Lanthanum-Promoted Electrocatalyst for the Oxygen\nEvolution Reaction: Unique Catalyst or Oxide Deconstruction?

A conventional performance metric for electrocatalysts\nthat promote\nthe oxygen evolution reaction (OER) is the current density at a given\noverpotential. However, the assumption that increased current density\nat lower overpotentials indicates superior catalyst design is precarious\nfor OER catalysts in the working environment, as the crystalline lattice\nis prone to deconstruction and amorphization, thus greatly increasing\nthe concentration of catalytic active sites. We show this to be the\ncase for La<sup>3+</sup> incorporation into Co<sub>3</sub>O<sub>4</sub>. Powder X-ray diffraction (PXRD), Raman spectroscopy and extended\nX-ray absorption fine structure (EXAFS) reveal smaller domain sizes\nwith decreased long-range order and increased amorphization for La-modified\nCo<sub>3</sub>O<sub>4</sub>. This lattice deconstruction is exacerbated\nunder the conditions of OER as indicated by operando spectroscopies.\nThe overpotential for OER decreases with increasing La<sup>3+</sup> concentration, with maximum activity achieved at 17% La incorporation.\nHRTEM images and electron diffraction patterns clearly show the formation\nof an amorphous overlayer during OER catalysis that is accelerated\nwith La<sup>3+</sup> addition. O 1s XPS spectra after OER show the\nloss of lattice-oxide and an increase in peak intensities associated\nwith hydroxylated or defective O-atom environments, consistent with\nCo(O)<sub><i>x</i></sub>(OH)<sub><i>y</i></sub> species in an amorphous overlayer. Our results suggest that improved\ncatalytic activity of oxides incorporated with La<sup>3+</sup> ions\n(and likely other metal ions) is due to an increase in the number\nof terminal octahedral Co(O)<sub><i>x</i></sub>(OH)<sub><i>y</i></sub> edge sites upon Co<sub>3</sub>O<sub>4</sub> lattice deconstruction, rather than enhanced intrinsic catalysis.