Structural and electrocatalytic properties of La-Co-Ni oxide thin films

La-Co-Ni oxides were fabricated in the form of thin-film materials libraries by combinatorial reactive co-sputtering and analyzed for structural and functional properties over large compositional ranges: normalized to the metals of the film they span about 0 - 70 at.-% for Co, 18 - 81 at.-% for La and 11 - 25 at.-% for Ni. Composition-dependent phase analysis shows formation of three areas with different phase constitutions in dependance of Co-content: In the La-rich region with low Co content, a mixture of the phases La₂O₃, perovskite, and La(OH)₃ is observed. In the Co-rich region, perovskite and spinel phases form. Between the three-phase region and the Co-rich two-phase region, a single-phase perovskite region emerges. Surface microstructure analysis shows formation of additional crystallites on the surface in the two-phase area, which become more numerous with increasing Ni-content. Energy-dispersive X-ray analysis indicates that these crystallites mainly contain Co and Ni, so they could be spinels growing on the surface. The analysis of the oxygen evolution reaction (OER) electrocatalytic activity over all compositions and phase constitutions reveals that the perovskite/spinel two-phase region shows the highest catalytic activity, which increases with higher Ni-content. The highest OER current density was measured as 2.24 mA/cm 2 at 1.8 V vs. Reversible Hydrogen Electrode (RHE) for the composition (La 28 Co 49 Ni 23 )O x .