFeyO4+δ cathode and the LSGM electrolyte the latter was preliminary coated by a layer of Ce0.8Sm0.2O1.9 buffer and sintered at 1300∘C in air for 1 h. Sr2Ni0.75Mg0.25MoO6−δ (SNMM), as well as NiO-SDC cermet was used as an anode.
The cathode materials were preliminary sonicated in ethanol media for ten minutes to reduce particle size and, thus, intensify a sintering process. The cathode and the anode materials were painted on the opposite sides of the electrolyte pellet and sintered at 1250∘C in air for 1 h. In order to separate the anode side, a single fuel cell sandwich with attached Pt-wire current collectors was placed on an YSZ tube and sealed using special high-temperature glass. The fuel H2 gas was fed inside of the tube with the rate of 250 ml/min. The current-voltage (I–U) characteristics of the cells were measured at 700, 750, 800, and 850∘C using a resistance box and the Agilent 34401A multimeter.
XRPD confirmed that all La1.5Sr0.5Ni1− yFeyO4+δ were single phase (no evidence of impurities or starting materials was detected) possessing tetragonal structure (space group I4/mmm). The refined lattice parameters are listed in Table 1.
XRPD pattern of the La1.5Sr0.5Ni0.9Fe0.1O4+δ -SDC mixture after calcination at 1250∘C for 1 h exhibits no impurity peaks, indicating that there was no reaction between the cathode and SDC (Fig. 1). On the contrary, XRPD profile of the La1.5Sr0.5Ni0.9Fe0.1O4+δ -LSGM mixture reveals appearance of the lanthanum enriched products related to the La3Ni2O7 and La4Ni3O10 structures (Fig. 1). Because of this interaction, SDC was used as a buffer layer between the cathode and the electrolyte.
Figure 2 illustrates temperature dependencies of total conductivity represented in the form of the ln(σ T) = f(1/T) plots. Total conductivity of the studied oxides shows semiconductor behavior within the whole temperature range studied. The linear lnσ T = f(1/T) dependencies indicate that conductivity in La1.5Sr0.5Ni1− y Fe y O4+δ is thermally activated. The activation energy values lay within the 9.7-13 kJ mol−1 range, which is characteristic of a small-polaron mechanism . The total conductivity significantly decreases with iron doping, which can be attributed to the hole trapping by iron cations forming stable Fe3+ states .
The TEC values obtained from the HT XRPD results were calculated in approximation of non-textured polycrystalline materials with randomly oriented crystallites  according to the formula presented elsewhere . The intermediate temperature range TEC values calculated from HT XRPD results for La1.5Sr0.5Ni1− yFeyO4 +δ are listed in Table 1. One can observe that the TEC values firstly increase with iron doping and then slightly decline at higher iron concentrations. The former phenomenon can be explained by the size factor. Iron substitution leads to an expansion of the unit cell due to larger radius of Fe3+ (r=0.645 Å) ions comparing to Ni3+ (r=0.56 Å) . As a result, the unit cell volume increases with y, which is followed by a growth of the TEC values. The decrease of the TEC values in case of compositions with y=0.3, 0.4 is due to higher interstitial oxygen concentrations: the release of interstitial oxygen from the structure at elevated temperature suppresses thermal expansion of the material. The TGA results show that all Fe-substituted La1.5Sr0.5Ni1− yFeyO4+δ accommodate oxygen excess in comparison to La1.5Sr0.5NiO4+δ  (Table 1). The significant increase of oxygen content with y can be attributed to the donor-type nature of the dopant: introduction of iron (Fe3+) into the nickel (Ni2+) sublattice increases the trend to incorporate negatively charged interstitial oxygen into the crystal structure in order to preserve electroneutrality. The electrochemical performances of single fuel cells are shown in Figure 3. The open circuit voltage (OCV) decrrature for all fuel cells. The experimental OCVs were close to the theoretical values for all cells indicating a good densification of the LSGM electrolyte, no gas leakage, and absence of electronic conduction across the electrolyte.
Figure 3 shows typical linear dependencies of the cell voltage vs. current density for the tested cells. One can observe that the output power density for each cell rises with temperature. The maximum values of power density at 850∘C in the case of SNMM anode have reached the values: 218, 274, 222, and 390 mW/cm2 for La1.5Sr0.5Ni0.9Fe0.1O4+δ (LSFN1), La1.5Sr0.5Ni0.8Fe0.2O4+δ (LSFN2), La1.5Sr0.5Ni0.7Fe0.3O4+δ (LSFN3), and La1.5Sr0.5Ni0.6Fe0.4O4+δ (LSFN4) cathodes, respectively. The maximum power density at 850∘C in the case of NiO-SDC anode was equal to 341 mW/cm2 for the La1.5Sr0.5Ni0.6Fe0.4O4+δ (LSFN4) cathode. It can be concluded that despite the significant decrease of total conductivity, the performance of the La1.5Sr0.5Ni1-yFeyO4+δ cathodes has been improved with the iron doping. This could be attributed to an increase of interstitial oxygen concentration with y, which, in its turn, benefits the oxygen-ion transport in the oxides . The performance of the La1.5Sr0.5Ni0.6Fe0.4O4+δ /SDC/LSGM/SDC/NiO-SDC cell is slightly improved in comparison with reported earlier results on the La2NiO4/LSGM/SDC /Ni-SDC system (319 mW/cm2 at 850∘C) . However, it is important to notice that thickness of the electrolyte in Ref.  was 0.4 mm, which is almost twice thinner than that used in our study (0.7 mm). The reducing of the electrolyte thickness could significantly increase the power densities of SOFC with the La1.5Sr0.5Ni0.6Fe0.4O4+δ cathode.
The La1.5Sr0.5Ni1-yFeyO4 + δ complex oxides (y=0.1-0.4) have been investigated as cathode materials for potential application in SOFC with the LSGM electrolyte. The XRPD results showed chemical incompatibility of the oxides with LSGM. In order to avoid chemical reaction between the cathode and the electrolyte material, using of the Ce0.8Sm0.2O1.9 buffer layer was necessary. Total conductivity of La1.5Sr0.5Ni1-yFeyO4 + δ decreases with iron doping, but the obtained values are still adequate for that required for cathode materials at SOFC . The highest conductivity equal to 130 S/cm at 800∘C was achieved for La1.5Sr0.5Ni0.9Fe0.1O4 + δ . The thermal expansion coefficients are close to that for LSGM electrolyte (12×10−6 K−1). Although the TEC values increase with iron content, high thermal stability is still retained. The oxygen content was shown to increase with y. The LSGM electrolyte-based fuel cells with the La1.5Sr0.5Ni1-yFeyO4 + δ (y=0.1-0.4) cathodes have been assembled. The best performances of 390 mW/cm2 and 341 mW/cm2 at 850∘C were achieved for the La1.5Sr0.5Ni0.6Fe0.4O4+δ cathode in pair with Sr2Ni0.75Mg0.25MoO6 and NiO-SDC cermet anodes, respectively. Finally, it can be concluded that simultaneous strontium and iron doping can improve the performance of the La2NiO4-based cathode material. One would expect a significant increase of power densities in comparison with the prototype while using a thin-film electrolyte.