Crystallographic and geometrical dependence of water oxidation activity in Co-based layered hydroxides

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 remain poorly understood. Thus, to address this topic we have conducted a thorough experimental and in silico study on the most important divalent Co-based LHs (i.e.: ɑ-LH, β-LH and LDH) which allows us to understand the role of the layered structure and coordination environment of divalent 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 the more favourable reconstruction into active Co(III)-based oxyhydroxide-like phase (dehydrogenation process) as well as the significantly lower calculated overpotential across the OER mechanism for the ɑ-LH structure (exhibiting lower Egap). Furthermore, ex-situ X-ray diffraction and absorption spectroscopy reveal the permanent transformation of ɑ-LH phase into a highly reactive oxyhydroxide-like stable 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.