Influence of Crystallographic Structure and Metal Vacancies on the Oxygen Evolution Reaction Performance of Ni-based Layered Hydroxides**

Nickel-based layered hydroxides (LHs) are a family of efficient electrocatalysts for the alkaline oxygen evolution reaction (OER). Nevertheless, fundamental aspects such as the influence of the crystalline structure and the role of lattice distortion of the catalytic sites remain poorly understood and typically muddled. Herein, we carried out a comprehensive investigation on ɑ-LH, β-LH and layered double hydroxide (LDH) phases by means of structural, spectroscopical, in-silico and electrochemical studies, which suggest the key aspect exerted by Ni-vacancies in the ɑ-LH structure. Density functional theory (DFT) calculations and X-ray absorption spectroscopy (XAS) confirm that the presence of Ni-vacancies produces acute distortions of the electroactive Ni sites (reflected as the shortening of the Ni−O distances and changes in the O−Ni−O angles), triggering the appearance of Ni localised electronic states on the Fermi level, reducing the Egap, and consequently, increasing the reactivity of the electroactive sites in the ɑ-LH structure. Furthermore, post-mortem Raman and XAS measurements unveil its transformation into a highly reactive oxyhydroxide-like phase that remains stable under ambient conditions. Hence, this work pinpoints the critical role of the crystalline structure as well as the electronic properties of LH structures on their inherent electrochemical reactivity towards OER catalysis. We envision Ni-based ɑ-LH as a perfect platform for hosting trivalent cations, closing the gap toward the next generation of benchmark efficient earth-abundant electrocatalysts.