Neodymium nickelate Nd_2-xSr_xNi_1-yCo_yO_4 ± d (x and y = 0 or 0.05) as cathode materials for the oxygen reduction reaction

The oxygen reduction reaction (ORR) was investigated using two new cathode materials based on neodymium nickelate with formula Nd_2-xSr_xNi_1-yCo_yO_4 ± δ (with x and y = 0 or 0.05) prepared by the citrate method. The ORR was measured in 0.5 M NaOH with a rotating disk electrode (RDE) at different rotation speeds after purging with pure oxygen. The synthesized materials were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS), whereas the electrochemical analysis was obtained by linear voltammetry and impedance spectroscopy. The results show that before the ORR, the grains of the undoped material are spherical in shape with a size around 400 nm, while after the oxygen reduction reaction, the grain size increases. In the doped material, on the contrary, the results are reversed and the grain size becomes smaller. In addition, we show that the ORR mechanism proceeds in a single step in the undoped material while it is carried out in two successive steps in the case of the doped one. XPS analysis confirms that the sites responsible for the ORR are the OH⁻ groups. Doping with Sr or Co does not improve the electrochemical properties: this dopant adversely affects the formation of the sites responsible for the dissociation of molecular oxygen. For larger hydroxide groups content (2.30, Nd₂NiO_4 ± δ) the electrochemical currents are much greater relative to that of the other Nd_1.95Sr_0.05Ni_0.95Co_0.05O_4 ± δ (0.98) material. This work helps to understand the role of OH⁻ groups, and the reduction mechanisms with 2 and 4 electrons.