Iron-Rich Perovskite Electrodes Push Symmetric Fuel Cells Into the Intermediate-Temperature Zone

Solid oxide fuel cells promise some of the highest efficiencies of any electrochemical energy-conversion technology, yet their commercial breakthrough has long been held hostage by a stubborn materials problem: the electrodes. Now a research team from Argentina, Chile, and Spain reports that a family of nanocrystalline strontium titanate-ferrite perovskites, SrTi1−xFexO3−δ, can serve as cobalt-free symmetric electrodes that remain highly active in both oxidizing and reducing atmospheres, opening a practical route toward intermediate-temperature solid oxide fuel cells that operate below 800 °C.

The study, published in Frontiers in Energy Research, was led by Esteban Asto-Ramos, María Belén Arcentales-Vera, and Diego G. Lamas of the Instituto de Tecnologías Emergentes y Ciencias Aplicadas (CONICET-Universidad Nacional de San Martín), with collaborators from Universidad Austral de Chile, the Centro Atómico Constituyentes, and the ALBA Synchrotron Light Source. The work targets symmetric solid oxide fuel cells, in which the same material functions as both anode and cathode. This design simplifies cell fabrication and minimizes compatibility issues arising from differences in thermal expansion, chemical interactions, and degradation mechanisms between the two electrodes, but it demands a material that can survive and perform under both air and fuel.

The researchers synthesized three compositions, STF20, STF35, and STF50, corresponding to x = 0.20, 0.35, and 0.50, using a gel-combustion route with citric acid and glycine as fuels and complexing agents, followed by calcination at 800 °C. X-ray diffraction with Rietveld refinement confirmed that all as-prepared powders form single-phase cubic perovskites with space group Pm-3m, and the refinements yielded coherent diffraction domain sizes of roughly 47 to 51 nm. Field-emission scanning electron microscopy revealed mean primary particle sizes of 52 ± 5, 43 ± 4, and 53 ± 4 nm for STF20, STF35, and STF50 respectively, values consistent with the diffraction results and confirming the nanoscale character of the combustion-derived powders.

Increasing the Fe content produced a monotonic contraction of the cubic lattice parameter from 3.90588(12) Å for STF20 to 3.89699(10) Å for STF50. The authors attribute this contraction not to ionic size alone, since Fe3+ in octahedral coordination has an ionic radius of 0.605 Å, but to the combined effects of cation substitution, mixed Fe3+/Fe4+ valence, oxygen-vacancy formation for charge compensation, and modifications of the local Fe–O bonding environment that alter the covalent character of the bonds and promote local distortions of the octahedra. X-ray photoelectron spectroscopy showed that titanium remains predominantly Ti4+ across the series, while the average Fe oxidation state rises from approximately 3.2 to 3.4 with increasing Fe substitution, indicating that charge compensation occurs mainly through changes in Fe valence and oxygen non-stoichiometry rather than through titanium reduction.

The XPS analysis also revealed a systematic surface-chemical evolution. The relative intensity of a higher-binding-energy Sr 3d doublet, assigned to surface-associated Sr species consistent with carbonate or hydroxide formation, is greater for STF20 and STF35 and substantially lower for STF50, together with a corresponding decrease in the higher-binding-energy O 1s contribution. This trend suggests that increasing Fe content reduces the abundance of carbonate-containing surface species, a finding relevant because carbonate formation has been shown to lower the electrocatalytic activity of perovskite oxides. Energy-dispersive spectroscopy mapping confirmed a homogeneous spatial distribution of Sr, Ti, Fe, and O throughout all samples, with no evidence of elemental segregation or local enrichment.

Temperature-programmed reduction experiments probed the redox behavior of the series. Reduction peaks centered near 356 °C and 407 °C are consistent with partial reduction of Fe cations through the Fe4+/Fe3+ and Fe3+/Fe2+ redox couples, while a broad feature above 650 °C, whose intensity grows with Fe content, is associated predominantly with the initial stages of reduction toward metallic iron. Ex situ X-ray diffraction after reduction treatments revealed the structural consequences. After reduction at 750 °C, the perovskite phase remains dominant in all samples, although metallic Fe is detected in every composition, and STF50 additionally exhibits the emergence of a Ruddlesden–Popper layered phase. After reduction at 900 °C, the transformation becomes far more pronounced: STF50 evolves into an RP-dominated multiphase system with phase fractions of 22.65 percent perovskite, 10.50 percent metallic Fe, and 66.85 percent RP phase, whereas STF20 remains almost entirely perovskite.

Electrochemical impedance spectroscopy on symmetric cells with LSGM electrolytes demonstrated that the Fe-rich compositions markedly outperform STF20 under both atmospheres. In air at 750 °C, the total area-normalized polarization resistance falls from approximately 0.125 Ω·cm2 for STF20 to roughly 0.02–0.03 Ω·cm2 for STF35 and STF50, and the fitted Warburg transport contribution drops dramatically from 0.1024 Ω·cm2 for STF20 to only 0.0040 and 0.0060 Ω·cm2 for STF35 and STF50 respectively. Under humidified diluted hydrogen (7 vol% H2/Ar), STF50 displays the smallest impedance response across the entire temperature range, with a polarization resistance of approximately 0.56 Ω·cm2 at 750 °C. Distribution of relaxation times analysis confirmed that Fe incorporation attenuates the dominant intermediate-frequency contribution and the secondary high-frequency processes, consistent with accelerated charge-transfer and surface-reaction kinetics. Increasing the hydrogen partial pressure to 97 percent further reduced the polarization resistance of STF50 to approximately 0.16 Ω·cm2 at 750 °C, indicating that higher fuel availability mitigates fuel-side polarization losses.

Arrhenius analysis yielded apparent activation energies for the overall area-specific resistance that increase with Fe content, rising from 0.95 eV for STF20 to 1.19 eV for STF50 in air, and from 0.94 eV to 1.20 eV under 7 percent hydrogen, with 1.31 eV for STF50 under 97 percent hydrogen. Despite these higher thermal activation barriers, STF50 consistently exhibits the lowest polarization resistance in both oxidizing and reducing environments, indicating that the enhanced mixed ionic-electronic conductivity and the larger density of active sites associated with Fe-rich defect chemistry offset the effect of the higher activation energy. Preliminary button-cell tests employing humidified hydrogen fuel and air oxidant confirmed the functional viability of STF50, delivering maximum power densities of approximately 60, 110, 210, and 330 mW·cm−2 at 700, 750, 800, and 850 °C respectively, with open-circuit voltages close to 1.05–1.08 V indicating adequate gas separation and high electrolyte density.

The authors emphasize that the electrochemical advantage of the Fe-rich compositions is accompanied by a greater sensitivity to reducing conditions, with STF50 undergoing the most pronounced structural transformation through Fe exsolution and Ruddlesden–Popper domain formation. Similar redox-driven transformations have been reported for other Fe-containing perovskites, and RP phases formed under reducing conditions in STF-based materials have been shown to reversibly transform back into the parent perovskite upon reoxidation, together with partial reincorporation of the exsolved nanoparticles into the oxide lattice. The researchers therefore regard STF50 as the most electrochemically active composition within the studied series rather than as an unequivocally optimal formulation for long-term operation, and they note that future work should focus on long-term testing, redox cycling, and post-test structural characterization to establish the stability limits of the most active compositions and clarify the role of reduction-induced phase transformations during cell operation.

Overall, the study demonstrates that the Fe/Ti ratio is a key parameter governing the structural, surface-chemical, redox, and electrochemical behavior of nanocrystalline STF electrodes, and positions Fe-rich titanate perovskites as a promising cobalt-free platform for next-generation intermediate-temperature symmetric solid oxide fuel cells. The compositional simplicity, tunable Fe/Ti ratio, and nanoscale character of the combustion-synthesized powders make this electrode family attractive for further development, particularly because operating within the intermediate-temperature range preserves the nanometric microstructure that provides a high density of active surface sites and relatively short transport pathways, whereas higher temperatures would promote particle coarsening and surface-area loss, reducing the benefits associated with the nanoscale architecture.

Subject of Research: Materials Science

Article Title: Nanocrystalline SrTi1-x Fex O3-δ perovskites as high-performance electrodes for intermediate-temperature symmetric SOFCs

Article References: Asto-Ramos, E., Arcentales-Vera, M. B., Troncoso, L., Gard, F. S., Huck-Iriart, C., & Lamas, D. G. (2026). Nanocrystalline SrTi1-x Fex O3-δ perovskites as high-performance electrodes for intermediate-temperature symmetric SOFCs. Frontiers in Energy Research, 14, Article 1948765. https://doi.org/10.3389/fenrg.2026.1948765

Image Credits: AI Generated

DOI: 10.3389/fenrg.2026.1948765

Keywords: solid oxide fuel cells, perovskites, electrodes, intermediate-temperature SOFC, cobalt-free materials, mixed ionic-electronic conductors, nanomaterials, Nanocrystalline, SrTi1-x, high-performance, intermediate-temperature, symmetric