The key to efficient green hydrogen production may lie in oxygen atoms deep within the catalyst itself.
Imagine a world powered by clean hydrogen fuel, produced by splitting water using only sunlight. The main obstacle? The oxygen evolution reaction (OER)—a complex, energy-intensive process that creates oxygen gas at the anode of water-splitting devices.
For decades, scientists believed this reaction occurred only on the surface of catalysts, but recent breakthroughs reveal a hidden world where oxygen atoms from within the catalyst lattice itself actively participate in the reaction. This discovery of lattice oxygen exchange is revolutionizing our approach to designing electrocatalysts and bringing us closer to a sustainable energy future.
OER occurs only on catalyst surfaces through the Adsorbate Evolution Mechanism (AEM).
Lattice oxygen atoms actively participate in OER through the Lattice Oxygen Oxidation Mechanism (LOM).
Metal sites on catalyst surface bind reaction intermediates
Oxygen molecules form entirely from electrolyte's water molecules
Limited by scaling relations for earth-abundant catalysts
Oxygen atoms from catalyst lattice directly participate in bond formation
Bypasses scaling relations that limit AEM-based catalysts 6
Explains unexpectedly high OER activity in some materials
"The OER proceeds via the lattice oxygen oxidation mechanism pathway on metal oxyhydroxides only if two neighbouring oxidized oxygens can hybridize their oxygen holes without sacrificing metal–oxygen hybridization significantly," researchers noted in a seminal Nature Energy paper 6 .
Breaks the scaling relations that limit traditional AEM-based catalysts, enabling higher efficiency with earth-abundant materials.
The discovery of LOM prompted a search for catalyst structures that could optimize this mechanism. Enter Metal Hydroxide-Organic Frameworks (MHOFs)—innovative materials synthesized by transforming layered hydroxides into two-dimensional sheets crosslinked by aromatic carboxylate linkers 1 .
Precise control through metal substitution and linker modification
Through π-π interactions between adjacent stacked linkers
Direct control of metal redox through organic-inorganic interface
Researchers demonstrated this tunability by incorporating acidic cations or electron-withdrawing linkers into nickel-based MHOFs, enhancing their oxygen evolution reaction activity by over three orders of magnitude per metal site 1 .
Iron substitution proved particularly effective, achieving a mass activity of 80 A g⁻¹ at an overpotential of just 0.3 V with sustained performance for 20 hours.
| Catalyst Material | Overpotential at 10 mA cm⁻² (mV) | Stability | Mechanism |
|---|---|---|---|
| NiFe-based MHOFs | ~300 | 20+ hours | LOM-dominated |
| CoOOH | 320 | Not specified | Mixed AEM/LOM |
| Zn₀.₂Co₀.₈OOH | Optimal | Good | LOM-optimized |
| IrO₂ (reference) | ~340 | Moderate | AEM-dominated |
To convincingly demonstrate lattice oxygen participation, researchers designed an elegant experiment using isotope labeling and cutting-edge analytical techniques. The study focused on rutile Ir¹⁸O₂, chosen for its well-defined crystal structure and relevance to commercial electrolyzers 4 .
Researchers first synthesized thin films of Ir¹⁸O₂ through reactive magnetron sputtering in an ¹⁸O₂ atmosphere, achieving 99.6% isotopic purity in the catalyst lattice 4 .
The ¹⁸O-labeled electrode was subjected to oxygen evolution reaction conditions in H₂¹⁶O-based electrolyte (0.1 M HClO₄), applying a constant current density of 1 mA cm⁻² for 10 minutes 4 .
Immediately after electrochemical treatment, a protective Cr layer was deposited via electron-beam physical vapor deposition to preserve the catalyst's surface state and prevent artifactual isotope exchange during air exposure 4 .
The critical evidence came from atom probe tomography (APT), a technique capable of mapping isotopes in three dimensions with near-atomic resolution. This allowed researchers to directly visualize and quantify the incorporation of electrolyte-derived ¹⁶O into the catalyst lattice 4 .
The APT results provided unambiguous evidence of lattice oxygen exchange. The three-dimensional reconstruction revealed a significant increase in ¹⁶O-containing species in the top 2.5 nm of the material, with the highest concentration (averaging 3.5% of total oxygen content) in the uppermost 0.5 nm 4 .
This gradient of isotope exchange demonstrated that the reaction wasn't limited to the immediate surface but penetrated multiple atomic layers into the catalyst. When correlated with the minimal iridium dissolution detected during the experiment (approximately 0.7 ng cm⁻² over 10 minutes), the results pointed toward a genuine exchange mechanism rather than simple catalyst degradation 4 .
| Depth from Surface (nm) | ¹⁶O Percentage of Total Oxygen |
|---|---|
| 0-0.5 | 3.5% (±0.7%) |
| 0.5-1.0 | ~2.5% |
| 1.0-1.5 | ~1.5% |
| 1.5-2.0 | ~0.8% |
| >2.5 | Background levels |
| Reagent/Material | Function in Research | Example Application |
|---|---|---|
| Isotope-labeled oxides (Ir¹⁸O₂) | Tracing oxygen pathways | Direct visualization of lattice oxygen exchange 4 |
| Aromatic carboxylate linkers | Building blocks for MHOFs | Creating tunable metal-organic frameworks 1 |
| Alkaline earth metals (Ca, Sr, Ba) | Modifiers of lattice oxygen activity | Enhancing oxygen diffusivity in perovskite oxides 8 |
| Potassium persulfate (K₂S₂O₈) | Oxidizing agent in synthesis | Enabling ambient-temperature synthesis of MOOH structures 3 |
| Ammonia vapor diffusion method | Controlled hydroxide source | Facilitating hierarchical structure growth 3 |
| Atom probe tomography | Atomic-scale 3D elemental mapping | Quantifying isotope distribution in catalysts 4 |
The recognition of lattice oxygen participation fundamentally changes how we approach electrocatalyst design. Instead of focusing exclusively on surface metal sites, researchers can now strategically engineer bulk composition and structure to enhance lattice oxygen reactivity.
With redox-inactive elements (like Zn²⁺ in CoOOH) to create optimal non-bonding oxygen states 6
Designs that balance stability with sufficient lattice flexibility for oxygen exchange
That combine the tunability of molecular catalysts with the stability of solid-state materials
As research progresses, the challenge remains to harness the enhanced activity of LOM-based catalysts while ensuring long-term structural stability. The very process of oxygen exchange that boosts performance may eventually lead to catalyst degradation if not properly managed.
What makes this field particularly exciting is that we're witnessing a paradigm shift in our fundamental understanding of electrochemical reactions—one that bridges traditional boundaries between solid-state chemistry and molecular catalysis. As research continues to unravel the intricate dance of atoms at catalyst surfaces, the dream of efficient, sustainable hydrogen production through water splitting comes increasingly within reach.
The hidden world of lattice oxygen, once a silent spectator in the oxygen evolution reaction, has now taken center stage—and its performance may well power our clean energy future.