Identifying the Origin of the Limiting Process in a Double Perovskite PrBa_0.5Sr_0.5Co_1.5Fe_0.5O_5+δ Thin-Film Electrode for Solid Oxide Fuel Cells

Abstract

Oxygen reduction reaction in a double perovskite material, PrBa 0.5 Sr 0.5 Co 1.5 Fe 0.5 O 5+δ (PBSCF), was studied for application as a cathode in a solid oxide fuel cell (SOFC). Electrochemical measurements were performed on a geometrically welldefined dense thin-film (0.8−2 μm thickness) electrode, fabricated as a symmetric cell. In combination with density functional theory (DFT) and molecular dynamics (MD) simulations, experiments provided an insight into the operating mechanism of the SOFC material tested at an open-circuit voltage. The dense thin-film electrode of PBSCF showed a thickness-dependent electrochemical performance, suggesting bulk diffusion limitation. To understand the origin of this diffusion-limiting electrochemical performance, DFT calculations were utilized to calculate the surface (γ) and oxygen vacancy formation (E OV ) energies. For example, E OV in the Pr plane (190 kJ/mol) of PBSCF was measured to be lower than that of the BaSr plane (E OV = 297 kJ/ mol). In addition, oxygen vacancies were difficult to be created in the BaSr/CoFe terminal surface (E OV = 341.6 kJ/mol) as compared to other terminal surfaces. MD simulations further elaborated on the nature of cation disordering in the surface and subsurface regions, consequently leading to the preferential segregation of the Ba cations to the surface, which is a known phenomenon in such double perovskite materials. Because of cation disordering and segregation of Ba species, the oxygen anion diffusivity (∼10 −12 cm 2 s −1 ), calculated from MD, in the near-surface region was observed to be 2 orders of magnitude lesser than that of the bulk (D = 2.98 × 10 −10 cm 2 s −1 ) of the material at 973 K. Surface characterization of the thin-film electrode using X-ray photoelectron spectroscopy was indicative of a nonperovskite Ba 2+ phase on the electrode surface. The segregation of Ba cations was linked with the transport of oxygen anions, which was limiting the electrochemical performance of the electrode.

Double Perovskite Oxides Bringing a Revelation in Oxygen Evolution Reaction Electrocatalyst Design

“…Thus, the ordered oxygen vacancies showed an alternative organization of pyramidal Co 3 + O 5 and octahedral Co 3 + O 6 along the B-axis (Figure 6). [72] PrBaCo 2 O 5.75 displays an excellent OER activity with a low overpotential of 0.36 V to achieve a current density of 10 mA cm À 2 , Nevertheless, as the oxygen vacancies are increased to d = 0.50, the OER activity is significantly reduced, where the overpotential increases up to 0.42 V, corresponding Tafel slopes are ~80 mV dec À 1 (PrBaCo 2 O 5.5 ) that is higher than 70 mV dec À 1 (PrBaCo 2 O 5.75 ). [71] A trove of Perovskite-based materials has been done, as summarized in Table 1, and the following section will present a brief summary of the modification of A and B-site, oxygen deficiency and other factors influencing the catalytic OER activity.…”

“…The simulated terminal surfaces for (a) BaSr/CoFe, (b) Pr/CoFe, (c) CoFe/BaSr, and (d) CoFe/Pr present in the PBSCF structure in the (001) direction.Here, Pr, Ba, Sr, Co, Fe, and O atoms are represented by white, green, pink, blue, black, and red spheres, respectively. Reproduced from Ref [72]…”

Boosting the Electrochemical Performance of Fe-Based Layered Double Perovskite Cathodes by Zn2+ Doping for Solid Oxide Fuel Cells

“…Recently, layered double perovskites, i.e., ReBaM 2 O 5+δ (Re = La, Pr, Sm, Nd, Gd, Y; M = Fe, Co), have attracted extensive research attention due to their fast oxygen-ion diffusion and surface exchange. − Owing to the huge difference in ionic radii, both rare earth and Ba 2+ ions tend to occupy different positions and generate a stacking sequence of −[BaO]–[MO 2 ]–[ReO δ ]–[MO 2 ]–[BaO]– along c -axis. This unique structure renders oxygen vacancies in [ReO δ ] layers and forms a two-dimensional migration path for oxygen ions. − At the same time, [MO 2 ] layers provide the pathway for the transport of electrons or holes due to the presence of metallic ions with different valence states.…”

Promotion of the oxygen evolution reaction via the reconstructed active phase of perovskite oxide

“…Based on a previous study, in the LP structure, the formation of oxygen vacancies is most preferred in the PrO plane. 28 Especially, an oxygen vacancy is preferentially created in the vicinity of Co than Fe. 25 However, in the RP structure, the PrO plane disappears due to the random arrangement of Pr cations at the A-site.…”