The Materials Project has announced an exciting new tool – the Computational Pourbaix App! This app helps users assess stability in aqueous environments, including multicomponent systems. Below, we briefly introduce Pourbaix diagrams and how the Pourbaix App can be used.
A Pourbaix diagram, also frequently called a potenial-pH diagram, or E-pH diagram, is a representation of solid-aqueous phase electrochemical equilibria. It is a two-dimensional representation of a three-dimensional free energy-pH-potential diagram, depicting the stable phase (or combination of stable phases) as a function of pH and potential. Typically, it also displays the conditions under which water is stable, where it either oxidizes (O2 evolution) or reduces (H2 evolution) – see orange dashed lines in Fig 1.
The Pourbaix diagram is very useful tool to estimate aqueous stability of solids and for guiding synthesis of solids through precipitation as a function of the aqueous solution. To target certain conditions, the pH and potential of the solution can be modified adding acid (e.g. HCl), alkaline salts (e.g. NaOH) as well as oxidants (e.g. permanganate salts) or reducing agents (e.g. carbon, hydrazine).
The Pourbaix App is capable of plotting elemental (e.g. Ni), and multi-element Pourbaix diagrams (e.g. Ni-Al). The default ion concentration is 10-6 M and note that oxygen and hydrogen are included by default, since these elements are always “open” in a Pourbaix diagram as they are available in the water environment.
Example 1 – single element (Fe)
To know which solid forms of iron are likely to form under normal aqueous conditions, construct the Fe diagram in the Computational Pourbaix App by entering “Fe” in the Element field, and click on the Generate button.
In Figure 1 above you can see that Fe2O3 and Fe3O4 (also called ‘rust’ !) form as the only stable solid phases when Fe reacts with water at potentials higher than 0 V vs the standard hydrogen potential. We also observe (as is the case for many elements and materials) that the stable phases of iron in acid as well as alkaline conditions are dissolved ions (Fe2+, FeO22-, etc).
Example 2 – multi element (Fe-Cr)
Let’s try to generate and interpret a multi-element Pourbaix diagram. For example, enter “Fe Cr” in the Element field, and click the Generate button (Fig 2). Note that the default composition for solids is 50-50, e.g. equal amounts Fe and Cr. The solid composition axis is really a third dimension and we show slices for each composition in the Pourbaix App, depending on what you specify. By moving the bar (boundary) between the Cr (blue) and Fe (green) – you can change the composition rendered.
The multi-element Pourbaix diagram shows which phases (or combinations of phases) are stable as a function of aqueous conditions. For example, we can study which element is most prone to dissolve as a function of pH and potential. The passivation region (where only solid phases are stable) extends up to about 1 V and spans the region of 5 < pH and we observe that Cr generally resists dissolution more than Fe. Actually, stainless steels take advantage of this as they contain sufficient chromium to form a passive film of chromium oxide, which then prevents further surface corrosion by blocking oxygen diffusion to the steel surface and blocks corrosion from spreading into the alloy’s internal structure.