Experimental validation of high electrical conductivity in Ni-rich LaNi_1−xFe_xO₃ solid solutions (x ≤ 0.4) in high-temperature oxidizing atmospheres

Abstract

LaNi_1−xFe_xO₃ solid solutions are an interesting system exhibiting a composition-controlled metal–insulator transition and are also potential cathode materials for solid oxide fuel cells, but the composition dependence of their electrical conductivity is still an open question due to the difficulty in synthesis and sintering. Here, in contrast to previous studies, it is demonstrated that the electrical conductivity of LaNi_1−xFe_xO₃ monotonically increases with decreasing x (increasing Ni content), reaching as high as 1.0 × 10⁴ S cm⁻¹ at room temperature and 2.5 × 10³ S cm⁻¹ at 800 °C in 0.2 bar O₂ when x = 0. The accurate electrical conductivity measurements of LaNi_1−xFe_xO₃ with high Ni contents (0 ≤ x ≤ 0.4) are realized using fully dense single-phase polycrystalline samples prepared by the post-sintering oxidation process. The results suggest that LaNi_1−xFe_xO₃ with a higher Ni content might be more suitable as the cathodes than the widely-studied composition LaNi_0.6Fe_0.4O₃. Furthermore, LaNiO₃ is now considered to have the highest electrical conductivity among precious-metal-free oxides in high-temperature oxidizing atmospheres and can find more applications.

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LaNiO₃, LaFeO₃, and their solid solutions LaNi_1−xFe_xO₃ (LNF, 0 ≤ x ≤ 1) have been extensively studied in the field of solid state chemistry/physics for over 50 years owing to their interesting electrical properties.^1–21 LaNiO₃ is a metallic oxide with high electrical conductivity, which is rather unusual for undoped 3d-transition-metal-based perovskite oxides.^6,10 LaNiO₃ contains a low-spin configuration of Ni³⁺ (t_2g⁶e_g¹).^1,7,8,11 Due to the strong covalent bonding between Ni³⁺ and O²⁻, the e_g orbitals are transformed into one-quarter filled itinerant σ* band states, and it results in the metallic behaviors. By contrast, LaFeO₃ is a charge-transfer insulator which contains a high-spin, localized electron configuration of Fe³⁺ (t_2g³e_g²).^1,11 Consequently the LaNi_1−xFe_xO₃ system exhibits a composition-controlled metal–insulator (m–i) transition at x = ∼0.3.^{2,3,5,12–14,22,23} Although no existing theory is sufficient to describe entirely the evolution of the electronic structure across the transition, a number of experimental and theoretical studies suggest that the metal–insulator transition is driven by increasing disorder effects arising from the random substitution for Ni with Fe in a metallic system with long–range Coulomb interactions.^12,16

In recent decades, LaNi_1−xFe_xO₃ (in particular x = 0.4) has also been recognized as a promising cathode material for solid oxide fuel cells (SOFCs)^22–42 because of its high electrical conductivity,^{22–26,28,32,34,37} good catalytic activity for oxygen reduction reaction,^24,27,29,40 high durability against chromium poisoning,³⁰ and thermal expansion coefficient (TEC) close to those of typical electrolyte materials.^{24–26,28,31–33,37}

Here, the structural and physical properties of LaNi_1−xFe_xO₃ is briefly reviewed, which vary rather monotonically with iron content x. At room temperature, the solid solutions with 0 ≤ x < 0.5 are isostructural with LaNiO₃ and have a rhombohedral perovskite structure, while those with composition 0.5 < x ≤ 1 are isostructural with LaFeO₃ and have an orthorhombic perovskite structure.^{2,14,15,20,22,24,31,32,43,44} At 800 °C, the stable region of the rhombohedral phase extends up to x = ∼0.9.³¹ The molar volume of LaNi_1−xFe_xO₃ increases with x due to the larger ionic radius of Fe³⁺ compared with that of Ni³⁺.^{14,15,22,24,31,32,33,38,43} The thermal expansion coefficient is normally reported to be in the range of 8–14 × 10⁻⁶ K⁻¹, close to those of typical electrolyte materials yttrium-stabilized zirconia (YSZ) and gadolinium-doped ceria (GDC), and it decreases with x.^{24–26,28,31–33,37} LaNiO₃ decompose in air at around 1000 °C or higher to form phases with oxidation states of Ni below +3 according to the following equation:⁴⁵

4LaNiO₃ (s) → La₄Ni₃O₁₀ (s) + NiO (s) + 1/2O₂ (g) (1)

This makes synthesis and sintering of LaNiO₃ difficult. The decomposition temperature of LaNi_1−xFe_xO₃ becomes higher with x due to the higher stability of Fe³⁺ compared with Ni³⁺, and thus LaNi_0.6Fe_0.4O₃ (x = 0.4) is stable up to around 1200 °C in air.^{15,22,38,43,44}

Despite many works to characterize LaNi_1−xFe_xO₃, the composition dependence of the electrical conductivity of LaNi_1−xFe_xO₃ is still an open question. Some studies have shown a monotonic increase of the electrical conductivity with decreasing x (increasing Ni content),^3,12,13,28 while other recent studies have indicated a conductivity maximum at x = 0.4.^19,22,24,32 The overpotential of LaNi_1−xFe_xO₃ cathodes on scandia-stabilized zirconia electrolytes reached a minimum also at x = 0.4, which suggested a correlation between the electrical conductivity and the cathode performance.²⁴ Based on this, most of the studies aiming at the development of LaNi_1−xFe_xO₃ cathodes for SOFCs have focused on LaNi_1−xFe_xO₃ with x = 0.4.^{26,27,29,33,34,36,37,40,41} However, it is worth mentioning that the decrease in the electrical conductivity while entering into the metallic region (x < ∼0.3) is rather unusual. In this context, the origin of the apparent conductivity maximum has been suggested to be lower sintering density²² or decomposition^24,32 of LaNi_1−xFe_xO₃ samples with x < 0.4 due to their poor thermal stability. Therefore, it is quite important to re-evaluate accurately the electrical conductivity of LaNi_1−xFe_xO₃ with x < 0.4 to validate the picture of the electronic conduction in the LaNi_1−xFe_xO₃ system and to find the optimal composition as cathodes.

In this study, we have prepared dense polycrystalline samples of single-phase LaNi_1−xFe_xO₃ with high Ni contents (0 ≤ x ≤ 0.4) by virtue of the post-sintering oxidation process.⁴⁶ In this process, pre-sintered dense tablets of mixed oxides were converted to single-phase LaNi_1−xFe_xO₃ under high oxygen partial pressures. Then their accurate electrical conductivity has been evaluated to reveal the composition dependence.

Dense samples of LaNi_1−xFe_xO₃ with x = 0.4 and 0.36 were readily prepared by conventional solid-state reaction of La₂O₃, NiO, and Fe₂O₃ and sintering under ordinary pressure, while that with x = 0.2 could not be obtained in the same way. Fig. 1(a) shows powder X-ray diffraction patterns and SEM images of LaNi_1−xFe_xO₃ tablets with compositions x = 0.4, 0.36, and 0.2 after sintered at 1300 °C in air, oxygen gas, and air, respectively. The sintered tablets with x = 0.4 and 0.36 were almost single-phase perovskite, while the tablet with x = 0.2 contained impurity phases La₄Ni₃O₁₀ and NiO in addition to the perovskite phase.§ The upper temperature limit of the stability of the perovskite phase LaNi_1−xFe_xO₃ has been reported to become lower as x or oxygen partial pressure (p_O2) decreases.^38,44 The present results follow this trend and reconfirm the difficulty in sintering LaNi_1−xFe_xO₃ with high Ni contents (x ≤ 0.2) under ordinary pressure.

Fig. 1 Powder X-ray diffraction patterns and SEM images of LaNi_1−xFe_xO₃ tablets (a) after sintered at 1300 °C in air or oxygen gas and (b) after HIP treatments at 1250 °C under p_O2 = 392 bar. The SEM images are those of fracture surfaces of the tablets. Their relative densities measured by the Archimedes method are indicated on the SEM images. The data of LaNi_1−xFe_xO₃ with x = 0 are from our previous work.⁴⁶ Part of the figure was presented at conferences.^47,48

The post-sintering oxidation realized dense samples of LaNi_1−xFe_xO₃ with x = 0.2 and 0. The above mixed-phase tablet with x = 0.2 was subsequently oxidized at 1250 °C under p_O2 = 392 bar using hot isostatic pressing (HIP). As evident from its powder X-ray diffraction pattern in Fig. 1(b), the sample with x = 0.2 was converted to almost single-phase perovskite owing to this high-pressure oxidation treatment. A dense sample of single-phase LaNiO₃ (x = 0) was also obtained similarly by oxidizing a pre-sintered tablet composed of fine La₂NiO₄ and NiO grains as reported in our previous paper.⁴⁶ Its XRD pattern in Fig. 1(b) confirms the almost single-phase nature. SEM images of the fracture surfaces of the samples with x = 0, 0.2, 0.36, and 0.4, and their relative densities measured by the Archimedes method are also shown in Fig. 1(a) and (b). There are few pores in their fracture surfaces after the final heat treatments, and their relative densities finally exceeded 98%.

Electrical conductivity measurements using the dense LaNiO₃ sample revealed that the electrical conductivity of LaNiO₃ has been much underestimated in the literature due to insufficient sintering. The electrical conductivity measurements of LaNi_1−xFe_xO₃ were carried out by the four-probe technique using bar-like tablets with silver electrodes (illustrated in Fig. 2) in air or a mixture of O₂ and Ar gases. Fig. 3 shows the electrical conductivity values of LaNiO₃ either obtained in this study using the dense sample or reported in the literature.^{5,12,14,18,28,49,50,51,52,53,54} While the temperature dependences (or the slopes) of the present and the reported data are consistent, significant discrepancies are seen in the electrical conductivity values. The present data and those obtained from a single-crystal LaNiO₃⁵³ are almost one order of magnitude higher than the other literature values. In general, the apparent electrical conductivity of a pressed/sintered material is heavily dependent on its relative density:^32,55 The apparent electrical conductivity varies from the inherent value of the material to almost an order of magnitude lower values as its relative density changes from 100% to 50%. The LaNiO₃ samples used in the cited studies in Fig. 3 except the single crystal were only pressed or sintered at around 1000 °C or lower^{5,12,14,18,28,49,50,51,52,54} due to the poor thermal stability of LaNiO₃ under an ordinary partial pressure of oxygen (1 bar O₂ or less). Based on previous studies^12,49,51,56 and our recent study on the La–Ni–O system,⁴⁶ it is thought to be difficult to obtain fully dense samples (relative density over 95%) under such conditions. Therefore, the literature values of the electrical conductivity of LaNiO₃ are likely to be much underestimated due to the lower relative densities. It is worth mentioning that this large discrepancy cannot be attributed to the difference in the atmosphere where the electrical conductivity measurements were carried out, as the variation of the electrical conductivity of LaNiO₃ is less than 12% in the p_O2 range of 10⁻³–1 bar at 800 °C (see Fig. S1, ESI†).

Fig. 2 (a) Schematic of LaNi_1−xFe_xO₃ samples used for electrical conductivity measurements. (b) Photograph of a LaNi_0.6Fe_0.4O₃ sample used for electrical conductivity measurements.

Fig. 3 Temperature dependence of the electrical conductivity (σ) of LaNiO₃ measured using a dense sample in this study or published in the literature.^{5,12,14,18,28,49–54} Atmospheres where measurements were carried out are indicated in the figure. Preparation conditions and relative densities of the specimen are summarized in the legend. The sintering condition of Świerczek et al.²⁸ was given in their paper as “1000–1200 °C range, depending on the sample chemical composition”. As LaNiO₃ is considered to be thermally most unstable composition in their paper, we guess the sintering temperature of LaNiO₃ was approximately 1000 °C.

The composition dependence of the electrical conductivity of LaNi_1−xFe_xO₃ is proved to be monotonic. Fig. 4(a) and (b) shows the iron content dependence of the electrical conductivity of LaNi_1−xFe_xO₃ at room temperature and 800 °C in 0.2 bar O₂. The electrical conductivity values at x = 0.4 obtained in this study agree reasonably with literature values. At x < 0.4, the electrical conductivity increases with decreasing x (increasing Ni content), reaching 1.0 × 10⁴ S cm⁻¹ and 2.5 × 10³ S cm⁻¹ at x = 0 at room temperature and 800 °C, respectively, while the literature data do not show a clear trend.¶,|| This discrepancy can be due to the underestimation of the conductivity of Ni-rich samples in the past studies due to the lower relative densities as discussed in the previous paragraph or the existence of secondary phases.^24,32 The increasing trend of the electrical conductivity with Ni content could be attributed to the previously proposed picture that the metal–insulator transition is driven by disorder effects arising from the substitution for Ni in metallic LaNiO₃ with Fe. However, further experimental and theoretical studies using appropriately synthesized samples are desirable to elucidate the nature of M–I transition in this system. Referring also to literature data on Ni-poor compositions, we can see that the electrical conductivity of LaNi_1−xFe_xO₃ increases monotonically with decreasing x in the whole composition range.

Fig. 4 (a) Iron content dependence of the electrical conductivity of LaNi_1−xFe_xO₃ at 25 ± 5 °C. The literature values by Chainani et al.,¹² Jung and Iguchi,¹³ Kharton et al.,²⁵ Basu et al.,²⁶ Zhou et al.,¹⁹ Iwasaki et al.,²⁰ and Zhang et al.⁵³ are also shown. The figure shows only the electrical conductivities of the samples whose relative densities were over 80% or that of a single crystal.⁵³ (b) Iron content dependence of the electrical conductivity of LaNi_1−xFe_xO₃ at 800 °C. The literature values by Chiba et al.,³² Kharton et al.,²⁵ Wærnhus et al.,⁵⁷ Zhou et al.,¹⁹ Iwasaki et al.,²⁰ Berenov et al.,⁵⁸ Kiselev and Cherepanov,⁵⁹ Niwa et al.,²³ and Sereda et al.³⁷ are also shown. The figure shows only the electrical conductivities of the samples whose relative densities were over 80%. The inset shows a zoom around x = 0.4. Part of the figure was presented at a conference.⁴⁷

In Fig. 5(a), the electrical conductivity of LaNi_1−xFe_xO₃ is compared with those of simple perovskite or perovskite-related (layered-perovskite) oxides based on 3d transition metals Cr, Mn, Fe, Co, Ni, and/or Cu. These oxides are stable in air and at high temperatures (∼500–800 °C), and have potential applications as the cathodes and interconnects of SOFCs. The electrical conductivity of LaNiO₃ (LaNi_1−xFe_xO₃ with x = 0) is found to be the highest among the above oxides including popular cathode materials such as La_0.6Sr_0.4Co_0.2Fe_0.8O_3−δ (LSCF), Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ (BSCF), La_0.5Sr_0.5MnO_3−δ (LSM), and La_0.6Sr_0.4CoO_3−δ (LSC). More broadly, Fig. 5(b) compares the electrical conductivities of LaNi_1−xFe_xO₃ and other representative electrically conducting oxides out of the above category, based on 3d, 4d, or 5d transition metals. ReO₃ exhibits 1–2 orders of magnitude higher conductivity than that of LaNiO₃. Also, RuO₂ with rutile structure, TiO_x (x = ∼1) with NaCl structure, Ti₂O₃ with corundum structure, and Na_xWO₃, La_1−xSr_xVO₃, SrMoO₃, and SrRuO₃ with perovskite structure exhibit equal or higher conductivity than LaNiO₃. However, many of them (ReO₃,⁶⁰ NaCl-type TiO_x,⁶¹ Ti₂O₃,⁶¹ Na_xWO₃,^62,63 La_1−xSr_xVO₃,^64,65 and SrMoO₃⁶⁶) are unstable in high-temperature air and the rest of them (SrRuO₃ and RuO₂) are based on precious metal, both can be drawbacks for practical use. Therefore LaNiO₃ is now considered as the precious-metal-free oxide with the highest electrical conductivity in high-temperature air.

Fig. 5 Comparison of the electrical conductivities of LaNi_1−xFe_xO₃ (x = 0, 0.2, 0.4) with those of representative electrically conducting oxides at temperatures between room temperature and 800 °C. Measurement conditions are summarized in the legend. (a) Comparison with simple perovskite or perovskite-related oxides based on 3d transition metals Cr, Mn, Fe, Co, Ni, and/or Cu: La_0.7Ca_0.3CrO_3−δ,⁶⁷ La_0.5Sr_0.5MnO_3+δ,⁶⁸ LaNi_0.6Fe_0.4O₃,³² La_0.6Sr_0.4Co_0.2Fe_0.8O_3−δ,⁶⁹ Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ,⁶⁹ LaCoO₃,⁷⁰ La_0.6Sr_0.4CoO_3−δ,⁷⁰ La_0.7Sr_0.3CuO_2.5−δ,⁷¹ NdSrCo₂O_5+δ,⁷² LaSrCoO₄,⁷³ La₄Co₃O₁₀,⁷⁰ La_1.4Sr_0.6NiO₄,⁷³ La₃Ni₂O₇,⁷⁴ La₄Ni₃O₁₀,⁷⁴ La_1.9Sr_0.1CuO₄,⁷⁵ and YBa₂Cu₃O_7−δ.⁷⁶ (b) Comparison with other representative electrically conducting oxides out of the above category, based on 3d, 4d, or 5d transition metals: TiO_1.22,⁷⁷ Ti₂O₃,⁷⁸ V₂O₃,⁷⁹ VO₂,⁸⁰ RuO₂,⁸¹ Fe₃O₄,⁸² La_0.4Sr_0.6TiO₃,⁸³ SrVO₃,⁸⁴ La_0.6Sr_0.4VO₃,⁸⁵ SrMoO₃,⁸⁶ SrRuO₃,⁸⁷ Na_0.851WO₃,⁸⁸ and ReO₃.⁸⁹ Part of the figure was presented at a conference.⁴⁸

Conclusions

In conclusion, the electrical conductivity of Ni-rich LaNi_1−xFe_xO₃ (0 ≤ x ≤ 0.4) is found to monotonically increase with decreasing x (increasing Ni content) at both room temperature and 800 °C by use of the fully dense single-phase polycrystalline samples prepared by the post-sintering oxidation process. The confirmed trend suggests that LaNi_1−xFe_xO₃ with smaller x (higher Ni content) might be more suitable for the cathodes of SOFCs than the widely-studied composition LaNi_0.6Fe_0.4O₃ (x = 0.4). It should be noted, however, that a higher conductivity does not necessarily lead to a better electrode performance. Therefore, it is desirable to investigate the electrochemical properties of Ni-rich LaNi_1−xFe_xO₃ (0 ≤ x ≤ 0.4). One concern with Ni-rich LaNi_1−xFe_xO₃ is the thermal instability during operation at high temperatures,³⁵ but it may be less problematic for the applications in low-temperature SOFCs, which are under active development. In addition, Ni-rich LaNi_1−xFe_xO₃ can be more readily applicable as oxygen electrodes in solid oxide electrolyzer cells (SOECs) because the electrodes are operated in highly oxidative conditions, where LaNi_1−xFe_xO₃ is more stable.^38,46 (see Fig. S2, ESI†) Furthermore, in a broader context, LaNiO₃ is now considered as the precious-metal-free oxide with the highest electrical conductivity in high-temperature air (2.5 × 10³ S cm⁻¹ at 800 °C in 0.2 bar O₂) and can find other applications as an oxidation-resistant, high-temperature electrical conductor.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This study was supported by Grant-in-Aid for JSPS Research Fellow Grant Number 16J09427. We are grateful to Shin-Etsu Chemical Co., Ltd for providing lanthanum oxide.

Notes and references

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Footnotes

† Electronic supplementary information (ESI) available: Experimental details, oxygen partial pressure dependence of the electrical conductivity of LaNi_1−xFe_xO₃, thermodynamic stability of lanthanum nickelates. See DOI: 10.1039/d0ma00983k

‡ Deceased.

§ Fe may partially occupy the Ni-sites in La₄Ni₃O₁₀ and NiO.⁴⁴

¶ Based on XRD patterns (Fig. 1), the amount of secondary phases (NiO and an unknown phase) is estimated to be ∼1 wt% or less. Therefore the secondary phases would not constitute a long-range conduction network and the high electrical conductivity observed in this study can be attributed to the main phase, LaNi_1−xFe_xO₃. Also, the fact that the conductivity value of the LaNiO₃ polycrystal obtained in this study are almost identical to that of a single crystal at room temperature (Fig. 3(a)) suggests that the influence of secondary phases is not significant.

|| In LaNi_1−xFe_xO₃ (0 ≤ x ≤ 0.4) solid solutions, electronic conduction should dominate over ionic conduction. Although they contain oxygen vacancies^9,35,54 and can conduct oxide ions, the oxide ion conductivity at 800 °C is assumed to be less than 1 S cm⁻¹ as in the case of other best oxide-ion-conducting perovskite oxides (LSC, LSGM, etc.).^90.

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