Recent Progress in LaNi_1-xFe_xO_3-δ Perovskite Oxides for SOFC Cathodes and Energy Catalysis: The Role of the Ni/Fe Ratio

Abstract

LaNi_1-xFe_xO_3-δ (LNF) perovskite oxides are promising Sr- and Co-free materials for solid oxide fuel cell (SOFC) cathodes and energy-related catalytic applications. The Ni/Fe ratio strongly influences their electronic structure, defect chemistry, transport properties, reducibility, and structural stability. This review examines how these composition-dependent properties affect LNF synthesis and performance across different applications. For SOFC cathodes, LaNi_0.6Fe_0.4O₃ is an important reference composition because of its favorable balance of conductivity, thermal compatibility, and stability. In oxygen evolution electrocatalysis and other applications, however, the preferred Ni/Fe ratio varies substantially with the required reaction properties and operating conditions. Synthesis, microstructure, surface reconstruction, reduction, and exsolution further affect the functional state of LNF. Therefore, no universally optimal Ni/Fe ratio exists. Composition should instead be selected for the intended application, followed by appropriate synthesis and surface/interface engineering to further improve performance.

1. Introduction

Perovskite oxides with the general formula ABO₃ have been widely investigated for electrochemical energy conversion, electrocatalysis, photocatalysis, and high-temperature catalytic reactions because their flexible crystal chemistry enables effective tuning of bulk composition, defect chemistry, electronic structure, and surface properties [1,2]. In these materials, B-site substitution is particularly effective for modifying transition-metal valence, metal–oxygen bonding, and oxygen-vacancy formation, thereby influencing charge transport, reducibility, structural stability, and surface reactivity [1,3]. Among these materials, LaNi_1-xFe_xO_3-δ (0 ≤ x ≤ 1, LNF) is of particular interest because it provides a continuous Ni-Fe compositional platform in which the B-site chemistry can be systematically varied while retaining within the perovskite structural framework. This makes LNF particularly suitable for examining how composition variation translates into changes in structure, physicochemical properties, and ultimately application performance.

The two end members, LaNiO₃ and LaFeO₃, exhibit distinctly different structural, electronic, and redox characteristics. LaNiO₃ is commonly described as a rhombohedrally distorted perovskite, whereas LaFeO₃ typically adopts an orthorhombic structure [4]. Relative to LaFeO₃, LaNiO₃ generally exhibits higher electronic conductivity and greater reducibility, while increasing Fe content tends to improve the structural and high-temperature stability of LNF compositions [5,6]. These differences originate from the different electronic configurations and metal–oxygen interactions associated with Ni and Fe, and therefore LNF should not be regarded simply as a linear interpolation between the two end members. Spectroscopic and first-principles studies have shown that partial Ni substitution in LaFeO₃ introduces strongly hybridized O 2p–Ni 3d electronic states, demonstrating that B-site composition can directly modify the electronic structure of the Ni–Fe–O framework [6]. Consequently, changing the Ni/Fe ratio can simultaneously alter lattice distortion, transition-metal valence, metal–oxygen covalency, oxygen-defect chemistry, reducibility, and electronic transport. The Ni/Fe ratio therefore provides a fundamental compositional variable for balancing properties that are not necessarily optimized at the same composition.

A systematic study of the LNF series showed that LaNi_0.6Fe_0.4O₃ (LNF64) exhibited the highest electrical conductivity among the investigated compositions, reaching approximately 580 S cm⁻¹ at 800 °C. Its average thermal expansion coefficient between 30 and 1000 °C was approximately 11.4 × 10⁻⁶ K⁻¹, close to that of yttria-stabilized zirconia (YSZ) [5]. This combination of high electrical conductivity, thermal compatibility, and relatively good high-temperature stability has made LNF64 an important Sr- and Co-free cathode candidate for solid oxide fuel cells (SOFCs). LNF cathodes have also shown good chromium tolerance in the presence of metallic interconnects [7], while an anode-supported SOFC using an LNF64 cathode and metallic interconnects was operated for 10,000 h, supporting the potential long-term durability of LNF64 under SOFC conditions [8]. Nevertheless, relatively limited oxygen transport and cathodic activity remain important challenges for LNF compared with several high-performance Co-containing cathode materials [9]. These results establish LNF64 as an important benchmark composition for SOFC cathodes rather than a universally optimal composition, because the preferred balance among conductivity, oxygen-reduction activity, stability, and thermal compatibility depends on operating temperature and electrode configuration.

Importantly, the preferred Ni/Fe ratio can differ substantially between SOFC cathodes and other applications. This reflects the different property requirements imposed by different functions. For SOFC cathodes, electrical conductivity, thermal compatibility, phase stability, and oxygen-reduction kinetics must be balanced simultaneously. In alkaline oxygen electrocatalysis, by contrast, surface electronic structure, transition-metal oxidation state, adsorption chemistry, and surface reconstruction under operating conditions become increasingly important. Partial Fe substitution has been reported to enhance both oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) activity by altering the surface oxidation state of Ni and the local Ni–O bonding environment [10]. For OER, different studies have identified different preferred Ni/Fe ratios under their respective experimental conditions. Depending on catalyst preparation and testing conditions, high activity has been reported for compositions ranging from Ni-rich LaNi_0.9Fe_0.1O₃ (LNF91) and LaNi_0.8Fe_0.2O₃ (LNF82) to intermediate LaNi_0.625Fe_0.375O₃ and Fe-richer LaNi_0.4Fe_0.6O₃ (LNF46) [11,12,13,14]. The spread in the reported preferred compositions should therefore not be interpreted simply as inconsistency among studies. Fe substitution can modify metal–oxygen hybridization and transition-metal d states [13,15], while surface reconstruction, lattice-oxygen participation, and dynamic redistribution of Ni and Fe can further change the catalytically relevant surface during operation [11,16,17]. Thus, for oxygen electrocatalysis, the nominal bulk Ni/Fe ratio establishes the initial electronic and defect-chemical environment, whereas synthesis, morphology, electrolyte environment, and operating-state evolution determine the catalytically active surface under reaction conditions.

The application-dependent role of the Ni/Fe ratio extends beyond oxygen electrocatalysis. In pseudocapacitive energy storage, LaNi_0.45Fe_0.55O_3-δ was used to separate Ni- and Fe-related redox processes and enable high-power charge storage [18]. In visible-light photocatalysis, LaNi_0.95Fe_0.05O₃ showed the best performance within the investigated single-phase composition series [19], while further improvement of a selected LNF composition through heterojunction formation was demonstrated using LaNi_0.8Fe_0.2O₃/g-C₃N₄ [20]. In high-temperature catalytic applications, the influence of composition is expressed differently. The Ni/Fe ratio influences reducibility and the formation of reduced Ni-containing phases [21], and it can also determine the composition of exsolved Ni–Fe nanoparticles generated under reaction conditions [22]. In coal-tar cracking, retention of the perovskite lattice was beneficial because lattice oxygen participated directly in catalytic oxygen transfer [23]. Chemical-looping studies further show that relatively Fe-rich compositions with limited Ni substitution can provide a favorable balance among reactivity, reversible oxygen transfer, carbon resistance, and cyclic stability [24,25]. These findings demonstrate that the preferred Ni/Fe ratio depends not only on the specific application but also on the dominant reaction pathway and the active material state developed under operating conditions.

Composition, however, represents only the first level of LNF materials design. Even at the same nominal Ni/Fe ratio, different synthesis routes can produce substantial differences in phase homogeneity, particle morphology, surface area, densification, and measured electrical or electrochemical properties [26,27,28]. The material can also evolve substantially under operating conditions. Surface reconstruction, cation redistribution, lattice-oxygen exchange, reduction, and exsolution can transform the as-prepared material into a different active state during reaction. Surface and interface engineering therefore provide a further level of optimization after the bulk composition has been selected. In SOFC cathodes, incorporation of ionic conductors, infiltration, functional layers, and defect engineering can improve oxygen-reduction kinetics without changing the basic Ni/Fe ratio [29,30,31]. Similar post-synthetic optimization has also been demonstrated in photocatalytic heterojunctions and Ni–Fe perovskite-type core–shell catalysts [20,32]. Accordingly, the Ni/Fe ratio is used in this review as the primary compositional axis rather than as an exclusive descriptor of LNF performance. Synthesis, microstructure, processing conditions, operating-state evolution, and surface/interface engineering are treated as coupled factors that determine how composition-dependent properties are ultimately expressed.

Despite the broad literature on LNF, previous studies have often focused on individual applications, specific compositions, or isolated modification strategies. As a result, a cross-application understanding of how the Ni/Fe ratio influences LNF performance remains limited. Likewise, the composition showing the highest activity in one reaction is not necessarily optimal for another application because the required electronic structure, defect chemistry, reducibility, oxygen-transfer behavior, and active state under operating conditions may be different. A comparison across different applications is therefore necessary to distinguish intrinsic composition effects from those arising from synthesis, microstructure, operating-state evolution, and surface/interface engineering.

This review focuses on LNF with particular attention to the role of the Ni/Fe ratio in determining structure, functional properties, and application performance. Representative synthesis strategies and their influence on phase formation and microstructure are first discussed. The composition–property–performance relationships of LNF as SOFC cathodes are then examined, followed by its use in oxygen evolution electrocatalysis. Other applications, including pseudocapacitive energy storage, photocatalysis, reforming, tar cracking, and chemical-looping processes, are subsequently considered. The review also emphasizes how the active material state can differ from that implied by the initial bulk composition under operating conditions. Finally, the major challenges in establishing intrinsic composition–property relationships, understanding operating-state evolution, achieving long-term stability and scalable synthesis, and applying surface/interface engineering are discussed. The central perspective of this review is that there is no universally optimal Ni/Fe ratio for LNF. Instead, the appropriate composition should be selected according to the property window required for a specific application, while synthesis, operating-state evolution, microstructure, and surface/interface engineering determine how the selected composition ultimately performs under operating conditions.

2. Synthesis Strategies and Their Influence on LaNi_1-xFe_xO_3-δ Properties

The synthesis method can significantly influence the structural, microstructural, and functional properties of LNF, even when the nominal composition is unchanged. Differences in precursor mixing, compositional homogeneity, particle formation, and thermal treatment can affect phase evolution and microstructure, thereby influencing the electrical, electrochemical, and catalytic properties of the final material. LNF has been prepared using a range of routes, including conventional solid-state reaction (SSR), citrate- and Pechini-based methods, the glycine nitrate process (GNP), co-precipitation, sol–gel synthesis, and, more recently, mechanochemically assisted solid-state processing. Therefore, differences in reported LNF performance should not be attributed to the Ni/Fe ratio alone; the synthesis route and resulting microstructure must also be considered.

A direct comparison of different synthesis routes for the same LNF composition was reported by Bevilacqua et al., who prepared LNF64 using GNP, gel–citrate complexation, and co-precipitation . The GNP route produced relatively inhomogeneous powders, and higher-temperature treatment improved phase homogeneity and electrical conductivity. In contrast, the gel–citrate route produced homogeneous LNF64 but showed relatively poor densification, whereas co-precipitation produced homogeneous powders that could achieve a similar porosity at a lower sintering temperature . For an estimated porosity of approximately 30%, the corresponding sintering temperatures were about 1070 °C for the co-precipitated sample, 1150 °C for the gel–citrate sample, and 1250 °C for the GNP-derived sample [27]. These differences are particularly relevant to SOFC cathode fabrication because higher sintering temperatures can promote reactions between LNF and ZrO₂-based electrolytes.

The same study also showed that the synthesis route strongly affected the microstructural evolution of LNF64 during thermal treatment . The co-precipitated LNF64 showed a relatively narrow grain-size distribution, with characteristic grain sizes increasing from approximately 0.2 μm at 1000 °C to 0.6 μm at 1200 °C. The GNP sample also showed a relatively narrow grain-size distribution but with larger grains, whereas the gel–citrate sample exhibited a broader grain-size distribution and a different pore-evolution behavior [27]. Electrical conductivity increased with densification and showed an approximately linear relationship with estimated porosity. For the gel–citrate and co-precipitated samples, similar conductivity was obtained at comparable porosity, indicating that the synthesis route mainly affected the temperature required to reach a given microstructural state. For example, co-precipitated LNF64 calcined at 1000 °C showed electrical conductivities of approximately 259–262 S cm⁻¹ at 200–400 °C and 238 S cm⁻¹ at 700 °C. These results show that the synthesis route can influence the measured electrical properties of LNF through its effects on phase homogeneity and densification. This relationship was further examined by Bevilacqua et al. using co-precipitated LNF64. Their subsequent study showed that thermal processing affected the local structure and microstructure of the material, with corresponding changes in its electrochemical behavior [33]. A similar influence of thermal processing was observed for Pechini-derived LNF64. Niwa et al. showed that the specific surface area, pore-size distribution, and pore volume changed systematically with sintering temperature, with two pore-size maxima at approximately 2.5 and 250 nm for the sample sintered at 900 °C. Both features were retained at 1000 and 1100 °C but decreased in intensity with increasing sintering temperature, indicating a progressive decrease in pore volume. Sintering below approximately 1100 °C was therefore suggested for maintaining sufficient pore volume [34].

Solution-based methods are widely used for LNF synthesis because they can promote intimate precursor mixing and compositional homogeneity. A later comparison of a two-step ceramic route, which corresponds to a conventional solid-state reaction (SSR) process, glycine–nitrate combustion, and modified Pechini routes further showed that the synthesis method affected phase formation and powder surface area. The Pechini method produced single-phase LNF64 at the lowest synthesis temperature and gave a specific surface area of 6.50 m² g⁻¹, compared with 1.50 and 1.57 m² g⁻¹ for the two-step ceramic (SSR) and glycine–nitrate routes, respectively [28]. Co-precipitation has also been used to prepare LNF over a relatively broad composition range (x = 0.4–0.8), allowing systematic comparison of structure, morphology, and electrochemical performance [35]. These solution-based routes generally involve several processing steps, including solution preparation, precipitation or gel formation, drying, and calcination. This number of processing steps is therefore an additional consideration when such routes are evaluated for larger-scale powder preparation.

High-energy ball-milling-assisted solid-state synthesis provides another route for LNF preparation using oxide precursors. A recent study prepared LNF by high-energy ball milling of La₂O₃, NiO, and Fe₂O₃, followed by calcination at 800 °C and sintering at 1000 °C [26]. For LNF64, transmission electron microscopy (TEM) analysis showed particle sizes of approximately 50–400 nm after calcination at 800 °C, increasing to about 100–600 nm after sintering at 1000 °C. Particle agglomeration was also observed after milling and calcination, suggesting that precise control of particle size and morphology may be limited by agglomeration and particle growth during subsequent thermal treatment [26]. In this route, high-energy ball milling was used to mix the oxide precursors before calcination. Because the route uses oxide precursors and avoids solution preparation, precipitation, or gel formation, it provides a comparatively simple alternative for LNF powder synthesis. A useful comparison was reported by Solovyev et al., who prepared LNF64 powders using a ceramic solid-state route and a modified Pechini method [36]. The powders showed similar lattice parameters but different specific surface areas, approximately 1.6 m² g⁻¹ for the solid-state-derived powder and 5.5 m² g⁻¹ for the Pechini-derived powder [36]. Both powders were subsequently used in LNF-based SOFC cathodes, illustrating how the synthesis route can influence powder characteristics relevant to electrode fabrication. Representative synthesis routes and their main effects on LNF and related oxide materials are summarized in

Table 1. Representative synthesis routes for LNF and related oxide materials.

Material	Synthesis Route	Key Condition	Main Observation	Ref.
LNF64	GNP	Sintering behavior	Lower phase homogeneity; ~30% porosity at ~1250 °C	[27]
LNF64	Gel–citrate	Solution route	Good phase homogeneity; poorer densification; ~30% porosity at ~1150 °C	[27]
LNF64	Co-precipitation	Precipitation followed by calcination	Good phase homogeneity; easier sintering; ~30% porosity at ~1070 °C	[27]
LNF64	Co-precipitation	Calcination/thermal treatment	Local structure, microstructure, and electrochemical behavior depended on thermal processing	[33]
LNF64	Pechini method	Sintering-temperature variation	Pore-size maxima of ~2.5 and ~250 nm at 900 °C; pore volume decreased with increasing sintering temperature	[34]
LNF64	Two-step ceramic route (SSR); GNP; modified Pechini	Different thermal treatments	Pechini gave single-phase LNF at the lowest synthesis temperature and the highest BET surface area (6.50 vs. 1.50–1.57 m2 g−1)	[28]
LNF64	Ceramic solid-state route; modified Pechini	Solid-state oxide processing vs. nitrate-based solution route	Pechini-derived powder showed a higher specific surface area (5.5 vs. 1.6 m2 g−1)	[36]
LaNi1-xFexO3	High-energy ball milling	800 °C calcination; 1000 °C sintering	LNF64: ~50–400 nm after calcination and ~100–600 nm after sintering; particle agglomeration observed	[26]
LSCM	Water-based gel-casting	Oxide/carbonate slurry	Shorter diffusion distance; lower phase-formation temperature	[37,38,39]
LSM	Gel-casting	Slurry route	Lower phase-formation temperature; improved electrode microstructure and electrochemical performance	[40]
LaCoO3	Nitrate-based gel-casting	Solution-derived precursor	Nanopowders, ~31–60 nm	[41]
Lanthanum silicate apatite	Gel-casting	Gel-casting; sintering	Improved phase formation, sinterability, ionic conductivity	[42,43]

Note: Gel-casting studies on LSCM, LSM, LaCoO₃, and lanthanum silicate apatites are included as related processing examples rather than direct studies of LNF.

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Comparison of these studies reveals a clear trade-off among different LNF synthesis routes. Solution-based methods can provide good precursor mixing and phase homogeneity, whereas solid-state routes based on oxide precursors can involve fewer solution-processing steps but require effective mixing and appropriate thermal treatment. For practical use of LNF as an electrode or catalyst, a practical synthesis route should combine simple precursor chemistry with homogeneous multicomponent mixing, controlled powder properties, suitable thermal-processing conditions, good reproducibility, and potential for scale-up.

Gel-casting provides another approach for improving multicomponent mixing and phase formation. Although reported gel-casting studies have mainly focused on related SOFC perovskites rather than LNF itself, they provide useful evidence for the influence of this synthesis route on phase formation and powder properties. In gel-casting, the precursor components are homogeneously distributed and immobilized within a polymer network, reducing the diffusion distance required for solid-state reaction during calcination and thereby facilitating perovskite phase formation [37,38,39,40,42,43]. This can lower the phase-formation temperature compared with conventional solid-state synthesis. For La_0.75Sr_0.25Cr_0.5Mn_0.5O_3-δ (LSCM)-based materials, water-based gel-casting was reported to reduce the phase-formation temperature compared with conventional solid-state processing [37,38,39].

For La_1-xSr_xMnO₃ (LSM), gel-casting similarly reduced the phase-formation temperature compared with conventional solid-state synthesis and produced powders with improved electrode microstructure and electrochemical performance [40]. Gel-casting has also been applied to LaCoO₃ using nitrate precursors, producing nanoscale powders with particle sizes of approximately 31–60 nm [41]. Studies on lanthanum silicate apatites further showed that gel-casting improved phase formation, sinterability, and ionic conductivity compared with conventional solid-state processing [42,43].

The relevance of gel-casting to LNF lies mainly in its processing principle rather than in any single reported performance value. By combining homogeneous precursor mixing with immobilization before drying and calcination, gel-casting can help maintain precursor homogeneity and shorten the diffusion distance required for phase formation, while allowing the use of oxide, carbonate, or solution-derived precursors. These features make gel-casting a potentially useful route for future LNF powder preparation, particularly where phase purity, controlled microstructure, reproducibility, and scalability need to be considered together.

Overall, these studies show that the synthesis route strongly affects phase formation, particle characteristics, densification, and the measured functional properties of LNF and related oxides. However, synthesis alone does not determine the intrinsic behavior of LNF. The Ni/Fe ratio remains a fundamental compositional parameter because it strongly influences the bulk structure, electronic state, defect chemistry, and transport properties of the material. These composition-dependent effects are particularly important for SOFC cathodes, where electrical conductivity, oxygen reduction activity, thermal compatibility, and interfacial stability must be balanced simultaneously.

3. LNF for SOFC Cathodes: Role of the Ni/Fe Ratio

3.1. Intrinsic Properties of LNF Cathode Materials

The Ni/Fe ratio is a key compositional variable determining the suitability of LNF as an SOFC cathode. An early systematic study by Chiba et al. showed that substitution of Fe for Ni strongly affected the phase stability, crystal structure, and electrical conductivity of the LNF system [5]. Ni-rich compositions showed a greater tendency toward multiphase behavior, whereas LNF64 formed a single rhombohedral perovskite phase under the investigated conditions. The electronic conductivity also showed a strong composition dependence. LNF64 exhibited the highest conductivity in the series, reaching approximately 580 S cm⁻¹ at 800 °C [5]. Its average thermal expansion coefficient between 30 and 1000 °C was approximately 11.4 × 10⁻⁶ K⁻¹, close to the value of 10.0 × 10⁻⁶ K⁻¹ reported for YSZ [5]. However, a definitive monotonic relationship between the Ni/Fe ratio and TEC has not been established across the complete LNF series. At elevated temperatures, the measured expansion can also be influenced by chemical expansion associated with oxygen loss and transition-metal reduction [9]. These results identified LNF64 as an important reference composition for SOFC cathodes because it combines high electronic conductivity with favorable thermal compatibility and a relatively stable single-phase perovskite structure. Composition-dependent defect studies indicate that LNF64 exhibits the highest oxygen-vacancy concentration within the investigated LNF series, although its absolute oxygen non-stoichiometry remains relatively low compared with many conventional mixed ionic–electronic conducting cathodes. However, high electronic conductivity does not necessarily translate into high oxide-ion transport. LNF64 exhibits predominantly p-type electronic conduction and relatively limited oxygen non-stoichiometry, which helps explain its comparatively modest oxygen-transport properties [9,44]. Oxygen non-stoichiometry in LNF64 is also strongly dependent on temperature and oxygen partial pressure [45,46].

However, the composition with the highest bulk conductivity is not necessarily the optimal cathode composition under all operating conditions. Li et al. compared LNF compositions with x = 0.4–0.8 and observed clear composition-dependent SOFC performance [35]. LaNi_0.2Fe_0.8O₃ (LNF28) showed a maximum power density of approximately 497 mW cm⁻² at 650 °C, whereas LNF46 reached approximately 227 mW cm⁻² at 450 °C [35]. These results indicate that the preferred Ni/Fe ratio for SOFC operation can depend on temperature and on the balance among several composition-dependent properties, including electronic conductivity, phase stability, and oxygen-reduction activity. Thus, bulk electronic conductivity alone is insufficient to predict cathode performance, particularly when oxygen transport and surface oxygen-reduction kinetics become rate-limiting. Therefore, LNF64 should be regarded as an important reference composition rather than a universally optimal composition for all SOFC conditions. Figure 1 summarizes how changes in the Ni/Fe ratio influence the main SOFC-relevant properties of LNF.

The Ni/Fe ratio therefore plays a central role in determining the intrinsic properties of LNF for SOFC cathode applications. Phase stability generally improves with increasing Fe substitution, whereas electronic conductivity shows a non-monotonic dependence on composition and reached its maximum near x = 0.4 in the series investigated by Chiba et al. [5]. LNF64 represents an important compromise between these properties, but the preferred composition can shift with operating temperature and cathode requirements. The practical performance of LNF cathodes therefore depends not only on bulk composition but also on electrode processing and electrolyte compatibility.

3.2. Electrochemical Performance and Electrolyte Compatibility of LNF64

LNF64 was subsequently investigated extensively as an SOFC cathode because of its favorable electronic conductivity and thermal compatibility [5]. Orui et al. demonstrated high cell performance using an LNF64 cathode with a scandia- and alumina-stabilized zirconia (SASZ) electrolyte [47]. Their study also highlighted an important processing trade-off. Increasing the cathode sintering temperature improved electrode consolidation, but at the same time promoted interfacial reactions with the zirconia-based electrolyte. In the LNF64/SASZ system, formation of a highly resistive La₂Zr₂O₇ interfacial phase became increasingly evident above approximately 1000 °C, and the reaction layer exceeded 100 nm after sintering at 1200 °C [47]. Similar LNF–zirconia interfacial reactions have been reported in other studies, although the temperature at which La₂Zr₂O₇ becomes detectable depends on the specific electrode composition, electrolyte, and thermal-treatment conditions [5,48]. After optimization of cathode processing and electrochemical preloading, the cell with the LNF64 cathode sintered at 1000 °C reached a maximum power density of 1.56 W cm⁻² at 800 °C [47]. These results illustrate the processing trade-off between achieving sufficient cathode consolidation and suppressing the formation of resistive interfacial phases.

The relatively limited oxide-ion transport of LNF also motivated the development of composite cathodes [9]. Bevilacqua et al. compared pure LNF64 with LNF–SDC composite electrodes and found significantly lower area-specific resistance for the composite electrode [33]. The improvement was attributed to the combination of the high electronic conductivity of LNF and the high ionic conductivity of SDC. The similar apparent activation energies of pure LNF and LNF–SDC suggested that the addition of SDC did not substantially alter the dominant electrochemical process, but instead improved ionic transport and extended the electrochemically active region [33]. Thus, the electrochemical performance of a fixed LNF64 composition can be further improved by addressing its limited oxide-ion transport at the electrode level. Representative properties and SOFC performance of LNF-based cathode materials are summarized in

Table 2. Representative properties and SOFC performance of LNF-based cathode materials.

LNF Composition	Electrode/Cell Strategy	Temperature (°C)	Representative Result	Ref.
LaNi1-xFexO3 x = 0–1	Bulk property screening	800 (conductivity); 30–1000 (TEC)	Maximum conductivity ≈580 S cm−1 at x = 0.4; TEC ≈ 11.4 × 10−6 K−1 for LNF64	[5]
LaNi1-xFexO3 x = 0.4–0.8	Cathode composition screening	450–650	LNF28: 497 mW cm−2 at 650 °C; LNF46: 227 mW cm−2 at 450 °C	[35]
LNF64	LNF/SASZ cathode	800	Maximum power density 1.56 W cm−2 after optimized processing/preloading	[47]
LNF64-SDC	Composite cathode	—	Lower area-specific resistance than single-phase LNF; extended electrochemically active region	[33]
LNF64–GDC	Composite cathode	650–850	Best performance near 50:50 LNF:GDC among the tested compositions	[30]
GDC-infiltrated LNF64	GDC-infiltrated cathode	750	Rp ≈ 0.115 Ω cm2	[29]
La0.94Ni0.6Fe0.4O3	A-site deficiency	750	Rp ≈ 0.61 Ω cm2; peak power ≈ 0.37 W cm−2	[31]
LNF64/LNO	Nanostructured infiltrated H-SOFC cathode	700	Rp ≈ 0.027 Ω cm2; peak power ≈ 969 mW cm−2	[49]
LNF64 nanofiber	H-SOFC cathode	700	Rp ≈0.128 Ω cm2; peak power ≈551 mW cm−2	[50]
LNF46	Fe–Cr interconnect exposure	900	Higher Cr-poisoning tolerance than LSM	[7]
LNF64	Anode-supported cell/metallic interconnect	—	Stable operation up to 10,000 h; later-stage degradation ≈ 0.5% per 1000 h	[8]

Note: The reported values were obtained using different cell configurations, electrolytes, electrode architectures, and testing conditions and therefore should not be directly compared unless the experimental conditions are equivalent.

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3.3. Chromium Tolerance and Long-Term Durability

Another important advantage of LNF is its tolerance to chromium-containing species released from metallic interconnects. Zhen et al. investigated LNF46 in the presence of a Fe–Cr alloy interconnect and compared its behavior with that of LSM [7]. After operation at 900 °C and 200 mA cm⁻² for 20 h, little chromium deposition was observed on the LNF surface or at the LNF/YSZ interface, whereas substantial chromium deposition occurred at the LSM/YSZ interface [7]. These results highlight chromium tolerance as an important advantage of Sr-free LNF cathodes. However, the available studies on selected LNF compositions do not yet establish a systematic relationship between the Ni/Fe ratio and chromium tolerance.

Long-term operation has also been demonstrated for LNF cathodes under specific cell conditions. An anode-supported SOFC unit using an LNF64 cathode and uncoated metallic interconnects was operated for 10,000 h, with a reported voltage degradation rate of approximately 0.5% per 1000 h during the later stage of operation [8]. Nevertheless, LNF should not be considered completely immune to chromium-related degradation. Under accelerated testing at high current density, Komatsu et al. observed increased cathodic resistance and the formation of a Cr-related interfacial layer near the cathode/electrolyte interface [51]. Thus, the major advantage of LNF is better tolerance to chromium poisoning rather than complete resistance to chromium deposition. To date, a direct long-term comparison of LNF64, LNF28, and LNF46 under identical cell configurations and operating conditions has not been reported [8,35].

3.4. Electrode Engineering Beyond the Ni/Fe Ratio

Once a suitable Ni/Fe ratio has been selected, further improvement of LNF cathodes mainly relies on electrode, interface, and defect engineering. The main strategies reported for LNF-based cathodes are summarized in Figure 2.

For the widely studied LNF64 composition, subsequent studies increasingly focused on electrode engineering rather than further changes in the Ni/Fe ratio. One major strategy is to introduce an ionic conductor into the LNF electrode. In LNF/GDC composite electrodes, the electrochemical response improved as the GDC fraction increased up to approximately 50 vol%, whereas further addition reduced performance because of decreased connectivity of the electronically conducting LNF phase [30]. At a 50:50 LNF:GDC ratio, the electrode resistance was approximately 30 times lower than that of single-phase porous LNF, illustrating the balance between improved ionic-conducting pathways and retention of electronic connectivity [30]. Electrode architecture provided further improvement. At 750 °C, Huang et al. reported polarization resistances of 0.581, 0.452, and 0.115 Ω cm² for a conventional LNF/GDC composite, a functionally graded electrode, and a GDC-impregnated LNF electrode, respectively [29]. The improvement after impregnation was attributed to nanosized GDC distributed within the porous LNF network, which extended ionic-conduction pathways and the effective triple-phase-boundary (TPB) region [29]. Similar improvements have been reported using other ceria- or bismuth-oxide-containing composite architectures [52,53].

A second strategy is to modify the cathode surface or the cathode/electrolyte interface. Thin functional layers and ceria-based active layers can reduce interfacial polarization while leaving the bulk Ni/Fe ratio unchanged [54,55,56,57]. For example, the use of Pr-doped ceria in an LNF–ceria active layer reduced the interface resistance to approximately one-third of that obtained with Sm- or Gd-doped ceria under the reported conditions, which was associated with the mixed conductivity of Pr-doped ceria and an expanded electrochemically active region [55]. Infiltration provides another route for controlling the active surface and microstructure. In an LNF64-infiltrated LNO cathode architecture, an intermediate LNF loading of approximately 31 wt% produced a polarization resistance of 0.027 Ω cm² and a peak power density of 969 mW cm⁻² at 700 °C, whereas higher loading increased the resistance again [49]. These results show that surface and interface engineering can substantially improve reaction kinetics and interfacial transport without changing the underlying bulk composition.

Defect engineering provides another route for improving the performance of LNF64. Mei et al. introduced controlled La-site deficiency into La_1-yNi_0.6Fe_0.4O₃ (y = 0, 0.02, 0.04, 0.06, and 0.08) and found that La_0.94Ni_0.6Fe_0.4O₃ showed substantially lower polarization resistance than stoichiometric LNF [31]. The improvement was associated with changes in Fe valence and oxygen-vacancy-related surface chemistry. At 750 °C, the peak power density increased from 0.11 W cm⁻² for stoichiometric LNF to 0.37 W cm⁻² for La_0.94Ni_0.6Fe_0.4O₃ [31]. This result shows that, after the Ni/Fe ratio is selected, defect chemistry provides an additional means of improving ORR activity without changing the basic LNF framework.

The SOFC literature therefore supports a two-stage design strategy for LNF cathodes. The Ni/Fe ratio establishes the bulk property basis, whereas electrode architecture, interface design, and defect chemistry determine how effectively these properties are translated into cathode performance. Once an appropriate bulk composition is selected, composite formation, infiltration, interface engineering, microstructure control, and defect modification provide additional routes for improving electrochemical performance. This distinction between composition selection and post-compositional optimization also provides a useful basis for understanding LNF in other electrochemical and catalytic applications.

4. Oxygen Evolution Reaction Electrocatalysis

Beyond SOFC applications, LNF has also been investigated as a bifunctional oxygen electrocatalyst in alkaline media. Zhang et al. studied sol–gel-derived LaNi_1-xFe_xO₃ and showed that partial Fe substitution enhanced both ORR and OER activity in 0.1 M KOH [10]. Fe substitution altered the surface oxidation state of Ni, suppressed Ni²⁺ species, and modified the local Ni–O bonding environment [10], indicating that the Ni/Fe ratio can influence both directions of oxygen electrocatalysis. For OER, however, the preferred Ni/Fe ratio varies considerably among reported studies. OER activity depends not only on bulk electronic properties but also on metal–oxygen covalency, surface redox chemistry, catalyst morphology, and electrochemical surface reconstruction [4,11,13]. The following discussion therefore examines how the Ni/Fe ratio, together with synthesis and surface evolution under operating conditions, influences the OER behavior of LNF.

The beneficial effect of partial Fe substitution in LaNiO₃ was recognized in early studies. Singh et al. investigated LaNi_1-xFe_xO₃ with x = 0, 0.25, and 0.50 and found that LaNi_0.75Fe_0.25O₃ showed the highest OER activity among the tested compositions [58]. At 100 mA cm⁻² in 1 M KOH, its overpotential was approximately 395 mV, compared with 428 mV for LaNiO₃ and 437 mV for LaNi_0.5Fe_0.5O₃ (LNF55) [58]. These results provided early evidence that the effect of Fe substitution on OER activity is non-monotonic. Representative composition-dependent OER studies of LNF-based catalysts are summarized in

Table 3. Representative studies of composition-dependent OER performance in LNF-based catalysts.

Composition Range	Synthesis/Catalyst Form	Best Reported Composition	Representative OER Performance	Ref.
LaNi1-xFexO3, x = 0, 0.25, 0.50	Low-temperature precursor route	LaNi0.75Fe0.25O3	η100 = 395 mV; LaNiO3: 428 mV; LaNi0.5Fe0.5O3: 437 mV	[58]
LaNi1-xFexO3, x = 0–0.9	Co-precipitation	LaNi0.4Fe0.6O3	η10 = 439 mV; Tafel = 52 mV dec−1	[12]
LaNi1-xFexO3 thin films	Epitaxial thin films	LaNi0.625Fe0.375O3	η ≈ 330 mV at 0.05 mA cm−2 (oxide area basis); Tafel ≈ 60 mV dec−1	[13]
LaNixFe1-xO3, x = 0, 0.3, 0.5, 0.7, 0.8, 1.0	Sol–gel	LaNi0.8Fe0.2O3	η10 = 391 mV; Tafel = 102.8 mV dec−1	[14]
LaNi1-xFexO3, x = 0.1, 0.2, 0.5	Nanorods	LaNi0.8Fe0.2O3	η10 = 302 mV; Tafel = 50 mV dec−1	[59]
LaNi1-xFexO3, x = 0–1	Perovskite powders/reconstructed surface	LaNi0.9Fe0.1O3	Highest intrinsic activity before and after reconstruction	[11]
LaNiO3 + Fe incorporation	Electrochemical Fe incorporation	Dynamic Ni–Fe surface	η10 ≈ 340 mV	[16]
LaNi1-xFexO3, x = 0, 0.25, 0.50, 0.75, and 1.00	Carbon templating	No clear single optimum	Representative OER; performance: Similar η10 (~330–340 mV) among Fe-containing compositions.	[4]

Note: OER metrics are reported under different experimental conditions and should not be compared directly across studies.

.

Later studies over broader composition ranges further demonstrated that OER performance can vary substantially with the Ni/Fe ratio. Gozzo et al. prepared LaNi_1-xFe_xO₃ with x = 0, 0.3, 0.6, and 0.9 by co-precipitation and found that LNF46 showed the highest OER activity among the investigated compositions [12]. It exhibited an overpotential of approximately 439 mV at 10 mA cm⁻² and a Tafel slope of 52 mV dec⁻¹, compared with 465 mV and 76 mV dec⁻¹ for LaNiO₃ [12]. In contrast, Wang et al. reported that sol–gel-derived LNF82 showed the best performance in their LNF series, with an overpotential of 391 mV at 10 mA cm⁻² and a Tafel slope of 102.8 mV dec⁻¹ [14]. A similar preference for LNF82 was reported for LNF nanorods. LNF82 nanorods reached an overpotential of 302 mV at 10 mA cm⁻² with a Tafel slope of 50 mV dec⁻¹, and the enhanced activity was attributed to both increased accessible surface area and modification of the intrinsic electronic structure and O-Ni hybridization [59]. Both studies show that partial Fe substitution can improve the OER activity of LaNiO₃, while also demonstrating that the composition giving the highest activity is not fixed.

The origin of this composition dependence has been investigated more directly using epitaxial thin films. Wang et al. studied LaNi_1-xFe_xO₃ thin films over a broad Fe-substitution range and observed a volcano-like dependence of OER activity on Fe content, with x = 0.375 showing the highest activity [13]. Spectroscopic measurements and first-principles calculations showed that Fe substitution changed the Ni oxidation state and modified the electronic structure through Ni–O–Fe interactions. High-valent Fe species increased the transition-metal 3d bandwidth and enhanced 3d–O 2p hybridization, whereas at higher Fe contents, reduced 3d–O 2p hybridization was associated with the declining side of the volcano-type activity trend [13]. These results indicate that the beneficial effect of Fe substitution is not simply due to the introduction of additional Fe sites but is closely related to modification of the electronic structure of the mixed Ni–Fe–O framework.

A complementary thin-film study further highlighted the electronic origin of the beneficial Fe effect. Yun et al. compared trivalent dopants on well-defined (001) LaNiO₃ epitaxial surfaces and found that Fe incorporation enhanced OER activity [15]. Using well-defined epitaxial surfaces to minimize structural variations, the authors linked the activity enhancement to changes in the transition-metal d states near the Fermi level. This result further supports the view that Fe incorporation can enhance LaNiO₃-based OER activity by modifying the electronic states involved in charge transfer near the Fermi level.

More recent studies have shown that the Ni/Fe ratio also influences the surface chemistry that develops during OER. An et al. investigated LaNi_1-xFe_xO₃ with x = 0, 0.10, 0.25, 0.50, 0.75, and 1.00 and found that LNF91 showed the highest intrinsic activity in both the pristine and reconstructed states [11]. Importantly, the activity after reconstruction did not increase simply with the extent of surface reconstruction. Instead, the chemistry of the reconstructed surface was identified as the more important factor controlling OER activity. Fe substitution also lowered the O 2p energy level and stabilized lattice oxygen, thereby linking the bulk Ni/Fe composition to the chemistry and stability of the surface formed under OER conditions [11]. Consistent with this stabilization mechanism, LNF91 also showed negligible potential increase after the initial activation period during a 100 h chronopotentiometric durability test.

The involvement of lattice oxygen in LaNiO₃-based OER has also been demonstrated directly by isotope-labeling experiments. Using ¹⁸O-enriched epitaxial LaNiO₃ films, Liu et al. observed dynamic lattice-oxygen exchange within the near-surface region during OER, while the surface largely retained its crystalline perovskite structure [17]. This result provides a mechanistic basis for considering lattice-oxygen behavior when evaluating how Ni/Fe composition and metal–oxygen bonding influence LNF OER catalysis. More broadly, studies of other perovskite systems have shown that greater lattice-oxygen participation can correlate with higher intrinsic OER activity [60], although the extent of this relationship remains composition- and system-dependent. More directly for LNF, An et al. compared ¹⁸O-labeled LaNiO₃ and LNF91 using in situ differential electrochemical mass spectrometry and observed substantially lower lattice-oxygen participation after Fe substitution, indicating that Fe stabilizes lattice oxygen and suppresses lattice-oxygen involvement during [11].

The catalytically relevant Ni/Fe ratio under OER conditions may therefore differ from the nominal bulk composition. Yu et al. demonstrated this point by introducing trace Fe³⁺ ions into the electrolyte during electrochemical activation of LaNiO₃ [16]. Fe-containing surface species gradually formed during activation, and the activated catalyst reached an overpotential of approximately 340 mV at 10 mA cm⁻². The double-layer capacitance also increased from 0.10 to 2.18 mF cm⁻² after Fe incorporation [16]. The authors proposed a dynamic equilibrium between Fe³⁺ in the electrolyte and Fe-containing species at the catalyst surface, which could help maintain active Ni–Fe surface species during OER. These results indicate that the catalytically relevant Ni/Fe ratio should not always be treated as a fixed bulk parameter because the surface composition can evolve under electrochemical conditions. The activated catalyst also showed good durability: negligible overpotential degradation was observed after 6000 accelerated cycles, while the current density remained close to its initial value during 24 h chronoamperometric operation [16].

The apparent optimum Ni/Fe ratio can also be affected by synthesis and morphology. Kubo et al. prepared high-surface-area LNF materials using a modified carbon-templating route and found only small differences in OER activity among the Fe-containing compositions [4]. In this series, the Ni/Fe ratio had only a limited effect on the measured OER activity. Composition-dependent differences in coating density and surface accessibility were also observed, indicating that morphology and catalyst utilization can influence the apparent composition–activity relationship [4].

The reported studies therefore do not support a single universal optimum Ni/Fe ratio for OER. Preferred compositions range from Ni-rich LNF91 and LNF82 to intermediate LaNi_0.625Fe_0.375O₃ and Fe-richer LNF46 [11,12,13,14]. These differences likely reflect variations in synthesis route, catalyst morphology, surface area, crystallinity, strain, electrode preparation, electrolyte conditions, and surface reconstruction rather than simple inconsistencies among individual studies. A more consistent conclusion is that partial Fe substitution can improve the OER activity of LaNiO₃, while the composition giving the highest activity depends strongly on the catalyst state and operating conditions.

The role of the Ni/Fe ratio in LNF OER catalysts can therefore be understood at two interconnected levels. In the bulk perovskite, Fe substitution modifies Ni valence, metal–oxygen covalency, and the electronic structure of the Ni–O–Fe framework. Under OER conditions, the bulk composition also influences surface reconstruction and the formation and stability of Ni–Fe-containing active species. Synthesis and morphology further influence how these composition-dependent properties are expressed at the catalyst surface. The Ni/Fe ratio should therefore be selected together with consideration of catalyst microstructure and the operating surface state, rather than treated as an isolated compositional variable.

5. Other Applications of LNF

Beyond SOFC cathodes and OER electrocatalysis, LNF has also been explored in several other applications, including pseudocapacitive energy storage, photocatalysis, methane reforming, tar cracking, and chemical-looping processes. Although the literature on these applications is less extensive than that on SOFC cathodes and OER electrocatalysis, these studies further demonstrate the application-dependent role of the Ni/Fe ratio. Each application requires a different balance among redox properties, oxygen-defect chemistry, electronic structure, lattice-oxygen mobility, reducibility, and structural stability. Consequently, the preferred Ni/Fe ratio can differ substantially from those identified for SOFC cathodes or OER catalysts.

5.1. Pseudocapacitive Energy Storage

The mixed redox chemistry of Ni and Fe makes LNF attractive for pseudocapacitive energy storage. Alexander et al. investigated LaNi_1-xFe_xO₃ with x = 0, 0.15, and 0.55 and used Fe substitution to tune the relative potentials of the Ni²⁺/Ni³⁺ and Fe³⁺/Fe⁴⁺ redox couples [18]. Fe incorporation shifted the Ni-related redox process toward higher potentials and the Fe-related process toward lower potentials through an inductive effect associated with Ni–O–Fe interactions. The pseudocapacitive charge storage of these LNF materials was dominated mainly by surface-based Ni and Fe redox processes rather than bulk ion intercalation [18].

Among the investigated compositions, LaNi_0.45Fe_0.55O₃ provided the largest separation between the Ni- and Fe-related redox processes, with a potential difference of approximately 0.95 V [18]. A symmetric pseudocapacitor using this composition delivered an energy density of 4.3 Wh kg⁻¹ at a power density of 250,000 W kg⁻¹. When LaNi_0.45Fe_0.55O₃ was combined with brownmillerite SrFeO_2.5 (BM-SFO) in an asymmetric configuration, the device reached an energy density of 19.5 Wh kg⁻¹ at 394 W kg⁻¹, with a cell voltage of 1.8 V [18]. These results show that, for pseudocapacitive energy storage, the Ni/Fe ratio can be used to tune the relative potentials of multiple redox processes rather than simply to maximize electronic conductivity.

5.2. Photocatalysis

LNF has also been investigated as a visible-light-responsive photocatalyst. Zeng et al. prepared a series of LNF materials by a sol–gel method and found a clear non-monotonic dependence of photocatalytic activity on Fe content [19]. Partial Fe substitution modified the crystal structure and increased the concentration of oxygen-vacancy-related defects. Among the investigated compositions, LaNi_0.95Fe_0.05O₃ showed the highest photocatalytic activity, achieving 98.89% degradation of methyl orange after 120 min of visible-light irradiation [19].

The composition dependence was also closely related to particle characteristics and defect chemistry. At relatively low Fe contents, no pronounced changes in particle morphology and size were observed, whereas higher Fe contents promoted particle growth, agglomeration, and morphological changes [19]. The authors attributed the high activity of LaNi_0.95Fe_0.05O₃ to its relatively small particle size, moderate crystallinity, and suitable concentration of oxygen vacancies [19]. At higher Fe contents, excessive oxygen-vacancy formation and lattice distortion could instead promote recombination of photogenerated electrons and holes, reducing photocatalytic activity [19]. Thus, the optimum composition resulted from a balance between defect chemistry and particle characteristics rather than from continuously increasing the Fe content.

After selection of the LNF composition, photocatalytic performance can be further improved through interface engineering. Bao et al. prepared LaNi_1-xFe_xO₃ with x = 0, 0.2, 0.4, 0.6, 0.8, and 1.0 and found that LNF82 showed the highest photocatalytic activity among the investigated LNF compositions, reaching a tetracycline degradation efficiency of 24.9% [20]. Fe substitution increased the concentration of oxygen vacancies, which was associated with improved trapping and separation of photogenerated carriers. The selected LNF82 composition was then coupled with g-C₃N₄ to form a type-II heterojunction. The optimized 1:1 LNF82/g-C₃N₄ composite achieved 86.1% tetracycline removal after 120 min, with an apparent rate constant approximately 4.8 times that of pure LNF82 and also achieved 98.2% Cr(VI) removal within 60 min under visible-light irradiation [20]. The heterojunction promoted the separation and transfer of photogenerated charge carriers, while the photocatalytic performance remained relatively stable after three cycles [20]. This study provides a clear example of a two-step design strategy in which a suitable Ni/Fe composition is first selected, and photocatalytic performance is then further enhanced through interface engineering.

5.3. Thermocatalytic and Chemical-Looping Applications

The reducibility and lattice-oxygen chemistry of LNF make it useful for high-temperature catalytic reactions. Jahangiri et al. investigated LNF-based materials for combined reforming of methane with CO₂ and O₂ and found that the Ni/Fe ratio strongly affected reducibility and catalytic performance [21]. Ni-rich compositions showed higher reforming activity, which was associated with the easier formation of metallic Ni under reaction conditions, whereas increasing Fe content made reduction progressively more difficult [21]. These results indicate that, in methane reforming, the Ni/Fe ratio controls the balance between reducibility and oxide stability, with sufficient Ni being required to generate the reduced Ni-containing active phase.

More recent work has shown that the initial Ni/Fe ratio can also influence the active phase formed under reaction conditions. Coronado-Delgadillo et al. investigated glycerol dry reforming over LaNi_1-xFe_xO₃ with x = 0, 0.25, 0.50, 0.75, and 1.00 [22]. Under reducing conditions, Ni nanoparticles were exsolved from LaNiO₃, whereas Ni–Fe nanoparticles formed from the Fe-containing LNF compositions; LaFeO₃ showed no evidence of Fe exsolution. The exsolved Ni–Fe nanoparticles showed no evidence of carbon deposition or thermal agglomeration during the investigated reaction period [22]. Among the investigated compositions, LNF55 showed the highest catalytic performance. At steady state, its glycerol consumption rate was approximately three times that of LaNiO₃ in the absence of co-fed CO₂ and approximately 1.7 times higher at a CO₂/glycerol feed ratio of 1 [22]. The reaction rate increased with Fe substitution up to x = 0.50 and decreased at higher Fe contents, showing a clear non-monotonic dependence on the Ni/Fe ratio. A similar trend was observed for the estimated surface Ni⁰/Ni²⁺/³⁺ ratio, suggesting a possible relationship between catalytic activity and the presence of metallic Ni in the exsolved Ni–Fe nanoparticles, although the authors noted that the extent of this relationship remains uncertain [22]. The improved performance of LNF55 was further associated with its greater resistance to deactivation under reaction conditions. These results show that the Ni/Fe ratio influences not only the properties of the initial perovskite but also the composition, stability, and evolution of the active metallic phase generated during operation.

Metallic-phase formation, however, is not always beneficial. Cui et al. compared fresh and reduced LaNi_1-xFe_xO₃ catalysts for coal-tar cracking at 700 °C and found that the fresh perovskites generally gave higher gas yields, whereas the reduced catalysts produced more tar and carbon deposition [23]. Among the fresh compositions, LNF82 showed the best overall performance, giving the highest total gas yield of 34.8 mmol g_coal⁻¹ and H₂ yield of 20.9 mmol g_coal⁻¹, together with the lowest tar production of 0.05% and carbon deposition of 10.9% [23].

The superior performance of the fresh perovskite catalyst was closely related to oxygen transfer through the perovskite lattice. Lattice oxygen migrated from the bulk to the surface and participated directly in tar oxidation, while water formed during the reaction could replenish oxygen in the perovskite and help maintain an oxygen-transfer cycle [23]. Reduction disrupted the original perovskite structure and generated surface metallic phases, changing the dominant reaction pathway and decreasing gas conversion while increasing tar and carbon deposition [23]. These results show that reduction and metallic Ni formation are not universally beneficial; when lattice oxygen participates directly in the catalytic reaction, retention of the perovskite structure can instead be important for maintaining high activity and suppressing carbon deposition.

Surface and interface engineering provides another level of optimization for related Ni–Fe perovskite-type catalysts. Wu et al. prepared La₂Ni_2-xFe_xO₆@CeO₂ core–shell materials with x = 0.2, 0.4, 0.6, 0.8, and 1.0 for dry reforming of methane [32]. Among the investigated compositions, La₂Ni_1.6Fe_0.4O₆@CeO₂ showed the highest performance, reaching CH₄ and CO₂ conversions of 93.12% and 89.95%, respectively, with an H₂/CO ratio close to 1.0. During a 41 h stability test, both conversions decreased by less than 10%, while the average H₂/CO ratio remained approximately 1.01 [32]. The spent catalyst showed a thermogravimetric analysis (TGA) weight loss of only 5.82%, further supporting its good resistance to carbon deposition [32]. The authors attributed the improved behavior to the combined effects of Ni/Fe tuning and the CeO₂ shell, which increased the specific surface area and promoted oxygen-vacancy- and lattice-oxygen-related properties. These results illustrate how Ni/Fe composition tuning can be combined with core–shell interface engineering to improve reforming activity, stability, and carbon resistance.

Chemical-looping processes place a different requirement on the Ni/Fe ratio because the oxide must repeatedly transfer lattice oxygen while maintaining sufficient reaction activity and structural stability. Zhang et al. investigated glycerol chemical-looping steam reforming using LaNi_1-xFe_xO_3-δ oxygen carriers with different Ni contents [25]. Among the investigated Ni-doped compositions, LaNi_0.1Fe_0.9O_3-δ (LNF19) showed the best overall hydrogen-production performance, with an H₂ selectivity of approximately 91%. During 10 redox cycles, the H₂ selectivity remained close to 90% [25]. These results indicate that relatively limited Ni substitution can improve the reactivity of Fe-rich oxygen carriers while retaining good cyclic hydrogen-production performance.

A similar preference for relatively Fe-rich compositions was observed in chemical-looping reforming of landfill gas followed by water splitting. Yao et al. investigated LaNi_1-xFe_xO₃ with x = 0, 0.2, 0.4, 0.6, and 0.8 and found that LaNi_0.2Fe_0.8O₃ (LNF28) showed the best overall performance [24]. CH₄ and CO₂ conversions reached approximately 87.2% and 94.9%, respectively, with syngas selectivity close to 99%, while the subsequent water-splitting step produced hydrogen with a purity of approximately 98.9%. After 20 redox cycles, CH₄ and CO₂ conversions remained approximately 84.6% and 92.4%, respectively, syngas selectivity remained about 95.1%, and the hydrogen purity remained close to 94.7% [24].

Moderate Ni incorporation enhanced Fe–Ni interactions and lattice-oxygen mobility, whereas excessive Ni promoted methane cracking, carbon deposition, and metal agglomeration [24]. Together with the glycerol chemical-looping results, these studies suggest that Fe-rich LNF compositions with limited Ni substitution can provide a useful balance among reaction activity, reversible lattice-oxygen transfer, carbon resistance, and cyclic stability. Representative applications and preferred compositions of LNF and related Ni–Fe perovskite systems are summarized in

Table 4. Representative applications of LNF and related Ni–Fe perovskite systems beyond SOFC cathodes and OER electrocatalysis.

Application	Material/Composition Range	Preferred Composition	Representative Performance	Ref.
Pseudocapacitive energy storage	LaNi1-xFexO3-δ, x = 0, 0.15, 0.55	LaNi0.45Fe0.55O3	4.3 Wh kg−1 at 250,000 W kg−1 for the symmetric LNF55/LNF55 device; 19.5 Wh kg−1 at 394 W kg−1 for asymmetric BM-SFO/LNF55 device	[18]
Visible-light photocatalysis	LaNi1-xFexO3, x = 0, 0.05, 0.10, 0.15, 0.20, and 1.0	LaNi0.95Fe0.05O3	98.89% methyl-orange degradation after 120 min	[19]
Heterojunction photocatalysis	LaNi1-xFexO3, x = 0–1	LaNi0.8Fe0.2O3/g-C3N4	86.1% TC removal in 120 min; 98.2% Cr(VI) removal in 60 min	[20]
Methane reforming with CO2/O2	LaNi1-xFexO3	Ni-rich compositions	Higher Ni content favored reduction and methane-reforming activity	[21]
Glycerol dry reforming/exsolution	LaNi1-xFexO3, x = 0–1	LaNi0.5Fe0.5O3 (LNF55)	Highest performance; steady-state glycerol consumption rate ≈ 3× that of LaNiO3 without co-fed CO2 and ≈1.7× at CO2/glycerol = 1	[22]
Coal-tar cracking	LaNi1-xFexO3-δ	LaNi0.8Fe0.2O3	Gas yield 34.8 mmol gcoal−1; H2 yield 20.9 mmol gcoal−1, tar 0.05%; carbon deposition 10.9%	[23]
Dry reforming of methane *	La2Ni2-xFexO6@CeO2	La2Ni1.6Fe0.4O6@CeO2	CH4 conversion 93.12%; CO2 conversion 89.95%; H2/CO ≈ 1.0; <10% conversion loss over 41 h	[32]
Glycerol chemical-looping reforming	LaNi1-xFexO3-δ	LaNi0.1Fe0.9O3-δ	H2 selectivity ≈ 91%; ≈90% after 10 cycles	[25]
Chemical-looping reforming/H2 production	LaNi1-xFexO3-δ, x = 0–0.8	LaNi0.2Fe0.8O3	CH4 conversion ≈ 87.2%; CO2 conversion ≈ 94.9%; syngas selectivity ≈ 99%; H2 purity ≈ 98.9%	[24]

* The La₂Ni_2−xFe_xO₆ notation and “double-perovskite-type” designation follow the original study [32]. Because clear evidence of long-range Ni/Fe B-site ordering was not provided, these materials are treated here as related Ni–Fe perovskite-type core–shell systems rather than as confirmed ordered double perovskites.

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These applications further demonstrate that there is no universally optimal Ni/Fe ratio for LNF-based materials. A relatively Fe-rich composition was useful for separating Ni- and Fe-related redox processes in pseudocapacitive energy storage, whereas only a small amount of Fe was required for the highest activity in the reported single-phase photocatalytic series. In thermocatalytic reactions, the preferred composition depends strongly on whether the reaction benefits from the formation of metallic Ni–Fe active phases or from retention of the perovskite lattice for oxygen transfer. The initial Ni/Fe ratio can also influence the nature of the active phase generated through exsolution under reaction conditions. In chemical-looping processes, relatively Fe-rich compositions with lower Ni contents have shown favorable balances among reaction activity, reversible oxygen transfer, carbon resistance, and cyclic stability. Beyond composition itself, heterojunction and core–shell structures further demonstrate that surface and interface engineering can improve performance after a suitable Ni/Fe ratio has been selected. Therefore, the Ni/Fe ratio should be selected according to the dominant reaction pathway and required functional properties, while synthesis, operating-state evolution, microstructure, and surface/interface engineering provide additional opportunities for optimization.

6. Challenges and Perspectives

The future development of LNF should focus on translating its highly tunable Ni/Fe chemistry into application-specific and practically useful materials. Because different applications require different balances among electronic conductivity, oxygen transport, reducibility, structural stability, and catalytic activity, future studies should focus on defining application-specific composition windows under well-controlled synthesis and testing conditions rather than searching for a universally optimal LNF composition.

A second challenge is to reproducibly realize the selected composition through synthesis and scale-up. Different preparation routes can strongly affect phase formation, particle size, surface area, densification, and measured functional properties even at the same nominal Ni/Fe ratio [27,28]. Future synthesis studies should therefore consider not only laboratory-scale performance but also compositional homogeneity, batch-to-batch reproducibility, precursor utilization, thermal-processing requirements, and scalability. Mechanically assisted solid-state processing and other simplified routes may reduce processing complexity [26], while gel-casting studies on related multicomponent oxides illustrate how homogeneous precursor distribution can facilitate phase formation [37,38,39,40,42,43]. Establishing scalable routes that preserve the desired phase, microstructure, and functional properties will be essential if LNF is to progress beyond laboratory studies. To date, well-documented pilot-scale production or industrial collaborations specifically focused on LNF powder synthesis remain scarce, highlighting the need for systematic scale-up studies.

The third challenge is that the as-prepared composition may not represent the material that operates during reaction. Surface reconstruction, cation redistribution, lattice-oxygen exchange, reduction, and exsolution can substantially change the active state. This behavior has already been observed during OER and high-temperature catalysis, where dynamic Fe incorporation, surface reconstruction, and formation of Ni or Ni–Fe nanoparticles can strongly influence activity [11,16,17,22]. Operando or quasi-operando spectroscopy, diffraction, microscopy, and oxygen-transport measurements should therefore be increasingly combined with controlled composition studies to connect the nominal Ni/Fe ratio with the true active structure. Such measurements are particularly important for distinguishing intrinsic Ni/Fe-composition effects from changes caused by the operating environment.

Long-term stability remains another critical challenge. Although LNF64 has demonstrated operation for up to 10,000 h in an SOFC configuration, measurable degradation remained, and chromium-related degradation can still occur under demanding operating conditions [8]. For OER and high-temperature catalytic applications, durability is further complicated by surface reconstruction, cation redistribution, particle coarsening, and phase evolution. Future studies should therefore combine activity measurements with standardized long-duration testing under clearly defined operating conditions so that the effects of Ni/Fe composition on both activity and stability can be compared more reliably across materials.

Performance enhancement should then be treated as a second stage of materials design rather than as a substitute for composition optimization. In SOFC cathodes, composite formation, infiltration, functional interlayers, defect engineering, and microstructure control have substantially improved LNF performance without changing the basic Ni/Fe framework [29,30,31]. Similar concepts are emerging in photocatalytic heterojunctions and Ni–Fe perovskite-type core–shell catalysts [20,32]. These studies suggest a practical hierarchy for future LNF development: first identify the Ni/Fe composition that provides the required bulk property window; next establish a reproducible and scalable synthesis route; then determine how the material evolves under operating conditions; and finally use surface, interface, defect, and microstructure engineering to improve application-specific activity and stability. This composition–synthesis–operating-state–engineering framework may provide a more rational path from empirical LNF screening toward scalable and predictive materials design.

To provide a cross-application perspective for future LNF design, the relationships among Ni/Fe composition, synthesis/processing, key functional properties, and targeted applications are summarized in

Table 5. Cross-application summary of Ni/Fe composition, synthesis/processing, key properties, and design considerations for LNF-based materials.

Targeted Application	Representative Ni/Fe Composition or Trend	Representative Synthesis/Processing	Key Composition-Dependent Properties	Practical Design Consideration	Ref.
SOFC cathodes	LNF64 as an important reference composition; preferred ratio may shift with operating temperature	Co-precipitation, Pechini/gel–citrate, GNP, solid-state routes; followed by electrode/interface engineering	Electronic conductivity, phase stability, thermal compatibility, oxygen transport	Select a composition balancing conductivity and stability; subsequently optimize ionic transport, microstructure, and electrode/electrolyte interface	[5,27,29,30,31]
OER electrocatalysis	No single optimum; reported preferred compositions range from Ni-rich LNF91/LNF82 to intermediate or more Fe-rich compositions	Co-precipitation, sol–gel, thin-film growth, nanostructuring; electrochemical surface reconstruction during operation	Ni/Fe valence, metal–oxygen covalency, electronic states, oxygen-defect chemistry, surface reconstruction	Ni/Fe ratio should be optimized together with morphology and the operating surface state rather than treated as an isolated variable	[11,12,13,14,15,16,17,59]
Pseudocapacitive energy storage	LaNi0.45Fe0.55O3-δ	Pechini synthesis followed by controlled reduction	Relative potentials of Ni2+/Ni3+ and Fe3+/Fe4+ redox processes; surface anion-redox behavior	Optimize the Ni/Fe ratio to separate complementary redox processes rather than simply maximize conductivity	[18]
Visible-light photocatalysis	LaNi0.95Fe0.05O3 for the reported single-phase series; LNF82 used for heterojunction engineering	Sol–gel synthesis; subsequent coupling with g-C3N4	Oxygen-vacancy-related defects, particle characteristics, lattice distortion, carrier recombination	Limited Fe substitution can provide a favorable defect concentration; after composition selection, heterojunction engineering can further improve charge separation	[19,20]
Reforming/exsolution catalysis	Ni-rich compositions or intermediate LNF55, depending on reaction conditions	Reduction under reforming conditions; in situ formation/exsolution of Ni or Ni–Fe active particles	Reducibility, hydrocarbon activation, active-metal formation, carbon resistance	Higher Ni generally facilitates reduction and fuel activation, but the active state and resistance to carbon formation must be considered simultaneously	[21,22]
Coal-tar cracking	LaNi0.8Fe0.2O3	Retention of the fresh perovskite state rather than prior reduction	Lattice-oxygen transfer, tar oxidation, structural stability, carbon formation	Retaining the perovskite lattice can be more important than maximizing metallic Ni formation when lattice oxygen participates directly in the reaction	[23]
Chemical-looping reforming/H2 production	Fe-rich LNF with limited Ni substitution, e.g., LNF19 and LNF28	Sol–gel-derived oxygen carriers; repeated reduction/reoxidation cycles	Reversible lattice-oxygen transfer, reducibility, carbon resistance, cyclic stability	A relatively Fe-rich matrix with moderate Ni provides a favorable balance between reaction activity and reversible oxygen-transfer stability	[24,25]

.

7. Conclusions

LaNi_1-xFe_xO_3-δ provides a versatile compositional platform in which the Ni/Fe ratio can be used to tune electronic structure, defect chemistry, transport properties, reducibility, and structural stability. The studies reviewed here show that these composition-dependent properties lead to markedly different preferred Ni/Fe ratios for different applications. For SOFC cathodes, LNF64 remains an important reference composition because of its favorable balance of electronic conductivity, thermal compatibility, and structural stability, although other compositions can become preferable under different operating conditions. In OER electrocatalysis, the reported preferred compositions span a much broader range because activity depends not only on the bulk Ni/Fe ratio but also on surface chemistry, morphology, and reconstruction during operation. Pseudocapacitive energy storage, photocatalysis, reforming, tar cracking, and chemical-looping studies further demonstrate that the preferred composition depends on the specific redox, electronic, and oxygen-transfer properties required by each process.

The nominal bulk Ni/Fe ratio should therefore be regarded as the starting point rather than the sole determinant of LNF performance. Synthesis route and thermal processing influence phase formation and microstructure, while the material may further evolve through reconstruction, reduction, lattice-oxygen exchange, or exsolution under operating conditions. After an appropriate composition is selected, electrode architecture, defect control, and surface/interface engineering can provide further performance improvements. Thus, future LNF development should focus on matching the Ni/Fe ratio to the requirements of the intended application, followed by suitable preparation and application-specific optimization. This composition-centered but application-specific strategy can support the development of LNF materials with improved performance, stability, and practical applicability.