Investigating reduction and oxidation of
La0.72Sr0.18Fe0.9Ni0.1O3–δ as a Co-free perovskite
electrode for symmetrical solid oxide cells

Siavash Alizadeh, Pascal Roussel, Mohammad Golmohammad, Duncan Fagg, Oleg I Lebedev, Elise Berrier, Yury Kolen'ko

► To cite this version:

Siavash Alizadeh, Pascal Roussel, Mohammad Golmohammad, Duncan Fagg, Oleg I Lebedev, et al. Investigating reduction and oxidation of La0.72Sr0.18Fe0.9Ni0.1O3–δ as a Co-free perovskite electrode for symmetrical solid oxide cells. Journal of physics, 2025, 7 (3), pp.035010. 10.1088/2515-7655/add76a. hal-05109464

HAL Id: hal-05109464

https://hal.science/hal-05109464v1

Submitted on 2 Dec 2025

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# Investigating the Link between Reduction and Oxidation in Co-free La<sub>0.72</sub>Sr<sub>0.18</sub>Fe<sub>0.9</sub>Ni<sub>0.1</sub>O<sub>3-δ</sub> Perovskite Electrode for Symmetrical Solid Oxide Cells

Siavash M. Alizadeh<sup>a, b</sup>, Pascal Roussel<sup>c</sup>, Mohammad Golmohammad<sup>d</sup>, Duncan Fagg<sup>e</sup>, Oleg I. Lebedev<sup>f</sup>, Elise Berrier<sup>b</sup>, and Yury V. Kolen'ko<sup>a, *</sup>

a. International Iberian Nanotechnology Laboratory, Avenida Mestre José Veiga, 4715-330 Braga, Portugal

b. Université de Lille, CNRS, Centrale Lille, ENSCL, Université Artois – UCCS, Lille, France

c. Université de Lille, CNRS, Centrale Lille, Univ. Artois, UMR8181, UCCS-Unité de Catalyse et Chimie du Solide, Lille F-59000, France

d. Renewable Energy Research Department, Niroo Research Institute (NRI), Tehran, Iran

e. TEMA – Centre for Mechanical Technology and Automation, Department of Mechanical Engineering, University of Aveiro, 3810-193 Aveiro, Portugal

f. Laboratoire CRISMAT, UMR 6508, CNRS-ENSICAEN, Caen, 14050 France

## Abstract

Iron-based perovskites are of increasing interest for application in symmetrical reversible solid oxide cells used for hydrogen production via steam electrolysis and power generation in fuel-cell configuration. We have synthesized new La<sub>0.72</sub>Sr<sub>0.18</sub>Fe<sub>0.9</sub>Ni<sub>0.1</sub>O<sub>3-δ</sub> compound employing rapid non-equilibrium auto-combustion synthesis. Using powder X-ray diffraction and electron microscopy, we revealed that the material is ABO<sub>3</sub>-type perovskite with orthorhombic crystal structure. This complex compound can undergo exsolution of metallic phase upon the thermal treatment in hydrogen-containing reductive atmosphere, and the resultant exsolved FeNi nanoparticles possess cubic crystal structure with predominantly exposed [001] facets. The exsolution is assisted by structural transformation of parent orthorhombic ABO<sub>3</sub>-type perovskite into Ruddlesden-Popper A<sub>2</sub>BO<sub>4</sub>-type perovskite with tetragonal structure. Conveniently, the exsolution is tailored to be either partially or completely reversible depending upon the temperature of the subsequent thermal treatment in oxidative atmosphere, correspondingly affording either perovskite plus nickel ferrite composite material or going back to parent La<sub>0.72</sub>Sr<sub>0.18</sub>Fe<sub>0.9</sub>Ni<sub>0.1</sub>O<sub>3-δ</sub> perovskite. The newly synthesized La<sub>0.72</sub>Sr<sub>0.18</sub>Fe<sub>0.9</sub>Ni<sub>0.1</sub>O<sub>3-δ</sub> material was electrochemically investigated as air electrode in symmetrical solid oxide fuel cell, demonstrating good performance with total polarization resistances of 2.42 and 0.33 Ω.cm<sup>2</sup> at intermediate (650 °C) and conventional (800 °C) operating temperatures of the cell, respectively. Finally, we show that the polarization resistance at the intermediate temperature can be significantly reduced from 2.42 to 1.63 Ω.cm<sup>2</sup> through the observed formation of nickel ferrite.

**Keywords:** auto-combustion, exsolution, nanoparticles, microstructure, redox, polarization resistance

<footnote>* Corresponding email: yury.kolenko@inl.int</footnote>
# 1. Introduction

Reversible solid oxide cells (rSOCs) are electrochemical devices which can work either as a solid oxide fuel cell (SOFC) or as a solid oxide electrolysis cell (SOEC). They have been considered as promising solution for sustainable energy generation due to their high efficiency and fuel flexibility [1,2]. The electrode of rSOCs requires high electrochemical activity, mixed ionic-electronic conductivity, chemical stability under both reducing and oxidizing operational conditions, compatibility with other cell components, and coke resistance in case of using hydrocarbon fuels. In context of developing critical raw materials (CRMs) free electrodes for rSOCs, the materials should be also abundant, economically sustainable, and environmentally benign. The challenge is to provide the requirements while enhancing the performance of traditionally used electrode materials such as lanthanum strontium iron cobaltite (LSCF) or lanthanum strontium manganite (LSM) [3,4].

An innovative approach in rSOC technology is symmetrical cell configuration, where the same electrode material is used as anode and as cathode. Such simplified cell configuration can potentially facilitate the fabrication process, reduce manufacturing costs, and improve scalability. This also enhances durability, as the identical electrode materials respond uniformly to operational conditions, minimizing the risks associated with mechanical and chemical mismatches and failure. Notably, the ability to switch seamlessly between generating electricity (i.e., SOFC) and producing fuels (i.e., SOEC) requires an electrode material that performs effectively in both modes [5].

The intrinsic capability of perovskite $ABO_3$ structures for various doping strategies at both A-site and B-site, enables the fine-tuning of their properties, thus makes perovskites interesting for rSOC applications. At first, perovskites were considered mainly for air electrode application, due to their high catalytic activity for oxygen reduction (ORR) and evolution (OER) reactions [6,7]. Over the past years, examples have been reported in adopting perovskites for fuel electrode application as well, affording viable alternative to conventional Ni-based cermets [8].

Exsolution is an interesting technique for producing stable, well-dispersed, and well-socketed metallic nanoparticles (NPs) on the surface of a host oxide material. The resultant composites could be further employed for the preparation of high-performing nanostructured electrodes of rSOCs. In contrast to the traditional synthesis methods of the supported metal catalysts, such as infiltration and impregnation, where the metallic NPs are formed through an additional phase, exsolution relies on cations' nucleation and growth mechanism that are repelled from the parent structure because of their reduction [9,10]. Hence, exsolved NPs exhibit stronger metal-support interactions, enhanced stability, less prone to coalescence, and demonstrate uniform distribution over supporting material, which is advantageous for long-term operation in reducing environments [11,12]. Moreover, the exsolved NPs are accounted as preferred catalytically active sites leading to the enhanced electrode efficiency in rSOC [13]. Importantly, the exsolved NPs are potentially redox-reversible, making them particularly interesting for symmetrical rSOCs [14].

Fe-based perovskites have been extensively investigated as the electrode of rSOCs. Prior studies suggest that they are more stable than Co- and Mn-based perovskites due to the stable $3d_5$ electronic state of Fe [15]. Moreover, Fe-based perovskites have demonstrated good thermal expansion coefficient (TEC) compatibility and chemical stability with gadolinium doped ceria (GDC) solid electrolyte, typically used in rSOC [16]. Yunfeng and co-authors investigated the performance of the cobalt-free $La_{0.6}Sr_{0.4}Fe_{0.8}Ni_{0.2}O_{3-\delta}$ composition as the air electrode in rSOC. They reported a good peak power density of 961 mW/cm² at 800°C in fuel cell mode and a current density of 1.09 A/cm² at 1.3 V and 850°C under absolute humidity of 50 vol.% [17]. In another study, Ting and co-authors synthesized a novel $K_2NiF_4$-type structure of $Nd_{0.8}Sr_{1.2}Ni_{1-x}Fe_xO_{4+\delta}$ and adopted this material as the air electrode in rSOC, achieving a remarkable peak power density of 1330 mW/cm² at 800°C [18]. Notably, promising performance of Fe-based perovskites has been reported for symmetrical rSOC configuration as well. For instance, a symmetrical cell made of $Sr_2FeMo_{0.65}Ni_{0.35}O_{6-\delta}$ mixed with GDC electrolyte demonstrated at 800°C a peak power density of 507 mW/cm² and a current density of 803 A/cm² at 1.5 V [19]. ($La_{0.8}Sr_{0.2})_{0.95}Fe_{0.8}Ti_{0.2}O_{3-\delta}$ synthesized by spray pyrolysis mixed with GDC electrolyte resulted in 617 mW/cm² peak power density at 800°C [20].
In context of exsolution, iron-based perovskite materials hosting pure Fe or Fe-based alloy exsolved NPs showed interesting results as the fuel electrode. For example, peak power density of 710 mW/cm² at 800°C was achieved using a fuel electrode made of La₀.₄Sr₀.₆Fe₀.₇₅Ni₀.₁Nb₀.₁₅O₃₋δ with exsolved Fe/Ni NPs socketed on the surface of the parent perovskite. Moreover, the cell showed good performance stability over 105 h of operation [21]. Another work reported Fe@Cu core@shell NPs exsolved from Sr₁.₉Fe₁.₅Mo₀.₄Cu₀.₁O₆₋δ, and the resulting composite was investigated as the fuel electrode. The peak power density of 1200 mW/cm² at 800°C was reached [22].

In this study, cobalt-free La₀.₇₂Sr₀.₁₈Fe₀.₉Ni₀.₁O₃₋δ complex perovskite composition was successfully prepared and then subjected to various thermal treatments. The perovskite phase stability was probed by *ex situ* and *in situ* powder X-ray diffraction, while the microstructure and chemical composition of parent perovskite and derivative compositions were characterized by transmission electron microscopy. Further, this study shows the application of as-synthesized materials as the air electrode for fuel-cell mode. For this purpose, the polarization resistance of a symmetrical cell at air was measured by electrochemical impedance spectroscopy. The results provide an interesting insight into the structure-property relationship in the materials prepared through exsolution.

## 2. Materials and Methods

### 2.1. Synthesis

A flowchart of the sample preparation is illustrated in Fig. 1. The perovskite was synthesized through straightforward auto-combustion route using metal nitrate precursors, namely, La(NO₃)₃·6H₂O (99.99%), Sr(NO₃)₂ (99.995%), Fe(NO₃)₃·9H₂O (≥98%), Ni(NO₃)₂·6H₂O (99.99%), as well as citric acid (99.5%) and glycine (≥99.5%) fuel. All the reagents were purchased from Sigma-Aldrich and used as received. The calculated amounts of metal nitrates, targeting the nominal 0.1 A-site deficient La₀.₇₂Sr₀.₁₈Fe₀.₉Ni₀.₁O₃₋δ composition, were dissolved in ultrapure water (produced by Milli-Q Advantage A10 system, Millipore) while stirring at room temperature. Glycine and citric acid were then added in a 1 : 1 molar ratio to total metal ions, while keeping a glycine to citric acid ratio of 0.35 : 0.65. The solution was then transferred to a crystallizing dish, which was placed on a hot plate. The dish was then heated to 140 °C, held at the temperature until full evaporation of the solvent, and then the temperature was raised to 220 °C wherein an ultra-rapid combustion occurred. The resulting ash-like powder was collected and grinded in an agate mortar and pestle. The as-synthesized material is denoted as *LSFN-comb* (Fig. 1). Next, the calcination of the as-derived LSFN-comb powder was performed at 850 °C for 4 h in air with heating rate of 5 °C/min, and the resultant sample is denoted as *LSFN-cal* (Fig. 1). To exsolve metallic species from LSFN-comb oxide material, the powder was heat treated at 800 °C for 4 h with heating rate of 5 °C/min under 100 mL/min flow of forming gas (5% H₂ + 95% N₂), and the atmosphere of forming gas was reminded during the cooling down to the room temperature. The resultant exsolved sample is denoted as *LSFN-red* (Fig. 1). Finally, LSFN-red powder was then oxidized at two different temperatures, namely at 650 °C (denoted as *LSFN-ox650*) and at 800 °C (denoted as *LSFN-ox800*) for 2 h in air with heating rate of 5 °C/min (Fig. 1).

<figure><img src="image_2.png" /><figcaption>**Figure 1.** Sample preparation flowchart.</figcaption></figure>
## 2.2. Characterization

Simultaneous thermogravimetric and differential scanning calorimetry analyses (TGA-DSC; SDT Q600, TA Instruments) were performed on LSFN-comb powder over a temperature range from 25 °C to 1000 °C at a heating rate of 5 °C/min in air. Powder X-ray diffraction analysis (PXRD; X'Pert PRO diffractometer, PANalytical) was conducted with an angular $2\theta$ range of 20°-80° and a step size of 0.01°. Temperature programmed reduction followed by temperature programmed oxidation analyses (TPR/TPO; Autochem II, Micromeritics) were carried out for evaluation of redox behaviour of the synthesized LSFN-cal material. The recorded profiles were subtracted for the background, and then the peaks were fitted by Gaussian model for TPR and Lorentzian model for TPO. The consumed quantities of H₂ and O₂ were calculated by considering ideal gas law. Morphological, microstructural, and compositional investigations were carried out using scanning (SEM; Quanta FEG 650, FEI) and transmission electron microscopy (TEM; JEM-ARM200F, Jeol).

Variable-atmosphere/temperature *in situ* PXRD was carried out using a D8 Advance diffractometer (Bruker) equipped with a XRK900 reaction chamber (Anton Paar) and a 1D PSD LynxEye detector (Bruker). This experiment involved three sequential heating (25 °C → 800 °C) and cooling (800 °C → 25 °C) cycles in air, forming gas (3% H₂ + 97% N₂), and air again, respectively. The heating and cooling rates in all three cycles were 5 °C/min and 10 °C/min, respectively, and the data was collected at 25°C intervals during heating and at 100°C intervals during cooling. The test sample was held at 800°C for 1 h, 4 h, and 1 h in first, second, and third cycles, respectively. The purpose of designing such cycling was the evaluation of perovskite's phase stability at high temperature in air (cycle 1) and in forming gas (cycle 2), as well as the investigation of the potential structural changes after oxidization of the forming-gas-treated sample (cycle 3). These cycles are intended to somewhat mimic the operational condition of rSOC in SOFC and SOEC modes. The acquired XRD patterns were refined by GSAS II software package using the sequential fitting tool [23]. Note that an alumina sample holder was used for *in situ* PXRD analysis, so the diffraction peaks of alumina inevitably present in the collected PXRD data, but fortunately, the peaks do not overlap with perovskite phase investigated.

## 2.3. Electrochemical cell fabrication and testing

The synthesized LSFN-cal La<sub>0.72</sub>Sr<sub>0.18</sub>Fe<sub>0.9</sub>Ni<sub>0.1</sub>O<sub>3-δ</sub> powder was used to make electrode ink based on a terpineol solution (Vehicle, Fuel Cell Materials). La<sub>0.6</sub>Sr<sub>0.4</sub>Fe<sub>0.8</sub>Co<sub>0.2</sub>O<sub>3</sub> powder (LSCF; *d*<sub>50</sub> = 0.49 µm, Fiaxell) was used as the reference material. Yttria-stabilized zirconia (YSZ; Fuel Cell Store) pellets were prepared by tape casting in 13 mm diameter and 400 µm thickness followed by sintering at 1450°C for 6 h in air. The detailed methodology has been described elsewhere [24,25]. Symmetrical solid oxide cells were prepared using the as-fabricated YSZ electrolyte pellet, hosted a gadolinium doped ceria (GDC; Fuel Cell Store) thin film. This film was applied by screen printing of an ink on both sides of the YSZ electrolyte pellet to serve as a buffer layer between the electrolyte and perovskite material. The perovskite ink was then printed on the GDC layers. The as-derived symmetrical perovskite | GDC | YSZ | GDC | perovskite solid oxide cells were then sintered at 1080 °C for 2 h in air with 4°C/min heating rate. Finally, the resultant symmetrical solid oxide cells were subjected to electrochemical impedance spectroscopy measurements (EIS; Autolab PGSTAT302N potentiostat/galvanostat, Metrohm) at high temperatures (Tube furnace type Losa, HTM Reetz) in the frequency range from 10 mHz to 1 MHz with a single sinusoidal signal of 10 mV. Silver mesh (Fuel Cell Material) was used as current collector and silver wires were used for electrical connection. An Alumina tube hosted the whole set-up. The mechanical pressure on the sample was provided by a small spring. The results were fitted with equivalent circuits using Nova 2.1 software package (Metrohm).

## 3. Results

We started by investigation of the thermal behaviour of the synthesized material. According to the TGA-DSC analysis, the total mass loss of LSFN-comb derived directly by auto-combustion is < 1.5%, indicating a very small amount of residual water/carbon content in the as-synthesized powder (Fig. 2a). Such a low weight loss highlights the effectiveness of our robust auto-combustion conditions for the direct preparation of thermally stable crystalline phase. As both DSC (red line in Fig. 2a) and TGA (black line in Fig. 2a) profiles continuously decrease with temperature, one could deduce that no phase transitions occurred during the heating of the LSFN-
comb material in air. The peak at around 200–250 °C in the derivative weight profile (blue line in Fig. 2a), corresponding to the mass increase in the TGA curve, indicates some oxygen uptake during the measurement, which could be due to the change in Fe oxidation state from Fe$^{2+}$ to Fe$^{3+}$ [26].

<figure><img src="image_3.png" /><figcaption>**Figure 2.** (a) TGA–DSC profiles of LSFN-comb sample synthesized by auto-combustion. (b) XRD patterns of the synthesized powders at different preparation stages (Fig. 1) showing the evolution of the phase composition as a function of</figcaption></figure>
reduction/oxidation at high temperature. (c) Heatmap plot of *in situ* PXRD data of LSFN-cal sample as a function of temperature and atmosphere of the respective treatment. (d) The evolution of unit cell volume as a function of the respective treatment on the course of *in situ* PXRD.

We next explored the phase composition of the synthesized samples by ex situ PXRD (Fig. 2b) at different preparation stages (Fig. 1), i.e., right after auto-combustion (LSFN-comb), and subsequent calcination (LSFN-cal), reduction (LSFN-red), and oxidization at 650 °C (LSFN-ox650) and 800°C (LSFN-ox800). The structural data obtained by Rietveld refinement of PXRD patterns is summarized in the Supporting Information (Table S1, Figs. S1–S4). LSFN-comb and LSFN-cal were established to be nearly phase pure compounds with ABO₃-type perovskite crystal structure (ICDD card no. 01-075-9500, hexagonal, R3c space group). As can be seen in Fig. 2b, the PXRD patterns of LSFN-comb and LSFN-cal are quite similar, suggesting nearly identical phase composition and crystallinity of auto-combustion-derived LSFN-comb and derivative LSFN-cal, obtained by the calcination of LSFN-comb. According to the PXRD analysis (Fig. 2b), LSFN-red is a phase mixture of perovskite with Ruddlesden-Popper A₂BO₄-type crystal structure (ICDD card no. 01-075-9500), lanthanum oxides (ICDD card no. 04-013-9392), and exsolved metallic Fe–Ni alloys (ICDD card no. 01-091-1716 and no. 04-008-8469). Regarding oxidized samples, the PXRD results (Fig. 2b) demonstrate that by oxidizing the LSFN-red composite material at 650 °C, exsolved metallic Fe–Ni transforms into nickel ferrite NiFe₂O₄ secondary phase (ICDD card no. 01-086-8861). In contrast, by increasing the oxidation temperature of LSFN-red to 800°C, the composite material was fully reconstructed to the initial ABO₃-type perovskite structure of LSFN-cal (Fig. 2b).

Encouraged by the success of auto-combustion synthesis of perovskite and exsolution of metallic NPs from the well-defined perovskite, we have investigated heat- and atmosphere-induced phase transformation of the La0.72Sr0.18Fe0.9Ni0.1O3-δ perovskite by means of in situ variable-atmosphere/temperature PXRD. Interestingly, while the ex situ PXRD experimental data above clearly evidences the exsolution process upon the reduction of complex La0.72Sr0.18Fe0.9Ni0.1O3-δ perovskite in forming gas leading to the formation of metallic Fe–Ni species, heatmap plot of the in situ PXRD shows no such evident structural transformation during air–forming gas–air heating and cooling (25 °C → 800 °C → 25 °C) cycling (Fig. 1c). Lack of the detection of exsolution process during in situ PXRD, reflected by the formation of metallic Fe–Ni, is likely to stem from the inherent lower resolution of the measurement, which is associated with fast scan rate used. At the same time, while equal unit cell expansion and contraction are expected due to the thermal effect, Fig. 1d demonstrates unequal periodic unit cell volume evolution of perovskite phase as a function of atmosphere/temperature, which indirectly supports the occurrence of reverse transformation events during in situ follow-up of redox cycling using PXRD.
<figure><img src="image_4.png" /><figcaption>**Figure 3.** Successive H₂-TPR and O₂-TPO profiles recorded for complex La₀.72Sr₀.18Fe₀.9Ni₀.1O₃-δ perovskite synthesized by auto-combustion followed by calcination.</figcaption></figure>

To get more insights into the redox behaviour of the synthesized La₀.72Sr₀.18Fe₀.9Ni₀.1O₃-δ perovskite hinted by the results above, we conducted successive H₂-TPR and O₂-TPO experiments, aiming at capturing reduction and oxidation events for constituent Fe and Ni species (Fig. 3, Table S2). Expectedly, the H₂-TPR profile of such perovskite was found to be rather complex, and the reduction behaviour seems to depend on the chemical composition. In low temperature range, three TPR peaks were identified at 321 °C, 401 °C, and 436 °C. The first small peak at low temperature could be either related to the generation of surface oxygen vacancies [27] or to the partial reduction of Fe⁴⁺ to Fe³⁺ [28]. Notably, the presence of Fe⁴⁺ is plausible in complex La₀.72Sr₀.18Fe₀.9Ni₀.1O₃-δ, as it facilitates charge compensation resulting from the incorporation of Ni dopant at Fe-site [29] and Sr dopant at La-site of the perovskite [30]. The next two peaks at low temperatures are attributed to the reduction of Ni³⁺ → Ni²⁺, and partial reduction of Ni²⁺ → Ni accompanied by partial reduction of Fe³⁺ → Fe²⁺, respectively [31,32]. At high temperatures, one peak was detected starting from about 700°C, which is attributed to the full reduction of Fe/Ni cations into metallic state [28,33]. The estimated amount of consumed H₂ (Table S2) for the overall H₂-TPR profile as well as the individual peaks are in a good agreement with the reported in the literature for similar systems [34].

The subsequent O₂-TPO exhibited oxidation events only in low temperature range, namely, at 318 °C, 390 °C, and 444°C. Interestingly, the temperatures of the observed H₂-TPR and O₂-TPO peaks are almost coincident (Fig. 3), indicating good redox reversibility of the synthesized La₀.72Sr₀.18Fe₀.9Ni₀.1O₃-δ perovskite. Overall, our findings from TPR/TPO analyses support the observed above exsolution of Fe/Ni species from complex La₀.72Sr₀.18Fe₀.9Ni₀.1O₃-δ perovskite, as well as indicate the reversibility of the observed transformations.

To understand the exact microstructural and compositional changes during the exsolution process, high resolution TEM (HRTEM), high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), energy-dispersive X-ray spectroscopy in STEM mode (STEM-EDX) and selected area electron diffraction (SAED) were employed to investigate parent complex La₀.72Sr₀.18Fe₀.9Ni₀.1O₃-δ perovskite (LSFN-cal) and derivative exsolved composite (LSFN-red). Fig. 4 shows the representative STEM-EDX element maps of LSFN-cal synthesized by auto-combustion followed by calcination. Although the maps reveal uniform distribution of key constituting elements La, Sr, Fe, and O through the La₀.72Sr₀.18Fe₀.9Ni₀.1O₃-δ, however, inclusions of segregated Ni-rich phase were clearly identified by STEM-EDX. This secondary phase is most likely LaNiO₃ perovskite. Notably, the minor admixture of this perovskite was barely manifested in the
respective PXRD pattern of LSFN-cal through the appearance of a very small peak at around $2\theta = 31.5^{\circ}$ (Fig. 2b), and therefore, the admixture of LaNiO$_3$ phase was omitted in Rietveld refinement (Fig. S1, Table S1). Representative HAADF-STEM images along the [101] and [100] zone axes of LSFN-cal are shown in Fig. 5, and the corresponding SAED patterns along with overlaid simulated images are given as the insets. All reveal high crystallinity of the obtained La$_{0.72}$Sr$_{0.18}$Fe$_{0.9}$Ni$_{0.1}$O$_{3-\delta}$ perovskite without structural defects being present. The spots of the SAED patterns can be completely indexed with reference to the orthorhombic *Pbnm* (62) structure (a=5.52 Å, b=7.72 Å, c=5044 Å ISCD 247633), using the LSFN unit cell parameters estimated by Rietveld refinement (Table S1). The image simulation was performed using the same orthorhombic *Pnma* structure and shows perfect fitting with an experimental HAADF-STEM images (Fig. 5).

<figure><img src="image_5.png" /><figcaption>**Figure 4.** Low-magnification HAADF-STEM image of the synthesized La$_{0.72}$Sr$_{0.18}$Fe$_{0.9}$Ni$_{0.1}$O$_{3-\delta}$, together with the simultaneously collected STEM-EDX maps of La L, Sr L, Fe K, Ni K, O K, as well as Fe, Ni, and O superposition.</figcaption></figure>

<figure><img src="image_6.png" /></figure>
<figure><img src="image_7.png" /><figcaption>**Figure 6.** Low-magnification HAADF-STEM image of exsolved composite sample, together with the simultaneously collected STEM-EDX maps of La L, Sr L, Fe K, Ni K, and O K, as well as Fe, Ni, and O superposition.</figcaption></figure>

<figure><img src="image_8.png" /><figcaption>**Figure 7.** Low-magnification (a,b) and high-resolution (c,d) HAADF-STEM images of singular exsolved FeNi particle along [001] zone axis, together with the corresponding SAED and FT data. The observed Fe<sub>3</sub>O<sub>4</sub> surface oxide layer is indicated by arrows.</figcaption></figure>
For the case of exsolved LSFN-red produced by heat treatment of La<sub>0.72</sub>Sr<sub>0.18</sub>Fe<sub>0.9</sub>Ni<sub>0.1</sub>O<sub>3-δ</sub> at 800 °C for 4 h under flow of forming gas, the PXRD and TPR results presented above indicate that the exsolution give rise to the formation of metallic FeNi NPs. Figs. 6 and 7 confirm that exsolution is indeed took place. Specifically, the STEM-EDX maps recorded for LSFN-red evidence that the sample consists of well-isolated FeNi NPs, which are supported on oxide material (Fig. 6). Analysis of EDX data reveals that the chemical composition of exsolved metallic NPs is mainly close to Fe<sub>0.9</sub>Ni<sub>0.1</sub>. The size distribution of the FeNi NPs was found to be rather broad ranging from around 5 nm to 200 nm. SAED reveals a high crystallinity of the resultant FeNi NPs (Fig. 7d), consistent with PXRD (Fig. 2b). The SAED patterns can be completely indexed in the cubic *Im*3*m* space group, using the unit cell parameter of α-Fe (ISCD 9008536). Not surprisingly, the HRTEM imaging also highlights the formation of a thin crystalline oxide layer (up to 5 nm thick) on the surface of exsolved metallic FeNi NPs, which is clearly seen in Fig. 7 (indicated by red arrows). According to the respective Fourier transform (FT) patterns, this layer is magnetite Fe<sub>3</sub>O<sub>4</sub> oriented along [110] (Fig. 7c), originating from natural partial surface oxidation for metallic NPs upon their exposure to air.

It is interesting to analyze the structure of the resultant perovskite phase after reductive exsolution. Fig. 8 shows typical high-resolution HAADF-STEM images of the perovskite phase in the resultant LSFN-red composite material. The phase is highly crystalline and devoid of any structural defects being present (Fig. 8, right panels). Notably, there are grain boundaries between perovskite particles (Fig. 8, left bottom panel), which is typical for such type of thermally-treated materials. SAED analysis also reveals high crystallinity of perovskite particles with [100] prominent growth direction (Fig. 8, left top panel). Nevertheless, some uncommon high index [411] zone axis of perovskite was also observed in the exsolved LSFN-red sample (Fig. 8, bottom panel). The spots of the SAED patterns can be completely indexed with reference to the tetragonal *I4/mmm* space group using unit cell parameters *a* = 3.89 Å and *c* = 12.71 Å, as estimated by Rietveld refinement (Table S1). This indicates that perovskite phase in LSFN-red sample is Ruddlesden-Popper A<sub>2</sub>BO<sub>4</sub>-type, consistent with PXRD (Table S1).

<figure><img src="image_9.png" /><figcaption>**Figure 8.** Representative [100] and [401] HAADF-STEM images((a) and (b) respectively), together with the corresponding SAED patterns and overlaid simulated images (insets) of perovskite La<sub>0.72</sub>Sr<sub>0.18</sub>Fe<sub>0.9</sub>Ni<sub>0.1</sub>O<sub>3-δ</sub> structure in exsolved LSFN-red composite material.</figcaption></figure>
Notably, the synthesized La<sub>0.72</sub>Sr<sub>0.18</sub>Fe<sub>0.9</sub>Ni<sub>0.1</sub>O<sub>3-δ</sub> perovskite is great potential candidate for anode and cathode in symmetrical fuel-cell and steam electrolysis modes, respectively. Considering this, we explored the performance of the synthesized material as air electrode for the oxygen reduction reaction (ORR) in fuel cell mode. EIS was carried out on symmetrical cells made of YSZ electrolyte, La<sub>0.72</sub>Sr<sub>0.18</sub>Fe<sub>0.9</sub>Ni<sub>0.1</sub>O<sub>3-δ</sub> layer on both sides as electrodes, and a GDC thin layer between the electrolyte and electrode to inhibit the formation of unwanted lanthanum zirconate and/or strontium zirconate phases at the perovskite/YSZ interfaces. The cross-sectional SEM image of the fabricated La<sub>0.72</sub>Sr<sub>0.18</sub>Fe<sub>0.9</sub>Ni<sub>0.1</sub>O<sub>3-δ</sub> | GDC | YSZ | GDC | La<sub>0.72</sub>Sr<sub>0.18</sub>Fe<sub>0.9</sub>Ni<sub>0.1</sub>O<sub>3-δ</sub> cell is shown in the Fig. S5. We observed a good contact between the porous La<sub>0.72</sub>Sr<sub>0.18</sub>Fe<sub>0.9</sub>Ni<sub>0.1</sub>O<sub>3-δ</sub> electrode and the electrolyte which plays significant role on the total polarization by facilitating the charge transfer across the electrolyte-electrode interface. Moreover, the density of the electrolyte was observed to be adequate for solid oxide cell. After the first EIS measurement in air, the cell was reduced at 800 °C in 5%H<sub>2</sub>/N<sub>2</sub> for 4 h, and then oxidized during the second EIS measurement in air. At this stage, the electrochemical testing was carried out after holding the cell at the targeted temperature for 2 h.

<figure><img src="image_10.png" /><figcaption>**Figure 9.** Nyquist plots measured at 650 °C and 800 °C of pristine La<sub>0.72</sub>Sr<sub>0.18</sub>Fe<sub>0.9</sub>Ni<sub>0.1</sub>O<sub>3-δ</sub> (red) and reduced-oxidized La<sub>0.72</sub>Sr<sub>0.18</sub>Fe<sub>0.9</sub>Ni<sub>0.1</sub>O<sub>3-δ</sub> (green) electrodes, as well as commercial benchmark LSCF sample (blue). The insets show an equivalent electrical circuit model used to fit EIS data.</figcaption></figure>

Fig. 9 shows the collected electrochemical EIS data of the as-synthesized La<sub>0.72</sub>Sr<sub>0.18</sub>Fe<sub>0.9</sub>Ni<sub>0.1</sub>O<sub>3-δ</sub> and reduced-oxidized La<sub>0.72</sub>Sr<sub>0.18</sub>Fe<sub>0.9</sub>Ni<sub>0.1</sub>O<sub>3-δ</sub> measured at 650 °C and 800 °C. For comparison, the commercial benchmark electrode material La<sub>0.6</sub>Sr<sub>0.4</sub>Fe<sub>0.8</sub>Co<sub>0.2</sub>O<sub>3</sub> (LSCF, Fiaxell) was also characterized in symmetrical LSCF|GDC|YSZ|GDC|LSCF cell under the same conditions. The distance between the first and second intercepts of the arc with the Z' axis indicates the total polarization resistance (R<sub>p</sub>) of the electrode. Normally, the electrode polarization resistance in such type of SOCs typically originates from three processes occurring at high, medium, and low frequencies: (i) charge transfer in the triple phase boundary region, (ii) gas diffusion in the porous media, and (iii) the conversion of gas into lattice ions, respectively [35]. The synthesized La<sub>0.72</sub>Sr<sub>0.18</sub>Fe<sub>0.9</sub>Ni<sub>0.1</sub>O<sub>3-δ</sub> material was estimated to exhibit R<sub>p</sub> = 2.42 Ω cm<sup>2</sup> at 650 °C, which is almost twice higher than the value of R<sub>p</sub> = 1.30 Ω cm<sup>2</sup> calculated for the reference LSCF material. By increasing the temperature to 800 °C, polarization resistance of our La<sub>0.72</sub>Sr<sub>0.18</sub>Fe<sub>0.9</sub>Ni<sub>0.1</sub>O<sub>3-δ</sub> and reference LSCF samples were estimated to be comparable, namely, R<sub>p</sub> = 0.35 Ω cm<sup>2</sup> and R<sub>p</sub> = 0.29 Ω cm<sup>2</sup>, respectively. After successive reduction of the La<sub>0.72</sub>Sr<sub>0.18</sub>Fe<sub>0.9</sub>Ni<sub>0.1</sub>O<sub>3-δ</sub> cell and oxidation at 650 °C, the R<sub>p</sub> value was found to significantly decrease to R<sub>p</sub> = 1.63 Ω cm<sup>2</sup>. In contrast, the difference between polarization resistance of our pristine La<sub>0.72</sub>Sr<sub>0.18</sub>Fe<sub>0.9</sub>Ni<sub>0.1</sub>O<sub>3-δ</sub> (R<sub>p</sub> = 0.35 Ω cm<sup>2</sup>) and one reduced-oxidized at 800 °C (R<sub>p</sub> = 0.33 Ω cm<sup>2</sup>) was found to be marginal.
## 4. Discussion

The straightforward synthesis of chemically complex La<sub>0.72</sub>Sr<sub>0.18</sub>Fe<sub>0.9</sub>Ni<sub>0.1</sub>O<sub>3-δ</sub> perovskite was carried out by auto-combustion route according to the Scheme 1. This non-equilibrium approach is rapid and omits the need for intermediate preparation steps, such as (re)grinding and (re)calcination. The data presented here show that the as-prepared LSFN-comb sample is phase pure and thermally stable perovskite. Interestingly, various degree of crystallinity has been reported for the end products of auto-combustion synthesis, which is attributed to methodological differences in auto-combustion protocols, namely, the type and quantity of the fuel. For example, Idrees and co-workers [36] reported gradual crystallization of the auto-combustion-derived material during increase of calcination temperature, while Tarrago and co-workers [37] obtained fully crystalline powders directly after auto-combustion, in good agreement with the results of this study.

Since our La<sub>0.72</sub>Sr<sub>0.18</sub>Fe<sub>0.9</sub>Ni<sub>0.1</sub>O<sub>3-δ</sub> perovskite is envisioned to be used in rSOC at T ≤ 800 °C, the auto-combustion derived LSFN-comb sample was subjected to the calcination at 850 °C to avoid any undesired structural and compositional changes possible during high temperature cell operation. Structurally, the resultant LSFN-cal sample was established to be nearly the same as pristine LSFN-comb one. Further, we thought that the as-prepared La<sub>0.72</sub>Sr<sub>0.18</sub>Fe<sub>0.9</sub>Ni<sub>0.1</sub>O<sub>3-δ</sub> LSFN-cal sample can potentially be employer as electrode in symmetrical rSOC, and hence, we investigated the redox behavior of the material. The presence of metallic phase confirms that the heat treatment of LSFN-cal sample at 800 °C under reducing atmosphere of forming gas provides access to the exsolution of FeNi NPs from parent La<sub>0.72</sub>Sr<sub>0.18</sub>Fe<sub>0.9</sub>Ni<sub>0.1</sub>O<sub>3-δ</sub> perovskite according to the Scheme 2. Notably, it was established that the exsolution process is accompanied by the phase transformation from ABO<sub>3</sub>-type La<sub>0.72</sub>Sr<sub>0.18</sub>Fe<sub>0.9</sub>Ni<sub>0.1</sub>O<sub>3-δ</sub> perovskite to Ruddlesden-Popper A<sub>2</sub>BO<sub>4</sub>-type La<sub>1.44</sub>Sr<sub>0.36</sub>Fe<sub>0.9</sub>Ni<sub>0.1</sub>O<sub>4</sub> perovskite. The generation of Ruddlesden-Popper phase was not surprizing and most likely induced by the crystal distortion caused by Ni dopant, as well as the generated oxygen vacancies during reduction. Other researchers also have reported such Ruddlesden-Popper phase formation after the reduction [38–40].

$$La(NO_3)_3 \cdot 6H_2O + Sr(NO_3)_2 + Fe(NO_3)_3 \cdot 9H_2O + Ni(NO_3)_2 \cdot 6H_2O \xrightarrow{T} La_{0.72}Sr_{0.18}Fe_{0.9}Ni_{0.1}O_{3-\delta} + NO_2 + O_2 + H_2O \quad (Scheme 1)$$

$$La_{0.72}Sr_{0.18}Fe_{0.9}Ni_{0.1}O_{3-\delta} + H_2 \xrightarrow{T} La_{1.44}Sr_{0.36}Fe_{0.9}Ni_{0.1}O_4 + La_2O_3 + Fe_{0.9}Ni_{0.1} \quad (Scheme 2)$$

$$La_{1.44}Sr_{0.36}Fe_{0.9}Ni_{0.1}O_4 + La_2O_3 + Fe_{0.9}Ni_{0.1} + O_2 \xrightarrow{T} La_{1.44}Sr_{0.36}Fe_{0.9}Ni_{0.1}O_4 + La_{0.72}Sr_{0.18}Fe_{0.9}Ni_{0.1}O_{3-\delta} + La_2O_3 + NiFe_2O_4 \quad (Scheme 3)$$

$$La_{1.44}Sr_{0.36}Fe_{0.9}Ni_{0.1}O_4 + La_2O_3 + Fe_{0.9}Ni_{0.1} + O_2 \xrightarrow{T} La_{0.72}Sr_{0.18}Fe_{0.9}Ni_{0.1}O_{3-\delta} \quad (Scheme 4)$$

To investigate the reversibility of the observed exsolution, the composite exsolved LSFN-red sample was subjected to oxidation in air at two different temperatures, namely, 650 °C and 800 °C, thus somewhat mimicking the operational temperatures of intermediate and conventional rSOC, respectively. At 650 °C, FeNi NPs were established to transform into nickel ferrite NiFe<sub>2</sub>O<sub>4</sub>, along with partial transformation of A<sub>2</sub>BO<sub>4</sub>-type perovskite into ABO<sub>3</sub>-type one (Scheme 3). In contrast, at 800°C, a full reversibility of the exsolution process was found to appear according to the Scheme 4, indicating that La<sub>0.72</sub>Sr<sub>0.18</sub>Fe<sub>0.9</sub>Ni<sub>0.1</sub>O<sub>3-δ</sub> material undergoes reverse reduction/oxidation transformations at 800 °C. Some other groups have reported that FeNi NPs still remained after the oxidization of the material at moderate temperature. For instance, Lopez-Garcia and co-authors [41] studied the redox reversibility of exsolved ternary alloy particles in Sr<sub>2</sub>FeCo<sub>0.2</sub>Ni<sub>0.2</sub>Mn<sub>0.1</sub>Mo<sub>0.5</sub>O<sub>6-δ</sub> by reduction/oxidization cycling conducted in near-atmosphere X-ray photoelectron spectroscopy. They observed that a major fraction of the metallic NPs has been re-incorporated into perovskite structure, but some of them still existed as separate phase after oxidization at 600°C for 2 h. The authors speculated that full dissolution of metallic phase in oxide might need higher oxidization temperatures or prolonged time of the heat treatment. In another report, Lie and co-workers [42] carried out a reduction/oxidization cycling of Sr<sub>2</sub>Fe<sub>1.4</sub>Ni<sub>0.1</sub>Mo<sub>0.5</sub>O<sub>6-δ</sub> with double perovskite structure, wherein they observed remained FeNiO<sub>x</sub> phase even after oxidization at 800 °C for 10 h.
Having in hands novel complex perovskite exhibiting partially (at 650 °C) and fully (at 800 °C) reversible redox behaviour, we further developed $La_{0.72}Sr_{0.18}Fe_{0.9}Ni_{0.1}O_{3-\delta}$ | GDC | YSZ | GDC | $La_{0.72}Sr_{0.18}Fe_{0.9}Ni_{0.1}O_{3-\delta}$ cell for the investigation of electrochemical performance of the perovskite towards oxygen reduction reaction (ORR) in symmetrical fuel-cell mode. EIS measurements in air indicated that the electrode reduced, oxidized and measured at 800 °C possess $R_p = 0.35 \ \Omega \ cm^2$, which is closed to $R_p = 0.33 \ \Omega \ cm^2$ of pristine electrode (i.e., non-reduced, non-oxidized). Similarity of these total polarization resistances is an addition prove of fully reversible reduction/oxidation transformations in our $La_{0.72}Sr_{0.18}Fe_{0.9}Ni_{0.1}O_{3-\delta}$ at 800 °C. Remarkably, the polarization resistance measured at 650 °C of the electrode reduced at 800°C and then oxidized at 650 °C was found to be reasonable lower $R_p = 1.63 \ \Omega \ cm^2$ compared to that of pristine electrode $R_p = 2.42 \ \Omega \ cm^2$. We believed that such improvement is related to the presence of $NiFe_2O_4$ metal oxide particles in the electrode material. Specifically, the current study showed that the reduction of $La_{0.72}Sr_{0.18}Fe_{0.9}Ni_{0.1}O_{3-\delta}$ at 800 °C leads to the exsolution of FeNi NPs, which then oxidize to $NiFe_2O_4$ at 650 °C. Notably, the ORR activity is strongly determined by the oxygen activation at the electrode surface, i.e., oxygen adsorption and dissociation. Previous studies proposed that metal oxide particles on the surface of perovskite electrode enhance the activation of oxygen, thus improving ORR activity of such composite metal oxide/perovskite electrode [43].

The general importance of our work is that newly synthesized complex $La_{0.72}Sr_{0.18}Fe_{0.9}Ni_{0.1}O_{3-\delta}$ perovskite could be considered as promising air electrode in intermediate and high temperatures symmetrical rSOC, just slightly underperforming compared to commercial LSCF benchmark, but comparing favorably to other reported Fe-based perovskite electrodes (Table S3). Our future work will focus on further optimization of titled $La_{0.72}Sr_{0.18}Fe_{0.9}Ni_{0.1}O_{3-\delta}$ material and the investigation of its performance as fuel electrode for the symmetrical rSOC.

## 5. Conclusion

We have designed and synthesized a novel $La_{0.72}Sr_{0.18}Fe_{0.9}Ni_{0.1}O_{3-\delta}$ material featuring $ABO_3$-type perovskite structure. A comprehensive study employing *ex situ* and *in situ* X-ray diffraction, TPR/TPO, and electron microscopy was carried out to investigate the link between the reduction and oxidation in this complex perovskite composition. The A-cation deficiency and B-cation doping in $La_{0.72}Sr_{0.18}Fe_{0.9}Ni_{0.1}O_{3-\delta}$ are keys to its ability towards exsolution of bimetallic FeNi NPs upon reductive thermal treatment, which is accompanied by the structural transformation of parent orthorhombic $ABO_3$-type perovskite to tetragonal $A_2BO_4$-type perovskite. The transformation was established to be either partially or fully reversible depending upon the temperature of the subsequent oxidative thermal treatment.

As an air electrode for solid oxide fuel cell with symmetrical perovskite | GDC | YSZ | GDC | perovskite configuration, $La_{0.72}Sr_{0.18}Fe_{0.9}Ni_{0.1}O_{3-\delta}$ demonstrates the electrochemical performance comparable to that of commercial LSCF benchmark, and it has total polarization resistances of 2.42 and 0.33 $\Omega \cdot cm^2$ at 650 and 800 °C, respectively. Leveraging on the established redox trends in $La_{0.72}Sr_{0.18}Fe_{0.9}Ni_{0.1}O_{3-\delta}$, we have also demonstrated an interesting pathway for further reduction of a total polarization resistance in this electrode material.

## Data availability statement

All data that support the findings of this study are included within the article and supplementary file.
Acknowledgment

This work was supported by the European Union's Horizon 2020 research and innovation program through the KNOWSKITE-X Project under grant agreement no. 101091534.

Conflict of interest

The authors declare no conflict of interest.

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