Hydrogen-related technologies are gaining increased attention as a solution to achieve carbon neutrality. Solid oxide fuel cells (SOFCs) can generate electricity with high efficiency (about 60-80%) by reacting hydrogen and oxygen at high temperatures (generally over 800℃) without using precious metal catalysts. Low- and medium-temperature fuel cells that operate in the 500~800℃ range are presently being studied due to the rapid deterioration and the high cost of the specialized stack and system materials needed to enable sustained operation at high temperatures. While lower temperature operation is highly desired, the oxygen reduction reaction (ORR) in the cathode slows down when the operating temperature is lowered. Therefore, there is a need to develop cathode materials with high efficiency in the intermediate- and low-temperature range [1, 2].

Oxygen vacancies promote ORR by providing an oxygen path in the crystal structure and enhancing the oxygen ion conductivity. Charge transfer to the adsorbed oxygen ions is caused by the electronic structure and promotes ORR. The charge transfer energy along with oxygen vacancy concentration is an important screening metric that can be used to identify promising perovskite oxides with potentially improved electrocatalytic activity [3, 4]. Recently, the charge transfer energy has received increasing attention because it directly affects the charge transfer between adsorbed oxygen and the cathode surface, which is the rate-determining step of ORR.

We investigated the ORR reaction mechanism and electrochemical properties of PrBa_0.9A_0.1Co_1.9Cu_0.1O_5+δ (PBACCu, A=Ca, Nd) from the perspective of valence electronic structure with those metrics. Each composition has a tetragonal crystal structure, and the lattice shrinks with Ca and Nd doping, which have smaller ionic radii than Ba. XPS analysis reveals an average oxidation number of 3.3 for both PBCaCCu and PBCCu. However, a charge disproportionation (Co³⁺ → Co²⁺ + Co⁴⁺) was observed. The average oxidation number of PBNdCCu decreased to 3.19, indicating the formation of additional oxygen vacancies. Deconvolution of the O 1s XPS spectrum revealed a decrease in lattice oxygen for the Ca and Nd doped compositions, indicating that A site doping enhances covalency. The oxygen vacancy peak in PBNdCCu increased from 54.27% to 62.68%, consistent with the average oxidation number analysis of the metal ions. Additionally, the B-O charge transfer energy was calculated from the XPS valence band spectra, yielding -3.60 eV for PBCCu, -3.45 eV for PBCaCCu, and -4.00 eV for PBNdCCu. Symmetric cells of all three electrode materials were examined by electrochemical impedance spectroscopy (EIS). The EIS results indicate that PBCaCCu has the smallest polarization resistance (0.062 Ω cm²) compared to PBCCu (0.113 Ω cm²) and PBNdCCu (0.101 Ω cm²) at 700°C, consistent with its lower charge-transfer energy. The effects of oxygen vacancy and charge transfer energy on the oxygen reduction reaction mechanism are further illuminated with the use of DRT analysis.