The reduction of operating temperature of solid oxide fuel cells (SOFCs) down to the intermediate temperature range (600–800 ) is one of the main challenges for materials scientists today, which facilitates the development of new materials with improved performance. The LaNiO -related compounds have attracted considerable attention as an alternative to cobaltite due to their high electronic conductivity . A relatively low thermodynamic stability of LaNiO can be improved by a partial substitution of Ni with such metals as Fe, Mn, or Ti [1,2]. The studies of the LaMn B O perovskites (B = Co, Ni, Mg, Li) revealed that the catalytic activity can be enhanced by the substitution of Ni . Moreover, the synergetic effect due to coexistence of two 3d-metals at the B-site in perovskite structure was observed when their concentrations were approximately equal . This can be illustrated by a significant increase in catalytic activity in the LaMn Cu O , La Sr Mn Co O , and La Sr Co Fe O systems .
Recently, La NiTiO and Pr NiMnO were proposed as cathode materials for intermediate-temperature SOFCs (IT-SOFCs) [4,5]. These compounds possessed perovskite-type monoclinic (sp.gr. P2 /n) crystal structure showing oxygen deficiency at room temperature. Both oxides exhibited relatively low values of total conductivity yielding 0.002 S/cm for La NiTiO and 3 S/cm for Pr NiMnO at 800 C in air. The activation energy of polarization resistance in the oxides was equal to 1.33 eV suggesting that the rate-limiting step of electrode processes was the charge transfer . As a result, Pr NiMnO showed a better electrochemical performance with area-specific resistance (ASR) equal to 0.38 Ω cm at 700 C against of 0.5 Ω cm at 800 C for La NiTiO . The obtained ASR values were slightly better than those for the state-of-the-art LSM-based cathodes .
In this work, NdNi Mn O was studied as a potential cathode material for IT-SOFCs: crystal structure and functional properties were determined in the temperature range of 25–1000 C in air in order to assess the feasibility for practical application.
NdNi Mn O was synthesized by a citrate-nitrate combustion technique described elsewhere . The obtained powder was calcined at 1100 C for 20 h, uniaxially pressed into pellets under a pressure of 20 bar and sintered at 1200 C for 15 h in air. The phase composition and crystal structure were analyzed by X-ray powder diffraction (XRPD) using XRD-7000 Maxima instrument (Shimadzu) with Cu-K radiation at RT. High-temperature XRPD (HT-XRPD) was performed in air within the 30–1000 C temperature range using HTK 1200N (Anton Paar) HT-chamber. The XRPD patterns were refined by the Rietveld method using FullProf software.
The temperature dependence of oxygen non-stoichiometry for NdNi Mn O was obtained by thermogravimetric analysis (TGA) using a Netzsch STA 409 PC instrument in the temperature interval of 25–1100 C. The absolute value of oxygen content at room temperature was calculated from the results of iodometric titration performed with Akvilon (АTP-02) as described previously . The total conductivity and Seebeck coefficient measurements were carried out simultaneously using the standard 4-probe DC technique with Pt wire leads. The data were collected in the temperature range of 25–1000 C on cooling with steps of 50 C in air.
The XRPD pattern of as-sintered NdNi Mn O refined by the Rietveld method is shown in Figure 1. The initial structural model was based on the data reported in  and included the main monoclinic phase (sp.gr. P2 /n) and an impurity of NiO (sp.gr. Fm3m). Figure 1 demonstrates that calculated curve satisfactorily describes the experimental data with R and R factors 5%. The amount of NiO in the sample was shown to be 1%. The structural and lattice parameters, together with the selected bond lengths are presented in Table 1. The refined crystal structure parameters are in good agreement with those reported earlier [4,5]. Previously, NdNi Mn O was studied by neutron powder diffraction (NPD) at 210 K in air . It crystallized in a monoclinic P2 /n structure containing two different octahedral positions (occupied by Ni and Mn) and was shown to be oxygen stoichiometric. Sánchez–Benítez et al. explained it by the charge disproportionation process Ni + Mn Ni + Mn and stated that from the structural point of view this compound could be considered as a double perovskite of composition Nd NiMnO . However, the degree of long-range ordering between Ni and Mn sites was equal to 88.4%. In this work, the obtained bond lengths (Table 1) also suggested that the majority of cation species existed in a form of Ni and Mn , although small amounts of Ni and Mn could be present. Taking into account these observations, one should conclude that subtle monoclinic distortions in NdNi Mn O occur due to different size of MO octahedra surrounding Ni and Mn, which are statistically located one after another (as a result of their equal concentrations) in three directions of the perovskite network.
The HT-XRPD profiles of NdNi Mn O (Fig. 2) indicated that the monoclinic phase was stable up to 1000 C in air with no structural transitions observed in the temperature range studied. The overlapping of peaks with temperature near 32.8 (inset in Figure 2) can be explained by an increase in unit cell parameters resulting in the shifting of peaks toward the smaller 2θ values. The refined lattice parameters as a function of temperature are presented in Figure 3. As can be seen from Figure 3, the unit cell expands predominantly in a and c directions. The Me–O bond lengths analysis revealed that strong distortion of the MnO octahedra might be responsible for the observed anisotropy. The thermal expansion coefficients (TECs) for unit cell parameters were determined from the slopes of the corresponding temperature dependencies as previously described in . The LTEC was estimated in approximation of non-textured polycrystalline material with randomly oriented crystallites [6,9]. The calculated TEC values are summarized in Table 2, together with TECs of Pr NiMnO  and La Ni Ti O . The LTEC value for NdNi Mn O correlates well with LTECs of Pr NiMnO and La Ni Ti O showing moderate thermal expansion compatible with that for well-known electrolyte materials .
|NdNi Mn O||T = 25–200 C||T = 200–1000 C|
|α 10 , К||25.2||29.98 0.5|
|α 10 , К||11.87||13.73 0.05|
|α 10 , К||2.05||4.58 0.17|
|α 10 , К||12.04 0.07||12.04 0.07|
|α 10 , К||8.65||10.12 0.19|
|Pr NiMnO ||T = 50–800 C|
|α 10 , К||10.6|
|La Ni Ti O ||T = 250–650 C||T = 750–950 C|
|α 10 , К||9.99 0.01||11.96 0.01|
The oxygen non-stoichiometry in NdNi Mn O was essentially temperature-independent in whole temperature range studied with average value of 0.15 0.02 at room temperature. Similar temperature dependence was also found in La Ni Ti O , although the oxygen deficit observed in NdNi Mn O was higher compared to that reported in , where NdNi Mn O was shown to be oxygen stoichiometric within standard deviations at 210 K in air by means of NPD. The increased oxygen deficiency in the obtained sample could be a result of cation non-stoichiometry at the A- and/or B-site, which was earlier observed for La NiTiO [4,10]. Indeed, such A-site deficiency can be assumed, since trace amounts of NiO were observed in the studied sample.
The total conductivity (σ) of NdNi Mn O showed semiconducting-like behavior in the whole temperature range studied (Fig. 4). The σ value of 7.3 S/cm was obtained at 800 C, which was slightly higher than 5.7 S/cm reported for Pr NiMnO at this temperature . The maximal value of 15 S/cm was reached at 1000 C. The ln(σT) = f(1/T) plot (inset in Figure 4) revealed that charge transfer was thermally activated suggesting a small polaron hopping mechanism. The activation energy increased with temperature from 0.34 eV (T 350 C) to 0.45 eV (T 350 C). Similar behavior was also observed for Pr NiMnO at 550 C . It could be explained by the gradual elongation of Ni–O–Mn bond lengths with temperature (for instance, the Ni–O1–Mn bond length increased from 3.83 Å at room temperature to 3.85 Å and 3.88 Å at 400 and 1000 C, respectively). The Seebeck coefficient (S) of NdNi Mn O (inset in Figure 4) possessed complex temperature dependence and adopted negative values at all studied temperatures showing that predominant charge carriers in the oxide were electrons localized on Ni cations forming Ni .
NdNi Mn O possessed monoclinic crystal structure (sp. gr. P2 /n) in the temperature range of 25–1000 C. This complex oxide showed moderate thermal expansion with LTEC value equal to 10.12 10 K (200–1000 C), which is compatible with such well-known electrolytes as (Y O ) (ZrO ) , Ce Sm O , and La Sr Ga Mg O . Moreover, the chemical expansion contribution is negligible due to temperature-independent oxygen non-stoichiometry. On the one hand, the latter fact is a notable advantage of this material compared with known cobalt-based cathodes. On the other hand, one could expect low oxygen-ion transport [10,11] in NdNi Mn O due to temperature-independent oxygen content.
NdNi Mn O showed semiconductor-like behavior in the whole temperature range studied with maximum value of conductivity 15 S/cm at 1000 C. The total conductivity in NdNi Mn O was slightly higher than that for Pr NiMnO , although it was still significantly lower compared with a preferable value ( 100 S/cm) for the IT-SOFCs cathodes . Bulk oxygen diffusion, surface oxygen exchange, and chemical compatibility of NdNi Mn O with well-known electrolytes should be studied in the future in order to make a conclusion on possibility of its application as a cathode material for IT-SOFCs.
The authors would like to acknowledge that the equipment of the Ural Center for Shared Use `Modern nanotechnology' SNSM UrFU was used in this research.