Protonic ceramic fuel cells (PCFCs) are promising electrochemical devices for the efficient and clean conversion of hydrogen and low hydrocarbons into electrical energy. Their intermediate operation temperature (500–800 °C) proffers advantages in terms of greater component compatibility, unnecessity of expensive noble metals for the electrocatalyst, and no dilution of the fuel electrode due to water formation. Nevertheless, the lower operating temperature, in comparison to classic solid oxide fuel cells, places significant demands on the cathode as the reaction kinetics are slower than those related to fuel oxidation in the anode or ion migration in the electrolyte. Cathode design and composition are therefore of crucial importance for the cell performance at low temperature. The different approaches that have been adopted for cathode materials research can be broadly classified into the categories of protonic–electronic conductors, oxide-ionic–electronic conductors, triple-conducting oxides, and composite electrodes composed of oxides from two of the other categories. Here, we review the relatively short history of PCFC cathode research, discussing trends, highlights, and recent progress. Current understanding of reaction mechanisms is also discussed.

La_0.98Cr_0.75Mn_0.25O_3−δ–Ce_0.9Gd_0.1O_1.95 (LCM-CGO) nanocomposite layers with different LCM contents, between 40 and 60 wt %, are prepared in a single step by a spray-pyrolysis deposition method and evaluated as both air and fuel electrodes for solid oxide fuel cells (SOFCs). The formation of fluorite (CGO) and perovskite (LCM) phases in the nanocomposite electrode is confirmed by different structural and microstructural techniques. The intimate mixture of LCM and CGO phases inhibits the grain growth, retaining the nanoscale microstructure even after annealing at 1000 °C with a grain size lower than 50 nm for LCM-CGO compared to 200 nm for pure LCM. The synergetic effect of nanosized LCM and CGO by combining their high electronic and ionic conductivity, respectively, leads to efficient and durable symmetrical electrodes. The best electrochemical properties are found for 50 wt % LCM-CGO, showing polarization resistance values of 0.29 and 0.09 Ω cm² at 750 °C in air and H₂, respectively, compared to 2.05 and 1.9 Ω cm² for a screen-printed electrode with the same composition. This outstanding performance is mainly ascribed to the nanoscale electrode microstructure formed directly on the electrolyte at a relatively low temperature. These results reveal that the combination of different immiscible phases with different crystal structures and electrochemical properties could be a promising strategy to design highly efficient and durable air and fuel electrodes for SOFCs.

Advances in doping strategies have significantly improved the properties of SrFeO₃-based electrodes. However, challenges such as high thermal expansion coefficients and limited redox stability remain critical issues that require further investigation. This study focuses on the optimization of (Sr_1-xPr_x)_0.95FeO_3-δ (0 < x ≤ 1) series, evaluating the effects of praseodymium content on thermal expansion, redox stability, and electrochemical performance for potential application as both air and fuel electrodes in symmetrical solid oxide fuel cells. Rietveld refinements of X-ray and neutron diffraction data reveal a phase transformation from tetragonal to cubic symmetry with Pr content (0.2 ≤ x ≤ 0.4), followed by a transition to orthorhombic symmetry (x ≥ 0.6). Thermogravimetric and dilatometric analyses demonstrate that higher Pr content effectively reduces both oxygen nonstoichiometry and the thermal expansion coefficients, which decrease from 31 × 10^-6 K^-1 for x = 0.2 to 8.4 × 10^-6 K^-1 for x = 1. Meanwhile the electrical conductivity remains relatively unaffected by the Pr-content up to x = 0.8, reaching values as high as 116 S cm^-1 at 700 °C in air. Additionally, the electrode polarization resistances are relatively low across the series, e.g. 0.11 Ω cm² in air and 0.09 Ω cm² in H₂ for x = 0.6 at 700 °C, while exhibiting excellent redox cycling stability. These findings indicate that (Sr_1-xPr_x)_0.95FeO_3-δ (x ≥ 0.6) materials are promising electrodes, offering tunable thermal expansion and electrochemical properties for reliable performance in both oxidizing and reducing environments.

The use of active layers is a promising strategy to improve the properties of air electrodes for solid oxide fuel cells (SOFCs). In this work, La_0.8Sr_0.2MnO_3‐δ is combined with different oxide ion conductors Ce_0.9Gd_0.1O_1.95, Bi_1.5Y_0.5O₃ and Pr₆O₁₁ to form highly efficient nanocomposite active layers in a single step by spray‐pyrolysis deposition. One of the main advantages of these nanocomposite layers is that the nanoscale microstructure is retained at relatively high sintering temperatures. As a consequence, the interfacial area between the electrode and the electrolyte increases significantly, leading to fast oxide ion transport at the interface, as well as more active sites for the electrochemical reactions. An exhaustive structural, microstructural, and electrochemical characterization of the layers are performed to evaluate their potential use in SOFCs. The incorporation of a La_0.8Sr_0.2MnO_3‐δ‐Ce_0.9Gd_0.1O_1.95 (LSM‐CGO) interlayer decreases the polarization resistance of LSM cathode from 1.71 to 0.46 Ω cm² at 700 °C. Moreover, a single cell with LSM‐CGO interlayer generates a peak power density of 1.20 W cm^–2 at 800 °C compared to 0.79 W cm^–2 for the same cell without active layer. These results demonstrate the great benefits of using nanocomposite layers to improve the properties of air electrodes.