In the past few years a novel concept of Solid Oxide Fuel Cells (SOFC) has been developed: the symmetrical Solid Oxide Fuel Cells (SSOFC). In this configuration, the same electrode material is used as anode and cathode. Several state-of-the-art SOFC electrode materials have been tested and optimised to operate in such a novel configuration, as is the case of the lanthanum chromites traditionally used as interconnect materials, lanthanum chromium manganites and strontium titanates generally used as anodes and lanthanum manganites typically used as cathodes. Fuel cell performances of approximately 800 mW cm⁻² and ∼500 mW cm⁻² under H₂ and CH₄ atmospheres have been obtained using some of these materials in symmetric configuration. Moreover, promising performances have also been reported for interconnect-based materials under city gas containing the impurity H₂S. An interesting feature of the compositions evaluated is their potential use as symmetrical electrodes for Solid Oxide Electrolysis Cells (SOEC) with efficiencies in the range of the conventional SOEC systems, i.e. LSM-YSZ/YSZ/Ni-YSZ. Hence, they may be considered all-in-one Symmetric and Reversible SOFCs (SR-SOFCs) with significant advantages compared to traditional configurations, regarding both fabrication and maintenance/operation. In this review, we provide an insight into the most common materials tested as symmetrical electrodes for SOFCs and SOECs, electrolytes employed, configurations tested, new fabrication processes and procedures for microstructural engineering.

In this paper we report the successful incorporation of high valence transition metals, i.e. Cr, Mo, W, V, Nb, Ti, Zr into SrFeO3−δ perovskite materials, for potential applications as symmetric electrode materials for Solid Oxide Fuel Cells. It is observed that the doping leads to a change from an orthorhombic structure (with partial ordering of oxygen vacancies) to a cubic one (with the oxygen vacancies disordered). These electrodes are chemically compatibles with Ce0.9Gd0.1O1.95 (CGO) and La0.8Sr0.2Ga0.8Mg0.2O3−δ (LSGM) electrolytes at least up to 1100 °C. Thermal annealing experiments in 5% H2–Ar at 800 °C also show the stability of the doped samples in reducing conditions, suggesting that they may be suitable for both cathode and anode applications. In contrast, reduction of undoped SrFeO3−δ leads to the observation of extra peaks indicating the formation of the brownmillerite structure with the associated oxygen vacancy ordering. The performance of these electrodes was examined on dense electrolyte pellets of CGO and LSGM in air and 5% H2–Ar. In both atmospheres an improvement in the area specific resistances (ASR) values is observed for the doped samples with respect to the parent compound. Thus, the results show that high valence transition metals can be incorporated into SrFeO3−δ-based materials and can have a beneficial effect on the electrochemical performance, making them potentially suitable for use as cathode and anode materials in symmetrical SOFC