Concentration and Element Ratio in Multi-Element Pourbaix Diagrams

Hello, my name is Hideshi. It’s my first time posting here or in any other internet thread (github, stackexchange, etc) so please forgive me if I don’t adhere to community common sense.

I am currently interested in the Pourbaix diagram app, especially those of multiple components, such as Ru Ir mixed oxides for OER. However, I am confused with one thing:

If I change the ratio of Ru:Ir from 50%:50% to for example, 80%:20% using the composition slider bar, I am expecting the concentration [mol/kg] to also be affected. However, in the web application, the concentration slider remains the same as before.

  1. Is this because the concentration slider only shows the magnitude (1e-8, 1e-7, etc) and not the exact concentration?
  2. If so, how can I tell the concentration of the ions which the Pourbaix app calculates for me?

I tried reading the documentation of the Pourbaix application, but I still don’t really feel like I understand how to use the “concentration slider” and “composition slider” of this tool. I would really appreciate it if someone could help me out.

As a side note, congratulations on this really successful database. Last month, I saw it had 70,000 inorganic compounds, but as I am writing this, it has more than 83,000. Amazing!


Hi Hideshi, thanks for using the Materials Project,

The input to the pourbaix diagram formalism for the ion concentration is exactly as shown. This ion concentration influences the chemical potential term associated with the ion entries containing a given element, but it’s not necessary to couple this with composition, which is a constraint on the thermodynamic analysis of allowed phases in the pourbaix analysis. A more thorough discussion of the multielement formalism can be found here (from the citation in the documentation).

Let us know if you have further questions.

Dear Joseph,

Thanks for the detailed reply and the PDF. It seems my misunderstanding came not from the app but from the concept of multi-element phase diagrams itself. I understand better now, and I appreciate your help.

Thank you.