Electrochemical properties of La2BO4+δ/C electrocatalysts and study of the mechanism of the oxygen reduction reaction in alkaline medium

Kinetics and mechanism of the oxygen reduction reaction (ORR) in an alkaline medium have been studied on La2BO4±δ (B: Ni, Co or Mn) various mixed conducting oxides (MIEC: mixed electronic and ionic conductor). The preparation of these materials was carried out using the citrate method. The purity, the surface chemical state and the morphology were investigated by X-ray diffraction, X-ray photon-electron spectrometry (XPS) and scanning electron microscopy, respectively. An iodometric titration was also performed to evaluate the oxygen (over)-stoichiometry. The catalytic activity of the La2NiO4±δ (L2NO), La2CoO4±δ (L2CO) and La2MnO4±δ (L2MO) oxides towards ORR in alkaline medium was evaluated using the rotating disc electrode in 0.5 M NaOH solution completed by various electrochemical techniques such as cyclic voltammetry, linear voltammetry, the Koutecky–Levich diffusion model and impedance spectroscopy. The La2NiO4±δ sample is pure and more porous, but the La2CoO4±δ and La2MnO4±δ samples show secondary phases and are less porous. The XPS analysis shows that the surface of the La2NiO4±δ material is rich in hydroxide ions (OH−) and the Ni3+/Ni2+ ratio is very low unlike La2CoO4±δ and La2MnO4±δ materials in which OH¯ are low. Porosity and percentages of OH¯ and B3+/B2+ ratio are parameters influencing the ORR mechanism and therefore favouring, for this purpose, a one-step mechanism on La2NiO4+δ and a two-step mechanism on La2CoO4±δ and La2MnO4±δ electrocatalysts.