Design of an inherently-stable water oxidation catalyst

Abstract

While molecular water-oxidation catalysts are remarkably rapid, oxidative and hydrolytic processes in water can convert their active transition metals to colloidal metal oxides or hydroxides that, while quite reactive, are insoluble or susceptible to precipitation. In response, we propose using oxidatively-inert ligands to harness the metal oxides themselves. This approach is demonstrated by covalently attaching entirely inorganic oxo-donor ligands (polyoxometalates) to 3-nm hematite cores, giving soluble anionic structures, highly resistant to aggregation, yet thermodynamically stable to oxidation and hydrolysis. Using orthoperiodate (at pH 8), and no added photosensitizers, the hematite-core complex catalyzes visible-light driven water oxidation for seven days (7600 turnovers) with no decrease in activity, far exceeding the documented lifetimes of molecular catalysts under turnover conditions in water. As such, a fundamental limitation of molecular complexes is entirely bypassed by using coordination chemistry to harness a transition-metal oxide as the reactive center of an inherently stable, homogeneous water-oxidation catalyst.

Visible‐Light‐Driven Water Oxidation with a Polyoxometalate‐Complexed Hematite Core of 275 Iron Atoms

“…In this context, we recently reported that the oxidatively inert heteropolytungstate (POM) cluster anion [α‐PW 11 O 39 Fe III ] 4− ‐O − could serve as a covalently attached oxo donor ligand for hematite Fe 2 O 3 NCs, giving water‐soluble complexes 1 (Figure b), uniquely positioned between molecular catalysts with iron active sites (Figure a) and α‐Fe 2 O 3 photoanodes (Figure c) . Notably, 1 is not only water‐soluble, but also inherently stable to oxidation and hydrolysis.…”

“…Notably, 1 is not only water‐soluble, but also inherently stable to oxidation and hydrolysis. As such it can catalyze visible‐light‐driven water oxidation for an entire week under turnover conditions with no decrease in activity …”

“…The hematite core of 1 , while only 3 nm in diameter and comprising 275±10 Fe atoms (complexed by ca. 15 POM ligands), is large enough to retain the visible‐light semiconductor properties of bulk hematite . As such, no added photosensitizer is required.…”

The Reactivity and Stability of Polyoxometalate Water Oxidation Electrocatalysts

“…Note that while the POMs in these systems are currently not considered the reactive WOC sites, they are nevertheless the sine qua non, without which the whole system would not be able to operate. Given the high technological potential of these composites for surface deposition, electrode immobilization or photoelectrode incorporation, a wide array of chemical modification possibilities has now become accessible and requires urgent research attention [102][103][104]. In addition, recent ground-breaking studies have shown that external stimuli, such as radio-frequency electromagnetic fields [105] or permanent magnetic fields [106], could be used to enhance the activities of WOCs.…”

Reactivity and Stability of Polyoxometalate Water Oxidation Electrocatalysts

“…Given the high technological potential of these composites for surface deposition, electrode immobilization or photoelectrode incorporation, a wide array of chemical modification possibilities has now become accessible and requires urgent research attention. [102][103][104] In addition, recent ground-breaking studies have shown that external stimuli such as radio-frequency electromagnetic fields [105] or permanent magnetic fields [106] could be used to enhance the activity of WOCs. These and similar concepts could in future be used to fine-tune and optimize industrial water electrolysis.…”