Electrocatalytic Mechanisms for Oxygen Reduction Reactions

The oxygen reduction reaction (ORR) lies at the heart of fuel cells, metal–air batteries and other clean energy technologies. Mechanistically, ORR proceeds via either a four-electron pathway, directly yielding water, or a two-electron route that forms peroxide intermediates. Electrocatalytic performance hinges on the ability of a catalyst to adsorb O₂, activate the molecule through sequential proton–electron transfers, and desorb final products while minimising side reactions. Key descriptors such as the d-band centre, coordination environment and electronic structure dictate adsorption energies of intermediates (OOH*, O*, OH*). Recent advances have focused on atomically dispersed metal–nitrogen–carbon frameworks, single-atom catalysts and heteroatom-doped carbons, all designed to enhance active site density and stability. In situ and operando spectroscopies now reveal dynamic transformations at working potentials, guiding rational design. The global push for decarbonisation has stimulated exploration of non-precious metal systems that combine high activity, durability and selectivity for practical ORR deployment.

Research from Nature Portfolio

Recent theoretical and experimental studies have demonstrated that hybridisation between metal d-orbitals and O₂ π* orbitals underpins activity at single Fe–N₄ centres. Advanced density functional theory calculations established descriptors—Fe–O bond length, d-band centre separation and magnetic moment—that accurately predict catalytic performance. Complementary experiments employing divacancy engineering in carbon matrices confirmed these relationships, enabling targeted tuning of active site electronics.

An axial coordination approach applied to iron phthalocyanine catalysts anchored on oxidised carbon supports has been shown to induce electronic localisation along the Fe–O axis. This strategy enhances O₂ adsorption and activation, yielding ultralow Tafel slopes of 27 mV dec⁻¹ and half-wave potentials up to 0.90 V. The work bridges fundamental insights and practical catalyst engineering.

In parallel, an in situ electrochemical protocol based on nitrite adsorption and reductive stripping has been developed to quantify active site density in Fe–N–C electrocatalysts under acidic conditions. A direct correlation between measured site density and turnover frequency provides a standardised method for rapid assessment and optimisation of emerging non-precious metal ORR catalysts.

Electrocatalytic Mechanisms for Oxygen Reduction Reactions publication trend

The graph below shows the total number of articles in electrocatalytic mechanisms for oxygen reduction reactions across all publications each year (not limited to Nature Index journals).

Technical terms

Active site density: Number of catalytic centres per unit mass or surface area participating in ORR.

Half-wave potential (E₁/₂): Potential at which the current reaches half its maximum, indicating ORR activity.

Tafel slope: Parameter describing the relationship between overpotential and current density, reflecting reaction kinetics.

Single-atom catalyst (SAC): Catalyst featuring isolated metal atoms on a support, offering maximal atom utilisation and unique electronic properties.

Four-electron pathway: Reaction mechanism reducing O₂ directly to water, minimising peroxide formation.

References

1. Advances on Axial Coordination Design of Single-Atom Catalysts for Energy Electrocatalysis: A Review. Nano-Micro Letters (2023).
2. In situ electrochemical quantification of active sites in Fe–N/C non-precious metal catalysts. Nature Communications (2016).
3. Iron phthalocyanine with coordination induced electronic localization to boost oxygen reduction reaction. Nature Communications (2020).
4. Zinc‐Mediated Template Synthesis of Fe‐N‐C Electrocatalysts with Densely Accessible Fe‐Nx Active Sites for Efficient Oxygen Reduction. Advanced Materials (2020).
5. Insights into the activity of single-atom Fe-N-C catalysts for oxygen reduction reaction. Nature Communications (2022).
6. Recent Progress on MOF‐Derived Heteroatom‐Doped Carbon‐Based Electrocatalysts for Oxygen Reduction Reaction. Advanced Science (2017).