# Oxygen evolution reaction
A critical consideration in designing improved battery electrodes is structural stability at and slightly above operating temperatures. Oxygen evolution reactions (OERs) can lead to structural degradation of the electrode, reducing the number of cycles a potential electrode can withstand.
The MP API client has a feature to generate OER data for the intercalation electrode collection, and the analysis tools used there can be extended further to study electrode materials not in our database.\
\
**NB: the OER data in MP only includes thermodynamic contributions, and neglects kinetic factors.**
#### Methods
Following Ong *et al.*, we use the thermodynamic[ phase diagram](/methodology/materials-methodology/thermodynamic-stability/phase-diagrams-pds.md) to trace possible decomposition products, while permitting structural changes, as a function of the effective O2 chemical potential *μ\*(T)*. To map *μ\*(T)* for gaseous O2 to a temperature, we approximate the *PV* contribution to Gibbs energy as that of an ideal gas \[1]:
$$
\mu^\*(T) \approx \mu(T\_0, p\_0) + k\_B T\[1 - S(T)/k\_B + \ln(p/p\_0)]
$$
We use the NIST JANAF thermochemical tables for oxygen \[2], which provide entropies *S(T)* measured at *p*=0.1 MPa, and reference ensemble conditions at room temperature: *T**0*=298.15 K and *p**0* = 0.21 atm \[1]. Using a spline fit of *T(μ\*)*, we can invert this relationship reliably for a large range of temperatures.
#### Examples
For olivine FePO4 ([mp-19017](https://next-gen.materialsproject.org/materials/mp-19017)), the website shows OER mechanisms for all stable entries in the insertion electrodes entry for the corresponding electrode material, [mp-19017\_Li](https://next-gen.materialsproject.org/batteries/mp-19017_Li)
You can retrieve the same data using the API client `mp_api` as follows:\\
```python
from mp_api.client import MPRester
with MPRester("your_api_key") as mpr:
oxyevo = mpr.get_oxygen_evolution("mp-19017","Li")
```
The keys of `oxyevo` are the formulas of the stable entries in the electrode entry, and its values are `pandas`-friendly dicts of the OER thermodynamic data:
```python
import pandas as pd
print(pd.DataFrame(oxyevo["Fe4 P4 O16"]))
>>> mu reaction evolution temperature
0 -8.334987 FePO4 -> FeP + 2 O2 8.000000e+00 1395.096762
1 -7.054396 FePO4 -> FeP + 2 O2 8.000000e+00 925.094009
2 -7.054396 4 FePO4 -> 2 Fe2P2O7 + O2 1.000000e+00 925.094009
3 -6.761139 4 FePO4 -> 2 Fe2P2O7 + O2 1.000000e+00 812.674113
4 -6.761139 6 FePO4 -> Fe3(P2O7)2 + Fe3(PO4)2 + O2 6.666667e-01 812.674113
5 -4.947961 6 FePO4 -> Fe3(P2O7)2 + Fe3(PO4)2 + O2 6.666667e-01 0.000000
6 -4.947961 FePO4 -> FePO4 -3.151621e-15 0.000000
```
#### References
\[1] S.P. Ong, A. Jain, G. Hautier, B. Kang, and G. Ceder, "Thermal stabilities of delithiated olivine MPO4 (M = Fe, Mn) cathodes investigated using first principles calculations", *Electrochem. Commun.* 12, 427–430 (2010),
\[2] M.W. Chase, NIST-JANAF Thermochemical Tables, vol. 12, American Chemical\
Society, New York, 1998.
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