Reversible amorphization and the catalytically active state of crystalline Co3O4 during oxygen evolution

Abstract

Water splitting catalysed by earth-abundant materials is pivotal for global-scale production of non-fossil fuels, yet our understanding of the active catalyst structure and reactivity is still insufficient. Here we report on the structurally reversible evolution of crystalline Co3O4 electrocatalysts during oxygen evolution reaction identified using advanced in situ X-ray techniques. At electrode potentials facilitating oxygen evolution, a sub-nanometre shell of the Co3O4 is transformed into an X-ray amorphous CoOx(OH)y which comprises di-μ-oxo-bridged Co3+/4+ ions. Unlike irreversible amorphizations, here, the formation of the catalytically-active layer is reversed by re-crystallization upon return to non-catalytic electrode conditions. The Co3O4 material thus combines the stability advantages of a controlled, stable crystalline material with high catalytic activity, thanks to the structural flexibility of its active amorphous oxides. We propose that crystalline oxides may be tailored for generating reactive amorphous surface layers at catalytic potentials, just to return to their stable crystalline state under rest conditions.

An Operando Investigation of (Ni–Fe–Co–Ce)Ox System as Highly Efficient Electrocatalyst for Oxygen Evolution Reaction

“…In addition, a similar Co XANES shift has been observed in different cobalt-based electrocatalysts in literature between OCP and oxygen evolution conditions. 56,[82][83][84][85] This shift has been interpreted as the formation of a Co(IV) fraction at catalytic conditions, following a Co(II)  Co(III)  Co(IV) redox scheme. The EXAFS spectrum collected under the catalytic condition (SI, Figure S3a) also resembles the one reported by Kanan et al 56 Therefore, we hypothesize the presence of a mixed configuration at the surface at high current densities (~ 8 mA cm -2 ), where we observe the partial oxidative conversion to cobalt oxyhydroxide (Co(II)  Co(III)) and the presence of Co(IV) (Co(III)  Co(IV)) as an active species in the catalytic cycle.…”

Undoped and Ni-Doped CoOx Surface Modification of Porous BiVO4 Photoelectrodes for Water Oxidation

“…They are all consistent with the published peaks for Co3O4, indicating the CoOx prepared in this work mostly likely has the short range order of Co3O4. [57][58][59][60] There are two types of cobalt ions with different oxidation states in spinel Co3O4: two Co 3+ ions in the octahedral site and one Co 2+ ion in the tetrahedral site. 61 Reported fitting results…”

“…58,59 The second peak (II in Figure 11c) corresponds to the Co 3+ -Co 3+ distance. 58,60 The third peak (III in Figure 11c) is assigned to Co 3+ -Co 2+ and Co 3+ -Co 2+ distances.…”

In situ characterization of cofacial Co(IV) centers in Co 4 O 4 cubane: Modeling the high-valent active site in oxygen-evolving catalysts

“…These metalate clusters are ubiquitous and likely the active catalytic species of conventional metal oxide oxygen evolution reaction (OER) catalysts. High-resolution transmission electron microscopy of crystalline cobalt oxides in neutral and alkaline solutions reveals that the surface of the oxide is indeed an amorphous overlayer comprising the metalate clusters (15)(16)(17)(18). Electrochemical kinetics (19) and spectroscopic measurements (20,21) support a mechanism consisting of a minor equilibrium proton-coupled electron transfer process to generate effectively a Co(III)Co(IV) precatalyst, followed by a subsequent oxidation to generate a doubly oxidized state that drives the turnover-limiting O-O bond-forming step (22).…”