Compared with conventional solid oxide fuel cell, using one material as both the cathode and anode to construct a symmetrical solid oxide fuel cell (SSOFC) configuration can simplify the fabrication processes and reduce the cost, while mitigating the chemical incompatibility and thermal mismatching issues. A-site double perovskite PrBaMn₂O_5+δ, with a matched thermal expansion coefficient with electrolytes, good structural stability in both oxidizing and reducing atmospheres as well as a modest catalytic activity is a promising symmetrical electrode. However, PrBaMn₂O_5+δ has a low electrical conductivity in reducing atmosphere and an inferior catalytic activity towards fuel oxidation. To address these issues, a lattice doping strategy was employed to regulate the charge carrier and oxygen vacancy concentrations, as well as the energy band structure. In addition, theoretical calculation could predict the properties of the material, and screen doping elements for high-performance material. The A site elements have a great impact on the properties of material in terms of structure, conductivity, and catalytic activity. In this paper, first-principles calculation was employed to investigate the effect of A-site Re (rare-earth) element species (i.e., La, Pr, Nd, Sm, Gd, and Y) on the structure and properties of ReBaMn₂O_5+δ materials, and the calculated results were verified by experimental data.

The Cambridge sequential total energy package (CASTEP) module in Material Studio software was utilized to calculate the binding energy, formation energy from oxides, and density of states (DOS) of LaBaMn₂O_5+δ (LBM), PrBaMn₂O_5+δ (PBM), NdBaMn₂O_5+δ (NBM), SmBaMn₂O_5+δ (SBM), GdBaMn₂O_5+δ (GBM), and YBaMn₂O_5+δ (YBM) materials, respectively. The selected materials were prepared by a sol-gel method. The phase structure of the synthesized powder was examined by X-ray diffraction (XRD). The electrical conductivities of the samples were measured by the DC four-terminal method in air and 5% H₂/Ar atmosphere, respectively. The electrochemical impedance spectroscopy (EIS) and current-voltage curves of the La_0.8Sr_0.2Ga_0.8Mg_0.2O_3-δ (LSGM, 300 μm) electrolyte supported symmetrical cell were obtained using Solartron 1260 with 1287.

The theoretical lattice parameters of ReBaMn₂O₆ and ReBaMn₂O₅ were obtained through geometric optimization in DFT calculation. The ReBaMnO₅ shows an enlarged lattice volume rather than that of ReBaMnO₆. Also, the lattice volume of RBM shrinks with decreasing Re³⁺ ion radius. The binding energy results show that GBM has the maximum absolute value of binding energy, indicating an intense structural stability, which is related to the half-filled electronic configuration (4f⁷) of Gd³⁺. YBM has the minimum binding energy, indicating a poor structural stability. For the formation energy, SBM and GBM show a negative formation energy from corresponding oxides, inferring an intense synthesis preference. LBM and YBM exhibit a positive formation energy, and they prefer to maintain two phases, i.e., La(Y)MnO₃ and BaMnO₃. The energy difference between the O 2p orbital center and the Fermi surface of perovskite materials is related to the catalytic activity. Based on the conduction mechanism of B—O—B small polaron of electrons in perovskite structure, the energy difference between O 2p and Mn 3d orbital center can be related to the conductivity of the material. In this regard, the DOS of ReBaMn₂O₆ and ReBaMn₂O₅ is calculated to predict the corresponding properties. The results show that ReBaMn₂O_5+δ exhibits a better catalytic activity towards oxygen reduction than fuel oxidation because of the lower energy difference between the O 2p orbital center and Fermi surface of ReBaMn₂O₆ than that of ReBaMn₂O₅. In addition, PBM shows the minimum O 2p-Fermi energy difference, indicating a superior catalytic activity. While YBM and LBM possess high these values, indicating a poor catalytic activity. For the energy difference between O 2p and Mn 3d orbital center of the material, ReBaMn₂O₆ shows a smaller value than that of ReBaMn₂O₅, inferring that a higher electrical conductivity can be obtained in oxidizing atmosphere than in reducing atmosphere. PBM and NBM exhibit lower O 2p-Mn 3d energy difference values, indicating that they can realize a high conductivity. While YBM and LBM show the opposite results, indicating a low conductivity of the material. To verify these calculation results, ReBaMn₂O_5+δ with La, Nd, and Gd A-site elements are synthesized for properties characterization.

The XRD patterns demonstrate that NBM and GBM show a tetragonal structure with P4/mmm space group in both air and 5% H₂/Ar atmosphere, while LBM sample consists of BaMnO₃, La_1-xM_1-zO₃, and MnO phases. This result is consistent with the calculation results that LBM possesses a positive formation energy, showing a thermodynamic instability. NBM and GBM exhibit a higher conductivity in air rather than in 5% H₂/Ar, and NBM displays higher conductivities than GBM. At 900 ℃, the conductivities of NBM are 48.8 S·cm^-1 and 10.0 S·cm^-1 in air and 5% H₂/Ar, respectively, which are higher than those of GBM (i.e., 38.9 and 8.8 S·cm^-1). For the EIS results, NBM and GBM-based symmetrical cells exhibit lower polarization resistance in air than that in 3%H₂O/H₂. In addition, NBM shows superior catalytic activity than GBM, as confirmed by a lower polarization resistance of NBM than GBM (i.e., 1.78 vs. 3.4 Ω·cm² in air, 2.60 vs. 3.7 Ω·cm² in 3% H₂O/H₂). The conductivity and EIS results both are in accordance with the theoretical calculation results.

The computation results indicated that LBM is not stable at single phase layered perovskite structure. GBM showed the maximum binding energy, indicating the optimum structural stability, while NBM exhibited a smaller energy difference between Mn 3d and O 2p orbital center, and a lower energy difference between the O 2p orbital center and Fermi surface, implying the good conductivity and excellent catalytic activity among the investigated materials. Based on the calculation results, La, Nd and Gd were selected as A-site elements for verification. The results manifested that LBM was difficult to form single-phase perovskite, but it was easy to synthesize the single phases NBM and GBM. They showed a good structural stability in both reducing and oxidizing atmospheres. NBM exhibited higher conductivities in both air and 5% H₂/Ar, compared to GBM. In addition, NBM electrode displayed much lower polarization resistance than GBM, demonstrating a superior catalytic activity. The LSGM electrolyte (300 μm) supported symmetrical cell with NBM electrode delivered a maximum power density of 335 mW·cm^-2 at 850 ℃.