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

Abstract

The Co 4 O 4 cubane is a representative structural model of oxidic cobalt oxygen-evolving catalysts (Co-OECs). The Co-OECs are active when residing at two oxidation levels above an all-Co(III) resting state. This doubly oxidized Co(IV) 2 state may be captured in a Co(III) 2 (IV) 2 cubane. We demonstrate that the Co(III) 2 (IV) 2 cubane may be electrochemically generated and the electronic properties of this unique high-valent state may be probed by in situ spectroscopy. Intervalence charge-transfer (IVCT) bands in the near-IR are observed for the Co(III) 2 (IV) 2 cubane, and spectroscopic analysis together with electrochemical kinetics measurements reveal a larger reorganization energy and a smaller electron transfer rate constant for the doubly versus singly oxidized cubane. Spectroelectrochemical X-ray absorption data further reveal systematic spectral changes with successive oxidations from the cubane resting state. Electronic structure calculations correlated to experimental data suggest that this state is best represented as a localized, antiferromagnetically coupled Co(IV) 2 dimer. The exchange coupling in the cofacial Co(IV) 2 site allows for parallels to be drawn between the electronic structure of the Co 4 O 4 cubane model system and the high-valent active site of the Co-OEC, with specific emphasis on the manifestation of a doubly oxidized Co(IV) 2 center on O-O bond formation.water splitting | renewable energy | solar-to-fuels | electrocatalysis | oxygen evolution reaction T he overall efficiency of the solar-to-fuels conversion process of water splitting in large part is determined by the overpotential required to drive the oxygen evolution half-reaction (i.e., 2H 2 O → O 2 + 4H + + 4e -) (1-3). This half-reaction may be driven at high activity by Earth-abundant catalysts, which are formed by self-assembly upon anodic deposition from buffered cobalt, nickel, and manganese salt solutions (4-10). As determined by in situ structural measurements (11-14), the heterogeneous films consist of aggregates of metalate clusters of molecular dimension. 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 (15-18). 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). Isotope labeling studies of active oxidic cobalt OER catalysts (23, 24) establish direct coupling of oxygens on neighboring sites, thus identifying one path for O-O bond formation from a Co(IV) 2 state,The generation of the reactive Co(IV) 2 intermediat...

Electrochemical trapping of metastable Mn 3+ ions for activation of MnO 2 oxygen evolution catalysts

“…Such an electronic structure has been generally correlated to high oxygen evolution activity (70). Mechanistically, in such a structure, electronic excitations create oxygen holes (Mn n -O → Mn n−1 -O • ), forming highly reactive oxyl radicals, which are known to be critical intermediates in the proton-coupled electron transfer transformation of H 2 O to O 2 (71)(72)(73)(74)(75)(76) and in the proposal of reductive coupling (77,78) and excess charging in Li batteries (79). The generation of oxyl radicals resulting from a high lying O2p valence band, with the population of oxygen radicals inversely proportional to the size of the HOMO-LUMO energy gap, is an emerging precept for the origin of enhanced OER activity in oxidic metal catalysts (80).…”

A review of transition metal‐based bifunctional oxygen electrocatalysts

“…Among these methods, the coupling effect can play an enormous role in the modification of electronic structure and conductivity of an electrocatalyst, in which enhanced OER and ORR activities can arise from the as‐synthesized materials due to the synergetic effect between Co and Ni (Figure b,c). Furthermore, the active sites for OER activity of multivalent Co (Co 2+ , Co 3+ , or Co 4+ ) are still unclear, and although different research groups have suggested that different surface states, such as Co 2+ , Co 3+ , or Co 4+ can potentially be the OER active sites, reaction mechanisms as well as the active surface of Co require further investigation. Researchers have reported that in the normal spinel crystal structure of Co 3 O 4 with a close‐packed face‐centered cubic configuration in which Co 2+ ions occupy the tetrahedral A sites and Co 3+ ions occupy the octahedral B sites, exposed active sites associated with cations possessing higher oxidation states (Co 3+ ions) can play a dominant role as ORR active sites .…”

Mechanistic Insights into Light‐Activated Catalysis for Water Oxidation

“…In particular, irradiation of a Ru(bpy) 3 2+ /S 2 O 8 2– solution in the presence of the initially diamagnetic cubane (all cobalt centers in oxidation state III), induces the rising of a signal with axial line shape ( g ⊥ = 2.33 and g ∥ = 2.07) . This is ascribed to the one‐electron oxidized form of the cubane (known also as “cubium”), characterized by a S = 1/2 and a formal Co(III) 3 Co(IV)‐oxo core, although the spin character is equally distributed among the four metal centers , . This evidence complements the kinetic information obtained by flash photolysis, and discussed in paragraph 3, on electron transfer from cubane to Ru(III).…”