Edge reactivity and water-assisted dissociation on cobalt oxide nanoislands

Abstract

Transition metal oxides show great promise as Earth-abundant catalysts for the oxygen evolution reaction in electrochemical water splitting. However, progress in the development of highly active oxide nanostructures is hampered by a lack of knowledge of the location and nature of the active sites. Here we show, through atom-resolved scanning tunnelling microscopy, X-ray spectroscopy and computational modelling, how hydroxyls form from water dissociation at under coordinated cobalt edge sites of cobalt oxide nanoislands. Surprisingly, we find that an additional water molecule acts to promote all the elementary steps of the dissociation process and subsequent hydrogen migration, revealing the important assisting role of a water molecule in its own dissociation process on a metal oxide. Inspired by the experimental findings, we theoretically model the oxygen evolution reaction activity of cobalt oxide nanoislands and show that the nanoparticle metal edges also display favourable adsorption energetics for water oxidation under electrochemical conditions.

Gold-supported two-dimensional cobalt oxyhydroxide (CoOOH) and multilayer cobalt oxide islands

“…The second point of view agrees well with our observation of the labyrinth structure (figure 4, c) which appears to be originating from additional dissociative adsorption from the rest gas, as described above. The strong dissociative properties are also in line with our recently described study of the dynamics in water dissociation and hydroxylation on Co-O bilayer edges 12 , however the process on trilayers leading to the initial formation of the √3 × √3 30° superstructure must take place at a much faster rate.…”

“…This overlayer structure (referred to as a "labyrinth" structure) is estimated to reach an OH coverage of up to 0.5 (i.e. now a total stoichiometry of CoOOH 0.5 ), similar to the case of our recent study of water dissociation on Co-O bilayer islands reaching saturation at exactly 50% hydroxylation on the island basal plane 12 , where the islands exhibit similar linear features in STM together with an underlying √3 × √3 30° superstructure. The additional hydroxylation compared to the density of 0.33 in the OH superstructure proposed in figure 4b is likely to arise from further dissociative adsorption of H 2 O or H 2 from the rest gas on the pristine islands, whereas such an additional hydroxylation would not result in a "labyrinth" structure starting from the alternative √3 × √3 30° with an OH density of 0.66.…”

“…At low oxygen pressure (~1×10 -6 mbar O 2 ) the island structure is a rocksalt Co-O bilayer exposing the (111) plane, coexisting with a minority island type comprising a wurtzite terminated Co-O-Co-O double bilayer (see the STM image in figure 6, left). Furthermore, the edge structure of these bilayer and double bilayer CoO x islands revealed a high activity for water 3 dissociation at room temperature, resulting in an extraordinary degree of hydroxylation of the cobalt oxide structures 12 .…”

Operando X‐Ray Absorption Spectroscopy Shows Iron Oxidation Is Concurrent with Oxygen Evolution in Cobalt–Iron (Oxy)hydroxide Electrocatalysts

“…[15,16] Both are comprised of layers of edgesharing MO(H) 6 octahedra and show increased OER activity and am ore-positive (nominal) M 2+/3+ reduction potential upon Fe-incorporation. [20,21] Herein we use operando XAS to study Co(Fe)O x H y .W e find clear evidence for Fe oxidation and FeÀOb ond shortening at OER relevant potentials,c onsistent with the hypothesis that high-valent Fe is involved in the OER mechanism. [6] Gong et al found that CoO x with Fe 3+ ions adsorbed from solution was more active than co-deposited Fe 4.4 Co 95.6 O x .…”

“…This is consistent with conversion from the hydroxide to oxyhydroxide phase during the Co 2+/3+ oxidation wave (Supporting Information, Figure S1C). [17,18,21] Upon Fe addition, Co oxidation during the OER is suppressed, evidenced by the lack of significant change in the Co K-edge energy when stepping the potential to OER conditions ( Figure 2B,C). E O 2 =OH À where the OER occurs.There is no change in the pre-edge feature.This shift is most likely due to afraction of Co being oxidized from Co 3+ to Co 4+ ,c onsistent with reports of Co 4+ during the OER.…”