Introduction
The transition toward sustainable energy systems and decarbonization has driven the development of energy conversion technologies integrated with renewable sources. In this context, solid oxide electrolysis cells (SOECs) are promising technologies for the electrochemical conversion of CO₂ and H₂O through co-electrolysis, producing synthesis gas and fuels from renewable electricity. Conventional electrodes, particularly Ni-YSZ anodes, suffer from degradation due to coking, sulfur poisoning, and loss of stability under variable operating conditions. Advanced mixed ionic–electronic conductors (MIECs), such as PBMO (PrBaMn₂O₅+δ), LSCM (La₀.₇₅Sr₀.₂₅Cr₀.₅Mn₀.₅O₃-δ), LSF (La₁₋ₓSrₓFeO₃-δ), LSCF (La₁₋ₓSrₓFe₁₋ᵧCoᵧO₃-δ), and LST (La₀.₂Sr₀.₈TiO₃+δ) [1], exhibit robust structural stability and versatile functionality. Among them, double-layer perovskites such as PBMO stand out due to their high electrical conductivity, fast oxygen exchange kinetics, excellent redox tolerance, and stability against carbon and sulfur. In this way, symmetric solid oxide cells have emerged as an alternative, employing a single ceramic material as both electrodes, which simplifies fabrication processes, reduces costs, and improves material compatibility, while also enabling their application as reversible electrodes in SOFC/SOEC systems [2].
Materials and methods
In this work, different PBMO samples were synthesized via mechanosynthesis of binary oxides using high-energy planetary ball milling. Symmetrical cells with a PBMO–GDC / GDC / YSZ / GDC / PBMO–GDC configuration were fabricated using YSZ electrolytes prepared by the tape-casting technique. A protective GDC layer and the electrode layers were deposited by semi-automatic ultrasonic spray coating. The electrode powders, mixed with 50 wt.% GDC, were symmetrically deposited onto the YSZ electrolyte and subsequently sintered. The performance of PBMO electrodes was evaluated through electrochemical and microstructural characterization, considering their activity in electrolysis and H2O/CO2 co-electrolysis. Electrochemical measurements were carried out using an open-flanges test set-up, which allowed a rigorous assessment of the activity of the electrodes in reversible operation, both in fuel cell and electrolyzer modes. Durability and redox stability during operation cycles were also analyzed, correlating post-mortem microstructural changes with electrochemical performance. Microstructural characterization included electrode morphology by scanning and transmission electron microscopy (SEM/TEM), and elemental mapping by energy-dispersive X-ray spectroscopy (EDX).
Results
The results showed that in steam electrolysis mode (75% H₂O / 25% H₂), the cells exhibited a clear temperature-dependent behavior, with a decrease in operating voltage as the temperature increased from 800 to 900 °C at a given current density. Current densities above 250 mA cm⁻² were achieved at voltages below ~1.35 V at 900 °C. Electrochemical impedance spectroscopy (EIS) revealed a significant reduction in polarization resistance with increasing temperature, indicating enhanced electrode kinetics. Under co-electrolysis conditions (45% H₂O / 45% CO₂ / 10% H₂), similar trends were observed, with improved performance at higher temperatures and current–voltage characteristics comparable to those obtained under steam electrolysis. The EIS spectra showed a decrease in total resistance with temperature, confirming the positive effect of thermal activation on charge-transfer processes and surface reactions during co-electrolysis. Short-term constant-current tests conducted at 850 °C under 50% H₂O and a current of 800 mA showed a stable voltage response over time, indicating good operational stability of the PBMO–GDC electrodes under high current density conditions. Post-mortem characterization by scanning electron microscopy (SEM) revealed a dense and homogeneous microstructure, good interfacial adhesion between layers, and the absence of delamination or fracture after electrochemical operation.
Conclusions
Mechanochemical synthesis enabled the production of phase-pure PBMO materials with a uniform morphology and properties suitable for electrode processing and integration into electrochemical cells. PrBaMn₂O₅+δ, employed simultaneously as both anode and cathode in symmetric SOECs, exhibited promising electrochemical performance under H₂O/CO₂ electrolysis and co-electrolysis conditions, with a clear enhancement in performance at higher operating temperatures. Symmetric cells with PBMO electrodes showed a homogeneous microstructure, adequate porosity, and excellent interfacial adhesion between layers, evidencing high structural integrity. In addition, constant-current tests confirmed the electrochemical operational stability of the PBMO-based symmetric configuration, with no signs of degradation observed during prolonged operation.
Acknowledgements
This work was carried out within the Renewable Energy and Hydrogen Program, included in the Complementary R&D&I Plans of the Spanish Ministry of Science, Innovation and Universities (MICIN), and was funded by the European Union's NextGenerationEU, under Component 17 of the Recovery, Transformation and Resilience Plan (C17.I01.P01).
It is also framed within the research project PID2024-162053OB-C31, funded by the Spanish Ministry of Science, Innovation and the State Research Agency (AEI), as well as the project SBPLY/24/180225/000095, funded by the Regional Government of Castilla-La Mancha (JCCM).
References
[1] Wang, M., Wang, J., Du, J., “A symmetrical solid oxide electrolysis cell supported by nanostructured electrodes for highly efficient CO2 electrolysis”, Journal of Power Sources 2024, 610, 234742.
[2] Garcia-Garcia, F.J., Sayagués, M.J., Gotor, et al., “A Novel, Simple and Highly Efficient Route to Obtain PrBaMn2O5+δ Double Perovskite: Mechanochemical Synthesis”, Nanomaterials 2021, 11, 380.
