Cobalt-based layered hydroxides (LHs) stand out as one of the best families of electroactive materials for the alkaline oxygen evolution reaction (OER). However, fundamental aspects such as the influence of the crystalline structure and its connection with the geometry of the catalytic sites remains poorly understood. Thus, to address this we have conducted a thorough experimental and in silico study on the most essential Co-LHs (i.e.: ɑ-LH, β-LH and LDH) which allows us to understand the role of the layered structure and coordination environment of Co atoms on the OER performance. The ɑ-LH, containing both octahedral and tetrahedral sites, behaves as the best OER catalyst in comparison to the other phases, pointing out the role of the chemical nature of the crystalline structure. Indeed, density functional theory (DFT) calculations confirm the experimental results which can be explained in terms of both, the significant reduction of the Egap, due to the presence of tetrahedral sites, as well as the more favourable reconstruction of the ɑ-LH structure into active Co(III)-based oxyhydroxide-like phase. Furthermore, ex-situ X-ray diffraction and absorption spectroscopy reveal the permanent transformation of ɑ-LH phase into an unprecedented highly reactive oxyhydroxide-like structure under ambient conditions. Hence, our findings highlight the key role of tetrahedral sites on the electronic properties of the LH structure as well as their inherent reactivity towards OER catalysis, paving the way for the rational design of more efficient and low-maintenance electrocatalysts.
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 LDH phases, analysing the role exerted by Ni-vacancies by means of structural, spectroscopical, in-silico and electrochemical studies. Indeed, density functional theory (DFT) calculations, in agreement with X-ray absorption spectroscopy (XAS), confirm that the presence of Ni-vacancies produces acute distortions of the electroactive Ni sites (shortening in 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 of Egap, and therefore increasing the reactivity of the electroactive sites. Furthermore, post-mortem Raman and XAS measurements unveil the transformation of ɑ-LH phase into a highly reactive oxyhydroxide-like structure stable under ambient conditions. Hence, this work pinpoints the critical role of cationic vacancies on the structural and electronic properties of the LH structures, which controls their inherent reactivity towards OER catalysis. We envision Ni-based ɑ-LH as a perfect platform for trivalent cations hosting, closing the gap toward the next generation of benchmark efficient earth-abundant electrocatalysts.
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