Researchers set out to answer whether water oxidation on amorphous iridium oxide (IrOx) is driven more by metal-centered or oxygen-centered oxidation. They used a mix of theory and time-resolved operando spectroscopy to get there.
By combining density functional theory (DFT), Bader-charge analyses, and surface Pourbaix diagrams with operando optical and X-ray spectroscopies, they finally tackled a long-standing question in OER catalysis. Their results show a potential-dependent shift—from Ir-centered to oxygen-centered oxidation—basically, a dual-site oxidation mechanism.
As the catalyst forms its active oxidizing species, the band alignments shift, putting the spotlight on strong Ir–O covalency. The team ran experiments on porous IrOx films, which they designed to mimic active sites.
What’s interesting is that these conclusions hold up across different surface motifs, including modeled rutile-like and hollandite-like structures. This work could have a big impact on how we design better water-splitting catalysts and clarifies how oxygen ligands keep the OER running.
Dual-site oxidation emerges from potential-dependent electronic rearrangements
With a blend of theory and operando spectroscopy, researchers found that at lower electrochemical potentials, electrons mostly leave the Ir 5d states. As the potential ramps up, electrons start departing from O 2p states instead.
This shift marks a move from metal-centered to oxygen-ligand oxidation—a dual-site mechanism in action. The changing electronic structure shows evolving band alignments, tightly coupling Ir and O states during the reaction.
That covalent Ir–O interaction? It’s central to forming the real active species for water oxidation. Surface Pourbaix diagrams, in parallel, predict how adsorbate coverages change with potential and time, outlining three distinct redox transitions.
These transitions involve coordinated water, surface hydroxyls, and surface oxo species. The diagrams help tie specific spectroscopic events to chemical states on the IrOx surface, giving us a framework for how the catalyst adapts under operating conditions.
Three redox transitions mapped by time-resolved operando spectroscopy
Time-resolved operando optical spectroscopy breaks down three redox transitions during potential sweeps. The first two transitions link to Ir oxidation, from about +3.1 to +4.7.
This marks the gradual removal of electrons from Ir, matching up with metal-centered oxidation steps that lead into the plateau for metal oxidation. The third redox transition pops up after Ir oxidation saturates and is tied to the oxidation of oxygen ligands, forming electrophilic O− species instead of further metal oxidation.
This result helps clear up conflicting interpretations in earlier studies by showing a separate, later-stage oxidation event that’s rooted in the lattice oxygen—not the metal center.
Corroborating spectroscopies and surface chemistry
Ir L-edge XANES shows white-line shifts that follow Ir oxidation up to about +4.7. After that, the signal flatlines, suggesting no further metal oxidation during the final redox event.
At the same time, operando O K-edge NEXAFS picks up a new pre-edge feature near 529 eV at OER-relevant potentials. This feature grows as Ir oxidation maxes out, which fits with more oxygen ligands joining the reaction.
The third redox step, tied to O− species formation, matches up with measured time constants that link these electrophilic oxygen holes to the overall oxygen evolution rate. Oxygen-based holes, rather than extra Ir oxidation, seem to do the heavy lifting for OER at high potentials.
These observations pull together spectroscopic evidence from different methods and point to a direct connection between oxygen-ligand oxidation and catalytic turnover.
Implications for catalyst design and broader relevance
The study uses amorphous, porous IrOx films as representative active sites. These films show similar behavior across different structures, including rutile and hollandite-like surfaces.
Researchers took a convergent multimodal approach. They found that covalent Ir–O interactions drive the formation of the active species, which honestly gives some solid direction for designing catalysts with better OER performance.
By mapping out the order of oxidation events and highlighting the part oxygen-centered charges play, the work points to ways we might stabilize electrophilic oxygen species. It also hints at how to strike a better balance between metal and oxygen redox processes for more durable, high-rate water splitting.
For folks working in electrochemistry or renewable energy, these ideas could help shape how we tweak catalyst composition, structure, and even operating conditions. That might just be the key to squeezing more efficiency and longer life out of electrolyzers and similar systems.
Here is the source article for this story: Key role of oxidizing species driving water oxidation revealed by time-resolved optical and X-ray spectroscopies