# The OER mechanism: four proton–electron steps

> Oxygen evolution on an oxide takes four proton–electron steps on one metal site, through HO*, O* and HOO*. Together they need 4.92 eV, 1.23 eV per electron; on a real catalyst the steps are uneven and the largest one sets the overpotential. Because HOO* and HO* bind together, oxides fall on one volcano, with a floor under their overpotential.

- Page: https://www.labcanvas.com/templates/electrocatalysis/oer-mechanism-free-energy-diagram
- Open in the LabCanvas editor (LabCanvas account): https://www.labcanvas.com/editor.html?template=oer-mechanism-free-energy
- Updated: 2026-10-14
- Category: Electrocatalysis
- Method: Reaction mechanism
- Starting materials: A metal site on an oxide surface, water (or OH⁻), an applied potential
- Product: O₂ and four protons and electrons (in acid), one per step
- Key result: Step energies illustrative (they add up to 4.92 eV); volcano schematic. Oxide surface from a perovskite (001) cell, a = 3.9 Å
- Figure: 1920 × 1080 px, 82 editable layers (5 generated objects with settings)

## Steps

1. **Step 1**: H₂O + * → HO* + H⁺ + e⁻
2. **Step 2**: HO* → O* + H⁺ + e⁻
3. **Step 3**: O* + H₂O → HOO* + H⁺ + e⁻
4. **Step 4**: HOO* → * + O₂ + H⁺ + e⁻ (Rossmeisl et al. 2007)
5. **Overpotential**: The four ΔG add up to 4.92 eV; the largest sets η = max ΔGᵢ/e − 1.23 V
6. **Scaling**: HOO* and HO* binding energies scale together on oxides: one volcano, one descriptor, a limit for planar oxides (Man et al. 2011)

## Strengths

- A simple model that explains activity trends across oxides
- Points to one descriptor for design

## Limitations

- Thermodynamics only: no barriers, no solvent
- Some oxides evolve oxygen from their lattice instead (Grimaud et al. 2017)

## Uses

- Screening OER catalysts
- Water electrolyzers
- Metal–air batteries

## References

1. Rossmeisl, Qu, Zhu, Kroes & Nørskov, “Electrolysis of water on oxide surfaces”, J. Electroanal. Chem. 2007. https://doi.org/10.1016/j.jelechem.2006.11.008 (primary)
2. Man, Su, Calle-Vallejo, Hansen, Martínez, Inoglu, Kitchin, Jaramillo, Nørskov & Rossmeisl, “Universality in Oxygen Evolution Electrocatalysis on Oxide Surfaces”, ChemCatChem 2011. https://doi.org/10.1002/cctc.201000397 (primary)
3. Suntivich, May, Gasteiger, Goodenough & Shao-Horn, “A Perovskite Oxide Optimized for Oxygen Evolution Catalysis from Molecular Orbital Principles”, Science 2011. https://doi.org/10.1126/science.1212858 (further reading)
4. Grimaud, Diaz-Morales, Han, Hong, Lee, Giordano, Stoerzinger, Koper & Shao-Horn, “Activating lattice oxygen redox reactions in metal oxides to catalyse oxygen evolution”, Nat. Chem. 2017. https://doi.org/10.1038/nchem.2695 (further reading)
5. Seh, Kibsgaard, Dickens, Chorkendorff, Nørskov & Jaramillo, “Combining theory and experiment in electrocatalysis: Insights into materials design”, Science 2017. https://doi.org/10.1126/science.aad4998 (review)

Credit: After Rossmeisl et al., J. Electroanal. Chem. 2007 (doi:10.1016/j.jelechem.2006.11.008) and Man et al., ChemCatChem 2011 (doi:10.1002/cctc.201000397). Oxide slab built from a perovskite (001) cell; step energies illustrative, volcano schematic.

## Questions

**What does the “The OER mechanism: four proton–electron steps” template show?** Oxygen evolution on an oxide takes four proton–electron steps on one metal site, through HO*, O* and HOO*. Together they need 4.92 eV, 1.23 eV per electron; on a real catalyst the steps are uneven and the largest one sets the overpotential. Because HOO* and HO* bind together, oxides fall on one volcano, with a floor under their overpotential.

**What are the steps shown in “The OER mechanism: four proton–electron steps”?** 1. Step 1: H₂O + * → HO* + H⁺ + e⁻. 2. Step 2: HO* → O* + H⁺ + e⁻. 3. Step 3: O* + H₂O → HOO* + H⁺ + e⁻. 4. Step 4: HOO* → * + O₂ + H⁺ + e⁻ (Rossmeisl et al. 2007). 5. Overpotential: The four ΔG add up to 4.92 eV; the largest sets η = max ΔGᵢ/e − 1.23 V. 6. Scaling: HOO* and HO* binding energies scale together on oxides: one volcano, one descriptor, a limit for planar oxides (Man et al. 2011).

**What are the starting materials and the product?** Starting materials: A metal site on an oxide surface, water (or OH⁻), an applied potential. Product: O₂ and four protons and electrons (in acid), one per step.

**Which papers is “The OER mechanism: four proton–electron steps” based on?** Rossmeisl, Qu, Zhu, Kroes & Nørskov, “Electrolysis of water on oxide surfaces”, J. Electroanal. Chem. 2007 (doi:10.1016/j.jelechem.2006.11.008); Man, Su, Calle-Vallejo, Hansen, Martínez, Inoglu, Kitchin, Jaramillo, Nørskov & Rossmeisl, “Universality in Oxygen Evolution Electrocatalysis on Oxide Surfaces”, ChemCatChem 2011 (doi:10.1002/cctc.201000397); Suntivich, May, Gasteiger, Goodenough & Shao-Horn, “A Perovskite Oxide Optimized for Oxygen Evolution Catalysis from Molecular Orbital Principles”, Science 2011 (doi:10.1126/science.1212858); Grimaud, Diaz-Morales, Han, Hong, Lee, Giordano, Stoerzinger, Koper & Shao-Horn, “Activating lattice oxygen redox reactions in metal oxides to catalyse oxygen evolution”, Nat. Chem. 2017 (doi:10.1038/nchem.2695); Seh, Kibsgaard, Dickens, Chorkendorff, Nørskov & Jaramillo, “Combining theory and experiment in electrocatalysis: Insights into materials design”, Science 2017 (doi:10.1126/science.aad4998).

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