The co-firing stage is an unavoidable step in the fabrication process of solid oxide fuel cells (SOFCs), and avoiding unwanted interfacial reactions is crucial for cathode construction during the co-firing process. In this study, LiMn₂O₄, (LiMO), a traditional electrode material for Li-ion batteries, is discovered to have protonation and proton diffusion properties, showing significant promise as a cathode for proton-conducting SOFCs (H-SOFCs). However, obvious interactions between the LiMO cathode and the BaCe_0.7Zr_0.1Y_0.2O_3-δ (BCZY) electrolyte can be identified during the co-firing process using the conventional sintering method, resulting in poor performance and making the use of LiMO in H-SOFCs challenging. To address this issue, the Joule heating process is used to produce the LiMO cathode for H-SOFCs. In contrast to the traditional co-firing process, which takes a few hours, the Joule heating method, which completes the co-sintering procedure in a few seconds, can successfully bind the LiMO to the BCZY electrolyte with no visible interlayer reactions or elemental diffusions. As a result, the full potential of LiMO for H-SOFCs is realized, resulting in a high fuel cell output of 1426 mW cm^-2 at 700 ^oC, about double that of the cell utilizing the normally sintered LiMO cathode. To the best of our knowledge, this is the first study to use Joule heating to prevent the cathode/electrolyte interfacial reaction in H-SOFCs, which presents an interesting approach for manufacturing and may also breathe new life into some materials that are previously incompatible with H-SOFCs. 

Designing high-performance cathodes is crucial for proton-conducting solid oxide fuel cells (H-SOFCs), as the cathode heavily influences cell performance. Although manganate cathodes exhibit superior stability and thermal compatibility, their poor cathode performance at intermediate temperatures renders them unsuitable for H-SOFC applications. To address this issue, Sc is utilized as a dopant to modify the traditional La_0.5Sr_0.5MnO₃ cathode at the La site. Although the solubility of Sc at the La site is restricted to 2.5%, this modest quantity of Sc doping can improve the material's oxygen and proton transport capabilities, hence improving cathode and fuel cell performance. Furthermore, when the doping concentration exceeds 2.5%, the secondary phase ScMnO₃ forms in situ, resulting in La_0.475Sc_0.025Sr_0.5MnO₃ (LScSM)+ScMnO₃ nanocomposites. Although the secondary phase is often considered undesirable, the high protonation capacity of ScMnO₃ can compensate for the low proton diffusion ability of LScSM. These two phases complement each other to provide high-performance cathodes. The nominal La_0.4Sc_0.1Sr_0.5MnO₃ is the optimal composition, which takes advantage of the excellent electronic conductivity and fast oxygen diffusion rates of LScSM, as well as the good proton diffusion capacity of ScMnO₃, to produce a high fuel cell output of 1529 mW·cm⁻² at 700 °C. Furthermore, the fuel cell exhibited good operational stability under working conditions, indicating that La_0.4Sc_0.1Sr_0.5MnO₃ is a viable cathode choice for H-SOFCs.

Nb-doped SrFeO_3−δ (SFO) is used as a cathode in proton-conducting solid oxide fuel cells (H-SOFCs). First-principles calculations show that the SrFe_0.9Nb_0.1O_3−δ (SFNO) cathode has a lower energy barrier in the cathode reaction for H-SOFCs than the Nb-free SrFeO_3−δ cathode. Subsequent experimental studies show that Nb doping substantially enhances the performance of the SrFeO_3−δ cathode. Then, oxygen vacancies (V_O) were introduced into SFNO using the microwave sintering method, further improving the performance of the SFNO cathode. The mechanism behind the performance improvement owing to V_O was revealed using first-principles calculations, with further optimization of the SFNO cathode achieved by developing a suitable wet chemical synthesis route to prepare nanosized SFNO materials. This method significantly reduces the grain size of SFNO compared with the conventional solid-state reaction method, although the solid-state reaction method is generally used for preparing Nb-containing oxides. As a result of defect engineering and synthesis approaches, the SFNO cathode achieved an attractive fuel cell performance, attaining an output of 1764 mW·cm⁻² at 700 °C and operating for more than 200 h. The manipulation of Nb-doped SrFeO_3−δ can be seen as a “one stone, two birds” strategy, enhancing cathode performance while retaining good stability, thus providing an interesting approach for constructing high-performance cathodes for H-SOFCs.

A new medium entropy material LiCo_0.25Fe_0.25Mn_0.25Ni_0.25O₂ (LCFMN) is proposed as a cathode for proton-conducting solid oxide fuel cells (H-SOFCs). Unlike traditional LiXO₂ (X = Co, Fe, Mn, Ni) lithiated oxides, which have issues like phase impurity, poor chemical compatibility, or poor fuel cell performance, the new LCFMN material mitigates these problems, allowing for the successful preparation of pure phase LCFMN with good chemical and thermal compatibility to the electrolyte. Furthermore, the entropy engineering strategy is found to weaken the covalence bond between the metal and oxygen in the LCFMN lattice, favoring the creation of oxygen vacancies and increasing cathode activity. As a result, the H-SOFC with the LCFMN cathode achieves an unprecedented fuel cell output of 1803 mW·cm⁻² at 700 ℃, the highest ever reported for H-SOFCs with lithiated oxide cathodes. In addition to high fuel cell performance, the LCFMN cathode permits stable fuel cell operation for more than 450 h without visible degradation, demonstrating that LCFMN is a suitable cathode choice for H-SOFCs that combining high performance and good stability.

A La_0.5Ba_0.5MnO_3−δ oxide was prepared using the sol–gel technique. Instead of a pure phase, La_0.5Ba_0.5MnO_3−δ was discovered to be a combination of La_0.7Ba_0.3MnO_3−δ and BaMnO₃. The in-situ production of La_0.7Ba_0.3MnO_3−δ+BaMnO₃ nanocomposites enhanced the oxygen vacancy (V_O) formation compared to single-phase La_0.7Ba_0.3MnO_3−δ or BaMnO₃, providing potential benefits as a cathode for fuel cells. Subsequently, La_0.7Ba_0.3MnO_3−δ+BaMnO₃ nanocomposites were utilized as the cathode for proton-conducting solid oxide fuel cells (H-SOFCs), which significantly improved cell performance. At 700 ℃, H-SOFC with a La_0.7Ba_0.3MnO_3−δ+BaMnO₃ nanocomposite cathode achieved the highest power density (1504 mW·cm⁻²) yet recorded for H-SOFCs with manganate cathodes. This performance was much greater than that of single-phase La_0.7Ba_0.3MnO_3−δ or BaMnO₃ cathode cells. In addition, the cell demonstrated excellent working stability. First-principles calculations indicated that the La_0.7Ba_0.3MnO_3−δ/BaMnO₃ interface was crucial for the enhanced cathode performance. The oxygen reduction reaction (ORR) free energy barrier was significantly lower at the La_0.7Ba_0.3MnO_3−δ/BaMnO₃ interface than that at the La_0.7Ba_0.3MnO_3−δ or BaMnO₃ surfaces, which explained the origin of high performance and gave a guide for the construction of novel cathodes for H-SOFCs.

A pure phase BaCo_0.5Fe_0.5O_3–δ (BCF), which cannot be obtained before, is successfully prepared in this study by using the calcination method with a rapid cooling procedure. The successful preparation of BCF allows the evaluation of this material as a cathode for proton-conducting solid oxide fuel cells (H-SOFCs) for the first time. An H-SOFC using the BCF cathode achieves an encouraging fuel cell performance of 2012 mW·cm^–2 at 700 ℃, two-fold higher than that of a similar cell using the classical high-performance Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3–δ (BSCF) cathode. First-principles calculations reveal the mechanism for the performance enhancement, indicating that the new BCF cathode significantly lowers the energy barriers in the oxygen reduction reaction (ORR) compared with the BSCF cathode. Therefore, improved cathode performance and fuel cell output are obtained for the BCF cell. The fuel cell using the BCF cathode also shows excellent long-term stability that can work stably for nearly 900 h without noticeable degradations. The fuel cell performance and long-term stability of the current BCF cell are superior to most of the H-SOFCs reported in previous reports, suggesting that BCF is a promising cathode for H-SOFCs.

LiCoO₂, a widely used electrode material for Li-ion batteries, was found to be suitable as a cathode material for proton-conducting solid oxide fuel cells (H-SOFCs). Although the evaporation of Li in LiCoO₂ was detrimental to the Li-ion battery performance, the Li-evaporation was found to be beneficial for the H-SOFCs. The partial evaporation of Li in the LiCoO₂ material preparation procedure led to the in-situ formation of the LiCoO₂+Co₃O₄ composite. Compared to the cell using the pure phase LiCoO₂ cathode that only generated moderate fuel cell performance, the H-SOFCs using the LiCoO₂+Co₃O₄ cathode showed a high fuel cell performance of 1160 mW·cm^–2 at 700 ℃, suggesting that the formation of Co₃O₄ was critical for enhancing the performance of the LiCoO₂ cathode. The first-principles calculation gave insights into the performance improvements, indicating that the in-situ formation of Co₃O₄ due to the Li-evaporation in LiCoO₂ could dramatically decrease the formation energy of oxygen vacancies that is essential for the high cathode performance. The evaporation of Li in LiCoO₂, which is regarded as a drawback for the Li-ion batteries, is demonstrated to be advantageous for the H-SOFCs, offering new selections of cathode candidates for the H-SOFCs.

A high-entropy ceramic oxide is used as the cathode for the first time for proton-conducting solid oxide fuel cells (H-SOFCs). The Fe_0.6Mn_0.6Co_0.6Ni_0.6Cr_0.6O₄ (FMCNC) high-entropy spinel oxide has been successfully prepared, and the in situ chemical stability test demonstrates that the FMCNC material has good stability against CO₂. The first-principles calculation indicates that the high-entropy structure enhances the properties of the FMCNC material that surpasses their individual components, leading to lower O₂ adsorption energy for FMCNC than that for the individual components. The H-SOFC using the FMCNC cathode reaches an encouraging peak power density (PPD) of 1052 mW·cm⁻² at 700 ℃, which is higher than those of the H-SOFCs reported recently. Additional comparison was made between the high-entropy FMCNC cathode and the traditional Mn_1.6Cu_1.4O₄ (MCO) spinel cathode without the high-entropy structure, revealing that the formation of the high-entropy material allows the enhanced protonation ability as well as the movement of the O p-band center closer to the Fermi level, thus improving the cathode catalytic activity. As a result, the high-entropy FMCNC has a much-decreased polarization resistance of 0.057 Ω·cm² at 700 ℃, which is half of that for the traditional MCO spinel cathode without the high-entropy design. The excellent performance of the FMCNC cell indicates that the high-entropy design makes a new life for the spinel oxide as the cathode for H-SOFCs, offering a novel and promising route for the development of high-performance materials for H-SOFCs.