Pourbaix diagrams


How come when I run a Pourbaix for a single element in water (Cu in this case) on EPH Web and then here on MP I dont get the same EPH diagram. The Cu+2 vertical boundary seem wrong on the chart generated here … could this be a bug ?


I don’t know about the provenance of EpH-Web diagrams, but you can read about ours here: https://materialsproject.org/docs/pourbaixdiagram.

One thing I noticed is that the default molality for EpH-Web vs MP is different (1e-6 vs 1e-8), and that certainly accounts for a large shift in the Cu[2+] vertical boundary position.

Thank you for your interest in and use of the Materials Project.


The problem is that the diagram isnt matching any of the diagrams at similar graphs in text books as well at same concentrations – the Cu system is one that is well studied and documented so there are lots of examples of what it should look like —the line should be vertical – its not in your app results…– to me it looks like there might be bug in the application or the data … but I will run some of the math and if I find out anything interesting I will certainly share it with you.

Thank you for making this program available … it is very impressive.


Thanks for the good question!

Indeed, there are differences, mainly due to two factors:

  1. EPH uses only experimental information, and most solids do not have a measured thermodynamic data. The Materials Project uses a combination of experimentally thermodynamic data for aqueous ions with computed enthalpies of formation for the solid states. Hence, the Materials Project has an order of magnitude more data on solids than any other resource. Please see the paper on Pourbaix diagram formalism for more details.

  2. That being said, one area where the Materials Project may have less accurate data is for hydroxides (which is one reason for the differences between the EPH and the Materials Project Cu diagram); we tend not to have good information on the proton positions, which can cause errors in the energy calculation of these compounds.

I hope this helps!


Ta doesn’t have aqueous ion information. I wonder if you can add this one or add me to the list of interested people!

Also I didn’t find the contact button at the bottom of the page but only the forum button. I guess they’re synonymous?


We revisited the elemental Pourbaix diagrams in order to update as well as improve the descriptions of the Pourbaix diagrams where needed.

We made the following changes:

  • Added more ions for Ni, Cu, S and Te.
  • Decided to change the oxide references of Se (from Se2O5 to Se metal) and Tc (from Tc2O7 to TcO2).
  • Added ions for Hf, Ta, Rh and Ir, which previously had no ion energies stored.

Thank you @as2362 and @kapersson for this update. See this post for information on how to download the updated reference data for use e.g. with pymatgen.